Calcination Temperature-Induced Morphology Transformation in WO₃ Flower-like Thin Films for Photocatalytic Wastewater Treatment

Abstract

WO₃ nanoflowers were synthesized via anodization and subsequently calcined in air at different temperatures (200–700 °C) to evaluate their photocatalytic activity. The samples were characterized in terms of their morphological, crystallite, and optical properties. Anodization produced WO₃ hydrate with a layer thickness of ~1.2 µm, which was transformed into WO₃ after heating. All samples exhibited monoclinic phase, with Raman shift intensity increasing with the calcination temperature. Some residual WO₃·H₂O was detected at certain temperatures. The calculated bandgap energy ranged from 2.49 to 2.67 eV, with higher calcination temperatures leading to lower absorbance in the UV region. The photodegradation of phenol under UV-Vis radiation reached 35% in 60 min for WO₃_700 °C, where the photocatalyst suffered a morphological transformation from a nanoflower to nanogranular structure, accompanied by increased crystallinity. Under visible light, the phenol abatement was limited, achieving 1–3% degradation. The WO₃ surface is likely negatively charged at the solution’s pH (5.6), which may explain the low phenol adsorption (~1%).

1. Introduction

Water scarcity and water pollution are two major problems of the 21st century. The persistence of complex organic compounds in water is a global issue that must be addressed urgently [1]. Biorecalcitrant organic compounds remain in water resources because conventional wastewater treatment technologies do not efficiently remove them [1,2]. However, the use of advanced oxidation processes (AOPs) can promote the degradation of these pollutants by transforming them into H₂O and CO₂ when complete mineralization is achieved.

Among AOPs, photocatalysis require a semiconductor photocatalyst to initiate and enhance the generation of oxidative radicals, particularly the •OH radical [3]. Therefore, the study and development of novel photocatalytic materials is a key research focus, particularly with the objective of achieving higher photocatalytic performance.

Regarding the materials, tungsten trioxide (WO₃) can be an interesting option to consider since it has already been widely studied by the scientific community for various applications such as gas sensors, conducting electrodes, hydrogen generation, electrocatalysts, or photocatalysts [4,5]. This material has an indirect bandgap energy of 2.3–2.8 eV, which allows WO₃ to be active under visible light and absorb up to 12% of the solar spectrum (visible spectrum up to 500 nm) [6]. This characteristic reduces dependence on artificial radiation, making WO₃ a promising candidate for photo-driven AOPs and a viable option for water and wastewater treatment solutions.

Currently, most of the photocatalysis-based solutions that are being studied and developed rely on the traditional TiO₂ powder (bandgap energy of 3.08–3.20 eV). However, TiO₂ presents several drawbacks for photocatalytic oxidation processes, namely: (i) powder materials are difficult to separate from aqueous solutions and reuse, posing challenging issues for industrial implementation; (ii) the bandgap energy of TiO₂ requires UV radiation, which is often provided artificially, leading to higher energy consumption and costs; (iii) while TiO₂ can be activated by solar radiation (due to UVA fraction in the solar spectrum), it cannot be activated under visible light, which constitutes the largest portion of the solar radiation spectrum. Thus, WO₃ offers certain advantages over TiO₂, but its application is also predominantly reported in powder form, which still presents similar limitations.

A potential solution to overcome the challenges associated with powder photocatalysts involves either supporting the powder on a substrate or producing the material in a form that can be easily separated from water [7]. In this regard, the anodization method is a simple and cost-effective process that enables the generation of metal oxides from a metallic foil (for example, Ti, W, Zn, Mo, etc.) forming photocatalysts that are easier to recover from water [8]. Additionally, this technology allows the formation of different nanostructures (such as nanotubes, nanorods, nanoparticles, nanosheets, nanoflowers, etc.) [9] whose presence and modification can enhance the material’s properties and improve the photocatalytic process. For example, TiO₂ nanotubes exhibit superior photocatalytic performance when compared to compacted TiO₂ [10,11,12].

In previous studies, Pancielejko et al. [13] obtained WO₃ thin films for air purification, obtaining WO₃ nanoflowers with bandgap energies ranging from 2.40 to 2.75 eV and capable of removing 100% of toluene (200 ppm) in 60 min under visible light (λ_max = 415 nm). However, in that study, the calcination conditions were not optimized for the obtained films. It is known that the calcination conditions play a crucial role in enhancing the material’s activity, and therefore, these conditions should be considered and optimized. With this, Fernández-Domene et al. [14] investigated different calcination temperatures (400 and 600 °C) for the preparation of WO₃ nanorods used in the photoelectrocatalytic removal of an insecticide at acidic pH. Their results indicated that calcination at 600 °C promotes a higher degree of dehydration and crystallinity. Similarly, Mzimela et al. [15] evaluated the calcination temperature (300, 500, and 700 °C) of WO₃ nanoparticles synthesized via the acidic precipitation method, concluding that the photocatalytic oxidation of rhodamine B was only achieved with samples calcined at 300 °C.

Although previous studies from literature examined WO₃ samples calcined at two or three different temperatures, no research has reported the optimization of the photocatalytic activity of anodized WO₃ by evaluating a wide range of calcination temperatures. Therefore, to the best of our knowledge, this work reports for the first time the impact of various calcination temperatures (from 200 to 700 °C) on WO₃ nanoflowers synthesized via anodic oxidation. The impact of calcination temperature and time were evaluated based on the photocatalytic oxidation of a phenol solution using the different materials.

2. Results and Discussion

2.1. Photocatalyst Characterization Results

SEM analysis was performed to examine the tungsten oxide film morphology and to determine the dimensions of the oxide layer. Figure 1 presents the morphologies obtained for the anodized W foil and the tungsten oxides calcined at different calcination temperatures, with the corresponding film color shown as insets.

The anodization resulted in the formation of flower-like bundles with an oxide layer thickness of approximately 1.2 µm, which agrees with the study of Pancielejko et al. [13]. However, this analysis also confirms that the annealing process can influence the nanostructure’s morphology, leading to a transformation from a flower-like structure to platelet and nanogranular shapes at temperatures above 400 °C. Comparing the as-anodized sample with the samples annealed in air from 200 to 400 °C (Figure 1a–d), no noticeable morphological differences were observed. Additionally, increasing the annealing temperature caused a change in the oxide layer color, as shown in the SEM image insets (Figure 1).

For the sample calcined at 700 °C, it is possible to visually note the partial destruction of the W foil, with oxide formation also observed on the non-anodized regions. A further increase in temperatures above 700 °C would likely result in the complete W foil disintegration, and due to this, higher temperatures were not explored. Similar behavior was reported by Syrek et al. [16], who observed grain formation and metallic W foil destruction at 700 °C. The development of these nanostructures may be associated with the transition from the amorphous to the monoclinic phase, as is discussed further. Figures S1–S3 provide additional SEM images of WO₃ nanostructures obtained after calcination at 500, 600, and 700 °C at different magnifications, offering a more detailed view of their structural morphology. Moreover, although not explicitly analyzed in this study, it is likely possible that the morphological changes induced by calcination temperature also influence the particle size as well as the specific surface area [15,17].

Different WO₃ nanostructures have been reported in previous studies. As example, Roselló-Márquez et al. [18] reported the anodization of a W foil using 0.05 M H₂O₂ with 1.5 M of H₂SO₄, forming a spongy layer with very small nanoplatelets. When H₂SO₄ was replaced by CH₄O₃S, tiny nanowires were obtained. In another study, Fernández-Domene et al. [14] synthesized nanorods and nanosheets that evolved into more defined nanorods, as the calcination temperature increased from 400 to 600 °C. Additionally, the anodization at 50 V in a 0.25 wt% HF electrolyte solution followed by calcination at 500 °C resulted in WO₃ nanopores with a blue oxide layer [5], as also observed in this study for the sample calcined at the same temperature (Figure 1). Mena et al. [17] reported that WO₃ microspheres obtained by the sol–gel method began to lose their shape as the calcination temperature increased from 500 to 600 and 700 °C, also promoting a crystallite phase change from orthorhombic (500 °C) to monoclinic (600 and 700 °C).

The crystallite phases were analyzed using Raman spectroscopy, as presented in Figure 2a. Higher annealing temperatures led to increased crystallinity, evidenced by more intense peaks. Therefore, the Raman spectra were divided into two separate figures (Figure 2b,c). Moreover, the increase in temperature was also accompanied by the conversion and higher definition of WO₃ crystallite phases.

The as-anodized sample and the ones annealed up to 300 °C appear to exhibit amorphous structure or poor crystallinity (Figure 2c). The WO₃_A sample presents Raman bands at 191, 661, and 947 cm⁻¹ that are attributed to the antisymmetric stretching vibrations of (W–O–W) bonds, the symmetric stretching vibrations of (O–W–O), and the symmetric stretching mode of the short terminal W⁶⁺=O bond, respectively [19]. Similar observations are reported in the literature, with these bands indicating the presence of WO₃·H₂O, WO₃·2H₂O, and amorphous WO₃·n(H₂O) hydrated oxides [19]. These hydrated species typically form during WO₃ synthesis and are associated with the anodization process in liquid electrolytes, particularly under acidic conditions [19].

Interestingly, in their work, Fernández-Domene et al. [19] suggest the presence of WO₃·2H₂O based on the observation of a doublet bond near 660 and 680 cm⁻¹ as well as the existence of Raman bands near 210 and 270 cm⁻¹. However, this is not observed in the present study. Instead, the existence of singular bands at 191 and 661 cm⁻¹ are consistent with the existence of WO₃·H₂O [19]. In the WO₃_300 °C sample, new peaks begin to appear at 133, 272, 325, 718, and 805 cm⁻¹. The 718 and 805 cm⁻¹ peaks result from the partition of the 661 cm⁻¹ peak, while the others (133, 272, 325 cm⁻¹) are characteristic signals of monoclinic WO₃ [19,20]. The 718 and 805 cm⁻¹ peaks correspond to the symmetric stretching modes arising from O–W–O bonds, whereas the peaks at 272 and 325 cm⁻¹ are associated with the bending of δ (O–W–O) bonds [19,20]. The low frequency peak at 133 cm⁻¹ is attributed to the lattice modes of monoclinic crystallite phase [19].

The samples annealed at 400, 500, and 600 °C exhibit the same peaks as the WO₃_300 °C sample, but with slight deviations and increased intensities as the temperature rises. The asymmetric peak at 718 cm⁻¹ observed in the samples calcined at 300 and 400 °C can indicate the existence of residual traces of tungsten hydrate [21], which could explain these variations. Although the Raman spectra of the samples share similar peaks, a noticeable increase in intensity is observed from 300 to 400 °C. The literature indicates that a monoclinic WO₃ phase is present at 400 °C, but at 500 °C, the structure may transition to either a monoclinic or orthorhombic WO₃ phase, or even a mixture of both [17,19,22,23]. It seems that the synthesis method may influence the crystallite structure at 500 °C [17]. Since no additional peaks are detected and only the intensity increases, it suggests that the samples calcined at 500, 600, and 700 °C remain in the monoclinic WO₃ phase, consistent with findings from other studies [16,17,20]. Moreover, at elevated temperatures, the monoclinic structure is the most thermodynamically stable WO₃ phase due to the minimization of the surface Gibbs energy/entropy, which enhances the structural stability [24]. It is also expected an increase in the oxide layer thickness and in the crystallite size as the annealing temperature (increases [16,20].

X-ray diffraction (XRD) analysis further corroborated the Raman spectroscopy results, confirming the presence of monoclinic WO₃ (ICDD entry 04-005-4272, space group P2₁/n), as shown in Figure 3 [22,24,25].

The sample calcined at 700 °C exhibits weak signals of metallic tungsten, which may be attributed to the formation of tungsten oxides and the structural destruction observed in the entire W foil. Interestingly, the XRD confirms the observations of Raman spectroscopy findings, confirming the presence of WO₃·H₂O in the as-anodized sample [26]. Based on this observation and supporting literature [13,19,27], it appears that the anodization of W foil under the specified conditions can be summarized by the following reactions (Equations (1)–(3)). The detailed mechanism regarding the formation of WO₃ nanoflowers through anodization has been described by Ng et al. [27].

W + 2 H₂O → WO₂²⁺ + 4 H⁺ + 6 e⁻

(1)

WO₂²⁺ + 2 H₂O → WO₃·H₂O + 2 H⁺

(2)

WO₃·H₂O + heat (>300/400 °C) → WO₃ (monoclinic) + H₂O

(3)

During anodization, an intermediate species (WO₂²⁺) is formed, which subsequently reacts with H₂O to produce WO₃·H₂O as the final anodization product. Upon calcination, dehydration of OH⁻ groups occurs, leading to the formation of WO₃, which in this case, adopts a more defined monoclinic structure for temperatures above 400 °C [21,27,28]. Furthermore, the presence of WO₃·H₂O appears to play a crucial role in maintaining the nanoflower-like morphology. This is supported by Raman analysis, which shows that when the peaks become more symmetric (at 500 °C and above) the flower-like shape is lost, converting to a different nanostructure.

The optical properties of the samples calcined at different temperatures were also evaluated. The indirect bandgap energy was determined using Tauc’s plot based on the direct transition between 2p electrons present in the WO₃ valence band of oxygen and the 5d conduction band of WO₃ [29,30]. Tauc’s plot is a widely used method used for estimating the bandgap energies of semiconductors based on the assumption that the absorption coefficient (α) can be expressed by (αhv)^1/y = B(hv − E_BG), in which the “y” depends on the nature of the electron transition (1/2 and 2 for direct and indirect transitions, respectively), the “hv” is the photon energy, “B” is a constant, and the “E_BG” is the bandgap energy of the material [31]. The absorption coefficient α is proportional to the Kubelka–Munk function (F(R) = (1−R²)/2R) that can be calculated based on the samples reflectance (R), obtained by DRS measurements.

With this, by plotting (αhv)^1/y versus the photon energy (hv) it is possible to estimate the bandgap energy [24,31]. Figure 4 presents the DRS UV-Vis spectra (a) and the corresponding Tauc’s plot (b).

As the calcination temperature increases, a decrease in the absorption intensity of the WO₃ samples is observed. The same behavior was reported by Abbaspoor et al. [24], where the DRS measurements showed that the as-anodized WO₃ exhibited the highest absorption intensity, while the sample calcined at 700 °C presented the lowest intensity, which is consistent with the findings of the present study. Additionally, the authors reported a “red shift” in the absorption edge with the increase in the calcination temperature increases, which is also observed in this work. This “red shift”, along with variations in the DRS absorption profiles, is attributed to an increase in the crystallite size [24]. Therefore, the differences observed in the DRS measurements (Figure 4a) can be directly linked to the morphological changes in the WO₃ thin films induced by calcination.

The bandgap energies calculated from the Tauc plot (Figure 4b) are presented in

Table 1. Summary of the performance of synthetized WO₃ films.

Sample	Type of Nanostructure	Bandgap Energy (eV)	Phenol Photodegradation—UV-Vis (%)	Phenol Photodegradation—Vis (%)
WO3_A	Flower-like	2.42	30	1
WO3_200 °C	Flower-like	2.65	27	2
WO3_300 °C	Flower-like	2.49	30	3
WO3_300 °C_1h	Flower-like	2.53	29	1
WO3_300 °C_2h	Flower-like	2.45	30	2
WO3_300 °C_6h	Flower-like	2.43	31	1
WO3_400 °C	Flower-like	2.50	27	1
WO3_500 °C	Platelet-like	2.64	32	1
WO3_600 °C	Granular	2.67	34	2
WO3_700 °C	Granular	2.64	35	0

and ranged between 2.42 and 2.67 eV, consistent with the literature [13,22,32]. The bandgap energy depends on bonding–antibonding interactions, and variations in this energy can be attributed to structural organization induced by thermal treatments, lattice parameters, crystallite sizes, defects, and shapes [22].

Photoluminescence measurements were conducted to obtain more details about the carrier’s generation through photoexcitation of WO₃ samples annealed at different temperatures. The results are presented in Figure 5.

In this experiment, lower PL intensity is associated with reduced recombination, which, when minimized, generally benefits the photocatalytic reactions [33,34]. The peaks observed at around 420, 438, and 481 nm are attributed to the presence of intrinsic defects, such as oxygen vacancies, while the peak observed near 527 nm is pointed to the radiative recombination of photogenerated charges [13]. In this case, it appears that as the calcination temperature increases, the recombination also increases. This phenomenon becomes particularly pronounced above 500 °C, which is the temperature at which the morphology of the WO₃ flowers changes (Figure 1). This suggests that the flower-like structure exhibits lower recombination compared to the grain-like morphology observed at calcination temperatures of 500, 600, and 700 °C.

It can be observed that when analyzing the temperatures in which the flower-like morphology is preserved (200–400 °C), the PL intensities are similar. However, the material calcinated at 300 °C seems to present slightly higher recombination than the other samples. Interestingly, the as-anodized sample shows the lowest recombination among all the synthesized WO₃ samples. Combined with its favorable optical properties and estimated bandgap energy (2.42 eV), this suggests that it could be a promising material for photocatalysis.

2.2. Phenol Photodegradation

WO₃ can be an interesting material for application in the photocatalytic oxidation of water contaminants due to its low bandgap energy, which is compatible with visible light activity. In this study, the photocatalytic activity of the material was evaluated for the abatement of phenol (20 mg/L), since it is a common contaminant in industrial wastewater [35].

Figure 6 presents the phenol abatement by photocatalysis with UV-Vis and visible light radiation after 60 min reaction (maximum error of ±1%). Figure S4 presents the degradation profile of phenol for the WO₃ samples calcined at different temperatures.

The results show a negligible adsorption on the photocatalyst’s surface, with only about 1% phenol removal in 30 min. However, after 60 min of irradiation, the highest phenol removal (under UV-Vis) was achieved for the WO₃_700 °C sample. Regarding adsorption, the pH zero point of charge (pH_zpc) of WO₃ is approximately 2.5 [36,37], while the phenol solution has a pH of 5.6. When the solution pH > pH_zpc, the photocatalyst surface becomes negatively charged, while for pH < pH_zpc, the surface becomes positively charged [38]. Thus, the anodized WO₃ has a negatively charged surface. At this pH, phenol is neutral (pK_a = 10), so the electrostatic interaction with WO₃ is minimal, leading to very poor adsorption, which agrees with the present results.

The phenol degradation by direct photolysis was reported by Mazierski et al. [39], showing 3% degradation under UV-Vis radiation and 1% under visible radiation. Analyzing the UV-Vis photodegradation reaction along with the sample’s crystallinity (Figure 2), it is possible to see that as the temperature increases, the phenol degradation rate is also higher (excepting the case of 400 °C). The increase in the calcination temperature leads to an increase in the sample’s crystallite intensity. Furthermore, from 500 to 700 °C, the oxide morphology changes, which may also be accompanied by an increase in the crystallite size [15,16,20]. The XRD analysis shows only a small presence of metallic tungsten at 700 °C (Figure 3), suggesting higher formation of tungsten oxides.

Therefore, this may explain the observed differences and the higher activity of the sample calcined at 700 °C, although the photocatalytic activity order is contrary to what is observed in the PL measurements (Figure 5). The fact that the 400 °C sample does not follow this trend may be related to the fact that the 400 ºC is a transition temperature between the flower-like morphology and the new granular/platelet-like structure. This assumption helps explain why the 300 °C sample performs the best under UV-Vis (just analyzing the flower-shape structures).

Additionally, under visible light radiation, the phenol removal is very low. The WO₃_300 °C has a relatively low bandgap energy of 2.49 eV (Table 1) compared to the other calcined samples, which can explain the best removal rate of 3%. The as-anodized sample has the lowest bandgap (2.42 eV), but its poor performance can be related to its amorphous composition.

Under visible light, Mzimela et al. [15] obtained about 78% dye removal (mostly related to adsorption—72%) using WO₃ powders calcined at 300 °C. Increasing the calcination temperature to 500 or 700 °C resulted in negligible removal of 1–2%, mainly attributed to adsorption, which aligns with the observations of the present study. All the samples exhibited monoclinic crystallite structure. Interestingly, as the calcination temperature increased, the authors observed a decrease in surface area, which could explain the minimal degradation observed at higher temperature, as this decrease negatively impacts adsorption. However, by optimizing the reaction conditions (calcination temperature, solution pH, pollutant concentration, catalyst loading, and irradiation time), the authors were able to improve the photocatalytic degradation by achieving 96% removal after 4 h of irradiation, with adsorption having no contribution to this degradation. Similar process conditions were considered by other authors [40], who also found that well-defined structural morphology (in the case, nanorods) leads to better performance when compared to nanoparticles (monoclinic phase present).

Therefore, although the present study and the results of Mzimela et al. [15] suggest that the monoclinic phase may not be efficient under visible light, the work of Adhikari et al. [40] and the process optimization carried out by Mzimela et al. [15] demonstrate that it is possible to obtain good performance using monoclinic WO₃. However, this low performance of WO₃ can also be attributed to other crystallite phases and be a consequence of the inherent characteristics of WO₃. For instance, Aslam et al. [41] reported a phenol degradation of about 2% and 8% in 30 and 180 min using powder disc-shaped WO₃ (~2.55 eV) under solar radiation. For other phenol derivatives, the degradation was almost complete for chlorophenol and nitrophenol, while about 11% degradation was observed for resorcinol, in 180 min. The authors also mention that the formation of superoxide anion radical (•O₂⁻) via the reduction of dissolved or adsorbed oxygen by conduction band electrons (e⁻) is not possible for WO₃, as its conduction band potential is +0.74 V, while the formation of •O₂⁻ requires the potential to be at −0.28 V. However, the authors suggest the presence of this oxidizing species through some alternative mechanism.

In another study involving solar radiation, only the incorporation of Au nanoparticles onto the WO₃ surface improved the photocatalytic activity (due to plasmonic effect), leading to enhanced degradation of some pollutants [42]. In fact, in the work of Desseigne et al. [42], the WO₃ was composed of an orthorhombic crystallite phase (calcination in air at 500 °C) and the presence of gold nanoparticles resulted in better performance due to a longer lifetime of the photogenerated charges. The rate of charge recombination depends on the electrical properties of the material, but the Schottky barrier created at the interface between the gold nanoparticles and the semiconductor allows to increase the recombination time, resulting in a reduced recombination. This leads to a higher generation of radicals, which are responsible for the increased degradation of pollutants [42].

Although WO₃ presents a low bandgap energy compatible with visible light irradiation, it appears to have a high recombination rate (as observed in Figure 4 and Figure 5), which may explain the low efficiency for photocatalysis, particularly under visible light. Furthermore, the work of Aslam et al. [41] and Desseigne et al. [42] support the notion of easy recombination in WO₃, as the electrons are not capable of generating •O₂⁻ radicals due to the conduction band energy level of WO₃. As a result, these electrons are available to react with the holes (h⁺). This recombination process is hindered by the incorporation of Au nanoparticles, which causes a stronger electron trapping effect, ultimately enhancing the photocatalytic activity by increasing the generation of oxidative species.

Therefore, using more efficient electron trappers should increase the charge carrier’s lifetime and reduce the recombination effect. This can be achieved by replacing oxygen by ozone [17,32,43,44,45,46], decorating WO₃ with noble metals (as Au or Ag) [42,47], or employing photoelectrocatalysis [48,49,50].

Interestingly, for the photocatalytic oxidation of toluene in the gaseous phase, Pancielejko et al. [13] achieved complete removal in 60 min of visible light irradiation (λ_max = 415 nm). This suggests that WO₃ can present high activity for the treatment of gaseous streams by photocatalysis, while its application for the treatment of aqueous contaminants may not be as effective or may require better optimization of the process conditions [15]. The high activity observed in the work of Pancielejko et al. [13] is also attributed to the contribution of this type of nanostructure, since the flower-like morphology enhances the surface area and light absorption due to multiple light scattering. This, in turn, reduces the recombination of electron-hole pairs, thus improving the photoactivity of WO₃ [13,51,52].

To analyze the influence of calcination time, the WO₃_300 °C was chosen since it is the sample with the highest photoactivity for phenol degradation and retains the flower-like nanostructure (Table 1). The calcination time was varied for 1, 2 or 6 h (characterization data are presented in Supplementary Materials). Figure S5 presents the SEM images of WO₃_300 °C calcined for different times, and Figure S6 presents different magnifications for WO₃_300 °C_1h to better understand the oxide layer morphology. Similar shapes are observed for the other flower-like structures. To evaluate the crystallite phases, the Raman and XRD spectra are also reported in Figures S7 and S8, respectively, demonstrating that the samples annealed at 300 °C for different time periods maintain the monoclinic structure. However, different Raman intensity is observed (Figure S7).

Regarding their optical properties, the samples exhibit similar DRS absorbances (Figure S9), although the WO₃_300 °C_1h shows higher absorbance, while the WO₃_300 °C_4h shows the lowest. The corresponding bandgap energy was calculated using the Kubelka–Munk function (Figure S10) and found to be 2.53, 2.45, 2.49, and 2.43 eV for the sample annealed for 1, 2, 4, and 6 h, respectively (Table 1). Additionally, the photogenerated electron-hole pairs recombination was evaluated using PL analysis (Figure S11), with WO₃_300 °C_6h presenting the lowest recombination. These results also suggest that samples with higher crystallinity (as indicated by Raman intensity—Figure S6) experience greater recombination.

The photocatalytic degradation of phenol was tested under UV-Vis (Figure S12) and Vis (Figure S13) radiation. The results suggest that increasing the calcination time does not significantly change the phenol degradation kinetics. Table 1 summarizes the results involving the synthesized WO₃ films. To highlight the findings from this study,

Table 2. Performance of anodized WO₃ in photocatalysis.

Radiation	Contaminant	Photocatalyst Form	Annealing Conditions	Main Results	Reference
Visible radiation—>400 nm	Methyl orange (20 ppm, 20 mL)	Nanoporous WO3 (3 × 4 cm2)	Air, 500 °C, 2 h, 2 °C/min	Monoclinic crystallite structure 70.4% degradation in 180 min	[5]
UV radiation—325 nm	Acid orange 7 (~5 ppm, 2 mL)	Nanoporous WO3	Air, 500 °C, 1 h,	Orthorhombic crystallite structure About 48% removal in 150 min	[23]
Visible radiation—455 nm	Methylene blue (~16 ppm, 3 mL)	Nanoporous WO3 (1.5 × 0.9 cm2)	Argon, 500 °C, 3 h	Monoclinic and orthorhombic crystallite structure About 3%, 5%, 6% and 7% in 60, 120, 180, and 240 min, respectively	[53]
UV radiation	Methyl orange (30 ppm)	WO3 nanopores (4 × 1 cm2)	Air, 500 °C, 1 h	Monoclinic crystallite structure About 2%, 18%, 25%, 38%, and 45% in 60, 120, 180, 240, and 300 min, respectively	[54]
UV-Vis-Infrared radiation	Methyl orange (30 ppm, 100 mL)	WO3 nanoplates (5 × 1 cm2)	Argon, 400 °C, 4 h	Orthorhombic WO3⋅H2O crystallite structure About 17%, 37%, 52%, 64%, 75% in 60, 120, 180, 240, and 300 min	[55]
Solar simulated radiation	Congo red (~7 ppm, 2 mL)	Nanoporous WO3	Air, 450 °C, 1 h, 2 °C/min	Orthorhombic crystallite structure About 65% and 86% for 60 and 90 min, respectively	[56]
Visible radiation	Methyl orange (30 ppm, 100 mL)	Nanoporous WO3 (4 × 1 cm2)	Air, 400 °C, 4 h	Monoclinic crystallite structure 50% degradation in 300 min	[57]
Visible radiation	Methyl orange (10 ppm, 20 mL)	Nanoporous WO3 (1 × 1 cm2)	Air, 500 °C, 3 h, 10 °C/min	Monoclinic crystallite structure About 10%, 22%, 32%. 40%, and 45% degradation in 60, 120, 180, 240, and 300 min, respectively	[58]
Visible radiation—>400 nm	Methylene blue (~10 ppm, 25 mL)	Nanoporous WO3 (0.63 cm2)	-	About 40 and 60% degradation in 60 and 120 min, respectively	[59]
Solar simulated radiation	Methyl orange (30 ppm, 100 mL)	WO3 nanotubes WO3 nanopores (4 cm2)	-	About 27%, 48%, 63%, and 73% for WO3 nanotubes and about 17%, 46%, 57%, and 60% for WO3 nanopores in 120, 180, 240, and 300 min, respectively	[60]
Solar simulated radiation	Methyl orange (30 ppm, 200 mL)	WO3 nanotubes WO3 oxide layer (2.5 × 2.5 cm2)	-	About 55% and 72% degradation for WO3 oxide layer and WO3 nanotubes in 300 min, respectively	[61]
Visible radiation—>420 nm	Methyl orange Rhodamine B Methylene blue 4-chlorophenol Bisphenol A (5 ppm)	WO3 nanoporous (2 × 2 cm2)	Air, 550 °C, 3 h	Monoclinic crystallite structure MO: ~3% and ~7% RhB: ~18% and ~38% MB: ~30% and ~58% 4-CP: ~14% and ~29% degradation in 60 and 120 min, respectively BPA: ~1% degradation in 120 min	[62]
Visible radiation—>420 nm	Methylene blue (~3 ppm, 20 mL)	WO3 nanotubes WO3 nanoplates (1 × 1 cm2)	Air, 450 °C, 4 h, 5 °C/min	Orthorhombic crystallite structure About 55%, 75%, and 82% in 60, 120, and 150 min for both nanostructures, respectively	[63]
UVA radiation—365 nm	Pentachlorophenol (20 ppm)	Nanoporous WO3 Thin-film WO3	Oxygen, 450 °C, 4 h, 5 °C/min	Monoclinic crystallite structure About 65% and 97% for nanoporous WO3 and 35% and 66% degradation in 60 and 120 min for thin-film WO3, respectively	[64]
Visible radiation	Pentachlorophenol (20 ppm)	Nanoporous WO3 Thin-film WO3	Oxygen, 450 °C, 4 h, 5 °C/min	Monoclinic crystallite structure About 21% and 34% for nanoporous WO3 and 6% and 13% degradation in 60 and 120 min for thin-film WO3, respectively	[64]
Solar simulated radiation	Tetracycline hydrochloride (20 ppm, 30 mL)	Nanoporous WO3 (2.3 cm2)	Air, 450 °C, 2 h, 3 °C/min	Monoclinic crystallite structure About 43% degradation in 60 min	[65]
UV-Vis radiation	Phenol (20 ppm, 8 mL)	Nanoflower-like WO3 (1.9 cm2)	Air, 300 °C, 4 h, 4 °C/min	Monoclinic crystallite structure 30% degradation in 60 min	This work
Visible radiation—>420 nm	Phenol (20 ppm, 8 mL)	Nanoflower-like WO3 (1.9 cm2)	Air, 300 °C, 4 h, 4 °C/min	Monoclinic crystallite structure 3% degradation in 60 min	This work
UV-Vis radiation	Phenol (20 ppm, 8 mL)	Nanoflower-like WO3 (1.9 cm2)	Air, 700 °C, 4 h, 4 °C/min	Monoclinic crystallite structure 35% degradation in 60 min	This work

presents a state-of-the-art comparison of anodized WO₃ photocatalysts for the degradation of different contaminants.

3. Materials and Methods

3.1. Synthesis of WO₃ Nanoflowers

The detailed synthesis of WO₃ nanoflowers by anodization is provided in our previous work [13]. In general, tungsten foil was cut into 2.5 × 2.5 cm pieces and ultrasonically cleaned in acetone, isopropanol, methanol, and deionized water for 10 min in each solvent, followed by drying with a nitrogen stream. The W foil was anodized in an O-ring anodization cell using the foil as anode and a platinum mesh as cathode, at room temperature (20 °C).

The electrolyte solution was composed of 0.5 wt% NaF in 1 M H₂SO₄ and the anodization was performed at 40 V during 1.5 h. These parameters were selected based on our previous work [13], as they achieved the highest toluene degradation. After anodization, the samples were carefully immersed in deionized water to remove any electrolyte residues and then dried at 60 °C overnight. Calcination was performed at various temperatures (200, 300, 400, 500, 600, and 700 °C) during 4 h with a heating step of 4 °C/min in air. For the best-performing sample, the effect of the calcination time was also evaluated. The samples are labeled as “WO₃_T”, where “T” represents the calcination temperature (200–700 °C). The as-anodized sample is denoted as “WO₃_A”.

3.2. WO₃ Photoactivity Evaluation

The photocatalytic activity of the prepared material was assessed by the degradation of a 20 mg/L phenol solution (pH = 5.6). The photocatalytic reaction was carried out in a quartz reactor with 8 mL of phenol solution and the WO₃ sample (2 × 2 cm, with an anodized layer of 1.9 cm²). The adsorption capacity of the material was tested under dark conditions with an oxygen stream (10–12 L/min) for 30 min. After this period, the irradiation (UV-Vis, 1000 W, Oriel 66021, Oriel Instruments, Irvine, CA, USA) was applied for 1 h, with sampling at each 20 min. For the visible light experiments, a GG420 filter was placed near the reactor to cut the UV radiation below 420 nm. The irradiation intensity was measured with a UV power meter (C9536-01, Hamamatsu Photonics K.K., Hamamatsu, Japan) being kept constant at 100 mW/cm². The phenol concentration was determined by HPLC-DAD (Shimadzu, Nakagyo-ku, Japan) using a flow rate of 0.55 mL/min of a 10%:90% mixture of acetonitrile and acidified water. The compound separation was performed in a C18 silichrom column at 40 °C, with phenol detection at 225 nm.

3.3. WO₃ Analytical Characterization

The WO₃ materials were analyzed regarding their morphology, crystallite structure, crystals dimensions, and radiation absorption. The morphology was evaluated using high-resolution scanning electron microscopy (JSM-7610F, JEOL, Akishima, Japan). The crystal structures were analyzed by Raman spectra (DXR2 SmartRaman, Thermo Scientific, Basingstoke, UK) equipped with a 532 nm DXR laser. This was also evaluated by X-ray diffraction (XRD, Phillips Xpert PROMDP, Farnborough, UK) with Cu Kα radiation (λ = 1.5404 Å). The radiation absorbance was measured using a DRS UV-Vis spectrophotometer (UV 2600, Shimadzu, Kyoto, Japan) equipped with an integrating sphere, using barium sulfate as the baseline. The bandgap energy was calculated through Tauc plot analysis (error ± 0.01 eV), based on the DRS data. Photoluminescence (PL) measurements were conducted to assess the recombination phenomenon occurring in the samples, using a photoluminescence spectrometer (LS-50B, PerkinElmer, Waltham, MA, USA) equipped with a Xe lamp and R928 photomultiplier. The determinations occurred at room temperature with excitation radiation at 300 nm, irradiating directly the sample surface at an angle of 90°.

4. Conclusions

The results show that increasing the calcination temperature causes a transformation in the WO₃ morphology, transitioning from flower-like to platelet/granular nanostructures at temperatures above 400 °C. This transformation may be related to the conversion of WO₃·H₂O into WO₃ due to the heating treatment.

After calcination, all the samples exhibited monoclinic phase and the Raman shift intensity increasing with the calcination temperature increase. For certain temperatures, there may still be the presence of residual WO₃·H₂O. The PL measurements suggest that the nanoflower shape reduces the recombination rate, and as the calcination temperature increases, the recombination rate becomes higher when other nanostructures are present.

In general, the photodegradation of phenol increased as the crystallite intensity also increased, with the best result under UV-Vis being achieved for WO₃_700 °C with a 35% degradation in 60 min. These results are probably influenced by the high recombination rate characteristic of WO₃ (as proven by PL), especially in the presence of oxygen. Thus, using powerful oxidants or better electron trapping agents (such as ozone), provide surface decoration with noble metals (as Au), or even employing photoelectrocatalysis, should be considered in future studies to enhance the photocatalytic activity of anodic WO₃ nanostructures.

The effect of calcination time was also evaluated for the best sample that maintained the nanoflower shape (WO₃_300 °C), with no significant improvement in phenol degradation being observed, despite changes in Raman intensities and radiation absorbance (accompanied by different bandgap energy) being noted. Therefore, from this work, the following points can be highlighted:

• Anodization produces WO₃·H₂O that is converted to monoclinic WO₃ after calcination in air;
• WO₃·H₂O forms a flower-like nanostructure, which transforms into a platelet-like and granular nanostructures when heated above 500 °C;
• As the calcination temperature increases, the oxide layer presents different colors;
• The monoclinic WO₃ exhibited bandgap energies ranging from 2.43 to 2.67 eV, depending on the calcination temperature and time;
• The flower-like morphology showed lower recombination compared to platelet-like or granular morphologies;
• The highest photoactivity was observed for the WO₃ granular-like morphology (700 °C), with the highest activity for the nanoflower-like structure being observed for the WO₃_300 °C sample.