Bismuth (III) nitrate pentahydrate (Bi(NO₃)₃·5H₂O ≥ 98%) from SIGMA-Aldrich (USA), triethanolamine (TEA, N(CH₂CH₂OH)₃ 97%), and thioacetamide (TA, CH₃CSNH₂ 99%) from Alfa Aesar (UK); ammonium hydroxide (NH₄OH 29%), and NH₃ from Fermont (MX). All analytical-grade chemicals were utilized, without any purification. Microscope glass slides (25 × 75 mm) were used as substrates to synthesize the different thin films.

Bi-based thin films were synthesized by CBD, varying the number of immersion baths. The first step in the deposition process involved a surface treatment of the microscope glass slides used as substrates. For that, glass slides were immersed in 10% HNO₃ (v/v) for 48 h and washed with deionized water to eliminate impurities and activate the surface, dried at room temperature, and carefully stored.

The bath solution is a mixture of an aqueous solution of bismuth (III) nitrate pentahydrate (0.01 M, 10 mL), triethanolamine (1 M, 5 mL), ammonium hydroxide (1 M, 1 mL), and thioacetamide (0.1 M, 2 mL) in deionized water to get a final volume of 100 mL.

For producing Bi-based thin films, the surface-activated glass substrates (two substrates in each bath) were immersed in the bath solution and placed in a water bath at 60°C for 3 h, as represented in Fig. 1. After completing the deposition period, the films were rinsed with deionized water and allowed to dry in the air at room temperature. To investigate the effect of different deposition layers, this procedure was consistently executed for the CBD technique’s one-immersion bath, two-immersion bath, and three-immersion bath, each time in fresh bath solutions. Throughout each immersion, the pH of the final volume was maintained at 9.

The UV-Vis technique was employed to characterize the Bi-based thin films across the 200–900 nm range using a Perkin Elmer Lambda 19 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was implemented to perform chemical analysis using a Perkin Elmer Phi 5600 ESCA system that was equipped with a magnesium (Mg) X-ray source. The X-ray diffractograms (XRD) were acquired using a Bruker D8 Advance X-ray diffractometer with CuKα radiation (1.5418 Å) with Bragg–Brentano geometry. A HITACHI TM3030Plus system was implemented to acquire the SEM micrographs and energy-dispersive X-ray spectroscopy (EDS) for la determination of the elemental composition. The surface morphology of all Bi-based thin films synthesized was analyzed using scanning electron microscopy (SEM) with a FEI model Scios operating at 10 kV. To enhance conductivity, all samples were sputter-coated with a thin gold layer before imaging. Both secondary electron (SE) and backscattered electron (BSE) modes were employed to reveal surface texture and compositional contrast.

Fig. 2 depicts the optical properties of Bi-based thin films fabricated by the CBD method with one, two, or three immersion baths. UV-Vis spectroscopy was measured in the wavelength range of 200–900 nm. The optical band gap energy (Eg) and absorption coefficient (α) were calculated from the transmittance data region. The Tauc plot (αhν)² versus photon energy (hν) was obtained by analyzing the UV-V is absorption spectra. The linear region of the plot was extrapolated to determine the optical band gap of the Bi-based thin films.

When comparing the results obtained for the one-, two-, and three-immersion bath thin films, a considerable absorbance increase was observed as the number of immersion baths increased. The three-immersion bath thin film exhibited the highest absorbance across the visible range, indicating a high light absorption, with a sharp absorption edge observed around 500–600 nm, as depicted in Fig. 2. These results indicate the influence of different refractive indices for phases present related to optical properties. Absorption coefficient is highly related to optical properties as absorption index and extinction coefficients, which are also influenced by thin-film fabrication method and structural characteristics of microstrain, interplanar spacings, phase, defects, and crystalline symmetry. It is evident that three-immersion bath thin film showed higher absorbance in the visible region compared to one- and two-immersion bath thin films, as depicted in Fig. 2.

To determine the band gap, the linear region of the plot was extrapolated to intersect the energy axis, (Fig. 2 (inset)), yielding an Eg value of approximately 2.07 eV and 2.3 eV, for the three- and two-immersion bath, respectively. These Eg values obtained for Bi₂O₂S thin films confirm its semiconductor nature [9]. These results indicate that the band gap decreases while the number of layers increases. This behavior suggests an influence of the crystal structure or electronic structure of the thin films, or an increase in defects of sites or oxygen vacancies, which in turn could influence the absorption properties. These results demonstrated that controlling the number of layers deposited effectively tunes the optical properties of Bi-based thin films, in this case, thin films of Bi₂O₂S, as demonstrated in Section 3.2, which is critical for optimizing performance in optoelectronic devices. From our findings, we demonstrate that the Bi₂O₂S thin films possess favorable optical properties, making them promising candidates for use in photovoltaic devices.

Fig. 3 depicts the XRD patterns of the Bi-based thin films prepared by chemical bath deposition with one-, two-, and three-immersion bath. The crystallinity and phase purity of the film were confirmed by distinct diffraction peaks corresponding to the orthorhombic phase of Bi₂O₂S (space group Pnnm 58), consistent with the standard JCPDS card No. 34-1493 and previous literature. Secondary phases of Bi₂O₃ and Bi₂S₃ were also detected, as evidenced by additional distinguishable peaks. Phase quantification based on rigorous peak deconvolution allowed the determination of the relative amounts of Bi₂O₂S, Bi₂O₃, and Bi₂S₃ present.

Table 1 summarizes the diffraction peak position (2θ), full width at half maximum (FWHM), and crystallite sizes calculated using the Scherrer equation. The main peaks observed at approximately 2θ = 22.21°, 25.74°, 31.23°, 43.79°, 52.67°, and 66.96° correspond to the (220), (310), (040), (520), (351), and (470) crystal planes of the Bi₂S₃ mixed with the Bi₂O₃ phase in the one-immersion bath thin film. Similarly, in the two-immersion bath thin film, peaks at 2θ = 24.91°, 31.29°, 46.16°, and 53.24° that correspond to the (110), (040), (060), and (112) crystal planes, confirming Bi₂O₂S formation. In the three-immersion bath thin films, the peaks at 2θ = 25.85°, 29.91°, and 53.45° well correspond to the (110), (040), and (112) crystal planes of Bi₂O₂S [6,10,11,12]. Notably, the Bi₂O₂S phase was absent in the one-immersion bath thin film, indicating that at least two immersion baths are required to induce the crystallization of this phase. These results demonstrate the successful formation of crystalline Bi₂O₂S by chemical bath deposition after two or more immersion baths.

The main diffraction peak corresponding to the (040) plane remains consistent for the two- and three-immersion bath thin film samples, appearing at approximately 30° (2θ). This consistency suggests that the underlying Bi₂O₂S crystal structure is stable across these deposition conditions. However, a significant decrease in full width at half maximum (FWHM) values was observed as the number of immersion baths increased, indicating improved crystallinity and grain growth. Additionally, as the immersion baths increase from two to three, the main diffraction peak shifts slightly toward lower 2θ angles, implying a slight expansion in the interlayer spacing. This shift may arise from interlayer interactions or strain induced by the stacking of layers. Furthermore, peak intensities become sharp and more pronounced with the increasing number of layers, reflecting larger average crystallite sizes and enhanced crystallinity in thicker films (Table 2). The average crystallite size increases from the two- to the three-immersion bath samples from approximately 19.33 nm to 48.27 nm as revealed by the Scherrer analysis of the (040) peak. These results suggest that grain growth improves as the number of layers increases. This is in accordance with the agglomeration of nanosheets observed in vacuum-dried samples of Bi₂O₂S and is correlated with a decrease in peak broadening in the two-immersion bath thin film (for additional details, refer to the Supplementary Materials (Figs. S1 and S2)) [13].

The occurrence of mixed phases such as Bi₂O₃ and Bi₂S₃ introduces heterointerfaces and lattice mismatches known to serve as recombination centers, which could be responsible for potentially reducing photovoltaic efficiency. Each phase exhibits a distinct electronic bandgap (Bi₂O₃: 2.6–3.0 eV, Bi₂S₃: 1.56–2.10 eV, Bi₂O₂S: 1.12–2.5 eV), which modifies optical absorption edges and carrier dynamics, thereby influencing device optimization [14,15,16,17,18,19]. In photovoltaic applications, phase purity and microstructural quality directly influence charge separation and transport [20,21]. Therefore, Williamson-Hall analysis is essential for precise structural understanding and elucidating stress-induced modifications in band structure and defect recombination pathways, which significantly affect device performance.

The appropriate bandgap of Bi-based thin films (1.5 to 3.0 eV) has garnered significant research interest as a promising semiconductor for solar energy conversion, enabling efficient absorption of visible light. The layered structure of Bi₂O₂S facilitates effective charge separation and transport, thereby enhancing photo-response and photovoltaic efficiency [22,23]. Moreover, Bi₂O₂S exhibits exceptional charge carrier lifetimes and high chemical stability, both critical for the long-term operation of solar devices [24,25]. Recent studies have demonstrated that prototype Bi₂O₂S-based solar cells and photodetectors can achieve high photocurrent densities and rapid response times, attributed to the material’s favorable optoelectronic properties and crystalline quality [26]. In this study, structural characterization of the synthesized Bi-based thin films was performed by integrating unit cell volume calculations derived from X-ray diffraction (XRD) data with stoichiometric and oxidation state information obtained through X-ray photoelectron spectroscopy (XPS, in Section 3.3). Unit cell volumes were calculated for the three phases present Bi₂O₃, Bi₂S₃, and Bi₂O₂S, in all the thin films synthesized (one-, two-, and three-immersion bath), using lattice parameters and crystal symmetries appropriate to each phase. These calculations allowed accurate estimation of phase-specific structural parameters (Table 3). Variations in unit cell volumes correlated subtly with the number of immersion baths, reflecting lattice expansions or contractions associated with phase growth dynamics.

As the number of immersion baths increases, distinct volumetric behaviors emerge, linked to atomic arrangements and bonding environments. Bi₂O₃ exhibits substantial unit cell volume expansion up to approximately 1280 Å after three immersion baths, likely driven by structural defects, internal stresses, and interlayer interactions during film growth. It is conceivable that Bi₂O₃ serves as a template for Bi₂O₂S formation in the two- and three-immersion bath thin films, where the ordered Bi₂O₂ base promotes the oriented growth and crystallinity of Bi₂O₂S. The unit cell volume of Bi₂O₂S after two immersion baths (~1317 Å) signifies notable lattice expansion at this stage.

Further stabilization in three-immersion bath samples may arise from a passive Bi₂O₂ layer (~169 Å), acting as a protective barrier that mitigates mechanical stress and degradation while enhancing adhesion and structural integrity. This templating and passivation mechanism aligns with phenomena in oxides and oxychalcogenides where hydroxylated intermediate phases encourage ordered growth and improved stability. So, the physical properties and structural stability of these bismuth-based compounds are significantly influenced by crystal structure and the number of immersion baths.

In comparison, Bi₂S₃ demonstrated a more moderate volume increase (~437 Å after three immersion baths), maintaining better dimensional integrity, likely due to reduced lattice distortion or stronger atomic bonding. Bi₂O₂S, crystallizing orthorhombically, exhibits the smallest unit cell volumes among the phases, with a dense layered architecture (~169 Å after three immersion baths) that supports efficient growth with minimal expansion. These structural characteristics underpin superior stability and reduce susceptibility to stress-induced lattice distortions of Bi₂O₂S. While all three compounds undergo volume increases with the number of immersion baths increases, the magnitude and growth patterns differ by crystallographic symmetry and atomic configuration, impacting their electronic and mechanical properties.

These volumetric trends robustly indicate phase stability and structural evaluation during film deposition. Simultaneously, as shown in Section 3.3, XPS-derived stoichiometric ratios (Bi:O:S) enabled empirical formula calculation for each phase (Table 3) confirming their presence and relative abundance. The oxidation states resolved from XPS core-level spectra correlated strongly with stoichiometric shifts, reflecting phase transformations-Bi (III) states aligned with oxygen-rich phases (Bi₂O₃, Bi₂S₃, Bi₂O₂S), while sulfur states correlated to sulfide phases.

The Sillén structure of Bi₂O₂S, characterized by alternating [Bi₂O₂] layers and sulfur planes, directly influences the material’s crystallographic and morphological properties as revealed by X-ray diffraction (XRD) and scanning electron microscopy (SEM, in Section 3.4) analyses. XRD patterns confirm the presence and evolution of the Sillén phase, showing progressive sharpening and intensification of diffraction peaks as the number of immersion baths increases, indicating enhanced crystallinity and lattice ordering. Complementary, SEM images illustrate the morphological transformation from isolated and scattered structures in the initial deposition stages to densely packed and well-aligned nanostructures upon completion of multiple immersion baths. This morphological evolution correlates with the layered crystal framework of the Sillén phase, which promotes anisotropic growth and directional assembly of nanostructures. Together, XRD and SEM analyses validate the growth mechanism rooted in the Sillén structure, providing a comprehensive understanding of the structural and morphological properties essential for optimizing Bi₂O₂S thin films in photovoltaic applications.

This comprehensive structural analysis established that the number of immersion baths critically influences crystallite size, interlayer spacing, and crystallinity of Bi₂O₂S nanosheets, with implications for their optoelectronic properties. To optimize photovoltaic efficiency, controlling phase purity is essential, as secondary phase formation introduces detrimental electronic and optical heterogeneities.

The XPS measurements provide important information on the oxidation state of Bi in the bismuth-based thin films. The XPS signals were calibrated using the C 1s peak of the adventitious carbon at 285 eV, and the signals were deconvolved using the Shirley method. The contributions of Bi 4f are identified at different energies (BE) and have distinctly different features as shown in Fig. 4a–c. The elemental composition and chemical states of the elements present in the Bi-based thin films were determined. The XPS survey showed the occurrence of Bi 4f, S 2p, and O 1s core-level (for additional details, refer to the Supplementary Materials, Fig. S3). High-resolution spectra of Bi exhibited the characteristic peaks corresponding to Bi 4f_5/2 and Bi 4f_7/2 spin-orbit doublet. These peaks are indicative of Bi +3 and +5 oxidation states, consistent with the formation of Bi-based compounds [24].

The O 1s spectrum showed a peak around 530 eV, attributed to lattice oxygen in the bismuth oxide structure. No signals related to metallic bismuth were found, indicating a predominantly oxidized surface layer. Quantitative analysis revealed a Bi:O atomic ratio close to the stoichiometric ratio expected for bismuth oxide and bismuth oxysulfide. The S 2p feature of sulfur in the S²⁻ oxidation state confirmed the sulfide ions for forming the Bi-based film formation. He et al. reported that the spectra for Bi³⁺ are characterized by a narrow, single line around 158.38 eV and 163.75 eV, which can be used to confirm the presence of Bi (III) oxidation state from the Bi₂S₃ and Bi₂O₃. These XPS results complement structural analysis performed by X-ray diffraction [27].

For the one-immersion bath thin film, the high-resolution spectra of Bi 4f_5/2 corresponding to Bi⁵⁺ exhibited two peaks at 168.56 eV and 163.19 eV, respectively. Also, the Bi 4f_7/2 attributed to Bi³⁺, exhibited two peaks 167.14 eV and 161.75 eV, with a spin-orbit of 5.37 eV. This peak corresponds to Bi in the 3+ oxidation state and is consistent with the occurrence of oxidized bismuth in the film [28]. The O 1s spectrum shows a single feature centered at 534.86 eV, oxygen bonded in a Bi-O structure, and oxygen vacancies [29]. The S 2p spectrum has a peak at 165.76 eV characteristic of S²⁻ oxidation state, which confirms the incorporation of sulfur into the lattice.

For the two-immersion bath thin film, the high-resolution spectra for Bi 4f_5/2 corresponding to Bi⁵⁺ exhibited two peaks at 163.86 eV and 158.49 eV, respectively. The O 1s spectrum shows a single feature centered at 531.85 eV, attributed to oxygen vacancies within the lattice oxygen bonded in the Bi-O structure. The S 2p spectrum has a peak at 160.87 eV characteristic of S²⁻ oxidation state, which confirms the incorporation of sulfur into the lattice.

For the three-immersion bath thin film, the high-resolution spectra of Bi 4f_5/2 corresponding to Bi⁵⁺ exhibited two peaks at 167.92 eV and 162.4 eV, respectively. Also, the Bi 4f_7/2 attributed to Bi³⁺ exhibited two peaks at 166.54 eV and 161.2 eV. The S 2p spectrum has a peak at 165.21 eV characteristic of S²⁻ oxidation state, which confirms the incorporation of sulfur into the lattice. The O 1s spectrum shows a single feature centered at 529.27 eV, attributed to lattice oxygen bonded in the Bi-O structure and oxygen vacancies. XPS confirmed the oxidation states of bismuth, with in the case of Bi₂O₃, Bi₂S₃, and Bi₂O₂S, the Bi 4f spin-orbit doublet was successfully assigned to Bi 4f. Further deconvolution of each of the Bi 4f doublet peaks for one, two, and three immersion baths allows us to assign these features to the Bi (III) state. The results are consistent with those reported by Samanta and Biswas [30] in the case of homologous (Bi₂)_m(Bi₂S₃)_n heterostructures.

When comparing the XPS signals of the three types of synthesized thin films, a notable shift is observed in the bismuth doublet (Bi 4f), strongly dependent on the number of immersion baths performed using the CBD technique. However, the most critical shift corresponds to the film obtained after two immersion baths. The binding energy shift to lower values (~158 eV) observed in the two-immersion bath thin film can be attributed to Bi³⁺ state coordinated with sulfur, confirming the successful crystallization of the bismuth sulfide Bi₂S₃ in the bismuthinite phase. Thermodynamically, the second immersion cycle promotes the lateral coalescence of initial isolated islands into a continuous and highly homogeneous layer. From an electronic perspective, replacing lattice oxygen with the less electronegative sulfur increases the electron density shielding around the bismuth atomic cores, which effectively decreases the binding energy associated with the core electrons. On the other hand, the pronounced shift toward higher binding energies exceeding 163 eV in the one-immersion bath sample is intrinsically linked to its discontinuous, ultrafine morphology. In this initial stage, nucleated islands leave exposed substrate areas, leading to a highly de-shielded electronic environment. This effect is driven by the strong electron-withdrawing nature of oxygen and the stabilization of higher oxidation states (such as Bi⁵⁺) within the uncoalesced Bi₂O₃ network [31]. Furthermore, a distinct intermediate trend at 162 eV appears for the three-immersion bath thin film. At this stage, the over-deposition of material shifts the growth vertically, increasing surface roughness and grain size. This thicker film introduces mass diffusion limitations for the incoming sulfur species, restricting a complete topotactic anion exchange down to the deepest layers and forcing the coexistence of both, where the binding energy difference (see Table 4) is attributed to the bismuth, which is predominant in the +3 oxidation state in the successful formation of the Bi₂O₂S phase [32].

Zatsepin et al. reported the study of solid-state interactions between Bi and the oxygen sublattice by Bi-implanted samples against those of the native metal and stable metal oxide phases, specifically examining the Bi 4f core-level spectra [33]. As is well known, XPS core-level spectra of not oxidized metals have strong asymmetrical line shapes, which is a key signature to identify the metal phase in thin films. The formation of oxidized bismuth species occurs when bismuth is fully oxidized to the +3 oxidation state and can be produced from bismuth in its metallic state. At this stage, several polymorphs of Bi₂O₃ exist, with their stability dependent on temperature and synthesis conditions. A partially oxide state will depend on spatial constraints due to ionic radius mismatch in Bi₂S₃ or Bi₂O₂S. The formation of oxidized bismuth species implies enough oxygen availability for oxygen and Bi atoms that can occupy substitutional or interstitial sites, allowing full oxidation and crystal restructuring. The formation of Bi₂O₂S requires sulfur incorporation in addition to oxygen, and the existence of oxidized Bi is detected by XPS.

The valence states of Bi, S, and O detected by XPS are consistent with Bi³⁺, S²⁻, and O²⁻, confirming the formation of the Bi₂O₂S compound. Observed shifts in BE values are attributed to the lattice changes due to sulfur incorporation and consequent variations in local chemical environment and electric potential. Rong et al. [28]. reported the formation of Bi₂O₂S flowers, and from their XPS findings, it was possible to corroborate the production of Bi₂O₂S flowers-like structures in our results. As can be seen from Fig. 4b, the peak at approximately 162.40 eV and 157.06 eV with spin-orbit splitting of 5.34 eV well corresponds to the binding energy of Bi 4f_7/2 and Bi 4f_5/2, respectively, and are assigned to the Bi³⁺. It is evident from the O 1s spectrum that oxygen is part of the lattice oxygen. The S 2p in Fig. 4 depicts a peak at 160.87 eV that is in good agreement with the S²⁻ oxidation estate, see Table 4 for the corresponding values of one-, two-, and three-immersion bath thin films.

Fig. 5a–c, corresponding to the SEM analysis, depicts the evolution of Bi-based thin films. In the case of the one-immersion bath thin film, the surface morphology displays small and discontinuous features, with sparse nucleation sites observed as irregular aggregates. Distinct nucleation centers create flower-like structures. A partial substrate coating and minimal growth are suggested by the absence of continuous coverage. Contrastingly, the two-immersion bath sample exhibits a significant morphological transformation. The hierarchical, porous, and high-surface-area structures observed are associated with the formation of the Bi₂O₂S phase. These results are consistent with those obtained by X-ray diffraction (XRD). The preferential growth along defined crystallographic directions is facilitated by the layers of Bi₂O₂S, which leads to a more uniform and anisotropic morphology. Concerning the three-immersion bath sample, a denser and more compact film was obtained. Nanostructures coalesce to diminish the definition of flower-like structures and enhance the uniformity of surface coverage. Compared to the two-immersion bath sample, this evolution results in a smoother topography with reduced porosity. According to the XRD analysis, the three-immersion bath sample is mainly composed of an oxidized bismuth phase that is distinctive from Bi₂O₂S. This phase is likely responsible for the denser particle growth and morphological changes that were observed.

At last, the deposition cycle number significantly impacts the morphology of the film, progressing from sparse nucleation (one-immersion bath) to hierarchical growth (two-immersion bath) and compact films (three-immersion bath). XRD results are consistent with these morphological transitions, which have the potential to remarkably influence the optical and electronic properties of the films. Due to its uniformity and phase purity, the two-immersion bath Bi₂O₂S phase thin film exhibits promising characteristics for photovoltaic applications.

The growth mechanism of Bi₂O₂S thin films is intrinsically linked to the Sillén phase crystal structure, which consists of alternating layers of Bi₂O₂ units and sulfur planes arranged in a tetragonal lattice. This distinctive layered architecture facilitates a layer-by-layer growth process, where the initial formation of a metastable oxide-rich layer transitions into a well-ordered Bi₂O₂S phase upon progressive sulfur incorporation. The strong coordination within the Bi₂O₂ layers and the weak electrostatic interactions with the sulfur planes enable anisotropic crystal growth and lattice ordering characteristic of the Sillén structure. This layered framework acts as a structural template that governs the phase transformation and the morphological evolution observed experimentally by XRD and SEM, promoting the formation of nanoneedle arrays with enhanced crystallinity and reduced defects. Thus, the unique features of the Sillén phase are crucial for understanding and controlling the growth dynamics, as well as optimizing the properties of Bi₂O₂S thin films for high-performance optoelectronic applications.

Fig. 6 shows a comparison of the measurements of SEM and EDS for the cross-section of the three-immersion bath thin film, prepared by the CBD technique. The visualization of the sample undergoes significant changes. It is possible to precisely determine their widths of 35 to 53 nm. However, it was impossible to distinguish all the individual layers.

In this study, a sequential, number of immersion baths dependent transformation process is facilitated by a metastable intermediate phase during the chemical bath deposition (CBD) of Bi-based thin films. As shown in the sections described above, this multistep evolution is evidenced by structural, morphological, and microstructural changes detected through complementary techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The growth mechanism is driven by progressive sulfur incorporation, increased crystallinity, and morphological refinement induced by successive immersion baths. Initially, the high oxygen activity in the chemical bath promotes nucleation of a metastable Bi₂O₂ phase during the initial deposition stage. Early SEM observations display isolated, short Bi₂O₂S flower-like structures randomly distributed on the substrate, see Fig. 5a, characteristic of initial nucleation with limited surface coverage and connectivity, reflective of a porous early-stage film morphology. Backscattered electron imaging, see Fig. 7, enhances compositional contrast, confirming the bismuth-rich nature of these flower-like structures. This phase seems to act as a structural intermediate, alongside transient secondary phases of Bi₂O₃ and Bi₂S₃, facilitating sulfur incorporation to form Bi₂O₂S. The Bi₂O₂ lattice offers a compatible environment, minimizing disruption during anion exchange and enabling transition to the Bi₂O₂S structure.

The first immersion bath occurs in an oxygen-rich environment, resulting in the formation of a laminar, metastable Bi-based oxide phase, likely Bi₂O₂(OH)₂ and/or Bi₂O₂, arising from partial hydrolysis of Bi³⁺ ions as confirmed by XPS. This phase serves as a structural template for subsequent sulfur incorporation. XRD patterns at this stage exhibit broad, low-intensity reflections corresponding to the (040) and (112) planes of Bi₂O₂S, suggesting poor crystallinity and incomplete sulfurization. Residual Bi₂O₃ may also be present as a secondary phase. SEM images reveal isolated, short nanoneedle-like structures randomly distributed on the substrate, characteristic of initial nucleation with limited surface coverage and connectivity, typical of early-stage, porous film morphology.

During the second immersion bath, the Bi₂O₂(OH)₂ scaffold undergoes a topotactic transformation driven by the increased availability of sulfide ions (S²⁻) released from thiosulfate or thiourea precursors in the chemical bath, as supported by XPS evidence indicating progressive sulfur incorporation replacing hydroxide or oxygen anions within the interlayer space. This substitution occurs while maintaining the underlying crystallographic orientation, facilitating the transition to the crystalline Bi₂O₂S phase.

The XRD patterns of two-immersion bath films exhibit more distinct and sharper peaks indexed to the (040), (060), and (112) planes of Bi₂O₂S, confirming improved crystallinity. Minor Bi₂S₃ phases may coexist, indicating ongoing competition between sulfide and oxide phase formation. SEM analysis, see Fig. 5b, shows that the nanoneedles become longer, more aligned, and more densely packed, reflecting enhanced anisotropic growth and grain connectivity. This intermediate stage is thus characterized by partial phase conversion and increased structural ordering.

The incorporation of sulfur is completed upon the third immersion bath, favoring the formation of well-crystallized orthorhombic Bi₂O₂S. The partially formed Bi₂O₂S layers act as a structural template, facilitating epitaxial-like or oriented growth during subsequent deposition. At this stage, XRD patterns exhibit sharp, intense reflections at 2θ values of approximately 30.46°, 45.23°, and 58.11°, corresponding to the (040), (414), and (132) crystallographic planes, respectively, indicative of the dominant Bi₂O₂S phase presence. A systematic shift toward higher angles, alongside a pronounced reduction in peak full width at half maximum (FWHM), suggests enhanced crystalline, decreased interplanar spacing, and reduced lattice strain.

SEM images, see Fig. 5c, reveal a continuous, interconnected network of elongated, densely packed nanoneedles. The Williamson-Hall analysis confirms a reduction in microstrain and dislocation density consistent with the dense morphology and anisotropic growth. The film thus becomes more compact, uniform, and structurally coherent following three immersion baths.

A step-by-step reaction mechanism is proposed as follows: (1)

The initial hydroxylation of the bismuth precursor and the formation of the bismuth oxide (Eq. (1))

(2)

On the other hand, the chemical decomposition of the sulfur source (ionic hydrolysis of thioacetamide) in the bath to yield active ions (Eq. (2))

(3)

Then, the subsequent topotactic anion exchange leading to the formation of crystalline bismuth oxyhydroxide (Eq. (3))

(4)

Alternatively, the formation of the bismuth sulfide (Eq. (4)), and finally

(5)

The formation of the bismuth oxysulfide (Eq. (5))

A clear phase transformation mechanism is evidenced by the gradual transition from a disordered, oxide-rich initial layer to a well-defined Bi₂O₂S nanoneedle array with successive immersion baths. Each additional immersion bath promotes morphological refinement, lattice ordering, and improved phase purity. These progressive enhancements culminate in nanostructured films that exhibit optimized morphologies, reduced lattice defects, and superior crystallinity. Such structurally coherent films are highly promising for optoelectronic applications, particularly in solar energy conversion, where their enhanced electrical and optical properties can significantly improve device performance.

The combined analysis of XRD and SEM results supports the proposed phase transformation and growth mechanism of Bi₂O₂S thin films with increasing immersion baths. XRD patterns reveal a progression from broad, low-intensity peaks in early immersion bath, indicating poor crystallinity and incomplete sulfur incorporation to the structure, intense reflections characteristic of well-crystallized orthorhombic Bi₂O₂S at higher immersion baths, confirming improved lattice ordering and purer phase. These structural improvements coincide with SEM observations, which show an evolution from isolated, short, flower-like structures with low surface coverage to densely packed, elongated, and interconnected nanoneedles forming a uniform network. This morphological refinement reflects anisotropic grain growth and coalescence, consistent with reduced lattice strain and defect densities evidenced by Williamson-Hall analysis. Together, the structural and morphological data elucidate a immersion baths-dependent growth pathway wherein a metastable oxide-rich phase serves as a template that transforms via sulfur incorporation into a well-ordered Bi₂O₂S array. This sequential transformation is directly linked to improved crystallinity, reduced microstrain, and the formation of a compact, uniform film morphology suitable for high-performance optoelectronic applications.