Copper doping in titanium oxide catalyst film prepared by dc reactive magnetron sputtering45

Effect of surface species on Cu-TiO 2photocatalytic activityBaifu Xin a ,b ,Peng Wang a ,*,Dandan Ding b ,Jia Liu b ,Zhiyu Ren b ,Honggang Fu baSchool of Municipal and Environmental Engineering,Harbin Institute of Technology,Harbin 150090,ChinabSchool of Chemistry and Materials Science,Heilongjiang University,Harbin 150080,ChinaReceived 20June 2007;received in revised form 11August 2007;accepted 3September 2007Available online 6September 2007AbstractThe Cu-TiO 2nanoparticles with different Cu dopant content were prepared by sol–gel method.The structure of the as-prepared catalysts and the surface species of Cu-TiO 2were determined using X-ray diffraction (XRD),X-ray photoelectron spectroscopy (XPS)and diffuse reflection spectroscopy (DRS).The relationship between the photocatalytic activity and the surface species of Cu-TiO 2was revealed via the measurement of surface photovoltage spectroscopy (SPS)as well as the degradation of the rhodamine B (RhB).The experimental results suggest that the Cu-TiO 2photocatalysts with appropriate content of Cu (about 0.06mol%)possess abundant electronic trap,which effectively inhibits the recombination of photoinduced charge carriers,improving the photocatalytic activity of TiO 2.While at high Cu dopant region (>0.06mol%),the excessive oxygen vacancies and Cu species can become the recombination centers of photoinduced electrons and holes.Meanwhile,at heavy Cu doping concentration,excessive P-type Cu 2O can cover the surface of TiO 2,which leads to decrease in the photocatalytic activity of photocatalyst.The photocatalytic experimental results are in good agreement with the conclusions of SPS measurements,indicating that there is a close relationship between the photocatalytic activity and the intensity of SPS spectra.#2008Published by Elsevier B.V .Keywords:TiO 2;Cu doping;Photocatalysis;SPS1.IntroductionTitanium dioxide is broadly used as a photocatalyst because it is photochemically stable,non-toxic and cost little [1–5].However,the efficiency of photocatalytic reactions is limited by the high recombination rate of photoinduced electron–hole pairs formed in photocatalytic processes and by the absorption capability for visible light of photocatalysts.Many studies have been devoted to the improvement of photocatalytic efficiency of TiO 2,such as depositing noble metals [6–14]and doping metal or nonmetal ions [15–22].Generally,the introduction of doped ions can result in the formation of a doping energy level between the conduction and valence bands of TiO 2.In principle,it should be possible for doped TiO 2to extend its absorption into the visible region effectively.The effects of copper ions have been studied on the photodegradation of organic pollutant [23–29].A number ofpostulations have been put forward with regard to the role of copper ions play in the mechanism of the photocatalytic degradation of these organics.Some studies have suggested that observed enhancement of the photocatalytic degradation rates was due to electron trapping by the copper ions leading to the prevention of electron hole-recombination [23,26,30–35].Despite a large number of studies have been carried out on these Cu-TiO 2systems,many aspects with regard to the role of copper species with multivalent states in the photocatalytic reaction remain unclear.In this work,the copper-doped TiO 2nanoparticles were prepared and the effects of the surface species on the photocatalytic activity were investigated by means of XPS,SPS and DRS technology.2.Experimental2.1.Preparation of Cu-TiO 2photocatalystThe photocatalyst was prepared by means of sol–gel method.7.6mL of tetrabutyl titanate was slowly added into 8mL of ethanol under vigorously stirring (solution A),a certain amount of Cu(NO 3)2dissolved in 2mL of ethanol was then/locate/apsuscAvailable online at Applied Surface Science 254(2008)2569–2574*Corresponding author.Tel.:+8645186283801;fax:+8645186673647.E-mail address:pwang73@ (P.Wang).0169-4332/$–see front matter #2008Published by Elsevier B.V .doi:10.1016/j.apsusc.2007.09.002added dropwise to solution A,and the mixed solution was continuously stirred until the gel was formed.The gel was dried at1008C,calcined at4008C,and ground to obtain the Cu-TiO2 nanoparticles.The Cu concentrations in the samples were0, 0.02,0.04,0.06,0.08,0.10,0.15,0.20,0.40,0.60,0.80,1and 3mol%,respectively.2.2.Characterization of samplesXRD analysis of TiO2powders was carried out on a Rigaku D/MAX-rA X-ray diffractometer,employing Cu K a(Ni filtered)radiation l=0.15418nm.The patterns were recorded in a range of20–658(2u).The surface compositions and chemical states of the samples were examined by a VG ESCALAB MKII X-ray photoelectron spectrometer using a monochromatic aluminum X-ray source. All binding energies(BE)were calibrated by the BE(284.6eV) of C1s,which gave BE values within an accuracy ofÆ0.1eV.Quantitative analysis was carried out using the sensitivity factors supplied by the XPS instrument.The SPS instrument was assembled at Jilin University,and monochromatic light was obtained by passing light from a 500W xenon lamp(CHF-XQ500W,China)through a double prism monochromator(SBP300,China).The slit widths of entrance and exit were2and1mm,respectively.A lock-in amplifier(SR830,USA),synchronized with a light chopper (SR540,USA),was employed to amplify the photovoltage signal.The powder sample was sandwiched between two ITO glass electrodes.The diffuse reflection spectra were obtained in the wavelength range300–700nm using a Shimadzu UV-2550 UV–vis spectrometer equipped with the integrating sphere accessory for diffuse reflectance spectra.BaSO4was used as a reference.2.3.Evaluation of the photocatalytic activity of Cu-TiO2The photocatalytic degradation of RhB over Cu-TiO2was carried out in a home-built reactor.A160W high-pressure mercury lamp was used as light source,whose intensity was 17.1mW/cm2.In each run0.10g Cu-TiO2catalyst was added into20mL RhB solution of10mg/L.After premixing for 20min,the light was turned on to initiate the reaction.A Shimadzu UV-2550UV–vis spectrometer was used to determine the concentration of RhB solution before and after photocatalytic degradation.3.Results and discussion3.1.XRD measurementFig.1shows the XRD patterns of3mol%Cu-TiO2powders calcined at different temperatures for4h.From Fig.1,it can be seen that a little rutile phase appeared when the calcining temperature was5508C,a considerable amount of anatase transformed into rutile in the powder while the sample was calcined at6008C,and all anatase transformed into rutile after calcined paring with the XRD patterns of pure TiO2samples[36],it can be found that Cu dopant could promote the phase transformation from anatase to rutile.There were two possible reasons for the decrease in the phase transformation temperature.One was that the density of surface defects of Cu-TiO2would increase with increasing Cu doping content,which would promote the phase transformation since the surface defects were considered as the rutile nucleation sites[37,38], Another was that the surface oxygen vacancy concentration of anatase grains is increased with increase of Cu dopant content, which favored the rearrangement of ions and reorganization of structure for rutile phase[39].Moreover,no peaks due to various Cu species were detected in Fig.1,which suggests that Cu species were uniformly dispersed in TiO2powder.3.2.XPS characterizationThe XPS spectra of O1s of TiO2(A)and the0.06%Cu-TiO2 (B)are shown in Fig.2.They werefitted with the non-linear least squarefit program using Gauss–Lorentzian peak shapes and three O1s peaks appear after deconvolution,which are attributed to lattice oxygen(O L,530.3eV),surface hydroxyl oxygen(O–OH,532.1eV)and adsorbed oxygen(O S,533.6eV) in TiO2[40].According to the principle method and handbook of the XPS instrument,the relatively quantitative analysis can be performed by utilizing the XPS peak area of different elements and their own sensitivity factor according to the following equation:nðE1ÞnðE2Þ¼AðE1Þ=SðE1ÞAðE2Þ=SðE2Þ;where n is the atomic number,E an element and S is elemental sensitivity factor.Thus,some important XPS data could be obtained.The approximate formulas for calculating the per-centage of oxygen vacancies are as follows:V O}%¼½ðThe atomic number ratio of TiÂ4ÞÀðThe atomic number ratio of O LÂ2Þ2sÂ100; Fig. 1.The XRD patterns of the3mol%Cu-TiO2calcined at different temperature for4h.B.Xin et al./Applied Surface Science254(2008)2569–2574 2570where V O ¨is the oxygen vacancies.Table 1shows the XPS data of O and Ti elements on the surface of TiO 2and the 0.06%Cu-TiO 2.Based on the data of Fig.2and Table 1,it can be found that the amount of surface hydroxyl oxygen,adsorbed oxygen and oxygen vacancies of the 0.06%Cu-TiO 2increased but the amount of crystal lattice oxygen decreased.The oxygen vacan-cies are a very active group,which can easily combine with other atoms or groups to become stable,and it accounts for an increase in the amount of the adsorbed oxygen and surface hydroxyl oxygen [41,42].Fig.3shows the Cu2p 3/2XPS spectrum of the 3mol%Cu-TiO 2.It was fitted with the non-linear least square fit program using Gauss–Lorentzian peak shapes.The XPS spectrum of Cu2p 3/2indicates that there are two components after deconvolution,attributed to Cu 2O (928.9eV)and CuO (931.1eV)on the surface of TiO 2,respectively.According to the position and the shape of the XPS peaks,the copper on the surface of TiO 2may exist in multiple-oxidation states.These experimental data are less than the reference data of Cu2p binding energies of Cu 2O (932.1eV)and CuO (933.4eV)reported by Wagner et al.[43].Meanwhile,the Ti2p binding energy of the 0.06mol%Cu-TiO 2sample is increased comparing with that of pure TiO 2as shown in Fig.4.This is because the Fermi levels of Cu 2O and CuO are lower than that of TiO 2so that the electrons of TiO 2may transfer to the Cu 2O and CuO dispersed on the surface of TiO 2,which results in changes in the outer electron cloud density of Ti and Cu ions.So,the Ti2p binding energy increases and Cu2p binding energies decrease.This fact suggests that there is an intense interaction between TiO 2and Cuspecies.Fig.2.XPS spectra of O 1s on the surface of TiO 2(A)and the 0.06%Cu-TiO 2(B)nanoparticles,respectively.Table 1XPS data of different chemical states of O and Ti elements on the surface of TiO 2and the 0.06%Cu-TiO 2SamplesBinding energy (eV)Atomic number ratio of O L and Ti Percentage of O L Percentage of O –OH Percentage of O S Evaluated percentage of oxygen vacancies O 1s (O L )O 1s (O –OH )O 1s (O S )Ti2p 3/2TiO 2530.1531.6533.0457.6100:58.377.015.57.5170.06%Cu-TiO 2530.1531.4533.1458.1100:61.564.625.59.923Fig.3.XPS spectrum of Cu2p on the surface of the 3%Cu-TiO 2nanoparticles.Fig.4.XPS spectra of Ti2p of TiO 2and the 0.06%Cu-TiO 2nanoparticles,respectively.B.Xin et al./Applied Surface Science 254(2008)2569–257425713.3.DRS analysisFig.5is DRS spectra of pure TiO 2and Cu-TiO 2with different doping ratio.Three response regions can be seen after Cu doping.It is well known that Cu 2O and CuO are P-type semiconductors with a small band gap of 2.1and 1.7eV,respectively [44,45].The absorption in the visible region from 400to 700nm can be attributed to the absorption of Cu 2O and CuO on the TiO 2surface.The Eg of TiO 2is approximately 3.2eV which corresponds to the absorption of wavelength <380nm.The DRS spectra of Cu-TiO 2exhibit a progressive increase in absorbance with an increase in Cu content.It can be concluded that Cu modified TiO 2extends its absorption into the visible region effectively.3.4.SPS analysisThe surface photovoltage (SPV)method is a well-establish contactless technique for the characterization of semiconduc-tors.It can offer important information about semiconductor surface,interface and bulk properties,mainly reflecting the carrier separation and transfer behavior with the aid of light [46].The SPS principle can be explained as shown in Chart 1.The absorbed photons induce the formation of free carriers by creating electron–hole pairs via band-to-band transitions.The photoinduced electrons may be transferred from the surface to the bulk and the photogenerated holes can be moved to the surface under the built-in electric field.Thus,the surfacepotential across the surface space charge region (SCR)change from V s to V s 0.It is the signal of SPS that variational value of surface potential before and after illumination during the test,i.e.D V .Fig.6shows the SPS spectra of pure TiO 2and Cu-TiO 2with different Cu dopant content.It can be found that the SPS spectra of pure TiO 2is different from that of doped TiO 2,The SPS intensity is increased with increase in Cu dopant content from 0up to 0.06mol%,and then decreased.This is because that oxygen vacancies and Cu (II)ions can act as traps to capture the photoinduced electron.Meanwhile,due to the Fermi levels of CuO and Cu 2O are lower than that of TiO 2,which leads to the photoinduced electrons transfer from TiO 2to CuO and Cu 2O at low Cu dopant region ( 0.06mol%).These factors result in that the surface potential of Cu doped TiO 2is much less than that of pure TiO 2during the illumination.Thus,the recombination of photoinduced electron–hole pairs is inhibited so as to make the SPS response signal increase.This is of benefit to the photocatalytic reaction.While at high Cu dopant region (>0.06mol%),the excessive oxygen vacancies and Cu species can become the recombination centers of photoinduced electrons and holes.So,it can be seen that the intensity of SPS signal is decreased.Moreover,from Fig.7,SPS spectra of pure TiO 2and the mol%Cu-TiO 2,a new negative response can be found.It can be attributed to the SPS response of P-type Cu 2O.It is well known that P-type semiconductor has poor photocatalytic activity.3.5.Evaluation of photocatalytic activityFig.8shows the photocatalytic degradation curves of RhB over Cu-TiO 2photocatalysts with different Cu dopant content calcined at 4008C for 4h.It can be found that the degradation ratio of RhB is increased gradually with increase in Cu dopant content.Over 0.06mol%Cu-TiO 2,the degradation ratio is the highest,which could be attributed to the following:(1)appropriate amount of oxygen vacancies and doped Cu (II)ions on the surface layer of TiO 2can effectively capture the photoinduced electrons;(2)photoinduced electrons can transfer to the Cu 2O and CuO on the surface of TiO 2;(3)the amount of surface hydroxyl is increased.While at highCuFig. 5.The DRS spectra of (a)pure TiO 2,(b)0.02mol%Cu-TiO 2,(c)0.2mol%Cu-TiO 2,(d)0.6mol%Cu-TiO 2,(e)0.8mol%Cu-TiO 2,(f)1.0mol%Cu-TiO 2and (g)3.0mol%Cu-TiO 2.Chart 1.Schematic diagram of surface photovoltage spectroscopyprinciple.Fig.6.SPS spectra of pure TiO 2and Cu-TiO 2with different Cu content.B.Xin et al./Applied Surface Science 254(2008)2569–25742572dopant region (>0.06mol%),the excessive oxygen vacancies and Cu species can become the recombination centers of photoinduced electrons and holes.Meanwhile,excessive P-type Cu 2O can cover the surface of TiO 2,leading to decrease in the photocatalytic activity of photocatalyst.The photocatalytic experimental results are in good agreement with the conclu-sions of SPS measurements,indicating that there is a close relationship between the photocatalytic activity and the intensity of SPS spectra.So the activity of photocatalyst may be estimated by SPS measurements.4.Conclusions(1)The Cu-TiO 2nanoparticles with different Cu dopant content have been prepared via sol–gel method.The Cu-TiO 2photocatalysts with appropriate content of Cu (about 0.06mol%)possess abundant electronic trap,such as the oxygen vacancies and Cu (II)ions.Meanwhile,the Cu 2O and CuO also act as the roles of capture the photoinduced electrons.As a result,the recombination of photoinduced charge carriers can be effectively inhibited.In addition,the content of surface hydroxyl on the surface of the 0.06mol%Cu-TiO 2is increased remarkably in contrast to that of pure TiO 2.These factors can improve the photocatalytic activity of TiO 2.While at high Cu dopant region (>0.06mol%),the excessive oxygen vacancies and Cu species can become the recombination centers of photoinduced electrons and holes.Meanwhile,at heavy Cu doping concentration,excessive P-type Cu 2O can cover the surface of TiO 2,which leads to decrease in the photocatalytic activity of photocatalyst.(2)The photocatalytic experimental results are in good agreement with the conclusions of SPS measurements,indicating that there is a close relationship between the photocatalytic activity and the intensity of SPS spectra.So the activity of photocatalyst may be estimated by SPS 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