Mild benzene-thermal route to GaP nanorods and nanospheres

Article abstract of DOI:10.1021/ic010431y

GaP nanorods and nanospheres were synthesized from a mild benzene-thermal route at 240 and 300 °C, respectively, using Na, P, and GaCl₃ as the starting materials. The structure of the products was identified as zinc blende phase by X-ray powder diffraction (XRD). Transmission electron microscopy (TEM) images showed that, when the reaction temperature was 240 °C, the products were nanorods with widths of 20-40 nm and lengths of 200-500 nm and nanospheres with diameters of 20-40 nm. However, when the reaction temperature was increased to 300 °C, the products were only nanospheres, and the diameters increased to 40-60 nm. The reaction proceeded through a metallic gallium intermediate, and a solution-liquid-solid (SLS) mechanism was proposed for the one-dimensional growth. The products were also investigated by UV-vis absorption and X-ray photoelectron spectroscopy.

Full text of DOI:10.1021/ic010431y

Inorg. Chem. 2002, 41, 1850−1854
Mild Benzene-Thermal Route to GaP Nanorods and Nanospheres
Shanmin Gao,† Yi Xie,* Jun Lu, Guoan Du, Wei He, Deliang Cui, Baibiao Huang, and
Minhua Jiang
,‡
,†
†
†
†
§
§
§
Structure Research Laboratory, UniVersity of Science and Technology of China,
Hefei, Anhui 230026, PRC, Scientific Research Institute of Shandong Ocean Chemical Industry,
Shouguang, Shandong 262737, PRC, and National Key Laboratory of Crystal Materials,
Institute of Crystal Materials, Shandong UniVersity, Jinan, Shandong 250100, PRC
Received April 24, 2001
GaP nanorods and nanospheres were synthesized from a mild benzene-thermal route at 240 and 300 °C, respectively,
using Na, P, and GaCl as the starting materials. The structure of the products was identified as zinc blende phase
3
by X-ray powder diffraction (XRD). Transmission electron microscopy (TEM) images showed that, when the reaction
temperature was 240 °C, the products were nanorods with widths of 20−40 nm and lengths of 200−500 nm and
nanospheres with diameters of 20−40 nm. However, when the reaction temperature was increased to 300 °C, the
products were only nanospheres, and the diameters increased to 40−60 nm. The reaction proceeded through a
metallic gallium intermediate, and a solution−liquid−solid (SLS) mechanism was proposed for the one-dimensional
growth. The products were also investigated by UV−vis absorption and X-ray photoelectron spectroscopy.
Introduction
The techniques of chemical synthesis and theoretical calcula-
tion of them have been developed greatly.
Recently, several experimental techniques have been
developed for the preparation of III-V nanocrystalline
semiconductor compounds. Wells and co-workers reported
7
-11
The dependence of physical properties of materials on
grain size is a well-known phenomenon. Because the
nanometer semiconductor quantum dots exhibit optical and
electronic properties from their corresponding bulk semi-
conductor materials, some of the current research in the
materials processing field has focused on the design and
synthesis of compound semiconductor nanometer materials.
The quantum confinement effects are the basis of new and
the reaction of InX
P(SiMe3) to synthesize InAs and InP nanocrystalline by
heating the products, respectively. Micic et al. synthesized
nanocrystalline InP by using P(SiMe and other organo-
metallics in a solution of trioctylphosphine oxide at 270 °C
3
(X) Cl, Br, I) with As(SiMe
3 3
) and
3
1
2
3 3
)
1
unusual molecular electronic devices. Current directions in
13
for several days. Lian Butter and co-workers confined GaAs
nanocrystals by refluxing GaCl with As(SiMe in decane
at 180 °C for 72 h. In addition, there are other routes, such
this field of research include the different new appropriate
preparation techniques of semiconductor nanoparticles and
the modifications of quantum particles by means of surface
3
3 3
)
1
4
1
5
as solve-thermal reaction and pyrolysis of single-source
2
chemistry, but most studies in this field have focused on
3-6
II-VI semiconductor nanocrystals, such as CdS and CdSe.
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8, 4109.
*
To whom correspondence should be addressed. E-mail: yxie@
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ustc.edu.cn.
†
University of Science and Technology of China.
Scientific Research Institute of Shandong Ocean Chemical Industry.
Shandong University.
‡
§
(
(
(
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1850 Inorganic Chemistry, Vol. 41, No. 7, 2002
10.1021/ic010431y CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/05/2002

Mild Route to GaP Nanorods and Nanospheres
1
6
precursors, to synthesize III-V compound nanocrystallines,
and each of these techniques has advantages and disadvan-
tages. The properties and theoretical calculation of these
compound nanocrystallines were also studied. For example,
the spectroscopic behavior of colloidal InP quantum dots has
analogous to the well-known vapor-liquid-solid (VLS)
mechanism. The reaction employed in the SLS growth is
described in eq 1.
3
5
(
t-Bu) M + EH f ME + 3(t-Bu)H
(1)
3
3
1
7
been investigated as a function of quantum dot diameter
and the electronic structure, surface effect, and red-shifted
emission.18 The theory of size-dependent resonance Raman
(M ) In, Ga; E ) P, As)
Our group reported another solution phase method for the
19
intensities in InP nanocrystals and optical properties of
GaAs nanocrystals 20 were also studied, but most of the
studies have focused on InP, InAs, and GaAs, with respect
to GaP nanocrystals, while there are relatively fewer studies
of their syntheses and properties.
The most important way to prepare GaP is the metal-
organic route. Dougall and co-workers21 have reported that
GaP semiconductor clusters can be synthesized in zeolite
36
synthesis of InAs nanofibers at low temperature. The route
is described in eq 2.
2
InCl + 2As + 6KBH f
3 4
2
InAs + 6KCl + 3B H + 3H (2)
2 6 2
The preparation of III-V compound semiconductor materials
is an arduous task, because toxic and dangerous precursor
materials are usually used. The use of EH in eq 1 is of
3
particular concern because of its carcinogenic and mutagenic
properties. Equation 2 is a safer route, and the further
advantage of this route is that the reactants are available,
2 2 6
but the products contain H and B H , which increase the
danger of the reaction process. As to the previous work, the
by using the reaction (Me
refluxing temperature, GaP nanocrystallines were obtained
from GaCl and P(SiMe or (Na/K) P and GaX in toluene
or other solvent, respectively. Another important method
for preparing GaP is the solid-state metathesis, in which GaX
3 3 4
)Ga + PH f 3CH + GaP. At
3
3
)
3
3
3
22,23
3
2
4,25
(
3
X ) F, Cl, I) and Na P are used as the raw materials,
but this process requires an extra washing step to remove
starting materials and byproducts.
synthesis of one-dimensional III-V compound semiconduc-
37,38
tors almost always focused on InP, InAs, GaAs, and GaN,
Very recently, a lot of work on the preparation of one-
dimensional structures, such as nanowires or nanowhiskers,
has been done, and important progress has been made.
Nanotubes and nanowires exhibit a wide range of electrical
and we have never seen the special topic of the synthesis of
GaP nanorods. In this paper, we exploit another route to
prepare GaP nanorods and nanospheres. Taking the toxicity
of phosphorus into account, we selected red phosphorus as
the phosphorus feedstock in the present process. If white
phosphorus was used as the phosphorus source, a warning
on the possibility of sparking in air should be noticed;
furthermore, a more extensive warning on its toxicity is
needed.
and optical properties, which depend sensitively on their sizes
and shapes.2
6-31
Buhro and co-workers have reported a
solution-liquid-solid (SLS) mechanism for the growth of
InP, InAs, and GaAs nanofibers,3
2-34
which is closely
(15) Xie, Y.; Qian, Y.; Wang, W.; Zhang, S.; Zhang, Y. Science 1996,
2
72, 1926.
Experimental Section
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16) Green, M.; O’Brien, P. Chem. Commun. 1998, 2459.
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A. Synthesis of GaP Nanorods and Nanospheres. All analytical
grade solvents used in our experiments were dried by sodium chips
and distilled in flowing nitrogen gas. Reactions were carried out
in a Teflon-lined autoclave. A 0.68 g (0.03mol) portion of sodium,
(
(
19) Shiang, J. J.; Wolter, R. H.; Heath, J. R. J. Chem. Phys. 1997, 106,
8981.
(
(
(
(
(
20) Uchida, H.; Curtic, C. J.; Kamat, P. V.; Jones, K. M.; Nozik, A. J. J.
0.309 g (0.01mol) of red phosphorus, and 1.76 g (0.01mol) of
Phys. Chem. 1992, 96, 1156.
gallium chloride were mixed in benzene, and then the mixture was
added to a Teflon-lined autoclave. Concurrently, benzene was added
to raise the filling ratio to ∼60-75%. After the air in the solution
was expelled by bubbling with nitrogen gas (99.999% pure), the
autoclave was sealed and heated to 240 and 300 °C for 6 h,
respectively, and then cooled to room temperature naturally. The
precipitates were filtered out, in which a few of spherules with
metallic sheen were observed. Then, the precipitates were washed
with absolute ethanol, 2 M HCl, and distilled water in sequence.
Finally, they were dried at 80 °C under vacuum for 5 h. The yield
of GaP based on gallium is about 87%.
21) Dougall, J. E. M.; Eckert, H.; Stucky, G. D.; Herron, N.; Wang, Y.;
Moller, K.; Bein, T. J. Am. Chem. Soc. 1989, 111, 8006.
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J. K. Chem. Mater. 1994, 6, 82.
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R. B. Inorg. Chem. 1993, 32, 2745.
(
(
(
25) Treece, R. E.; Macala, G. S.; Kaner, B. R. Chem. Mater. 1992, 4, 9.
26) Yan, P.; Xie, Y.; Qian, Y.; Liu, X. Chem. Commun. 1999, 1293.
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1
969.
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19, 5455.
(
1
(
(
(
29) Duan, X. F.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188.
30) Gudiksen, M. S.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 8801.
31) Su, H.; Xie, Y.; Gao, P.; Lu, H.; Xiong, Y.; Qian, Y. Chem. Lett.
B. Characterization of the Products. The X-ray diffraction
pattern of the dried powders was collected on a D/max-γA model
X-ray diffractometer with Ni-filtered Cu KR radiation. The X-ray
2000, 790.
(
32) Trentler, T. J.; Hickman, K. M.; Geol, S. C.; Viano, A. M.; Gibbons,
P. C.; Buhro, W. E. Science 1995, 270, 1791.
(35) Wagner, R. S. In Whisker Technology; Levitt, A. P., Ed.; Wiley: New
York, 1970, chapter 3.
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2619.
(
33) Trentler, T. J.; Goel, S. C.; Hickman, K. M.; Viano, A. M.; Chiang,
M. Y.; Beatty, A. M.; Gibbons, P. C.; Buhro, W. E. J. Am. Chem.
Soc. 1997, 119, 2172.
(
34) Buhro, W. E.; Hickman, K. M.; Trentler, T. J. AdV. Mater. 1996, 8,
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685.
Inorganic Chemistry, Vol. 41, No. 7, 2002 1851

Gao et al.
Figure 1. XRD pattern of the samples obtained by reacting 6 h at (A)
40 °C and (B) 300 °C.
2
beam voltage was 45 kV, and the beam current was 40 mA. The
optical absorption spectra were collected at room temperature with
a Hitachi 340 UV-vis-NIR recording spectrophotometer with 1
cm quartz cuvettes. The powders were milled and ultrasonically
dispersed in benzene. A blank solution of benzene was used as the
reference. Transmission electron microscopy (TEM) images of the
samples were obtained with a Hitachi model H800 transmission
electron microscope, using an accelerating voltage of 150 kV.
Samples were deposited from benzene solutions of the products
onto thin amorphous carbon films supported by copper grids. X-ray
photoelectron spectra (XPS) were recorded on an ESCALab MKII
instrument with Mg KR radiation as the exciting source. The binding
energies obtained in the XPS analysis were corrected by referencing
the C 1s peak to 284.60 eV.
Figure 2. (A) TEM image for GaP nanorods prepared at 240 °C. (B)
Selected area electron diffraction pattern for GaP nanorods. (C) TEM image
for GaP nanospheres at 300 °C. (D) Selected area electron diffraction pattern
for GaP nanospheres.
Among the three well-known whisker-growth mechanisms,
that is, vapor-liquid-solid (VLS), vapor-solid (VS), and
solution-liquid-solid (SLS), the SLS mechanism is most
likely to function under the present conditions because both
cases involved reactions in organic solvent, and in both cases,
gallium coexisted with the products. In this system, gallium
Results and Discussion
3
atoms easily form from the reduction of GaCl by sodium.
3
Reaction of GaCl , sodium, and red phosphorus at 240
The newly reduced gallium atoms are active enough to react
with phosphorus immediately. However, if the surrounding
phosphorus is insufficient, the reduced gallium atoms will
aggregate into spherules because of its very low melting point
and 300 °C in benzene yielded a fine gray-black powder.
Figure 1 shows the X-ray diffraction patterns of the GaP
powders. Pattern A is the sample which reacted at 240 °C,
and pattern (B) is the sample which reacted at 300 °C. The
peaks correspond to the (111), (220), and (311) planes of
GaP with a zinc blende structure, respectively. Broadening
of the peaks is due to the decreasing particle size. According
to the Scherrer equation, it could be estimated that the
average particle sizes are about 32 nm for A and 36 nm for
B. TEM images for the sample prepared at 240 °C show
nanorods with diameters of 20-40 nm and lengths of 200-
00 nm and nanospheres with diameters of 20-40 nm
Figure 2A). Selected area diffraction within a linear segment
of a rod (marked with an arrow) gives single-crystal patterns,
which could be indexed for the [134] zone axis of crystalline
GaP and suggests that the rod axis was the [111] direction
(302.65 K) and coexist with the final products. The molten
gallium can presumably be regarded as the intermediate
phase in the process of preparing GaP nanocrystals. When
the precipitate is filtered, a few spherules with metallic sheen
can be observed, and its XRD patterns indicated that these
spherules were metallic gallium, supporting the idea that
molten gallium is an intermediate in the growth process of
GaP nanocrystals. In addition, if benzene and water in the
filtered solution were vaporized, a white solid substance
appeared, which is NaCl, as proved by its XRD patterns.
These two facts further confirmed the proposed mechanism
of rod growth. According to the SLS mechanism, the reaction
should be
5
(
(
Figure 2B). Rods of cubic compounds (such as III-V
3
2-34
38
semiconductors)
and elements (such as Si) obtained
3
Na + GaCl f 3NaCl + Ga
(3)
(4)
3
from VLS and SLS processes typically grow along the [111]
direction. Only nanospheres with an average diameter of 40
nm were clearly seen for the sample prepared at 300 °C
Ga + P f GaP
(
Figure 2C). Figure 2D is the electron diffraction pattern of
So, when the reactions were carried out at 240 °C, the
products were GaP nanorods, but an interesting phenomenon
was discovered: there were some nanospheres coexisting
in the products prepared at 240 °C and only nanospheres in
sample B. The diffraction rings illustrate the existence of
some amorphous composition in the product, which is in
accordance with the XRD pattern (Figure 1B).
1852 Inorganic Chemistry, Vol. 41, No. 7, 2002

Mild Route to GaP Nanorods and Nanospheres
Figure 3. Optical absorption spectra of samples prepared at (A) 240 °C
and (B) 300 °C.
the products prepared at 300 °C. The similar phenomenon
36
has been observed previously. However, there has been no
suitable explanation for this phenomenon in the previous
work. Here, we propose a reasonable explanation for it as
follows. It is well-known that the physical properties of
materials, such as the melting point, depend sensitively on
the particle size. The melting point of nanocrystals falls
quickly with decreasing particle size. For example, the
melting point of Pb bulk material, which is 600 K, falls to
Figure 4. X-ray photoelectron spectra of samples prepared at 240 °C.
absorption onset at 460 nm was estimated to be ap-
proximately 32 nm using the effective mass approximation
(EMA).
288 K when the particle size decreases to 20 nm. Ag
X-ray photoelectron spectra (XPS) analysis was used to
measure the elemental composition of the nanocrystals.
Higher resolution spectra taken of the Ga 2p region and P
2p region were shown in Figure 4. The gallium core is spin-
orbit split to 2p3/2 and 2p1/2 with the 2p3/2 peak at 1117.2
eV, both corresponding to Ga from GaP. The P 2p core
shows two peaks, one at 128.5 eV corresponding to P from
GaP and the other at 133.2 eV corresponding to oxidized P
species. Because of the higher surface areas and reactivity
of the nanocrystals, the atoms on the surface are much more
easily oxidized when they are exposed to air. Thus, the
phosphorus atoms on the surface of the products may turn
to oxide phosphorus gradually, resulting in the oxidized
phosphorus species in the XPS spectrum. In our previous
nanocrystals begin to melt when the temperature is lower
than 373 K, while the melting point of Ag bulk material is
1173 K. According to these facts, we hypothesize that 300
°
C may be the melting point or over the melting point of
GaP nanorods at the present size under the current benzene-
thermal condition. At this temperature (300 °C), the GaP
nanorods may melt and produce GaP nanospheres. That may
be why only nanospheres were obtained in the final products
prepared at 300 °C. With regard to the nanospheres in the
products prepared at 240 °C, the GaP nanorods with smaller
size may melt and produce relative GaP nanospheres;
meanwhile, GaP nanorods with larger size remain, resulting
in the coexistence of larger GaP nanorods and smaller GaP
nanospheres in the final product.
36
work for InAs, similar phenomena have been reported, and
3
9
Our UV-vis spectroscopic investigations provided insight
into the quantum confinement in GaP nanocrystals. Figure
Alivisatos et. al have also observed asimilar phenomenon
in InP nanocrystals. Peak areas of these high-resolution scans
were measured and used to calculate the Ga-to-P ratio for
the nanocrystals. Quantification, in which only the P 2p peak
corresponding to GaP was used, gave a Ga-to-P ratio of 1.09:
1. As mentioned previously, there was trace gallium removed
by hydrochloric acid, so it is no wonder that more phosphorus
atoms than gallium atoms occupied the surface sites of the
product. This excess phosphorus is also responsible for the
oxidized P 2p peak at 133.2 eV. Thus, we conclude that the
nanocrystals are close to stoichiometric.
3
shows blue-shifted UV-vis absorption spectra of GaP
nanorods prepared at 240 °C and nanospheres prepared at
00 °C dispersed in benzene. Both absorption spectra show
3
long tails near the absorption edge because of defect or
particle size distribution. From the absorption spectra, one
can clearly see that curve B of the GaP nanospheres has a
more obvious shoulder peak and its wavelength region is
narrower than curve A of GaP nanorods. This means that
GaP nanospheres are perfect in crystal grains and have better
uniformity in particle size than GaP nanorods, which grow
with high orientation. The result is consistent with those of
the TEM images. The particle size giving rise to the
(
39) Guzelian, A. A.; Katari, J. E. B.; Kadavanich, A. V.; Banin, U.; Hamad,
K.; Juban, E.; Alivisatos, A. P.; Wolters, R. H.; Arnold, C. C.; Heath,
J. R. J. Phys. Chem. 1996, 100, 7212.
Inorganic Chemistry, Vol. 41, No. 7, 2002 1853

Gao et al.
Conclusion
Acknowledgment. Financial support from the Chinese
National Foundation of Natural Science Research through
the Outstanding Youth Science Fund and Huo Yingdong
Foundation for Young Teachers is gratefully acknowledged.
Samples of 20-40 nm × 200-500 nm GaP nanorods and
2
0-60 nm GaP nanospheres were successfully synthesized
3
via a novel reaction of GaCl , P, and Na at 240 and 300 °C
under benzene-thermal conditions. The reaction proceeds
with gallium as the likely intermediate. The SLS mechanism
is likely responsible for the nanorod growth.
IC010431Y
1854 Inorganic Chemistry, Vol. 41, No. 7, 2002