Photocatalytic Fibers for Environmental Purification: Challenges and Opportunities in the Post‐Pandemic Era

The COVID‐19 pandemic has underscored the paramount importance of maintaining clean and safe environments. Strengthening environmental purification measures is pivotal for better preparedness and resilience against future health crises. Photocatalytic fibers, due to their versatility and adaptability to various application scenarios, play a pivotal role in the field of environmental purification. In this perspective, light is shed on the pivotal advancements in employing photocatalytic fibers to tackle environmental issues encompassing dye degradation, antibiotics decomposition, heavy metal removal, indoor air purification, and microbial disinfection. These applications hold significant promise in promoting cleaner and healthier environments while aligning with sustainability objectives. However, the implementation of photocatalytic fibers in practical scenarios is not without its challenges. Issues, such as near infrared responsiveness, all‐day active capacity, reusability, and scalability, must be addressed to ensure their reliability and long‐term effectiveness. Potential solutions for surmounting these challenges are also delved, aiming to provide a comprehensive overview for researchers and stakeholders. Exploring new avenues and innovative approaches may pave the way for the widespread deployment of photocatalytic fibers in environmental purification, especially in the post‐pandemic era where the need for clean and safe environments is more evident than ever.

The COVID-19 pandemic has underscored the paramount importance of maintaining clean and safe environments.Strengthening environmental purification measures is pivotal for better preparedness and resilience against future health crises.Photocatalytic fibers, due to their versatility and adaptability to various application scenarios, play a pivotal role in the field of environmental purification.In this perspective, light is shed on the pivotal advancements in employing photocatalytic fibers to tackle environmental issues encompassing dye degradation, antibiotics decomposition, heavy metal removal, indoor air purification, and microbial disinfection.These applications hold significant promise in promoting cleaner and healthier environments while aligning with sustainability objectives.However, the implementation of photocatalytic fibers in practical scenarios is not without its challenges.Issues, such as near infrared responsiveness, all-day active capacity, reusability, and scalability, must be addressed to ensure their reliability and long-term effectiveness.Potential solutions for surmounting these challenges are also delved, aiming to provide a comprehensive overview for researchers and stakeholders.Exploring new avenues and innovative approaches may pave the way for the widespread deployment of photocatalytic fibers in environmental purification, especially in the post-pandemic era where the need for clean and safe environments is more evident than ever.
removal, [14] indoor air purification, [15] and microbial disinfection. [16,17]he emergence of the COVID-19 pandemic in 2019 has had a profound and far-reaching global impact.This highly contagious virus spreads primarily through close-range aerosols and respiratory droplets generated during activities like coughing, sneezing, and even normal breathing. [18]According to data from the World Health Organization as of September 2023, COVID-19 cases have been reported in 233 countries and regions, resulting in a staggering 770 million infections and tragically, 6.87 million fatalities.Unfortunately, these numbers continue to rise at an alarming rate.So far, wearing masks and maintaining social distance remain the most effective self-protection measures.Among the various types of masks, surgical masks, which incorporate a layer of polypropylene fabric for effective particle and droplet filtration, are the primary and most commonly used option.The polypropylene fabric is typically electrostatically charged to create a negative surface charge, enhancing the mask's filtration efficiency.However, it is important to note that masks should not be reused beyond a maximum of five times and must be replaced daily.This is because their filtration efficiency for bacteria, viruses, and other droplets significantly diminishes over time.Consequently, the disposal of a large number of used masks, often through incineration, poses significant environmental challenges.Furthermore, the issue of used masks from infected individuals presents a dual problem, i.e., plastic pollution and the potential for secondary infections due to improper handling.In response to these challenges, Li et al. have developed a novel fiber material incorporating a photocatalyst. [19]This material can be used to create masks that can be effectively sterilized with just 10 min of exposure to light.Upon light irradiation, these masks can be safely reused and eventually discarded, thus conserving raw materials used for mask production, preventing the risk of secondary infections from contaminated masks, and reducing the environmental impact associated with the disposal of used masks.In the context of the ongoing efforts to combat the spread of COVID-19, the development of versatile photocatalytic fibers capable of performing microbial disinfection is indispensable.Such innovations hold the potential to address pressing issues related to mask usage, disposal, and environmental sustainability during the pandemic and beyond.
Over the past decade, a great deal of strides has been made in developing innovative approaches to enhance the efficiency of photocatalytic fibers.A cursory search on the Web of Science using just two keywords, "photocatalytic" and "fiber," reveals a remarkable surge in the number of published articles and citations received over the last 10 years, as depicted in Scheme 1.The clear and substantial growth in both publications and citations since 2013 underscores the growing importance and impact of photocatalytic fiber systems.While some reviews have summarized recent developments in photocatalytic fibers, [20][21][22] there remains a dearth of instantaneous and comprehensive retrospectives on this subject.The present juncture presents an opportune moment to conduct an assessment of the most pivotal research achievements in this emerging field to bridge existing knowledge gaps.In this perspective, we introduce the critical advancements in photocatalytic fibers, particularly focusing on their advanced applications in environmental purification.We delve into the challenges and prospects associated with photocatalytic fibers to provide researchers with a comprehensive overview and inspire the exploration of new avenues for their widespread adoption in environmental purification, especially in the postpandemic era.

Synthesis and Optimization
Photocatalytic fibers leverage the principles of photocatalysis, wherein certain semiconductor materials, upon exposure to light of adequate energy, can generate electron-hole pairs.These transient charged species can then participate in redox reactions with pollutants, converting them into less harmful or inert substances. [3]Utilized in the form of fibers, these photocatalysts provide an extended surface area for enhanced light absorption and pollutant degradation and removal.Such systems are particularly promising for environmental purification, offering a sustainable approach to detoxify pollutants in air and water.However, for effective real-world applications, it is crucial to optimize these systems.Factors, such as the synthetic methods of photocatalytic fibers and the compositions of composite photocatalysts, both influence photocatalytic efficiency.Properly optimized photocatalytic fiber systems can maximize pollutant removal rates, reduce energy consumption, and extend the operational life of the fibers, ensuring more efficient and economical environmental purification.

Synthetic Approaches
The fabrication strategy of photocatalytic fiber systems is crucial in determining their physical and chemical properties, which, in turn, affect their application in environmental purification.Synthetic methods for photocatalytic fibers can be broadly categorized into two approaches, i.e., top-down and bottom-up approaches.The top-down approach, also known as surface decoration, involves applying pre-synthesized photocatalysts onto existing fibers.This method capitalizes on the hierarchical pore structure and large surface area of the fibers to achieve high photocatalyst loading while maintaining the flexibility and stability of the fibers.The abundance of voids and pores enhances the interaction between the fibers and pollutants.Popular top-down synthetic techniques include hydrothermal methods, [23] coating methods, [24] in-situ growth, [25] and layer-by-layer assembly. [26]hese methods are cost-effective and do not necessarily require specialized equipment.They allow for precise control of crystal type, size, morphology, and surface area.However, challenges remain, including ensuring strong adherence between the fibers and photocatalysts to prevent shedding, as well as addressing the potential decline in fiber properties when using aggressive solvents or under operation at high pressures and high temperatures.
In contrast, the bottom-up approach integrates photocatalyst powders into the fiber preparation process or simultaneously forms them with fibers.This approach results in fibers with embedded photocatalyst particles, ensuring better adhesion and thereby, improved stability compared to the top-down method.Furthermore, the structure, porosity, morphology, and diameter of the fibers can be more easily modulated, offering possibilities for optimizing the resultant photocatalytic properties.The main techniques in the bottom-up approach include electrospinning, [27] casting method, [28] and co-extrusion. [29]lthough some of these techniques necessitate specialized equipment, they offer advantages, such as a wide range of usable raw materials, simplicity in production, and scalability.Challenges for this approach include the potential for photocatalyst aggregation within the fibers, which can reduce their effectiveness and brittleness in fibers with a high catalyst content.

Composite Photocatalysts
At the forefront of research, there has been an intensified focus on the engineering of composite photocatalysts to optimize photocatalytic efficiency.[32] This section delineates the fundamental design principles of these composites for performance optimization.Metal-semiconductor composites are distinguished by their enhanced charge separation and plasmonic augmentation.[35][36][37] Certain metals exhibit localized surface plasmon resonance (LSPR), [38][39][40][41] amplifying light absorption and thus, elevating photocatalytic activity.Nonetheless, only certain wavelengths can be harnessed due to the restricted bandwidth of LSPR.Semiconductor-semiconductor composites feature either a type-II [42][43][44][45][46][47] or a p-n heterojunction [48,49] at their interface.While one semiconductor orchestrates electron transport, the other aids hole movement, collectively curtailing charge recombination and bolstering carrier utilization.Merging two semiconductors with divergent bandgaps enables the capture of a broader light spectrum.Band edges must align properly for efficient charge transfer, implying that not all semiconductor combinations are viable.Charge entrapment at the interface can also pose detrimental effects on photocatalytic efficiency.52][53] With its superior electron mobility, graphene acts as an electron acceptor, thereby reducing recombination.Additionally, graphene provides a protective shield against the photocorrosion of semiconductors.However, ensuring a robust interface between graphene and semiconductors is non-trivial.Excessive or bulky graphene layers may obstruct light penetration to the underlying semiconductor, compromising overall efficiency.[56][57][58][59] Analogous to semiconductorsemiconductor composites, Z-scheme composites can incorporate semiconductors with diverse bandgaps for expanded light absorption.Yet, crafting an efficacious Z-scheme system necessitates a comprehensive grasp of the associated semiconductors and their energetics.Occasionally, an external electron mediator, derived from metal [60][61][62][63] or graphene, [64] might be indispensable to enable vectorial charge transfer, adding complexity to the configuration.While each composite offers distinct photocatalytic advantages, their appropriateness is contingent upon the specific pollutants targeted and the irradiation sources at disposal.Moreover, factors, such as synthetic method, stability, and economic considerations, critically influence the feasibility of deploying these composite photocatalysts in photocatalytic fibers for large-scale environmental purification.

Current Development
Environmental purification through the utilization of photocatalysts involves harnessing the photoexcited charge carriers to break down specific pollutants.When exposed to light, photocatalysts generate electrons and holes.If the energy levels of the photocatalyst's conduction band and valence band are appropriately aligned with the redox potentials of O 2 /•O 2 À (À0.046V vs NHE) and H 2 O/•OH (þ2.80 V vs NHE), the photoexcited electrons and holes can engage in reactions with dissolved O 2 and H 2 O, resulting in the formation of highly reactive •O 2 À and •OH radicals.These free radicals are also termed reactive oxygen species (ROS).They can be coupled with free electrons and holes, engaging with the adsorbed molecules on the semiconductor surface.This interaction instigates the transformation of pollutants into innocuous byproducts, underlining the pivotal role of free radicals in photocatalytic reactions. [3]his advanced oxidation process has been widely employed for various photocatalytic reactions related to environmental purification.22] Instead of reiterating these developments, the following discussion spotlights the utilization of photocatalytic fibers for other intriguing and practical applications.74][75]

Degradation of Antibiotics
Pharmaceutical antibiotics play a pivotal role in combatting infectious diseases across various aspects of human life and are often produced as secondary metabolites. [76]Among these antibiotics, the penicillin and tetracycline (TC) classes are the most widely used.However, due to their slow metabolism and overuse, often without due consideration of potential side effects, antibiotic resistance has emerged as a substantial public health concern.As an illustration, in 2020, humans consumed over 33 305 tons of TC around the world. [77]Unfortunately, nearly 80% of the used TC found its way into rivers and soil as remnants and metabolites, posing a significant threat to human health. [78]Effectively removing the discharged TC in an environmentally friendly manner has become an urgent and pressing task.Shi et al designed an in-situ growth method to deposit g-C 3 N 4 /BiOBr nanostructures on carbon fibers (CFs). [66]As displayed in Figure 1a, the photocatalytic fibers comprising CFs/g-C 3 N 4 /BiOBr were weaved into a cloth in a dimension of 5 cm Â 5 cm.The thusfabricated functional cloth was employed to conduct photocatalytic degradation of TC from tetracycline hydrochloride (TC-HCl) solution under visible light illumination.The comparative results are shown in Figure 1b.Among all the samples tested, CFs/g-C 3 N 4 /BiOBr exhibited the highest activity of TC degradation along with considerable chemical and structural stability.In Figure 1b, a typical type-II band alignment was considered for g-C 3 N 4 /BiOBr.Under light illumination, the photoexcited electrons and holes were transported in an opposite direction, leading to the spatial separation of electrons from holes.
The results of scavenger experiments revealed that •O 2 À and holes were mainly responsible for the photodegradation of TC over CFs/g-C 3 N 4 /BiOBr.The complete mineralization of TC into CO 2 and H 2 O was confirmed by liquid chromatography-mass spectrometry (LCMS) and total organic carbon (TOC) analysis.With this examination, the photocatalytic capacity of CFs/g-C 3 N 4 /BiOBr for decomposing TC can be validated.Importantly, the photocatalytic fibers displayed great mechanical strength.As visualized in Figure 1d, the bundle of fibers can hang a lock with a weight much heavier than itself.This feature is important to extend the application scenarios of photocatalytic fibers.
Moving beyond the degradation of TC, it becomes imperative to explore the potential of these advanced photocatalytic fibers for the decomposition of other antibiotics.For instance, antibiotics like levofloxacin (LVFX) and norfloxacin (NOR) are among the widely used fluoroquinolones, with extensive applications in the clinical sector to counteract a plethora of infections.Yet, analogous to TC, these antibiotics have also been frequently identified in aquatic environments due to incomplete metabolism and indiscriminate dumping, contributing to the adverse ecological impacts and the upsurge in antibiotic-resistant strains.One noteworthy research, led by Zhang et al., focused on the synthesis of NH 2 -MIL-125(Ti) deposited CF/MoS 2 -based weavable photocatalysts. [79]Characterized by its multifaceted structure, this cloth manifested a robust adsorption capacity.Specifically, the experimental results elucidated that the CF/MoS 2 /NH 2 -MIL-125(Ti) cloth could efficiently remove 81.1% of LVFX within 2 h.Such a pronounced removable capability indicates a promising avenue  [66] Copyright 2020, Elsevier.
for wastewater treatment, where rapid removal of antibiotics is crucial to prevent their prolonged exposure to the environment.On another related note, the endeavors of Liu et al. in fabricating BiOCl-deposited CoFe 2 O 4 (CFO/BOC) fibers brought forth another groundbreaking result in the realm of antibiotics degradation. [67]These fibers, with their intricate architecture and enhanced photo-reactive properties, showcased a remarkable efficiency in degrading NOR.As evidenced by the experimental outcomes, CFO/BOC fibers could degrade as much as 75.5% of NOR within just 2 h of visible light illumination.This outstanding performance underscores the potential of CFO/BOC fibers in mitigating the risk associated with antibiotic contaminants.The examples above elucidate the strides being made in the synthesis and utilization of photocatalytic fibers.Not only do they exhibit strong photocatalytic activity, but their mechanical strength, as illustrated by the CFs/g-C 3 N 4 /BiOBr fibers, underscores their potential for scalable applications.As researchers continue to explore the myriad of possibilities, these fibers might soon become a mainstay in environmental remediation, especially in contexts that necessitate the removal of persistent pharmaceuticals.Such advancements, holistically viewed, are pivotal steps towards realizing a sustainable environment and mitigating public health risks associated with antibiotic residues.

Removal of Heavy Metal Ions
The contamination of water sources by heavy metals represents a significant global environmental concern, posing threats to both aquatic ecosystems and human health.Hexavalent chromium (Cr 6þ ) stands out as one of the most potent heavy metal pollutants, often released into water sources by industrial activities.In 2019, Cr 6þ was categorized as the first class of hypertoxic water contaminants due to its carcinogenic and cytotoxic properties. [80]n reality, it is quite common to encounter coexisting pollutants of Cr 6þ and antibiotics in wastewater, primarily due to the simultaneous discharge of domestic sewage and pharmaceutical effluents.Effectively addressing the removal of both Cr 6þ and antibiotics in water treatment has become an urgent and pressing international environmental challenge.Recent advancements have shown promise in converting hazardous Cr 6þ into the more environmentally benign Cr 3þ while simultaneously degrading antibiotics using photocatalytic fibers.This demonstrates a promising approach to mitigate the toxic impact of these pollutants, showcasing the potential for innovative solutions in environmental purification.Chen et al. designed photocatalytic fibers by immobilizing g-C 3 N 4 @cellulose aerogel composites (g-C 3 N 4 @CA) on blended polyester fibers (B-PET) to conduct simultaneous Cr 6þ removal and antibiotics degradation. [14]Sulfaquinoxaline sodium (SQX) was selected as the antibiotics model for examination.

Indoor Air Purification
Indoor air pollution has a significant and far-reaching impact on public health.With the onset of the COVID-19 pandemic, an increasing number of employees have transitioned to remote work to minimize in-person contact.Consequently, people now spend more than 90% of their time indoors, where various pollutants, including volatile organic compounds (VOCs), particulate matter, and CO, are prevalent and pose detrimental health risks. [81,82]Notably, the coronavirus primarily spreads through respiratory droplets, which can linger in the air for extended periods.Consequently, indoor air pollution has gained recognition as a pressing issue that requires immediate attention.Traditional methods employed for indoor air purification, such as adsorption and filtration, are cost-effective but tend to be less effective at low pollution concentrations. [83]Furthermore, without proper post-treatment, adsorbents and filters can become sources  [14] Copyright 2019, Elsevier.
of secondary pollutants within ventilation systems. [84]hotocatalysis offers a promising technology for air purification because it can completely decompose pollutants without generating secondary pollution.This makes it an attractive and environmentally friendly option for addressing indoor air quality concerns.In Mamaghani's study, TiO 2 was prepared and supported on two supporting materials, nickel foam filters (NFF) and activated carbon fibers (ACF), to fabricate photocatalytic air filters (10 cm Â 10 cm Â 0.1 cm) for indoor air purification. [70]igure 3a depicts the experimental setup used to examine the effectiveness of the fabricated photocatalytic air filters in the removal of methyl ethyl ketone (MEK), a ubiquitous oxygenated VOC that has a long atmospheric lifetime to pose a threat to health.In a typical procedure, the MEK-laden air stream with 2 ppm MEK and 20% relative humidity was introduced into the reactor.The concentrations of MEK and by-products (acetone, acetaldehyde, formaldehyde) in the outlet were determined by HPLC. Figure 3b,c compare the removal efficiency of MEK and by-product generation under UV illumination among relevant filter samples.The photocatalytic filters comprising TiO 2 supported on ACF exhibited a maximal MEK removal efficiency of around 65% and an intermediate by-product concentration of 175 ppb.The superiority of ACF over NFF in promoting the MEK removal efficiency for the supported TiO 2 was believed to result from the high adsorption affinity associated with the porous structure of ACF.Because of the high degree of porosity, the adsorbed MEK and by-products can more easily access •OH radicals at the TiO 2 surface to mineralize MEK.This would promote the reaction kinetics of MEK decomposition and facilitate the regeneration of ACF, ensuring efficient and consistent performance toward MEK removal.Copyright 2021, Elsevier.

Microbial Disinfection
Microbial infections pose a significant and ongoing threat to public health, as they can originate from diverse sources and affect various parts of the human body.The transmission of microorganisms to humans primarily occurs through multiple pathways, including airborne particles, contaminated water, consumed food, and living vectors, such as insects and animals. [85]everal well-known bacterial pathogens, including Streptococcus pyogenes (S. pyogenes, gram-positive), Staphylococcus aureus (S. aureus, gram-positive), Escherichia coli (E.coli, gram-negative), Pseudomonas aeruginosa (P.aeruginosa, gram-negative), and Salmonella typhi (S.Typhi, gram-negative), have the potential to cause severe infectious diseases, resulting in substantial morbidity and mortality. [86]These bacteria can target different bodily systems, leading to a range of symptoms that vary in severity and duration.Photocatalysis has emerged as an efficient and sustainable method for sterilization, capable of eradicating bacterial pathogens through advanced oxidation processes involving radicals, such as •OH, •O 2 À , and SO 4 • À . [87]This approach shows promise in combatting microbial infections and represents a valuable tool in the fight against infectious diseases.Wu et al.
proposed a photocatalytic fiber paradigm by embedding PdOmodified N-doped TiO 2 (PdO/TiON) in ACF. [73]As Figure 4a The bactericidal mechanism of photocatalytic fibers primarily hinges on the generation of ROS during the photocatalytic process. [7]When photocatalysts are irradiated with light at wavelengths within their absorption range, electrons are excited from the valence band to the conduction band, leaving behind photogenerated holes in the valence band.These excited electrons and holes are highly reactive and can participate in redox reactions at the surface of the photocatalyst.The primary reactions involve the interaction of these excited electrons and holes with H 2 O and dissolved O 2 molecules.The holes can oxidize H 2 O, producing •OH radicals, while the excited electrons can reduce O 2 , generating •O 2 À radicals. [3]These radicals, especially •OH radicals, are among the most potent oxidative agents, which can attack organic molecules, including the cell walls and membranes of bacteria, leading to their inactivation. [88]In microbial cells, the primary detrimental effects induced by ROS encompass the initiation of lipid peroxidation processes, which compromise membrane integrity leading to cellular lysis. [89]Additionally, oxidative perturbations to proteins and peptidoglycans within the cell wall further degrade its structural cohesion. [90]Intracellularly, the infiltration of radicals can culminate in the oxidation of DNA, proteins, and other essential cellular constituents, subsequently inhibiting bacterial replication processes. [91]he novel coronavirus (SARS-CoV-2), responsible for the COVID-19 pandemic, has had a profound impact on millions  and d) after photocatalytic treatment.Reproduced with permission. [73]Copyright 2009, Elsevier.
of people worldwide.To prevent the transmission of the virus, it is crucial to develop disinfectants that can be easily incorporated into daily life.Antiviral chemicals, such as alcohol and hydrogen peroxide, have been widely used for disinfecting SARS-CoV-2.However, their effectiveness is limited over the long term due to the evaporation of volatile chemicals.Photocatalysis offers an effective and sustainable approach to combating SARS-CoV-2.In 2021, a study demonstrated for the first time that TiO 2 can deactivate 99.9% of SARS-CoV-2 within 20 min under LED illumination. [92]This research revealed that photocatalysis can be applied in indoor environments to reduce the risk of SARS-CoV-2 transmission.Subsequently, studies focusing on the design of effective photocatalytic systems for disinfecting SARS-CoV-2 have been ongoing, [93][94][95] highlighting the potential of this technology in enhancing public health and safety.Maślana et al. designed photocatalytic self-disinfection papers composed of cellulose fibers modified with g-C 3 N 4 photocatalysts and Ag particles. [72]In this study, the antiviral activity was evaluated by using Φ6 phage virus as SARS-CoV-2 mimics.The fabricated papers not only exhibited a photocatalytic effect to degrade organic dyes, but also displayed antiviral properties to inactivate Φ6 phage virus.These promising results illustrated that the fabricated photocatalytic papers can be adapted to cope with the current pandemic crisis.

Challenges and Opportunities
Photocatalytic fibers harness the power of sunlight to initiate chemical reactions, providing a sustainable and environmentally friendly approach to purification.These fibers have demonstrated their potential across various practical application scenarios, encompassing dye degradation, antibiotics decomposition, heavy metal removal, indoor air purification, and microbial disinfection.Despite their promise, the widespread implementation of photocatalytic fibers in realistic application scenarios faces several critical challenges.Scheme 2 outlines the current obstacles impeding the broad deployment of photocatalytic fibers for environmental purification in the postpandemic era.In the following section, we delve into potential opportunities and strategies that can help address these challenges and pave the way for the effective and extensive utilization of photocatalytic fibers in environmental purification applications.

Near Infrared Responsiveness
The first significant challenge revolves around the limited near infrared absorption capability of photocatalysts supported on fibers.It is essential to note that the spectrum distribution of sunlight comprises approximately 6.8% in the UV range (λ < 400 nm), 38.9% in the visible range (λ = 400-700 nm), and a substantial 54.3% in the near infrared range (λ = 700-3000 nm).Unfortunately, the majority of photocatalysts developed to date can only absorb UV and visible light from the solar spectrum.To achieve full-spectrum-driven environmental purification using photocatalytic fibers, there is an urgent need to develop photocatalysts that are responsive to near infrared light.Conventional near infrared responsive photocatalysts are typically limited to materials with small bandgaps, such as lead and mercury chalcogenides.However, these materials are plagued by high toxicity, poor stability, and limited redox capabilities, rendering them impractical for many applications.A Scheme 2. Challenges and opportunities facing the photocatalytic fibers for environmental purification in the post-pandemic era.potential solution to this challenge lies in leveraging the unique property of LSPR exhibited by non-stoichiometric semiconductors, e.g., n-type WO 3Àx [96,97] and p-type Cu 2Àx S. [98,99] Different from the LSPR of noble metals which usually absorb visible light, the plasmon resonance of non-stoichiometric semiconductors is commonly situated in near infrared region.By harvesting the untapped near infrared energy to harness the entire solar spectrum, the performance of photocatalytic fibers toward environmental purification can be promoted to an advanced level.
As a proof of concept, Zhang et al. prepared for plasmonic CuS/BiOCl composites and demonstrated their near infrareddriven photocatalytic activity for the degradation of organic dyes (methyl orange, MO) and antibiotics (sulfamethoxazole, SMX). [100]In this study, BiOCl with two different shapes, i.e., nanosheets (NSs) and nanoplates (NPLs), was employed to study the effect of the exposed {110} facets on the extraction of hot holes from plasmonic CuS for photocatalytic reactions.Figure 5a,b showed the uniform dispersion of CuS on BiOCl NPLs.As displayed in Figure 5c, both CuS/BiOCl NSs and CuS/BiOCl NPLs exhibited a noticeable absorption band across 600-1400 nm.This additional band was attributed to the LSPR of CuS as was confirmed by the spectral shift at different dielectric media.Figure 5d,e summarized the resultant photocatalytic performance.Both CuS/BiOCl NSs and CuS/BiOCl NPLs displayed noticeable activities of MO and SMX degradation under near infrared illumination.The higher activity of CuS/BiOCl NPLs was ascribed to the more exposed {110} facets of BiOCl NPLs, which may induce more efficient interfacial charge transfer.A plausible charge transfer mechanism was proposed in Figure 5f to account for the observations.Upon LSPR excitation by near infrared, hot electrons and hot holes can be generated by CuS.Because of the upward band bending at the interface, the hot holes of plasmonic CuS were injected into BiOCl to achieve charge separation.The scavenger experiments revealed that the separated hot holes participated in MO and SMX degradation.This study has paved the way for the development of near infrared responsive photocatalysts utilizing non-stoichiometric semiconductors.These innovative photocatalysts can be incorporated into the fabrication of versatile photocatalytic fibers, enabling environmental purification driven by broadband sunlight.

All-Day Active Capacity
Continuous light irradiation is a prerequisite for conducting photocatalytic reactions using a specific photocatalyst.When light exposure ceases, the activity of the photocatalyst wanes because the generation of charge carriers in the absence of light is halted.Given the persistent and uninterrupted occurrence of environmental pollution, it becomes imperative to develop photocatalytic fibers with the capacity for continuous operation during the day and at night.In response to this need, substantial efforts have been directed toward the design of photocatalysts capable of conducting photocatalytic reactions under both illuminated and dark conditions, often referred to as a round-the-clock photocatalytic system.These unique systems can operate  [100] Copyright 2022, Elsevier.
seamlessly throughout the day and night, and they have found widespread applications in tasks, such as dye degradation, heavy metal removal, microbial disinfection, and hydrogen generation. [101] study by Chiou reported that Se nanorods exhibited significant activity in degrading methylene blue (MB) in the dark after undergoing a pre-irradiation treatment. [102]The sustained generation of •OH radicals on the Se surface after irradiation cessation was attributed to the memory catalytic effect.More recently, Chiu et al. demonstrated an all-day-active photocatalyst model using TiO 2 nanowires decorated with Au@Cu 7 S 4 (TiO 2 -Au@Cu 7 S 4 ). [48]As shown in Figure 6a, TiO 2 -Au@Cu 7 S 4 can act as a photocatalyst for MO degradation under light illumination.In dark conditions, both Au and Cu 7 S 4 could generate •OH radicals, allowing continuous MO degradation.It's worth noting that both Au and Cu 7 S 4 have peroxidase-like features, enabling them to catalyze the decomposition of H 2 O 2 to produce •OH in the absence of light.By combining photocatalytic capability with peroxidase-like features, TiO 2 -Au@Cu 7 S 4 can exhibit all-day active MO degradation, as illustrated in Figure 6b.Under light illumination, all samples displayed significant MO degradation activity, which continued even after irradiation was stopped for up to 3 h.This remarkable all-day active capability can also be achieved with Au@Cu 2 O core-shell nanocrystals. [33]As depicted in Figure 6c, the peroxidase-like function of Au and the Fenton reactivity of Cu 2 O cooperate to sustainably produce •OH radicals in the dark.Figure 6d demonstrates that Au@Cu 2 O, by combining its photocatalytic ability, can also exhibit all-day active capability in inactivating E. coli.

Reusability and Scalability
Photocatalytic fibers are susceptible to losing their activity due to catalyst detachment, which often occurs because of weak adhesion and severe corrosion during photocatalytic processes.This can result in a decline in performance and limit their reusability.Some strategies can be employed to address these reusability issues.These include grafting chemical moieties onto the fibers to enhance the adhesion of photocatalysts to the supporting fibers, [103] introducing protective layers to prevent catalyst detachment, [104] and exploring advanced composites with inherent high corrosion resistance. [105]Scalability is another critical factor that must be taken into account when designing versatile photocatalytic fibers.Thus far, the sizes of photocatalytic fibers fabricated in laboratory settings (ranging from hundreds of micrometers to a few centimeters in length and width) fall short of the dimensions required for practical implementation (several tens of centimeters in length and width).The creation of large-scale prototypes of photocatalytic fibers that meet the demands of environmental purification remains a significant challenge.Moreover, photocatalytic fibers must be designed to seamlessly integrate with materials commonly used in real-world applications, including textiles, filters, and papers, without compromising their catalytic activity.This compatibility is Figure 6.a,b) TiO 2 -Au@Cu 7 S 4 as all-day active photocatalysts for MO degradation.Reproduced with permission. [48]Copyright 2017, Elsevier.c,d) Au@Cu 2 O as all-day active photocatalysts for E. coli inactivation.Reproduced with permission. [33]Copyright 2019, Elsevier.essential to ensure their successful adoption in various practical scenarios.

Summary and Outlooks
The post-pandemic era places a strong emphasis on sustainability.Photocatalytic fibers align with this focus by offering eco-friendly purification solutions that can reduce the need for chemical treatments and energy-intensive processes.With the escalating concerns over antibiotic residues in water systems due to their potential to induce antibiotic resistance in microbes, photocatalytic fibers could serve as a sustainable solution.Future research might focus on tailoring the specific surface properties of these fibers to ensure complete mineralization of the antibiotic compounds rather than partial degradation.Conventional methods for removing heavy metal ions have limitations.By modifying the photocatalytic fiber surface with selective functional groups or integrating bimetallic systems, enhanced selectivity and efficiency in capturing and reducing various heavy metal ions could be achieved.Photocatalytic fibers, when integrated into indoor textiles, curtains, or air filters, could provide a passive and continuous purification mechanism to treat indoor air pollution.The key lies in enhancing the sensitivity of the fibers to visible or indoor light and ensuring they remain effective in a variety of environmental conditions, such as varying humidity and temperature.The current global health landscape underscores the necessity for efficient and eco-friendly disinfection methods.Further research is required to understand the interaction between photocatalytic fibers and various microbial species and to engineer fibers that can work under low light conditions, ensuring round-the-clock disinfection.
Photocatalytic fibers may not be equally effective for all types of pollutants.Ensuring that they can efficiently target pollutants, including pathogens such as SARS-CoV-2, is a priority.In particular, treating multiple refractory waste species at the same time will be a future study direction for the development of photocatalytic fibers.Compatibility and integration with other technologies also need to be addressed.For instance, incorporating smart and sensor technologies into photocatalytic fibers can enhance their versatility and allow for real-time monitoring and control of purification processes.Integrating photocatalytic fibers into existing infrastructure, such as air filtration systems and wastewater treatment plants, can also be challenging.To achieve these goals, collaboration between materials scientists, environmental engineers, public health experts, and policymakers is crucial for advancing the advanced use of photocatalytic fibers.In the post-COVID-19 era, there is a growing awareness of the importance of clean and safe environments.Photocatalytic fibers have the potential to play a significant role in addressing environmental challenges, but ongoing research and development are necessary to overcome current challenges and fully realize their potential for environmental purification.

Figure 1 .
Figure 1.a) Photocatalytic fibers comprising CFs/g-C 3 N 4 /BiOBr weaved into a cloth.b) Results of photocatalytic TC degradation under visible light illumination over relevant samples.c) Proposed mechanism of photocatalytic TC degradation on g-C 3 N 4 /BiOBr.d) Photo of the fabricated photocatalytic fibers under hanging test.Reproduced with permission.[66]Copyright 2020, Elsevier.
Figure 2a illustrates the photocatalytic mechanism derived from scavenger experiments.Under visible light illumination, the photoexcited electrons of g-C 3 N 4 can remove Cr 6þ by reducing it to a Cr 3þ state.In contrast, the •O 2 À and holes mediated the degradation of SQX.As examined from the high performance liquid chromatography-mass spectrometry (HPLC-MS) HPLC-MS analysis, completing the mineralization of SQX into SO 4 2À and NO 3 À ions can be achieved, collaborating withthe capacity of the fabricated photocatalytic fibers for degrading antibiotics.Figure 2a,b compare the photocatalytic performance of simultaneous Cr 6þ removal and SQX degradation over relevant samples.Because of the robust structure, the photocatalytic fibers comprising g-C 3 N 4 @CA/B-PET exhibited the highest photocatalytic activity.The water impact test shown in the inset of Figure 2b revealed the excellent mechanical strength of the fabricated photocatalytic fibers.The fibers remained intact after 24 h of water impact at a flow rate of 0.56 m s À1 .

Figure 2 .
Figure 2. a) Photocatalytic fibers comprising g-C 3 N 4 @CA/B-PET and the proposed photocatalytic mechanism.b,c) Results of simultaneously photocatalytic Cr6þ removal and SQX degradation under visible light illumination over relevant samples.Inset in (c) shows the photo of the fabricated photocatalytic fibers under water impact test.Reproduced with permission.[14]Copyright 2019, Elsevier.

Figure 3 .
Figure 3. a) Experimental setup for MEK removal over the fabricated photocatalytic filters.b,c) Results of photocatalytic MEK removal and by-product generation under UV illumination among relevant filter samples.Reproduced with permission.[70]Copyright 2021, Elsevier.
, b show, the fabricated photocatalytic fibers exhibited remarkable inactivation efficiency for both gram-positive (S. aureus) and gram-negative (E.coli) cells under visible light illumination.Upon 30 min of irradiation, the fibers can kill more than six orders of magnitude cells.The extent of cell damage by the photocatalytic effect was further examined from morphological observation.As observed in Figure4c,d, the surface of healthy E. coli was smooth with evident flagella, whereas the morphology of the treated E. coli was characteristic of rumples and holes.Confirming whether the bacterial cells were indeed decomposed and killed by photocatalysts was a significant matter.The observation from Figure4dcan verify the actual bactericidal effect exerted by the fabricated photocatalytic fibers.

Figure 4 .
Figure 4. Results of photocatalytic inactivation of a) S. aureus and b) E. coli under visible light illumination on PdO/TiON embedded in ACF.Surface morphology of E. coli cells c) beforeand d) after photocatalytic treatment.Reproduced with permission.[73]Copyright 2009, Elsevier.

Figure 5 .
Figure 5. a,b) TEM images of CuS/BiOCl NPLs.c) Absorption spectra of relevant samples.d,e) Results of photocatalytic MO and SMX degradation under different illumination conditions among relevant samples.f ) Proposed mechanism of interfacial charge transfer for plasmonic CuS/BiOCl.Reproduced with permission.[100]Copyright 2022, Elsevier.