光催化Mixed oxide sulfide systems for photocatalysis

Res.Chem.Intermed.,V ol.29,No.5,pp.467–475(2003)

óVSP2003.

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Mixed oxide/sul de systems for photocatalysis

AGA TINO DI PAOLA¤,MAURIZIO ADDAMO and LEONARDO PALMISANO Dipartimento di Ingegneria Chimica dei Processi e dei Materiali,Universitàdi Palermo,

Viale delle Scienze,90128Palermo,Italy

Received8April2003;accepted23April2003

Abstract—Mixed WO3/WS2and ZnO/ZnS powders have been prepared by sul dation of H2WO4 or ZnO.Photocatalytic degradation of phenol has been employed to test the photoactivity of the samples.The resultsindicated that the coupling of the two tungsten chalcogenidesenhanced the rate of disappearanceof the organic substrate.Unlikely,the ZnO/ZnS systems exhibited lower photocatalytic activitiy than pure ZnO.The different photocatalytic behaviour of the two mixed oxide/sul de systems can be explained by a simultaneous displacement of electrons and holes between two different semiconductors.

Keywords:Photocatalysis;electron displacement;mixed oxide/sul de systems. INTRODUCTION

Heterogeneous photocatalysis is a promising and alternative method for the treat-ment of gaseous and liquid wastes[1–4].Many different semiconductor particu-lates have been used for the degradation of environmental pollutants and TiO2and ZnO have been proven to be very ef cient catalysts for the photodegradation of several organic substrates[5–10].

One of the limiting factors that control the ef ciency of a photocatalyst is the quick recombination between photogenerated electrons and holes as it impedes the charge transfer at the semiconductor surface.An interesting approach for achiev-ing an ef cient charge separation involves coupling two separate semiconductors possessing different energy levels for their corresponding conduction and valence bands[10].The improvement of photocatalytic activity has been attributed to the transfer of electrons and holes from a semiconductor to another.

To date,a large variety of coupled semiconductors has been investigated[10–20]. Recently,polycrystalline WO3/WS2systems[21–23]have shown signi cantly higher activities than pure WO3and WS2.An improved charge separation has

¤To whom correspondence should be addressed.E-mail:dipaola@dicpm.unipa.it

468 A.Di Paola et al.

been also observed in ZnO/ZnS colloids[15,19].In view of the interesting features of such oxide/sul de systems we have prepared and characterized mixed WO3/WS2and ZnO/ZnS powders.The photocatalytic activity of the samples has been determined by photooxidation of phenol in aqueous suspensions and compared with that of the pure semiconductors.

EXPERIMENTAL

The chemicals used were all of analytical grade.Mixed WO3/WS2and ZnO/ZnS catalysts were prepared by sul dation of H2WO4and ZnO powders with gaseous H2S at different temperatures and for different times[21].

H2WO4(Fluka)was placed in a cylindrical Pyrex reactor and allowed to react with a ux of H2S(Matheson).ZnO(Fluka)was sul dized with pure H2S or with H2S/H2(20:80,v/v)mixture.Before use,the resulting powders were ground in an agate mortar until particles with a grain size<451m were obtained.The mixed samples were denoted by three numbers,e.g.6.5/0.5/523,where the rst number represents the amount of initial oxide powder(g),the second the time(h)and the third the temperature of sul dation(K).The samples denoted by mix were obtained with the mixture H2S/H2.

Powder composition was gravimetrically determined by calcining the samples in air at873K for3h.The sulfur content was calculated from the weight loss, assuming WO3or ZnO as the only products of calcination.The samples were characterized by X-ray diffraction and determination of BET speci c surface area. A0.5-l batch cylindrical Pyrex reactor was used for the photocatalytic experi-ments.The reaction mixture,containing2g powder/l and a2001M phenol solution (pH4.5),was magnetically stirred and oxygen was continuously bubbled through the solution before and during the irradiation.A125W medium-pressure Hg lamp (Helios Italquartz)was placed within the inner part of the photoreactor and cooling water was circulated through a Pyrex jacket surrounding the lamp.The Pyrex thim-ble,together with the water circulation,allowed to eliminate the IR component of the incident beam as well as any radiation below300nm.

The phenol concentration was determined spectrophotometrically by a standard colorimetric method[24].The mineralization of the substrate was followed by determining the total organic carbon(TOC)with a Carlo Erba TCM480analyzer. The concentration of Zn ions in solution was measured by atomic absorption spectroscopy using a Varian SpectrAA-20Plus spectrometer.

RESULTS

Different mixed oxide/sul de systems were prepared by sul dation of H2WO4or ZnO.In both cases the powder composition was tailored by varying the amount of starting powder,time,or the reaction temperature.

Mixed oxide/sul de systems for photocatalysis469 Table1.

Speci c surface area values,sul de molar fractions and photocatalyticactivity results of bare H2WO4, WS2and mixed WO3/WS2samples

0.￡108/r 0

.￡108/

(m2g?1)(molar fraction)(mol l?1s?1)(mol l?1s?1m?2) H2WO411.500.050.2

WS2 1.710.010.3

H2WO4/6.5/0.5/67313.00.44 2.959.6

H2WO4/6.5/1/62312.90.56 1.28 3.9

H2WO4/6.5/1/67313.10.670.260.4

Table2.

Speci c surface area values,sul de molar fractions and photocatalytic activity results of bare ZnO, ZnS and mixed ZnO/ZnS samples

0.108/r 00.108/

(m2g?1)(molar fraction)(mol l?1s?1)(mol l?1s?1m?2)

ZnS8.11 1.30.2

ZnO/6.5/0.5/47311.10.47 3.80.5

ZnO/6.5/1/47313.20.50 2.50.3

ZnO/6.5/0.5/52312.10.58 1.50.2

ZnO/6.5/1/52310.90.68 3.20.4

ZnO/6.5/1/6739.10.74 1.40.2

ZnO/6.5/2/6738.90.970.30.1

ZnO mix/6.5/1/673 6.70.1014.0 3.0

ZnO mix/6.5/1/723 6.60.1611.7 2.5

ZnO mix/6.5/1/773 6.10.1810.3 2.4

X-ray diffraction patterns of the powders revealed the contemporary presence of WO3and WS2or ZnO and ZnS.The speci c surface areas(SSA)of the various samples are reported in Tables1and2together with the molar fraction of WS2 or ZnS determined by chemical analysis.The amount of sul de increased with increasing temperature or duration of sul dation,in agreement with the results obtained by X-ray diffraction.

The photoreactivity experiments revealed different behaviours of the samples, depending on the amounts of oxide and sul de found in the powders.Figure1 shows the kinetics of disappearance of phenol for different samples obtained by sul dation of H2WO4.Both H2WO4and WS2exhibited a low photoreactivity.The mixed powders were more photoactive than the pure tungsten chalcogenides and their ef ciency depended on the relative amounts of WO3and WS2in the various samples.In particular,as already reported in previous works[21,22],the best results were obtained when the surface WS2molar fraction was near to0.5.It is worth noting that mixed WO3/WS2systems obtained by sul dation of WO3[21] or by oxidation of WS2[23]were generally more active.The photoactivity results

470 A.Di Paola et al.

Figure1.Photocatalyzed conversion of phenol.(e)H2WO4,(E)WS2,(f)H2WO4/6.5/0.5/673, (u)H2WO4/6.5/1/623,(F)H2WO4/6.5/1/673.

are summarized in Tables1and2in the form of initial reaction rates(r0)and initial reaction rates per square meter of photocatalyst(r00).

As shown in Fig.2,only a small fraction of the phenol was mineralized to CO2and H2O in the presence of the tungsten chalcogenide powders.In particular,although phenol completely disappeared after6h of irradiation,only50%mineralization was reached with the sample6.5/0.5/673containing44%of WS2.The largest extents of mineralization were obtained by using the samples that were photocatalytically more ef cient.

Figure3shows the results of phenol oxidation in the presence of zinc chalco-genide powders.ZnO is very ef cient for phenol degradation[9,10],whilst ZnS is a poorer photocatalyst.The presence of ZnS quenches the degradation of phenol on ZnO and the mixed ZnO/ZnS systems are less active than ZnO(see Table2). Mixed systems with a high molar fraction of ZnS revealed rates of phenol oxidation even smaller than that observed with pure ZnS.In order to reduce the content of ZnS in the powders,some samples were obtained by sul dizing ZnO with a mixture H2S/H2.These powders containing high amounts of ZnO were signi cantly more ef cient than those obtained by sul dizing with only H2S. Figure4shows the total organic carbon as a function of the irradiation time for the zinc chalcogenide systems.Very low mineralization percentages were obtained both with pure ZnS or with the mixed powders whilst ZnO mineralized the substrate after only2h.

Mixed oxide/sul de systems for photocatalysis471

Figure2.Variation of total organic carbon.(e)H2WO4,(E)WS2,(f)H2WO4/6.5/0.5/673, (u)H2WO4/6.5/1/623,(F)H2WO4/6.5/1/673.

Figure3.Photocatalyzed conversion of phenol.(e)ZnO,(E)ZnS,( )ZnO mix/6.5/1/673, (f)ZnO mix/6.5/1/773,(s)ZnO/6.5/1/473,(F)ZnO/6.5/1/523,(u)ZnO/6.5/1/673.

472 A.Di Paola et al.

The stability of the various samples under UV illumination was also studied. No appreciable release of HS?was found in solution after8h of irradiation of the suspensions containing WS2or mixed WO3/WS2powders.Conversely,the suspensions of zinc chalcogenides showed a certain degree of instability,as shown by the molar percentages of zinc dissolved after2h of irradiation(Table3).

Figure4.Variation of the total organic carbon.(e)ZnO,(E)ZnS,(s)ZnO/6.5/1/473, (F)ZnO/6.5/1/523,(u)ZnO/6.5/0.5/523.

Table3.

Photocorrosion of the zinc chalcogenide samples after2h of irradiation

Sample ZnS Molar percentage of dissolved zinc

(molar fraction)

ZnO 1.8

ZnS1 1.5

ZnO/6.5/0.5/4730.4715.0

ZnO/6.5/1/4730.508.5

ZnO/6.5/0.5/5230.5811.8

ZnO/6.5/1/5230.68 4.8

ZnO mix/6.5/1/6730.10 3.7

ZnO mix/6.5/1/7730.18 3.4

ZnO mix/6.5/2/7730.21 4.9

Mixed oxide/sul de systems for photocatalysis473 DISCUSSION

Previous studies have shown that mixed WO3/WS2systems prepared by sul dation of WO3[21,23]or by oxidation of WS2[22]exhibit an enhanced photoactivity

compared with that of the pure tungsten chalcogenides.

As schematized in Fig.5a,the conduction band of WS2[25,26]lies at a potential more negative than the conduction band of WO3[27],whilst the valence band

of WO3is located at a potential more positive than the valence band of WS2. Under UV irradiation,electrons pass from the conduction band of WS2to the conduction band of WO3and hole transfer occurs from the valence band of WO3to

the corresponding band of WS2.The simultaneous transfer of electrons and holes on different semiconductors increases the lifetime of the charge carriers improving the ef ciency of the interfacial charge-transfer processes.The charge separation is

very ef cient because in these systems WS2and WO3are adjoined to each other. Unfortunately the speci c surface areas of the powders are rather low[21,22]and not much different from those of WO3or WS2(2.4and1.7m2g?1,respectively). The results of this work con rm the increased photoef ciency of the mixed WO3/WS2systems.The sul dation of H2WO4allows to obtain powders with higher surface areas but less ef cient than those obtained by sul dation of WO3

or by oxidation of WS2.

The differences in photoactivity may be related to morphological and structural differences between the WO3phases present in the different mixed particles.As shown in Table1,the SSA values of the sul dized powders are not much different from that of H2WO4,indicating that the volume of the solid changes very little during the sul dation.This means that the substitution of the atoms of oxygen with those of sulfur does not cause a signi cant distortion of the lattice and consequently both WO3obtained by dehydration of H2WO4,namely WO3?H2O,and WS2 formed by sul dation of WO3or WO3?H2O substantially keep the same crystalline

(a)(b)

Figure5.Schematic diagrams representing the charge transfer processes between(a)WO3/WS2and (b)ZnO/ZnS-coupled particles.

474 A.Di Paola et al.

structure of H2WO4[28],rather different from that of WO3[29].Obviously other

physico-chemical factors as geometry of the particles,surface texture or particle

size distribution can play a signi cant role.

The photoreactivity results have revealed that the mixed ZnO/ZnS systems

obtained by sul dation of ZnO are less ef cient photocatalysts than ZnO.Figure5b shows the position of the valence and conduction bands of ZnO and ZnS.Since

the valence band of ZnO[27]is more anodic than that of ZnS[30,31],ZnO acts

as a sink for the photogenerated holes that are quickly transferred into the valence band of ZnS.Contemporaneously,electron transfer occurs from the more cathodic

conduction band of ZnS to the corresponding band of ZnO.The photoinduced charge separation in the ZnO/ZnS system should enhance the ef ciency of phenol

degradation.Kamat and Patrick[19]found an increase of the quantum yield for

the oxidation of SCN?to(SCN)?2in ZnO/ZnSe and ZnO/ZnS systems obtained by exposing a ZnO colloidal suspension to an atmosphere of H2Se or H2S,respectively.

The increased oxidation of SCN?was explained by the simultaneous migration of electrons toward ZnO and holes toward ZnS.

Rabani[15]investigated the photocatalytic behaviour of colloid particles,com-

posed of ZnO and ZnS particles in contact or ZnO colloid particles coated with ZnS.ZnS quenched the ZnO visible emission by hole transfer from ZnO to ZnS, but the yield of reduction of methyl viologen was lower than that obtained in the presence of only ZnO.This effect was attributed to a relatively ef cient electron-hole recombination that takes place at the interface between ZnO and ZnS before localization of the holes occurs.

As shown in Table3,both bare ZnO and ZnS undergo photocorrosion.This

means that,if the photogenerated holes are not consumed rapidly by phenol, photoanodic corrosion ensues,leading to the release of Zn2C ions in solution.When the mixed ZnO/ZnS powders are activated by irradiation,electron transfer from ZnS to ZnO and hole transfer from ZnO to ZnS may occur,in competition with the electron–hole pair recombination in the different semiconductors.The excess holes on ZnS ultimately oxidize phenol but also anodically autocorrode the ZnS particulates.The larger quantity of Zn2C ions measured for the mixed ZnO/ZnS powders indicates that the photocorrosion of ZnS is increased,con rming the displacement of both electrons and holes from one semiconductor to another[10]. The coupling of ZnO and ZnS inhibits the photocatalytic activity of ZnO,since ZnS is a poorer photocatalyst than ZnO and the migration of electrons and holes between the two different semiconductors decreases the concentration of holes on ZnO.Obviously,the activity of the mixed ZnO/ZnS powders becomes less signi cant with increasing the amount of ZnS.

Acknowledgements

This work was nancially supported by the Ministero dell’Istruzione,dell’Universitàe della Ricerca(Roma).

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