Controlled synthesis of ZnGa2O4 nanorod arrays from hexagonal ZnO microdishes and their photocatalytic activity on the degradation of RhB

Article abstract of DOI:10.1039/c4ra05863a

Novel ZnGa₂O₄ nanorod arrays with desirable photocatalytic activity for organic dye degradation have been synthesized by an associated solution-liquid-solid growth and topological morphology-conversion synthetic strategy.

Full text of DOI:10.1039/c4ra05863a

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Controlled synthesis of ZnGa O nanorod arrays
2
4
from hexagonal ZnO microdishes and their
Cite this: RSC Adv., 2014, 4, 48590
photocatalytic activity on the degradation of RhB†
Received 17th June 2014
Accepted 22nd September 2014
a
a
a
a
b
b
Zesheng Li,* Bolin Li, Zhenghui Liu, Dehao Li, Chunyu Ge and Yueping Fang*
DOI: 10.1039/c4ra05863a
www.rsc.org/advances
2 4
Novel ZnGa O nanorod arrays with desirable photocatalytic activity applications in photocatalysis, solar cells, sensors, nano-
for organic dye degradation have been synthesized by an associated generators, room-temperature ultraviolet (UV) lasers, optical
9
solution–liquid–solid growth and topological morphology-conver- waveguides, and so on. Recently, ZnGa O , a bimetallic semi-
2
4
sion synthetic strategy.
conductor with spinel structure, has been investigated for its
technical application in the photocatalytic eld. Because its
10
hybridized orbits of Ga4s4p and Zn4s4p and wider band gap
The manipulation of physical dimensions and chemical
compositions of semiconductors has attracted intensive atten-
tion due to their important roles in determining the physico-
2 4
(4.4 eV), ZnGa O can promote the mobility of photogenerated
electrons and improve the absorption efficiency in UV light
radiation, which is the most available light source for waste
1
chemical properties of the materials. The emergence and
11
water purication by photocatalytic degradation. Generally,
application of nanotechnology has provided great exciting
advances in controlling the structure and morphology of
1
-D ZnGa O could be synthesized by chemical vapor deposition
2 4
12
(CVD), which requires a thin-lm ZnO template and multiple
2
semiconductors. Among various nanostructures, one-dimen-
ꢀ
steps of high-temperature processes (>1000 C). It is believed
that complicated methods of material synthesis are not
advantageous for further applications. For this purpose, a
sional (1-D) nanostructures, such as wires, tubes, rods and belts
have attracted intense research interest because they exhibit
13
outstanding properties in the optical, electronic, and catalytic
facile synthesis method for 1-D ZnGa
highly desirable for their scalable application in photocatalytic
elds.
It is well known that the solution–liquid–solid (SLS) growth
2 4
O nanostructures is
3

elds. Compared with conventional bulk materials and 0-D
structures (e.g., nanoparticles), 1-D nanostructures can improve

the transport of charge carriers and thus reduce the recombi-
4
nation losses at grain boundaries. Particularly, the aligned 1-D
mechanism has been widely applied to prepare various
continuous 1-D crystalline nanostructures via hydrothermal
synthesis routes, in which low-melting point metals (e.g., In, Ga
and Sn, etc.) are used to induce the growth of 1-D nano-
nanostructures formed by the directed self-assembly of nano-
rods, nanowires and nanotubes are a class of novel 1-D oriented
architectures that gives rise to extraordinary collective proper-
5
ties. Such self-supported 1-D arrays are demonstrated to have
14
structures. Previously, we have developed the SLS method
high performance in catalysis systems for the photocatalytic
under hydrothermal conditions for the growth of Sn-lled
6
7
degradation of organic dyes and for air purication due to
15
In(OH)3 nanotubes
and the twinned nanotowers and
their large surface area, integrated charge-transfer paths, as well
16
nanodendrites of HgSe. Recently, we further developed a
Ga-mediated SLS growth strategy for the synthesis of novel
Ga-doped, self-supported, independent, aligned ZnO nanorods
8
as morphological stability.
As an important functional semiconductor with a direct wide
band gap (3.37 eV) and a large exciton binding energy (60 meV),
ZnO has attracted great interest owing to its potential
17
by one-pot hydrothermal synthesis. Furthermore, the topo-
logical morphology conversion (TMC) process has proven to be
a promising technique for the synthesis of newly nano-
structured materials by means of chemical and structural
transformation from established precursors with similar
a
Development Center of Technology for Petrochemical Pollution Control and Cleaner
Production of Guangdong Universities, College of Chemical Engineering, Guangdong
18
University of Petrochemical Technology, Maoming, Guangdong, 525000, China
b
morphologies. Most recently, we reported the synthesis of
Institute of Biomaterial, College of Science, South China Agricultural University, novel SnO
-doped SiC hollow nanochains by TMC process from
2
Guangzhou, 510642, China. E-mail: lzs212@163.com; ypfang@scau.edu.cn
19
2
a precursor of SnO @C core–shell nanochains.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra05863a
1
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In this study, we demonstrate the controlled synthesis of
2 4
Schematic illustrations of crystal structures for ZnGa O
ZnGa O nanorod arrays from hexagonal ZnO microdishes by are illustrated in Fig. 2. The ball-and-stick model (Fig. 2A)
2
4
using an efficient strategy utilizing SLS growth and TMC shows the typical cubic unit cell of spinel ZnGa O , corre-
2
4
process. Firstly, the Ga-doped ZnO nanorod arrays were sponding to the XRD data. Fig. 2B displays the crystallo-
synthesized by the regular Ga-mediated SLS hydrothermal graphic shear of ZnGa , where zinc/oxygen tetrahedrons
2 4
synthesis, in which the hexagonal ZnO microdishes were are shown in yellow. For one unit cell of ZnGa O ,
2 4
O
17
2
ꢁ
transformed into aligned ZnO nanorods. Secondly, ZnGa
2
O
4
32O constitute the close-packed subunit cell with a face-
2
+
nanorod arrays were further synthesized by a heat-treating TMC centred-cubic structure, 8Zn occupy 8 tetrahedral vacancies
process from the Ga-doped ZnO nanorod arrays (see ESI† for (accounting for 1/8 of the total number of tetrahedral
3
+
detailed synthetic steps). The newly designed 1-D semi- vacancies) to form 4 A-sites, and 16Ga occupy 16 octahedral
conductor nanostructures of ZnGa O nanorod arrays showed a vacancies (accounting for 1/2 of the total number of octahe-
2
4
very high activity for the photocatalytic degradation of Rhoda- dral vacancies) to form 4 B-sites, where one complete unit
mine B (RhB) under UV light radiation. cell of spinel ZnGa is made up of the 4 A-sites and
The phase structures of the as-synthesized samples have 4 B-sites by alternate permutation.
2 4
O
been characterized by XRD. Fig. 1 shows the XRD patterns of the
To investigate the detailed nanostructures and morpholog-
hexagonal ZnO microdishes (black pattern), Ga-doped ZnO ical evolution of the as-prepared samples, FE-SEM and TEM
nanorod arrays (red pattern) and the ZnGa
2 4
O nanorod arrays characterization were further implemented. Fig. 3 shows the
(blue pattern). From the black and red patterns, it can be readily representative SEM images of the hexagonal ZnO microdishes.
concluded that the two samples have the hexagonal phase The lower-magnication images (Fig. 3A and B) indicate that
structures of ZnO (JCPDS card no. 36-1451 (ref. 9)). The sharp the sample has an interesting conguration comprising
and clear diffraction peaks clearly indicate the good crystalline homologous 3-D hexagonal microdishes. The higher-magni-
nature of the two products. It suggests that the ideal ZnO cation images (Fig. 3C and D) reveal that the owerpot is made
structures have been obtained by the hydrolyzation of ZnSt
2
up of a hexagonal pot on one side and short nanorods on the
under hydrothermal treatment. Expressly, from the red pattern, other (just like one owerpot).
no obvious Ga diffraction peak can be observed for the Ga-
Fig. 4A and B show the representative SEM images of
doped ZnO sample, which is mainly due to the amorphous Ga-doped ZnO nanorod arrays. Evidently, the sample has highly
existence of Ga dopant as indicated by one of our previous self-supported, aligned 1-D nanostructures, where the ZnO
17
works. As illustrated by the blue pattern, the XRD pattern of nanorods are vertically well-assembled into micro-architectures
the ZnGa
2
O
4
sample can be indexed as cubic spinel ZnGa
2
O
4
without extraneous supporting substrates. These results
˚
with a cell parameter of a ¼ 8.335 A, agreeing well with the demonstrate that ZnO 1-D nanostructures have been achieved
10
JCPDS Card no. 38-1240. Several distinct peaks at 30.31, 35.71, by the Ga-mediated SLS hydrothermal process. Fig. 4C and D
3.41, 57.41 and 63.01 match well with the (220), (311), (400), show the representative SEM images of ZnGa nanorod
511) and (440) crystal planes of ZnGa , respectively. There is arrays, which reveal the loosely aligned 1-D nanostructures
negligible ZnO component that remained in the ZnGa O relative to the Ga-doped ZnO sample. Fig. 5 shows the SEM-EDS
sample. Therefore, the results indicate that the ZnGa O phase elemental mapping of Ga, Zn and O from the individual
4
(
2 4
O
2 4
O
2
4
2
4
has been successfully synthesized from the Ga-doped ZnO ZnGa O nanorod array. It is found that the three elements, Ga,
2
4
phase via the heat-treating conversion process.
Zn and O (Fig. 5B–D), are homogeneously distributed, corre-
sponding to the SEM image (Fig. 5A).
Fig. 6 shows the TEM images of Ga-doped ZnO nanorods
2 4
(Fig. 6A and B) and ZnGa O nanorods (Fig. 6C and D). It can be
observed that the two samples both have dense, aligned nano-
rod architectures with single nanorods of 50–80 nm diameter. A
spot of nanopores exists in the ZnGa O nanorods (see Fig. 6D),
2
4
Fig. 2 Schematic illustrations of the crystal structures for ZnGa
Fig. 1 XRD pattern of hexagonal ZnO microdishes (black), Ga-ZnO a ball-and-stick model and (B) the crystallographic shear, zinc/oxygen
nanorod arrays (red) and ZnGa nanorod arrays (blue). tetrahedrons are showed in yellow.
2 4
O : (A)
2 4
O
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Fig.
2 4
5 SEM-EDS elemental mapping images from the ZnGa O
nanorod array.
Fig. 3 Representative SEM images of hexagonal ZnO microdishes.
Fig. 6 TEM images of the (A) and (B) Ga-ZnO nanorods and (C) and (D)
ZnGa nanorods.
2 4
O
Fig. 4 Representative SEM images of the nanorod arrays: (A) and (B)
Ga-ZnO, (C) and (D) ZnGa O .
2 4
The possible formation process of the ZnGa O4 nanorod
2
arrays, involving the evolution of Ga-doped ZnO nanorods and
ZnGa O nanorods, are schematically shown in Fig. 7. In the
absence of Ga during the hydrothermal process, hexagonal ZnO
microdishes were obtained (see Fig. 3 for details), indicating the
which might originate from crystal defects during the structure
conversion process. The insets in Fig. 6B and D further indicate
the (101) planes of ZnO (d ¼ 0.248 nm) and the (220) planes of
2
4
ZnGa
2
O
4
(d ¼ 0.295 nm). The above-mentioned results largely
Ga species plays an important role in the formation of the ZnO
demonstrate the feasibility of topological morphology conver-
sion of Ga-doped ZnO nanorods to ZnGa O nanorods by the
17
1
-D framework. Fig. 7 (i) illustrates a formation process of the
2
4
Ga-doped ZnO nanorods with the SLS growth route. The SLS
TMC heat-treating process. In addition, the BET specic surface
areas of ZnGa nanorods, Ga-doped ZnO nanorods and
hexagonal ZnO microdishes were determined as 65.7, 61.2 and
14
process may consist of four steps: formation of the liquid Ga
2 4
O
droplets in hydrothermal solution (S), production of ZnO by
ZnSt hydrolysis and its diffusion to liquid Ga droplet to form a
2
2
ꢁ1
3
8.4 m g , respectively.
liquid–solid (LS) interface, 1-D growth of the Ga-doped ZnO
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concentration of dye, C
0
the initial concentration of dye, k the
reaction rate constant, and t the irradiation time. The degra-
dation rate constant k of ZnGa O nanorods was estimated to be
2
4
ꢁ
1
0
.0397 min , which was 2.51 times and 4.22 times higher than
ꢁ
1
those of Ga-doped ZnO nanorods (0.0158 min ) and hexagonal
ꢁ
1
ZnO microdishes (0.0094 min ), respectively (Fig. 8B). Fig. 8C
Fig. 7 Schematic illustration for the preparation of ZnGa
rods: (i) solution–liquid–solid (SLS) growth for Ga-doped ZnO nano-
rods and (ii) topological-morphology-conversion (TMC) process for
2 4
ZnGa O nanorods.
2
O
4
nano- shows the change of absorption spectra of the RhB solution
when exposed to UV light at different times in the presence of
the ZnGa photocatalyst sample, which clearly shows that the
2 4
O
absorption peak of RhB drops gradually with an increase of
exposure time to 90 min (conrming the degradation of RhB as
shown in Fig. 8A). Furthermore, stability testing on the ZnGa O4
2
nanorods at the LS interface, and continuous 1-D growth of
ZnO, consuming the liquid Ga droplet and thus completing the
SLS assembly of the Ga-doped ZnO nanorods. Fig. 7 (ii) illus-
trates the latent evolution of the ZnGa O nanorods from
2 4
Ga-doped ZnO nanorods in the TMC process. In this process,
the initial nanorod morphology of the Ga-doped ZnO precursor
sample illustrates that the degradation rate shows slight
decrease aer four runs of irradiation within 360 min (Fig. 8D).
The above mentioned results demonstrate that the obtained
ZnGa O nanorods can be used as a promising photocatalyst
2 4
with excellent activity and desirable stability for the degradation
of RhB in aqueous solution under UV light radiation.
2 4
would be inherited by the ZnGa O product by means of the
The physical dimensions and chemical compositions of
semiconductors have signicant effects on their physicochem-
potential chemical and structural transformation (ZnO reacting
with Ga to form ZnGa O ) under the heat-treating process.
2
4
1
ical properties. The smaller dimension of semiconductors
Further study on the accurate reaction mechanism is in
progress.
The photocatalytic activities and stability of the as-prepared
2 4
photocatalysts (the ZnGa O nanorods, Ga-doped ZnO nano-
rods and hexagonal ZnO microdishes) were evaluated by
degradation of RhB in aqueous solution under UV light radia-
2 4
tion (see Fig. 8). Obviously, the ZnGa O nanorods exhibit a
much higher photocatalytic activity than the Ga-doped ZnO
nanorods and hexagonal ZnO microdishes (Fig. 8A). RhB was
largely degraded (97.2%) in 90 min under UV light radiation,
while the degradation rates of Ga-doped ZnO nanorods and
hexagonal ZnO microdishes were only 75.9% and 57.1%,
respectively. The rst-order reaction rate constant can be
oen leads to higher photocatalytic activity because of their
higher specic surface area along with optimized utilization
2
efficiency. Obviously, the specic surface area of Ga-doped ZnO
2
ꢁ1
nanorods (61.2 m g ) is much higher than that of hexagonal
2
ꢁ1
ZnO microdishes (38.4 m g ). Hence, the improved activity of
Ga-doped ZnO nanorods relative to hexagonal ZnO microdishes
can be attributed to the high surface area of nanorod structures.
2 4
However, the specic surface areas of ZnGa O nanorods
2
ꢁ1
2
ꢁ1
(
65.7 m g ) and Ga-doped ZnO nanorods (61.2 m g ) are
more approximate, so the high activity of the ZnGa nanorods
2 4
O
sample is largely due to its favorable material structures.
It is generally accepted that the superior photocatalytic
activity of ZnGa O originates from its unique electronic and
2
4
calculated by the plots of the ln(C/C
obtained rate law may be ln(C/C
0
) vs. radiation time (t). The
) ¼ ꢁkt, where C is the
band structures. For one thing, the bottom of the conduction
band (LUMO) is formed by Ga4s4p–Zn4s4p hybridized atomic
orbits, which promote the mobility of photogenerated electrons
0
20
and are benecial for charge separation. For another, the
wider band gap (4.4 eV) endows the photogenerated charge with
stronger reductive capability, which results in stronger redox
ability of the photogenerated electron–hole pairs, thus facili-
21
tating the photocatalytic activity. In addition, the possible
existence of trace heterojunctions of ZnO–ZnGa O4 may
2
contribute to photocatalytic activity by the improved charge
transfer. Accurate results are expected by further advanced
characterization techniques.
To analyze the catalytic mechanism, the band structures of
2 4
ZnO and ZnGa O and possible pathway of RhB photocatalytic
degradation are schematically illustrated in Fig. 9. In the case of
ZnGa O , the E is 3.15 V, which is much higher than the 2.38 V
2
4
VB
ꢁ
of E(HOc/OH ); the E is ꢁ1.25 V, which is much lower than
CB
ꢁ
11
the ꢁ0.33 V of E(O /O c). In the case of ZnO, only the E is
2
2
CB
2
/O c). As a result, HOc can be more
under UV light in RhB aqueous
photocatalysts' much higher
2ꢁ
lower than that of E(O
readily generated for ZnGa
solution, leading to the ZnGa
photocatalytic properties than that of ZnO. The possible reac-
tion process is as follows: once the ZnGa catalyst generates
2 4
O
Fig. 8 (A) Photocatalytic activities and (B) rate constant k of RhB
degradation for the as-prepared photocatalysts; (C) absorption spec-
trum of RhB solution and (D) cycling runs for the degradation of RhB in
2 4
O
the presence of ZnGa
O
2 4
photocatalyst under UV light radiation.
2 4
O
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the advanced application of photodegradation in water puri-
cation. The as-prepared ZnGa O sample has highly self-sup-
2
4
ported, aligned 1-D nanostructures, where ZnGa O nanorods
2
4
with 50–80 nm diameter are vertically assembled into micro-
architectures without extraneous supporting substrates. The
ZnGa O nanorod arrays showed much higher BET specic
2 4
surface areas over the hexagonal ZnO microdishes. Due to
favorable 1-D nanostructures and unique electronic band
2 4
structures, the ZnGa O photocatalyst exhibited a much higher
photocatalytic activity relative to the hexagonal ZnO microd-
ishes, in which the degradation rate of RhB can be 4.22 times as
high as that of the ZnO photocatalyst. The ndings suggest that
this newly fashioned architecture of ZnGa
2 4
O could be used as a
promising photocatalytic material for organic dye degradation.
Fig. 9 Schematic illustration for the band structures of ZnO and Acknowledgements
ZnGa O
2 4
and the proposed pathway of RhB photocatalytic
degradation.
This research was supported by the National Natural Science
Foundation of China (21443006, 21173088 and 21105030),
Natural Science Foundation of Guangdong Province
the electron–hole pairs by UV light radiation, the photo- (S2013040015162), Guangdong Province and Chinese Academy
ꢁ
generated electrons (e ) on the conduction band (CB) bond with of Sciences Strategic Cooperative Project (2012B090400003),
adsorbed oxygen and water to form strong oxidizing species of Science and Technology Project of Maoming (2014006) and
ꢁ
+
O
2
c and HOc, while the holes (h ) on the valence band (VB) also Doctor Startup Project of School (513086).
bond with adsorbed oxygen and water to form HOc. Under the
action of substantial, strong oxidizing species, the structure of
RhB is destroyed and nally decomposed into CO and H O. All
Notes and references
2
2
22
these processes might be described as follows:
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RSC Adv., 2014, 4, 48590–48595 | 48595