Photocatalytic Activity of Cu/TiO

Notes Bull. Korean Chem. Soc. 1999, Vol. 20, No. 8 957

Photocatalytic Activity of Cu/TiO

with Oxidation State of Surface-loaded Copper

Kang YongSong,YoungTaeKwon,Guang JinChoiJandWanIn Lee*

Department of Chemistry andCenter for Chemical Dynamics,Inha University, Inchon 402-751, Korea tDivisionof Chemical Engineering, KoreaInstituteofScienceand Technology,P.O.Box 131, Seoul130-650, Korea

ReceivedFebruary 8, 1999

Loadingof some metalsonTiO2 surface has beenknown as one of the successful methods for the improvement of photocatalytic oxidation reaction. 1~4 The role of loaded metal is trapping and subsequent transfer of photoexcited electrons on TiO2 surface. However, all conductive metals are not effective for the improvement of photocatalytic activityofTiO2. It hasbeensofarreported that Au,Agand Pd are effective,2〜4 but Pt or Ru is detrimental for the photo­

catalytic oxidation reaction ofTiO2.5〜7 One ofthe important properties required to the loaded metal wouldbe a suitable workfunction value. It is proposed that photocatalyticoxida­ tion reaction is an electrochemicalreaction amongtheTiO2

conduction band,loadedmetal, and electrolytesadsorbedon TiO2 surface. Energy level of the loaded metal may be defined as its workfunction.Then,thepositionof workfunc­

tionwill be critical for theeffective transferofelectrons on the conduction band ofTiO2 toward adsorbed electrolytes.

Secondly, the loaded metals on TiO2 should be chemically stableduringthephotocatalyticoxidation reaction or storage in atmosphericcondition. If theloadedmetals are oxidized, conductivity of metal will be decreased down and their energylevelmaybealtered from its metallic state. Then,the photocatalytic activity willbe degenerated, since they can­ not traportransferthephotoexcitedelectron efficiently.

In thiswork, we investigatetheeffect of oxidationstate of loaded Cu on the photocatalytic oxidation reaction. Cu is one ofextraordinary metals, which exist stably as metallic form as well as metal oxides in atmospheric condition. In addition, it is expected that theloaded Cu mayimprovepho- tocatalyticactivity of TiO2, since the workfunction of Cuis quite similar to TiO2 conduction band and the standard reduction potential of oxygenmolecule, which is considered tobe aprobable electronacceptorin TiO2 aqueous suspen- sion.8 In addition,little study has sofarbeen carried outon thephotocatalyticactivityof Cu-loaded TiO2.9,10

Experiment지 Section

The TiO2 usedin this study was Degussap25 (47 m2/g, 70% of anatase). Cu was deposited onto TiO2 by photo­ chemicaldecomposition of CuSO4 - 5压。.A reagent grade ofCuSO4 • 5H2O was obtainedfrom AldrichChemical Co.

andused as received. 1.0 g of TiO2 wasdispersed in aqueous solutionscontaining various amountsofthe CuSO4 -5H2。.

Distilledwaterwasadded to achieve atotal volume of 140 mL. In order to expeditethe photodecomposition, 10mL of ethanol was added in this suspension as a hole scavenger.

Then the mixture wastransferredtothe silica reaction vessel

and 300W Xe lamp (LX300 UV) without UVcut-off filter was usedto irradiate the suspensions. After the irradiation for 30 min,the aqueous suspension wasthen centrifugedand the recovered precipitates werevacuum-desiccated for sev­ eral hours. The concentration of CuSO4 - 5压。，which was not decomposed during the photochemical decomposition reaction, was evaluated from the characteristic Cu2+(aq) absorptionpeakataround 800nm and thisresult was used for thecalibration of theloadedCuconcentrationonTiO2. It was found thatmost ofCuSO4 - 5压。was decomposed to metallic Cu. The deposition of Ag on TiO2 powders was achievedbyphotodecompositionof AgF (AldrichChemical Co.). Detailed preparation procedure is given in previous work.3

For the evaluation of photocatalytic efficiencies of sus­

pended powder samples inthe aqueous solution, 1,4-dichlo- robenzene(DCB)wasutilized as a model compound.1.5 mg ofTiO2 ormetal-loaded TiO2 powders were added in 100 mL ofdistilled water and suspendedby ultrasonic homoge­

nizer(Cole-Palmer) operatedat 20 kHz. 14 mL of dispersed samplewas transferred to a Pyrex reactor and2 mL of 200 卩 M DCBaqueous solutionwas added. Total volume ofthe solution was adjusted to 16 mL and the concentration of DCBto 25)1M. Sampleswerethenirradiated with a 300W Xelamp(LX300UV) with UV cut-offfilter. Afterthe irra­

diation of every 2.5 min, the concentration of remnantDCB in the solution was measured with a Perkin-Elmer 552A UV-visible spectrophotometer.

XRD patterns of TiO2 powders in glancing angle mode wereobtained using aPhilipsdiffractometer(PW3020)with a monochromated high intensity CuKai radiation (入=1.5405 A). Theoxidation state ofCu for theCu/TiO2 samples, was examined by X-ray photoelectron spectroscopy (XPS, SSI- 2803S). X-ray absorption near edge spectra (XANES) for CuK-edge were obtainedusing the 3C1 EXAFS beamline in Pohang Light Source (PLS). Each spectrumwasobtained at thefluorescencemode with a solid detector.

Results andDiscussion

Photocatalytic activity ofCu/TiO2 samples with the con­

centration ofloaded Cu is described in Figure 1. Catalytic activity was defined as overall degradation rate constant, since the photocatalytic oxidation reaction of aqueous organic molecules with TiO2 suspensions approximated to first-order kinetics. The optimum concentration of Cupro­ viding the maximized photocatalytic efficiency is 0.5-1.0 mole percent relative to TiO2. Photocatalytic activity of

958 Bull. Korean Chem. Soc. 1999, Vol. 20, No. 8 Notes

0.050

0.025 0.075 -

0.000

0.0 0.5 1.0 1.5 2.0

Mole % of Loaded Cu vu

沱)

1U5SUO。

욺

uDr 으

a5pe」₆

<DQ

=e

」

(D>

o

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Figure 1. Photocatalytic activity of Cu/TiO² samples with the concentration of surface-loaded Cu (Model compound: DCB, 25 I

丄

0.8% Cu/TiO2 is approximately 1.4 times of that of pure TiO2.

Previously, we reported that Ag is greatly effective in improving photocatalytic activity of TiO2.3 Photocatalytic activityof 1.5 mole % Ag/TiO2 isabout2.0 times of that of pure TiO2. It is considered that the high efficiency of Ag/

TiO2 originates from the effective electron-transfercapabil­ ity of Ag clusters. The workfunction of Ag is 4.26 eV.

Hence,the energy levelof Ag isdenotedto-0.24VinNHE, whichis locatedat slightlyhigherlevel than the conduction band ofTiO2 (-0.1V inNHE), and in addition it is close to the electrochemical potential of oxygen reduction in aqueoussolution [O2 (aq) + e_ T O2— (aq), E° = -0.56V in NHE],11,12 as is describedinFigure2. Thus,itis considered that the loadedAg onTiO2 canpromotethetransfer of elec­ tronsin TiO2 conductionband to outeroxygen, which isdis­ solved in water. The same argument can also be appliedto Cu/TiO2 system. The workfunctionof Cu is 4.65 eV,which isrelativelyclosetothat of Ag, much differently from other noblemetals suchas Pt, Ru, or Rh. Therefore,the role of Cu on TiO2 surface is considered to be expedition ofelectron transferasAg does.Relativelylowerphotocatalytic activity of Cu/TiO2 than that of Ag/TiO2 isascribed to thelocation of Cu energy level, which isnot so much appropriate in trans­

ferringthe electrons to outer oxygenas that ofAg (see Fig­

ure2).

Itwas observed that Cu/TiO2 samples left in atmospheric conditionwere gradually changedintheircolorfrompurple to pale-gray, and the photocatalyticefficiencies were accord- in이ydecreased down with theelapseofstoringtimeinair.It is speculated that the decrease ofphotocatalytic activity is closely related to the oxidation of surface-loaded Cu on TiO2. Inthis work, wepreparedseveral Cu/TiO2 oxidizedat differentconditionsandcharacterizedthe oxidation states of Cu for these samples, toinvestigate thecorrelationbetween theoxidationstate of loadedCuand thephotocatalytic activ-

Figure 2. Schematic energy diagram of TiO², loaded metals, and several electrolytes.

ity. 0.8 mole % Cu/TiO2, presenting optimized photocata­ lyticactivity, wasannealed at200and 300oC for30 min in oxygenatmosphere, respectively. Photocatalytic activity of annealedCu/TiO2 samples was greatly decreased,compared with that of as-preparedCu/TiO2, asisdescribedin Figure3.

The colors ofthesamples annealedat200-300oC inoxygen were changed to gray. Cu clustersonTiO2 maynot be metal­

lic Cu. Forthe X-raydiffraction measurement, Cu has been loaded on a low-crystallized anatase TiO2 film, since the crystallographicphase ofCu in 0.8 mole % Cu/TiO2 can not be identified by XRD and in addition CuO (111) peak is overlapped with anatase (112) peakof TiO2 powders. As is shown in Figure 4, the loaded Cu in as-prepared Cu/TiO2

sampleis identifiedtometallic copper.In theXRD patterns for the sample annealed at 200oC in oxygen, 35.6° peak is indexed to Cu2O (100) phase and a small peak at around 38.7° is denoted to CuO (111) and anatase (112). For the samples annealed at 300oC, Cu species on TiO2 is mainly CuO phase. This indicates that most ofmetallic Cu clusters

0.100 - V듵

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o 0.025- 0.050 - 0.075-

一•一

—▲ Cu/TiO2

• O - Ag/TiO2

0.000 4­

0 100 200

Annealing Temperature (°C) 300

Figure 3. Photocatalytic activity of TiO2, Cu/TiO2, Ag/TiO2

samples with the oxidation temperature (For Cu/TiO2 the concentration of Cu is 0.8 mole %, and for Ag/TiO2 that of Ag is 1.5 mole % relative to TiO?).

Notes Bull. Korean Chem. Soc. 1999, Vol. 20, No. 8 959

00 2

W-U

CDC-

Cu/TiO2 (as-prepared)

“I 如kt#"” 쌔나I o

o o

o 6

4

o -

Anatase Anatase

1 I U I J --- --- --

Pure TiO2 filrn^^ An^ase (200) I (211) a서쌔*허뿨"너*

Anatase (101)

10 20 30 40

Two Theta

50 60

Figure 4. XRD patterns of Cu-loaded TiO^? films annealed at several temperatures (TiO? film in anatase phase was deposited on a silica substrate at 450 oC with MOCVD method. Cu was deposited on TiO^? film by photodecomposition method with 0.001 M CuSO4 • 5H?O aqueous solution.).

in C니/TiO? samples are oxidized to Cu?O by annealing at

?00 oC and they areconvertedto CuO phasebyannealingat 300 oC. However, the XRD resultshere do not provide the direct proof forthe oxidation state of Cu, becausethe XRD patterns arenot obtainedfromthe Cu/TiO? powdersamples presenting optimized photocatalytic activity. Hence, XPS spectrawereobtained forthe 0.8 mole % Cu/TiO? samples.

As is shown in Figure 5 (a), the binding energy of Cu ?p3/?

for fresh C나/TiO? is 93?.7 eV and that of the samples annealedat ?00oC and 300 oC is933.?and933.4eV,respec­ tively. This indicates that Cu in as-prepared Cu/TiO? is metallic and the samples oxidized at ?00 oC and 300 oC involveCuOphase.However,thepresence of Cu?Ophaseis unclear. Oxidation state of Cuwas also analyzed by X-ray absorption spectroscopy. XANES spectra of Cu K-edge regionwereobserved for thetwoCu/TiO? samples(see Fig­

ure6). Compared to freshCu/TiO?, samplesannealedat 300

oC show extra absorption peak at around 8978 eV, even though it isnot clear with poor intensity ofsignal, which is caused by thelowconcentration of Cu in the samples. The peakataround8978eVisidentified tothetransition from 1s to 3d.^13,14 This is anotherproof thattheoxidationstate ofCu is +? forthesampleannealedat 300 ^oC.

As a reference experiment, pure TiO? and Ag/TiO? sam­ ples were annealed at the same condition. Photocatalytic activityof pureTiO? was slightlydecreased with theanneal­ ing process as is shown in Figure 3. Ag/TiO? was also decreasedin photocatalytic efficiency,but theirpercentage of decrease is considerably smaller than that of Cu/TiO?. XPS spectrainFigure 5(b) present that thebindingenergies of Ag 3d5/? line were not appreciably changed with the annealingtreatmentat ?00oC or 300oC. Thisindicates that the loaded Ag onTiO? still existsas metallic state and this explains why photocatalytic activity of Ag/TiO? sample is

5000 -

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으 은

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4000 -

3500- 4500 -

으( oe CD>_느 七은

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366 368 370 372 374 376

Binding Energy (eV)

Figure 5. XPS spectra of Cu/TiO2 and Ag/TiO^? samples annealed at several temperatures (The concentration of Cu is 0.8 mole % and that of Ag is 1.5 mole %.); (a) Cu/TiO?; (b) Ag/TiO?.

less decreased thanthat of Cu/TiO?.

Itis derivedthatthemetallic CuonTiO? improvesphoto­

으( roo cn>

느 호

<

u 으

은

。s q<

Figure 6. XANES spectra of Cu K-edge region for Cu/TiO?

samples (The concentration of Cu is 0.8 mole %).

960 Bull. Korean Chem. Soc. 1999, Vol. 20, No. 8 Notes

catalyticactivity of TiO2. When Cu is oxidized to CuO,pho- tocatalyticefficiency isgreatly decreaseddown. The activity is evenpoorerthan thatof pureTiO2 sample. It is expected that CuOclustersonTiO2 are not effective in the transfer of photoexcited electrons on the TiO2 conduction band. This supportsourhypothesis, thatis, the role ofloaded metal on TiO2 surface is theexpedition of electron transfer fromTiO2

conduction bandto outersystem.

Acknowledgment. The authors wish to acknowledge the financial support of the Korea Research Foundation in 1997 (97-003-D00157) and Inha University 97-98 fund.

Experimentsat PLSweresupported by MOST and POSCO.

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