Study of Calcination Temperature Influence on Physicochemical Properties and Photodegradation Performance of Cu₂O/WO₃/TiO₂

Abstract

Photodegradation is a sustainable green technology that has been studied worldwide, especially for wastewater treatment. The calcination temperature has an important impact on the physicochemical properties of the prepared photocatalysts. In this study, a ternary photocatalyst of Cu₂O/WO₃/TiO₂ (CWT) was successfully synthesized using an ultrasonic-assisted hydrothermal technique, and the calcination temperature was varied from 500 to 800 °C. The characterization outcomes proved that the anatase phase titanium dioxide (TiO₂) in the CWT composite transformed to rutile phase TiO₂ when the calcination temperature reached 700 °C and 800 °C. The surface area of the CWT composite decreased from 35.77 to 8.09 m².g⁻¹ and the particle size of the CWT composite increased from 39.11 to 180.25 nm with an increasing calcination temperature from 500 to 800 °C. Photoelectrochemical (PEC) studies showed the charge-transfer resistance of 208.10 Ω, electron lifetime of 32.48 ms, current density of 1.40 mA.cm⁻², transient photovoltage of 0.53 V, and p-n heterojunction properties for CWT-500. Reactive Black 5 (RB5) was used as the model pollutant to examine the photodegradation performance. The photodegradation rate of CWT-500 was the highest (0.70 × 10⁻² min⁻¹) due to its large surface area, effective separation of photoexcited electron-hole pairs, and low photoexcited charge carrier recombination rate.

1. Introduction

Titanium dioxide (TiO₂) has gained momentous attention from worldwide researchers due to its broad range of applications, especially its auspicious photocatalytic performance. When TiO₂ is irradiated with light, reactive species of superoxide anion and hydroxyl radicals will be produced to photodegrade the Reactive Black 5 (RB5) into harmless products of water, carbon dioxides, and mineral acid(s) [1]. Despite its advantages, such as its low cost, photochemical stability, non-toxicity, reusability, and wide availability [2,3], a large band gap of 3.20 eV for anatase and 3.00 eV for rutile [2] restricts the full utilization of sunlight [4]. Sunlight consists of approximately 5% UV light and 45% visible light [5]. Thus, the formation of TiO₂ composites [2,6] has been produced to absorb visible light. In addition, the separation and recombination rate of photogenerated charge carriers will be enhanced with the formation of a heterojunction structure between the metal oxides and TiO₂.

Heat treatment is an important step in inducing the crystallization and properties of the TiO₂ [7] composite, where the calcination temperature tends to affect both the physicochemical and photoelectrochemical (PEC) properties of the photocatalyst [8]. Several studies have been conducted to develop TiO₂ composites under different calcination temperature conditions to enhance the photodegradation performance. A study performed by Norouzi et al. [5] showed that all calcined Ag/TiO₂ exhibited higher phenol degradation performance compared to uncalcined Ag/TiO₂. Yang et al. [4] explored the influence of calcination temperatures on the formation of anatase, rutile, and anatase–rutile mixed phases of C-TiO₂. They concluded that C-TiO₂ synthesized at 550 °C displayed the most efficient photodegradation performance due to the larger surface area and effective separation of photoexcited charge carriers. Oliveira et al. [7] found out that an increase in the calcination temperature promoted the sintering of mesopores, thus decreasing the total volumes and surface area of the prepared TiO₂.

In the previous study, we constructed a novel ternary Cu₂O/WO₃/TiO₂ (CWT) composite photocatalyst using an ultrasonic-assisted hydrothermal method, with a proposed direct Z-scheme heterojunction and the proposed photocatalytic mechanism preserved the redox potential of the CWT composite [9]. However, the effect of the calcination temperature on the physicochemical characteristics and photodegradation performance of the CWT composite had not been considered. Therefore, in this study, we synthesized a CWT composite with different calcination temperatures and focused on the impact of the calcination temperature on the structural, morphological, and PEC properties, which affect the RB5 photodegradation performance of the prepared CWT composites. The outcome showed that it is important to optimize the calcination temperature since it can cause several issues, including a reduction in the total surface area, alteration of the pore structure, phase transformation, and increase in crystal and particle sizes.

2. Experimental

2.1. Chemicals

All chemicals, reagents, and solvents were of analytical grade and used as received without further purification. Reactive Black 5 (RB5) (C₂₆H₂₁N₅Na₄O₁₉S₆, dye content ≥50%) was supplied by Sigma Aldrich, United States. Copper (ii) nitrate trihydrate (Cu(NO₃)₂•3H₂O), nitric acid (HNO₃, 65%), acetone (C₃H₆O, 99.80%), sodium tungstate dihydrate (Na₂WO₄•2H₂O, 99%), sodium hydroxide (NaOH, 97%), hydrochloric acid (HCl, 37%), sodium sulfate (Na₂SO₄, 99%), and propanol (C₃H₈O, 99.80%), and titanium (iv) isopropoxide (C₁₂H₂₈O₄Ti, 97%) were purchased from Merck, United States. D-(+)-glucose anhydrous (C₆H₁₂O₆, 99%) was supplied by Alfa Aesar, United Kingdom. Ethanol (C₂H₅OH, 95%) and ammonium oxalate monohydrate (C₂H₈N₂O₄•H₂O, 99.50%) were supplied by Systerm Chemicals, Malaysia. Double distilled water was utilized throughout the study.

2.2. Synthesis of CWT

The ternary composite CWT was prepared via an ultrasonic-assisted hydrothermal technique according to our previous study [10]. A more detailed synthesis method is provided in the supplementary information. The final product of CWT powder was calcined at different calcination temperatures. The prepared photocatalysts were named CWT-500, CWT-600, CWT-700, and CWT-800 according to the calcination temperatures of 500 °C, 600 °C, 700 °C, and 800 °C, respectively.

2.3. Characterization

Thermogravimetric analysis (TGA) was determined by using a simultaneous thermal analyzer-6000 (STA-6000) from Perkin Elmer, United States. The working temperature region was set from 30 °C to 800 °C. The thermal processing speed was set at 10 °C.min⁻¹ during the heating process and 20 °C.min⁻¹ for the cooling process. In the current experiment, inert argon gas was first introduced to the system at a rate of 20 mL.min⁻¹ to flush out combustible products and prevent the condensation process. The properties of X-ray diffraction (XRD) were studied with a Shimadzu XRD-6000 from Shimadzu Scientific Instruments, Japan, operated with an X-ray source of Cu Kα radiation (step size of 0.02°.s⁻¹; scanning 2θ in the range of 20 to 85°). The Scherrer equation, as shown in Equation (1), was used to determine the crystallite size of the prepared photocatalysts:

$$D_{hkl}={\displaystyle \frac{0.90\lambda }{\beta cos\theta }}$$

(1)

where $D_{hkl}$ represents the crystallite size in the direction perpendicular to the lattice planes. Next, $\lambda$ (0.15 nm) represents the wavelength of the incident Cu Kα radiation. $\beta$ represents the full width at half maximum of the XRD peak in radians, and $\theta$ is Bragg’s angle in degrees. A scanning electron microscope (SEM) (SU8010) from Hitachi, Japan, paired with a Silicon Drift X-ray detector (X-max) from Horiba, Japan, was used to study the surface morphology and element compositions of the prepared photocatalyst. Transmission electron microscope (TEM), (Zeiss LEO Libra-120) from Germany was used to further characterize the morphology of the prepared photocatalysts. The specific surface area was studied using Brunauer–Emmett–Teller (BET) (Micromeritics ASAP 2020) from Malvern Panalytical Company, Georgia, United States. The pore diameter and pore volume of the prepared samples were determined using the Barrett–Joyner–Halenda (BJH) technique. The band gap energy (eV) of the prepared photocatalysts was obtained with a Lambda 35 UV-Vis spectrophotometer (Perkin Elmer, United States) with Tauc’s plot and Equation (2):

$$\alpha hv=A{(hv-E_g)}^n$$

(2)

where $\alpha$, $h$, and $v$ represent the absorption coefficient, Planck constant (6.63 × 10³⁴ Js), and light frequency, respectively. In addition, $A$ is a constant, $E_g$ is the band gap energy (eV), and the reading of n is 0.50 for indirect transition mode [11].

2.4. PEC Study

For the PEC study, both the silver chloride and platinum plate acted as a reference electrode and counter electrode, respectively. On the other hand, fluorine-doped tin oxide (FTO) glasses coated with the prepared photocatalysts were used as the working electrode; 0.50 M sodium sulfate (pH 6.50) was used as the electrolyte. PEC tests, including the electrochemical impedance spectrum (EIS), linear sweep voltammetry (LSV), photocurrent, photovoltage, and Mott–Schottky (MS), were examined using an Autolab PGSTAT 302N (Metrohm from Switzerland) coupled with NOVA 2.10 software. EIS was operated with conditions of a 0 V potential, 10 mV AC amplitude, and frequencies ranging from 0.10 Hz to 100 kHz. The charge transfer resistance (Rct) reading was determined using ZView 4.0g software. LSV was carried out with a potential range of −1 to 2 V (step size of 0.05 V). The photocurrent was determined with a 0.50 V bias. MS was performed with a potential range of −1 to 1 V (vs. silver/silver chloride) and a frequency of 100 Hz. A Xenon lamp with an intensity of 20 mW.cm⁻² was placed 15 cm away from the working electrode for LSV, transient photocurrent, and photovoltage response tests. The electron lifetime of the CWT composites can be estimated using Equation (3) as below:

$$t_e={\displaystyle \frac{1}{2\pi f_{peak}}}$$

(3)

where $t_e$ and $f_{peak}$ represent electron lifetime and peak frequency [12]. The charge carrier density was determined using Equations (4) and (5) below by using the obtained gradient value of the MS plot:

$$N_D=({\displaystyle \frac{2}{e\mathsf{\epsilon }{\mathsf{\epsilon }}_0}})/{\displaystyle \frac{d(1/C^2)}{dV}}$$

(4)

$$N_A=-({\displaystyle \frac{2}{e\mathsf{\epsilon }{\mathsf{\epsilon }}_0}})/{\displaystyle \frac{d(1/C^2)}{dV}}$$

(5)

where $e$, $\mathsf{\epsilon }$, ${\mathsf{\epsilon }}_0$, and ${\displaystyle \frac{d(1/C^2)}{dV}}$ represent the electron charge with a reading of 1.60 × 10⁻¹⁹ C, the relative dielectric constant of the photocatalyst, the permittivity of the vacuum with a reading of 8.85 × 10⁻¹² F.m⁻¹, and the gradient determined from the MS slope (F⁻².cm⁻⁴), respectively [13]. According to the previous experiment, the prepared bare TiO₂ from the precursor of titanium isopropoxide has an $\mathsf{\epsilon }$ reading of approximately 1283 measured at a frequency of 100 Hz [14].

2.5. Photodegradation Study

A xenon lamp (150 W) was utilized as the light source throughout the photodegradation study. RB5 solution with a concentration of 20 ppm was used to study the photodegradation performance of the prepared photocatalysts. The mixture of the photocatalyst and RB5 solution was stirred for half an hour in dark conditions to enable adsorption-desorption equilibrium between the RB5 solution and the prepared photocatalysts. A 3 mL solution was taken for a fixed interval of time and centrifuged before analyzing the concentration at a wavelength of 597 nm using a UV-Vis spectrophotometer. The RB5 photodegradation performance of the prepared CWT composites with different calcination temperatures was calculated using Equation (6):

$$RB5photodegradationremoval\left(\%\right)=[{\displaystyle \frac{\left(C_0{–C}_t\right)}{C_0}}]\times 100\%$$

(6)

where $C_0$ represents the initial RB5 concentration (mg.L⁻¹) after reaching the adsorption–desorption equilibrium condition and $C_t$ represents the RB5 concentration (mg.L⁻¹) at the specific time interval (min). First-order kinetics (Equation (7)) were applied to fit the photodegradation data:

$$-\ln \left({\displaystyle \frac{C_0}{C_t}}\right)=kt$$

(7)

where $k$ represents the photodegradation rate constant (min⁻¹). A reusability test was carried out where the filtered powder sample was reused without any prior activation and washing processes. The photocatalyst powder was dried in the oven overnight before each cycle.

3. Characterizations

3.1. TGA

The thermal behavior of the prepared TiO₂ was studied. Figure 1 displays the TGA plot of the prepared TiO₂. The sample was heated in an air atmosphere with a heating rate of 10 °C.min⁻¹ from 30 to 800 °C. According to the result, an overall weight loss of 7.24% (from 99.92 to 92.68%) occurred in the TiO₂ nanoparticles. According to the TGA plot, an initial weight loss occurred during the first thermal event of 30 to 140 °C due to the decomposition of water physically adsorbed on the surface of TiO₂ or desorption of TiO₂ [15,16] followed by an endothermic peak in the Differential Thermal Analysis (DTA) slope. The second thermal event of 140 to 500 °C was due to the decomposition of intermediates [7]. The weight was maintained nearly constant when the heating temperature was above 500 °C, which indicated that the TiO₂ was finally converted to stable oxides at a temperature above 500 °C [7,16]. Therefore, temperatures of 500 °C, 600 °C, 700 °C, and 800 °C were used as the calcination temperatures to study the effect of the calcination temperature on the photocatalyst samples.

3.2. XRD

Figure 2 shows the XRD data of the CWT composite with different calcination temperatures together with the International Centre for Diffraction Data (ICDD) profiles of anatase and rutile TiO₂. Peaks representing monoclinic WO₃ (ICDD card no. 00-043-1035) at 37.34° (103), 38.28 (131), 38.89° (310), 40.86° (311), and 56.27° (241) were observed for the CWT composites. In addition, peaks representing Cu₂O (ICDD card no. 00-076-0317) at 36.42° (111) were also observed in Figure 2. There was only one peak related to Cu₂O in the XRD data; this might be attributed to the reason for either the low Cu₂O content or highly dispersed Cu₂O over the CWT composite [17]. The formation of CWT composites was proven by the presence of all Cu₂O, WO₃, and TiO₂ peaks throughout the XRD spectra.

When the calcination temperature was 500 °C and 600 °C, as shown in Figure 2, the synthesized CWT showed the anatase phase of TiO₂ (ICDD card no. 01-078-2486) with main peaks located at 25.31°, 36.95°, 37.79°, 38.57°, 48.04°, 53.89°, 55.07°, 62.11°, 62.69°, 68.76°, 70.30°, 75.05°, 76.04°, 82.68°, and 83.17°, respectively. However, when the calcination temperature was increased to 700 °C and 800 °C, as shown in Figure 2, the anatase phase of TiO₂ shift to the rutile phase of TiO₂ (ICDD card no. 01-073-1765) with main peaks located at 27.47°, 36.13°, 39.23°, 41.30°, 44.10°, 54.39°, 56.69°, 62.87°, 64.12°, 69.09°, 69.92°, 72.51°, 76.67°, 82.45°, and 84.36°, respectively. This observation corresponds to the research carried out by Sarngan et al. [18] where the anatase crystals are transformed into rutile crystals when the temperature reaches 700 °C and above.

On the other hand, an increase in the calcination temperature increased the crystallinity of the produced CWT photocatalysts, proved by an increase in the sharpness of the XRD peak [19] from CWT-500 to CWT-800 and the calculated crystallite sizes of 21.84 nm (CWT-500), 48.80 nm (CWT-600), 63.45 nm (CWT-700), and 170.13 nm (CWT-800), respectively. This can be explained by the reason that calcination causes the growth of crystal structures resulting in a larger size of the synthesized materials. CWT-500 has broad peaks compared to others, which is characteristic of smaller crystallite sizes [3,18,20].

3.3. SEM

From the SEM images in Figure 3, the average particle size of the CWT composite was increased when the calcination temperature increased from 500 to 800 °C. The homogenous and uniform nanostructures of CWT in Figure 3a were replaced by less uniform and irregular plate-like particles (Figure 3b–d) with wide-size distributions when the calcination temperature was increased from 500 to 800 °C. The results revealed that the calcination temperature has a great influence on the morphologies of the prepared photocatalysts. This situation can be explained by the relationship between the kinetic energy and calcination temperature where the higher the temperature, the more kinetic energy [17,20]. Thus, the nanoparticles will tend to pile up and fuse, producing less uniform nanoparticles and forming irregular large-scale particles [17]. Therefore, the calcination temperature affects both the particle size and shape of the photocatalyst [20], and the particle sizes of the CWT composites grow when the calcination temperature increases [21].

3.4. TEM

Figure 4 demonstrates the TEM images of the CWT composite with different calcination temperatures of 500 °C, 600 °C, 700 °C, and 800 °C. When the calcination temperature is raised from 500 to 800 °C, the particle sizes become larger. This may be due to particle growth because of the higher calcination temperature. The observation was in good agreement with the obtained outcomes from both the SEM and XRD, where the CWT particles and crystallite sizes increased from the temperature of 500 to 800 °C. The average particle sizes of CWT-500, CWT-600, CWT-700, and CWT-800 in TEM images were determined to be 39.11 nm, 54.75 nm, 73.56 nm, and 180.25 nm, respectively. In addition, the rutile phase of TiO₂ in CWT-700 and CWT-800 has a bigger size compared to anatase phase TiO₂ in CWT-500 and CWT-600 [22].

3.5. BET

The textural properties of the prepared photocatalysts were studied using BET. Figure S1 in the supplementary information displays the nitrogen (N₂) adsorption–desorption isotherms of CWT composites with calcination temperatures ranging from 500 to 800 °C. All the photocatalysts showed a type IV isotherm, in which adsorption curves are separated from the desorption curves [6], followed by hysteresis loops ranging from 0.01 to 1. It is well known that the loop formed between 0.40 to 0.80 represents the presence of mesopores [23]. This observation is in good agreement with the obtained pore diameter ranging from 7.73 to 16.92 nm as mesopores [6]. Mesoporous photocatalysts have a good adsorption capacity for pollutants, enhancing the photodegradation performance [6]. From

Table 1. Nitrogen (N₂) physisorption data of the prepared photocatalysts.

Sample	Surface Area (m2.g−1)	Pore Volume (cm3.g−1)	Pore Diameter (nm)
CWT-500	35.77	0.09	12.96
CWT-600	12.81	0.06	16.92
CWT-700	15.01	0.02	7.73
CWT-800	8.09	0.01	9.34

, the overall surface area of CWT was decreased from 35.77 to 8.09 m².g⁻¹ when the calcination temperature was increased from 500 to 800 °C. This could be attributed to the agglomeration of particles [7,17,24].

Figure S2 in the supplementary information displays the pore distributions of CWT composites with different calcination temperatures. The pore diameter of CWT-600 (16.92 nm) is larger than CWT-500 (12.96 nm), and the surface area of CWT-600 (12.81 m².g⁻¹) is smaller than that of CWT-500 (35.77 m².g⁻¹). This may be due to the reason that at a higher temperature, the pores are expanded causing the surface area to reduce as a result of the enlargement of the existing pores, which breaks the crosslinks in the composite matrix and results in the composite aggregate rearranging and pores collapsing [17]. This result also occurred for both the CWT-700 and CWT-800 when the calcination temperature increased from 700 to 800 °C. The surface area of both the CWT-700 and CWT-800 are smaller than CWT-500.

It is well known that anatase phase TiO₂ (CWT-500) has a higher surface area compared to rutile phase TiO₂ (CWT-700 and CWT-800) [17,22]. The outcome of the surface area was in good agreement with the particle size of the prepared photocatalysts obtained through the previous TEM outcomes, where increasing the calcination temperature decreases the surface area due to an increasing particle size. The overall pore diameter ranged from 7.73 to 16.92 nm, confirming the existence of a mesoporous structure in the prepared CWT composites [6]. The pore volume of all the composites decreased from 0.09 to 0.01 cm³.g⁻¹ when the calcination temperature increased from 500 to 800 °C. This may be due to the strong interactions among Cu₂O, WO₃, and TiO₂ [25].

3.6. Optical Characteristics

Figure 5 displays the plot of (αhv)^1/2 versus the photon energy for the band gap determination. The band gap energies of CWT-500, CWT-600, CWT-700, and CWT-800 were determined to be 2.65 eV, 2.60 eV, 2.25 eV, and 1.95 eV, respectively. The result shows that the absorption edges of all prepared CWT composites are located within the visible light region, which is beneficial for the photocatalytic reaction under visible light excitation. With an increase in the calcination temperature from 500 to 800 °C, the absorption edge shifted to a lower band gap (with a longer wavelength) [26]. The reduction in the band gap reading can be attributed to the presence of rutile phase TiO₂ [24], which affects the shifting of the absorption band of the prepared CWT composites. This observation is in good agreement with the previous XRD data.

3.7. PEC Study

An excellent photocatalyst should reduce the recombination rate of the photoexcited charge carrier and enhance the charge carrier transportation to the photocatalyst surface. The separation and transfer efficiency of photoexcited charge carriers played an important role in the photodegradation performance of the prepared CWT photocatalyst [27,28]. The PEC study was carried out to explore light responsiveness, light absorption, and charge carrier characteristics [11]. EIS was conducted to confirm the charge separation at the interface of the synthesized photocatalysts. The reduction in the arc radius of the photocatalyst in the Nyquist plot of EIS represents a lower resistance of electron transfer and high mobility of electrons [29] and vice versa [27].

Figure 6a displays the Nyquist plot, where the arc radius of CWT-500 was the smallest, indicating its fastest photoexcited charge carrier transfer rate compared to other photocatalysts [30]. The reading of charge transfer resistance (Rct) can be determined based on the arc radius of the Nyquist plot using ZView software. The Rct in ascending order was CWT-500 (208.10 Ω) < CWT-600 (5120 Ω) < CWT-700 (9335 Ω) < CWT-800 (22,227 Ω), respectively. CWT-500 has proven to have the lowest resistance of charge carrier transfer, and this may be due to the high surface area of CWT-500, which tends to absorb more light and increases the number of photogenerated charges [31,32], as CWT-500 has a larger active surface area compared to other photocatalysts. In addition, the particle size of CWT-500 is the smallest and shows a uniform distribution, which could provide more conductive channels, leading to enhanced electrochemical performance [19].

Furthermore, EIS can be used to investigate the cooperation between WO₃ and Cu₂O loadings on the charge separation efficiency of the ternary CWT-500 composite under light irradiation. As shown in Figure 6a, the arc radius and Rct reading of CWT-500 under light irradiation conditions (208.10 Ω) were smaller than CWT-500 in the dark condition (997.30 Ω). This is due to the higher mobility of photoexcited charge carriers and more efficient interfacial charge transfer, indicating a small charge transfer resistance under light irradiation. In addition, this observation proved that the CWT-500 is light-reactive.

Figure 6b depicts that the EIS Bode phase plot concurs with the Nyquist plot results. The phase angle of CWT-500 located at a low-frequency range was low, confirming the higher rate of electron transfer compared to other CWT composites [12]. In addition, the maximum peak frequency of CWT-500 was located at a low frequency as compared to other photocatalysts. CWT-500 has the smallest frequency peak reading (4.90 Hz) compared to CWT-600 (575.44 Hz), CWT-700 (1071.52 Hz), and CWT-800 (3019.95 Hz). This confirmed the fast electron transfer in the heterojunction formed among Cu₂O, WO₃, and TiO₂ [12] in the CWT-500 composite. The electron lifetimes of 32.49 ms, 0.28 ms, 0.15 ms, and 0.05 ms were obtained for CWT-500, CWT-600, CWT-700, and CWT-800, respectively. The lifetime of the photoexcited electron of anatase TiO₂ (CWT-500 and CWT-600) is longer than the rutile TiO₂ (CWT-700 and CWT-800) [33].

An LSV measurement (Figure 6c) was performed to investigate the photocurrent densities of the prepared CWT composites initiated by light irradiation. The photocurrent reading was generated after the photocatalyst was illuminated with light, indicating that under the condition of light, the photoexcited charge carries were generated successfully from the photocatalyst entering the electrolyte, thus producing the photocurrent reading [34]. In the LSV curve, the current density reading increased when the applied potential increased, this implies that all the prepared CWT composites are photocatalytically active [35]. Our results showed that the photocurrent densities at the potential of 0.50 V (vs. silver/silver chloride) of all samples lie in the elevated order of CWT-800 (0.32 mA.cm⁻²) < CWT-700 (0.60 mA.cm⁻²) < CWT-600 (0.68 mA.cm⁻²) < CWT-500 (1.05 mA.cm⁻²). The photocurrent density reading of CWT-500 confirmed less resistance in electron transportation and, therefore, reduced the recombination rate of charge carriers [12]. The LSV outcome corresponds to the EIS results.

The descending photocurrent reading of 1.05 (CWT-500) to 0.32 mA.cm⁻² (CWT-800) followed the trend of the reduction in surface area (35.77 m².g⁻¹ for CWT-500 and 8.09 m².g⁻¹ for CWT-800) of the photocatalyst. More light absorption on the large surface of CWT-500 led to the production of a higher photocurrent density. Moreover, a higher surface area provides more contact area between the photocatalyst and electrolyte, which enhances the transfer of photoexcited charge carriers from the photocatalyst to electrolyte [36]. The LSV measurement helps to prove that the addition of WO₃ and Cu₂O enhanced the light-trapping efficiency of ternary CWT-500 composites, and this characteristic promotes photodegradation activity. The LSV test was performed to determine the bias voltage used to study the photocurrent density of the prepared photocatalyst [37], and 0.50 V was used in the next studies.

To explore the factors affecting the photocatalytic performance of the prepared photocatalysts, a transient photocurrent response test was carried out. The transient photocurrent response of the prepared CWT composites was carried out under irradiation of light with five on–off cycles. Figure 6d shows that all the prepared CWT composite photocatalysts showed immediate photocurrent responses upon light irradiation and the responses disappeared immediately after the light was shut off, indicating the presence of the photo-response for the prepared photocatalyst [38]. The instantaneous photocurrent response has a positive relationship with the photoexcited charge carrier separation and transfer efficiency, which promote better photodegradation activity [6]. As shown in Figure 6d, the fast and uniform photocurrent response indicated that the prepared photocatalyst could act as a high-quality photosensitive catalyst. The photocurrent density was measured from the difference between photocurrent densities in the presence and absence of light irradiation.

The photocurrent density in ascending order was CWT-800 (0.38 mA.cm⁻²) < CWT-700 (0.57 mA.cm⁻²) < CWT-600 (0.70 mA.cm⁻²) < CWT-500 (1.40 mA.cm⁻²). The CWT-500 composite displays the highest photocurrent intensity, representing the greater separation efficiency of the photoexcited charge carrier in the composite [39]. The high efficiency of CWT-500 compared to other composites can be due to the high surface area, larger pore volume, and effective charge carrier separation [29,40]. On the other hand, CWT-800 exhibited a relatively low photocurrent density reading due to the fast recombination of photoexcited charge carriers [40]. The photocurrent density of CWT-600, CWT-700, and CWT-800 was slightly reduced from the first cycle to the fifth cycle of light irradiation. This may be due to the accumulation of holes [6].

The result of the transient photovoltage response (Figure 6e) was consistent with the outcome of the transient photocurrent response where CWT-500 immediately reached the lowest value once turning on the light. The potential difference in both light and dark conditions was calculated to be 0.53 V, 0.40 V, 0.35 V, and 0.16 V, corresponding to CWT-500, CWT-600, CWT-700, and CWT-800, respectively. The MS measurements were performed under open circuit potential in a dark environment. Figure 6f displays the MS plot of the prepared CWT composites with different calcination temperatures.

V_FB can be evaluated using the MS plot by extrapolating the MS plot to 1/C² = 0. A more negative V_FB value for photocatalysts represents a higher electron-hole pair density and better separation performance of photoexcited electron-hole pairs [41]. CWT-500 has the highest charge carrier density and better charge separation performance compared to other CWT composites due to its V_FB reading (−0.25 V), obtained in Figure 6f. The V_FB readings of CWT-600, CWT-700, and CWT-800 were determined to be −0.15 V, −0.05 V, and −0.05 V, respectively.

The charge carrier density reading of CWT-500 (N_A: 5.25 × 10¹⁶ cm⁻³ and N_D: 6.06 × 10¹⁶ cm⁻³) was larger compared to CWT-600 (N_A: 3.45 × 10¹⁶ cm⁻³ and N_D: 5.94 × 10¹⁶ cm⁻³), CWT-700 (N_A: 1.54 × 10¹⁶ cm⁻³ and N_D: 5.51 × 10¹⁶ cm⁻³), and CWT-800 (N_A: 1.13 × 10¹⁶ cm⁻³ and N_D: 5.46 × 10¹⁶ cm⁻³). Thus, CWT-500 has a smaller recombination rate of photoexcited charge carriers than other photocatalysts and subsequently enhances the photodegradation performance [42]. CWT-500 has the highest density of photoexcited charge carriers and provides better transfer performance of charge carriers [43]. Figure 6f also shows an inverted V-shape MS plot representing n-type TiO₂ and WO₃ (positive slope) and p-type Cu₂O (negative slope). This outcome proves the formation of a p-n junction for the CWT composites [10]. The formation of a p-n heterojunction among Cu₂O, TiO₂, and WO₃ develops a charge layer that reduces charge recombination and improves the photogeneration of electron-hole pairs [44].

4. Results and Discussion

Photodegradation Performance

Figure 7a,b show the RB5 photodegradation performance of CWT produced with different calcination temperatures. RB5 was slightly degraded under solar light irradiation without a photocatalyst, proving the photostability of RB5 under solar light. All the prepared CWT composites followed pseudo-first-order kinetics (Figure 7b). The RB5 photodegradation rate was determined to be 0.70 × 10⁻² min⁻¹, 0.41 × 10⁻² min⁻¹, 0.13 × 10⁻² min⁻¹, and 0.09 × 10⁻² min⁻¹ for CWT-500, CWT-600, CWT-700, and CWT-800, respectively. From the result, CWT-500 and CWT-600 with anatase phase TiO₂ have higher RB5 photodegradation performance than CWT-700 and CWT-800 with rutile phase TiO₂.

The larger band gap of anatase TiO₂ of CWT-500 and CWT-600 increases the oxidation strength of photoexcited electrons, thus promoting the photodegradation activity [22,33]. CWT-500 showed the highest photodegradation performance, which corresponds to the effective separation of the photoexcited charge carrier and reduction in the recombination rate of the photoexcited charge carrier. In addition, CWT-500 has the largest surface area (35.77 m².g⁻¹), which provides more active sites for the adsorption of RB5 and light [24,26], compared to CWT-600. Light absorption enhances the production of more oxidizing agents to photodegrade the RB5 dye molecules and thus increases the photodegradation rate of CWT-500 [24]. The RB5 photodegradation rate of CWT-700 was lower compared to CWT-600, although the reactive surface area of CWT-700 (15.01 m².g⁻¹) was larger than CWT-600 (12.81 m².g⁻¹). This observation proves that the anatase phase is more effective in enhancing the reaction rate compared to the surface area of the prepared CWT composite [5].

A reusability test was conducted to determine the photostability of the prepared CWT-500 photocatalyst, ensuring its cost-effectiveness and consistent photodegradation performance for the repeated treatment of RB5 wastewater. Figure S3 in the supplementary information illustrates the results of the reusability test using the CWT-500 photocatalyst to treat 20 mg.L⁻¹ RB5. The RB5 photodegradation rate remained at 0.61 × 10⁻² min⁻¹ after five cycles, indicating the consistent high performance of the photocatalyst. A minimal, unavoidable reduction in the photodegradation rate was observed during the fifth cycle compared to the first cycle (0.70 × 10⁻² min⁻¹). This decline may be due to the partial loss of the photocatalyst during the recovery processes including transfer, filtration, centrifugation, and drying, and the preparation of the RB5 solution for absorbance measurements using the UV-Vis spectrophotometer.

5. Conclusions

In conclusion, the calcination temperature has a significant impact on the physicochemical characteristics of the produced CWT composite photocatalyst. Phase transformation of anatase to rutile TiO₂ occurred when the calcination temperature was increased from 500 to 800 °C. The superiority of the prepared CWT-500 with the highest photodegradation performance is generally attributed to its large surface area, effective separation of photoexcited electron-hole pairs, and low charge carrier recombination rate.