Advances in TiO₂ Nanoparticles for Rhodamine B Degradation ^†

Abstract

Titanium dioxide (TiO₂) nanoparticles (NPs) have garnered significant attention as photocatalysts for degrading organic pollutants, particularly synthetic dyes such as rhodamine B (RhB), methylene blue, methyl orange, and others. The impact of several synthesis methods, including sol–gel, hydrothermal, and chemical vapor deposition (CVD) techniques, on the electrical and morphological properties of TiO₂ NPs has been studied, emphasizing the distinctive physicochemical properties of TiO₂ NPs, including their extensive surface area, significant oxidative capacity, and remarkable chemical stability, which are important in the recent advancements in their use for RhB degradation. A detailed examination of TiO₂’s photocatalytic mechanism shows that it is based on the generation of reactive oxygen species (ROS) by photoinduced electron–hole pair formation under ultraviolet (UV) light exposure. In wastewater treatment, TiO₂ degrades RhB into less harmful byproducts by the generation of electron–hole pairs that initiate redox reactions under sunlight. This study includes a thorough overview of significant factors influencing photocatalytic efficacy. The parameters include particle size, crystal phase (anatase, rutile, and brookite), surface changes, and the incorporation of metal or non-metal dopants to enhance visible light absorption. Researchers continually seek methods to overcome challenges, including restricted visible-light responsiveness and rapid electron–hole recombination. The investigated approaches include heterojunction generation, composite development, and co-catalyst insertion. This review article aims to address the deficiencies in our understanding of TiO₂-based photocatalysis for the degradation of RhB and to propose enhancements for these systems to enable more efficient and sustainable wastewater treatment in the future.

1. Introduction

Access to pure water is recognized as a serious issue in the modern world because of quick population growth, increased water consumption, and a rise in the quantity of wastewater discharged by chemical industries. RhB is a xanthene dye and has many uses in the leather, paper, textile, and paint industries. Therefore, it is crucial to find efficient techniques for RhB degradation before it is released into the environment. Unfortunately, traditional techniques are not useful at degrading chemical compounds completely, as most chemical compounds have a high chemical stability and are not easily biodegradable [1]. Semiconductor photocatalysis has gained recognition in the last twenty years as a result of its applications to pollution control. Among various frequently used oxide semiconducting photocatalysts, TiO₂ is a widely used photocatalyst, as it shows outstanding photocatalytic performance, chemical durability, inexpensiveness, and safety. Mainly, TiO₂ has anatase (Eg = 3.2 eV) and rutile (Eg = 3.0 eV) crystalline phases. The anatase crystalline phase consistently shows enhanced photocatalytic performance compared to any other crystalline phase [2]. The anatase crystalline phase of TiO₂ exhibits a large band gap (3.2 eV) and enhanced electron–hole separation capability throughout UV illumination. TiO₂ NPs are not able to capture light with a wavelength exceeding 398 nm and show activity under UV light. Sadly, industrial-scale UV lights are expensive and have a high energy demand when it comes to implementing this method at a large scale. Under solar light, nearly 5% of sunlight consists of UV light, and pure TiO₂ remains inactive. Doping TiO₂ with nonmetallic species (F, S, B, and N) or transition metal species (Cu, Cs, Cr, Fe, and Au) is a significant technique that boosts light capture on the surface and enables TiO₂ performance in visible light. Under optimal dosing conditions, iron ions are recognized as a high-quality dopant of TiO₂ because they have the ability to enhance the total number of catalytically active sites on the TiO₂ surface, decrease the rate of electron–hole pair recombination, and reduce the TiO₂ band gap, as indicated in the literature [1]. Relevant research indicates that the massive energy emitted by ultrasonic-assisted cavitation is able to meet the requirements of TiO₂ electron-filled band electron shift, thus causing catalytic activity comparable to the photocatalytic process. The preferential site nucleation and catalytic performance on the surface of TiO₂ solid-phase particles is appropriate for catalytic decomposition of toxic organic species [3].

2. Mechanism

Photocatalytic oxidation using semiconductor materials like TiO₂ is a light-induced process that involves the generation of reactive species capable of degrading organic pollutants and microorganisms. As the photocatalyst is exposed to UV light, its electrons become excited, creating electron–hole pairs that migrate to the surface (Figure 1). The holes react with hydroxide ions or adsorbed water and produce very reactive hydroxyl radicals, which are powerful, non-selective oxidants responsible for breaking down a wide range of organic contaminants. Simultaneously, the excited electrons reduce oxygen molecules on the catalyst surface, forming superoxide radicals. These radicals not only participate in further oxidation reactions but also help prevent reassociation of the charge transporters and maintain the performance of the process. The superoxide radicals can then undergo a series of reactions to generate additional reactive oxygen radicals, particularly hydrogen peroxide and hydroperoxyl radicals, which further decompose into more hydroxyl radicals. Together, these reactive species contribute to the full decomposition of organic contaminants and the neutralization of microbes, making the entire photocatalytic system highly effective for environmental remediation applications [4,5].

Figure 1. Photocatalytic degradation of RhB.

3. Synthesis Methods

In the sol–gel method, titanium alkoxide precursors like titanium (IV) n-butoxide (TNBT) or titanium isopropoxide (TTIP) are mixed with alcohol-based solvents with continuous stirring, followed by the dropwise mixing in of acidified liquid water to initiate hydrolysis and condensation reactions, forming a TiO₂ sol (Figure 2). After aging, the sol transitions into a gel, which can be dried to obtain xerogel in the form of a powder, dense ceramic, or fibers. Additives such as complexing agents or metal salts can be incorporated to enhance the material’s properties based on the intended application [6]. The sol–gel technique is highly flexible and enables accurate control over NP characteristics [7]. TiO₂ NPs were synthesized via hydrothermal synthesis under an applied magnetic field. Fifty milliliters of a 0.1 M TTIP mixture was placed inside a low-temperature water bath until a temperature range of 3–5 °C was reached. Subsequently, 5.0 M KOH was incrementally mixed into the low-temperature solution until a pH of 7.0 was reached. The resulting mixture was subsequently placed into a Teflon autoclave and placed under a temperature of about 160 °C for 5 h. Then, the chemical mixture was exposed to a magnetic field. The resulting powder was dried overnight in the oven and subsequently thermally treated in air at a temperature of 500 °C for 30 min, with a heating rate of 5 °C/min [8]. Photocatalytic TiO₂ thin films were deposited using a vertical low-pressure CVD reactor designed as a custom cold-wall system. The titanium and oxygen sources were TTIP and oxygen gas (O₂), respectively. The deposition temperature was varied between 287 °C and 362 °C, while the operating pressure was maintained at 1 Torr. Argon was employed as the carrier gas for TTIP, with flow rates of 80 cm³·min⁻¹ for Ar and 10 cm³·min⁻¹ for O₂. The TTIP delivery rate was maintained at 4.464 × 10⁻⁵ mol·min⁻¹. The microstructural properties of the deposited TiO₂ films were characterized by X-ray diffraction (XRD) using Cu Kα radiation operated at 40 kV and 20 mA. The surface morphology and cross-sectional structures of the films were examined using scanning electron microscopy (SEM). The photocatalytic degradation of benzene was evaluated using 10 × 10 cm² TiO₂-coated glass substrates placed in a quartz reactor with an internal volume of 610 mL. The experiments were carried out under UV illumination (Sankyo BLB lamp, λ = 365 nm) with an intensity of 0.71 mW·cm⁻² at ambient temperature. The initial benzene concentration in the reactor was approximately 120 ppm.

Figure 2. Various synthesis methods.

The degradation products were analyzed via gas chromatography (HP6890 series) under constant-flow conditions. The operational parameters were maintained at a pressure of 18.5 psi, a helium flow rate of 0.9 mL·min⁻¹, and an average linear velocity of 29 cm·s⁻¹ [9].

4. Key Factors Influencing Photocatalysis

Particle size plays a critical role in determining the effectiveness of nano-photocatalytic materials. The dimensions and configuration of the catalyst affect its catalytic performance and surface structure [10,11]. Tiny particles of the photoactive material enhance the yields of products as compared to the bulk particles [12]. In contrast to bulk titanium dioxide, granular TiO₂ NPs exhibit an increased surface area and an expanded band gap. Their composition includes a greater number of active sites, leading to enhanced photocatalytic activity, superior photoelectrochemical properties, and increased gas sensitivity [13,14]. Rutile represents the stable phase, while anatase and brookite are considered metastable and can convert to rutile upon heating [15]. Rutile by itself has hardly ever been found to be active for the photodegradation of organic compounds in aqueous solutions, but satisfactory levels of photocatalytic activity have been exhibited by samples containing mixtures of anatase and rutile [16,17]. In contrast, brookite has received relatively little attention as a photocatalyst, mainly because obtaining it in pure form is quite challenging [18]. High anatase crystallinity and large crystallite size mean low surface energy, and a low surface energy is unfavorable for the phase transformation process, consequently leading to an enhancement in the initial phase transformation temperature [2]. Owing to the large surface area of NP-based semiconductor systems and the sufficient spectrum property of the standard dyes, the integrated systems may capture a high percentage of the incoming solar radiation, as high as 46%. However, efficiencies achieved in simulated sunlight in an AM 1.5 environment are comparatively low, approximately 7–11% [19]. It is expected that in light-assisted catalysis, the adsorption of hazardous compounds ahead of time is a key step [20]. Thus, high surface area is a crucial factor in enhancing the light-assisted activity of TiO₂. It is suggested that large surface area and high-quality anatase crystals are vital for effective photocatalytic activity [2]. The ionic interactions between the surface of the catalyst, radicals, and substrates during the decomposition process are determined by the pH of the contaminated water containing organic dye. If the pH is lower than 2, then TiO₂ shows strong oxidation [21]. If the pH is less than 5, dye photodegradation can be stopped by excess proton concentration, reducing efficiency. On the other hand, if the pH is more than 10, hydroxyl ions can neutralize acidic products produced by photocatalytic degradation [22].

5. Challenges

TiO₂ photocatalysts have some problems, including limited sunlight utilization, poor reaction efficiency, and ineffective treatment of concentrated wastewater [3]. The anatase form of TiO₂ possesses a wide band gap of approximately 3.2 eV and experiences a high rate of electron–hole recombination under UV irradiation. TiO₂ particles are unable to absorb light with wavelengths exceeding 398 nm and therefore exhibit photocatalytic activity solely in the UV region [1]. Furthermore, TiO₂ has a large band gap, and it can use only UV light for H₂ production. UV light accounts for fewer than 5% of the solar spectrum, and visible light contributes approximately 50%. H₂ production may decrease if visible light is not deployed [23]. However, the multiple-step electron energy exchange process in the photocatalysis process is a longstanding challenge, and it should be solved [24]. In addition, extra charges, surface additives such as metals or metal oxides, and the states of adsorbed species play crucial roles in the photocatalysis process. These elements open chances to explore model systems and increase challenges in finding significant insights into the basic process of TiO₂ photocatalysis [25].

6. Result

Natarajan et al. synthesized UV-LED/TiO₂ NPs with a 30 nm size in order to apply them in the degradation of RhB dye and found 96% efficiency after 180 min [26]. Slimani et al. prepared Ce and Sm co-doped TiO₂ NPs with a 15.1–17.8 nm size for RhB degradation to achieve 98% removal efficiency in 30 min [27]. Abdel-Messih et al. developed SnO₂/TiO₂ NPs with a size of 19 nm and achieved 92% efficiency within 189 min [28]. Ruíz-Santoyo et al. formulated TiO₂-ZrO₂ as NPs of 10.1 nm exhibiting 84.6% RhB removal after 150 min [29]. Indira et al. constructed 2.28 nm Ce-TiO₂ NPs that removed 95% RhB after 100 min [30]. Safajou et al. engineered Pd/TiO₂ NPs with a 50 nm size that removed 76% of RhB within 40 min [31]. Wang et al. obtained RGO–TiO₂ NPs of 5 nm that could degrade RhB with an efficiency rate of 86% after 5 h [32]. Karimi et al. found 20–30 nm TFNA/TiO₂ NPs were 86% efficient in RhB removal after 100 min [33]. Gatou et al. synthesized 1.82 nm TiO₂/SiO₂ NPs that degraded 100% of RhB after 210 min [34]. Malika et al. generated 20 and 30 nm Cu-Ni (75:25)-TiO₂/MMT NPs that achieved 97% degradation efficiency after 50 min [35]. Baruah et al. processed GO/PANI/15%TiO₂ NPs of 15.91 nm, which degraded 98% of RhB within 80 min [36]. Mekprasart et al. yielded CuPc/TiO₂ NPs with a 20–30 nm size and achieved 87.5% efficiency after 35 min [37]. Ali et al. found that TiO₂@rGo NPs of 25 nm could degrade 96% of RhB within 60 min [38]. Mufti et al. fabricated Fe3O4@TiO₂ NPs with a size of 38.5–72.8 nm, degrading 67.7% of RhB within 120 min [39]. Rasalingam et al. synthesized Ti-HTS-240 of 12 nm, which degraded more than 97% of RhB within 180 min [40].

7. Conclusions

It can be seen that the best synthesis method for TiO₂ NPs is sol–gel. Sol–gel, CVD, and hydrothermal are a few of the main approaches, but the sol–gel method has superior control over crystallinity, size of particles, and surface area. The reviewed literature shows that doping and heterojunction formation can enhance the degradation rate of RhB. Controlled synthesis and tuning by surface engineering of TiO₂ NPs will be useful for energy-efficient and sustainable photocatalytic systems for RhB degradation.

8. Strategies to Overcome Challenges

Adding QAD molecules to the TiO₂ surface causes a two-fold improvement in the photoactivity of the QAD/TiO₂ nano-heterostructure in comparison with undoped TiO₂ NPs, and QAD/TiO₂ achieved nearly complete photocatalytic degradation (~98%) of RhB dye within 3 h under only UV-A light irradiation [41]. Now, doping with non-metallic elements, such as S, C, and N, is receiving a lot of research interest. Researchers showed that the p atomic orbitals of these elements considerably overlap with the band of valence electrons of O 2p-orbitals, which helps the movement of light-induced charge particles to the surface of the catalytic material [42]. When In₂O₃ and TiO₂ are closely combined with each other under photoexcitation, the photoinduced electrons from In₂O₃ will efficiently move to the excited-state band of TiO₂, while the holes in TiO₂ will quickly migrate to the valence electronic band of In₂O₃. This action will obviously promote the detachment of the charge carriers and produce the generation of many light-sensitive ·OH free radicals throughout the decomposition procedure, which are the main reactive particles for enhancing the light-induced decomposition [43]. Normally, crystal facets with a large surface energy display a high light-driven activity due to their strong affinity, effective electron/hole partition, and rich reactive active sites. It is a practical approach to increase the photocatalytic activity at the atomic scale, forming a strongly reactive crystal facet. For example, it has been broadly confirmed that the (001) facets of anatase TiO₂ photoactive catalysts display improved photocatalytic performance compared to the other facets because of the high proportion of reactive, not saturated, pentacoordinated Ti atom (Ti5c) sites, which show a high tendency for reactants. Integrating photoactive semiconductors in combination with co-catalysts efficiently adjusts electronic band structure, improves surface electron transfer, and provides more reactive sites. This is another path to enhancing photoactive characteristic semiconductor compounds. For photocatalytic CO₂ reduction, H₂ evolution, and carbon-based pollutant decomposition, noble metals are commonly used as co-catalysts to pave the way. When combining noble metals with a semiconductor, a Schottky barrier is created at the surface between the noble metals and the semiconductor due to their high work function, enabling the interfacial transfer of charge. The photoexcited electrons move to the surface of noble metal NPs and participate in the reduction reaction because the semiconductor is irradiated by light. Meanwhile, the generated holes migrate to the surface, where they participate in photocatalytic oxidation reactions. Consequently, incorporating a noble metal co-catalyst into the semiconductor can promote charge separation and enhance photocatalytic performance [44].