Fabrication of High-Crystallinity ZnO Nanorods for Photocatalytic Application

Abstract

ZnO nanorods were synthesized on AZO substrates by chemical bath deposition, and were subsequently annealed under an air and vacuum ambient. Both annealing processes could improve the crystallinities of ZnO nanorods. The air-annealed ZnO nanorods showed higher crystallinity and partial reduction of oxygen-vacancy-related defects. The air-annealed ZnO nanorods exhibited a much higher photodegradation efficiency of 70% degradation for methyl red. In addition, as-grown ZnO nanorods were coated with undoped and Al-doped ZnO by mist chemical vapor deposition. Both coated thin layers modified the surface of ZnO nanorods, while the AZO-coated ZnO nanorods showed higher crystallinity and light absorption which resulted in the improvement in the photodegradation rate of methyl red. These findings demonstrate that appropriate annealing treatment and AZO surface engineering for ZnO nanorods are effective approaches for improving crystallinity, which leads to improvement of the photocatalytic efficiency of ZnO-based materials.

1. Introduction

In the recent decades, the extensive use of organic dyes in various industrial processes has resulted in the generation of large amounts of wastewater. Among various organic dyes, azo dyes account for more than half of all industrial dyes [1]. These dyes can pose serious risks to human health and the ecological environment. For example, the aromatic amines produced by the cleavage of methyl red (MR) have been identified as having carcinogenic, mutagenic, hepatotoxic, and bladder toxicity risks [2]. Semiconductor photocatalysis, such as ZnO, CuO [3], TiO₂ [4], WO₃ [5] and Fe₂O₃ [6], have been demonstrated to be effective, sustainable, and versatile technology for degrading azo dyes [7,8,9]. Among them, ZnO is a lower-cost option than many others, making it more suitable for large-scale industrial production. ZnO is a promising semiconductor material because of its unique properties, including a wide direct bandgap of 3.37 eV, a large exciton binding energy of 60 meV, high optical transparency, high surface reactivity, and favorable electrical properties [10]. In addition to its intrinsic properties, ZnO is also an attractive material because of its low cost, wide availability, and versatile synthesis routes [11]. Among various fabrication methods for ZnO, including hydrothermal growth [12], sol–gel processing [13], vapor-phase deposition [14], and sputtering techniques [15], each method has both advantages and disadvantages. Vapor-phase and vacuum-based methods can provide high crystallinity and excellent structural control; however, they generally require expensive equipment, high temperatures, and complicated fabrication procedures. In contrast, chemical bath deposition (CBD) offers a simple, low-cost, and low-temperature process suitable for large-area and flexible substrates [16]. Although CBD-grown ZnO nanorods may exhibit relatively lower crystallinity, the method provides excellent scalability, easy morphology control, and compatibility with industrial applications.

In recent years, many studies have focused on improving the photocatalytic performance of ZnO [17,18,19]. Various modification strategies have been developed for ZnO-based materials, including annealing treatment [20], elemental doping [21], surface-area engineering [22], etc. Annealing treatment has been widely reported as an effective strategy to modify the structural properties and photocatalytic performance of ZnO materials. Appropriate thermal treatment can improve crystallinity [23] and adjust defect states [24]. These previous studies on defect modulation have mainly focused on annealing treatments conducted at relatively high temperatures, typically above 500 °C. However, excessive annealing temperatures may also cause undesirable effects, such as grain growth, reduced surface area, and the fusion of ZnO nanorods into larger particles [25], which may lead to reduced degradation efficiency. The effects of annealing-induced changes in crystallinity and defect states on the photocatalytic performance of ZnO materials had not been sufficiently discussed. Moreover, surface engineering has been recognized as an effective strategy to enhance the photocatalytic performance of ZnO materials [16]. Increasing the specific surface area can expose more catalytically active sites [26], enhance the adsorption of organic pollutants on the catalyst surface, and shorten the migration distance of photogenerated charge carriers to surface reaction sites [27], thereby improving photocatalytic degradation efficiency. Recently, several studies have attempted to modify the surface of ZnO materials through coating strategies [28], including coating different materials to form composite or shell-layer structures [29], as well as coating similar materials to achieve secondary growth [30,31]. However, most of these studies have mainly focused on ZnO powders or nanoparticles, which are less favorable for practical recycle and reuse. In contrast, surface modification of the recyclable nanorod-structured ZnO films has been comparatively less explored.

In our previous study, well-aligned ZnO nanorods could be fabricated on Al-doped ZnO (AZO) substrates, and the influence of deposition time on the photocatalytic performance of ZnO nanorods was investigated [32]. In addition, the mist chemical vapor deposition (mist CVD) method was used to deposit ZnO thin film and AZO film with precise thickness control on glass substrates. To further improve the photodegradation efficiency for azo dyes, in this study, thermal annealing in different ambient and surface coating methods was proposed to improve the crystallinity of ZnO nanorods. The photodegradation effects of MR dye using obtained ZnO nanorods were investigated.

2. Results and Discussion

2.1. Effect of Annealing Ambient

2.1.1. Structural Properties of ZnO Nanorods

Figure 1 shows the scanning electron microscope (SEM) images of as-grown ZnO nanorods (NRs), annealed at 300 °C and 400 °C in a vacuum and air ambient respectively. Further statistical analysis of the nanorod diameter distribution was carried out with area (2.5 × 2.5 µm²) about 100 nanorods for each sample from top view of a SEM image. The length measurement was from a cross-section view of SEM images as shown in Figure 2b. It can be clearly observed that the ZnO nanorods formed vertically aligned arrays on the AZO substrates, with a length of 1153 nm and a diameter distribution mainly in the range of 50~60 nm after the CBD process. After annealing in the vacuum, the length of nanorods was slightly decreased to 1024 nm at 300 °C and 1095 nm at 400 °C, while the diameters distribution mainly increased to the ranges of 70~80 nm and 60~70 nm respectively. However, the morphology of nanorods was significantly changed after annealing in an air ambient. The length of nanorods was estimated to decrease to approximately 988 nm at 300 °C after air annealing with a diameter mostly distributing in the range of 60~70 nm. The length of ZnO nanorods was significantly increased to 1191 nm with the distribution diameter mostly in the range of 60~70 nm after annealing at 400 °C. In addition, the sheet resistance of the samples was measured to be 8.985 Ω/□ (as-grown), 8.158 Ω/□ (300 °C vacuum annealing), 8.177 Ω/□ (400 °C vacuum annealing), 15.85 Ω/□ (300 °C air annealing), and 26.85 Ω/□ (400 °C air annealing).

In order to study the effects of annealing ambient on elemental composition, energy-dispersive X-ray spectroscopy (EDX) analysis was conducted. The atomic ratios of Zn/O, Zn/(Zn + O) and O/(Zn + O) are shown in Figure 3. The as-grown ZnO nanorods exhibited Zn/O, Zn/(Zn + O), and O/(Zn + O) ratios of 79.03%, 44.14%, and 55.86%, respectively. After vacuum annealing at 300 °C and 400 °C, the Zn/(Zn + O) ratios increased to 44.71% and 46.55%, while the O/(Zn + O) ratios decreased to 55.29% and 53.45%, respectively. Therefore, the Zn/O ratio increased to 87.08% after vacuum annealing at 400 °C, indicating a Zn/O ratio increase resulting in the ZnO compound increasing more than those the 300 °C vacuum-annealed samples. During vacuum annealing, more oxygen atoms could escape from the ZnO lattice with a temperature increase, resulting in a Zn/O ratio increase toward stoichiometric ZnO. After air annealing, Zn/(Zn + O) and O/(Zn + O) showed a similar tendency to those observed after vacuum annealing. The 300 °C air-annealed sample exhibited Zn/(Zn + O), and O/(Zn + O) ratios of 46.78%, and 53.22%, respectively. At 400 °C, the Zn/(Zn + O) and O/(Zn + O) ratios slightly changed to 46.87%, and 53.13%, while the Zn/O ratio increased to 88.20% indicating an increase in stoichiometric ZnO, because the oxygen-containing ambient may promote further reaction and improve the crystallinity of ZnO during air annealing.

The XRD patterns revealed the effect of annealing on the crystal structure, as shown in Figure 4. The structural properties of the annealed ZnO nanorods are exhibited in

Table 1. Structural properties of the annealed ZnO nanorods obtained from XRD analysis.

Sample	2θ (°)	Intensity (cps)	FWHM (°)	d (Å)	Crystallite Size (nm)	Stress (GPa)	Lattice Constant c (Å)
AZO substrate	34.49	1291	0.262	2.600	31.8	−0.303	5.200
As-grown	34.46	142,849	0.184	2.601	46.8	−0.256	5.201
Vacuum 300 °C	34.47	115,952	0.199	2.600	41.8	−0.303	5.200
Vacuum 400 °C	34.44	144,285	0.181	2.602	46.0	−0.117	5.204
Air 300 °C	34.50	158,752	0.171	2.598	48.5	−0.513	5.195
Air 400 °C	34.46	179,353	0.172	2.601	48.2	−0.256	5.201

. Only (002) diffraction peaks were obtained for these five samples. The intensity of the (002) peak increased almost in order of as-grown nanorods, vacuum-annealed nanorods, and air-annealed nanorods. In contrast, the FWHM values slightly decreased in the same order. A slight asymmetric broadening with a sligh shoulder observed close to the ZnO (002) peak in several samples might be due to the different crystallite sizes and higher compressive stress from the AZO seeds layer. The c-axis crystallite sizes were estimated using the Scherrer equation [33]; the calculated results were 46.8 nm for the as-grown nanorods, and 41.8 nm and 46.0 nm for the 300 °C and 400 °C vacuum-annealed nanorods respectively. The c-axis crystallite size was much higher for air-annealed nanorods, 48.5 nm and 48.2 nm for the 300 °C and the 400 °C air-annealed nanorods respectively. Based on Bragg’s law, the calculated lattice constants of all samples were nearly 5.2 Å, indicating there was negligible lattice distortion. These results indicate that the annealing process could improve the crystallinity of ZnO nanorods, and the improvement was more pronounced with air annealing compared to a vacuum ambient. Compared with 300 °C annealed ZnO nanorods, the 400 °C annealing process provided sufficient thermal energy to induce atomic rearrangement within the ZnO nanorods, resulting in crystallinity improvement after both vacuum and air annealing. In addition, annealing in air introduced an additional oxygen source which could promote regrowth of ZnO, leading to higher crystallinity.

2.1.2. Optical Properties of ZnO Nanorods

The transmittance spectra of the as-grown and annealed ZnO nanorods in different ambient are shown in Figure 5. After annealing in both the vacuum and air ambient, the ZnO nanorods showed decreased transmittance of about 61% compared with 67% for the as-grown ZnO nanorods. In order to investigate the bandgap energy change, Tauc plots were applied to estimate the optical bandgap as shown in Figure 5b. The bandgap was estimated using Equation (1) [34].

$${\left(\alpha hv\right)}^{1/n}=A\left(hv-E_g\right)$$

(1)

where α is the absorption coefficient, hv is the photon energy, A is a constant and E_g is the optical bandgap. The optical bandgap energies are estimated to be 3.32 eV for the as-grown sample, 3.29 eV for the vacuum-annealed sample, and 3.28 eV for the air-annealed sample. The negligible variation in bandgap energy suggests that annealing had a limited influence on the intrinsic electronic structure of ZnO.

To further investigate the defect-related photoluminescence (PL) behavior of ZnO nanorods, PL measurements were performed on the as-grown, vacuum-annealed and air-annealed 400 °C samples, which exhibited relatively better crystallinity. The PL spectra are shown in Figure 6. All samples exhibited a near-band-edge (NBE) ultraviolet emission centered around 380 nm, which is generally associated with band edge emission. In addition, a broad visible emission peak from 580 nm to 660 nm was observed in both the as-grown sample and air-annealed sample, which were associated with oxygen interstitials or surface oxygen-related defects [35]. This red shift in the visible emission after air-annealing suggests the air annealing promoted stoichiometric ZnO reducing the oxygen interstitials. Compared with the as-grown sample, the vacuum-annealed sample exhibited an improved NBE emission together with a strongly suppressed visible emission band. This result suggests that vacuum annealing improved the crystallinity of ZnO nanorods and reduced defect-related recombination centers.

2.1.3. Photocatalytic Performance

Since the samples annealed at 400 °C showed relatively better crystallinity, only the ZnO nanorods annealed at 400 °C in two different ambient conditions were selected for subsequent photocatalytic performance evaluation. Figure 7 shows the UV-vis absorption spectra of MR solution after photocatalytic degradation under UV irradiation, together with the corresponding normalized concentration (C/C₀) and degradation efficiency over the as-grown and annealed ZnO nanorods. According to the Langmuir–Hinshelwood(L-H) model [36], the degradation rate of MR could be estimated by a pseudo-first-order reaction at low concentrations, as shown in Equation (2).

$$\ln \left(C_0/C\right)=-kt$$

(2)

Prior to UV irradiation, a dark adsorption test was conducted to evaluate the adsorption behavior of the samples. After 30 min of dark adsorption, the MR concentration decreased by 9.8% for the air-annealed ZnO nanorods, indicating that adsorption contributed only slightly to the initial concentration decrease. After 5 h of UV irradiation, the air-annealed ZnO nanorods exhibited the highest photocatalytic activity, followed by the vacuum-annealed and as-grown ZnO nanorods. The highest degradation efficiency was 70% for the air-annealed ZnO nanorods with a degradation rate of 0.0041 min⁻¹, followed by 53% for the vacuum-annealed ZnO nanorods with a degradation rate of 0.0025 min⁻¹ and 45% for the as-grown sample with a rate of 0.0020 min⁻¹. To further evaluate the reusability of the photocatalyst, the air-annealed ZnO nanorods with the highest photocatalytic activity were subjected to a total of three repeated degradation cycles, as shown in Figure 7c. The degradation efficiencies remained relatively stable at approximately 68~75% with values of 70%, 75% and 68% for the 1st, 2nd and 3rd cycles, respectively. These results indicate that the prepared samples exhibited a certain degree of reusability and potential practical applicability for photocatalytic degradation.

When UV light irradiated ZnO nanorods which were put into the MR solution, electrons in the valence band of ZnO were excited to the conduction band, leaving holes in the valence band and generating electron–hole pairs. These electrons and holes could migrate to the nanorod surface and react with adsorbed species, leading to the formation of reactive oxygen species such as hydroxyl radicals (·OH) and superoxide radicals (·O₂⁻) as shown in Figure 8. These reactive oxygen species played an important role in the degradation of MR dyes. Previous studies have also reported that superoxide radicals (·O₂⁻) play a dominant role in the photocatalytic degradation of azo dyes [37]. MR molecules are first attacked by ·OH and ·O₂⁻ radicals to form aromatic intermediates [38], which are subsequently oxidized into smaller species and finally degraded into CO₂, H₂O, and inorganic ions. Improved crystallinity is generally associated with reduced charge carrier recombination and enhanced charge transport, which can facilitate the photocatalytic process. In addition, the PL results suggest that the air-annealed ZnO nanorods retained oxygen-related surface defect states. These defects could act as carrier-trapping or surface-reaction sites, promoting charge separation and the generation of reactive oxygen species. Therefore, the superior photocatalytic performance of the air-annealed ZnO nanorods could be attributed to both effect of improved crystallinity and modified oxygen-related defect states.

2.2. Effect of Surface Coating of ZnO Nanorods

2.2.1. Structural Properties of ZnO Nanorods

ZnO nanorods were synthesized on AZO substrates using the CBD method, which was followed by a mist CVD coating process. Figure 9 shows the SEM images of the as-grown ZnO nanorods and the nanorods coated with ZnO and AZO thin layers respectively for 10 min at 400 °C. It could be observed that the vertically grown ZnO nanorods exhibit a height of about 1560 nm with a diameter of 147 nm. After coating ZnO for 10 min, the diameter of nanorods were 162 nm, but became much more uniform and the length of nanorods was slightly increased to 1691 nm. The ZnO nanorods exhibited improved diameter uniformity with a larger size of 176 nm, and the length was increased to 1714 nm after AZO coating for 10 min.

To further verify the formation of the AZO coating layer, Transmission Electron Microscopy (TEM) and EDX analysis were performed, as shown in Figure 10. As shown in Figure 10a, the nanorod exhibited a clear coating layer, where the ZnO nanorod as a core was surrounded by a lighter coating layer with a uniform layer of approximately 54.8 nm. In Figure 10(b-1–b-4), elemental mapping reveals the distribution of Zn, O and Al. It could be observed that Zn and O were uniformly distributed in the center area, while Al was uniformly distributed on the surface area of nanorod, as shown in Figure 10(b-4), indicating the successful formation of the AZO coating layer on a ZnO nanorod.

The XRD patterns of as-grown and coated ZnO nanorods are shown in Figure 11, and the structural properties of the ZnO nanorods obtained from XRD analysis are shown in

Table 2. Structural properties of the coated ZnO nanorods obtained from XRD analysis.

Sample	2θ (°)	Intensity (cps)	FWHM (°)	d (Å)	Crystallite Size (nm)	Stress (GPa)	Lattice Constant c (Å)
As-grown	34.49	224,919	0.184	2.598	45.3	−0.443	5.197
ZnO coating	34.50	265,615	0.187	2.598	44.4	−0.513	5.195
AZO coating	34.50	273,921	0.181	2.598	46.0	−0.513	5.195

. Only (002) peaks were obtained in the three samples, located at 34.49° for the as-grown sample and 34.50° for both ZnO-coated and AZO-coated samples, indicating a strong preferred orientation along the c-axis, same as ZnO nanorods, because the ZnO coating and AZO coating were the re-growth ZnO layers on ZnO nanorods which were the growth core. Compared to as-grown nanorods, both ZnO- and AZO-coated nanorods exhibited an increased (002) peak intensity. Although the AZO-coated sample exhibited the highest peak intensity and the narrowest FWHM of 0.181°, the FWHM among the three samples was not significantly changed, with values of 0.184° and 0.187° for as-grown and ZnO-coated samples, respectively. Therefore, the crystallite sizes were calculated using the Scherrer equation, which mainly reflects the coherent crystalline domain size along the c-axis. The calculated crystallite sizes are very similar, ranging from 44.4 to 46.0 nm: 45.3 nm for the as-grown sample, 44.4 nm for the ZnO-coated sample, and 46.0 nm for the AZO-coated sample. All calculated lattice constants c were close to the value of 5.20 Å. These results indicate that coated nanorods maintained high crystalline quality after the mist CVD coating process, with slight crystallinity improvements observed for AZO-coated samples.

2.2.2. Optical Properties of ZnO Nanorods

The transmittance and absorbance spectra of as-grown nanorods and ZnO-coated and AZO-coated nanorods are shown in Figure 12. The corresponding Tauc-plot images are displayed in Figure 12c. The as-grown nanorods exhibited a transmittance of 47.04% in the visible region. After ZnO coating, the transmittance slightly decreased to 44.1%, and further decreased to 33.94% after AZO coating. In contrast, the absorbance spectra showed an opposite tendency. The AZO-coated sample exhibited higher absorbance than the as-grown and ZnO-coated samples, especially in the visible region, indicating enhanced light utilization after AZO coating. However, the absorption edges of the three samples were similar, suggesting that the coating process had a limited influence on the intrinsic bandgap structure. The corresponding bandgap energies were calculated to be 3.30 eV for the as-grown sample, 3.28 eV for the ZnO-coated nanorods, and 3.29 eV for the AZO-coated nanorods.

2.2.3. Photocatalytic Performance

ZnO-coated and AZO-coated ZnO nanorods were used for photodegradation measurement. As-grown ZnO nanorods was also used as reference. The absorbance spectra of MR are shown in Figure 13. The MR absorption band at 520 nm was used to calculate the degradation rate of obtained ZnO nanorods. Prior to UV irradiation, the dark adsorption test showed that the MR concentration decreased by 7% for the AZO-coated sample, indicating a slight adsorption effect. All the samples showed significant absorbance intensity reduction at the MR characteristic wavelength of 520 nm. The as-grown ZnO nanorods showed the lowest photocatalytic efficiency of 39% with a photocatalytic degradation rate of 0.0016 min⁻¹, followed by ZnO-coated nanorods which obtained a degradation efficiency of 41% with a rate of 0.0018 min⁻¹. The AZO-coated sample exhibited the highest photocatalytic degradation efficiency of 64% and the degradation rate was calculated to be 0.0034 min⁻¹. Furthermore, the reusability test results of the AZO-coated nanorods are shown in Figure 13c. The degradation efficiencies remained close at approximately 64~72%, with values of 64%, 72% and 68% for 1st, 2nd and 3rd cycles, respectively.

To further confirm the structural stability of the AZO-coated sample after repeated use, XRD and SEM characterizations were carried out. As shown in Figure 13d, the diffraction peak of the AZO-coated sample after reaction remained at 34.50°, which was consistent with that before the reaction. The FWHM after reaction was 0.192°, showing no significant change. Moreover, the SEM images showed that the ZnO nanorods were hardly morphologically changed after the reaction. These results indicate that the AZO-coated ZnO nanorods maintained good structural stability after repeated photocatalytic degradation.

Among these three samples, the AZO-coated sample exhibited the best photocatalytic performance. This improvement could be attributed to the combined effect of enhanced crystallinity and higher light absorption. The improved crystallinity may facilitate charge transport and ROS generation, while the higher absorbance of the AZO-coated nanorods could improve the utilization of incident light, thereby enhancing photocatalytic activity.

3. Materials and Methods

3.1. ZnO Nanorods Grown on AZO Substrates by CBD Method

ZnO nanorods were fabricated by chemical bath deposition (CBD) using a 300 nm-thick AZO substrate. A solution was prepared, combining zinc nitrate hexahydrate (Zn (NO₃)₂·6H₂O, FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan; 0.015 mol/L) with hexamethylenetetramine (HMTA, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan; 0.0075 mol/L), which was dissolved in 200 mL of deionized water to form a precursor solution for the CBD process, as shown in Table 3. The AZO substrates were vertically immersed in reaction vessels containing the prepared growth solutions at 95 °C for 5 h and 10 h respectively, with the vessels covered with plastic film to minimize evaporation. Finally, the synthesized ZnO samples were rinsed with deionized water to eliminate the remaining salts, and dried with N₂ gas.

Table 3. Experimental parameters for the CBD growth of ZnO nanorods.

Substrate	Zn(NO3)2·6H2O (mmol)	HMTA (mmol)	Solvent (mL)	Temperature (°C)	Time (h)
AZO	3	1.5	200	95	5, 10

Table 3. Experimental parameters for the CBD growth of ZnO nanorods.

Substrate	Zn(NO3)2·6H2O (mmol)	HMTA (mmol)	Solvent (mL)	Temperature (°C)	Time (h)
AZO	3	1.5	200	95	5, 10

3.2. Annealing Effects for ZnO Nanorods in Different Ambients

To investigate the influence of annealing conditions, the as-grown ZnO nanorods were annealed at 300 °C and 400 °C for 1 h under vacuum and air ambient respectively. Vacuum annealing was carried out in a quartz tube furnace, where the pressure was maintained at 2.4~4.8 × 10⁻³ Pa. Air annealing was performed in a rapid thermal annealing (RTA, MILA-3000, ULVAC-RIKO Inc., Yokohama, Japan) system.

3.3. ZnO Nanorods Coated AZO and ZnO Layers by Mist Chemical Vapor Deposition

AZO and ZnO layer coatings on as-grown nanorods were fabricated using mist chemical vapor deposition (Mist CVD). The deposition conditions are summarized in Table 4. For the ZnO coating, zinc acetate (ZA, Zn (CH3COO)₂·2H₂O, Sigma-Aldrich, St. Louis, MO, USA) was dissolved in a mixed solvent of deionized water and methanol (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) at a volume ratio of 90:10 to obtain a total metal concentration of 0.04 mol/L. For the AZO coating, aluminum acetylacetonate (AA, Al (C₅H₇O₂)₃, Sigma-Aldrich, St. Louis, MO, USA) was additionally introduced into the zinc acetate precursor, corresponding to an Al content of 2 at.%. The mist droplets are generated from the precursor solution in the solution chamber using an ultrasonic transducer with a frequency of 2.4 MHz. The droplets are transported to the reaction chamber by N₂ gas serving as a carrier gas (2.5 L·min⁻¹) and a dilution gas (4.5 L·min⁻¹). The substrates were placed in the reaction chamber maintained at 400 °C, and the coating time was 10 min.

Table 4. Deposition conditions for the mist CVD coating process.

Deposition Parameters	Conditions
Solute	Zinc acetate, aluminum acetylacetonate
Solvent composition	Methanol and DI water (10:90, v/v)
Total metal concentration (mol·L−1)	0.04
Al content (at.%)	0, 2
Ultrasonic frequency (MHz)	2.4
Carrier gas (L·min−1)	N2, 2.5
Dilution gas (L·min−1)	N2, 4.5
Substrate	ZnO nanorods/AZO
Substrate temperature (°C)	400
Deposition time (min)	10

Table 4. Deposition conditions for the mist CVD coating process.

Deposition Parameters	Conditions
Solute	Zinc acetate, aluminum acetylacetonate
Solvent composition	Methanol and DI water (10:90, v/v)
Total metal concentration (mol·L−1)	0.04
Al content (at.%)	0, 2
Ultrasonic frequency (MHz)	2.4
Carrier gas (L·min−1)	N2, 2.5
Dilution gas (L·min−1)	N2, 4.5
Substrate	ZnO nanorods/AZO
Substrate temperature (°C)	400
Deposition time (min)	10

3.4. Photocatalytic Degradation Experiment

To evaluate the photocatalytic degradation performance of ZnO nanorods, azo dye methyl red (MR, Hayashi Pure Chemical Ind., Ltd., Osaka, Japan ) was used as a model pollutant. The initial concentration of MR solution was fixed at 1 × 10⁻⁴ mol·L⁻¹ throughout all photocatalytic experiments to ensure consistent comparison among different samples. A solution was prepared by dissolving 13.5 mg of the dye in 500 mL of deionized water. The solution was magnetically stirred in the dark until complete dissolution. Prior to irradiation, the samples (1.5 × 1.5 cm) were immersed in 70 mL of MR solution and kept in the dark for 30 min to establish adsorption–desorption equilibrium. Subsequently, the sample and solution were irradiated under ultraviolet light (254 nm) for 5 h with magnetic stirring. After irradiation, the solution was stored in the dark prior to analysis. In addition, to evaluate the reusability of the photocatalysts, the degradation experiment was repeated for three cycles under the same conditions, with each cycle lasting 5 h under UV irradiation.

3.5. Characterizations

Morphological, structural and optical characterizations were carried out for all the samples. The morphology and elemental compositions of ZnO nanorods were studied using a field-emission scanning electron microscope (FE-SEM, SU-8020, Hitachi, Tokyo, Japan) equipped with energy-dispersive X-ray spectroscopy (EDX) and Transmission Electron Microscopy (TEM, Titan ETEM G2, FEI, Hillsboro, OR, USA). The structural properties of the prepared samples were examined by X-ray diffraction (XRD, SmartLab, Rigaku Corporation, Tokyo, Japan). The optical properties and photocatalytic degradation performance of the samples were evaluated using a UV-visible spectrophotometer (U-4100, Hitachi, Tokyo, Japan). The sheet resistance was measured using a four-point probe system (Loresta-GP MCP-T610, Mitsubishi Chemical Corporation, Tokyo, Japan). Photoluminescence (PL) spectra were measured at room temperature using a He-Cd laser (IK3201R-F, KIMMON KOHA Co., Ltd., Tokyo, Japan) with an excitation wavelength of 325 nm as the excitation source.

4. Conclusions

The crystallinity of ZnO nanorods was improved by both annealing-treatment processes and surface-coating processes. Compared with vacuum annealing, air annealing contributed to the improvement of crystallinity significantly due to the oxygen supply during the annealing process and the partial reduction of oxygen-vacancy-related defects. Air annealing at 400 °C resulted in the highest crystallinity of ZnO nanorods, resulting in a higher degradation efficiency of 70% and degradation rate of 0.0041 min⁻¹ toward the MR solution.

Both ZnO- and AZO-coated ZnO nanorods contributed to enhance the degradation rate of the MR solution. The AZO-coated sample exhibited the highest photocatalytic activity, with a degradation efficiency of 64%, which was attributed to improved crystallinity, enhanced light absorption capability, and effective surface modification rather than significant bandgap variation.

These results demonstrate that appropriate annealing treatment and Al-doped ZnO surface engineering are effective strategies for enhancing the photocatalytic performance of ZnO nanorods. Notably, the obtained photocatalytic efficiency was considered promising under the low catalyst-loading nanostructured thin-film photocatalytic system employed in this study. The present study provides a practical route for the development of ZnO-based thin-film photocatalysts for wastewater treatment applications.