Utilization of TiO₂ Nanoparticles for Methylene Blue Degradation ^†

Abstract

Titanium dioxide (TiO₂) nanoparticles (NPs) are useful as a potential photocatalyst for the degradation of dyes such as methyl orange, rhodamine B, and methylene blue (MB). Understanding the mechanism of photocatalysis and the factors influencing photocatalysis is important for engineering TiO₂ NPs to achieve an unprecedented photocatalysis rate. For TiO₂ NPs, their unique physicochemical qualities, such as small size, large surface area, optimum semiconductor bandgap, substantial oxidative potential, and outstanding chemical stability are factors which influence the MB degradation rate. The electron–hole pair separation in TiO₂ NPs allows for photocatalysis, which is not possible in their bulk form. The formation of reactive oxygen species (ROS) via photoinduced generation of electron–hole pairs under light irradiation is the starting point of the mechanism of photocatalysis for TiO₂ NPs. By generating ROS, TiO₂ NPs catalyze the degradation of MB. The photocatalytic performance of TiO₂ NPs is also different for different crystal phases, such as anatase, rutile, and brookite. The addition of metal or non-metal dopants into TiO₂ NPs enhances photocatalysis by enhancing light absorption, which enhances the generation of electron–hole pairs and of ROS. This review article will explain the mechanism of photocatalysis, the parameters influencing photocatalytic activity, active sites and recombination rates, disadvantages, and strategies to overcome these challenges that can improve TiO₂ NPs for a future wastewater treatment that is both efficient and sustainable.

1. Introduction

Photocatalysts have been extensively studied for their potential to convert solar energy and remove environmental pollutants [1]. TiO₂ NPs have received particular attention because of their high oxidizing capacity under UV irradiation, as well as their superior chemical and thermal stability and environmental friendliness [2]. MB is one of the most widely used dyes for wood, silk, and cotton. It can cause methemoglobinemia, nausea, vomiting, excessive perspiration, mental confusion, and even permanent burns to the eyes of both humans and animals [3]. Most textile dyes are refractory to chemical oxidation and photocatalytically stable, making them impervious to decolorization using standard physicochemical and biological techniques. Using nano-semiconductors that possess high photocatalytic reactivity, such as TiO₂, Fe₂O₃, ZnO, and CdS, offers a successful approach for dye removal [4]. TiO₂ NPs have been regarded as the most promising photocatalyst for the degradation and elimination of various toxic organic contaminants [5]. Studies demonstrate that combining activated carbon (AC) with TiO₂ enhances MB degradation due to the high porosity and self-photoactivity of the carbon support. The synergistic TiO₂/AC composite effectively degrades organic dyes through simultaneous adsorption and photocatalytic oxidation [6].The photocatalytic process typically begins when incident UV radiation reaches oxide materials, promoting electron excitation from the filled valence band (VB) to the empty conduction band (CB) and generating electron (e⁻) and hole (h⁺) pairs on the photocatalyst surface [4]. Variables, including dye adsorption on the catalyst surface, surface area, bandgap energy, particle size, electron–hole recombination rate, and crystallinity, affect TiO₂’s photocatalytic activity during the degradation process [7]. Diatomite’s porous structure combined with anatase–rutile TiO₂ phases enhances MB degradation, achieving 80% efficiency in 270 min under natural sunlight [8]. The absorption spectra are significantly reduced with reaction time, and the color changes from blue to colorless. The reduction in absorption spectra is most likely caused by the degradation of MB. Advanced Oxidation Processes (AOPs) have been developed to handle organic pollutants. Strong redox reactions are the basis of these methods, which produce extremely reactive radicals that can degrade pollutants like MB without releasing more hazardous byproducts [9]. Ozonation [10], electrochemical oxidation/degradation [11], UV/H₂O₂ oxidation [12], heterogeneous photo-Fenton processes, catalytic oxidation, and photocatalytic degradation [9] are AOP techniques frequently utilized in MB photodegradation processes. This review article aims to explain why TiO₂ NPs are a great photocatalyst that can effectively degrade MB using photocatalysis.

2. Materials and Methods

TiO₂ NP synthesis starts with a titanium (Ti) salt, a solvent, and a reducing agent. The reducing agent can be NaOH, NaHCO₃, chitosan, or any other compound which can reduce Ti salt to TiO₂ NPs. Isopropanol (analytical grade) serves as the solvent [1,3].

2.1. Synthesis Methods

In the sol–gel process, titanium alkoxide precursors are mixed with alcohol solvents and are then hydrolyzed using acidified water to form a TiO₂ NPs. With aging, the sol converts into a gel, which can be dried to produce xerogel in various forms [13,14]. Titanium isopropoxide (TIP) can be added to an acetic acid–ethanol mixture and stirred for 1 h before being transferred for solvothermal treatment at 180 °C for 24 h. The dried product is then finely ground and calcined at 550 °C and 950 °C to obtain the final TiO₂ powder [15]. In a hydrothermal process, TIP is hydrolyzed in an acidic isopropanol–water mixture using nitric acid as the peptizing agent. Precursor solutions containing different TIP concentrations are stirred vigorously and then annealed at 60 °C for 8 h to form stable TiO₂ colloidal NPs [16]. In green synthesis, fresh P. guajava leaves are washed, boiled in distilled water, and filtered to obtain an aqueous extract. A previous study synthesized TiO₂ NPs by adding 20 mL of this extract to 80 mL of TiO(OH)₂ (0.1 mM) and stirring at room temperature for 24 h. The reaction produced light green TiO₂ NPs, whereas the individual extract and TiO(OH)₂ showed no color change [17]. In top-down synthesis, GO/TiO₂ nanocomposites can be prepared by dispersing 0.25 mg/mL of graphene oxide in water and mixing it with 100 mg of NH₄TiOF₃. The mixture can be processed following a procedure similar to that used for MWCNTs. The resulting precipitates are filtered, washed with water and ethanol, and dried in a vacuum oven at 60 °C for 12 h [18].

2.2. Photocatalytic Activity Test

Photocatalytic activity tests are performed under light, for example, UV light, visible light, or sunlight. The degradation reaction is run by introducing TiO₂ NPs into the dye solution under stirring in a dark system to reach an absorption equilibrium. The system is then placed in a light system for degradation for a certain amount of time. Dye concentration at various time intervals can be measured by a UV–Vis spectrophotometer. If the concentration shows a significantly lower value after a time interval, it can be said that dye degradation is catalyzed by the NPs [1,3].

3. Photocatalytic Mechanism in TiO₂

The semiconducting substance TiO₂, sometimes referred to as titania, is a member of groups IV–VI of the periodic table. In nature, it exists in three primary polymorphs: brookite, rutile, and anatase [19]. TiO₂ functions as an n-type semiconductor because of the oxygen vacancies present [20]. With bandgap energies of roughly 3.03 eV for the rutile phase and 3.2 eV for the anatase phase, TiO₂ is a semiconductor with a wide bandgap [20]. Its electrical conductivity lies between that of an insulator and a conductor. Its conductivity is governed by its band structure, composed of a valence band (VB) and a conduction band (CB). The CB corresponds to the molecular LUMO, while the VB arises from interactions involving the HOMO [21]. While titanium 3d, 4s, and 4p orbitals make up the conduction band in TiO₂, oxygen 2p orbitals make up most of the VB instead. Figure 1 shows the bandgap refers to the energy difference between the CB and VB [20]. In TiO₂ NPs, the semiconductor takes up photons with energy at least as high as its bandgap. An electron from the VB is excited by this photon absorption and moves to the CB, creating an electron–hole pair (e⁻/h⁺). Redox reactions on the TiO₂ surface can then involve the excited electron in the CB and the hole in the CB. In Equation (1), when TiO₂ absorbs UV light (hν), an electron is promoted to the CB, leaving a hole in the VB. This creates a reactive pair (e⁻ + h⁺), which triggers photocatalytic processes. This basic procedure can be illustrated as follows [22]:

TiO₂ + hν → e⁻ + h⁺

(1)

4. Electrons and Holes Participate in Redox Process

Photogenerated electrons and holes act as the primary charge carriers that initiate redox reactions leading to the mineralization of hazardous contaminants. These charge carriers, however, are also susceptible to recombination, which lowers the likelihood that electrons will reach the CB and drastically lowers the photocatalytic process’s overall efficiency [21]. Figure 2 presents the basic mechanism of TiO₂ photocatalysis, which involves light absorption, charge separation, and surface reactions. When TiO₂ absorbs photons with energy greater than its bandgap, electrons are excited from the VB to the CB, leaving holes in the VB. The separated charges can follow productive redox routes: in pathway 1, electrons migrate to the surface and reduce electron acceptors (A → A⁻), generating species such as superoxide radicals, and in pathway 2, holes oxidize electron donors (D → D⁺), leading to the formation of hydroxyl radicals from water. These radicals actively degrade pollutants like MB. However, electrons and holes may also recombine either at the surface (pathway 3) or within the bulk (pathway 4), lowering the overall photocatalytic efficiency. Therefore, enhancing charge separation and extending TiO₂’s optical absorption toward the visible and infrared regions are essential for improved photocatalysis [22].

Non-recombining electrons and holes can move to the surface of TiO₂ and engage in redox reactions with molecules that have been adsorbed on the surface. In Equation (2), superoxide radicals (∙O₂⁻) or hydroperoxide radicals (HO₂∙) are produced when electrons (e⁻) in the CB engage in reduction processes with molecular oxygen (O₂):

O₂ + e⁻ → ^∙O₂⁻

(2)

The VB’s photogenerated holes (h⁺) participate in oxidation processes with water molecules or surface-adsorbed hydroxyl ions, resulting in the production of extremely reactive hydroxyl radicals (∙OH) [21]. FeO₄@TiO₂ NPs effectively demonstrate this photocatalytic mechanism. When oxidant radicals recombine, H₂O₂ can be formed, while the photogenerated holes may either directly oxidize organic compounds or be trapped by electron donors. The superoxide radical anion can further react with the produced H₂O₂ to generate additional hydroxyl radicals. Under artificial sunlight irradiation, these ROS, including superoxide and hydroxyl radicals, degrade MB dye into non-toxic byproducts such as CO₂, H₂O, and mineral acids [23]. Figure 3 provides a graphical representation of these procedures, where (a) illustrates the oxidation process and (b) illustrates the reduction process. Together, the radicals HO₂∙, ∙O₂⁻, and ∙OH are known as ROS, and they play an essential role in the degradation of organic materials. The effectiveness of photocatalytic degradation is strongly influenced by both the properties of the semiconductor and the characteristics of the target organic pollutants [24].

5. Parameters of TiO₂ NPs Influencing Photocatalytic Activity

The photocatalytic efficiency of TiO₂ NPs is strongly influenced by factors such as particle size, surface area, crystal phase, and charge carrier dynamics. Nanoscale TiO₂ offers increased surface area, improved adsorption of dye molecules, and reduced electron–hole recombination, all of which enhance photocatalytic activity. Quantum confinement and optimized interactions with MB further improve charge separation and reactivity. Among TiO₂ polymorphs, anatase shows the highest activity due to its favorable bandgap and superior charge carrier properties. Overall, the combined effects of small particle size, high surface area, and anatase composition significantly enhance the photocatalytic performance of TiO₂-based materials [25]. These fundamental parameters are not independent but rather act synergistically to determine the overall photocatalytic efficiency of TiO₂ systems [2]. Among its phases, anatase shows the highest activity due to its larger surface area, higher surface hydroxyl density, and more negative conduction band potential, all of which promote efficient charge separation and ROS formation. Mixed anatase–rutile systems also perform well because interfacial charge transfer reduces electron–hole recombination. In MB degradation, anatase achieves nearly full degradation within 1 h, while rutile reaches only about 85% after 4 h. Mixed-phase TiO₂ shows intermediate activity, but its activity is higher than that of pure rutile, confirming that particle size and phase composition strongly influence photocatalytic efficiency [25,26].

6. Active Sites and Recombination Rates in TiO₂ Photocatalysis

The photocatalytic efficiency of TiO₂ depends on electron–hole recombination rates, surface area, and active site density. An initial increase in efficiency is attributed to a higher number of active sites and increased surface area, whereas excessive photocatalyst loading can reduce efficiency by promoting aggregation, increasing light scattering, and limiting light penetration [27]. Electron–hole recombination reduces photocatalyst activity, but decreasing TiO₂ particle size shortens the distance charge carriers must travel to reach surface reaction sites, lowering recombination likelihood. A smaller particle size also increases the surface area, enhancing interactions between the photocatalyst and pollutant, which leads to higher photodegradation efficiency [25]. Electron–hole recombination, occurring in the bulk or on the surface, reduces photocatalytic efficiency because it happens much faster than charge carriers can reach the surface. Smaller particles (~10 nm) enhance carrier mobility and lower recombination, while impurities and lattice defects act as trapping centers that further affect recombination [2]. Photocatalytic efficiency is affected by pollutant concentrations and by the number of available active sites. In one study, the kinetic rate constant decreased from 0.05 to 0.009 min⁻¹ when the concentration of bisphenol A (BPA) rose from 10 to 100 mg/L. The low availability of e⁻ and h⁺ species, which generate ROS and degrade pollutants, may have been the cause of this decline. Frontistis et al. [27] observed similar results during the degradation of the endocrine disruptor BPA, explaining that the decrease in efficiency was due to limited active sites for pollutant and byproduct adsorption, as well as decreased production of oxidizing species caused by partial occupation of the photocatalyst surface [28]. To enhance MB degradation, the nitrogen content in 5%N_T/TiO₂ NPs gradually increased. MB degradation efficiency changed with N-doping level. After 150 min of visible light exposure, undoped TiO₂ degraded only about 15% of MB, while 5%N_T/TiO₂ showed the highest activity, achieving nearly 56% degradation in the same period. [29].

7. Disadvantages of TiO₂ as a Photocatalyst

Although TiO₂ is the most widely used photocatalyst, its use has several drawbacks. These drawbacks include a wide bandgap energy that restricts light absorption to the ultraviolet region (wavelengths less than 390 nm) and the rapid recombination of photogenerated electron holes, which impacts quantum efficiency. As a result, this semiconductor has limits when used in photocatalytic processes in the visible light spectrum [22].

8. Improvements in TiO₂ NPs’ Photocatalytic Activity

Extensive research has been conducted to extend the light absorption capabilities of TiO₂ into the visible area to enhance its photocatalytic activity. Generally speaking, the strategies created for this purpose fall into two main categories: (i) bandgap engineering, which narrows the bandgap of TiO₂ by using special synthesis techniques or by introducing foreign elements into its crystal structure, and (ii) surface sensitization, which is the use of visible light-active materials as sensitizers to improve light harvesting and encourage photoactivation of TiO₂ under visible irradiation [30].

9. Strategies to Overcome Challenges

Photocatalytic dye degradation can be performed with both natural and artificial light, but artificial light is better because it stays the same and does not change with the weather [9]. Textile industries release a lot of MB dye into bodies of water, which is very toxic and can be bad for both people and microbes. MB is a major pollutant because it causes cancer, does not break down in the environment, and stays there for a long time. Also, using nanomaterials to clean water raises health and environmental issues. Long-term studies are necessary to evaluate the release of NPs into aquatic systems and their effects on human health and ecosystems [31]. To get more improved photocatalysis, TiO₂ doped materials, composite of TiO₂ NPs with CNT or nanocellulose can be used.

10. Conclusions

TiO₂ NPs are widely recognized for their excellent photocatalytic ability to degrade MB. This efficiency comes from their suitable bandgap, large surface area, and strong generation of ROS under light irradiation. Their overall activity is strongly influenced by factors such as particle size, crystal phase, degree of crystallinity, and the rate of electron–hole recombination. Among the different polymorphs, anatase TiO₂ typically shows the highest photocatalytic performance. A variety of synthesis methods—such as sol–gel, hydrothermal, green synthesis, and top-down approaches—enable precise control over the structural and optical properties of TiO₂, leading to improved photocatalytic behavior. Additionally, modification strategies like metal or non-metal doping, mixed-phase TiO₂, and surface sensitization can significantly enhance visible light absorption. Despite these advantages, TiO₂ still faces challenges, including its wide bandgap and fast electron–hole recombination. Environmental concerns related to NP stability, recovery, and reuse are also important considerations.