Hexagonal nanoplates of high-quality γ-gallium oxide: Controlled synthesis and good heterogeneous catalytic performance for thiophenes

Article abstract of DOI:10.1039/c7ta09913d

Hexagonal nanoplates of high-quality γ-gallium oxide (γ-Ga₂O₃) were successfully synthesized by using a competitive and cooperative interaction model based on multiple equilibria, including a precipitation interaction, a coordination interaction and two binding interactions. The synthetic method is straightforward and affords the desired product in very high yield. It is important to note that only a combination of the coordination interaction and binding interactions can contribute to the formation of hexagonal nanoplates by effectively suppressing the generation of gallium oxide hydroxide and directly limiting the assembly of the nanoplates into microflowers. This model is significant because it allows us to understand how the disassembly process of nanostructures is related to the synergistic mechanism of multiple interactions. Furthermore, the γ-Ga₂O₃ hexagonal nanoplates exhibit good heterogeneous catalytic performance for the oxidation reaction of thiophenes. A possible mechanism was proposed for the catalytic process, and this may open up a new perspective in the study of oxidative desulfurization.

Full text of DOI:10.1039/c7ta09913d

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SONG, Y. Q. Wang, M. M. Ruan, Y. Teng, J. Xia, J. Yang, S. S. Chen and F. Wang, J. Mater. Chem. A, 2018,
DOI: 10.1039/C7TA09913D.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA09913D
Hexagonal Nanoplates of Highꢀquality γꢀGallium Oxide: Controlled Synthesis and Good
Heterogeneous Catalytic Performance for Thiophenes†
Zun Yang,a Le Xin Song,*a,b Ya Qian Wang,a Mao Mao Ruan,a Yue Teng,c Juan Xia,a Jun Yang,a Shan Shan Chena and Fang Wanga
Received (in Cambridge, UK) 6th January 2018, Accepted: Xth XXXXXXXXX 2018
First published on the web Xth XXXXXXXXX 2018
DOI: 10.1039/x0xx00000x
Hexagonal nanoplates of highꢀquality γꢀgallium oxide (γꢀGa2O3) were
successfully synthesized by a competitive and cooperative interaction
model based on multiple equilibria, including a precipitation interaction, a
coordination interaction and two binding interactions. The synthetic
mechanism is critical for applying this technique to achieve the
dimensionꢀcontrolled nanostructures. This is one objective of the
present study.
A second objective is to investigate the catalytic property of the
asꢀobtained nanoplates for the oxidative desulfurization of thiophenes
(TPs), a class of major sulfurꢀcontaining organic compounds in
petroleum. It was reported that γꢀGa2O3 presented a promising
prospect as a versatile catalyst in numerous reactions, like NO
reduction,14 dehydrogenation of paraffin,15 methanol conversion,16
cyclization of hydrocarbons9 and oxidative degradation of organic
effluents.17 As we know, the oxidative removal of thiophenes is of
great importance and interest in petroleum industry.18~21 In the past
decades, a wide variety of metal oxide catalysts have been developed
to accomplish this task.22,23 Although some of them, such as MoO3,
showed high catalytic activities for the oxidation of TPs,24,25 the
disadvantages of such catalysts include the requirement of high
temperature and/or long reaction time, large catalyst amounts and
poor catalyst recyclability,26 which have severely limited their
application. In this work, we hope to demonstrate that these
disadvantages can be circumvented by using the highꢀquality γꢀGa2O3
hexagonal nanoplates as a heterogeneous catalyst.
method is straightforward and affords desired product in very high yield.
It is important to note that only a combination of the coordination
interaction and binding interactions can contribute to the formation of
hexagonal nanoplates by effectively suppressing the generation of gallium
oxide hydroxide and directly limiting the assembly of the nanoplates into
microflowers. This model is significant because it allows us to understand
how the disassembly process of nanostructures is related to the synergistic
mechanism of multiple interactions. Furthermore, the γꢀGa2O3 hexagonal
nanoplates exhibit good heterogeneous catalytic performance for the
oxidation reaction of thiophenes. A possible mechanism was proposed for
the catalytic process, and this may open up a new perspective in the study
of oxidative desulfurization.
Controlled synthesis, structure and catalytic performance of gallium
oxide (Ga2O3) nanomaterials are a current hot topic for research in
advanced functional materials.1~3 However, as an important member
of the family of Ga2O3, the metastable γꢀGa2O3 has not been
sufficiently studied in the field of controlled synthesis, because of its
harsh synthesis conditions such as the use of organic solvents, toxic
reagents, high temperatures, high pressures and long reaction times.4~6
Although different γꢀGa2O3 architectures in the forms of 1D (tubes,
wires, rods)7~9 and 3D (flowers, spheres)10,11 nanostructures have been
developed, highꢀquality 2D structures have rarely been reported,
mainly because they are easily assembled together into complete 3D
structures.10,12 In a recent attempt to fabricate a 2D γꢀGa2O3
architecture through a oneꢀpot hydrothermal method, freestanding
singleꢀlayered nanosheets was obtained, but associated with a low
yield of only 32%.13 For these reasons, until now, it has still been a
great challenge to design and develop a new, green synthesis method
for the acquisition of highꢀquality 2D γꢀGa2O3 with a high yield.
To overcome the 2D selfꢀassembly problem, in this work we
applied a synergistic effect by combining the binding interaction
between Ga3+ ions and polyvinyl pyrrolidone (PVP) with the
coordination interaction between Ga3+ ions and oxalate ions (Ox2–) in
water. Our results show that the highꢀquality γꢀGa2O3 ultrathin
hexagonal nanoplates were successfully constructed with a high yield
of 99%. This is the first example, to our knowledge, of a synergistic
combination of multiple interaction mechanisms mediating
selfꢀassembly/disassembly of γꢀGa2O3 nanostructures, and this finding
could have wideꢀranging and profound effects on the controlled
growth of nanomaterials. Thus, clarifying the assembly/disassembly
In a word, this study not only provides a successful pathway for
fabricating the γꢀGa2O3 hexagonal nanoplates in water, but also
reveals that the nanoplates exhibit a high catalytic activity for the
oxidation of TPs. We believe that the present results offer exciting
prospects for future studies of this material.
Initially, metal gallium (0.05 g, 0.70 mmol) was dissolved in HCl
(30 mL, 0.40 M) to form an aqueous solution of Ga3+ ions (Equation
1). Then, urea (0.50 g, 8.30 mmol), potassium oxalate monohydrate
(POM, 0.39 g, 2.10 mmol) and PVP (Kꢀ30, 78 mg, 0.70 mmol PVP
units ) were added to the solution under vigorous stirring for 0.5 h at
room temperature. The role of urea is that its slow hydrolysis results
in a homogeneous release of hydroxyl ions, thus gradually raising the
pH (Equation 2).27 Subsequently, the solution was transferred into a
sealed Teflonꢀlined stainless steel autoclave and heated at 453 K for 3
h. Finally, a white crystal was obtained in a high yield (99%) by
filtration (pH of filtrate, 7.5), washing with water and absolute ethyl
alcohol and drying at 373 K for 10 h (see the ESI† for the detailed
synthesis procedure).
Figure 1a shows the Xꢀray diffraction (XRD) pattern of the white
crystal, having a faceꢀcentered (fcc) structure of γꢀGa2O3 phase with a
space group of Fd3m (JCPDS 20ꢀ0426).28 No peak of impurity was
detected. It was reported that γꢀGa2O3 has a cubic defect spinelꢀtype
structure, and its unit cell contains two formula units: GaO6
octahedron and GaO4 tetrahedron.29 The fieldꢀemission scanning
electron microscopy (FEꢀSEM) image in Figure 1b indicates that the
γꢀGa2O3 exhibits a 2D nanostructure like nanoplates, and thus the
crystal was marked as γꢀGa2O3ꢀnp. The nanoplates have an ultrathin
hexagonal structure with a side length of about 300~350 nm and a
thickness of 10~30 nm (Figure S1). Figure 1c displays a largeꢀscale
SEM image of the sample, revealing a highly monodispersed
distribution nature. Figures 1d and 1e are the SEM and transmission
aDepartment of Chemistry, University of Science and Technology of
China, Jin Zhai Road 96, Hefei 230026, P. R. China
Eꢀmail: solexin@ustc.edu.cn; Fax: +86ꢀ551ꢀ63601592;
bNational Synchrotron Radiation Laboratory, University of Science
and Technology of China, Hefei 230026, P. R. China
cState Grid Auhui Electric Power Research Institute, Zi Yun Road
299, Hefei 230601, P. R. China
†
Electronic supplementary information (ESI) available:
Experimental section, XRD, FEꢀSEM, Raman, NMR, XPS, FTꢀIR,

Journal of Materials Chemistry A
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DOI: 10.1039/C7TA09913D
electron microscopy (TEM) images of a single hexagonal nanoplate,
respectively, clearly illustrating the feature of the plate structure with
d
i
s
t
i
n
c
t
e
d
g
e
s
a
n
d
Figure 1. Left column: a) the XRD pattern, b) FEꢀSEM and c) large scale SEM images of the γꢀGa2O3ꢀnp obtained at 453 K for 3 h in the presence of PVP (78 mg)
and urea (0.50 g); d) The FEꢀSEM, e) TEM, f) HRꢀTEM and g) SAED images of a single nanoplate. Right column: the equations involved in the synthesis of
γꢀGa2O3 are summarized.
corners. The high resolution TEM (HRꢀTEM) in Figure 1f shows that
the fringe spacing is 0.248 nm, corresponding to the (311) interplanar
distance of the fcc structure. The selectedꢀarea electron diffraction
(SAED) pattern in Figure 1g emphasizes the singleꢀcrystalline nature
of this material, possessing a six fold symmetry, and the dots in the
pattern are corresponding to the (440) and (311) planes of the fcc
complex. The 13C nuclear magnetic resonance (NMR) spectroscopy
(Figure S3) show that the generation of TOG results in a large upfield
shift of the carbons of Ox2– ions from 173.8 to 165.1 ppm due to the
presence of coordination bonds of GaꢀO. The Raman spectra further
demonstrate this (Figure S4). The molar ratio of Ga3+ to Ox2− in the
complex was determined to be 1:3 by continuous variation (Job's
plot)31 method (Figure S5). These results strengthen our recent claim
that the coordination interaction between Ga3+ ions and organic acid
radicals is an effective method to avoid the generation of GaOOH in
water.32
ꢀ
γꢀGa2O3 structure orientated in the [112] direction.
To the best of our knowledge, in the few examples of 2D γꢀGa2O3
synthesis, although very thin nanosheet structures were reported,12,13
they all have common problems, such as poor monodispersity, bad
structural stability and high tendency to selfꢀassemble into 3D
structures. Also, these synthesis methods have some drawbacks such
as long operation time, low yields and the use of organic solvents. In
contrast, our γꢀGa2O3 nanoplates show great superiority in the
structure and property. Structurally, they exhibit the typical
characteristic of a perfect singleꢀcrystal structure, having an intact
A
significant difference between this work and our recent
approach32 is that we introduce a new variable, i.e. the binding
interaction of PVP with Ga3+ into the reaction system in this work,
thereby forming a multicomponent system composed of Ga3+ (center
ion), Ox2– (ligand), PVP (host, binding agent) and OH– (precipitation
agent, Scheme 1). It results in two important consequences. Firstly,
hexagonal morphology with an extremely smooth surface. In our view, the multiple interaction process allows γꢀGa2O3 nanoparticles to be
this structure is one of the most wellꢀestablished structures of 2D
γꢀGa2O3. In the view of overall properties, they indicate the special
character of a very high 2D structural stability, a fairly uniform
particle size distribution and a very good monodispersity. As far as we
know, these features are unprecedented. Moreover, our synthetic
method is novel, green and efficient.
formed in the form of 2D nanoplates. Secondly, and more importantly,
the 2D nanoplates can be stabilized in a monodispersed form rather
than selfꢀassembling into 3D nanospheres or nanoflowers directly.
Therefore, the second issue is why the 2D hexagonal nanoplates
were not selfꢀassembled into 3D structures as described before.10 This
is a major unresolved issue that has huge scientific and commercial
These results from experimental and structural analysis, together
with previous observations,10 will allow us to address several key
problems with regard to the creation of the γꢀGa2O3ꢀnp.
The first issue is the formation route of the γꢀGa2O3ꢀnp. This fact
that no gallium oxide hydroxide (GaOOH) could be observed even at
earlier time points (1 and 2 h) is completely different from previous
reports that the generation of γꢀGa2O3 in solution nearly always
originated from the formation (Equations 3 and 4) and decomposition
(Equation 5) of GaOOH.30,10 Here, we propose that the γꢀGa2O3ꢀnp
was formed from the direct decomposition of
a complex
[tris(oxalato)gallium(III), TOG, Equation 6] of Ga3+ with Ox2– in
water, as shown in Equation 7. A competitive process of Ox2− and
OH− for Ga3+ was depicted in Scheme 1. The competitive relationship
was clearly observed from our experimental phenomena. For example,
the addition of small amounts of POM into a GaCl3 solution leads to a
quick change from turbid to transparent (Figure S2). Thus, we suggest
that the coordination interaction between Ox2− and Ga3+ ions is
stronger than the precipitation interaction between OH− and Ga3+ ions.
Indeed, our data provide clear evidence for the formation of the
Scheme 1. A schematic model regarding multiple interactions for the creation
of the γꢀGa2O3ꢀnp. A) The Ox2− displays a dominant position both in the
competitive process of Ox2− and OH− for Ga3+ and in the competitive process
of Ox2− and PVP for Ga3+. B) The formation process of the γꢀGa2O3ꢀnp was

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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA09913D
summarized: i) the generation of Ga3+; ii) the formation of the complex TOG
and the supramolecule PVPꢀGa3+; iii) the formation of the supramolecule
The contribution of synergistic effect of the coordination interaction between
Ga3+ and Ox2– and the binding interactions of PVP to Ga3+ and TOG to the
PVPꢀTOG; iv) the decomposition of TOG in PVPꢀTOG into the γꢀGa2O3ꢀnp. C) creation of the γꢀGa2O3ꢀnp.
Figure 2. The XRD patterns and FEꢀSEM images of the samples obtained by the same method as in Figure 1, but using A) different concentrations of PVP: a1) 0,
a2) 40, a3) 120 and a4) 160 mg; B) different reaction times: b1) 1, b2) 2, b3) 6 and b4) 10 h; and C) different pHs: c1) 2.0, c2) 3.5, c3) 6.0 and c4) 9.0.
implications because it is involved in the dimensional regulation of
inorganic nanomaterials. In order to address this, we performed a
series of experiments to examine the importance of PVP in the
formation process of hexagonal nanoplates.
for example, the average decomposition activation energies of TOG
and PVPꢀTOG were determined to be 152 and 167 kJ·mol−1,
respectively. Therefore, we concluded that in the multicomponent
system, PVP acts as a double binding host, playing a crucial role in
decreasing the formation and decomposition rate of TOG. Such a rate
decrease would directly contribute to the overall decrease of primary
particle formation, nucleation and microstructural growth of γꢀGa2O3
since it was originated from the decomposition of TOG.
Interestingly, the intensity of the binding interactions is closely
associated with the concentration of PVP. We noticed that when the
concentration of PVP was changed from 0 to 160 mg, the structure of
γꢀGa2O3 evolves dramatically from an aggregation state of
microflowers (named as γꢀGa2O3ꢀmf, Figure 2Aꢀa1) assembled by
nanopetals, a mixture state of some microflowers and numberless
nanopetals falling from other microflowers (Figure 2Aꢀa2), to a final
form consisted of nanopetals (Figures 1b, 2Aꢀa3, ꢀa4). We consider
that the nanopetals were loosely stacked in the final structure,
resulting in each particle making a large contribution to the external
surface area. This is completely different from the flowerꢀlike
It is known that PVP has been used extensively as a stabilizer,
protecting agent or shape directing agent to control the growth of
inorganic materials such as Au, Ag, ZnS, Cu2O and BiVO4.33~37 Some
authors have suggested that PVP may act as a ligand by donating the
lone pair of electrons of the nitrogen atom or carbonyl moiety in
pyrrolidone ring.38 Nevertheless, in this work there was no evidence
for the formation of a PVPꢀGa3+ coordination compound since no
absorption due to GaꢀN or GaꢀO coordination bonds was detected in
the UVꢀVis region (200~600 nm, Figure S5). Moreover, when POM
was completely substituted by PVP, only GaOOH was formed (Figure
S6). Therefore, it is concluded that PVP do not play a ligand role in
changing the formation route of the γꢀGa2O3.
1
All the same, independent experiments including H NMR spectra
(Figure S3), conductivity measurements (Figure S5) and Fourier
transform infrared spectra (Figure S7) have demonstrated the presence
of the interactions between PVP and Ga3+, and between PVP and TOG. assembled structure since a number of nanopetals were connected
Further, the nature of the interaction of PVP with Ga3+ was considered
to be an ionꢀdipole interaction between the positively charged Ga3+
and the electronegative oxygen atom in the C=O bond (Figure S7),
while the interaction of PVP with TOG is due to the binding of PVP
chains to the coordination ion. Here we tentatively define the two
kinds of interactions as binding interactions. The Job's plot analyses
indicate that for the PVPꢀGa3+ and PVPꢀTOG supramolecules,
saturation corresponded to approximately 1:1 and 1:4 molar ratios of
Ga3+ to PVP units and TOG to PVP units, respectively (Figure S5).
Scheme 1 illustrates a competitive equilibrium among the multiple
interactions including the precipitation interaction between Ga3+ and
OH–, the coordination interaction between Ga3+ and Ox2– and the two
binding interactions. It is reasonable to assume that the competitive
process of Ox2− and PVP for Ga3+ decreases the formation rate of
TOG. Our experimental evidence supports this assumption (Figure
S5). As the process progresses, more TOG was formed with the
gradual release of Ga3+ from its PVP supramolecule. This is partly
explained by the equilibrium shift from the weak ionꢀdipole
interaction of PVP with Ga3+ to the strong coordination interaction of
Ox2– with Ga3+. As a result, the binding of PVP to Ga3+ was being
gradually substituted by its binding to TOG. The formation of the
PVPꢀTOG supermolecule is important because it can contribute to
decreasing the decomposition rate of TOG. The decomposition
kinetics from thermogravimetric data demonstrates this (Figure S8),
densely to each other in the microflowers. Clearly, the optimum
concentration is about 78 mg, and the concentration was used in the
subsequent assays. This finding is highly indicative that hexagonal
nanoplates were derived from the disassembly of microflowers. From
an opposite point of view, it is strongly suggested that the presence
and concentration of PVP is of fundamental importance in inhibiting
selfꢀassembly of the nanoplates. This result, together with Scheme 1,
allows us to create a direct link between the disassembly of 3D
microstructures and the decrease of formation and decomposition rate
of TOG.
Further, we investigated the growth mechanism of γꢀGa2O3 over
time in the presence of PVP (78 mg). Figure 2B illustrates the images
of the samples obtained as described in Figure 1, but with different
reaction times. Taken together with Figure 1b, these images reveal
two important features. On the one hand, with the increase of time
from 1, 2 to 3 h, a continuous structural evolution occurs from
spherical particles with rough surface, to flowerꢀlike particles formed
by interleaving nanoplates, and eventually to hexagonal nanoplates.
On the other hand, with the increase of time from 3, 6 to 10 h, we
noted that γꢀGa2O3 was gradually transformed into rodꢀlike GaOOH,
suggesting that a longer time results in the hydrolysis of γꢀGa2O3
(Equation 8).39 This implies that reaction time is a driving factor to
promote the application of PVP in the disassembly process of 3D
microstructures to form 2D nanoplates.

Journal of Materials Chemistry A
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Also, a series of pHꢀdependent experiments by changing the initial
concentration of urea were conducted in the presence of PVP (78 mg).
We found that the γꢀGa2O3ꢀnp can be created only in a narrow interval
of pH from 6.0 to 7.5. On the one hand, when the pH is lowered to 2.0
or 3.5, coralꢀlike aggregates of GaOOH were produced with a
diameter of 1~1.5 ꢁm as shown in Figures 2Cꢀc1 and c2). We
consider that the coralꢀshaped GaOOH was originated from the direct
hydrolysis of Ga3+ ions (Equation 9). This is because high acidity
inhibits the coordination of Ox2– ions to Ga3+. On the contrary, as the
pH increases to 9.0, a mixture of spherical γꢀGa2O3 and rodꢀlike
GaOOH occurred as can be seen in Figure 2Cꢀc4. It may be explained
by Equations 10 and 11 since Ga2O3 is an amphoteric compound.
These results indicate that pH plays an adjusting role to improve the
application of PVP in mediating the phase transition, dimension
evolution and morphology adjustion of the Ga(III) oxides. Too high or
too low pH results in an advantage of the precipitation interaction to
generate GaOOH. We have tried to use different alkali sources such as
NaOH and ammonia (Figure S9), but none of them can avoid the
problem of generation of GaOOH even in the case of a moderate pH,
highlighting the vital role of slow hydrolysis of urea to continuously
produce OH− ions in influencing the progress of reactions 3 to 6. This
role is indeed a reflection of the competitive relation between the
precipitation interaction and coordination interaction of Ga3+ ions.
In addition, we performed the hydrothermal experiments at
different temperatures (Figure S10) with the same conditions as in
Figure 1. Our results show that when the temperature decreased to 433
K, flowerꢀlike γꢀGa2O3 microparticles were obtained, whereas when
the temperature was increased to 473 K, nanoplateꢀlike γꢀGa2O3
crystals with broken edges were formed. The structural change
induced by increasing temperature may be explained by the fact that
the disassembly process is highly related to the formation energy of a
structure, as reported by other authors.40 Thus, it is difficult to
effectively achieve the disassembly from the 3D flowerꢀlike structure
to the 2D plateꢀlike structure at a relatively low temperature, since the
energy of the former is usually lower than that of the latter.41 On the
contrary, when the temperature is too high, the edges of the 2D
plateꢀlike structure can be partially destroyed. Thus, the temperature
of 453 K was chosen as the optimum reaction temperature for the
multiple interaction model.
using the same method as in Figure 1, similar γꢀGa2O3ꢀnp were
obtained (Figure S11). It suggests that this method can be efficiently
used for organic diacid ligands. However, no precipitate was formed
for citric acid (CA) at 3 h or for disodium ethylenediaminetetraacetate
(EDTA) at 3 and 6 h. In the case of CA, γꢀGa2O3 nanospheres
(diameter, 100~150 nm, Figure S12) was formed only at the end of 6
h. We think that this is due to the fact that the strong coordination
ability of the polydentate ligands leads to increased stability of the
Ga(III) chelates, causing a decreased decomposition rate. Additionally,
when we used ethylenediamine (EDA) or ethylene glycol (EG) as a
ligand, only GaOOH microrods were observed (Figure S11). The
former is due to increased alkalinity in the solution, whereas the latter
is the result of a relatively weak coordination ability. Further, the
concentration ratio of ligands to Ga3+ also has an impact on the
formation of γꢀGa2O3. For instance, when the molar ratio of POM to
Ga3+ is higher than 3:1, such as 5:1, no precipitate was generated at
the end of 3 h. Contrarily, when the ratio is lower than 3:1, such as
1.5:1, GaOOH appeared (Figure S13). Thus, we concluded that in the
multiple interaction model, a ligand with appropriate coordination
capability and concentration is a prerequisite for creating γꢀGa2O3.
On the other hand, as shown in Scheme 2 (right part), the nature of
hosts exerts profound effects on the morphology of γꢀGa2O3. For
example, PEGꢀ4000 (PEG) can play a similar role as PVP, but the
presence of βꢀcyclodextrin (βꢀCD) and polyvinyl chloride (PVC)
results in the formation of rattan ballꢀlike and 3D flowerꢀlike
microstructures consisting of 2D nanopetals, respectively (Figure S14).
This means that the surface feature of γꢀGa2O3 is directly related to
the structure of the hosts. It can be explained by the fact that different
hosts may have different binding interactions (including interaction
sites, intensity, and so on) to Ga3+ and POM, leading to different
effects on the formation and decomposition of POM to produce
γꢀGa2O3. This observation that βꢀCD and PVC do not play a role in
limiting the selfꢀassembly of 2D nanopetals into 3D structures allows
us to suggest that the selection of hosts against selfꢀassembly is very
important in the multiple interaction model.
The multiple interaction model for the synthesis of the γꢀGa2O3ꢀnp
is
summarized in Scheme 3. In this model, we illustrate a consequence
of the interactions (Scheme 3A) and highlight the disassembly role of
PVP in the generation of the γꢀGa2O3ꢀnp (Scheme 3B). Based on this
model, we are able to not only achieve the controlled growth of
γꢀGa2O3 and GaOOH in aqueous solution, but also regulate the
assembly and disassembly between the 2D and 3D structures of
γꢀGa2O3, which are generally difficult to handle by using traditional
techniques. It is hoped that this model presented here could be
extended to a wide variety of other metal oxide nanomaterials.
Scheme 2. The synergistic effect of hosts and ligands on the creation of
γꢀGa2O3 nanoplates.
It is worth highlighting that all the effects of reaction time,
temperature and pH on the creation of the γꢀGa2O3ꢀnp are based on
the premise of the synchronous presence of ligands and hosts.
Therefore, it is significant to clarify the role of the nature of ligands
and hosts in mediating the coordination and binding interactions,
which will contribute to the understanding how the multiple
interaction model can function as a tool in promoting the controlled
synthesis of γꢀGa2O3.
Scheme 3. A) A multiple interaction model for the controlled synthesis of the
γꢀGa2O3ꢀnp; B) The contribution of disassembly of PVP to the generation of
the γꢀGa2O3ꢀnp.
On the one hand, as shown in Scheme 2 (left part), when we used
oxalic acid dihydrate (OAD) or malonic acid (MA) to replace POM

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The second objective of this study is to evaluate the catalytic
activity of the γꢀGa2O3 nanoplates for the oxidative desulfurization of
TPs. The solutions consisting of nꢀoctane and TPs (TP; 2ꢀmethylꢀTP,
2ꢀMTP; 3ꢀmethylꢀTP, 3ꢀMTP; benzoꢀTP, BTP and dibenzoꢀTP,
DBTP) were used as model fuels. The optimal oxidation reaction time,
temperature, and amount of the Ga2O3 in the oxidation process of TP
to its dioxide by H2O2 (Equation 12) were determined to be 2 h, 343 K,
and 0.75 g·L−1 (Figure S15), respectively. In order to make a
comparison of the catalytic properties, we selected three samples
(γꢀGa2O3ꢀnp, γꢀGa2O3ꢀmf, and βꢀGa2O3ꢀnp) as the parallel catalysts.
The βꢀGa2O3ꢀnp was obtained by sintering the γꢀGa2O3ꢀnp at 1073 K
for 3 h in air (Figure S16). It has a similar 2D hexagonal nanoplate
structure to the original, and matches with an endꢀcentered monoclinic
structure (space group C2/m; JCPDS 43ꢀ1012).42
Figure 3 shows the UVꢀVis curves of the residual TP components
after stirring the reaction for 2 h at 343 K in the organic phase. Each
curve exhibits a maximum absorbance at a wavelength of about 230
nm. It was observed that the absorbance peak intensity sharply
decreased upon addition of the catalysts. Further, the catalytic
efficiency (ζ) was determined to be 66.3, 61.0 and 42.1% for the
γꢀGa2O3ꢀnp, βꢀGa2O3ꢀnp and γꢀGa2O3ꢀmf, respectively, based on
Equation 13 by measuring the residual concentrations of TP in the
absence (C0) and presence of (C) the catalysts. The N2
adsorptionꢀdesorption isotherms (Figure S17) exhibit that all the three
catalysts have the type IV pattern43 with a small hysteresis loop at a
relative pressure range of 0.43~0.96. Also, our data show that the
γꢀGa2O3ꢀnp has an average pore diameter of 3.3 nm, while the pore
diameters of βꢀGa2O3ꢀnp and γꢀGa2O3ꢀmf are 1.8 and 1.2 nm,
respectively. This indicates there is no significant difference in pore
diameters between the samples. We noticed that the specific surface
area (SSA, Figure S17) of βꢀGa2O3ꢀnp (9.6 m2/g) is the smallest
among the three catalysts, but its catalytic efficiency is more close to
that of the γꢀGa2O3ꢀnp (SSA, 31.2 m2/g), both of which are much
higher than that of the γꢀGa2O3ꢀmf (SSA, 27.8 m2/g). Also, our data
indicate that for β phase of Ga2O3, its 2D plate structure has a higher ζ
value than its 3D flower structure (40%, Figure S18). These
observations allow to suggest that the catalytic oxidation activity of
the Ga2O3 catalysts for TP increases with decreasing dimensionality,
since a crucial advantage of 2D materials is that all of the catalytically
active sites are exposed because of the thin nature of the nanoplates.44
Further, we found that the recovered γꢀGa2O3ꢀnp (after five cycles)
shows almost the same catalytic performance as the fresh one, and no
visible structural difference between the fresh catalyst and the used
one (ζ, 64.2%, Figure S18), which means that the γꢀGa2O3ꢀnp can well
maintain their structural integrity during the recycles (the recycling
procedure of the catalyst was shown in the experimental section of the
ESI†). These results not only hightlight that the catalyst is easily
recycled and reused without any significant loss in this desulfurization
reaction, but also are an indication that the TP analogs adsorbed on
Figure 3. UVꢀVis curves of the residual TP components in nꢀoctane after
reaction for 2 h in the organic phase: a) only TP, b) TP with H2O2, c) TP with
H2O2 and the γꢀGa2O3ꢀmf, d) TP with H2O2 and the βꢀGa2O3ꢀnp, and e) TP with
H2O2 and the γꢀGa2O3ꢀnp. The inset in this figure is a proposed catalytic
mechanism for the oxidative desulfurization of TP with aqueous H2O2 over the
γꢀGa2O3ꢀnp.
Table 1. The ζ values of different catalysts in the oxidative desulfurization.
ζ /%
Catalysts
Solvents Ma /g·L−1
TP
BTP
DBTP
γꢀGa2O3ꢀnp
water
water
water
ACd
AC
0.75
0.6
0.16
3.3
−
66.3
30
33
−
76.2
−
80.6
−
45
MoO3
AMTb,46
−
−
47
MoO3/Al2O3
−
70.5
46
98.7
60.5
−
48
Mn3O4
65
0.2
−
57
64.2
45
91
−
24
Mo/Al2O3
AC
1.36
−
MoꢀLDHc,49
AC
50
WOx/ZrO2
AC
1.3
2
65
−
51
ZrO2
AC
17
37
−
51
V2O5
AC
2
−
−
52
CeO2/TiO2
water
water
water
AC
1
−
90
< 40
< 45
−
52
CeO2
1
−
−
52
TiO2
1
−
−
53
NiꢀCuO/BiVO4
1
94
43
42
40
−
53
Ni/BiVO4
AC
1
−
−
53
CuO/BiVO4
AC
1
−
−
53
BiVO4
AC
1
−
−
a Amount of catalysts; b Ammonium molybdate tetrahydrate; c Moꢀcontaining
layered double hydroxide; d Acetonitrile.
surface of the catalyst only led to a weak poisoning effect.
A possible catalytic mechanism is described in Figure 3. First, the
H2O2 adsorbed on the surface of Ga2O3 nanoparticles was activated by
coordinatively unsaturated Ga(III) on this surface to generate a
Ga(III)ꢀHO2 coordination intermediate (Step I). This activation has
been proved by other authors,9 leading to a higher electron density of
Ga(III) due to the interaction between the Ga(III) and the oxygen
atoms of O–O bond in the intermediate. Our XPS analysis confirms
this tendency as there is a clear decrease in the binding energy of
Ga(III) 3d core level (Figure S19). Then, the activation polarization
and subsequent cleavage of the O–O bond resulted in introduction of
an oxygen atom into the TP (Step II), generating the TPꢀmonoꢀoxide.
The deoxy intermediate Ga(III)ꢀOH was then further contacted with
H2O2 to form the Ga(III)ꢀHO2 again (Step III). Finally, the
TPꢀmonoꢀoxide was completely oxidized into the TPꢀdioxide in the
catalysis of the Ga(III)ꢀHO2 (Step IV, Figures S20).
t
h
e
Furthermore, we found that the ζ values of the γꢀGa2O3ꢀnp for the
TPs were gradually decreased in the order of DBTP (80.6%) > BTP
(76.2%) > TP (66.3%) > 3ꢀMTP (58.7%) > 2ꢀMTP (45.5%) (Figure

Journal of Materials Chemistry A
Page 6 of 8
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DOI: 10.1039/C7TA09913D
S21). This indicates that the conjugation effect of thiophene ring with
benzene ring is important for promoting the oxidation of TPs to
TPsꢀdioxide, whereas the steric hindrance of methyl group seems to
prospect in the future. We hope that our catalytic results will
contribute to the industrial desulfurization of petroleum.
Conflicts of interest
p
l
a
y
a
negative effect on the catalytic reaction. The data reveal that the
γꢀGa2O3ꢀnp not only provides a gradually decreasing catalytic activity
from aromatic TPs to methyl TPs, but also, in a sense, presents a
generally high activity level for all the TPs.
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Synchrotron Radiation
Laboratory (No. UN2016LHJJ) and the Natural Science Foundation of
Anhui Province (No. 1508085MB30).
Overall, the catalytic results of the γꢀGa2O3ꢀnp, including short
reaction time, low reaction temperature, small catalyst amount, high
ζ
values and good cycle ability, are comparable to those obtained using
other catalysts45~53 (Table 1 and S1). In particular, these results were
achieved without the use of organic solvents, cocatalysts and supports,
implying that the catalytic process is accessible, green and
environmentꢀfriendly. Very recently, several metal oxide catalysts
with high catalytic desulfurization efficiency for some certain
thiophene analogues were reported by some researchers.25c,54,55
However, most of them required the use of organic solvents or phase
transfer reagents. Especially, their high catalytic desulfurization
efficiency are based on the use of specific supporters and/or
cocatalysts,52,53 thus increasing the cost and difficulty of
desulfurization. In contrast, our γꢀGa2O3 nanoplates show great
advantages not only in overcoming the above disadvantages of the use
of traditional catalysts but also in providing good desulfurization
performance for a wide range of thiophene analogues. It should be
noted that this is the first report on the catalysis of γꢀGa2O3 in the
oxidation removal of TPs, exhibiting that it has a significant
application potential in the oxidative desulfurization of various TPs
from fuels.
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DOI: 10.1039/C7TA09913D
Graphical Abstract
Hexagonal nanoplates of highꢀquality γꢀgallium oxide
were successfully synthesized by a competitive and
cooperative interaction model
Zun Yang, Le Xin Song, Ya Qian Wang, Mao Mao Ruan, Yue
Teng, Juan Xia, Jun Yang, Shan Shan Chen and Fang Wang