Photocatalytic Upcycling of Plastic Waste: Mechanism, Integrating Modus, and Selectivity

Plastics are ubiquitous and indispensable in our daily life because of their low cost, portability, durability, and processability. However, due to the overuse, short service life, and chemical inert character, the accumulated discarded plastics pose a great threat to the sustainable development of ecology and environment. Photocatalysis represents highly promising technology in transforming plastic wastes into value‐added products via green and mild method. In this perspective, the advantages of photocatalysis are discussed and compared with other catalysis technologies including thermal catalysis, electrocatalysis, and enzyme catalysis. Then the possible photocatalytic upcycling path of plastic wastes is clarified under different experimental conditions. The types of plastic wastes that can be upcycled by photocatalysis, the integrating modus between plastic wastes and the photocatalysts as well as the modulation of the product selectivity are also emphasized. Finally, the challenges and insights into the future development of photocatalytic plastic waste upcycling fields are presented. It is expected that this timely and critical review provides the instructive guidance for the design of photocatalysts with high efficiency and high selectivity toward plastic waste upcycling.

generally occurred at the specific site of enzyme.41][42][43][44][45][46][47] Hence, it is considered as cheaper and greener route to transform plastic wastes into value-added products.More importantly, the specific chemical bonds in the waste plastic can be precisely activated by controlling the electronic band structure of photocatalysts.Thus, photocatalysis is promising in regulating the selectivity of plastic wastes during the transforming process.
Currently, several reviews on the photocatalytic upcycling of plastic wastes have been published.For example, Zhang and co-workers [2] highlighted the photosynthesis of plastic wastes to various value-added products, whereas Ouyang et al. [4] emphasized the photocatalytic degradation of plastics and plasticderived chemicals.Besides, Duan and co-workers [43] focused on the valorization of plastic wastes into functional materials, such as metal-organic frameworks or multiwall nanotubes.These reviews focus the discussions mainly on the reactants and products of the photocatalytic upcycling reactions while ignoring the reaction process.Generally, for the photocatalytic upcycling reaction process, the photocatalysts absorb the irradiated light to generate photoexcited electrons and holes which then participate in the reduction or oxidation reaction of the plastic wastes on the surface of the photocatalysts.The factors, such as the integrating modus between the photocatalysts and the plastic wastes, the surface structure of the photocatalysts and the reaction environment, could affect the photocatalytic reaction path, which hence could influence the photocatalytic efficiency and product selectivity of the photocatalytic upcycling process.As the photocatalytic efficiency and product selectivity could determine whether the photocatalytic upcycling process is feasible for practical application, it is of great importance to discuss  1) Reproduced with permission. [2]Copyright 2022, Wiley-VCH.
(2) Reproduced with permission under the terms of the Creative Commons CC BY license. [108]Copyright 2021, the Authors.Published by Springer Nature.(7) Reproduced with permission. [109]Copyright 2003, The American Association for the Advancement of Science.
the photocatalytic upcycling reaction process with the emphasize on the photocatalytic efficiency and product selectivity.In this review, the possible photocatalytic upcycling reaction paths under different reaction environment together with available types of plastic wastes that can go through photocatalytic valorization are clarified.After that, we discussed the advantages and disadvantages of different integrating modus over plastic wastes including solid-phase modus, solvent-assisted modus, and magnetic-driving modus.The modulation of the product selectivity via modifying the surface structure of the photocatalysts and the reaction environment were also emphasized.Finally, the challenges and future insights of this technique are discussed with the expectation of intriguing new thoughts toward the development of plastic waste upcycling industry and carbon circular economy.

Photocatalytic Upcycling Mechanism of Plastic Wastes
60][61][62] Methods such as constructing photocatalysts with vacancies and integrating narrow bandgap photocatalysts with wide bandgap photocatalysts could help to widen the solar absorption spectrum; [63][64][65] Strategies such as constructing heterogenous structure and strain could improve the charge separation and migration properties. [66,67]These different approaches can be combined together to improve the photocatalytic efficiency, and they can also be applied to improve the photocatalytic upcycling properties of plastic wastes.More importantly, photocatalytic upcycling of small molecules from plastic wastes requires less energy. [68]For example, the Gibbs free energy changes for the photocatalytic conversion of ethylene glycol and lactic acid from polyethylene terephthalate (PET) and polylactic acid (PLA) are about þ9.2 and þ27 kJ mol À1 , respectively, which are much smaller than that of the photocatalytic water splitting process (about 237 kJ mol À1 ), [69] indicating that photocatalytic upcycling of small molecules from plastic wastes are thermodynamically feasible.The photocatalytic upcycling routes are commonly affected by the reaction environment during the photocatalytic process (Figure 2).With the appearance of H 2 O and O 2 , the photocatalytic upcycling of plastic wastes follows route: 1). the photogenerated holes and electrons can be transferred into •OH and •O 2 À , respectively.These oxygen active species can trigger C─C bond cleavage in plastic wastes and then produce CO 2 .Additionally, as the photocatalytic oxidation of small molecules from plastic wastes is thermodynamically feasible than that of water, the photocatalytic upcycling of plastic waste also could follow route; 2) in which the photogenerated holes are prone to directly attack the chain of plastics to produce fuel or other value-added chemicals while the photogenerated electrons are prone to act with H þ to produce H 2 .Simultaneously, when O 2 is absent, the photocatalytic upcycling of plastic wastes follows the route; 3) combining the oxidation component of (1).Photoexcited holes combine with H 2 O to generate •OH which is applied to promote the cleavage of C─C bond in plastic wastes and produce CO 2 .The obtained CO 2 combines with H þ and photoexcited electrons to yield •COOH and then C─C bond coupling is occurring with the assistance of H þ and e À to produce valueadded chemicals of CH 3 COOH.Overall, it is easily observed that the products of the photocatalytic upcycling process can be regulated via modulating the solvent, the atmosphere, and other related reaction environment.

Available Types of Plastic Wastes to Be Upcycled Via Photocatalysis
Currently, the reported plastic wastes that can be upcycled by photocatalysts are classified into two groups according to the structural feature of the backbone as demonstrated in Figure 3. [70,71] One is homobackbone type in which the backbone structure is C─C with or without other elements or groups replacing H in the C─H bonds.This kind of plastic wastes includes polypropylene(PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), et al. [72,73] The other one is heterobackbone type in which other functional groups such as carboxylic acid group, amide group, and ester group appear in the backbone except the general C─C bond.This type of plastic wastes consists of PLA, PET, polyamide 6 (PA6), polyurethane (PUR), and so on. [74,75]Typically, C─C bond is chemically stable and hard to break.[78][79] Photocatalysts with high-catalytic activities like TiO 2 , [80] Nb 2 O 5 [81] have been reported to present photocatalytic upcycling properties in phototransformation of PE, PP, PVC, and PS.[84][85] For the heterobackbone plastic wastes, the functional groups are easy to undergo hydrolysis, alcoholysis, alkaline hydrolysis, etc. [86][87][88] Most of the current research works prefer to transform the heterobackbone plastic wastes into monomers or oligomers and then undergo oxidation or reduction reactions.As the functional groups in the backbone of heterobackbone plastic wastes are prone to be cleaved, the photocatalytic upcycling process over heterobackbone plastic wastes are easy to be carried out and tend to achieve rather higher phototransforming efficiency.

Solid-Phase Modus
According to the current research results, the modus of the photocatalytic upcycling system can be divided into three types including solid-phase modus, solvent-assisted modus, and magnetic-driving modus based on the integrating and interaction modes between the plastic wastes and the photocatalysts.They have different advantages and disadvantages.[91] The preparation of the film and the setup for the photocatalytic reaction system are demonstrated in Figure 4a. [82]In the solid-phase modus, the photogenerated electrons and holes can be directly migrated to the adjacent plastic wastes and trigger the cleavage of the long chain.The photocatalytic degradation properties are evaluated via examining the changes of the morphology by SEM, the chemical bonds by Fourier transform infrared (FT-IR), the weight loss and the average molecular weight of the photocatalyst-plastic films.As shown in Figure 4b-h, [83] after a period of irradiation time with the presence of photocatalysts, the prepared film presents more holes on the surface, new CO and C─O stretching vibrations in the FT-IR spectrum.Additionally, the weight loss of the films is increasing, and the average molecular weight is decreasing, indicating that the photocatalytic transforming reaction occurs and new products are generated.The solid-phase modus can be employed to degrade the chemical inert plastic wastes, such as PS, PE etc.However, this modus needs to dissolve the plastic wastes in certain organic solvent which suppress its further application.Besides, the migration rate of photogenerated electrons and holes is low and less amount of high-active oxidative species are produced in the system without the assistance of solvent during the photocatalytic reaction, resulting in low transforming efficiency of plastic wastes.Furthermore, the gas product can be detected via gas chromatography whereas the liquid product is difficult to be detected as only less amount is produced and is difficult to collect.These drawbacks might limit its wide applications and new technologies need to come into being.

Solvent-Assisted Modus
For solvent-assisted modus, proper solvent is employed during the photocatalytic upcycling process.As the simple illustration for the setup of the photocatalytic system displayed in Figure 5a, [86] photocatalysts, plastic wastes, and the solvent are mixed together to undergo photocatalytic upcycling reaction.Since the plastic wastes are long-chain polymers, it is difficult to have efficient contact with the photocatalysts.To promote the photocatalytic upcycling property, the plastic wastes can be first hydrolyzed or pyrolyzed into monomers or oligomers by alkali, acid, or alcohol with the assistance of heat.For example, Erwin Reisner, and co-workers have converted PET into ethylene glycol in KOH solution at 40 °C.And then they employed CN x /Ni 2 P (Figure 5b) [69] as the catalyst to catalyze the water splitting reaction for H 2 (Figure 5c) [69] with the converted ethylene glycol as the electron donor to undergo photooxidation reaction toward the production of value-added organic products (such as formate, glyoxal, glycolate, etc.).Except the strategy of hydrolysis or pyrolysis to convert the plastic wastes into monomers or oligomers, some researchers adopt the powders derived from the plastic wastes to carry out photocatalytic upcycling reactions.These powders have same chemical composition with the original plastic wastes, but their sizes are much smaller, which also could enhance the efficient contact between the polymers and the photocatalysts.Xie and co-workers have fabricated Nb 2 O 5 atomiclayer photocatalyst (Figure 5d) [81] and applied it in the direct photocatalytic phototransforming of PE, PP, and PVC.As shown in Figure 5e-g, [81] the yield of CO 2 is continuously increasing with the irradiation time increasing.Additionally, the evolution of CH 3 COOH during the photocatalytic upcycling of PE, PP, and PVC are similar with the photoreduction of pure CO 2 in water, indicating that the photocatalytic upcycling process first converts PE, PP, and PVC to CO 2 and then undergoes the photoreduction reaction of CO 2 to CH 3 COOH.Even though the photocatalytic upcycling efficiency of solvent has been improved compared with the solid-phase modus and the liquid products as well as the gas products can be detected by the NMR and gas chromatography, the mechanism of how the plastic wastes are absorbed and activated on the photocatalysts are still unclear and the selective cleavage of the backbone together with the selectivity of certain products still cannot be well controlled.

Magnetic-Driving Modus
94][95][96][97][98][99][100][101][102] 6a). [97]Under the action of magnetic field,the as-prepared photocatalytic microrobots could move toward specific direction which facilitates the directional processing of plastic wastes and textile fibers according to practical needs.Additionally, with the assistance of magnetic field, the contact between the plastic wastes and the photocatalytic active particles can be promoted which is conductive for the improvement of the photocatalytic upcycling efficiency.What is more, the photocatalytic microrobots are feasible to be collected and upcycled.As demonstrated in Figure 6b, with the light irradiation and magnetic field, the BiVO 4 /Fe 3 O 4 microrobots move from one end to the other end of the homemade channel with the plastic fragments detached from the aqueous solution. [99]The photocatalytic plastic upcycling property thus could be enhanced with the treatment of these specific microrobots as shown in Figure 6c-f in which the weight loss of the plastics is increased and the surface wettability as well as the surface morphology of the plastics have been changed.Magnetic-driving modus is a kind of emergent methodology as it can achieve targeted removal of waste plastic polymers under the driving of magnetics.However, its efficiency is not high, and it is also not so intelligent.Advanced efforts have to be drawn in improving its photocatalytic upcycling efficiency and its intelligence, such as synergy with the integrated circuits.
To clearly demonstrate the advantages and disadvantages, the characteristics of the above discussed integrating modus have been summarized in Table 1.

Product Selectivity Modulation
opyright 2013, Elsevier Ltd. b,d) SEM images of PS and PS-TiO 2 samples before irradiation.c,e) SEM images of PS and PS-TiO 2 samples after irradiation for 40 h.f ) a-c refers to the FT-IR spectra of PS sample before irradiation, PS sample after 30 h irradiation and PS-TiO 2 sample after 30 h irradiation, respectively.g) Weight loss of PS and PS-TiO 2 samples with the dependence irradiation time.f ) Changes in the average molecular weight of PS and PS-TiO 2 samples with the dependence irradiation time.Reproduced with permission. [83]Copyright 2003, Elsevier Ltd.
It determines whether the photocatalytic upcycling of plastic wastes could enter into practical application.Hence, it is of great significance to investigate the product selectivity of plastic waste photocatalytic upcycling process.

Surface Structure Modification of the Photocatalysts
The surface of the photocatalysts is the main place where the catalytic reaction takes place.It could affect the adsorption of the reactant molecules, the reaction path, the selective evolution of the intermediates as well as the desorption of the product molecules.Hence, the surface structure of the photocatalysts could have significant influence on the product selectivity of the plastic waste photocatalytic upcycling process.Doping metal atoms could modify the surface-active sites of the photocatalysts and thus affect the product selectivity of the photocatalytic upcycling process.Jiao et al. doped Zr into CoFe 2 O 4 quantum dots to generate charge-asymmetrical dual metal sites and employed it in the photoreforming process of PE (Figure 7a). [103]They found that this special charge-asymmetrical dual metal sites could promote the adsorption of *CH 2 CH 2 and thus facilitate the production of C2 value-added chemicals.The selectivity of CH 3 COOH from the photoreforming of PE could arrive at 100% with the evolution rate of 1.10 mmol g À1 h À1 over Zr-CoFe 2 O 4 , which is much higher than that over CoFe 2 O 4 (Figure 7b).The species of the cocatalyst also could affect the plastic waste photocatalytic upcycling process and the product selectivity.Han et al. prepared carbonized polymer dots-graphitic carbon nitride (CPDs-CN) with Pt as the cocatalyst and adopted it in the photoreforming process of PET pretreated with 1 M KOH aqueous solution (Figure 7c). [86]The main obtained products are glycolic acid and acetic acid with the yields of 383 and 554 μmol after 8 days of irradiation (Figure 7d).In contrast, Uekert et al. applied CN x with Ni 2 P as the cocatalysts (Figure 7e) in the photoreforming process of similar pretreated PET and the product is mainly glyoxal (Figure 7f ). [69]As a result, the product selectivity of the plastic waste photocatalytic upcycling process can be regulated by modulate the surface structure of the photocatalysts.

Solvent
As discussed above, the experimental conditions determine the reaction path of the photocatalytic upcycling process.For the solvent-assisted modus, the photocatalytic upcycling reaction occurs in the solvent.Hence, solvent plays significant role in the final product selectivity.Cao et al. carried out the photoreforming reaction of PS in the solvent of acetonitrile and tetrahydrofuran over C 3 N 4 , and achieved the selectivity to value-added organics of 61% in acetonitrile whereas 11% in tetrahydrofuran. [104]Simultaneously, Du et al. employed MoS 2 /CdS (Figure 8a,b) in the photoreforming reaction of PLA pretreated with 10 M KOH [105] , and Zhang et al. applied d-NiPS 3 /CdS (Figure 8d,e) in the photoreforming reaction of PLA pretreated with 2 M KOH. [106]As demonstrated in Figure 8c-f, the product Figure 5. a) Schematic illustration for the setup of the photocatalytic system in solvent assisting-modus.Reproduced with permission. [86]Copyright 2022, Elsevier Ltd. b) TEM images of CN x /Ni 2 P. c) H 2 evolution yield with the dependence of irradiation time over CN x /Ni 2 P. Reproduced with permission. [69]opyright 2019, American Chemical Society.d) Low-resolution TEM image of single-unit-cell thick Nb 2 O 5 layers.e,f ) The yields of CO 2 and CH 3 COOH over Nb 2 O 3 atomic layers during the photocatalytic upcycling of PE, PP, and PVC as well as the photoreduction of pure CO 2 in water with the dependence of irradiation time.g) The evolution rates of CO and CH 3 COOH during the photocatalytic upcycling of PE, PP, and PVC as well as the photoreduction of pure CO 2 in water.Reproduced with permission. [81]Copyright 2020, Wiley-VCH.
over MoS 2 /CdS is mainly formate whereas the product over d-NiPS 3 /CdS is mainly pyruvate.In these two photocatalytic systems, MoS 2 and d-NiPS 3 involved in the photocatalytic reduction reaction of water splitting for H 2 whereas CdS involved in the photocatalytic oxidation reaction of PLA to organics.The photocatalytic body in the photocatalytic upcycling process is same and the solvent is different.Hence, it can be concluded that the difference in the product and the selectivity is originated from the solvent environment.

Atmosphere
Except solvent, other experimental conditions, such as the atmosphere, also have close relationship with the product selectivity of the photocatalytic upcycling process.The atmosphere which contains O 2 could involve in the generation of oxygen active species, thus affecting the photocatalytic reaction path.As demonstrated in Figure 8g, [81] [99] Copyright 2021, American Chemical Society.e) Schematic illustration and different magnified SEM images of the microfiber network after irradiation without the treatment of microrobots.f ) Schematic illustration and different magnified SEM images of the microfiber network after irradiation with the treatment of microrobots.Reproduced with permission. [97]Copyright 2023, Nature Publishing Group.upcycling reactions.Following these results, we could control the photocatalytic upcycling paths and products via regulating the surface structure of the photocatalysts as well as the reaction environment in the photocatalytic systems.It provides guidance for our future design of photocatalytic systems with high efficiency and high-product selectivity toward the upcycling treatment of plastic wastes.

Summary and Perspectives
Photocatalysis represents a kind of green technology in transforming plastic wastes into value-added chemicals.There are various chemical bonds, such as C─H bond, C─C bond, C─O bond, etc., existing in the plastic wastes.The activation of these chemical bonds requires different energy.Generally, different photocatalysts possess different bandgaps and they can absorb different range of sunlight to generate photoexcited electrons and holes.Additionally, the band energy levels of different photocatalysts are different, which provides the ability to trigger oxidation/reduction reactions with different energy levels.Thus, photocatalysis possesses the possibility to convert plastic wastes into different high-value products.Besides, the reaction routes for photocatalytic upcycling process of plastic wastes can be modulated via regulating the experimental conditions such as solvents and atmospheres, resulting in controllable product selectivity.Furthermore, different modus to integrate plastic wastes and photocatalysts could produce different upcycling efficiency and be suitable to different application scenarios.Despite so many progresses on photocatalytic upcycling of plastic wastes, there are still some challenges to be addressed.First, the influence by the surface structure of photocatalysts on the photocatalytic upcycling routes of plastic wastes is unclear.High-value chemicals, such as formate, acetic acid, and methane, are the aimed products for the photocatalytic transforming of plastic wastes.The evolution efficiency and selectivity of these high-value chemicals determine whether the photocatalytic upcycling reaction is feasible for practical application.As the surface of photocatalysts is the place where the catalytic reaction takes place, it is of great importance to investigate how the surface structure of the photocatalysts affect the photocatalytic upcycling path of plastic wastes and how to efficiently convert the plastic wastes into high-value chemicals via regulating the surface structure of photocatalysts.Although the construction of dual active sites on the surface of photocatalysts has been reported to facilitate the evolution of C2 products, it is still far from satisfaction for the practical application of photocatalytic upcycling process of plastic wastes.Other surface modification strategies, including the doping of nonmetal atoms or the construction of vacancies to modulate the surface electronic structure, the construction of lateral heterostructure to redistribute the surface electron density, can be employed to further explore the mechanism of the surface structure modification on the photocatalytic upcycling reaction.
Second, the plastic wastes are always polymers which mainly consist of C─C long chains and C─H bonds.Due to the intrinsic feature of plastics, they always have the characteristic of Reproduced with permission. [103]Copyright 2022, American Chemical Society.c) High-resolution TEM image of CPDs-CN-7.d) Time-dependent yields of various products from photoreforming of pretreated PET over CPDs-CN-7 with Pt as the cocatalyst under light irradiation.Reproduced with permission. [86]Copyright 2022, Elsevier Ltd. e) Schematic illustration of pretreated PET photoreforming process over CN x with Ni 2 P as the cocatalyst.f ) Quantification of the products from the pretreated PET over CN x with Ni 2 P as the cocatalyst after 5 days photoreforming.Reproduced with permission. [69]Copyright 2019, American Chemical Society.
hydrophobicity while most of the photocatalysts are hydrophilic which is determined by the preparation method.In addition, the relatively atomic mass of C and H atoms are rather small, which makes the relative molecular weight of plastics such as PE and PP smaller than water or other commonly used solvents.The combined effect is that there is no effective contact between the photocatalyst and plastic waste to ensure the catalytic reaction proceeding smoothly and efficiently, especially for the photocatalytic upcycling reactions carried out in solutions.From this point of view, it is of great importance to improve the wettability between photocatalyst and plastic waste.The current general method via hydrolysis or pyrolysis of plastic wastes into monomers or oligomers can have a certain effect.However, this pretreatment method requires strong acid or base and high temperature.Some other researchers adopted plasma to pretreat plastic wastes to generate oxygenated groups on the polyolefin chains, and thus improve the hydrophilicity of plastic wastes. [107]owever, this technique requires specific instrument and the species as well as the obtained oxygenated groups are uncontrollable.To make the photocatalytic upcycling process suitable to be efficiently carried out in simple and mild conditions, new strategy to achieve controllable wettability between photocatalysts and plastic wastes is indispensable.Employing organic surfactants such as oleic acid, oleylamine to modify the surface of photocatalysts or prepare photocatalysts can be explored to make the surface of photocatalyst hydrophobic.Similarly, functional moiety modification of the plastic wastes such as hydroxyl or carboxyl moieties can be adopted to make the surface of plastic waste hydrophilic.Copyright 2022, American Chemical Society.d) Schematic illustration for the photoreforming of pretreated PLA over d-NiPS 3 /CdS.e) TEM image of d-NiPS 3 / CdS.f ) Quantification of various products over CdS, NiPS 3 /CdS, and d-NiPS 3 /CdS after 9 h of photoreforming.Reproduced with permission. [106]opyright 2023, American Chemical Society.g) Schematic illustration for the production ofCH 3 COOH over Nb 2 O 5 atomic layer under simulated natural environmental conditions.h,i) Quantification of CO 2 and CH 3 COOH under O 2 and Ar atmosphere over Nb 2 O 5 atomic layer.Reproduced with permission. [81]Copyright 2020, Wiley-VCH.
Finally, more details on the photocatalytic upcycling routes need to be further explored.For traditional catalytic reactions, the small molecules are adsorbed on the surface of catalysts and then transferred to various intermediates and final products.Different catalytic reactions possess different rate-determining step and these steps are tightly associated with the structure of the catalysts.Exploring detailed reaction routes and constructing relationship between structure and properties could provide guidance for the design of efficient catalysts.For the catalytic reaction of plastic wastes, it is unknown how the polymers are adsorbed on the surface of the photocatalysts, which kind of sites in the photocatalysts are prone to be adsorbed by the polymers and which kinds of atoms in the polymers intend to adsorb on the surface of the photocatalysts.Theoretical simulation is a kind of common technology in exploring the catalytic reaction routes, and it is proposed to be applied to systematically investigate the photocatalytic upcycling reaction routes.Furthermore, various in situ characterization techniques, such as in situ Raman, in situ FTIR, and in situ ESR can be adopted to detect various intermediates, facilitating the analysis on the photocatalytic upcycling reaction routes of plastic wastes.
In conclusion, recent advances in photocatalytic upcycling of plastic wastes, including the mechanism, the modus between the photocatalysts and the plastic wastes as well as the product selectivity, have been summarized in this contribution.Despite these achievements, the product efficiency and selectivity are still far from satisfactory.It is sincerely expected to explore the influence of the surface structure on the photocatalytic upcycling process, efficient methods to improve the contact between the photocatalysts and plastic wastes as well as deep investigation on the photocatalytic upcycling routes, aiming at constructing systematic relationship between the structure of the photocatalysts and the property of photocatalytic valuation toward plastic wastes.This review helps to provide critical insight for the future design of efficient photocatalysts and promote the practical application of photocatalytic upcycling process of plastic wastes.

Figure 1 .
Figure 1.Advantages and disadvantages of different strategies to treat plastic wastes.(1) Reproduced with permission.[2] Copyright 2022, Wiley-VCH.(2)Reproduced with permission under the terms of the Creative Commons CC BY license.[108]Copyright 2021, the Authors.Published by Springer Nature.(7) Reproduced with permission.[109]Copyright 2003, The American Association for the Advancement of Science.

Figure 2 .
Figure 2. Illustration for the photocatalytic routes of plastic wastes.

Figure 3 .
Figure 3. Available types and structure of plastic wastes to be upcycled via photocatalysis.

Figure 4 .
Figure 4. a) Schematic illustration for the preparation of PE-TiO 2 films and the setup of the photocatalytic reactors.Reproduced with permission.[82]Copyright 2013, Elsevier Ltd. b,d) SEM images of PS and PS-TiO 2 samples before irradiation.c,e) SEM images of PS and PS-TiO 2 samples after irradiation for 40 h.f ) a-c refers to the FT-IR spectra of PS sample before irradiation, PS sample after 30 h irradiation and PS-TiO 2 sample after 30 h irradiation, respectively.g) Weight loss of PS and PS-TiO 2 samples with the dependence irradiation time.f ) Changes in the average molecular weight of PS and PS-TiO 2 samples with the dependence irradiation time.Reproduced with permission.[83]Copyright 2003, Elsevier Ltd.

Figure 6 .
Figure 6.a) Schematic illustration for the preparation of photoactive microrobots.Reproduced with permission. [97]Copyright 2023, Nature Publishing Group.b) Schematic illustration of the movement of BiVO 4 /Fe 3 O 4 microrobots under the light irradiation and magnetic field in homemade channel.c) Weight loss of microplastics with the dependence of treatment time over BiVO 4 /Fe 3 O 4 microrobots in aqueous solution.d) The surface wettability change of microplastics before and after the treatment by BiVO 4 /Fe 3 O 4 microrobots.Reproduced with permission.[99]Copyright 2021, American Chemical Society.e) Schematic illustration and different magnified SEM images of the microfiber network after irradiation without the treatment of microrobots.f ) Schematic illustration and different magnified SEM images of the microfiber network after irradiation with the treatment of microrobots.Reproduced with permission.[97]Copyright 2023, Nature Publishing Group.

Figure 7 .
Figure 7. a) Schematic illustration of PE photoreforming into C2 value-added product via Zr doped CoFe 2 O 4 dual-site photocatalyst.b) The evolution yield of CH 3 COOH from PE photoreforming process over Zr-CoFe 2 O 4 and CoFe 2 O 4 .Reproduced with permission.[103]Copyright 2022, American Chemical Society.c) High-resolution TEM image of CPDs-CN-7.d) Time-dependent yields of various products from photoreforming of pretreated PET over CPDs-CN-7 with Pt as the cocatalyst under light irradiation.Reproduced with permission.[86]Copyright 2022, Elsevier Ltd. e) Schematic illustration of pretreated PET photoreforming process over CN x with Ni 2 P as the cocatalyst.f ) Quantification of the products from the pretreated PET over CN x with Ni 2 P as the cocatalyst after 5 days photoreforming.Reproduced with permission.[69]Copyright 2019, American Chemical Society.

Figure 8 .
Figure 8. a) Schematic illustration for the photoreforming of pretreated PLA over CdS nanorods with MoS 2 tip.b) TEM image of MoS 2 /CdS.c) Concentration variation of formate products during the photoreforming process over MoS 2 /CdS.Reproduced with permission.[105]Copyright 2022, American Chemical Society.d) Schematic illustration for the photoreforming of pretreated PLA over d-NiPS 3 /CdS.e) TEM image of d-NiPS 3 / CdS.f ) Quantification of various products over CdS, NiPS 3 /CdS, and d-NiPS 3 /CdS after 9 h of photoreforming.Reproduced with permission.[106]Copyright 2023, American Chemical Society.g) Schematic illustration for the production ofCH 3 COOH over Nb 2 O 5 atomic layer under simulated natural environmental conditions.h,i) Quantification of CO 2 and CH 3 COOH under O 2 and Ar atmosphere over Nb 2 O 5 atomic layer.Reproduced with permission.[81]Copyright 2020, Wiley-VCH.
For example, Kim et al. has integrated Fe 3 O 4 with Bi 2 O 3 /Ag to prepare 1D Fe 3 O 4 / Bi 2 O 3 /Ag microrobots (Figure Jiao et al. found that the C─C bond cleavage in PE is triggered by •OH to form CO 2 with the presence of O 2 , and then the obtained CO 2 undergoes C─C bond coupling through •COOH and HOOC─COOH with the assistance of photoexcited electrons to produce CH 3 COOH.By controlling the atmosphere with O 2 or with Ar during the photocatalytic upcycling process, it is observed that the intermediate CO 2 and the final CH 3 COOH product could only be generated with the presence of O 2 , indicating that O 2 plays significant role in the photocatalytic

Table 1 .
Summary for the characteristics of different integrating modus.