Fundamentals and some applications of ...static.tongtianta.site/paper_pdf/a4e2f140-8508-11e9-9296-00163e08bb86.pdf · moted the field of photocatalysis was the work of Fujishima and

REVIEW

Fundamentals and some applications of photoelectrocatalysisand effective factors on its efficiency: a review

Ebrahim Zarei1 & Reza Ojani2

Received: 18 May 2016 /Revised: 12 August 2016 /Accepted: 2 September 2016 /Published online: 23 September 2016# Springer-Verlag Berlin Heidelberg 2016

Abstract Effluents of a large variety of industries usuallycontain important quantities of synthetic organic compounds.The discharge of these compounds in the environment causesconsiderable non-aesthetic pollution and serious health riskfactors. Since conventional wastewater treatment plants can-not degrade the majority of these pollutants, powerfulmethods for the decontamination of dye wastewaters havereceived increasing attention over the past decade. In thiswo rk , f undamen t a l s and ma in app l i c a t i on s o fphotoelectrocatalysis as one of the most powerful and recentprogresses of emerging photoassisted electrochemical treat-ments with UV irradiation are studied. The effect of variouseffective factors such as photoanode type, light source and itsintensity, pH solution value, type and concentration ofsupporting electrolyte, type of cathode electrode, to be mov-ing of photoanode or solution, thicknesses of semiconductorfilm on the electrode surface, and applied potential on thedestruction of pollutants is described. Furthermore, variousmethods used for TiO2 modification are mentioned. Also, ap-plication of photocatalysts except semiconductors is presentedfor photoelectrocatalytic aims. Finally, application ofphotoelectrocatalysis in determination of materials as a newmethod is discussed.

Keywords Photoelectrocatalysis . Effective factors .

Destruction . Determination

Introduction

In 1964, Kato et al. [1] published their work on the photocat-alytic oxidation of tetralin (1,2,3,4-tetrahydronaphthalene) bya TiO2 suspension [1], which was followed by McLintocket al. [2] investigating the photocatalytic oxidation of ethyleneand propylene in the presence of oxygen adsorbed on TiO2.However, the most important discovery that extensively pro-moted the field of photocatalysis was the work of Fujishimaand Honda on photoelectrochemical water splitting in 1972[3]. After this discovery, titania-based photocatalysts were fre-quently used for heterogeneous photocatalysis, in contrast tothe photoelectrochemical approach, and no bias potential isneeded [4]. Since then, research efforts in understanding thefundamental processes and in enhancing the photocatalyticefficiency of TiO2 have come from extensive research per-formed by chemists, physicists, and chemical engineers.Such studies are often related to energy renewal and energystorage [5–9]. In recent years, applications to environmentalcleanup have been one of the most active areas in heteroge-neous photocatalysis. This is inspired by the potential appli-cation of TiO2-based photocatalysts for the total destruction oforganic compounds in polluted air and wastewater [10–12]. Ina heterogeneous photocatalysis system, photoinduced molec-ular transformations or reactions take place at the surface of acatalyst. The research of powerful and practical treatments todecolorize and degrade dyeing wastewaters to decrease theirenvironmental impact has then attracted increasing interestover the past two decades.

Figure 1 summarizes the main technologies utilized for theremoval of these pollutants. An extensive literature reportingthe characteristics and applications of most important conven-tional technologies developed for this purpose including phys-icochemical and chemical methods, advanced oxidation pro-cesses (AOPs), microbiological treatments, and enzymatic

* Ebrahim [email protected]

1 Department of Basic Science, Farhangian University, Tehran, Iran2 Department of Analytical Chemistry, Faculty of Chemistry,

Mazandaran University, Babolsar, Iran

J Solid State Electrochem (2017) 21:305–336DOI 10.1007/s10008-016-3385-2

http://crossmark.crossref.org/dialog/?doi=10.1007/s10008-016-3385-2&domain=pdf

decomposition has been collected in several critical reviews[13–18]. In contrast, only little information on the interest ofphotoelectrochemical technologies for destroying dyes fromwastewaters has been shown in some previous reviews [14,15, 17, 18], without considering the recent advances of thesepromising methods, mainly developed since 2002.

Traditional physicochemical treatments applied to thepurification of dyeing wastewaters include adsorption withinorganic (mainly, activated carbon materials) and organicsupports, coagulation by lime, aluminum or iron salts, fil-tration, and ion exchange (see Fig. 1). These procedureslead to effective decolorization, but their application is re-stricted by the formation of sludge to be disposed or by theneed to regularly regenerate the adsorbent materials [13,14, 18]. More powerful chemical methods such as ozona-tion and oxidation with hypochlorite ion, as well as AOPssuch as Fenton’s reagent and photocatalytic systems

involving TiO2/UV, H2O2/UV, and O3/UV, have been col-lected in Fig. 1; also, fast decolorization has been providedalong with degradation of dyes. However, the use of thesemethods is not completely accepted at present because theyare quite expensive and have operational problems [13–15,17]. On the other hand, the application of microorganismsto the biodegradation of synthetic dyes is an attractive andsimple method by operation. A large number of activatedsludge processes, mixed cultures with aerobic and anaero-bic decomposition, and pure cultures with white-rot fungusand bacteria (Fig. 1) have been tested for decolorizationand destruction of dyes. Unfortunately, these treatmentsare very inefficient because the majority of these com-pounds are chemically stable and resistant to microbiolog-ical attack [13, 14, 18]. The characteristics of enzymes inmicroorganisms that are suitable for the decomposition ofdyes have also been extensively investigated, requiring a

Fig. 1 Mainmethods used for theremoval of organic dyes fromwastewaters

306 J Solid State Electrochem (2017) 21:305–336

greater knowledge of the enzymatic processes involved inthem for application to environmental protection [13](Fig. 1).

Main methods used for the removal of organic dyes fromwastewaters are discussed. Over the past 10 years, the electro-chemical technology has been largely developed for its alter-native use for wastewater remediation. It currently offerspromising approaches for the prevention of pollution prob-lems from industrial effluents. The application of electro-chemistry to environmental pollution abatement has been thetopic of several books and reviews [15, 19–27]. The mainadvantage of this technology is its environmental compatibil-ity, due to the fact that its main reagent, the electron, is a cleanreagent [19]. Other advantages are related to its versatility,high-energy efficiency, amenability of automation, and safetybecause it operates at mild conditions [19, 23, 26]. The strat-egies of electrochemistry include both the treatment of efflu-ents and waste and the development of new processes or prod-ucts with less harmful effects, often denoted as process-integrated environmental protection. The main electrochemi-cal procedures utilized for the remediation of dyestuffs waste-waters are also given in Fig. 1.

Electrocoagulation (EC), electrochemical oxidation (EO)with different anodes, and indirect electrooxidation with ac-tive chlorine are typical methods for the removal of thesepollutants. The treatment by emerging technologies such aselectro-Fenton (EF) and photoassisted systems likephotoelectro-Fenton (PEF) and photoelectrocatalysis has re-cently received great attention, but the possible role of elec-trochemical reduction has been clarified in much lesser extent.Note that electrochemical oxidation, electro-Fenton, andphotoassisted electrochemical systems have been classifiedas electrochemical advanced oxidation processes (EAOPs).In this paper, we present a general review of relatedness toprinciples, modification methods, and different effective fac-tors on photoelectrocatalysis efficiency and its applicationsespecially in determination.

Photoelectrocatalysis

Fundamentals of photoelectrocatalysis

Unlike metals which have a continuum of electronic states,semiconductors possess a void energy region where no energylevels are available to promote recombination of an electronand hole produced by photoactivation in the solid [28].Figure 2 depicts the mechanism of the electron–hole pair for-mation when the TiO2 particle is irradiated with light of ade-quate energy. The light wavelength for such photon energyusually corresponds to λ < 380 nm. In photocatalysis process,when the TiO2 nanoparticles in colloidal suspensions or de-posited as a thin film on a solid carrier was illuminated withUV light, a great number of electrons would be excited from

the valence band (VB) to the conduction band (CB) by ab-sorbing UV light quanta, leaving highly oxidative holes in VB(hVB

+) and forming negative sites in CB (eCB−), as shown in

Fig. 3a(1) and reaction (2) [29]:

TiO2 þ hν→eCB− þ hVB

þ ð1Þ

Organics can then be directly oxidized by the hole or byheterogeneous hydroxyl radical formed from the followingreaction between the photogenerated vacancy and adsorbedwater:

hVBþ þ H2O→

∙OHþ Hþ ð2Þ

On the other hand, the chemisorption properties on TiO2

surfaces have been extensively studied. Particular interest hasbeen given to the influence of defect sites on thechemisorptive behavior of the surface, since these defect sitesare also found as the active sites for photocatalytic processes[30, 31]. In addition, other weaker oxidizers (superoxide rad-ical ion O•−

2 , HO•2, and H2O2) and more •OH can be produced

Fig. 2 Schematic photoexcitation in a solid followed by deexcitationevents

Fig. 3 Schematic representation of photocurrent generation from themicroscopic (a) and macroscopic (b) views

J Solid State Electrochem (2017) 21:305–336 307

from reaction between chemisorbed O2 on the TiO2 surfaceand the photoinjected electron by the following equations[29]:

eCB− þ O2→O2

∙− ð3Þ

O2∙− þ Hþ→HO2

∙ ð4Þ2HO2

∙→H2O2 þ O2 ð5ÞH2O2 þ O2

∙−→∙OHþ OH− þ O2 ð6Þ

The quantum yield for an ideal system, Φ, is given by thesimple relationship:

ϕ ¼ kCTkR þ kCT

ð7Þ

The quantum yield is proportional to the rate of the chargetransfer processes (kCT) and inversely proportional to the sumof the charge transfer rate (kCT) and the electron–hole recom-bination rate (kR) (bulk and surface). The major loss in effi-ciency of photocatalysis is due to the recombination of elec-trons promoted to the valence band either with unreacted holesor with adsorbed hydroxyl radicals as observed in Fig. 3a(2)and the following reactions [29]:

eCB− þ hVB

þ→TiO2 þ heat ð8ÞeCB

− þ ∙OH→OH− ð9Þ

The electrochemical technology can provide much higherefficiency for wastewater remediation by means ofphotoelectrocatalysis. This photoassisted method consists inthe application of either a suitable constant current, I, or aconstant bias anodic potential (Eanod) to a TiO2-based thin-film anode subjected to UV illumination. In this scenario,the photogenerated holes could oxidize the organic com-pounds at the anode surface (Fig. 3a(3)), while the photoin-duced electrons are continuously extracted from the anode by

an external electrical circuit (Fig. 3a(4)). This causes the inhi-bition of reactions (4)–(9) and favors the production of higheramount of holes from reaction (2) and heterogeneous •OHfrom reaction (3), thus largely enhancing organic oxidationin comparison to photocatalysis [32]. As indicated inFig. 3b, thin semiconductor particulate films prepared fromparticulate suspensions consist of small particles which arein close contact with each other and are capable of exhibitingphotoelectrochemical properties similar to polycrystallinesemiconductor films. The whole reaction process viewed onboth macroscopic andmicroscopic scales was shown in Fig. 3.

Ojani et al. [33] compared rhodamine B removal in thevarious degradation processes, pheotoelectrocatalysis (PEC),photocatalysis (PC), electrochemical oxidation (EC), and di-rect photolysis (DP) (Fig. 4a). The relative concentration ofrhodamine B, C/C0, in this figure and other figures was ob-tained by evaluating the ratio of rhodamine B or other sub-stance concentration C at time t and the initial rhodamine B orother substance concentration C0 in the solution, at t = 0. Theexperimental results shown in this figure demonstrated thatthe removal rate of rhodamine B in the PEC oxidation wasmore than that the PC, DP, and EC oxidation. In this work, itwas firstly reported that electrochemical monitoring of con-centration changes in various mentioned processes. Figure 4bshows the differential pulse voltammograms of rhodamine Bat various times at the surface of carbon paste electrode forPEC process as an example. Rhodamine B concentration var-iations were determined by monitoring the Ip value at the Ep

oxidation of rhodamine B over different times. From Fig. 4b,it can be clearly seen that the peak current decreases as afunction of time during photoelectrocatalysis. This means thatrhodamine B disappeared quickly with the increase of reactiontime in PEC degradation process.

Also, Hou et al. constructed a photoelectrochemical reactor asa proton exchange membrane fuel cell was utilized to electro-chemically enhance the photocatalytic decomposition of gas-

Fig. 4 a Comparison of (a) PEC, (b) PC, (c) DC, and (d) EC degradationof rhodamine B (RB). Phosphate buffer solution (pH 7.0), initial RBconcentration 4.2 mg L−1, external bias 0.65 V. b Differential pulsevoltammograms of RB in PEC degradation process at the surface of the

carbon paste electrode in PEC degradation process at various times: (a) 0,(b) 20, (c) 40, (d) 60, (e) 80, ( f ) 100, and (g) 120 min. Scan rate10 mV s−1


phase isopropanol (ISP) [34]. The TiO2-coated steel plate wasused as a photoanode in this experiment, as shown in Fig. 5a.Another drilled stainless steel plate (24 × 24 mm) immersed inethanol, acetone, and deionized water in sequence was utilizedas a cathode. The photoelectrochemical reactor depicted inFig. 10b was designed to follow the structure of a proton ex-change membrane (PEM) fuel cell. The commercial membraneelectrode assembly consisted of a Nafionmembrane was used assolid electrolyte and electrode separator to divide thephotoelectrochemical reactor into the anode and cathode cham-bers. The photoanode and cathode electrodes were positioned onopposite sides of the Nafion membrane and tightly sandwichedby two acrylic flow-field plates to give a good contact. Theledges of the photoanode and cathode were separately attachedby alligator clips to establish an electrical contact with apotentiostat. The decomposition of gaseous IPA by electrolytic,photocatalytic, and photoelectrocatalytic processes for experi-ments conducted with various applied bias potentials in the pres-ence of 10 % relative humidity depicted in Fig. 6 showed thatmerely about 10 % IPAwas decomposed by electrolytic processfor experiments conducted even with 4 and 5 V bias potentials.The decomposition of IPA by photocatalytic process withoutany applied bias potential was approximately 40 %. The IPA

decomposition was remarkably increased by roughly 10 to 20%with increasing applied bias potential from 3 to 5 V forphotoelectrocatalytic process because the photogenerated elec-trons could be efficiently driven out of photoanode by the ap-plied bias potential.

The electrochemical cells used in photoelectrocatalysis areusually tank or flow reactors that permit the pass of UV lightthrough a quartz glass to reach the exposed surface of theTiO2-based anode with the minimum loss of incident irradia-tion. Examples of systems and cells for treating synthetic or-ganic compounds can be seen in Figs. 7 [35] and 8 [33].

Fig. 5 a The TiO2-coated photoanode and b the photoelectrochemicalreactor used in decomposition of gaseous isopropanol

Fig. 6 Effects of applied bias on the IPA decomposition by theelectrolytic, photocatalytic, and photoelectrocatalytic processes indecomposition of gaseous isopropanol

Fig. 7 Schematic diagram of the electrochemical cell employed to carryout photocatalytic degradation of 4-chlorophenol (4-CP)


The effective factors on efficiency of photoelectrocatalysisprocess

Photoanode type

The most typical photoanodes are Ti meshes or plates coatedwith TiO2 [36–38] or Ti/Ru0.3Ti0.7O2 [39–43]. The former isutilized in a three-electrode cell under potentiostatic conditionssince it is stable at low current density, whereas the latter haslarge stability at high I and its properties permit its use in two-electrode reactors. When a Ti/Ru0.3Ti0.7O2 photoanode isemployed, greater dye concentrations can be treated because itpermits the pass of higher current density with larger stability.On the other hand, Comninellis [44] explained the differentbehavior of electrodes in electrooxidation considering two lim-iting cases: the so-called active and non-active anodes. Typicalexamples are Pt, IrO2, and RuO2 for the former and PbO2 andSnO2 for the latter. The proposed model assumes that the initialreaction in both kinds of anodes (generically denoted as M)corresponds to the oxidation of water molecules leading to theformation of physisorbed hydroxyl radical (M(•OH)):

Mþ H2O→M ∙OHð Þ þ Hþ þ e− ð10Þ

Therefore, for Ti/Ru0.3Ti0.7O2 electrode, the synergistic ef-fect found in photoelectrocatalysis was related to the simulta-neous formation of heterogeneous •OH on RuO2 from waterdischarge by reaction (10) and on TiO2 via reaction (2) owingto photogenerated holes by reaction (3). The major disadvan-tages of Ti/Ru0.3Ti0.7O2 anode is this point that coupled pro-duction of heterogeneous •OH on RuO2 and TiO2 in a Ti/Ru0.3Ti0.7O2 anode only takes place in large extent when itis irradiated with an UVC light, since smaller energeticallyphotons supplied by UVB and UVA lights do not promoteefficiently the formation of electron–hole pairs in the mixedoxide [39, 41].

It should be mentioned TiO2 thin-film modified graphite[34, 45], conducting glass [46–48], and Cu2O plates [49] asphotoanodes have been applied for photoelectrocatalytic deg-radation of some organic compounds. Also, carbon nanotubes(CNTs) were selected as the support materials for catalystdeposition to obtain a large surface area and by mixing TiO2

a n d CNT h a s b e e n p r e p a r e d e l e c t r o d e s f o rphotoelectrocatalytic purposes [50, 51]. Janáky et al. [52]

Fig. 8 Schematic diagram of the reactor system of photoelectrocatalysis

Fig. 9 SEM micrograph of raw Ti mesh and TiO2/Ti mesh in which a isthe raw Ti mesh, b is the 120-V TiO2/Ti mesh, and c is the 180-V TiO2/Timesh in 0.5 M H2SO4 solution


described nanostructured carbon-based electrodes as tem-plates for the electrodeposition of inorganic oxides (in manycases, oxide semiconductors) and mentioned that nanostruc-tured carbon-based assemblies are useful in solar photovoltaiccells, heterogeneous photocatalysis, and so forth because oftheir good electronic conductivity and other properties.

A wide variety of preparation methods of TiO2 exist,and extensive efforts are being devoted to fabrication ofhigh surface area TiO2 photocatalyst. One of the mostcommon methods to obtain a high surface area is by usingpowdery TiO2. In this regard, it is generally well ac-knowledged that titania of Degussa P-25, produced byhydrolysis of TiCl4, is considered to be a superior com-mercial photocatalyst. Two limiting problems arise whenTiO2 powder is being used: environmental problems frominsufficient filtration [53, 54] and a lack of ability to reuseand regenerate the material. Those problems can be effec-tively solved using embedded catalyst fabricated via im-plementation of a few methods, such as pasting procedure[55], gelation [56, 57], pulse laser deposition [58],sputtering deposition [59], sol–gel [60, 61], and electro-chemical oxidation [34, 62, 63]. Of these, electrochemicalpolarization of titanium is an effective method for grow-ing TiO2 because of the good mechanical adhesion andthe electrical conductivity of the titanium metal substrate.

The parameters that most affect the TiO2 characteristics arethe electrolyte type, concentration, and pH value. Using dif-ferent electrolytes and applied potentials can alter the micro-structure, morphology, and thickness of the grown oxide layer[22–25]. Phosphoric and sulfuric acids are very common elec-trolytes in various degrees of dilution (Fig. 9) [64]. Moreover,over the past decades, nanostructured materials derived fromTiO2 with unique structural and functional properties haveextensively been investigated for many promising applica-tions, including solar cells, supercapacitors, photocatalysis,and photoelectrocalysis [65]. Ge et al. [66] have describedpreparation of one-dimensional TiO2 nanomaterials with var-ious morphology, including nanotubes, nanorods, nanowires,nanobelts, nanosheets, and nanofibers, and six main prepara-tion methods, hydrothermal, electrochemical anodization, va-por deposition, sol–gel, template-assisted, and electrospinningmethods.

For the first time, in 2001, Grimes et al. [67] haveshowed that the addition of fluoride ions to electrolyteduring anodization changes the mechanism of growthleading to the formation of long and smooth nanotubularmorphology (Fig. 10). Huang et al. [68] have introducedelectrochemical techniques of TiO2 nanotube array fabri-cation and modification and their applications. This sub-ject has been investigated by other researchers [69–71].Baram et al. [72] evaluated photoelectrochemical behaviorof three TiO2 electrodes (Fig. 11). Two electrodes werealso prepared by anodizing in 0.5 M sulfuric acid at aconstant current of 100 mA/cm2 until final potential of110 V (HS 110 V TiO2) and 150 V (HS 150 V TiO2)was reached. The third electrode was fabricated by Tianodization in 1 M sodium sulfate (Na2SO4) + 0.5 % wtNaF solution at a constant of 20 V (nanotubular TiO2).Similar behavior was observed in all forms of TiO2: alinear increase in photocurrent until saturation. Of thethree TiO2 electrodes, the nanotubular TiO2 possessesthe highest saturation photocurrent. This behavior indi-cates that the charge transfer and conduction is more ef-fective through the nano tubes than mesoporous film,most probably due to the increased surface area of thenanotubular structure.

Fig. 10 Top (left) and cross(right) sectional high-resolutionSEM micrographs of TiO2 grownvia anodization in 1 M Na2SO4 +0.5 % wt NaF in a constant po-tential of 20 V for 2 h

Fig. 11 The photocurrent dependence on the potential for the HS 110 Vand HS 150 Valong with the previous results for the nanotubular TiO2


Light source and its intensity

The effect of the light source and its intensity on the decolor-ization efficiency of Methyl Orange has been studied byZainal et al. [37] using the photoelectrochemical reactor ofFig. 12a. Solutions of 120 cm3 with 10 mg dm−3 of this azodye in 0.1 M NaCl at 40 °C were treated under illumination ofthe 10-cm2 Ti/TiO2 anode by several UVA and visible lampsat an anodic bias potential of 1.0 V vs. SCE. Figure 12b showsthat at 2 h of photoelectrocatalysis, only 6 and 13 % fractionalconversion was found for 50-W tungsten halogen and 30-Wfluorescent lamps, respectively, whereas the increase in poten-cy of the tungsten halogen lamp to 300 W leads to 94 % colorremoval, slightly higher than 93 % obtained for the 100-WUVA lamp.

These findings evidence the production of more photonswith suitable energy for electron excitation in TiO2 by theUVA lamp, although they are generated in larger extent bythe fluorescent lamp than the tungsten halogen one at similarintensity. An enhancement of dye decolorization is then ex-pected at higher intensity since more significant amounts of

electron–hole pairs should be formed, as found when the po-tency of the tungsten halogen lamp rises from 50 to 300 W.The fractional conversion of the Methyl Orange solution onlyunderwent a slight decrease after 20 runs of 2 h using the 300-W tungsten halogen lamp that induced a photocurrent of ca.0.5 mA. This indicates that TiO2 is a good anode material forremoving dyes with a long lifetime at low Eanod and currentvalues.

Also, Chen et al. [73] investigated the photoelectrocatalyticperformance of benzoic acid on TiO2 nanotube array elec-trodes’ (TNAs) different illumination intensity (Fig. 13). Asit is shown in this figure, the more intense the light is, theshorter is the time for complete degradation, indicating thatan increase in the intensity of light is conducive to promotingthe photoelectrocatalytic degradation rate.

pH solution value

In water or wastewater treatment, pH is a common factor thatinfluences the removal of pollutants in many processes ofcharged organic pollutants because the pH value of the solu-tionwill change the existing configuration of degraded speciesand surface charges of catalysts. Under different pH valueconditions, hydroxyl groups on TiO2 surface undergo the fol-lowing equilibrium through Lewis acid–base reaction [74]:

pH < PZC : TiOHþ Hþ⇄TiOH2þ ð11Þ

pH > PZC : TiOHþ OH−⇄TiO− þ H2O ð12Þ

Fig. 12 a Sketch of the photoelectrochemical systemwith a stirred three-elec t rode two-compar tment tank reac tor ut i l ized for thephotoelectrocatalytic decolorization of Methyl Orange. The anode wasa 5-cm × 2-cm plate of Ti coated with TiO2, the cathode was a 1-cm

2 Ptsheet, and the light source was placed 8 cm away from the sample. bFractional conversion for 120 cm3 of a 10-mg dm3 Methyl Orange solu-tion in 0.1 M NaCl at Eanod = 1.0 V vs. Ag/AgCl and 40 °C underillumination with a (black circle) 50-W tungsten halogen lamp, (blacksquare) 30-W fluorescent lamp, (black up-pointing triangle) 100-WUVAlamp, and (black diamond) 300-W tungsten halogen lamp

Fig. 13 Photocurrent responses of a 2-mol/L NaNO3 blank solutioncontaining benzoic acid (0.4 mmol/L) in the thin-cell reactor based onthe TNA electrodes at the fixed applied bias potential of 2.5 V underdifferent light intensities


Since the isoelectrical point of TiO2 is at pH 4–6, hence, thecatalyst surface is positively charged at more acidic pH, whileit is negatively charged at pH value almost above 6 [75]. pHinfluences the PEC process in many ways, such as TiO2 flat-band potential variation and changes in adsorption ability ofthe target compound on the TiO2 film [76]. It is should benoted that pH has doublet effect in reaction, one is pH effectin adsorption of substrate molecules and second is pH effect inthe OH− and •OH concentration that affected degradation per-formance that resulted from reaction of holes with H2O.Therefore, in some cases, these two effects are alien and re-versely affected degradation performances. In these cases, ad-sorption of substrate molecules at the surface of electrode ismore effective than •OH concentration [77].

On the other hand, the value of pH could influence thecharge carried by the organic molecules. The most organiccompounds with the change of pH were converted as cationicor anionic forms. Therefore, its adsorption on the catalystsurface became difficult or easy because of electrostatic repul-sive or attractive forces, respectively. As a result, pH effectand optimal pH for PEC degradation could be differentiatedfor different compounds. For example, Quan et al. [78]showed that pentachlorophenol (PCP) degradation kineticsin aqueous solution using a TiO2 nanotube film electrode areoptimal at pH 3.25 compared with pH 6.29 and pH 8.00(Fig. 14). Also, Sonia et al. [79] presented that pH 4.0 isoptimum pH for photoelectrocatalytic degradation of p-nitro-phenol using Ti/TiO2 thin-film electrode (Fig. 15). In otherstudy, Ojani et al. investigated photoelectrocatalytic degrada-tion of 3,4-dichlorophenol using TiO2 thin-film modifiedgraphite electrode and obtained that pH 8.0 is the best pHfor this purpose [45]. Because 3,4-dichlorophenol existed inwater as weak organic acid (pKa 8.63), therefore, 3,4-

dichlorophenol may be presented in different forms dependingon the pH value in the solution. Thus, the pH value wouldhave a significant effect on the adsorption–desorption proper-ties of 3,4-dichlorophenol at the catalyst’s surface.

Type and concentration of supporting electrolyte

This factor influences photoelectrocatalytic degradation ofpollutants. A research group reported an interesting influenceof the supporting electrolyte on the photoelectrocatalysis ofthe azo dye Reactive Orange 16 as a function of solution pH[38]. These experiments were carried out with 0.04–0.05 mMsolutions of this dye in 0.025–0.5 M NaCl or Na2SO4 in thepH range 2.0–12.0 using the photoelectrochemical cell con-taining three-electrode divided cell containing a 10-cm2 Ti/TiO2 anode biased at Eanod = 1.0 V vs. SCE. In all cases,overall decolorization was obtained in approximately20 min. However, faster degradation was determined atpH < 6.0 for chloride media and at pH > 10.0 for sulfateeffluents. The superiority of NaCl in acidic media was as-cribed to the large ability of the holes formed at thephotoanode from reaction (1) to oxidize Cl− to active chlorinespecies (Cl2, HClO, and ClO−) and probably other radicalssuch as Cl• and Cl2

•, which oxidize organics more quicklythan heterogeneous •OH produced in Na2SO4 via reaction(2). The opposite trend in alkaline solutions was related tothe strong inhibition of the generation of chlorinated oxidantsfrom Cl− decelerating the destruction of pollutants. After 3 hof photoelectrocatalysis of 0.04 mM Methyl Orange, 62 and56 % mineralization was achieved in 0.5 M NaCl at pH 4.0and 0.5 M Na2SO4 at pH 12.0, respectively.

Zanoni et al. [80] performed photoelectrocatalytic degrada-tion of reactive dye Remazol Brilliant Orange 3R (RBO) at pH

Fig. 15 Photocurrent–potential curves for TiO2 thin-film electrode in1 × 10−4 mol L−1 pNP/HClO4 solutions under dark conditions (a) underUV illumination at pH = 5 (b) and pH = 2 (c). Scan rate = 10 mV s−1

Fig. 14 PEC degradation of PCP as affected by pH (initial concentrationof PCP 20 ppm, bias potential 0.2 V, concentration of Na2SO4 0.01 M,light intensity 620 mW/cm2)


6.0 when the photoelectrode was biased at +1 V (vs. SCE).The photoelectrocatalytic degradation experiments were per-formed in a two-compartment reactor. A Nafion 117 mem-brane was used to separate both compartments while allowingelectrolyte contact. Also, the photoactive area of the anode(TiO2) was 10 cm2 and was illuminated by a 450 W Xe–Hgarc lamp.

The influence of the type of electrolyte on the removal ofcolor was investigated through experiments conducted with5.7 × 10−5 mol L−1 RBO in 0.25 mol L−1 NaCl, NaClO4,KNO3, and Na2SO4. Dye absorbance was monitored during30 min of photoelectrocatalytic degradation (Fig. 16). Theresults obtained showed that the selection of a supportingelectrolyte for the complete removal of color is critical to theprocess. The degree in discoloration was 28 and 19 % whenNaClO4 and Na2SO4 were used as electrolytes, respectively.On the other hand, KNO3 promoted 75 % of color removalafter 30 min. This occurred due to photolytic reaction of NO3

−

under irradiation of λ < 380 nm by the overall reaction [81]:

NO3− þ H2Oþ hν→∙NO2 þ ∙OHþ OH− ð13Þ

Therefore, the photocatalytic oxidation properties of NO3−

also are likely for increasing the concentration of hydroxylradicals and thereby increasing the discoloration rate of thedye. Nevertheless, in subsequent experiments, we did not con-sider KNO3 as a supporting electrolyte since the UV–Visspectrum obtained after photocatalysis of the dye showedthe occurrence of extra peaks in the UV region, suggestingother pathways for the photooxidation route or, alternatively,the corrosion of the coated catalyst. Most importantly, thehighest discoloration rate is observed in chloride media, indi-cating that the mechanism of RBO degradation in NaCl

solution may be different than that operating in the othersupporting electrolytes.

Also, Zanoni et al. [80] carried out the degradation exper-iment of reactive dye RBO using a titanium dioxide thin-filmphotoelectrode both in 0.5 and 1.0 mol L−1 NaCl, at E = +1 V,followed by measuring the absorbance decay at λ = 496 nmover 30min (Fig. 17). It reveals that 100% of color removal isattained after 25 min of experiment when the concentration ofNaCl is 1.0 mol L−1, which suggests that the dye degradationdepends on the ratio [NaCl]/[dye] in terms of concentration.

Quan et al. [78] selected Na2SO4 as the electrolyte, andthree levels of concentration (0, 0.005, and 0.01 M) werestudied to investigate the influence of electrolyte concentra-tion. The results show (Fig. 18) that PCP degradation wasenhanced when Na2SO4 was present, and there was a trendof increasing PCP removal as Na2SO4 concentration rose. Therate constants were 0.54, 0.67, and 0.84 h−1, corresponding to0, 0.005, and 0.01 M Na2SO4 concentrations. This observa-tion may have been anticipated as the presence of electrolyteincreases the conductivity of solutions. As a result, the PECprocess was enhanced.

Type of cathode electrode

In the electrochemical study of TiO2 slurries, Gerischer andHeller have highlighted the rate of O2 reduction (indicated byEq. 3) as the limiting step in the oxidation of organic mole-cules at the irradiated TiO2 particles [82]. Since the efficiencyof a semiconductor photocatalyst obviously depends on elec-tron and hole recombination, therefore, presence of oxygen orother substances is important as a good scavenger in the PECsetup. It is worth that in PEC, photogenerated electrons were

Fig. 16 Results from the photoelectrocatalytic oxidation of5 × 10−5 mol L−1 RBO dye at pH 6.0 in 0.5 mol L−1 of (white circle)NaCl, (white diamond) KNO3, (white triangle) NaClO4, and (blackcircle) Na2SO4 using UV light and E = +1 V (vs. SCE)

Fig. 17 Effect of concentration of NaCl on absorbance decay of8.0 × 10−5 mol L−1 RBO dye at pH 6.0 as a function of time ofphotoelectrocatalytic oxidation over a TiO2 thin-film electrode biased atE = +1 V (vs. SCE): (white circle) 0.5 mol L−1 NaCl and (black circle)1.0 mol L−1 NaCl


kept from recombining with the holes using the external biasapplication; however, there is a low quantum yield arisingfrom the very fast recombination of photogenerated electronsand holes with lifetimes of between picoseconds and nanosec-onds. Therefore, other factors such as scavengers that alongwith the external bias could reduce electron and hole recom-bination is useful in PEC. On the other hand, it is alreadymentioned, the mechanism of photocatalysis in the presenceof TiO2 is the enhanced formation of hydroxyl radicals (•OH)which are active in oxidation processes and have a significanteffect on the chemical oxidation of organic compounds in theenvironment. Complete mineralization of many organic sub-stances is possible in aqueous systems, when sufficient •OHradical flux can be generated.

In the most PEC oxidation systems, a particulate TiO2 filmelectrode is usually used as the photoanode, while a counterelectrode such as a Pt electrode is used as the cathode. The roleof the cathode beyond a counter electrode is usuallydisregarded in this kind of PEC reaction systems. Actually,some future advantages of utilizing the cathode for generatinguseful oxidant species such as hydrogen peroxide (H2O2)within the photoreactor could be realized, if the counter elec-trode is used as a functional cathode. In fact, addition of H2O2

chemical as a sacrificial oxidant to scavenge the photoinducedTiO2 conduction band electrons (eCB

−) has been proven to bebeneficial for improving the efficiency of photocatalytic (PC)oxidation in a TiO2 suspension system by capturing the eCB

−

to form the hydroxyl radical (•OH) [83, 84] as follows:

H2O2 þ eCB−→OH− þ ∙OH ð14Þ

Furthermore, H2O2 may generate hydroxyl radical directlyunder UV illumination according to the following reaction:

H2O2 þ UV→2∙OH ð15Þ

At high concentrations, H2O2 can act as a hydroxyl radicalscavenger instead of a free radical generator and decrease theamount of hydroxyl radicals in the liquid. Subsequently, anyoverdosing of H2O2 can inhibit the photocatalytic oxidation oforganics [85]. Because of the advantage that the electrochem-ical reaction is easily controlled by current or potential, theappropriate and continuous supply of H2O2 can be achievedby an electrochemical method instead of the chemical dosingmethod. Therefore, it is possible to utilize an electrical currentto generate H2O2 continuously on the surface of the cathodeduring the photoreaction in a PEC reaction system to furtherimprove the efficiency of the TiO2-based PEC oxidation.

It is known since 1882 that hydrogen peroxide can be ac-cumulated in aqueous medium from the cathodic reduction ofdissolved O2 gas at carbonaceous electrodes with high surfacearea [86], such as carbon felt, reticulated vitreous carbon(RVC), graphite, activated carbon fiber, and graphite–polytetrafluoroethylene (PTFE) as follows:

O2þ2Hþþ2e−→H2O2 EÅ¼ 0:695 V.NHE ð16Þ

Li et al. developed an E-H2O2/TiO2 photoelectrocatalyticoxidation system oxidation for 2,4,6-trichlorophenol (TCP)degradation [87]. A TiO2/Ti mesh electrode was applied inthis photoreactor as the anode to conduct PEC oxidation,and a RVC electrode was used as the cathode toelectrogenerate hydrogen peroxide simultaneously. To studythis E-H2O2-assisted reaction system for TCP degradation, thefour experiments of (a) E-H2O2 (RVC as a cathode and Pt asan anode in the dark), (b) E-H2O2/UV (RVC as a cathode andPt as an anode under UVA illumination), (c) TiO2 PEC (TiO2/Ti as a photoanode and Pt as a cathode under UVA illumina-tion), and (d) E-H2O2/TiO2 PEC (TiO2/Ti as a photoanode and

Fig. 18 PEC degradation of PCP as affected by concentration of Na2SO4

(initial concentration of PCP 20 ppm, pH 7.03, bias potential 0.2 V, lightintensity 620 μW/cm2)

Fig. 19 Degradation of TCP in different reactions: a E-H2O2, b E-H2O2/UV, c TiO2 PEC, and d E-H2O2/TiO2 PEC


RVC as a cathode under UVA illumination) (Fig. 19). Thedegradation of TCP has been considered to follow pseudo-first-order kinetics and the kinetic constants, k, along withremaining TCP in the experiments have been calculated(Table 1). It is clear that the E-H2O2-assisted TiO2 PEC inthe RVC-TiO2/Ti-UV system achieved the best TCP degrada-tion performance of up to 99 % after 60 min. These experi-mental results confirmed that in many cases the photocatalyticdegradation of a water contaminant can be enhanced byadding H2O2.

Xie et al. [88] applied a TiO2 or Au-TiO2 film electrode asthe photoanode to degrade bisphenol A (BPA) in aqueoussolution under UVor visible illumination and a RVC electrodeas the cathode to generate H2O2 simultaneously. Also, anotherinteresting study made by Xie et al. [32] has shown the supe-riority of coupling electro-Fenton with photoelectrocatalysisfor removing the azo dye Orange G with respect to EO andother single photoassisted methods. Comparative experimentswere performed with the quartz reactor of Fig. 20 for 5 h. Thecell was filled with 30 cm3 of 0.1 mM solution of the dyecontaining 0.01 M Na2SO4 at pH 6.2, and an 8-W UVA lampwas used as light source. The results show that Orange G isnot directly photolyzed under UVA irradiation, being slightlydestroyed in about 3 % by EO with a TiO2/Pt cell atEanod = 0.71 V vs. SCE and in 8 % by TiO2 photocatalysis,as expected if small amounts of oxidizing species (•OH and/orholes) are formed at the TiO2 surface. Photoelectrocatalysis

using the TiO2/Pt cell enhances dye removal to 25 % owing tothe photogeneration of more amounts of oxidant holes by thepass of current. The oxidation power of this technique can beimproved to yield 50 % dye destruction if the Pt cathode isreplaced by a RVC electrode at Ecat = −0.54 V vs. SCE since itis also oxidized with H2O2 produced from O2 reduction viareaction (16). In contrast, Orange G disappears completelywhen applying photoelectro-Fenton with a Fe/RVC cell usingca. 17 mg dm−3 Fe2+, pH 3.0, and Ecat = −0.71 V vs. SCE andits decolorization rate even slightly increases if electro-Fentonunder the same conditions is coupled to photoelectrocatalysiswith a TiO2/RVC cell, as a result of the additional formation oflarge amounts of homogeneous •OH from Fenton’s reaction(17) and/or photo-Fenton reaction (1). The latter method alsoprovided the greatest mineralization of 74 % in 5 h.

Fe2þ þ H2O2→Fe3þ þ ∙OHþOH− ð17ÞFe OHð Þ2þ þ hν→Fe2þ þ ∙OH ð18Þ

To be moving of photoanode or solution

The quantum yield of holes, or oxidation capability, was di-rectly related to the radiation power of the illumination. Inconventional PEC reactor design, where the photoanode iscompletely immersed in solution, the radiation has to passthrough the wall of the reactor and sample solution before itreaches the photoanode surface [83, 84, 89], causing signifi-cant loss of radiation power due to absorption and, conse-quently, low treatment efficiency. High power light sourceshave been employed in laboratory experiments [90].However, the setup was complex (generally with a jacket tocirculate cooling water), and considerable amount of lightpower was lost in the form of heat dissipation. Obviously, thismethod would not be applicable in industrial applications dueto the high operating cost. Invalid light consumption is a well-known problem that restricts the practical application of TiO2

PEC oxidation in real organic especially of colored wastewa-ter treatment; however, only a few studies are available toaddress this problem [91].

In order to improve the light utilization of TiO2 electrode,Xu et al. proposed two methods [92, 93], termed thin-filmPEC process, in which organic degradation occurs on the sur-face of a rotating TiO2/Ti disk photoelectrode coated with an

Fig. 20 Experimental setup of the aerated three-electrode undividedquartz cell with 5-cm2 electrodes used for the photoassistedelectrochemical oxidation of 30 cm3 of a 0.1 mM Orange G solution in0.01 M Na2SO4 of pH 6.2 with an 8-W UVA lamp of λmax = 365 nm

Table 1 Pseudo-first-orderkinetic constant and TCPremaining (calculated based ondata of Fig. 19) after 60 min atvarious reaction conditions

Reaction system Kinetic constant, k (min−1) Correlation coefficient, R2 TCP remaining, %

E-H2O2 0.0019 0.9973 89

E-H2O2/UV 0.0063 0.9937 69

TiO2 PEC 0.0321 0.9981 15

E-H2O2/TiO2 PEC 0.0733 0.9952 0.6


aqueous film. On the basis of Beer’s law [94]:

A ¼ εbc ð19Þ

where A is the absorbance, b is the path length, and c is themolarity of solution. The absorbance, or radiation power lossdue to solution absorption, can be reduced with the decreaseof path length. Forming a thin aqueous film on TiO2 surfacewas possible because TiO2 is super hydrophilic when illumi-nated by UV light [95, 96]. In thin-film PEC, the quantumyield of photogenerated electron–hole pairs significantly in-creases because of the enhanced radiation; consequently, or-ganic compounds can be oxidized more efficiently.

In the first method, Xu et al. [92] applied the TiO2/Ti ro-tating disk PEC reactor consisted of a semicircle quartz cell(85 mm diameter) using the TiO2/Ti disk as anode and a Cusheet (15-mm long, 10-mm wide, and 1.5-mm thick) as cath-ode (Fig. 21a). The distance between the anode and cathodewas about 1 cm, which was chosen based on the size of thereactor cell and the optimum current obtained. When the re-actor was filled with sample solution, the TiO2/Ti electrodewas arranged so that 24 cm2 was exposed in air and 18 cm2

was immersed in the solution (Fig. 21b), and the Cu electrodewas arranged in the way that half of it was immersed in thesolution and the other half was exposed to the open air. ThisTiO2/Ti disk electrode was rotated at a controlled speed, driv-en by a motor.

Such a configuration made it possible to perform threetypes of PEC modes in a single reactor. They were (1) thin-film PEC, in which only the upper half of the electrode disk in

open air was irradiated (effective illumination surface area24 cm2) and the lower part of the electrode was blocked; (2)conventional PEC, in which only the lower half of the elec-trode (effective illumination surface area 18 cm2) was exposedto irradiation and the upper half was blocked; and (3) a com-bined PEC, in which the whole disk was exposed to light(effective illumination surface area 42 cm2) and it combinedboth thin-film PEC and conventional PEC on one single elec-trode. Because thin-film PEC and conventional PEC wereswitched by rotating, this combined PEC mode was referredto as rotating disk PEC. The thin film is dynamically refreshedby rotating the photoelectrode disk, which also promotes themass transfer of the organic pollutants and the degradationproducts. This reactor is very powerful because it not onlyemploys highly effective thin-film PEC process but it alsocombines this process together with conventional PEC to si-multaneously oxidize pollutants in a single treatment system.

In the second method, Xu et al. [93] placed the anode at 60°slant angle (Fig. 22) and the wastewater was circulated by apump from the bulk solution to a reservoir. When the reactorwas filled with sample solution, the TiO2/Ti anode was ar-ranged that 21 mm in length was immersed in the solutionand the other part was exposed to the open air. In an interestingstudy, Su et al. [97] applied a nano-TiO2-modified platinumrotating ring–disk electrode (RRDE) as versatile working elec-trode for study of photoelectrocatalytic degradationmechanismof p-nitrophenol (pNP) (Fig. 23). In the mentioned work, it wasdemonstrated that the use of hydrodynamic differential pulsevoltammetry (HDPV) technique is effective for the study of thephotoelectrocatalytic degradation mechanism of pNP. By de-tecting the main electroactive intermediate product, hydroqui-none (HQ), which was produced through the direct reactionbetween highly powerful oxidant (•OH) and pNP, the conclu-sive proof of the photoelectrocatalytic degradation mechanismof pNP is along with a radical oxidation scheme. Our findingsindicate that the use of HDPV is a promising method in the insitu investigation of photoelectrocatalytic degradation mecha-nism of toxic nitrophenols.

Fig. 21 a Schematic diagram of the side view of the TiO2/Ti rotating diskPEC reactor. The figure is not to scale. 1, speed controller; 2, motor; 3,electrolytic cell; 4, TiO2/Ti rotating disk anode; 5, cathode; 6, UV lamp; 7,aluminum foil; 8, DC power supply. b The front view of the TiO2/Tirotating disk electrode. The cell is filled with sample solution

Fig. 22 Schematic diagram of the side view of the thin-film PEC reactor.The figure is not to scale. 1, TiO2/Ti anode; 2, cathode; 3, pump; 4,reaction cell; 5, reservoir; 6, UV lamp; 7, aluminum foil; 8, DC powersupply; 9, water outlet; 10, water inlet


Figure 24 shows the corresponding photoelectrochemicalHDPV response of 1 × 10−4 mol L−1 pNP containing0.1 mol L−1 H2SO4 solutions with different illumination timeor without UV illumination. The platinum-disk electrodemodified by nano-TiO2 retained the potential of +1.0 V here-in. When potential scan was carried out from +0.4 to +0.7 V,no appreciable electrochemical behavior was observed (curvea) at the platinum-ring electrode in the dark, but the responsebaselines increased gradually after 15 min (curve b–d) due tothe change of photocurrent under UV illumination.Subsequently, the anodic peak at +0.55 V was observed re-markably as a result of the formation of electroactive interme-diate species through the direct reaction betweenphotogenerated powerful oxidant (hydroxyl radicals, •OH)and pNP [98]. The obvious increase of the anodic peak cur-rents during 40 min (curve d–g) can be attributed to cumula-tive effects of intermediate product which was detected in-stantly at the platinum-ring electrode via hydrodynamic com-pulsive transport in the reaction vessel.Where after, the anodicpeak current decreased gradually and shifted to more positivepotential in 40 min (curve g–i). The results imply that the

formation of the intermediate product is made from the mix-ture of some compounds, and simultaneously the further deg-radation of this intermediate product occurs during the pro-cess. However, when the potential was swept from +0.7 to+0.4 V under the same conditions, the cathodic peak couldnot be observed clearly. Apparently, some electroactive inter-mediates which can be oxidized were formed in the abovephotoelectrocatalytic degradation process of pNP [98, 99].Therefore, the intermediate product is a mixture ofelectroactive HQ and nonelectroactive species.

Thicknesses of semiconductor film on the electrodesurface

In a TiO2 slurry system, there is an optimal amount of thephotocatalyst for organic destruction. Too little or too muchTiO2 will cause either insufficient absorption of the UV lightor scattering or screening of the UV light, both leading toinefficient reaction processes. There appears to be little inthe literature about the variation of the reaction rate with thethickness of TiO2 films. However, the thickness of a thin-filmsemiconductor would be expected to be an important param-eter in determining organic destruction efficiency, particularlywhen the light has to penetrate the film before reaching thecatalytic surface.

The maximum penetration depth of the incident light intoTiO2 (e.g., TiO2 modified on SnO2-coated glass electrode) is1/α, where α is the absorption coefficient of the TiO2 at thewavelength of the incident light (Fig. 25). Any hole generatedin the depletion layer widthWwill be efficiently transported tothe surface. Holes generated in a depth between W and(W − Lp), where Lp is the minority carrier length, which is100 nm for TiO2 and is the distance the holes move into afield-free region before recombination with an electron [100],may diffuse to the depletion layer boundary and will then alsobe efficiently transported to the surface. Absorption of radia-tion at depths >(W − Lp) will result in charge recombination[100, 101]. The internal electric field is obviously dependentupon the degree of doping and can arise from the presence ofimmobilized negative charge on the semiconductor surface, as

Fig. 23 The schematic of nano-TiO2-modified Pt rotating electrode as aworking electrode

Fig. 24 The HDPVof1 × 10−4 mol L−1 pNP solutionscontaining 0.1 mol L−1 H2SO4 atthe platinum-ring electrode withdifferent UV illumination time(a–i) 0, 5, 10, 15, 20, 30,40, 50;60 min rotation rate 1600 rpm


well as from the application of a positive bias, which is alsodepicted in Fig. 25.

Hitchman and Tian [102] investigated the experimentaleffects of the thickness of CVD thin-film anodes on the deg-radation efficiency of 4-chlorophenol (4-CP) (Fig. 26) wherethe amounts of 4-CP remaining are plotted as a function oftime. Figure 26a shows results for films deposited at 370 °Cand ranging in thickness from 133 to 1310 nm. There is anonmonotonic variation with film thickness in the amountdestroyed at any given time. This is more clearly seen inFig. 27a where the [4-CP] destroyed at 120 min is plotted as

a function of film thickness. It can be seen from Fig. 27a thatthe [4-CP] destroyed increases with the film thickness from133 to 275 nm, appears to remain effectively constant until athickness of 698 nm, and then decreases significantly for afilm thickness of 1310 nm. Clearly in this case, film thicknessinfluences PEC efficiency. For the films deposited at 400 °C,though, there is very different behavior (Figs. 26b and 27b)since there is very little variation in the amount of 4-CPdestroyed with film thickness. These results can be consideredin terms of the interaction of the light with the films. In thecase of the results shown in Fig. 26a, for the 1310-nm film,one could consider that the thickness is near the maximumpenetration depth of the incident light in which case most ofthe holes will be generated in the bulk of the film and willhardly reach the surface, resulting in a low destruction per-centage of 4-CP.

Conversely, if the film is very thin, such as 133 nm, thenlittle of the incident light will be absorbed and hence only afew holes will be generated and reach the surface. As the filmthickness increases, then provided that the depletion layerwidth W does not exceed the light penetration length of 1/α,the number of photogenerated holes should increase sincethey are within the depth of W or (W − Lp), resulting in anincrease of the PEC activity of the film. However, althoughthe degradation rate does increase with the film thickness up to275 nm (Fig. 27a), there is then no observed further changewith film thickness up to 698 nm. This could be understood bywater oxidation competing with that of 4-CP. That is, most ofthe holes are consumed in oxidation of water, and only a smallfraction of the increased holes generated will participate in the4-CP oxidation.

Heikkila et al. [103] studied atomic layer deposited TiO2 filmsfor photoelectrocatalysis of for the first time and used these filmswith different thicknesses for investigation of methylene blue

Fig. 25 Schematic of an n-type TiO2 semiconductor on SnO2-coatedglass illuminated with ultra-band gap light. The penetration depth of thelight is 1/α, where α is the absorption coefficient of the semiconductor atthe wavelength of the light; W is the depletion layer width; Lp is thedistance the minority carriers (i.e., holes in n-TiO2) move in a field-freeregion before recombination

Fig. 26 PEC degradation of 4-CP by CVD TiO2 thin-filmanodes with different thicknesses:deposition temperatures a 370and b 400 °C


(MB) degradation rate. Photoelectrochemical measurementswere performed with a three-electrode setup with Pt as a counterelectrode and 3 M Ag/AgCl as a reference electrode. All poten-tialsmentioned hereafter are related to this electrode. TiO2-coatedindium tin oxide (ITO) was used as the working electrode. Thedecrease ofMB absorbancemaximum at 665 nm as a function ofdegradation time was studied for photocatalysis (PC) andphotoelectrocatalysis (PEC). The exponential decrease was ob-served especially for PEC, which suggests first-order kinetics.According to equation −ln(A / A0) = kt, a straight line can befitted to the data and the apparent first-order rate constants k areacquired from the slope. The half-life for MB degradation wasacquired using equation T1/2 = ln2/k and was depicted for bothPC and PEC in Fig. 28. As it can be shown, PC degradation ratestays quite constant above a film thickness of 150 nm, whereasPEC degradation rate increases as a function of film thicknessuntil it saturates at approximately 350 nm. The difference be-tween PC and PEC degradations is huge, half-lives being 22 hfor PC and slightly less than 2 h for PEC.

Applied potential

It is obvious that the degradation rate of pollution increased dra-matically as the applied cell voltage increased. Ojani et al. [45]investigated the effect of bias potential ranged between 0.4 and1.4 Von photoelectrocatalytic degradation of 3,4-dichlorophenol(Fig. 29). The results demonstrated that the amount of 3,4-dichlo-rophenol degraded was increased with the increase of potential.This is due mostly to a decrease in the electron–hole recombina-tion rate. The application of positive potential across the graphiteplate-supported TiO2 photoelectrode could produce a potentialgradient inside the film that forced the photogenerated holes andelectrons to move in opposite directions. Subsequently, the con-centration of photogenerated holes (or hydroxyl radicals formedby subsequent oxidation of water) on the surface increasedwhichin turn caused the amount of 3,4-dichlorophenol degraded toincrease. Most of the photogenerated electrons were removed

Fig. 27 Photocurrents anddestruction efficiencies for PECdegradations of 4-CP withdifferent thicknesses of CVDTiO2 thin-film anodes: depositiontemperatures a 370 and b 400 °C

Fig. 28 Methylene blue degradation half-lives for photocatalysis (hollowspheres) and for photoelectrocatalysis (solid spheres)

Fig. 29 Effect of different applied potentials on degradation of 3,4-dichlorophenol. Phosphate buffer solution (pH 8.0), initial 3,4-dichlorophenol concentration 6.7 mg L−1, time experiment 40 min


either by the electric field or by reaction with dissolved oxygen.Further increasing the applied potential beyond 1.2 V leads to adecrease in degradation. This can be explained by more wateroxidized by photogenerated holes [102]. Bias potential 1.2Vwasselected as optimal bias potential for PEC degradation of 3,4-dichlorophenol solution.

Figure 30 showed the effect of applied cell voltage on thedegradation of Brilliant Orange K-R (RBOKR) byphotoelectrochemical process [104]. When the applied cell volt-age increased from 2 to 30 V, the degradation rate constants ofRBOKR increased rapidly from 0.0213 to 0.0539 min−1, whichmeans that the apparent rate constant of RBOKR degradationenhanced by 2.5-fold.

Modification methods of TiO2

Modification necessity

TiO2-based photocatalytic process has shown a great potentialas a low-cost, environmentally friendly, and sustainable treat-ment technology to remove organic pollutants in sewage to

overcome the shortcomings of the conventional technologies.However, Dong et al. [105] described the limitations in theapplication of TiO2-based particles for photocatalytic degra-dation of organic pollutants (Fig. 31). Also, they mentionedmethods to overcome these limitations. One of the importantTiO2 limitations is TiO2 band gap being width (anatase, ∼3.2 eV) compared to some semiconductors allows it to absorbonly the UV light (λ < 380 nm) which account for merely 4–5 % of the solar energy, as shown in Fig. 32, thereby hamper-ing its wide application [106].

Therefore, one of the most active fields of research in het-erogeneous photocatalysis using semiconductor particles isthe development of a system capable of using natural sunlightto degrade a large number of organic and inorganic contami-nants in wastewater [107, 108]. The overall photocatalyticactivity of a particular semiconductor system for the statedpurpose is measured by several factors including the stabilityof the semiconductor under illumination, the efficiency of thephotocatalytic process, the selectivity of the products, and thewavelength range response. For example, small band gapsemiconductors such as CdS are capable of receiving excita-tion in the visible region of the solar spectrum but are usuallyunstable and photodegrade with time [109].

The limitations of a particular semiconductor as aphotocatalyst for a particular use can be surmounted by mod-ifying the surface of the semiconductor. To date, three benefitsof modifications to photocatalytic semiconductor systemshave been studied: (1) inhibiting recombination by increasingthe charge separation and therefore the efficiency of the pho-tocatalytic process; (2) increasing the wavelength responserange (i.e., excitation of wide band gap semiconductors byvisible light); and (3) changing the selectivity or yield of aparticular product. A few examples will be given in the fol-lowing sections illustrating the large body of work conductedin the area of photocatalyst surface modification. Various at-tempts have been made to enhance the separation ofphotogenerated electron–hole pairs and extend the optical ab-sorption of TiO2 into the visible range, including doping me-tallic ions or non-metal into the TiO2 lattice, modification with

Fig. 30 Effect of applied cell voltage on the degradation of BrilliantOrange K-R (RBOKR)

Fig. 31 Limitations inapplication of TiO2-basedparticles for photocatalyticdegradation of organic pollutants


other semiconductors, dye sensitization, and noble metalloading.

Compositing with semiconductors

Coupled semiconductor photocatalysts provide an interestingway to increase the efficiency of a photoelectrocatalytic pro-cess by increasing the charge separation and extending the

energy range of photoexcitation for the system. Yu et al.[110] prepared a novel CdS/TiO2 nanorod/TiO2 nanotube ar-ray (CdS/TNR/TNT) photocatalyst. The self-organized highlyoriented TNTs were first synthesized by anodizing Ti sheets.The rutile TNRs were then grafted on the TNTs by a hydro-thermal method. Subsequently, the CdS quantum dots (CdSQDs) were deposited on the surface of the resulting TNRs/TNTs using a sequential-chemical bath deposition (S-CBD)method (Fig. 33a). Compared with TNTs, the photocurrentdensities of CdS/TNTs and CdS/TNRs/TNTs) increased 6.7-and 17-fold, respectively. This result further indicates thatCdS deposition significantly improves the photocurrent asthe electron photogenerated in CdS can rapidly migrate to-wards the TiO2 nanotubes, which effectively prevents theelectron–hole pairs from recombining (Fig. 33b). The energyof the excitation light is too small to directly excite the TiO2

portion of the photocatalyst, but it is large enough to excite anelectron from the valence band across the band gap of CdS(Eg = 2.5 eV) to the conduction band. According to this ener-getic model in Fig. 34, the hole produced in the CdS valenceband from the excitation process remains in the CdS particlewhile the electron transfers to the conduction band of the TiO2

particle. The electron transfer from CdS to TiO2 increases thecharge separation and efficiency of the photocatalytic process(Fig. 34). Recently, it was reported, fabrication ofheterojunction composites based on n-type TiO2 TNAscoupled with p-type Cu2O nanoparticles using a square wavevoltammetry deposition method (Fig. 35) [66]. The Cu2O–TNAs prepared compared to bare photocatalyst showed largerphotodegradation rate of methyl orange (88.8 %) at the ap-plied potential of 0.5 V under visible light irradiation. Theenhanced photoelectrocatalytic activity can be attributed toreducing the recombination rate of the photoexcited elec-tron–hole pairs in TNAs when coupled with Cu2Onanoparticles.

Surface sensitization

Surface sensitization of a wide band gap semiconductorphotocatalyst (TiO2) via chemisorbed or physisorbed dyes thatcan increase the efficiency of the excitation process. The pho-tosensitization process can also expand the wavelength rangeof excitation for the photocatalyst through excitation of thesensitizer followed by charge transfer to the semiconductor.Organic dyes are usually transition metal complexes with low-lying excited states, such as polypyridyl complexes, phthalo-cyanine, and metalloporphyrins. The metal centers for thedyes include Ru(II), Zn(II), Mg(II), Fe(II), and Al(III), whilethe ligands include nitrogen heterocyclic compounds withdelocalized σ or aromatic ring systems [111]. The mechanismof the photoassisted degradation of dyes under visible radia-tion follows a different pathway from that of UV radiation.Excitation of an electron in the dye molecule occurs to either

Fig. 33 a Schematic diagram of the synthesis of the CdS/TNRs/TNTsphotoanode. b J–t (a bias of −0.5 V (vs. SCE)) curves of TNTs,TNRs/TNTs, CdS/TNTs, and CdS/TNRs/TNTs under visible lightirradiation

Fig. 32 Solar spectrum at sea level with the sun at zenith


the singlet or triplet excited state of the molecule. If the oxi-dative energy level of the excited state of the dye moleculewith respect to the conduction band energy level of the semi-conductor is favorable (i.e., more negative), then the dye mol-ecule can transfer the electron to the conduction band of thesemiconductor. The surface acts as a quencher accepting anelectron from the excited dye molecule. Subsequently, elec-tron–hole recombination is reduced (Fig. 36) [112].

Wahyuningsih et al. [112] fabricated dye sensitization ofthe TiO2 electrode. Sensitization of titanium dioxide was per-formed using a dye, i.e., Fe(II)–polypyridyl complexes. Thephotoelectrocatalytic degradation of rhodamine B (RB) usingITO/TiO2/dye as electrode was investigated via a series ofpotentials, from +1.0 to −1.0 V, and at various pH and NaClconcentration values (ITO is indium tin oxide conductiveglass) under visible illumination.

Qin et al. [113] enhanced photoelectrocatalytic degradationof phenols using bifunctionalized dye (cis-Ru(dcbpy)2(NCS)2as dye) sensitized TiO2 film. The bifunctionalized TiO2 filmelectrode is composite of an area of dye-sensitized TiO2 film,electrolyte, and counter electrode, which is similar to the struc-ture of dye-sensitized solar cells (DSSCs). A schematic mech-anism for the degradation reaction is shown in Fig. 37. In dye-sensitized region, light absorption takes place in the dye underirradiation, with subsequent electron transfer from the dye’s

excited states (dye*) into the conduction band (CB) of theTiO2. The electrons transport to the degradation zone via dif-fusion of electrons through the network of TiO2 nanoparticlesand react with oxygen in solution to produce O2

•−, followedby reaction with H+ to form highly active species of hydroxylradicals (•OH). The oxidized dyes are regenerated via the re-dox couple I−/I3

−, which means that the positive charges trans-fer from the oxidized dyes (dyes+) to the redox couple. Therichness of I− would make regeneration of the oxidized dyesimmediately. The regenerated dyes producedmuchmore elec-trons, which injected into the conduction band of TiO2 underlight irradiation. Themore active species •OH could be formedin the reactor A through reaction of oxygen and electron,leading to higher degradation efficiency. On the other hand,as dyes repeat the process of light absorption and regeneration,the oxidized voltage on the counter electrode becomes higherand higher with continuous arrival of positive charges. Sincethe anode electrode is connected with the counter electrode,

Fig. 34 Schematic diagram ofthe photoelectrocatalytic processof CdS/TNRs/TNTs

Fig. 36 Scheme of photoelectrocatalytic degradation system. CE is Cufoil as counter electrode

Fig. 35 The schematic diagram of the photoelectrocatalytic mechanismof Cu2O/TiO2 TNA p-n heterojunction composite


the oxidized voltage on the anode electrode will arrive at highvalue enough for production of •OH through water oxidation.

Wang et al. [114] fabricated TiO2 nanotube arrays by an-odization and focused on immersion method synthesis ofsalicylic acid (SA)-modified TiO2 nanotube array electrode.Figure 38 displayed the surface photovoltage (SPV) spectra ofthe TiO2 nanotube arrays and SA-modified TiO2 nanotubearrays. A redshift of the excitation band edge was observed,and for SA-modified TiO2 nanotube arrays, the surfacephotovoltage signal was stronger than that of TiO2 nanotube

arrays over the whole tested wavelength range, indicating thata high photocatalytic activity could be expected for SA-modified TiO2 nanotube arrays.

The degradation efficiencies of SA-modified TiO2 nano-tube array electrode were much higher than that of the non-modified one. Under UV light irradiation, in 2 h, 100 % ofpNP was degraded by SA-modified TiO2 nanotube array elec-trode, while only 63 % of pNP was degraded by TiO2 nano-tube array electrode (Fig. 39a). Meanwhile, under visible lightirradiation, in 3 h, 100 % of pNP was degraded by SA-modified TiO2 nanotube array electrode, and in contrast,79 % of pNP was degraded by TiO2 nanotube array electrode(Fig. 39b). These results were consistent with that of SPV,which would be attributed to the improved utilization of vis-ible light and the increased amounts of surface hydroxylgroups which accepted holes generated by solar or UV illumi-nation to form hydroxyl radicals thus preventing electron–hole recombination.

Doping with metal and non-metal elements

The catalytic activity of TiO2 nanoparticles can be improvedby doping with impurities such as metals and non-metal,which can effectively inhibit the recombination of thephotogenerated electron–hole pairs during the photocatalyticreaction (Fig. 40) [68]. Rajeshwar et al. stated that this chem-ical modification may be performed either in the

Fig. 37 Possible mechanism forthe degradation reaction

Fig. 38 Surface photovoltage spectra of TiO2 nanotube arrays and SA-modified TiO2 nanotube arrays


semiconductor bulk or on its surface. They emphasize that theterm Bdoping^ should not been considered as equation withthe term metal modification [115]. They mentioned that dop-ing concept is the controlled introduction of trace-level

impurities into the host lattice that change the optical responseof semiconductor; however, in metal modification, metal par-ticles present as discontinuous islands on the semiconductorsurface. Therefore, based on their opinion, doping concept isdifferent to modification term.

Xing et al. [116] prepared lanthanide-doped TiO2 nanopar-ticle (La-TiO2NP)-modified electrode by the sol–gel and dip-coating methods on the indium tin oxide electrode (Fig. 41).They showed that the degradation efficiency of methyl orangecould be increased by 26.17 % with the La-TiO2NP-modifiedelectrode compared with that using the pure TiO2NP-modifiedelectrode under applied voltage of 0.3 V with illumination.

Cheng and Han [117] reported to prepare an N-dopedTiO2-loaded NaY zeolite membrane (N-doped TiO2/NaY ze-olite membrane). This electrode was applied for PEC degra-dation of phenol. The UV–Vis result showed that the N-dopedTiO2/NaY zeolite membrane exhibited a more obvious red-shift than that of N-TiO2 nanoparticles (Fig. 42).

Also, highly ordered boron-doped TiO2 nanotube arrayswere fabricated via a facile electrodeposition method by Liet al. [118]. The photoelectrocatalysis of phenol under simu-lated solar irradiation was performed using boron-doped orundoped TiO2 nanotube arrays and the resulting concentra-tion–reaction time curves showed noticeable differences in

Fig. 39 Variation of PNP concentrations by photoelectrocatalysis (PEC)technology with TiO2 nanotube array electrode and SA-modified TiO2

nanotube array electrode a under UV light irradiation (1.0 mW/cm2,0.8 V vs. SCE) and b under visible light (λ > 400 nm, 27 mW/cm2,0.8 V vs. SCE) irradiation

Fig. 40 Schematic of energylevel of pure TiO2 (a) and dopedTiO2 (b)

Fig. 41 XRD pattern of the La-TiO2NP-modified electrode


the removal efficiencies among these tests. Clearly, 35 % ofthe phenol was removed via direct photolysis (DP) degrada-tion after 2 h. In contrast, via PEC degradation for 2 h, 56 and66 % of the phenol were removed with undoped and boron-doped samples, respectively (Fig. 43).

Loading with metal nanoparticles

Addition of noble metals to a semiconductor surface canchange the photocatalytic process by changing the semicon-ductor surface properties. The Fermi level of some noblemetals (Au, Ag, Pt, etc.) is lower than the conduction bandof TiO2. Upon visible light illumination, the lower energy ofFermi levels of a noble metal nanoparticle can induce thetransfer of photoexcited electrons from TiO2 to the metal par-ticles, thus significantly restraining the electron–hole

recombination and lead to efficient charge separation followedby remarkably enhanced photocatalytic performance [119].

In an interesting work, Xie et al . investigatedphotoelectrocatalytic properties of Ag nanoparticle-loadedTiO2 nanotube arrays (Ag/TiO2NTs) prepared by pulse cur-rent deposition [120]. The UV and visible photocatalytic ac-tivities of Ag/TiO2NTs were investigated and compared withthose of TiO2NTs by degradation of methyl orange. Figure 44illustrates the main charge transfer processes between TiO2

and Ag nanoparticles after Ag/TiO2NTs is activated by UVlight. The electrons are excited to the CB and the holes are leftin the VB. Work-function was defined as follows [121]:

W ¼ E0 E f ð20Þ

where E0 and Ef are vacuum energy and Fermi energy, respec-tively. Since the work-function of TiO2 is lower than that ofmetallic Ag, the Ef of Ag is lower than that of TiO2. TheSchottky barriers are formed between Ag nanoparticles andTiO2 nanotubes after Ag nanoparticles are loaded on the TiO2

surface [122, 123]. The photogenerated electrons facilelytransfer from the conduction band of TiO2 to Ag (as thedotted arrows shown in Fig. 44). Therefore, the loaded Agnanoparticles could efficiently separate photogenerated elec-trons and holes and reduce their recombination. In the photo-catalytic process, the photogenerated electrons could be trans-ferred to surface-absorbed oxygen rapidly to form activatedO2

•− (as the dotted arrows shown in Fig. 44). The activated O2•

− further produces •OH via a series of reaction with H+. Thisstep is the photoreduction process. The photogenerated holescould also react with H2O to give rise to •OH radicals, which is

Fig. 44 Schematic diagram of the interface charge carrier transfer ofphotocatalysis (dotted arrows) and photoelectrocatalysis (solid arrows)for Ag/TiO2NTs under UV light irradiation

Fig. 43 Concentration variation of phenol in direct photolysis (DP) andphotoelectrocatalysis (PEC) degradation

Fig. 42 UV–Vis spectra of TiO2, N-TiO2, and N-doped TiO2/NaY zeo-lite membrane


the photooxidation process. The photoreduction and photoox-idation steps both generate •OH which is responsible for thedegradation of pollutant. The photocatalytic reaction undervisible light irradiation can be shown as follows:

Agþ e−→Agþ ð21Þe− þ TiO2 CBð Þ→TiO2 eCB

−ð Þ ð22ÞTiO2 eCB

−ð Þ þ O2→O2∙− ð23Þ

Figure 45 illustrates the plasma-induced photo-electrochemistry on the Ag/TiO2NT system. As a kind of vis-ible light-sensitive photocatalyst, Ag/TiO2 can absorb visiblelight due to surface plasma resonance of Ag [124]. Whenirradiated by visible light, the Ag nanoparticles are activatedinto excited state, a higher energy level than ECB of TiO2.Then the photoexcited electrons are injected into the conduc-tion band of the TiO2 semiconductor, while Ag is converted tothe Ag+. In turn, the injected electrons on the TiO2NTs reactwith adsorbed molecular oxygen to produce O2

•−, which ef-fectively degrades the dye molecules to various products.However, the visible PC activities of Ag/TiO2NTs are lowerthan its UV PC activities. Further studies on the enhancementof visible light activity for TiO2NTs are being carried out inour laboratory at present.

Gu et al. [125] fabricated TiO2NTs loaded with Au nano-particles as an electrode for enhanced photoelectrocatalyticactivity towards partially hydrolyzed polyacrylamide(HPAM) degradation. The photocurrent response of TiO2

NTs and Au/TiO2NTs was investigated for further evaluationof the enhancement of PEC performance (Fig. 46). The re-duced recombination of photogenerated charges in Au/

TiO2NTs would make them good photocatalysts for organicpollutant degradation.

Photoelectrocatalysis without using semiconductors

In most studies about photoelectrocatalysis process, semicon-ductors were used as photocatalyst. However, photocatalystsexcept semiconductors were applied for photoelectrocatalyticaims [126, 127]. Ramhirez et al. prepared [126] a supramo-lecular electrode made of stacked CoII(tetrabenzoporphyrin).The stack was held together by π–π interactions, and the firstlayer of the porphyrin complex was anchored to the glassycarbon surface through the porphyrin’s 4-aminopyridinegroup. An electrode formed by CoII(tetrabenzoporphyrin)adsorbed on the glassy carbon surface was inactive towardsthe reduction of carbon dioxide. In contrast to this electrode,the supramolecular electrode exhibited good electrocatalyticactivity towards the same reaction. Illumination of the supra-molecular electrode enhanced its electrocatalytic activity to-wards the reduction of carbon dioxide.

For excitation of the complex, the wavelengths chosenwere 420, 525, and 625 nm, in the spectrum of the porphyrincomplex (Fig. 47). When 625 nm light was used on the elec-trode, it caused a significant shift in the wave of the CO2

reduction towards more positive values and no such a dis-placement of the wave occurred when the electrode was ex-posed to 420 or 525 nm light (Fig. 48). Since the absorptivitiesof the porphyrin complex are large at all the irradiation wave-lengths, it is not possible to relate the lack of the effect atλexc = 420 or 525 nm to the absorption of light by the com-plex. It is reasonable to assume that different excited states areproduced when the light absorbed by the compound has men-tioned wavelengths. It must also be considered that differentrelaxation paths will be available to the pp singlet excited

Fig. 45 Schematic diagram of the interface charge carrier transfer ofphotocatalysis for Ag/TiO2NTs under visible light irradiation

Fig. 46 Amperometric I–t curves of (a) pristine TiO2NTs and (b) Au/TiO2NTs (1.24 wt%) at an applied potential of 0.5 V under UV lightirradiation. The potential of the working electrode was set at 0.5 V vs.Ag/AgCl electrode


states produced in each case. For example, irradiation at620 nm could form the lowest lying porphyrin-centered ππ*triplet state, while the excited states produced by irradiating atwavelengths of the 420 nm band could relax into inactive Co-centered, d-d excited states.

In an interesting study by Dilgin et al. [127], the MB onzirconium phosphate (ZrP) was used as a modifier material incarbon paste electrode (CPE). The immobilization of MB insalt form on ZrP may also be explained as an exchange pro-cess represented by the following reaction:

ZrP− þ Hþð Þ þ MBþ þ Cl−ð Þ→ZrPMBþ HCl ð24Þ

This electrode has been applied for determination of ascor-bic acid in a flow injection (FI) system by photocatalytic pro-cess using irradiation from a 100-W halogen lamp. Figure 49shows cyclic voltammograms recorded in solution pH 7.00containing 2.5 mM AA for the bar CPE and modified CPE(MCPE). A broad oxidation peak of AAwas observed with a

peak at +490 mV s−1 (Fig. 49b), whereas for the modifiedCPE, two main waves are shown with the peak potentialsaround −160 and +110 mV (Fig. 49d). The peak potentialsof these two catalytic oxidation peaks reflect the existence ofseveral forms of MB to ZrP. The difference between the an-odic peak currents in the absence and presence of AA for themain voltammetric wave is not as high as for the less visiblebut more positive wave (at around +110 mV) reflecting theinfluence of the thermodynamic driving force on the efficien-cy of the electrocatalytic reaction. When the surface electrodewas irradiated using a 100-W lamp, a much further increase ofthe new anodic peak at +110mVwas observed in the presenceof 2.5 mM AA (Fig. 49e). The future increase can be ex-plained by photoexcitation of MB on the electrode surface,and then the excited MB can more easily react with AA asshown below:

MBþ hν→MB* photoexcitation ð25ÞMB* þ AA→L�MBþ DHAA chemical reaction ð26ÞL�MB⇄MBþ 2e electrode reaction ð27Þ

Photoelectrocatalysis application in determination

Many research reports about TiO2 are related to its applicationfor degradation of most kinds of organic pollutants, such as

Fig. 49 Cyclic voltammograms for (a) unmodified CPE, (b) 2.5 mMAAat unmodified CPE, (c) MCPE (MB-ZrP), (d) 2.5 mMAA at MCPE, and(e) 2.5 mMAA at MCPE with irradiation by using 100 W halogen lamp,scan rate 100 mVs−1, supporting electrolyte pH 7.00, phosphate buffercontaining 0.5 M KCl

Fig. 47 UV–Vis spectrum of 5 × 10−5 M CoII(tetrabenzoporphyrin) indimethylformamide (DMF)

Fig. 48 Cyclic voltammograms recorded at 5 mV s−1 showing theresponse of the supramolecular electrode towards the reduction of CO2

with and without illumination at different wavelengths


detergents, dyes, pesticides, herbicides, etc., under UV lightirradiation [128–131]. Only in a few articles thatphotoelectrocatalytic determination of some materials withelectrodes has been investigated. Recently, a novelphotoelectrochemical immunosensor based on TiO2/CdShybrid-modified electrode was developed [132]. The TiO2/CdS hybrid-modified electrode was obtained by alternatelydipping the TiO2-modified ITO electrode into the[Cd(NH3)4]

2+ and S2− solution repeatedly (Fig. 50). As canbe seen in Fig. 50, after the ITO/TiO2/CdS electrode was coat-ed with chitosan (CS), α-fetoprotein (AFP) antibodies werecovalently conjugated on the surface of the electrode. Thus, alabel-free photoelectrochemical immunosensor for the detec-tion of AFP was developed by monitoring the changes in thephotocurrent signals of the electrode resulting from theimmunoreactions (Fig. 51). The immunosensor displayed alinear response to AFP in the ranges from 50 pg mL−1 to50 ng mL−1 with a relatively low detection limit of40 pg mL−1. The photoelectrochemical results for the detec-tion of AFP in five human sera showed acceptable accuracy.

By other researcher groups, tin oxide nanoparticle elec-trode for selective photoelectrochemical detection of DNA

[133] and TiO2/PbO2/Ti electrode to determine the chemicaloxygen demand (COD) values [134] have been applied. Moet al. developed a three-dimensional (3-D) structured sensor ofmicrocrystallineβ-PbO2-coated carbon felt (CF) consisting ofcrisscrossing carbon fibers to determine COD in wastewater[135]. Each carbon fiber in the felt is coated by perfect octa-hedral β-PbO2 microcrystals (Fig. 52). The unique structuremakes it possess huge surface area and abundant active sitesfor generating hydroxyl radicals, thus exhibiting excellent per-formances. Eleven representative organic compounds classi-fied into three types were selected as the standard samples todraw a calibration curve. The first type was the chemicalsoften used for COD analysis: glucose, potassium hydrogenphthalate (KHP), xylose, and glutamic acid; the second typewas the pollutants usually exiting in chemical or pharmaceu-tical wastewater: hydroquinone, pNP, p-hydroxybenzoic acid,cysteine, and lactic acid; and the third type was intermediateproducts during electrolysis of organics: oxalic acid and aceticacid, but their sodium salts were used for the present studybecause the stability in concentration was taken into account(Fig. 53). The calibration plot of Iresp (Isignal − Iblank) andCODTh (CODTh is theoretical value of COD calculated on

Fig. 50 Schematic diagram ofthe immunosensor constructionprocess

Fig. 51 a Electrochemicalimpedance Nyquist plot and bphotocurrent response of themodified ITO electrodes: (a)TiO2/CdS, (b) TiO2/CdS/CS, (c)TiO2/CdS/CS/anti-AFP, (d) TiO2/CdS/CS/anti-AFP/BSA, and theTiO2/CdS/CS/anti-AFP/BSAelectrode after incubation with (e)100 pg/mL AFP and ( f ) 5.0 ng/mL AFP


the basis of organic compound concentration) of these stan-dard samples was drawn as follows: various concentrations ofeach sample were measured separately. All values of the 11samples were drawn in a plot and regressed into a calibrationcurve through curve fitting. The result is given in Fig. 53. Theregression line in this figure demonstrates a good linearityover the range of 50–5000 mg L−1.

Also, the modified carbon paste electrode was preparedby adsorption of MB on zirconiumphosphate [127] andmuscovite [136]. These modified electrodes were success-fully used for the photoelectrocatalytic oxidation of theascorbic acid in a flow injection analysis (FIA) system.

Moreover, Xu et al. [137] reported that the dopamine co-ordinated nanoporous TiO2 film electrode covered on anITO electrode showed to be a new photoelectrochemicalmethodology for sensitive NADH determination. Also,other research groups have been interested in thephotoelectrocatalytic oxidation of NADH using a newpolymeric phenothiazine-modified graphite electrode[138], poly(toluidine blue O)-modified glassy carbonelectrode [139], poly-hematoxylin-modified glassy carbonelectrode [140], and poly(neutral red)-modified glassycarbon electrode [141] since the photoelectrocatalyticmethod has better sensitivity than the electrocatalyticmethod for the determinat ion of NADH. Also,electropolymerized methylene blue-modified glassy car-bon electrode [142] has been used for photoelectrocatalyticdetermination of NADH (Fig. 54). This figure shows the cur-r e n t – t i m e c u r v e s f o r t h e amp e r om e t r i c a n dphotoamperometric FIA responses to various concentrationsof NADH. Although the peak current was increased, depend-ing on NADH concentration for both the amperometric andthe photoamperometric methods, the responses of thephotoamperometric method were higher than that of the am-perometric in all NADH concentrations. Zhang et al. [143]developed a photoelectrochemical detector for determinationof glucose and sucrose using FIA and high-performance liquidchromatography (HPLC) based on the oxidation power ofnanostructured TiO2 coated onto the ITO substrate.

Ojani et al. reported the photoelectrocatalytic oxidationusing TiO2/Ti foil electrode and application of this elec-trode for formaldehyde determination as a novel and sim-ple method [144]. The experiments were carried out in thephosphate buffer solution (pH 7.0) using the three-electrode undivided quartz cell at Eanod = 0.8 V vs. Ag/

Fig. 52 A schematic diagram ofthe unique structure of CF/β-PbO2 electrode

Fig. 53 Calibration plot of Iresp and CODTh of standard samples inelectrochemical cell containing a corresponding amount of (1) sodiumacetate, (2) lactic acid, (3) sodium oxalate, (4) glucose, (5) xylose, (6)hydroquinone, (7) cysteine, (8) glutamic acid, (9) p-hydroxybenzoic acid,(10) p-nitrophenol, and (11) potassium hydrogen phthalate for CF/β-PbO2 electrode at 1.45 V


AgCl/KCl (3 mol L−1) as shown in Fig. 55. Also, in otherwork , Ojan i and Zare i [145] us ing the abovephotoelectrochemical cell and similar conditions deter-mined hydrazine.

Conclusion and outlook

Efficient methods for destroying synthetic organic com-pounds from wastewaters at lab and pilot plant scale havebeen recently developed from the electrochemical tech-nology. The most suitable electrode materials and opti-mized experimental conditions have been established inmost electrochemical techniques for different cell config-urations. Electrochemical methods have demonstrated to

be an effective and economical technology for almost to-tal degradat ion of pollutants from wastewaters .Photoelectrocatalysis with generation of holes and hetero-geneous •OH as oxidants at the surface of a TiO2-basedthin-film photoanode under UV irradiation has higherability for dye destruction than photocatalysis, electro-chemical oxidation, and direct photolysis. Also,nonsemiconductor substances have been used forphotoelec t rocata ly t ic a ims. On the other hand,photoelectrocatalysis as a successful and new methodhas been applied for the determination of analytes. Thiswork has presented the recent progress of fundamentals,modification, and application of electrochemically anod-ized TiO2 materials in photoelectrocatalysis and variouseffective factors on its efficiency. Moreover, for the first

Fig. 54 Current–time curves of injected NADH solution with differentconcentrations (a) 0.1, (b) 1, (c) 10, (d) 40, (e) 100, ( f ) 400, and (g)1000 μM, using poly-MB/GCE in FIA system. Experimentalconditions: applied potential, +150 mV vs. Ag/AgCl, carrier stream,0.1 M phosphate buffer containing 0.1 M KCl (pH 7.0); flow rate,

1.3 mL min−1; sample loop, 100 μL; transmission tubing length, 10 cm;injected NADH solution, 0.1 mM. Inset shows the catalytic current curvevs. the concentration of injected NADH solution for (a) amperometricand (b) photoamperometric methods


time, in this review, photoelectrocatalysis application indetermination has been described. It should be mentionedT i O 2 n a n o t u b e a r r a y s a r e w i d e l y u s e d i nphotoelectrocatalytic degradation of pollutants due to ex-cellent physical and chemical properties. However, exten-sive challenges on fabrication of high-quality TiO2 nano-tube arrays continued. On the one hand, it is urgent toseek new modification method to improve the transferefficiency of photocarriers and suppress the recombina-tion of electron and holes. Since applied bias potentialcan significantly facilitate the transfer of photocarriersand suppress the recombination of photogenerated elec-trons and holes, photoelectrocatalysis showed much betterphotoelectric activity than bare photocatalysis due to thesynergetic effect of light irradiation and external electricfield. Suffered from serious environmental problems,photoelectrocatalysis for degradation of pollutants showedpromising prospects to solve these problems in the future.Also, the alternative use of inexpensive and renewablesunlight in photoelectrocatalysis should also be

investigated to make much more attractive photoassistedprocesses in practice.

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