Strategic pathway selection for photocatalytic degradation: roles of holes and radicals

Abstract

As global resources become scarce and environmental issues become increasingly severe, developing photocatalytic technology for efficiently and cleanly degrading pollutants has become a research trend. Radical degradation pathways are highly regarded owing to their wide application and efficiency in handling pollutants. Comparatively, direct oxidation by holes exhibits unique advantages in dealing with specific types of pollutants, and both degradation pathways have their own characteristics and strengths. However, past research on pollutant degradation has mainly focused on radical degradation, with little recognition of the role of direct hole oxidation in pollutant degradation, and there is a lack of attention toward the transition between the two pathways. This has made it difficult to select the most effective degradation strategy for different types of pollutants. To fill the knowledge gap in photocatalytic degradation pathways and overcome the predicament of blindly dealing with pollutants, the characteristics of these two oxidation pathways and their transition mechanisms are systematically explored. Additionally, this study provides the first summary of the types of pollutants that are suitable for degradation by holes and radicals. This paper offers a clear basis for selecting the most appropriate photocatalytic strategy according to the characteristics of different pollutants and reaction conditions, aiming to enhance researchers’ understanding of pollutant degradation and promote the development of environmental management technology towards higher efficiency and precision.

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1. Introduction

As industrialization expands worldwide with rapid population growth, environmental pollution has emerged as a pressing global challenge. Organic pollutants in water bodies and the atmosphere, such as dyes in industrial wastewater, pesticide residues from agricultural activities, pharmaceutical waste, and various chemicals from daily life, have significantly impacted ecosystems, posing threats to human health and biodiversity. Thus, innovating materials and techniques for pollutant detection and management is a trending research area.^1–4 Photocatalysis technology has gained significant attention for its potential in efficiently degrading organic pollutants using sunlight or artificial light sources at ambient temperature and atmospheric pressure. Employing this strategy is a potent manifestation of eco-friendly chemistry, holding vast prospects for utility in environmental remediation and harnessing energy, making it a promising solution for addressing organic compound contamination in water bodies.^5–8 Recent years have witnessed the development of a wide array of photocatalysts, each varying in composition, structure, and properties. Every photocatalytic reaction starts with a shared initial step: the excitation of a semiconductor photocatalyst generates free electrons in the conduction band (ecb⁻) and oxidized holes in the valence band (hvb⁺).^9,10 These active species can effectively degrade organic pollutants and transform them into harmless small molecules, thereby achieving the mineralization of pollutants.

In the photocatalytic process, pollutants are primarily degraded through oxidation–reduction reactions involving holes, superoxide radicals (˙O₂⁻), and sulfate radicals (SO₄⁻) present on the catalyst's surface.^11,12 Owing to the predominant presence of hydroxyl and superoxide radicals, photocatalytic pollutant degradation commonly relies on radicals generated as primary oxidizing agents.^13–15 Among these, hydroxyl radical (˙OH⁻)-mediated processes are particularly favored by researchers owing to their capability to non-selectively decompose diverse organic pollutants. Hydroxyl radicals possess extremely high oxidation potential, capable of breaking chemical bonds in organic pollutants, thereby achieving effective degradation.¹⁶ Nevertheless, by-products may be formed due to the non-discriminatory nature of this reaction. Additionally, the production of hydroxyl radicals is impacted by environmental conditions including light intensity, reaction medium pH, and the presence of oxidants, which can restrict their applicability.

On the other hand, holes, which act as strong oxidizing agents, can oxidize pollutant molecules through direct contact. The efficiency of this process is influenced by the interaction between the pollutants and the surface of the photocatalyst. When the pollutants have good contact with the outer layer of the photocatalyst, the holes can effectively oxidize them into harmless products. This oxidation pathway offers the possibility of selectively degrading specific types of pollutants, especially under certain conditions, making it possible to effectively degrade difficult-to-treat pollutants. However, this potential advantage has rarely been reported. Therefore, despite the significant theoretical advantages of the direct oxidation method via holes, its application in actual environmental remediation still requires further research.

However, determining the most suitable photocatalytic degradation pathway for different types of pollutants is a key research topic. Pollutants exhibit significant differences in photocatalytic reactions due to their unique chemical structures and states. These reaction characteristics are influenced not only by the properties of the photocatalyst but also by the chemical reactivity of the pollutants. For example, PHCAs and benzoic acid are more likely to be directly oxidized by holes, while hydroxyl radicals have lower degradation efficiency for PHCAs. However, when treating chloroacetic acid and benzoic acid, the radical pathway is more efficient than hole degradation.¹⁷ This highlights the specificity and efficiency of different photocatalytic degradation pathways in treating specific pollutants, emphasizing the importance of optimizing the selection of photocatalytic degradation methods for targeted pollutants. Despite some foundational research, a deep understanding of the dependency of photocatalytic degradation reactions on specific pollutants remains insufficient. Future research should focus on how to select or customize photocatalytic degradation pathways based on the reaction characteristics of pollutants to enhance the degradation efficiency and selectivity. This approach not only advances scientific understanding but also has practical implications for environmental remediation.¹⁸

In light of this, the objective of this paper is to conduct a comprehensive analysis on the adaptability and efficiency of two oxidation pathways, namely, radical degradation and direct hole oxidation, towards different types of pollutants. The objective is to investigate the optimal photocatalytic approach for effectively decomposing pollutants in specific circumstances. A systematic review will be performed, summarizing current applications, advantages, and limitations of radical degradation and direct hole oxidation in photocatalytic pollutant degradation. Furthermore, the regulatory mechanisms governing the interconversion between these two pathways will be explored, highlighting the potential of a single photocatalyst to alternate between radical-mediated degradation and direct hole oxidation pathways. These discussions provide novel perspectives for research on photocatalytic pollutant degradation. Through this comprehensive analysis, the goal is to offer practical guidance to researchers and engineers in selecting appropriate pathways for photocatalytic degradation based on pollutant characteristics. This approach aims to reduce trial-and-error processes in material development and application while advancing environmental remediation technologies towards greater efficiency and precision.

2. Mechanisms of pollutant degradation: radical-mediated and direct hole oxidation

Both radical-mediated degradation and direct hole oxidation are essentially forms of photocatalysis. To clarify their similarities and differences, it is necessary to delve into their mechanisms. The process of degrading pollutants through photocatalysis is compartmentalized into four distinct stages: light absorption, generation of charge carriers and their separation, production of reactive oxygen species, and pollutant breakdown. This research aims to offer a thorough comprehension of the photocatalytic process, specifically examining the impact of radical and hole on the overall process of photocatalysis.

2.1. Light absorption

In both radical-mediated degradation processes and hole-direct degradation processes of photocatalysis, light absorption is the initial stage of the photocatalytic reaction, which directly determines the efficiency and effectiveness of the photocatalysis. When a photocatalytic material irradiated with light, electrons in the material can absorb the energy of the photons. If the photon's energy matches the energy difference required for the electrons to transition from a lower energy level to a higher one, the electrons will jump from the ground state to an excited state. In this process, the transition of electrons is not only a manifestation of energy absorption but also the starting point of the entire photocatalytic reaction chain. The formation of electron–hole pairs through light absorption significantly impacts the subsequent responses, thus determining the reactivity and overall performance of the photocatalytic material.^19,20

However, due to the inherent band structure of materials, many materials have wide band gaps, limiting them to absorb only ultraviolet light rather than visible light. Therefore, enhancing the efficiency and range of light absorption is crucial for photocatalysis, and the morphology of the photocatalyst plays a decisive role in this efficiency. Reducing the size can increase the specific surface area and quantum size effect, thereby improving the ability to contact and absorb high-energy light. Specific shapes such as nanorods or nanosheets enhance light capture by increasing the scattering and light path length. Surface roughness and crystal defects increase light scattering, while porous structures enhance the absorption efficiency by providing more active sites. Different crystal structures and orientations, as well as adjustments in chemical composition and doping, can broaden the wavelength range of light absorption, especially extending into the visible light region.^21,22

2.2. Interfacial charge transfer

After light absorption, the generated electron–hole pairs are very unstable and tend to quickly recombine due to their inherent charge attraction, preventing these charge carriers from actually participating in catalytic reactions. Charge recombination can occur through various mechanisms, with the primary pathway being the relaxation process where photoexcited electrons directly return from the conduction band to the valence band. Alternatively, following photoexcitation, electrons may be captured by electron traps located on or just below the semiconductor surface, thereby promoting subsequent recombination (as shown in Fig. 1b).²³

Fig. 1 (a) Photocatalytic mechanism. (b) After photoexcitation, radiative recombination and non-radiative recombination can occur, denoted by straight and wavy lines, respectively. In this illustration, as electrons ascend and holes descend, they each gain stability; e_tr⁻: electron trap, h_tr⁺: hole trap, e_r⁻: active electron surface site, h_r⁺: active hole surface site. (c) The photocatalytic process generates free radicals.

Michael R. Hoffmann et al. stated that charge carrier recombination is primarily affected by two factors.²⁴ The primary factor is the struggle between the recombination of charge carriers and their entrapment, while the secondary factor involves the competition between recombination of trapped carriers and charge transfer at the interface. These two factors correspond to two methods of suppression: first, increasing the bandgap to make charge carrier recombination more difficult, and second, accelerating interfacial charge transfer so that the charge carriers are consumed before they can recombine.

Usually, controlling the photocatalyst's morphology can optimize charge carrier separation and transport. 1D nanostructures, like nanowires and nanorods, provide direct and efficient transport channels for electrons due to their linear pathways, effectively reducing the rate of electron–hole pair recombination. Two-dimensional nanosheets and nanobelts offer increased surface area and edge effects, providing more sites for charge separation. Heterojunction structures utilize interfacial electric fields to promote charge separation, thus optimizing the dynamics of carriers. The regulation of surface defects and states, such as by loading noble metal nanoparticles, further enhances charge separation efficiency and reduces carrier recombination. Additionally, metallic nanoparticles exhibit a notable effect known as surface plasmon resonance of the local electric field, which aids in the rapid separation and transport of charges.

Besides morphology control, the choice of different degradation pathways also affects charge carrier separation. In the photocatalytic process, the creation of free radicals, such as ˙O₂⁻ and ˙OH, is typically achieved through the reaction of photogenerated electrons and holes with sacrificial agents like water or dissolved oxygen. This free radical-mediated pathway not only directly participates in pollutant degradation but also enhances the productivity of charge carrier separation. This occurs due to the transfer of electrons and holes to the sacrificial agents, reducing their direct recombination, thereby efficiently harnessing the charges generated by light and enhancing the efficiency of degradation. For instance, in the process of generating hydroxyl radicals, the holes in the valence band interact with water. Although this process consumes holes, it simultaneously reduces their recombination with conduction band electrons. In 2024, A. Rebekah et al. discovered that hydroxyl radicals produced through a Fenton-like reaction not only effectively degrade organic dyes but also promote the participation of more charge carriers in the reaction due to their strong oxidizing properties, thereby reducing recombination.²⁵

In 2024, Che and colleagues synthesized two different supramolecular photocatalysts, namely, PDI-SE and PDI-IS, by introducing serine and isopropylamine into benzodiimide. Experiments show that compared with PDI-SE, PDI-IS is less prone to produce free radicals, and its degradation pathway relies more heavily on the direct oxidation by holes. Concurrently, transient absorption spectroscopy, photocurrent density tests, and electrochemical impedance measurements demonstrated that PDI-IS has a disadvantage over PDI-SE in terms of charge carrier separation and recombination.²⁶ This suggests that the degradation pathway involving the direct participation of holes competes with the pathway where the holes transfer to the radicals on the limited surface of the catalyst. The radical-mediated pathway, which involves the migration of charge carriers during radical generation, can effectively reduce charge carrier recombination, also confirming that the radical pathway promotes charge carrier separation more effectively than the hole pathway.

2.3. Formation of reactive oxygen species

The generation of active oxygen species is a crucial stage in the process of photocatalysis as it plays a pivotal role in effectively breaking down pollutants. These reactive oxygen species primarily include ˙OH, ˙O₂⁻, and H₂O₂. Their crucial function lies in promoting the effective breakdown of pollutants during the photocatalytic process, which are formed through interactions with the excited holes and electrons in the photocatalyst. The generation of radicals relies heavily on dissolved oxygen,²⁷ In contrast, the oxidation action of holes remains unaffected by dissolved oxygen or other oxidizing agents, which is particularly advantageous in environments with limited oxygen supply or insufficient stability of radicals under the reaction conditions.

Hydroxyl radicals are recognized for their exceptional potency as oxidizing agents, capable of reacting non-selectively with most organic substrates.²⁸ The primary locations for hole capture on metal oxide surfaces are surface hydroxyl groups. The interaction between positive charges (holes) and the presence of surface hydroxyl groups or absorbed water molecules has the potential to generate hydroxyl radicals. Upon the absorption of light energy, photocatalysts like titanium dioxide (TiO₂) exhibit this phenomenon. The promotion of electrons from the valence band to the conduction band leads to the formation of positively charged vacancies, commonly referred to as holes. These holes exhibit a significant oxidation potential, allowing them to directly oxidize water molecules or surface hydroxyl groups, leading to the generation of hydroxyl radicals. Additionally, in some cases, if suitable oxidizing agents are present, conduction band electrons can indirectly influence the generation of hydroxyl radicals.^27,29,30 The process is as follows.

H₂O + h⁺ → ˙OH + H⁺ (1)

OH⁻ + h⁺ → ˙OH (2)

Indeed, in this process, holes act as strong oxidizing agents and can directly react with water molecules or surface-bound hydroxyl ions (OH⁻) to generate hydroxyl radicals (˙OH). This process does not depend on external oxygen but directly uses water as the substrate for the oxidation reaction.

O₂ + e⁻ → ˙O₂⁻ (3)

2˙O₂⁻ + 2H⁺ → H₂O₂ + O₂ (4)

H₂O₂ + ˙O₂⁻ → ˙OH + OH⁻ + O₂ (5)

Electrons within the conduction band have the ability to engage in interactions with dissolved oxygen, resulting in the generation of ˙O₂⁻ (superoxide anions), which is a typical reduction process and highly dependent on the existence of dissolved oxygen. Superoxide anions facilitate the creation of supplementary hydroxyl radicals via a range of mechanisms, such as reactions with H₂O₂, which itself can be produced by the catalytic reactions of ˙O₂⁻.³¹

Superoxide anion is another important ROS; the presence of electrons in the conduction band can lead to a reduction in the concentration of dissolved oxygen.³¹

O₂ + e⁻ → ˙O₂⁻ (6)

The superoxide anion itself also has a certain degree of oxidizing ability and can directly or indirectly contribute to the oxidation of organic substances. Additionally, the superoxide anion can further react to produce hydrogen peroxide and more hydroxyl radicals, thereby enhancing the overall oxidative efficiency through a series of chain reactions.

The formation of hydrogen peroxide is typically the result of indirect reactions involving superoxide anions. This process involves the dismutation of two superoxide anions.

2˙O₂⁻ + 2H⁺ → H₂O₂ + O₂ (7)

While hydrogen peroxide is less reactive than hydroxyl radicals, in the presence of appropriate catalysts, it can break down to generate extra hydroxyl radicals, thus enhancing the oxidation process.

In summary, throughout the photocatalysis process, there is a consistent relationship between the concentration of dissolved oxygen and the occurrence of reactive oxygen species such as ˙OH, ˙O₂⁻, and H₂O₂. Tsutomu Hirakawa and other researchers utilized a dissolved oxygen (DO) sensor to monitor during the photocatalytic degradation of ethanol (EtOH) in a TiO₂ suspension under UV light and noted that with increasing concentrations of ethanol, the rate of dissolved oxygen depletion also significantly increased. At an EtOH concentration of 5 mM, the complete depletion of dissolved oxygen occurred within 20 minutes. At low EtOH concentrations, adding H₂O₂ can accelerate the photocatalytic oxidation of EtOH, while at high EtOH concentrations, the effect of H₂O₂ is negligible. This may be because at low EtOH concentrations, H₂O₂ promotes the generation of hydroxyl radicals, thereby accelerating EtOH oxidation. However, at high EtOH concentrations, EtOH adsorption on the TiO₂ surface replaces H₂O₂. The experiment also found that H₂O₂ undergoes both oxidation and reduction reactions during photocatalysis but its reduction rate is lower than its oxidation rate, indicating a competitive O₂ reduction process. This suggests that H₂O₂ is a crucial intermediate in photocatalysis, capable of acting as an electron acceptor to separate charges but it is also an intermediate product in the generation of radicals from dissolved oxygen, with O₂ reduction still playing a dominant role in promoting radical generation and charge separation. The study highlights that radical generation depends on dissolved oxygen consumption and, by monitoring dissolved oxygen and H₂O₂, quantitatively and kinetically analyzes the radical-mediated photocatalytic degradation of pollutants, emphasizing the central role of dissolved oxygen in this process.³¹ In 2023, Yi Hu and colleagues observed that under hypoxic conditions, no measurable signals for ˙OH or H₂O₂ were detected (Fig. 2a–c), affirming that their generation is initiated by the decrease in dissolved oxygen.³²

Fig. 2 By measuring (a) the formation of 2-HTA to assess the photocatalytic generation of ˙OH radicals and (b) 7-HC under air-equilibrated and hypoxic conditions. (c) DPD was employed as a probe molecule to estimate the photocatalytic production of H₂O₂. (d) The relationship between the logarithm of the observed rate constants (log k_obs) for the oxidation of 12 phenols by holes and their respective E_½ values in MCN suspensions and (e) PPACN photocatalysts.³² Reprinted with permission from ref. 32. Copyright 2023, American Chemical Society.

In contrast, the pathway for direct degradation by holes does not depend on dissolved oxygen. Yi Hu and colleagues studied the pathway of direct hole transfer in anoxic aqueous suspensions, and the reduction pathway of molecular oxygen (O₂) hindered the production of reactive oxygen species (ROS). Their findings demonstrated a logarithmic relationship between the observed reaction rate constants and the Hammett constants (σ) as well as the midpoint oxidation potentials (E_½) of aromatic substances, as depicted in (Fig. 2d and e). This discovery supports the existence of a direct hole oxidation pathway.³² In this mechanism, photoexcited holes directly interact with organic pollutant molecules attached to the surface of the catalyst, resulting in the direct oxidative degradation of the pollutants. As highly effective oxidizers, these holes can disrupt the chemical structure of the pollutants directly, thereby facilitating their degradation. This direct oxidation method is particularly important in low-oxygen or anoxic environments, which are often encountered in natural water bodies and industrial wastewater treatment. In such environments, the generation of radicals that depend on dissolved oxygen may be very limited or nonexistent.

2.4. Degradation of pollutants

2.4.1. Radicals. In the photocatalytic process, radicals play crucial roles as mediators in pollutant degradation and significantly impact the overall efficiency and broad applicability of the photocatalytic system. Free radicals break down pollutant molecules through various reaction mechanisms during the photocatalytic process, leading to their degradation and eventual mineralization. These reaction mechanisms can be categorized and are discussed in detail according to the type of degradation as follows.

Free radicals can undergo addition reactions with double bonds or aromatic rings in pollutant molecules, forming hydroxylated or peroxide intermediates. This type of reaction is common in organic pollutants with unsaturated bonds, such as phenols, dyes, and polycyclic aromatic hydrocarbons (PAHs).

R = R + ˙OH → R(OH)–R (8)

For example, phenol reacts with ˙OH to form hydroxyphenol.

C₆H₅OH + ˙OH → 2C₆H₄(OH)₂ (9)

Free radicals can substitute hydrogen atoms or other functional groups in pollutant molecules, forming new radical intermediates. This type of reaction is common in organic pollutants with alkyl chains, such as organic solvents and pesticides.

R–H + ˙OH → R˙ + H₂O (10)

For example, toluene reacts with ˙OH to form the benzyl radical.

C₆H₅CH₃ + ˙OH → C₆H₅CH₂˙ + H₂O (11)

Free radicals can engage in electron transfer reactions with pollutant molecules, forming oxidized products. This type of reaction is common in organic pollutants with easily oxidizable functional groups, such as alcohols and aldehydes.

RH + ˙OH → R˙ + H₂O (12)

RH + ˙O₂⁻ → R˙ + HO₂⁻ (13)

For example, ethanol reacts with ˙OH to form the ethoxy radical.

Free radicals possess diffusivity in degrading pollutants, thereby holding unique advantages in pollutant degradation. This diffusivity is crucial for expanding the reaction zone beyond the surface into the solution, allowing for the mineralization of non-adsorbed substrates. Because of their small molecular size and high reactivity, radicals can quickly react with surrounding pollutant molecules the moment they are formed.^33,34 Free hydroxyl radicals (˙OH) can diffuse through air or water and react with remote substrates that are not directly in proximity to the TiO₂ surface, such as through an organic polymer membrane. This diffusivity allows radicals to operate independently of the particular circumstances present on the catalyst surface, such as the availability of surface sites or particular surface chemical environments. They can be active over a broader reaction area, thus enhancing the overall processing capacity of the system. In 2024, Xiaohui Li et al. discussed the importance of radical diffusivity, particularly hydroxyl radicals (˙OH), in improving the photocatalytic efficiency.³⁵ The article points out that small hydroxyl radicals (˙OH) possess strong oxidative capabilities and can diffuse into the medium, subsequently oxidizing lignocellulose efficiently. The key to this strategy lies in the effective management of the photocatalytic reaction kinetics, which are significantly enhanced by the process of isolating and relocating electrons and holes that are generated through the action of light, thus crucially involving the generation and diffusion of ˙OH radicals.

In order to verify the hydroxyl free radicals in the process of degradation of invasive pollutants by TiO₂, Ji Young Hwang et al. studied the crystal structure of TiO₂ samples in photocatalytic oxidation (PCO) of TMA at pH 3 and 11.²⁷ The results show that at pH 3, the TiO₂ sample did not show any degradation activity toward TMA, whereas at pH 11, it did (Fig. 3a). According to the measurements (Fig. 3b), TMA adsorption on the TiO₂ sample was almost negligible at pH 3, while adsorption was significantly enhanced at pH ≥ 11. However, even in the absence of the surface adsorption of TMA, at pH 3, anatase TiO₂ can degrade TMA, which shows that TMA degradation can exceed the TiO₂ surface area, which embodies that ˙OH radicals have a diffusion effect in the degradation of the pollutants.

Fig. 3 (a) Under UV light irradiation, photocatalytic decomposition rates of trimethylamine (TMA) at pH 3 and 11 were studied, employing varying anatase/rutile ratios (with a total amount fixed at 1.5 g L⁻¹). (b) Anatase and rutile TiO₂ were examined using ATR-FTIR to study the absorption band at 1490 cm⁻¹ for trimethylamine (TMA) across varying pH levels. Experimental parameters included [TiO₂] = 1.5 g L⁻¹, initial [TMA] = 100 μM, and λ > 320 nm for UV light irradiation.²⁷ Reprinted with permission from ref. 27. Copyright 2021, Elsevier. (c) ADN degradation during UV/PDS treatment with and without SRNOM. (d) Measured (k_obs) and calculated degradation rate constants for ADN, indicating contributions from SO₄˙⁻ (k_SO4^˙–, red columns) and ˙OH (k_˙OH, pale blue columns). Conditions: I₀ = 0.16 mW cm⁻², pH = 7, [ADN] = 1 μM, [K₂S₂O₈] = 290 μM.⁴¹ Reprinted with permission from ref. Copyright 2023, American Chemical Society.

Further evidence suggests that the stronger diffusivity of radicals leads to a more favorable degradation process. In 2022, Ke Zhang et al. discovered that enhancing radical diffusion can significantly increase the rate of degradation. In 2016, Yoshio Nosaka et al. found that rutile exhibits a higher adsorption capacity for ˙OH radicals, resulting in decomposition only on the surface molecules. Conversely, hydroxyl radicals generated by anatase have greater diffusivity, making anatase TiO₂ more active due to the production of more diffusible hydroxyl radicals.³³

The diffusivity and high reactivity of radicals result in their non-selectivity during oxidation. This means that radicals react not only vigorously with organic pollutants in water but also readily with other organic or inorganic components. This can lead to resource wastage and quenching of radicals, thereby reducing the degradation efficiency of pollutants.^36–38

For example, during advanced oxidation processes aimed at degrading trace organic contaminants (TrOCs), dissolved organic matter (DOM) can inhibit their degradation. DOM competes with photosensitizers for photon absorption, induces internal filtering effects, and scavenges reactive radicals, thereby reducing their concentration.^39,40 In 2022, Shuangshuang Cheng and colleagues studied the application of UV/PDS technology on ADN. Without the addition of SRNOM, the degradation rate of ADN was 76% within 10 minutes. However, the presence of SRNOM led to a decrease in the degradation rate as its concentration increased (Fig. 3c). Therefore, the SRNOM had a supplementary function in preventing the breakdown of AND (Fig. 3d).⁴¹ During the photocatalytic process, radicals might be reduced due to excess oxidants, inorganic anions, and pH changes. These factors include consumption of radicals by oxidants, the reaction of inorganic anions with radicals to form less reactive substances, and under alkaline conditions, the potential transformation of hydroxyl radicals into less active radicals. These situations reduce the efficiency of photocatalysis and decrease the degradation of pollutants.⁴²

2.4.2. Holes. Direct hole oxidation exhibits high selectivity and can oxidize specific chemical bonds or functional groups. In certain applications, this method is more efficient than the radical pathway. The adsorption capacity of photocatalysts for different pollutants varies, leading to increased contact opportunities with holes for certain pollutants. Additionally, the selectivity of holes is related to their energy levels; effective oxidation occurs only if the hole's energy level is higher than the pollutant's redox potential. Therefore, the oxidation efficiency of different pollutants varies based on the adsorption characteristics and hole energy levels. Fig. 2d and e shows a logarithmic correlation between the observed rate constants and Hammett constants as well as half-wave oxidation potentials. This indicates that the structure of different aromatic compounds affects their rate of hole oxidation and electronic properties (reflected by Hammett constants and half-wave oxidation potentials) significantly influence their reaction rates. Specifically, Hammett constants describe the electronic effects of different substituents in aromatic compounds. The electron-attracting or donating ability of substituents affects the reactivity of the entire molecule, with compounds containing strong electron-attracting groups being more susceptible to hole oxidation because they are more prone to losing electrons. This structure–activity relationship supports that direct hole oxidation has better selectivity compared to free radical degradation pathways, and holes tend to degrade the pollutants adsorbed on the catalyst surfaces such as surface-bound dyes and some persistent organic pollutants.³² Therefore, holes exhibit stronger oxidative capabilities and higher selectivity towards pollutants with strong interactions on photocatalyst surfaces.²⁴ Rational material design can enhance selective actions in direct hole oxidation processes in photocatalysis, promoting pollutant removal in complex aqueous matrices and benefiting other types of industrial production.⁴³ The presence of holes allows for the selective and efficient oxidation of specific organic and halide anions, addressing the issue of selective oxidation of free radicals.⁴⁴ For instance, in a free radical system, chloride ions in wastewater are easily converted into chlorine radicals, resulting in both a decrease in the breakdown speed of contaminants and the generation of harmful by-products through chlorination. However, holes can prevent this problem by avoiding such reactions. Similarly, sulfate oxidation has the potential to generate bromate free radicals by oxidizing the bromide present in water; nevertheless, utilizing holes is more effective in dealing with strong pollutant oxidation without generating toxic by-products.^45–47

3. Bidirectional mechanism switching

In the photocatalytic process, effectively controlling and regulating the activities of radical pathways and hole degradation pathways is crucial for the selective treatment of various environmental pollutants. In many cases, direct oxidation through holes and radical-mediated oxidation are two distinct processes that do not overlap.^48,49 The precise regulation of the photocatalyst to flexibly switch between these two pathways, adapting to the degradation needs of specific pollutants, is essential for achieving higher treatment efficiency and selectivity. Consequently, it is necessary to explore how to modulate these two degradation pathways by modifying the photocatalysts to optimize pollutant handling performance during photocatalysis.

3.1. Switching from the radical pathway to hole pathway

3.1.1. Constructing a heterojunction. In the modification of photocatalytic materials, it is generally believed that the downward shift of the valence band can enhance the oxidizing power of holes. However, for the free radical-mediated degradation pathway, the upward shift of the valence band may be detrimental to photocatalytic oxidation.⁵⁰ Nevertheless, this view is not always applicable. In 2011, research by Chunying Wang and colleagues showed that Bi_3.84W_0.16O_6.24, although having weaker valence band oxidation capacity and being unable to form hydroxyl radicals compared to P25 TiO₂, had a higher efficiency in direct hole degradation.⁵¹ In 2023, Yi Hu and others found that despite the lower oxidation potential of the valence band of carbon–nitrogen compounds, which is insufficient to directly oxidize water to produce hydroxyl radicals, they could effectively degrade a variety of pollutants through hole adsorption.³² These findings indicate that in some cases, adjusting the valence band upwards will not inhibit photocatalytic oxidation but may instead promote degradation. This is because the standard reduction potential for the oxidation of water to produce hydroxyl radicals is about +2.7 V under standard conditions (25 °C, 1 atm, pH = 0), and the upward shift of the valence band hinders the transfer pathway of holes to water, thus favoring their transfer to pollutants. Therefore, constructing a heterojunction to achieve an upward shift of the valence band is an important strategy to promote the transition from the free radical pathway to the hole pathway.

In 2022, Fengjie Chen and his team created a p–n heterojunction interface by immobilizing BiOBr on P25, designing a novel spherical composite material, and demonstrating its effectiveness in the photodegradation of PFOA. Under irradiation, electron vacancies from P25's valence band are transferred to that of BiOBr. Various techniques confirmed the retention of holes in the valence band of BiOBr. P25/BiOBr retained a large number of holes, which directly reacted with PFOA, significantly surpassing the removal rate of In₂O₃ (Fig. 4a). The unique orbital structure of BiOBr nanoparticles caused the valence band to move upward, reducing the band gap (Fig. 4b) and improving the charge separation efficiency. This resulted in more holes being retained on the surface of BiOBr, facilitating the direct oxidation of PFOA, thereby optimizing the photocatalytic process.⁵²

Fig. 4 (a) Photodegradation pathway of P25/BiOBr. (b) Photodegradation of 100 mg L⁻¹ PFOA under simulated sunlight using different photocatalytic systems at an initial pH of 3.5, with a photocatalyst concentration of 0.5 g L⁻¹.⁵² Reprinted with permission from ref. 52. Copyright 2022, Elsevier.

Thus, in the research on photocatalytic materials, adjusting the upward shift of the valence band position can effectively promote the direct involvement of holes in oxidation reactions, thereby reducing the dependence on radical generation. This strategy enhances the oxidative capacity of the valence band, allowing photo-generated holes to react directly and more readily with organic pollutants, thereby optimizing the pathways and efficiency of the photocatalytic reaction. Additionally, this adjustment helps improve the charge separation efficiency and minimize carrier recombination, thereby enhancing the overall photocatalyst performance.

3.1.2. Enhanced adsorption. Holes have the capability to react with water, producing hydroxyl radicals (˙OH), which are utilized in the degradation of pollutants, while they can also directly react with pollutants, leading to their oxidative degradation. These two pathways are competitive as the utilization direction of holes influences the choice of the reaction pathway. By modifying the surface to increase the likelihood of contact between the catalyst and pollutants, the interaction between pollutants and holes can be enhanced. This directly boosts the hole-induced degradation process, thereby shifting the degradation pathway from radical-based to hole-based.

In 2021, Yang Jin and colleagues synthesized BiOBr-cg composites by coating graphitized carbon on the surface of BiOBr. This composite material was effective against bisphenol A (BPA), ciprofloxacin, ibuprofen, and diphenhydramine. The π–π interactions on the surface of the graphitized carbon enhanced the adsorption capacity for organic pollutants such as BPA. DFT calculations indicated that BPA exhibited stronger adsorption energy on the carbon component of the graphitized BiOBr-cg compared to BiOBr (Fig. 5a and b). Consequently, BPA was primarily adsorbed in the HOMO distribution region, facilitating its oxidative degradation via surface valence band holes. Additionally, strong orbital interactions were formed between the graphitized carbon and BiOBr. This interaction created a new surface valence band (Fig. 5c–f), resulting in an upward shift of the HOMO level, which significantly promoted charge segregation (Fig. 5g). Therefore, this alteration enhanced both the light absorption capacity and the photogenerated charge carrier separation efficiency.⁵³

Fig. 5 (a)Molecular structure of the BiOBr portion and (b) graphitized C portion in BiOBr-Cg (the model displays spheres colored purple (Bi), red (O), brown (Br), gray (C), and white (H), respectively). DFT calculations of BiOBr showing (c) the molecular orbital with the lowest energy that is unoccupied (LUMO) and (d) the molecular orbital with the highest energy that is occupied (HOMO). (e) The LUMO of the BiOBr portion, (f) the HOMO of the graphitized C portion in BiOBr-Cg, and (g) the transformation of the HOMO and LUMO orbitals in BiOBr, BiOBr-Cg, and graphitized carbon (Bi, O, H, C, and Br atoms are denoted by purple, white, brown, gray, and red spheres, respectively).⁵³ Reprinted with permission from ref. 53. Copyright 2022, Elsevier.

Based on the aforementioned mechanisms, it can be inferred that enhancing the adsorption capacity of photocatalysts towards pollutants can effectively promote the shift from radical-mediated degradation to direct oxidation pathways via holes. This is because increased contact between pollutants and the photocatalyst surface makes it more likely for the catalyst surface to facilitate the direct reaction between photogenerated holes and pollutants, eliminating the need for sacrificial agents in the solution. Although this enhanced adsorption may not favor radical generation, it improves the direct utilization rate of holes; by doing so, the selectivity and efficiency of the oxidation reaction can be enhanced, which is especially beneficial for effectively breaking down complex or stubborn pollutants that necessitate specific chemical reaction pathways. Such a strategy enhances the specificity of photocatalytic treatment, leading to the refinement and increase in the use of photocatalytic technology for environmental remediation.

3.2. Transition from hole pathway to radical pathway

3.2.1. Constructing a heterojunction. Constructing heterojunctions is an important strategy to promote the transition from a radical pathway to a hole pathway. In the field of photocatalysis, by constructing heterojunctions, the pathways for the separation and movement of charges produced by light can be controlled and directed, thereby facilitating the transition of photocatalytic degradation reactions from hole-dependent direct oxidation to radical oxidation. This strategy is based on a deep understanding of the electrochemical properties of hydroxyl radical (˙OH) generation from water oxidation. Under standard conditions (25 °C, 1 atm, pH = 0), the standard redox potential for generating ˙OH is approximately +2.7 V. By designing heterojunctions with suitable band structures, the valence band position can be lowered, thereby promoting the process of water molecule oxidation to generate ˙OH radicals. Additionally, the introduction of oxygen vacancies or other defects at the heterojunction interface can enhance the electron capture capability at the interface, further promoting the generation of ˙OH radicals. The vacancies for oxygen also capture electrons and attract water molecules, thereby speeding up their oxidation process. Therefore, through carefully designed heterojunctions, the effective separation of charges generated by light can also be accomplished, but the pathway of photocatalytic degradation can also be regulated, thus enhancing the application potential of photocatalysts in environmental purification.

In 2021, Jiangli Sun and colleagues developed an innovative VL-driven Z-scheme Zn₃In₂S₆/AgBr composite photocatalyst for the photocatalytic degradation of MNZ. Through the spin capture experiment and free radical capture experiment, it was discovered that the catalyst could produce hydroxyl radicals, which predominantly contribute to the breakdown of MNZ through photocatalysis (Fig. 6a and b). However, before the construction of the heterojunction, the valence band potential of Zn₃In₂S₆ is not sufficient to generate hydroxyl radicals and can only be degraded by holes. After the construction of the Zn₃In₂S₆/AgBr heterojunction, the valence band potential decreases, which leads to the conversion of the holes to hydroxyl radicals. Therefore, the construction of heterojunctions is an important means to control the transition of the hole degradation pathway to the free radical degradation pathway.⁵⁴

Fig. 6 (a) The diagram for MNZ oxidation and Cr(VI) reduction mechanism over the Zn₃In₂S₆/AgBr photocatalyst under visible light. (b) The ESR spectra of ˙OH. (c) Effects on MNZ degradation over Zn₃In₂S₆/AgBr with different scavengers.⁵⁴ Reprinted with permission from ref. 54. Copyright 2021, Elsevier.

In recent years, the strategy of constructing heterojunctions to lower the valence band potential, thereby promoting the transition from hole degradation pathways to radical degradation pathways, has been widely applied to achieve efficient pollutant degradation. In this field, Shijie Li and his team have made continuous breakthroughs, and in 2023, they developed photocatalysts with S-type heterostructures, such as Au/MIL-101(Fe)/BiOBr, Mn_0.5Cd_0.5S/BiOBr, and Cd_0.5Zn_0.5S/Bi₂MoO₆. These photocatalysts’ unique S-type heterostructures can generate a large number of active radicals like ˙O₂⁻ and ˙OH, showing excellent performance in antibiotic degradation. The application of this method further emphasizes the importance of heterojunctions in regulating the photocatalytic process, providing valuable strategies for designing novel and efficient photocatalysts.^55–57

3.2.2. Constructing radical generation sites. Promoting the transition from hole pathways to radical pathways can be achieved by modifying the semiconductor surface to create radical generation sites. The simplest approach may involve the adsorption of anions from inorganic sources, including fluorides, phosphates, and sulfates, on the surface.^58,59

In 2004, Hyunwoong Park et al. discovered that the impact caused by surface fluorination on the photocatalytic activity varied significantly depending on the type of substrate being degraded. F–TiO₂ exhibited superior photocatalytic oxidation performance for Acid Orange 7 and phenol compared to pure TiO₂, but it showed poorer photocatalytic degradation efficiency for dichloroacetic acid. The research findings revealed that on F–TiO₂, the ˙OH radical-mediated oxidation pathways were enhanced, whereas hole transfer-mediated oxidation pathways were mainly inhibited. This inhibition was considered to overcome the substance adsorption or complexation on F–TiO₂ surfaces, resulting in a shift of the photocatalytic degradation mechanism from direct hole oxidation to oxidation mediated by hydroxyl radicals.⁵⁸

In 2020, Liyong Ding et al. discovered that photocatalysts with Brønsted acid/base sites form strong hydrogen bonds with water, enhancing H₂O adherence and interaction with surface vacancies, thus increasing ˙OH production. Without these sites, ˙OH generation is hindered. Spin-trapping EPR spectroscopy showed that BiOCl, despite its theoretical oxidative ability, is less effective than TiO₂ in generating ˙OH radicals (Fig. 7a). Solid-state ¹H MAS NMR explored the surface hydroxyl groups along with water molecules that were adsorbed onto BiOCl and TiO₂ (Fig. 8b and c). The surface hydroxyl groups on TiO₂ exhibited robust interactions and swift proton exchange with water molecules that were adsorbed onto the surface. BiOCl had weaker water adsorption due to the absence of surface hydroxyl groups essential for hydrogen bonding. This is due to Lewis acidic bismuth metal layers on BiOCl, which lack Brønsted acid/base sites, making water adsorption molecular and hindering ˙OH generation.¹⁷

Fig. 7 (a) The EPR spectra from spin-trapping experiments reveal the ˙OH radicals generated by various photocatalysts in water or PFOA solutions under dark conditions (b and c) ¹H MAS NMR spectra of dehydrated TiO₂ and BiOCl at temperatures of 393 K, 473 K, and 673 K; (d) spin-trapping EPR spectra of ˙OH radicals generated by different photocatalysts in PFOA or phenol solutions (under dark conditions). Photocatalytic degradation behavior of phenol (e) and PFOA (f) under simulated sunlight exposure using different photocatalysts, specifically BiOCl and TiO₂, each at a concentration of 1 g L⁻¹, with a phenol concentration of 20 mg L⁻¹.¹⁷ Reprinted with permission from ref. 17. Copyright 2020, Elsevier.

Fig. 8 (a) Transmission electron microscopy (TEM) was used to capture the images of BiOCl nanosheets. (b) Photocatalytic defluorination of PFOA was conducted on BiOCl in the presence of scavengers, namely, KI, ascorbic acid (AA), or tert-butanol (TBA), each at a concentration of 0.4 mmol L⁻¹, to eliminate holes, oxygen, and hydroxyl radicals (˙OH), respectively.⁶⁶ Reprinted with permission from ref. 66. Copyright 2019, Elsevier. (c) Transmission electron microscopy (TEM) images showing Fe/TNTs@AC. (d) The impact of various scavengers on the defluorination of pre-adsorbed PFOA on Fe/TNTs@AC.⁶⁷ Reprinted with permission from ref. 67. Copyright 2020, Elsevier. (e) TEM images of titanium dioxide nanotubes (TNTs). (f) Comparative evaluation of X3B degradation rates over 3 hours under various conditions employing titanium dioxide nanotubes (TNTs) as photocatalysts (X3B: 0.2 mM, tert-butanol (BuOH): 1% (v/v), EDTA: 0.1 M).⁷² Reprinted with permission from ref. 72. Copyright 2012, Elsevier.

To address this issue, researchers grafted phosphate ions onto the surface of BiOCl, artificially creating new Brønsted acid/base sites. Electron paramagnetic resonance (EPR) results (Fig. 7d) showed that the addition of phosphate increased the generation of hydroxyl radicals during the photocatalytic degradation of PFOA and phenol. Notably, after introducing Brønsted acid/base sites, the capability of BiOCl to degrade phenol significantly improved, whereas the degradation of perfluorooctanoic acid (PFOA) through photocatalysis faced obstacles (Fig. 8e and f). This phenomenon is the result of competition between the radical generation pathway of water oxidation and the direct pore oxidation pathway.

The aforementioned research indicates that altering the surface properties of semiconductor photocatalysts to construct radical generation sites is a crucial method for regulating the transition from hole pathways to radical pathways. By introducing or modifying adsorbed inorganic anions and surface active sites such as Brønsted acid/base sites on the semiconductor surface, the adsorption and activation of water molecules can be significantly influenced, thereby regulating the generation rate of hydroxyl radicals. This surface modification not only impacts the activity of the photocatalyst but also guides the photocatalytic reaction pathway from a direct hole-mediated to a radical-mediated reaction pathway. This strategy provides an important approach for optimizing the photocatalyst design, enhancing their application efficiency in the field of ecological rehabilitation and energy conversion.

4. The suitable pathways for radicals and holes

Based on the summary of the mechanisms, it has been concluded that the radical degradation pathway is versatile, widely diffusive, and exceptionally effective in treating a broad spectrum of pollutants. In contrast, the hole degradation pathway is highly selective, possesses strong oxidizing capabilities, and operates independently of dissolved oxygen, making it particularly effective for degrading recalcitrant pollutants or those with specific chemical properties. Consequently, these degradation pathways have been matched with the types of pollutants they are most suitable for and practical examples have been provided to substantiate these findings.

4.1. Holes

4.1.1. Pollutants with high chemical stability. Pollutants with high chemical stability are challenging to since the effectiveness of photocatalysis lies in its remarkable efficiency bond dissociation energy and strong resistance to radical oxidation. The high bond energy makes these molecules very stable, requiring substantial energy to break.⁶⁰ Additionally, the strong antioxidative properties of these pollutants allow them to effectively withstand the attack of reactive oxygen species generated by photocatalysts, such as ˙OH and ˙O₂⁻.⁶¹ In contrast, holes have stronger oxidizing properties, enabling the degradation of stable pollutants that are otherwise difficult to degrade under conventional conditions.^62–64

Perfluorinated compounds, because of their energetically strong C–F bonds (631.5 kJ mol⁻¹) and the high electronegativity exhibited by halogen atoms, resist most traditional reduction and oxidation processes.⁶⁵ In 2017, Zhou Song et al. showed that the effective defluorination of PFOA was achieved using BiOCl nanosheets, with the process being enhanced in the presence of positively charged cations.⁶⁶ Their research demonstrated that the rate of defluorination was 1.7 and 14.6 times higher compared to that of commercially available In₂O₃ and TiO₂, respectively, which primarily degrade through radical-mediated pathways. Subsequently, AA, TBA, and KI acted to quench superoxide radicals, while the existence of hydroxyl radicals and the positive charge carriers produced by light excitation were noted. Experimental findings (Fig. 8b) show that the degradation efficiency of PFOA was reduced by 10.9% with the addition of ascorbic acid and by 17.8% with tert-butanol. In contrast, the breakdown of PFOA was notably hindered with the incorporation of potassium iodide. This discovery indicates that photogenerated positive charges (holes) are the main active components accountable for the deterioration of PFOA.

In 2020, researchers including Fan Li crafted a novel composite material, Fe/TNTs@AC, that integrates TiO₂ with activated carbon, specifically for eliminating PFOA.⁶⁷ Photogenerated positive charges (h+) were captured using a combination of potassium iodide (KI), isopropanol (IP), and p-benzoquinone (BQ), hydroxyl radicals (˙OH), and superoxide radicals (˙O₂⁻), respectively. The findings (Fig. 8d) demonstrated that the addition of KI decreased the defluorination rate of PFOA from 62% to 28%. In contrast, other scavengers had minimal impact on the defluorination rate, indicating that photogenerated positive charges are primarily responsible for PFOA degradation, with ˙OH and ˙O₂⁻ playing secondary roles. Calculations based on the density functional theory (DFT) and the analysis of degradation intermediates revealed that the initial breakdown of PFOA involves the direct oxidation of the activated molecule by photogenerated positive charges, leading to decarboxylation, cleavage of C–F bonds, and chain-shortening reactions. These mechanisms underscore the critical role of photogenerated positive charges in the degradation of PFOA.

In 2015, Zhang et al. investigated the degradation of persistent environmental pollutants in watery environments, such as the antibiotic ciprofloxacin, exposed to visible light; BiOBr suspensions facilitate the oxidation process directly through hole activity. Ciprofloxacin, which contains epoxy ring structures and fluorinated groups, is typically challenging to degrade. The research revealed that direct hole oxidation is a potent strategy for breaking down highly persistent contaminants.⁶⁸

Additionally, azo dyes, which commonly contain azo linkages (–N=N–) and aromatic rings, possess high bond energies, making them difficult to oxidize through radical processes. Instead, they are more amenable to degradation via hole oxidation.^69–71 In 2013, Sun J et al. documented the first instance of degradation of the anionic azo dye Reactive Red X-3B (X3B) through direct hole oxidation in a tnt/UV/O₂ system.⁷²Fig. 8f displays the degradation rate of X3B under various conditions employing TNT as a photocatalyst. The results indicate that under UV irradiation with O₂ as the oxidant, the degradation rate of X3B reached 80.38% (Fig. 8f(a)).

Contrarily, under identical experimental circumstances, with EDTA added as a hole scavenger, only a minimal degradation (as depicted in Fig. 8f(a and b)) of X3B was observed. The addition of the hydroxyl radical (˙OH) scavenger BuOH had little effect on the decomposition of X3B, with a degradation rate of 79.60% (Fig. 8f(c)). These results affirm that the degradation of X3B through photocatalysis, with O₂ serving as the electron acceptor, primarily occurs via direct hole-mediated oxidation. The redox potential of holes in this system is approximately 3.4 V, which is higher than that of hydroxyl radicals (2.8 V), ozone (2.0 V), and O₂ (1.2 V). Consequently, the strong oxidizing capacity of holes is responsible for the rapid mineralization of X3B dye.

The degradation rates of halogenated carboxylic acids are summarized in Table 1, considering both hole and radical pathways. The comparison of different reactive oxygen species for the degradation of PFOA revealed that systems dominated by direct hole oxidation exhibited superior degradation rates, defluorination rates, and mineralization rates compared to most radical-based degradation systems. Although the UV/Cu–TiO₂ system utilizing hydroxyl radicals shows a commendable degradation rate, its defluorination rate is significantly lower than that of direct hole degradation. This suggests that while radical pathways can attack terminal carboxyl groups, causing C–C bond cleavage and formation of short-chain intermediates, they are ineffective in breaking C–F bonds to completely degrade the pollutants. Thus, direct hole oxidation proves to be more advantageous for treating highly chemically stable pollutants.

Table 1  Efficiency of degradation of halogenic carboxylic acids by different pathways

Pollutants	Catalyst	Degradation pathways	Degradation rate	Citation
Perfluorooctanoic acid	BiOCl	Holes	16.53 μmol L^−1 h^−1	66
	Fe/TNTs@AC	Holes	90% degradation in 4 h	67
			62% mineralization in 4 h
	In2O3/PS	Holes and ˙SO4^−	50% defluorination in 4 h	73
	P25/BiOBr	Holes	99.7% degradation in 100 min	52
	BiOBr	Holes	70% defluorination in 12 h	18
Trichloroacetic acid	BiOBr	Holes	K = 0.89 h^−1
Dichloroacetic acid	BiOBr	Holes	K = 0.40 min^−1
Perfluorooctanoic acid (PFOA)	Graphene/MnO2–H2O2	˙OH and ˙O2^−	5.58 μmol L^−1 h^−1	74
	Birnessite-1 M H2O2	˙O2^−	22% degradation in 150 min	75
	Iron(III)–EDTA–H2O2–ethanol	˙O2^−	41% degradation in 150 min
	UV/Cu–TiO2	˙OH	91% degradation in 12 h	76
	TiO2(P25)	˙OH	19% defluorination in 12 h	18
	UV/Fe–TiO2	˙OH	0.0048 mg L^−1 min^−1	76
	Pb–BiFeO3	˙OH	5% degradation in 12 h	77
Trichloroacetic acid (TCA)	TiO2(P25)	˙OH	K = 0.0022 h^−1	18
Dichloroacetic acid	TiO2(P25)	˙OH	K = 0.0032 min^−1

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4.1.2. Pollutants with strong interfacial interactions with catalysts. Initial substrate adsorption on the catalyst surface is critical for effective oxidation. It is widely acknowledged that adsorption significantly influences the photocatalytic deterioration pathways of pollutants.^78,79 When pollutants are strongly adsorbed onto the catalyst surface, the minimal distance between them significantly enhances the electron transfer efficiency from the pollutants to the holes. Consequently, highly adsorptive pollutants favor direct hole oxidation.^24,58,79,80

In 2022, Qiang Zhang and his team synthesized carbon nitride with a graphitic structure (g-C₃N₄) via direct thermal polymerization and utilized it for the breakdown of flumequine (FLU).⁸¹ They conducted experiments using triethanolamine (TEOA), isopropanol (IPA), and argon (Ar) as active species scavengers (Fig. 9c). TEOA and IPA were typically used in the past, and research has been conducted on the investigation of h⁺ cavities and ˙OH hydroxyl radicals. Ar indirectly confirmed the role of superoxide radicals (˙O₂⁻) by removing oxygen (O₂) from the solution. When introduced separately into the FLU-containing solution, Ar and IPA caused a slight reduction in the FLU degradation rate. Notably, the addition of TEOA significantly suppressed FLU degradation, highlighting the predominant role of holes (h⁺) in driving FLU degradation, with ˙O₂⁻ and ˙OH playing secondary roles. Subsequent analysis using the interaction between g-C₃N₄ and FLU was found to exhibit substantial π–π interactions, as evidenced by the application of the Reduced Density Gradient (RDG) method due to their conjugated structures (Fig. 9b). These interactions enhance FLU adsorption on the g-C₃N₄ surface, reinforcing the smooth flow of electrons from FLU to the holes of g-C₃N₄ through π–π electron channels, thereby promoting direct interactions with photo-generated holes.

Fig. 9 (a) TEM image, (b) g-C₃N₄ and FLU (hydrogen is represented by the color white, cyan represents carbon, blue signifies nitrogen, red denotes oxygen, and brown indicates fluorine) form π–π interactions between their respective structures. (c) The investigation focused on the impact of various scavengers on the degradation process of FLU.⁸¹ Reprinted with permission from ref. 81. Copyright 2022, Elsevier. (d) SEM and TEM images (insets) have been included for g-C₃N₄ samples U, M, MC, and MCB0.07. Detailed tables in the insets provide data on the surface area, pore volume, and carbon-to-nitrogen (C/N) mass ratios of each sample. Additionally, the binding free energies (E_b) of phenol and atrazine on the undoped samples are also reported. (e) Carbon-doped (f) C₃N₄ sheets were analyzed for binding affinities. The reaction coordinate R_CS and the evaluation of their binding affinities involved measuring the displacement of pollutants from the center of mass (COM) towards the g-C₃N₄ sheet in a direction perpendicular to its surface.⁸² Reprinted with permission from ref. 82. Copyright 2016, American Chemical Society.

In 2016, Qinmin Zheng et al. conducted a comparative study on the photocatalytic degradation of atrazine and phenol using C₃N₄ and carbon-doped C₃N₄. They observed that undoped C₃N₄ exhibited higher degradation efficiency for phenol. Experimentally, the reactivity of undoped C₃N₄ towards phenol was 3.9 times that towards atrazine. In contrast, for carbon-doped C₃N₄, the reactivity towards phenol decreased, and the reactivity towards atrazine was 1.8 times than that towards phenol.⁸² To explain this phenomenon, they found through experiments in 2019 that regardless of whether C₃N₄ was undoped or carbon-doped, the amount of ˙OH and 3C₃N₄* produced was minimal. Minimal contributions of reactive oxygen species, such as ¹O₂ and ˙O₂⁻, were observed in the degradation process of phenol and atrazine. Instead, it was determined that hole oxidation is the key process driving the breakdown of pollutants. Computational simulations employing molecular dynamics (MD) and density functional theory (DFT) were subsequently employed to investigate differences in the degradation pathways of phenol and atrazine before and after carbon doping. In carbon-doped C₃N₄, the binding energy with phenol decreased from 1.52 kcal mol⁻¹ to 0.54 kcal mol⁻¹, while the binding energy with atrazine increased from 0.68 kcal mol⁻¹ to 0.90 kcal mol⁻¹ (refer to Fig. 9e and f). Additionally, atrazine, phenol, and C₃N₄ all contain aromatic ring structures, and their binding relies on π–π interactions. This implies that doping enhances the interfacial interaction between atrazine and the photocatalyst while weakening the interaction between phenol and the photocatalyst.⁴³ This indicates that in photocatalytic degradation, the strength of the interfacial interactions between the efficiency of hole-mediated direct degradation is influenced by the catalyst and pollutant present.

Additionally, X3B, a negatively charged anionic dye, can adsorb onto the positively charged TiO₂ surface through electrostatic attraction with the sulfonic groups in X3B molecules.⁷² In contrast, polyoxometalates like H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀ are negatively charged in degradation environments,^80,83 leading to weaker interfacial interactions with X3B and slower degradation rates compared to TiO₂ utilizing hole oxidation (Table 2). Thus, in photocatalytic degradation, pollutants with strong adsorption tendencies are more effectively degraded via hole oxidation.

Table 2  Efficiency of X3B degradation by different pathways

Pollutants	Catalyst	Degradation pathways	Degradation rate	Citation
X3B	Bi2WO6@Bi2S3	Holes and ˙O2^−	0.0066 min^−1	84
	TNT/UV/O3	Holes	0.084 min^−1	72
	TNT/UV/O2	Holes	0.038 min^−1	85
	H3PW12O40	˙OH	0.0114 min^−1	83
	H4SiW12O40	˙OH	0.000681 min^−1
	H4GeW12O40	˙OH	0.000543 min^−1

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4.1.3. Strongly nucleophilic pollutants. Furthermore, nucleophilic pollutants such as alcohols, phenols, and anilines are widely present in the environment and have diverse sources.⁸⁶ These pollutants exhibit nucleophilic characteristics due to their electron-rich nature. Holes are positively charged defects with electrophilic properties, tending to accept electrons.^87,88 Therefore, nucleophilic pollutants readily react with the electrophilic holes generated by the photocatalyst, resulting in high reactivity during photocatalytic degradation.

In 2023, Yi Hu and colleagues investigated the degradation of 12 phenols and 6 anilines under anaerobic conditions, focusing on hole-mediated processes. They scrutinized the molecular orbital arrangements of these aromatic substances and pinpointed the energy levels of the highest occupied molecular orbital, denoted as EHOMO. The presence of electron-donating groups elevated the EHOMO values (thereby increasing nucleophilicity), which enhanced their reactivity toward electrophilic attacks. The discrepancies in EHOMO values between phenols and anilines were linked to differences in the electron density of their –OH and –NH₂ groups, thereby directly impacting their electron-donating capabilities. The log k_obs values of aromatic compounds show a positive correlation with their respective EHOMO energies (Fig. 10a–d), indicating that the HOMO levels of these compounds influence their direct hole oxidation. The HOMO energy is positively correlated with the oxidation reactivity of aromatic compounds, implying that the more nucleophilic the electronic structure of the compound, the higher its efficiency in direct hole oxidation.³²

Fig. 10 The study investigated the correlation between the observed rate constants (k_obs) of 12 phenolic compounds and 6 aniline compounds, and their respective EHOMO (highest occupied molecular orbital energy) values. The k_obs values were measured under visible light illumination in (a and c) MCN and (b and d) PPACN suspensions.³² Reprinted with permission from ref. 32. Copyright 2023, American Chemical Society. (e) Carbon/defect-modified polymerized carbon nitride porous nanotubes synthesized via dual-strategy synthesis. (f) They quantified the amounts of hydrogen (H₂) produced and bisphenol A (BPA) degraded by PCN-SA-d in the presence of triethanolamine (TEOA) and tert-butanol (t-BuOH).⁸⁹ Reprinted with permission from ref. 89. Copyright 2021, Elsevier.

In 2021, Tingting Huo and her team synthesized commercial porous carbon nitride nanotubes (cd-PCN) with carbon and nitrogen vacancy defects using carbon modification and solvent dispersion treatment. These nanotubes were designed to break down bisphenol A (BPA). tert-Butanol (t-BuOH) and triethanolamine (TEOA) were employed as hydroxyl radical scavengers and positively charged particle scavengers, respectively (Fig. 10e). The presence of t-BuOH resulted in nearly unchanged rates of BPA degradation and hydrogen evolution, suggesting the minimal involvement of hydroxyl radicals in BPA degradation. Conversely, with TEOA, BPA degradation decreased and hydrogen evolution significantly increased, indicating that TEOA scavenges holes and inhibits BPA degradation. This suggests that TEOA can enhance BPA degradation. Additionally, introducing hydrogen peroxide (H₂O₂) into the BPA solution showed no oxidation reaction under ambient conditions, indicating that BPA degradation primarily occurs through direct hole oxidation rather than by hydroxyl radicals or H₂O₂.⁸⁹

In 2022, Xindan Zhang and colleagues developed a peroxide-free resin photo-Fenton system capable of completely degrading BPA under visible light within 120 minutes. To investigate the degradation mechanism, formic acid (FA) was used as a hole scavenger. The findings indicated that FA significantly inhibited BPA degradation, with an inhibition rate of 51.5%, confirming that photogenerated positive charges (holes) play a critical role in the degradation of bisphenol A (BPA).⁹⁰ Similarly, Yawei Feng and his team demonstrated through the use of scavengers in the piezo-photocatalytic process that holes are the primary active oxidizing species in BPA degradation.⁹¹

Bisphenol A (BPA), characterized by its two phenolic hydroxyl groups, is a strongly nucleophilic pollutant.^92,93 Analyzing the degradation rates of BPA through various pathways (Table 3) indicates that the photocatalytic oxidation of BPA mainly depends on direct hole-induced degradation. These instances underscore the crucial role of positive charges (holes) in the degradation of nucleophilic pollutants.

Table 3  Efficiency of bisphenol A degradation by different pathways

Pollutants	Catalyst	Degradation pathways	Degradation rate	Citation
Bisphenol A (BPA)	Peroxide-free resin photo-Fenton system	Holes and ˙OH	K = 1.84 h^−1	90
	B-140-20	Holes	99.5% degradation in 90 min	51
	N–TiO2/AC (400M–700T)	Holes and ˙OH	50% defluorination in 8 h	94
	CeO2–AgI	Holes and ˙O2^−	78% degradation in 120 min	95
	D–Π–A polymer	Holes and ˙OH	99% degradation in 10 min	96
	P25 TiO2	˙OH	60.8% degradation in 90 min	51
	CeO2–0.2AgI	˙O2^−	K = 0.78 h^−1	95
	NixFeyO4–BiOBr	˙OH	75% degradation in 100 min	97
	BaFe12O19/Ag3PO4	˙OH	79.9% degradation in 30 min	98

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4.1.4. Pollutants in complex environments. In complex environmental conditions, pollutant degradation strategies must account for various factors, particularly pH levels and the presence of inorganic ions, as these significantly influence the efficiency and suitability of free radical and hole oxidation processes. In environments with high pH (greater than 9), the concentration of hydroxide ions (OH⁻) in water increases, which rapidly reacts with ˙OH radicals to form water and oxygen; as a result, the availability of ˙OH radicals for pollutant oxidation is decreased.^99–101 Moreover, inorganic ions such as chloride, commonly found in industrial wastewater, can inhibit advanced oxidation processes due to the high reactivity and non-selectivity of radicals, resulting in their interaction with unintended substances present in the aqueous environment, thus decreasing the oxidation rates and efficiency.^44,102 In comparison, the hole degradation pathway exhibits high selectivity, enabling the oxidation of specific chemical bonds or functional groups, which enhances the efficiency and selectivity of pollutant removal in complex aqueous matrices.⁴³

In 2006, Niyaz Mohammad Mahmoodi and colleagues demonstrated that direct hole degradation is more resistant to interference than free radical degradation by investigating the degradation of Acid Red 14 (AR 14) dye using TiO₂ nanoparticles in complex environments characterized by high pH and the presence of various anions (such as CO₃²⁻, Cl⁻, NO₃⁻, HCO₃⁻, SO₄²⁻). Photo-generated holes possess strong oxidizing capabilities, allowing them to directly oxidize AR 14 adsorbed on the surface of TiO₂. Even under different pH conditions and in the presence of anions, the oxidizing power of these holes remains stable, ensuring continuous and effective photocatalytic degradation of AR 14. In contrast, free radicals are susceptible to interference from anions, and reactive by-products are generated, resulting in a reduction of the oxidation efficiency. Compared to free radicals, holes exhibit greater resistance to interference and higher stability by directly oxidizing pollutants adsorbed on the TiO₂ surface in complex environments, thus achieving the effective photocatalytic degradation of pollutants.⁴³

In 2017, Jeffrey Farner Budarz et al. explored the photocatalytic reactions of TiO₂ nanoparticles in complex environments with high pH and specific anions (such as carbonates and phosphates). Their study revealed that carbonates and phosphates significantly inhibit the formation and oxidizing ability of hydroxyl radicals under these conditions. These anions occupy the active sites at the interface of the catalyst, preventing the generation of hydroxyl radicals. In contrast to hydroxyl radicals, organic pollutants adsorbed on TiO₂ surfaces can be directly oxidized by holes generated from photons. This oxidation capability remains stable in high pH environments and in the presence of anions, thus reducing interference effects. Experiments using potassium iodide (KI) as a probe to detect the oxidizing ability of photo-generated holes showed that carbonate and phosphate anions can modify the oxidation rate but they do not significantly diminish the overall oxidation efficiency.¹⁰³

In 2023, Yuxi Guo used BiOBr as a model semiconductor to investigate the relationship between pH-induced surface charge and dye degradation mechanisms. Methanol, TBA, and BQ were employed in capture experiments to selectively target holes, ˙OH, and ˙O₂⁻, respectively (Fig. 11b and c). At pH = 1.8, the presence of methanol did not hinder the effectiveness of methylene blue (MB) degradation, indicating that the involvement of positive charges (holes) is negligible under low pH conditions. Conversely, at pH 10, methanol significantly inhibited the degradation of MB, while incorporating TBA and BQ, which impeded the ˙OH and ˙O₂⁻ routes, showing minimal impact on MB degradation. This demonstrates that at high pH, the role of free radicals is considerably less significant than the direct oxidation by holes.¹⁰⁵

Fig. 11 (a) SEM image of BiOBr. (b) Capture curve of photogenerated carriers during MB photodegradation at pH 1.8. (c) Capture curve of photogenerated carriers during MB photodegradation at pH 10. Owing to the instability of BQ under highly alkaline conditions, extensive nitrogen (N₂) solution was used to scavenge oxygen radicals, effectively removing dissolved O₂.¹⁰⁵ Reprinted with permission from ref. 105. Copyright 2023, Elsevier.

In 2016, Jaime Carbajo and colleagues investigated the influence of anions on the generation of photo-induced holes (h⁺) and hydroxyl radicals (˙OH). They utilized P25 TiO₂ and custom-made TiO₂ catalysts to assess the degradation efficiency of phenol, dichloroacetic acid (DCA), and pyridine carboxamide (PA) in both deionized water and simulated natural water conditions (see Table 4). The study revealed that the degradation of dichloroacetic acid was primarily driven by photo-generated holes, with minimal influence observed in the presence of anions. In contrast, carbonates and phosphates significantly inhibited the production of ˙OH, resulting in reduced photocatalytic efficiency. Chlorides and nitrates had a less significant impact. While P25 TiO₂ exhibited higher efficiency in deionized water, the presence of anions in simulated natural water greatly inhibited the degradation of phenol but had minimal effect on pyridine carboxamide. This suggests that inorganic anions profoundly affect ˙OH generation but have a lesser influence on photo-generated holes, underscoring the importance of anions in practical water treatment applications.¹⁰⁴

Table 4  Degradation pathways and rates of dichloroacetic acid and phenol in different environments

Pollutants	Water quality	Catalyst	Degradation pathways	Degradation rate	Citation
Dichloroacetic acid	Deionized water	TiEt-450	Holes	0.0237 min^−1	104
Phenol	Deionized water	TiEt-450	˙OH	0.0119 min^−1
Dichloroacetic acid	Natural water	TiEt-450	Holes	0.0049 min^−1
Phenol	Natural water	TiEt-450	˙OH	0.0018 min^−1

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Therefore, while designing and selecting strategies for treating environmental pollutants, considering the impact of these environmental conditions on oxidation processes is crucial. The appropriate selection of oxidation techniques, especially in light of the behavior and reaction characteristics of pollutants in complex environments, indicates that hole oxidation processes can operate effectively over a broad pH range and demonstrate strong resistance to various anions, avoiding the conversion of chloride ions into chlorine radicals. This approach can notably enhance the treatment efficiency and mitigate potential environmental risks.

4.1.5. Pollutants that are more competitive than water. In photocatalytic systems, water molecules are ubiquitous and have strong nucleophilicity. The adsorption of water can significantly alter the physicochemical properties of the material,¹⁰⁶ enabling their interaction with vacancies present on the surface of the photocatalyst. This reduces the oxidation of pollutants, thereby significantly affecting the rate and pathway of the photocatalytic reaction.¹⁰⁷ Therefore, when degrading pollutants in water, it is necessary to consider the ability of the pollutants to compete with water molecules for holes and to assess whether their hole competitiveness is stronger than that of water molecules.

In a photoelectrochemical study, Villarreal et al. developed a model to differentiate between the direct and indirect photooxidation of the dissolved pollutants in water on TiO₂.¹⁰⁸ Through photoelectrochemical measurements (Fig. 12a), the results indicate the distinct mechanisms for methanol and formic acid degradation. Methanol's photocurrent density shows a linear dependence on the square root of light intensity, suggesting its indirect degradation mediated by radicals. In contrast, formic acid exhibits a linear relationship with light intensity, indicating direct degradation by holes. The strong interaction between formic acid and TiO₂ surface enables it to competitively adsorb with water molecules, thus facilitating direct oxidation by holes. Conversely, methanol's weaker interaction with the surface impedes effective competition with polar carboxyl-containing pollutants (–COOH) for adsorption on titanium dioxide surfaces. Consequently, these pollutants primarily undergo photooxidation indirectly via surface-bound hydroxyl radicals (˙OH). Nonetheless, specific adsorption under direct hole oxidation enhances interfacial charge transfer processes.

Fig. 12 The graph (a) shows the variation of the slope of the photocurrent density (j_pn/a[RH₂]) of methanol with respect to φ and φ_½ under different light intensities.¹⁰⁸ Reprinted with permission from ref. 108. Copyright 2004, American Chemical Society. Graph (b) shows the same for formic acid as [RH₂] approaches zero. (c) The zeta potentials of aqueous suspensions containing TiO₂ particles (50 mg L⁻¹) were measured at different pH levels, both with and without 2 mM anions present (b. SO₄²⁻, c. PO₄³⁻, d. F⁻).¹⁰⁷ Reprinted with permission from ref. 107. Copyright 2013, Elsevier. (d) On the left, water molecules are positioned distantly from acetic acid molecules to simulate separate adsorption. On the right, water molecules are initially positioned near the Ti sites where acetic acid is captured by a surface to demonstrate molecule interaction. (e) Photocatalytic degradation of adsorbed saturated CH₃COOH and CD₃OOD on TiO₂ surfaces after dehydration at 303 K as well as H₂O and D₂O post-dehydration at 303 K. (f) The PDOS showcases different potential locations on the TiO₂{101} surface where holes can be trapped. The dashed lines depict the overall density of states (DOS) for the corresponding slab. All DOS are compared to the centrally located Ti 3s states within the slab.¹⁰⁹ Reprinted with permission from ref. 109. Copyright 2018, Elsevier.

Trichloroacetic acid, dichloroacetic acid, acetic acid, and formic acid, all of which feature polar carboxyl groups (–COOH), exhibit stronger binding affinity with titanium dioxide than water. Studies have indicated that the presence of surface anions on titanium dioxide may impede the breakdown process of dichloroacetic acid and formic acid contaminants.⁵⁸ In 2013, Hua Sheng et al. revealed the impact of surface anionization on the breakdown of pollutants by presenting empirical and theoretical proof.¹⁰⁷ They observed a notable reduction in the degradation rate of formic acid following surface anionization using PO₄³⁻ and F⁻ ions, while the degradation rate of phenol and benzene increased (Table 5). Zeta potential measurements (Fig. 12b) showed that these anions impart a negative charge to the surface of TiO₂, thus enhancing its Lewis basicity and increasing water molecule adsorption. Electron paramagnetic resonance (EPR) experiments (Fig. 12c) revealed that anionization significantly increased the generation of ˙OH radicals, indicating that the oxidation ability of the holes had shifted to ˙OH radicals, thereby weakening the direct hole-mediated oxidation ability of formic acid. This suggests that formic acid relies more on direct hole oxidation. Before modification, its adsorption was superior to water, but after modification, its adsorption competitiveness weakened. This confirms that pollutants are more likely to undergo direct hole degradation when their adsorption competitiveness is stronger than that of water.

Table 5  Effect of catalyst surface anionization on the degradation rate of pollutants in different degradation pathways

Pollutants	Catalyst	Degradation pathways	Adsorption behavior	Degradation rate	Citation
Formic acid (FA)	TiO2	Holes	Stronger than water	122 min^−1	107
Formic acid (FA)	Anionized TiO2	Holes	Stronger than water	54 min^−1
Phenol	TiO2	˙OH	Weaker than water	57 min^−1
Phenol	Anionized TiO2	˙OH	Weaker than water	187 min^−1
Benzene	TiO2	˙OH	Weaker than water	107 min^−1
Benzene	Anionized TiO2	˙OH	Weaker than water	238 min^−1

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In 2017, Hongna Zhang et al. discovered that when water is present, the primary mechanism for the photocatalytic oxidation of acetic acid and trichloroacetic acid involves direct oxidation by positive charges (holes) rather than being targeted by hydroxyl radicals (˙OH).¹⁰⁹ Through kinetic isotope effect (KIE) studies (Fig. 12e), they discovered that substituting H₂O with D₂O did not markedly alter the degradation rate of acetic acid; the findings imply that the rate is not constrained by the generation of hydroxyl radicals (˙OH), hence providing evidence for the hypothesis that acetic acid undergoes direct oxidation through positive charges (holes). The analysis of the kinetic isotope effect on intermediate products was also conducted. Fig. 12f shows that acetic acid and trichloroacetic acid compete for holes more effectively than water. The location of the valence band maximum (VBM) is where one can locate the partial density of states (PDOS) for acetic acid, while the PDOS of adsorbed water molecules is positioned noticeably lower than the VBM. Furthermore, the transfer of holes from TiO₂ ⁶⁴ surfaces and the formation of ˙OH radicals from adsorbed water molecules is thermodynamically unfavorable. This finding also confirms the aforementioned views.

4.2. Free radicals

4.2.1. Weakly adsorbed pollutants. Contaminants that are difficult to be weakly adsorbed by photocatalysts typically exhibit higher degradation difficulty. This is due to the fact that the breakdown of these pollutants relies on the presence of surface sites that are actively engaged in the photocatalytic process and their degradation kinetics are influenced by the adsorption and mass transfer rates of the contaminants on the photocatalyst surface. Such pollutants may not remain on the photocatalyst surface for an extended period, thereby reducing the efficiency of hole-mediated degradation. Conversely, radicals, owing to their high diffusivity, can freely move in the aqueous medium, thus facilitating the effective contact and degradation of these weakly adsorbed pollutants.^33,34 Therefore, pollutants with weak adsorption characteristics are more suitable for oxidative degradation mediated by hydroxyl radicals.^24,58,110

Phenol is a common pollutant in industrial wastewater, characterized by its high hydrophilicity and low octanol–water partition coefficient (K_oc), which makes it readily dispersible in water and difficult to adsorb.¹¹¹ In 2016, Wanwan Meng et al. efficiently synthesized BiFeO₃ and lanthanum-doped BiFeO₃ for studying phenol degradation under simulated solar illumination conditions (Fig. 13a). It was found that effective lanthanum doping greatly improves the photocatalytic performance of BiFeO₃. Subsequently, tert-butyl alcohol (TBA) was introduced as a scavenger of hydroxyl radicals (˙OH) in phenol photodegradation experiments. The addition of TBA hindered the degradation of phenol, indicating that the decomposition of phenol was mainly affected by hydroxyl radicals (Fig. 13b). The presence of hydroxyl oxygen on the surface of lanthanum-doped BiFeO₃ with lanthanum was found to be significantly increased by O 1s XPS spectroscopy, especially for BiFeO₃ doped with 15%–51.89% lanthanum (Fig. 14c and d). The presence of lanthanum in BiFeO₃ resulted in increased hydroxyl radical generation and enhanced photocatalytic performance, highlighting the importance of hydroxyl radicals in the mechanism of phenol degradation.¹¹²

Fig. 13 (a) The catalytic performance of the lanthanum-doped BiFeO₃ catalyst and the uncatalyzed control experiment after 180 minutes of exposure to artificial sunlight. (b) The performance of 15% lanthanum-doped BiFeO₃ nanoparticles was assessed under simulated sunlight exposure, while incorporating different additives at a concentration of 4 mmol L⁻¹; figures (c) and (d) O 1s spectra of BiFeO₃ catalysts exhibiting varying degrees of lanthanum doping.¹¹² Reprinted with permission from ref. 112. Copyright 2016, Elsevier. (e) The investigation focused on the degradation of PCE through the utilization of nano-Fe₃O₄, both in the presence and absence of glutathione. The initial concentrations employed were as follows: PCE = 0.01 mM, Fe₃O₄ = 2.33 g L⁻¹, glutathione = 0.5 mM, while maintaining an initial pH level of 7. (f) The influence of TBA as a scavenger on the oxidative deterioration of PCE was examined in suspensions containing nano-Fe₃O₄-GSH and nano-Fe₃O₄, with varying concentrations of TBA at 10 mM, 20 mM, and 30 mM.¹¹⁴ Reprinted with permission from ref. 114. Copyright 2020, Elsevier.

Fig. 14 (a) Comparative degradation percentages of Rhodamine B using various ZnO microparticles and nanoparticles through sonochemical treatment. (b) Quantitative EPR spectroscopy. (c) Sono-photocatalytic decomposition of Rhodamine B using ZnO DR in the presence of salt or DMSO.¹¹⁷ Reprinted with permission from ref. 117. Copyright 2019, Elsevier. (d) The photodegradation of Rhodamine B was investigated using hybrid materials in the presence of different radical scavengers, namely, 1,4-benzoquinone (BQ), isopropanol (IPA), and ammonium oxalate (AO).¹¹⁸ Reprinted with permission from ref. 118. Copyright 2021, Elsevier. (e) Changes in the fluorescence intensity of TAOH in the prepared samples and commercial SnO₂, P25 suspensions with irradiation time. (f) Kinetics of MO degradation on different photocatalysts.¹¹⁹ Reprinted with permission from ref. 119. Copyright 2017, Elsevier.

Tetrachloroethylene is a low-polarity compound, which makes its adsorption ability weak when interacting with the surface of polar photocatalysts. The surface of photocatalysts is typically highly hydrophilic, thus making it less effective at adsorbing tetrachloroethylene.¹¹³ In 2020, Nur Dalila Mohamad et al. demonstrated through a series of experiments that tetrachloroethylene (PCE) is suitable for degradation by hydroxyl radicals (˙OH).¹¹⁴ Under the synergistic effect of nano-magnetite (nano-Fe₃O₄) and glutathione (GSH), the degradation efficiency of PCE is significantly improved. Firstly, experiments (Fig. 13e) in the nano-magnetite-GSH suspension show that PCE is entirely degraded within a span of 4 hours, demonstrating a degradation rate constant of 0.035 ± 0.01 h⁻¹. In contrast, in the suspension with only nano-magnetite, the degradation rate constant of PCE is only 0.003 ± 0.08 h⁻¹. Control experiments indicate that the effect of GSH alone on PCE degradation is minimal, with a degradation rate constant of only 0.0001 ± 0.09 h⁻¹, demonstrating that the presence of GSH greatly improves the oxidative breakdown of PCE. Following this, the utilization of a nitrobenzene (NB) probe reveals the significant production of ˙OH within the suspension of nano-magnetite-GSH. To validate the predominant role of ˙OH, tert-butanol (TBA) was introduced as a revolutionary ˙OH scavenger in the experiments. The findings (Fig. 13f) indicate that upon TBA addition, the degradation rate constant of PCE in the nano-sized magnetite particles coated with the glutathione (GSH) system exhibited a substantial decrease, dropping from 0.035 h⁻¹ to 0.006 h⁻¹, providing further evidence of the critical role played by ˙OH in PCE degradation. SEM and XRD analysis were utilized to verify the degradation of PCE during its decomposition process; nano-magnetite was partially oxidized to maghemite (γ-Fe₂O₃) rather than being completely oxidized to hematite (α-Fe₂O₃). This indicates that GSH continuously generates ˙OH by reducing Fe³⁺ to Fe²⁺. During the degradation process, PCE undergoes hydroxylation and oxidation reactions, leading to complete degradation and the formation of oxalic acid and carbon dioxide as the predominant products. Through the comparison of PCE degradation rates under various experimental conditions, the outcomes obtained from the radical probe experiment, and the radical scavenging experiment, the paper strongly demonstrates that PCE is suitable for degradation by ˙OH. The productivity of PCE degradation is greatly improved by the nano-magnetite-GSH system as it effectively increases the production of OH⁺ ions, thus conclusively demonstrating the pivotal role of OH⁺ radicals in PCE degradation.

Furthermore, chloroform, characterized by a high hydrophilic partition coefficient, is a pollutant readily degraded by hydroxyl radicals during photocatalysis (Table 6). This underscores the significant advantage of the radical degradation pathway in treating weakly adsorbed pollutants. These pollutants, due to their chemical properties, enable radicals to effectively oxidize and degrade them, which enhances the efficiency of the photocatalytic process.

Table 6  Chloroform is degraded by hydroxyl radicals

Pollutant	Catalyst	Degradation pathways	Degradation rate	Citation
Chloroform	Montmorillonite KSF	˙OH	54% degradation in 10 h	115
Chloroform	CuO/TiO–PES	˙OH	90% degradation in 40 h	116

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4.2.2. Pollutants that are easily oxidized. Pollutants containing easily oxidizable functional groups, due to their inherent chemical structure and electronic properties, become efficient reaction sites in oxidation reactions. Therefore, theoretically, they can undergo oxidation by both radical oxidation and hole oxidation mechanisms. However, because radical degradation is not limited by adsorption and has stronger reactivity, it has traditionally been the primary method used to degrade such substances.

Methyl orange, methylene blue, and Rhodamine B contain easily oxidizable amino and hydroxyl functional groups, making them typical examples of easily oxidizable pollutants. In 2018, Carmine Lops et al. proposed a sonophotocatalytic degradation method for Rhodamine B using ZnO micro-nanomaterials.¹¹⁷ The generation of short-lived radicals was detected using EPR spectroscopy and spin trapping techniques. The findings boldly demonstrated that under ultrasonic and light illumination, ZnO micro-nanomaterials produced the most hydroxyl radicals (˙OH) (Fig. 14a), and the generation of hydroxyl radicals (Fig. 14b) exhibited a direct positive correlation with the rate of degradation. During the photodegradation process of Rhodamine B, different active species scavengers were introduced in scavenging experiments. When dimethyl sulfoxide (DMSO) was added as a scavenger of hydroxyl radicals (˙OH), the degradation of Rhodamine B was significantly inhibited, while the degradation effect was not significantly reduced when other scavengers were added (Fig. 14c). This suggests that hydroxyl radicals possess the capability to effectively degrade Rhodamine B.

In 2021, Chinh Van Tran prepared Cu_0.5Mg_0.5Fe₂O₄–TiO₂ composites using co-precipitation and sol–gel methods.¹¹⁸ This substance exhibited the efficient degradation of Rhodamine B when exposed to simulated solar radiation. The composite exhibited a high degradation rate constant of 13.96 × 10⁻³ min⁻¹, achieving a remarkable degradation efficiency of 98.4% within 180 minutes. Radical scavengers were employed during the process: superoxide radicals (˙O₂⁻) can be scavenged using 1,4-benzoquinone (BQ), hydroxyl radicals (˙OH) can be eliminated with isopropanol (IPA), and holes (h⁺) can be neutralized by ammonium oxalate (AO). The results indicated that after adding IPA, the degradation of Rhodamine B was markedly reduced, highlighting ˙OH as the predominant reactive oxygen species (Fig. 14d).

In 2016, Jinghui Wang and colleagues synthesized SnO_X/Zn₂SnO₄via a one-pot hydrothermal method for degrading methyl orange.¹¹⁹ Within 30 minutes, the SnO_x/Zn₂SnO₄ composite completely decolorized the methyl orange solution, with a degradation rate constant of 8.34 × 10^−2% per minute. Photoluminescence (PL) technology was then used, wherein using terephthalic acid (TA) as a probe molecule enables the detection of ˙OH generation. The results showed (Fig. 14e) that the samples produced a large amount of 2-hydroxyterephthalic acid (TAOH) upon irradiation. The formation rate of ˙OH followed the sequence SnO₂ < SZTO-S < SZTO-HS < ZTO < P25 < SZTO-H. This order is closely correlated with the photocatalytic degradation performance of MO (Fig. 14f).

This suggests that the primary reactive oxygen species were hydroxyl radicals, and the material's exceptional effectiveness can be attributed to its proficient production of ˙OH. Consequently, the decomposition process of methyl orange may be significantly influenced by the pivotal role played by hydroxyl radicals.

Furthermore, methylene blue is enriched with readily oxidizable amino and hydroxyl functional groups, making it highly susceptible to oxidation as a pollutant. Like Rhodamine B and methyl orange, it undergoes rapid degradation by hydroxyl radicals during photocatalysis (Table 7). This underscores the pivotal role of radicals in effectively treating these easily oxidizable pollutants, thereby significantly enhancing the degradation efficiency.

Table 7  Rhodamine b(RhB), methyl orange (MB), and methylene blue (MB) are degraded by hydroxyl radicals

Pollutant	Catalyst	Degradation pathways	Degradation rate	Citation
RhB	α-Fe2O3/ZnFe2O4 @Ti3C2	˙OH	100% degradation in 150 min	120
RhB	Bi2Fe4O9/g-C3N4	˙OH	88.3% degradation in 50 min	121
RhB	α-Fe2O3/g-C3N4	˙OH	96% degradation in 90 min	122
MO	Fe(III)/CQDs/Fe-doped g-C3N4	˙OH	93% degradation in 60 min	123
MB	P25–rGO–PAM (PGP)	˙OH and ˙O2^−	K = 0.0276 min^−1	124

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4.2.3. Highly unsaturated pollutants. Highly unsaturated pollutants have double or triple bond structures, making them prone to attack by free radicals, which can lead to addition or cleavage reactions. Free radicals can initiate chain reactions and easily attack the π-electron cloud of unsaturated bonds to generate radical intermediates that drive chain reactions.¹²⁵

PAHs such as naphthalene, phenanthrene, anthracene, and benzo[a]pyrene feature numerous conjugated double bonds, making them highly unsaturated pollutants. In 2020, Guang Lu et al. utilized CeVO₄ nanoparticles for the photocatalytic degradation of naphthalene.¹²⁶ Experimental data indicated a rate constant of degradation of 0.469 h⁻¹ for naphthalene. Different scavengers were used to identify the main reactive oxygen species (ROS) (Fig. 15a) and isopropanol (IPA) was added to scavenge ˙OH, triethanolamine (TEOA) to scavenge holes, and benzoquinone (BQ) to scavenge ˙O₂⁻ radicals.

Fig. 15 (a) The samples were subjected to scavenging experiments. ESR experiments were conducted on DMPO-O₂ (b) and DMPO-OH (c) in the samples.¹²⁶ Reprinted with permission from ref. 126. Copyright 2020, Elsevier. (d) Under solar irradiation, the photoluminescence (PL) spectra of different materials containing titanium were examined. (e) Degradation of phenanthrene using various titanium-based materials through photocatalysis.¹²⁷ Reprinted with permission from ref. 127. Copyright 2016, Elsevier. (f) Capture experiments of the photocatalytic degradation on Bi₂S₃/CeVO₄ with the same quenchers.¹²⁸ Reprinted with permission from ref. 128. Copyright 2023, Elsevier.

The findings emphasize that the degradation process is primarily driven by ˙OH and ˙O₂⁻, with holes making minimal contributions. Subsequently, in the electron spin resonance (ESR) studies, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was employed as a scavenger (Fig. 15b and c). No significant indications were detected when light was not present. However, when exposed to visible light, the intensity of the characteristic peaks for ˙O₂⁻ and ˙OH increased. The results unequivocally indicate that the signal intensity of ˙OH surpasses that of ˙O₂⁻, providing further confirmation of the predominant role of ˙OH in the photochemical degradation process.

In 2016, Xiao Zhao et al. utilized TiO₂ (P25) as a precursor to prepare a Co-TNTs-600 nanomaterial for catalyzing the degradation of phenanthrene,¹²⁷ a polycyclic aromatic hydrocarbon (PAH), with a decay rate of 0.39 per hour. Using p-benzoquinone as a probe molecule, the generation of ˙OH by various titanium-based materials under sunlight irradiation was measured via photoluminescence (PL) technique. Experimental results (Fig. 15d) showed a fluorescence intensity order of TNTs < Co-TNTs < TNTs-600 < P25 < Co-TNTs-600, consistent with the apparent rate constants (k₁) of phenanthrene degradation (Fig. 15c). This consistency highlights that the enhanced generation of ˙OH radicals plays a vital role in facilitating the enhanced photocatalytic degradation of phenanthrene.

In 2023, Jianghua Huang et al. developed a novel S-type heterojunction photocatalyst consisting of Bi₂S₃/CeVO₄ for catalyzing the degradation of naphthalene,¹²⁸ achieving a degradation rate of 0.02834 min⁻¹. In the experiments, triethanolamine (TEOA) was used to scavenge h⁺, isopropanol (IPA) to scavenge ˙OH, and benzoquinone (BQ) to scavenge ˙O₂⁻. Experimental results indicate (Fig. 15f) that upon the addition of TEOA, the efficiency of NAP degradation experiences a minor decline; however, when IPA or BQ was added, the degradation efficiency decreased significantly to 38% and 58%, respectively. This implies that the involvement of ˙O₂⁻ and ˙OH is essential in the degradation mechanism of NAP.

In addition, tetracycline and quinolone antibiotics, due to their high content of unsaturated bonds, have been proven suitable for degradation via radical pathways. Shijie Li and his team have made continuous breakthroughs in this field. In 2023, they developed a novel S-type heterostructure photocatalyst, Au/MIL-101(Fe)/BiOBr, which effectively degraded norfloxacin and reduced its ecological toxicity, with experiments demonstrating that the main active species were ˙OH and ˙O₂⁻. In the same year, they synthesized Cd_0.5Zn_0.5S/Bi₂MoO₆, which produced ˙O₂⁻ that played a crucial role in the degradation of oxytetracycline. In 2024, they further introduced Mn_0.5Cd_0.5S/BiOBr, a catalyst that rapidly degraded tetracycline hydrochloride using ˙OH. These studies demonstrate the high efficiency of radical degradation pathways in the treatment of antibiotic pollution.^55–57

Table 8 lists additional examples of pollutant degradation through the use of highly unsaturated free radicals. These examples further illustrate the efficacy and wide-ranging application of free radicals in treating such pollutants. The extensive confirmation of the breakdown of these pollutants through the action of unbound radicals has been observed in previous experimental studies, showing significant degradation efficiency and reaction characteristics.

Table 8  Degradation of contaminants with high unsaturated bond content by free radicals

Pollutant	Catalyst	Degradation pathways	Degradation rate	Citation
Benzopyrene	FeO2	˙OH	K = 1.11 × 10^−2 h^−1	129
Sixteen-PAH	ZnO/PAM–PVA	˙OH	100% degradation in 2 h	130
Phenanthrene	g-C3N4/Fe3O4	˙OH	92% degradation in 2 h	131
Benzopyrene	Pt/TiO2–SiO2	˙OH	99.7% degradation in 72 h	132
Anthracene	ZnO	˙OH	96% degradation in 2 h	133
Benzopyrene	goethite	˙OH	K = 2.38 × 10^−1 h^−1	129
Norfloxacin	Au/MIL-101(Fe)/BiOBr	˙OH and ˙O2^−	K = 2.14 × 10^−2 min^−1	55
Tetracycline hydrochloride	Mn0.5Cd0.5S/BiOBr	˙O2^−	K = 2.99 × 10^−2 min^−1	57
Oxytetracycline	Cd0.5Zn0.5S/Bi2MoO6	˙O2^−	K = 3.87 × 10^−2 min^−1	56
Levofloxacin	Ag/Ag3PO4/C3N5	˙OH and ˙O2^−	K = 3.62 × 10^−2 min^−1	134

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5. Conclusions

This review explores the pathways of free radical degradation and direct hole oxidation in photocatalytic technology, revealing their mechanisms, advantages, and limitations in pollutant treatment. While the free radical degradation pathway is highly regarded for its wide applicability and efficiency, its non-selectivity and dependence on specific environmental conditions restrict its scope of application. In contrast, the direct hole oxidation pathway exhibits excellent selectivity and high oxidative capacity, but its limited contact efficiency with pollutants hinders its widespread application. To overcome these challenges, the various pollutant degradation pathways are analyzed and summarized by exploring both mechanisms, supported by practical examples.

To clarify the similarities and differences between these two pathways, the investigation covered four processes: light absorption, charge separation, reactive oxygen species generation, and pollutant degradation. By examining the mechanisms of each process in the two photocatalytic degradation pathways, the distinct characteristics of each pathway were identified. Free radical degradation pathway exhibits excellent treatment effectiveness across a wide range of pollutant types due to its good applicability, wide diffusion range, and promotion of charge separation. On the other hand, the hole degradation pathway is particularly suitable for treating recalcitrant or specific chemical properties of pollutants because it possesses strong selectivity, high oxidative capacity, and does not require dissolved oxygen. This study addresses the gap in the previously unexplored aspect of photocatalytic degradation of contaminants, enhancing researchers’ understanding of the unique properties associated with each degradation pathway.

Furthermore, for the first time, the regulatory mechanisms for the interconversion between these two pathways have been summarized. To regulate the transition from the free radical pathway to the hole pathway, it is necessary to enhance pollutant adsorption and reduce the valence band in order to improve the hole utilization efficiency. Conversely, for regulating the transition from the hole pathway to the free radical pathway, incorporating branched acid–base sites onto the catalyst surface can enhance charge carrier separation and transfer efficiency. This provides the possibility of transitioning from the same photocatalyst to either the free radical-mediated pathway or the direct hole degradation pathway. It offers corresponding strategies for the rational regulation of photocatalysts, allowing researchers to select the most suitable oxidation pathway based on the characteristics of pollutants. This enhances the effectiveness and specificity of photocatalytic decomposition.

Finally, based on the classification of the two types of biodegradation pathways for pollutant degradation, this study aims to enable researchers to more accurately devise and optimize specific design strategies, thereby enhancing their understanding of hole and free radical mechanisms in pollution control. The establishment of this classification method not only reduces researchers’ blind trial and error in environmental applications but also deals with the previous neglect of the hole pathway in pollution degradation research.

For hole-mediated degradation:

(1) Chemically inert pollutants.

(2) Highly adsorptive pollutants.

(3) Nucleophilic pollutants.

(4) Pollutants in complex matrices.

(5) Pollutants with higher reactivity than water.

For radical-mediated degradation:

(1) Less adsorptive pollutants.

(2) Easily oxidizable pollutants.

(3) Pollutants with high unsaturation levels.

The aforementioned discoveries not only establish a robust scientific foundation for the development and selection of photocatalysts but also provide valuable insights into advancing environmental remediation technologies. The research results of this study offer strong theoretical support and experimental evidence for the future applications of photocatalytic technology, heralding the tremendous potential of photocatalytic technology to evolve towards more efficient and precise environmental governance.

Despite some progress in the pathways of free radical degradation and direct hole oxidation, there are still some urgent challenges to be addressed. Currently, the technology for identifying the reactive oxygen species (ROS) primarily responsible for degradation is still limited. The main method is to use hole or free radical scavengers to explore the degradation pathways. However, this method has several significant drawbacks: low selectivity, where scavengers may act on multiple reactive oxygen species at the same time, making it difficult to clearly distinguish the specific reactive species; interference from side reactions, where scavengers themselves may participate in side reactions, thus complicating the experimental results; and the impact on reaction kinetics, where the introduction of scavengers may change the kinetic behavior of the reaction system, thereby affecting the identification of the main degradation pathways.

To address these issues, we believe that improvements can be made through the following measures. First, the development of highly selective probe molecules that can specifically recognize and react with specific reactive oxygen species, thereby improving the accuracy of detection and avoiding confusion from the simultaneous detection of multiple species. Second, the application of advanced characterization techniques (such as electron paramagnetic resonance spectroscopy, mass spectrometry, and nuclear magnetic resonance) can directly detect and characterize reactive oxygen species, thus reducing the interference from the side reactions introduced by scavengers and obtaining clearer experimental results. Finally, computational chemistry simulations can predict the behavior and reaction pathways of different reactive oxygen species, assisting in experimental design and result interpretation, thereby reducing the impact of scavengers on the reaction kinetics.

In addition, in our research, the co-degradation of pollutants by different reactive oxygen species has a high rate and effect, which may indicate that multi-pathway synergy has great potential in pollutant degradation. Unfortunately, a systematic understanding and comprehensive experimental verification of these complex synergistic mechanisms and their functional characteristics are still lacking, and further research is needed to fully understand the synergistic effects of multiple degradation pathways in complex situations, including an examination of their mechanisms and resultant effects.

Solving the above issues can help to better identify and understand the reactive oxygen species primarily responsible for degradation as well as the synergistic mechanisms of multiple degradation pathways, thus providing a theoretical basis and technical support for the efficient degradation of pollutants.

Author contributions

The authors contributed equally to this manuscript.

Data availability

No primary research results, software or code has been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (22479042, 22075071), Heilongjiang Province double first-class disciplines collaborative innovation achievement project (LGXCG2023-004), Postdoctoral Research Grant of Heilongjiang Province (LBH-Z23265), Harbin Manufacturing Science and Technology Innovation Talent Project (2022CXRCCG016).

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Footnote

† These authors make equal contributions to this work.

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