Microwave-assisted synthesis of few-layered TaTe2 and its application as supercapacitor

Article abstract of DOI:10.1002/ejic.201403220

We report a simple and rapid microwave-assisted synthesis of tantalum telluride (TaTe₂) nanosheets. The ratio of tantalum pentachloride (TaCl₅) and elemental tellurium (Te) powder were adjusted in the presence of NaBH₄ in such a way as to obtain the TaTe₂ nanosheet. The samples were characterized by various techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), UV/Vis spectroscopy, photoluminescence (PL) spectroscopy, and XRD. Our SEM, TEM, and AFM results show the formation of sheet-like morphology, while the XRD data confirms the high crystalline quality and stable phase of the TaTe₂ formed. The supercapacitor cells were fabricated by using TaTe₂ nanosheets as anode material, platinum metal wire as a counterelectrode, and Ag/AgCl as reference electrode. The calculated coulombic efficiency is more than 95 , while the cycle-to-cycle decrease in capacity is less than 5 . The maximum discharge or charging capacity is below 2.4 Wh/kg, which is an ideal characteristic for achieving supercapacitor behavior. The first experimental investigations on the use of microwave to synthesize atomically thin few-layer TaTe₂ nanosheets are described. This material also shows good performance as a supercapacitor.

Full text of DOI:10.1002/ejic.201403220

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DOI:10.1002/ejic.201403220
Microwave-Assisted Synthesis of Few-Layered TaTe2 and
Its Application as Supercapacitor
Disha Chakravarty,[a] Praveen Kumar,[a] Vaishali S. Ugale,[a] and
Dattatray J. Late*[a]
Keywords: Tantalum / Tellurium / Microwave chemistry / Nanostructures / Supercapacitors
We report a simple and rapid microwave-assisted synthesis
phology, while the XRD data confirms the high crystalline
of tantalum telluride (TaTe2) nanosheets. The ratio of tanta- quality and stable phase of the TaTe2 formed. The superca-
lum pentachloride (TaCl5) and elemental tellurium (Te) pow-
der were adjusted in the presence of NaBH4 in such a way
as to obtain the TaTe2 nanosheet. The samples were charac-
terized by various techniques such as scanning electron mi-
croscopy (SEM), transmission electron microscopy (TEM),
atomic force microscopy (AFM), UV/Vis spectroscopy, photo-
luminescence (PL) spectroscopy, and XRD. Our SEM, TEM,
and AFM results show the formation of sheet-like mor-
pacitor cells were fabricated by using TaTe2 nanosheets as
anode material, platinum metal wire as a counterelectrode,
and Ag/AgCl as reference electrode. The calculated cou-
lombic efficiency is more than 95%, while the cycle-to-cycle
decrease in capacity is less than 5%. The maximum dis-
charge or charging capacity is below 2.4 Wh/kg, which is an
ideal characteristic for achieving supercapacitor behavior.
InSe.[24,25] Each of them has a variety of properties de-
pending upon the number of layers and heterostructure, of-
fering considerable potential for the realization of future
optoelectronic devices. Further, it has been reported that
bulk materials of these semiconducting TMDCs are indirect
band gap semiconductor materials, for example, MoS2 has
a band gap of about 1.2 eV. However, the single-layer MoS2
has a direct and wide band gap of 1.8 eV.[5,9,12] Devices
based on such novel systems show promising applications,
for example field-effect transistors fabricated by using sin-
gle-layer MoS2 offer good Ion/off ratios of about 108 and
mobilities of about 200 cm2/Vs.[5,7,9,12] Further, the intrinsic
band gap in these TMDCs can be easily tuned depending
upon the number of layers and choice of material, which
can determine the possible application, such as sensing,
photonics devices, and energy harvesting.
The progress in the synthesis of new layered TMDC ma-
terials will be an important factor for propelling this new
material field forward. The synthesis of large-area and high-
quality nanosheets of metal dichalcogenides is still a chal-
lenge and one of the important parameters that can affect
their various applications. The TaTe2 nanosheet has not yet
been synthesized, because of its high melting point and the
deficiency of the phase diagrams. Their crystals can exhibit
ionic, covalent, and metallic bonding interactions.[26–31] In
particular, the relatively low electronegativity of tellurium
in transition metal tellurides frequently leads to complex
competition between metals and nonmetals for the valence
electrons and, as a consequence, to valence electron local-
izations. It is interesting to note that TaTe2 has been re-
ported to have a distorted 1T structure with space group
Introduction
1D and 2D nanomaterials have played a fundamental
role in shaping the future nanotechnology and have been
used in a variety of optoelectronic and nanoelectronic de-
vices. The 2D form of carbon, commonly known as graph-
ene, has received major interest due to its huge potential for
use in future photonic and nanoelectronic devices.[1–4]
These include miniaturized nanodevices based on inorganic
2D layered materials, which work in a wide range of the
visible to infrared (IR) and near-IR spectrum and can oper-
ate at low power.[5–17] The transition metal dichalcogenides
(TMDCs) are a new class of materials having direct and
tunable band gap. They are made up of hexagonal layers of
metal atoms (M = Mo, W, V, Ga, Ta, Sn) sandwiched be-
tween two layers of chalcogen atoms (X = S, Se, Te). The
interatomic binding in the TMDCs is strong enough; this is
essentially because of the covalent in-plane bonding. But
the successive layers in TMDC materials are interrelated
through weaker van der Waals interlayer forces, which en-
ables these layered materials to be simply exfoliated me-
chanically.[5,9,12] Further, depending on the mixture of tran-
sition metal atom and chalcogen atom, a wide variety of
TMDCs are possible such as MoS2,[5,9,12,14,17] WS2,[13,15,18]
MoSe2,[16] WSe2,[19] MoTe2,[20,21] WTe2,[22] GaTe,[23]
[a] Physical and Materials Chemistry Division,
CSIR-National Chemical Laboratory,
Dr. Homi Bhabha Road, Pashan,
Pune- 411008, Maharashtra State, India
E-mail: dj.late@ncl.res.in
datta099@gmail.com
http://academic.ncl.res.in/dj.late/profile
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C2/m.[28b,28c] Among all the metal dichalcogenides, Te- the standard JCPDS data card No. 71–2197. Figure 5a
based nanosheets of nanometer thickness have remained shows the Energy-Dispersive X-ray analysis (EDXA) for
practically unexplored. In particular, this is the case for the elemental composition investigations of the TaTe2 nano-
TaTe2. Although the thermoelectric and electronic proper-
ties of this material were studied in the past,[28–30] the focus
of past research work has been on the synthesis of bulk
crystals.[28–30] There are few reports in the literature that
describe the synthesis of bulk TaTe2 materials.[28–31] In this
manuscript, we attempted a rapid synthesis of TaTe2 nano-
sheets, using a microwave-assisted technique with elemental
tellurium powder and TaCl5 as a source of Te and Ta,
respectively. Further, as synthesized TaTe2 nanosheets were
also tested as supercapacitors.
Results and Discussion
TaTe2 nanosheets were prepared by using a microwave-
assisted method. Tantalum pentachloride (TaCl5) is found
to be an effective reagent for growing the TaX2 (chalcogen
element X = S, Se, Te) layered materials.[28,28c,28d] It is pos-
sible that each of the polymorphic phases (either the trigo-
Figure 1. SEM images of the TaTe2 nanosheets synthesized by a
nal “2H” phase with prismatic coordination or the ortho-
rhombic “1T” phase with octahedral coordination) can be
carefully grown by controlling the growth param-
eters.[28,28c,28d] In a typical reaction, tellurium powder is dis-
solved in ethylene glycol with TaCl5 and sodium boro-
hydrate (NaBH4). The reaction mixture is then kept in a
microwave oven for 2 min. We expect the reaction shown in
the equation to occur.
microwave-assisted method.
The elemental tellurium undergoes reduction by sodium
borohydride, a strong reducing agent, and forms tellurated
sodium borohydride (NaBH2Te3), which also acts as a re-
ducing agent for the next step of the reaction. TaCl5 further
undergoes reduction by the in situ generated tellurated so-
dium borohydride and forms TaTe2. According to the pro-
posed mechanism, the products HCl and Cl2 evolve as
gases. Furthermore, the boron complex formed during the
reaction is washed out by using ethylene glycol, a polar,
protic solvent that also helps in the stabilization of the
boron complex.[32]
Figure 1 shows the typical SEM images of TaTe2 materi-
als having sheet-like morphology with lengths of 1–10 μm.
Figure 2 depicts the typical TEM images of the TaTe2 nano-
sheets. The inset of Figure 2d shows the typical selected
area electron diffraction (SAED) pattern, which confirms
the high crystallinity of the TaTe2 nanosheets. Figure 3
shows the typical AFM image of few-layer-thick TaTe2
nanosheets (left) and the corresponding AFM height profile
(right). It is clearly observed from the AFM image that the
TaTe2 nanosheets have lateral dimensions of a few micro-
meters and thicknesses of about 4 nm as shown in AFM
height profile, which clearly indicates the few-layered nature
of the TaTe2 nanosheets. Figure 4 shows the typical XRD
Figure 2. TEM images of the TaTe2 nanosheets. The inset of (d)
shows the selected area electron diffraction (SAED) pattern of the
TaTe2 nanosheets.
Figure 3. AFM image of few-layer-thick TaTe2 nanosheets (left)
pattern of TaTe2 nanosheets, which matches very well with and the corresponding AFM height profile (right).
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sheets. The EDXA analysis of the TaTe2 sample revealed showed an absorption onset of 231 nm. The room-tempera-
that the average Ta/Te ratio is close to 1:2. It is important ture PL spectra of solid TaTe2 nanosheet samples recorded
to note that the ternary phase diagram Cl–Te–Ta is still at excitation wavelengths of 350 and 400 nm are shown in
unknown in the literature, and no such ternary compound Figure 5c and Figure 5d, respectively. The broad PL emis-
has been reported so far. However, more accurate and de- sion peaks can be seen at 627 and 728 nm, respectively.
tailed investigations by use of slow-scan XRD, high-resolu- Supercapacitor cells were fabricated by using the synthe-
tion TEM, and XPS analysis are still needed (not reported sized few-layered TaTe2 nanosheets as anode materials.
in this manuscript).
Platinum metal was used as counterelectrode, whereas Ag/
AgCl was employed as reference electrode. The electro-
chemical properties of the TaTe2 nanosheet samples were
measured by means of a three-electrode system in 0.5 m
aqueous KOH electrolyte. The specific capacitance of the
TaTe2 nanosheet electrode was calculated from the follow-
ing equation:
In the equation, m is the mass of a single electrode, v is
the scan rate (mV/s), ν1 and ν2 are the integration limits
(potential window), and I(ν) denotes the response current
(A). Figure 6a shows the comparative cyclic voltammog-
rams (CVs) of the TaTe2 nanosheet sample at different scan
rates of 10, 20, 50, 100, 500, and 1000 mV/s. The capaci-
tance value observed is lower for higher scan rates, while
Figure 4. XRD pattern of TaTe2 nanosheets (JCPDS data card No.
71–2197).
The as-synthesized TaTe2 nanosheets were also charac- for lower scan rates the capacitance value increases. The
terized by using UV/Vis and photoluminescence spectra. observed coulombic efficiency is more than 95%, while the
Figure 5b shows the UV/Vis absorption spectrum of the cycle-to-cycle decrease in capacity is less than 5%. The
TaTe2 nanosheets. It is evident that the TaTe2 nanosheets maximum discharge or charging capacity is below 2.4 mV/s,
Figure 5. EDS spectrum (a), UV/Vis spectrum (b), and photoluminescence spectrum of TaTe2 nanosheets obtained with an excitation
wavelength of 350 nm (c) and 400 nm (d).
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Figure 6. Electrochemical properties of the TaTe2 nanosheet electrodes synthesized by a microwave-assisted method: (a) Cyclic voltammog-
ram at different scan rates (outside curve at 1000 mV/s and inside curve at 10 mV/s), (b) galvanostatic charging–discharging profile at
5 μA current, (c) cycling stability of the TaTe2 nanosheets for 400 cycles, and (d) electrochemical impedance spectroscopy data (the Rct
value is measured as 132 Ω).
which is a preferable characteristic for achieving superca- is a competent in assessing the capability of the material to
pacitor behavior. The area under the CV curve represents transfer and exchange charge with molecules in the vicinity.
the charge storage capacity of the electrode; therefore, the Therefore, EIS was performed to find out the resistance of
value of specific capacitance calculated at a scan rate of the TaTe2 nanosheets materials towards heterogeneous
100 mV/s is 1.4 F/g (see Table 1 for specific capacities at dif- charge transfer, with the help of a Nyquist plot obtained
ferent scan rates). Figure 6b shows the typical charge–dis- from the EIS measurements as depicted in Figure 6d. It is
charge profile of TaTe2 at 5 μA. For the practical applica- well established that the diameter of the semicircle in a
tion, the stability of the electrode is an essential and signifi- Nyquist plot represents the charge transfer resistance value,
cant parameter. Therefore, the response of the capacitor which is inversely proportional to the rate of charge trans-
was recorded for more than 400 cycles for TaTe2 nanosheets fer. The EIS spectra (Nyquist plots) of TaTe2 nanosheets
in the range –0.3 to 0.2 V at 5.0 μA/g and is depicted in were measured under the influence of some initial voltage
Figure 6c. The cyclic stability of TaTe2 was measured by amplitude in the frequency range (0.1–1.0ϫ106 Hz) at an
recording the charge–discharge of the supercapacitor over open circuit potential. A typical plot is shown in Figure 6d.
400 cycles; the superior storage capacity was confirmed. It It consist of a semicircular arc at higher frequencies and a
is notable that the TaTe2 materials retain their good cyclic straight line at lower frequencies. A charge transfer limiting
stability over a huge number of charging–discharging cycles. process can be measured by considering the diameter of the
The long-term stability tests suggest that the stability of semicircular arc, and it is a combination of Faradic charge
TaTe2 solid-state supercapacitor is suitable for devices fabri- transfer resistance (Rct) in parallel with capacitances
cated using the drop casting of the sample. It has also been (double layered) at electrode–electrolyte interfaces. In ad-
observed that the TaTe2 nanosheet sample takes nearly ten dition to this, the straight line helps in determining the War-
cycles to stabilize its storage capacity, after which the value burg impedance. The slope of the line is the Warburg im-
of specific capacitance is nearly stable. Electrochemical im- pedance, which shows the diffusion of electrolyte ions into
pedance spectroscopy (EIS) represents a very powerful, sen- the electrodes, and a slope of zero to the real axis corre-
sitive, and nondestructive technique to investigate the in- sponds to zero diffusion resistance.[33] Figure 6d shows a
terfacial properties of various oxides, sulfides, carbides, Nyquist plot of TaTe2 nanosheet electrode materials mea-
nitrides, and a range of other materials. Specifically, the EIS sured in 0.5 m aqueous KOH electrolyte. The Rct value is
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times with ethanol at 20 °C and at 4500 rpm. The product was then
dried under vacuum for 50 min. The solvent helps in stabilizing the
intermediate side product, which facilitates the formation of TaTe2.
132 Ω, which demonstrates the good electrical conductivity
in aqueous electrolytes. An almost vertical line was ob-
served in the Nyquist plot that represents the dominance of
double-layer charge-storage at low frequencies. A vertical
Characterization: The as-synthesized products of TaTe2 were fur-
line in the Nyquist plot represents good electrical double- ther characterized by SEM, TEM, AFM, XRD, and UV/Vis and
PL spectroscopy. SEM images were acquired with a FEI SEM
QUANTA 200 3D instrument. High resolution (HR) TEM images
were obtained with a FEI TECNAI TF-30 (FEG) instrument. The
XRD spectrum was obtained with a device from Bruker Instru-
ments (XRD-D8, Avance, Bruker-AXS). The AFM images were
acquired by using an ICON instrument. The absorption capacity
of the samples was analyzed by a UV/Vis spectrophotometer (Cecil,
Model No. CE3021). The photoluminescence spectrum was ac-
quired by exciting the samples with 400 nm laser light with a HOR-
IBA Flurolog 3 spectrofluorometer instrument.
layer capacitor behavior. In summary, we have proposed
and realized the microwave synthesis approach to grow
TaTe2 nanosheets. Further, the proper contact between the
Pt electrode and the TaTe2 nanosheets can considerably en-
hance the electrochemical properties including high area,
gravimetric capacitance, and excellent rate capability for the
supercapacitor.
Table 1. Specific capacitances of TaTe2 nanosheets.
Scan rate (mV/s)
Specific capacitance (F/g)
Fabrication and Characterization of the Supercapacitor Device: The
electrochemical performance of the TaTe2 nanosheet sample was
investigated by CV, galvanostatic charge–discharge tests, and EIS
with a C-H electrochemical workstation (CHI660D, Austin, TX),
using a three-electrode set-up. The working electrodes were made
by drop casting TaTe2 nanosheet samples. The EIS measurements
were recorded over a frequency range of 100 kHz to 10 MHz with
an amplitude of 10 mV at the open-circuit potential. The superca-
pacitor cells were fabricated by using few-layered TaTe2 nanosheets
as anode materials and platinum metal as counterelectrode,
whereas Ag/AgCl was used as reference electrode. The electrochem-
ical properties of the TaTe2 nanosheet sample were measured in a
three-electrode system in 0.5 m aqueous KOH electrolyte.
10
50
100
200
500
1000
4.09
1.60
1.40
1.10
0.87
0.71
4. Conclusions
We report the simple and rapid microwave-assisted
method for the synthesis of TaTe2 nanosheets. The Ta and
Te source powder ratio were optimized to obtain nano-
sheet-like morphology. The as-synthesized TaT2 samples
were characterized by using SEM, TEM, AFM, XRD, and
PL and UV/Vis spectroscopic analyses. The SEM, TEM,
Acknowledgments
The authors would like to thank Prof. C. N. R. Rao (FRS) (Direc-
and AFM results show the formation of sheet-like mor- tor ICMS, Bangalore, India) and Prof. Vinayak P. Dravid (Director,
NUANCE NWU, USA) for support and encouragement. One of
the authors, Dr. D. J. L., would like to thank the Department of
Science and Technology (Government of India) for a Ramanujan
Fellowship (Grant No. SR/S2/RJN-130/2012). The research work
was primarily supported by the National Chemical Laboratory –
Major Laboratory Project (NCL–MLP) (project grant 028626), the
Department of Science and Technology – Science and Engineering
Research Board (DST-SERB) Fast-Track Young Scientist Project
(grant no. SB/FT/CS-116/2013), and partially by the Department
of Electronics and Information Technology, Ministry of Communi-
phology, while the XRD analysis confirms the highly crys-
talline and stable phase formation and the EDXA results
support the elemental ratio of 1:2. The supercapacitor cells
were fabricated by using few-layered TaTe2 nanosheets as
anode material and platinum metal as counterelectrode.
The observed coulombic efficiency was more than 95%,
while the cycle-to-cycle decrease in capacity was less than
5%. The maximum discharge or charging capacity was be-
low 2.4 mV/s, which is a preferable characteristic for achiev-
ing supercapacitor behavior. Further experimental work is cations and Information Technology (DeitY, MCIT), Government
of India [Indian Nanoelectronics Users Program, Indian Institute
of Technology, Bombay (INUP IITB) project].
needed to control the growth of nanosheets and proper 1T
TaTe2 phase formation.
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Published Online: February 12, 2015
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