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Chemical Physics Letters 394 (2004) 387–391

Photocatalytic deposition of gold nanoparticleson electrospun nanofibers of titania

Dan Li, Jesse T. McCann, Matthew Gratt, Younan Xia *

Department of Chemistry, University of Washington, Seattle, WA 98195-1700, USA

Received 11 June 2004; in final form 11 June 2004

Available online 29 July 2004

Abstract

In this letter, we have demonstrated that gold nanostructures could be selectively deposited on electrospun titania nanofibers

through the photocatalytic reduction of HAuCl4 in the presence of an organic capping reagent. Depending on the type and concen-

tration of capping reagent used, gold nanoparticles, fractal nanosheets or nanowires could be obtained. This approach provides a

simple route to fabricate metal-decorated titania nanofibers that hold much promise for use in catalysis and chemical sensing.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

Metallic nanostructures with various shapes (includ-

ing spherical particles, wires, plates or hollow structures)

have received extensive attention due to their potential

applications in catalysis, biological labeling, information

storage, photonics and surface enhanced Raman scatter-ing (SERS) [1–7]. These nanostructures are often

prepared through the chemical, radiolytic, or photo-

chemical reduction of metal salts in solutions in the pres-

ence of organic stabilizers or templates, where the

resultant products are collected in the form of either col-

loids or powders [1,8–16]. For practical applications

such as heterogeneous catalysis and SERS, it is often

necessary to support these nanostructures on solid sub-strates (e.g. silicon wafers or porous matrices) [7]. Selec-

tive deposition of these structures on specific areas is of

great potential utility in that nanoscale devices can be

built from structures patterned in such a manner [17].

Deposition of metal nanoparticles on other nanostruc-

tures made of carbon or other semiconducting materials

is of great interest because the resultant metal/semicon-

0009-2614/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2004.07.044

* Corresponding author. Fax: +206 685 8665.

E-mail address: xia@chem.washington.edu (Y. Xia).

ductor composites may exhibit some intriguing proper-

ties [18–22]. For example, multicolor photochromism

has been observed in TiO2 films loaded with Ag nano-

particles [23]. Extensive research has shown that the

attachment of gold or other metal nanoparticles to

TiO2 nanoparticles could cause the Fermi level of titania

to be shifted to more negative potentials and prevent therecombination of electron–hole pairs, therefore leading

to significant improvement in photocatalytic activity

and photoelectrochemical response [24–27]. Such com-

posite nanostructures hold great promise for the photo-

degradation of organic contaminants, for water

splitting, and for the fabrication of solar cells [26,27].

As for the preparation of Au/TiO2 nanocomposites,

one of the approaches involves attaching pre-synthe-sized gold nanoparticles to TiO2 nanoparticles via the

linkage of bifunctional molecules (e.g. mercaptoprop-

ionic acid) [28]. Because the synthetic chemistry of gold

particles has been well-established, gold nanoparticles

with a variety of sizes and morphologies are available

for decorating TiO2 nanoparticles. However, the pres-

ence of the linkagemoleculesmay influence the properties

of the resultant nanocomposites since the organic linkersare often susceptible to decomposition due to the photo-

oxidation of TiO2 nanoparticles. The preparation of

Fig. 1. (a) UV–Vis absorption spectrum of a solution that contained

1.0 · 10�4 M HAuCl4 and 1.0 · 10�4 M poly(vinyl pyrrolidone) (PVP)

after 1 h exposure to UV light; (b), (c) UV–Vis absorption spectra of a

glass slide coated with anatase nanofibers (b) before and (c) after UV

irradiation for 1 h in the same HAuCl4/PVP solution.

388 D. Li et al. / Chemical Physics Letters 394 (2004) 387–391

linker-free Au/TiO2 composite nanoparticles could be

achieved via in situ photoreduction of gold salts on

the surface of TiO2 nanoparticles [29]. This method is

relatively simple and has been adopted to deposit vari-

ous metal nanoparticles (e.g. Ag, Pt or Pd) on TiO2

nanoparticles [29,30]. However, morphological controlof photodeposited nanoparticles has not been demon-

strated previously. Recently, we developed a simple

method to prepare titania nanofibers by electrostatic

spinning (known commonly as electrospinning) [31].

We have demonstrated that these nanofibers could be

directly assembled into three-dimensional porous mats.

We believe that these nanofibers can serve as catalysts

and/or catalyst supports. Here we demonstrate that goldnanostructures can be selectively deposited on electro-

spun anatase nanofibers by utilizing the photocatalytic

feature of titania. The morphology of these gold nano-

structures could be readily controlled by varying the

experimental parameters.

2. Experimental

In a typical experiment, TiO2 nanofibers were pre-

pared by electrospinning from a solution containing

0.5 g titanium tetraisopropoxide (Aldrich), 2 ml acetic

acid, 7.5 ml ethanol and 0.3 g poly(vinyl pyrrolidone)

(PVP) (Aldrich, Mw � 1,300,000) using the procedure

we have previously reported [31]. The as-spun nano-

fibers were collected on glass slides or silicon wafers

Fig. 2. (a), (b) SEM images of TiO2 nanofibers that have been immersed in a

pyrrolidone) (PVP) and irradiated by UV light for (a) 15 min and (b) 60 min,

from sample-b.

and then calcined in air at 500 �C for 1 h. X-ray and

electron diffraction studies indicated that the final prod-

ucts were in the anatase phase. The average diameter of

the calcined nanofibers was around 30 nm. The

substrate coated with a layer of calcined titania nanofi-

bers was immersed into a solution containing

1.0 · 10�4 M HAuCl4 and various types and concentra-

tions of organic capping reagents. The concentration ofthe polymeric capping reagents was calculated as the

concentration of the respective monomer subunits in

solution containing 1.0 · 10�4 M HAuCl4 and 1.0 · 10�4 M poly(vinyl

respectively. (c), (d) TEM image and electron diffraction pattern taken

D. Li et al. / Chemical Physics Letters 394 (2004) 387–391 389

solution. The substrate was then irradiated with UV

light for specified intervals. A long-wave UV lamp (k�365 nm) with two 15 W tubes (Model XX-15A, Spec-

tronics Corp., Westbury, NY) was used as the UV light

source. After UV irradiation for a specified interval, the

samples were rinsed with copious amount of water anddried in air. The samples on silicon wafers were directly

employed for scanning electron microscopy (SEM) im-

aging. The samples were then transferred onto copper

grids for transmission electron microscopy (TEM) and

electron diffraction (ED) studies. SEM images were tak-

en using a field-emission scanning electron microscope

(Sirion, FEI, Portland, OR) operated at an accelerating

voltage of 5 kV. TEM images were taken using a PhilipsEM-430 microscope operated at 80 kV. Optical absorp-

tion spectra were obtained by using a Cary 5E (Varian,

Walnut Creek, CA) spectrometer.

Fig. 3. SEM images of TiO2 nanofibers that have been irradiated by

UV light in a solution that contained 1.0 · 10�4 M of HAuCl4 and

different types of capping reagents for 1 h: (a) poly(vinyl alcohol)

(PVA); (b) poly(ethylene glycol) (PEG); and (c) sodium citrate. The

concentration of all capping reagents was 1.0 · 10�4 M. The scale bar

in the insets is 50 nm.

3. Results and discussion

It has been reported that HAuCl4 can be directly re-duced to form well-dispersed Au colloids under UV ir-

radiation in the presence of some organic stabilizers

such as PVP, poly(ethylene glycol) (PEG), poly(vinyl

alcohol) (PVA) or other surfactants [13–15]. These or-

ganic stabilizers might also act as scavengers (that is,

they are oxidized when HAuCl4 is reduced) thereby fa-

cilitating the reduction of HAuCl4. Our experiment

showed that the photoreduction process was fairly slowunder the irradiation of the long-wave UV lamp if the

concentration of the capping reagent was lower than

2.0 · 10�4 M. As shown in Fig. 1a, no detectable color

change occurred if a solution containing 1.0 · 10�4 M

HAuCl4 and 1.0 · 10�4 M PVP (Aldrich, Mw � 55,000)

55,000) was irradiated by the UV lamp for 1 h. Howev-

er, if a solid substrate covered with anatase nanofibers

was immersed in the solution, the substrate turned pur-ple after 1 h of UV irradiation while the solution re-

mained colorless. Fig. 1c shows the UV–Vis

absorption spectrum of a sample deposited on a glass

slide. The absorption band around 540 nm is character-

istic of the surface plasmon band of gold nanoparticles

[1]. Fig. 2a, b show SEM images of two samples sup-

ported on silicon wafers that have been irradiated for

two different periods of time. It is clear that the surfacesof the anatase nanofibers have been decorated with

nanoparticles 10–30 nm in diameter. Both size and

number (or density) of the nanoparticles increased di-

rectly with the duration of irradiation. Fig. 2c, d show

a typical TEM image and an electron diffraction (ED)

pattern of the same sample, respectively. The ED pat-

tern could be indexed to that of face centered cubic

(fcc) gold and anatase, respectively. These results clear-ly indicate that Au nanoparticles have been formed on

the surfaces of TiO2 nanofibers.

It is well-known that TiO2 is a very useful oxide

semiconductor for photocatalysis [32]. With the irradi-

ation of UV light (>3.2 eV), the absorption of pho-

tons by TiO2 promotes electrons from the valence

band to the empty conduction band, thus generating

electron–hole pairs. The photogenerated holes can ox-idize water to produce hydroxyl radicals, which can

further oxidize organic contaminants. The excited elec-

trons have the ability to reduce some metal ions. Our

experiment shows that this photocatalytic reduction is

well-suited for decorating TiO2 nanofibers with gold

nanoparticles. Furthermore, we found that the adhe-

sion of these nanoparticles to the nanofibers was rela-

tively strong, resisting continuous water rinsing for afew hours.

390 D. Li et al. / Chemical Physics Letters 394 (2004) 387–391

We also found that the morphology of the deposited

gold nanostructures could be controlled by varying the

type of capping reagent, as well as its concentration.

Fig. 3 shows typical SEM images of TiO2 nanofibers

after 1 h of UV irradiation in HAuCl4 solutions contain-

ing different types of organic capping reagents but withthe concentration held constant (1.0 · 10�4 M). In the

presence of PVA (Aldrich, Mw � 30,000), the deposited

gold nanoparticles were like crumpled sheets. When

PEG (Aldrich, Mw � 20,000) was used, irregular gold

nanowires were formed. When sodium citrate was ap-

plied, similar to the case of PVP (Fig. 2b), gold nanopar-

ticles were obtained but with larger sizes and a lower

density. The concentration of a capping reagent alsohad a considerable effect on the morphology of the

photoreduced gold nanostructures. If the concentration

of organic capping regents was higher than 1.0 · 10�3

M, the resultant gold nanoparticles exhibited a semi-

spherical shape exclusively (see Fig. 4b, d). The particle

size decreased slightly as the concentration of a capping

reagent increased. It is worth noting that if the concen-

tration was beyond 0.01 M, the solutions also turnedpurple or red with 1 h of UV irradiation using the same

lamp. In this case, the high concentration of the organic

scavenger greatly enhanced the photoreduction of

HAuCl4 in the solution and led to the formation of con-

siderable amount of gold nanoparticles in the liquid

phase. Due to the action of the capping reagent, the

as-formed gold colloids in the solution phase were very

Fig. 4. SEM images of TiO2 nanofibers that have been irradiated by UV lig

capping reagents at various concentrations for 1 h: (a) poly(vinyl pyrrolidon

5.0 · 10�6 M; (d) PVA, 0.1 M. The scale bar in the insets is 50 nm.

stable and could not be deposited onto the silicon wafer

or the wall of the reactor.

If the concentration of the capping reagents was less

than 5.0 · 10�6 M, fractal nanosheets or nanowires were

obtained, irrespective of the type of capping reagent

used (see Fig. 4a, c for two typical SEM images). Thismorphology is quite similar to the recent results report-

ed by Sastry and co-workers, where they found that

fractal gold plates were produced when the reduction

of chloroaurate ions was constrained to a monolayer

at the air–water interface [33]. It is believed that the

highly localized reduction of the constrained ions might

be responsible for the formation of the flat, highly aniso-

tropic shape. The generation of gold nanosheets ornanowires in our experiments might involve a similar

mechanism. It has been reported that if gold nanoparti-

cles are in physical contact with TiO2 surfaces, the

photogenerated electrons tend to migrate to the surface

of gold nanoparticles [24,25]. Therefore, the reduction of

HAuCl4 should be restricted primarily to the surfaces of

pre-deposited gold nanoparticles, particularly when low

concentrations of organic scavengers were used. Furtherwork is needed to elucidate the exact mechanism leading

to the formation of these structures.

We also attempted the photocatalytic reduction ex-

periment with other types of nanofibers such as SiO2,

Fe2O3, Al2O3, and SnO2, and found that these materials

could not catalyze the reduction of HAuCl4. In fact, we

also found that gold nanoparticles were not deposited

ht in a solution that contained 1.0 · 10�4 M of HAuCl4 and different

e) (PVP), 5.0 · 10�6 M; (b) PVP, 0.1 M; (c) poly(vinyl alcohol) (PVA),

D. Li et al. / Chemical Physics Letters 394 (2004) 387–391 391

on the substrates (silicon or glass) used to support the

anatase nanofibers or on the walls of the reactors, even

in the presence of high concentration of capping re-

agents, indicative of the high selectiveness to titania

surfaces.

4. Conclusion

In summary, we have demonstrated that gold nano-

structures including particles, sheets and wires could be

selectively deposited on the surfaces of electrospun

anatase nanofibers through photocatalytic reduction

of HAuCl4 in the presence of organic capping reagents.In addition to gold nanostructures, our preliminary

results showed that Ag, Pt, and Pd nanoparticles could

also be deposited on TiO2 nanofibers using similar

reactions. As we and other groups have recently dem-

onstrated, electrospun nanofibers could also be manip-

ulated and assembled into uniaxially aligned arrays and

multilayered films [34]. The selective deposition scheme

described in this paper might provide a new route to thefabrication of highly oriented, wire-like assemblies

of metal nanoparticles for chemical or biological

sensing [35].

Acknowledgements

This work has been supported in part by an AFOSR-MURI grant awarded to the University of Washington

and a research fellowship from the David and Lucile

Packard Foundation. Y.X. is a Camille Dreyfus Teacher

Scholar (2002) and an Alfred P. Sloan Research Fellow

(2000).

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