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Applied Catalysis A: General 466 (2013) 153– 160

Contents lists available at SciVerse ScienceDirect

Applied Catalysis A: General

j ourna l h om epage: www.elsev ier .com/ locate /apcata

ynergism of oxygen vacancy and carbonaceous species on enhancedhotocatalytic activity of electrospun ZnO-carbon nanofibers: Chargearrier scavengers mechanism

orasae Samadia, Hossein Asghari Shivaeea, Ali Pourjavadia,b, Alireza Z. Moshfegha,c,∗

Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 11365-8639, Tehran, IranDepartment of Chemistry, Sharif University of Technology, P.O. Box 11555-9516, Tehran, IranDepartment of Physics, Sharif University of Technology, P.O. Box 11555-9161, Tehran, Iran

a r t i c l e i n f o

rticle history:eceived 19 March 2013eceived in revised form 22 May 2013ccepted 18 June 2013vailable online xxx

eywords:

a b s t r a c t

Novel ZnO-carbon and ZnO nanofibers were fabricated by electrospinning of polymer precursor followedby subsequent annealing in nitrogen and air, respectively. Field emission scanning electron microscopy(FESEM) and X-ray diffraction (XRD) indicated the smooth and beadless nanofibers with wurtzite crystalstructure. X-ray photoelectron spectroscopy (XPS) showed the presence of oxygen vacancies (VO) andchemisorbed O2 on the surface of the samples. Band gap narrowing of the ZnO-carbon nanofibers in com-parison to ZnO were measured by diffuse reflectance spectroscopy (DRS). Photo-degradation of azo dye

lectron–hole recombinationarbon doped ZnOn O Clectrospinninghotosensitization

under the UV and visible light was evaluated and ZnO-carbon showed an enhancement in photocatalyticactivity due to the presence of carbonaceous species and oxygen vacancies. Formation of hydroxyl radi-cal during the photocatalytic process was verified by the formation of carbon using photoluminescence(PL) analysis. The photocatalytic mechanism was investigated by measuring the degradation rate in thepresence of tert-butyl alcohol (t-BuOH) and I− anion as •OH radical and hole (h+) scavenger. The resultsindicated photo-generated electrons are the main photo-oxidation pathway.

. Introduction

Water purification from various kinds of chemical and biologi-al contaminations is a vital issue for human beings [1]. There areany wastewater treatment methods, and among them heteroge-

eous photocatalytic process using nanomaterials has received aonsiderable attention because of its low-cost and inert nature ofhe catalyst [2]. It is well known that the photocatalytic reaction onhe surface of a semiconductor is initiated by electron and hole gen-ration that produce hydroxyl (•OH) and superoxide (•O2

−) radicalsor decomposition of organic pollutant in water [3].

Zinc oxide (ZnO) is a n-type semiconductor with a wide band gapnd large exciton binding energy [4]. To date, it has been demon-trated that ZnO nanomaterials have better performance in the

hotocatalytic degradation of dyes as compared to TiO2 [5–7] and

t has been practically applied for purification of wastewater. Theain issues in the photocatalytic property of ZnO are the efficiency

∗ Corresponding author at: Department of Physics and Institute for Nanosciencend Nanotechnology, Sharif University of Technology, P.O. Box 11365-8639, Tehran,ran. Tel.: +98 21 6616 4516; fax: +98 21 6601 2983.

E-mail addresses: Samadi@mehr.sharif.edu (M. Samadi),hivaee@alum.sharif.edu (H.A. Shivaee),urjavad@sharif.edu (A. Pourjavadi), moshfegh@sharif.edu (A.Z. Moshfegh).

926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2013.06.024

© 2013 Elsevier B.V. All rights reserved.

improvement and the activity under the visible light. To achievethese goals, two main approaches must be considered: (a) theband gap narrowing to maximize and control photons absorption inthe visible region by ZnO and (b) retardation in the electron–holerecombination rate which enhances the photocatalyst efficiency.Recently, Rehman et al. [4] have reviewed various techniques tomodify electronic and optical properties of ZnO for efficient waterand air purification under visible light irradiation. In our recentwork, we prepared and applied MWCNT-doped ZnO nanofiberswhich fulfilled these two challenges [8].

It has been reported that the existence of defects in a ZnO crystalleads to electrical and optical changes [9] and among these defects,oxygen vacancies (VO) are known to be effective in the photocat-alytic degradation [10]. Schmidt-Mende and MacManus-Driscoll[11] calculated the defect energy levels in the ZnO and demon-strated that oxygen vacancy defects induce new donor energylevels below its conduction band. The creation of electron in thedonor state due to the oxygen vacancy was explained by the fol-lowing reaction [12–14]:

V → V 2+ + 2e− (1)

O O

These defect states tuned the band gap of ZnO and lead to thephotocatalytic activity under visible light. Recently, researchershave focused their attention to the creation of oxygen vacancy

1 ysis A:

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54 M. Samadi et al. / Applied Catal

o improve photocatalytic degradation of toxic waste on the ZnOanomaterials using different techniques [15–19]. Recently Bay-ti et al. [20] have considered the creation of oxygen vacancies inhe TiO2 film through annealing under the oxygen-poor condition.hey studied the relationship between the improvement of photo-atalytic performance and the amount of oxygen vacancies in theamples.

To date, it has been proven that carbonous materials in the formf CNT, activated carbon, amorphous carbon, and atomic carbonn the metal oxides influence their photocatalytic properties. Inecent years, ZnO-carbon nanocomposites have been fabricated byifferent methods in order to enhance the photocatalytic activity

n comparison to pure ZnO [5,6,21]. The higher activity of theseanocomposites is due to the introduction of new empty energytate of carbon in the band gap of ZnO. These created energy lev-ls accommodate photoelectrons in the photocatalytic process.herefore, it causes electron–hole separation and increases thehotocatalyst efficiency [22–25]. Valentin et al. [26] investigatedifferent structural models of carbon species into the TiO2 latticesing Densiy Function Theory (DFT) and studied the relative stabil-

ty of them as a function of oxygen pressure during synthesis. Theyoncluded, under oxygen-poor condition, carbon prefers to bind toxygen ions and C O Ti bonds are energetically more favorable.he carbon species are stabilized by excess electrons brought by aattice oxygen vacancy.

Here, we introduced electrospinning technique [27] as a facilend inexpensive method for the fabrication of novel ZnO-carbonanofibers. It is known that annealing of a polymer in nitrogentmosphere lead to the carbonization and creation of carbon inhe sample [22]. Polyvinyl alcohol and zinc acetate fibers werennealed in nitrogen and air to prepare ZnO-carbon and ZnO,espectively. The aim of this study is to clarify the effect of botharbonaceous species and oxygen vacancies presence on the photo-egradation of methylene blue (MB) as a test substance under bothV and visible illumination. The relationships between PL, DRS,nd XPS results and oxygen vacancy as well as carbon contentere discussed. The •OH radical production was detected by 2-ydroxyterephthalic acid formation as a fluorescent product. To theest of our knowledge, this work is the first photocatalytic mech-nistic study of the ZnO-carbon nanocomposites using •OH radicalnd hole (h+) scavengers. A possible mechanism of dye degradationccording to the role of carbon and oxygen vacancies was proposed.

. Experimental methods

Nanofibers were prepared by electrospinning method. Torepare polymer precursor, 1 g of polyvinyl alcohol (PVAW = 740,000) was dissolved in 12 mL deionized water and stirred

or 1 h at 80 ◦C in an oil bath. Then 2 g zinc acetate dihydrate wasdded to the polymer solution with constant stirring for 2 h at0 ◦C to obtain a homogenous solution. The precursor solution was

oaded into a plastic syringe with a 0.7 mm diameter stainless steeleedle positioned in horizontal direction respect to collector. Theistance and the voltage between the needle and the collector were2 cm and 8.5 kV, respectively. The optimum flow rate was selectedt 0.4 mL/h. The as-spun fibers were annealed using a tube furnacet 460 ◦C for 150 min in nitrogen to obtain ZnO-carbon nanofibers.n order to comparison, ZnO nanofibers were also prepared bynnealing the as-spun fibers in the air.

The surface morphologies of fibers were characterized byESEM (S4160 Hitachi). Phase identification and the crystallinity

f nanofibers were investigated by XRD (PW 3710 Philips) with Cu� radiation source (� = 1.54056 A). Thermal Garavimetry Analysis

TGA) and Differential Thermal Analysis (DTA) (Rheometric Scien-ific STA1500) were used to determine the thermal behavior of the

General 466 (2013) 153– 160

as-spun fibers. Measurements were conducted from 100 to 600 ◦Cwith the rate of 10 ◦C/min in both air and nitrogen atmosphere. Thephase and crystallinity of the nanofibers were characterized usingXRD (PW 3710 Philips) with Cu Ka radiation (� = 1.54056 A). DRS ofthe samples were recorded by spectrometer (Ava Spec-2048TEC).XPS equipped with an Al K� X-ray energy source of 1486.6 eV wasemployed at lower pressures than 10−7 Pa to investigate the sur-face chemical composition of the nanofibers. The C (1s) core levelline was fixed at the binding energy of 285.0 eV for energy cali-bration of the other peaks. The peak deconvolutions were carriedout using SDP software with Gaussian (80%)–Lorentzian (20%). Pho-toluminescence spectroscopy (Varian Cary Eclipse FluorescenceSpectrophotometer) was used to study electrons and holes recom-bination process.

The photocatalytic activity of pure ZnO and ZnO-carbonnanofibers were examined using MB degradation in aqueous solu-tion under both UV and visible light irradiation. The experimentswere carried out in 50 mL beaker charged with 15 mL of 10−5 M MBand 6 mg of nanofibers. Prior to irradiation, the mixture solutionwas kept in a dark place for 60 min to obtain adsorption–desorptionequilibrium. The light source illuminated to the MB solution andthen a few milliliters of solution were drawn from the reac-tion mixture by syringe at a constant time interval and loadedin a UV–vis spectrophotometer (Perkin Eelmer Lambda 950). Thephoto-degradation of MB under the UV irradiation was carried outby UV lamp (Philips 15 W) that was placed at about 130 mm abovethe dye solution. Solar simulator (Luzchem’s SolSim) with a poly-carbonate filter (for elimination of any UV irradiation) was used asa visible light source by the power of 80 lux and the source-sampledistance of 180 mm.

Formation of •OH radical on the sample surface under UV irradi-ation was investigated by fluorescence technique. The investigationwas carried out by using terephthalic acid (TA) as a probe moleculethat readily react with •OH radicals to produce a highly fluorescentproduct of 2-hydroxyterephthalic acid (TAOH). For this experiment,6 mg of photocatalyst added to 15 mL of 5 × 10−4 M TA solution thatwas prepared in a diluted NaOH aqueous solution with a concen-tration of 2 × 10−3 M. A 15 W UV lamp (Philips) fixed at a distance of130 mm above the surface solution, was used as a UV light source.The PL spectra were measured by sampling in every 10 min illumi-nation.

3. Results and discussion

3.1. Structure and morphology of ZnO-carbon nanofibers

The FESEM images of the as-spun, ZnO and ZnO-carbon fibersare shown in Fig. 1a–c. The surface of the nanofibers is smoothand beadless with several centimeters long. As depicted in theinset of Fig. 1b and c, although the ZnO nanofibers are completelywhite, ZnO-carbon nanofibers are black due to the presence of car-bonaceous species. According to the fiber diameter distributionhistogram shown in Fig. 1d, the as-spun nanofibers have the aver-age size of 320 nm. After annealing, the average size of the ZnOnanofibers reduced to 140 nm (Fig. 1e) due to the decomposition ofPVA and acetate group of the as-spun fibers. The average diameterof the ZnO-carbon nanofibers was about 200 nm (Fig. 1f), which isgreater than the ZnO nanofibers due to the existence of carbon.

Thermal behavior of as-spun nanofibers was studied using TGAunder the air and nitrogen atmosphere. As depicted in Fig. 2, theweight loss occurred up to ∼460 ◦C and after that the plot became

plateau which shows most of the organic compound are decom-posed at this temperature. TGA curves revealed three steps ofweight loss; dehydration, decomposition of the acetate group andPVA, respectively. DTA curve of the as-spun samples annealed in

M. Samadi et al. / Applied Catalysis A: General 466 (2013) 153– 160 155

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generated. Carbon is a small size atom and in the interstitial position

ig. 1. FESEM images of different nanofibers and corresponding histograms: as-sphotos.

itrogen shows an exothermic peak at about 270 ◦C for the decom-osition of acetate group and the formation of ZnO crystal inonsistent with TGA results; the peaks at 370 and 450 ◦C belong tohe thermal decomposition of the side chain and main chain of theVA, respectively. Comparison of the TGA plots in air and nitrogenn Fig. 2 clarified that the total weight loss at 460 ◦C are 41.3% and5.0% under nitrogen and air, respectively. The difference betweenhe remaining weights is due to the carbon contents in the ZnO-arbon nanofibers as proposed by Zhang et al. [5] for ZnO-carbonanoparticles. Similar TGA and DTA results were also reported [28].

Crystal structure of ZnO and ZnO-carbon nanofibers were deter-ined by XRD technique, as shown in Fig. 3. The XRD patterns of

oth nanofibers were indexed as a hexagonal wurtzite structure

rom (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) planes.he averaged particle sizes were estimated from the Scherrer equa-ion as ∼154 A for both ZnO and ZnO-carbon nanofibers using the

ore intense (1 0 1) peak. In addition, there is no change in the

ig. 2. TGA of the as-spun nanofibers in nitrogen and air. DTA curve is obtained initrogen atmosphere.

d), ZnO (b, e), and ZnO-carbon (c, f) nanofibers. Insets show corresponding optical

crystalline lattice parameter of ZnO for both samples. It is worthto note that the diffraction peaks of both samples are sharp andintense without the characteristic peak of impurity. There are threekinds of carbon doping in the ZnO crystal lattice as reported bySakong and Kratzer [29]. C substitutes for oxygen, zinc, or at aninterstitial lattice site. For the first state (C substitutes for oxy-gen) the Zn C and in the second state (C substitutes for zinc) theZn O C bond formed in the ZnO crystal lattice. They showed thatcarbon substituting for zinc or oxygen in the ZnO lattice caused thedisplacements of the neighboring atoms and changed the latticeconstants. In the third state (C in an interstitial site) interstitial Cformed a covalent bond with neighboring O and carbonate species

induces no strain in the crystal lattice [26,29]. In the XRD resultsof the ZnO and ZnO-carbon, there were no changes in the lattice

Fig. 3. XRD patterns of ZnO and ZnO-carbon nanofibers.

156 M. Samadi et al. / Applied Catalysis A: General 466 (2013) 153– 160

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ig. 4. The C (1s) core level of ZnO-carbon nanofibers (a), the O (1s) core level photd) and ZnO-carbon nanofibers (e) and insets show corresponding deconvolution.

arameters and peak positions; therefore carbon is in an intersti-ial position and bond with neighboring O in the ZnO crystal latticend C O Zn bond formed.

XPS technique was employed to investigate the surface chemicalomposition of the nanofibers. The wide survey scan of XPS spectranot shown) indicated just the peaks corresponding to Zn, O and Cithout any impurities in both ZnO and ZnO-carbon nanofibers. Theigh resolution XPS spectrum of the C (1s), the O (1s) and the Zn (2p)egions are shown in Fig. 4. Deconvolution of the C (1s) core leveln Fig. 4a exhibited two dominant peaks at 284.6 and 287.8 eV forhe ZnO-carbon nanofibers. But, there was only one single peak at84.6 eV for ZnO nanofibers (not shown). The C (1s) peak at 284.6 eV

s due to the presence of adventitious elemental carbon and C Cond and the peak at 287.8 eV is assigned to the C O Zn bond,s reported by other researchers [30,31]. There was no XPS signalround 281 eV corresponding to Zn C bond for C substitute O in therystal structure of the ZnO-carbon nanofibers. The presence of the

O Zn bond is attributed to the interaction between PVA and zinccetate in the precursor solution and resultant ZnO-carbon productFig. 5). As proposed by He et al. [32] the Zn+2 ions of zinc acetatenteract with OH in PVA and after annealing these ions transfer tohe ZnO structure followed by polymer conversion to carbon.

The XPS spectra of the O (1s) core level of the pure ZnO andnO-carbon nanofibers are shown in Fig 4b and c. The peak decon-olution shows three different peaks in both samples; first peakentered at 530.3 eV is associated to the Zn O bond in the ZnOurtzite structure. The second peak at 531.9 eV is attributed to the

xygen vacancies on the surface of the samples. The third peakt 533.2 eV is attributed to the adsorbed or chemisorbed O2, H2Or C O Zn bond. Similar XPS results were also reported by otheresearchers [12,31,33,34]. Comparison of the area under the decon-oluted peaks shows the amount of oxygen vacancies, C O Zn

ond and chemisorbed O2 in the ZnO-carbon is higher than ZnOanofibers. The oxygen vacancies are an active site and caused O2dsorption on the surface of the ZnO-carbon, similar observationas also reported [7,35,36].

ron spectra of ZnO (b) and ZnO-carbon nanofibers (c), the Zn (2p) core level of ZnO

Fig. 4d and e shows the Zn (2p) region of the ZnO and ZnO-carbonnanofibers, respectively. The binding energies of the Zn (2p3/2) andZn (2p1/2) in both samples are at about 1021.9 eV and 1045.1 eV,respectively. The Zn (2p3/2) core level spectrum in ZnO-carbon canbe resolved by using two peaks while the same spectrum in the ZnOfits only to one peak. The peak at lower binding energy of 1021.9 eVis believed to be the Zn2+ in ZnO wurtzite structure, while the peakat a higher energy of 1023.7 eV is attributed to C O Zn bond. Thesecond peak represent in the Zn (2p3/2) deconvoluted peak the for-mation of carbonaceous species on the surface of the ZnO-carbonnanofibers.

Therefore, the XPS data of the O (1s) core level are in good agree-ment with analysis of the Zn (2p3/2) and C (1s) core levels spectra,and show the interaction between carbon and ZnO and the presenceof the oxygen vacancies in the ZnO-carbon. The role of these oxy-gen vacancies will be discussed in the photocatalytic degradationof synthesized nanofibers (Section 3.2).

Optical properties of the ZnO and ZnO-carbon nanofibers weremeasured by UV–vis diffuse reflectance spectroscopy as shown inFig. 6. The inset of the figure shows the UV–vis absorption spectra ofthe ZnO and ZnO-carbon nanofibers. Although the ZnO nanofibershave no significant absorbance in the visible light, the ZnO-carbonnanofibers exhibit a higher absorbance in both UV and visibleregions. Similar behavior was also reported by other researcher forZnO-carbon nanoparticles [5]. Comparing the both spectra showsthe absorption edge in the ZnO-carbon nanofibers shifts toward thelower energy. The optical band gap energy of the nanofibers is cal-culated by using the optical absorption edge obtained from UV–visDRS spectra and the Tauc model [34]:

(˛hv)2 = A(hv − Eg) (2)

where is the absorption coefficient, h� is the photon energy, A is

a constant and Eg is the band gap. Fig. 6 shows the plot of (˛h�)2

vs. h� using the optical absorption data where the extrapolatedintercept gives the corresponding Eg. The band gap energies wereestimated about 3.1 and 2.8 eV for ZnO and ZnO-carbon nanofibers,

M. Samadi et al. / Applied Catalysis A: General 466 (2013) 153– 160 157

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Fig. 5. Schematic of the ZnO-carbon nanofib

espectively. This result obviously reveals that the band gap of thenO-carbon is narrower than the ZnO nanofibers. The band gapeduction is consistent with the corresponding XPS results wherehows the ZnO-carbon nanofibers have a larger amount of oxygenacancy as compared to ZnO that causes band gap narrowing. Asentioned above, oxygen vacancy induces new energy state under

he conduction band and causes the band gap reduction in the ZnO-arbon [10,15,17]. The relevance between oxygen vacancies andand gap narrowing were also represented by other researchers innO and TiO2 [19,20,34].

.2. The kinetics and mechanism of MB degradation under the UVnd visible light

The photocatalytic activity of the ZnO and ZnO-carbonanofibers was evaluated by measuring the degradation rate of MBqueous solution under the similar condition. The approximatelyinear relationship between ln(C0/C) and irradiation time (t) in Fig. 7ndicated that the first order reaction kinetics that describes thehotocatalytic degradation of MB by the samples. This model is aorm of reduced Langmuir–Hinshelwood equation [8]:( )

nC0

C= kt (3)

here C0 is the initial concentration of MB, C is the MB concen-ration at any time (ppm), and k is the first-order degradation

ig. 6. Plot of (ah�)2 vs. h� for ZnO and ZnO-carbon nanofibers and correspondingV–vis DRS spectra (inset).

brication from the precursor as-spun fibers.

rate constant. The degradation rate constant could be obtainedfrom the slope of the straight line by linear fitting of ln(C0/C)vs. irradiation time. As shown in Fig. 7, MB degradation underthe UV illumination as a result of photolysis is negligible [37];therefore, the MB degradation is mainly attributed to the pho-tocatalytic property of the nanofibers. The MB degradation rateconstants for the ZnO-carbon and ZnO nanofibers are about 0.0020and 0.0013 min−1, respectively. It is clear that the difference in pho-tocatalytic activity between the samples is due to the higher valuesof carbon and oxygen vacancy in the ZnO-carbon nanofibers. Var-ious reasons proposed by Guo et al. [21] for enhancement of MBphoto-degradation of ZnO-carbon nanorods in comparison to pureZnO. First, carbon enhances the MB adsorption on the surface ofZnO-carbon and promotes the interaction between photocatalystand MB dye. Second, as depicted in Fig. 6, ZnO-carbon nanofibersshowed higher UV absorption in comparison to the ZnO. Third, thesynergy between carbon and ZnO in the ZnO-carbon nanofibersleads to the retardation in electron–hole recombination and photo-degradation enhancement that will be discussed extensively later.The other reason for the enhancement of photocatalytic activity isrelated to the oxygen vacancy in the ZnO-carbon nanofibers. Recentreports showed that the oxygen vacancy in the ZnO nanostruc-

tures promotes photocatalytic activity [10,15,16]. As discussedabove, the surface oxygen vacancies induce new energy levelsbelow the conduction band. The photo-excited electrons, captureby this new energy levels which will retard the recombination of

Fig. 7. Variation of ln(C0/C) vs. UV irradiation time for the ZnO, ZnO-carbon, and dyesolution (no photocatalyst).

158 M. Samadi et al. / Applied Catalysis A: General 466 (2013) 153– 160

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ZnO-carbon nanofiber surface under the UV irradiation. Gradualincrease of the PL intensity with UV irradiation time clarified the•OH radical production on the surface of the ZnO-carbon during the

ig. 8. Photoluminescence (PL) spectra of the ZnO and ZnO-carbon nanofibers.

hoto-generated electrons and holes and therefore, resulting in themprovement of the photocatalytic activity.

It is well established that one of the main strategies tonhance photocatalytic activity is the retardation of electron–holeecombination. PL technique has been used to investigate theecombination rate between the photo-generated electrons andoles. Fig. 8 shows the PL spectra of ZnO and ZnO-carbon nanofibers

n the 350–600 nm wavelength range with an excitation at 320 nm.he pure ZnO spectrum has a prevalent appearance which containsn ultraviolet PL peak at 380 nm and a visible emission peak. TheV peak corresponds to the recombination of free excitons betweenalence and conduction band and the visible peak is due to the tran-ition in defect states [38]. The PL intensity of ZnO-carbon was muchower than the ZnO nanofibers, which represents the quenching ofhe electron–hole recombination. It has been previously reportedhat oxygen vacancy caused the increase of the PL intensity dueo the binding of the photo-electrons with this defect to createree or binding excitons [7,35,36]. Although we demonstrated theresence of oxygen vacancies in the ZnO-carbon by XPS and DRSesults, the reduction of PL intensity is in contrast to the other pub-ished reports. Thus, the PL signal quenching could be related to thexistence of carbonaceous species in the samples. Some researchersanifested the synergistic effect between carbon and ZnO or TiO2

or PL intensity reduction [6,23,25,39]. Liao et al. [39] describedhe band structure diagrams of the ZnO and carbon. They showedarbon as a weak p-type material with a positive electron affinitynduces new energy levels under the ZnO conduction band. Othereports [6,21,22,24,25] have also confirmed that carbon introducesew empty electronic states and the photo-electron transfers fromonduction band of ZnO to this new energy level by non-radiativeecay. Therefore, on the basis of our results, carbon accommodateshe photo-electron and causes retardation in electron–hole recom-ination leads to an enhancement in the photocatalytic activity ofnO-carbon nanofibers.

Further investigations were performed for understanding theechanism of dye degradation under UV light. To verify whether

OH radicals are involved in the photocatalytic degradation of MBn the ZnO-carbon nanofibers, the photocatalytic activity were car-ied out in the presence of tert-butyl alcohol (t-BuOH), as a widely

sed •OH radical scavenger by the following reaction [37]:

-BuOH + •OH → t-BuO• + H2O (4)

Fig. 9. Comparison of the MB photocatalytic degradation by ZnO-carbon nanofibersin the presence of t-BuOH and I− .

If the •OH radicals play a major role in the degradation of MB,the reaction rate is expected to decrease greatly. But our resultin Fig. 9 depicts a small decrease of the MB photo-degradationrate in the presence of t-BuOH. The ZnO-carbon rate constant is0.0016 min−1 and in the presence of 0.01 M t-BuOH slightly reducesto 0.0012 min−1. Therefore, the MB degradation partially occursthrough the •OH radicals. This issue is considered here that perhaps•OH radicals are not produced during the photocatalytic activity.Thus, the production of the •OH radicals should be clarified by thefollowing experiment during the photocatalytic reaction.

To verify the formation of •OH radicals, we used terephthalicacid (TA) that readily reacts with •OH radicals in aqueous solu-tion and produced 2-hydroxyterephthalic acid (TAOH) which emitsa unique photoluminescence (PL) spectrum [37]. The PL peak at425 nm attributed to the TAOH and its intensity is proportional tothe amount of formed •OH radicals. Fig. 10 shows the intensityof the PL signal corresponding to the production of TAOH on the

Fig. 10. PL spectra of TAOH generated in the presence of ZnO-carbon nanofibersunder the different UV irradiation time.

M. Samadi et al. / Applied Catalysis A: General 466 (2013) 153– 160 159

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ig. 11. Schematic energy band diagram and proposed photocatalytic mechanismf the ZnO-carbon under the UV light.

hotocatalytic activity. Therefore, it is obvious the dye degradationechanism occurs through another pathway.Further investigation was performed to clarify the mechanism

f MB photo-degradation in the presence of ZnO-carbon nanofibers.odide ion (I−) is a scavenger of both •OH radical and positive holesh+) by the following reactions [37]:

I− + •OH → I2 + OH− + e− (5)

I− + 2h+ → I2 (6)

Fig. 9 shows the photocatalytic decomposition of MB in the pres-nce of 0.01 M KI as a source of I− ions. The rate constant increasesore than ten times from 0.0016 min−1 to 0.017 min−1 and 80%

f the dye solution degraded in 90 min. As clarified above the •OHadicals have a little effect on the photocatalytic degradation, there-ore, the change in the photocatalytic property in the presencef I− should be related to the effect of holes. It is believed that

− acts as a hole scavenger, therefore, the photo-degradation shouldecrease if the photocatalytic activity pathway controlled via h+

articipation. But, our obtained results show a different behav-or which indicates the expense of h+ in the KI solution leads toecrease in the electron–hole recombination. Therefore, the photo-egradation improvement in the presence of I− ZnO-carbon is dueo the increase in the e− amount. Similar behavior for the photo-atalytic activity was also reported by the other groups [40,41].

As reported by the other researchers the electrons in the CB andO of ZnO transfer to the new band state of the carbon [6,15,17].ccording to the Density Functional Theory (DFT) calculations, theew carbon state lie deep in the band gap of the ZnO [29]. Theeduction potential of superoxide radical is −0.28 V vs. NHE and its just below the conduction band of the ZnO. Therefore the photo-enerated electrons that transfer to these carbon states could notransfer to the O2 to produce superoxide radicals. Therefore the

echanism of dye decomposition is induced directly by electrons.According to the obtained results the synergy between ZnO,

xygen vacancy, and carbon in the enhancement of photocatalyticctivity is elucidated in more detail. As mentioned above, oxygenacancies induce new electron donor states beneath the conduc-ion band of the ZnO and the photo-generated electrons in the CBf the ZnO transfer to them [12–14]. On the other hand, the oxy-en vacancy and photo-generated electrons transfer to the newand state of carbon with minimum recombination by holes. Fur-hermore, as discussed above carbonaceous material caused moredsorption of the MB molecules to the surface of the photocatalyst21]. Therefore, the electrons that transfer to the carbon decomposehe MB on the surface of the ZnO-carbon nanofibers. The schematicf the photocatalytic activity pathway of ZnO-carbon is illustratedn Fig. 11 [6,15,17].

In order to study the photo-stability of the ZnO and ZnO-carbonanofibers, the nanofibers recovered from the methylene blue solu-ion and the photocatalytic activity investigated after two weeks.he photo-stability activity results showed the decrease of 22% and

Fig. 12. Variation of ln(C0/C) vs. visible irradiation time for the ZnO, ZnO-carbon,and dye solution (no photocatalyst).

3% for ZnO and ZnO-carbon nanofibers, respectively. The resultshowed that carbonous species improve the stability of the ZnOin the photocatalytic reaction. As reported by Xie et al. carbonousmaterials have improved the stability of photocatalysts due totheir relatively low work function, good stability and corrosionresistance [42]. As shown in the inset of Fig. 1, the ZnO-carbonnanofibers are completely dark and the carbonaceous speciessurrounded the ZnO completely. The carbon species prevent thedirect contact of ZnO to the photocatalytic solution and therefore,avoid the photo-corrosion and cause the promotion in the stabilityof it. Other researcher reports showed similar results and confirmedthe improvement of photo-stability of ZnO by using carbonaceousspecies (ref #5, 6, 22, 31).

The activity of the ZnO and ZnO-carbon nanofibers under the vis-ible light was also investigated and the results depicted in Fig. 12.As could be seen, the dye degradation as a result of photolysis(absence of photocatalyst) is negligible. It was observed that undervisible light, the ZnO nanofibers have a slight photo-degradationactivity that could be related to the photosensitization effect of themethylene blue [34]. On the other hand, the MB degradation rateconstant increased to 0.003 min−1 in the presence of ZnO-carbonnanofibers. The photo-degradation enhancement of ZnO-carbonnanofibers under the visible light can be explained by the factthat carbonaceous species act as a photosensitizer as suggestedby the other groups for TiO2–carbon composites [43–45]. In thisprocess, carbonaceous species excited by visible light illumina-tion and transfer electrons into the conduction band of ZnO in theZnO-carbon nanofibers. Then, the electrons caused the MB photo-degradation under the visible light.

4. Conclusions

ZnO-carbon nanofibers were fabricated by electrospinning ofthe PVA/zinc acetate precursors and subsequent annealing in nitro-gen at 460 ◦C. The average diameter of the nanofibers was about200 nm with wurtzite crystal structure. TGA revealed a total weightloss of 41.3% of ZnO-carbon with about 6% carbon content ascompared with the ZnO. The band gap energy of the ZnO-carbonreduced as compared to the ZnO nanofibers due to the oxygen

vacancies. The XPS results confirmed the formation of C O Znbond on the surface of ZnO-carbon. The peak deconvolution anal-ysis showed a more intense oxygen vacancies and chemisorbed O2peaks in the ZnO-carbon in comparison to ZnO nanofibers. •OH

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adical formation during the photocatalytic activity was tracedy production of 2-hydroxyterephthalic acid as a probe molecule.he kinetics studies showed the higher photocatalytic decompo-ition rate of the ZnO-carbon as compared to the ZnO nanofibersnder both UV and visible light due to the presence of carbona-eous species and oxygen vacancies. The possible mechanism of thehoto-degradation was studied by examination of photocatalyticroperty in the presence of tert-butyl alcohol (t-BuOH) and I− anions •OH radical and hole (h+) scavengers. The results showed t-BuOHas a little effect on the MB degradation rate, therefore, •OH radicalas not the source of dye decomposition during the photocatalytic

ctivity. The addition of KI as a source of I− anion leads to increase inhotocatalytic performance due to the retardation of electron–holeecombination rate by trapping the valence band holes. The resultshowed that the mechanism of the photo-degradation of MBolecules on ZnO-carbon nanofibers is attributed to the photo-

enerated electrons. The electrospun ZnO-carbon nanofibers coulde easily separated and recovered by sedimentation, and wouldreatly promote their practical application. In comparison to ZnO,nO-carbon showed higher photo-stability due to the good stabilitynd corrosion resistance of carbonaceous species.

cknowledgments

The authors wish to thank Research and Technology Council ofhe Sharif University of Technology for financial support. Usefuliscussion with Dr. M. Zanetti from University of Torino (Italy) isreatly acknowledged.

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