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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: geeta.mutta@gmail.com

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.

References

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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.

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Experimental Simulated

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2θ CuKα1 (O)

Experimental Simulated

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(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.

.