Materials Sciences and Applicatio ns, 2011, 2, 1188-1193
doi:10.4236/msa.2011.29160 Published Online September 2011 (http://www.SciRP.org/journal/msa)
Influences of Silver-Doping on the Crystal
Structure, Morphology and Photocatalytic
Activity of TiO2 Nanofibers
Nasser A. M. Barakat1,2*, Muzafar A. Kanjwal3, Salem S. Al-Deyab4, Ioannis S. Chronakis3,
Hak Yong Kim1
1Orgnanic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju, Republic of Korea; 2Chemical Engineer-
ing Department, Faculty of Engineering, El-Minia University, El-Minia, Egypt; 3Technical University of Denmark, DTU Food,
Lyngby, Denmark; 4Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia.
Received May 10th, 2011; revised May 20th, 2011; accepted May 27th, 2011.
ABSTRACT
Doping of titanium dioxide nanofibers by silver nanoparticles revealed distinct improvement in the photocatalytic activ-
ity; however other influences have not been investigated. In this work, effect of sliver-doping on the crystal structure,
the nanofibrous morphology as well as the photocatalytic activity of titanium oxide nanofibers has been studied. Sil-
ver-doped TiO2 nanofibers having different silver contents were prepared by calcination of electrospun nanofiber mats
consisting of silver nitrate, titanium isopropoxide and poly(vinyl acetate) at 600˚C. The results affirmed formation of
silver-doped TiO2 nanofibers composed of anatase and rutile when the silver nitrate content in the original electrospun
solution was more than 3 wt%. The rutile phase content was directly proportional with the AgNO3 concentration in the
electrospun solution. Negative impact of the silver-doping on the nanofibrous morphology was observed as increase the
silver content caused to decrease the aspect ratio, i.e. producing nanorods rather nanofibers. However, silver-doping
leads to modify the surface roughness. Study of the photocatalytic degradation of methylene blue dye clarified that in-
crease the silver content strongly enhances the dye oxidation process.
Keywords: Titanium Oxide Nanofibers, Nanostructured Ceramics, Inorganic Materials, Crystal Structure,
Electrospinning
1. Introduction
Enormous efforts have been devoted to the research of
TiO2 material, which has led to many promising applica-
tions in areas ranging from photovoltaics and photocata-
lysis to photo-/electrochromics and sensors [1-5]. New
physical and chemical properties emerge when the size
of the TiO2 becomes smaller and smaller, and down to
the nanometer scale. The main advantage of the nanos-
tructural materials is the high surface to volume ratio,
however much literature indicated that the one-dimen-
sional (ID) nanostructures have special characteristics
due to the axial ratio aspect [6-8]. Among the ID shapes,
nanofibers have novel features because of the long axial
ratio; consequently good reports were introduced about
fabrication of TiO2 nanofibers/nanotubes [9-11]. Re-
cently, metal-doping strategy has been invoked to im-
prove the physical and chemical properties of TiO2 nano-
particles/nanofibers [12-14]. Noble metal incorporation
into the titania dielectric provides an absorption feature
due to the surface Plasmon resonance (SPR) occurring
over the visible range of the spectrum [15]. In particular,
silver and gold metals are the most popular materials due
to the strong SPR character [16]. Metal-doping may also
have an influence on the titania crystal structure leading
to distinct improving in the application fields [13,17].
Here, we study effect of the silver-doping on the crystal
structure, the morphology and the photocatalytic activity
of TiO2 nanofibers. Several techniques have been intro-
duced to fabricate the nanofibers,however electrospin-
ning is the most widely utilized process due to its sim-
plicity, high yield and low cost [18-21]. Accordingly,
silver-doped TiO2 nanofibers were prepared by calcina-
tion of electrospun mats consisting of silver nitrate, tita-

Influences of Silver-Doping on the Crystal Structure, Morphology and Photocatalytic Activity of TiO Nanofibers1189
2
nium isopropoxide and poly(vinyl acetate) at 600˚C.
Control of silver content in the prepared nanofibers could
be achieved by modifying the silver nitrate concentration
in the original electrospun solution. Physiochemical cha-
racterizations indicated that the silver content does have
distinct influences on the morphology, crystal structure
and photocatalytic activity of the TiO2 nanofibers.
2. Experimental Section
2.1. Materials
Silver nitrate (99.8 assay), methylene blue dihydrate
(95.0 assay), acetic acid (99.7 assay) and N,N-dimethyl-
formamide (DMF, 99.5 assay) were obtained from Sho-
wa, Co. Japan. Titanium (1V) isopropoxide (Ti(Iso)),
98.0 assay) was purchased from Junsei Co. Ltd., Japan.
Poly(vinyl acetate) (PVAc, MW = 500,000 g/mol) ob-
tained from Aldrich USA. These materials were used
without any further purification.
2.2. Electrospinning Set-up
A sol-gel composed of titanium isopropoxide (Ti(Iso))
and poly(vinyl acetate) (14% wt, in N,N-dimethylfor-
mamide; DMF) was prepared by mixing these ingredi-
ents for 10 min, and then few drops of acetic acid were
added till the solution became transparent. The final so-
lution contained 60 wt% Ti(Iso). Ten gram of the pre-
pared solution was mixed with silver nitrate/DMF solu-
tion (AgNO3 was dissolved in a minimum amount of
DMF) to prepare the silver-doped titanium oxide nanofi-
bers [14], several solutions containing different contents
of silver nitrate have been prepared. In the electrospin-
ning process, a high voltage power supply (CPS-60
K02V1, Chungpa EMT Co., Republic of Korea) was
used as a source of electric field. The sol-gel was sup-
plied through a plastic syringe attached to a capillary tip.
A graphite rod connected with the positive electrode
(anode) was inserted into the sol-gel while the negative
one (cathode) was attached to a metallic collector cov-
ered with polyethylene sheet. Briefly, the solutions were
electrospun at 6 kV voltage and 15 cm working distance
(the distance between the needle tip and the collector).
The formed nanofiber mats were initially dried for 24 h
at 80˚C under vacuum and then calcined in air atmos-
phere at 600˚C for 1 h with a heating rate of 5˚C/min.
The photocatalytic degradation of methylene blue dye
was carried out in a simple photo reactor. The reactor
was made of glass (1000 ml capacity, 23 cm height and
15 cm diameter), covered with alumina foil and equipped
with 365 nm (wavelength radiation) ultra-violet lamp.
The initial dye solution and the photocatalyst were pla-
ced in the reactor and continuously stirred for complete
mixing during the photocatalytic reaction. Typically, 100
ml of the dye solution (10 ppm, concentration) and 50
mg of the catalyst were used. At specific time intervals, 2
ml samples were withdrawn from reactor and centrifuged
to separate the nanofibers (catalyst), and then the ab-
sorbance intensity was measured at wavelength of 664
nm [14].
2.3. Characterization
The phase and crystallinity were characterized by using
Rigaku X-ray diffractometer (Rigaku Co, Japan) with Cu
Kα (λ = 1.54056 Å) radiation over a range of 2θ angles
from 20 to 800. Surface morphology of the nanofibers
was studied by JEOL JSM-5900 scanning electron mi-
croscope, JEOL Ltd, Japan and field-emission scanning
electron microscope (FESEM, Hitachi S-7400, Japan).
High resolution image was observed by JEOL JEM 2010
transmission electron microscope (TEM) operating at
200 kV, JEOL Ltd., Japan. The concentration of the dyes
during the photodegradation study was investigated by
spectroscopic analysis using HP 8453 UV-visible spec-
troscopy system, Germany. The spectra obtained were
analyzed by HP ChemiStation software 5890 series.
3. Results and Discussion
Titanium oxide has three popular crystal structures; an-
tase, rutile and brookite. Anatase is a polymorph with the
two other minerals. The minerals rutile and brookite as
well as anatase all have the same chemical formula, TiO2,
but they have different structures. Rutile is the more
common and the more well known mineral of the three,
while anatase is the rarest. Anatase shares many of the
same or nearly the same properties as rutile such as luster,
hardness and density. However due to structural differ-
ences, anatase and rutile differ slightly in the crystal
habit. The phase change from anatase polymorph of tita-
nia to rutile one has been the subject of considerable in-
terest and the focus of many groups’ activities over the
years. Figure 1 shows the effect of silver-doping on the
crystal structure of the obtained powder after the calcina-
tion process. In case of silver-free solution, the results
affirm formation of pure anatase titanium dioxide, exis-
tence of strong diffraction peaks at 2
 values of 25.09˚,
37.65˚, 38.44˚, 47.89˚, 53.89˚, 55.07˚, 62.40˚, 68.70˚,
70.04˚ and 75.00˚ corresponding to the crystal planes of
(101), (004), (112), (200), (105), (211), (204), (220),
(220) and (215), respectively indicates formation of ana-
tase titanium dioxide [JCPDS card no 21-1272]. Addition
of small amount of silver nitrate (2 wt%) did not affect
the crystal structure of the titania, however pure silver
metal was formed due to decomposition of the silver ni-
trate upon heating. The extra peaks at 2
 values of 38.11˚,
44.29˚, 64.43˚ and 77.48˚, corresponding to the crystal
planes of (111), (200), (220) and (311), respectively affirm

Influences of Silver-Doping on the Crystal Structure, Morphology and Photocatalytic Activity of TiO2 Nanofibers
1190
Figure 1. Effect of silver content on the crystal structure of TiO2.
presence of silver metal [JCPDS card no 04- 0783].
In Figure 1, the main peaks of anatase and silver were
marked as A and S, respectively. Increase the silver ni-
trate content to 3 wt% in the original electrospun solution
led to partial phase change in the titania crystal structure.
The standard peaks of rutile can be observed at 2
 values
of 27.45˚, 36.09˚, 41.23˚, 54.32˚, 56.64˚, and 69.08˚ cor-
responding to the crystal planes of (110), (101), (111),
(211), (220), and (301), respectively [JCPDS card no
21-1272]. Interestingly, the results indicate that increase
the amount of silver enhances formation rutile phase as
shown in Figure 1 (the main rutile peaks were marked
by R letter). Addition of 5 wt% silver nitrate led to in-
crease the rutile content to be higher than the anatase.
According to the XRD data, the ratio of rutile to anatase
is ~ 75% and 440% when the added silver nitrate was 3
wt% and 5 wt%, respectively. The electrospinning tech-
nique involves the use of a high voltage to charge the
surface of a polymer solution droplet and thus to induce
the ejection of a liquid jet through a spinneret. Due to
bending instability, the jet is subsequently stretched by
many times to form continuous, ultrathin fibers. It is
widely used for production of many polymeric nanofi-
bers. Moreover, the electrospinning process has been
exploited to produce metal oxides nanofibers by calcina-
tion of electrospun mats obtained from completely mis-
cible sol-gel solutions. Electrospinning of a sol-gel com-
posed of Ti(IsO) and PVAc/DMF solution has been car-
ried out in our lab in previous study [22], the resultant
electrospun nanofibers have good morphology. Addition
of silver nitrate to the sol-gel does not affect the mor-
phology [14]. Accordingly, well morphology electrospun
nanofibers mats were obtained from AgNO3/Ti(Iso)/PVAc
solutions at all the utilized AgNO3 contents (data are not
shown). Figure 2 shows the morphology of the resultant
powder after the calcination process. As can be observed
in Figures 2(a) and 2(b) which demonstrate the obtained
product from silver-free electrospun solution, well and
smooth TiO2 nanofibers were obtained, these nanofibers
composed of pure anatase as could be concluded from
the XRD results (Figure 1). Incorporation of small amount
of silver does not have considerable impact on the nano-
fibrous morphology as shown in Figure 2(c) and 2(d)
which represent the FE-SEM images of the powder ob-
tained from calcination of an electrospun solution con-
taining 2 wt% AgNO3. However, the surface became
little rough (Figure 2(d)). Figure 3 demonstrates the
TEM analysis results for TiO2 nanofibers originated from

Influences of Silver-Doping on the Crystal Structure, Morphology and Photocatalytic Activity of TiO Nanofibers1191
2
Figure 2. Effect of silver content on the nanofibrous morphology of TiO2.
Figure 3. TEM image for the TiO2 nanofibers originated
from electrospun solution containing 2 wt% silver nitrate.
an electrospun solution containing 2 wt% silver nitrate.
Due to the large atomic radius of the silver atom, they
cannot interpenetrate in TiO2 crystals to form solid solu-
tion alloy. Therefore, silver metal distributes in the form
of separated nanoparticles as shown in the normal TEM
and HR TEM images. Of course, the average size of
those nanoparticles is directly proportional with the
amount of silver. Therefore, more sharp and strong peaks
were observed in the XRD spectra representing the nano-
fibers containing high silver content. This might be an
acceptable explanation of obtaining nanorods rather than
nanofibers when the amount of the utilized silver nitrate
was 3 wt% (Figure 2(e) and 2(f)), in other words, the big
size silver nanoparticles led to break the nanofibers.
However, the surface became much rougher (Figure 2(f))
compared with the pristine and low silver content TiO2
nanofibers. More increase in the silver nitrate content in
the electrospun solution led to complete annihilation of
the 1D structure. Much literature has been reported ex-
plaining the positive effect for silver incorporation on the
photocatalytic of TiO2 nanostructures. Generally, these
studies have drawn this conclusion. The Ag nanoparticles
deposited on TiO2 surface act as electron acceptors, en-
hancing the charge separation of electrons and holes and
consequently the transfer of the trapped electron to the
adsorbed O2. The sufficient dye molecules are adsorbed
on the surface of Ag-TiO2 more than on the TiO2 surface,
increasing the photoexcited electron transfer from the
sensitized dye molecule to the conduction band of TiO2
and consequently increasing the electron transfer to the
adsorbed O2. Figure 4 affirms this conclusion. As shown
in Figure 4 which represents the effect of silver content

Influences of Silver-Doping on the Crystal Structure, Morphology and Photocatalytic Activity of TiO Nanofibers
1192 2
Figure 4. Effect of silver content on the photocatalytic ac-
tivity of TiO2 nanofibers.
on photcatalytic degradation of methylene blue dye, the
rate of degradation enhances with increasing the silver
content in the TiO2 nanofibers. Moreover, the surface
roughness might have an influence due to increase the
total surface area.
4. Conclusions
Calcination of electrospun nanofiber mats composed of
silver nitrate, titanium isopropxide and poly(vinyl acetate)
in air at 600˚C leads to produce silver-doped TiO2 nano-
fibers having both of antase and rutile phases if the silver
nitrate content is more than 3 wt%. Rutile phase content
can be increased by increasing the silver content in the
original electrospun solution. However, the nanofibrous
morphology is strongly affected by the silver-content,
excess silver causes to destroy the 1D structure. Sil-
ver-doping of the titanium oxide nanofibers greatly en-
hances the photocatalytic activity of this interesting ma-
terial.
5. Acknowledgements
This paper was supported by research funds of chonbuk
National University 2010. We thank Mr. T. S. Bae and J.
C. Lim, KBSI, Jeonju branch, and Mr. Jong-Gyun Kang,
Centre for University Research Facility, for taking high-
quality FESEM and TEM images, respectively.
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