Photocatalytic Degradation of Plastic Waste: a Mini Review

micromachines

Review Photocatalytic Degradation of Plastic Waste: A Mini Review

Qian Ying Lee 1 and Hong Li 1,2,3,*

1 School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore; [email protected] 2 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore 3 CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, Singapore 637553, Singapore * Correspondence: [email protected]

Abstract: Plastic waste becomes an immediate threat to our society with ever-increasing negative impacts on our environment and health by entering our food chain. Sunlight is known to be the natural energy source that degrades plastic waste at a very slow rate. Mimicking the role of sunlight, the photocatalytic degradation process could signiﬁcantly accelerate the degradation rate thanks to the photocatalyst that drastically facilitates the photochemical reactions involved in the degradation process. This mini review begins with an introduction to the chemical compositions of the common plastic waste. The mechanisms of photodegradation of polymers in general were then revisited.

Afterwards, a few photocatalysts were introduced with an emphasis on titanium dioxide (TiO2), which is the most frequently used photocatalyst. The roles of TiO2 photocatalyst in the photodegradation process were then elaborated, followed by the recent advances of photocatalytic degradation of various plastic waste. Lastly, our perspectives on the future research directions of photocatalytic plastic degradation are present. Herein, the importance of catalytic photodegradation is empha- sized to inspire research on developing new photocatalysts and new processes for decomposition of plastic waste, and then to increase its recycling rate particularly in the current pandemic with the Citation: Lee, Q.Y.; Li, H. ever-increasing generation of plastic waste. Photocatalytic Degradation of Plastic Waste: A Mini Review. Micromachines Keywords: plastic waste; photocatalytic degradation; photodegradation mechanisms; titanium 2021, 12, 907. https://doi.org/ dioxide catalyst 10.3390/mi12080907

Academic Editor: Nam-Trung Nguyen 1. Introduction Received: 6 July 2021 Plastic is a widely used product globally due to its versatile properties such as low Accepted: 28 July 2021 weight, user-friendly designs, chemical resistance, excellent thermal stability, and outstand- Published: 30 July 2021 ing electrical insulation. The resin identification code (RIC) system created by the Society of Plastic Industry (SPI) is often used to categorize the type of plastic during production Publisher’s Note: MDPI stays neutral and to facilitate post-consumer plastic recycling. The plastics with SPI number 1 to 7, with regard to jurisdictional claims in including polyethylene terephthalate (PET), polyethylene (PE), polyvinyl chloride (PVC), published maps and institutional affil- polypropylene (PP), polystyrene (PS), etc., are present in Table1[1]. iations. The global plastic production was about 2 million tons in the year 1950, which is trivial compared to the annual production nowadays (368 million tons as of 2019) [2]. According to data from Plastic Europe, the most produced plastic type is polyolefin including high- density polyethylene (HDPE), low-density polyethylene (LDPE), and PP, which are mainly Copyright: © 2021 by the authors. used in the packaging industry. Many of those plastics produced in 1950 still exist as Licensee MDPI, Basel, Switzerland. plastic waste in the environment. One of the reasons for the increasing global plastic This article is an open access article production over the years in various industries is the benefit of one-time use, and thus is distributed under the terms and very convenient and cost-effective. As a result, the global plastic market size is expected to conditions of the Creative Commons reach USD 722.6 billion by 2027 [3]. In addition, the recent COVID-19 outbreak worldwide Attribution (CC BY) license (https:// has caused a surge in the demand for medical personal protective equipment (PPEs) such creativecommons.org/licenses/by/ as single-use disposable face masks, medical face shields, and gloves, most of which are 4.0/).

Micromachines 2021, 12, 907. https://doi.org/10.3390/mi12080907 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, x 2 of 21 Micromachines 2021, 12, x 2 of 21

Micromachines 2021, 12, x 2 of 21

Micromachines 2021, 12, x which are made of polymer materials [4]. For instance, a single-use disposable face mask2 of 21 which are made of polymer materials [4]. For instance, a single-use disposable face mask MicromachinesMicromachines 2021 ,2021 12, x, 12 , x consists of three layers: An inner layer (made of fibrous material), a middle 2layer of 21 2(melt of 21 consists of three layers: An inner layer (made of fibrous material), a middle layer (melt whichgusted are filter made part), of polymerand an outer materials layer [4]. (nonwoven) For instance, [5]. aThe single-use middle disposable filtering layer face ofmask the gusted filter part), and an outer layer (nonwoven) [5]. The middle filtering layer of the Micromachines 2021, 12, x consistsmask can of be three nanofibers layers: Anand/or inner microfibers, layer (made which of fibrous is manufactured material), a usingmiddle raw layer material (melt2 of 21 maskwhich can are bemade nanofibers of polymer and/or materials microfibers, [4]. For which instance, is manufactured a single-use disposable using raw face material mask gustedsuchwhich as filter PP.are madeThe part), outer of andpolymer layer an outerofmaterials the layer mask [4]. (nonwoven) canFor instance,be made [5].a of single-use polyethyleneThe middle disposable filteringterephthalate face layermask (PET).of the suchconsistswhich as are PP. of made threeThe outer oflayers: polymer layer An of innermaterials the masklayer [4]. can(made For be instance, madeof fibrous of polyethylenea single-usematerial), disposablea terephthalate middle layer face (PET). mask(melt maskTheconsists demand can ofbe three fornanofibers disposable layers: Anand/or innerplastic microfibers, layer products (made ofinwhich cludingfibrous is material),manufactured food containe a middle rs,using plasticlayer raw (melt bags, material and Thegustedconsists demand filter of three part),for disposablelayers: and anAn outer plasticinner layer layer products (nonwoven) (made in cludingof fibrous [5]. foodThe material), containemiddle afilteringrs, middle plastic layerlayer bags, of(melt andthe Micromachines 2021, 12, 907 maskgustedsuchwhichpolygusted as mailbagscan are filterPP. filter bemade The part), nanofiberspart), inouter of the polymerandand layerpackaging an and/or outer outerof materials the layermicrofibers, layermaskindustry (nonwoven) [4]. (nonwoven) can For whichbe which instance, made [5]. are [5]. isTheof manufactured made polyethyleneaThe middlesingle-use middlefrom filtering plastic disposable filtering terephthalateusing layer has raw oflayer facealsothe material (PET).of maskbeen 2the of 20 polymask mailbags can be nanofibers in the packaging and/or microfibers, industry whichwhich is aremanufactured made from using plastic raw hasmaterial also been suchmaskTheconsistsgrowing demand as can PP. ofrapidly be threeThe fornanofibers outer disposablelayers:since layer more Anand/or of plasticinner people the microfibers, masklayer products are can(made staying be inwhich madecludingof at fibrous home, isof manufactured polyethylenefood material), calling containe for a terephthalaters, usingfoodmiddle plastic delivery,raw layer bags, material (PET). (melt and growingsuch as PP.rapidly The outersince layer more of thepeople mask are can staying be made at of home,polyethylene calling terephthalate for food delivery, (PET). and Thesuchpolygustedonline demand asmailbags shopping filterPP. The part),for inouter disposableparticularly the and layerpackaging an outerof plastic in the the layermaskindustry productscurrent (nonwoven) can pandemicwhichbe in madecluding are [5]. of [4]. made polyethylenefoodThe containemiddlefrom plastic filteringterephthalaters, plastic has layer alsobags, (PET).of been and the onlinegrowingThe demand shopping rapidly for particularlydisposablesince more plastic inpeople the products current are staying inpandemiccluding at home,food [4]. containe callingrs, for plastic food bags, delivery, and and polyThemaskmadepoly demand mailbagscan mailbags of be polymer fornanofibers inin disposable thethe materials packaging and/or plastic [4]. microfibers,industry Forindustry products instance, which which inwhich acluding are single-use are madeis manufacturedmadefood from disposable containefrom plastic plastic rs,hasusing face plastic also maskhas raw been alsobags, consistsmaterial been and of Table 1. SPI number onlinefor different shopping types particularlyof plastic and in the the reco currentvery rate pandemic from the total[4]. solid plastic waste [1]. Table 1. SPI number growingpolysuchforthreegrowing different asmailbags layers:PP. rapidly rapidly typesThe An inouter of since sincethe innerplastic layer packagingmore layer and of people people thethe (made reco maskindustry are veryare of staying can fibrous stayingrate whichbe from atmade material), home,at theare home, of total madecallingpolyethylene a solidcalling middle fromfor plastic food for layerplastic terephthalate wastefooddelivery, (melt has[1].delivery, gustedandalso (PET). been and filter onlinegrowingThepart),online demand shopping andshoppingrapidly anfor outer particularly disposableparticularlysince layer more (nonwoven) plasticin inpeople the the current currentproducts are [stayingpandemic5]. pandemic Theincluding at middle [4]. home, [4]. food filtering calling containe layerforrs, food ofplasticCurren thedelivery, mask bags,can and be Table 1. SPI number for different types of plastic and the recovery rate from the total solid plastic wasteCurren [1]. Recove SPI Full onlinepolynanofibers mailbags shopping and/or in particularly the microfibers, packaging in the whichindustry current is manufactured pandemicwhich are [4]. made using from raw materialplastic hastly such also asRecove PP.been The SPI TableFullTable 1. SPI 1. SPInumber number growingfor for different different rapidly types Chemicaltypes of sinceof plasticplastic Structuremore andand the peoplethe reco recovery veryare rate stayingrate from from the at thetotal home, total solid solid callingplastic Uses plastic waste for waste[1].food Curren [1].delivery,tly ry Rate and Number Name outer layerChemical of the mask Structure can be made of polyethylene terephthalate Uses (PET).Recycla TheryRecove demand Rate NumberSPI TableNameFull 1. SPI number onlineforfor different disposable shopping types plasticparticularlyof plastic products and inthe the reco including currentvery rate foodpandemic from containers, the total[4]. solid plastic plastic bags,Curren waste andRecycla [1].tly poly mailbags(%) Chemical Structure Uses Currenble?Recove ry(%) Rate NumberSPI NameFull in the packaging industry which are made from plastic has also beently Recycla growingble? Recove rapidly SPI Full Chemical Structure UsesDisposable Currenrytly Rate (%) NumberTable Name 1. SPI number forsince different more typesChemical people of plastic are Structure stayingand the reco at home,very rate calling from the for total food solidDisposable delivery, Uses plastic Recycla andwaste online [1]. shoppingRecovery Rate NumberSPI NameFull bottles for Recyclable?tly(%) Polyethy particularlyChemical in the current Structure pandemic [4]. bottles Uses for ble? ry(%) Rate Number PolyethyName Disposabledrinks, RecyclaCurrenble? lene Disposabledrinks, Recove(%) SPI Full bottles for ble?tly 1 PolyethyleneTable 1. SPI number for differentChemical types of plastic Structure and the recovery rate from the totalbottlesmedicines,Disposable solid Usesfor plastic and waste Yes [1]. ry19.5 Rate Number1 terephthNamePolyethy medicines,drinks, and RecyclaYes 19.5 terephth drinks,manyDisposablebottles other for Polyethyalatelene many other (%) 1 SPI lene PETE/PET medicines,Currently and ble?Yes Recovery19.5 1 terephthalate Full Name Chemical Structure Usesmedicines,bottlesconsumerdrinks, and for Yes 19.5 NumberPolyethyleneterephth PETE/PET consumerRecyclable? Rate (%) 1 manymedicines,manyDisposableproducts otherdrinks, other and Yes 19.5 alatealate terephthlene PETE/PETPETE/PET consumerproducts 1 consumermedicines,Moremanybottles durable other for and Yes 19.5 Polyethyterephthalate Disposable bottlesMore for durable PETE/PET productsmanyproductsdrinks, other High-lene Polyethylene drinks, medicines,containers,consumer and such 1 High-alate Morecontainers,More durable durable suchYes 19.5 1 densityterephthalate PETE/PET many other consumermedicines,asconsumerproducts those for and Yes 19.5 2 terephthdensityHigh- containers,as those such for Yes 10 2 polyethyHigh- productscontainers,Moremanydetergent,products durable other such Yes 10 polyethyalatedensity HDPE as thosedetergent, for 2 density PETE/PETHDPE Moreasconsumer those durable for Yes 10 2 High-lenepolyethy detergent,bleach,containers, shampoo such Yes 10 polyethylene HDPE More durablebleach,detergent,products shampoo densityHigh-lene HDPE bleach,containers,oras shampoo motorthose for suchoil 2 lene containers, suchbleach,or as motor shampoo oil Yes 10 polyethydensity High-density or Piping,motorMoreasdetergent, those oildurable cables, for 2 2 Polyviny HDPE those for detergent,Piping, cables,Yes Yes 10 PolyvinypolyethyHigh-lene polyethylene Piping,gardenbleach,containers,ordetergent, cables,motor furniture,shampoo oilsuch 3 Polyvinyl HDPEHDPE bleach, shampoogarden or furniture, No 0 3 densitylenel motorgarden oilbleach,Piping,orfencingas furniture, motorthose shampoo cables, and foroil No 0 2 3Polyviny chloridel fencing and No Yes0 10 polyethy PVC fencinggardenordetergent, motor and furniture, oil 3 chloridelchloride HDPEPVC carpetPiping, backing cables, No 0 Polyviny PVC Piping, cables, gardencarpet backing lene Polyvinyl carpetgardenbleach,Piping,Plasticfencing backing furniture,shampoo cables,bags, and 3 Polyvinychloride PVC furniture, fencing and No 0 3 Low-l chloride PlasticcarpetPlasticor bags,motor backing bags, oil No 0 Low-Low- carpet backinggardenwrappingfencing furniture, andfilms, 3 chloridedensityl PVCPVC wrappingwrapping films, films, MostlyNo 0 4 densitydensity carpetPiping,Plasticfencingtrays backing cables,andbags, andMostly Mostly 5 4 4Polyviny polyethychlorideLow- traystrays and and no5 5 polyethy LDPEPVC Plastic bags, wrappinggardenwrappingcarpet furniture,backing films,no 3 polyethydensityl Low-density LDPELDPE computerPlasticcomputer bags, MostlyNono 0 4 4 Low-lenelene films, trays andfencingcomputertrays and andMostly no5 polyethychloridelene polyethylene componentswrappingPlasticcomponents bags, films, no densityLow- LDPELDPEPVC computer componentscarpetcomponentscomputer backing Mostly 4 lene BottlewrappingBottletrays caps, caps,and films, 5 polyethydensity componentsBottle caps, Mostlyno 4 PolyproPolypro LDPE Bottle caps,reusable reusablereusablePlasticcomputertrays food andbags, foodSometi Someti 5 5 5polyethy PolyproLow-lene reusable food Sometino1 1 5pylene Polypropylene LDPE food containerscontainerswrapping andBottlecomputer and caps, films,Sometimes mes 1 5 densitypylene containerscomponents and Mostlymes 1 Polypropylenelene PP car partscontainersreusable food and Sometimes 4 PP carcomponentsBottle partstrayscar parts caps,and 5 5 polyethy car parts no 1 pylene LDPE PlasticcontainersBottle computerutensils, caps, and mes Polyprolene PP Plasticreusable utensils, food Someti 5 Polystyr Plastic utensils,packagingPlasticcar partsutensils, Someti 1 6 PolystyrPolypropylene containersreusablecomponentspackaging food and Sometimes1 56 6Polystyrene Polystyrene packaging peanutspeanutspackaging and Sometimes mes Someti 1 pylene PP PlasticcontainersBottle utensils, caps, and mes 6 ene PS and Styrofoam (EPS)peanutscar parts and mes 1 PolystyrPolyproene PPPS Styrofoamreusablepeanutspackaging (EPS) foodand Sometimes 6 StyrofoamPlasticcar partsutensils, (EPS) 1 5 Other: PS MultilayerStyrofoam (EPS) 1 Polystyrpyleneene Other: for Plasticcontainerspeanutspackaging utensils, and and Sometimes Other:for PP Multilayerbarrier barrierMultilayer films, 6 Other: example, PS StyrofoamMultilayercar parts (EPS) 1 Polystyreneforexample, films, toothbrushes,toothbrushes,barrierpeanutspackaging films, and Sometimes 6 77 for polycarbonate barrier films,No Varies Varies1 example,Other:enepolycarb PS some food containers,someStyrofoamPlastictoothbrushes,peanutsMultilayer food utensils, and(EPS) mes 7 example,and polymethyl toothbrushes, No Varies 7 polycarbPolystyrOther:foronate PS CDs, andcontainers, DVDsStyrofoambarrierMultilayersomepackaging CDs, foodfilms, (EPS) SometiNo Varies 6 polycarbmethacrylate PMMAPMMA PC PC some food 1 example,Other:onateenefor and andcontainers,toothbrushes,barrierpeanuts MultilayerDVDs films, and CDs, mes 7 onate PMMAPS PC containers, CDs, No Varies example,polycarbandfor PMMA PC Styrofoamtoothbrushes,barriersomeand DVDs foodfilms, (EPS) 7 and In 2020 alone, about 52 billion single-use disposableand face DVDs masks haveNo been producedVaries example,polycarbOther:onate PMMA PC containers,toothbrushes,Multilayersome food CDs, 7 owing to the pandemic, and about 1.6 billion (3%) of them enter the oceansNo thatVaries need polycarbonateandfor containers,barrierandsome DVDs foodfilms, CDs, 40–50 centuriesPMMAfor complete PC degradation. Tons of microplastics will be released in the example,onate containers,toothbrushes, CDs, 7 and oceans duringPMMA the natural PC degradation of these masks, whichand DVDs will likelyNo enter Varies our food polycarband andsome DVDs food chain. Moreover, other plastic wastes have also caused a variety of environmental issues, onate directly imposingPMMA an immediate PC threat to our health andcontainers, survival. As CDs, a result, excessive use and of plastic has caused a flood of plastic wastes entering theand natural DVDs environment, bringing adverse effects to humans and the environment [6]. On top of that, more environmental issues have emerged, especially when there is a lack of proper management and handling of plastic wastes. Micromachines 2021, 12, 907 3 of 20

Recycling is one of the promising methods to reduce the detrimental effects caused by plastic wastes. PET and HDPE show a recovery rate of 19.5% and 10%, respectively, as depicted in Table1. However, plastic such as PVC, LDPE, PP, and PS that has a recovery rate of less than 5%, depending on their applications, are barely recyclable. PVC has a recovery rate of 0%, suggesting that it is completely non-recyclable which is mainly due to the high chlorine content in its raw material and the high level of hazardous additives in it. One can see that only less than 10% of the plastic wastes are recycled on average, leaving more than 80% to accumulate in the natural environment [7]. Therefore, engineering plastic degradation processes such as photodegradation [6,8], thermo-oxidative degradation [9,10], and biodegradation [11] have attracted intensive research interest. Photodegradation has attracted our attention as it strikes an excellent balance of energy consumption, time consumption, and cost-effectiveness, holding great promise to end plastic wastes. Most importantly, it can capitalize on abundant and renewable solar energy. Herein, we revisit the concept of catalytic photodegradation of plastic wastes. We start with the discussion of the mechanisms involved in photocatalytic degradation of polymer, followed by summarizing the recent advances of catalytic photodegradation of various types of plastic polymers to shed light on the degradation of the entire spectrum of plastic waste, as illustrated in Figure1. Plastic polymers including PE (LDPE, HDPE), PP, PS, and Micromachines 2021, 12, x 4 of 21 PVC are described in our review as they constitute the world’s most common plastic and most of them are challenging to be recycled. Lastly, we present our prospective on the future research directions on photocatalytic degradation of plastic wastes.

Figure 1. Focus of this review on catalytic photodegradation for various types of plastic polymers. Figure2. 1. Photodegradation Focus of this review of on Polymers catalytic photodegradation for various types of plastic polymers. 2.1. Background of Photodegradation

The sun emits energy over a broad range of wavelengths from ultraviolet spectrum (UV), visible spectrum (Vis-), to infrared spectrum (IR) with decreasing energy. Ultraviolet radiation consists of electromagnetic waves with a wavelength between 100 and 400 nm [12], Micromachines 2021, 12, x 5 of 21

2. Photodegradation of Polymers 2.1. Background of Photodegradation Micromachines 2021, 12, 907 The sun emits energy over a broad range of wavelengths from ultraviolet spectrum4 of 20 (UV), visible spectrum (Vis-), to infrared spectrum (IR) with decreasing energy. Ultraviolet radiation consists of electromagnetic waves with a wavelength between 100 and 400 nm [12], which is further divided into three major sub-regions. With the wavelengthwhich is further ranging divided from into100 threeto 280 major nm, UV-C sub-regions. is fully Withabsorbed the wavelength by the ozone ranging in the from atmosphere.100 to 280 nm,UV-B UV-C has a wavelength is fully absorbed from 280 by to the 315 ozonenm, whereas in the UV-A atmosphere. has a wavelength UV-B has a fromwavelength 315 to 400 from nm. 280 Most to 315 of the nm, UV-B whereas is also UV-A absorbed has a by wavelength the atmosphere, from 315 while to 400 UV-A nm. is Most completelyof the UV-B reachable is also absorbed to the by Earth’s the atmosphere, surface. whileVisible UV-A light is is completely the section reachable of the to the electromagneticEarth’s surface. spectrum Visible light that is is the visible section to ofhumans, the electromagnetic covering the spectrumrange of wavelength that is visible to fromhumans, 400 coveringto 700 nm. the rangeInfrared of wavelengthradiation co fromnsists 400 of to electromagnetic 700 nm. Infrared waves radiation with consists a wavelengthof electromagnetic from 700 waves to 1 mm. with a wavelength from 700 to 1 mm. UVUV radiation radiation is isknown known as as the the most most damaging damaging source source to polymers to polymers owing owing to its to high its high energy.energy. Hence, Hence, polymers polymers that that are are continuous continuouslyly exposed exposed to UV to UVradiation radiation will willundergo undergo deteriorationdeterioration in in their physical and chemicalchemical structure,structure, resultingresulting inin photodegradation.photodegradation. Pho- Photodegradationtodegradation of polymerof polymer includes includes chain chain scission, scission, alteration alteration of molecule’s of molecule’s shape, shape, reduction reductionin molecule’s in molecule’s weight, and weight, deterioration and deterioration of polymer of polymer properties properties typically typically in the presence in the of presenceUV radiation of UV and radiation oxygen and [8 ].oxygen [8].

2.2.2.2. Photodegradation Photodegradation Mechanisms Mechanisms ThereThere are twotwo well-regardedwell-regarded photodegradation photodegradation mechanisms, mechanisms, i.e., i.e., singlet singlet oxygen oxygen induced inducedoxidation oxidation [8,13] and [8,13] free radical and free caused radical oxidation caused [14 ],oxidation as detailed [14], in theas followingdetailed in sections. the following sections. 2.2.1. Singlet Oxygen Mechanism of Oxidation 2.2.1. Singlet Oxygen oxygen mechanismMechanism of of Oxidation oxidation involves the direct reaction of singlet oxygen withSinglet polymer. oxygen The mechanism singlet oxygen of oxidation is produced involv duees the to thedirect quenching reaction of of singlet the excited oxygen triplet withstate polymer. of suitable The sensitizers singlet oxygen (3S), asis produced depicted indue Equation to the quenching (1). of the excited triplet state of suitable sensitizers (3S), as depicted in Equation (1). 3 3 1 1 S + O2 → S + O2 (1) S O →SO (1) AA singlet singlet oxygen oxygen could could react react with with the the prod productuct of a NorrishNorrish reaction,reaction, which is aa pho- photochemicaltochemical reaction reaction taking taking place place with with ketones ketonesand and aldehydes.aldehydes. A A Norrish Norrish reaction reaction can can be besubdivided subdivided into into type type I andI an typed type II reactions,II reactions, and and Norrish Norrish Type Type II II reaction reaction causes causes inter- intermolecularmolecular rearrangement rearrangement of the of carbonylthe carbonyl group group (C=O) (C=O) to form to form the vinyl the vinyl group group (-CH=CH (- 2). CH=CHThereafter,2). Thereafter, a generated a generated singlet oxygen singlet that oxygen reacts that with reacts the with vinyl the group vinyl could group then could further thendecompose further thedecompose molecule, the leading molecule, to chain leading scission to andchain formation scission ofand hydroperoxides formation offunc- hydroperoxidestional group (ROOH), functional as showngroup (ROOH), in Figure 2asa. shown Likewise, in Figure the singlet 2a. Likewise, oxygen producedthe singlet also oxygenleads to produced the formation also leads of ROOH to the by formation the oxidation of ROOH of an oleﬁnby the containing oxidation allylicof an olefin hydrogen, containingas depicted allylic in Figure hydrogen,2b. as depicted in Figure 2b.

FigureFigure 2. 2. SingletSinglet oxygen oxygen mechanism mechanism of ofoxidation. oxidation. (a) (Singleta) Singlet oxygen oxygen oxidation oxidation of a vinyl of a vinyl group group to to formform the the hydroperoxide hydroperoxide func functionaltional group group (ROOH) (ROOH) [13]; [13 reprinted]; reprinted from from [13] [with13] with permission permission from from Elsevier. (b) Singlet oxygen oxidation of an oleﬁn group to form the hydroperoxide functional group (ROOH) [8].

2.2.2. Free Radical Mechanism of Oxidation The free radical mechanism of oxidation involves producing free radicals that react with the oxygen. The high energy in the UV radiation breaks the C-C and C-H bonds in polymers to create free radicals. The free radicals then react with the oxygen to create the Micromachines 2021, 12, x 6 of 21

Elsevier. (b) Singlet oxygen oxidation of an olefin group to form the hydroperoxide functional group (ROOH) [8].

Micromachines 2021, 12, 907 2.2.2. Free Radical Mechanism of Oxidation 5 of 20 The free radical mechanism of oxidation involves producing free radicals that react with the oxygen. The high energy in the UV radiation breaks the C-C and C-H bonds in polymers to create free radicals. The free radicals then react with the oxygen to create the hydroxyl group (O-H) and carbonyl group (C=O). The mechanism involves three main hydroxyl group (O-H) and carbonyl group (C=O). The mechanism involves three main steps, i.e., initiation, propagation, and termination, as summarized in Figure3. steps, i.e., initiation, propagation, and termination, as summarized in Figure 3.

FigureFigure 3. 3.Photodegradation Photodegradation mechanism mechanism of of polymer polymer involving involving free free radical radical oxidation. oxidation. Purple Purple dotted dotted box: box: Initiation Initiation step; step; black dashed boxes: Propagation step; orange dash-dotted boxes: Termination step. black dashed boxes: Propagation step; orange dash-dotted boxes: Termination step.

In the initiation step, the existence of chromophore group, which absorbs the incident light due to energy matching, in the polymer’s structure is important to act as the photo- initiator [8]. As a result, the absorbed light-excited charge carriers break the chemical bonds in the polymer chains to produce hydrogen radicals and polymer radicals (Path 1). In principle, non-absorbing polymers such as polyethylene (PE) and polypropylene (PP) are almost perfectly stable on exposure to solar radiation with a wavelength greater than Micromachines 2021, 12, 907 6 of 20

290 nm, since the structure contains only bonds of C-C and C-H [15]. Moreover, they have no unsaturated chromophores that can absorb light to form free radicals. However, the impurities or structural defects in the polymers, as well as carbonyl groups within the backbone of the polymer can act as chromophores to carry out the photo-initiation [6]. The potential sources for producing free radicals are carbonyl groups such as ketones and hydroperoxides. Hence, different initiation steps have been undertaken for various conditions in different polymers to form the free radicals. Those reactions can be initiated by physical factors such as direct UV initiated photolysis of C-C and C-H bond or by chemical factors such as the residues of catalyst used, incorporation of carbonyl groups, and introduction of peroxides or unsaturation site [13]. The free radicals produced can extract hydrogen atoms from the other polymers and thus initiate the photodegradation activity. In the propagation step [8,16], polymer macro radicals formed through photo-initiation react with the oxygen to form polymer peroxyl radicals (Path 2). Subsequent reactions of polymer peroxyl radicals with another polymer produce hydroperoxides and polymer macro radicals (Path 3). Afterwards, the polymer macro radicals produced undergo auto-oxidation to repeat the formation of polymer peroxyl radicals (Path 4). In addition, the hydroperoxides react with another hydroperoxide to form polymer alkoxy radicals, polymer peroxide radicals, and water (Path 5). Moreover, the photodecomposition of hydroperoxides leads to the formation of polymer alkoxy radicals and hydroxyl radicals (Path 6). Propagation ultimately leads to chain scission to form an oxygen-containing functional group including olefin and ketone (Path 7). The hydroxyl radicals produced will extract hydrogen from another polymer to form polymer macro radicals and water (Path 8). Lastly, in the termination step [8], the free radicals produced react with each other to undergo the crosslinking reaction for the formation of inert products (Path 9). When the oxygen pressure is high, polymer peroxyl radicals react with themselves to form inert products and oxygen (Path 10). In contrast, when sufficient oxygen cannot be maintained, polymer peroxyl radicals react with polymer macro radicals (Path 11). Conclusively, olefins and ketones are the expected products of the termination reactions [16]. The whole process causes a reduction in the molecule’s weight, and the polymer becomes more brittle, leading to further photodegradation.

3. Photocatalytic Degradation In photocatalytic degradation, which is a photochemical reaction process with the help of photocatalysts, a semiconductor is often used to absorb light and to accelerate the photoreaction rate [17]. Photocatalysis is used in many applications such as removal of pollutants and bacteria [18], energy conversion [19], and water splitting for green hydrogen generation [20]. An ideal photocatalyst should be able to absorb light at room temperature, and have high stability towards photo corrosion, as well as non-toxicity.

3.1. Titanium Dioxide (TiO2) as Photocatalyst

TiO2 is the most widely used photocatalyst due to its versatile properties such as high level of oxidation-reduction ability [21,22], chemical stability [21], high-temperature stability, cost-effectiveness, and environmental friendliness [23,24]. The oxidation-reduction ability of a photocatalyst depends on its energy band position w.r.t. redox potentials. As illustrated in Figure4, using water splitting redox potentials as the reference, TiO 2 has a more positive electrochemical potential with respect to the normal hydrogen electrode (NHE) potential. Having the valance band maximum (VBM) more positive than 1.23 eV, showing that the oxidation ability is sufﬁcient in oxidizing water. The lower the position of VBM of a semiconductor, the higher the oxidation capability it has. In contrast, the conduction band minimum (CBM) should be more negative than the hydrogen reduction potential for affecting the reduction of water. The higher the position of CBM of a semiconductor, the higher the reduction capability it has. Thus, TiO2 has one of the highest oxidation capabilities among all of the semiconductors listed in Figure4. However, one Micromachines 2021, 12, x 8 of 21 Micromachines 2021, 12, x 8 of 21

reference, TiO2 has a more positive electrochemical potential with respect to the normal reference, TiO2 has a more positive electrochemical potential with respect to the normal hydrogen electrode (NHE) potential. Having the valance band maximum (VBM) more hydrogen electrode (NHE) potential. Having the valance band maximum (VBM) more positive than 1.23 eV, showing that the oxidation ability is sufficient in oxidizing water. positive than 1.23 eV, showing that the oxidation ability is sufficient in oxidizing water. The lower the position of VBM of a semiconductor, the higher the oxidation capability it The lower the position of VBM of a semiconductor, the higher the oxidation capability it has. In contrast, the conduction band minimum (CBM) should be more negative than the has. In contrast, the conduction band minimum (CBM) should be more negative than the hydrogen reduction potential for affecting the reduction of water. The higher the position Micromachines 2021, 12, 907 hydrogen reduction potential for affecting the reduction of water. The higher the position7 of 20 of CBM of a semiconductor, the higher the reduction capability it has. Thus, TiO2 has one of CBM of a semiconductor, the higher the reduction capability it has. Thus, TiO2 has one of the highest oxidation capabilities among all of the semiconductors listed in Figure 4. of the highest oxidation capabilities among all of the semiconductors listed in Figure 4. However, one should decrease the band gap to increase light adsorption. For instance, However, one should decrease the band gap to increase light adsorption. For instance, ZnSshould has decrease a too large the band gap to increaseabsorb light light efficiently adsorption. though For instance, it has high ZnS oxidation has a too large and ZnS has a too large band gap to absorb light efficiently though it has high oxidation and reductionband gap tocapability. absorb light Put efﬁcientlytogether, TiO though2 has ita hasgreat high balance oxidation between and oxidation-reduction reduction capability. reduction capability. Put together, TiO2 has a great balance between oxidation-reduction capabilityPut together, and TiO band2 has gap a size, great resulting balance betweenin an excellent oxidation-reduction photocatalyst. capability and band capabilitygap size, resultingand band in gap an size, excellent resulting photocatalyst. in an excellent photocatalyst.

FigureFigure 4. 4. BandBand gap gap potential potential diagram diagram of of the the most most commo commonn semiconductors, used as photocatalysts Figureat NHE 4. scale Band [22]; gap reproduced potential diagram from [22] of thewith most permission commo nfrom semiconductors, the Springer Nature.used as photocatalysts atat NHE NHE scale scale [22]; [22]; reproduced reproduced from from [22] [22] wi withth permission permission from from the the Springer Springer Nature. Nature.

TiOTiO2 hashas three three crystalline crystalline forms: forms: Anatase, Anatase, ruti rutile,le, and and brookite brookite [25–27], [25–27], as as depicted depicted in TiO2 2has three crystalline forms: Anatase, rutile, and brookite [25–27], as depicted in 6 FigureFigure 55a–c.a–c. Different phasesphases havehave differentdifferent symmetriessymmetries ofof thethe octahedral-shapedoctahedral-shaped TiOTiO6 Figure 5a–c. Different phases have different symmetries of the octahedral-shaped TiO6 fundamental building blocks [28]. Anatase and rutile forms are commonly used in fundamentalfundamental building building blocks [[28].28]. AnataseAnatase andand rutile rutile forms forms are are commonly commonly used used in pho- in photocatalytictocatalytic degradation, degradation, while while brookite brookite is is uncommonly uncommonly used used as as a a photocatalystphotocatalyst due to photocatalytic degradation, while brookite is uncommonly used as a photocatalyst due to itsits unstable unstable structure. structure. Anatase Anatase can can be be converted converted into into rutile rutile by by heating heating above above 700 700 °C◦C[ [25].25]. its unstable structure. Anatase can be converted into rutile by heating above 700 °C [25]. TiOTiO22 can appearappear inin various various geometry geometry including including nanoparticles nanoparticles [29 ],[29], nanowires nanowires [30], [30], nan- TiO2 can appear in various geometry including nanoparticles [29], nanowires [30], nanotubesotubes [31 ],[31], and and other othe nanostructuresr nanostructures [32]. [32]. nanotubes [31], and other nanostructures [32].

Figure 5. Atomic structure ofof ((aa)) anataseanatase TiOTiO22,(, (bb)) rutilerutile TiO TiO2,2, and and ( c(c)) brookite brookite TiO TiO2.2. Green Green and and red red balls balls represent represent Ti Ti and and O Figure 5. Atomic structure of (a) anatase TiO2, (b) rutile TiO2, and (c) brookite TiO2. Green and red balls represent Ti and 2 − − + + Oatoms, atoms, respectively. respectively. Schematic Schematic carrier carrier recombination recombination process process in ( din) anatase(d) anatase and and (e) rutile (e) rutile TiO2 TiO. The. The e and e and h represent h represent the O atoms, respectively. Schematic carrier recombination process in (d) anatase and (e) rutile TiO2. The e− and h+ represent photogenerated electron and hole, respectively. Reproduced from [27] with permission from the PCCP Owner Societies.

the photogenerated electron and hole, respectively. Reproduced from [27] with permission from the PCCP Owner Societies.

Anatase TiO2 has a CBM at the energy level of ECB = −0.51 V, while rutile TiO2 has CBM at ECB = −0.31 V [33]. In contrast, their VBMs are similar at an energy level of EVB = +2.69 V. Consequently, the anatase form has a slightly larger bandgap Eg = 3.2 V than that of rutile (Eg = 3.0 V), leading to a higher energy of photogenerated charges. As a result, anatase has a relatively higher photocatalytic activity, however, a relatively narrower absorption bandwidth. Moreover, anatase exhibits an indirect bandgap, while rutile and brookite show direct bandgaps, as illustrated in Figure 5d,e. Hence, the lifetime of photogenerated electron-hole pair in anatase is longer since a direct recombination of electron-hole pair is forbidden theoretically. The detailed energy band structure and density of state (DOS) of anatase and rutile TiO2 are similar, as shown in Figure 6 [27]. Owing to strong O 2p-Ti 3d hybridizations, the valance bands contain O 2p and some Ti 3d states. Such strong p-d hybridizations broaden Micromachines 2021, 12, 907 the valance bands and enhance the photo carrier transfer. In contrast, the conduction8 of 20 bands consist of primarily Ti 3d states with minor O 2p and Ti 3p states. It is worth noting that the calculated band gaps of anatase and rutile TiO2 are smaller than the measured onesof photogenerated due to the limitation electron-hole of basic pairdensity in anatase functional is longer theory since calculations a direct [34,35]. recombination However, of thiselectron-hole deficiency pair can is forbiddenbe corrected theoretically. by the GGA+U approach with more appropriate determinedThe detailed U parameter energy values band structure [36]. The and GGA+U density approach of state is (DOS) improved of anatase in terms and of rutile the energy gap of material and impurity state in the gap. It yields an energetic sequence TiO2 are similar, as shown in Figure6[ 27]. Owing to strong O 2p-Ti 3d hybridizations, the consistentvalance bands with contain the experiments, O 2p and some Erutile Ti< E 3danatase states., for Such5 < U strong< 8 eV. p-d hybridizations broaden the valanceMoreover, bands the and effective enhance mass the photoof photogenerated carrier transfer. charge In contrast, carriers the can conduction be obtained bands by parabolicconsist of primarilyfitting to Tithe 3d calculated states with CBM minor and O 2pVBM and Tialong 3p states. a specific It is worthdirection noting in thatthe reciprocalthe calculated space. band Notably, gaps ofthe anatase average and effective rutile mass TiO2 ofare photogenerated smaller than the charge measured carriers ones in anatasedue to the phase limitation is lower of basic than density those functionalin rutile and theory brookite calculations phases, [34 resulting,35]. However, in faster this migrationdeﬁciency of can the be photogenerated corrected by the electron GGA+U and approach holes to with the more surface appropriate of TiO2 [27]. determined Therefore, U moreparameter photogenerated values [36]. charge The GGA+U carriers approach in anatase is can improved participate in terms in the of thesurface energy reactions. gap of Notably,material andthe impurityphotocatalytic state inactivity the gap. also It yieldsgreatly an depends energetic on sequence other factors consistent such with as the geometryexperiments, [32], E therutile substrates< Eanatase ,[33], for 5and < U the < 8 oxygen eV. vacancy/defects density [37].

Figure 6. EnergyEnergy band band structure structure (left (left pane panel)l) and and DOS DOS (right (right panel) panel) for for(a) anatase (a) anatase and and(b) rutile (b) rutile TiO2 TiO[27];2 reproduced[27]; reproduced from [27]from with [27] permission with permission from fromthe PCCP the PCCP Owner Owner Societies. Societies.

3.2. OtherMoreover, Photocatalysts the effective mass of photogenerated charge carriers can be obtained by parabolic ﬁtting to the calculated CBM and VBM along a speciﬁc direction in the reciprocal Zinc oxide (ZnO) having a comparable bandgap to that of TiO2 is often used as an space. Notably, the average effective mass of photogenerated charge carriers in anatase alternative to TiO2, as presented in Figure 7a. Many reports have focused on the use of phase is lower than those in rutile and brookite phases, resulting in faster migration ZnO as the photocatalyst, and some results have also shown a high photocatalytic of the photogenerated electron and holes to the surface of TiO [27]. Therefore, more degradation rate [38–42]. Iron oxide [43], cadmium sulphide [44],2 zinc sulphide [45], photogenerated charge carriers in anatase can participate in the surface reactions. Notably, tungsten oxide [46], tin oxide [47], bismuth vanadate [48], and non-metallic carbon nitride the photocatalytic activity also greatly depends on other factors such as the geometry [32], [49,50] are also often employed as the photocatalysts for photodegradation too, as the substrates [33], and the oxygen vacancy/defects density [37]. depicted in Figure 7b–d. Though the photocatalyst plays the vital role in 3.2. Other Photocatalysts

Zinc oxide (ZnO) having a comparable bandgap to that of TiO2 is often used as an alternative to TiO2, as presented in Figure7a. Many reports have focused on the use of ZnO as the photocatalyst, and some results have also shown a high photocatalytic degradation rate [38–42]. Iron oxide [43], cadmium sulphide [44], zinc sulphide [45], tungsten oxide [46], tin oxide [47], bismuth vanadate [48], and non-metallic carbon nitride [49,50] are also often employed as the photocatalysts for photodegradation too, as depicted in Figure7b–d. Though the photocatalyst plays the vital role in photodegradation, the degradation rate also greatly depends on the polymer structure, photocatalyst mass loading, as well as the experimental setup. Micromachines 2021, 12, x 10 of 21

Micromachines 2021, 12, 907 9 of 20 photodegradation, the degradation rate also greatly depends on the polymer structure, photocatalyst mass loading, as well as the experimental setup.

Figure 7. AtomicFigure structure 7. Atomic of structure typical photocatalysts of typical photocatalysts for photodegradation. for photodegradation. (a) ZnO wurtzite (a) ZnO cell, wurtzite Zn atoms cell, Zn in yellow, O atoms in greyatoms [51]. in (b yellow,) The hexagonal O atoms in representation grey [51]. (b) ofThe unite hexagonal cell of ferric representation oxide (hematite) of unite with cell of Fe ferric atoms oxide in blue and O atoms in red(hematite) [52]; reprinted with fromFe atoms [52] within blue permission and O atoms from in IOP red Publishing. [52]; reprinted Structure from [52] of (c with) s-triazine permission and ( dfrom) tri-s-triazine IOP Publishing. Structure of (c) s-triazine and (d) tri-s-triazine as the primary building blocks of as the primary building blocks of graphitic carbon nitride with C atoms in grey and N atoms in blue [50]; reprinted from [50] graphitic carbon nitride with C atoms in grey and N atoms in blue [50]; reprinted from [50] with with permission from Elsevier. permission from Elsevier. 3.3. Photocatalytic Degradation Mechanism 3.3. Photocatalytic Degradation Mechanism When the photocatalyst such as TiO2 is exposed to the UV light with irradiation energy When the photocatalyst such as TiO2 is exposed to the UV light with irradiation that is equal to or more than its bandgap, the electrons (e−) are excited from its VB to CB, energy that is equal to or more than its bandgap, the electrons (e−) are excited from its VB creating an energetic electron (e−)-hole (h+) pair, as depicted in Equation (2). to CB, creating an energetic electron (e−)-hole (h+) pair, as depicted in Equation (2). hv − + TiO2→ e + h (2) (2) TiO e h Part of the electronsPart of will the electronsrecombine will with recombine holes swiftly with in holes femtoseconds, swiftly in femtoseconds, and the rest and the rest has a longer lifetimehas a longer [24]. lifetimeThe recombination [24]. The recombination of electrons-holes of electrons-holes reduces the number reduces of the number of photogeneratedphotogenerated electrons and electrons holes andtransported holes transported to the TiO to the2 surface TiO2 surface for oxidation- for oxidation-reduction reduction chemicalchemical reactions. reactions. Therefore, Therefore, the thepresence presence of scavengers of scavengers and and incorporation incorporation of of trap sites, trap sites, whichwhich help help reduce reduce the the recombination recombination of of electrons electrons and holesholes ininthe the bulk bulk of of semiconductor, semiconductor,are are attractive attractive strategies strategies to increaseto increase the the catalytic catalytic activity activity [25 ].[25]. Thereafter, Thereafter, the electrons and the electrons andholes holes transferred transferred to the to the surface surface of the of the semiconductor semiconductor undergo undergo interfacial interfacial charge-transfers charge-transfersto carryto carry out out the the oxidation-reduction oxidation-reduction chemical chemical reaction reaction as follows.as follows. In the reductiveIn thereaction, reductive electrons reaction, in the electrons CB of photocatalyst in the CB of photocatalystreact with oxygen react to with oxygen to produce superoxide,produce as superoxide, illustrated asin illustratedEquation (3). in EquationThen, the (3). superoxide Then, the will superoxide undergo will undergo further reductionfurther to produce reduction hydrogen to produce peroxides, hydrogen which peroxides, may generate which mayhydroxides generate and hydroxides and hydroxyl radicalshydroxyl through radicals the reaction through with the reactionthe electrons, with theas shown electrons, in Equation as shown (4) in [33]. Equation (4) [33]. − − O e →OO2+ e → O2 (3) (3) HO e →OH ⋅ OH (4) H O + e− → OH− + ·OH (4) In the oxidative reaction, holes in the VB of2 photocatalyst2 react with the water in the air to form protonIn and the hydroxyl oxidative radicals, reaction, as holes depicted in the in VB Equation of photocatalyst (5). Consequently, react with the water in hydroxyl radicalsthe airwill to decompo form protonse organic and hydroxyl pollutants radicals, into th ase water depicted and in carbon Equation dioxide. (5). Consequently, Hence, hydroxylhydroxyl radical radicals is known will as decomposethe essential organic element pollutants in photocatalysis. into the waterMoreover, and carbonit dioxide. is regarded thatHence, the hydroxylinterfacial radical charge is tran knownsfer asat the essentialsurface of element the semiconductor in photocatalysis. is Moreover, important init isproducing regarded thathydroxyl the interfacial radicals, charge affecting transfer the atoverall the surface photocatalytic of the semiconductor is degradation rateimportant [33]. in producing hydroxyl radicals, affecting the overall photocatalytic degradation rate [33]. HO h →H ⋅ OH (5) + + H2O + h → H + ·OH (5) Micromachines 2021, 12, x 11 of 21

Micromachines 2021, 12, 907 10 of 20

Since UV light only represents 5–8% of the solar spectrum (at sea level), it limits the efficiency of photocatalytic degradation for most photocatalysts such as TiO2 and ZnO. Hence,Since photocatalytic UV light only degradation represents 5–8%under of visible the solar light spectrum is crucial (at seato utilize level), itmore limits solar the energy.efﬁciency To ofincrease photocatalytic the photocatalytic degradation degradat for mostion photocatalystsactivity under visible such as light, TiO 2theand energy ZnO. bandHence, gap photocatalytic needs to degradationbe reduced. under This visiblecan be light achieved is crucial by to utilizeintroducing more solar structural energy. imperfectionsTo increase the into photocatalytic the crystal, a degradation process known activity as chemical under visible doping. light, This the is energy possible band by varyinggap needs the tosynthesis be reduced. conditions This can and/or be achieved adding controlled by introducing amounts structural of impurities imperfections such as C,into N, theFe3+ crystal,, Cr3+, etc. a process[53]. known as chemical doping. This is possible by varying the synthesis conditions and/or adding controlled amounts of impurities such as C, N, Fe3+, Cr3+, etc. [53]. 4. TiO2-Based Photocatalyst in Plastic Degradation

4.1.4. TiO Polystyrene2-Based Photocatalyst(PS) in Plastic Degradation 4.1. PolystyreneThe photocatalytic (PS) degradation of PS plastic [(C8H8)n, as shown in the inset of Figure 2 8a] withThe TiO photocatalytic as photocatalyst degradation was ofinvestigated PS plastic [(C in8 Hthe8) nambient, as shown air in under the inset UV of irradiationFigure8a] [54].with The TiO 2resultsas photocatalyst showed that was the investigated degradation in theof PS-TiO ambient2 composite air under UVexists irradiation at a much [54 higher]. The weightresults showedloss rate that(Figure the 8a), degradation lower average of PS-TiO molecular2 composite weight exists (Figure at a much8b), and higher more weight voids generatedloss rate (Figure (Figure8a), 8c,d), lower as average compared molecular to that weight of pure (Figure PS8 b),film and degradation more voids under generated UV irradiation.(Figure8c,d), as compared to that of pure PS film degradation under UV irradiation.

Figure 8. Photodegradation of PS-TiO2 ﬁlm. (a) Weight loss comparison between pure PS (curve a) and PS-TiO2 samples Figure 8. Photodegradation of PS-TiO2 film. (a) Weight loss comparison between pure PS (curve A) and PS-TiO2 samples (curve b) over the UV irradiation time. Inset: Chemical formula of PS. (b) The variations of average molecular weight M of (curve B) over the UV irradiation time. Inset: Chemical formula of PS. (b) The variations of average molecular weightw Mw ofpure pure PS PS (curve (curve a) andA) and PS-TiO PS-TiO2 samples2 samples (curve (curve b) over B) over the the UV UV irradiation irradiation time. time. SEM SEM images images of (c of) pure (c) pure PS sample PS sample (A3), (A3), and and(d) PS-TiO (d) PS-TiO2 sample2 sample (B3) (B3) after after UV irradiationUV irradiation for 40 for h 40 [54 h]; [54]; reprinted reprinted from from [54] with[54] with permission permission from from Elsevier. Elsevier.

VariousVarious modifiedmodified TiO TiO2 catalysts2 catalysts have have been been employed empl foroyed the for photocatalytic the photocatalytic degrada- degradationtion of PS including of PS copperincluding phthalocyanine copper phthalocyanine (CuPc) sensitized (CuPc) TiO 2 sensitized[PS-(TiO2/CuPc] TiO2 [PS- [55], grated TiO (PS-G-TiO )[56,57], iron phthalocyanine (FePc) sensitized TiO (PS-FePc- (TiO2/CuPc]2 [55], grated2 TiO2 (PS-G-TiO2) [56,57], iron phthalocyanine (FePc)2 sensitized TiO )[58], and hindered amine modified aromatic polyamide dendrimer/ polystyrene- TiO22 (PS-FePc-TiO2) [58], and hindered amine modified aromatic polyamide dendrimer/ grafted TiO (PS-HADPG-TiO )[59]. From the experimental results, modified TiO pho- polystyrene-grafted2 TiO2 (PS-HADPG-TiO2 2) [59]. From the experimental results, modified2 tocatalysts showed a much higher photocatalytic degradation efficiency than pure TiO . TiO2 photocatalysts showed a much higher photocatalytic degradation efficiency than2 When the photocatalytic degradation of PS plastic was carried out with FePc-TiO , HADPG- pure TiO2. When the photocatalytic degradation of PS plastic was carried out with2 FePc- TiO2 or TiO2/CuPc as photocatalyst, it was found that these modified TiO2 had a widened TiO2, HADPG-TiO2 or TiO2/CuPc as photocatalyst, it was found that these modified TiO2 absorption under visible light. For instance, the UV-VIS absorption spectrum of TiO /CuPc had a widened absorption under visible light. For instance, the UV-VIS absorption2 shows a broad absorption peak in the visible range of 500 to 650 nm thanks to CuPc, as spectrum of TiO2/CuPc shows a broad absorption peak in the visible range of 500 to 650 depicted in Figure9a. This enhanced the ability for PS degradation under visible light, nm thanks to CuPc, as depicted in Figure 9a. This enhanced the ability for PS degradation subsequently increasing the photocatalytic degradation efficiency under solar irradiation under visible light, subsequently increasing the photocatalytic degradation efficiency (Figure9b). under solar irradiation (Figure 9b). As illustrated in Figure 9c, the UV portion of the incident light (below 388 nm) is absorbed by TiO2, while the visible portion of the incident light (below 685 nm) is absorbed by CuPc. Upon illumination in the visible range, an electron in CuPc is excited to a singlet state (S1), leaving a hole in the ground state (S0). As the energy level of S1 (−0.63 V vs. NHE) is higher than that of TiO2 (−0.5 V vs. NHE), the excited electron will be swept to TiO2 by the built-in electric field at the interface of TiO2 and CuPc. As such, the photogenerated electron and hole are separated and then utilized to reduce oxygen and to oxidize PS molecule, respectively. Notably, the anatase crystalline form of TiO2 shows Micromachines 2021, 12, 907 11 of 20

Figure 9. Photodegradation of PS-(TiO2/CuPc) ﬁlm. (a) The UV-VIS absorption spectra of TiO2, TiO2/CuPc, and CuPc samples. (b) Weight loss curve of PS-TiO2 and PS-(TiO2/CuPc) samples. (c) The schematic charge separation mechanism of TiO2/CuPc sample under visible and UV light radiation [55]; reprinted (adapted) from [55] with permission from the American Chemical Society.

As illustrated in Figure9c, the UV portion of the incident light (below 388 nm) is absorbed by TiO2, while the visible portion of the incident light (below 685 nm) is absorbed by CuPc. Upon illumination in the visible range, an electron in CuPc is excited to a singlet state (S1), leaving a hole in the ground state (S0). As the energy level of S1 (−0.63 V vs. NHE) is higher than that of TiO2 (−0.5 V vs. NHE), the excited electron will be swept to TiO2 by the built-in electric ﬁeld at the interface of TiO2 and CuPc. As such, the photogenerated electron and hole are separated and then utilized to reduce oxygen and to oxidize PS molecule, respectively. Notably, the anatase crystalline form of TiO2 shows higher activity than rutile TiO2 for photocatalytic degradation, owing to the lower recombination rate and longer lifetime, as discussed in the previous section [58,59].

4.2. Polyvinyl Chloride (PVC) Investigations have been also carried out to determine the photocatalytic degradation efficiency of pure PVC [(C2H3Cl)n, as depicted in the inset of Figure 10a], and PVC film with TiO2 or modified TiO2 as photocatalyst. As expected, the degradation efficiency is always relatively higher for the latter. According to Cho and Choi [60], light penetration into the PVC composite film depends on the concentration and size of TiO2, as well as the film thickness. The higher the amount of TiO2, the greater the light absorbance of the PVC composite film is, as displayed in Figure 10a. However, the optical transparency decreases with more TiO2, and thus shallower light penetration depth, as shown in Figure 10b. Moreover, the weight loss percentage is higher in air ambient compared to that in nitrogen ambient (Figure 10c). It suggests that the presence of oxygen in the air was essential for the efficient photocatalytic degradation of the polymer, agreeing with the reaction mechanism discussed in Equation (3). Interestingly, photocatalytic degradation of PVC film with the vitamin-C (VC)-modified TiO2 photocatalyst was investigated [61]. The specific binding of VC to the TiO2 surface was attributed to the fact that the Ti ions can easily form complexes with oxygen-containing ligands. The UV-VIS spectrum shows that the absorption range of PVC-VC-TiO2 has significantly broadened compared to that of PVC-TiO2 and PVC-VC, as depicted in Figure 11a. The broadened absorption was ascribed to TiO2 and the unsaturated bonds of VC. The redshift of the absorption threshold wavelength up to 600 nm was explained by the bidentate binding of α-substitute surface modifiers, leading to the five-membered ring at the surface of Ti atoms, as illustrated in Figure 11b. It was found that the photocatalytic activity was greatly enhanced by the formation of TiIV-VC charge-transfer complex having a five-member chelate ring structure. Moreover, the optimum mass ratio of VC to TiO2 is 0.5 for the highest efficiency of photocatalytic activity, as shown in Figure 11c.

Micromachines 2021, 12, x 12 of 21

higher activity than rutile TiO2 for photocatalytic degradation, owing to the lower recombination rate and longer lifetime, as discussed in the previous section [58,59].

Figure 9. Photodegradation of PS-(TiO2/CuPc) film. (a) The UV-VIS absorption spectra of TiO2, TiO2/CuPc, and CuPc samples. (b) Weight loss curve of PS-TiO2 and PS-(TiO2/CuPc) samples. (c) The schematic charge separation mechanism of TiO2/CuPc sample under visible and UV light radiation [55]; reprinted (adapted) from [55] with permission from the American Chemical Society.

4.2. Polyvinyl Chloride (PVC) Investigations have been also carried out to determine the photocatalytic degradation efficiency of pure PVC [(C2H3Cl)n, as depicted in the inset of Figure 10a], and PVC film with TiO2 or modified TiO2 as photocatalyst. As expected, the degradation efficiency is always relatively higher for the latter. According to Cho and Choi [60], light penetration into the PVC composite film depends on the concentration and size of TiO2, as well as the film thickness. The higher the amount of TiO2, the greater the light absorbance of the PVC composite film is, as displayed in Figure 10a. However, the optical transparency decreases with more TiO2, and thus shallower light penetration depth, as shown in Figure 10b. Moreover, the weight loss percentage is higher in air ambient compared to that in nitrogen ambient (Figure 10c). It suggests that the presence of oxygen in the air was essential for Micromachines 2021, 12, 907 the efficient photocatalytic degradation of the polymer, agreeing with the reaction12 of 20 mechanism discussed in Equation (3). Micromachines 2021, 12, x 13 of 21

TiO2 has significantly broadened compared to that of PVC-TiO2 and PVC-VC, as depicted in Figure 11a. The broadened absorption was ascribed to TiO2 and the unsaturated bonds of VC. The redshift of the absorption threshold wavelength up to 600 nm was explained by the bidentate binding of α-substitute surface modifiers, leading to the five-membered ring at the surface of Ti atoms, as illustrated in Figure 11b. It was found that the photocatalytic activity was greatly enhanced by the formation of TiIV-VC charge-transfer complex having a five-member chelate ring structure. Moreover, the optimum mass ratio of VC to TiO2 is 0.5 for the highest efficiency of photocatalytic activity, as shown in Figure 11c. Upon illumination, the TiIV-VC charge-transfer complex promoted the synergetic effect between VC and TiO2, as illustrated in Figure 11b. The photogenerated electrons are FigureFigure 10. 10. PhotodegradationPhotodegradation of of PVC–TiO PVC–TiO2 2film.film. (a ()a )UV–VIS UV–VIS absorption absorption spectra spectra of of the the PVC–TiO PVC–TiO22 compositecomposite films films with with transferred from VC to the conduction band of TiO2 to form the superoxide radicals, varyingvarying TiO TiO22 contents.contents. Inset: Inset: Chemical Chemical formula formula of of PVC. PVC. ( b)) Apparent absorbance ofof thethe compositecomposite filmsfilms atatλ λ= =350 350nm nm as as a according to Equation (3). The superoxide 2radicals attack the polymer chains nearby, afunction function of of the the film film thickness. thickness. ( c()c) Weight Weight loss loss of of the the pure pure PVC PVC and and PVC-TiO PVC-TiO2 (1.5 (1.5 wt%) wt%) composite composite film film during during irradiation irradiation in in air (filled symbols) or N2 which(open symbols) accelerate atmosphere the PVC [60];degradation reprinted viafrom one-electron [60] with permission reduction from of Elsevier.surface oxygen. air (filled symbols) or N2 (open symbols) atmosphere [60]; reprinted from [60] with permission from Elsevier.

Interestingly, photocatalytic degradation of PVC film with the vitamin-C (VC)- modified TiO2 photocatalyst was investigated [61]. The specific binding of VC to the TiO2 surface was attributed to the fact that the Ti ions can easily form complexes with oxygen- containing ligands. The UV-VIS spectrum shows that the absorption range of PVC-VC-

FigureFigure 11. 11. PhotodegradationPhotodegradation of of PVC-VC-TiO PVC-VC-TiO2 2film.film. (a ()a )UV–VIS UV–VIS absorption absorption spectra spectra of of di differentfferent films films before before irradiation: irradiation: 2 2 2 2 PVC–VC–TiOPVC–VC–TiO2 filmfilm with 2 wt% TiOTiO2 andand 11wt% wt% VC VC (curve (curve a), A), PVC–TiO PVC–TiO2 film film with with 2 wt% 2 wt% TiO TiO2 (curve(curve b), PVC–VCB), PVC–VC film film with with2 wt% 2 wt% VC (curve VC (curve c), and C), pure and PVCpure film PVC (curve film (curve d). (b) TheD). ( proposedb) The proposed mechanism mechanism of photocatalysis of photocatalysis process forprocess VC modified for VC modified TiO2. (c) The weight loss rate at different mass ratios of VC to TiO2 in PVC-VC-TiO2 film; [61] reprinted from [61] TiO2.(c) The weight loss rate at different mass ratios of VC to TiO2 in PVC-VC-TiO2 film; [61] reprinted from [61] with with permission from Elsevier. permission from Elsevier.

AsUpon the photocatalytic illumination, theactivity TiIV-VC of TiO charge-transfer2 photocatalyst complex has low promoted efficiency theunder synergetic visible light [62], a perchlorinated iron (II) phthalocyanine modified TiO2 (FePcCl16-TiO2) was effect between VC and TiO2, as illustrated in Figure 11b. The photogenerated electrons developed and showed greatly improved absorption ability of visible light [63]. Similarly, are transferred from VC to the conduction band of TiO2 to form the superoxide radicals, theaccording photocatalytic to Equation degradation (3). The superoxide of PVC with radicals bismuth attack oxyiodide-modified the polymer chains nearby, TiO2 (PVC- which BiOI/TiOaccelerate2) the[64], PVC polyoxometalate-modified degradation via one-electron TiO reduction2 (PVC-POM/TiO of surface2) oxygen. [65], and nano- 2 2 graphite-dopedAs the photocatalytic TiO [PVC-(Nano-G/TiO activity of TiO2)]photocatalyst [66] also showed has low im efficiencyproved undervisible visiblelight activitylight [62 due], a perchlorinatedto their broadened iron (II)absorption phthalocyanine spectra. modifiedThe modified TiO2 (FePcClphotocatalysts16-TiO2) can was effectivelydeveloped improve and showed the migration greatly and improved separation absorption of TiO2 abilityphotogenerated of visible electrons light [63 thanks]. Sim- toilarly, the built-in the photocatalytic electric field degradationat the heterojunction, of PVC withwhich bismuth inhibits oxyiodide-modifiedthe recombination of TiO the2 photogenerated(PVC-BiOI/TiO charge2)[64], carriers, polyoxometalate-modified and thus further improves TiO2 (PVC-POM/TiO the photocatalytic2)[65 degradation], and nano- rate.graphite-doped TiO2 [PVC-(Nano-G/TiO2)] [66] also showed improved visible light activity due to their broadened absorption spectra. The modified photocatalysts can effectively im- 4.3.prove Polypropylene the migration (PP) and separation of TiO2 photogenerated electrons thanks to the built-in electricTiO field2 with at mixed the heterojunction, crystalline forms which (anatase inhibits and the rutile) recombination exhibits the of highest the photogenerated activity for photodegradationcharge carriers, and of thus PP further[(C3H6)n improves, as depicted the photocatalytic in the inset degradationof Figure 12a] rate. with high consumption of oxygen [67]. Furthermore, it was found that the particle size of TiO2 plays 4.3. Polypropylene (PP) an important role, and extra fine TiO2 is more favourable than large particles for photocatalyticTiO2 with degradation. mixed crystalline Similar forms to the (anatase other polymers, and rutile) modified exhibits TiO the2 highestphotocatalysts activity enhancefor photodegradation the photocatalytic of PP degradation [(C3H6)n, as effi depictedciency. inMeng the insetet al. of [68] Figure investigated 12a] with TiO high2 immobilizedconsumption organoclay of oxygen (TiO [67].2-OMT) Furthermore, photocatalysts. it was foundSamples that with the increasing particle size mass of ratio TiO2 ofplays TiO2 an-OMT important photocatalyst role, and in the extra clay fine from TiO 2,2 5,is and more 10 favourable mmol Ti/g thanclay were large prepared particles foras PP/OMTTi2, PP/OMTTi5, and PP/OMTTi10, respectively. Pure PP, PP/OMT, and PP/TiO2 composites as control samples were prepared for comparison. Apparently, PP/OMTTi5 Micromachines 2021, 12, 907 13 of 20

photocatalytic degradation. Similar to the other polymers, modified TiO2 photocatalysts enhance the photocatalytic degradation efficiency. Meng et al. [68] investigated TiO2 immobilized organoclay (TiO2-OMT) photocatalysts. Samples with increasing mass ratio Micromachines 2021, 12, x 14 of 21 of TiO2-OMT photocatalyst in the clay from 2, 5, and 10 mmol Ti/g clay were prepared as PP/OMTTi2, PP/OMTTi5, and PP/OMTTi10, respectively. Pure PP, PP/OMT, and PP/TiO2 composites as control samples were prepared for comparison. Apparently, PP/OMTTi5 shows the highest degradation rate as reflectedreflected by the largest carbonylcarbonyl bandband absorbanceabsorbance area, as depicted in Figure 1212a.a. Differential scanning calorimetry (DSC) thermal analysis shows that PP/OMTTi5 has has double double endothermic endothermic melting melting peaks peaks after after irradiation irradiation over 80 h, as presentedpresented in Figurein Figure 12b. This12b. observationThis observation was attributed was toattributed the decreased to the crystallization decreased crystallizationof PP during photodegradation, of PP during photodegradation, which caused which the metastable caused the crystal metastable phase crystal to appear phase as tothe appear lower as DSC the peak. lower The DSC quantitative peak. The quantita moleculetive weight molecule characterized weight characterized by gel permeation by gel permeationchromatography chromatography indicates a 2-order-of-magnitudeindicates a 2-order-of-magnitude reduction of molecularreduction weightof molecular of PP weightafter 300-h of PP of after irradiation. 300-h of The irradiation. photogenerated The photogenerated electrons and holeselectrons in TiO and2 can holes react in withTiO2 − canthe absorbedreact with H the2O absorbed or O2 to produce H2O or O various2 to produce reactive various oxygen reactive species oxygen (ROSs) species including (ROSs)O2 , • • • • includingHOO , HO O ,, HOO etc. These , HO , ROSsetc. These capture ROSs hydrogen capture atomshydrogen in PP atoms polymer in PP chains polymer and chains then • • andgenerate then –CHgenerate2(CH CH3)C –CHmacroC moleculemacro molecule radicals, radicals, followed followed by chain by scission, chain scission, as detailed as detailedin Figure in 12 Figurec. 12c.

Figure 12. Photodegradation of PP. (a) Irradiation time dependent carbonyl band area of composite of pure PP, PP/OMMT,

PP/OMTTi2, PP/OMTTi5, PP/OMTTi5, PP/OMTTi10, PP/OMTTi10, and and PP/TiO PP/TiO2. 2.Inset: Inset: Chemical Chemical formula formula of of PP. PP. (b (b) )Melting Melting endotherms of PP/OMTTi5 film ﬁlm irradiated for 0 hh (curve(curve a),A), 80 80 h h (curve (curve b), B), 170 170 h (curveh (curve c), C), 300 300 h (curve h (curve d), asD), well as well as that as that of pure of pure PP ﬁlm PP film irradiated for 0 h (curve E) and 300 h (curve F). (c) Proposed mechanism for degradation of PP/TiO2-OMT composite irradiated for 0 h (curve e) and 300 h (curve f). (c) Proposed mechanism for degradation of PP/TiO2-OMT composite [68]; [68]; reprinted from [68] with permission from John Wiley and Sons. reprinted from [68] with permission from John Wiley and Sons.

Interestingly, carbon coating on TiO2 increases the photocatalytic activity compared Interestingly, carbon coating on TiO2 increases the photocatalytic activity compared to pure PP but not PP-TiO2 composite film [69]. Three types of samples were prepared by to pure PP but not PP-TiO2 composite ﬁlm [69]. Three types of samples were prepared mixing carbon-coated TiO2 and PP with the decreasing carbon content from PP5Ti (30.6 by mixing carbon-coated TiO2 and PP with the decreasing carbon content from PP5Ti wt%),(30.6 wt%),PP10Ti PP10Ti (11.6 wt%), (11.6 wt%),PP30Ti PP30Ti (2.7 wt%) (2.7 to wt%) PPDTi to (0 PPDTi wt.%), (0 with wt.%), reference with reference to pure PP. to Directpure PP. evidence Direct evidencefrom scanning from scanningelectron microscopy electron microscopy (SEM) images (SEM) are images shown are in Figure shown 13, in whereFigure the13, wheremorphology the morphology of the degraded of the samples degraded are samples presented. are presented. One can see One that, can after see that,500- hafter of illumination, 500-h of illumination, PP30Ti and PP30Ti PPDTi and show PPDTi a ve showry rough a very surface rough with surface very with large very cavities. large Incavities. contrast, In contrast,the rest theof the rest samples of the samples show a show much a muchsmoother smoother surface, surface, suggesting suggesting mild degradation.mild degradation. The authors The authors attributed attributed the decreas the decreaseded efficiency efﬁciency to the to carbon-coated the carbon-coated layer decreasing the amount of UV light reaching the surface of the particles, and in turn reducing the hydroxyl groups on the surface of TiO2. These observations thus emphasize the importance of TiO2-PP interface for PP photodegradation. Micromachines 2021, 12, 907 14 of 20

layer decreasing the amount of UV light reaching the surface of the particles, and in turn Micromachines 2021, 12, x 15 of 21 reducing the hydroxyl groups on the surface of TiO2. These observations thus emphasize the importance of TiO2-PP interface for PP photodegradation.

Figure 13 13.. SEMSEM images images of of PP PP nano-composite nano-composite samples samples after after 500 500 h of h ofUV UV irradiation. irradiation. (a) Pure (a) Pure PP, PP,(b) (PP5Ti,b) PP5Ti, (c) PP10Ti, (c) PP10Ti, (d) (PP30Ti,d) PP30Ti, and and (e) PPDTi (e) PPDTi [69]. [69 ].

ComparedCompared withwith the the abovementioned abovementioned photocatalysts photocatalysts including including TiO2-OMT TiO2-OMT and carbon- and carbon-coatedcoated TiO2, the TiO reduced2, the reduced graphene graphene oxide oxide (rGO) (rGO) coated coated TiO2 TiO(TiO2 2(TiO-rGO)2-rGO) is the is the best best for forphotocatalytic photocatalytic degradation degradation thanks thanks to the to introduction the introduction of rGO, of asrGO, illustrated as illustrated in Figure in 14Figure[ 70]. 14Brieﬂy, [70]. uponBriefly, illumination, upon illumination, the photogenerated the photogenerated electrons electrons and holes and in TiO holes2 cause in TiO reduction2 cause reductionand oxidation and reactions,oxidation respectively,reactions, respectively, leading to leading the formation to the offormation ROSs. The of ROSs. generated The generatedROSs attack ROSs the C-Hattack bonds the inC-H PP bonds and generate in PP and PP macrogenerate radicals PP macro for further radicals chain for scission.further chainThe high scission. rate ofThe degradation high rate wasof degradati attributedon towas (1) rGOattributed extending to (1) the rGO absorption extending range the of TiO to a visible region due to the presence of Ti-O-C bond; (2) rGO acting as a good absorption2 range of TiO2 to a visible region due to the presence of Ti-O-C bond; (2) rGO π actingelectron as acceptora good electron thanks acceptor to its 2D thanks-conjugation, to its 2D facilitating π-conjugation, electron-hole facilitating separation, electron-hole and separation,thus decreasing and thethus recombination decreasing the rate; recombination and (3) rGO offeringrate; and more (3) adsorptionrGO offering sites more and adsorptioncatalytic sites. sites and catalytic sites. 4.4. Polyethylene (PE) The photocatalytic degradation rate of PE (see inset of Figure 15a for molecular structure) is found not linearly proportional to the TiO2 concentration [71]. The experiment using TiO2/CuPc as photocatalyst to degrade the PE plastic has been conducted, and the results showed that PE-TiO2/CuPc has a higher photocatalytic degradation rate than PE-TiO2 [72]. However, there was an optimal concentration of CuPc, i.e., 0.8 wt% for the highest photocatalytic degradation rate, probably caused by the trade-off between absorption in unit area and light penetration depth, as displayed in Figure 15a. The author furthers characterized the photovoltage using surface photovoltage spectroscopy (SPS), as depicted in Figure 15b. In the range of 300–400 nm, TiO2/CuPc shows a much higher photovoltage, suggesting a higher charge separation efﬁciency and longer excitations lifetime than that of TiO2. Moreover, the TiO2/CuPc sample displays a broader range of photo response. However, there is no response in the visible region, indicating that there is no charge transfer between TiO2 and CuPc in the visible range. The current-potential curve further shows that visible light illumination did not make a difference in photocurrent on

Figure 14. Schematic illustration of photocatalytic degradation of polypropylene by TiO2-rGO nanocomposite [70]; reprinted from [70] with permission from Elsevier. Micromachines 2021, 12, x 15 of 21

MicromachinesFigure 202113. SEM, 12, 907 images of PP nano-composite samples after 500 h of UV irradiation. (a) Pure PP, (b) PP5Ti, (c) PP10Ti, (d15) of 20 PP30Ti, and (e) PPDTi [69].

Micromachines 2021, 12, x Compared with the abovementioned photocatalysts including TiO2-OMT16 ofand 21

TiOcarbon-coated2/CuPc, as shownTiO2, the in reduced the inset graphene of Figure 15oxideb. However, (rGO) coated the photocurrent TiO2 (TiO2-rGO) of TiO is 2the/CuPc best isfor much photocatalytic larger than degradation that of TiO2 .thanks Furthermore, to the in duetroduction to the different of rGO, molecularas illustrated structures, in Figure it 4.4.is14 found [70].Polyethylene Briefly, that the (PE)upon degradation illumination, of PS-TiO the 2photogenerated/CuPc was slower electrons than PE-TiO and holes2/CuPc. in TiO2 cause reductionFa et al.and studied oxidation the photocatalytic reactions, respectively, degradation leading of the polyethylene-oxidizedto the formation of ROSs. polyethy- The The photocatalytic degradation rate of PE (see inset of Figure 15a for molecular lenegenerated wax-TiO ROSs2 (PE-OPW-TiO attack the C-H2)[ 73bonds]. The in presence PP and generate of OPW helpsPP macro improve radicals the interactionfor further structure) is found not linearly proportional to the TiO2 concentration [71]. The betweenchain scission. PE and The modified high rate TiO 2ofparticles, degradati furtheron was improving attributed theto degradation(1) rGO extending efficiency. the experiment using TiO2/CuPc as photocatalyst to degrade the PE plastic has been Photodegradationabsorption range of of TiO doped2 to a TiO visible2 composite region due films to includingthe presence Fe/Ag of Ti-O-C dually bond; doped (2) TiO rGO2, conducted, and the results showed that PE-TiO2/CuPc has a higher photocatalytic Ag-dopedacting as a TiOgood2, electron and Fe doped acceptor TiO thanks2 were to investigated its 2D π-conjugation, [74]. Overall, facilitating the degradation electron-hole of degradation rate than PE-TiO2 [72]. However, there was an optimal concentration of CuPc, dopedseparation, TiO2 andwas greaterthus decreasing than that ofthe the recombination undoped TiO rate;2, and and Fe/Ag (3) rGO dually offering doped TiOmore2 i.e., 0.8 wt% for the highest photocatalytic degradation rate, probably caused by the trade- showsadsorption the highest sites and degradation catalytic sites. rate under UV light among all, as shown in Figure 15c. off between absorption in unit area and light penetration depth, as displayed in Figure 15a. The author furthers characterized the photovoltage using surface photovoltage spectroscopy (SPS), as depicted in Figure 15b. In the range of 300–400 nm, TiO2/CuPc shows a much higher photovoltage, suggesting a higher charge separation efficiency and longer excitations lifetime than that of TiO2. Moreover, the TiO2/CuPc sample displays a broader range of photo response. However, there is no response in the visible region, indicating that there is no charge transfer between TiO2 and CuPc in the visible range. The current-potential curve further shows that visible light illumination did not make a difference in photocurrent on TiO2/CuPc, as shown in the inset of Figure 15b. However, the photocurrent of TiO2/CuPc is much larger than that of TiO2. Furthermore, due to the different molecular structures, it is found that the degradation of PS-TiO2/CuPc was slower than PE-TiO2/CuPc. Fa et al. studied the photocatalytic degradation of the polyethylene-oxidized polyethylene wax-TiO2 (PE-OPW-TiO2) [73]. The presence of OPW helps improve the interaction between PE and modified TiO2 particles, further improving the degradation efficiency. Photodegradation of doped TiO2 composite films including Fe/Ag dually doped TiO2, Ag-doped TiO2, and Fe doped TiO2 were investigated [74]. Overall, the degradation of doped TiO2 was greater than that of the undoped TiO2, and Fe/Ag dually 2 FigureFigure 14.14.Schematic Schematic illustration illustrationdoped of photocatalyticTiOof photocatalytic shows degradationthe highestdegradation ofdegradation polypropylene of polypropylene rate by TiOunder2 -rGOby UVTiO nanocomposite light2-rGO amongnanocomposite [ 70all,]; reprintedas shown [70]; in fromreprinted [70] with from permission [70] with permission fromFigure Elsevier. 15c. from Elsevier.

Figure 15. Photodegradation of PE. ( a) CuPc concentration wt% dependent degradation rate. Inset: Chemical Chemical formula formula of of PE. (b) Surface photovoltage spectroscopy spectra of TiO2 and TiO2/CuPc photocatalyst. Inset: The I-V curves of different PE. (b) Surface photovoltage spectroscopy spectra of TiO2 and TiO2/CuPc photocatalyst. Inset: The I-V curves of different photocatalysts, i.e., TiO2/CuPc without irradiation (curve A), TiO2/CuPc under visible irradiation (curve B), TiO2 under photocatalysts, i.e., TiO2/CuPc without irradiation (curve A), TiO2/CuPc under visible irradiation (curve B), TiO2 under UV irradiation (curve C), and TiO2/CuPc under UV radiation (curve D) [72]; reprinted from [72] with permission from UV irradiation (curve C), and TiO /CuPc under UV radiation (curve D) [72]; reprinted from [72] with permission from Elsevier. (c) Effect of simulated sunlight2 on the photocatalytic degradation of PE film [74]. Elsevier. (c) Effect of simulated sunlight on the photocatalytic degradation of PE ﬁlm [74].

Moreover, the photocatalytic activity of PE-TiO2 can be improved using modified Moreover, the photocatalytic activity of PE-TiO2 can be improved using modiﬁed TiO2 TiO2 photocatalysts such as multiwalled carbon nanotube (MWCNT)-TiO2 composite photocatalysts such as multiwalled carbon nanotube (MWCNT)-TiO2 composite (TiO2- (TiO2-MWCNTs) [75], polypyrrole(PPy)-TiO2 composite (PPy/TiO2) [76], and MWCNTs) [75], polypyrrole(PPy)-TiO2 composite (PPy/TiO2)[76], and polyacrylamide polyacrylamide grafted TiO2 (PAM-g-TiO2) [77]. PAM absorbs moisture from the grafted TiO2 (PAM-g-TiO2)[77]. PAM absorbs moisture from the atmosphere due to its atmosphere due to its high hydrophilicity, which results in the weight loss of PAM-g-TiO◦ 2 high hydrophilicity, which results in the weight loss of PAM-g-TiO2 sample before 100 C sampleon the thermogravimetricbefore 100 °C on the analysis thermogravimetri curve, as shownc analysis in Figure curve, 16a. as The shown absorbed in Figure moistures 16a. The absorbed moistures produced more hydroxyl radicals which would accelerate the photodegradation. The carbonyl index measurement in Figure 16b increases with the increasing UV exposure duration due to the initiation of chain scission caused by Micromachines 2021, 12, 907 16 of 20

Micromachines 2021, 12, x 17 of 21

produced more hydroxyl radicals which would accelerate the photodegradation. The carbonyl index measurement in Figure 16b increases with the increasing UV exposure photocatalyticduration due tooxidation, the initiation which of chain produces scission carbonyl caused bycompounds photocatalytic with oxidation, low molecular which weightsproduces (e.g., carbonyl ester, compounds acid, and aldehyde). with low molecular The degraded weights PAM (e.g., also ester, generates acid, and amide aldehyde). and acidThe degradedthat promote PAM the also degradation generates amide of LDPE. and As acid displayed that promote in Figure the degradation 16c, the molecular of LDPE. weightAs displayed (Mw) distribution in Figure 16 c,curve the molecular peak shifts weight to the (M lowerw) distribution value from curve LDPE peak towards shifts to the the LDPE/PAM-g-TiOlower value from LDPE2-UV sample, towards indicating the LDPE/PAM-g-TiO that the PAM-g-TiO2-UV sample,2 has the indicating highest thatactivity the towardsPAM-g-TiO LDPE2 has degradation the highest under activity UV towards irradiation. LDPE degradation under UV irradiation.

2 2 2 Figure 16. PhotodegradationPhotodegradation of of PE-PAM-g-TiO PE-PAM-g-TiO2 film.film. ( (aa)) Thermogravimetric Thermogravimetric analysis analysis of of TiO TiO2 (curve(curve A), a), MPS-TiO2 (curve(curve B), and PAM-g-TiO2 (curve C). (b) The carbonyl index of various samples under UV irradiation. (c) Molecular weight b), and PAM-g-TiO2 (curve c). (b) The carbonyl index of various samples under UV irradiation. (c) Molecular weight distribution of films LDPE (curve A), LDPE-UV (curve B), LDPE/TiO2-UV (curve C), LDPE/MPS-TiO2-UV (curve D), and distribution of films LDPE (curve a), LDPE-UV (curve b), LDPE/TiO2-UV (curve c), LDPE/MPS-TiO2-UV (curve d), and LDPE/PAM-g-TiO2-UV (curve E) with irradiation time of 520 h [77]; reprinted from [77] with permission from Elsevier. LDPE/PAM-g-TiO2-UV (curve e) with irradiation time of 520 h [77]; reprinted from [77] with permission from Elsevier. 5. Conclusions and Outlook 5. Conclusions and Outlook In recent years, plastic pollution has increasingly shown a negative impact on our In recent years, plastic pollution has increasingly shown a negative impact on our environment and health. The recycling rate of plastic waste is very low (~10%) mainly environment and health. The recycling rate of plastic waste is very low (~10%) mainly caused by the contamination of plastic waste by other solid waste, as well as the fact that caused by the contamination of plastic waste by other solid waste, as well as the fact the mixing nature of plastic waste consists of various polymers. Additionally, the that the mixing nature of plastic waste consists of various polymers. Additionally, the widespread plastic waste makes collection and transportation necessary for centralized widespread plastic waste makes collection and transportation necessary for centralized recycling technology such as mechanical and thermal catalytic recycling. Thus, a key to recycling technology such as mechanical and thermal catalytic recycling. Thus, a key to increase the recycling rate of plastic waste, and thus less disposal of plastic waste to increase the recycling rate of plastic waste, and thus less disposal of plastic waste to landfill, landfill, is to develop a decentralized degradation process that could leverage renewable is to develop a decentralized degradation process that could leverage renewable energies energies as the driving force. To this end, photodegradation is perfect as sunlight is known as the driving force. To this end, photodegradation is perfect as sunlight is known to be to be an effective energy source to degrade plastic. Therefore, we present this short review an effective energy source to degrade plastic. Therefore, we present this short review to torecap recap the the photocatalytic photocatalytic degradation degradation mechanism mechanism and andto to discussdiscuss thethe researchresearch efforts on photodegradationphotodegradation of of various various pl plasticastic polymers, polymers, aiming aiming to to inspire inspire more more research research ideas to addressaddress this this urgent urgent challenge of plastic waste. Despite the advantages of photocatalysis andand extensive extensive efforts efforts devoted devoted to to plastic plastic photodegradation photodegradation to to date, date, there there are still several majormajor challenges challenges as as follows: follows: (1)(1) PhotocatalyticPhotocatalytic degradation degradation mechanism. mechanism. Deco Decompositionmposition of ofa large a large plastic plastic polymer polymer to smallto small molecules molecules is mechanistically is mechanistically complicated. complicated. There There could could be be a adozen dozen or or more differentdifferent reaction pathways.pathways. How How to identifyto identify and thenand tothen control to control the reaction the pathwayreaction pathwayis a paramount is a paramount challenge. challenge. Some in Some situ/operando in situ/operando characterizations characterizations could be could useful, be useful,such as such Raman as Raman spectroscopy, spectroscopy, photoluminescence photoluminescence spectroscopy, spectroscopy, and high-resolution and high- resolutionsoft X-ray soft absorption X-ray absorption spectroscopy. spectroscopy It is urgent. It is to urgent develop to adevelop suitable a andsuitable effective and effectivecharacterization characterization tool/method tool/method for in situ/operando for in situ/operando monitoring monitoring of the degradation of the degradationprocess. Theoretical process. investigation Theoretical includinginvestigation first-principle including modelling first-principle and microkinetic modelling andmodelling microkinetic complementary modelling complementary to the in situ/operando to the in situ/operando studies could studies be powerful could forbe powerfulreaction pathwaysfor reaction investigation. pathways investigation. (2)(2) Contamination-tolerantContamination-tolerant degradationdegradation technology.technology. Most Most of theof plasticthe plastic wastes wastes are gener- are generatedated in a widespreadin a widespread manner manner and contaminated and contaminated by various by various other wastes other wastes such as such food aswastes, food wastes, wood waste,wood andwaste, chemical and chemical waste. waste. Though Though the photocatalytic the photocatalytic process process shows shows good tolerance to these contaminations, it is a surface reaction. Thus, its efficiency would be low if light penetration is blocked by the non-transparent Micromachines 2021, 12, 907 17 of 20

good tolerance to these contaminations, it is a surface reaction. Thus, its efﬁciency would be low if light penetration is blocked by the non-transparent contaminations. Thus, a photocatalyst that could degrade these contaminations should be developed and used together with the plastic degradation photocatalyst. (3) Multifunctional photocatalyst. Very often, a plastic product consists of various plastic components, e.g., electronic plastic may contain polyimide, ABS, etc. Therefore, a multifunctional photocatalyst that consists of individual elements working for a speciﬁc plastic could be very useful for addressing the plastic waste issue. Moreover, the selectivities of these catalysts are crucial from an economical perspective. Most of the studied catalyst generates CO2 as the major product. Though less harmful than plastic waste, CO2 is also a serious environmental issue to be addressed. (4) Facile processes to introduce photocatalyst into plastic waste. Most of the current work involves dissolving plastics followed by mixing with photocatalyst to make a composite, which is then photodegraded. Considering the low economic viability of such a process, a new method to introduce photocatalyst into plastic waste is needed. For instance, dispersion of photocatalyst particles that can stick on bulk plastic waste could be attractive as it is facile and cost-effective. However, an intensive research effort may be necessary for developing such a photocatalyst.

Author Contributions: Writing, review and editing, Q.Y.L. and H.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Singapore Ministry of Education under grant Tier 1 RG101/18 (grant number: 2018-T1-001-051). Acknowledgments: The authors extended their appreciation to Nanyang Technological University Singapore for support via ACE award. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Rahimi, A.; García, J.M. Chemical recycling of waste plastics for new materials production. Nat. Rev. Chem. 2017, 1, 0046. [CrossRef] 2. Plastics—The Facts 2020. Available online: https://www.plasticseurope.org/en/resources/publications/4312-plastics-facts-2020 (accessed on 21 February 2021). 3. Plastics Market Size, Share & Trends Report, 2020–2027. Available online: https://www.grandviewresearch.com/industry- analysis/global-plastics-market (accessed on 21 February 2021). 4. Mehran, M.T.; Naqvi, S.R.; Haider, M.A.; Saeed, M.; Shahbaz, M.; Al-Ansari, T. Global plastic waste management strategies (Technical and behavioral) during and after COVID-19 pandemic for cleaner global urban life. Energy Source Part A Recovery Util. Environ. Eff. 2021, 1–10. [CrossRef] 5. Aragaw, T.A. Surgical face masks as a potential source for microplastic pollution in the COVID-19 scenario. Mar. Pollut. Bull. 2020, 159, 111517. [CrossRef] 6. Zhang, K.; Hamidian, A.H.; Tubi´c,A.; Zhang, Y.; Fang, J.K.H.; Wu, C.; Lam, P.K.S. Understanding plastic degradation and microplastic formation in the environment: A review. Environ. Pollut. 2021, 274, 116554. [CrossRef] 7. Liang, Y.; Tan, Q.; Song, Q.; Li, J. An analysis of the plastic waste trade and management in Asia. Waste Manag. 2021, 119, 242–253. [CrossRef] 8. Yousif, E.; Haddad, R. Photodegradation and photostabilization of polymers, especially polystyrene: Review. SpringerPlus 2013, 2, 398. [CrossRef] 9. Sharuddin, S.D.A.; Abnisa, F.; Daud, W.M.A.W.; Aroua, M.K. A review on pyrolysis of plastic wastes. Energy Convers. Manag. 2016, 115, 308–326. [CrossRef] 10. Qureshi, M.S.; Oasmaa, A.; Pihkola, H.; Deviatkin, I.; Tenhunen, A.; Mannila, J.; Minkkinen, H.; Pohjakallio, M.; Laine-Ylijoki, J. Pyrolysis of plastic waste: Opportunities and challenges. Anal. Appl. Pyrolysis 2020, 152, 104804. [CrossRef] 11. Shah, A.A.; Hasan, F.; Hameed, A.; Ahmed, S. Biological degradation of plastics: A comprehensive review. Biotechnol. Adv. 2008, 26, 246–265. [CrossRef] 12. Radiation. In IARC Monographs on the Evaluation of Carcinogenic Risks to Humans Volume 100D; The International Agency for Research on Cancer: Lyon, France, 2012. 13. Singh, B.; Sharma, N. Mechanistic implications of plastic degradation. Polym. Degrad. Stab. 2008, 93, 561–584. [CrossRef] 14. Geuskens, G.; David, C. The photo-oxidation of polymers. A comparison with low molecular weight compounds. Pure Appl. Chem. 1979, 51, 233–240. [CrossRef] Micromachines 2021, 12, 907 18 of 20

15. Fairbrother, A.; Hsueh, H.; Kim, J.H.; Jacobs, D.; Perry, L.; Goodwin, D.; White, C.; Watson, S.; Sung, L. Temperature and light intensity effects on photodegradation of high-density polyethylene. Polym. Degrad. Stab. 2019, 165, 153–160. [CrossRef] 16. Ali, S.S.; Elsamahy, T.; Koutra, E. Degradation of conventional plastic wastes in the environment: A review on current status of knowledge and future perspectives of disposal. Sci. Total Environ. 2021, 771, 144719. [CrossRef] 17. Ohtani, B. Photocatalysis A to Z—What we know and what we do not know in a scientific sense. J. Photochem. Photobiol. C Photochem. Rev. 2010, 11, 157–178. [CrossRef] 18. Chatterjee, D.; Dasgupta, S. Visible light induced photocatalytic degradation of organic pollutants. J. Photochem. Photobiol. C Photochem. Rev. 2005, 6, 186–205. [CrossRef] 19. Luo, J.; Zhang, S.; Sun, M.; Yang, L.; Luo, S.; Crittenden, J.C. A Critical Review on Energy Conversion and Environmental Remediation of Photocatalysts with Remodeling Crystal Lattice, Surface, and Interface. ACS Nano 2019, 13, 9811–9840. [CrossRef] 20. Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future Challenges. J. Phys. Chem. Lett. 2010, 1, 2655–2661. [CrossRef] 21. Lee, S.Y.; Park, S.J. TiO2 photocatalyst for water treatment applications. J. Ind. Eng. Chem. 2013, 19, 1761–1769. [CrossRef] 22. Rajaambal, S.; Sivaranjani, K.; Gopinath, C.S. Recent developments in solar H2 generation from water splitting. J. Chem. Sci. 2015, 127, 33–47. [CrossRef] 23. Nakata, K.; Fujishima, A. TiO2 photocatalysis: Design and applications. J. Photochem. Photobiol. C Photochem. Rev. 2012, 13, 169–189. [CrossRef] 24. Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D.W. Understanding TiO2 photocatalysis: Mechanisms and materials. Chem. Rev. 2014, 114, 9919–9986. [CrossRef][PubMed] 25. Zhang, J.; Tian, B.; Wang, L.; Xing, M.; Lei, J. Photocatalysis: Fundamentals, Materials and Applications; Springer Nature: Singa- pore, 2018. [CrossRef] 26. Bickley, R.I.; Gonzalez-Carreno, T.; Lees, J.S.; Palmisano, L.; Tilley, R.J.D. A structural investigation of titanium dioxide photocatalysts. J. Solid State Chem. 1991, 92, 178–190. [CrossRef] 27. Zhang, J.; Zhou, P.; Liu, J.; Yu, J. New understanding of the difference of photocatalytic activity among anatase, rutile and brookite TiO2. Phys. Chem. Chem. Phys. 2014, 16, 20382–20386. [CrossRef][PubMed] 28. Cromer, D.T.; Herrington, K. The Structures of Anatase and Rutile. J. Am. Chem. Soc. 1955, 77, 4708–4709. [CrossRef] 29. Shi, H.; Magaye, R.; Castranova, V.; Zhao, J. Titanium dioxide nanoparticles: A review of current toxicological data. Part. Fibre Toxicol. 2013, 10, 15. [CrossRef] 30. Lee, J.; Lee, Y.; Song, H.; Jang, D.; Choa, Y. Synthesis and characterization of TiO2 nanowires with controlled porosity and microstructure using electrospinning method. Curr. Appl. Phys. 2011, 11, S210–S214. [CrossRef] 31. Mack, J.M.; Tsuchiya, H.; Ghicov, A.; Yasuda, K.; Hahn, R.; Bauer, S.; Schmuki, P. TiO2 nanotubes: Self-organized electrochemical formation, properties and applications. Curr. Opin. Solid State Mater. Sci. 2007, 11, 3–18. [CrossRef] 32. Yu, Y.; Zhang, P.; Guo, L.; Chen, Z.; Wu, Q.; Ding, Y.; Zheng, W.; Cao, Y. The Design of TiO2 Nanostructures (Nanoparticle, Nanotube, and Nanosheet) and Their Photocatalytic Activity. J. Phys. Chem. C 2014, 118, 12727–12733. [CrossRef] 33. Park, H.; Park, Y.; Kim, W.; Choi, W. Surface modification of TiO2 photocatalyst for environmental applications. J. Photochem. Photobiol. C Photochem. Rev. 2013, 15, 1–20. [CrossRef] 34. Finazzi, E.; Di Valentin, C.; Pacchioni, G.; Selloni, A. Excess electron states in reduced bulk anatase TiO2: Comparison of standard GGA, GGA+U, and hybrid DFT calculations. J. Chem. Phys. 2008, 129, 154113. [CrossRef][PubMed] 35. Labat, F.; Baranek, P.; Domain, C.; Minot, C.; Adamo, C. Density functional theory analysis of the structural and electronic properties of TiO2 rutile and anatase polytypes: Performances of different exchange-correlation functionals. J. Chem. Phys. 2007, 126, 154703. [CrossRef] 36. Arroyo-de Dompablo, M.E.; Morales-García, A.; Taravillo, M. DFT+U calculations of crystal lattice, electronic structure, and phase stability under pressure of TiO2 polymorphs. J. Chem. Phys. 2011, 135, 054503. [CrossRef][PubMed] 37. Pan, X.; Yang, M.; Fu, X.; Zhang, N.; Xu, Y. Defective TiO2 with oxygen vacancies: Synthesis, properties and photocatalytic applications. Nanoscale 2013, 5, 3601–3614. [CrossRef] 38. Tofa, T.S.; Kunjali, K.L.; Paul, S.; Dutta, J. Visible light photocatalytic degradation of microplastic residues with zinc oxide nanorods. Environ. Chem. Lett. 2019, 17, 1341–1346. [CrossRef] 39. Yong, R.Y.; Li, J.H.; Chen, L.L.; Liu, D.Q.; Li, H.Z.; Zheng, Y.; Ding, J. Synthesis, surface modification and photocatalytic property of ZnO nanoparticles. Powder Technol. 2009, 189, 426–432. [CrossRef] 40. Štrbac, D.; Aggelopoulos, C.A.; Goran, Š.; Dimitropoulos, M.; Novaković,M.; Ivetić,T.; Yannopoulos, S.N. Photocatalytic degradation of Naproxen and methylene blue: Comparison between ZnO, TiO2 and their mixture. Process Saf. Environ. Prot. 2018, 113, 174–183. [CrossRef] 41. Dindar, B.; Içli, S. Unusual photoreactivity of zinc oxide irradiated by concentrated sunlight. J. Photochem. Photobiol. A Chem. 2001, 140, 263–268. [CrossRef] 42. Ameur, S.B.; Hadjltaief, H.B.; Barhoumi, A.; Duponchel, B.; Leroy, G.; Amlouk, M.; Guermazi, H. Physical investigations and photocatalytic activities on ZnO and SnO2 thin films deposited on flexible polymer substrate. Vacuum 2018, 155, 546–552. [CrossRef] Micromachines 2021, 12, 907 19 of 20

43. Muraro, P.C.L.; Mortari, S.R.; Vizzotto, B.S.; Chuy, G.; dos Santos, C.; Brum, L.F.W.; da Silva, W.L. Iron oxide nanocatalyst with titanium and silver nanoparticles: Synthesis, characterization and photocatalytic activity on the degradation of Rhodamine B dye. Sci. Rep. 2020, 10, 3055. [CrossRef][PubMed] 44. Senasu, T.; Hemavibool, K.; Nanan, S. Hydrothermally grown CdS nanoparticles for photodegradation of anionic azo dyes under UV-visible light irradiation. RSC Adv. 2018, 8, 22592–22605. [CrossRef] 45. Rao, H.; Lu, Z.; Liu, X.; Ge, H.; Zhang, Z.; Zou, P.; He, H.; Wang, Y. Visible light-driven photocatalytic degradation performance for methylene blue with different multi-morphological features of ZnS. RSC Adv. 2016, 6, 46299–46307. [CrossRef] 46. Arai, T.; Horiguchi, M.; Yanagida, M.; Gunji, T.; Sugihara, H.; Sayama, K. Reaction Mechanism and Activity of WO3-Catalyzed Photodegradation of Organic Substances Promoted by a CuO Cocatalyst. J. Phys. Chem. C 2009, 113, 6602–6609. [CrossRef] 47. Bhattacharjee, A.; Ahmaruzzaman, M.; Sinha, T. A novel approach for the synthesis of SnO2 nanoparticles and its application as a catalyst in the reduction and photodegradation of organic compounds. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2015, 136, 751–760. [CrossRef] 48. Zou, L.; Wang, H.; Wang, X. High Efficient Photodegradation and Photocatalytic Hydrogen Production of CdS/BiVO4 Het- erostructure through Z-Scheme Process. ACS Sustain. Chem. Eng. 2016, 5, 303–309. [CrossRef] 49. Yan, S.C.; Li, Z.S.; Zou, Z.G. Photodegradation Performance of g-C3N4 Fabricated by Directly Heating Melamine. Langmuir 2009, 25, 10397–10401. [CrossRef][PubMed] 50. Wen, J.; Xie, J.; Chen, X.; Li, X. A review on g-C3N4-based photocatalysts. Appl. Surf. Sci. 2017, 391, 72–123. [CrossRef] 51. Borysiewicz, M.A. ZnO as a Functional Material, a Review. Crystals 2019, 9, 505. [CrossRef] 52. Guo, Y.; Clark, S.J.; Robertson, J. Electronic and magnetic properties of Ti2O3, Cr2O3, and Fe2O3 calculated by the screened exchange hybrid density functional. J. Phys. Condens. Matter 2012, 24, 325504. [CrossRef] 53. Khlyustova, A.; Sirotkin, N.; Kusova, T.; Kraev, A.; Titov, V.; Agafonov, A. Doped TiO2: The effect of doping elements on photocatalytic activity. Mater. Adv. 2020, 1, 1193–1201. [CrossRef] 54. Shang, J.; Chai, M.; Zhu, Y. Solid-phase photocatalytic degradation of polystyrene plastic with TiO2 as photocatalyst. J. Solid State Chem. 2003, 174, 104–110. [CrossRef] 55. Shang, J.; Chai, M.; Zhu, Y. Photocatalytic Degradation of Polystyrene Plastic under Fluorescent Light. Environ. Sci. Technol. 2003, 37, 4494–4499. [CrossRef][PubMed] 56. Zan, L.; Tian, L.; Liu, Z.; Peng, Z. A new polystyrene–TiO2 nanocomposite film and its photocatalytic degradation. Appl. Catal. A Gen. 2004, 264, 237–242. [CrossRef] 57. Zan, L.; Wang, S.; Fa, W.; Hu, Y.; Tian, L.; Deng, K. Solid-phase photocatalytic degradation of polystyrene with modified nano-TiO2 catalyst. Polymer 2006, 47, 8155–8162. [CrossRef] 58. Fa, W.; Zan, L.; Gong, C.; Zhong, J.; Deng, K. Solid-phase photocatalytic degradation of polystyrene with TiO2 modified by iron (II) phthalocyanine. Appl. Catal. B Environ. 2008, 79, 216–223. [CrossRef] 59. Lei, Y.; Lei, H.; Huo, J. Innovative controllable photocatalytic degradation of polystyrene with hindered amine modified aromatic polyamide dendrimer/ polystyrene-grafted-TiO2 photocatalyst under solar light irradiation. Polym. Degrad. Stab. 2015, 118, 1–9. [CrossRef] 60. Cho, S.; Choi, W. Solid-phase photocatalytic degradation of PVC–TiO2 polymer composites. J. Photochem. Photobiol. A Chem. 2001, 143, 221–228. [CrossRef] 61. Yang, C.; Gong, C.; Peng, T.; Deng, K.; Zan, L. High photocatalytic degradation activity of the polyvinyl chloride (PVC)-vitamin C (VC)-TiO2 nano-composite film. J. Hazard. Mater. 2010, 178, 152–156. [CrossRef][PubMed] 62. Gupta, A.; Lakshmp, Y.N.; Manivannan, R.; Victoria, S.N. Visible range photocatalysts for solid phase photocatalytic degradation of polyethylene and polyvinyl chloride. J. Chil. Chem. Soc. 2017, 62, 3393–3398. [CrossRef] 63. Fa, W.; Gong, C.; Tian, L.; Peng, T.; Zan, L. Enhancement of photocatalytic degradation of poly(vinyl chloride) with perchlorinated iron (II) phthalocyanine modified nano-TiO2. J. Appl. Polym. Sci. 2011, 122, 1823–1828. [CrossRef] 64. Yang, C.; Deng, K.; Peng, T.; Zan, L. Enhanced Solid-Phase Photocatalytic Degradation Activity of a Poly(vinyl chloride)-TiO2 Nanocomposite Film with Bismuth Oxyiodide. Chem. Eng. Technol. 2011, 34, 886–892. [CrossRef] 65. Yang, C.; Tian, L.; Ye, L.; Peng, T. Enhancement of photocatalytic degradation activity of poly(vinyl chloride)-TiO2 nanocomposite film with polyoxometalate. J. Appl. Polym. Sci. 2010, 120, 2048–2053. [CrossRef] 66. Zhang, Y.; Sun, T.; Zhang, D.; Shi, Z.; Shi, Z.; Zhang, X.; Li, C.; Wang, L.; Song, J.; Lin, Q. Enhanced photodegradability of PVC plastics film by codoping nano-graphite and TiO2. Polym. Degrad. Stab. 2020, 181, 109332. [CrossRef] 67. Ohtani, B.; Adzuma, S.; Miyadzu, H.; Nishimoto, S.; Kagiya, T. Photocatalytic degradation of polypropylene film by dispersed titanium dioxide particles. Polym. Degrad. Stab. 1989, 23, 271–278. [CrossRef] 68. Meng, X.; Wang, H.; Qian, Z.; Gao, X.; Yi, Q.; Zhang, S.; Yang, M. Preparation of photodegradable polypropylene/clay composites based on nanoscaled TiO2 immobilized organoclay. Polym. Compos. 2008, 30, 543–549. [CrossRef] 69. Kamrannejad, M.M.; Hasanzadeh, A.; Nosoudi, N.; Mai, L.; Babaluo, A.A. Photocatalytic degradation of polypropylene/TiO2 nano-composites. Mater. Res. 2014, 17, 1039–1046. [CrossRef] 70. Verma, R.; Singh, S.; Dalai, M.K.; Saravanan, M.; Agrawal, V.V.; Srivastava, A.K. Photocatalytic degradation of polypropylene film using TiO2-based nanomaterials under solar irradiation. Mater. Des. 2017, 133, 10–18. [CrossRef] 71. Zhao, X.; Li, Z.; Chen, Y.; Shi, L.; Zhu, Y. Solid-phase photocatalytic degradation of polyethylene plastic under UV and solar light irradiation. J. Mol. Catal. A Chem. 2007, 268, 101–106. [CrossRef] Micromachines 2021, 12, 907 20 of 20

72. Zhao, X.; Li, Z.; Chen, Y.; Shi, L.; Zhu, Y. Enhancement of photocatalytic degradation of polyethylene plastic with CuPc modified TiO2 photocatalyst under solar light irradiation. Appl. Surf. Sci. 2008, 254, 1825–1829. [CrossRef] 73. Fa, W.; Yang, C.; Gong, C.; Peng, T.; Zan, L. Enhanced photodegradation efficiency of polyethylene-TiO2 nanocomposite film with oxidized polyethylene wax. J. Appl. Polym. Sci. 2010, 118, 378–384. [CrossRef] 74. Asghar, W.; Qazi, I.A.; Ilyas, H.; Khan, A.A.; Awan, M.A.; Aslam, M.R. Comparative Solid Phase Photocatalytic Degradation of Polythene Films with Doped and Undoped TiO2 Nanoparticles. J. Nanomater. 2010, 2011, 1–8. [CrossRef] 75. An, Y.; Hou, J.; Liu, Z.; Peng, B. Enhanced solid-phase photocatalytic degradation of polyethylene by TiO2–MWCNTs nanocom- posites. Mater. Chem. Phys. 2014, 148, 387–394. [CrossRef] 76. Li, S.; Xu, S.; He, L.; Xu, F.; Wang, Y.; Zhang, L. Photocatalytic Degradation of Polyethylene Plastic with Polypyrrole/TiO2 Nanocomposite as Photocatalyst. Polym. Plast. Technol. Eng. 2010, 49, 400–406. [CrossRef] 77. Liang, W.; Luo, Y.; Song, S.; Dong, X.; Yu, X. High photocatalytic degradation activity of polyethylene containing polyacrylamide grafted TiO2. Polym. Degrad. Stab. 2013, 98, 1754–1761. [CrossRef]