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Original Article

Recent Res. Devel. Mat. Sci., 10 (2013): 45-58 ISBN: 978-81-308-0518-4

3. Synthesis and characterization of anatase

TiO2 nanofibers using Stevia rebaudiana leaf

aqueous extract Mirla Rodríguez1, Ángela B. Sifontes1, Franklin J. Méndez 1, Edgar Cañizales2

Andrea Mónaco1, María Tosta1 and Joaquín L. Brito1 1Centro de Química, Instituto Venezolano de Investigaciones Científicas, Apartado 20632, Caracas 1020-A, Venezuela; 2Área de Análisis Químico Inorgánico. PDVSA. INTEVEP. Los Teques 1070-A,

Venezuela

Abstract. Anatase TiO2 nanofibers with high surface area

(252 m2/g), diameter in the range of 25-60 nm and length of

100-150 nm, have been synthesized at room temperature by

template method using titanium isopropoxide and Stevia rebaudiana

leaf extract. Characterization was carried out using scanning

electron microscopy (SEM), transmission electron microscopy

(TEM), N2adsorption porosimetry, X-ray diffraction (XRD), Fourier

transform infrared (FT-IR), ultraviolet–visible diffuse reflectance

spectroscopy (UV-vis DRS) and thermogravimetric analysis (TGA).

The template effect of the diterpene glycosides molecules present in

S. rebaudiana leaf aqueous extract was confirmed by TEM and

SEM. The band gap energy was determined using Kubelka-Munk

function. The TiO2 nanofibers synthesized offer potential

applications in catalysis and photocatalysis.

Correspondence/Reprint request: Dr. Ángela B. Sifontes, Centro de Química, Instituto Venezolano de

Investigaciones Científicas, Apartado 20632, Caracas 1020-A Venezuela. E-mail: angelasifontes@gmail.com

Mirla Rodríguez et al.

46

Introduction

Titanium dioxide (TiO2, Eg = 3.2 eV ) has been widely studied due to its

promising applications in photocatalyst, solar cells, environmental pollution

degradation, biomaterials, optical devices, sensors and catalyst processes

[1- 4]. Titanium dioxide has three different structure types: rutile, anatase

and brookite. All of these crystalline forms of TiO2 occur in nature as

mineral, but only rutile and anatase have been able to be synthesized in pure

form at low temperature until recent days [5]. Anatase TiO2 is

thermodynamically metastable and can be easily transformed into the stable

rutile phase when it is heated to 500–600 °C. Such transformation of the

TiO2 crystalline phase is usually accompanied with a severe sintering or

growth of TiO2 crystallites, resulting in a severe decrease in photocatalytic

activity [6]. Anatase TiO2 with higher crystallinity is preferred for

photocatalysis, since higher crystallinity would mean fewer defects for the

recombination of photogenerated electrons and holes [6-7].

Nanosized TiO2 powders are prepared by several methods such as

hydrothermal, sol gel, microemulsion, and thermal decomposition of alkoxides

[7]. On the other hand, nanometric TiO2 compounds with different morphologies

have been reported: nanotubes of 10-20 nm diameter and 50-80 nm length [8],

nanofibers [9], nanoparticles of 30-50 nm in diameter [10].

Functional properties of TiO2 are influenced by many factors such as

crystallinity, particle size, and surface area [11]. From the point of view of

surface area nanosheets and nanotubes of TiO2 offer high surface areas

(about 100-400 m2/g) [12].

Nowadays, many researches are giving great importance to the

development of environmentally friendly synthesis methods using products

less toxic and of low cost. In recent years, S. rebaudiana leaf extract which

contains a complex mixture of eight sweet diterpene glycosides including

stevioside, rebaudioside A, rebaudioside B, rebaudioside C, rebaudioside D,

rebaudioside E, dulcoside and stelviol bioside [13] has been successfully

used for the synthesis of gold and silver nanoparticles [14-16] and most

recently, the synthesis of mesoporous hollow Al2O3 nanorods [13].

Therefore, in this study, we describe for synthesizing anatase TiO2

nanofibers using S. rebaudiana leaf extract as template in aqueous medium.

Nanofibers offer highest surface areas (per unit volume and mass) that the

bulk materials and approaching those of nanoparticles [17]. The materials

prepared by this method exhibit properties potentially useful for many

applications in nanotechnology, electronics, optics, catalysis and other fields.

TiO2 nanofibers

47

Materials and methods

Preparation of the S. rebaudiana leaf extract

Portions of 1.2 g of leaves of S. rebaudiana were extracted with 50 mL

of hot water (65 ºC) for 3 h, as described previously by Nishiyama et al.

[18]. The crude extract was filtered through a Whatman Qualitative filter

paper No. 1.

Synthesis of TiO2 nanofibers

Synthesis of porous titania was carried out from aqueous solutions

employing titanium isopropoxide (Sigma-Aldrich) as metal precursor and

S. rebaudiana leaf extract as template. In a typical preparation, 5.84 g of

titanium isopropoxide was dissolved in 54 mL of distilled water. The

resultant solution was magnetically stirred at room temperature for 2 h.

Subsequently, the extract was added dropwise. The pH value was adjusted to

5 using a diluted acid nitric aqueous solution. The obtained solution was

evaporated and dried at 80 ºC for 48 h. Finally, the resulting solid was

calcined to remove the template. This was carried out in a tubular furnace

under air atmosphere, with a heating rate of 5 ºC/min up to 500 ºC, 600 ºC

and 750 ºC and kept at the maximum temperature for 6 h.

Characterization and instrumentation

Characterization was carried out by X-ray diffraction, using a Bruker

D-8 Focus diffractometer and CuKα radiation in the 2θ range between 5°

and 90°, operating at 40 kV and 30 mA. Thermogravimetric analysis was

performed from room temperature to 750 °C in a Mettler Toledo

thermogravimetric analyzer under air flow (100 mL/min) at a heating rate of

10 °C/min. Fourier transform infrared (FT-IR) spectra, of samples prepared

before and after calcinations, were recorded with a Perkin Elmer 100

spectrometer in the range of 2000-400 cm-1

. The textural properties of the

calcined oxides were characterized by N2 adsorption porosimetry

(Micromeritics, ASAP 2010). The samples were degassed at 300 °C under

vacuum. Nitrogen adsorption isotherms were measured at liquid N2

temperature (77 K) and N2 relative pressures ranging from 10-6

to 1.0 P/Po.

Specific surface areas were calculated according to Brunauer-Emmett-Teller

(BET) method and the pore size distribution were obtained according to the

Barret-Joyner-Halenda (BJH) method [19]. The morphologies were observed

Mirla Rodríguez et al.

48

by field emission scanning electron microscopy (FE-SEM), using a Quanta

250 FEG scanning electron microscope (accelerating voltage of 30 kV). The

evaluation by transmission electron microscopy was performed in a

HITACHI 7100 microscope. The samples were prepared by suspending the

powders in an ethanol-based liquid and pipetting the suspension onto a

carbon/collodion-coated 200 mesh copper grid.

The optical absorption edge of the titania nanofibers was measured using

a Lambda 35 UV−vis spectrophotometer (Perkin Elmer) equipped with an

integrating sphere, employing BaSO4 as the reflectance reference sample.

Reflectance spectra was recorded at 190 – 800 nm wavelength. The band gap

energy of the titania nanofibers was determined from the reflectance using

Kubelka-Munk function, F(R), and the extrapolation of Tauc plot, plot of

(F(R).hν)1/2

against hν [20].

Results and discussion

Figure 1 shows the wide angle X-ray diffraction pattern of the

synthesized samples calcined at different temperatures (500, 600 and

750 ºC). XRD analyses were conducted to investigate the relationship

between the thermal stability and anatase–rutile phase transformation. The

peaks of samples calcined were identified by comparison with JCPDS-84-

1286 according 2θ which confirmed that an anatase structure at 2θ=25.40. The XRD diffractograms corresponding to the treated samples at

500 and 600 °C, indicated that the obtained TiO2 had relatively high

crystallinity, attributable to the anatase phase (101), (004), (105), (200) and

(211) Miller indexes [5, 21-22]. It is noteworthy that these diffractograms do

not present any peaks assigned to the rutile phase (2θ=27.36 o

). On the other

hand, the sample calcined at 750 ºC gave diffraction peaks of anatase

nanocrystal along with minor peaks corresponding to rutile phase.

Figure 2 shows the TGA and DSC curves of the as-synthesized sample.

The TGA of the as-synthesized sample showed weight losses between

40 and 450 °C, while no changes were evident above 450 °C. The first zone

between 40 and 150 °C corresponds to the desorption of physisorbed water.

Weight loss in the 150-450 °C region, was associated to removal of most of

the template (thermal degradation of the diterpene glycosides). The

“plateau” observed above 450 ºC in the TGA curve indicated that a stable

phase had been formed. Figure 2 shows the DSC profile for the

as-synthesized sample. The Figure displays a broad peak from 240 to 360 ºC

which was attributed to the decomposition of the organic template and the

crystallization of TiO2.

TiO2 nanofibers

49

20 30 40 50 60

2θ/Degree

Rel

ativ

e in

ten

sity

(a.

u.)

500°C

600°C

750°C

TiO2 (anatase)

TiO2 (rutile)

30

00

a.u.

(101)

(004) (105)

(200)(211)

Figure 1. XRD patterns of the synthesized samples with different calcination

temperature.

-100

-80

-60

-40

-20

0

60

70

80

90

100

30 230 430 630 830

Wei

gh

tlo

ss(%

) HF

(mW

)

Temperature (°C)

Figure 2. TGA and DSC curves of the as-synthesized sample.

Mirla Rodríguez et al.

50

In the synthesis medium, titanium alcoxides hydrolyze and subsequently

polymerize to form a three-dimensional oxide network. These reactions can

be schematically represented as follows:

Ti(OR)4 + 4H2O→ Ti(OH)4 + 4ROH (hydrolysis), (Ι)

Ti(OH)4→ TiO2 xH2O + (2− x)H2O (condensation), (ΙΙ)

where R is ethyl, i-propyl, n-butyl, etc.[23]. Even so, hydrolysis in the

presence of excess water is rapid and complete within seconds. The size,

stability and morphology of the sol produced from alcoxides is strongly

affected by the water-to-titanium molar ratio (x = [H2O]/[Ti]). High x ratios

would favors the formation of colloidal TiO2. Frequently, particles of small

size are formed under these conditions [23].

The condensation and hydrolysis products possess proton–donors

centers (OH-groups, as well as water molecules) adsorbed on the surface.

OH-groups are well-known to have ability for H-bond creation and can

appear as centers for the physical adsorption of molecules (especially of

polar ones), which are acceptors of protons [24-26]. On the other hand, the

hydrolysis products of the diterpene glycosides molecules present in

S. rebaudiana leaf aqueous extract (stevioside, rebaudioside A, rebaudioside

B, rebaudioside C, rebaudioside D, rebaudioside E, dulcoside and stelviol

bioside) can act as proton acceptors. These molecules would be bound to the

three-dimensional titanium oxide forming a supramolecular structure. The

literature provide similar examples of supramolecular structures obtained on

the basis of diterpene glycosides derivatives [24-26].

Figure 3 shows TEM and SEM images of the titania powders

synthesized at 500 ºC, indicating the fibers-like morphology. The nanofibers

have diameter in the range of 25-60 nm and length of 100-150 nm.

The observed morphology was similar to the morphology of alumina

nanorods reported in our previous publication [13], which was synthesized

employing aluminum isopropoxide. The formation of structures based on the

template effect of the diterpene glycosides molecules present in

S. rebaudiana leaf aqueous extract was confirmed by these results. An

explanation of the template effect in the course of the synthesis was

proposed as the result of a complex system of intermolecular interactions

between the diterpene glycosides molecules and the metal alcoxide [13]. The

TEM image (Figure 3A) of the S. rebaudiana dried extract is evidence of the

template effect. Figure 3 clearly demonstrates the morphology of the template,

which was replicated by the synthesized metal oxides.

TiO2 nanofibers

51

Figure 3. TEM image of the S. rebaudiana dried extract (A); TEM (B) and SEM

(C, D) images of the titania powders calcined at 500 ºC.

It is interesting to reference the formation of molecular complex of

isosteviol with the substituted organic molecules proposed by Andreeva et al.

[26]. In this study it was demonstrated that these compounds can form

crystalline molecular complexes, whose supramolecular structures look like a

double chiral helix cross-linked [25-26]. This would be consistent with the

morphology of the titania obtained after calcination at 500 ºC.

Figure 4 and 5, show the TEM image of samples calcined at 600 and

750 ºC. It is seen that the morphology of the synthesized titania powders at

higher calcination temperature has a change. The nanofibers are completely

destroyed and only crystallites with size of 25-100 nm and morphology of

sphere-like form are observed. It would be attributed to the high temperatures

which cause the sintering of the nanocrystallites.

In order to identify the chemical nature of the synthesized nanofibers,

FTIR measurements were carried out yielding spectra as shown in Figure 6.

The characteristic vibrational bands in the as-synthesized sample appeared in

the range of 1650-1519 cm-1

for (COO-) stretching bands, 1470-1300 cm-1

for

Mirla Rodríguez et al.

52

50 nm

Figure 4. TEM image of the titania powders calcined at 600 ºC.

154 nm

Figure 5. TEM image of the titania powders calcined at 750 ºC.

C-H stretching, scissoring and bending. These carboxilate groups were attributed

to diterpenic molecules like steviol and isosteviol generated in the synthesis

medium [13, 27]. A broad band between 400 cm−1

and 700 cm−1

should be

due to the bands of Ti –O- Ti bond. After calcination, the transmittance peaks

TiO2 nanofibers

53

400800120016002000

Rel

ativ

e tr

ansm

itta

nce

(%

)

Wavenumber (cm-1)

500°C

600°C

750°C

Uncalcined

16

28

.36

13

84

.24

10

66

.09

54

1.1

3

15

21

.28

Figure 6. FT-IR spectra of titania nanoparticles before and after calcination

(500, 600 and 750 ºC).

of organic groups disappeared due to the removal of the template. In the

FTIR spectrum corresponding to the calcined sample at 500 ºC, the presence

of titania was supported by the appearance of a transmittance peak at about

500 cm-1

. The band at 1625 cm−1

was assigned to the bending mode of

adsorbed water [27].

Figure 7 exhibits nitrogen sorption isotherms (A) and the pore size

distributions (B) of the anatase titania nanopowders synthesized with

different calcination temperatures (500, 600 and 750 oC). The hysteresis loop

of titania shifted toward higher P/Po value with increasing calcination

temperature from 500 to 750 °C.

Mirla Rodríguez et al.

54

Figure 7. Nitrogen sorption isotherms (A) and the pore size distributions (B) of the

anatase titania nanopowders synthesized with different calcination temperatures

(500, 600 and 750 ºC).

Table 1. Surface area, pore volume and pore diameter of TiO2 samples

calcined at 500, 600 and 750 ºC.

The titania synthesized showed the maximum of adsorbed nitrogen

volume at 500 °C, and the adsorbed nitrogen volume rapidly decreased from

500 to 750 °C. The titania calcined at 750 °C almost lost its hysteresis loop.

Table 1 presents pore properties calculated from the nitrogen sorption

isotherms shown in Figure 7.

TiO2 nanofibers

55

The pore size of titania nanofibers increased from ~5 to ~24 nm with an

increase in the calcination temperature from 500 to 750 °C. The maximum

pore volume was 0.33 cm3/g at 500 °C, and the surface area decreased from

253 to 18 m2/g with an increase in the temperature from 500 to 750 °C. The

isotherm for titania calcined at 500 ºC exhibits typical type IV pattern

with inflection point of nitrogen adsorbed volume at P/P0 about 0.42 (type

H2 hysteresis loop), indicating the existence of mesopores [19]. Adsorption

isotherm of TiO2 sample calcined at 600 °C also exhibit hysteresis loop.

However, an increase in adsorption volume of N2 was observed and located

in the P/P0 range of 0.70- 0.95, indicating collapse and loss of mesoporosity.

The pore size distribution obtained by BJH approach (Figure 7B) is

noticeably broad, in the range of 4-45 nm. The mesostructure of the material

totally collapses in the calcination process at 600-750 ºC, indicating a

substantial loss of porosity.

The UV-vis DRS spectra for titania powders calcined at 500 ºC is shown

in Figure 8. Diffuse reflectance spectra was used to study the possible

transitions between the valence band and the conduction band, as well as

other possible transitions due to impurities. For semiconductor materials, the

direct band gap can be derived by establishing the Tauc Plot of transformed

Kubelka-Munk function versus the absorbed light energy [20]. As can be

seen in Figure 8C, the so-called Tauc optical band gap is obtained at the

intercept between the extension line of slop and the base line. This indicates

that titania nanofibers have a band gap of 3.2 eV, which corresponds to

approximately 385 nm. This calculated band gap energy is consistent with

data reported in the literature for other forms of titania materials [28].

0

20

40

60

80

100

200 300 400 500 600 700

0,0

0,4

0,8

1,2

1,6

0 1 2 3 4 5 6 7 80,0

0,4

0,8

1,2

1,6

200 300 400 500 600 700

Ref

lect

ance

(%

)

Wavelength (nm) Wavelength (nm) Eg (eV)

1.6

1.2

0.8

0.4

0.0

Ku

bel

ka-

Mu

nk

1.6

1.2

0.8

0.4

0.0

Ku

bel

ka-

Mu

nk

(A) (B) (C)

Figure 8. Diffuse reflectance spectra (A), the absorption spectra obtained by

applying a Kubelka-Munk function to the diffuse reflectance spectra (B), and plot of

transformed Kubelka Munk function vs the energy of the light absorbed (C) for

titania powders calcined at 500 ºC.

Mirla Rodríguez et al.

56

Conclusions

Titania nanofibers with high surface area (252 m2/g), diameter of

25-60 nm and length of 100-150 nm could be prepared at room temperature in

an aqueous medium by using titanium isopropoxide as precursor and the Stevia

rebaudiana leaf extract. The prepared titania nanofibers exhibit a mesoporous

structure and are composed of nanocrystalline anatase titania. Due to their

porous structure and band gap of 3.2 eV, these materials may find numerous

applications in photocatalysis, solar energy conversion and catalysis.

Acknowledgements

The authors would like to thank the Microbiology Center (IVIC) and

M.Sc. Freddy Sanchez for the TEM micrographs; Diffraction and

Fluorescence Laboratory, Dr. Reinaldo Atencio and Miguel Ángel Ramos

García (INZIT) for DRX analysis; Dra. Tamara Zoltan and Ing. Maibelin

Rosales (IVIC) for the UV-vis DRS analysis.

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