九州大学大学院総合理工学府報告第 37 巻 第 2 号 1-5 頁 平成 28 年 2 月Engineering Sciences Reports, Kyushu University
Vol. 37, No. 2, pp. 1-5, FEB, 2016

Structural analysis of mesoporous ZrO2 and TiO2
nanofiber mats prepared by electrospinning methods
Chanmin LEE*1 Yong-Gun SHUL*2 and Hisahiro EINAGA*3,†
E-mail of corresponding author: einaga.hisahiro.399@m.kyushu-u.ac.jp†(Received December 29, 2015, accepted January 9, 2016)
Ag particles loaded mesoporous zirconia oxide (ZrO2) and titanium oxide (TiO2) nanofibers wereprepared by electrospinning methods followed by calcination at 500-600ºC. The crystal structure ofthe Ag particles were investigated by X-ray diffraction (XRD) and scanning electron microscopy(SEM). The fiber diameters for the ZrO2 and TiO2 mats were in the range of 60-160 nm and 50-450
nm, respectively. X-ray line broadening analysis showed that they were composed of smaller sized
crystallites. Nitrogen-adsorption studies revealed that the ZrO2 and TiO2 fibers have both
mesoporous and macroporous structures, which were originated from the aggregation of smallercrystallites than the fiber diameters and the interspaces formed by randomly oriented nanofibers.
The deposition of Ag on the fibers followed by calcination at 500ºC changed the distribution of themesopore sizes.
Key words: ZrO2, TiO2, mesoporous materials, electrospinning method

1.

Introduction

Zirconium dioxide (ZrO2) and titanium
dioxide (TiO2) are widely used in the catalysts
for supporting materials because they arechemically and physically stable and havegenerally high surface areas. These materials
are generally prepared by liquid-phase andgas-phase synthesis processes1,2). As for thecatalyst supports, mesoporous materials whichhave the pores whose diameters are in therange of 2-50 nm because they generally havelarge surface areas and intrinsic properties
caused by the porous structures. Over thedecades, mesoporous materials have beendeveloped using liquid-phase preparation
methods using surfactants as the template
molecules3). The preparation of mesoporous
ZrO2 and TiO2 has been reported by several
groups and the efficacy of these materials forcatalytic reactions has been discussed4,5). In
addition, much attention has been recently
focused on the metal oxides that havemacroporous structures because the catalyst
Department of Molecular and Material Sciences
Department of Chemical and BiomolecularEngineering, Yonsei University*3 Department of Energy and Material Sciences*1*2

properties also depended on the shape andmorphologies of the catalysts. For instance,
the control of the catalyst shape andstructures improved the oxidation of dieselsoot emitted from the diesel engines. Therefore,
the fabrication of metal oxides having
macropores is one of the important targets in
the research area of catalysis science.
The electrospinning phenomenon was firstlyobserved by Rayleigh in 1897. Since then, thishas been utilized for fabricating polymeric andinorganic fibrous materials6-8), due to theirconvenient and versatile method. In particular,
electrospun ceramic fibers are often in thespotlightdue
totheoutstanding
characteristics such as high surface area andhierarchical pore9,10). Additionally, they arewidely used in a wide variety of applications,for example, as filtration membranes11),
delivery of bioactive agents12), batteries or
solar cells13,14), proton exchange membrane
fuel cells15) and catalytic reaction16).
We herein report that the ZrO2 and TiO2nanofibers which have both mesoporous andmacroporous structures have been successfully
prepared by the electrosppining method. The
characteristic porous and crystallographic
structure of ZrO2 and TiO2 nanofibers are

2Structural analysis of mesoporous ZrO2 and TiO2 nanofiber mats prepared by electrospinning methods

investigated by scanning electron microscopy(SEM), N2 adsorption desorption isotherm,
Barrett-Joyner-Halenda (BJH) pore size
distribution and X-ray diffraction (XRD)
analyses. We also investigated the effect ofAg loading on the porous structures of ZrO 2and TiO2 fibers because supported silver
catalysts draw much attention due to theirpotential
applicationforsolid-solid
heterogeneous catalytic reactions such as
catalytic soot oxidation and catalyticsterilization.

2.

Experimental

2. 1. Chemicals and preparation of ZrO2 andTiO2 nano-fibrous support and catalysts
Zirconium oxychloride (ZrOCl2·8H2O) andTitanium isopropoxide were purchased from
Kishida Chemical. Silver nitrate (AgNO3,99.8%) and N, N-dimethylformamide (DMF,99.5%) were obtained from Wako Pure Chem
Ind Ltd. Polyvinylpyrrolidone (PVP, MW=1.3×106) were obtained from Sigma–Aldrich
Japan.

Fig. 1. Schematic illustration of ZrO2 and TiO2nanofibers synthesis sequence.
Figure 1 shows the schematic illustration ofZrO2 and TiO2 nanofibers synthesis sequence.
To prepare ZrO2 nanofiber, we used a generalelectropinning method followed by a heat
treatment. 3g of ZrOCl2·8H2O and 1g of PVP
were dissolved in 9g DMF. Then, thisprecursor mixture was vigorously stirred atroom temperature for 6 h. The preparedprecursor solution was electronspun onto arotating drum with a distance between thedrum surface and spinneret tip of 12 cm. High
voltage of 18 kV was applied at the stainlesssteel needle (27 G, inner diameter = 0.21 mm)
and feeding rate was maintained at 0.5 mL h-1.
The electrospun Zr/PVP nanofibers were
calcined at 600ºC for 3 hrs under the air
atmosphere to perfectly eliminate PVP.
For the synthesis of TiO2 fibers, 0.73 g ofPVP and 0.35 g Acetic acid were dissolved in 7g EtOH. Then, 1.8 g Titanium isopropoxide
was added to the prepared solution, and thismixture
vigorously
stirredatroomtemperature for 30 min. The electrospinning ofTiO2 fiber was performed with the preparedprecursor solution at a voltage of 14 kV anddistance between the collector (rotating drum)and syringe needle (30 G, inner diameter =0.15 mm) of 15 cm. During electro-spinning, afeeding rate and humidity were kept at 0.6 mL
hr-1 and 30 % RH, respectively. To remove thepolymer substance (PVP) and form the pureTiO2 fiber, the electrospun TiO2/PVP fibers
were calcined at 500ºC for 6 hrs in airatmosphere.
5 wt% silver was loaded on the calcined ZrO2and TiO2 fibers by the following procedures; (1)calculated amount of AgNO3 (5 wt% Ag) was
dissolved in 100 mL of pure water; (2) 0.1 g offibers were added to solution; (3) to evaporate
water and decompose the metal salts, they
were stirred at 250ºC for 3 h; (4) finally thesample was calcined at 500ºC for 5 h.
2. 2. Characterization
The morphologies of ZrO2 and TiO2
nanofibers were observed by the scanningelectron microscopy (SEM; JEOL). X-ray
diffraction were performed by means of RINT
2000 (RIGAKU) recorded from 25 to 100 o withscan
speedof10min-1.
The
Brunauer-Emmett-Teller (BET) surface areaand pore diameter were analyzed by nitrogen
sorption at 77 K (BELSORP II; BEL). To
discuss the chemical condition on the surfaceof
thecatalyst,X-ray
photoelectron
spectroscopy (XPS; Thermo VG Scientific, UK)
was conducted using a monochromated Al
X-ray source (Al kα line at 1486.6 eV) at X-ray
power of 36 W under a vacuum (7.8 × 10 -9mbar).

3. Results and discussion
3. 1. Structural properties of ZrO2 nanofibers
The shapes and morphological structures
were investigated using a scanning electron
microscopy (SEM). Figure 2 displays SEM
images and fiber diameter distribution of
eletrospun ZrO2 nanofibers. The ZrO2nanofibers exhibited an average diameter of
105 nm (selected 200 samples) with range ofabout 60 to160 nm and a rough surface
morphology, as shown in Fig. 2a. Figure 2c also
shows that the ZrO2 nanofiber mats have theporous structures, which were composed of the平成 27 年度

九州大学大学院総合理工学報告

randomly oriented ZrO2 fibers. The interspace
sizes are in the range of 1-20 µm.

第 37 巻第2号

observed, respectively. According to peak band
width, line-broadening analyses were carriedout to estimate the crystalline sizes of the ZrO2fibers using the Scherrer equation (1).
D = 0.94λ / (β1⁄2 cos θ)

Fig. 2. SEM images of electrospun nanofibers(a) ZrO2 and (b) TiO2 and fiber diameter
distribution of (c) ZrO2 and (d) TiO2 nanofibers.
. Figure 3a exhibits the XRD patterns of ZrO2,
which confirms the crystal structure. The
major diffraction peaks corresponding to (11Ī),(111), (002), (110), (21Ī), (220), (131), (311) and(400) of crystalline monoclinic structure(JCPDS No. 74-0815) and (101), (110), (112),(211), (202) and (220) of crystalline tetragonalphase (JCPDS No. 79-0815) were clearly3(1)Where D is the crystallite size, λ is the X-ray
wavelength of 1.54056 Å, β1⁄2 is the full widthat half maximum (FWHM) of (101) peak and θis the Bragg angle. The average size of theZrO2 crystallines thus determined were 12 nm,
which was much smaller than that of the ZrO2fibers determined by the SEM images. Thisfindings indicates that the fibers werecomposed of the aggregated crystallites ofmonoclinic and tetragonal phases.
The pore structure of ZrO2 and Ag/ZrO2
nanofibers was investigated by nitrogen
adsorption studies. Figure 4 shows thenitrogen
adsorption-desorption
isotherms
obtained at 77 K (Fig. 4a and b). ZrO 2 andAg/ZrO2 nanofibers showed type III isotherms
and H3 hysteresis loops, which are thecharacteristics of N2 adsorption-desorption on
mesoporous materials. Moreover, pore size
distribution measured from Barrett, Joyner
and Halenda (BJH) method exhibited themaximum peaks in range of 4.63 – 7.98 nm,
further confirming that the ZrO2 fibers havemesoporous structures (see Fig. 5a).
Figure 4 also shows the N2 adsorption-desorption isotherms for Ag deposited ZrO 2(Ag/ZrO2) nanofibers. The material has also
exhibit the hysteresis loop in the relative
pressure range of 0.6-0.9. The pore size
distribution obtained by the BJH method
shows that the Ag/ZrO2 nanofibers have thepores in the range of 2-20 nm with a maximum
at 10 nm. Thu, the pore sizes distribution wasslightly changed after deposition of AgNO 3 on
the ZrO2 fibers followed by the calcination.
BET surface areas of ZrO2 and Ag/ZrO2
nanofibers were calculated to be 15.5, and 9.5
m2 g-1, respectively. Because the Ag loading
was 5wt%, the decrement of the BET surface
area by Ag deposition cannot be explained interms of the decrement of the ZrO 2 content. Itis likely that the sintering of ZrO2 in thecalcination process gave rise to the decrease inZrO2 surface area.
3. 2. Structural properties of TiO2 fibers

Fig. 3. XRD patterns of (a) ZrO2 and (b) TiO2and Ag supported TiO2 nanofiber.

Figure 2 shows the SEM images of the TiO2nanofibers prepared by the electrospinning
method. After calcination at 500◦C, the4

Structural analysis of mesoporous ZrO2 and TiO2 nanofiber mats prepared by electrospinning methods

diameter of TiO2 nanofiber ranged from about
50 to 450 nm (average diameter of 240 nm)
and exhibited a rough surface morphology (Fig.
2b and d). The sizes are much larger thanthose for ZrO2 fibers. The difference ispresumably due to the combustion ofpolymathic
substance(PVP)athightemperature. Figure 2 also shows that theTiO2 nanofiber mats have the porous
structures, which were composed of therandomly oriented TiO2 fibers, as in the case ofZrO2 fibers.

The crystalline structure of the TiO2 wasinvestigated using of X-ray diffraction (XRD)
analysis and the XRD patterns TiO2 fiber inthe range of 20 to 80º is presented in Fig. 3b.
The diffraction peaks (black line) showed (101),(103), (004), (112), (200), (105), (211), (213),(116), (220) and (107) planes, indicating theformation of anatase phase of titanium oxide(JCPDS No. 84-1286). The crystallite size wascalculated using the Scherrer equation (1). The
crystallite size of TiO2 was 19.5 nm. This value
was much smaller than that of TiO2 fiberdiameters, indicating that the fibers werecomposed of the crystallites of anatase phases.
Figures 4 and 5 show the N2 adsorption-desorption isotherms and the BJH pore size
distributions obtained from the desorption
branch of the isotherms for TiO2 fibers. The N2
adsorption-desorption isotherm also confirmedthat TiO2 and Ag/TiO2 fibers also showed typeIII isotherms and H3 hysteresis loops,
corresponding to a mesoporous structure, asshown in Fig. 4c and d. One major pore-size
peak was observed for the TiO2 fibers at about
10.5 nm with range from 2 to 12 nm,
indicating the presence of mesopores (see Fig.
5b). The pore size distribution was not so muchaffected by Ag deposition, whereas the porevolume was lowered due to the pore blocking
by the Ag deposition and lowered amount ofTiO2 fibers contained in the catalyst. Thus, theFig. 4. N2 adsorption-desorption isotherm of(a) ZrO2, (b) Ag/ZrO2, (c) TiO2 and (d)Ag/TiO2 nanofibers.

Fig. 5. BJH pore size distributions of (a)ZrO2 and Ag/ZrO2 and (c) TiO2 and (d)Ag/TiO2 nanofibers.

平成 27 年度

九州大学大学院総合理工学報告

calcination of TiO2 in the presence of AgNO3
did not change the pore sizes, although thevolume of mesopore decreased. This is inmarked contrast with the results for Ag/ZrO2.
According to the N2 adsorption isotherms,
the surface areas of the fibers were determinedby the BET method. The surface areas of TiO 2and Ag/TiO2 fibers were calculated at 34.4 m2
g-1 and 24.1 m2 g-1, respectively. Therefore, thesurface area of the fiber catalyst also
decreased by the Ag deposition, which werelocated in the mesopores of the TiO2 fibers
blocking the pores and reduced the amount offibers contained in the catalyst.
3.3 Comparison of ZrO2 and TiO2 nanofibers
As described above, the ZrO2 and TiO2 fiber
materials prepared by the electrospinning
method have both mesopores. The mesoporesin the range of 2-10 nm with the maximum of 5nm were located on the ZrO2 fibers, whereas
those in the range of 5-20 nm with themaximum of 10 nm were formed in the TiO2fibers. Because the mesopores were originatedfrom the aggregation of the crystallites of ZrO2and TiO2, the difference in the mesopore sizeswere due to the sizes of these crystallites. As
the crystallites consisting the fibers increased,
the interstitial spaces, the mesopores of thefibers, which were formed by the aggregationof the crystallites increased. These findings
also imply that the changes in crystalline sizes
can control the mesoporous structures.
The ZrO2 and TiO2 fibers showed different
average diameters of 105 and 240 nm,
respectively. This difference can be attributed
to the difference in the electrospinningparameters such as precursor solution
concentration, feeding rate, spinneret innersize and applied voltage for the formation of
low lever diameter (100-300 nm), which
control the structures of fiber materials. On
the other hand, the macroporous structures ofZrO2 and TiO2 fiber materials, which wereoriginated from the interspaces formed by thenanofibers were not so much different forthese two fibers. It is likely, however, that theinterspace sizes can be controlled by changing
the density of the nanofibers, which is also
affected by the preparation conditions for fibermaterials.
It is noteworthy that the post-treatment of
the fiber materials changed the porous
structures of the ZrO2 fibers. When the Ag
particles were deposited on the ZrO2 fibers, thepore sizes of Ag/ZrO2 nanofibers apparently

第 37 巻第2号

5increased. One of the explanations for thisbehavior is that the calcination of ZrO2 in thepresence of AgNO3 promoted the sintering andcoarsening of the ZrO2 crystallites, leading to
the increase in mesopores formed by theaggregation of the crystallites.

4.

Summary and Conclusion

In this paper, we showed that the ZrO2 andTiO2 fiber materials were successfully
prepared by the electrospinning method,
followed by calcination at 600 and 500ºC,respectively. Both the ZrO2 and TiO2 fibers
have the mesoporous and macroporousstructures. The former structures wereoriginated from the aggregation of crystallites.
The latter structures were due to theinterspace of the fibers. These fiber materials
are good candidates for catalytic processes inwhich the adsorption, diffusion, and themorphology strongly affect the catalyticperformances.
Acknowledgments
This work was partly supported by theGrant-in-Aid for Encouragement of Young
Researchers of the Faculty of EngineeringSciences, Kyushu University.
References
1) Brian L. Cushing, Vladimir L. Kolesnichenko, Charles
J. O’Connor, Chem. Rev. 104 (2004,) 3893−3946
2) F. E. Kruis, H. Fissan, A. Peled, J Aero. Sci, 29 (1998)
511–535
3) A. Corma, Chem. Rev. 97 (1997) 2373−2419
4) M. P. Kapoor, Y. Ichihashi, W. Shen, Y. Matsumura,
Catal. Lett. 76 (2001) 139-142
5) D. Chen, F. Huang, Y. Cheng, R. A. Caruso, Adv. Mater.
21 (2009) 2206–2210
6) C. Burger, B.S. Hsiao, B. Chu, Annu. Rev. Mater. Res.
36 (2006) 333–368.
7) D.H. Reneker, I. Chun, Nanotechnology 7 (1996) 216.
8) D. Li, Y. Xia, Nano Lett. 3 (2003) 555.
9) W. Li, C.T. Laurencin, E.J. Caterson, R.S. Tuan, F.K.
Ko, J. Biomed. Mater. Res. 60 (2001) 613–621.
10) D. Li, Y. Xia, Adv. Mater. 16 (2004) 1151–1170.
11) X.H. Qin, S.Y. Wang, J. Appl. Polym. Sci. 102 (2006)
1285–1290.
12) T.J. Sill, H.A. von Recum, Biomaterials. 29 (2008)
1989–2006.
13) B.C. Kim, K.S. Yang, M. Kojima, K. Yoshida, Y.J. Kim,Y.A. Kim, Adv. Funct. Mater. 16 (2006) 2393–2397.
14) S. Chuangchote, T. Sagawa, S. Yoshikawa, Appl. Phys.
Lett. 93 (2008).
15) C. Lee, S. Jo, J. Choi, K.-Y. Baek, Y. Truong, I.
Kyratzis, et al., J. Mater. Sci. 48 (2013) 3665–3671.
16) C. Lee, J.-I. Park, Y.-G. Shul, H. Einaga, Y. Teraoka,Appl. Catal. B Environ. 174-175 (2015) 185–192.