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Single step hydrothermal synthesis of mixed valent V6O13 nano-architectures: A case study of the possible applications inelectrochemical energy conversion
Citation for published version:Mutta, GR, Popuri, SR, Ruterana, P & Buckman, J 2017, 'Single step hydrothermal synthesis of mixedvalent V
6O
13 nano-architectures: A case study of the possible applications in electrochemical energy
conversion', Journal of Alloys and Compounds, vol. 706, pp. 562–567.https://doi.org/10.1016/j.jallcom.2017.02.272
Digital Object Identifier (DOI):10.1016/j.jallcom.2017.02.272
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Accepted Manuscript
Single step hydrothermal synthesis of mixed valent V6O13 nano-architectures: Acase study of the possible applications in electrochemical energy conversion
Geeta R. Mutta, Srinivasa R. Popuri, Pierre Ruterana, Jim Buckman
PII: S0925-8388(17)30721-1
DOI: 10.1016/j.jallcom.2017.02.272
Reference: JALCOM 41000
To appear in: Journal of Alloys and Compounds
Received Date: 16 September 2016
Revised Date: 13 January 2017
Accepted Date: 26 February 2017
Please cite this article as: G.R. Mutta, S.R. Popuri, P. Ruterana, J. Buckman, Single step hydrothermalsynthesis of mixed valent V6O13 nano-architectures: A case study of the possible applicationsin electrochemical energy conversion, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.02.272.
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Single step hydrothermal synthesis of mixed valent V6O13 nano-architectures: A case study of the possible applications in electrochemical
energy conversion Geeta R. Mutta1*, Srinivasa R. Popuri1, Pierre Ruterana2, Jim Buckman3
1School of Engineering & Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom 2CIMAP, UMR 6252, CNRS-ESICAEN-CEA-UCBN, 6, Bd Maréchal Juin, 14050 Caen, France
3Institute of Petroleum Engineering & Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, United
Kingdom
*Corresponding author: [email protected]
Abstract
Availability of several stable vanadium oxidation states offers numerous advantages in terms
of applications but at the same time it poses great challenge to isolate them in single phase
during their synthesis process. In this study, we developed a facile single step hydrothermal
synthesis of V6O13 nano-architectures using environmental friendly citric acid as a reducing
agent. By means of time dependant hydrothermal conditions study, we observed evolution of
several stable vanadium oxide phases. The as-synthesized powder samples phase confirmation
was thoroughly studied by powder X-ray diffraction and transmission electron microscopy.
The morphology of the products formed was investigated at each hydrothermal reaction time.
Finally, these V6O13 nano-architectures were employed as counter electrode (CE) in dye
sensitized solar cells (DSSCs). The photovoltaic performance and the electrical impedance
studies of the DSSC device made use of V6O13 nano-architectures are reported here. It is
believed that with further optimization, this relatively inexpensive material can act as efficient
CE for DSSCs, which thereby open up a new avenue for vanadium oxides as a new class of
materials for clean energy generation.
Keywords: Hydrothermal synthesis; V6O13 nano-architectures; Counter electrode; DSSCs; Pt
free; Chemical synthesis
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1. Introduction
Over the past few decades, vanadium oxides have attracted much research attention
due to existence of multiple valence states from two in VO, three in V2O3, four in VO2 to five
in V2O5.[1] Along with these multiple valence states, different co-ordination geometries and
their layered structured nature made them promising candidates for potential energy related
applications, such as cathode in lithium batteries, super capacitors.[2-7] The vanadium-oxygen
(V-O) phase diagram is complex due to existence of a wide range of ordered and disordered
defect structures with mixed valent compounds, so-called Magneli (VnO2n-1) and Wadsley
(V2nO5n-2) phases.[8, 9] It is worth noting that the mixed-valence chemistry of these vanadium
oxides is considerably less well established, especially in terms of solution based synthesis
processes such as hydrothermal methods.[10]
V6O13 belongs to the Wadsley phase category taking the mixed valences of V4+ (d1)
and V5+ (d0) and has been considered as an excellent candidate for the battery cathode material
due to its high electrochemical capacity.[11, 12] So far, numerous synthetic procedures are
proposed to prepare the pristine V6O13 such as solid state reaction at elevated temperatures for
few days, annealing the hydrated vanadium pentoxide aerogels under pure argon, thermal
decomposition of NH4VO3 in a stream of high purity argon/nitrogen atmosphere, an electro-
deposition thermal process, and a hydrothermal process.[13-19] In addition, often the V6O13
phase is formed as an intermediate during VO2 synthesis under reducing conditions.[20, 21]
Among these, solution-based hydrothermal synthesis has many advantages due to its selective
nature, the number of adjustable parameters during reaction, low cost and suitability for large-
scale synthesis, low temperature and no special equipments required. In general, the main
strategy to prepare V6O13 under hydrothermal conditions is to reduce stable high oxidation
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degree of vanadium (V5+) using reducing agents. Some of the recent hydrothermally processed
V6O13 results are summarized in Table 1, where in some cases four preparation steps are
involved and even hydrogen peroxide (H2O2) is essential to obtain the desired V6O13 phase.
Due to complex V-O phase diagram, isolation of one single phase compounds are often very
challenging particularly mixed valent compositions. Hence, one needs to work out an
elaborate synthesis protocol to ensure the formation of a pure phase and to avoid other
undesirable vanadium oxides.
Dye sensitized solar cells (DSSCs) have evolved as commercially realistic generators
of electric power from sunlight. Due to high electrocatalytic activity and excellent
conductivity, platinum (Pt) is a conventional choice as a counter electrode (CE) which is an
important component of DSSCs. Pt costs about 40% of the total manufacturing cost of
DSSCs.[22] Hence there is an ongoing effort to discover alternative CEs. Since the vanadium
oxides have both good catalytic activity and a significantly lower precursor price, we have
selected and employed V6O13 nano-architectures as a CE in DSSCs. To the best of our
knowledge, this is the first time V6O13 nano-architectures have been synthesized using V2O5
and citric acid as the starting materials by a facile single step hydrothermal process and
explored as a CE for DSSCs.
2. Synthesis and characterization
2.1 Materials synthesis
V6O13 nano-architectures were synthesized by a single step hydrothermal synthesis.
An aqueous solution of V2O5 (Sigma Aldrich analytical grade) was prepared by dissolving 2.6
mmol V2O5 in double distilled water under magnetic stirring. Later same molar quantity of
citric acid monohydrate (C6H8O7·H2O; Sigma Aldrich) was added into the above solution and
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stirring was prolonged for 15 min. After complete dissolution, the resulting solution was
transferred to a Teflon-lined stainless steel autoclave of 30 mL volume for thermal treatment
at 180oC for different duration of reaction times. Once the reaction time was complete, the
autoclaves were cooled down to ambient temperature naturally; the resultant powder samples
were collected and rinsed with distilled water and then with ethanol several times; the
products were finally dried in an oven at 50oC for 6 hrs.
2.2 Characterization
The phase purity and crystallographic structure of the as-synthesized powder specimens
were confirmed by X-ray diffraction (XRD; PANalytical X’ Pert Diffractometer).
Conventional transmission electron microscopy (CTEM) and as well as high resolution
transmission electron microscopy (HRTEM) were carried out on V6O13 nano-architectures in
order to determine the local structure of the material. The followed procedures can be found in
our previous reports.[23, 24] The surface morphology of powder specimens was investigated
by scanning electron microscopy (Quanta 650 FEG SEM). The V6O13 film was fabricated by
doctor blade technique, as detailed in section 3.4.1, and then directly used as CE of DSSC.
Photovoltaic performance and electrochemical impedance spectroscopy of these DSSCs were
recorded on a Keithley 2400 source meter under AM1.5 Global solar simulator and with an
Autolab electrochemical workstation.
3. Results and discussion
3.1. Phase formation studies using X-ray diffraction
Unless specified all the hydrothermal reactions were performed at 180ºC. Fig. 1
represents typical XRD patterns of the samples prepared by the hydrothermal process at 180ºC
for different reaction durations. The dissolution of V2O5 in water gives rise to a yellow
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coloured aqueous solution. The hydrothermal reaction for 5 hrs at this stage (absence of citric
acid) results in yellowish V2O5 only and shows the phase stability of V2O5 under these
hydrothermal conditions (Fig. 1a).[25] In the second step, by the addition of citric acid and
prolongation of the stirring time, a light green/blue solution forms, which indicates that the
valence state of vanadium in the solution was partly reduced from V5+ to V4+ (V5+ and V4+
solutions are yellow and blue colours, respectively). This indicated that citric acid acts as the
reducing regent. It is noted that in this study only the duration of hydrothermal reaction was
varied by keeping other parameters fixed. XRD studies after hydrothermal reaction for 1 hr
shows a greenish coloured mixed valence hydrate V10O24.9H2O (Fig. 1b) (JCPDS card
number 25-1006). This shows that V2O5 structure allows H2O molecules to be embedded in
the intercalation sites without a far-reaching restructuring; in the same time the presence of a
reducing agent as well as the increase in reaction duration promotes the reduction of V5+ to
V4+ and accelerates the hydrate formation. Further increasing the duration to 5 hrs results in
bluish coloured V6O13 and simulated pattern obtained using JCPDS card number 01-075-1140
(Fig. 1c). All the XRD reflections can be readily indexed as the monoclinic crystalline phase
with space group: C2/m (12) and lattice parameters of a=11.934(2) Å, b=3.6775(3) Å,
c=10.142(2) Å and = 100.81(1)o of V6O13 and this is in agreement with the literature. [26]
No peaks of any other phases are detected, indicating that the as-prepared V6O13 is phase-
pure. In the final step, hydrothermal reaction for 7 hrs bluish-black coloured VO2 (B) phases
(Fig. 1d) (JCPDS card number 01-81-2392). These results express the conversions among
vanadium oxides in these reducing hydrothermal conditions as follows:
V2O5 V10O24.9H2O V6O13 VO2(B). The V6O13 crystal structure contains slightly distorted
VO6 octahedra. These octahedra are joined by corner sharing single sheets and edge sharing
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double sheets parallel to (010) plane. Corner share single sheets form a “V2O5-layer” of
octahedra, and by contrast, edge share double sheets form a “VO2(B)-layer” of octahedra.
Obviously V2O5, VO2 (B) and V6O13 have a close resemblance among their crystal structures.
They have similar polyhedral connectivity and a similar structural repeat unit along the a-b
plane. It is expected from the adjacent area in V-O binary phase diagram that the free energy
of V6O13 is close to that of V2O5 and VO2 (B), suggesting a rather facile inter-convertibility
among V2O5, V6O13 and VO2(B) through oxidation/reduction.
3.2 Morphological study by SEM
In order to investigate the morphology of the synthesized products of the time
dependant hydrothermal reactions, SEM characterization was performed on samples at
different reaction times. Fig. 2 shows the representative SEM images of the precursor
(hydrothermally treated V2O5; without reducing agent), intermediate sample (V10O24.9H2O),
desired phase (V6O13) and the end product (VO2(B)). The morphology of V2O5 precursor
consists of rod-like structures shown in Fig. 2a whose size ranging from a few nm-1 µm.
After 1 hr of reaction flower like particles (Fig. 2b) formed whose phase is confirmed as
V10O24.9H2O from XRD. When the reaction carried out to 5 hrs the desired V6O13 phase was
formed which was confirmed by XRD and TEM in the next section. The morphology of
V6O13 powder sample is predominantly consists of a large quantity of layered nano-structure
stackings. These morphologies are consistent with the layered crystal structure nature of
V6O13. It can be seen from the SEM micrograph in Fig. 2c that high density bundle-like V6O13
nano-architectures uniformly grow. Here each nano-architecture has a characteristic size of
about 1-2 µm and consists of several thin nanosheets, which are stepwise stacked. A
magnified view of these bundles can be seen in Fig. 2c inset, which shows the stacking of
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these nanosheets. With further reaction upto 7 hrs, VO2 (B) spherical particles with diameter
range from ~1.5 to 4 µm were formed.
3.3 Phase confirmation of V6O13 by TEM
The phase confirmation of the as-synthesized V6O13 powder samples at 180oC/5 hr
was further characterized by TEM. Fig. 3a shows a typical low resolution TEM image of an
individual nanosheet which was scraped off the nano-architecture, which are made up of
several nanosheets. The single nanosheet is of good transparency to the electron beam and is
homogenous, which indicates that the nanosheets are very thin and their surfaces are very flat.
On the other hand, a nano-architecture is not transparent. A set of well resolved parallel lattice
fringes are observed in high resolution TEM, as shown in Fig. 3b. The interplanar spacing is
measured to be 0.58 nm and corresponding to that of (200) planes of V6O13. This is confirmed
in Fig. 3c which shows its corresponding diffraction pattern and exhibits visible bright spots
corresponding to the crystal planes of the monoclinic V6O13 as imaged along the [001] zone
axis, indicating the individual preferentially oriented grains are of good crystal quality.
3.4. DSSC device fabrication and characterization
3.4.1. DSSC electrodes preparation and device assembly
V6O13 films have been used as CEs. The detailed experimental procedures of CE
preparation are described as follows. Initially fluorine doped tin oxide (FTO) coated glass
substrates were cleaned with soapy water followed by deionised water and isopropyl alcohol.
Adhesive tapes were stuck on the edges of the substrate to obtain an area of 1cm2 shown in
Fig. 4 step1. Later a smooth viscous paste was prepared by mixing V6O13 powder with ethyl
cellulose and α-terpineol. A portion of this paste was applied near the top edge of the FTO
glass and the paste spread across from one end to the other by sliding the microscopic glass
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slide, as shown in step 2. The step 3 image shows that the area not covered by the adhesive
tape has been coated by V6O13. Later the adhesive tapes were removed and the printed
electrodes were sintered under inert atmosphere at 450ºC for 30 mins to remove the organic
binders which ensure good electrical contact between the particles and good adhesion to the
FTO substrate. The resulting sintered V6O13 film (schematic of final CE is shown in step 4)
was placed in a sealed environment to protect these films from absorbing moisture from
ambient air.
The dye (N719) sensitized TiO2 photo-electrode (PE) was prepared in the same way as
reported elsewhere,[27] except the deposition method was doctor blade technique. Later a
prototype DSSC was fabricated by assembling PE and CE into a sandwich type cell. Both the
electrodes were fastened using paper binder clips and the liquid electrolyte (Iodine redox
couple[28]) was drawn into the cell by capillary effect and the final device was set for
characterization.
3.4.2. DSSC device characterization
Photovoltaic performances of DSSCs were tested at a light intensity of 100 W/m2 with
an active cell area of 0.25 cm2, which was defined by a mask. The photocurrent density–
voltage (J-V) curve of DSSC is displayed in Fig. 5a and corresponding photovoltaic
parameters are as follows: the short circuit current (Jsc) is 1.83 mA/cm2, open circuit voltage
(Voc) is 0.55 V, fill factor (FF) is 0.57 and cell efficiency is 0.57%. The lower power
conversion efficiency of this DSSC compared with more conventional ones is attributed to the
electro catalytic activity of the CEs.
Electrochemical impedance spectroscopy (EIS) measurements were performed to
understand the electrocatalytic performance of the CE towards reduction of l3-. The impedance
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spectra were collected under dark conditions by varying the frequency from 0.1 Hz to 1 MHz
at AC amplitude of 10 mV. The obtained Nyquist plot of full cells at DC bias 0.9 V is shown
in Fig. 5b. The charge transfer resistance at the CE/electrolyte interface (Rct) in the Nyquist
plots is our main focus since the CE used for DSSCs is responsible for this. The Rct value of
DSSC at a bias of 0.9 V was found to be approximately 120 Ω. Furthermore, Rct is a measure
of the catalytic activity of counter electrode for reducing I3−, therefore it is predicted that
changing the morphology would increase the catalytic sites of the counter electrodes for
reducing I3− and hence improve the performance. Although the performance of V6O13 based
DSSCs is presently inferior to that of conventional Pt based DSSCs, this material has great
potential to be used as a CE in DSSCs due to its unique chemical stability, selectivity, good
catalytic activity and electrical conductivity.
4. Conclusions and future directions
In summary, nano-architectures of pure phase V6O13 were successfully synthesized by
a single step hydrothermal synthesis using citric acid as reducing agent for the first time. The
phase formation and purity were confirmed by XRD and TEM and to the best of our
knowledge; V6O13 has been implemented for the first time as a CE in DSSC devices. The
photovoltaic performance of 0.57% is achieved with a fill factor of about 57%. As this device
performance is far from being optimized, profound benefits would come with this
inexpensive, facile synthesis and scalable materials, suggesting the potential of V6O13 as a
candidate for alternative CEs in DSSCs. Further improvements can be expected by tuning the
particle size, increasing electric conductivity by means of doping, making composites with
graphene, and testing against alternative redox mediators and dyes.
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5. Acknowledgements
Dr. M. Vasundhara of CSIR-NIIST, India is gratefully acknowledged for the access to their
facilities. The authors acknowledge Prof. John I. B. Wilson of Heriot-Watt University, UK for
critically reading the manuscript. This work was not associated with any funding.
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Table. 1 V6O13 recent synthesis literature (LIBs: Li-ion batteries; NIBs: Na-ion batteries)
Year Precursors Conditions Morphology Application Reference
2011
V2O5, H2O2, Octylamine,
Acetone, Absolute ethanol
160°C/48 hrs Hollow-flowers Super-capacitor [13]
2014 V2O5,
Ethanol
160°C/24 hrs Annealed at 350°C/2 hrs
Belt-like particles
LIB cathode [14]
2014 V2O5,
Ethanol 180/24 hrs Nanosheets LIB cathode [15]
2014
NH4VO3, Oxalic acid
180°C/44 hrs 450°C/8 hrs
Nanosheets LIB cathode [16]
2014
NH4VO3, Oxalic acid,
LiNO3
200°C/24 hrs Micro-flowers Cathode in LIB
and NIBs [17]
2015 V2O5,
Oxalic acid 260°C/48 hrs Nanobelts
Thermal decomposition of NH4ClO4
[18]
2015 V2O5, H2O2,
Absolute ethanol 250°C/48 hrs Belt-like LIB cathode [19]
2016
V2O5, Citric acid
180oC/5 hrs Nano-
architectures CE in DSSCs Present work
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Fig. 1. Room temperature powder XRD patterns of (a) V2O5 powder, (b) intermediate stage V10O24.9H2O, (c) as-synthesized V6O13 powder and (d) VO2 (B) powder. Here the XRD patterns are simulated using corresponding JCPDS data cards.
10 20 30 40 50 600
500
1000
1500
2000
2500
Inte
nsity
(a.
u.)
2θ CuKα1 (O)
Experimental Simulated
(a)
5 10 15 20 25 30 35 40 45 50 55 60 650
1000
2000
3000
4000
2θ CuKα1 (O)
(b)
Inte
nsity
(a.
u.)
Experimental Simulated
10 20 30 40 50 60 700
300
600
900
1200
1500
1800
2θ CuKα1 (O)
Experimental Simulated
(c)
Inte
nsity
(a.
u.)
10 20 30 40 50 60 700
100
200
300
400
Inte
nsity
(a.
u.)
2θ CuKα1 (O)
(d) Experimental Simulated
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Fig. 2. SEM image of (a) V2O5 powder, (b) intermediate stage V10O24.9H2O, (c) as-synthesized V6O13
powder, the inset figure shows the closer view and (d) VO2 (B) powder.
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Fig. 3. (a) TEM micrograph of V6O13 powder, (b) HRTEM micrograph of V6O13 powder, (c) diffraction pattern corresponding to micrograph (b) along the [001] zone axis.
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Fig. 4. An illustration of fabrication steps of V6O13 counter electrode.
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Fig. 5. (a) The J-V curves of the DSSCs fabricated with V6O13 as counter electrode. (b)Nyquist plots of full cells assembled DSSC with V6O13 counter electrode.
0.0 0.1 0.2 0.3 0.4 0.50.0
0.5
1.0
1.5
2.0 (a)
Cur
rent
den
sity
(m
Acm
-2)
Voltage (V)
60 90 120 150 1800
5
10
15
20
25 (b)
-Z''
(Ω)
Z' (Ω)
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Highlights
1. Hierarchical nano-structures of V6O13 are synthesized by a facile one step
hydrothermal process.
2. Phase formation mechanism investigated under hydrothermal conditions by
using time dependant study.
3. For the first time, we show the feasibility of V6O13 in electrochemical energy
conversion applications.
.
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