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ZnO-photocatalysis

Journal of Solid State Chemistry 178(2005)3500–3506

Preparation and photocatalytic activity of ZnO/TiO 2/SnO 2mixture

Cun Wang a ,Bo-Qing Xu a,?,Xinming Wang b ,Jincai Zhao c

Innovative Catalysis Program,Key Lab of Organic Optoelectronics &Molecular Engineering,Department of Chemistry,

Tsinghua University,Beijing 100084,China

Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China

Institute of Chemistry,Chinese Academy of Sciences,Beijing 100080,China

Received 12June 2005;received in revised form 29August 2005;accepted 6September 2005

Available online 5October 2005

Abstract

ZnO/TiO 2/SnO 2mixture was prepared by mixing its component solid oxides ZnO,TiO 2and SnO 2in the molar ratio of 47171,followed by calcining the solid mixture at 200–13001C.The products and solid-state reaction process during the calcinations were characterized with powder X-ray diffraction (XRD),thermogravimetric and differential thermal analysis UV-Vis diffuse re?ectance spectroscopy (UV-Vis DRS)and Neither solid-state reaction nor change of crystal phase composition took place among the ZnO,TiO 2and SnO 2powders on the calcinations up to 6001C.However,formation of the inverse spinel Zn 2TiO 4and Zn 2SnO 4was detected at 700–900and 1100–12001C,respectively.Further increase of the calcination temperature enabled the mixture to form a single-phase solid solution Zn 2Ti 0.5Sn 0.5O 4with an inverse spinel structure in the space group of O 7h àFd 3m .The ZnO/TiO 2/SnO 2mixture was photocatalytically active for the degradation of methyl orange in water;its photocatalytic mass activity was 16.4times that of SnO 2,2.0times that of TiO 2,and 0.92times that of ZnO after calcination at 5001C for 2h.But,the mass activity of the mixture decreased with increasing the calcination temperature at above 7001C because of the formation of the photoinactive Zn 2TiO 4,Zn 2SnO 4and Zn 2Ti 0.5Sn 0.5O 4.The sample became completely inert for the photocatalysis after prolonged calcination at 13001C (42h),since all of the active component oxides were reacted to form the solid solution Zn 2Ti 0.5Sn 0.5O 4with no photocatalytic activity.

Keywords:Zinc oxide;Titanium dioxide;Tin dioxide;Mixed oxides;Solid-state reaction;Photocatalysis;Inverse spinel;Zn 2Ti 0.5Sn 0.5O 4

1.Introduction

Photocatalytic degradation of organic pollutants in water and air using semiconductors,such as TiO 2and ZnO,has attracted extensive attention in the past two decades [1].Previous studies have proved that such semiconductors can degrade most kinds of persistent organicpollutants,suc h as detergents,dyes,pestic ides and volatile organicc light irradiation.A photocatalytic process is based on the generation of electron–hole pairs by means of band-gap radiation that can give rise to redox reactions the species adsorbed on the surface In

principle the coupling of different semiconductor oxides seems useful in order to achieve a more ef?cient electron–hole pair separation under irradiation and,consequently,a higher photocatalytic activity.A large variety of coupled polycrystalline or colloidal semiconductor systems,in which the adhere each other in so-called ‘‘sandwich structure’’or in ‘‘core-shell geometry’’,have been prepared and used for many photocatalytic reactions [2–9].Typical examples of such couplings are SnO 2/TiO 2[2,3],ZnO/TiO 2[4]and ZnO/SnO 2[5,6].For SnO 2/TiO 2,a stearicac id method was used by Yang et al.to prepare a SnO 2/TiO 2binary oxide [2].It was found that the binary oxide showed higher photocatalytic activity than Degussa P-25TiO 2in the photocatalytic degradation of methyl orange (MO).Shi et al.reported that ultra?ne SnO 2/TiO 2coupled particles prepared by a homogenous precipitation method https://www.sodocs.net/doc/455460142.html,/locate/jssc

Corresponding author.Fax:+861062792122.

E-mail addresses:wangcun@https://www.sodocs.net/doc/455460142.html, (C.Wang),bqxu@https://www.sodocs.net/doc/455460142.html, (B.-Q.Xu).

exhibited higher activity than that of the corresponding ultra?ne pure TiO2for the photocatalytic degradation azo dye active red X-3B[3].

case of ZnO/TiO2,Marci et al.[4]discovered that the polycrystalline SnO2/TiO2powders prepared by loading the anatase or rutile TiO2with ZnO from Zn(NO3)2á6H2O or32á2H2O precursors were not so bene?cial,would be expected on the basis intrinsic properties,for the photocatalytic degradation of4-nitrophenol in aqueous medium, although of the showed

slightly higher than those of the corresponding single TiO2 and ZnO.

For ZnO/SnO2,the composite ZnO/SnO2nanocrystal-line particles prepared with colloidal SnO2and ZnO precursors by Bandara et al.[5]exhibited improved photocatalytic activity for the sensitized of dyes(e.g.,Eosin Y),compared to those single oxides SnO2or TiO2).In an earlier work of Wang et al. coupled nanosized ZnO/SnO2samples prepared by co-precipitation method also showed higher activity than ZnO and SnO2the photocatalytic degradation MO in aqueous solution[6].

An increase of the lifetime of photogenerated electron–hole pairs in the coupled oxides,due to the hole and transfer between the two coupling semiconductors, is crucial to the catalytic activity enhancement in many photocatalytic reaction systems.Nevertheless,it should be considered that the photocatalyitc activity also strongly depends on the bulk surface physicochemical

ties of the catalysts,such as phase composition,defects(both in the bulk and at the surfaces) and surface https://www.sodocs.net/doc/455460142.html,eful information on the photocatalysts can also be obtained by the characteriza-tions that are frequently used in conventional thermal catalysts.

In this work,polycrystalline ZnO/TiO2/SnO2mixture powders were prepared by mixing stoichiometric amounts of the ZnO,TiO2and SnO2powders heat-treatment at elevated temperatures.The mixture powders were then characterized by X-ray diffraction(XRD), thermogravimetricand differential thermal analysis(TG-DTA),speci?c surface area determination with the Brunauer–Emmett–Teller(BET)method and UV-visible diffuse re?ectance spectroscopy(UV-Vis DRS).The results were used to explain the activity change of the calcined ZnO/TiO2/SnO2mixtures for the photocatalytic degrada-tion of MO in aqueous solution.

2.Experimental

2.1.Preparation of the ZnO/TiO2/SnO2mixtures

The ZnO/TiO2/SnO2mixtures were prepared from a powder mixture of the constituent oxides,at the molar ratio of ZnO:TiO2:SnO2?47171,by the solid-state reac-tion at elevated temperatures.The starting materials,ZnO (zincite,499.9wt%,particle size ca.0.5m m and BET surface area of 3.3m2/g),TiO2(anatase,499.8wt%, particle size ca.200nm and BET surface area of11.4m2/g) and SnO2(cassiterite,499.9wt%,particle size ca.250nm and BET surface area of10.5m2/g),were analyticgrade reagents purchased from Beijing Chemical Factory(China) and were used as supplied.The mixture was ground thoroughly in an agate mortar about2h,and then divided into corundum crucibles,followed by calcination for different in the range200–13001C.The calcined samples were hand-ground again to prepare the powdered mixture for photo-catalysis measurements.

2.2.Characterizations of the ZnO/TiO2/SnO2mixtures XRD patterns of the ZnO/TiO2/SnO2mixtures were recorded in ambient air at room temperature with a Bruker D8Advance with Cu K a radiation (l?1:5406A);the accelerating voltage,emission current, and scanning speed were40kV,and61/min, respectively.The TG-DTA analysis was conducted with a sample loading of44.33mg on a Setaram TGA/DTA92 instrument in staticambient air heated at a heating rate of 101C/min from50to14001C.The speci?c surface areas (S BET)were determined by using nitrogen adsorption data at77K obtained by a Micromeritics ASAP2010system with multipoint BET method.In order to increase the accuracy of the measured surface area data,we intention-ally higher sample loading(up to5g for the sample with the lowest speci?c surface area)to ensure that the measured sample surface area was larger than 1.0m2, which is within the accuracy of the instrument.The UV-Vis DRS were recorded on a Hitachi U-3010spectrophot-ometer,using pure BaSO4powders as the reference sample.

2.3.Evaluation of photocatalytic activity

The photodegradation MO in water was used to evaluate the catalytic of the ZnO/TiO2/SnO2 mixture powders.reactor consists of two parts:a100mL columned glass bottle a500W high pressure Hg lamp(UV light)with a intensity of 2.9?104m W/cm2at the middle of the of the bottle and a maximum wavelength of365nm. The lamp positioned parallel to the glass bottle. In all the reaction temperature was kept at 25721C.

Reaction suspensions were prepared by adding the photocatalyst powders the aqueous MO solutions of 100mL.Prior to the photoirradiation,the suspensions were ultrasonically sonicated for30then magne-tically stirred dark for10min to establish an adsorption/desorption equilibrium.The suspensions con-taining MO and photocatalysts were then irradiated by the UV light.

C.Wang et al./Journal of Solid State Chemistry178(2005)3500–35063501

During the photodegradation,the concentration of MO in the solution was monitored by sampling a (ca.2mL)of the reaction suspension at different times.After separation of the solid catalysts from the suspensions by centrifuging at 9000rpm for 10min followed by ?ltration through a 0.2m m millipore ?lter,the MO concentrations in the ?ltrates analyzed by UV-Vis spectroscopy with a Unico PC UV-Vis spectro-photometer at its maximum absorption wavelength (ca.464nm).

3.Results and discussion

The XRD patterns of the ZnO/TiO 2/SnO 2mixture powders after different calcinations are shown in Fig.1.The data show that both the phase composition and the growth of crystals in the ZnO/TiO 2/SnO 2mixture are sensitive to the calcination condition.After the calcinations for 2h at 3001C (not shown),500and 6001C (not shown),the mixture showed no other phases except those of the starting materials:ZnO (zincite),TiO 2(anatase)and SnO 2(cassiterite).A reaction between ZnO and TiO 2to form Zn 2TiO 4(cubic)(JCPDS 77-0014)took place during the calcinations at 700–9001C,which agrees with the observa-tions in a number of earlier reports [10–12].However,the formation of ZnTiO 3(hexagonal)and/or Zn 2Ti 3O 8(cubic),which were reported in those earlier works [10–12],was not detected in the XRD patterns of Fig.1.The characteristic of anatase TiO 2were hardly detectable after the calcination at 8001C for 2h (not shown),and they disappeared completely after the calcination at 9001C for 2h.To understand the disappearance of the anatase TiO 2,independent calcinations were made on the anatase TiO 2and the XRD data of this pure TiO 2(not shown)indicated,

in agreement with the data reported in Ref.[13],that an anatase-to-rutile transformation occurred in between 1000and 11001C.phase transformation temperature (1000–11001C)is higher than the disappearance tempera-ture of TiO 2in the mixture (800–9001C).Since no formation of the rutile 2phase was detected in the calcined mixtures,it that the disappearance of anatase 2was not due to a phase transformation of TiO 2.Also,there is least possibility that the TiO 2could exist in a XRD invisible amorphous state after the calcination at the temperature as high as 9001C.The disappearance of anatase TiO 2in the calcined mixture can most reasonably be accounted for by a solid-state reaction between the anatase TiO 2and ZnO to form Zn 2TiO 4.Although a reaction between the anatase TiO 2and SnO 2to form a SnO 2-rich solid solution TiO 2–SnO 2of the rutile structure [13–17]cannot be completely ruled out,this reaction is little possible since the disappearance of TiO 2in Fig.1was apparently accompanied with a continuous reduction of the ZnO signals and the formation of Zn 2TiO 4in the calcined mixture.The XRD peaks of Zn 2TiO 4were intensi?ed when the calcination temperature was raised up to 10001C,due to a sintering of the Zn 2TiO 4crystals.

A further calcination of the mixture at 11001C for 2h resulted in the formation of another new phase Zn 2SnO 4(cubic)(JCPDS 74-2184),which is consistent with the result in Ref.[18].A still further increase of the calcination temperature (12001C)caused both Zn 2TiO 4and Zn 2SnO 4phases to react to form a completely new phase with a set of diffractions at 2y ?18:001,29.661,34.911,36.471,42.401,52.641,56.081,61.571,which may be assigned to a solid solution Zn 2Ti x Sn 1àx O 4(0o x o 1).The solid-state reaction leading to the formation of Zn 2Ti x Sn 1àx O 4(0o x o 1)from Zn 2TiO 4and Zn 2SnO 4can be expressed as Zn 2TiO 4tZn 2SnO 4!Zn 2Ti x Sn 1àx O 4

e0o x o 1T.

The formation and growth of the Zn 2Ti x Sn 1àx O 4(0o x o 1)crystallites continued when the calcination temperature was further increased to 13001C when the residual ZnO (zincite)and SnO 2(cassiterite)almost implying that the ?nal product can be approximately viewed as Zn 2Ti 0.5Sn 0.5O 4.

The XRD pattern of the solid solution Zn 2Ti 0.5Sn 0.5O 4was indexed by referencing to the Zn 2SnO 4and Zn 2TiO 4inverse spinels (Fig.1).It seems that the calcination for 2h at 13001C was not long enough to induce a complete transformation of the ZnO/TiO 2/SnO 2mixture into the solid solution Zn 2Ti 0.5Sn 0.5O 4,which should show a diffraction pattern at 2y ?18:041,29.591,34.841,36.441,42.291,46.321,52.421,55.861,61.241,64.401,69.421.Therefore,the calcination period was extended to 6,10,14,18,22,26,30,34,38and 42h at 13001C,respectively,and it was found that a period of 42h is suf?cient for such a purpose (see Fig.1).The inverse spinel-like face-centered cubic structure (space group O 7h àFd 3m )has 8A 2BX 4units (A ?Zn,B ?Ti and Sn,X ?O)(Z ?8)that involve

I n t e n s i t y (a .u .)

2 (°)

1520253035

40455055606570

(111)

(220)

(311) (222)

(400)

(331)

(422)

(511)

(440)

(620)

(531)

(a)

(b) (c)

(d) (e) (f) ???????

????

????????

????????????×××

××

××××

××××++++

+++××

××××××

××××

??????????

???

????

??????????

??????

???

Fig.1.XRD patterns of the calcined ZnO/TiO 2/SnO 2powder mixtures.The calcination temperature and duration are 5001C and 2h (a),7001C and 2h (b),9001C and 2h (c),11001C and 2h (d),13001C and 2h (e),and 13001C and 42h (f),https://www.sodocs.net/doc/455460142.html,rmation on the crystal phase:(+)anatase TiO 2;(?)cassiterite SnO 2;(K )zincite ZnO;(E )Zn 2TiO 4;(B )Zn 2SnO 4;and (W )Zn 2Ti 0.5Sn 0.5O 4.

C.Wang et al./Journal of Solid State Chemistry 178(2005)3500–3506

3502

a total of 56atoms [19].In these inverse spinel structures the cations (Zn 2+,Ti 4+and Sn 4+)could be arranged as Zn tetra [ZnTi x Sn 1àx ]octa O 4,i.e.,one half of the Zn 2+ions are in the tetrahedral sites;the other half of the Zn 2+ions are mixed randomly with the Ti 4+and Sn 4+ions in the octahedral sites [19,20].The cubic lattice constant of Zn 2Ti 0.5Sn 0.5O 4,calculated with the software (Diffrac plus Win-MetricVersion 3.0)on the X-ray diffrac tometer,is

8.54770.003A

,which is larger than that of Zn 2TiO 4(8.4450A

,JCPDS 77-0014)but smaller than that of Zn 2SnO 4(8.6500A

,JCPDS 74-2184)due to the difference in radius between Ti 4+(0.68A

)and Sn 4+(0.71A ).Fig.2shows the TG-DTA

curves of the starting ZnO/TiO 2/SnO 2mixture.The TG curve no signi?cant weight change in the temperature range of 400–14001C.A little weight loss of 0.11wt%from 99.87wt%at 4001C to 99.76wt%at 14001C could imply a formation of low concentration oxygen vacancies during the heat treatment,due to the escape of part crystal lattice oxygen from the component single oxides (ZnO,TiO 2,SnO 2)and/or the solid solutions Zn 2TiO 4,Zn 2SnO 4and Zn 2Ti x Sn 1àx O 4(0o x o 1).While on the DTA curve,endothermiceffec ts were apparent at above 7001C,not to mention the strong endothermicpeak at c a.11901C.Since the accompanying TG curve no signi?cant formation of volatile products,the endothermic DTA features must be with the thermal effects of the solid-state reactions inducing to the formation of Zn 2TiO 4,Zn 2SnO 4and Zn 2Ti x Sn 1àx O 4(0o x o 1)from the mixed oxides.This explanation is consistent with the XRD data (Fig.1)that Zn 2TiO 4and Zn 2SnO 4were formed at about 700and 11001C,respectively.In fact,the formation of Zn 2TiO 4from the ZnO and TiO 2,and that of Zn 2SnO 4from the ZnO and SnO 2were found endothermicin the literatures [21,22].As we mentioned in presenting the XRD data,the thermal effects occurred in the temperature range of 700–13001C could not include the anatase-to-rutile

transformation of TiO 2because all of the anatase TiO 2component had reacted with ZnO (at 700–9001C)to form the Zn 2TiO 4crystalllites before the anatase-to-rutile transformation could take place (at 1000–11001C).Since the Zn 2TiO 4and Zn 2SnO 4reacted to form Zn 2Ti x Sn 1àx O 4(0o x o 1)solid solution at about 12001C and the solid solution was found to be the only detectable product after the calcination at 13001C,the endothermicpeak at about 11901C should be related with the formation of Zn 2Ti x Sn 1àx O 4(0o x o 1)from Zn 2TiO 4and Zn 2SnO 4.

Table 1gives that the speci?c surface areas of the calcined ZnO/TiO 2/SnO 2mixture samples.It can be seen that the speci?c surface area of the ZnO/TiO 2/SnO 2mixture decreased with increasing the calcination tempera-ture due to the changes in the sample particle size (sintering)and in the sample crystal phases.But the effect was little when the calcination temperature is p 5001C,indicating that no signi?cant sintering occurred among the sample particles below 5001C.It can be seen that duration of the calcination is also a factor,but not an important one,affecting the speci?c surface area,since the sample surface area decreased slowly with extending the calcina-tion time.

Fig.3shows the UV-Vis diffuse re?ectance spectra of the calcined ZnO/TiO 2/SnO 2mixtures.The wavelengths of the absorption edges in the UV-Vis spectra were determined by extrapolating the sharply rising portions and horizontal portions of the UV-Vis curves,de?ning the edges as the wavelengths at the intersections according to the literatures [6,23].The samples calcined at 5001C for 2h,7001C for 2h,and 13001C for 42h displayed a one-edge absorption,respectively.By contrast,those samples cal-cined at 900,1100and 13001C for 2h displayed a two-edge absorption,respectively.The absorption edges are at 388.8nm for the sample calcined at 5001C for 2h and 389.5nm for the one calcined at 7001C for 2h,correspond-ing to the apparent band-gap energies of 3.19and 3.18eV,respectively.It was reported that both the band-gap energy of ZnO and that of TiO 2were of 3.20eV [24],and the absorption edge of SnO 2occurred in between 1.64and 3.7eV [6,25–28].Therefore,for the sample calcined at 5001C for 2h,the absorption curve contains the contribu-tions from each of the oxide components ZnO,TiO 2and SnO 2,according to the XRD results.The UV-Vis absorp-tion of the sample calcined at 7001C for 2h could also be ascribed to the absorptions from ZnO,TiO 2and SnO 2because the content of Zn 2TiO 4in the mixture was not signi?cant.The two absorption edges are at 354.3and 387.4nm for the sample calcined at 9001C for 2h,364.3and 397.9nm for the one calcined at 11001C for 2h,and 352.0and 398.6nm for the one calcined at 13001C for 2h;the corresponding apparent band-gap energies are 3.50and 3.20eV,3.40and 3.12eV,and 3.52and 3.11eV,respec-tively.The absorption edges of Zn 2TiO 4and Zn 2SnO 4prepared by calcination of ZnO with the stoichiometric amounts of TiO 2and SnO 2,respectively,are at 355.3and

W e i g h t (%)

H e a t f l o w (m W ) Temperature (°C)

99.65

99.7099.7599.8099.8599.9099.95100.00400

600

800

1000

1200

1400

-4-3-2

-1012345Fig. 2.TG-DTA curves in static ambient air of the ZnO/TiO 2/SnO 2mixture.Starting sample weight:44.33mg;heating rate:101C/min.

C.Wang et al./Journal of Solid State Chemistry 178(2005)3500–3506

3503

368.0nm (not shown).These absorptions correspond to the band-gap energies of 3.49eV for the Zn 2TiO 2and 3.37eV for Zn 2SnO 4.Therefore,the absorptions at lower wave-length may be ascribed to the Zn 2TiO 4for the sample calcined at 9001C for 2h,the Zn 2TiO 4and Zn 2SnO 4for the sample calcined at 11001C for 2h,and to the Zn 2Ti 0.5Sn 0.5O 4for the sample calcined at 13001C for 2h.The absorptions at higher wavelength might be attributed to the unreacted ZnO and SnO 2in these three samples,respectively.

Using the UV-Vis DRS data in Fig.3for the solid solution Zn 2Ti 0.5Sn 0.5O 4,which was the only product after the calcination of the starting mixture at 13001C for 42h,the absorption edge of the Zn 2Ti 0.5Sn 0.5O 4sample is measured at 352.3nm,corresponding to a band-gap energy of 3.52eV.

Fig.4shows the dependence of the photocatalytic activity of the ZnO/TiO 2/SnO 2mixture on its calcination temperature.The photocatalytic activity decreased slightly with increasing the calcination temperature up to 5001C,

but decreased signi?cantly when the mixture was calcined at higher temperatures,especially at above 9001C.The mixture became almost inactive after the calcination at 13001C.

The photocatalytic activities of the calcined mixtures were normalized with respect to their speci?c surface area and the obtained areal activities are given in Table 2.The data were expressed by (C 0àC )/(At ),where C 0and C were,respectively,the equilibrium concentrations of MO (mg/L)before and after the light irradiation.And,A was the speci?c surface area (m 2/g)or loading (g)of the sample for the calculation of the areal or mass activity;t was the reaction time (min)under light irradiation.The reaction time was set as 10min in the calculation of the areal or mass activities.It is seen that the areal activity remained almost unchanged when the mixture was calcined at temperatures no higher than 9001C,considering the measurement errors in speci?c area and photocatalytic activity.The areal activity of the mixture decreased dramatically after it was calcined at 1100or 13001C for 2h.When the calculation at 13001C was prolonged up to 42h,the calcined mixture became inactive for the photo-catalytic reaction.

It is well known that the areal activity of a photocatalyst depends on the density of surface active sites.For the calcinations at temperatures no higher than 5001C,the

Table 1

Speci?c surface areas (S BET )of the calcined ZnO/TiO 2/SnO 2mixtures with the molar ratio of Zn:Ti:Sn ?4:1:1Calc.temp.(1C)As mixed 200300500700900110013001300a S BET (m 2/g)

6.67

6.59

6.43

6.25

5.92

4.32

2.32

0.37

0.23

The duration of the calcination was 42h for this sample but 2h for the others.

250300350400450500550600

A b s o r b a n c e (a .u .)

Wavelength (nm)

1300 °C, 42 h

1300 °C, 2 h

1100 °C, 2 h

900 °C, 2 h

700 °C, 2 h

500 °C, 2 h

Fig.3.UV-Vis DRS of the ZnO/TiO 2/SnO 2mixture after the calcinations at the indicated

temperatures.

0.0

0.20.40.6

0.81.00

202530354045

Irradiation time (min)

C /C 0

Fig.4.Dependence of the photocatalytic activity of the ZnO/TiO 2/SnO 2mixture on the calcination temperature and duration.The calcination temperature and duration are raw mixture with no calcinations (B ),2001C and 2h (&),3001C and 2h (J ),5001C and 2h (W ),7001C and 2h (’),9001C and 2h (E ),11001C and 2h (m ),13001C and 2h (?),and 13001C and 42h (K ),respectively.Loading of the photocatalysts:2.5g/L;concentration of methyl orange:20mg/L.

C.Wang et al./Journal of Solid State Chemistry 178(2005)3500–3506

3504

composition and speci?c surface area of the ZnO/TiO 2/SnO 2mixture remained unchanged.Therefore,the areal activity of the mixture changed little after the calcinations at those low temperatures.We found that the separately prepared pure Zn 2TiO 4as well as Zn 2SnO 4solid solutions were totally inactive for photocatalytic reaction (not shown).As indicated by the XRD results in Fig.1,the mixture calcined at 7001C contained some Zn 2TiO 4from the reaction of TiO 2and ZnO.Nevertheless,the areal activity of the mixture calcined at 7001C was just a little lower than those calcined at the lower temperatures (Table 2).This little changed photocatalytic activity suggests that the amount of Zn 2TiO 4formed in the mixture after the calcination at 7001C was not very signi?cant.When the mixture was calcined up to about 9001C,all of the anatase TiO 2had been converted to the photocatalytically inactive Zn 2TiO https://www.sodocs.net/doc/455460142.html,pared with the sample calcined at lower temperatures (p 7001C),the slightly improved areal photocatalytic activity as well as the decreased speci?c surface area of this sample (calcined at 9001C)would imply that a considerable amount of the photoactive ZnO still remained unreacted at the sample surface after the calcination at 9001C,which could be the main contribution for the photocatalysis (the remaining SnO 2was not enough active as shown in Fig.5).When the calcination temperature was raised to 11001C,the majority of the photoactive ZnO reacted with SnO 2to form Zn 2SnO 4and only a very small amount of ZnO was left unreacted,therefore,both the mass (Fig.4)and areal (Table 2)photocatalytic activities of this calcined mixture became signi?cantly lower than those of the samples calcined at lower temperatures.Although the calcination at 13001C for 2h signi?cantly reduced the mass activity (Fig.4)of the mixture compared to the calcination at 11001C,the areal activity (Table 2)of the mixture did not show signi?cant difference after the calcinations at 1100and 13001C,both for 2h.Such an insigni?cant difference in the areal activity is probably arisen from the very small speci?c surface area (0.37m 2/g),only a little unreacted photoactive ZnO would enable a meaningful areal activity.However,the mass (Fig.4)and areal photocatalytic activity data (Table 2)were in agreement to show that the prolonged calcination at 13001C (42h)?nally made the mixture inactive for the photocatalysis,which gives an independent evidence that all of the components in the mixture were reacted to form the solid solution Zn 2Ti 0.5Sn 0.5O 4with no photocatalytic activity.Thus,it could be conclusive that the conduction band energy (E CB )of Zn 2Ti 0.5Sn 0.5O 4is not positive enough to oxidize the H 2O/OH àinto the OH radicals,

which is necessary to initiate a photocatalytic reaction in aqueous solution [1,29].

A comparison of the photocatalytic activities of the ZnO/TiO 2/SnO 2mixture,and the individual pure ZnO,TiO 2and SnO 2,all calcined at 5001C for 2h,is made in Fig.5.The speci?c surface areas were,respectively,6.25m 2/g for ZnO/TiO 2/SnO 2, 3.20m 2/g for ZnO,11.36m 2/g for TiO 2and 10.18m 2/g for SnO 2.Theoreti-cally,the coupled ZnO/TiO 2/SnO 2mixture would exhibit higher photocatalytic activity than any of the correspond-ing single oxides ZnO,TiO 2or SnO 2[29,30],but the data were not totally that case in Fig.5.The mass photo-catalytic activity of the ZnO/TiO 2/SnO 2mixture,obtained according to the method de?ned previously,is 2.0times that of TiO 2,16.4times that of SnO 2and 0.92times that of ZnO,i.e.,the ZnO/TiO 2/SnO 2mixture was photocatalyti-cally more active than the pure TiO 2and SnO 2but slightly less active than ZnO.This could be caused by the co-presence of the highly active ZnO and little active SnO 2in the ZnO/TiO 2/SnO 2mixture.Therefore,the present data failed to show the coupling effect in increasing the photocatalytic activity with the ZnO/TiO 2/SnO 2mixture.4.Conclusions

ZnO/TiO 2/SnO 2mixtures with the molar ratio of ZnO:TiO 2:SnO 2?4:1:1were prepared from the compo-nent oxide powders ZnO,TiO 2and SnO 2by solid-state reaction at elevated temperatures.Formation of the inverse

Table 2

Areal photocatalytic activities of the calcined ZnO/TiO 2/SnO 2mixtures with the molar ratio of Zn:Ti:Sn ?4:1:1Calc.temp.(1C)

As mixed 200300500700900110013001300a Areal activity (mg/(L min m 2))

2.54

2.56

2.63

2.70

2.26

2.84

0.88

0.94

0.043

The duration of the calcination was 42h for this sample but 2h for the others.

0.0

0.20.40.60.81.0

C /C 0

Irradiation time (min)

https://www.sodocs.net/doc/455460142.html,parison of the photocatalytic activities of ZnO,TiO 2,SnO 2and ZnO/TiO 2/SnO 2after the calcination at 5001C for 2h.Loading of the photocatalysts:2.5g/L;concentration of methyl orange:20mg/L.

C.Wang et al./Journal of Solid State Chemistry 178(2005)3500–3506

3505

spinel Zn2TiO4and Zn2SnO4was detected at700–900and 1100–12001C,respectively.Further calcinations at still higher temperatures enabled the mixture to form a single solid solution Zn2Ti0.5Sn0.5O4.The ZnO/TiO2/SnO2mix-ture was photocatalytically active for the degradation of methyl orange in water,but its mass activity decreased with increasing the calcination temperature because of the formation of the photoinactive Zn2TiO4,Zn2SnO4and Zn2Ti0.5Sn0.5O4.The sample became completely inert for the photocatalysis after prolonged calcination at13001C (42h)since all of the active component oxides were reacted to form the solid solution Zn2Ti0.5Sn0.5O4with no photocatalytic activity.After the calcination at5001C for 2h,the photocatalytic mass activity of the ZnO/TiO2/SnO2 mixture was16.4times that of SnO2,2.0times that of TiO2, and0.92times that of ZnO.

Acknowledgments

The authors would thank Profs.Ruji Wang,and Jianyuan Yu,Mr.Jingzhi Wei,and Yu Liang,Mrs. Xiaoyan Ye,Meijuan Zhao,and Feng’en Chen for their helps in the characterizations of the samples.We also thank the?nancial support of this work from NSF China (Grant:20125310)and National BasicResearc h Program of China(Grant:2003CB615804).

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ZnO-photocatalysis

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