A Comprehensive Study of the Optical, Structural, and Morphological Properties of Chemically Deposited ZnO Thin Films

Abstract

Zinc oxide (ZnO) is a wide bandgap semiconductor with optoelectronic and photocatalytic properties, which depend on its optical, structural, and morphological characteristics. In this study, we synthesized ZnO thin films by chemical bath deposition (CBD) and then thermally annealed them at 400 °C and 600 °C to evaluate the effect of thermal treatments. We characterized their structural, optical, morphological, and chemical properties using X-ray diffraction (XRD), UV–Vis spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The optical bandgap values were 3.20 eV for the as-grown thin films, and 3.23 eV and 3.21 eV after annealing at 400 °C and 600 °C, respectively. SEM micrographs revealed a change from elongated agglomerates in the as-grown thin films to uniform flower-like structures after annealing at 600 °C. XPS analysis confirmed ZnO formation in all samples, and we detected residual precursor species only in the as-grown thin films, which were completely removed by annealing at 600 °C. These results demonstrate that the CBD synthesis of ZnO can tune its optical and morphological properties through thermal annealing, making it suitable for optoelectronic, sensing, and photocatalytic applications.

1. Introduction

Zinc oxide (ZnO) is an n-type semiconductor with a direct bandgap from 3.20 to 3.40 eV [1,2], making it an attractive material for a wide variety of optoelectronic applications, including light-emitting diodes (LEDs), lasers, and photodetectors, due to its high emission efficiency and thermal stability, which allow it to operate at elevated temperatures [3,4,5,6]. Furthermore, ZnO exhibits piezoelectric properties, allowing its use in gas sensors and biosensors [7,8]. Moreover, its broad applicability also extends to renewable energy technologies implemented in solar cells [9] and as a photocatalyst for the degradation of environmental contaminants [10,11,12,13]. One challenge commonly reported for ZnO thin films is the aging effect caused by oxygen adsorption at the active surface on the thin film [14,15,16]. This phenomenon can cause rapid variations in both optical and electrical properties depending on ambient humidity and exposure time [15]. In practical device applications, such effects are typically mitigated through surface passivation strategies such as encapsulation [14,15] or protective coatings [16], which ensure the long-term stability of ZnO-based devices.

Structurally, ZnO is an interesting material that can crystallize in the wurtzite, zincblende, and rocksalt phases. The hexagonal wurtzite phase, being the most thermodynamically stable under ambient conditions, is the most common [17,18]. The literature reports preferred orientations along the (100), (002), and (101) crystalline planes. However, the preferential orientation is not a fixed characteristic of ZnO. The growth parameters, such as temperature and precursor concentration, strongly influence it. For example, the (002) orientation typically dominates at temperatures above 140 °C, whereas the (101) orientation becomes more predominant at lower synthesis temperatures [18].

A variety of synthesis routes have been reported for ZnO thin films, including thermal evaporation [19], spray pyrolysis [8,13], sol–gel [12,20], SILAR (Successive Ionic Layer Adsorption and Reaction) [21,22], spin coating [23], chemical bath deposition (CBD) [5,7,24,25], and sonochemical synthesis [6,10]. However, CBD shows up for its simplicity, low cost, scalability, and ability to produce uniform thin films without the need for complex equipment or high-vacuum environments. Also, CBD offers facile control over sample thickness, stoichiometry, and crystal growth by adjusting parameters such as pH, precursor concentration, and deposition time, qualities that make it attractive for both fundamental research and industrial applications.

In this work, ZnO thin films were synthesized by chemical bath deposition (CBD) using zinc acetate as the metal precursor. We investigated their structural, optical, chemical, and morphological properties using UV–vis diffuse reflectance spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Unlike many previous CBD-based studies, this work highlights how annealing not only removes hydroxide-related residuals but also induces morphological reorganization, transforming elongated agglomerates into compact flower-like structures with higher symmetry and uniformity after thermal annealing. These morphological changes, attributed to ZnO complete crystallization and enhanced atomic mobility at higher temperatures, directly impact light scattering, surface reactivity, and carrier transport, which are key factors that govern the performance of ZnO thin films in optoelectronic, sensing, and photocatalytic applications [3,4,5,6,7,8,9,10,11,12,13,26,27]. In the present study, we avoided encapsulation to isolate and analyze the intrinsic effects of thermal annealing on the structure, composition, and morphology of ZnO growth by CBD. However, it is important to note that in device fabrication, encapsulation remains a common strategy to enhance long-term stability. The novelty of this study lies in demonstrating the combined role of precursor selection and thermal annealing conditions in enhancing the structural crystallinity, chemical purity, and surface morphology stability of ZnO samples.

2. Materials and Methods

2.1. Materials

This study synthesized ZnO thin films using zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O, purity 99%, CAS No. 5970-45-6) as the zinc precursor. Triethanolamine (TEA, CAS No. 102-71-6) was employed as the complexing agent, while ammonium hydroxide (NH₄OH, 29%, CAS No. 1336-21-6) was used as the hydroxide ion source. All reagents, except NH₄OH, were dissolved in deionized water to prepare homogeneous aqueous solutions and were used without further purification.

2.2. Synthesis Procedure

The chemical bath deposition (CBD) method promotes nucleation and thin-film growth on glass substrates by chemical reaction of a metal ion source, a complexing agent, and a hydroxide ion source.

The synthesis of ZnO thin films involves adding 12 mL of 0.5 M zinc acetate solution and 3 mL of 29% ammonium hydroxide. The ammonium hydroxide ensures a strongly alkaline medium (pH~11), generating hydroxide ions (OH⁻) and promoting the precipitation of Zn(OH)₂ as an intermediate phase. A high pH drove the hydrolysis of Zn²⁺ ions and prevented uncontrolled precipitation, favoring the subsequent dehydration of Zn(OH)₂ into ZnO. Subsequently, the addition of 5 mL of 1 M triethanolamine (TEA) into the solution as a complexing agent regulated molecular assembly and promoted controlled film growth.

All reagents were dissolved in 40 mL of deionized water, initiating deposition on glass substrates at 70 °C for two hours. Following this stage, the substrate was removed from the chemical bath and placed into a newly prepared solution for an additional hour at 70 °C under identical conditions. Although we did not monitor the pH continuously during the process, the initial pH measurement confirmed alkaline conditions throughout the thin film deposition.

The proposed reaction mechanism consists of the following steps:

• Dissociation of zinc acetate

  Zn(CH₃COO)₂∙2H₂O → Zn²⁺ + 2CH₃COO⁻ +2H₂O,

  (1)

2.

Dissociation of ammonium hydroxide

NH₄OH ↔ NH₃ + H₂O ↔ (NH₄)⁺ + OH⁻,

(2)

3.

Formation of Zn(OH)₂

Zn²⁺ + 2OH⁻ → Zn(OH)₂,

(3)

Zn(OH)₂ + 2NH₄OH → [Zn(OH)₄]²⁻ + 2NH₄⁺,

(4)

[Zn(OH)₄]²⁻ + 2NH₄⁺ → Zn(OH)₂ + 2NH₃ + 2H₂O,

(5)

4.

Formation of the zinc–TEA complex ion to avoid precipitation and obtain ZnO thin films.

Zn²⁺ + C₆H₁₅NO₃ → [Zn(C₆H₁₅NO₃)₂]²⁺,

(6)

[Zn(C₆H₁₅NO₃)₂]²⁺ + 2OH⁻ → Zn(OH)₂ + 2C₆H₁₅NO₃,

(7)

Zn(OH)₂ → ZnO + H₂O,

(8)

Although the reaction involves the dehydration of Zn(OH)₂ into ZnO and H₂O, XPS and XRD analyses revealed residual precursor species in the as-grown thin films, which indicates that secondary phases or impurities can coexist under low-temperature CBD conditions. The presence of hydroxide and intermediate species (e.g., Zn(OH)₂ and TEA-derived organics) remained in the as-deposited films because chemical bath deposition occurs below the boiling point of water. To eliminate these residues and enhance crystallization of the wurtzite ZnO phase, thermal annealing at 400 °C and 600 °C was applied to the samples. After synthesis, all ZnO samples were dried and stored under ambient laboratory conditions, with a relative humidity of approximately 35–45%. Post-deposition thermal annealing was then applied in each case for two hours, followed by natural cooling to ambient temperature. The resulting thickness of the ZnO films was 5.37 ± 0.11 mm for the as-deposited films, and 4.50 ± 0.30 mm and 4.23 ± 0.21 mm for the samples annealed at 400 and 600 °C.

Table A1. Precursors, concentrations, and volumes used for the chemical bath deposition of ZnO thin films.

Reagent	Molar Concentration	Volume
Zn(CH3COO)2	0.5 M	12 mL
NH4OH (29%)	-	3 mL
C6H15NO3	1 M	5 mL
Deionized water	-	40 mL
Zn(CH3COO)2	0.5 M	12 mL

. Shows the order of addition, molar concentrations, and volumes of the reagents used in ZnO synthesis.

2.3. Characterization

The structural, optical, chemical, and morphological properties of the ZnO thin films were analyzed using various characterization techniques. Optical properties were measured using a Lambda 19 PERKIN ELMER double-beam UV–Vis spectrophotometer (PerkinElmer Inc., Waltham, MA, USA), equipped with an integrating sphere, which is ideal for measuring diffuse reflectance spectra of optically opaque samples in the wavelength range of 300 to 800 nm. X-ray diffraction (XRD) measurements were performed with a Rigaku Geigerflex D/Max-B diffractometer (Rigaku Corporation, Tokyo, Japan), employing a CuKα radiation source (λ = 1.5406 Å) and a graphite monochromator, operated at 40 kV and 20 mA. The chemical composition and surface analysis of the ZnO thin films were performed using a Φ Phi 5100 X-ray Photoelectron Spectrometer (LVAC-PHI, Chigasaki, Kanagawa, Japan; distributed by ULVAC-PHI, Inc., Chanhassen, MN, USA). This system operates under ultra-high vacuum (UHV) conditions, with a base pressure of 2.1 × 10⁻⁹ Torr and is equipped with a magnesium X-ray source (Perkin Elmer), operating in the energy range of 0 to 1100 eV. Prior to measurement, argon ion sputtering cleaned the surface of ZnO thin films under UHV conditions. However, the presence of carbon and oxygen peaks appeared due to their adsorption onto the sample surface from the ambient environment. The thickness of the thin films was measured by optical profilometry using a Bruker Contour GT-i optical profilometer (Bruker Nano Surfaces Division, Tucson, AZ, USA). Finally, morphology was analyzed using a Thermo Scientific Phenom ProX scanning electron microscope (Thermo Fisher Scientific, Eindhoven, The Netherlands), which offers an electronic magnification range from 80× to 130,000×, a spatial resolution of ≤10 nm, and operates at selectable accelerating voltages of 5 kV, 10 kV, and 15 kV.

The characterization sequence was as follows: diffuse reflectance spectroscopy (UV–Vis) was performed immediately after drying, XRD and SEM analyses were obtained within the following 48 h, and XPS characterization within the next 48 h. This procedure ensured consistent exposure and minimized variability due to environmental factors. By applying an identical protocol for all samples, variations in the results could be attributed to synthesis and thermal annealing conditions rather than environmental exposure.

3. Results and Discussion

The optical properties of the thin films were investigated using the diffuse reflectance technique, from which absorption, reflectance, and transmittance spectra were obtained. In addition, X-ray diffraction (XRD) was employed to determine the stoichiometry and crystalline structure of the ZnO thin films, and the diffraction patterns were compared with standard crystallographic databases (JCPDS). SEM provided a detailed analysis of the surface morphology, revealing changes induced by thermal annealing. Furthermore, XPS analysis was used to identify the oxidation states of the chemical elements present in the ZnO films, as well as to detect residual compounds associated with impurities or secondary phases formed during the synthesis process.

3.1. Optical Properties via Diffuse Reflectance

The spectrum obtained through the diffuse reflectance technique corresponds to the reflectance of the material as a function of wavelength. From these data, the Kubelka–Munk function and transmittance can be obtained, with the Kubelka–Munk function as an approximation of the absorption coefficient of optically opaque samples [28].

The normalized optical spectra of ZnO thin films in the as-grown state and after thermal annealing at 400 °C and 600 °C are presented in Figure 1. Figure 1a shows the absorption spectra, where negligible absorption is observed above ~420 nm, while a sharp absorption edge appears at shorter wavelengths. The position of the edge shifts with annealing temperature, occurring at ~410 nm for the as-grown film, ~402 nm after annealing at 400 °C, and ~400 nm after annealing at 600 °C. Figure 1b displays the transmission spectra, where all films exhibit high transparency (>60%) in the visible range with a steep absorption edge between ~350 and 400 nm. The as-grown sample shows the highest transmittance, while the 600 °C annealed film exhibits the lowest transmittance, consistent with the reflectance increase shown in Figure 1c. The reflectance spectra reveal a sharp rise in the near-UV region associated with the fundamental absorption edge of ZnO. The thin film as grown presents the lowest reflectance across the visible range, whereas annealing at 400 °C leads to a moderate increase, and annealing at 600 °C results in the highest reflectance, particularly above ~450 nm, suggesting changes in morphology and defect concentration [29,30,31,32,33].

Moreover, to determine the optical band gap, the data were analyzed using both the Tauc and derivative methods, which provide an estimate of the energy necessary for electronic transitions between the valence and conduction bands. In the Tauc method, the linear portion of the $(\alpha h\upsilon {)}^n$ vs. $h\upsilon$ plot is extrapolated to its intersection with the energy axis, yielding the optical band gap value. In contrast, the derivative method involves plotting ${\displaystyle \frac{d\left(\mathrm{l}\mathrm{n}\alpha \right)}{d\left(hv\right)}}$ vs. $h\upsilon$, where the maximum of the band corresponds to the band gap energy.

Figure 2 shows the results obtained for ZnO thin films: as-grown, thermal annealed at 400 °C, and annealed at 600 °C. Figure 2a–c correspond to the derivative method, with the maximum value indicating the band gap value for as-grown (3.20 eV), annealed at 400 °C (3.23 eV), and annealed at 600 °C (3.21 eV) ZnO thin films. Figure 2d presents the Tauc analysis [F(R)hυ]² vs. hυ from which the band gap energies were determined as 3.21 eV, 3.23 eV, and 3.20 eV for the same respective conditions, in agreement with the observed in the absorption edge positions. Minor differences between the visual estimation from the absorption edge and the calculated E_g values can be attributed to band tailing and defect-related states within the gap, as well as spectral broadening effects that smooth the absorption onset. The results obtained are consistent with literature values of band gap for ZnO thin films, which report a direct band gap between 3.20 and 3.40 eV [3,34,35,36,37].

Considering the experimental precision of the Tauc method (±0.1 eV), the band gap values of 3.20–3.23 eV can be regarded as independent of thermal annealing temperature. The slightly lower value of 3.20 eV lies at the lower limit of the reported range for ZnO. It is most likely associated with localized states within the band gap, originating from intrinsic defects such as oxygen vacancies [31,32], zinc interstitials [31,32], or other structural imperfections [33]. Although the optical band gap does not change significantly with thermal annealing, complementary XRD and SEM results demonstrate that thermal treatment improves the quality of the ZnO thin films.

3.2. Structural Properties

Figure 3 shows the X-ray diffraction (XRD) patterns of the ZnO thin films. The horizontal axis corresponds to the diffraction angle 2θ (in degrees), while the vertical axis represents the intensity in arbitrary units. The diffraction peaks are well defined and indexed to the (100), (002), (101), (102), (110), (103), (200), (112), and (201) crystallographic planes, with a preferred orientation along the (101) plane [24,37]. Comparison with the crystallographic reference card JCPDS 36-1451 confirms that all samples correspond to a polycrystalline hexagonal ZnO structure [24,34,37], with peak positions falling within the experimental tolerance of ±0.2° in 2θ [24,34]. For the as-grown and 400 °C thin films, some peaks appear slightly shifted relative to the JCPDS 36-1451 card. These shifts are related to lattice strain and residual hydroxyl-related species formed during low-temperature CBD growth, consistent with the XPS evidence of –OH groups adsorbed on the surface. These features do not indicate the presence of a separate crystalline Zn(OH)₂ phase but rather point to stress and defect incorporation in the wurtzite ZnO lattice. In contrast, the 600 °C annealed films exhibit well-defined peaks that align closely with the JCPDS 36-1451 reference card, reflecting improved phase purity and crystallinity. The higher intensity of these peaks confirms the beneficial effect of thermal treatment, which enhances atomic mobility and eliminates hydroxyl-related defects.

2dsinΘ = nλ,

(9)

Using Wulff-Bragg’s law (Cu Kα, λ = 1.5406 Å, n = 1), the interplanar distances, d, were calculated in

Table 1. Experimental diffraction data (2θ, interplanar distances d, and relative intensities) of ZnO thin films as-grown and thermal annealed at 400 °C and 600 °C, compared with reference JCPDS for ZnO (36-1451, P6₃mc).

ZnO	Experimental Peak Position 2θ (°)	dhkl (Å)	Intensity (a.u.)	hkl	a dhkl (Å)
As grown	31.74	2.8169	784	(1 0 0)	2.8143
	34.34	2.6093	604	(0 0 2)	2.6033
	36.22	2.4781	1085	(1 0 1)	2.4759
	47.46	1.9141	197	(1 0 2)	1.9111
	56.5	1.6274	311	(1 1 0)	1.6247
	62.84	1.4776	224	(1 0 3)	1.4771
	66.3	1.4087	69	(2 0 0)	1.4072
	67.9	1.3793	205	(1 1 2)	1.3782
	68.98	1.3603	107	(2 0 1)	1.3583
TA 400 °C	31.74	2.8169	973	(1 0 0)	2.8143
	34.44	2.602	967	(0 0 2)	2.6033
	36.26	2.4755	1648	(1 0 1)	2.4759
	47.54	1.9111	375	(1 0 2)	1.9111
	56.62	1.6243	445	(1 1 0)	1.6247
	62.88	1.4768	427	(1 0 3)	1.4771
	66.48	1.4053	78	(2 0 0)	1.4072
	67.98	1.3779	300	(1 1 2)	1.3782
	69.12	1.3579	145	(2 0 1)	1.3583
TA 600 °C	31.77	2.8143	2268	(1 0 0)	2.8143
	34.42	2.6034	1739	(0 0 2)	2.6033
	36.26	2.4755	3735	(1 0 1)	2.4759
	47.56	1.9103	820	(1 0 2)	1.9111
	56.62	1.6243	1097	(1 1 0)	1.6247
	62.88	1.4768	838	(1 0 3)	1.4771
	66.44	1.406	189	(2 0 0)	1.4072
	68.06	1.3765	702	(1 1 2)	1.3782
	69.08	1.3585	360	(2 0 1)	1.3583

TA: thermal annealing, ^a: interplanar distances for ZnO obtained from JCPDS 36-1451.

for all diffraction peaks. The experimental d values match those of wurtzite ZnO (JCPDS 36-1451) for the diffraction peaks (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes. In addition, the relative intensity and sharpness of the peaks (i.e., their sensitivity) provide evidence of the influence of thermal annealing. For the as-grown and 400 °C thin films, the diffraction peaks are of lower intensity, indicating the presence of lattice strain and structural defects [38]. In contrast, the increase in the intensity of diffraction peaks in the sample annealed at 600 °C is attributed to improved crystallinity and structural quality.

Table 2. Lattice parameters (a, c), c/a ratios, and structural phase of as-grown ZnO thin films, annealed at 400 °C and 600 °C, obtained from experimental XRD data.

ZnO Sample 1	a (Å) ± σa	c (Å) ± σc	c/a	Structural Phase
As grown	3.220 ± 0.016	5.199 ± 0.045	1.614	ZnO (strained)
TA 400 °C	3.247 ± 0.001	5.205 ± 0.003	1.603	ZnO (strained, improved crystallinity)
TA 600 °C	3.247 ± 0.002	5.202 ± 0.005	1.602	ZnO (wurtzite)

¹ Lattice parameters were refined from multiple diffraction peaks using Wulff–Bragg’s law and the hexagonal lattice equation. Uncertainties (±σ_a, ±σ_c) were obtained by least-squares fitting.

summarizes the lattice parameters a and c, their uncertainties, and the calculated c/a ratios for the ZnO thin films synthesized as-grown and after thermal annealing at 400 °C and 600 °C. For a hexagonal system, d is related to the Miller indices (h k l) and the lattice constants a and c by:

$${\displaystyle \frac{1}{d^2}}={\displaystyle \frac{4}{3}}\cdot {\displaystyle \frac{h^2+hk+k^2}{a^2}}+{\displaystyle \frac{l^2}{c^2}}.$$

(10)

The experimental lattice parameters confirm the wurtzite structure in all samples. For reference, the crystallographic card JCPDS 36-1451 reports lattice constants of a = 3.24982 Å and c = 5.20661 Å, corresponding to a c/a ratio of 1.602. The experimental values obtained for the as-grown films differ slightly from reference lattice constants, which suggests the presence of residual strain and hydroxide-related incorporation during CBD growth at low temperature. Thermal annealing at 400 °C partially relaxes this strain, while annealing at 600 °C yields lattice parameters almost identical to the reference values reported, confirming improved crystallinity, reduced defect density, and phase purity.

3.3. Morphological Properties

According to the literature, a variety of morphologies have been reported for ZnO depending on the synthesis method and processing conditions. Similar morphologies of ZnO thin films obtained have been observed, including hexagonal or tubular nanorods [5,39,40], plate-like [41,42], granular-shaped [40,42], as well as flower-like [43,44,45,46,47,48], depending on the thermal treatment and synthesis parameters [32,39,40,41,43,44]. Figure 4 shows scanning electron microscopy (SEM) images of the ZnO thin films. Figure 4a corresponds to surface micrographs acquired at a working scale of 1 μm. The images reveal clusters of elongated microcrystals with approximate sizes of 5 µm, growing in random directions. Figure 4b micrograph shows a reorganization of the surface morphology compared to the as-grown samples. A more homogeneous distribution of the microcrystal can be observed, which appears to be more densely packed and better adhered to the substrate after the thermal treatment, i.e., well-defined flower-like structures are visible, indicating improved order and compaction. This transformation is associated with the removal of hydroxide-related residues and the enhancement of atomic rearrangements, which promote the growth of larger, more stable ZnO crystals [35,41].

Figure 4c reveals a notable evolution in the morphology of ZnO samples subjected to thermal annealing at 600 °C. The flower-like structures appear more defined and have greater uniformity in shape and arrangement. The improvement in definition and uniformity is associated with the complete conversion to the wurtzite ZnO phase and an increase in crystallinity. The observed structures suggest increased atomic mobility during the thermal treatment, favoring the formation of crystals with structural quality. Such distinctive flower-like morphologies are particularly attractive due to their large surface area and structural tunability, which broaden the scope of potential applications. In the biomedical field, ZnO flower-like morphology is promising for biosensing [43], antibacterial coatings [43,45], and tissue engineering [43], while in optoelectronic and sensing devices, their unique surface features can improve light scattering, UV shielding [46,47], photocatalytic activity [44,48], and gas detection [49,50,51].

3.4. Compositional Properties

The low-resolution X-ray photoelectron spectroscopy (XPS) analysis, also known as a survey scan, was performed for the ZnO thin film in the binding energy range from 1100 to 0 eV, as shown in Figure 5. The presence of zinc (Zn), carbon (C), and oxygen (O) was detected. However, carbon and oxygen are considered atmospheric contaminants adsorbed onto the sample surface, and therefore also contribute to the O 1s and C 1s spectral regions.

To obtain more precise information on the chemical states of the elements, a high-resolution XPS analysis was performed. In this technique, the photoelectron detector is focused on specific binding energy regions associated with the region of the main elements of interest, allowing precise identification of their oxidation states and chemical environments. The high-resolution spectra of the C 1s, O 1s, and Zn 2p states for the ZnO thin films in the as-grown state and after thermal annealing (TA) at 400 °C and 600 °C are presented in Figure 6.

In the C 1s region, illustrated in Figure 6a,d,g, the spectra were deconvoluted into two main components: a peak at 284.5 eV, attributed to adventitious carbon contamination, and another at 285.5 eV, associated with C–OH [52]. The O 1s state, shown in Figure 6b,e,h, reveals three oxygen species: attributed to the ZnO formation at ~530.2 eV, OH⁻ species adsorbed on the surface at ~531.4 eV, and molecular H₂O adsorbed at ~532.5 eV [53,54,55]. Changes in the relative intensities of the OH⁻ and H₂O peaks with increasing annealing temperature indicate modifications in surface chemistry and in the degree of hydroxylation or hydration of the ZnO thin films. In the Zn 2p region, presented in Figure 6c,f,i, the spectra exhibit the Zn 2p₃/₂ and Zn 2p₁/₂ spin–orbit doublet, located at ~1021.35 eV and ~1044.35 eV, respectively, with an experimental spin–orbit splitting of 23.0 eV for all cases [56,57,58]. This doublet splitting is consistent with Zn²⁺ in ZnO. The binding energies and the spin–orbit separation after thermal annealing confirm the Zn²⁺ oxidation state, although variations in peak intensities may reflect changes in surface composition or defect density.

4. Conclusions

ZnO thin films were successfully synthesized on glass substrates by chemical bath deposition (CBD) under alkaline conditions adjusted with NH₄OH to maintain a high pH environment for the ZnO formation. Optical, structural, morphological and chemical characterizations confirmed the formation of the hexagonal wurtzite phase of ZnO. XRD analysis revealed that the as-grown thin films exhibit wurtzite ZnO diffraction peaks slightly shifted with respect to the JCPDS reference, attributed to lattice strain and defect incorporation during low-temperature growth. After thermal annealing at 400 °C, diffraction peaks became more intense, indicating a reduction in the strain stress and improved crystallinity, while the sample annealed at 600 °C exhibits a wurtzite ZnO structure with higher intensities of diffraction peaks, confirming crystalline quality.

Tauc plot and derivative methods yielded direct band gap values of 3.20 eV, 3.23 eV, and 3.21 eV for the as-grown ZnO thin films, thermally annealed at 400 °C and at 600 °C, respectively, in agreement with reported literature values. The band gap values remain nearly constant, but the absorption edge sharpness is improved with thermal annealing, which we attribute to reduced defect density and relaxation of residual strain in the ZnO lattice.

SEM micrographs revealed changes in the morphology of the ZnO thin films. The as-grown thin films exhibited elongated agglomerates oriented in random directions, while thermal annealing promoted their reorganization into flower-like structures. At 400 °C, the agglomerates became more densely packed and firmly adhered to the substrate, and at 600 °C, well-defined, highly symmetrical formations were obtained, consistent with enhanced atomic mobility.

XPS confirmed ZnO formation in all samples and detected residual precursor species only in the as-grown thin films. These residues were removed entirely upon thermal annealing. It is important to clarify that the detection of OH species by XPS does not necessarily imply the formation of a crystalline Zn(OH)₂ phase. XPS is highly surface-sensitive and can detect adsorbed hydroxyl groups or residual precursor-related species, whereas XRD requires long-range crystallographic periodicity to produce diffraction peaks. Therefore, hydroxyl groups clearly observed in the O 1s spectra do not translate into Zn(OH)₂ diffraction peaks in the diffractogram. The main contribution is attributed to wurtzite ZnO, and the possible presence of surface hydroxyl residues is discussed by XPS results.

The thickness of the ZnO films was 5.37 ± 0.11 µm for the as-grown sample and decreased slightly to 4.50 ± 0.30 µm and 4.23 ± 0.21 µm after annealing at 400 °C and 600 °C, respectively, consistent with densification and morphological compaction during thermal treatment.

These results emphasize that while the band gap is stable mainly across thermal annealing conditions, improvements in crystallinity, defect reduction, and morphological reorganization are decisive for enhancing ZnO thin films for optoelectronic, sensing, and photocatalytic applications.