Concave trisoctahedral Ag3PO4 microcrystals with high-index facets and enhanced photocatalytic properties

Article abstract of DOI:10.1039/c2cc37324f

Herein, we demonstrate for the first time the fabrication of concave trisoctahedral Ag₃PO₄ microcrystals enclosed by {221} and {332} facets based on the heteroepitaxial growth procedure, which exhibit much higher photocatalytic activ

Full text of DOI:10.1039/c2cc37324f

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Concave trisoctahedral Ag3PO4 microcrystals with
high-index facets and enhanced photocatalytic
properties†
Cite this: Chem. Commun., 2013,
49, 636
Received 8th October 2012,
Zhengbo Jiao,a Yan Zhang,a Hongchao Yu,a Gongxuan Lu,a Jinhua Yeb and
Yingpu Bi*a
Accepted 26th November 2012
DOI: 10.1039/c2cc37324f
www.rsc.org/chemcomm
Herein, we demonstrate for the first time the fabrication of concave fast growth rates. Thereby, the synthesis of semiconductor photo-
trisoctahedral Ag3PO4 microcrystals enclosed by {221} and {332} facets catalysts exposed to high-index facets and exploration of their
based on the heteroepitaxial growth procedure, which exhibit much potential applications are still a great challenge.
higher photocatalytic activities than cubic Ag3PO4 and commercial
N-doped TiO2.
Herein, we demonstrate for the first time the fabrication of
TOH Ag3PO4 microcrystals enclosed with {221} and {332} high-
index facets based on the heteroepitaxial growth procedure by
Semiconductor photocatalysts have attracted considerable attention utilizing Au@Ag core–shell nanorods (NRs) as starting materials.
during the past few decades as a result of their great potential for Furthermore, their photocatalytic performance studies reveal that
resolving the current energy and environmental problems.1,2 It has the TOH Ag3PO4 exhibits much higher photocatalytic properties
been gradually acquired that the photocatalytic as well as photo- than cubes for the degradation of organic contaminants under
electric properties of semiconductors can be further enhanced or visible-light irradiation. The theoretical calculation suggests that
optimized through tailoring the morphology and the exposed crystal Ag3PO4 {221} and {332} facets possess higher surface energy,
facets.3–5 For example, Lu et al. demonstrated that anatase TiO2 with which should be a significant contribution to the enhanced
{001} facets are more reactive than the thermodynamically stable photocatalytic activity. Moreover, the Au@Ag nanorods embedded
{101} facets for the photocatalytic production of H2 from water inside TOH Ag3PO4 can result in the fast separation of photo-
splitting.6,7 Huang et al. reported that the {110} facets of Cu2O excited electron–hole pairs from their hetero-interface and thus
exhibit higher photocatalytic activities than the {111} and {100} improve the photocatalytic efficiency.
facets as a result of their higher surface energy.8 More recently,
Firstly, note that the morphology, structure, and exposed crystal
our group have revealed that single-crystalline Ag3PO4 rhombic facets of as-prepared Ag3PO4 crystals could be rationally tailored by
dodecahedrons enclosed by {110} facets exhibit much higher photo- adjusting the composition of metal starting materials. As shown in
catalytic activities than cubes bound by {100} facets for the degrada- Scheme 1, Fig. S2, and Fig. S3 (ESI†), when the Au@Ag nanorods
tion of organic contaminants.9 However, note that the as-prepared with high aspect ratio were utilized in this heteroepitaxial growth
semiconductor crystals are generally bound by relatively low-index process, novel TOH Ag3PO4 crystals with sharp corners and edges
crystal planes rather than high-index facets. More specifically, the have been fabricated. In the case of the trepang-like Au@Ag
high-index facets possess the high density of atomic steps, ledges, nanorods, TOH Ag3PO4 with an obtuse boundary could be obtained,
kinks, and dangling bonds, which usually exhibit much higher
chemical reaction activities than the low-index planes.10 Although
such high-index facets are fascinating and significant for the
enhancement of reactivity, they usually disappear rapidly during
the growth process because of their high surface energies as well as
a State Key Laboratory for Oxo Synthesis & Selective Oxidation, and National
Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou
Institute of Chemical Physics, CAS, Lanzhou 730000, China.
E-mail: yingpubi@licp.cas.cn
b International Center for Materials Nanoarchitectonics (MANA), and Research Unit
for Environmental Remediation Materials, National Institute for Materials Science
(NIMS), Tsukuba, 305-0047, Japan
Scheme 1 Schematic illustration of the growth process of Ag3PO4 with different
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c2cc37324f
morphologies fabricated by the seed-mediated method using different seeds.
c
636 Chem. Commun., 2013, 49, 636--638
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which possesses round edges and planes. Additionally, tetrahedral
Furthermore, the crystal facets of fcc metallic TOH structures,
Ag3PO4 could be fabricated when Au@Ag nanorods with a short such as Au or Ag nanocrystals (NCs), have been widely investigated
aspect ratio were introduced in this synthetic system. Furthermore, in recent years.10,14 However, the crystal facets index of bcc TOH
considering the body-centered cubic (bcc) crystalline structure and crystals has rarely been discussed so far. Although some differences
the regular tetrahedron morphology of Ag3PO4, it can be speculated exist between these two kinds of Bravais lattice, the relationship
that the tetrahedrons are bound entirely by {111} facets.11 Moreover, between Miller indices and projection angles should be suitable for
in the presence of pure Ag nanowires with a length of about 10 mm, both bcc and fcc nanocrystals because they all belong to an isometric
novel necklace-like Ag3PO4 were fabricated, and their exposed facets system. Moreover, as far as the isometric system is concerned, the
should be indexed to {100} facets.12 It is well known that the metallic angles between two crystal facets have no relationship with Bravais
TOH crystal structures such as Au, Au–Pd, and Ag have been lattice but Miller indices themselves, just as expressed as follows.15
extensively fabricated by many research groups,10,13,14 however,
TOH semiconductor crystals with high index facets have rarely been
reported until now as a result of their high instability. Thereby, in
this work, most attention has been focused on the TOH Ag3PO4 to
investigate the effects of high-index facets on their photo-
catalytic properties.
h1h2 þ k1k2 þ l1l2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
cos F ¼
2
2
2
2
2
2
ðh1 þ k1 þ l1 Þðh2 þ k2 þ l2
Þ
According to the above equation, if a TOH nanocrystal is
composed of a certain crystal facet, the projection angles should
be confirmed. Thereby, vice versa, the Miller indices can be deter-
mined from the projection angles when the TOH is viewed along a
certain crystallographic direction. The optimal projection direction
for TOH is along [110], which results in an octagon with 2-fold
symmetry.16 The four edge-on facets can be determined by three
different angles a, b and g, and the relationship between Miller
indices and the projection angles can be expressed as follows.13,16
Fig. 1A and B show the typical scanning electron microscopy
(SEM) images of TOH Ag3PO4 microcrystals fabricated by reacting a
[Ag(NH3)2]+ complex with Na2HPO4 in the presence of Au@Ag
nanorods. It can be clearly seen that Ag3PO4 microcrystals with
regular concave TOH morphology have been fabricated in high
yield and their dimensions are generally in the range of 1.5–2 mm
(Fig. S3A, ESI†). The randomly oriented microcrystals in SEM
images are consistent with the geometrical model of concave
TOH when viewed from different angles, which further confirms
the TOH morphology of Ag3PO4 microcrystals. Because no Au@Ag
nanorod can be directly observed in SEM images, it could be
concluded that they are embedded inside the Ag3PO4 microcrystals.
Furthermore, the X-ray diffraction (XRD) patterns (Fig. 1C) match
well with the body-centered cubic phase of Ag3PO4 (JCPDS. No.
06-0505) and several smaller diffraction peaks can be indexed to the
facet-centered cubic (fcc) phase of Au or Ag. More specifically, the
h ꢀ l
2h
pffiffi
a ¼ 180 ꢀ 2 arctan
g ꢀ a
b ¼ 90 þ
2
l
pffiffi
g ¼ 180 ꢀ 2 arctan
2h
Fig. 2A shows a SEM image of a single TOH Ag3PO4, and red
diffraction peaks of metallic Au and Ag are very similar, which lines are used to accurate the edges that form the outline of the
cannot be obviously distinguished. Therefore, the characterization TOH Ag3PO4. A model of TOH projected from the h110i direc-
of X-ray photoelectron spectroscopy (XPS) has also been performed tion has been shown in Fig. 2B, and the four edge-on facets are
to further investigate their composition and clarify the existence of indicated by the black arrows. The projection angles corre-
Au and Ag in Ag3PO4 microcrystals (Fig. 1D). The survey spectrum sponding to ideal TOH crystals enclosed by high-index facets
clearly confirms the presence of both gold and silver in the with different Miller indices are given in Table S1 (ESI†).
as-prepared product. Fig. S6 (ESI†) shows the high-resolution Comparing the measured angles with the calculated ones, it
spectra of the TOH Ag3PO4 microcrystal, which gives more informa- can be concluded that the four edge-on facets consist of {221}
tion about their chemical components.
and {332} facets, which can be further confirmed by the average
values of a, b and g as shown in Fig. 2C. The inset of Fig. 2C
shows an ideal model, which is made up by the average values
of projection angles of ten samples ordered in the clockwise
direction depending on a1 Z a2 and g1 Z g2. Based on the above
discussion, it can be concluded that the TOH Ag3PO4 microcrystals
are bound by high-index facets, such as {221} and {332}.
The ultraviolet visible diffuse reflectance spectrum of TOH
Ag3PO4 has been shown in Fig. 2D, indicating that an absorbance
peak edge around 550 nm has been observed. Furthermore, it
exhibits stronger optical absorption at around 500–700 nm com-
pared to cubic Ag3PO4, which may be due to the LSPR of gold.
According to Kubelka–Munk function, the relationship between the
absorption coefficient and band gap energy can be described by the
equation (ahn)1/2 = A(hn ꢀ Eg), in which a, n, A, and Eg are absorption
coefficient, light frequency, proportionality constant and band gap,
respectively. The plot (ahn)1/2 vs. hn as well as the bandgap can be
Fig. 1 (A and B) SEM images and corresponding models of TOH Ag3PO4 viewed
along different directions. (C) XRD patterns and (D) XPS spectra of TOH Ag3PO4.
c
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96 atoms with a 15 Å vacuum region (Fig. 3). Each surface was
scissored to form a nonpolar surface with atomic stoichiometry
satisfied within the each side of the slab. Geometry relaxations
were performed by fixing the midlayers. The surface energy (g)
could be computed by the formula:
g = (Eslab ꢀ nEbulk)/2A
in which Eslab is the total energy of the slab, Ebulk is the total
energy of the bulk per unit cell, n is the number of bulk unit
cells contained in the slab and A is the surface area of each side
of the slab. The calculation results showed that the surface energy
of both {221} (2.01 J mꢀ2) and {332} (1.40 J mꢀ2) facets are much
higher than that of {100} (1.02 J mꢀ2) facets. Correspondingly,
{221} and {332} facets should be more reactive than {100} facets
attributing to the presence of more defects and a higher density
of unsaturated atomic steps and kinks, which can serve as active
sites for breaking chemical bonds. Moreover, Au@Ag nanorods
embedded inside the TOH Ag3PO4 could facilitate the separation
of photoexcited electrons and holes, which may also contribute
greatly to the enhancement of photocatalytic performance of
TOH Ag3PO4.
Fig. 2 (A) SEM image and (B) model of a TOH Ag3PO4 microcrystal viewed along
the h110i direction. (C) SEM images of TOH Ag3PO4 and the inset is an ideal
model fabricated by the average values of ten TOH microcrystals whose edges
are highlighted by blue lines. (D) UV-vis diffusive reflectance spectra and the inset
shows photographs and plots of (ahn)1/2 vs. hn. (E) Photocatalytic activities of
TOH Ag3PO4 for RhB degradation under visible-light irradiation.
In summary, we have demonstrated a facile and efficient
process for the shape-selective synthesis of uniform and perfect
TOH, TOH with an obtuse boundary, tetrahedron and necklace-
like Ag3PO4 by the heteroepitaxial growth method. Importantly,
there exist high-index facets, such as {221} and {332}, on the
surface of TOH Ag3PO4, which exhibit superior photocatalytic
performance for the degradation of RhB dye. This fundamental
study shows a novel morphological control strategy, which may
be adapted for the preparation of other semiconductor materials
for photocatalytic and photoelectric applications.
evaluated by extrapolating the straight line to the hn axis intercept.
As shown in the inset of Fig. 2D, the bandgap of TOH and cubic
Ag3PO4 is 2.26 eV and 2.40 eV, respectively, indicating that the TOH
structure causes the band gap narrowing.
Furthermore, the photocatalytic behaviors of TOH Ag3PO4 for
the degradation of RhB under visible-light irradiation were
explored. For comparison, the performances of cubic Ag3PO4 and
N-doped TiO2 were also investigated. As shown in Fig. 2E, except for
N-doped TiO2, both these Ag3PO4 photocatalysts exhibited excellent
photocatalytic activities for the RhB degradation reaction. Although
owning much larger dimensions than cubes (about 500 nm), the Notes and references
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Ag3PO4 needed 8 min. Moreover, it can be clearly observed that
the specific surface areas of TOH Ag3PO4 should be much smaller
than cubes as a result of their larger dimensions. Therefore, the
significant difference in photocatalytic activities should not be
attributed to their surface areas.
To obtain further insight into the high photocatalytic perfor-
mance of TOH Ag3PO4, the surface structures and surface energies
have also been investigated through density functional theory
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Fig. 3 Relaxed geometries for the (A) {100}, (B) {221} and (C) {332} facets of
Ag3PO4 based on a 96-atom slab model. The vacuum region was set as 15 Å.
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638 Chem. Commun., 2013, 49, 636--638
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