Dual-Functional Photocatalytic and Photoelectrocatalytic Systemsfor Energy- and Resource-Recovering Water TreatmentTae Hwa Jeon, Min Seok Koo, Hyejin Kim, and Wonyong Choi*

Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673,Korea

ABSTRACT: The solar-driven photo(electro)catalytic proc-ess is a key technology for utilization of solar energy. It isbeing intensively investigated for application to environmentalremediation and solar fuel production. Although bothenvironmental and energy applications operate on the basisof the same principle of photoinduced interfacial chargetransfer, most previous studies have focused on either theenvironmental or energy process only since these twoprocesses require very different catalyst properties andreaction conditions. This Perspective describes a dual-functional photo(electro)catalytic process that enables watertreatment along with the simultaneous recovery of energy(e.g., H2 and H2O2) or resources (e.g., metal ions) and discusses the status and perspectives of this emerging technology. Theessential feature of the process is to utilize the hole oxidation power for the degradation of water pollutants and the electronreduction power for the recovery of energy and resources from wastewaters at the same time. Various PC, PEC, andphotovoltaic-driven electrochemical (PV-EC) processes with different dual-functional purposes (e.g., pollutant removalcombined with H2 or H2O2 production, heavy-metal recovery, denitrification, fuel cell) are introduced and discussed. Thereviewed technology should offer chances for the development of next-generation water treatment processes based on thewater−solar energy nexus.

KEYWORDS: water−energy nexus, solar fuel, solar water treatment, photocatalysis, advanced oxidation processes

1. INTRODUCTION

Photo(electro)catalysis has attracted great interests for variousapplications such as the production of solar fuels, water and airpurification, organic synthesis, and recovery of resourcesutilizing solar photon energy.1−8 Various redox processesthat are thermodynamically either spontaneous or non-spontaneous can be achieved in photo(electro)catalyticsystems by coupling of photoactive materials (usually semi-conductors) and suitable reactants and subsequent initiation ofinterfacial charge transfer reactions on the semiconductorsurface.9,10 Most research works in this area have focused onthe development of photocatalysts and photoelectrodes havinghigh efficiency, selectivity, and stability under specific catalyticreaction conditions.11,12 The application of photo(electro)-catalytic process for water treatment has been extensivelyinvestigated as an advanced oxidation process (AOP)2,13 that isinitiated by photoinduced charge transfers on semiconductormaterials with subsequent generation of reactive oxygenspecies (ROS) such as superoxide radical ion (O2

•−), hydrogenperoxide (H2O2), and hydroxyl radical (·OH).14−22 At thesame time, photoexcited electrons and holes can directlyparticipate in the redox transformation reactions of targetpollutants,1 which include the degradation of various organicpollutants,23−25 conversion of toxic anions (e.g., chromate,26,27

arsenite,28 bromate,29,30 and nitrate31−33) and heavy-metal

ions,34 and even inactivation of microorganisms.35−37 Theconcurrent reactions of electrons and holes on a photoexcitedsemiconductor can achieve the transformation of a wide varietyof aquatic pollutants into less toxic or nontoxic forms underambient conditions as long as energetic photons are available.On the other hand, solar fuel production via photo(electro)-

catalysis has also received intense attention as a viable methodof solar energy storage since the early study of photo-electrocatalytic hydrogen production was reported.38−40 Thereaction mechanism for fuel production by photo(electro)-catalysis is basically the same as that in the above watertreatment application in that the overall process is initiated byinterfacial photoinduced charge transfers. However, thecharacteristics of the interfacial charge transfers are verydifferent. The environmental photo(electro)catalysis dependslargely on the generation of ROS, which is achieved by singleelectron transfer (because ROS are usually radical species) inthe presence of dissolved O2 (because O2 is both a commonprecursor of ROS and a reagent needed for mineralization). Incontrast, the solar fuel photo(electro)catalysis requires multi-ple electron transfers because cheap and abundant precursors

Received: September 3, 2018Revised: October 23, 2018Published: October 26, 2018

Perspective

pubs.acs.org/acscatalysisCite This: ACS Catal. 2018, 8, 11542−11563

© 2018 American Chemical Society 11542 DOI: 10.1021/acscatal.8b03521ACS Catal. 2018, 8, 11542−11563

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such as H2O, CO2, and N2 should be converted (or reduced)into energy-rich fuel molecules (e.g., H2, CH3OH, HCOOH,and NH3) by a series of electron transfers.41−45 The solarenergy storage process should be carried out in the absence ofO2 because it is a good scavenger of photogenerated electronsand hinders the overall fuel synthesis process. Therefore,environmental photo(electro)catalysis and solar fuel photo-(electro)catalysis require very different reaction conditions andcatalytic materials and are difficult to achieve simultaneouslyusing the same photocatalyst.The utilization of photoactive semiconductor materials in

solar conversion has employed three different methods, whichare classified into photocatalytic (PC), photovoltaic-drivenelectrochemical (PV-EC), and photoelectrochemical (PEC)systems. PC systems are based on semiconductor particlesuspensions in which photocatalysts and target reactants aremixed together in a single reaction medium (usually anaqueous phase).46 PC systems have been widely investigated inlaboratories over the past decades because they are easy toimplement.47 PV-EC systems are devices in which photovoltaiccell and an electrolytic cell are coupled.48,49 Since the catalyticreaction is separated from the light absorption part in thissystem, it offers the great advantage of easier optimization ofboth electrocatalysis (electrode) and light harvesting (photo-voltaic) with higher efficiencies.50 In addition, both photo-voltaic and electrolytic devices are commercially available on alarge scale, which makes these systems highly practical.51 PECsystems are hybrids of PC and EC systems that combine thelight-harvesting and electrocatalysis functions simultaneouslyon the same electrode. They are composed of two electrodesimmersed in electrolytes, and at least one of the electrodesshould be a semiconductor material to act as a photoanode orphotocathode. Common PEC systems consist of a semi-conductor photoanode and a metallic cathode.52 Since thephotoelectrode is the key part for the overall conversionprocess, it is essential to develop a proper semiconductorphotoelectrode for the desired purpose, such as environmentalremediation or fuel synthesis.53−55

In this Perspective, we focus on integrated semiconductorphotochemical systems that are designed for achieving watertreatment and energy/resource recovery simultaneously. Thebasic concept is illustrated in Scheme 1. In most cases, thewater treatment part utilizes the hole oxidation power whilethe energy/resource recovery part is driven by the electronreduction power. Although each application alone (watertreatment or energy/resource recovery) has been extensivelyinvestigated, the combined systems with dual purposes haveattracted attention only recently. The published literatureexamples on this topic are listed in Table 1. In typical water-splitting studies, the reducing power of electrons is convertedto H2 energy but the oxidizing power of holes is wasted inproducing useless O2. Similarly, in photocatalytic watertreatments, the hole oxidation power drives the degradationof pollutants but the electron reduction power is largelywasted. In essence, the key concept in the dual-functionalprocess is to use both the hole oxidation power for thedegradation of water pollutants and the electron reductionpower for the recovery of energy and resources fromwastewaters at the same time. The main challenge in dual-functional photo(electro)catalysis is that the desired photo-catalyst properties of the two systems are different. Dual-functional photo(electro)catalysis should make solar-drivenwater treatment more viable and sustainable since the removal

of aquatic pollutants happens concurrently with the recovery ofenergy and resources from the polluted water, which isconceptually ideal for the water−energy nexus.

2. HYDROGEN PRODUCTION COUPLED WITH WATERTREATMENT2.1. Photocatalytic Systems. The photocatalytic reac-

tions are based on interfacial charge transfers of excitedelectrons and holes in semiconductors. For highly efficient PCconversions, enhancing the light-harvesting efficiency ofsemiconductor materials presents the biggest challenge. Sincebare semiconductor materials have limited efficiencies inabsorbing and utilizing solar light, semiconductor materialshave been modified or hybridized in various ways such as bandgap engineering, interfacial heterojunction formation, surfacecomplexation, impurity doping, and sensitization.1,2,9 Highlight absorption efficiency is necessary but not sufficient forhigh solar conversion efficiency, which also strongly dependson the interfacial charge transfer characteristics. The interfacialcharge transfer efficiency depends on the energy levels of theredox couples in the electrolyte: only the redox couples ofwhich the energy levels are located within the band gap arethermodynamically allowed (e.g., the redox couples shown inScheme 2). In a typical mode of photocatalysis for watertreatment, the oxidative destruction of water pollutants isinitiated by hole transfer, which is concurrently coupled withelectron transfer to dissolved O2. On the other hand, a typical

Scheme 1. (a) Common Applications of SemiconductorPhotocatalysis for Environmental Remediation and WaterSplitting (The Red Dashed Line Represents the UncommonDual-Functional Photocatalysis for Simultaneous WaterTreatment and Energy Recovery); (b) PhotoelectrocatalyticProcesses for Simultaneous Water Treatment and Energy/Resource Recovery

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Table 1. Dual-Functional Water Treatment Systems for Oxidation of Pollutants Combined with Simultaneous Recovery ofEnergy or Resources (Reduction Process)

type ofsystema catalyst oxidation process (pollutant degradation)

reduction process (recovery offuel and resources) ref

PC F-TiO2/GO/Pt 4-chlorophenol (4-CP) H2 57PC F-TiO2/Pt 4-CP, bisphenol A, 2,4-dichlorophenoxyacetic acid H2 124PC (F or P)-TiO2/Pt 4-CP, urea H2 63PC SrTiO3/Rh/Cr2O3 4-CP H2 66PC F-TiO2/Pt 4-CP, bisphenol A H2 56PC Er3+:Y3Al5O12/Pt-TiO2 phenol, glycerol H2 125PC TiO2/CdS/Pt inorganic (S2−/SO3

2−) or organic (ethanol) H2 126PC TiO2/Pt formaldehyde H2 127PC TiO2/Pt alcohols (methanol, ethanol, 1-propanol, 1-butanol), organic acids

(formic acid, acetic acid), acetaldehydeH2 128

PC TiO2/agarose hydrogel heavy metal (Cd2+)-containing wastewater H2 129PC TiO2/AgX waste-activated sludge H2 129PC P-TiO2/Pt estrogenic activity H2 130PC Er3+:Y3Al5O12/MoS2−NaTaO3−PdS amaranth (dye) H2 131PC Cu2O cubooctahedra dyes (MO, RhB, MB) H2 132PC zinc oxysulfide (ZnO0.6S0.4) RV5 H2 133PC Pt/TiO2/Nf RhB H2 134PC Ti3+-doped TiO2 MB, MO, RhB, 4-CP H2 135PC TiO2/Pt AO7 H2 136PC MoS2/ZnIn2S4/RGO dyes (RhB, EY, MB), fulvic acid, p-nitrophenol H2 137PC Ru-doped LaFeO3 glucose H2 138PC F-TiO2/Pt glucose H2 139PC CNT/TiO2/Pt biomass-derived compounds (arabinose, fructose, glucose,

cellobiose)H2 140

PC Fe2O3 polymorphs biomass-derived compounds (ethanol, glycerol, glucose) H2 141PC TiO2/Pt glycerol H2 142PC TiO2/Pt bioethanol, glycerol, alcohols, saccharides, starch, cellulose H2 143PC N-doped ZnO H2S H2 144PC N-TiO2/graphene H2S H2 145PC CdS/TiO2 H2S H2 146PC CdS QD/GeO2 glass H2S H2 147PC CdxZn1−xS H2S H2 148PC TiO2 EDTA Fe, Hg, Ag, Cr 149PC TiO2 cyanide Cu 91PC TiO2 Cu−EDTA, glycerol H2 92PC TiO2 NH3 NO2

− to N2 150PC TiO2/Cu glycerol NO3

−, H2 151PC TiO2/Ag formic acid NO3

− 31PC TiO2/(Ag or Pd) formic acid NO3

− 152PC TiO2/Ag 4-CP Cr(VI) to Cr(III) 99PC g-C3N4/MoS2 RhB Cr(VI) to Cr(III) 153PC CdS/Sr(NbZn)O As(III) H2 154PC TiO2/Pt As(III), 4-CP − 155PEC photoanode: electrochromic TiO2

nanotube4-CP H2 54

cathode: stainless steelPEC photoanode: multilayer

BiOx−TiO2/Ti electrodesphenol H2 156

PEC photoanode: WO3 chlorine-oxidized species in seawater H2O2 87cathode: cobalt chlorin complex

PEC photoanode: (C,N)-doped TNT perfluorooctanoic acid (PFOA) H2 157cathode: Pt wire

PEC photoanode: TiO2 nanorods phenol, toluene, p-xylene, mesitylene, hydroquinone, phloroglucinol H2 158photocathode: carbon-coated Cu2Onanowire

PEC photoanode: TiO2 film nitrogen-containing water (urea, formamide, NH4+) H2 159

cathode: Pt foilPEC photoanode: TiO2 nanofiber/

Ag@AgClurban wastewater (17-β-ethynylestradiol) H2 160

cathode: Pt film

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Table 1. continued

type ofsystema catalyst oxidation process (pollutant degradation)

reduction process (recovery offuel and resources) ref

PEC anode: Cu2O/TiO2 nanotube ibuprofen H2 161cathode: Pt wire

PEC anode: TiO2 aniline, salicylic acid H2O2 162cathode: Pt wire

PEC photoanode: W-doped TiO2 nanotube RhB H2 163cathode: Pt foil

PEC photoanode: WO3/TiO2/Ti RB5 H2 164cathode: Pt mesh

PEC photoanode: CdSe/TiO2 nanotubes MO H2 165cathode: Pt foil

PEC photoanode: TiO2 nanotubes MB H2 166cathode: Pt foil

PEC photoanode: WO3 biomass derived organic wastes (ethanol, glycerol or sorbitol) H2 167cathode: Pt foil

PEC photoanode: Fe2O3/Ni(OH)2 glucose H2 168cathode: Pt wire

PEC photoanode: metal-doped Fe2O3 glucose H2 169cathode: carbon paste with Pt

PEC photoanode: WO3 H2S H2 170cathode: Si PVC

PEC photoanode: n-Si H2S H2 171cathode: Pt plate

PEC photoanode: n-Si H2S H2O2 83cathode: carbon

PEC photoanode: TiO2 film Cu−EDTA Cu 93cathode: stainless steel

PEC photoanode: IrOx-coated Ti mesh EDTA Cu 172cathode: stainless steel

PEC photoanode: TiO2/Ti Cu−EDTA Cu 95cathode: stainless steel

PEC photoanode: Bi2MoO6 Cu−cyanide Cu 96cathode: Ti plate

PEC photoanode: TiO2 nanotube ofloxacin Cu 173cathode: Ti plate

PEC photoanode: g-C3N4 Ag−cyanide Ag 174cathode: Ti plate

PEC photoanode: TiO2 acid dye, surfactant Cr(VI) to Cr(III) 175cathode: Pt gauze

PEFC photoanode: TiO2 nanotubes ornanorods

phenol H2 176

photocathode: carbon-coated Cu2Onanowire

PEFC photoanode: TiO2 nanotubes MO electricity, H2O2 177cathode: anthraquinone/polypyrrole/graphite felt

PEFC photoanode: WO3 phenol, RhB, Congo red electricity, H2 122photocathode: Cu2O

PEFC photoanode: WO3 phenol electricity 178photocathode: Cu2O

PV-EC anode: BiOx−TiO2 urea in human urine H2 179cathode: stainless steel

PV-EC anode: BiOx/TiO2 mixture of domestic wastewater and stored urine H2 180cathode: stainless steel

PV-EC anode: BiOx/TiO2 domestic wastewater H2 69cathode: stainless steel

EC anode: active carbon nanotube phenol H2O2 181cathode: active carbon nanotube

EC anode: BiOx/TiO2 domestic wastewater and human urine H2 182cathode: stainless steel

EC anode: boron-doped diamond cyanide-containing wastewater, 4-nitrophenol H2 183cathode: stainless steel

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water-splitting photocatalysis utilizes hole transfer to oxidizewater and electron transfer to produce H2. The dual-functionalphotocatalysis for H2 production coupled with water treatmentshould utilize hole transfer to degrade organic pollutants andelectron transfer to generate H2 at the same time. Scheme 1illustrates the overall process. The key problem is how tocontrol the charge transfers selectively. The electrons shouldbe consumed to reduce water (or protons) selectively for H2production via two-electron transfer (not to reduce O2),whereas the holes should be used to oxidize organics, mainlyvia one-hole transfer leading to the generation of carbon-centered radicals, not to oxidize water for producing O2 viamultiple hole transfer. The one-electron oxidation of water toOH radical requires a highly positive potential (2.70 V vsNHE) but can occur rapidly with little kinetic limitation.1 Onthe other hand, the four-electron oxidation of water to O2needs a much lower potential (1.23 V vs NHE) but iskinetically hindered. The control over the single versusmultiple electron transfer determines the overall process. Thedual-functional photocatalysts should enable single holetransfer and the multiple electron transfer at the same time,which differs from the charge transfer mechanism of typicalenvironmental photocatalysts (single hole and single electrontransfer) and water-splitting photocatalysts (multiple hole andmultiple electron transfer).Since photocatalysis is a surface chemical process, the

controlled charge transfer and catalysis should be related to thesurface properties, which can be modified in various ways. Kimand Choi56 developed a simple method for dual-functionalphotocatalysis by modifying the surface of TiO2 with platinumnanoparticles and surface fluorides (denoted as F-TiO2/Pt).

Figure 1a compares bare TiO2, F-TiO2, Pt/TiO2, and F-TiO2/Pt for the photocatalytic degradation of 4-chlorophenol (4-CP) and the concurrent generation of H2 in the deaeratedsuspensions. With F-TiO2/Pt, 4-CP degradation was accom-panied by notable production of H2, whereas the H2production was insignificant with other catalysts. Whenbisphenol A (BPA) was used instead of 4-CP as a substrate,the same behavior was observed. The production of H2 on F-TiO2/Pt was much enhanced in the presence of organiccompounds, and the H2 production was gradually reduced asthe organic compounds were depleted during the repeatedcycles of dual-functional photocatalysis (see Figure 1b). Thisindicates that the organic pollutants serve as a hole scavengers(or electron donors) for H2 generation, which was markedlyreduced as the organic pollutants were degraded. The overallprocess demonstrates that the surface-modified F-TiO2/Ptmakes holes oxidize organic pollutants and electrons reducewater to generate H2 at the same time, while the otherphotocatalysts (bare TiO2, F-TiO2, Pt/TiO2) cannot. Theproposed mechanism of the dual-functional photocatalysis onF-TiO2/Pt is illustrated in Figure 1c.57 The photogeneratedholes on bare TiO2 generate surface-bound OH radicals, whichrapidly recombine with electrons in the absence of an electronacceptor (e.g., O2). The Pt nanoparticles deposited on TiO2form a Schottky barrier at the interface that separates theelectrons from holes by accumulating electrons on Pt and serveas a cocatalyst for hydrogen production.58−60 This electrontrapping on Pt hinders the charge pair recombination andincreases the lifetimes of the charge carriers. On the otherhand, the fluorination treatment of the TiO2 surface reducesthe concentration of surface OH groups, which are the sites ofhole trapping.61 It also hinders the adsorption of organiccompounds on the TiO2 surface and subsequently retards thedirect hole transfer to organic compounds.62 As a result, thesurface fluorination decreases the efficiency of surface holetrapping, and then the holes on the fluorinated TiO2 surfacepreferentially react with the adsorbed water molecules instead.The oxidation of adsorbed water molecules (not surfacehydroxyl groups) generates weakly bound (mobile) OHradicals, which can easily desorb from the surface. Thisprocess essentially removes the holes from the TiO2 surfacewhile minimizing the charge recombination between theelectrons trapped in Pt and the surface-trapped holes.Consequently, the trapped electrons on Pt are much longerlived to reduce water and thus produce more H2 in thepresence of surface fluorides. As a result, the production of H2and the degradation of organic compounds are synergicallyenhanced on F-TiO2/Pt. Such unique photocatalytic behavior

Table 1. continued

type ofsystema catalyst oxidation process (pollutant degradation)

reduction process (recovery offuel and resources) ref

EC anode: IrO2/Ti Cu−EDTA Cu 184cathode: Pt/Ti

EC anode: Pt/Ti plate cyanide Cu 185cathode: copper cylinder

EC anode: BiOx−TiO2 As(III) H2 186cathode: stainless steel

EC anode: Ru-coated Ti As(III) Cr(VI) to Cr(III) 102cathode: Ru-coated Ti AuPd/CNTs(electrocatalyst)

aPC, photocatalytic; PEC, photoelectrochemical; PEFC, photoelectrochemical fuel cell; EC, electrochemical; PV-EC, photovoltaic-drivenelectrochemical.

Scheme 2. Energy Level Diagram for a TypicalPhotocatalyst (TiO2) and Various Redox Couples ThatShould Serve as Either Electron Acceptors (Right Side) orElectron Donors (Left Side)

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was also observed when phosphate ions were used instead offluoride ions in preparing P-TiO2/Pt (see Figure 1d).63 Thephosphate ions are adsorbed on the TiO2 surface bysubstitution of the surface OH groups as fluoride ions do,and the above-discussed mechanism of F-TiO2/Pt can also beapplied to the dual-functional photocatalysis on P-TiO2/Pt.The dual-component surface modification (Pt deposition andsurface fluorination) enables the dual-functional photocatalysisby hindering the surface-mediated recombination andenhancing electron transfer for H2 production at the sametime. Many other photocatalytic systems involving H2production coupled with the oxidation of various pollutantssuch as urea, dyes, biomass-derived products (e.g., glucose andglycerol), and H2S are listed in Table 1. Regardless of the kindof pollutant, the key mechanism should be related to theselective charge transfer: single hole transfer to initiate thedegradation of pollutants and multiple electron transfer for H2evolution.Most photocatalytic studies of H2 production have been

carried out under deaerated conditions since H2 is not evolvedin the presence of dissolved O2.

64 However, this dual-functional photocatalyst system cannot remove the totalorganic carbon (TOC) because the mineralization of organicpollutants requires the presence of dioxygen.65 To overcomethis problem, Cho et al.66 employed a hybrid Cr2O3/Rh/SrTiO3 catalyst to enable simultaneous H2 production andmineralization of 4-CP under deaerated conditions. Theyprepared rhodium nanoparticles covered with a thin chromiumshell as a cocatalyst, which had been originally developed as awater-splitting photocatalyst (see Figure 2a).67 The Cr2O3/

Rh/SrTiO3 catalyst exhibited far higher activity than Rh/SrTiO3 in both 4-CP removal and TOC removal, whichindicates that 4-CP could be mineralized in the suspension ofCr2O3/Rh/SrTiO3 even under deaerated conditions (Figure2b). The evolution of H2 and O2 concurrently occurrednonstoichiometrically on Cr2O3/Rh/SrTiO3 but not on Rh/SrTiO3 (Figure 2c). The fact that O2 was generated along withthe degradation of 4-CP on Cr2O3/Rh/SrTiO3 implies thatholes react with both 4-CP and water molecules at the sametime. The in situ-generated O2 can be immediately used for themineralization of 4-CP, which explains why 4-CP could bemineralized under deaerated conditions. The loading of theCr2O3 shell on the Rh nanoparticles changes the reactionmechanism by blocking the back reaction of H2 and O2 (toproduce H2O) on Rh and allowing selective electron transferto water/protons only, not to O2. Figure 2d shows that Cr2O3/Rh/SrTiO3 is more efficient for H2 production than F-TiO2/Ptas a dual-functional photocatalyst. It was proposed that 4-CPwas degraded via direct hole transfer and mineralized with insitu-generated O2. All of the above-mentioned dual-functionalphotocatalytic systems successfully demonstrated simultaneousgeneration of H2 and degradation of organics, which shouldserve as a conceptual model for the dual-functional watertreatment technology. However, its practical realization willrequire extensive efforts beyond the conceptual demonstration.

2.2. PEC and PV-EC Systems. The dual-functionalphotocatalytic water treatment based on a slurry processcannot be suitable for practical applications because thepostphotocatalysis separation and recovery processes of thephotocatalyst particles demand additional efforts and energy.

Figure 1. (a) Simultaneous degradation of 4-CP and production of H2 in UV-irradiated suspensions of bare TiO2, F-TiO2, Pt/TiO2, and F-TiO2/Pt. (b) Time profiles of H2 production from the degradation of organic substrates (4-CP and BPA) during repeated photocatalysis cycles in thesame batch reactor. Reproduced with permission from ref 56. Copyright 2010 Royal Society of Chemistry. (c) Schematic illustrations of interfacialcharge transfer and recombination occurring on F-TiO2/Pt. Reproduced with permission from ref 57. Copyright 2015 Elsevier. (d) Production ofH2 in suspensions of bare TiO2, F-TiO2, P-TiO2, Pt/TiO2, F-TiO2/Pt, and P-TiO2/Pt with 4-CP under λ > 320 nm irradiation. Reproduced withpermission from ref 63. Copyright 2012 Royal Society of Chemistry.

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Therefore, more practical dual-functional photocatalytic watertreatment should employ immobilized photocatalysts on theelectrode surface in a PEC device. The PEC system utilizes anexternal electrical bias under photoirradiation to separate theelectrons and holes to a cathode and an anode, respectively.This should be an ideal method to achieve the destructiveremoval of organic pollutants on the photoanode and thereduction of water to H2 on the cathode at the same time (seeScheme 1b). In a recent study, electrochromic titania nanotubearrays (denoted as Blue-TNTs) have been proposed as aphotoanode and exhibited higher charge carrier density andelectrical conductivity for its PEC application to simultaneouswater treatment and H2 production (Figure 3a).54 Theelectrochromic behavior is related to the surface defectgeneration, which changes the Ti oxidation state from +4 to+3 during the cathodic polarization. The formation of Ti3+ inthe TiO2 lattice accompanies the color change of the electrodeand the red shift of the absorption spectrum (Figure 3b). TheBlue-TNT photoanode coupled with a stainless steel cathodemarkedly enhanced the efficiency of pollutant degradation andH2 production, whereas intact TNTs or a TiO2 nanoparticlefilm did not exhibit such dual-functional PEC activities (Figure3c).The photoinduced holes on the photoanode can be also

utilized to generate reactive chlorine species (RCS) (viaoxidation of chloride) as an oxidant for water treatment.Recently, Kim et al.68 reported a hybrid PEC system thatachieved the desalination of saline water, RCS-mediated watertreatment, and hydrogen production at the same time (Figure4a). A hydrogen-treated TiO2 nanorod (H-TNR) photoanodeand a Pt foil cathode were placed in anode and cathode cells,

respectively, with a middle cell containing saline water facingthese cells through the membranes. Under irradiation,photogenerated charges initiate desalination of saline waterby transporting chloride and sodium ions in the middle celltoward the anode and cathode cells, respectively. RCS aregenerated by photogenerated holes on the H-TNR photo-anode, which can oxidize urea (and other organic compounds),while hydrogen is produced on the cathode with a Faradaicefficiency of ∼80% (see Figure 4b,c). Cho et al.69 reported thata PV-powered electrolysis system employing a multilayer Bi-doped TiO2 anode and a stainless steel cathode can treatdomestic wastewater with simultaneous H2 production (Figure5a). A low-cost polycrystalline PV panel was employed toapply a direct-current potential across the anode and thecathode. The removal of chemical oxygen demand (COD) wasachieved by the homogeneous reaction between the oxygen-demanding compounds and RCS under a static anodicpotential (+2.2 or +3.0 V vs NHE) (Figure 5b,c). Therefore,over 95% removal of COD and ammonium ions was achievedwithin 6 h with concurrent H2 production (with currentefficiencies ranging from 34% to 84%).While most of the studies on the simultaneous removal of

organic pollutants and production of H2 employed only one ortwo organic pollutants as test substrates, the successfulapplication to real domestic wastewater demonstrated thepractical feasibility of the prototype PV-powered wastewaterelectrolysis with simultaneous H2 production. Since chlorideions are ubiquitous in natural fresh waters, wastewaters, andseawater, the activation of chloride into RCS under solarirradiation is an appealing method to achieve dual-functionalwater treatment. The sustained production of RCS in PEC and

Figure 2. (a) HR-TEM image of Cr2O3/Rh/SrTiO3 with a schematic illustration. (b) Photocatalytic degradation of 4-CP and concurrent chlorideion production in a deaerated suspension of Rh/SrTiO3 and Cr2O3/Rh/SrTiO3. (c) Time profiles of H2 and O2 production in the presence of 4-CPin the irradiated suspension of Cr2O3/Rh/SrTiO3 and Rh/SrTiO3. (d) Comparison of the initial H2 production rates of Cr2O3/Rh/SrTiO3 and F-TiO2/Pt in the presence of 4-CP. Reproduced with permission from ref 66. Copyright 2016 Royal Society of Chemistry.

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(PV-)EC systems should serve many purposes, such aschlorination disinfection, tertiary treatment of wastewaters,toilet water treatment, and ship ballast water treatment.70−73

For example, a small-scale PV-EC reactor (treatment capacityof 0.13 m3/day) based on the simultaneous RCS generationand H2 production system has been proposed as a recyclingsystem for toilet water.74

3. IN SITU PRODUCTION OF H2O2 IN WATERTREATMENT

Hydrogen peroxide is a green oxidant that is being widely usedin water treatment, chemical synthesis, and bleaching.75 On theother hand, it can be considered as a green liquid fuel that canbe utilized for electricity generation via a fuel cell.76 SinceH2O2 decomposes to generate water and dioxygen only, it is aclean fuel that does not leave a carbon footprint. Thephotochemical synthesis of H2O2 as an alternative solar fuelusing photoactive semiconductor materials is being activelyinvestigated.77−79 Therefore, water treatment coupled withH2O2 production through dual-functional photocatalysis is anattractive option. The in situ-generated H2O2 can be recoveredas a fuel or be utilized as an in situ oxidant for water treatment

(e.g., the Fenton process).80 For example, a recent studyreported a ternary-hybrid photocatalytic system that consistedof modified carbon nitride, WO3, and Fe3+ for As(III)oxidation under irradiation with visible light (λ > 420 nm),which utilizes in situ-generated H2O2 as a Fenton reagent.81

The current industrial method of H2O2 synthesis (i.e., theanthraquinone process) requires H2 gas, organic solvents, andhigh energy input, which is not eco-friendly.75 The photo-catalytic production of H2O2 can be an alternative greensynthetic method that demands dioxygen, water, and lightonly. H2O2 can be generated through proton-coupled electrontransfer (PCET) to dioxygen through selective two-electrontransfer (eq 1):82

EO 2H 2e H O ( 0.695 V vs NHE)2 2 2+ + → ° =+ − (1)

To maximize the photonic efficiency of H2O2 production, thenumber of electrons transferred to O2 should be limited totwo, and the competing electron transfers to protons (2H+ +2e− → H2) should be hindered. To serve this dual-purposephoto(electro)catalysis, the ideal electron donors for H2O2production should be the pollutants (P) themselves, which canbe oxidatively converted or degraded in contaminated water(eq 2):

nP P eox→ + −(2)

Zong et al.83 developed a PEC system (Figure 6a) where thetoxic pollutant H2S can be utilized as an electron donor forH2O2 production without an external electrical bias (eq 3):

H S O H O S2 2 2 2+ → + (3)

In this PEC system, hazardous H2S gas (usually generated as awaste from coal and petroleum chemical processing) can beoxidized to elemental sulfur (S) on the photoanode, which canbe coupled with the reduction of O2 to H2O2 on the cathode.The carbon/p+n-Si coupled electrode was used for anunbiased system, and anthraquinone (AQ) and iodide wereadded in the cathode and anode cells, respectively. The use ofcarbon as the counter electrode generated higher photocurrentthan a Pt electrode, which reduces the cost of the system byeliminating the need for the expensive Pt electrode (Figure6b). The redox shuttle of I−/I3

− was employed for theoxidation of H2S in the photoanode compartment (to mediatethe hole transfer from the p+n-Si electrode to H2S), while theAQ/H2AQ redox shuttle was used for the two-electronreduction of O2 in the cathode cell. These redox shuttlespecies are more favorably oxidized and reduced than waterand protons, and the PEC device exhibited highly enhancedcurrent and a solar-to-chemical conversion efficiency of 1.1%.This combined PEC−chemical reaction system achievedsignificant photocurrent density (8 mA cm−2 in an unbiasedsystem), leading to a higher H2O2 generation rate comparedwith other PEC H2O2 production systems.84−86

It is interesting to note that seawater can be also utilizedinstead of pure water for the production of H2O2. Mase et al.87

reported a two-compartment PEC cell using an m-WO3/FTOphotoanode and CoII(Ch)/carbon paper cathode for theproduction of H2O2. The cobalt complex fixed on the carbonpaper (cathode) can generate H2O2 via O2 reduction whileprohibiting the coordination of chloride ion to the cobaltcomplex. The catalytic production of H2O2 on the cathode ismuch enhanced by the oxidation of chlorides (serving as anelectron donor) on the photoanode in seawater and NaClsolution compared with pure water (Figure 6c). Compared

Figure 3. (a) Schematic illustration of electrochromic TiO2 nanotubearrays (Blue-TNTs) combined with a stainless steel electrode for PECdegradation of organic compounds with simultaneous H2 production.(b) Diffuse-reflectance spectra of pristine TiO2 nanotube arrays(TNTs) and Blue-TNTs. (c) PEC degradation of 4-CP on TNTs andBlue-TNTs with concurrent H2 production for five repeated runs.Reproduced from ref 54. Copyright 2017 American Chemical Society.

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Figure 4. (a) Schematic illustration of a sunlight-driven hybrid PEC system that couples desalination, water treatment, and H2 production at thesame time. (b) Time profiles of urea removal and TOC (upper panel) and concomitant production of nitrate and ammonia (lower panel). (c) H2production in the cathode compartment and Faradaic efficiency. Reproduced with permission from ref 68. Copyright 2018 Royal Society ofChemistry.

Figure 5. (a) Schematic illustration of the PV-powered electrolysis system for wastewater treatment with simultaneous H2 production. (b, c) Timeprofiles of COD removal in domestic wastewater with different chloride concentrations (0, 10, 30, and 50 mM) and anodic potentials (L, 2.2 V vsNHE; H, 3.0 V vs NHE). Reproduced from ref 69. Copyright 2014 American Chemical Society.

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with four-electron oxidation of water molecules, two-electronoxidation of chloride (leading to Cl2 and HOCl) is greatlyfavored, so the overall PEC process is much improved withseawater conditions. The in situ-generated RCS might befurther utilized as an oxidant for seawater treatment (e.g., shipballast water). H2O2 produced by this PEC reaction could beutilized in a fuel cell to generate electrical energy. The H2O2

fuel cell with a Ni mesh anode and an FeII3[CoIII(CN)6]2/

carbon cloth cathode in one-compartment cell demonstrated asolar-to-electricity conversion efficiency of 0.28% using H2O2

produced in the PEC reactor.Shi et al.88 reported an interesting case of an unassisted PEC

system that produced H2O2 on both a BiVO4 photoanode anda carbon cathode (Figure 6d). The optimized system achieveda H2O2 production rate of 0.48 μmol min−1 cm−2, which is thehighest value among oxide−semiconductor-based PEC H2O2

production systems. The in situ-generated H2O2 can beutilized in water treatment through the reductive decom-

position into OH radicals,21,89 Since PEC water splitting hasefficiency limits due to various parameters such as fractionalsolar light absorption,90 a more efficient PEC system shouldutilize most of the charge carrier energy by converting bothholes and electrons into useful products. For example, Figure6e illustrates an efficient PEC system for H2O2 production thatutilizes not only electrons but also holes. The in situ-generatedH2O2 along with RCS can be utilized for water treatment.

4. REDOX-COUPLED CONVERSION AND RECOVERYOF TOXIC METAL IONS

Wastewater containing various heavy metals such as lead,copper, nickel, arsenic, and chromium is a major environ-mental problem for which various remediation technologiesare being developed. Since heavy-metal ions are non-degradable, it is very important to treat the wastewater toremove them before the water is discharged into theenvironment. PC and PEC processes are suitable for the

Figure 6. (a) Schematic illustration of the PEC cell with redox shuttles for the production of H2O2 and S from H2S. (b) Current−voltage curves ina two-electrode system on p+n-Si with carbon or Pt as the counter electrode under AM 1.5 irradiation (100 mW cm−2). Reproduced withpermission from ref 83. Copyright 2014 Royal Society of Chemistry. (c) Time profiles of PEC production of H2O2 in water (red circles), seawater(blue circles), and NaCl solution (blue squares) at pH 1.3 under AM 1.5 irradiation (100 mW cm−2). Reproduced with permission from ref 87.Copyright 2016 Nature Publishing Group. (d) Time profiles of H2O2 production on a BiVO4 photoanode and carbon cathode in a two-electrodesystem with O2 purging with an applied bias of 1.5 V. Reproduced with permission from ref 88. Copyright 2018 Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim. (e) Overall scheme for PEC production of H2O2 and reactive chlorine species (RCS) on a photoanode coupled with concurrentH2O2 production on the catalyst-loaded cathode.

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recovery of heavy-metal ions since the photoinduced reductionof metal ions leads to their deposition onto the surface of thecatalyst or electrode. However, the actual process is morecomplex than a simple reduction of metal ions. Toxic heavy-metal species are often found as stable complexes, which arerecalcitrant and difficult to remove. Many advanced oxidationprocesses (AOPs) have been widely investigated for thedegradation of common chelating agents such as ethyl-enediaminetetraacetic acid (EDTA).91,92 The photocatalyticdegradation of organometallic complexes releases heavy-metalions that should be further treated for removal or recovery.Zhao et al.93 investigated the simultaneous PEC oxidation of

Cu−EDTA on a TiO2-film as a photoanode and the reductiverecovery of Cu2+ ions on a stainless steel cathode (Figure 7a).Unlike the slurry photocatalytic system, in which Cu2+ ionsreleased from the degradation of Cu−EDTA must berecovered from the slurry phase by additional separationprocesses, the PEC system does not need the separationprocess because copper ions are directly deposited on thecathode.94 The PEC process showed enhanced activities fornot only the degradation of Cu−EDTA but also the recovery

of Cu2+ ions compared with the PC and EC systems (Figure7b). However, because of the recalcitrant nature of metalcomplexes, their oxidative degradation proceeds slowly, whichlimits the efficient treatment. Zeng et al.95 added persulfate(S2O8

2−) to the PEC reactor, which can be reductivelydecomposed into sulfate radicals (SO4

•−) on a cathode. Sulfateradicals with high oxidation power can decompose a widerange of recalcitrant pollutants. This persulfate−PEC systemenhanced the Cu−EDTA degradation efficiency from 47.5%for the PEC system without persulfate to 98.4% in 60 min andrecovered the Cu2+ ions quantitatively under a wide range ofpH conditions (Figure 7c). The heavy-metal ions complexedwith inorganic anions present a more difficult case for themetal recovery process. For example, the decomposition ofcopper cyanide (Cu(CN)3

2−) releases toxic cyanide ions,which could result in the generation of toxic HCN gas.Therefore, the treatment process should be carried out athigher pH (>10) to avoid the protonation of the cyanide ions(pKa(HCN) = 9.2).96 Under such alkaline conditions, theliberated metal ions are not only reductively deposited on thecathode but also easily precipitated. Zhao et al.96 reported the

Figure 7. (a) Schematic illustration of PEC oxidation of Cu−EDTA and recovery of Cu. (b) Removal profiles of Cu−EDTA complexes and theconcurrent profiles of Cu recovery with photocatalysis (PC), electrooxidation (EO), and PEC processes. Reproduced from ref 93. Copyright 2013American Chemical Society. (c) Variation of the residual ratio of Cu complexes and percentage of Cu recovery in UV/persulfate (S2O8

2−), PEC,EC/S2O8

2−, and PEC/S2O82− processes. Reproduced from ref 95. Copyright 2016 American Chemical Society. (d) Time profiles of intermediates

produced by PEC oxidation of Cu(CN)32− and (d, e) effects of (e) EDTA and (f) K4P2O7 on total cyanide removal and Cu recovery. Reproduced

from ref 96. Copyright 2015 American Chemical Society.

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visible-light-induced PEC degradation of copper cyanides inthe presence of EDTA or K4P2O7 using a Bi2MoO6

photoanode. Cyanide ions were oxidized to cyanate ions(CNO−) by holes (Figure 7d), and the concurrent oxidation of

Cu+ to Cu2+ also occurred on the Bi2MoO6 photoanode. Thisresulted in the deposition of CuO and CuOOH on thephotoanode, which inhibited the further PEC reaction.However, in the presence of EDTA or K4P2O7, liberated

Figure 8. (a) Schematic illustration of photocatalytic degradation of 4-CP and subsequent reductive removal of Cr(VI) in the dark using Ag/TiO2.(b) Time profiles of the photocatalytic degradation of 4-CP under UV irradiation and the subsequent Cr(VI) reduction in the dark (after the lightwas turned off). (c) Time profiles of the normalized open-circuit potentials (OCPs) of bare TiO2, Pt/TiO2, and Ag/TiO2 in 0.1 M NaClO4 (pH 3)with 300 μM 4-CP under λ > 320 nm irradiation. Reproduced from ref 99. Copyright 2017 American Chemical Society.

Figure 9. (a) Schematic illustration of the simultaneous transformation of Cr(VI) and As(III) using AuPd/CNTs as an electrocatalyst. (b)Rotating ring−disk electrode (RRDE) curves of CNTs, Pd/CNTs, Au/CNTs, and AuPd/CNTs in O2-saturated H2SO4 solution (pH 3). The insetshows the corresponding electron transfer numbers of AuPd/CNTs. (c) Redox conversion of Cr(VI) and As(III) in the presence of AuPd/CNTs,electrolysis, and electrolysis with AuPd/CNTs. (d) Effects of various free radical scavengers on redox conversion of Cr(VI) and As(III) after 15min of reaction. Reproduced from ref 102. Copyright 2015 American Chemical Society.

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copper ions were immediately complexed with EDTA orP2O7

4−, which could be reductively deposited on the cathode(Figure 7e,f). With this strategy, cyanide could be effectivelyremoved with simultaneous Cu recovery.For practical applications, the decomposition of EDTA or

CN− below the legal emission levels and the recovery of Cushould be economically feasible. However, the powerconsumption is one of the major parameters that should becritically evaluated for the application of this PEC process towater treatment. Most PEC processes require an additionalexternal bias, although they are photochemical processes.Therefore, electrical power consumption is needed. Inaddition, the use of lamps instead of sunlight needs additionalenergy for irradiation. Although the PEC process exhibitedsuperior performance for the degradation of pollutants andmetal recovery compared with the electrolytic system, the totalenergy consumption of the PEC system, including the lightsource, should be carefully compared with that for otherphotocatalytic and electrolytic systems through technoeco-nomic analysis.97,98

Most photocatalytic or PEC/EC water treatment applica-tions involve the removal of pollutants by oxidation orreduction alone. However, such processes have a limitationin that the thermodynamic driving force of only one chargecarrier is utilized, while that of the other charge carrier iswasted. For the better performance, the system should takeadvantage of both the oxidation and reduction processes ofpollutants simultaneously. Some examples have been reportedin the literature.Choi et al.99 studied sequential treatment by photocatalytic

oxidation and dark reduction using Ag/TiO2 (Figure 8a). Inthe sequential combination of photocatalysis with the darkthermal reaction, organic pollutants were first oxidized underUV irradiation with concurrent storage of electrons in Ag.Then the electrons stored in Ag/TiO2 and the intermediatesgenerated from the degradation of organic pollutants wereutilized for the reduction of hexavalent chromium (Cr(VI))under the dark conditions. Because of the superior electronstorage capacity of Ag metal nanoparticles, Ag/TiO2 exhibitedhigher electron storage compared with Au/TiO2 and Pt/TiO2,which improved the reduction process on the metal nano-particles and the reactivity of the remaining holes on TiO2 atthe same time.100 The photocatalytic oxidation efficiencies of4-CP decreased in the order Pt/TiO2 > Ag/TiO2 > TiO2, butAg/TiO2 exhibited higher activity for the removal of Cr(VI)than bare TiO2 and Pt/TiO2 in the post-irradiation dark period(Figure 8b). In the Ag/TiO2 system, the stored electrons werelong-lived even in the presence of dissolved O2, and the Cr(VI)removal on Ag/TiO2 in the dark period continued for hours.This was further confirmed by an open-circuit potentialmeasurement, which showed that a residual potential on theAg/TiO2 electrode persisted for over an hour in the darkperiod (Figure 8c).Simultaneous oxidation of As(III) to As(V) and reduction of

Cr(VI) to Cr(III) is an interesting treatment example for theredox-coupled removal of pollutants.101 Sun et al.102

demonstrated that the synergistic redox conversion of Cr(VI)and As(III) could be enhanced by H2O2 generated in situ in athree-dimensional electrocatalytic reactor employing AuPd/carbon nanotubes (CNTs) as an electrocatalyst (Figure 9a).Although the use of H2O2 increases the treatment cost, the insitu synthesis of H2O2 through O2 reduction is highly desirable,as discussed in the previous section. In Figure 9b, AuPd/CNTs

showed a higher peak current density and a more positiveonset potential compared with other samples. This implies thatthe AuPd/CNTs electrode is more kinetically favored for O2reduction and in situ H2O2 generation than the other electrodesamples. Therefore, the enhancement of Cr(VI)/As(III) redoxconversion on AuPd/CNTs was attributed to the electro-catalytic in situ generation of H2O2 (Figure 9c). In addition,hydroxyl radicals (·OH) and superoxide radicals (O2

•−) can bealso generated via the reduction of O2.

103 The Cr(VI)reduction was little influenced by ·OH and O2

•−, but thepresence of ·OH and O2

•− influenced the As(III) oxidationreaction (Figure 9d).The most outstanding advantage of PC- and PEC-based

treatment technologies compared with other AOPs is that theyenable the reductive conversion of pollutants as well as theoxidative degradation. The treatment of recalcitrant toxic metalcomplexes provides a good example in which such merit makesthe PC/PEC treatment versatile. Oxidative degradation of theligands induces the decomplexation of the metal ions, and theconcurrent reductive conversion of metal ions makes therecovery of metal species facile. The oxidative and reductiveconversions in the PC/PEC system can be also achievedsequentially, as shown in the example in Figure 8.101

5. NITROGEN CONVERSION COUPLED WITHORGANIC OXIDATION

Nitrogen is present in nature in various forms, includingnitrate, nitrite, nitrogen oxides, dinitrogen, and ammonium (inthe order of decreasing oxidation state). Among them, nitrateis the most oxidized form (+5 oxidation state) of nitrogenspecies in the environment, and it can be found in agriculturalor industrial wastewaters at high concentrations.104 Sincenitrate is highly dissolved in water and nonadsorbing on metaloxides and most adsorbents, the efficient conversion orremoval of nitrate in water treatment has been a crucialissue. The sequential reduction of nitrate should generatevarious products such as nitrite, dinitrogen, and ammonium,which should be coupled with oxidation of organic compounds(electron donors). The ideal scenario for nitrate removal inwater is to utilize organic pollutants as in situ electron donorsto reduce nitrate selectively to dinitrogen, which shouldcomplete the abiotic denitrification process in a single step.However, most of the reported studies of nitrate conversioninvestigated the transformation of nitrate to ammonium ions.For photocatalytic systems for nitrate reduction, metal-modified photocatalysts were generally investigated to usemetallic catalysts for nitrate reduction in the presence oforganic electron donors.105,106 For example, Kominami etal.107 reported photocatalytic reduction of nitrate to ammoniausing metal (Pt, Pd, Ni, Au, Ag, Cu, etc.)-loaded TiO2 in thepresence of oxalic acid. Photocatalytic activities for nitrateconversion to ammonia, production of hydrogen, andoxidation of oxalic acid using metal-loaded TiO2 wereinvestigated. Ag- or Cu-loaded TiO2 samples showed thehighest nitrate reduction with 96−99% selectivity for NH3. Ni-or Au-loaded TiO2 samples showed much lower NH3selectivity (less than 22%) with higher activities for H2production, whereas Pt- or Pd-loaded TiO2 samples showedthe highest activities for H2 production with no activities forgeneration of NH3. The photocatalytic reduction of nitratecompetes with photocatalytic hydrogen production, whichreduces the overall efficiency of nitrate conversion. Therefore,control of the catalyst selectivity is critical. In general, metals

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with high hydrogen overvoltage (HOV) favor nitrate reductionover H2 production. Similar to this study, most PC systems fornitrate reduction have been investigated with various metalliccatalysts, in particular Ag, Cu, or Cu-based alloys.106,108−110

However, the complete conversion of nitrate to dinitrogen gashas rarely been reported among the numerous studies of PCsystems.111

For denitrification in water treatment, electrochemicalsystems are among the emerging approaches.104 The directreduction of nitrate to dinitrogen gas is also difficult in ECsystems because complicated reaction pathways compete withnitrate reduction and specific conditions are needed in eachstep. Therefore, a typical strategy for EC denitrification is tocombine nitrate reduction to ammonia on the cathode andammonia oxidation to dinitrogen on the anode in the same ECreactor.112,113 For ammonia oxidation, the indirect oxidationprocess mediated by chlorine radical (Cl·) is an effectiveapproach.114 Zhang et al.115 reported an advanced concept forPEC denitrification using chlorination. In that study, theyprepared a WO3 nanoplate film as a photoanode for chlorineoxidation under visible-light irradiation and Pd−Cu alloydeposited on Ni foam (NF) as a cathode for selective nitratereduction to ammonia (Figure 10a). The Pd−Cu alloyelectrode showed the highest electrocatalytic activity fornitrate reduction compared with other electrodes (Pt, bareNF, Cu/NF, Pd/NF) (Figure 10b). They investigated nitrateremoval in a PEC−chlorine system and found that both nitrateand ammonia were effectively removed as the chlorine ionconcentration increased, demonstrating a high removalefficiency of total nitrogen (∼95%) (Figure 10c,d).

6. PHOTO(ELECTRO)CATALYTIC FUEL CELLS

Photo(electro)catalytic fuel cells (PEFCs) are a promisingtechnology that recovers electricity along with the PECoxidation of organic pollutants.116 These systems consist of aphotoanode and a (photo)cathode in the solution containingorganic substrates (e.g., glucose or alcohol) and operatethrough concurrent PEC oxidation of organic substrates on thephotoanode and dioxygen reduction on the cathode.117 Thebasic concept of PEFCs is very similar to that of a conventionalPEC system, but a PEFC should operate without an externalbias, driven by the difference in the Fermi levels of the twoelectrodes under irradiation. The Fermi level differencegenerates an internal bias so that photoexcited electrons inthe photoanode can be transferred to the cathode via anexternal circuit, generating electricity. Therefore, the net effectof the PEFC is to convert chemical energy contained inorganic pollutants into electrical energy with the assistance ofphotoenergy. Although simultaneous degradation of bio-organic fuels and recovery of energy has also been achievedby microbial fuel cells (MFCs), the current efficiency of MFCsis limited by the complex biochemical electron transfer processand complicated operation processes involving cultivation ofbacteria.118 In this regard, systems combining a PEC cell ordye-sensitized solar cell (DSSC) with an MFC have beeninvestigated to overcome the current limits of MFCs. For thePEC−MFC combined system, different configurations can bepossible: (i) microbial-modified electrode as a cathode in thePEC cell,119 (ii) enzyme-mediated biochemical oxidation inthe PEC cell,120 and (iii) assembled device between the PECcell and the MFC.121 Although all PEC−MFC combinedsystems are more efficient than conventional MFCs, the system

Figure 10. (a) Schematic illustration of the inorganic nitrogen removal mechanism in the PEC system based on a WO3 photoanode and Cu−Pd-loaded Ni foam cathode. (b) Electrocatalytic nitrate removal efficiencies on different cathode materials. (c, d) Effect of chlorine concentration onthe PEC (c) nitrate removal and (d) ammonia generation. Reproduced from ref 115. Copyright 2018 American Chemical Society.

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performance is still limited by the operating conditions formicroorganisms.PEFCs can overcome such limits of MFC-based systems. In

a PEFC, a rapid charge transfer process can be achieved byoxide-semiconductor-based photo(electro)catalysis so that thesystem can be effectively operated even without external bias.Chen et al.122 reported a wastewater treatment systemcombined with a PEFC to utilize chemical energy abstractedfrom wastewater to generate electricity and hydrogen gas. Inthis study, they prepared a WO3 photoanode and a Cu2Ophotocathode and induced an internal bias due to themismatched Fermi levels of the two electrodes (Figure 11a).In this way, electricity was generated without an external biasby transferring photogenerated electrons from the WO3photoanode to the Cu2O photocathode via an external circuit(Figure 11b,c). At the same time, organic compounds (phenol,rhodamine B, and Congo red) were simultaneously oxidized byphotogenerated holes on the WO3 photoanode (Figure 11d).Zhou et al.123 reported a solar-charged PEFC with a similarelectrode configuration (WO3 photoanode−Cu2O photo-cathode) for phenol oxidation and simultaneous hydrogenproduction. Hydrogen production on the Cu2O electrode was3 times higher when coupled with phenol oxidation on theWO3 electrode (compared with the case coupled with wateroxidation on WO3), which implies that phenol oxidation onWO3 provides more electrons that are transferred though theexternal circuit. In addition, hydrogen could be produced onCu2O even in the dark after the irradiation period because of

the stored electrons in the WO3 electrode, enablingsimultaneous wastewater treatment, energy recovery, andcharge storage.PEFCs might be a good candidate technology for

simultaneous wastewater treatment and hydrogen production,but there are obvious limits for choosing photoelectrodematerials. To drive effective internal electron flow between twoelectrodes, the Fermi level of the photoanode should be morenegative than that of (photo)cathode, and the Fermi leveldifference should be as high as possible. However, suitablematerial combinations that match such properties are few, andthe development of new photoactive materials for PEFCapplications are highly encouraged.

7. SUMMARY AND OUTLOOK

The solar chemical conversion processes initiated by band gapexcitation of semiconductor materials have been extensivelyinvestigated for decades by employing photocatalysis ofsuspended semiconductor nanoparticles and photoelectroca-talysis of semiconductor electrodes. Both processes are basedon photoinduced interfacial charge transfers, which utilize thereductive power of electrons and the oxidative power of holes.The semiconductor-based PC and PEC processes have manyadvantages since they can be environmentally benign (no toxicchemicals needed), sustainable (driven by solar power), facileand safe (working under ambient conditions), and econom-ically feasible (when low-cost and earth-abundant semi-conductor materials are employed). In most photocatalytic

Figure 11. (a) Energy level diagram of the WO3/W−Cu2O/Cu photoelectrocatalytic wastewater fuel cell (PWFC) system for organic degradationand H2 production or electricity generation (VP, photovoltage). (b) Open-circuit voltage of the WO3/W−Cu2O/Cu PWFC system in the dark andunder AM 1.5 irradiation (100 mW cm−2) in 0.1 M KH2PO4 (pH 7). (c) Photocurrent−time profiles of the PWFC system with no external bias in0.1 M KH2PO4 (pH 7). The photocurrent activities of other electrodes (WNA−Cu2O/Cu, WO3/W−Pt, and Pt−Cu2O/Cu) were also compared.(d) Removal efficiencies of phenol in photolysis and the WO3/W−Cu2O/Cu and WNA-Cu2O/Cu PWFC systems. Reproduced from ref 122.Copyright 2012 American Chemical Society.

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and PEC processes, the main purposes of the chemicalconversion processes are focused on either electron-drivenreduction (e.g., H2 production, CO2 conversion) or hole-driven oxidation (e.g., oxidation of organic compounds).Although any PC or PEC process must involve reductive andoxidative conversions simultaneously to make the overallcharge balance, most studies have focused on one process (thetarget conversion reaction), and the other counterpart reactionhas received less attention and been little utilized for usefulconversions.This Perspective aimed to introduce, analyze, and discuss

dual-functional PC, PEC, and PV-EC processes that utilizeboth electron reduction power and hole oxidation powerefficiently to achieve simultaneous water treatment (pollutantdegradation) and recovery of energy (e.g., H2 production) andresources (e.g., heavy-metal elements). Many studies have triedto design and modify photoactive materials for this purpose.The integration of water treatment with energy/resourcerecovery in a single PC/PEC system is an ideal example ofenergy and environmental applications of solar energy, butthere is a long way to go beyond the conceptual demonstrationtoward commercialization. Dual-functional water treatment isconsidered a suitable method that can solve the inextricablewater−energy nexus problem. The merits, demerits, andchallenges in developing dual-functional PC, PEC, and PV-EC processes are summarized in Table 2. The overall processcan be limited by low efficiency, instability, and high operationcost even at a laboratory scale. To solve these limitations andestablish practical solar-driven systems, we need to develop notonly highly efficient and durable photocatalytic materials but

also economical and scalable solar reactors for commercializa-tion. There has been little effort on this latter part despite theintensive interest in the former part, and therefore, thepractical realization of solar-powered dual-functional watertreatment technology needs more balanced approaches andefforts in both materials development and reactor design andengineering.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: wchoi@postech.edu. Tel: +82-54-279-2283.ORCIDWonyong Choi: 0000-0003-1801-9386NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the Global ResearchLaboratory (GRL) Program (NRF-2014K1A1A2041044), theBasic Science Research Program (NRF-2017R1A2B2008952),and the Framework of International Cooperation Program(NRF-2017K2A9A2A11070417), which were funded by theGovernment of Korea (MSIP) through the National ResearchFoundation (NRF).

■ REFERENCES(1) Park, H.; Kim, H.-i.; Moon, G.-h.; Choi, W. PhotoinducedCharge Transfer Processes in Solar Photocatalysis Based on ModifiedTiO2. Energy Environ. Sci. 2016, 9, 411−433.

Table 2. Comparison of Solar-Driven Technologies for the Dual-Functional Water Treatment Process

photocatalytic (PC) photoelectrochemical (PEC)photovoltaic-driven

electrochemical (PV-EC)

merits • single-phase reaction medium • redox potential controllable by an applied bias • mature status of PV and ECtechnology

• very simple design and setup (low capital cost) • easier control of reaction parameters andconditions

• most of the merits of PECsystems

• no need for electrolytes • relatively simple design and setup • separation of the light-collect-ing part from the reactor part

• good stability • separation of the water treatment cell from theenergy/resource recovery cell

• highest solar conversion effi-ciency

• direct energy supply from solar light • direct energy supply from solar light • easy scale-up and wide range ofapplications using commercial-ized PV

demerits • low efficiency and small-scale operation (difficult to scale up) • medium efficiency and medium-scale operation(difficult to scale up)

• high capital cost

• redox potentials limited by SC band gap and band positions • highly caustic electrolyte needed • need for electricity and elec-tricity loss during EC conver-sion

• difficult to optimize the reaction conditions for both oxidation andreduction simultaneously

• lack of long-term stability of the SC electrodedue to fouling and material deterioration

• highly caustic electrolyteneeded

• difficult to investigate the operating mechanisms • difficult to construct large-area SC electrodes • limited by PV and ECefficiency and cost

• need for separation of the catalyst and products

challengesand futureworks

• control and optimization of the catalyst and reaction conditions forboth oxidation and reduction simultaneously in a single-phasemedium

• development of SC photoelectrodes withimproved efficiency and durability

• system engineering to reducethe costs of PV and EC parts

• catalyst engineering for higher efficiency and selectivity • development of SC photoelectrodes thatoperate under mild electrolyte and circum-neutral pH conditions

• development of PV and ECtechnology

• development of efficient and economical solar reactors for PC, PEC, and PV-EC

• optimization of system and material engineering for the characteristics of the target water to be treated

• systematic investigation of the coupling between the kind of pollutants to be removed and the kind of energy/resources to be recovered

• antifouling strategy for practical water treatment applications

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DOI: 10.1021/acscatal.8b03521ACS Catal. 2018, 8, 11542−11563

11557

(2) Park, H.; Park, Y.; Kim, W.; Choi, W. Surface Modification ofTiO2 Photocatalyst for Environmental Applications. J. Photochem.Photobiol., C 2013, 15, 1−20.(3) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.;Mi, Q. X.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells.Chem. Rev. 2010, 110, 6446−6473.(4) Nakata, K.; Fujishima, A. TiO2 Photocatalysis: Design andApplications. J. Photochem. Photobiol., C 2012, 13, 169−189.(5) Lan, Y. C.; Lu, Y. L.; Ren, Z. F. Mini Review on Photocatalysis ofTitanium Dioxide Nanoparticles and Their Solar Applications. NanoEnergy 2013, 2, 1031−1045.(6) Friedmann, D.; Hakki, A.; Kim, H.; Choi, W.; Bahnemann, D.Heterogeneous Photocatalytic Organic Synthesis: State-of-the-art andFuture Perspectives. Green Chem. 2016, 18, 5391−5411.(7) Chong, M. N.; Jin, B.; Chow, C. W.; Saint, C. RecentDevelopments in Photocatalytic Water Treatment Technology: AReview. Water Res. 2010, 44, 2997−3027.(8) Lu, M.; Pichat, P. Photocatalysis and Water Purification: FromFundamentals to Recent Applications; John Wiley & Sons, 2013; pp 1−397.(9) Li, K.; Peng, B. S.; Peng, T. Y. Recent Advances inHeterogeneous Photocatalytic CO2 Conversion to Solar Fuels. ACSCatal. 2016, 6, 7485−7527.(10) Colmenares, J. C.; Luque, R. Heterogeneous PhotocatalyticNanomaterials: Prospects and Challenges in Selective Transforma-tions of Biomass-Derived Compounds. Chem. Soc. Rev. 2014, 43,765−778.(11) Takanabe, K. Photocatalytic Water Splitting: QuantitativeApproaches toward Photocatalyst by Design. ACS Catal. 2017, 7,8006−8022.(12) Kou, J. H.; Lu, C. H.; Wang, J.; Chen, Y. K.; Xu, Z. Z.; Varma,R. S. Selectivity Enhancement in Heterogeneous PhotocatalyticTransformations. Chem. Rev. 2017, 117, 1445−1514.(13) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D.W. Environmental Applications of Semiconductor Photocatalysis.Chem. Rev. 1995, 95, 69−96.(14) Gligorovski, S.; Strekowski, R.; Barbati, S.; Vione, D.Environmental Implications of Hydroxyl Radicals (•OH). Chem.Rev. 2015, 115, 13051−13092.(15) Chong, M. N.; Jin, B.; Chow, C. W. K.; Saint, C. RecentDevelopments in Photocatalytic Water Treatment Technology: AReview. Water Res. 2010, 44, 2997−3027.(16) Zhang, J.; Nosaka, Y. Mechanism of the OH RadicalGeneration in Photocatalysis with TiO2 of Different CrystallineTypes. J. Phys. Chem. C 2014, 118, 10824−10832.(17) Li, Q.; Zhang, N.; Yang, Y.; Wang, G. Z.; Ng, D. H. L. HighEfficiency Photocatalysis for Pollutant Degradation with MoS2/C3N4

Heterostructures. Langmuir 2014, 30, 8965−8972.(18) Nosaka, Y.; Nosaka, A. Understanding Hydroxyl Radical (·OH)Generation Processes in Photocatalysis. ACS Energy Lett. 2016, 1,356−359.(19) Gaya, U. I.; Abdullah, A. H. Heterogeneous PhotocatalyticDegradation of Organic Contaminants over Titanium Dioxide: AReview of Fundamentals, Progress and Problems. J. Photochem.Photobiol., C 2008, 9, 1−12.(20) Maurino, V.; Minero, C.; Mariella, G.; Pelizzetti, E. SustainedProduction of H2O2 on Irradiated TiO2 − Fluoride Systems. Chem.Commun. 2005, 2627−2629.(21) Sheng, J.; Li, X.; Xu, Y. Generation of H2O2 and OH Radicalson Bi2WO6 for Phenol Degradation under Visible Light. ACS Catal.2014, 4, 732−737.(22) Tsukamoto, D.; Shiro, A.; Shiraishi, Y.; Sugano, Y.; Ichikawa,S.; Tanaka, S.; Hirai, T. Photocatalytic H2O2 Production fromEthanol/O2 System Using TiO2 Loaded with Au−Ag Bimetallic AlloyNanoparticles. ACS Catal. 2012, 2, 599−603.(23) Sun, B.; Vorontsov, A. V.; Smirniotis, P. G. Role of PlatinumDeposited on TiO2 in Phenol Photocatalytic Oxidation. Langmuir2003, 19, 3151−3156.

(24) Bae, S.; Kim, S.; Lee, S.; Choi, W. Dye Decolorization Test forthe Activity Assessment of Visible Light Photocatalysts: Realities andLimitations. Catal. Today 2014, 224, 21−28.(25) Li, Z.; Cong, S.; Xu, Y. Brookite vs Anatase TiO2 in thePhotocatalytic Activity for Organic Degradation in Water. ACS Catal.2014, 4, 3273−3280.(26) Kim, G.; Lee, S.-H.; Choi, W. Glucose−TiO2 Charge TransferComplex-Mediated Photocatalysis under Visible Light. Appl. Catal., B2015, 162, 463−469.(27) Sun, B.; Reddy, E. P.; Smirniotis, P. G. Visible Light Cr(VI)Reduction and Organic Chemical Oxidation by TiO2 Photocatalysis.Environ. Sci. Technol. 2005, 39, 6251−6259.(28) Choi, W.; Yeo, J.; Ryu, J.; Tachikawa, T.; Majima, T.Photocatalytic Oxidation Mechanism of As(III) on TiO2: UniqueRole of As(III) as a Charge Recombinant Species. Environ. Sci.Technol. 2010, 44, 9099−9104.(29) Noguchi, H.; Nakajima, A.; Watanabe, T.; Hashimoto, K.Design of a Photocatalyst for Bromate Decomposition: SurfaceModification of TiO2 by Pseudo-Boehmite. Environ. Sci. Technol.2003, 37, 153−157.(30) Zhao, X.; Liu, H.; Shen, Y.; Qu, J. Photocatalytic Reduction ofBromate at C60 Modified Bi2MoO6 under Visible Light Irradiation.Appl. Catal., B 2011, 106, 63−68.(31) Zhang, F. X.; Jin, R. C.; Chen, J. X.; Shao, C. Z.; Gao, W. L.; Li,L. D.; Guan, N. J. High Photocatalytic Activity and Selectivity forNitrogen in Nitrate Reduction on Ag/TiO2 Catalyst with Fine SilverClusters. J. Catal. 2005, 232, 424−431.(32) Hirayama, J.; Kamiya, Y. Combining the Photocatalyst Pt/TiO2

and the Nonphotocatalyst SnPd/Al2O3 for Effective PhotocatalyticPurification of Groundwater Polluted with Nitrate. ACS Catal. 2014,4, 2207−2215.(33) Hirakawa, H.; Hashimoto, M.; Shiraishi, Y.; Hirai, T. SelectiveNitrate-to-Ammonia Transformation on Surface Defects of TitaniumDioxide Photocatalysts. ACS Catal. 2017, 7, 3713−3720.(34) Wang, D. W.; Li, Y.; Li Puma, G.; Lianos, P.; Wang, C.; Wang,P. F. Photoelectrochemical Cell for Simultaneous ElectricityGeneration and Heavy Metals Recovery from Wastewater. J. Hazard.Mater. 2017, 323, 681−689.(35) Watts, R. J.; Kong, S.; Orr, M. P.; Miller, G. C.; Henry, B. E.Photocatalytic Inactivation of Coliform Bacteria and Viruses inSecondary Wastewater Effluent. Water Res. 1995, 29, 95−100.(36) Ren, S.; Boo, C.; Guo, N.; Wang, S.; Elimelech, M.; Wang, Y.Photocatalytic Reactive Ultrafiltration Membrane for Removal ofAntibiotic Resistant Bacteria and Antibiotic Resistance Genes fromWastewater Effluent. Environ. Sci. Technol. 2018, 52, 8666−8673.(37) Akhavan, O.; Ghaderi, E. Photocatalytic Reduction ofGraphene Oxide Nanosheets on TiO2 Thin Film for Photo-inactivation of Bacteria in Solar Light Irradiation. J. Phys. Chem. C2009, 113, 20214−20220.(38) Tran, P. D.; Wong, L. H.; Barber, J.; Loo, J. S. C. RecentAdvances in Hybrid Photocatalysts for Solar Fuel Production. EnergyEnviron. Sci. 2012, 5, 5902−5918.(39) Fujishima, A.; Honda, K. Electrochemical Photolysis of Waterat a Semiconductor Electrode. Nature 1972, 238, 37.(40) Fujishima, A.; Kohayakawa, K.; Honda, K. HydrogenProduction under Sunlight with an Electrochemical Photocell. J.Electrochem. Soc. 1975, 122, 1487−1489.(41) Li, Z. S.; Luo, W. J.; Zhang, M. L.; Feng, J. Y.; Zou, Z. G.Photoelectrochemical Cells for Solar Hydrogen Production: CurrentState of Promising Photoelectrodes, Methods to Improve TheirProperties, and Outlook. Energy Environ. Sci. 2013, 6, 347−370.(42) Jing, D.; Guo, L.; Zhao, L.; Zhang, X.; Liu, H.; Li, M.; Shen, S.;Liu, G.; Hu, X.; Zhang, X.; Zhang, K.; Ma, L.; Guo, P. Efficient SolarHydrogen Production by Photocatalytic Water Splitting: FromFundamental Study to Pilot Demonstration. Int. J. Hydrogen Energy2010, 35, 7087−7097.(43) Moon, G. H.; Fujitsuka, M.; Kim, S.; Majima, T.; Wang, X. C.;Choi, W. Eco-Friendly Photochemical Production of H2O2 through

ACS Catalysis Perspective

DOI: 10.1021/acscatal.8b03521ACS Catal. 2018, 8, 11542−11563

11558

O2 Reduction over Carbon Nitride Frameworks Incorporated withMultiple Heteroelements. ACS Catal. 2017, 7, 2886−2895.(44) Li, S. N.; Dong, G. H.; Hailili, R.; Yang, L. P.; Li, Y. X.; Wang,F.; Zeng, Y. B.; Wang, C. Y. Effective Photocatalytic H2O2 Productionunder Visible Light Irradiation at g-C3N4 Modulated by CarbonVacancies. Appl. Catal., B 2016, 190, 26−35.(45) Kang, U.; Choi, S. K.; Ham, D. J.; Ji, S. M.; Choi, W.; Han, D.S.; Abdel-Wahab, A.; Park, H. Photosynthesis of Formate from CO2and Water at 1% Energy Efficiency via Copper Iron Oxide Catalysis.Energy Environ. Sci. 2015, 8, 2638−2643.(46) Litter, M. I. Heterogeneous PhotocatalysisTransition MetalIons in Photocatalytic Systems. Appl. Catal., B 1999, 23, 89−114.(47) Braham, R. J.; Harris, A. T. Review of Major Design and Scale-up Considerations for Solar Photocatalytic Reactors. Ind. Eng. Chem.Res. 2009, 48, 8890−8905.(48) Park, H.; Vecitis, C. D.; Hoffmann, M. R. Solar-PoweredElectrochemical Oxidation of Organic Compounds Coupled with theCathodic Production of Molecular Hydrogen. J. Phys. Chem. A 2008,112, 7616−7626.(49) Winkler, M. T.; Cox, C. R.; Nocera, D. G.; Buonassisi, T.Modeling Integrated Photovoltaic-Electrochemical Devices usingSteady-State Equivalent Circuits. Proc. Natl. Acad. Sci. U. S. A.2013, 110, E1076−E1082.(50) Chang, W. J.; Lee, K.-H.; Ha, H.; Jin, K.; Kim, G.; Hwang, S.-T.; Lee, H.-m.; Ahn, S.-W.; Yoon, W.; Seo, H.; Hong, J. S.; Go, Y. K.;Ha, J.-I.; Nam, K. T. Design Principle and Loss Engineering forPhotovoltaic−Electrolysis Cell System. ACS Omega 2017, 2, 1009−1018.(51) Jia, J. Y.; Seitz, L. C.; Benck, J. D.; Huo, Y. J.; Chen, Y. S.; Ng, J.W. D.; Bilir, T.; Harris, J. S.; Jaramillo, T. F. Solar Water Splitting byPhotovoltaic-Electrolysis with a Solar-to-Hydrogen Efficiency over30%. Nat. Commun. 2016, 7, 13237.(52) Gratzel, M. Photoelectrochemical Cells. Nature 2001, 414,338−344.(53) Yang, Y.; Niu, S. W.; Han, D. D.; Liu, T. Y.; Wang, G. M.; Li, Y.Progress in Developing Metal Oxide Nanomaterials for Photo-electrochemical Water Splitting. Adv. Energy Mater. 2017, 7, 1700555.(54) Koo, M. S.; Cho, K.; Yoon, J.; Choi, W. PhotoelectrochemicalDegradation of Organic Compounds Coupled with MolecularHydrogen Generation Using Electrochromic TiO2 Nanotube Arrays.Environ. Sci. Technol. 2017, 51, 6590−6598.(55) Zhang, H. J.; Chen, G. H.; Bahnemann, D. W. Photo-electrocatalytic Materials for Environmental Applications. J. Mater.Chem. 2009, 19, 5089−5121.(56) Kim, J.; Choi, W. Hydrogen Producing Water Treatmentthrough Solar Photocatalysis. Energy Environ. Sci. 2010, 3, 1042−1045.(57) Cho, Y. J.; Kim, H. I.; Lee, S.; Choi, W. Dual-FunctionalPhotocatalysis using a Ternary Hybrid of TiO2 Modified withGraphene Oxide along with Pt and Fluoride for H2-Producing WaterTreatment. J. Catal. 2015, 330, 387−395.(58) Khan, M. R.; Chuan, T. W.; Yousuf, A.; Chowdhury, M.;Cheng, C. K. Schottky Barrier and Surface Plasmonic ResonancePhenomena towards the Photocatalytic Reaction: Study of TheirMechanisms to Enhance Photocatalytic Activity. Catal. Sci. Technol.2015, 5, 2522−2531.(59) Yang, Y.; Chang, C.-H.; Idriss, H. Photo-Catalytic Productionof Hydrogen form Ethanol over M/TiO2 Catalysts (M = Pd, Pt orRh). Appl. Catal., B 2006, 67, 217−222.(60) Belhadj, H.; Hamid, S.; Robertson, P. K.; Bahnemann, D. W.Mechanisms of Simultaneous Hydrogen Production and Form-aldehyde Oxidation in H2O and D2O over Platinized TiO2. ACSCatal. 2017, 7, 4753−4758.(61) Park, H.; Choi, W. Effects of TiO2 Surface Fluorination onPhotocatalytic Reactions and Photoelectrochemical Behaviors. J. Phys.Chem. B 2004, 108, 4086−4093.(62) Minero, C.; Mariella, G.; Maurino, V.; Pelizzetti, E. Photo-catalytic Transformation of Organic Compounds in the Presence ofInorganic Anions. 1. Hydroxyl-Mediated and Direct Electron-Transfer

Reactions of Phenol on a Titanium Dioxide−Fluoride System.Langmuir 2000, 16, 2632−2641.(63) Kim, J.; Monllor-Satoca, D.; Choi, W. Simultaneous Productionof Hydrogen with the Degradation of Organic Pollutants using TiO2

Photocatalyst Modified with Dual Surface Components. EnergyEnviron. Sci. 2012, 5, 7647−7656.(64) Korzhak, A.; Kuchmii, S. Y.; Kryukov, A. Effects of Activationand Inhibition by Oxygen of the Photocatalytic Evolution ofHydrogen from Alcohol-Water Media. Theor. Exp. Chem. 1994, 30,26−29.(65) Schmelling, D. C.; Gray, K. A. Photocatalytic Transformationand Mineralization of 2, 4, 6-Trinitrotoluene (TNT) in TiO2 slurries.Water Res. 1995, 29, 2651−2662.(66) Cho, Y. J.; Moon, G. H.; Kanazawa, T.; Maeda, K.; Choi, W.Selective Dual-Purpose Photocatalysis for Simultaneous H2 Evolutionand Mineralization of Organic Compounds Enabled by a Cr2O3

Barrier Layer Coated on Rh/SrTiO3. Chem. Commun. 2016, 52,9636−9639.(67) Maeda, K.; Teramura, K.; Lu, D.; Saito, N.; Inoue, Y.; Domen,K. Noble-Metal/Cr2O3 Core/Shell Nanoparticles as a Cocatalyst forPhotocatalytic Overall Water Splitting. Angew. Chem., Int. Ed. 2006,118, 7970−7973.(68) Kim, S.; Piao, G.; Han, D. S.; Shon, H. K.; Park, H. SolarDesalination Coupled with Water Remediation and MolecularHydrogen Production: A Novel Solar Water-Energy Nexus. EnergyEnviron. Sci. 2018, 11, 344−353.(69) Cho, K.; Qu, Y.; Kwon, D.; Zhang, H.; Cid, C. A.; Aryanfar, A.;Hoffmann, M. R. Effects of Anodic Potential and Chloride Ion onOverall Reactivity in Electrochemical Reactors Designed for Solar-Powered Wastewater Treatment. Environ. Sci. Technol. 2014, 48,2377−2384.(70) Huang, X.; Qu, Y.; Cid, C. A.; Finke, C.; Hoffmann, M. R.; Lim,K.; Jiang, S. C. Electrochemical Disinfection of Toilet Wastewaterusing Wastewater Electrolysis Cell. Water Res. 2016, 92, 164−172.(71) Dunlop, P.; Byrne, J.; Manga, N.; Eggins, B. The PhotocatalyticRemoval of Bacterial Pollutants from Drinking Water. J. Photochem.Photobiol., A 2002, 148, 355−363.(72) Panizza, M.; Cerisola, G. Electrochemical Oxidation as a FinalTreatment of Synthetic Tannery Wastewater. Environ. Sci. Technol.2004, 38, 5470−5475.(73) Lacasa, E.; Tsolaki, E.; Sbokou, Z.; Rodrigo, M. A.;Mantzavinos, D.; Diamadopoulos, E. Electrochemical Disinfectionof Simulated Ballast Water on Conductive Diamond Electrodes.Chem. Eng. J. 2013, 223, 516−523.(74) Cid, C. A.; Qu, Y.; Hoffmann, M. R. Design and PreliminaryImplementation of Onsite Electrochemical Wastewater Treatmentand Recycling Toilets for the Developing World. Environ. Sci. WaterRes. Technol. 2018, 4, 1439−1450.(75) Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L.Hydrogen Peroxide Synthesis: An Outlook Beyond the Anthraqui-none Process. Angew. Chem., Int. Ed. 2006, 45, 6962−6984.(76) Fukuzumi, S. Production of Liquid Solar Fuels and Their Usein Fuel Cells. Joule 2017, 1, 689−738.(77) Moon, G.-h.; Fujitsuka, M.; Kim, S.; Majima, T.; Wang, X.;Choi, W. Eco-Friendly Photochemical Production of H2O2 throughO2 Reduction over Carbon Nitride Frameworks Incorporated withMultiple Heteroelements. ACS Catal. 2017, 7, 2886−2895.(78) Kim, H.-i.; Choi, Y.; Hu, S.; Choi, W.; Kim, J.-H. PhotocatalyticHydrogen Peroxide Production by Anthraquinone-AugmentedPolymeric Carbon Nitride. Appl. Catal., B 2018, 229, 121−129.(79) Kofuji, Y.; Isobe, Y.; Shiraishi, Y.; Sakamoto, H.; Tanaka, S.;Ichikawa, S.; Hirai, T. Carbon Nitride−Aromatic Diimide−GrapheneNanohybrids: Metal-Free Photocatalysts for Solar-to-HydrogenPeroxide Energy Conversion with 0.2% efficiency. J. Am. Chem. Soc.2016, 138, 10019−10025.(80) Bokare, A. D.; Choi, W. Review of Iron-Free Fenton-LikeSystems for Activating H2O2 in Advanced Oxidation Processes. J.Hazard. Mater. 2014, 275, 121−135.

ACS Catalysis Perspective

DOI: 10.1021/acscatal.8b03521ACS Catal. 2018, 8, 11542−11563

11559

(81) Moon, G.-h.; Kim, S.; Cho, Y.-J.; Lim, J.; Kim, D.-h.; Choi, W.Synergistic Combination of Bandgap-Modified Carbon Nitride andWO3 for Visible Light-Induced Oxidation of Arsenite Accelerated byIn-Situ Fenton Reaction. Appl. Catal., B 2017, 218, 819−824.(82) Kim, S.; Moon, G.-h.; Kim, H.; Mun, Y.; Zhang, P.; Lee, J.;Choi, W. Selective Charge Transfer to Dioxygen on KPF 6-ModifiedCarbon Nitride for Photocatalytic Synthesis of H2O2 under VisibleLight. J. Catal. 2018, 357, 51−58.(83) Zong, X.; Chen, H. J.; Seger, B.; Pedersen, T.; Dargusch, M. S.;McFarland, E. W.; Li, C.; Wang, L. Z. Selective Production ofHydrogen Peroxide and Oxidation of Hydrogen Sulfide in anUnbiased Solar Photoelectrochemical Cell. Energy Environ. Sci.2014, 7, 3347−3351.(84) Fuku, K.; Miyase, Y.; Miseki, Y.; Funaki, T.; Gunji, T.; Sayama,K. Photoelectrochemical Hydrogen Peroxide Production from Wateron a WO3/BiVO4 Photoanode and from O2 on an Au CathodeWithout External Bias. Chem. - Asian J. 2017, 12, 1111−1119.(85) Fuku, K.; Miyase, Y.; Miseki, Y.; Gunji, T.; Sayama, K.Enhanced Oxidative Hydrogen Peroxide Production on ConductingGlass Anodes Modified with Metal Oxides. ChemistrySelect 2016, 1,5721−5726.(86) Fuku, K.; Sayama, K. Efficient Oxidative Hydrogen PeroxideProduction and Accumulation in Photoelectrochemical WaterSplitting using a Tungsten Trioxide/Bismuth Vanadate Photoanode.Chem. Commun. 2016, 52, 5406−5409.(87) Mase, K.; Yoneda, M.; Yamada, Y.; Fukuzumi, S. SeawaterUsable for Production and Consumption of Hydrogen Peroxide as aSolar Fuel. Nat. Commun. 2016, 7, 11470.(88) Shi, X.; Zhang, Y.; Siahrostami, S.; Zheng, X. Light-DrivenBiVO4−C Fuel Cell with Simultaneous Production of H2O2. Adv.Energy Mater. 2018, 8, 1801158.(89) Chai, G.-L.; Boero, M.; Hou, Z.; Terakura, K.; Cheng, W.Indirect Four-Electron Oxygen Reduction Reaction on CarbonMaterials Catalysts in Acidic Solutions. ACS Catal. 2017, 7, 7908−7916.(90) Fountaine, K. T.; Lewerenz, H. J.; Atwater, H. A. EfficiencyLimits for Photoelectrochemical Water-Splitting. Nat. Commun. 2016,7, 13706.(91) Barakat, M. A.; Chen, Y. T.; Huang, C. P. Removal of ToxicCyanide and Cu(II) Ions from Water by Illuminated TiO2 Catalyst.Appl. Catal., B 2004, 53, 13−20.(92) Lee, S. S.; Bai, H. W.; Liu, Z. Y.; Sun, D. D. Green Approach forPhotocatalytic Cu(II)-EDTA Degradation over TiO2: TowardEnvironmental Sustainability. Environ. Sci. Technol. 2015, 49, 2541−2548.(93) Zhao, X.; Guo, L. B.; Zhang, B. F.; Liu, H. J.; Qu, J. H.Photoelectrocatalytic Oxidation of Cu(II)-EDTA at the TiO2Electrode and Simultaneous Recovery of Cu(II) by Electrodeposition.Environ. Sci. Technol. 2013, 47, 4480−4488.(94) Rhoads, K. R.; Davis, A. P. Metal Recovery and Catalyst Reusefrom the Photocatalytic Oxidation of Copper-Ethylenediaminetetra-acetic Acid. J. Environ. Eng. 2004, 130, 425−431.(95) Zeng, H. B.; Liu, S. S.; Chai, B. Y.; Cao, D.; Wang, Y.; Zhao, X.Enhanced Photoelectrocatalytic Decomplexation of Cu-EDTA andCu Recovery by Persulfate Activated by UV and Cathodic Reduction.Environ. Sci. Technol. 2016, 50, 6459−6466.(96) Zhao, X.; Zhang, J. J.; Qiao, M.; Liu, H. J.; Qu, J. H. EnhancedPhotoelectrocatalytic Decomposition of Copper Cyanide Complexesand Simultaneous Recovery of Copper with a Bi2MoO6 Electrodeunder Visible Light by EDTA/K4P2O7. Environ. Sci. Technol. 2015, 49,4567−4574.(97) Shaner, M. R.; Atwater, H. A.; Lewis, N. S.; McFarland, E. W. AComparative Technoeconomic Analysis of Renewable HydrogenProduction using Solar Energy. Energy Environ. Sci. 2016, 9, 2354−2371.(98) Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.;Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.;Wang, H.; Miller, E.; Jaramillo, T. F. Technical and EconomicFeasibility of Centralized Facilities for Solar Hydrogen Production via

Photocatalysis and Photoelectrochemistry. Energy Environ. Sci. 2013,6, 1983−2002.(99) Choi, Y.; Koo, M. S.; Bokare, A. D.; Kim, D. H.; Bahnemann,D. W.; Choi, W. Sequential Process Combination of PhotocatalyticOxidation and Dark Reduction for the Removal of Organic Pollutantsand Cr(VI) using Ag/TiO2. Environ. Sci. Technol. 2017, 51, 3973−3981.(100) Wood, A.; Giersig, M.; Mulvaney, P. Fermi Level Equilibrationin Quantum Dot-Metal Nanojunctions. J. Phys. Chem. B 2001, 105,8810−8815.(101) Wang, Z. H.; Bush, R. T.; Sullivan, L. A.; Liu, J. S.Simultaneous Redox Conversion of Chromium(VI) and Arsenic(III)under Acidic Conditions. Environ. Sci. Technol. 2013, 47, 6486−6492.(102) Sun, M.; Zhang, G.; Qin, Y. H.; Cao, M. J.; Liu, Y.; Li, J. H.;Qu, J. H.; Liu, H. J. Redox Conversion of Chromium(VI) andArsenic(III) with the Intermediates of Chromium(V) and Arsenic-(IV) via AuPd/CNTs Electrocatalysis in Acid Aqueous Solution.Environ. Sci. Technol. 2015, 49, 9289−9297.(103) Liu, R.; Sun, L.; Qu, J.; Li, G. Arsenic Removal throughAdsorption, Sand Filtration and Ultrafiltration: In Situ PrecipitatedFerric and Manganese Binary Oxides as Adsorbents. Desalination2009, 249, 1233−1237.(104) Duca, M.; Koper, M. T. M. Powering Denitrification: ThePerspectives of Electrocatalytic Nitrate Reduction. Energy Environ. Sci.2012, 5, 9726−9742.(105) Sa, J.; Aguera, C. A.; Gross, S.; Anderson, J. A. PhotocatalyticNitrate Reduction over Metal Modified TiO2. Appl. Catal., B 2009,85, 192−200.(106) Soares, O.; Pereira, M. F. R.; Orfao, J. J. M.; Faria, J. L.; Silva,C. G. Photocatalytic Nitrate Reduction over Pd-Cu/TiO2. Chem. Eng.J. 2014, 251, 123−130.(107) Kominami, H.; Furusho, A.; Murakami, S.; Inoue, H.; Kera,Y.; Ohtani, B. Effective Photocatalytic Reduction of Nitrate toAmmonia in an Aqueous Suspension of Metal-Loaded Titanium(IV)Oxide Particles in the Presence of Oxalic Acid. Catal. Lett. 2001, 76,31−34.(108) Li, L. Y.; Xu, Z. Y.; Liu, F. L.; Shao, Y.; Wang, J. H.; Wan, H.Q.; Zheng, S. R. Photocatalytic Nitrate Reduction over Pt-Cu/TiO2Catalysts with Benzene as Hole Scavenger. J. Photochem. Photobiol., A2010, 212, 113−121.(109) Gao, W. L.; Jin, R. C.; Chen, M. X.; Guan, X. X.; Zeng, H. S.;Zhang, F. X.; Guan, N. J. Titania-Supported Bimetallic Catalysts forPhotocatalytic Reduction of Nitrate. Catal. Today 2004, 90, 331−336.(110) Kobwittaya, K.; Sirivithayapakorn, S. Photocatalytic Reductionof Nitrate over TiO2 and Ag-Modified TiO2. J. Saudi Chem. Soc. 2014,18, 291−298.(111) Zhang, F.; Jin, R.; Chen, J.; Shao, C.; Gao, W.; Li, L.; Guan, N.High Photocatalytic Activity and Selectivity for Nitrogen in NitrateReduction on Ag/TiO2 Catalyst with Fine Silver Clusters. J. Catal.2005, 232, 424−431.(112) Reyter, D.; Belanger, D.; Roue, L. Optimization of theCathode Material for Nitrate Removal by a Paired ElectrolysisProcess. J. Hazard. Mater. 2011, 192, 507−513.(113) Li, M.; Feng, C. P.; Zhang, Z. Y.; Lei, X. H.; Chen, R. Z.;Yang, Y. N.; Sugiura, N. Simultaneous Reduction of Nitrate andOxidation of By-Products using Electrochemical Method. J. Hazard.Mater. 2009, 171, 724−730.(114) Ji, Y. Z.; Bai, J.; Li, J. H.; Luo, T.; Qiao, L.; Zeng, Q. Y.; Zhou,B. X. Highly Selective Transformation of Ammonia Nitrogen to N2based on a Novel Solar-Driven Photoelectrocatalytic-Chlorine RadicalReactions System. Water Res. 2017, 125, 512−519.(115) Zhang, Y.; Li, J. H.; Bai, J.; Shen, Z. X.; Li, L. S.; Xia, L. G.;Chen, S.; Zhou, B. X. Exhaustive Conversion of Inorganic Nitrogen toNitrogen Gas Based on a Photoelectro-Chlorine Cycle Reaction and aHighly Selective Nitrogen Gas Generation Cathode. Environ. Sci.Technol. 2018, 52, 1413−1420.(116) Han, L.; Guo, S. J.; Xu, M.; Dong, S. J. PhotoelectrochemicalBatteries for Efficient Energy Recovery. Chem. Commun. 2014, 50,13331−13333.

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DOI: 10.1021/acscatal.8b03521ACS Catal. 2018, 8, 11542−11563

11560

(117) Wang, K. Q.; Yang, J.; Feng, L. G.; Zhang, Y. W.; Liang, L.;Xing, W.; Liu, C. P. Photoelectrochemical Biofuel Cell usingPorphyrin-Sensitized Nanocrystalline Titanium Dioxide MesoporousFilm as Photoanode. Biosens. Bioelectron. 2012, 32, 177−182.(118) Logan, B. E.; Hamelers, B.; Rozendal, R.; Schroder, U.; Keller,J.; Freguia, S.; Aelterman, P.; Verstraete, W.; Rabaey, K. MicrobialFuel Cells: Methodology and Technology. Environ. Sci. Technol. 2006,40, 5181−5192.(119) Du, Y.; Feng, Y.; Qu, Y.; Liu, J.; Ren, N.; Liu, H. ElectricityGeneration and Pollutant Degradation using a Novel BiocathodeCoupled Photoelectrochemical Cell. Environ. Sci. Technol. 2014, 48,7634−7641.(120) Hambourger, M.; Kodis, G.; Vaughn, M. D.; Moore, G. F.;Gust, D.; Moore, A. L.; Moore, T. A. Solar Energy Conversion in aPhotoelectrochemical Biofuel Cell. Dalton Trans. 2009, 9979−9989.(121) Wang, H.; Qian, F.; Wang, G.; Jiao, Y.; He, Z.; Li, Y. Self-Biased Solar-Microbial Device for Sustainable Hydrogen Generation.ACS Nano 2013, 7, 8728−8735.(122) Chen, Q. P.; Li, J. H.; Li, X. J.; Huang, K.; Zhou, B. X.; Cai, W.M.; Shangguan, W. F. Visible-Light Responsive Photocatalytic FuelCell Based on WO3/W Photoanode and Cu2O/Cu Photocathode forSimultaneous Wastewater Treatment and Electricity Generation.Environ. Sci. Technol. 2012, 46, 11451−11458.(123) Zhou, Z.; Wu, Z.; Xu, Q.; Zhao, G. A Solar-ChargedPhotoelectrochemical Wastewater Fuel Cell for Efficient andSustainable Hydrogen Production. J. Mater. Chem. A 2017, 5,25450−25459.(124) Kim, J.; Lee, J.; Choi, W. Synergic Effect of SimultaneousFluorination and Platinization of TiO2 Surface on Anoxic Photo-catalytic Degradation of Organic Compounds. Chem. Commun. 2008,756−758.(125) Zhang, H. B.; Ma, C. H.; Li, Y.; Chen, Y.; Lu, C. X.; Wang, J.Sol-Gel-Hydrothermal Synthesis of Er3+:Y3Al5O12/Pt-TiO2 Mem-brane and Visible-Light Driving Photocatalytic Hydrogen Evolutionfrom Pollutants. Appl. Catal., A 2015, 503, 209−217.(126) Daskalaki, V. M.; Antoniadou, M.; Li Puma, G.; Kondarides,D. I.; Lianos, P. Solar Light-Responsive Pt/CdS/TiO2 Photocatalystsfor Hydrogen Production and Simultaneous Degradation of Inorganicor Organic Sacrificial Agents in Wastewater. Environ. Sci. Technol.2010, 44, 7200−7205.(127) Belhadj, H.; Hamid, S.; Robertson, P. K. J.; Bahnemann, D. W.Mechanisms of Simultaneous Hydrogen Production and Form-aldehyde Oxidation in H2O and D2O over Platinized TiO2. ACSCatal. 2017, 7, 4753−4758.(128) Patsoura, A.; Kondarides, D. I.; Verykios, X. E. PhotocatalyticDegradation of Organic Pollutants with Simultaneous Production ofHydrogen. Catal. Today 2007, 124, 94−102.(129) Xu, Y. F.; Zhang, C.; Lu, P.; Zhang, X. H.; Zhang, L. X.; Shi, J.L. Overcoming Poisoning Effects of Heavy Metal Ions againstPhotocatalysis for Synergetic Photo-Hydrogen Generation fromWastewater. Nano Energy 2017, 38, 494−503.(130) Zhang, W. L.; Li, Y.; Wang, C.; Wang, P. F.; Wang, Q.; Wang,D. W. Mechanisms of Simultaneous Hydrogen Production andEstrogenic Activity Removal from Secondary Effluent though SolarPhotocatalysis. Water Res. 2013, 47, 3173−3182.(131) Rong, Y.; Tang, L.; Song, Y. H.; Wei, S. N.; Zhang, Z. H.;Wang, J. A New Visible-Light Driving Nanocomposite PhotocatalystEr3+: Y3Al5O12/MoS2-NaTaO3-PdS for Photocatalytic Degradation ofa Refractory Pollutant with Potentially Simultaneous HydrogenEvolution. RSC Adv. 2016, 6, 80595−80603.(132) Lin, Z. Y.; Li, L. H.; Yu, L. L.; Li, W. J.; Yang, G. W. Dual-Functional Photocatalysis for Hydrogen Evolution from IndustrialWastewaters. Phys. Chem. Chem. Phys. 2017, 19, 8356−8362.(133) Chu, K. H.; Ye, L. Q.; Wang, W.; Wu, D.; Chan, D. K. L.;Zeng, C. P.; Yip, H. Y.; Yu, J. C.; Wong, P. K. EnhancedPhotocatalytic Hydrogen Production from Aqueous Sulfide/SulfiteSolution by ZnO0.6S0.4 with Simultaneous Dye Degradation underVisible-Light Irradiation. Chemosphere 2017, 183, 219−228.

(134) Kim, J.; Park, Y.; Park, H. Solar Hydrogen ProductionCoupled with the Degradation of a Dye Pollutant Using TiO2

Modified with Platinum and Nafion. Int. J. Photoenergy 2014, 2014,324859.(135) Wang, J. G.; Zhang, P.; Li, X.; Zhu, J.; Li, H. X. SynchronicalPollutant Degradation and H2 Production on a Ti3+-Doped TiO2

Visible Photocatalyst with Dominant (001) Facets. Appl. Catal., B2013, 134−135, 198−204.(136) Patsoura, A.; Kondarides, D. I.; Verykios, X. E. Enhancementof Photoinduced Hydrogen Production from Irradiated Pt/TiO2

Suspensions with Simultaneous Degradation of Azo-Dyes. Appl.Catal., B 2006, 64, 171−179.(137) Zhang, S. Q.; Wang, L. L.; Liu, C. B.; Luo, J. M.; Crittenden,J.; Liu, X.; Cai, T.; Yuan, J. L.; Pei, Y.; Liu, Y. T. PhotocatalyticWastewater Purification with Simultaneous Hydrogen Productionusing MoS2 QD-Decorated Hierarchical Assembly of ZnIn2S4 onReduced Graphene Oxide Photocatalyst. Water Res. 2017, 121, 11−19.(138) Iervolino, G.; Vaiano, V.; Sannino, D.; Rizzo, L.; Palma, V.Enhanced Photocatalytic Hydrogen Production from GlucoseAqueous Matrices on Ru-Doped LaFeO3. Appl. Catal., B 2017, 207,182−194.(139) Iervolino, G.; Vaiano, V.; Murcia, J. J.; Rizzo, L.; Ventre, G.;Pepe, G.; Campiglia, P.; Hidalgo, M. C.; Navio, J. A.; Sannino, D.Photocatalytic Hydrogen Production from Degradation of Glucoseover Fluorinated and Platinized TiO2 Catalysts. J. Catal. 2016, 339,47−56.(140) Silva, C. G.; Sampaio, M. J.; Marques, R. R. N.; Ferreira, L. A.;Tavares, P. B.; Silva, A. M. T.; Faria, J. L. Photocatalytic Production ofHydrogen from Methanol and Saccharides using Carbon Nanotube-TiO2 Catalysts. Appl. Catal., B 2015, 178, 82−90.(141) Carraro, G.; Maccato, C.; Gasparotto, A.; Montini, T.; Turner,S.; Lebedev, O. I.; Gombac, V.; Adami, G.; Van Tendeloo, G.;Barreca, D.; Fornasiero, P. Enhanced Hydrogen Production byPhotoreforming of Renewable Oxygenates Through NanostructuredFe2O3 Polymorphs. Adv. Funct. Mater. 2014, 24, 372−378.(142) Daskalaki, V. M.; Kondarides, D. I. Efficient Production ofHydrogen by Photo-Induced Reforming of Glycerol at AmbientConditions. Catal. Today 2009, 144, 75−80.(143) Kondarides, D. I.; Daskalaki, V. M.; Patsoura, A.; Verykios, X.E. Hydrogen Production by Photo-Induced Reforming of BiomassComponents and Derivatives at Ambient Conditions. Catal. Lett.2008, 122, 26−32.(144) Bhirud, A. P.; Sathaye, S. D.; Waichal, R. P.; Nikam, L. K.;Kale, B. B. An Eco-Friendly, Highly Stable and Efficient Nano-structured p-Type N-Doped ZnO Photocatalyst for EnvironmentallyBenign Solar Hydrogen Production. Green Chem. 2012, 14, 2790−2798.(145) Bhirud, A. P.; Sathaye, S. D.; Waichal, R. P.; Ambekar, J. D.;Park, C. J.; Kale, B. B. In-situ Preparation of N-TiO2/GrapheneNanocomposite and its Enhanced Photocatalytic Hydrogen Produc-tion by H2S Splitting under Solar Light. Nanoscale 2015, 7, 5023−5034.(146) Jang, J. S.; Li, W.; Oh, S. H.; Lee, J. S. Fabrication of CdS/TiO2 Nano-Bulk Composite Photocatalysts for Hydrogen Productionfrom Aqueous H2S Solution under Visible Light. Chem. Phys. Lett.2006, 425, 278−282.(147) Apte, S. K.; Garaje, S. N.; Valant, M.; Kale, B. B. Eco-FriendlySolar Light Driven Hydrogen Production from Copious Waste H2Sand Organic Dye Degradation by Stable and Efficient OrthorhombicCdS Quantum Dots-GeO2 Glass Photocatalyst. Green Chem. 2012,14, 1455−1462.(148) Garaje, S. N.; Apte, S. K.; Naik, S. D.; Ambekar, J. D.;Sonawane, R. S.; Kulkarni, M. V.; Vinu, A.; Kale, B. B. Template-FreeSynthesis of Nanostructured CdxZn1‑xS with Tunable Band Structurefor H2 Production and Organic Dye Degradation Using Solar Light.Environ. Sci. Technol. 2013, 47, 6664−6672.

ACS Catalysis Perspective

DOI: 10.1021/acscatal.8b03521ACS Catal. 2018, 8, 11542−11563

11561

(149) Chen, D.; Ray, A. K. Removal of Toxic Metal Ions fromWastewater by Semiconductor Photocatalysis. Chem. Eng. Sci. 2001,56, 1561−1570.(150) Kominami, H.; Kitsui, K.; Ishiyama, Y.; Hashimoto, K.Simultaneous Removal of Nitrite and Ammonia as Dinitrogen inAqueous Suspensions of a Titanium(IV) Oxide Photocatalyst underReagent-Free and Metal-Free Conditions at Room Temperature. RSCAdv. 2014, 4, 51576−51579.(151) Lucchetti, R.; Onotri, L.; Clarizia, L.; Di Natale, F.; DiSomma, I.; Andreozzi, R.; Marotta, R. Removal of Nitrate andSimultaneous Hydrogen Generation through Photocatalytic Reform-ing of Glycerol over “In Situ” Prepared Zero-Valent Nano Copper/P25. Appl. Catal., B 2017, 202, 539−549.(152) Herissan, A.; Meichtry, J. M.; Remita, H.; Colbeau-Justin, C.;Litter, M. I. Reduction of Nitrate by Heterogeneous Photocatalysisover Pure and Radiolytically Modified TiO2 Samples in the Presenceof Formic Acid. Catal. Today 2017, 281, 101−108.(153) Wang, X.; Hong, M. Z.; Zhang, F. W.; Zhuang, Z. Y.; Yu, Y.Recyclable Nanoscale Zero Valent Iron Doped g-C3N4/MoS2 forEfficient Photocatalysis of RhB and Cr(VI) Driven by Visible Light.ACS Sustainable Chem. Eng. 2016, 4, 4055−4062.(154) Zou, J. P.; Wu, D. D.; Bao, S. K.; Luo, J. M.; Luo, X. B.; Lei, S.L.; Liu, H. L.; Du, H. M.; Luo, S. L.; Au, C. T.; Suib, S. L. HydrogenEvolution from Water Coupled with the Oxidation of As(III) in aPhotocatalytic System. ACS Appl. Mater. Interfaces 2015, 7, 28429−28437.(155) Kim, J.; Kim, J. Arsenite Oxidation-Enhanced PhotocatalyticDegradation of Phenolic Pollutants on Platinized TiO2. Environ. Sci.Technol. 2014, 48, 13384−13391.(156) Park, H.; Bak, A.; Ahn, Y. Y.; Choi, J.; Hoffmannn, M. R.Photoelectrochemical Performance of Multi-Layered BiOx−TiO2/TiElectrodes for Degradation of Phenol and Production of MolecularHydrogen in Water. J. Hazard. Mater. 2012, 211−212, 47−54.(157) Peng, Y. P.; Chen, H. L.; Huang, C. P. The Synergistic Effectof Photoelectrochemical (PEC) Reactions Exemplified by ConcurrentPerfluorooctanoic acid (PFOA) Degradation and Hydrogen Gen-eration over Carbon and Nitrogen Codoped TiO2 Nanotube Arrays(C-N-TNTAs) Photoelectrode. Appl. Catal., B 2017, 209, 437−446.(158) Wu, Z. Y.; Zhou, Z. Y.; Zhang, Y. J.; Wang, J.; Shi, H. J.; Shen,Q.; Wei, G. F.; Zhao, G. H. Simultaneous PhotoelectrocatalyticAromatic Organic Pollutants Oxidation for Hydrogen ProductionPromotion with a Self-Biasing Photoelectrochemical Cell. Electrochim.Acta 2017, 254, 140−147.(159) Pop, L. C.; Tantis, I.; Lianos, P. PhotoelectrocatalyticHydrogen Production using Nitrogen Containing Water SolubleWastes. Int. J. Hydrogen Energy 2015, 40, 8304−8310.(160) Wang, D. W.; Li, Y.; Li Puma, G.; Wang, C.; Wang, P. F.;Zhang, W. L.; Wang, Q. Dye-Sensitized Photoelectrochemical Cell onPlasmonic Ag/AgCl @ Chiral TiO2 Nanofibers for Treatment ofUrban Wastewater Effluents, with Simultaneous Production ofHydrogen and Electricity. Appl. Catal., B 2015, 168−169, 25−32.(161) Chang, K. L.; Sun, Q. N.; Peng, Y. P.; Lai, S. W.; Sung, M. H.;Huang, C. Y.; Kuo, H. W.; Sun, J.; Lin, Y. C. Cu2O Loaded TitanateNanotube Arrays for Simultaneously Photoelectrochemical IbuprofenOxidation and Hydrogen Generation. Chemosphere 2016, 150, 605−614.(162) Leng, W. H.; Zhu, W. C.; Ni, J.; Zhang, Z.; Zhang, J. Q.; Cao,C. N. Photoelectrocatalytic Destruction of Organics using TiO2 asPhotoanode with Simultaneous Production of H2O2 at the Cathode.Appl. Catal., A 2006, 300, 24−35.(163) Gong, J. Y.; Pu, W. H.; Yang, C. Z.; Zhang, J. D. Novel One-Step Preparation of Tungsten Loaded TiO2 Nanotube Arrays withEnhanced Photoelectrocatalytic Activity for Pollutant Degradationand Hydrogen Production. Catal. Commun. 2013, 36, 89−93.(164) Guaraldo, T. T.; Goncales, V. R.; Silva, B. F.; de Torresi, S. I.C.; Zanoni, M. V. B. Hydrogen Production and SimultaneousPhotoelectrocatalytic Pollutant Oxidation using a TiO2/WO3 Nano-structured Photoanode under Visible Light Irradiation. J. Electroanal.Chem. 2016, 765, 188−196.

(165) Wang, W. C.; Li, F.; Zhang, D. Q.; Leung, D. Y. C.; Li, G. S.Photoelectrocatalytic Hydrogen Generation and Simultaneous Deg-radation of Organic Pollutant via CdSe/TiO2 Nanotube Arrays. Appl.Surf. Sci. 2016, 362, 490−497.(166) Wu, H. J.; Zhang, Z. H. Photoelectrochemical Water Splittingand Simultaneous Photoelectrocatalytic Degradation of OrganicPollutant on Highly Smooth and Ordered TiO2 Nanotube Arrays. J.Solid State Chem. 2011, 184, 3202−3207.(167) Raptis, D.; Dracopoulos, V.; Lianos, P. Renewable EnergyProduction by Photoelectrochemical Oxidation of Organic Wastesusing WO3 Photoanodes. J. Hazard. Mater. 2017, 333, 259−264.(168) Wang, G. M.; Ling, Y. C.; Lu, X. H.; Zhai, T.; Qian, F.; Tong,Y. X.; Li, Y. A Mechanistic Study into the Catalytic Effect of Ni(OH)2on Hematite for Photoelectrochemical Water Oxidation. Nanoscale2013, 5, 4129−4133.(169) Iervolino, G.; Tantis, I.; Sygellou, L.; Vaiano, V.; Sannino, D.;Lianos, P. Photocurrent Increase by Metal Modification of Fe2O3

Photoanodes and Its Effect on Photoelectrocatalytic HydrogenProduction by Degradation of Organic Substances. Appl. Surf. Sci.2017, 400, 176−183.(170) Luo, T.; Bai, J.; Li, J. H.; Zeng, Q. Y.; Ji, Y. Z.; Qiao, L.; Li, X.Y.; Zhou, B. X. Self-Driven Photoelectrochemical Splitting of H2S forS and H2 Recovery and Simultaneous Electricity Generation. Environ.Sci. Technol. 2017, 51, 12965−12971.(171) Zong, X.; Han, J. F.; Seger, B.; Chen, H. J.; Lu, G. Q.; Li, C.;Wang, L. Z. An Integrated Photoelectrochemical-Chemical Loop forSolar-Driven Overall Splitting of Hydrogen Sulfide. Angew. Chem., Int.Ed. 2014, 53, 4399−4403.(172) Chaudhary, A. J.; Donaldson, J. D.; Grimes, S. M.; ul Hassan,M.; Spencer, R. J. Simultaneous Recovery of Heavy Metals andDegradation of Organic SpeciesCopper and Ethylenediaminetetra-acetic acid (EDTA). J. Chem. Technol. Biotechnol. 2000, 75, 353−358.(173) Liu, L.; Li, R. Z.; Liu, Y.; Zhang, J. D. SimultaneousDegradation of Ofloxacin and Recovery of Cu(II) by Photo-electrocatalysis with Highly Ordered TiO2 Nanotubes. J. Hazard.Mater. 2016, 308, 264−275.(174) Qi, F. J.; Yang, B.; Wang, Y. B.; Mao, R.; Zhao, X. H2O2

Assisted Photoelectrocatalytic Oxidation of Ag-Cyanide Complexes atMetal-free g-C3N4 Photoanode with Simultaneous Ag Recovery. ACSSustainable Chem. Eng. 2017, 5, 5001−5007.(175) Paschoal, F. M. M.; Anderson, M. A.; Zanoni, M. V. B.Simultaneous Removal of Chromium and Leather Dye fromSimulated Tannery Effluent by Photoelectrochemistry. J. Hazard.Mater. 2009, 166, 531−537.(176) Wu, Z. Y.; Zhao, G. H.; Zhang, Y. J.; Liu, J.; Zhang, Y. N.; Shi,H. J. A Solar-Driven Photocatalytic Fuel Cell with Dual Photo-electrode for Simultaneous Wastewater Treatment and HydrogenProduction. J. Mater. Chem. A 2015, 3, 3416−3424.(177) Zhang, Y.; Li, J. H.; Bai, J.; Li, L. S.; Xia, L. G.; Chen, S.; Zhou,B. X. Dramatic Enhancement of Organics Degradation and ElectricityGeneration via Strengthening Superoxide Radical by using a Novel3D AQS/PPy-GF Cathode. Water Res. 2017, 125, 259−269.(178) Zhou, Z. Y.; Wu, Z. Y.; Xu, Q. J.; Zhao, G. H. A Solar-ChargedPhotoelectrochemical Wastewater Fuel Cell for Efficient andSustainable Hydrogen Production. J. Mater. Chem. A 2017, 5,25450−25459.(179) Kim, J.; Choi, W. J. K.; Choi, J.; Hoffmann, M. R.; Park, H.Electrolysis of Urea and Urine for Solar Hydrogen. Catal. Today 2013,199, 2−7.(180) Cho, K.; Hoffmann, M. R. Molecular Hydrogen Productionfrom Wastewater Electrolysis Cell with Multi-Junction BiOx/TiO2

Anode and Stainless Steel Cathode: Current and Energy Efficiency.Appl. Catal., B 2017, 202, 671−682.(181) Liu, Y. B.; Xie, J. P.; Ong, C. N.; Vecitis, C. D.; Zhou, Z.Electrochemical Wastewater Treatment with Carbon NanotubeFilters coupled with In Situ Generated H2O2. Environ. Sci. WaterRes. Technol. 2015, 1, 769−778.

ACS Catalysis Perspective

DOI: 10.1021/acscatal.8b03521ACS Catal. 2018, 8, 11542−11563

11562

(182) Cho, K.; Kwon, D.; Hoffmann, M. R. ElectrochemicalTreatment of Human Waste Coupled with Molecular HydrogenProduction. RSC Adv. 2014, 4, 4596−4608.(183) Jiang, J. Y.; Chang, M.; Pan, P. Simultaneous HydrogenProduction and Electrochemical Oxidation of Organics using Boron-Doped Diamond Electrodes. Environ. Sci. Technol. 2008, 42, 3059−3063.(184) Juang, R. S.; Lin, L. C. Treatment of Complexed Copper(II)Solutions with Electrochemical Membrane Processes. Water Res.2000, 34, 43−50.(185) Szpyrkowicz, L.; Zilio-Grandi, F.; Kaul, S. N.; Polcaro, A. M.Copper Electrodeposition and Oxidation of Complex Cyanide fromWastewater in an Electrochemical Reactor with a Ti/Pt Anode. Ind.Eng. Chem. Res. 2000, 39, 2132−2139.(186) Kim, J.; Kwon, D.; Kim, K.; Hoffmann, M. R. ElectrochemicalProduction of Hydrogen Coupled with the Oxidation of Arsenite.Environ. Sci. Technol. 2014, 48, 2059−2066.

ACS Catalysis Perspective

DOI: 10.1021/acscatal.8b03521ACS Catal. 2018, 8, 11542−11563

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