Synthesis and optical properties of Gallium phosphide nanotubes

Article abstract of DOI:10.1021/jp053058e

Gallium phosphide nanotubes with zinc blende structure were synthesized for the first time. The as-prepared GaP nanotubes are polycrystalline with diameters of 30-120 nm and occasionally partially filled. The growth has been reasonably proposed to follow vapor-liquid-solid (VLS) mechanism. The integration of the nanotubular structure with the unique intrinsic semiconducting properties of GaP might bring GaP nanotubes some novel optical and electronic properties and applications. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp053058e

J. Phys. Chem. B 2005, 109, 19719-19722
19719
Synthesis and Optical Properties of Gallium Phosphide Nanotubes
Qiang Wu, Zheng Hu,* Chun Liu, Xizhang Wang, and Yi Chen
Key Laboratory of Mesoscopic Chemistry of MOE and Jiangsu ProVincial Laboratory for NanoTechnology,
Department of Chemistry, Nanjing UniVersity, Nanjing 210093, China
Yinong Lu
Laboratory of Electron Microscopy, College of Materials Science and Engineering,
Nanjing UniVersity of Technology, Nanjing 210009, China
ReceiVed: June 8, 2005; In Final Form: August 13, 2005
Gallium phosphide nanotubes with zinc blende structure were synthesized for the first time. The as-prepared
GaP nanotubes are polycrystalline with diameters of 30-120 nm and occasionally partially filled. The growth
has been reasonably proposed to follow vapor-liquid-solid (VLS) mechanism. The integration of the
nanotubular structure with the unique intrinsic semiconducting properties of GaP might bring GaP nanotubes
some novel optical and electronic properties and applications.
1. Introduction
2. Experimental Methods
The GaP nanotubes are produced through a chemical reaction
Nanotubes have attracted increasing attention due to their
great prospects in the fundamental physical sciences and modern
nanotechnology arising from the unique nanotubular structures
and quantum confinement effects.1 Since the discovery of carbon
nanotubes,2 many efforts have been devoted to exploring new
nanotubular structures of other substances,3 and the nanotubular
family has been rapidly extended to numerous layered com-
pounds including nitrides,4 chalcogenides,5 halogenides,6 and
so on for evenly bent nanotubes, and very recently to nonlayered
compounds such as GaN,7 AlN,8 ZnO,9 and Te10 for faceted
nanotubes. Some promising properties such as hydrogen stor-
age,11 field emission,12 and superconductivity13 have been
discovered, and some prototypes of advanced nanodevices such
as field effect transistors,14 sensitive biochemical sensors,15 and
tips in scanning probe microscopy16 have also been developed.
with Ga and Ga2O3 as the Ga source, red phosphorus as the P
source, and AlN (e.g., AlN nanowires) as the reducer. The
preparation of GaP nanotubes was conducted in a tubular furnace
at elevated temperature. In a typical run, Ga, Ga2O3 powder,
and red phosphorus are mixed with the mole ratio of 4:1:6 and
grinded in mortar for 10 min. The mixture precursor (0.5 g)
was placed at the upstream end of a small quartz tube with the
inner diameter of 1 cm and length of 10 cm. About 0.1 g AlN
nanowires of about 15 nm in diameter, which were prepared
via the extended vapor-liquid-solid (VLS) growth25 (see
Figure S2 of the Supporting Information), were put on the
downstream end of the small quartz tube as the reducer. The
quartz tube with the two ends loosely plugged with glass fiber
was mounted in the tubular furnace, heated to 1030 °C in argon,
and held there for 3 h. Then the furnace was cooled to room
temperature, and the product was collected for characterization.
X-ray diffraction (XRD) experiments were operated on a
Philips X’pert Pro X-ray diffractometer with Cu KR radiation
of 1.5418 Å. The morphology, structure, and component of the
product were analyzed by transmission electron microscopy
(TEM, JEOL-JEM-1005 at 80 kV) and high-resolution trans-
mission electron microscopy (HRTEM, JEM2010 at 200 kV)
with an energy-dispersive X-ray analyzer (EDX, Thermo
NORAN) attached. The grid specimen was prepared by
ultrasonic treatment to the product in the aqueous solution of
ethanol for dispersion. The optical properties of the product were
investigated by Raman spectroscopy (JY-HR-800, excited with
an Ar+ line at 514 nm) and photoluminescence spectroscopy
(SLM48000DSCF, excited with a He-Cd laser at 442 nm) at
room temperature.
Among the nanotubular family, the semiconductor nanotubes
of III-V compounds are particularly interesting because these
series of compounds provide adjustable direct band gaps from
0.17 eV in InSb to 6.2 eV in AlN.17 Hence, ultimate nanoscale
optoelectronic devices for a wide range of wavelengths with
many advanced features such as high thermal conductivity and
superior stability may be realized by using these nanotubular
structures. However, to our knowledge, the obtained nanotubes
of this system are still restricted to a few members of GaN,7
AlN,8 InN,18 and InP19 to date. As a popular semiconductor with
a wide band gap of 2.26 eV, gallium phosphide has been found
to be an important candidate for light emission devices in the
visible range.20 GaP nanowires21-23 and nitrogen-doped nano-
belts24 have been obtained by different preparation methods
recently. In this letter, the synthesis and characterization of GaP
nanotubes are reported. The as-prepared GaP nanotubes are
polycrystalline with zinc blende structure, and the formation
mechanism is reasonably proposed.
3. Results and Discussion
Figure 1 is the typical TEM images of the product. In addition
to the unconsumed reducer of AlN nanowires with diameters
of about 15 nm, some nanotubes are clearly observed with a
diameter of about 30-120 nm and a thickness of about 10 nm.
* To whom the correspondence should be addressed. Phone: 0086-25-
83686015. Fax: 0086-25-83686251. E-mail: zhenghu@nju.edu.cn.
10.1021/jp053058e CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/30/2005

19720 J. Phys. Chem. B, Vol. 109, No. 42, 2005
Wu et al.
Figure 1. Typical TEM images of the as-prepared product. The filled
location is marked with an arrow. (a) Several nanotubes with different
diameters; (b) coexistence of nanotubes with unconsumed AlN nano-
wires. The diameters of the nanotubes (product) are clearly larger than
those of AlN nanowires (reactant).
Figure 3. Typical TEM images of the as-prepared GaP nanotubes.
(a) Overview of a section of a nanotube. (b) HRTEM image of the
nanotube. (c) Corresponding ED pattern of the nanotube.
Figure 4. Raman (a) and photoluminescence (b) spectra of the product.
Figure 2. XRD patterns of (a) the reducer of AlN nanowires and (b)
the as-prepared product. The indices corresponding to the diffraction
peaks of GaP with zinc blende structure are marked, respectively. The
unmarked peaks arise from hexagonal AlN.
phonon and longitudinal optical (LO) phonon at 360.2 and 395.6
cm-1, respectively, according with the corresponding values of
360 and 395 cm-1 for the zinc blende GaP nanorods with
diameters around 22 nm.22 Both peaks are broadened and
asymmetrical in shape and show significant downshifts of ca.
5 and 7 cm-1 with respect to the TO and LO phonon modes of
bulk or microcrystal GaP.23 The broadened, asymmetrical, and
downshift features of the Raman peaks may partially result from
the quantum confinement effect and surface defects due to the
Al2O3 coating.22,23 In addition, it is noticed that the intensity
ratio of the TO to LO peaks is unusually higher than that for
the GaP microcrystals, nanowires, or nanobelts,22-24 which may
be correlated with, e.g., the more intensive internal stress or
the lattice distortion due to the unique nanotubular structure.
Figure 4b presents the photoluminescence spectrum of the
product, which has a sharp peak centered at 2.14 eV. This green
peak can be attributed to the direct band gap emission of GaP,
a little bit red-shifted but in agreement with the emission of
GaP nanowires.23 The red shift of the emission peak may result
from the surface Al2O3 impurity on the nanotubes, rather than
from the size effects.23 Detailed studies will follow to elucidate
the energy downshift of the photoluminescence peaks. The
optical characterization result indicates that these GaP nanotubes
have potential applications in light emitting devices.
These nanotubes are easily distinguished from the AlN nanow-
ires with much smaller diameters. Some partially filled nano-
tubes are also noticed, which will be discussed later.
Figure 2 shows the XRD patterns of the reducer of AlN
nanowires (a) and the as-prepared product (b). It is seen that an
additional set of diffraction peaks which could be well-assigned
to GaP with zinc blende structure (JCPDS: 32-397) appear in
Figure 2b for the product in comparison with Figure 2a for AlN
nanowires. In combination with the TEM results, it is learned
that GaP nanotubes are produced.
To learn more about the novel tubular nanostructure of GaP,
high-resolution TEM was employed to characterize the as-
prepared nanotubes, as shown in Figure 3. The contrast between
the tube wall and the inside hollow region could be clearly
identified from Figure 3a. The wall thickness of the nanotube
is about 10 nm, and the wall surface is rather rough. The high-
resolution image (Figure 3b) shows that the nanotube wall is
composed of many tiny crystallites with grain sizes of ca. 5
nm with different orientations, indicating the polycrystalline
structure of the nanotube. This is confirmed by the starry
electron diffraction (ED) pattern as shown in Figure 3c. The
spacing between the neighboring fringes of a particle is 0.318
nm, corresponding to the d value of (111) planes of zinc blende
GaP. All of these results verify that the polycrystalline zinc
blende GaP nanotubes are obtained. An amorphous layer of the
Al2O3 byproduct was also observed on the surface of the GaP
nanotubes (see Figure S3 of the Supporting Information).
Raman spectrum of the as-prepared sample in Figure 4a
shows the Raman-active modes of transverse optical (TO)
As mentioned before, partially filled nanotubular morphol-
ogies were also observed in the product as typically shown in
Figure 5. Two typical picture contrasts are obvious, indicating
two different filled substances. The “grey” substance as shown
in Figure 5a has the same appearance in color as the nanotube
wall, indicating another morphology of the GaP product as
supported by the EDX characterization. The “dark” substance
as shown in Figure 5b was confirmed to be metal gallium by
the corresponding HRTEM image (see Figure S4 of the

Synthesis and Optical Properties of GaP Nanotubes
J. Phys. Chem. B, Vol. 109, No. 42, 2005 19721
Figure 5. (a-b) Typical TEM images of the partially filled GaP nanotubes, (c) a hollow nanotube grown from “ingredient supplier”, (d) two
partially filled nanotubes grown from ingredient suppliers, and (e) a detached ingredient supplier.
Supporting Information). Due to the low Ga content in the
product, no signal of Ga could be detected in the XRD pattern
in Figure 2.
due to the aforementioned factors such as complementing,
consuming, or soaking could be the main reason for the diameter
variations of the GaP nanotubes as observed, which favors the
formation of polycrystalline rather than single-crystalline nano-
tubes.
Special attention has been paid to the end parts of the
nanotubes as typically shown in Figure 5c-e, which could give
us some clue about the growth mechanism. It is seen that the
GaP nanotubes are grown from the “ingredient suppliers”, which
is similar to the case in VLS growth.26 However, the dimensions
of the ingredient suppliers are much larger than the diameters
of the corresponding nanotubes. Figure 5c and d represent the
two cases corresponding to the hollow and partially filled
nanotubes, respectively. The ingredient supplier in Figure 5e
might be detached from a nanotube during the cooling stage in
synthesis or subsequent ultrasonic treatment for the TEM
specimen.
According to the above experimental results, the growth
mechanism or process for the formation of GaP nanotubes is
proposed as follows. At a temperature of 1030 °C, the reaction
between the precursors of Ga and Ga2O3 produced the gaseous
species of Ga2O,27 which diffused to and were reduced by AlN.
Ga species were then generated and aggregated into small liquid
droplets in the interspaces of the AlN framework due to its low
melting point of 29.8 °C and high boiling point of 2403 °C.
Meanwhile, phosphorus vapor diffused into the Ga droplets and
reacted to form GaP species within the droplets. This process
could be represented as
This growth mechanism is supported by the results of the
comparison experiments. As discussed above, the Ga droplets
were generated via the reduction of Ga2O by AlN; hence the
reducer of AlN played an important role for the formation of
GaP nanotubes. We substituted AlN nanoparticles for AlN
nanowires under the same synthesis condition, and the GaP
nanotubes could still be obtained. However, when AlN was
substituted by γ-Al2O3 or Fe-Co/γ-Al2O3 powder, no GaP
nanotubes were synthesized, and only the GaP particles could
be observed in the product, which resulted from the gaseous
reaction of Ga2O with P (see Figure S5 of the Supporting
Information).28 In addition, after reaction, no GaP nanotubes
but particles could be observed in the region for putting the
mixture of Ga, Ga2O3, and P precursors although Ga and P
existed there. This is because the granular Ga precursor is too
big to reach the supersaturated level necessary for nanotubular
precipitation.
The GaP nanostructures in the product should rely on the
concentration of the GaP species in the ingredient supplier
droplet, which mainly depends on the pressure of the P vapor.
It is believed that the GaP nanostructure is controlled by the
competition of the crystal growth rate at the liquid-solid
interface with the diffusion of GaP species in the droplet.19 High
concentrations of the GaP species in the droplet would provide
enough ingredient for the nucleation front at the liquid-solid
interface through diffusion; hence, the growth takes place at
the entire liquid-solid junction (kinetically limited), giving rise
to GaP nanowires. On the contrary, a low concentration of GaP
species in the droplet could not provide enough ingredient for
the growth at the entire liquid-solid junction. The growth
becomes diffusion-limited and the nucleation front is ring-shaped
at the liquid-solid interface, giving rise to the growth of GaP
nanotubes. Indeed, GaP nanowires have been synthesized in our
comparison experiment, where excessive red phosphorus in the
precursor was used; hence, the concentration of the GaP species
in the droplet was increased. (see Figure S8 of the Supporting
Information) This has an analogy with the cases for the growth
of InP19 and carbon one-dimensional nanostructures.31
4Ga(l) + Ga2O3(s) f 3Ga2O(g) 27
3Ga2O(g) + 2AlN(s) f Al2O3(s) + 6Ga(l) + N2(g)
P(g) + Ga(l) f GaP(s) (nanotube) 28
When GaP species reached a supersaturated level, they were
precipitated out from the Ga droplets, i.e., the ingredient
suppliers, to form the GaP nanotubes in the manner similar to
VLS growth.26 Here, Ga droplets functioned as reactant as well
as reaction medium.29 P vapor and the newly produced Ga
species were continuously complemented into the Ga droplets
to sustain this growth, and finally the GaP nanotubes were
obtained. When the complementing rate is larger than the
consuming rate due to nanotube growth, the size of the Ga
droplet would increase. Sometimes, some liquid Ga or GaP-
containing Ga could be soaked inside the nanotubes through
the connecting part during growing, probably due to the
volumetric expansion of the Ga droplets or the capillarity
effect,30 leading to the partially filled nanotubes. The size
changes of the Ga droplets (ingredient suppliers) during growth
4. Conclusions
In summary, polycrystalline GaP nanotubes with zinc blende
structure were synthesized for the first time, and the growth

19722 J. Phys. Chem. B, Vol. 109, No. 42, 2005
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Acknowledgment. This work was financially supported by
the National Key Project for High-Tech (Grant No. 2003-
AA302150), NSFC (Grant Nos. 20471028 and 50302004), the
Foundation of Jiangsu Province (Grant Nos. BG2003029,
BK2003413, and BK2005416), and the Chinese Ministry of
Education (Grant Nos. 02110 and NCET-04-0449).
Supporting Information Available: Schematic experimen-
tal apparatus, the preparation process of AlN nanowire precursor,
EDX results of GaP nanotubes, HRTEM image of partially filled
GaP nanotube, the morphologies of GaP products prepared under
different conditions, and the brief analysis of the Raman and
photoluminescence spectra of the product. This material is
available free of charge via the Internet at http://pubs.acs.org.
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