Study of Phenol Red Photocatalytic Decomposition on KBrO₃-Supported TiO₂ Nanoparticles for Wastewater Treatment

Abstract

In this study, the enhanced photodegradation of a high-concentration phenol red (PR) using very fine TiO₂ nanocrystals by adding a KBrO₃ electron acceptor was reported for the first time. The structural study on TiO₂ nanocrystals using HRTEM, XRD, Raman, and EDX was performed and it confirmed the anatase phase of TiO₂ nanocrystals. UV–Vis absorbance of 20 mg.L⁻¹ PR was measured and the photodegradation was extracted. The KBrO₃ concentration effects exhibited an important enhancement in the degradation of PR dye. The efficiency of PR was increased during 110 min from 75% of pure TiO₂ to 92% and 98% of TiO₂ with 1 mg and 5 mg KBrO₃, respectively. For different samples, a first-order kinetic of dye degradation is confirmed. The instantaneous amount of degraded dye increased from 150 to 180 and 197 mg/g TiO₂ with 1 mg and 5 mg KBrO₃, respectively. The mechanism of the photodegradation reaction confirms the effect of OH^- radicals on increasing the photocatalytic activities. The addition of electron acceptors KBrO₃ improved the photocatalysis rate, where it prevented e-h recombination through conduction band electron capture, which increases the concentration of hydroxyl radicals. The proposed mechanism and results were supported by photocurrent measurements and a Raman spectra analysis of the final photodegraded products. The photocurrent of TiO₂ was observed at 1.2 µA, which was significantly improved up to 13.2, and 21.3 µA with the addition of 1 mg and 5 mg of KBrO₃. The Raman spectra of the final products confirmed that ${\mathrm{SO}}_4^{2-}$ and carbons are byproducts of PR degradation.

1. Introduction

The increasing demand for pharmaceutical, textile, and printing products containing colored dyes has led to negative impacts on the environment [1,2,3]. Based on different studies, the amount of wastewater released by the textile industry ranks among the highest in the world [4]. According to the United States “Color Index” commodity dyes have reached tens of thousands. About 60,000 tons of dyes are discharged into the environment in the form of waste each year worldwide [4], 80% of which are azo dyes. Dyes belong to organic pollutants. One of the most harmful dye-contaminated clean water is phenol red (PR). PR is particularly used in different industries such as pharmaceutical, coal processing, petroleum refineries, dye industries, and chemical, etc. PR is very harmful and hazardous for animals and plants living in water because it has carcinogenic and mutagenic effects [5]. That is why there is more demand to remove it from wastewater. The permissible limit of phenol is 1 mg/L for industrial effluents to be discharged into inland surface waters and 5 mgL⁻¹ for discharge into public sewers [6]. Organizations are providing more funding to develop cost-effective and environmental techniques to treat wastewater from dyes. The advanced oxidation process (AOP) is very promising to effectively treat the pollutants in wastewater by photodegradation.

Over the past decades, TiO₂ semiconductors have attracted enormous attention due to their interesting physical and chemical properties [7,8,9,10,11]. TiO₂ is a chemically stable and non-toxic material used in a wide range of applications such as dye-sensitized-based solar cells, batteries’ electrodes, photoanodes, catalysis, and gas reduction [12,13,14,15,16,17]. In most applications, scientists have tried different approaches to improve the electronic and optical properties of TiO₂, e.g., by tuning its optical band gap, slowing the recombination rate of hole–electron pair, controlling its phase, etc. These properties’ improvements were performed by decreasing the size of nanoparticles, changing the geometries, doping with metal/transition metal, adding a donor-acceptor, etc. Although adding/doping metal/transition metal to TiO₂ nanocrystals can increase its photocatalytic activities by extending its absorbed wavelength range. There are still major challenges related to the secondary pollution caused by the presence of metal with TiO₂ and the formation of complexes, especially for large-scale applications [18,19,20]. On the other hand, the affordability of new cutting-edge UV laser technologies such as UV laser diodes pushed the industry to use them easily in photodegradation applications. Another factor is the wider band gap of TiO₂ that can be photo-excited when using a UV laser. An alternative way is using an electron acceptor to avoid the formation of secondary pollutants. It is very known that adding donors/acceptors to TiO₂ increased the oxide-reduction reaction of photodegradation for dyes [4,19,21,22]. Wahab et al. studied the effect of H₂O₂ as an electron acceptor in enhancing the photodegradation efficiency of PR using TiO₂ nanoparticles [23]. In their study, it took 300 min to degrade 94% of PR. Asiri et al. fabricated TiO₂ nanocrystals with an average size of 77 nm that showed lower degradation time than 5 nm commercialized nanoparticles, which exhibited a higher photodegradation percentage of 95.2% during 100 min [24]. Other photocatalysts such as La-substituted bismuth ferrite (Bi_1-xLa_xFeO₃) and oxyhydroxide of Fe (III) (Goethite) have been used in the photodegradation of PR [25,26]. Ahlawat et al. synthesized a cerium oxide–titanium oxide nanocomposite (CeO₂-TiO₂ NC) for PR degradation and adsorption applications simultaneously; in their study, the photodegradation with the adsorption lasted 80 min with a 99.8% degradation rate [27]. In our previous work, a 95% degradation rate of PR was obtained using KBrO₃ as an acceptor with Nb-TiO₂ nanocomposites. The degradation percentage is increased by more than 3% using KBrO₃ pure TiO₂ nanoparticles. Although, the higher degradation rate and short duration in some studies, such as (CeO₂-TiO₂ NC), used half the concentration of PR and 5–10 times more concentrated photocatalysts compared to this work. The reported studies confirm that no one has reported the effect of KBrO₃ on the photodegradation of higher concentrations for phenol red using a very small size TiO₂ nanocrystals. The objective of this study is to explore the effect of KBrO₃ with different concentrations on the degradation efficiency of highly concentrated PR using very low concentrations of TiO₂ nanoparticles.

In this work, we performed a photodegradation study of highly concentrated phenol red of 20 mgL⁻¹ using very fine TiO₂ nanoparticles with an average size of 5 nm supported with two different concentrations of KBrO₃ as an electron acceptor. HRTEM and EDX mapping were carried out into the samples for the structural study. The degradation efficiency, the concentration decay, and the degraded quantity under the UV–Vis lamp were measured. The photo-induced current of TiO₂ was examined to demonstrate the effect of KBrO₃ in the recombination of the electron–hole. A detailed photocatalysis mechanism was proposed to understand the processes. The final products of PR degradation were investigated by Raman spectra with two modes of sampling, before and after stirring.

2. Materials and Methods

2.1. Materials Preparation

TiO₂ nanopowders were purchased from Skyspring Nanomaterials (product ID: 7930DL, Houston, TX, USA) with an average size of 5 nm. KBrO₃, ACS, 99.9% min, was ordered from Alfa Aesar (CAS number 40013), Kandel, Germany. A total of 1 mg and 5 mg of KBrO₃ were added to two solutions containing 5 mg of the photocatalysts (TiO₂), denoted by 20% and 100%, respectively. In this work, we used three samples: pure TiO₂, TiO₂ with 20 wt.% KBrO₃, and TiO₂ with 100 wt.% KBrO₃.

2.2. Experimental Method

High transmission electronic microscopy (HRTEM) and EDX measurement were carried out using the equipment (JEOL, JEM-2100F, Tokyo, Japan) working at 200 kV. A solution of 5 µL of nanoparticles was covered in a carbon-coated copper grid after sonication for 10 min. X-ray diffractometer (Philips Type PW 1710, Amsterdam, The Netherlands) with CuKα radiation was used to measure the crystal structure of the prepared samples. XRD patterns were obtained within the range of 10°–90° with a frequency of scanning 2°/min. Raman spectra were recorded at room temperature using a confocal Raman microscope (HORIBA; LabRAM HR800, France SAS) with a He-Ne wavelength laser of 632.8 nm and an output power of 20 mW.

We performed the photocatalysis measurement using a solution containing 20 mg.L⁻¹ PR with 5.0 mg of nanopowders in 50 mL of PR solution. A spectrophotometer with a scanning frequency of 5.0 nm/s was employed to measure the UV–Vis absorbance spectra of phenol red. To perform an effective photodegradation of PR, a UV lamp with a spectral range (315–400 nm) (UV 400 HL230 Fe Clear-A, UV Light Technology Limited, Birmingham, UK) with a power density of 100 mW/cm². The maximum absorbance peak located at 450 nm was used to measure the photodegradation of PR. The photocurrent measurement was carried out using a quartz cell attached with two electrodes coated with gold. Three aqueous samples were prepared for photocurrent measurements by dissolving 5 mg of TiO₂, TiO₂/1 mg-KBrO₃, and TiO₂/5 mg-KBrO₃ in 10 mL of DI water. The electrodes were connected to a DC power supply and Keithley multimeter (7.5 digits 2010 Series Low Noise Multimeter) for current measurements. The cell was irradiated with the same light source used for photocatalysis measurements (UV lamp with a spectral range (315–400 nm)).

3. Results

3.1. Structural Study

TEM images confirm the presence of 5 nm as an average size of TiO₂ nanocrystals (Figure 1a,b). TEM shows that TiO₂ nanoparticles are dispersed in agglomerated groups. Different crystal orientations were noticed in the HRTEM images (Figure 1a). A 0.33 nm interplane distance between (101) planes was observed (Figure 1c,d). Selected area electron diffraction (SAED) confirms the presence of different rings that correspond to (101), (004), (200), (211), and (204) planes, as shown in Figure 1e,f [28,29]. In Figure 2a–e, we can see the mapping of the element’s distribution using EDX techniques. Ti and O atoms are distributed with the quantities 69.88 and 30.12, and the weight ratio % of Ti and O is measured to be 43.66 and 56.34, respectively.

The XRD measurement was performed on the sample to study the structure and phase composition of TiO₂. Figure 3 shows clearly the XRD patterns related to pure TiO₂. Different diffraction peaks at 2θ angles 25.3, 37.9, 48.05, 53.9, 55.06, 62.4, 68.76, 70.3, and 75.06 attributed to the crystal planes of the anatase phase TiO₂ ((101), (004), (200), (105), (211), (204), (116), (220), and (215)), as reported in JCPDS card no. 21–1272. The sharpness of the observed peaks confirmed the higher crystallinity and purity of the anatase phase of the TiO₂ nanoparticles.

Raman spectra were measured on pure TiO₂, TiO₂/1 mg-KBrO₃, and TiO₂/5 mg-KBrO₃. KBrO₃ was mixed with TiO₂ nanoparticles and then milled. Afterward, a drop of water was added to dissolve KBrO₃ to form a gel. After 1 h of drying, we put a small quantity into glass slides to perform a Raman measurement. Figure 4a shows the Raman spectra of pure TiO₂, TiO₂/1 mg-KBrO₃, and TiO₂/5 mg-KBrO₃. The six basic Raman active modes $A_{1g}+2B_{1g}+3E_g$ that belong to $148$(E_g), 196 (E_g), 396 (B_1g), 515 (A_1g), 519 (B_1g), and 638 (E_g) are observed [30,31]. The sharp peak located at 148 cm⁻¹ assigned to the E_g mode is shifted relative to the bulk located at 144 cm⁻¹, the origin of the shift is the size confinement effect of the very small nanoparticles [27]. The very low blue shift of the E_g mode 148 cm⁻¹ relative to the bulk mode is due to the effect of the small size on the phonon confinement [32]. In addition, new bands related to the KBrO₃ vibrational modes located at 360 cm⁻¹ and 793 cm⁻¹ are observed (Figure 4a) [33,34]. A blue shift is observed with the increase in KBrO₃ quantity with a zoom-in on the band located at 793 cm⁻¹, which reflects the compressive stress due to the increasing number of KBrO₃ or the increasing of electrostatic force between the ions (K⁺ and BrO₃^-) and TiO₂ nanoparticles (Figure 4b). To confirm the interaction effect of KBrO₃ on TiO_2, a Lorentzian function was used to adjust the peak position of the E_g mode (148 cm⁻¹), and a small red shift of 0.5 cm⁻¹ is observed by increasing the KBrO₃ quantity (Figure 4c,d). This shift is due to the coulomb force between the ions of KBrO₃ and TiO₂ nanoparticles.

3.2. Phenol Red Photodegradation

The photocatalytic study of the three samples (pure TiO₂, TiO₂/1 mg-KBrO₃, and TiO_2/5 mg-KBrO₃) was performed by measuring the photo absorption in the range between 350 and 550 nm. The initial concentration of phenol red was 20 mg/L before the catalyst was placed. The absorption spectrum of phenol red was measured before the catalyst was placed; then, the catalyst was placed and the solution was placed directly in the dark for two hours to ensure that the dye degradation process was due to the photocatalyst effect. Figure 5 shows the absorption spectrum as a function of a wavelength when the solution is exposed to light rays. It appears from the absorption spectra that the adsorption that occurred after 2.0 h in the dark can be ignored, as shown by the red curve with an irradiation time equal to zero. It appears clearly that the red phenol dye decays in the presence of pure TiO₂, but it quickly decays on its surface in the presence of the KBrO₃ electron acceptor. However, the largest decomposition as a result of photocatalysis was observed when using an amount of 5.0 mg of electron acceptors, where the absorption spectrum almost reached zero, as shown in Figure 5c. In addition to the three samples, another experiment was carried out for phenol red in the presence of KBrO₃ only. The experiment was performed with different irradiation times. Figure 5d shows three curves measured between 0 and 70 min of irradiation. The result indicated that the photodegradation phenomenon is not available for the bare KBrO₃, which cannot stand alone to photodegrade the phenol red dye, as shown in Figure 5d. The result observed in this figure confirmed that the photodegradation properties of TiO₂ are fostered by the existence of KBrO_3, not vice versa.

The reported unique result observed in Figure 5 is reflected in Figure 6 for the decomposition of phenol red, by calculating the photodegradation efficiency of these three samples, as follows.

$$\eta \%=\frac{C_0-C_t}{C_0}\times 100$$

(1)

where $C_0$ and $C_t$ are the initial and instantaneous concentration of the dye at irradiation time, t. Since the absorbance peak seems to show a slight shift, the highest absorbance value was considered in each absorbance spectrum for any further calculations. Figure 6a shows the efficiencies in the dark after two hours and in light of the three samples during the photodegradation of phenol red between 0 and 110 min of irradiation time. At 0 min, the adsorption efficiencies on the semiconductor surface during the dark time were 3.0%, 3.2%, and 3.2% for TiO₂, TiO₂/1 mg-KBrO₃, and TiO₂/5 mg-KBrO₃, respectively. We considered the dye concentration after the dark adsorption as the initial concentration for the photocatalysis effect. Thus, after 5 min of irradiation, phenol red has been decomposed on the surface of TiO₂, TiO₂/1 mg-KBrO₃, and TiO₂/5 mg-KBrO_3, with the values of 33, 63, and 71%, respectively. It is clear from this that the presence of the KBrO₃ compound increased the decomposition process by a large difference from the pure sample. After 20 min of irradiation, the decomposition rate became slow in the two samples containing the electron acceptor KBrO₃ compared to the bare samples of TiO₂. However, the efficiency of the bare sample reached 33%, while the efficiency of TiO₂/1 mg-KBrO₃ reached 71%, and the efficiency of TiO₂/5 mg-KBrO₃ reached 79%. Therefore, the concentration of red phenol decreased significantly in the presence of the electron acceptor within only 20 min, which led to a slow process of adsorption of red phenol molecules on the surface of the catalyst to complete the photodegradation process. This behavior is reflected in Figure 6b, which shows a decrease in the concentration of phenol red in the solution with an increasing irradiation time. This is the case with any behavior of the decomposition processes, the decomposition process slows down with time as the decomposing material approaches its end. However, we find that after 110 min of irradiation, the decomposition efficiency of the pure sample reached 75%, while it reached 92 and 98% for the two samples, TiO₂/1 mg-KBrO₃ and TiO₂/5 mg-KBrO₃, respectively. The dye concentration decreased close to 0.2 mg/L for the highest active materials, as shown in Figure 6b.

Since the concentration of phenol decreases with time due to ultraviolet radiation, the linear relationship of ln(C/Co) versus irradiation time, t, can be calculated using the following equation [35,36]:

$$\ln \left(\frac{C}{C_o}\right)=-kKt+K{C}_o$$

(2)

where $C_o$ is the initial dye concentration and $C$ is the remaining dye concentration after the irradiation time (t), k is the first-order reaction rate constant, and K is the decomposition equilibrium constant. The semi-logarithmic relationship of phenol red concentration as a function of radiation time is shown in Figure 7. The decomposition rate constant and the degradation equilibrium constant can be calculated from Equation (2). The constants k and K were calculated and are listed in

Table 1. First-order reaction rate constants (k) and the degradation equilibrium constant (K) for the 20 mg/L of PR for TiO₂, TiO₂/1 mg-KBrO₃, and TiO₂/5 mg-KBrO₃ samples at 303 K.

Sample	k	K (s−1)	R2
TiO2	0.75	0.013	0.95
TiO2/1 mg-KBrO3	0.27	0.046	0.94
TiO2/5 mg KBrO3	0.28	0.057	0.96

. It was found that k decreased from 0.75 to 0.28 accompanied with an increasing KBrO₃ in the solution, while K increased from 0.013 to 0.057 s⁻¹ due to the addition of the presence of an electron acceptor. A first-order reaction in photocatalysis depends on the reaction rate linearly on the concentration of dye, so the rate varies according to changes in the concentration of the dye.

The higher results show that KBrO₃ has a significant effect on photodegradation when added to TiO₂. This effect is reflected by calculating the amount of instantaneous dye ($Q_t \mathrm{in} \mathrm{mg}/\mathrm{g}$) degraded per gram of metal oxide, as follows:

$$Q_t=\frac{\left(C_0-C_t\right)}{m}\mathrm{V},$$

(3)

where m is the mass of the adsorbed oxide measured in grams, and V is the volume of the solution used in L. Figure 8 shows the instantaneous amount of phenol red decayed on only one gram of the substance as a function of irradiation time. It was observed that the instantaneous amount increased with the irradiation time. The figure shows the ability of TiO₂ by adding KBrO₃ to decompose a larger amount of phenol red on its surface. The instantaneous amount of degraded dye increased from 150 mg/g for TiO₂ to 197 mg/g with the KBrO₃ electron acceptor at 110 min.

3.3. Photocatalysis Reactions and KBrO₃ Mechanism

The mechanism of degradation of phenol red on the surface of TiO₂ due to photocatalysis can be explained by comparing it with the presence of KBrO₃ in solution. Hence, it can show how the presence of KBrO₃ fostered the degradation of phenol red. Based on the advanced oxidation process, the phenomenon of phenol red degradation because of photocatalysis has been defined in some literature [24,37]. When the TiO₂ compound is photoexcited, forming a pair of electron holes $(e^{-}-h^{+})$, the photolysis of the dye in water begins, as follows:

$$Ti{O}_2+h\nu \to Ti{O}_2 (e^{-}+h^{+})$$

(4)

Direct oxidation reactions occur due to the oxidizing potential of the $h^{+}$.

$$h^{+}+dye\to dy{e}^{.+}\to \mathrm{oxidation} \mathrm{of} \mathrm{the} \mathrm{dye}$$

(5)

The decomposition of water molecules or the reactions of $O{H}^{-}$ with the exited holes enables the formation of hydroxyl radicals ($O{H}^{.}$), which are photolysis responsible.

$$h^{+}+H_{2}O\to H^{+}+O{H}^{.}$$

(6)

$$h^{+}+O{H}^{-}\to O{H}^{.}$$

(7)

Another way is the formation of peroxide anions by reacting the oxygen molecules with the exited electrons. These peroxide anions are in charge of obtaining hydroxyl radicals ($O{H}^{.}$).

$$e^{-}+O_2\to {}^{.}O_2^{-}$$

(8)

$${}^{.}O_2^{-}+dye\to dye-O{O}^{.}$$

(9)

The effect of KBrO₃ could be clarified based on the previous report [22]. The addition of electron acceptors improves the decomposition rate in several ways, as it prevents electron–hole recombination by picking up the conduction band electron, thus increasing the concentration of hydroxyl radicals. KBrO₃ is an ionic compound that dissolves in polar solvents like water. Thus, KBrO₃ undergoes ionization to form ${\mathrm{K}}^{+}$ and ${\mathrm{BrO}}_3^{-}$ ions in water.

$${\mathrm{KBrO}}_3\to {\mathrm{K}}^{+}+{ \mathrm{BrO}}_3^{-}$$

(10)

The enhancement of the removal rate is due to the reaction between the ${\mathrm{BrO}}_3^{-}$ ion and conduction band electron, which reduces the recombination of electron–hole [22].

$${\mathrm{BrO}}_3^{-}+6e_{CB}^{-}+6H^{+} \to B{r}^{-}+3H_{2}O$$

(11)

With adding KBrO₃ with 1 mg to 5 mg of TiO₂, a high improvement in the photodegradation properties of TiO₂ was observed. On the further increase in the KBrO₃ addition, a slight enhancement was observed.

Based on the above formation of ${}^{.}OH$ radicals, the organic dye decays, where ${}^{.}OH$ radicals are high oxidizers with 2.8 V in potential. There is no mechanism equation for PR in photocatalysis degradation. However, we have proposed that the mechanism and reactions pathways analysis may reveal that PR dye is first converted into different intermediate products and then completely mineralized into ${\mathrm{CO}}_2,{ \mathrm{H}}_2\mathrm{O},{ \mathrm{and} \mathrm{SO}}_4^{2-}$.

$${}^{.}OH+PR \left({\mathrm{C}}_{19}{\mathrm{H}}_{14}{\mathrm{O}}_5\mathrm{S}\right)\to \mathrm{intermediate} \mathrm{products} \to {\mathrm{CO}}_2,{ \mathrm{H}}_2\mathrm{O},{ \mathrm{SO}}_4^{2-}$$

(12)

The most important factor for obtaining a high photocatalytic efficiency is the reduction in the recombination rate of the electron–hole that is excited in the electronic structure. Reducing this rate reduces the waste of optical energy and increases the yield. Thus, increasing the yield of red phenol decomposition in water. This required efficiency or higher yield can be achieved by adding a KBrO₃ electron acceptor. The addition of the electron acceptor KBrO₃ in the current study prevented the e-h recombination in the valence band of the oxide. Hence, it leads to an increase in the OH concentration and hence, a rapid degradation of the dye. The Tauc plot to determine the TiO₂ bandgap and the scheme of the photocatalysis mechanism is proposed in Figure 9a,b, respectively. The scheme demonstrated the reactions of KBrO₃ with the excited e-h to prevent their recombination.

The results obtained in the present study are supported by the photocurrent measurements for pure TiO₂, TiO₂/1 mg-KBrO₃, and TiO₂/5 mg-KBrO₃, as shown in Figure 10. The photocurrent measurements were carried out at a bias voltage of 0.2 V for all samples. The sample solution was placed in an electrochemical cell containing two electrodes. The cell containing the sample solution was irradiated by the same source of UV used for the photocatalysis measurements. From Figure 10, when pure TiO₂ was irradiated, the photocurrent signal reached the steady state value within 40 s. The sample shows a basic current of 35.2 µA. This current increased by 1.2 µA due to UV radiation. After switching off the UV light, the sample takes about 40 s for full recovery, returning to the basic current. With the addition of 1 mg and 5 mg of KBrO₃, the basic current increased up to 43 and 66 µA, respectively. Samples TiO₂/1 mg-KbrO₃ and TiO₂/5 mg-KbrO₃ required a longer time, about 250 s, to reach the steady state, accompanied by a significant increase in the photo-induced current compared to the pristine TiO₂ sample. The photo-induced current or photocurrent was 13.2 µA and 21.3 µA for TiO₂/1 mg-KBrO₃ and TiO₂/5 mg-KBrO₃, respectively. The decay time of TiO₂/1 mg-KBrO₃ and TiO₂/5 mg-KBrO₃ was found to be 486 and 893 s, respectively. This is considered a significant improvement in the photocurrent in the presence of KBrO₃, indicating that the e-h recombination process for these two samples is very slow, which greatly improves the photocatalytic degradation of phenol red. In the Raman spectra, a small order of magnitude 0.5 cm⁻¹ corresponds to ~0.1 meV, which consists of more of an electrostatic interaction between the ions of KBrO₃ and the surface of TiO₂ nanoparticles. The latter confirms how KbrO₃ interacts with the surface of semiconductors and how it could capture electrons from the conduction band of TiO₂ during the photodegradation process and consequently, slow the recombination rate of hole–electron in the valence band of TiO₂.

A comparative study between this work and some reported results on the photodegradation of phenol red using TiO₂ nanocomposites and other photocatalysts with the effect of acceptors is presented in

Table 2. Comparison of the current work and some previous work for PR degradation using different photocatalysts.

Photocatalysts	Catalyst (gL−1)	PR Con. (M)	Acceptors/Donors Con. (M)	Degradation (%)/Duration (Min)	Refs
TiO2	0.5	2.8 × 10−5	H2O2 (3 × 10−3)	94/300	[23]
n-TiO2	0.6	3.76 × 10−5	no	95/100	[24]
Goethite	1.0	1.0 ×10−5	H2O2 (5 × 10−3)	80/100	[25]
Bi1-xLaxFeO3	1.0	1.0 ×10−5	no	90/102	[26]
CeO2-TiO2	1.0	2.8 × 10−5	No	99/80	[27]
Nb-TiO2	0.1	5.6 × 10−5	KBrO3(10−3)	95/110	[38]
TiO2	0.1	5.6 × 10−5	KBrO3(10−3)	98/110	This work

. We concluded that despite the higher degradation rate and short duration for some studies, such as (CeO₂-TiO₂ NC), in most of them, they used half the concentration of PR in this work and 5–10 times more concentrated photocatalysts. The latter makes KBrO₃ an excellent acceptor when simply mixing without a complex method of fabrication with very small nanosized-pure TiO₂ to degrade high concentrated solutions of PR.

3.4. Analysis of Photodecomposition Products

The photodecomposition of PR products was analyzed using Raman spectroscopy. The sample of a solution of 50 mL with 20 mg/L of PR containing TiO₂/5 mg-KBrO₃, which was irradiated for 110 min, was chosen for this study. Two samples were withdrawn from the solution carefully, before (top solution) and after stirring the solution. The solution samples were placed dropwise into glass slides and left to dry at room temperature. The Raman spectra were recorded into different points and then summed up, as shown in Figure 11. A comparison study with the two samples in addition to the KBrO₃ sample was performed to reveal more details on the photodecomposition products of PR. The three bands that appeared in the spectra of the sample before stirring, located at 793 cm⁻¹, 1346 cm⁻¹, and 1590 cm⁻¹, are attributed to KBrO₃ and carbon. The Raman peak of the KBrO₃ band coincides well with those observed in the Raman spectrum of KBrO₃. However, after stirring, the Raman spectra showed that the carbon band disappeared, KBrO₃ decreased, and that its band became wide; in addition, new five bands located at 148 cm⁻¹, 393.8 cm⁻¹, 547.4 cm⁻¹, 633.4 cm⁻¹, and 1091 cm⁻¹, attributed to TiO₂ and $S{O}_4^{2-}$, were observed [39,40]. The observation confirms that SO₄ components precipitated during the photodegradation process and carbon flitted in the solution due to its small weight compared to the SO₄ complexes. The Raman results observed here confirmed the proposed mechanism in Section 3.3.

4. Conclusions

In summary, we show that TiO₂ becomes more reactive in the degradation of phenol red in the presence of KBrO₃, whether in a small amount of about 1 mg or 5 mg. The results showed that KBrO₃ fosters the yield of the photoactive reactions and thus enhances the decay efficiency of phenol red on the oxide surface. In addition, increasing the concentration of KBrO₃ enhanced photodegradation. A good adjustment with the first-order kinetics for all the samples was concluded. The efficiency of PR was increased during 110 min from 75% of pure TiO₂ to 92% and 98% of TiO₂ with 1 mg and 5 mg KBrO₃, respectively. The instantaneous amount of degraded dye increased from 150 to 180 and 197 mg/g TiO₂ with 1 mg and 5 mg KBrO₃, respectively. The increase in hydroxyl radicals leads to a significant improvement in the photocatalytic activity. The electron acceptors significantly enhanced the photodegradation rate. It prevented electron–hole recombination through conduction band electron capture, which increases the concentration of hydroxyl radicals. The photocurrent measurements were performed to support the claims of the current study. The photo-induced current showed that the existence of KBrO₃ in the solution supported the reduction in the e-h recombination. The high photo-induced current of TiO₂ was observed by adding 1 mg and 5 mg of KBrO₃. The Raman spectra of the final products exhibited that PR was completely mineralized into ${\mathrm{CO}}_2,{ \mathrm{H}}_2\mathrm{O},{ \mathrm{and} \mathrm{SO}}_4^{2-}$.