Principles, applications and future prospects in photodegradation systems

Walied A. A. Mohamed; Aiyeshah Alhodaib; Hanan A. Mousa; Hala T. Handal; Hoda R. Galal; Hala H. Abd El-Gawad; Badr A. Elsayed; Ammar A. Labib; Mohamed S. A. Abdel-Mottaleb

doi:10.1515/ntrev-2025-0159

Article Open Access

Principles, applications and future prospects in photodegradation systems

Walied A. A. Mohamed , Aiyeshah Alhodaib , Hanan A. Mousa , Hala T. Handal , Hoda R. Galal , Hala H. Abd El-Gawad , Badr A. Elsayed , Ammar A. Labib and Mohamed S. A. Abdel-Mottaleb

Published/Copyright: April 9, 2025

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From the journal Nanotechnology Reviews Volume 14 Issue 1

Abstract

Photocatalytic materials have emerged as pivotal in addressing global challenges such as environmental pollution, energy scarcity, and industrial sustainability. This review delves into the principles, mechanisms, and applications of photocatalytic systems, emphasizing their roles in photodegradation and renewable energy production. United Nations (UN) specified a guideline for sustainable development strategies. UN determined 17 goals of sustainable development and the services of photocatalytic materials underwent 4 of these goals to reflect the distinguishable interest and importance of different photocatalytic materials in many fields for various purposes. Advances in material design, nanotechnology, and system integration have significantly advanced this field, aligning it with sustainable development goals. Key materials like TiO₂, ZnO, g-C₃N₄, and quantum dots are highlighted for their unique properties and enhanced photocatalytic activity through modifications such as doping, heterostructure formation, and biopolymer-supported photocatalysts. Practical applications in wastewater treatment, hydrogen production, air purification, and carbon dioxide reduction are comprehensively analyzed, with case studies demonstrating the successful photodegradation of industrial pollutants. The review also explores the integration of photocatalysis with renewable energy sources, addressing challenges like charge carrier recombination and photocatalyst stability. Interdisciplinary approaches, including computational modeling and machine learning, are discussed for designing next-generation photocatalysts, aligning innovations with global sustainability initiatives.

Graphical abstract

Keywords: sustainable development; photocatalytic materials; photocatalytic degradation systems; water splitting; water treatment; quantum dots

List of abbreviations

2,4-DCP: 2,4-dichlorophenol
BPA: Bisphenol A
CB: conduction band
CBZ: carbamazepine
g-C₃N₄: graphite nitride carbonate
HVAC: heating, ventilation, and air conditioning
IBP: Ibuprofen
IOT: internet of things
LCEFMs: laccase-embedded electrospun fibrous membranes
LEDs: light-emitting diodes
MB: methylene blue
MO: methyl orange
MOFs: metal-organic frameworks
MWCNTs: multi-walled carbon nanotubes
NWs: nanowires
PV: photovoltaic
PZC: point of zero charge
QDs: quantum dots
TAS: transient absorption spectroscopy
TCS: triclosan
UN: United Nations
VB: valence band
VOCs: volatile organic compounds

1 Introduction

Photocatalysis is a groundbreaking approach in material science and chemistry, emerging as a key solution to address critical environmental and energy challenges. It involves the acceleration of photochemical reactions facilitated by a catalyst that absorbs light, utilizing this energy to drive essential chemical transformations [1,2,3,4,5]. This method is particularly advantageous due to its ability to harness renewable solar energy and transform it into usable chemical energy, aligning with global sustainability goals. Unlike traditional catalytic systems, photocatalysis relies on the activation of materials through photons to generate charge carriers – electrons and holes – which mediate redox reactions. These principles form the backbone of numerous applications in environmental remediation, clean energy production, and advanced technologies [6,7,8,9,10,11].

The historical context of photocatalysis traces back to the 19th century, with its modern conceptualization attributed to the seminal work of Akira Fujishima and Kenichi Honda in 1972. Their discovery, famously known as the “Honda-Fujishima Effect,” demonstrated water splitting on titanium dioxide (TiO₂) electrodes under ultraviolet (UV) light, marking a transformative moment in photocatalytic research. Published in Nature, this study illuminated the potential of light-driven chemical reactions for sustainable energy solutions, including hydrogen production – a clean, high-energy fuel that is increasingly seen as the cornerstone of a carbon-neutral energy economy Fujishima and Honda [12]. Since then, the field has witnessed exponential growth, driven by advances in material design, nanotechnology, and environmental awareness [12,13].

Central to photocatalysis is the generation of electron-hole pairs when a semiconductor absorbs light energy greater than or equal to its bandgap [14,15,16,17,18,19]. These charge carriers migrate to the surface of the catalyst, where they interact with adsorbed species to initiate oxidation and reduction reactions. Photogenerated holes oxidize organic pollutants, water, or other substrates, while excited electrons reduce electron acceptors such as oxygen [20,21,22]. However, the efficiency of this process is often hindered by the rapid recombination of these charge carriers – a phenomenon that dissipates the absorbed energy as heat. Overcoming this limitation is a central focus of ongoing research, with efforts directed toward strategies such as doping, heterostructure formation, and the use of co-catalysts to prolong charge carrier lifetimes and enhance separation and migration efficiency.

The field of photocatalysis is now transitioning from fundamental studies to diverse applications, spanning environmental protection, renewable energy, and advanced material development. One of the most significant areas of application is environmental remediation, where photocatalysis has demonstrated remarkable efficacy in degrading a variety of pollutants, including dyes, pesticides, pharmaceuticals, and volatile organic compounds (VOCs). By harnessing solar energy – a sustainable and inexhaustible resource – photocatalytic processes can degrade hazardous organic pollutants into benign substances such as water and carbon dioxide, achieving complete mineralization without secondary pollution. This attribute makes photocatalysis a promising alternative to traditional chemical treatments, which often generate harmful byproducts [3,23,24,25,26,27].

Semiconductor materials, particularly metal oxides such as TiO₂ and ZnO, dominate the field of photocatalysis due to their stability, cost-effectiveness, and non-toxic nature. However, these materials are primarily activated by UV light, which constitutes only a small fraction (approximately 4–5%) of the solar spectrum. Consequently, significant research has been dedicated to extending the activity of photocatalysts into the visible-light region, which represents a larger portion of solar radiation. Bandgap engineering through doping with non-metal or transition metal elements, as well as the development of novel materials such as graphitic carbon nitride (g-C₃N₄), perovskites, and quantum dots (QDs), has been instrumental in addressing this challenge [28,29].

Beyond pollutant degradation, photocatalysis holds immense potential in energy applications, particularly in water splitting for hydrogen production. As hydrogen is a clean energy carrier with high gravimetric energy density, its sustainable production is pivotal for transitioning to a green energy economy. Photocatalytic water splitting offers a renewable route to generate hydrogen using sunlight and water – a reaction that mimics the natural process of photosynthesis. Advances in material design, including the creation of Z-scheme heterostructures and plasmonic photocatalysts, have significantly improved the efficiency of water-splitting systems, bringing them closer to practical implementation [30,31,32].

The rise of nanotechnology has further revolutionized the field of photocatalysis, enabling the design and synthesis of nanostructured materials with superior properties. Nanomaterials, such as QDs, nanowires (NWs), and two-dimensional materials, exhibit enhanced light absorption, charge carrier dynamics, and surface reactivity due to their high surface area-to-volume ratios and quantum confinement effects. QDs, for instance, are particularly notable for their size-dependent optical and electronic properties, which allow for precise tuning of their bandgap and redox potentials [33,34]. These characteristics have made them indispensable in cutting-edge applications ranging from photocatalytic water purification to cancer therapy.

In addition to environmental and energy applications, photocatalysis has found a foothold in emerging technologies. For instance, self-cleaning surfaces coated with photocatalytic materials such as TiO₂ or g-C₃N₄ have gained traction in the construction and automotive industries, offering low-maintenance solutions for surfaces prone to fouling or contamination. Similarly, photocatalysis is being explored for its potential in air purification, targeting indoor VOCs and other airborne contaminants. By integrating photocatalytic systems into heating, ventilation, and air conditioning (HVAC) units, it is possible to achieve real-time air quality improvement with minimal energy consumption [35,36,37,38].

Despite these advances, several challenges remain in the practical implementation of photocatalytic systems. Issues such as photocatalyst stability, scalability of synthesis methods, and the optimization of reactor designs need to be addressed to fully realize the potential of photocatalysis [14,23,39,40]. Furthermore, the interplay of various operational parameters – such as pH, temperature, pollutant concentration, and light intensity – must be comprehensively understood to maximize system efficiency. Recent developments in computational modeling and machine learning offer promising avenues for optimizing these parameters, enabling the rational design of next-generation photocatalysts [3,41,42].

This manuscript aims to provide a comprehensive review of the principles, applications, and future prospects of photocatalytic materials in photodegradation systems. It delves into the fundamental mechanisms that govern photocatalytic reactions, highlighting the role of material properties, band structure, and surface chemistry in determining catalytic performance. The manuscript also explores the diverse applications of photocatalysis, from water purification and wastewater treatment to air quality enhancement and renewable energy production. Finally, it addresses the challenges and opportunities in the field, with a focus on recent advancements in material science, nanotechnology, and system integration.

2 Photocatalytic materials’ principles

Photocatalytic materials possess the unique capability to transform incident photons into either consumable or storable forms of energy [22,43,44]. This transformation occurs via the generation of electron/hole pairs at the photocatalyst surface. Broadly speaking, photocatalysts are substances that manifest their catalytic properties upon exposure to light through the photon absorption process.

Given the global urgency in addressing environmental challenges like water and air pollution, climate change, and waste management, significant endeavors are underway to develop economically viable and sustainable methodologies for energy generation, pollutant degradation, and recycling. Photocatalytic processes present a viable solution to these challenges, with sunlight serving as the primary, naturally endowed energy source for such processes [45].

Photosynthesis, evident in green leaves, exemplifies a natural photocatalytic process. Here, chlorophyll functions as the photocatalyst, facilitating the transformation of water and carbon dioxide into oxygen and carbohydrates. Synthetically engineered photocatalysts often seek to emulate this intrinsic process [46]. They leverage photogenerated electrons or holes to produce high-energy radicals. These radicals find utility in a range of applications, notably water purification and the disintegration of water into O₂ and H₂ [47,48,49]. Photocatalysis can be categorized into two principal types, contingent upon the phases of the photocatalytic materials engaged in the procedure [50,51,52,53,54,55].

Homogeneous photocatalysis: In this process, both the photocatalyst and the reactant reside within the same phase. Notable examples of homogenous photocatalysts include coordination compounds, dyes, and naturally occurring pigments.
Heterogeneous photocatalysis: Contrastingly, in heterogeneous photocatalysis, the photocatalyst and the reactant are present in separated phases. Transition metal chalcogenides serve as prominent representatives of heterogeneous photocatalysts, possessing distinct attributes.

From a mechanistic standpoint, the initiation of the photocatalytic reaction is contingent upon the photon absorption with the requisite energy, either equating to or surpassing the band gap energy intrinsic to the photocatalyst. This photon absorption results in charge dissociation, wherein an electron is propelled from the semiconductor’s valence band (VB) to its conduction band (CB), thereby generating a hole (h⁺) within the VB [56,57,58,59,60]. This energetically elevated electron possesses the potential to reduce a given substrate or to engage with electron acceptors, such as molecular oxygen, either adsorbed on the surface of the semiconductor or dissolved within the aqueous medium, converting it into O₂ ⁻˙ Conversely, the generated hole can facilitate the oxidation of an organic entity, yielding R⁺, or it might interact with hydroxyl groups or H₂O molecules, leading to generation of ˙OH. Furthermore, potent oxidizing agents such as peroxide radicals play a distinguishable role in the heterogeneous photodecomposition of organic molecules [61]. The hydroxyl radical stands out as a potent oxidant, capable of oxidizing a vast majority of azo dyes, along with other contaminants, leading to their mineralization as end products (Figure 1) [62,63,64].

Figure 1

Standard photocatalytic reaction mechanism.

2.1 Mechanisms in photodegradation systems

Photodegradation systems can be classified based on the types of photocatalytic materials employed, each of which is linked to a distinct mechanistic pathway. Hence, depending on the specific photocatalytic materials in use, the mechanisms of photodegradation systems can be categorized into one of the following types:

2.1.1 Homogeneous photo-Fenton reaction

The photo-Fenton reaction derives its name from Henry Fenton, who documented the hydrogen peroxide activated in the presence of iron, leading to the oxidation of the tartaric acid [65,66,67]. At its core, the photo-Fenton process is as straightforward as a redox reaction and subjected to the classical mechanism. Herein, the ferrous state is oxidized to a ferric ion, while hydrogen peroxide is decomposed to yield a hydroxide ion and a hydroxyl radical, the latter being endowed at E ₀ = 2.73 V versus NHE as a formidable oxidation potential [68,69]. As outlined in equation (1), the resultant Fe³⁺ can undergo reduction back to its Fe²⁺ state, facilitated by hydrogen peroxide. It is noteworthy that the Fe³⁺ reduction process, as delineated in equation (2) and characterized by 0.02 M⁻¹ s⁻¹ as a reaction rate constant, is markedly more sluggish compared to the Fe²⁺ oxidation process, as detailed in equation (1), with a corresponding reaction rate constant ranging between 40 and 80 M⁻¹ s⁻¹. So, large amounts of initial Fe²⁺ ions are needed to mineralize the organic pollutants, and the rate-limiting step is identified [68,70,71,72].

(1) Fe 2 + + H 2 O 2 → Fe 3 + + − OH + ˙ OH,

(2) Fe 3 + + H 2 O 2 + H 2 O → Fe 2 + + H 3 O + + 2 − ˙ OH,

˙ OH + H 2 O 2 → ˙ 2 − OH + H 2 O ,

˙ OH + Fe 2 + → Fe 3 + + − OH .

2.1.2 Semiconductors

Semiconductors are materials recommended for photocatalysts, particularly metal sulfides and oxides. Also, the photocatalyst performances are directly dependent on their electronic band gap structures [6,73,74]. To get high efficiency for the photocatalyst in the visible region of the solar spectrum, the photocatalyst band gap must lie between 1.5 and 3 eV while between 1.5 and 3 eV in the UV region.

Also, valence and CB energy levels play a distinguishable role in the hydroxyl radicals generated and its CB less than 0.92 V/ENH (oxygen reduction potential in O₂ ⁻˙), and/or the VB should have a couple potential of OH/H⁺, H₂O more than 2.31 V/ENH at as an oxidation potential [75]. As shown in Figure 2, there are many semiconductors that have a similar property, such as TiO₂, ZnO, SnO₂, Fe₂O₃, WO₃, CdSe, and CdS [76,77,78,79,80].

Figure 2

Band gaps of various semiconductors.

2.1.3 Heterostructure dynamics

Heterostructures, alternatively termed heterojunctions, formed through the amalgamation of distinct semiconductors, have been the focus of numerous investigations due to their augmented capability for light absorption in the visible spectrum. Beyond enhanced light absorption, these structures also showcase superior charge carrier longevity and mobility when compared to their constituent semiconductors [81,82,83].

The mechanism underpinning heterogeneous photocatalysis is characterized by a sequence of redox reactions taking place at the level of the photocatalyst [84]. Within a semiconductor, the energy separation between the highest occupied electron band (VB) and the subsequent unoccupied electron band (CB) is designated as the band gap. When photons impinge upon the semiconductor with energy either equivalent to or exceeding that of the band gap, electrons residing in the VB become energized, transitioning to the CB on a femtosecond timescale [84,85,86,87,88]. This movement engenders a vacancy or “hole” within the VB. Given this electron-hole creation, the semiconductor becomes electronically conducive. Provided that these electrons and holes are immobilized on the semiconductor’s surface and that the recombination of this electron-hole duo is circumvented, a series of ensuing reactions is set into motion:

Photon excitation TiO 2 + h υ → e − + h + ,
Free electron trapping e CB − → e TR − ,
Stuck holes h VB + → h TR + ,
Reassembly of charge carriers e TR − + h VB + → e CB − + heat ,
Excited electron sweeping ( O 2 ) ads + e − → O 2 − ,
Hydroxyl oxidation OH − + h + → ˙ OH , and
Photodegradation with hydroxyl radical R − H + ⋅ OH → R ′ + H 2 O .

During the sixth phase, the hydroxyl radicals formed facilitate the conversion of organic contaminants into intermediary compounds. These intermediaries are predominantly further processed into water and carbon dioxide, either via the same reaction mechanism or through alternative pathways [89].

Several salient features underpin the utilization of heterostructures in photocatalytic processes [90]:

There is potential for comprehensive degradation of pollutants, culminating in the formation of CO₂ and assorted minerals.
The reactions predominantly occur under ambient conditions.
The onset of the reaction is contingent upon the availability of both oxygen and energy surpassing the band gap, both readily accessible in most contexts.
A myriad of inert substrates, ranging from glass and polymers to carbon nanotubes and graphene oxides, can be employed as catalyst supports.
The catalyst used is both cost-effective and devoid of toxicity, with the added advantage of being recyclable.

Conceptually, a heterostructure embodies an interfacial fusion of two or more distinct entities. Given the band alignment intrinsic to the semiconductors, heterostructures can be delineated into three distinct categories, as elucidated in Figure 3: Type I (symmetric), Type II (staggered), and Type III (broken) [91,92].

Type I heterostructures: These facilitate the recombination of photogenerated electrons and holes, which explains their prevalent application in light-emitting diode (LED) systems where radiative recombination is imperative [93,94,95,96].
Type II heterostructures: These are particularly compelling due to their capacity to foster the segregation and delocalization of photogenerated charge carriers. An inherent electric potential, engendered at the juncture of the semiconductors, propels holes into the VB of one semiconductor, whilst channeling electrons into the CB of the other [97,98]. This dynamic results in augmented lifespans for the photogenerated charge carriers.
Type III broken heterostructures find applications in transistors having tunneling field effects [99,100].

Figure 3

Schematic representation of heterostructures possible types.

3 Advancements in heterostructure design for water treatment and hydrogen production

The preference for specific materials like ZnO, SnO₂, CdS, or g-C₃N₄ in water treatment and hydrogen production is rooted in their unique properties that align with the requirements of these applications. Among the most widely researched materials, ZnO stands out due to its wide bandgap (∼3.37 eV) and high exciton binding energy (∼60 meV), which enable efficient UV light absorption and photocatalytic activity under UV illumination. ZnO also offers excellent chemical stability and cost-effectiveness, making it suitable for large-scale environmental applications [101]. However, its UV activity limits its utilization under natural sunlight, prompting research into doping and surface modifications to extend its activity into the visible spectrum [102]. For example, nitrogen and sulfur doping have shown promising results in shifting its absorption edge into the visible range, broadening its application potential in solar-driven systems [103].

SnO₂, another promising material, exhibits superior electron mobility compared to other semiconductors, enhancing charge separation and reducing recombination losses during photocatalytic reactions. This characteristic makes SnO₂ highly effective in heterojunction systems, where it is often paired with narrow bandgap materials like CdS or g-C₃N₄ to form Type-II heterojunctions or Z-scheme configurations. Such designs optimize charge carrier dynamics, improving efficiency in water splitting and pollutant degradation [104]. CdS, with its narrow bandgap (∼2.4 eV), absorbs visible light effectively, making it a strong candidate for sunlight-driven photocatalysis. Its use in hydrogen production is particularly notable, as it demonstrates excellent hydrogen evolution efficiency under solar irradiation. However, its stability under prolonged illumination is a limitation, as it suffers from photocorrosion. Strategies such as co-catalyst integration with noble metals, surface passivation, or coupling with stable semiconductors address this drawback [105,106].

g-C₃N₄, a metal-free polymeric photocatalyst, has emerged as an environmentally friendly alternative due to its tunable bandgap (∼2.7 eV), stability, and facile synthesis from inexpensive precursors. Its 2D structure facilitates efficient charge transfer and separation, critical for high photocatalytic efficiency. The ability of g-C₃N₄ to act as both a standalone photocatalyst and a component of heterostructures with materials like ZnO or TiO₂ further enhances its versatility. In water treatment, it effectively degrades organic pollutants, while in hydrogen production, coupling with co-catalysts or engineering surface defects has significantly improved its hydrogen evolution rates [107,108,109].

These materials are chosen based on a combination of their intrinsic properties, ease of modification, and compatibility with system designs for target applications. A balanced approach, leveraging their individual strengths and addressing their limitations, continues to drive innovations in photocatalytic systems.

In the realm of water treatment and hydrogen production, the academic community has shown significant interest in the application of Type II heterostructures, as evinced by numerous scholarly publications. Such junctions can be synthesized between a variety of materials including, but not limited to, metal oxides like ZnO, TiO₂, and SnO₂, metal sulfides such as CdS, WO₃, and carbon-infused structures like g-C₃N₄ [110,111,112,113,114,115]. An illustrative example would be the ZnO/SnO₂ heterostructure as shown in Figure 4.

Figure 4

Schematic representation of charge carrier behavior in type II heterostructure ZnO/SnO₂.

The enhanced photocatalytic efficiency witnessed in such a heterostructure can be attributed to the extended longevity of the charge carriers, a direct consequence of the electron/hole segregation towards divergent ends of the junction [116,117,118,119]. Specifically, electrons transition from the VB of ZnO to that of SnO₂, whilst holes undertake the inverse journey. This phenomenon accelerates the photocatalytic degradation kinetics of compounds, like methylene blue (MB), especially in the vicinity of such heterostructures.

4 Z-Scheme heterostructures

Venturing beyond the canonical types, the Z-Scheme heterostructure emerges as a variant bearing similar to the Type II configuration in terms of band alignments. Within a Z-Scheme configuration [120,121,122], the mechanism governing charge carrier migration deviates from its Type II counterpart. Electrons in the CB of semiconductor “B” are primed for recombination with holes originating in the VB of semiconductor “A,” as delineated in Figure 5.

Figure 5

Schematic representation of carrier recombination in (a) type II heterojunction, (b) Z-scheme heterojunction [120].

The Z-Scheme heterostructure paradigm can be dichotomized into three distinct categories: the liquid phase Z-Scheme, the all-solid-state Z-Scheme, and the direct Z-Scheme as shown in Figure 6. The historical chronology of these architectures traces back to 1979 with the inception of the liquid phase Z-Scheme [123,124]. Its synthesis involves the strategic amalgamation of two disparate semiconductors, using a shuttle redox mediator facilitated by an electron acceptor/donor (A/D) dyad, exemplified by pairs like Fe²⁺/Fe³⁺ or I⁻/IO₃ [125,126]. Subsequently, the all-solid-state Z-Scheme, a successor to the liquid phase variant, employs a solid electron mediator interposed between two semiconductors. Predominantly, this mediator assumes the form of noble metals such as Au, Pt, or Ag, manifested either as nanoparticles, thin films, or conductive carbon architectures encompassing graphene sheets, nanotubes, or QDs [127,128,129,130,131,132].

Figure 6

Schematic representation of different Z-schemes types.

The demarcation between a Type II heterostructure and a direct Z-Scheme stems from the electronic blueprint delineated by the Fermi level equilibrium between the semiconductors under study as shown in Figure 7. Comparatively, the heterojunction derived from ZnO and SnO₂, given the elevated Fermi level of SnO₂ [133,134], orchestrates a congruent band bending phenomenon, but the outcome manifests as a Type II heterostructure.

Figure 7

Schematic representation for the formation of direct Z-scheme and type II heterostructure between ZnO/SnO₂ and ZnO/g-C₃N₄ [138].

Indeed, akin to Type II heterostructures, the synthesis of a visible light-sensitive semiconductor in conjunction with an adept photocatalyst like TiO₂ or ZnO begets photocatalysts proficient under solar irradiation. Compounding this, Z-Scheme photocatalysts proffer an edge over their Type II counterparts, championing the retention of high-energy carriers within the lowest valence and highest CBs. Consequently, Z-Scheme heterostructures, such as TiO₂/g-C₃N₄ [135], ZnO/g-C₃N₄ [135], ZnIn₂S₄/Bi₂WO₆ [136], and ZnO/CdS [137], have been rigorously examined to ascertain their prowess in both photocatalytic water purification and hydrogen production through water dissociation.

5 Insights into photodegradation system dynamics

For a robust understanding of the photocatalytic paradigm, a profound grasp of the e⁻/h⁺ pair production, their interaction timelines with molecular targets, and their consequent recombination metrics are indispensable. This is pivotal for the judicious design of efficacious photocatalysts. The temporal dynamics of a typical photocatalytic operation span from milliseconds to a few seconds [138], as delineated in Figure 8. Generally, light absorption and e⁻/h⁺ pair diffusion ensue rapidly, within a temporal window of picoseconds. Subsequently, these charge carriers disperse within a timescale ranging from nanoseconds to microseconds. A plethora of materials demonstrate this same timescale for charge carrier recombination [139,140,141]. The redox processes, propelled by the migration of these carriers to the photocatalyst’s surface and synchronized with the water/pollutant interface interaction, manifest over milliseconds to seconds.

Figure 8

Timescale for photocatalytic water splitting.

The intricacies of the photodegradation mechanism, particularly carrier lifetimes and mobilities, are contingent on several parameters including chemical milieu, the energetic impetus for charge migration to reactant molecules, and the longevity of reactive intermediaries [142,143]. Contemporary methodologies, equipped to acutely characterize various kinetics and pathways integral to the process, have burgeoned in the recent past. These encompass optical spectroscopies, electron or scanning probe microscopies, transient absorption spectroscopy (TAS), and other state-of-the-art techniques. Notably, TAS leverages dual-energy sources wherein an initial pulse (typically a laser) stimulates atoms or electrons, and a subsequent, temporally delayed, and attenuated pulse quantifies the induced optical alterations [144]. This facilitates the computation of relaxation timelines for the generated species, with temporal resolutions intrinsically tied to the primary energy source [145]. The advent of cutting-edge laser technologies, which can generate pulses as brief as 10 fs, has significantly enhanced the temporal resolution of ultra-fast TAS, illuminating the incipient phases of photocatalytic operations.

6 Enhancing photocatalytic efficiency

Tremendous research has focused on enhancing efficiency through materials design, manipulation of electronic-optical properties, controlled nanostructuring, and composite fabrication. In this section, we will provide a comprehensive overview of the major strategies investigated to increase the efficiency of semiconductor photocatalysts including doping, heterojunctions, surface modifications, and deposition of cocatalysts [146].

6.1 Doping with metals and non-metals

Doping involves the intentional incorporation of small amounts of impurity atoms into the lattice of a semiconductor. This enables controlled manipulation of the electronic structure and optical properties to improve photoactivity. Important doping principles and approaches include bandgap narrowing where doping with metals or non-metals introduces intermediate states that decrease the bandgap promoting visible light absorption. Transition metals like Fe, Cr, and V are common dopants [147]. Also, Increased charge separation and Improved carrier mobility included where the ionic dopants create a space-charge field near the surface that induces charge carrier segregation suppressing recombination Such as common dopants W⁶⁺, Mo⁶⁺ and the co-doping with hetero-dopants like (N,F) or (N,S) in TiO₂ enhances both carrier density and mobility leading to more efficient transport [148,149,150].

Suppression of recombination and Oxygen vacancies were affected by the doping process where iso-electronic dopants like Y³⁺ and La³⁺ decrease local defects and traps resulting in lower charge carrier recombination rates and the metal ion dopants with lower oxidation states like Cr³⁺ and V⁴⁺ introduce oxygen vacancies that help trap electrons promoting separation [151,152].

However, excessive doping concentrations can increase recombination centers and inhibit photocatalytic activity. An optimal doping level that balances visible light absorption and charge carrier dynamics is required. Common doping levels range from 0.1 to 10 mol% [153].

6.2 Composite materials and heterojunctions

Composite materials combining different semiconductors, metals, or carbon nanomaterials as heterojunctions have emerged as an efficient approach to enhance photocatalytic performance and expand applicability [154,155]. Band alignment where Type II heterojunctions with staggered band energies promote charge carrier separation and migration to the interface due to built-in potential gradient while in visible light harvest, coupling UV-responsive photocatalysts like TiO₂ with narrow bandgap semiconductors like CdS or QDs enables broadband absorption [156,157,158,159,160].

Co-catalysis where integrating noble metals like Pt and Au introduces active sites for surface redox catalytic reactions like H₂ generation while vectorial transfer where nano-heterojunctions with 1D or 2D nanomaterials like graphene oxide establish vectorially migration pathways improve interfacial transfer but in synergism where hybrid nanocomposites combining photocatalysts with carbon nanotubes, metal-organic frameworks (MOFs), layered clays, etc., provide multifunctional attributes [161,162,163]. Key design criteria include optimal band alignment, epitaxial interfaces to minimize defects, suitable relative content, and dispersion. 1–10 wt% nanoscale cocatalyst loading is commonly employed. Excessive loading can increase recombination and reduce light penetration depth.

6.3 Surface modifications

Photocatalytic reactions occur at the semiconductor-electrolyte interface. Hence, surface modifications are extremely effective for enhancing activity. In co-catalysts where noble metal deposition enables Fermi-level equilibration providing catalytic sites for redox half-reactions. 1–5 wt% loading is optimal and in dye sensitization, dye molecules on the surface act as antennas improving visible light absorption and injecting electrons into the CB [59,160]. Ultra-thin porous zeolite or MOF membranes provide molecular sieving selectivity and mitigate electron-hole recombination used as selective membranes while coordination with ligands like polyvinyl propylene as a capping agent, citrate controls crystal growth and alters exposed reactive facets besides limiting aggregation.

Surface functionalization with −COOH, −NH₂, and −SH groups as acid-base sites create selective active sites for adsorption and charge transfer while graphene oxide coupling π–π stacking provides an extended π-conjugation network enabling better charge delocalization and separation [164,165,166,167,168].

Overall, rational surface and interface engineering provide a powerful tool kit for optimizing light harvesting, charge transfer, surface reactions, selectivity, and stability in photocatalyst systems.

In summary, strategies like doping, nanocomposite heterojunctions, controlled interfacial modifications, and cocatalyst deposition offer multiple complementary pathways for addressing the key limitations in photocatalytic efficiency. A multidimensional optimization and systems approach combining these various modulation principles promises to realize the full potential of semiconductor photocatalysis for renewable energy generation and sustainable environmental solutions [148].

7 Exploring the influence of operating parameters on photocatalytic degradation systems

Photocatalytic systems’ performance and efficiency in oxidation reactions can be influenced by a multitude of factors. This section provides an analytical overview of several critical parameters and their implications on photocatalytic processes [169,170].

7.1 Influence of pH on photocatalysis

The pH plays a pivotal role in governing the photocatalytic processes. It directly influences the ionization levels, the oxidation potential of the photocatalytic capacity band, agglomeration, and pollutant absorption [171,172]. Notably, the surface charge, which is integral to the photocatalytic process, varies with pH values. The pH value at which the surface charge neutralizes is termed by point of zero charge (PZC). Research using titanium dioxide (TiO₂) as a proxy has shown its PZC to be between 4.5 and 7, contingent upon its composition and type. At the PZC, electrostatic attractions between water contaminants and photocatalytic particles are at their nadir, given the absence of electrostatic forces. Titanium dioxide’s reactions concerning the PZC are delineated as follows [173,174]:

pH < pH PZC TiOH + H + → TiOH 2 + ,

pH < pH PZC TiOH + OH − → TiO − + H 2 O .

The photocatalytic efficiency of AgBr/Ag₂CO₃, synthesized via the in situ growth method under varied pH conditions was studied. Their findings underscored the superior photocatalytic performance of the AgBr/Ag₂CO₃ heterostructure compared to pure AgBr and Ag₂CO₃ [175]. They noted that pH variances, especially in the 7–10 range, influenced photocatalytic activity by affecting crystallization processes. Notably, at pH = 9 the AgBr/Ag₂CO₃ photocatalyst exhibited optimal photocatalytic efficiency, degrading 98.93% of rhodamine B (RhB) (initial concentration 5 mg L⁻¹) in a mere 20 min [175].

7.2 Role of temperature in photocatalytic reactions

Temperature is another salient factor affecting photocatalytic activity and pollutant degradation. While many photocatalytic reactions transpire at ambient temperatures, temperatures below 0°C tend to hinder the rate of desorption of the end product, thereby augmenting the activation energy. Conversely, elevating the reaction temperature beyond 80°C limits reactant adsorption [176,177]. Also, the impact of varying temperatures on the photocatalytic efficiency of Ag₂WO₄ nanorods in degrading methyl orange (MO) and MB. As temperatures rose, the nanorods’ structural attributes were altered, which enhanced their surface area and absorption rate. The stability and performance of these photocatalysts were rigorously assessed across multiple photocatalytic cycles [178,179].

7.3 Importance of oxidants in photocatalysis

External oxidants, such as H₂O₂, KBrO₃, and HNO₃, serve as irreversible electron acceptors, promoting the generation of intermediate radicals that expedite contaminant removal. These oxidants function by scavenging electrons from the VB, bolstering photocatalytic efficiency through: 1) curtailing electron/hole pair recombination time, 2) augmenting production of OH· to annihilate pollutants, and 3) generating oxidant species to amplify oxidation rates [7,180,181,182].

7.4 Implications of pollutant concentration on photocatalysis

Evaluating the interplay between pollutant concentrations and their photodegradation rates is paramount for the optimal photodegradation treatment systems design. Most contaminants’ degradation reactions align with pseudo-first-order kinetics, modifiable through the equation tailored for solid–liquid reactions (Langmuir–Hinshelwood equation).

(3) ln ( C / C 0 ) = k r K t = k 1 t .

In this relation, k ₁ signifies the first-order reaction coefficient, t represents the time span required for concentration reduction from the initial concentration (C ₀) to the final concentration (C) state, K denotes the equilibrium constant for adsorption of contaminant on the surface of the catalyst, and k _r is the reaction limiting rate [183].

For chromatic compounds, degradation might intensify with escalating color concentrations, but beyond a certain threshold, this rate diminishes. This decline can be attributed to the reduced UV radiation reaching the surface of the photocatalysts. Most studies have utilized pollutant concentrations ranging from 10 to 200 mg L⁻¹, analogous to typical concentrations observed in real wastewater samples [184,185,186].

Also, the photodegradation capabilities of MB, RhB, and methyl green were investigated [187]. This study also delved into Cr(vi) reduction in an aqueous solution utilizing a reusable magnetic catalyst, Fe³O⁴/reduced-graphene-oxide, under visible light irradiation. Among the key parameters analyzed was the initial concentration of pigment impact on the degradation rate. When maintaining a consistent photocatalyst concentration of 0.5 g L⁻¹ and a pH of 5, pigment molecule concentrations fluctuated between 8 and 5 × 10⁻⁴ mM. 0.1 mM as an initial concentration yielded an impressive degradation rate exceeding 98% for all the pigments studied. Interestingly, as the initial photocatalyst concentration augmented, the efficiency of the photodegradation saw a decline [187,188]. This observation suggests potential light penetration hindrance even at the minimum dye concentration.

7.5 Considerations for catalyst loading

Conceptually, augmenting the catalyst quantity should enhance the active sites available in the solution, leading to elevated photon absorption and a consequent boost in OH radical production and positive hole illumination. Also, an unwarranted catalyst concentration surge might inversely impact photocatalytic activities. The upsurge of the solution opacity and light scattering led to diminished photon absorption by the photocatalyst, which stands as a pivotal reason for this observed decline [189,190].

The influence of operational parameters on the photocatalytic degradation of MB employing urea-based graphite nitride carbonate (g-C₃N₄) [191,192]. Findings indicated that escalating the photocatalyst quantity from 0.01 to 0.05 g invigorated the photocatalyst’s activity, a phenomenon linked to the proliferation of active sites. Nonetheless, any further increase beyond 0.05 g curtailed the photocatalyst’s efficacy, primarily owing to diminished light accessibility [191,193,194].

7.6 Illumination intensity and spectral characteristics in photocatalytic systems

It has been previously postulated that the activation of a photocatalytic entity necessitates incident light possessing an energy that, at a minimum, corresponds to the energy defined by the band gap of the photocatalyst [195,196,197]. Such an energy threshold facilitates the generation of charge carriers, which in turn lead to the production of free radical’s instrumental in the abatement of contaminants. Consequently, the extent of degradation is significantly influenced by the light intensity. Moreover, the spatial distribution of illumination within a photocatalytic reactor critically dictates both the conversion efficacy of the contaminants and the resultant degradation magnitude. Numerous scholarly investigations have elucidated a linear correlation between the quantum of pollutant degradation and the intensity of illumination. Conversely, certain studies have proposed a linear relationship between degradation extent and the light intensity square. It is imperative to underscore that at elevated illumination intensities; the reaction kinetics tend to exhibit independence from light intensity variations [198,199].

Delving into empirical observations, the photodegradation rates of Dianix Blue and Vat Green 1 Dyes, in conjunction with various irradiation sources and in the presence of multi-walled carbon nanotubes (MWCNTs) as well as assorted MWCNTs/TiO₂ nanocomposites (with TiO₂ concentrations of 3, 6, and 10%), were found to be contingent upon the intrinsic energy characteristics of the employed irradiation sources [200]. Notably, amplification in the energy emanating from the irradiation source was concomitant with a heightened formation rate of charge carriers, specifically the electron-hole pairs. This phenomenon consequentially augmented the photocatalytic efficiency of the synthesized photocatalysts. Figure 9 provides a visual representation by depicting the pseudo-first-order linear plots, specifically ln(C ₀/C) versus irradiation duration, elucidating the photodegradation kinetics of Vat Green 1 Dye (panels a, c, and e) and Dianix Blue Dye (panels b, d, and f) under the influence of different light sources (sunlight, UV, and Xenon), respectively [201,202].

Figure 9

Pseudo first-order linear plots for the photodegradation of Vat Green 1 Dye (a, c, and e) and Dianix Blue Dye (b, d, and f) under different sunlight, UV, and Xenon irradiations, respectively [201,202].

There are many artificial photoreactor systems depending on their irradiation light sources (lamps) such as mercury, halogen, UV, visible, and Xenon. These photoreactors are used as simulators for the photodegradation process on a lab scale to optimize the experimental condition, and expecting the maximum photodegradation percentage can be applicable in the case of using solar reactors for wastewater and industrial wastewater treatment [203]. The efficiency of evaluation of this process is due to the photoreactor type and producing condition. Each type has different advantages and disadvantages than the other, but the Xenon photoreactor is considered the best one because it has the nearest light distribution (spectrum) to the sunlight than other photoreactors with different irradiation light sources and halogen lamps even with different filters farther than sun spectrum as shown in Figure 10.

Figure 10

Different irradiation light sources.

8 Optical and electronic factors influencing the light absorption characteristics

8.1 Bandgap

Bandgap dictates the light energy required for photocatalyst activation. Photocatalysts like TiO₂ with high bandgap (∼3.2 eV) can only utilize UV light [204]. Smaller bandgap semiconductors absorb visible light enabling solar spectrum utilization. Relative band positions decide the redox potentials for oxidation and reduction half-reactions. Straddling of CB and VB across water redox potentials enables water splitting. Oscillator strength – stronger oscillator strengths for electronic transitions increase molar extinction coefficient and light absorption while indirect bandgap semiconductors require phonon assistance for bandgap transitions. This makes them inefficient for light absorption and photocatalysis [205,206]. Overall, matching the photocatalyst bandgap to the target reactions, modifying the band energies through doping, and enhancing light harvesting through crystallinity and morphology modifications are important for optimizing light absorption and photocatalytic efficiency [7,16,207,208].

8.2 Surface area and particle size

The high surface area of nanoscale semiconductor particles maximizes the availability of surface-active sites for reactants and photon absorption cross-section per unit volume [209]. There are some key dependencies such as smaller particle size increases, surface area-to-volume ratio, and number of exposed active sites, but too small particles increase charge carrier recombination and specific crystal facets and surface defects act as active sites. Shape control selectively exposes reactive facets like [51] in anatase TiO₂ [210,211]. Optimal crystallite size balances the trade-off between higher surface area and lower recombination in larger nanoparticles, and porous and hollow nanostructures provide higher surface area and accessibility to active sites within internal cavities. Dispersion stability prevents aggregation, which reduces exposed area, and immobilization on substrates retains high exposed area [211,212,213,214]. Controlling photocatalyst size in the nanoscale, forming porous and hollow morphologies, and preventing aggregation are crucial for maximizing surface area and the number of active sites available for photocatalysis.

9 Applications of photodegradation systems

9.1 Photo-driven water splitting

The escalating global energy demand presents formidable challenges for contemporary and subsequent generations. Amidst the myriad strategies postulated as alternatives to fossil fuels, hydrogen stands out as a prime contender. However, a predominant obstacle remains: the natural paucity of gaseous hydrogen. Hence, sophisticated methodologies are requisite to efficiently and sustainably extract hydrogen from materials wherein it is entrenched. In this ambit, solar energy emerges as a propitious avenue for engendering eco-compatible hydrogen via the water-splitting process. Delving further, methodologies such as photocatalytic and photoelectrochemical water splitting are being extensively explored to harness solar energy for hydrogen production [215,216]. Also, there are many challenges for photocatalytic overall water splitting as shown in Figure 11 [217].

Figure 11

Challenges for photocatalytic overall water splitting [217].

9.2 Water purification and treatment

Purification of drinking water and treatment of industrial and domestic wastewater represent the most widely researched applications for semiconductor photocatalysis like TiO₂. There are many target pollutants such as dyes, phenolic compounds, pesticides, pharmaceutical residues, surfactants, and emerging water contaminants like perfluorinated compounds and microplastics. pH, dissolved oxygen, catalyst loading, light intensity and wavelength, pollutant structure, and concentration are influential parameters affecting water purification and treatment processes. Slurry reactors with suspended photocatalysts, immobilized catalyst films, fixed bed reactors, optical fiber reactors, and solar photoreactors are different reactor types used for different water purification and treatment processes [218,219,220].

Recently, visible light active photocatalysts like C, N co-doped TiO₂, graphene-based materials, and g-C₃N₄ enable the use of the solar spectrum. Novel reactors using LEDs and UV-transparent optical fibers provide efficient irradiation. Hybrid systems combining photocatalysis with other advanced oxidation processes like Fenton chemistry also enhance treatment kinetics. Developing stable visible light photocatalysts, integrating with renewable energy sources and artificial intelligence for smart optimization remain important directions for sustainable decentralized water purification [221,222,223].

Biopolymer-supported photocatalysts represent an emerging class of materials that align with the principles of green chemistry and sustainability, offering significant potential for wastewater treatment. Biopolymers such as cellulose, chitosan, and alginate are naturally abundant, biodegradable, and non-toxic, making them ideal candidates for supporting photocatalytic materials. These biopolymers act as substrates or matrices for anchoring photocatalysts, enhancing their stability, dispersion, and reusability in aqueous environments. Their hydrophilicity and porosity enable efficient adsorption of pollutants, which enhances the photocatalytic degradation process [224]. Furthermore, their functional groups, such as hydroxyl and amino groups, provide active sites for the immobilization of photocatalysts through chemical bonding or physical interactions.

One of the most studied biopolymer-supported photocatalysts is chitosan–TiO₂ composites. The combination of chitosan’s biocompatibility and TiO₂’s photocatalytic properties results in a synergistic system where the biopolymer not only stabilizes the photocatalyst but also aids in the adsorption of organic pollutants from water. This enhances the efficiency of TiO₂ in degrading contaminants under UV or visible light. Modifications such as doping TiO₂ with metal or non-metal elements and incorporating co-catalysts have further improved its performance in these composites [225].

Similarly, cellulose, being a renewable and abundant biopolymer, has been extensively used to support photocatalysts like ZnO and g-C₃N₄. Cellulose-based composites exhibit excellent mechanical strength and flexibility, allowing their application in various reactor configurations for continuous water treatment systems. For instance, cellulose-supported g-C₃N₄ has shown remarkable photocatalytic activity in degrading dyes and pharmaceutical pollutants under visible light, attributed to the uniform distribution of g-C₃N₄ on the cellulose matrix and the enhanced charge separation achieved in the system [226].

Alginate, another widely utilized biopolymer, is particularly advantageous for encapsulating photocatalysts due to its gel-forming ability in aqueous environments. Alginate-supported ZnO and CdS photocatalysts have been reported to achieve high efficiency in wastewater treatment applications. The porous structure of alginate beads ensures maximum exposure of the photocatalyst to light and pollutants, while the biopolymer matrix prevents agglomeration and leaching of the photocatalyst, thereby improving its durability and reusability [227].

Biopolymer-supported photocatalysts offer a sustainable and effective approach to addressing the limitations of conventional systems, such as low stability, aggregation, and difficulty in recovery. By combining the adsorption capabilities of biopolymers with the photocatalytic activity of semiconductors, these composites provide a multifunctional solution for wastewater treatment. Future research should focus on optimizing the synthesis, scalability, and integration of biopolymer-supported photocatalysts into industrial processes to advance their practical applicability.

9.3 Industrial wastewater treatment

A wide spectrum of toxic organic pollutants is discharged in wastewater from chemical, petrochemical, pharmaceutical, textile, paper, pesticide, and mining industries. Conventional biological processes are often inadequate for these complex and recalcitrant effluents. Photocatalysis is being increasingly applied for pre-treatment, primary treatment, and polishing of such industrial wastewater. There are many target pollutants such as dyes, aromatic compounds, phenols, amines, cyanides, sulfides, and nitrates originating from specific industries based on their manufacturing processes [218,228]. Wastewater matrix complexity, pH, temperature, catalyst selection, hydraulic residence time, and process integration affect treatment performance. There are many reactors used to treat industrial wastewater such as solar photocatalysis, thin film slurry reactors, sequencing batch reactors, and flow photoreactors. Hybridization with other chemical oxidation processes is also beneficial (Figure 12) [228,229,230].

Figure 12

Photocatalytic degradation diagram of MB and indigo carmine dyes in the presence of ZrO₂/ZnO.

Recently, selective photocatalysts, viable solar utilization through compact reactor design, real-time optimization through sensors, and artificial intelligence integration are emerging. Compliance with stringent regulatory discharge limits while minimizing energy, chemical, and operational costs is driving innovations in industrial wastewater photocatalysis.

Industrial wastewater treatment using photocatalytic technologies has gained momentum due to its ability to degrade a wide range of recalcitrant organic and inorganic pollutants. Specific industries such as textiles, petrochemicals, pharmaceuticals, and food processing generate complex effluents that often contain dyes, phenols, heavy metals, and persistent organic pollutants, making traditional treatment methods insufficient. Photocatalytic technologies provide a sustainable solution by utilizing semiconductor materials to achieve complete mineralization of these contaminants under light irradiation.

In the textile industry, effluents often contain high concentrations of synthetic dyes, which are non-biodegradable and toxic to aquatic ecosystems. Titanium dioxide (TiO₂)-based photocatalytic systems have been widely applied for dye degradation. For instance, TiO₂ immobilized on glass beads or membranes effectively degrades azo dyes like MO and indigo carmine under UV light, achieving over 90% removal efficiency within hours [231]. Advanced configurations like TiO₂-graphene oxide composites have further enhanced visible light absorption and degradation rates in textile wastewater [232].

In the petrochemical industry, wastewater often contains aromatic hydrocarbons and phenolic compounds, which are highly toxic and resistant to biological degradation. Zinc oxide (ZnO)-based photocatalysts have been employed to treat these effluents. ZnO nanoparticles supported on silica or alumina exhibit high efficiency in degrading phenolic contaminants, owing to their superior oxidation potential and stability [233]. Additionally, hybrid systems combining photocatalysis with Fenton oxidation or ozonation have demonstrated synergistic effects, significantly improving treatment kinetics [234].

Pharmaceutical wastewater is another major challenge, containing active pharmaceutical ingredients, antibiotics, and endocrine-disrupting compounds. Graphitic carbon nitride (g-C₃N₄) photocatalysts have been successfully utilized to degrade drugs like ibuprofen (IBP) and carbamazepine (CBZ). The addition of co-catalysts like gold nanoparticles to g-C₃N₄ has further enhanced its activity, achieving over 95% removal of pharmaceutical residues under visible light [235]. For complex effluents, Z-scheme photocatalysts like ZnO/g-C₃N₄ and CdS/TiO₂ have shown remarkable efficiency in the simultaneous degradation of multiple pollutants [236].

In the food processing industry, wastewater contains organic matter, oils, and nitrogen compounds, which can lead to eutrophication if untreated. Biopolymer-supported photocatalysts, such as alginate-ZnO beads, have been applied to degrade these contaminants effectively while ensuring easy recovery and reuse of the catalyst [224].

9.4 Harnessing solar energy

The sun, functioning as an intrinsic nuclear dynamo, disseminates quantized energy units termed photons. These quants embody formidable energy potential, sufficient to satiate a substantial fraction of Earth’s energy requirements. Photocatalytic materials are judiciously employed in diverse configurations to capture this photon energy, thereby facilitating electricity generation in solar cells [27,237,238,239]. The scientific community has ushered in a plethora of solar cell variants including organic, dye-sensitized, photoelectrochemical, and hybrid constructs as modalities to leverage solar energy.

9.5 Photocatalytic reduction of carbon dioxide

The deleterious consequences of fossil fuel combustion, predominantly manifesting as CO₂ emissions a potent greenhouse gas have been incontrovertibly acknowledged. This gas impedes the outward radiation of terrestrial heat, thus exacerbating global warming.

Consequently, a slew of endeavors is being orchestrated to attenuate atmospheric CO₂ concentrations, encompassing strategies like sequestration, storage, and valorization of CO₂. Transmuting CO₂ into value-enhanced chemicals emerges as an especially enticing approach to mollify its environmental burden [240,241]. Nevertheless, a paramount challenge ensues from the intrinsic stability of CO₂ and the robust energy profile of the CO bond, with a bond enthalpy of 805 kJ mol⁻¹. In this context, the application of photocatalysts as a remedy to this conundrum has galvanized significant research interest [242]. Typically, this modality entails a tripartite process: photon absorption, charge carrier differentiation, and transference, followed by reductive reactions (Figure 13).

Figure 13

Artificial photosynthesis scheme for the CO₂ reduction on photocatalysts (left side) and the conversion of absorbed carbon contaminations by the photogenerated charges on Photocatalysts (right side).

9.6 Air purification and VOC removal

Indoor air pollution caused by VOCs like aromatics, aldehydes, and amines possesses significant environmental and health risks. Photocatalytic oxidation using nanocrystalline TiO₂ and WO₃ offers an energy-efficient approach for purifying indoor air environments. Some of these target VOCs are formaldehyde, benzene, toluene, xylene, acetaldehyde, and trichloroethylene which originate from paints, adhesives, furniture, cleaners, and automotive exhaust. Humidity, light intensity, catalyst content, air flow rate, VOC molecular structure, and inlet concentration are influential factors effecting the air purification and VOC removal processes. Some of the configurations for these processes are air circulation chambers coated with photocatalyst thin films and hybrid designs combining photocatalysis with other air purification technologies like ozonation [243,244,245].

Recently, doped visible light-activated TiO₂ and WO₃ enable complete mineralization of VOCs under indoor ambient light. Novel catalyst deposition methods like electrophoretic deposition improve film quality and performance. Integrating air photocatalysis systems into building HVAC systems using renewable energy sources and internet of things-based monitoring for intelligent control represents a major opportunity for healthier indoor environments are considered future outlooks for air purification and VOC removal processes [246].

9.6.1 Phenolic contaminants

Traditional methodologies, encompassing adsorption via activated carbon, membrane-based filtration, and ion exchange when applied to the abatement of phenolic impurities, frequently culminate in a concentrated effluent stream, necessitating subsequent remediation phases [247]. Such ancillary interventions invariably augment the operational cost and concomitant ecological implications. In a contemporary context, the avenue of photocatalytic oxidation has emerged as a propitious stratagem in addressing these contaminants (Figure 14) [248].

Figure 14

Synergistic effect of membrane adsorption and laccase degradation for phenolic pollutants by MWCNTs-LCEFMs [248].

In an auxiliary exploration, MWCNT-augmented laccase-embedded electrospun fibrous membranes (MWCNTs-LCEFMs) were harnessed for the remediation of ubiquitous phenolic entities in aquatic systems, namely triclosan (TCS), bisphenol A (BPA), and 2,4-dichlorophenol (2,4-DCP). The abatement efficacies of MWCNTs-LCEFMs for TCS, BPA, and 2,4-DCP were discerned at 99.7 ± 0.02, 95.5 ± 0.46, and 92.6 ± 0.74%, in tandem with degradation rates of 85.6 ± 1.5, 90.5 ± 1.1, and 81.7 ± 1.9%, respectively [51,249].

9.6.2 Nitrogen-based contaminants

The presence of nitrogenous entities poses ecological challenges, underscored by their robust stability, water solubility, and their propensity to engender eutrophication, given their nutrient attributes. The synthesis and photocatalytic assessment of La/Fe/TiO₂ in the remediation of wastewater enriched with ammonia-based nitrogen was investigated [250]. La/Fe/TiO₂ was ingeniously crafted via sol–gel technique, and its resultant constitution exhibited superior chemical and physical attributes in photocatalytic prowess compared to pristine TiO₂. Noteworthy enhancements included an amplified visible light receptivity, an augmented surface interactivity, and a more geometrically defined morphology [251,252]. The empirical outcomes pertaining to ammonia nitrogen’s optical decomposition underscored the heightened catalytic activity of La/Fe/TiO₂ vis-à-vis both pristine TiO₂ and singularly TiO₂ doped with a metal. The denouement revealed that the tailored catalyst could effectively mitigate upwards of 70% of ammonia nitrogen, starting from 100 mg L⁻¹ as an inaugural concentration.

9.6.3 Sulfur-containing compounds

Sulfur-based compounds manifest properties that are inherently toxic, corrosive, and characterized by a distinct odor, engendering potential ecological perturbations. Within the gamut of industries, notably within the oil and petrochemical sectors, effluent streams enriched with sulfur derivatives emerge predominantly from desulfurization procedures. Elevated sulfide concentrations in wastewater precipitate a decline in dissolved oxygen levels, thereby imperiling aquatic biota [253,254]. Concurrently, the refining of sulfur-laden crude necessitates substantial hydrogen quantities. To address these intertwined challenges, innovative photocatalytic strategies exhibiting a commendable oxidation potential (+2.8 V) have been put forth. One such stratagem entails the doping of Cerium (Ce³⁺) onto titanium dioxide (TiO₂) particulate, synthesized via the sol–gel methodology, culminating in a narrowing of the band gap of TiO₂, transitioning from approximately 3.2 eV to a more favorable 2.7 eV, situated within the visible light spectrum. The efficacy of this composite was rigorously assessed in the context of sulfidic wastewater treatment.

9.6.4 Pharmaceuticals in effluents

Pharmaceutical residues, discerned within municipal wastewater, typically span a concentration gradient from nanograms to micrograms per liter [230]. While these drug moieties resist facile degradation, their relatively minuscule concentrations in typical wastewater do not unduly strain conventional treatment infrastructure. Nonetheless, effluents specifically emanating from pharmaceutical enterprises present a unique challenge due to augmented total organic carbon levels.

Photocatalytic interventions have been robustly advocated as potent remedies for these contaminants [255,256,257]. In an investigative tangent, the attenuation of IBP and CBZ in aqueous matrices employing TiO₂ and ZnO photocatalysts under both UV and visible irradiation was meticulously examined [256,258].

9.6.5 Agrochemical contaminants

Agrochemical derivatives, particularly pesticides, epitomize prominent pollutants within the agricultural sector. ZnO films have been identified as efficacious photocatalysts in the degradation of Temephos pesticide and its oxidation derivatives [259]. The NW morphology ostensibly amplifies the photocatalytic proficiency, attributed to an augmented surface interface, enhanced defect densities facilitating the capture of low-energy photons, and the synergy with gold nanoparticles, potentially inducing surface plasmon resonance. Collectively, these factors magnify the photocatalytic throughput under simulated solar illumination (Figure 15) [228,260].

Figure 15

Photocatalytic effect of ZnO on mineralization of temephos pesticide [260].

9.7 Contemporary recent applications in photodegradation systems

9.7.1 Promising photocatalysts

Also, in addition to the mainstay metal oxide semiconductors like TiO₂ and ZnO, a variety of other nanocrystalline semiconductor materials have shown promise as photocatalysts including WO₃, Fe₂O₃, and BiVO₄ as stable visible light responsive semiconductor photocatalysts for O₂ generation and organic oxidation and CdSe, Cu₂O and Ag₃PO₄ as sensitizer semiconductors to form heterojunctions with TiO₂ and ZnO for improved visible light harvesting [261]. GaP, GaN, and InP as III–V semiconductors were investigated for visible light water splitting and CuO and SnO₂ as developed as composite cocatalysts with TiO₂ to provide efficient interparticle charge transfer. Also, BiOX (X = halogen such as Cl, Br, I): Layered semiconductor photocatalysts combining Bi³⁺ with halides that show visible light activity for organic oxidation. While no single semiconductor matches the optimal bandgap, charge mobility, visible light absorption, and stability characteristics required for overall practical efficiency, combining different semiconductors as nanocomposites provides a route for the rational design of improved photocatalysts (Figure 16).

Figure 16

Graphitic carbon nitride precursor types.

9.7.2 Graphitic carbon nitride

An emerging class of metal-free polymeric photocatalysts is graphitic carbon nitrides, denoted as g-C₃N₄. They possess appealing properties including visible light response from a tunable bandgap of 2.7–2.8 eV based on heptazine subunits and decent charge separation and transport due to 2D conjugated structure. Also, strong oxidation power to degrade organics and split water under visible light. The 2D layered structure also allows easy integration with other nanomaterials to form composite photocatalysts. Limitations include fast recombination, low solar energy utilization, and insufficient reduction of power. These are being addressed through doping, defect engineering, plasmonic particles, and coupling with redox cocatalysts. The stability, visible light activity, and potential for scalable synthesis from cheap precursors make g-C₃N₄ a promising sustainable photocatalyst for continued research and development [262,263].

9.7.3 Perovskite photocatalysts

Organometal halide perovskites like CH₃NH₃PbI₃ have attracted tremendous interest as solar cell absorbers [264]. Recently, their application as photocatalysts has also emerged. Notable aspects include strong visible light absorption with a tunable bandgap around ∼1.5 to 2.3 eV, long charge carrier lifetimes, and diffusion lengths which provide efficient charge separation and strong reduction power from electrons. Also, oxidation power from holes, low-temperature solution-based synthesis, and structural flexibility are considered an interesting and notable aspect. Perovskites have shown the photocatalytic capability for dye degradation, water splitting, CO₂ reduction, and nitrogen fixation under visible NIR light activation. However, issues like toxicity, instability to humidity/heating, and lack of standard optimized synthetic procedures remain key challenges that need to be addressed before large-scale practical application.

9.7.4 QD-mediated photodegradation mechanisms

QDs as photocatalysts present a departure from conventional bulk semiconductors due to their distinct valence and CB positions. The optical-electronic characteristics of QDs are dictated by unique quantum principles, predominantly influenced by their diminutive dimensions that often align with their Bohr radii, typically being less than 10 nm. Beyond materials traditionally recognized as semiconductors, specific conductive materials, such as carbon and graphene, manifest electronic energy gaps in the visible spectrum (1.8–3.1 eV) as a consequence of quantum confinement effects.

An intriguing aspect of QDs is the capacity for fine-tuning their energy gap via size modulation proximal to the Bohr radius, affording customizable light absorption characteristics. The dynamics of excitons within QDs can also be altered by size variations. QDs have garnered significant attention in the realm of photocatalysis, elucidating opportunities for optimal energy level alignment with redox potentials of participating species in electrolytes. The incorporation of QDs in photocatalytic systems introduces several advantages, including:

Elevated surface area versus volume ratios.
Modifiable bandgaps facilitating efficient light absorption.
Tailorable band edge positions for specific chemical reactions.
Minimal particle size dimensions ensuring the short charge transfer distances and potentially minimizing the recombination process.

In a broader application context, titanium dioxide and zinc oxide QDs exhibit enhanced photocatalytic capabilities compared to their nano oxide counterparts, offering superior degradation rates for several industrial dyes as depicted in Figures 17 and 18. Notably, they demonstrate a heightened efficacy for dyes conventionally challenging to degrade using nano oxides, such as Dianix Blue Dye and Reactive Yellow 145 [265,266].

Figure 17

The photodegradation mechanism of Dianix Blue Dye in the presence of ZnO QDs [265].

Figure 18

Separation and transfer of photogenerated charges scheme for photodegradation of Reactive Yellow 145 dye in the presence of TiO₂ QDs [266].

9.7.5 QD-mediated self-cleaning

Simultaneously, QD-imbued photocatalysts are gaining traction in the self-cleaning domain, envisaged as coatings for various substrates ranging from textiles and woods to metals and photovoltaic (PV) cells, impacting both the efficiency and longevity of such panels as elucidated in accompanying Figure 19 (Table 1) [267].

Figure 19

Solar cells coated with self-cleaning materials.

Table 1

Some of the different photocatalytic materials and highlighting their distinctive applications

Photocatalytic materials	Distinctive application	Ref.
GaP, GaN, and InP	Water splitting	[215,216]
TiO₂, C, N co-doped TiO₂ and g-C₃N₄	Water purification and treatment	[218,219,220]
ZnO and ZrO₂/ZnO	Industrial wastewater treatment	[228,229,230]
TiO₂ and WO₃	Air purification and VOC removal	[243,244,245]
MWCNTs	Phenolic contaminants removal	[51,249]
La/Fe/TiO₂	Nitrogen-based contaminants removal	[243,244,245]
Ce³⁺/TiO₂	Sulfur-based compounds removal	[243,244,245]
TiO₂ and ZnO	Pharmaceuticals effluents treatment	[243,244,245]
ZnO (NW)	Agrochemical contaminants removal	[243,244,245]
CdSe, Cu₂O, and Ag₃PO₄	Sensitizer semiconductors	[243,244,245]
CH₃NH₃PbI₃	Solar cells	[264]
QD oxides	Photodegradation of organic pollutants	[265,266]
QD-imbued photocatalysts	Self-cleaning	[267]

10 Conclusion

The advancement of photocatalytic materials represents a critical scientific frontier, with profound implications for environmental remediation, energy conversion, and sustainable industrial processes. This manuscript presents an extensive review of the fundamental principles, material advancements, and emerging applications of photocatalytic technologies, emphasizing their potential in photodegradation systems and renewable energy applications.

A central focus of this study is the mechanism governing photocatalytic reactions, which involves charge carrier generation upon light absorption, followed by electron-hole separation and migration to catalytic sites. The efficiency of this process is often hindered by rapid charge recombination, necessitating strategies such as heterojunction formation, element doping, and surface functionalization. The incorporation of metallic and non-metallic dopants has been extensively explored, demonstrating significant improvements in charge carrier lifetime and catalytic efficiency. Notably, Z-scheme heterostructures have emerged as a promising approach to enhance charge separation while maintaining strong redox potentials.

This review also highlights the critical role of semiconductor materials such as TiO₂, ZnO, g-C₃N₄, and QDs, whose photocatalytic properties can be tuned through bandgap engineering and defect manipulation. The data presented underscore the importance of selecting materials with optimal band positions, allowing for efficient light absorption and charge transport. In particular, QDs have demonstrated superior photoresponse properties, enabling high-efficiency degradation of industrial pollutants such as Dianix Blue Dye and Reactive Yellow 145, achieving over 90% degradation rates under controlled experimental conditions.

The practical applications of photocatalysis in wastewater treatment have been explored through pilot studies conducted in the textile, pharmaceutical, and industrial sectors. The use of ZnO QDs and doped SnO₂/NiO nanoparticles in treating real industrial effluents has yielded promising results, with chemical oxygen demand reduction efficiencies exceeding 85% in some cases. Additionally, biopolymer-supported photocatalysts, such as chitosan-TiO₂ composites, have demonstrated enhanced stability and recyclability, making them viable for continuous wastewater treatment systems. The ability of g-C₃N₄-based heterostructures to achieve near-complete mineralization of pharmaceutical contaminants such as IBP and CBZ further underscores the applicability of these materials in advanced oxidation processes.

The review also discusses the integration of photocatalysis with other advanced technologies, including photoelectrochemical cells, bioelectrochemical reactors, and hybrid ozonation systems. These synergistic approaches have been found to significantly enhance degradation kinetics, particularly for recalcitrant pollutants such as nitrogen- and sulfur-containing organic compounds. Photo-bioelectrochemical cells integrating photocatalysts with microbial fuel cells have also been proposed as a novel avenue for sustainable wastewater treatment, leveraging the ability of microbial consortia to assist in pollutant breakdown while simultaneously generating electrical energy.

Another crucial aspect discussed in this manuscript is the scalability and commercialization of photocatalytic systems. The successful deployment of solar-driven wastewater treatment plants in Egypt highlights the feasibility of large-scale photocatalytic applications. The new & renewable energy authority has initiated several projects, including solar water heating installations for 30 hotels, with an annual operational cost of approximately $500,000. Additionally, the Benban Solar Park, one of the largest PV power stations in the world, has contributed significantly to Egypt’s renewable energy portfolio, producing 3.8 TW h annually. These real-world implementations demonstrate the economic and environmental viability of photocatalysis-driven processes.

Despite these advancements, several challenges remain in the widespread adoption of photocatalytic technologies. Material degradation over extended use, loss of catalytic activity due to fouling or deactivation, and high synthesis costs for engineered nanomaterials are major barriers that must be addressed. Furthermore, reaction kinetics in real wastewater matrices are often more complex than in laboratory conditions, necessitating a better understanding of operational parameters such as pH, temperature, and light intensity. This study also underscores the importance of computational modeling and machine learning in optimizing material properties and reaction conditions, allowing for the rapid discovery of next-generation photocatalysts.

Looking forward, several key research directions must be pursued to advance the field of photocatalysis. These include:

Development of Earth-abundant photocatalysts: The reliance on rare or toxic metals remains a challenge, necessitating the exploration of cost-effective, environmentally benign alternatives.
Enhancement of visible-light activity: Many current photocatalysts are primarily UV-active, limiting their practical applications. Doping, plasmonic enhancement, and heterojunction engineering will play a crucial role in extending light absorption into the visible and near-infrared regions.
Integration with renewable energy technologies: The coupling of photocatalysis with PV and electrochemical systems holds great potential for enhanced energy conversion efficiency and sustainability.
Advanced reactor designs: The development of thin-film, packed-bed, and LED-driven photocatalytic reactors will be critical for achieving scalable industrial applications.
Techno-economic assessments: Comprehensive life-cycle analyses are required to assess the economic viability and environmental impact of large-scale photocatalytic systems.

Photocatalysis remains a transformative technology with the potential to revolutionize environmental remediation, renewable energy production, and sustainable chemical synthesis. The advancements highlighted in this study – from material innovations to real-world applications – underscore the increasing relevance of photocatalysis in addressing global sustainability challenges. By integrating multidisciplinary research efforts, optimizing material properties, and improving scalability, photocatalytic technologies can play a pivotal role in shaping a cleaner, more sustainable future.

Acknowledgments

The authors extend their appreciation to the Deanship of Research and Graduate Studies at King Khalid University for funding this work through a Large Research Project under grant number RGP2/180/45.

Funding information: This work was funded by the Deanship of Research and Graduate Studies at King Khalid University through a Large Research Project under grant number RGP2/180/45.
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.
Data availability statement: Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

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Received: 2024-08-24

Revised: 2025-02-10

Accepted: 2025-03-16

Published Online: 2025-04-09

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/ntrev-2025-0159

Keywords for this article

sustainable development; photocatalytic materials; photocatalytic degradation systems; water splitting; water treatment; quantum dots

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