Materials Research Bulletin 44 (2009) 1102–1105

Fabrication and photocatalysis of mesoporous ZnWO4 with PAMAM as a template

Shen Lin a,b,*, Jiebo Chen a,b, Xiulan Weng a, Liuyi Yang a, Xinqin Chen a

a College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350007, Fujian, PR Chinab State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, PR China

A R T I C L E I N F O

Article history:

Received 7 April 2008

Received in revised form 31 July 2008

Accepted 7 October 2008

Available online 21 October 2008

Keywords:

A. Inorganic materials

B. Chemical synthesis

C. Electron microscopy

C. X-ray diffraction

D. Catalytic properties

A B S T R A C T

Mesoporous ZnWO4 was prepared with the template of PAMAM. The as-formed samples were

characterized by X-ray diffraction (XRD), nitrogen absorption, scanning electron microscopy (SEM),

transmission electron microscopy (TEM), and UV–vis diffuse reflectance spectroscopy (DRS). It is found

that the size of pore is in the range of 5–22 nm and that the porosity of ZnWO4 is composed of aggregated

ZnWO4 nanoparticles. The photocatalytic activities towards degradation of rhodamine B (RhB) and

malachite green (MG) under UV light has been investigated. The formation mechanism of mesoporous

structures is proposed.

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1. Introduction

In recent years, ZnWO4 has been attracted a great deal ofresearch attention due to its luminescence properties, andpotential applications such as optical fibers, scintillator materials,magnetic properties and heterogeneous catalysis [1–3]. It is also apromising material for X-ray scintillators because its luminescenceoutput and after glow are comparable to or better than thosematerials currently in use [4]. ZnWO4 has been prepared by severaldifferent processes such as Czochralski method [5], sintering ofWO3 and ZnO or ZnCO3 powder [6], reaction in aqueous solutionfollowed by heating of the precipitate [7], and heating of ZnO thinfilms with WO3 vapor [8], sol–gel reaction [9], and hydrothermalreaction over an extensive period [10]. ZnWO4 particles preparedby these processes are relatively large with inhomogeneousmorphologies or else the ZnWO4 particles should be preparedwith high temperature, almost above 1000 8C for 24 h.

Poly (amidoamine) (PAMAM) dendrimer is a kind of compoundwith their characteristic architectural feature [11]. It consists of adefined core, interior branch, terminating groups and exhibitmonodisperse, structurally well-defined, a definite molecular

* Corresponding author at: College of Chemistry and Materials Science, Fujian

Normal University, Fuzhou 350007, Fujian, PR China. Tel.: +86 591 22868169;

fax: +86 591 83465393.

E-mail address: shenlin@fjnu.edu.cn (S. Lin).

0025-5408/$ – see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2008.10.011

weight and size [12]. In addition, dendrimers of higher generations(G > 3) take a spherical shape, and they can be used as templatesor, nanoreactors to synthesize nano-clusters or nanocomposite[13].

It is well known that photocatalysic properties of solid greatlydepend on their particle size and morphology. Mesoporousmaterials are of particular importance as they have higher packingdensity, good permeation [14], and larger percentage of the activesites than bulk material.

In this paper, we wish to report the preparation of mesoporousZnWO4 with G4-PAMAM (Generation 4 PAMAM with –NH2 endgroups) as a template. To our knowledge, it is the first example forthe preparation of mesoporous photocatalysts using this method.The properties of the materials were characterized by XRD,HRTEM, SEM, DRS, and BET. The photocatalytic activities ofmesoporous ZnWO4 was evaluated by decomposition of RhB andMG.

2. Experimental

2.1. Fabrication of the mesoporous ZnWO4

2 � 10�5 mol G4-PAMAM was added to 100 mL of distilledwater with vigorous agitation till complete homogenization of thePAMAM solution, then the solid of H2WO4 was added until the pH8. Adequate solution of Na2WO4 was added into the above solutionslowly keeping stirred for 2 h. Afterwards, the white precipitate

Fig. 1. XRD pattern of the as-prepared mesoporous ZnWO4 heat-treated at (a)

773 K; (b) 873 K; (c) 973 K for 2 h.

Fig. 2. Nitrogen adsorption–desorption isotherms (inset) and BJH pore size

distribution plot for the mesoporous ZnWO4 heat-treated at 873 K for 2 h.

Fig. 3. HRTEM (a and b) and SEM (c) images of the mesoporous ZnWO4 prepared at 873 K.

S. Lin et al. / Materials Research Bulletin 44 (2009) 1102–1105 1103

Scheme 1. Preparation process of the mesoporous ZnWO4 with the template of PAMAM.

Fig. 4. DRS of the mesoporous ZnWO4 heat-treated at 873 K for 2 h.

S. Lin et al. / Materials Research Bulletin 44 (2009) 1102–11051104

was separated out and washed with distilled water, dried at 323 K,and then calcined at different temperature for 2 h.

2.2. Characterization

XRD patterns were obtained on a Philips X’pert–MPD X-raydiffractometer with Cu Ka. The HRTEM images were taken on aJEM2010 transmission electron microscopy instrument at anaccelerating voltage of 200 kV. The SEM image was taken on JSM-7500F. UV–vis absorption spectra were recorded on a Varian Cary100 Scan UV–vis system equipped with a labsphere diffusereflectance accessory. The Nitrogen absorption and desorptionisotherms were determined using OMNISORP100CX nitrogenadsorption apparatus.

2.3. Photocatalytic activity

UV-light photocatalytic activities of the samples were eval-uated by the decomposition rate of rhodamine B and malachitegreen (MG) in an aqueous solution under mercury 16 W high-pressure lamp with 254 nm, respectively. 50 mg of photocatalystwas suspended in a 50 mL aqueous solution of 1.0 � 10�5 Mrhodamine B. Prior to irradiation, the suspensions were magne-tically stirred in the dark for 30 min to ensure establishment ofadsorption/desorption equilibrium among the photocatalyst,rhodamine B and water. The concentrations of RhB and MG atdifferent irradiation time were monitored by HP 6010 UV–visspectrophotometer at absorption wavelength 554 nm and 620 nm.

3. Results and discussion

3.1. Formation process of crystalline phase

The crystalline structure of synthesized samples were character-ized by XRD. Fig. 1 shows the influence of the heat-treatedtemperature on crystalline of the porous ZnWO4. With the increaseof temperature, the XRD patterns became perfect and the intensitiesincreased gradually, but further increase of the calcinationstemperatures has little effects on the crystal phase of ZnWO4. Allthe peaks are indexed on basis of the crystallographic data of theknown structure of ZnWO4 (JCPDS No.15-0774), suggesting that the

as-samples are phase pure. The cell parameters are a = 4.691;b = 5.720; c = 4.925 A, space group P2/c. By applying the Scherrerformula on the (0 2 1) diffraction peaks, the average crystallite sizesof the samples is foundtobe 38.5 nm,40.2 nmand 40 nm, suggestingthat the calcination temperature has little effect on grain size.

3.2. Grain size, morphology and pore size distribution of mesoporous

ZnWO4

Fig. 2 shows pore size distribution curves calculated fromdesorption branch of the nitrogen adsorption–desorption iso-therms by the Barrett–Joyner–Halenda (BJH) method and thecorresponding isotherms (inset) of samples. The sharp decline indesorption curves and the isotherms of type IV with hysteresis-loop are associated with the presence of mesoporosity. It isreported that the diameter of G4-PAMAM is about 5 nm [11];however, the pore size distributions exhibit the range of 5.0–22.0 nm with the average pore diameter of ca.14.2 nm. It is mostlypossible due to aggregation of the PAMAM.

Fig. 3 shows HRTEM (a and b) and SEM (c) image of therepresentative as-prepared sample. It is clearly observed that there

Fig. 5. (a) Photocatalytic degradation of RhB and MG and (b) UV–vis spectral changes of RhB solution during the photodegradation process in the sample of ZnWO4 prepared at

873 K for 2 h.

S. Lin et al. / Materials Research Bulletin 44 (2009) 1102–1105 1105

is the contrast between gray edge and dark center of the spheres,which reveals that the nanoparticles exhibit well crystalline. Theaverage diameter of particles is estimated to be about 40 nm, inagreement with that obtained from XRD. Furthermore, no differentappearances were observed in the HRTEM image. It is clear that theZnWO4 nanoparticles are not porous. Further observed from theSEM image (Fig. 3c) show that the pores can be seen as black spotswith no-ordered wormhole-like structures and the nanosizedZnWO4 particles appear white. These results indicate that theporosity of the ZnWO4 consists of the aggregated ZnWO4

nanoparticles. Thereby, the formation of mesoporous ZnWO4

can be shown in Scheme 1.

3.3. Optical properties of the mesoporous ZnWO4

The photoabsorption abilities of the mesoporous ZnWO4

sample was detected by UV–vis DRS as shown in Fig. 4. For acrystalline semiconductor, it was shown that optical absorptionnear the band edge follows ahy = A(hy � Eg)n/2, where a, y, Eg, andA are absorption coefficient, the light frequency, the band gap, andconstants, respectively. For the ZnWO4, n is determined to be 1.Thus, the band gaps of the ZnWO4 were estimated to be 3.95 eV,which is larger than the reported value of 3.75 eV [2]. Because thecrystallite size of ZnWO4 is far larger than 5 nm, the blue shift isresulted from crystallization degree quantum rather than sizeeffect. For W-based semiconductors, it was already found thatexcitons are formed due to the transitions into the tungstate W5d

state hybridized with O2p and possess a very strong tendency forself-trapping [15]. Free electrons and holes can be created due tothe transitions into cationic states. An active photocatalyst for thedecomposition of the organic compounds must have a VB withstrong oxidizing ability and photogenerated holes with highmobility. The hybridized VB of ZnWO4 has shown strong ability tophotocatalytic degradation of the organic pollutants in the workdescribed herein.

3.4. Photocatalysis activity

The photocatalytic activities of the prepared ZnWO4 for thedegradation of RhB and MG are shown in Fig. 5a. It is amazing that

over 95% of RhB and MG was photocatalytically degraded after40 min irradiation.

The evaluation of photocatalytic activity was further carried outin aqueous solution for degradation of RhB. The temporal evolutionof the spectral change was shown in Fig. 5b. Tetraethylatedrhodamine shows a major absorption band at 554 nm.UV lightirradiation of the RhB/ZnWO4 dispersion leads to an apparentdecrease in absorption peaks, indicating that the chromophoricstructure of the RhB was destroyed. The adsorption peaks at259 nm and 354 nm were disappeared after 80 min of irradiationas comparison with the initial one, suggesting the destruction ofconjugated structure of RhB. These experiments results indicatethat ZnWO4 exhibited high photocatalytic performance on thedecomposition of dye and may become a promising materialsapplied in photocatalysis.

Acknowledgments

This project was financially supported by the National NaturalScience Foundation of China (No.20771024).

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