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Journal of Luminescence 110 (2004) 17–22

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doi:10.1016/j.jlu

ZnO nanoparticles included within all-silica MCM-41:preparation and spectroscopic studies

Ying Xiong, Lei Z. Zhang1, Guo-Qing Tang*, Gui-Lan Zhang, Wen-Ju Chen

Institute of Modern Optics, Optoelectronic Information Science and Technology Laboratory, MOE, Nankai University,

Tianjin 300071, PR China

Received 24 November 2003; received in revised form 1 April 2004; accepted 1 April 2004

Available online 19 July 2004

Abstract

ZnO nanoparticles have been successfully loaded in the all-silica molecular sieve MCM-41. Combined powder X-ray

diffraction and infrared spectral measurements show that ZnO particles have been formed and dispersed in the

nanoporous channels host. The marked blue shifts in the electronic absorption band and visible fluorescence

are ascribed to the quantum size effects. The visible photoluminescence is attributed to the defects that are related to the

anionic oxygen vacancies.

r 2004 Elsevier B.V. All rights reserved.

PACS: 78.30.�j; 78.55.�m; 78.67.�n

Keywords: Zinc oxide; Nanoparticles; MCM-41; Photoluminescence

1. Introduction

Since the discovery of buckminsterfullerene [1],nanostructured materials have attracted greatinterest due to their potential impact in manyareas such as electronics, photonics, catalysis, andsensing [2]. Many applications are possible includ-ing one-dimensional superconductors, high-den-sity semiconductor nanowire sensor arrays,

ing author. Tel./fax: +86-22-2350-1242.

ess: gqt@nankai.edu.cn (G.-Q. Tang).

dress: Arthur Amos Noyes Laboratory of

ics, Mail Code 127-72, California Institute of

sadena, CA 91125, USA.

e front matter r 2004 Elsevier B.V. All rights reserve

min.2004.04.002

gigahertz nanomechanical resonators, and high-density molecular electronics circuits [3]. Mostrecently, a general method for producing ultra-high-density arrays of aligned metal and semicon-ductor nanowires and nanowire circuits has beenshowcased by Heath and co-workers [4]. In 1992,the synthesis of a new family of nanoporousmolecular sieves designated as M41S was reported[5]. MCM-41, one member of this family, hasuniform cylindrical pores in hexagonal arrange-ment and is produced using rod-like micellesof cationic surfactant molecules as a template.The main characteristics of MCM-41 are itshigh pore volume (B1 cm3 g�1), large surfacearea (B1000m2 g�1) and very narrow pore-size

d.

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Y. Xiong et al. / Journal of Luminescence 110 (2004) 17–2218

distribution (2–10 nm). Due to the regularlyhexagonal arrays of nanoporous channels andchangeable pore diameters, MCM-41 has becomethe particular focus of extensive investigations,and these properties make MCM-41 materialamong the best candidates as the hosts for manyguest materials [6], especially semiconductor na-noparticles, which may prove useful in optoelec-tronic device applications.ZnO, a promising group II–VI semiconductor

with wide and direct band gap of ca. 3.37 eV atroom temperature [7], is one of the few metal oxidenanoparticles that show quantum confinementeffects in an experimentally accessible condition[8]. Hence, ZnO nanoparticles have shown con-siderable potentials as the starting material for thefabrication of short wavelength electrooptic de-vices, light-emitting diodes, transparent conduc-tive films, and so on [9]. A key stage for the studyand applications of ZnO nanoparticles is to obtainstable sample. The solution process is a main andeffective method to prepare nanometer-sized ZnO[8]; however, these ZnO colloids are not stable andgrow rapidly. In this contribution, we report anovel route of synthesizing ZnO nanoparticles byusing all-silica MCM-41 as the confinement hostand perform the spectroscopic studies. It isnoteworthy that all-silica sieves are chosen herebecause the interference of electrostatic effects dueto the exchanged cations and associated fieldsobserved in the aluminosilicate sieves can beavoided in this case [10].

2. Experimental

2.1. Sample preparation

The nanoporous MCM-41 sample used in thiswork was prepared and characterized according tothe literature [11] and calcined before use. MCM-41 absorbs metal ions of IB, IIB, and VIII groupsin water and ethanol mixed solutions because thethermoactivated hydroxyl groups in its intrachan-nel surface have a strong ability to complex theseions. The preparation process of ZnO nanoparti-cles involves a room-temperature stirring ofMCM-41 in 0.08mol/l Zn(CH3CO2)2 � 2H2O

water/ethanol solutions for 24 h. The resultedsample was filtered, washed with copious ethanolin order to remove the minute amount of Zn2+

that were not complexed in the channels and onthe outside surfaces of MCM-41. Finally, thesample was dried and calcined at 380�C in air for4 h to form ZnO-MCM-41 nanocomposites.

2.2. Physical measurements

The powder X-ray diffraction (XRD) wasperformed with a Rigaku X-ray diffractometer(model D/max-B) with CuKa radiation (40 kV,120mA). Diffuse reflectance UV-visible absorp-tion spectra of pure ZnO and its MCM-41complex system were both recorded on a Shimad-zu UV-2101 spectrophotometer. Infrared spectraon KBr pellets were measured on a Bio-Rad FTS135 FT-IR spectrophotometer. Xenon arc lamp(450W) is used as the excitation light source inperforming excitation and emission spectra. Allthe excitation and emission spectra were recordedon the Acton Research SpectroPro-300i spectro-meter with two channels, one being spectral CCDoperating at �15�C and the other being PMT.Chemical analysis was performed using an ICP9000 (N+m) spectrometer.

3. Results and discussion

A dominating strong peak at ca. 2y ¼ 2:2�

features the powder XRD diagram of the puresiliceous MCM-41, which is consistent with thereported MCM-41 structure [5]. A 3.0-nm-wideeffective mean pore diameter can be estimatedaccording to the reference [11]. The powder XRDpatterns of ZnO loaded MCM-41 are essentiallythe same as those of pure molecular sieves,indicating that the structure of MCM-41 isretained after the preparation process and ZnOshould be fully dispersed in its channels, as thecase of Fe2O3 inside the mesoporous host [12]. Thecontent of ZnO in MCM-41 is found to be ca.3.0wt% as revealed by ICP analysis.The infrared spectra of bulk ZnO, pure siliceous

MCM-41, and ZnO-MCM-41 nanocomposites onKBr pellets are presented in Fig. 1. IR spectrum of

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4000 3500 3000 2500 2000 1500 1000 500

Wavelength / cm-1

3500 3000 2500 2000 1500 1000 500

-0.2

-0.1

0.0

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0.93500 3000 2500 2000 1500 1000 500

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-0.1

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Fig. 1. The infrared spectra of bulk ZnO (line 1), pure siliceous

MCM-41 (line 2), and ZnO-MCM-41 nanocomposites (line 3)

at room temperature.

250 300 350 400 450 500 550 600 650

20

40

60

80In

tens

ity /

arbi

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y un

its

Wavelength / nm

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300 400 500 6000.0

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Fig. 2. The normalized excitation (monitoring at 520 nm) and

photoluminescence spectra of bulk ZnO at room temperature.

lex ¼ 330nm.

250 300 350 400 450 500 550 600 650

50

60

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80

90

100

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nsity

/ ar

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ary

units

Wavelength / nm

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300 400 500 6000.0

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Fig. 3. The normalized excitation (monitoring at 405 nm) and

photoluminescence spectra of ZnO-MCM-41 nanocomposite at

room temperature. lex ¼ 330 nm.

Y. Xiong et al. / Journal of Luminescence 110 (2004) 17–22 19

MCM-41 is consistent with that reported in theliterature [15], and remains nearly unchangedcomparing with that of ZnO-loaded sieves despitethe disappearance of the 964 cm�1 band and theslight broadening of the 463 cm�1 band. The lattershould be attributed to the 447 cm�1 strongabsorption band of ZnO [16], regardless of itslow loading content, other than the bands ofMCM-41. From the results of IR spectral mea-surement, it can be seen that the framework ofMCM-41 has been unaltered in the preparationprocess of ZnO nanoparticles, and the slightalternation of the spectrum can be ascribed tothe loading of trace amount ZnO.Diffuse reflectance UV-visible absorption spec-

tra of pure ZnO and its MCM-41 complex systemare also recorded. It is well known that thequantum size effect is one of the most strikingfeatures of nanometer-sized materials [13]. Inthe quantum size region, the band gap of thesemiconductor increases when the size of theparticles is decreased, which leads to the blueshiftof absorption bands. The evidence of quantumsize effect of confined ZnO nanoparticles isshowcased in some recent nice papers [14]. In ourstudy, the blueshift of absorption band of ZnOloaded in MCM-41 is not so much, which is linewith its excitation spectrum (see Fig. 3) and maybe due to the different preparation conditions(under poor oxygen atmosphere and low calciningtemperature).

The normalized excitation and photolumines-cence spectra (lex ¼ 330 nm) of ZnO bulk materialand ZnO-MCM-41 nanocomposite are shown inFigs. 2 and 3, respectively. The excitation spectraare measured by monitoring at 520 and 405 nm ofthe emission spectra, respectively, which corre-spond to the maximum emission intensities of thebulk material and the nanoparticles of ZnO. Aweak electronic absorption band centering atca. 359nm is observed in the excitation spectrumof bulk ZnO, while that of its nanoparticles isfeatured by a strong band dominating at ca. 340nm.

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The marked blue shift of this electronic absorptionsignature reflects an increasing band gap of thesemiconductor, which arises from the well-knownquantum size effect. Considering the large surfacearea of MCM-41, the nano-sized ZnO particlesshould be located in its channels and are possiblyformed as monolayers loosely attached onto theinternal surfaces of the molecule sieve.As seen in Fig. 2, bulk ZnO exhibits a

characteristic photoluminescence spectrum withtwo maxima around 393 and 515 nm. It isgenerally accepted that the UV emission atlem ¼ 393 nm is attributed to the exciton band,which is resulted from the radiative combinationof photogenerated conduction band electrons(e�CB) with valence band hole (h

þVB) [8,17]. How-

ever, no unanimous interpretation has yet beengiven for the green fluorescence peak, as stated inthe 1999 edition of the Phosphor Handbook [18].Henglein and colleagues put forward the conceptof ‘‘anion vacancies’’ [8a,b]. They describe thefluorescence centers as a cation-rich structure(anion vacancies) on the surface of a crystallitewhich can attract photogenerated electrons fromthe conduction band of particles such as ZnO[8a,b] and ZnS [19]. Contrarily, Hoffmann and co-workers attribute the green fluorescence to photo-generated, trapped electrons tunneling to preexist-ing, trapped holes [8c]. Their model differs fromthe previous one in that it does not require thepresence of surface anion vacancies as a prerequi-site for the occurrence of visible emissions.Although some reports can be found in supportof the latter mechanism [20], our present studyshowcases evidence confirming the former and thereasons are explained as follows.Before our discussion of the emission spectrum

of ZnO loaded in MCM-41, it should be notedthat we have tried in vain to record anyunambiguous emission from our pure MCM-41sample. However, such a MCM-41 emission isactually reported in some recent publications[21,22]. The red (1.8–2 eV) photoluminescence inMCM-41 has been commonly attributed to oxy-gen-related defects [21], while the origin of theblue-green (2.4–2.7 eV) photoluminescence inMCM-41 is still under debate and several modelshave been proposed [22]. However, it is found

from these papers that in most cases the MCM-41emission is very weak, which may be obtained bycontinuous accumulation of the faint emissionsignal. In the meantime, the intensity variation ofthe observed MCM-41 emission is dependent onthe preparation procedure and temperature. Forthe MCM-41 prepared in our laboratory, noemission can be observed under the excitationand measurement condition used to observephotoluminescence of the ZnO-MCM-41 nano-composite system. In addition, no MCM-41emission spectrum is reported when opticallyactive molecules are incorporated into the nano-porous channels of MCM-41.It is evident in Fig. 3 that ZnO nanoparticles

confined in the MCM-41 host display a strongfluorescent band centering at ca. 405 nm. Thenature of this emission band is unambiguouslyassigned to the blue-shifted visible fluorescenceaccording to some seminal papers [8]. It has beenobserved that concomitant to the decrease of thegrain size of ZnO particles, the visible fluorescenceblueshifts dramatically accompanying by an in-crease of its emission intensity, and the UVemission intensity decreases rapidly until unobser-vable [8]. In agreement with these observations, wehave similarly recorded that the intensity of thevisible fluorescence increases by a factor of ca. 5.We propose that the visible fluorescence is due tothe defects that are related to the anionic oxygenvacancies, which is correlated with our samplepreparation procedures. The visible luminescenceof bulk ZnO is characteristic of phosphors fired inair or under strong reduction conditions [23].Excess Zn has been detected by a chemicalanalysis. Interstitial zinc ions and oxygen vacan-cies are considered to be the dominant defects.Similar results have also been released regardingthe mechanistic studies of the photoluminescencein ZnO powders [24] and fine particles [25]dispersed in alkali halide crystals as transparentmatrices. Being further experiments to confirm ourproposal, we have calcined the as-synthesizedsample in oxygen flow and finally found that thevisible fluorescence intensity of the ZnO-MCM-41solid decreases drastically, which is because a richoxygen environment leads to the decrease of theamount of oxygen vacancies in ZnO nanoparticles.

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So the visible emission should be very likelyattributed to the defects caused by anion vacan-cies. The massive blue shift to short wavelength ofthe emission peak should arise from the sizequantization effects because of the extremely smallsize of the sieve-confined ZnO particles. It can beexpected that there should be more defects (oxygenvacancies) of ZnO nanoparticles in porous solidthan those in solution system, and therefore theabove effects should be more remarkable as wehave actually observed. In addition, due to asmaller crystal size, the phonon state densityresponsible for the radiationless relaxation shouldbe lower; that is, there will be fewer channels forradiationless recombination.In summary, we report in this letter the

successful preparation and spectroscopic proper-ties of ZnO nanoparticles included with all-silicamolecular sieve MCM-41. Combined powderXRD and IR spectral characterizations show thatZnO particles have been formed and dispersed inthe nanoporous channels host. The great blueshifts in the electronic absorption band and visiblefluorescence are assigned to the quantum sizeeffects. Anionic oxygen vacancies have beenassumed to be the most likely candidates for therecombination centers involved in the visibleluminescence of ZnO. The visible emission inten-sity of the ZnO-MCM-41 nanocomposite increasesby a factor of ca. 5 comparing with that of bulkZnO, showcasing promising potentials of usingnano-sized ZnO in the optoelectronic devicefabrications. Further studies on the optical non-linearity behaviors of ZnO are underway in ourlaboratory.

Acknowledgements

Financial support from the National NaturalScience Foundation of China (No. 59672015) isgreatly acknowledged.

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