07) 4784–4786www.elsevier.com/locate/matlet

Materials Letters 61 (20

A new synthesis process and characterization of three-dimensionallyordered macroporous ZrO2

Shi Li ⁎, Jingtang Zheng, Weiya Yang, Yucui Zhao

The State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Dongying 257061, China

Received 23 October 2006; accepted 7 March 2007Available online 15 March 2007

Abstract

A three-dimensionally ordered macroporous (3DOM) material ZrO2 has been successfully synthesized by using ZrOC12·8H2O as precursorand polystyrene beads with diameters of 480 nm as template. The merit of this process is that ZrOC12·8H2O is cheaper and has a high meltingpoint. SEM images show that precursor concentration has an important effect in fabricating 3DOM ZrO2. The sample prepared by using theprecursor solution with a concentration of 1.6 M displays a well long-ranged ordered structure and uniform pore sizes. Precursor concentrationbetween 1.3 M and 2.0 M is considered to be the most favorable to fabricate 3DOM ZrO2. XRD analysis indicates that the crystallinity of 3DOMZrO2 is monoclinic phase. Nitrogen adsorption and desorption measurements at 77.4 K show detailed pore structures of 3DOM ZrO2.© 2007 Elsevier B.V. All rights reserved.

Keywords: Macroporous; Catalysts; ZrOC12·8H2O; ZrO2; Surfaces

1. Introduction

In recent years, three-dimensionally ordered macroporous(3DOM) materials with uniform pore size and well-definedperiodic structure have been a hot research topic due to theirpotential applications, such as photonic crystals, catalysts,catalytic supports, chromatographic packing materials, chemi-cal sensors and adsorbents. Colloidal crystal templating methodis a very promising method to fabricate 3DOM materials.Generally, this method involves three steps: (1) assembly ofuniform monodispersed microspheres into organic templates,such as polystyrene (PS) beads; (2) the corresponding precursorsolution is infiltrated into the voids of templates and solidified;(3) removal of the templates. The pore sizes of 3DOM structurescan be controlled easily by using colloidal beads with variousdiameters and a lot of 3DOM materials ranging from polymers,metals to inorganic oxides [1–7] have been fabricated by thismethod.

Transition metal oxide ZrO2 is a very important ceramicmaterial. So far, commercial ZrO2 materials are mostly

⁎ Corresponding author. Tel.: +86 546 8395024; fax: +86 546 8395190.E-mail address: lishi19785460@163.com (S. Li).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.03.033

nanopowders or nanoparticles and have been successfullyapplied in the fields of photoelectric ceramics, dielectricceramics and fuel cell. The fabrication development of 3DOMZrO2 is significantly interesting because of its potentialapplication for catalyst or catalytic supports [8]. Someresearchers have successfully prepared 3DOM ZrO2 by usingthe corresponding metal alkoxide as precursor, such aszirconium butoxide [9–11]. In present study, 3DOM ZrO2 hasbeen prepared successfully by using transition metal chloride–ZrOC12·8H2O as precursor and PS beads as organic template.The significant merit of this method is that ZrOC12·8H2O isvery cheap and has a high melting point.

2. Experimental procedure

All reagents used in the experiment are analytically pure.Deionized water 200 ml with 0.0412 g sodium p-styrenesulfonate (SSS) and 0.1281 g NaHCO3 were poured into areactor and stirred constantly for 10 min, then 19.59 g styrenemonomers were added to the solution under N2 atmosphere.After 30 min, 0.288 g potassium persulfate (KPS) wasintroduced into the solution and the whole polymerizationsystem constantly reacted for 28 h to form PS beads. From

4785S. Li et al. / Materials Letters 61 (2007) 4784–4786

beginning to end, the temperature of the reactive system waskept at 75 °C. After the PS beads were dispersed ultrasonically,they were self-assembled into ordered three-dimensional close-packed organic templates with beads diameter of 480 nm bygravity deposition for 30 days. Fig. 1 shows the SEMmicrograph of the PS template. ZrOC12·8H2O was dissolvedin ethanol to make precursor solutions with different concentra-tions and the solutions were refluxed for 20 h at 70 °C. PStemplates were soaked in the solutions for 20 min, then theexcess solutions were removed by vacuum filtration. Theinfiltered templates were dried at 75 °C for 3 h. The infiltrationsteps were repeated 4 times. The composites were calcined in atube furnace under air flowing at 350 °C for 4 h, followed bycalcination at 580 °C for 6 h at a heating rate of 3 °C/min. Aftercooling naturally, the samples of 3DOM ZrO2 were obtainedeventually.

The surface morphology of the samples was characterized byscanning electron microscopy (FEI Quanta200, Holland).Powder X-ray diffraction (XRD) patterns of 3DOM ZrO2

were collected by using a diffractometer (X'Pert Pro, Holland).The pore structure of 3DOM ZrO2 was analyzed by means ofnitrogen adsorption by utilizing Micromeritics ASAP 2010sorption analyzer.

3. Results and discussion

The structural unit of ZrOC12·8H2O in solution is [Zr4(OH)8(H2O)16]

8+ which has eight hydroxyl groups. The PS beads have a lotof hydroxyl groups also, so PS templates can be wetted easily by [Zr4(OH)8(H2O)16]

8+. In this study, precursor concentrations haveimportant influences in fabricating 3DOM ZrO2. If the precursorconcentration was less than 1.3 M, the low viscosity of precursorsolution would lead to a very weak interaction between precursor andPS template. Consequently, filling matters were separated from PStemplates during vacuum filtration and it was very difficult to form3DOM structure in spite of good wetting ability between precursorsand PS templates. Precursor solutions with concentrations of 2.0 M and1.6 M were used to fill the PS templates respectively. The surfacestructures of the resulted ZrO2 were depicted by SEM in Fig. 2. Whenprecursor concentration was 2.0 M, Fig. 2(a) showed that themorphology of macropores could not be observed because the surface

Fig. 1. SEM micrograph of PS template with beads diameter of 480 nm.

Fig. 2. SEM images of ZrO2 prepared by using 2.0 M solution as precursor (a)and inner structure through slits (b); SEM image of ZrO2 prepared by using1.6 M solution as precursor (c).

of the sample was coated with a layer densified film. From Fig. 2(b),ordered macroporous structures could be observed in the slits of thecoated film but they did not display intact pore walls. Only the thickerparts of pore walls were observed. These phenomena indicated that theviscosity of the precursor solution was so high that precursors whichjust infiltrated into the surface layer of PS templates congregatedtogether and blocked further infiltration. Consequently, large cavitiesand the shrinkage of the surface layer in the products would be formedduring calcinations and they could cause the collapse of pore structure.

Fig. 3. XRD profile of 3DOM ZrO2 prepared by 1.6 M precursor solution.

Fig. 4. The adsorption/desorption isotherm of 3DOM ZrO2 (a); BJH poredistribution of 3DOM ZrO2 in desorption process (b).

4786 S. Li et al. / Materials Letters 61 (2007) 4784–4786

As indicated by Fig. 2(c), 3DOM ZrO2 was obtained successfully byusing the precursor concentration of 1.6 M. The 3DOM ZrO2 with aface-centered cubic (fcc) porous structure was matched with the size ofstarting PS templates and its structure was the negative replica of close-packed PS beads. The spherical pore diameter is about 330 nm and theshrinkage of the skeleton was approximately 31%. Each pore wasconnected by small windows of 90 nm and wall thickness was 100 nm.Precursor concentration between 1.3 M and 2.0 M is the most favorableto fabricate 3DOM ZrO2.

Fig. 3 shows the XRD pattern of the ZrO2 product prepared byusing the precursor solution with concentration of 1.6 M. Four sharppeaks can be seen at the positions of 28.15°, 31.41°, 34.10°,49.23°, which represent monoclinic crystal plane (111), (110), (112),(200), so the wall crystallinity of 3DOM ZrO2 is thermodynamicallystable monoclinic phase. Crystal size is 27.8 nm as calculated by theDebye–Scherrer formula.

The structures in 3DOM ZrO2 are further investigated by nitrogenadsorption and desorption measurements at 77.4 K, as showed in Fig.4. From Fig. 4(a), the curves show a very slow rise in the amount ofadsorbed volume ahead of relative pressure of 0.9, which indicates thatthere are a few micropores and the single molecular layer saturatedadsorption is finished rapidly. Then the shape of the isotherm increasessharply at a high pressure area near 1.0 and exhibits hysteresis.Hysteresis means capillary coagulation in mesopores and demonstratesthe existence of mesopores. Fig. 4(b) shows the pore size has acentralized distribution at the range of 3–4 nm and there is a broad poredistribution between 20 nm and 70 nm, which are consistent withadsorption/desorption isotherm separation in Fig. 4(a). BET specificarea of this 3DOM ZrO2 is 37.2 m

2/g and the mesopores with diameterless than 5 nm have the largest contributions to the specific area.

4. Conclusions

3DOM ZrO2 materials were obtained successfully by usingthe ZrOC12·8H2O solution instead of the corresponding metal

alkoxide as precursor and the structures of 3DOM ZrO2 wereinvestigated. The resulted 3DOM ZrO2 can be looked as theinverse replica of colloidal crystal template and macroporesconnect with each other by small windows. In this study,precursor concentration is a decisive factor to prepare 3DOMZrO2.

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