納米尺寸半導(dǎo)體材料的熒光性能的觀察和其與光催化活動(dòng)性的關(guān)系

1、 ÃCorresponding author.The Laboratory of Physical Chemistry,School of Chemistry and Material Sciences, Heilongjiang University,Harbin150080,PR China.Tel.:+86045186608458;fax:+86045186673647.E-mail address:JLQiang(F.Honggang.1.IntroductionA number of investigations have focused on semiconductor

2、photocatalysts for their application in solar energy conversion and environmental purication,since Fujishima and Honda discovered the photocatalytic splitting of water on the TiO 2electrodes in 1972110.Among various semiconductor photocatalysts,ZnO and TiO 2are very important for their strong oxidiz

3、ing power and non-toxicity.In general,one of the major limitations in semiconductor photocatalysis is the relatively efciency,because of the high recombination rate at or near its surface.Some success in enhancing the by several methods,such as using nano-sized semiconductor crystallites instead of

4、bulk materials 11,12and modifying photocatalysts by doping with ions 1315,or coupling TiO 2to other oxides 1618.These methods can greatly improve the separation rate of photo-induced charge carriers in semiconductor photocatalysts so that the photocatalytic activity increases.The photoluminescence (

5、PLtechnique has been widely used to investigate the structure and properties of the active sites on the surface of metal oxides and zeolites 19,20,because of its high sensitivity and non-destructive character.Also,the PL technique has been useful in the eld of photocatalysis over semiconductors for

6、understanding the surface processes.PL spectrum is an effective way to study the electronic structure,optical and photochemical properties of semiconductor materials,by which information such as surface oxygen vacancies and defects,as well as the efciency of charge carrier trapping,immigration and t

7、ransfer can be obtained 5,15,2128.Therefore,it is of great signicance for environmental photocatalysis.It is well materials result from the recombination of general,the lower the PL intensity,the lower the electronhole pairs,and the higher the photocatalytic activity of semiconductor photocatalysts

8、29,30.However,the photo-catalytic activity of semiconductor photocatalysts can increase as the PL intensity becomes stronger 31,32.These results seem to be incompatible.In fact,the inherent relationships between the PL spectrum and photocatalytic activity are determined mainly by the attributes of t

9、he PL signal.To the best of our knowledge,despite some work about the PL spectra and photocatalytic activity,papers devoted solely to a systematic description of the PL techinique of semiconductor nanomaterials have seldom been reported,especially about the relationships between the PL intensity and

10、 the photocatalytic activity on the basis of PL attributes.In this review,the mechanisms and inuencing factors of PL spectra of unmodied and modied semiconductor nanomaterials are mainly presented,together with some fundamental aspects,and the PL applications in environmental photocatalysis are emph

11、asized.In particular,our own contributions are highlighted.In other words,the inherent relationships between the PL intensity and photocatalytic activity are discussed in detail.This review can help us further understand transfer and separation behaviors of photo-induced charge carriers at or near s

12、emiconductor surfaces and their effects on photocatalytic activity,which are favorable to investigate and prepare functional semiconductor materials with high activity.Also,this review can help us understand the essential phenomenon of photophysics and photochemistry of nano-sized semiconductor mate

13、rials.J.Liqiang et al./Solar Energy Materials &Solar Cells 90(2006177317871774J.Liqiang et al./Solar Energy Materials&Solar Cells90(20061773178717752.PL mechanism of semiconductor nanomaterials the photon energy can be higher than the band gap energy sometimes,which mainly results from the t

14、ransitions of higher energy of excited electrons from the CB band to the VB top directly.This PL signal is also attributed to a kind of bandband PL phenomenon.Process III is the excitonic PL process in which the non-radiative transitions of excited electrons from the CB bottom to different sub-bands

15、 (or surface statesoccur rst,and subsequent radiative transitions from the sub-band to the VB top can take place 32.The energy of the radiative photon,which is the energy difference between the sub-band and VB top,is lower than the band gap energy.In general,the excitonic PL signal mainly results fr

16、om surface oxygen vacancies and defects of semiconductors.Our research group reported that there were lots of oxygen vacancies and defects on the surfaces of ZnO nanoparticles,and proved the oxygen vacancies and defects had a strong ability to bind electrons by means of EPR measurement 33.Moreover,t

17、he nanoparticle size is ne,so that the average distance which the electrons can move freely to is very short.These factors can make surface oxygen vacancies and defects very easily bind electrons to form excitons in the sub-band.Thus,the excitonic PL process can occur,and the smaller the nanoparticl

18、e size,the larger the oxygen vacancy and defect content,the higher the probability of exciton occurrence and the stronger the PL signal 31,34,35.In addition,the excited electrons at the CB bottom can come back to the VB directly or indirectly by non-radiative transitions,which is the process IV.The

19、above discussions demonstrate that only photophysical processes II and III can give rise to PL phenomena,the former is attributed to the bandband PL,and the latter is attributed to the excitonic PL.According to PL attributes,the bandband PL spectrum can directly reect the separation situation of pho

20、to-induced charge carriers,viz.the stronger the bandband PL signal,the higher the recombination rate of photo-induced carriers.The excitonic PL spectrum cannot directly reect the separation situation of photo-induced carriers.However,it can reveal some important information about surface defects,oxy

21、gen vacancies and surface states,which can strongly affect photocatalytic reactions.The PL performance of semiconductor samples can be examined with a uorescence spectrometer.In our experiment,the PL spectrum is recorded by a PE LS55spectrouorophotometer,using a xenon lamp of 350W as light source.Th

22、e energy of excitation light is higher than the band gap energy of the bulk semiconductor material.During the PL testing process,the sample preparation is carefully performed so that it can efciently be repeated.3.Main factors affecting PL performance of semiconductor nanomaterialIn fact,the PL proc

23、esses are very complicated,which can be greatly affected by some factors,such as excitation energy,material size,dopant species and form.Fig.2shows the PL spectra of different size of ZnO nanoparticles with excitation wavelength of 300nm (Aand 350nm (B32.Although the bandband PL phenomenon cannot be

24、 found,ZnO samples can exhibit a strong and wide PL signal at the range from 400to 550nm,with the excitation light of energy higher than the band gap energy,having two obvious PL peaks at about 420and 480nm,respectively.These PL signals areJ.Liqiang et al./Solar Energy Materials &Solar Cells 90(

25、2006177317871776attributed to excitonic PL,which mainly result from surface oxygen vacancies and defects of ZnO nanoparticles.The PL peak at 420nm is attributed to band edge free excitons,and that at 480nm is attributed to bound excitons 22,24,25.The excitonic PL intensity of ZnO nanoparticles decre

26、ases as the particle size increases,which is ascribed to the decrease in the content of surface oxygen vacancy and defect with increasing particle size.Therefore,a bulk semiconductor material can exhibit a weak PL signal in the usual condition,even no PL signal.In addition,it can be seen from Figs.2

27、(Aand (B,although the energy of used excitation light are both higher than the band gap energy of ZnO,the PL spectra are not completely similar.This is because there are different energy levels in the CB of semiconductor nanoparticles,to which the electrons in the VB can be promoted on the condition

28、 of different excitation energy,and the excited electrons with different energy in the CB can come back to the VB via different courses,further resulting in different PL emissions.These demonstrate that the PL mechanisms of semiconductor nanoparticles are very complicated.However,the peak positions

29、of PL signals do not change a little,indicating that there are several relatively stable energy levels of excitons and surface states.Fig.3shows the PL spectra of TiO 2nanoparticles calcined at different temperature with excitation wavelengths of 300nm (Aand 350nm (B.The excitonic PL intensity of Ti

30、O 2 I n t e n s i t y / a .u .Wavelength / nm I n t e n s i t y / a .u .Wavelength / nm I n t e n s i t y / a .u .Wavelength / nm I n t e n s i t y / a .u .Wavelength / nm (A(B1777To further improve the photoelectric and photocatalytic performance of semiconductor materials,doping with metal ion is

31、often used as an efcient modication method.However,the affecting mechanisms of different dopants on photo-induced carriers are different,which results in different effects on PL spectra.The dopant (I,whose stable chemical state possesses a half-lled or full-lled outer electronic structure,cannot cap

32、ture electrons,such as La 3+and Zn 2+.However,it can greatly inuence PL spectra of nano-sized semiconductor materials by changing material surface structure like defects and oxygen vacancies.In our experiment,the excitonic PL intensity of TiO 2nanoparticles increases after an appropriate amount of L

33、a or Zn is doped by utilizing a solgel process 31,36.Fig.4shows the PL spectra of different amount of Zn-doped (ATiO 2nanoparticles calcined at 5001C,and La-doped (BTiO 2nanoparticles calcined at 6001C with excitation wavelength of 300nm.It can be seen that the undoped and doped TiO 2nanoparticles c

34、an exhibit obvious excitonic PL signals with similar curve shape,demonstrating that Zn or La dopant both do not give rise to new PL phenomena.However,the intensity and response range of PL signals are greatly inuenced,viz.the excitonic PL intensity can increase by doping with an appropriate amount o

35、f Zn or La.This is because of the points that Zn or La dopant can have an inhibiting effect on TiO 2phase transformation from anatase to rutile,especially for La dopant,which can result in an increase in the content of surface oxygen vacancy and defect.Thus,during the dopant (Isystem,the stronger th

36、e excitonic PL spectrum,the higher the content of surface oxygen vacancy and defect.The dopant (II,whose stable chemical state easily captures electrons to become another relatively stable chemical states with a half-lled or full-lled outer electronic structure, I n t e n s i t y / a .u .Wavelength

37、/ nmI n t e n s i t y / a .u .W avelength / nm(A(B I n t e n s i t y / a .u .Wavelength / nmI n t e n s i t y / a .u .Wavelength / nm(A(B I n t e n s i t y / a .u .Wavelength / nm3504004505005506006507000500002000003%WO x -TiO 24%WO x -TiO 2TiO 21.5%WO x -TiO 2P L i n t e n s i t y (a .u .Wavelength

38、 (nm325nm 27.Although the noble metal dopant cannot result in new PL phenomena,it makes the excitonic PL intensity decrease,which is mainly attributed to the capture of noble metal ions.Thus,the photoelectrons and holes can be efciently separated.Therefore,during the noble metal-doped system,the wea

39、ker the excitonic PL spectrum,the higher the separation rate of photo-induced charge carriers.In a word,the dopant species have great effects on separation and recombination processes of photo-induced charge carriers of semiconductor nanomaterials,and can further inuence PL performance.Moreover,the

40、dopant amount is also one of important factors affecting PL signals.Semiconductor nanomaterials have two kinds of main formsparticle and lm.In general,for semiconductor nanoparticles,the excitonic PL signal can easily exhibit under the excitation energy higher than the band gap energy.However,the ba

41、ndband PL signal can seldom be observed.For nanoparticle lms,the bandband PL signal can be seen. I n t e n s i t y / a .u .Wavelength / nm Wavelength / nm P L i n t e n s i t y (a .u .350400450500550cbaI n t e n s i t y (a .u .Wavelength / nma : 500°Cb : 600°Cc : 700°CPL peaks appear

42、at about367and473nm wavelengths,respectively,and the former is attributed to bandband PL,the latter is attributed to excitonic PL.4.PL applications of semiconductor nanomaterial in environmental photocatalysis Delectrons and holes are trapped on the surfaces,respectively.Therefore,the dynamic behavi

43、ors of photo-induced charge carriers,such as transfer,capture,separation and recombination,can greatly affect photocatalytic activity of semiconductor materials.The PL phenomenon is a kind of photophysical process,and the photocalytic reaction is a kind of photochemical process.The PL and photocatal

44、ytic processes are closely related to dynamic behaviors of photo-induced charge carriers of semiconductor materials according to their mechanisms.Therefore,there are certain relationships between PL and photocatalysis,and the intrinsic relationships between PL intensity and photocatalytic activity c

45、an be revealed on the basis of PL attributes.For the bandband PL spectrum of semiconductor materials,the weaker the PL spetrum,the higher the separation rate of photo-induced charge carriers and,possibly,the higher the photocatalytic activity.In Fig.9,the bandband PL intensity of the sample (bcalcin

46、ed at 7001C is lower,which is responsible for higher photocatalytic activity 29.For the excitonic PL spectrum of semiconductor materials,the relationships between PL intensity and photocatalytic activity are very complicated,which are mainly dependent on dopant species.Fig.11reects the activity of d

47、ifferent size of ZnO nanoparticles during the photo-catalytic oxidation gas phase n -C 7H 1632,and Fig.12reects the activity of different amount of La-doped TiO 2nanoparticles calcined at 6001C during the photocatalytic oxidation liquid-phase phenol 31.According to Figs.2and 4,it can be found that t

48、he stronger the excitonic PL spectra,the higher the photocatalytic activity.During the PL process,oxygen vacancies and defects can easily bind photo-induced electrons to form excitons so that PL signal can easily occur,and the larger the content of oxygen vacancy or defect,the stronger the PL signal

49、.However,during the process of photocatalytic reaction, 24002800320036004000oxygen vacancies and defects can become centers to capture photo-induced electrons,so that the recombination of photo-induced electrons and holes can be effectively inhibited31.Moreover,oxygen vacancies can promote O 2adsorb

50、ing,and there is a strong interaction between the photo-induced electrons bound by oxygen vacancies and the adsorbed O 243.This indicates that oxygen vacancies can favor the adsorbed O 2to capture photo-induced electrons,simultaneously producing d O 2radical groups.The radical groups are active to p

51、romote the oxidation of organic substances 2,3.Thus,it can be suggested that oxygen vacancies and defects are in favor of photocatalytic reactions,and the stronger the excitonic PL spectrum,the larger the content of oxygen vacancy or defect,and the higher the photocatalytic activity.Fig.13reects the

52、 activity of different amount of Ce-doped TiO 2nanoparticles calcined at 6001C during the photocatalytic oxidation liquid-phase phenol 37,and Fig.14reects the activity of different amount of Ag-doped TiO 2nanoparticles calcined at 4001C during the photocatalytic oxidation liquid-phase rhodamine (RhB

53、41.According to Figs.5and 8,it can be seen that the lowest PL intensity of the samples corresponds to the highest photocatalytic activity,which is because of the point that the lower the excitonic PL D e g r a d a t i o n r a t e of P h e n o l La content / %intensity,the stronger the capacity of th

54、e dopants to capture photo-induced electrons,the higher the separation rate of photo-induced electrons and holes,and the higher the photocatalytic activity.Fig.15reects the activity of different amount of Sn-doped TiO 2nanoparticles calcined at 6001C during the photocatalytic oxidation liquid-phase

55、phenol 39,and Fig.16reects the activity of different amount of W-doped TiO 2nanoparticles calcined at 5001C during the photocatalytic oxidation liquid-phase methylene blue (MB30.According to Figs.6and 7,it can be seen that the photocatalytic activity of TiO 2samples increases as the excitonic PL spe

56、ctrum become weaker,viz.the weaker the excitonic PL spectrum,the higher the photocatalytic activity.On the basis of the relevant CB position of TiO 2,WO 3and SnO 2,the photo-induced electrons can easily transfer from TiO 2surface to WO 3or SnO 2CB,which can improve the separation rate of charge carr

57、iers and result in the decrease in excitonic PL intensity.Hence,the photocatalytic activity of TiO 2nanoparticles can be enhanced by doping W or Sn,and the weaker the excitonic PL spectrum,the higher the separation rate of photo-induced electrons and holes,and the higher the photocatalytic activity.

58、 Sn content / %In addition,the effects of dopant amount on PL and photocatalytic performance cannot be neglected.In general,when the dopant content is lower than its optimal ratio,the impurity energy level will be a separation center of photo-induced electrons and holes.On the contrary,when the dopant content i