Results in Physics 9 (2018) 1391–1402
Contents lists available at ScienceDirect
Results in Physics
journal homepage: www.elsevier.com/locate/rinp
Nanostructured stannic oxide: Synthesis and characterisation for potential T
energy storage applications
D. Dodoo-Arhina,b,⁎, R.A. Nuamaha,b, P.K. Jainb,c, D.O. Obadad, A. Yayaa
a Department of Materials Science and Engineering, University of the Ghana, Ghana
bAfrican Materials Science and Engineering Network (A Carnegie-IAS RISE Network), South Africa
c Department of Physics, University of Botswana, Botswana
d Department of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria
A R T I C L E I N F O A B S T R A C T
Keywords: SnO2 nanoparticles were synthesized using the hydrothermal technique. Well crystalline particles with different
SnO2 nanoparticles morphologies and crystallite size in the range of 2 nm–10 nm were obtained by using Urea and Soduim
Supercapacitor Borohydride as reducing agents, and deploying Dioctyl Sulfosuccinate Sodium Salt (AOT) and Cetyl Trimethyl
Hydrothermal technique ammonium bromide (CTAB) as the surfactants. Samples have been characterised by X-ray diffraction, Scanning
Cyclic voltammetry Electron microscopy, Energy Dispersive X-ray spectroscopy, specific surface area, porosity, and Fourier
Transform Infrared spectroscopy. Preliminary studies on the potential electrochemical properties of the as-
produced nanoparticles were investigated using cyclic voltammetry, electrochemical impedance spectroscopy
and potentiostatic charge-discharge in aqueous KOH electrolyte. The surfactant and reducing agents used in the
synthesis procedure of SnO2 nanoparticles influenced the particle size and the morphology, which in turn in-
fluenced the capacitance of the SnO2 nanoparticles. The SnO2 electrode material showed pseudocapacitor
properties with a maximum capacitance value of 1.6 Fg−1 at a scan rate of 5mVs−1, an efficiency of 52% at a
current of 1 mA and a maximum capacitance retention of about 40% after 10 cycles at a current of 1mA. From
the Nyquist plot, The ESR for the samples increase accordingly as SCA (31.5Ω) < SAA (31.85Ω) < SE
(36.3Ω) < SAT (36.92Ω) < SCT (40.41Ω) < SA < SC (53.97Ω). These values are a confirmation of the low
capacitance, efficiencies and capacitance retention recorded. The results obtained demonstrate the potential
electrochemical storage applications of SnO2 nanoparticles without the addition of conductive materials.
Introduction Supercapacitors can be grouped into two types (Electrical Double Layer
Capacitor and Pseudocapacitor) according to their charge/discharge
For some time now, considerable efforts have been made to develop mechanisms [2]. The Electrical Double Layer Capacitor stores energy by
new energy storage devices which have high energy and high power electrostatic adsorption/desorption between polarized solid electrode
density to meet the world’s demand for clean energy. Supercapacitors vertical line liquid electrolyte interfaces, and the Pseudocapacitor
(SCs) which are also known as Electrochemical Capacitors (ECs) are stores energy by surface faradaic redox reactions on the interface of the
promising energy storage devices. They can produce a large amount of electroactive material and the electrolyte. The large capacitance of the
energy in a short period of time and they are usually preferred for Pseudocapacitor is made possible by electrode material that can be
energy storage systems due to their excellent cyclability and very good reversibly oxidized and reduced over a wide potential range [2–4].
power performance as compared to conventional capacitors and bat- The major electrode materials that are used in supercapacitors are
teries [1]. Supercapacitors are currently used in a wide range of con- those made of graphene and other carbonaceous materials (carbon
sumer electronics, memory back-up systems, and industrial power and nanotubes, activated carbon, etc.). The desire to use carbon materials
energy management. A new development as far as supercapacitors are comes from the fact that they store the charges electrostatically using
concerned is their use in emergency doors on Airbus A380. Another reversible adsorption of ions of the electrolyte onto active materials that
promising application is their use in low-emission hybrid electric ve- are electrochemically stable and have a high accessible surface area.
hicles and fuel cell vehicles to serve as a temporary energy storage However, their low specific charge has made scientists look into finding
device with a high-power capability to store energies when braking. new materials which are environmentally friendly and have high
⁎ Corresponding author at: Department of Materials Science and Engineering, University of the Ghana, Ghana.
E-mail address: ddodoo-arhin@ug.edu.gh (D. Dodoo-Arhin).
https://doi.org/10.1016/j.rinp.2018.04.057
Received 17 January 2018; Received in revised form 9 April 2018; Accepted 20 April 2018
Available online 28 April 2018
2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license 
(http://creativecommons.org/licenses/BY/4.0/).
D. Dodoo-Arhin et al. Results in Physics 9 (2018) 1391–1402
Table 1
Experimental Parameters.
Sample Id Method SnCl2·2H2O (g) Reducing Agent pH Domain Size (nm) BET surface Area (m2/g) Pore Size (Å) Pore volume
cm3/g
SA SnCl2·2H2O+NaBH4 2.1 2.6 g NaBH4 9.03 2.5 88.3118 114.9269 0.253735
SAA SnCl2·2H2O+NaBH4+AOT 2.1 2.6 g NaBH4 9.18 3.1 63.9505 157.1270 0.251209
SAT SnCl2·2H2O + NaBH4+CTAB 2.1 2.6 g NaBH4 8.94 6.1 54.9821 126.0116 0.173210
SC SnCl2·2H2O+Urea 2.1 2.6 g Urea 8.27 10.5 50.9202 173.9906 0.232929
SCA SnCl2·2H2O+Urea+AOT 2.1 2.6 g Urea 8.42 6.2 26.6186 210.4345 0.140037
SCT SnCl2·2H2O+Urea+CTAB 2.1 2.6 g Urea 8.12 11.5. 18.6892 23.1262 0.010805
Fig. 1a. XRD patterns of SnO2 nanoparticles.
Fig. 1c. Lognormal Size Distribution Analysis of the SnO2 particles.
manganese oxide [7].
Tin oxide (SnO2), a transparent conducting oxide and a wide band
gap n-type semiconductor, has been used in many applications such as
gas sensors [8], electrodes in solid-state ionic devices [9], solar cells
[10], etc., due to its unique properties such as its being chemically
inert, mechanically hard, and thermally stable. Only one stable phase
(there is no metastable one) is known, which has a tetragonal ar-
rangement of the atoms exhibiting either a rutile or cassiterite structure
[11]. Tin oxide can be synthesized using a variety of techniques such as
sol-gel [12], the hydrothermal method [13], precipitation [14], spray
pyrolysis [15] and Chemical Vapour Deposition [16]. Techniques such
as precipitation provide particle sizes of less surface area and smaller
pore size as a result of agglomeration, which makes them unsuitable for
applications such as gas sensors and electrode materials for super-
capacitors and batteries as compared to hydrothermal and micro-
emulsions techniques [17–19].
In this paper tin oxide nanoparticles were synthesized via the hy-
Fig. 1b. SnO2-SCT modelled using WPPM. drothermal technique under mild conditions (120 °C) using sodium
borohydride and urea as the reducing agents. Physical and electro-
energy storage capacity [5]. Transition metal oxides have also been chemical characterizations were performed on the tin oxide nano-
studied widely for use as electrode materials for supercapacitors. Al- particles using X-ray Diffraction (XRD), Fourier Transform Infrared
though RuO2 has good capacitive properties as a supercapacitor elec- Spectroscopy, Energy Dispersive Spectroscopy, Scanning Electron
trode material (specific capacitance: 1300 Fg−1) [6], its toxic nature, Microscopy, and Cyclic Voltammetry to determine the phase, surface
rarity and high production cost will exclude it from wide and com- morphology, chemical bonds and charge and discharge properties of
mercial applications. Manganese oxide is another transition metal oxide the as-synthesized tin oxide particles. This is with a view of exploring
studied as an electrode material for supercapacitor application. Man- the potential of SnO2 nanoparticles for energy storage applications.
ganese can exist in three different valence states and its oxides are
highly complex. Theoretically, the capacitance of manganese oxides is
estimated to be up 1100 Cg−1 (from Mn (IV) to Mn (III)) but the Experimental
electrochemical reversibility of redox transition of manganese dioxide
is usually too low to be applicable, and pure manganese dioxide pos- Materials and chemicals
sesses a poor capacitive response due to the high resistance of bulk
All the reagents used in this work were of analytical grade and were
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Fig. 2. FTIR spectra of as-synthesized SnO2 nanoparticles: (a) SA, (b) SAA; (c) SAT, (d) SC; (e) SCA (f) SCT.
used without any further purification. The reagents and materials used (AOT, C20H37NaO7S) [Sigma Aldrich]; Cetyl Trimethyl ammonium
include Tin (IV) Chloride pentahydrate (SnCl4·5H2O), [98% Sigma bromide (CTAB, C19H42BrN), [Central Drug House Ltd]; Nickel foam
Aldrich], Urea (CO(NH2)2), [Paskem]; Sodium Borohydride (NaBH4) [Alantum-korea]; polyvinylidene difluoride (PVdF) [Sigma Aldrich]; 1-
[Lab. Tech chemicals]; Ethanol (C2H5OH), [96%, BDH laboratory sup- methyl-2-pyrrolidinone (NMP) [Sigma Aldrich].
plies (Analar)]; distilled water; Dioctyl Sulfosuccinate sodium salt
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Fig. 3. SEM Micrographs of SnO2 nanoparticles of the various treatments – (a) SA, (b) SAA, (c) SAT, (d) SC, (e) SCA, (f) SCT.
Synthesis of SnO2 nanoparticles furnace at 600 °C for 3 h. The reaction mechanism is represented by the
In a typical hydrothermal synthesis of the nanoparticles (see Eqs. (16). The final product was stored for further characterization. The
Table 1), 2.1 g of SnCl4·5H2O was added to a solution consisting of procedure was repeated for both the surfactant template assisted and
60ml of water and 60ml ethanol under constant stirring at room non-surfactant assisted hydrothermal processes.
temperature. 2.6 g of urea was then added drop-wise to the colourless
mixture under rigorous stirring for 15mins and the pH of the solution
determined. The greenish brown mixture was subsequently transferred Pretreatment of the Nickel foam substrate
into a 200ml Teflon-lined steel autoclave and heated at 120 °C for 7 h at The Nickel foam substrate was carefully etched with hydrochloric
a vessel pressure of 15psi to complete the hydrothermal reaction. After acid (HCl 2M) for thirty minutes to ensure the removal of NiO layer on
autoclaving, the white precipitates from the precursor solution were the surface. The etched Ni substrate was then rinsed in ethanol and
washed several times with distilled water and ethanol via a cen- distilled water several times via an ultrasonic cleaner.
trifugation process to rid the precipitates of any impurity. The washed Preparation of the SnO2 Working Electrodes. The working electrodes
precipitates were then dried overnight in an oven at 100 °C, after which for electrochemical evaluation were prepared by mixing 98wt% of
the dried precipitates were calcined in a Linn High therm electric SnO2 nanoparticles from the various treatments and 2wt% poly-
vinylidene difluoride (PVdF) binder in a mortar. The mixture was then
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Fig. 4. EDX spectra of SnO2 nanoparticles under the various treatments – (a) SA, (b) SAA, (c) SAT, (d) SC, (e) SCA, (f) SCT.
dissolved in 1-methyl-2-pyrrolidinone (NMP) to form a slurry. The Characterization of SnO2 nanoparticles
slurry was coated on the treated Ni foam with dimensions 3 cm×1 cm
and pore size of 450 µm and dried overnight at 80 °C to ensure complete The as-produced nanoparticles are then characterized via X-ray
evaporation of the NMP. Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier
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Table 2 system reducing it to Sn4+(H O) (x=1–4) ions. The excess of OH−2 x
EDX atomic % of the synthesized SnO2 nanoparticles. ions from the ethanol contribution and at temperature higher than
Sample Element (Atomic %) Atomic ratio of Sn:O room temperature, SnO2 can be form through the following scheme:
Sn4+ Cl− + − 2−O Sn (aq) + + C2H54OH(aq) → Sn(OH)4(aq) + C2H5Cl(aq) (3)
SA 66.29 30.21 1:1.19 The dissolved C2H4Cl impurities are removed by washing with a 1:1
SAA 54.95 30.33 1:1.81 water –ethanol solution. The resultant Tin hydroxide is then oven dried
SAT 63.31 33.04 1:1.92 and then calcined:
SC 81.06 61.54 1:1.32
SCA 58.37 36.45 1:1.60 2 Heat−
SCT 76.46 21.51 1:3.55 Sn(OH)4(aq) ⎯⎯⎯⎯→ SnO2(s) + 2H2O(2) (4)
The reaction equation for the precursor solution using urea as re-
Transform Infra-Red (FTIR), and Brunauer–Emmett–Teller (BET) surface ducing agent is shown as:
area analysis for their crystal structure, optical, microstructure, porosity CO(NH ) 3H O CO 2NH+ 2OH−2 2(S) + 2 → 2(g) + + (5)
and surface properties. To determine the phases present and the mi- 4
crostructure of the SnO2 nanoparticles, X-ray powder diffraction (XRD) Sn4+ Cl− ++ + NH + 4OH− → Sn(OH)2− + 2NH Cl
patterns were collected on an Empyrean diffractometer (Panalytical BV, (aq) 4 (aq) 4(aq) 4 (aq) (6)
Netherlands) with theta/theta geometry, operating a Cu Kα radiation The final product is washed with a 1:1 water –ethanol solution via
tube (λ=1.5418 Å) at 40 kV and 45mA. The XRD patterns of all the centrifugation and decantation procedures to remove the dissolved
randomly oriented powder specimens were recorded in the 20°–70° 2θ NH4Cl impurities. The resultant Tin hydroxide is then oven dried and
range with a step size of 0.017° and a counting time of 14 s per step. The then calcined as shown in Eq. (4).
diffraction patterns were matched against the ICSD’s PDF database and The surfactant assisted systems are frequently used to control nu-
qualitative phase analysis conducted using the X’Pert Highscore plus cleation and growth of inorganic particles [24–25]. In this approach,
search match software (Panalytical, Netherlands). The instrumental the CTAB and AOT surfactant templates simply served as scaffolds with
resolution function was characterized with the NIST SRM 640d (Si) or around which the inorganic particles are generated in situ and
standard [20] and all peak profiles were simultaneously fitted with shaped into nanostructures with its morphology complementary to that
symmetrical pseudo-Voigt functions whose width and shape were of the template. In this case of the as-prepared SnO2 nanoparticles, the
constrained according to the Caglioti formulae [21]. Microstructural surfactant templates resulted in nanorod and nanoflower-like structures
analysis was performed using the Whole Powder Pattern Modelling as shown in Fig. 3. From Table 1 it is observed that when sodium
(WPPM) method [22], with the aid of the PM2K software [23]. borohydride was used as the reducing agent (samples SA, SAA, SAT),
A high resolution JSM5900 scanning electron microscope operated the pH was higher than when urea was used as the reducing agent
at 2.0 kV was used in the surface morphological investigations of the as- (samples SC, SCA, SCT). It can also be observed that the addition of
produced particles. Prior to the SEM analysis, the samples were me- CTAB to both sodium borohydride and urea precursor solutions (sam-
tallized with carbon coating to render them conductive. ples SAT, SCT) decreased the pH and this could be attributed to the fact
Specific surface area (SSA) and porosity of the synthesized powders that CTAB in aqueous solution dissociates to produce NH+4 ions which
were measured by nitrogen chemisorption using an Accelerated Surface consume OH− ions in the aqueous medium. Hence, there is
Area and Porosimetry System, model ASAP 2010 (Micromeritics OH-deficiency and more H+ in solution; leading to a decrease in pH.
Instrument Corporation). In the BET/porosity characterization, the
samples (≈0.2 g) were outgassed at 300 °C in a vacuum for 24 h. The
data of the adsorption and desorption isotherms were used to evaluate X-ray diffraction analysis (XRD)
the porosity of the sample. Transmission FTIR spectra were recorded on
a Vertex 70 v (Bruker) spectrometer in the 4000–400 cm−1 range with a The diffraction peaks in Fig. 1a are markedly broadened, which
4 cm−1 resolution. Sample compartment was evacuated during acqui- indicates that the crystalline domain sizes of samples are very small,
sition and the contact between the sample and the diamond ATR crystal with the average size estimated from the WPPM (Fig. 1c) are in the
was of 2mm diameter. Spectra were recorded and analysed with the range of being 2.5 nm–11.5 nm (see Table 1. It can be seen from the
Opus software. XRD data that samples produced with sodium borohydride (SA, SAA
and SAT) have broader and less intense peaks than samples produced
with urea (SC, SCA and SCT), which means that the samples produced
Results and discussion with sodium borohydride have smaller crystallite sizes than particles
produced with urea. This can be attributed to the fact that samples
Synthesis produced with sodium borohydride have a higher pH than samples
produced with urea, and the higher the pH, the smaller the particles
Tin (IV) chloride molecules are known to adopt a near perfect tet- sizes [26]. In higher pH value solutions, the coordinated water mole-
rahedral symmetry with average Sn-Cl distance of 227.9 pm. In aqueous
4+ cules suppress the agglomeration among the freshly formed nanocrys-solutions, SnCl4 dissociates into [Sn(H2O)6] ions and Cl− anions that
4+ 4+ tallites [26–27]. The Whole Powder Pattern Modelling (WPPM) ap-can partially coordinate with the Tin (Sn ) ions. In [Sn(H2O)6] , the
4+ proach using the PM2K software which connects a physical model forsix water molecules completely surround the Sn providing a the microstructure with the diffraction pattern, allowing an extraction
shielding effect. The addition of NaBH4 to the water-ethanol-precursor of the microstructure parameters without recurring arbitrary peak
solution, causes the following reaction to occur: shapes to fit each diffraction peak, was used for the microstructure
NaBH4(s) + 2H2O(l) → NaBO2(aq) + 4H2(g) + heat (1) analysis. A typical diffraction pattern of sample SCT modelled with the
WPPM is shown in Fig. 1b. The modelling was carried out by assuming
The interaction of the produced hydrogen gas with the dissolved the presence of a lognormal distribution (Fig. 1c) of aligned spherical
hydroxyl ions generates solvated electrons: domains. The almost featureless nature of the residual line in Fig. 1b
H −2(g) + 2OH(aq) → 2H2O 2e− (2) indicates a good agreement between experimental data and model,(aq) + (aq)
which also suggests that the shape assumption is right for the domains
The solvated electrons can penetrate the hexaaquaTin(IV) ions investigated.
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Fig. 5. Linear Isotherm plots of the as-synthesized SnO2 nanoparticles; (a) SA, (b) SAA, (c) SAT, (d) SC, (e) SCA, (f) SCT.
Fourier Transform Infrared spectroscopy analysis (FTIR) are attributed to Sn–O stretching modes of Sn–O–Sn and Sn–OH [29].
The peak at about 1630 cm−1–1645 cm−1 belongs to H-OH bonds
The FTIR results in Fig. 2(a–f) indicate that synthesized powder is coming from the moisture content of the powders and the peak at
tin oxide containing some degree of moisture (possibly crystallized 3405 cm−1 and 3420 cm−1 is attributed to Sn-OH bonds [30]. Peaks at
water), residual hydroxide and some AOT content. Residual hydroxide about 2920–2949 cm−1 can also be attributed to hydroxyl groups that
content is an indication of either incomplete conversion of hydrated tin are absorbed on the Tin Oxide surface. There are no impurity peaks
oxide or some reaction between SnO2 and H2O due to very high surface from the surfactant used, which together the XRD data confirms that
area (see Fig. 4 and Table 1 of the synthesized powder and the residual phase pure SnO2 nanoparticles were synthesized using the water-in oil
AOT from inadequate washing of precipitates before drying [28]. microemulsions technique.
The peaks occurring at about 507 cm−1–1357 cm−1 in Fig. 2(a–f) Fig. 2a represents the FTIR spectra of SnO2 particles prepared using
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Fig. 6. Cyclic Voltagrams of the as-obtained SnO2 nanoparticles at different scan rates – (a) SA; (b) SAA; (c) SAT; (d) SC; (e) SCA; (f) SCT.
sodium borohydride as a reducing agent. The peak at 3405 cm−1 is peak at 993 cm−1 and 1114 cm−1. From the above result (Fig. 2b), it is
attributed to Sn-OH bonds. Peaks at 1008 cm−1, 644 cm−1 and obvious that AOT molecules are adsorbed in the pores of the powders
538 cm−1 are attributed to Sn–O stretching modes of Sn–O–Sn and [31]. 2449 cm−1, 3420 cm−1 and 1342 cm−1 peaks are attributed to
Sn–OH. Peak at 1645 cm−1 and 1357 cm−1 are attributed to H-OH Sn-OH bonds [30]. Peak at 1645 cm−1 is also assigned to H-OH bonds
bonds coming from the moisture content of the powder and Sn-OH from the moisture content of the powder. 1433 cm−1 peak corresponds
bonds respectively. to bending vibrations of –CH2. Which shows that a few organic groups
The hydrophobic group R-SO +3 Na of AOT is responsible for the are absorbed on the surfaces of SnO2 nanoparticles [32] which could be
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orientation.
Energy Dispersive X-ray spectrum (EDX) analysis of the as-synthesized SnO2
nanoparticles
According to the EDX analysis shown in Fig. 4 and Table 2, the
atomic ratio of Sn to O for all the treatments agrees with tin oxide being
an n-type semiconductor with intrinsic carrier density primarily de-
termined by oxygen vacancies. The EDX (Fig. 4) shows that Sn and O
are present, which is in agreement with the XRD results [18].
Specific surface area and porosity
The specific surface areas of the samples were calculated by using
the BET method [38]. The nanoparticles exhibit specific surface areas in
the range of 18.69m2/g–88.31m2/g; pore sizes of 23.13 Å–114.93 Å;
and pore volumes of 0.01 cm3/g–0.25 cm3/g (see Table 1. The corre-
sponding N2 adsorption-desorption isotherms are given in Fig. 5. The
relatively high specific area of the 2.5 nm particles is due to their high
Fig. 7. Speci c capacitance of the synthesized SnO particles. surface to volume ratio as compare to the 11.5 nm nanoparticles.fi 2
Electrochemical characterization of the obtained SnO2 nanoparticles
as a result of combustion of the organic groups.
From the Fig. 2c, the peak at 2920 cm−1 can be attributed to CH From the Fig. 6, it can be observed that the current under the curve
stretching [33]. The peak at 3420 cm−1, 1645 cm−1, and 993 cm−1 are increased with the increasing scan rate and this was due to the reaction
assigned to Sn-OH bonds, H-OH bonds from the moisture content of the time being shorter, and voltammetric current is increased if the rever-
powder and the hydrophobic group R-SO Na+3 of AOT respectively sibility is excellent [39]. Results also show two main peaks, a broad
[34]. 644 cm−1 542 cm−1 peaks are attributed to Sn–O stretching cathodic peak and anodic peaks corresponding to redox peak of Sn4+/
modes of Sn–O–Sn. Sn3+. The electron addition process for the SnO2 semiconductor elec-
The peaks at 3405 cm−1, 1630 cm−1, and 962 cm−1 in the Fig. 2d trode can be written as [40].
are assigned to Sn-OH bonds, H-OH bonds from the moisture content of
− +
the powder. 2920 cm−1 and 2859 cm−1 are attributed to alkyl CH SnIVO e III −2 + (external) + H(solution) → Sn O(OH ) (7)
stretching [34]. 644 cm−1 and 507 cm−1 are attributed to Sn–O From Fig. 7, the specific capacitance generally decreases with in-
stretching modes of Sn–O–Sn and Sn–OH, respectively [29,35].
−1 −1 creasing scan rate. The specific decrease of capacitance with increase inFrom Fig. 2e, peaks at 644 cm and 538 cm are attributed to scan rate can be attributed to electrolytic ions diffusing and migrating
Sn–O stretching modes of Sn–O–Sn and Sn–OH, respectively.
−1 −1 into the active materials at low scan rates. At high scan rates, the dif-2935 cm and 2874 cm are attributed to alkyl CH stretching. The
−1 - −1 −1 fusion effect, limiting the migration of the electrolytic ions, causes somepeak at 3420 cm , 2343 cm 1, 1630 cm, and 962 cm are assigned active surface areas to become inaccessible for charge storage [41].
to Sn-OH bonds, H-OH bonds from the moisture content of the powder Specific capacitance of the nanoparticles was calculated from the
[30].
−1 −1 cyclic voltammetry (CV) curves in Fig. 6 using the following equation:From Fig. 2f, the peaks at 644 cm and 538 cm are attributed to
Sn–O stretching modes of Sn–O–Sn and Sn–OH, respectively. C i=
2935 cm−1
s
and 2874 cm−1 peaks are attributed to alkyl CH stretching. sm (8)
The peak at 3420 cm−1, 2343 cm-1, 1630 cm−1, and 962 cm−1 are as- where Cs is the specific capacitance, i is the average cathodic current, s
signed to Sn-OH bonds, H-OH bonds from the moisture content of the is the scan rate and m is the mass of the electrode. It is reported that the
powder [30]. factors affecting the capacitance are particle sizes and electro-chemical
conditions, some of which are type of electrolyte, concentration of
SEM micrographs of the as-synthesized SnO nanoparticles electrolyte and scan rate. Some other factors affecting the capacitance2
are surface activation under the electrochemical conditions, oxygen
The SEM micrographs in Fig. 3 clearly show different morphologies content on the surface, surface oxides and lattice defects resulting from
for SnO prepared in different surfactants. The shape of the agglomer- the method of preparation [39]. The highest capacitance recorded for2
ated sample SC looked like florets of cauliflower (nanoflower-like the SnO2 nanoparticles was estimated to be 1.6F/g at a scan rate of
structures). Nanorods were formed for samples SA and SAA. For sam- 5mV/s. The low values of capacitance recorded could be attributed to
ples prepared with CTAB (SCT and SAT), agglomerated particles were low conductivity of the SnO2 nanoparticles, which is evident from the
formed and this is because, in an aqueous system, CTAB ionizes com- high internal resistance noticed in Fig. 7. The observed specific capa-
pletely and results in cation with a tetrahedral structure. The electro- citance values are comparable with reported values of SnO2 [41].
static interaction takes place between CTA+ cations and Sn(OH)2− It can also be seen that samples with AOT (SAA and SCA) as the6
anions, the CTA+ cations condense into aggregates in which counter surfactant have higher capacitance than samples with CTAB (SAT and
ions Sn(OH)2−6 are interrelated in the interfaces between the head group SCT) as the surfactant and samples without surfactant addition (SA and
to form CTA+ – Sn(OH)2−6 pair [19,36]. AOT being an anionic surfac- SC). This could be attributed to the fact that for samples prepared with
tant, it was found that there is no direct interaction between the CTAB there was agglomeration of particles, hence the porosity de-
Sn(OH)2−6 ions and the SO−7 head of the AOT since both are similarly creased as compared with samples prepared with AOT. This is con-
charged. Both the Sn(OH)2−6 nanoparticles and micelles coexist in- firmed by the SEM micrographs from Fig. 2. Enhanced porosity allows
dividually with no significant change in the structure of the micelles for more electrolyte penetration and hence increases charge storage
[37]. The different morphology reveals the role of individual reducing [43]. Although the average crystallite size for sample SA was smaller
agents and surfactants in controlling the nucleation and crystal and as such had a larger surface area compared to the other samples
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Fig. 8. Constant Current Charge-Discharge profile of the SnO2 nanoparticles at different current densities – (a) SA; (b) SAA; (c) SAT; (d) SC; (e) SCA; (f) SCT.
Table 1, it still had a lower capacitance value recorded. This could be materials does not linearly increase with increasing specific surface
attributed to the fact that there was less electro active surface for the area. The pore size of the electrode material plays an important role in
transfer of electrons and diffusion of ions [44]. Since not all the specific the electrochemical active surface area. In addition, the intrinsic elec-
surface area is electrochemically accessible when the material is in trical resistance of the material may increase because the bonded het-
contact with an electrolyte, the measured capacitance of various eroatom (O) possesses higher reactivity, resulting in barriers to electron
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with surfactant AOT exhibited better capacitance than samples with
surfactant CTAB due to less agglomeration. This observation suggest
that the addition of surfactant AOT aids in the improvement of the
capacitance performance of SnO2 nanoparticles. The general low ca-
pacitance recorded for the SnO2 nanoparticles from the various treat-
ments can be attributed to the low conductivity of SnO2.
The results obtained indicates that Sodium borohydride acts as a
good reducing agent in producing SnO2 particles with good crystal-
linity, high surface area and good pore structure for supercapacitor
electrode material than Urea. The type of surfactant and reducing agent
used also influenced the particle size and the morphology, which in
turn influenced the capacitance of the SnO2 nanoparticles.
Acknowledgement
The authors acknowledge support from the University of Ghana
BANGA-Africa programme, the African Materials Science and
Engineering Network and the Regional Initiative on Science and
Education.
Fig. 9. The Nyquist plot for the as-synthesized SnO2 nanoparticles. References
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