A composite thin film of simultaneously formed carbon and SnO2 QDs for supercapacitor application

Article abstract of DOI:10.1039/c8nj00375k

A uniform size and structure of a composite material are critical assets that determine the properties, such as charge transfer, thermal, photoluminescence, mechanical, etc., and consequently the applications of the material; herein, we report the concept of flame/combustion at a liquid-liquid interface for the first time to synthesize in situ a thin film of a composite consisting of two or more quantum dots. The synthesis of the thin films of a composite containing C and SnO₂ QDs having particle sizes below 2 nm was successfully carried out. As compared to single quantum dot systems, the formed composite showed significantly improved specific capacitance due to the synergistic effect arising from the strong interaction between C and SnO₂ QDs. This was confirmed by XPS, and UV visible spectroscopy. Moreover, it was confirmed that even after 1000 charge/discharge cycles, the interaction between C and Sn remained unaltered; this indicated significant stability of the capacitance. Some of the advantages of this method include a one-step eco-friendly process and use of ambient conditions. The generality of the method was established by synthesizing C-ZnO and C-TiO₂ composite thin films. This new approach can be extended to form many other valuable composite thin films for various applications.

Full text of DOI:10.1039/c8nj00375k

View Article Online
View Journal
NJC
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. P. Gaikwad, S.
H. Patil, K. R. Patil, S. D. sathaye and C. V. Rode, New J. Chem., 2018, DOI: 10.1039/C8NJ00375K.
This is an Accepted Manuscript, which has been through the
Volume 40 Number
1 January 2016 Pages 1–846
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
Accepted Manuscripts are published online shortly after
New Journal of Chemistry
www.rsc.org/njc
A journal for new directions in chemistry
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

Please do not adjust margins
New Journal of Chemistry
Page 1 of 10
View Article Online
DOI: 10.1039/C8NJ00375K
Journal Name
ARTICLE
Composite thin film of simultaneously formed carbon and SnO2
QDs for supercapacitance application
Aarti P. Gaikwad,a Sagar H. Patil,a,d Kashinath R. Patil,b Shivaram D. Sathayec and Chandrashekhar
V. Rodea*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Abstract
www.rsc.org/
Composite material with uniform size and structure is a critical asset that decides its properties like charge transfer,
thermal, photoluminescence, mechanical, etc. and consequently the applications. Here for the first time, we report the
concept of flame/combustion at liquid-liquid interface to synthesize in situ, thin film formation of composite consisting of
two or more quantum dots. Synthesis of thin films of a composite containing C and SnO2 QDs having particle sizes below 2
nm was successfully done. As compared to a single quantum dots system, the formed composite showed significantly
improved specific capacitance due to the synergistic effect arising from strong interaction between C and SnO2 QDs. This
was confirmed by XPS, UV visible spectroscopy and photoluminescence spectra. Moreover, it was confirmed that even
after 1000 charge/discharge cycles, the interaction between C and Sn remained unaltered; indicating significant stability of
capacitance. Some of the advantages of this method include one step, eco-friendly process at ambient conditions. The
generality of the method was established by synthesizing C-ZnO and C-TiO2 composite thin film. This new approach can be
extended to form many other valuable composite thin films for various applications.
innovative procedure altogether. Large number of composite
materials e.g. TiO2/SnO2,11 CdS/TiO2,12 TiO2/GO,13 CdS/ZnO14,
CuO/Co3O4,15 rGO/Au16, Ge/TiO2, PbSe/ZnSe17 etc., have been
Introduction
Composite materials such as carbon/metal oxide, metal/metal
oxide, polymer/metal oxide have wide range of applications in
various fields such as catalysis, sensing, electrocatalysis,
batteries, bioimaging, solar cells etc.1,2 Moreover, if particle
sizes of the respective components are in nanometer range,
bulk properties of the material get modified which become
suitable for contemporary applications.3-6 Therefore, the
methods to obtain solid materials in nanometer sizes are of a
sustaining interest of researchers for last four decades.3-6 In
this regard, quantum dots (QDs) are of significant importance
as their respective particle sizes are smaller than their
respective Bohr radius which then generate exclusive
properties.7-10
synthesized and studied. However synthesizing composite
materials suffer from drawbacks such as (i) a chronological
addition of different precursors, (ii) higher temperature, (iii)
costly instruments, (iv) use of dispersing/capping agents, (v)
inert atmospheric conditions by employing N2/Argon
surrounding etc. which render the synthesis cumbersome,
multi-step and time consuming. Such methods may be also
specific to the product and cannot be generalized, for wide
range of applications. Equally important is the reproducibility
of thin film formation required for their applications such as
supercapacitor. Since nanomaterials/QDs are formed normally
in powder form, additional steps also may be needed to
fabricate a film.18
In the recent times, rather than pure single phase materials,
composite consisting of QDs of different components are
found to be more beneficial for various applications. Such
composite formation normally requires more than one step for
a single phase end product and hence necessitates an
Therefore, the development of a rapid, one-pot, eco-friendly,
low cost, generalized method of QDs composite thin film
formation involving minimum number of steps is still required.
Previously, we have reported a method to make thin films of
QDs of various singular semiconductors.10 In the present work
we describe for the first time, one-pot method of
simultaneous synthesis of a thin film of the composite
comprising QDs of carbon and SnO2, at liquid-liquid interface.
Compared to our previous work on single component QDs, this
new route is more efficient and applicable to a wider variety of
simultaneous synthesis of more than single component QDs.
Other, important advantages of this method are; (i)
a.CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pashan, Pune-
411008, India.
Email: cv.rode@ncl.res.in
b.39, Yoganand Park I, Paud Road, Kothrud, Pune-411038, India.
c. 759/83 Deccan Gymkhana, Pune-411004, India.
KBP College Vashi, Navi Mumbal-400705, India.
d.
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/
x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 2 of 10
View Article Online
DOI: 10.1039/C8NJ00375K
ARTICLE
Journal Name
simultaneous synthesis of QDs of both C and SnO2 at room technique). The operation was repeated several times to get
temperature, (ii) the QDs formed on the surface can be the desired film thickness.
transformed into thin film on a suitable substrate, (iii) Synthesis of C QDs thin films
possibility to tailor the composition of various components in To prepare carbon QDs, 10-3 M anthracene, 1.8 mg of
the final composite, (iv) deposited composite is found to have anthracene was dissolved in 1 ml alcohol. A clear solution was
good adherence to the substrate without the use of binders or formed in which 9 ml petrol was added. From this solution, 10-
4
additives, (iv) eco-friendly, in-situ, low cost synthesis without
M anthracene was prepared by adding proper amount of
requiring harsh post treatment with better control over alcohol/petrol. This solution was spread on the surface of the
parameters like film thickness etc. The generality of this distilled water and immediately set on fire. The combustion of
method of QDs was established by synthesizing C-ZnO and C- anthracene led to the formation of thin layer of carbon on the
TiO2 composite thin films. The end-product formed following surface of the subphase. In this way, carbon QDs was formed
our method showed properties suitable for one of the most at the surface. To form a compact film, same procedure was
sought after application for supercapacitor.
Experimental
followed as used to make SnO2 films.
Synthesis of C-SnO2 QDs thin films
The materials used in the present work were all of analytical
For the preparation of carbon-SnO2 QDs composite thin
grade. Anthracene (C14H10, 99%), Tin (II) chloride dihydrate films, 10-4 M solution of SnCl2.2H2O was used as a subphase. A
(SnCl2.2H2O, 98%), hydrochloric acid (HCl), petrol (Sulfur free), precursor solution of carbon, 10-4 M of anthracene in petrol
zinc acetylacetonate (Zn(C5H7O2)2.xH2O 99%), ammonium which was spread on the subphase and subsequently followed
titanium oxalate ((NH4)2TiO(C2O4)2.H2O 99%) were purchased the same procedure as described above to obtain a composite
from Sigma-Aldrich and castor oil from local market. For the thin film.
synthesis of QDs films, all the chemicals were used as received, Synthesis of C-ZnO, C-TiO2 QDs thin films
without further purification. Throughout all the experiments
For the preparation of C-ZnO and C-TiO2 QDs composite
double-distilled, deionized water was used. The substrates thin films, 10-4 M solutions of Zinc acetyl acetonate and
used are glass slides, quartz plates and silicon wafers. Glass Ammonium titanium oxalate were used as a subphases
and quartz substrates were cleaned by pre-treating them with respectively; and the same procedure was followed as
freshly prepared piranha solution (H2SO4/H2O2 = 70:30, v/v) at described for C-SnO2 QDs composite thin film formation.
700C for 15 min, thoroughly rinsed with deionized water and Characterization
dried in nitrogen flow.
The size distribution and morphologies of as-prepared
products were examined by transmission electron microscopy
(TEM, FEI Tecnai F-20) at an acceleration voltage of 200 kV.
Synthesis of Quantum dots thin film
Synthesis of SnO2 QDs thin films
To synthesize SnO2 QDs, SnCl2.2H2O was used as a precursor. High resolution transmission electron microscopy (HRTEM, FEI
Petrol (sulfur free) was used as a fuel for the combustion of Tecnai F-30) was used at an acceleration voltage of 300 kV.
SnCl2.2H2O for the formation of SnO2. The experimental Specimens for TEM and HRTEM analysis, were prepared on
procedure is essentially the same as given earlier (Scheme carbon coated copper grids by depositing the respective film
S1).10
by the above described procedure by dipping it twice in the
In a typical procedure, 10-4 M solution of SnCl2.2H2O is formed solution followed by drying at room temperature. The electron
by dissolving tin (II) chloride in distilled water. A whitish diffraction facility was employed for assessment of the
solution was formed. To this solution, few drops of HCl were structure and the phases present. The absorption in the UV-
added until a clear solution was formed. This solution was vis. region was studied (JASCO dual beam spectrophotometer
placed in a Teflon tray (15 X 15 X 2 cm) in such a way that a (JASCO V-570 operated at a resolution of 1 nm). The chemical
meniscus formed above the edges of the tray. The surface of analysis of the film was done by surface characterization by
the solution was cleaned by sweeping the Teflon barrier to XPS analysis (ESCA-3000, VG Scientific Ltd, UK) with a base
avoid any dust contamination on the surface. Surface of pressure of lower than 1.0 x 10-9 Pa. Mg Kα radiation (1253.6
subphase was divided into two compartments by fixing a eV) was used as a X-ray source and operated at 150 W. The
teflon thread barrier on the edges of the tray. In one of the peak shift due to charging was corrected using the C1s level at
compartments, petrol was allowed to spread on the subphase 285 eV as an internal standard. The photoluminescence
with the help of a micro-syringe. In this process, stretching of spectra of film were recorded on Fluorescence
the thread formed the uniform layer of petrol which was spectrophotometer (F-2500) using UV blocking filters to cut
immediately set on fire. During this process, sufficient heat down the scattered light from the sample. Fourier transform
was generated to form SnO2 at the interface. The formed infrared spectra (FTIR) were acquired with Perkin Elmer
product was then compressed laterally by spreading a drop of spectrum.
castor oil on the surface of the subphase in the other Electrochemical Properties of SnO2, C and C- SnO2 QDs thin
compartment. The as-formed compressed film on the surface film
was transferred on to suitable substrate by immersing the Thin film synthesized by combustion at liquid-liquid interface
substrate vertically in the solution at a constant rate of 0.5 was studied on an Autolab PGSTAT 30 (Ecochemie)
cm/min and lifting it vertically at the same rate (Blodgett electrochemical workstation using cyclic voltammetry,
chronopotentiometry and electrochemical impedance test.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 3 of 10
View Article Online
DOI: 10.1039/C8NJ00375K
Journal Name
ARTICLE
The measurement was performed by configuring the samples independent of the reaction/s, provided it takes place
into a two electrodes system, where the sample used as the spontaneously at the temperature developed by the
working electrode as well as counter electrode. The working flame/combustion and within the time span of combustion.
electrodes were prepared as follows: C QDs or SnO2 QDs or C- Also, the particle characteristic morphology is independent of
SnO2 QDs composite at the interface of the liquid was collected precursors used, its concentration, small changes in the ratio
on the FTO and used as it is without the use of any binder or of precursor to fuel, number of simultaneous reactions, etc.
further processing. The electrolyte used was 1 M Na2SO4 The thin films of QDs of SnO2, C and composite C-SnO2
aqueous solution at room temperature. The electrochemical prepared by flame/combustion at liquid-liquid interface were
properties and capacitive behaviour of the supercapacitor characterized by XRD analysis. A multilayered film did not
electrodes were evaluated by cyclic voltammetry (CV) and show XRD pattern. The absence of XRD signal may be due to
galvanostatic charge−discharge (CD).
the very small particle size and insufficient material
The specific capacitance values of the samples are calculated concentration of the samples.20
from the CV curves using following equation:19
Further, each sample was examined by XPS analysis to know
elemental composition and purity of the material (Fig. 1). Fig.
1a and Table S1 indicate the absence of peaks other than
expected elements after the combustion of precursors. It was
obvious to expect carbon formation from organic precursors
and metal compounds leading to the formation of respective
oxides by high temperature combustion process. The
elemental mapping of the C, SnO2 and C-SnO2 QDs composites,
images are shown in Fig. S1, confirming this conclusion.
Where, Csp is the specific capacitance in farads per gram (Fg-1), m is
the active mass of the electrode material (g) obtained from quartz
crystal microbalance (QCM-200), I is the response current density
(Ag-1) and v is the potential scan rate in (Vs-1) and V is potential (V).
The specific capacitance, C (Fg-1), of the electrode material was
calculated from the galvanostatic charge-discharge according
to the following equation
Where, m is the active mass of the electrode material obtained
from quartz crystal microbalance (QCM-200) (10.4, 10.5 and
11.5 μg for SnO2, C and C-SnO2 composite respectively), i is the
response current density (Ag-1), ∆t is discharge time (s), ∆E is a
voltage difference in (V).
Results and Discussion
The growth mechanism of QDs is normally controlled by
thermodynamics wherein lowering of surface energy of
particles formed becomes more important. However, in the Fig. 1 XPS spectra of a) survey scan b) Sn 3d c and d) deconvoluted
peak of C1s for C QDs and C-SnO2 composite.
present method, growth of particles by this method is
prevented by the surface tension forces at the interface which
controls direct contact of particles. Also, practically the particle
formation process takes place at room temperature with
insufficient energy for growth of the particles in spite of small
particles’ active nature. Thus, kinetics must be considered to
explain control of growth/size of particles. Exactly at this
juncture, the present method differs from other
combustion/flame methods. Most of the methods in the
literature allow sufficient time for reaction to go to its
completion which allows changes in the characteristics of
particle morphology and growth of particle due to
agglomeration. As against this in the present method, although
reaction leads to end products, the time of reaction is so small
(few seconds) that kinetic effect on particle morphology of
end products, namely, growth in sizes/agglomeration is
restricted. Moreover, it occurs by the inherent parameters
(very small time span) of the method and therefore it is
The high-resolution XPS spectra of SnO2 and C-SnO2
composites showed two distinct peaks at 486.5 and 495 eV
attributed to Sn 3d5/2 and Sn 3d3/2, respectively, arising due to
spin-orbit splitting (Fig. 1b). The peak positions were in
agreement with the expected spin orbit separation of 8.5 eV
assignable to Sn+4, a formal oxidation state of Sn.21 However, a
slight shift in the peak for the composite was observed for Sn
3d; which could be assigned to the interaction between C and
SnO2. The high resolution spectra of C 1s (Fig. 1c) showed four
peaks at 284.2, 285.5, 286.5 and 288.2 eV which are attributed
to C-C sp2, C-C sp3, C-O and C=O species, respectively present
in the carbon QDs.22 The graphitic carbon (sp2) might be
formed during carbonization of aromatic carbon precursor
namely, anthracene wherein large number of sp2 carbon
bonds exist in the precursor itself; so that no rearrangement in
the product was needed at the expense of high energy. This
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 4 of 10
View Article Online
DOI: 10.1039/C8NJ00375K
ARTICLE
Journal Name
may be seen on the background of reported carbonization intensity of most of the peaks decreased considerably to the
products formed in non-aromatic carbon precursor such as extent that some peaks vanished. This was obvious as during
sucrose.23,24 This outcome is important as graphitic carbon the carbonization process, most of the anthracene structure
provides higher electronic conductivity to a composite which is collapsed. The end product was expected to be a solid carbon
vital for various relevant applications.25 Thus, the choice of having mostly C-C bonds in its structure. However, the reaction
aromatic precursor for the formation of C QDs and composite time was small to restrict the growth resulting in bigger carbon
based on it turned out to be the most appropriate one. Two particles. Thus, the end product was the formation of ‘C’ QDs.
other peaks at 286.5 and 288.2 eV were assigned to the The presence of peaks around 2924 and 2846 cm-1 were
presence of COOH and C=O groups of C QDs respectively. The assigned to C-H stretching, 1589 cm-1 for C=C stretching and
presence of such functional groups on ‘C’ based nanomaterials the peak around 1571 cm-1 further confirmed the presence of
is reported in the literature.26 In the present case, where the the sp2 and sp3 C in C QDs. Since the reaction takes place at
reaction medium was water and reaction time was very short liquid-liquid interface, some C-OH or C-OO- type bonds are
so as to eliminate ‘C’ completely as CO2.
expected to be formed on the surface of QDs which also need
The deconvolution of ‘C’ peak revealed the intricacies of the to be considered. The presence of C=O stretching around 1743
functionalization of products. A peak at 282.3 eV in the cm-1 in C and C-SnO2 composite confirmed that some of the C
composite was a newly formed peak which could be assigned QDs were oxidized during the formation. These results were
to metal-carbon bond formation (Fig. 1c).27 Simultaneously, consistent with the XPS studies. The peak at 575 and 663 cm-1
C=C i.e. C sp2 bond formation decreased while, C-C bond were assigned to the O–Sn–O and Sn–O vibrational modes,
formation seemed to rise. This may be accounted for by respectively; which further confirmed the presence of SnO2.
considering that many double bonds might have been UV- visible absorption spectra of SnO2, C and C-SnO2 QDs
transformed during the formation of Sn-O-C or/and Sn-C composite thin films on quartz plate are shown in Fig. 3. SnO2
bonds. This was supported by the peak observed at 282.3 eV, QDs showed the peak at around 225 nm, which was at much
attributable to M-C bond (Fig. 1c).27 Moreover, it was observed lower wavelength than the reported optical absorption edge
that during composite formation, the area under intensity for bulk SnO2
curve of C=O bond gradually deceased as compared to that ascribed to the smaller size of the particles in the film.29 C QDs
(∼
344 nm, 3.6 eV).28 This blue shift could be
observed for its counterpart in C QDs. Two important showed a broad absorption peak from 270 to 300 nm, which
inferences drawn from these observations were, 1) Composite was similar to that of the typical C QDs.30 The absorption peak
with less C sp2 contribution in bonding, and 2) an interaction at 270 nm corresponded to the π→π* transiꢀon of the
between metal and carbon; directly or through oxygen.
aromatic sp2 hybrid orbital and small peak around 300 nm
corresponded to n→ π* transitions of the sp3 hybrid orbital,
respectively.31,32 For the composite C-SnO2 QDs thin film, a
clear shift in the peak was observed. A shift in peak of
composite suggests that there was an interaction between C
QDs with SnO2. Moreover, from the graph it was clear that the
intensity of the peak at
∼300 nm increased in case of
composite film which confirmed increase in concentration of
sp3 C in the composite.22-24 These results are consistent with
XPS and IR. All these results confirmed that there was an
interaction between C and SnO2 QDs.
Fig. 2 FTIR spectra for Anthracene, C, SnO2 and C-SnO2
composite.
Fourier transform infrared (FTIR) spectrum for the C-SnO2
composite sample was compared with the corresponding
spectra of C QDs, SnO2 QDs and pure anthracene as presented
in Fig. 2. Pure anthracene showed more number of absorption
peaks due to the various sp2 and sp3 stretching vibrations in
benzene ring. The peaks around 1625-1575 cm-1 and 1525-
1475 cm-1 were attributed to the C=C stretching in benzene
ring. The peak in anthracene around 3554 cm-1 for aromatic -H
bond disappeared after its combustion. Moreover, the
Fig. 3 UV-visible absorption spectra of the sample SnO2, C QDs
and C-SnO2 QD composite.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 5 of 10
View Article Online
DOI: 10.1039/C8NJ00375K
Journal Name
ARTICLE
The optical band gap was calculated using the Kubelka–Munk The high-resolution TEM (HRTEM) provided as insets of Fig. S3
equation. From Fig. S2, it can be seen that the band gaps for for SnO2 and C-SnO2 QDs composite exhibited clear lattice
the C, SnO2, C-SnO2 QDs composite and SnO2 bulk were about fringes with a d-spacing of 0.26 nm corresponding to (101)
5.7, 4.24, 4.14, 4.76, and 3.67 eV respectively, which was lattice planes of SnO2. The lattice fringe spacing (in C and C-
higher than the respective bulk materials. It is not surprising SnO2 composite QDs) was 0.24 nm corresponding to the in-
that we observed two absorption band edges for composite, plane lattice constant in sp2 carbon or graphene like
which is similar to observed and explained by Umarao et al. for structures.34 The SnO2 QDs and C QDs were found to be evenly
TiO2 GO composite.33
distributed on the substrate. During the formation of C-SnO2
The size distribution of the particles in the samples under QDs composite, it was proposed that chemical/physical bonds
consideration was examined by using transmission electron formed between SnO2 and C would lead to the formation of
microscopy (Fig. 4). Typical TEM images for SnO2, C and C-SnO2 interface between them. This tight coupling would be
QDs composite showed the presence of un-agglomerated, favorable for the charge transfer between C and SnO2.
spherical shaped, distinct nanoparticles of C QDs and SnO2 Moreover, it is likely to promote the separation of electron–
with a size below 1.5 nm (bottom inset of histogram). In the hole pairs formed by exciting the semiconductor which is
TEM of the composite sample, the darker particles represent important for catalytic, sensor and other applications. All the
SnO2 and the lighter particle C QDs. Excellent dispersion of C above characterizations confirmed that C- SnO2 is not a
QDs and SnO2 QDs confirmed that agglomeration/ physical mixture of two separate phases of C and SnO2 but a
aggregation/growth of particles was restricted during the real composite.
synthesis process. This observation was important as no The weight fraction of C QDs in C-SnO2 QDs composite was
dispersion or capping agents were employed in the process. determined by themogravimetric analysis (TGA) (Fig. S4). The
Two fundamental causes of preventing the particle growth TGA curve shows a total weight loss of 32.4% at 400°C due to
appear to be 1) liquid-liquid interface of few nanometer widths the combustion of carbon component in composite. After
imposed confinement to the reaction zone, and 2) very fast reaching 800°C, the total fraction of SnO2 content in the
reaction time of few seconds. The selected area electron composite is calculated to be 67.6%.
diffraction (SAED) pattern of the film formed by two dips for Further, we could also show that the developed method was
SnO2, C and C- SnO2 QDs composite films on copper grid are potentially useful for the applications. It occurred to us that
shown at the bottom inset of each Fig. S3. SnO2 QDs show the conducting C QDs and SnO2 having high capacitance can form a
distinct spots on ring pattern which were indexed to composite which can be an electrode for value-added energy
tetragonal SnO2 structure (JCPDS Card No. 77-0452). However, storage devices. Therefore, the capacitive performance of the
C QDs shows ring pattern and C-SnO2 showed ring pattern with as-prepared SnO2, C and C-SnO2 composite QDs thin films on
spots indexed to tetragonal SnO2 structure (JCPDS Card No. 77- conducting glass was evaluated as an electrode material by
0452).
using standard cyclic voltammetry (CV) and charge-discharge
in two-electrode system. The CV measurements were done in
1 M Na2SO4 as an electrolyte. During fabrication of the
electrode, no binder material like Nafion was used, which is an
advantage of the procedural part of present method of
depositing film on conducting glass and using it as working
electrode. The galvanostatic charge–discharge technique was
used to check the stability and recycling of electrodes. Figs. 5
a, b and c shows the cyclic voltammogram (CV) of SnO2, C and
C- SnO2 QDs composite thin films at various scan rates ranging
from 20 to 200 mVs−1. The voltammogram of the C-SnO2
electrode was just about an ideal rectangle with a greater area
and better shape symmetrical shape than voltammogram of
pure carbon and SnO2 thin films respectively. This indicated
the enhanced capacitance of the hybrid matrix. Moreover, the
shape at high scan rates were similar to those at low scan rate,
indicating better energy storage performance of the C-SnO2
composite with excellent ionic and electronic transport
concerned with electrode material.35
The specific capacitance (Csp) of C, SnO2 and C-SnO2 QDs
calculated from the CV curves using equation 1, is shown in
(Table S2 in ESI†).19 The calculated Csp for the C, SnO2 and C-
SnO2 QDs thin films were 185, 220 and 576 Fg-1 at the scan
rate of 20 mVs-1 and 138, 159 and 547 Fg-1 at the scan rate of
200 mVs-1, respectively. Fig. 5d and Table S2, showed clearly
Fig. 4 Low magnification TEM image of SnO2 QDs (a), C QDs (b)
and C-SnO2 QDs composite (c); the insets shows particle size
histogram of respective sample (bottom).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 6 of 10
View Article Online
DOI: 10.1039/C8NJ00375K
ARTICLE
Journal Name
the higher capacitance values for C-SnO2 (hybrid electrode) i. 1). The capacitance (576–547 Fg-1) obtained in the present
e. 576 and 547 Fg-1 at scan rates of 20 and 200 mVs-1, study is considerably higher than those of Sn-based carbon
respectively, as against the capacitance values of SnO2 and C hybrid composites reported earlier (Table 1). These results
QDs, respectively.
indicate that the potential of the composite C-SnO2 QDs thin
film can offer high performance supercapacitors. The percent
retention of Csp in the case of composite is 95 which were
higher than the singular components of the hybrid namely, C
and SnO2.
The galvanostatic charge/discharge curves of the QDs thin film
of the samples measured at different current densities are
shown in Fig. 6. A symmetrical charge/discharge characteristics
and linear voltage vs. time profiles were observed which
represent the excellent capacitive characteristics of these
supercapacitors. The charge–discharge curves (Figs. 6 a to c)
were nearly symmetric, revealing a good capacitive behavior.
In addition, no voltage drop was observed indicating that the
electrode had low internal resistance. Csp values calculated on
the basis of charge/discharge results for SnO2, at 20 and 100
μA were 225 and 167 Fg−1, respectively. Moreover, the hybrid
electrode showed high values of 578 Fg-1 to 552 Fg-1 at the
high current densiꢀes from 20 μA to 100 μA, (Table S3, ESI†).
The observed Csp from the calculations of both CV and
charge/discharge study were found to be consistent.
Fig. 5 Typical CV curves of SnO2 QDs (a), C QDs (b) and C-SnO2
QDs composite (c) at different scan rates 20 to 200 mVs−1. (d)
Specific capacitance values calculated from eqn. (1) for SnO2,
C, and C-SnO2 composite as a function of scan rates mVs-1.
Table 1 Comparison of the electrochemical performance of the
Carbon, SnO2 and C-SnO2 composites.
Electrode
Materials
C QDs
Specific capacitance Reference
and
Moreover, it was clearly seen that as the scan rate increased,
the specific capacitance (Csp) decreased. The decreasing Csp
was observed due to part inaccessibility of electrode surface
for holding charge at high charging–discharging rates. This
might arise due to the presence of secluded active sites that
could not sustain the higher frequency of redox transitions.36
(Fg-1)
year
185.0
Present work
Present work
Present work
SnO2 QDs
C-SnO2
220.0
(QDs) 576.0
composite film
SnO2 QDs
SnO2 thin film
C QDs
241
119
106
247
420
201437
201638
201639
201540
201441
SnO2@C
SnO2@C
From Table 1, a significant value addition was observed by
making C-SnO2 QDs composite in terms of higher capacitance
at high current density with good retention over the individual
components of the composite.
Fig. 6 Typical galvanostatic charge discharge profile of SnO2
QDs (a), CQDs (b) and C-SnO2 QDs composite (c) device at
different current densities, (d) the specific capacitance of SnO2,
C, and C-SnO2 QDs composite as a function of current density.
The specific capacitance of the composite was 576–547 Fg-1,
nearly three times higher than that of the carbon QDs (185 Fg-
1) and almost two times higher than that of pure SnO2 (220 Fg-
Fig. 7 Electrochemical impedance spectroscopy for SnO2, C and
C-SnO2 QDs composite.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 7 of 10
View Article Online
DOI: 10.1039/C8NJ00375K
Journal Name
ARTICLE
Electrochemical impedance spectroscopy (EIS) gives by UV-visible, FTIR and XPS studies. The UV-visible and FTIR
information regarding the internal resistance of an electrode spectra of C-SnO2 composites after charge/discharge cycle
material as well as the resistance between the electrode and clearly showed that the peaks were unaltered (Fig. S6).
the electrolyte (Fig. 7). The spectra were analyzed using However, slight broad range of peak from 230 to 350 nm in
Nyquist plots. The solution resistance (Rs) for the C, SnO2 and UV-visible spectrum was seen, which might be due to the
C-SnO2 QDs composite was found to be 75.4, 60.3 and 47.6 Ω, presence of broad particle size range. XPS analysis in Fig. S7
respectively. The charge transfer resistance (Rct) for SnO2, C, and Table S1 clearly showed the presence of peaks after
and C-SnO2 QDs composite was measured to be 11.3, 15.4 and charge/discharge cycle of C-SnO2 composites. The peaks at
8.03 Ω, respectively. The equivalent series resistance (Re) can 282.1, 283.6, 284.7, 286.5 and 287.8 eV of deconvoluted C 1s
be obtained from the X-intercept of the Nyquist plots, which is (Fig. S7 (a)) spectrum could be assigned to metal-carbon, C-C
a
combined resistance comprising ionic resistance of sp3, C=C sp2 and carbon bonded to -OH and -COOH groups,
electrolyte, contact resistance of active material/current respectively. The presence of peak M-C after charge/discharge
collector interface and intrinsic resistance of substrate. This cycles confirmed the strong interaction between C and Sn (Fig.
indicates that the C-SnO2 hybrid electrode possessed low S7 b) and thus confirmed the stability of the composite.
charge transfer impedance, and therefore, the composite The enhanced electrochemical performance of the composite
showed higher capacitance than shown by the SnO2 and C could be attributed to the following features.
electrode respectively. These results are consistent with those i) The small size of component (QDs/0D) resulted in high
obtained by the CV measurements.
surface area, shorter diffusion path and lower charge-transfer
resistance. These features improved the access of the
electrolyte and enhanced the capacitance of the material.
Therefore, the electrode easily absorbed the electrolyte
cations (Na+) on the surface from the electrode:44
(SnO2)surface + Na+ + e-
(SnO2 - Na+ )surface
2-
.
This provides more charge storage and easy access of SO3
2-
Therefore, both Na+ and SO3 from the electrolyte are fully
utilized in SnO2 electrode.
ii) The composite prepared by our method offers more stable,
binder free (e.g. Nafion etc.) electrode for the capacitance
application than most of the methods reported earlier (Table
1). This method is also useful to synthesize other composites
as shown by the synthesis of C-ZnO and C-TiO2 composites for
which the detail characterization is given in supporting
information (Figs. S8-S10).
Fig. 8 Stability performances for SnO2, C and C-SnO2 QDs
composite.
Conclusions
The stability of the performance of the composite was also
studied up to 5000 cycles of charge–discharge processes (Fig.
8) which showed the retention of Csp up to 96% at high
current density of 20 μA. The higher percentage of SnO2 in
In summary, simultaneous synthesis of two materials in QD
form viz. C QDs and SnO2/ZnO/TiO2 QDs for the first time, was
achieved by an easy, low cost, eco-friendly, single step
flame/combustion at liquid-liquid interface without using any
surfactant. It was shown that a thin film of composite was
formed during the synthesis and the HRTEM of C-SnO2
composite showed uniformly well dispersed C and SnO2 QDs
on the substrate with particle size < 2 nm. UV-visible, FTIR and
XPS characterization evidenced the presence of sp2 and sp3
form of C in C QDs. Moreover, systematic investigation by XPS,
FTIR, TEM of C, SnO2 and C-SnO2 QDs composite showed the
successive interaction between C and SnO2 QDs. An electrode
of C-SnO2 composite exhibited high capacity, excellent rate
capability, and long cycle life revealed from its CV studies. The
maximum specific capacitance for the C-SnO2 QDs composite
film achieved was 576 Fg−1 at a scan rate of 20 mVs−1 and for
SnO2 and bare carbon; it was only 220 and 185 Fg−1,
respectively. TEM image of the composite after 1000
charge/discharge cycles showed distinct particles of both
carbon and SnO2. Moreover, UV-visible, FTIR and XPS study of
the composite after electrochemical measurements showed
composite ensured
a
higher reversible capacity for
composite.42 Also it can be seen that in case of C-SnO2
composite, the specific capacitance decreased only by 4% at
current density 20 μA. Thus, it was relatively more stable at
higher current density as well as at higher number of cycles,
and can be useful for specific applications.
To understand the encouraging electrochemical performance
of C-SnO2 QDs composites, their microstructure and
morphology was also examined by TEM after 1000
charge/discharge cycles. The TEM image (Fig. S5) clearly
showed that the distinct particles of C and SnO2 QDs were
visible even after electrochemical cycling. However, some
increase in particle size was also observed, which might be
responsible for the capacitance loss during cycling.43
Nevertheless, the size of most of the particles was found to be
less than 2.5 nm (histogram shown as inset of Fig. S5). The
stability of the composite after cycling was further confirmed
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 8 of 10
View Article Online
DOI: 10.1039/C8NJ00375K
ARTICLE
Journal Name
the strong interactions between carbon and tin establishing
the excellent stability at higher current density as well as at
higher number of cycles. The general applicability of our
methodology was proven for the formation of few other
composites of C-ZnO and C-TiO2.
[17] S. Abe, One-step physical synthesis of composite thin
film. Solar cells - New aspects and solutions. Prof. L. A.
Kosyachenko, Eds., In Tech 2011, 149–166.
[18] R. K. Selvan, I. Perelshtein, N. Perkas, A. Gedanken, J.
Phys. Chem. C, 2008, 112, 1825−1830.
[19] T. Lee, T. Yun, B. Park, B. Sharma, H. K. Song, B. S.
Kim, J. Mater. Chem., 2012, 22, 21092−21099.
[20] S. D. Sathaye, K. R. Patil, D. V. Paranjape, A. Mitra, S.
V. Awate, A. B. Mandale, Langmuir, 2000, 16, 3487–
3490.
Conflicts of interest
There are no conflicts to declare.
[21] F. Ye, B. Zho, R. Ran, Z. Shao, Chem. Eur. J,
2014, 20, 4055–4063.
Acknowledgements
[22] L. Ding, S. He, S. Miao, M. R. Jorgensen, S. Leubner, C.
Yan, S. G. Hickey, A. Eychmüller, J. Xu, O. G.
The author acknowledge Department of Science & Technology,
Government of India for financial support vide reference no.
SR/WOS-A/CS-1015/2015 under Women Scientist Scheme to
carry out this work. We thanks to Mr. R. S. Gholap, Mr. Pankaj,
Mr. Venkatesh and Mr. Harshwardhan for helping in TEM and
SEM characterization. Mr. S. S. Deo, Mrs. Ashwini, Mr.
Babasaheb and Mr. Prashant Gaikwad for their help and moral
support.
Schmidt, Science report, 2014,
[23] D. Banham, F. Feng, K. Pei, S. Ye, V. Birss, J. Mater.
Chem. A, 2013, , 2812–2820.
4, 1–6.
1
[24] C. H. Kim, D. K. Lee, T. J. Pinnavaia, Langmuir, 2004,
20, 5157–5159.
[25] S. Maass, F. Finsterwalder, G. Frank, R. Hartmann, C.
Merten, J. Power Sources, 2008, 176, 444–451.
[26] Y. Wang, A. Hu, J. Mater. Chem. C, 2014,
2, 6921–
References
6939.
[27] A. Furlan, J. Lu, L. Hultman, U. Jansson, M.
Magnuson, J. Phys.: Condens. Matter, 2014, 26
,
[1] V. Tjoa, J. Chua, S. S. Pramana, J. Wei, S. G.
415501:1–415501:11.
Mhaisalkar, N. Mathews, ACS Appl. Mater. Interface,
[28] A. D. Bhagwat, S. S. Sawant, B. G. Ankamwar, C.
2012,
4, 3447–3452.
M. Mahajan, J. Nano- Electron. Phys, 2015, 7, 04037-
[2] S. Xu, D. Li, P. Wu, Adv. Funct. Mater., 2015, 25
,
1–4.
1127–1136.
[29] L. E. Brus, J. Chem. Phys., 1984, 80, 4403–4409.
[30] S. N. Baker, G. Baker, Angew. Chem. Int. Ed., 2010, 49
6726– 6744.
[3] O. Adegoke, E. Y. Park, Scientific Reports, 2016, 6,
,
27288-1–9.
[4] X. Zhong, Y. Feng, W. Knoll, M. Han, J. Am. Chem.
Soc., 2003, 125, 13559–13563.
[31] D. Pan, J. Zhang, Z. Li, M. Wu, Adv. Mater., 2010, 22
734−738.
,
[5] M. D. Regulacio, M.–Y. Han, Accounts Chem. Res.,
2015, 43, 621–630.
[32] Y. P. Sun, B. Zhou, Y. Lin, W., Wang, K. A. S. Fernando,
P. Pathak, M. J. Meziani, B. A. Harruff, X. Wang, H. F.
Wang, P. J. G. Luo, H. Yang, M. E. Kose, B. L. Chen, L.
[6] R. Sardar, A. M. Funston, P. Mulvaney, R. Murray,
Langmuir, 2009, 25, 13840–13851.
M. Veca, S.
Y. Xie,
J.
Am.
Chem.
Soc.,
[7] S. S. Lo, T. Mirkovic, C. Chuang, C. Burda, G. D.
Scholes, Adv. Mater., 2011, 23, 180–197.
[8] P. Kambhampati, J. Phys. Chem. C, 2011, 115, 22089–
22109.
2006, 128, 7756–7757.
[33] S. Umrao, S. Abraham, F. Theil, S. Pandey, V.
Ciobota, P. K. Shukla, C. J. Rupp, S. Chakraborty, R.
Ahuja, J. Popp, B. Dietzek, A. Srivastava, RSC Adv.,
[9] C. D. M. Donega, Chem. Soc. Rev., 2011, 40, 1512–
1546.
2014,
[34] M.
Reckmeier, A. L. Rogach, J. K.
4, 59890-59901.
Fu, F. Ehrat, Y. Wang, K.
Z.
Milowska, C.
[10] A. H. Jadhav, S. H. Patil, S. D. Sathaye, K. R. Patil, J.
Colloid and Interface Sci., 2015, 439, 121–128.
[11] Z. Liu, D. D. Sun, P. Guo, J. O. Leckie, Nano Lett., 2007,
Stolarczyk, A.
S. Urban, J. Feldmann, Nano Lett., 2015, 15, 6030–
6035.
7, 1081–1085.
[35] M. Chen, H. Wang, L. Li, Z. Zhang, C. Wang, Y. Liu, W.
[12] D. Robert, Catal. Today, 2007, 122, 20−26.
Wang, J. Gao, Appl. Mater. Interface, 2014, 3, 14327–
[13] C. Chen, W. Cai, M. Long, B. Zhou, Y. Wu, D. Wu, Y.
14337.
Feng, ACS Nano, 2010, 4, 6425−6432.
[36] J. G. Wang, Y. Yang, Z. H. Huang, F. Y. Kang, J. Mater.
Chem., 2012, 22, 16943–16949.
[14] S. Ghosh, M. Ghosh, P. Kumar, A. S. Sarkar, S. K. Pal,
J. Phys. Chem. C, 2016, 120, 27717–27723.
[37] Y. Gao, J. Wu, W. Zhang, Y. Tan, J. Zhao, B. Tang,
Mater. Lett., 2014, 128, 208–211.
[15] H-J. Kim, S. Y. Kim, L. J. Lim, A. E. Reddy, C. V. V. M.
Gopi, New J. Chem., 2017, 41, 5493–5497.
[38] A. A. Yadav, J Mater Sci: Mater Electron, 2016, 27
,
[16] Q. Wang, Q. Wang, M. Li, S. Szunerits, R. Boukherrou,
New J .Chem., 2016, 40, 5473–5482.
1866–1872.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
New Journal of Chemistry
Page 9 of 10
View Article Online
DOI: 10.1039/C8NJ00375K
Journal Name
[39] G. Chen, S. Wu, L. Hui, Y. Zhao, J. Ye, Z. Tan, W. Zeng,
ARTICLE
Z. Tao, L. Yang, Y. Zhu, Scientific Reports, 2016, 6,
19028-1–9.
[40] Z. J. Li, T. X. Chang, G. Q. Yun, Y. Jia, Powder Technol.,
2012, 224, 306–310.
[41] Y. Zhang, Z. Hu, Y. Liang, Y. Yang, N. An, Z. Li, H. Wu, J
Mater Chem A Mater Energy Sustain, 2015, 3, 15057–
15067.
[42] X. Fan, J. Shao, X. Xiao, X. Wang, S. Li, H. Ge, L. Chen,
C. Wang, J. Mater. Chem. A, 2014, , 18367–18374.
[43] C. Guan, J. Liu, Y. Wang, L. Mao, Z. Fan, Z. Shen, H.
Zhang, J. Wang, ACS Nano, 2015, , 5198–5207.
2
9
[44] R. S. Mane, J. Chang, D. Ham, B. N. Pawar, T. Ganesh,
B. W. Cho, J. K. Lee, S. H. Han, Curr. Appl. Phys., 2009,
9, 87–91.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

New Journal of Chemistry
Page 10 of 10
View Article Online
DOI: 10.1039/C8NJ00375K
TOC/ Graphical Abstract
Composite thin film of simultaneously formed
carbon and SnO2 QDs for supercapacitance
application
Aarti P. Gaikwad, Sagar H. Patil, Kashinath R. Patil, Shivaram D. Sathaye and
Chandrashekhar V. Rode*
A simple, one step, low cost combustion method for simultaneous formation of two/more
components QDs thin film