J. Nig. Soc. Phys. Sci. 3 (2021) 74–81

Journal of the
Nigerian Society

of Physical
Sciences

Synthesis of SnO2/CuO/SnO2 Multi-layered Structure for
Photoabsorption: Compositional and Some Interfacial Structural

Studies

L. O. Animasahuna,b,∗, B. A. Taleatua, S. A. Adewinbia,c, H. S. Bolarinwab, A. Y. Fasasid

aDepartment of Physics and Engineering Physics, Obafemi Awolowo University, Ile Ife, Nigeria
bDepartment of Physics Electronics and Earth Sciences, Fountain University Osogbo, Nigeria

cDepartment of Physics, Osun State University, Osogbo, Nigeria
dCenter for Energy Research and Development, Obafemi Awolowo University, Ile Ife, Nigeria

Abstract

Many metal oxide heterostructures have been synthesized as mixed oxides or layered structures for photocatalytic, photodegradation of pollutants
and light-harvesting applications. However, in the layered structures the effects of interfacial properties and composition have largely not been
explored. Hence, the effects of interfacial mixing and diffusion of sandwiched thin CuO layer on optical absorption of as-deposited and heat-
treated multi-layered structured SnO2/CuO/SnO2 films were studied. The RBS analysis of the as-deposited films showed the presence of a minute
amount of Cu in the surface and bottom SnO2 layers of the structure. We attributed this to inhomogeneous layer thickness evidenced by very low
Sn/Cu atoms ratio of the CuO layer. However, the thermal treatment of the layered structure led to pronounced interlayer mixing and consequent
formation of SnO2-CuO solid solutions throughout the layered structure. The layer integrity of the inserted CuO of the as-deposited films was
very high and the as-deposited structure was far more optically absorbing. However, the annealed structure showed lesser optical absorption
because of the onset of interfacial mixing and improved crystallization. This reflected in the optical bandgap variations of the as-deposited and
annealed multilayered structures. The significance of this result is that the multi-layered films possess band narrowing – evidence of increased
photon absorption - making it a better candidate than pure SnO2 oxide for photocatalysis, photodegradation and photodetection applications. It
also pointed to the fact that attention must be paid to effects of heat treatments or annealing when inserting an absorbing layer into a photocatalyst
or a material meant for photodegradation or any light-harvesting material.

DOI:10.46481/jnsps.2021.160

Keywords: Rutherford backscattering spectroscopy, thin films, interfacial mixing, optical absorption, photocatalysis, photodegradation

Article History :
Received: 26 January 2021
Received in revised form: 12 March 2021
Accepted for publication: 26 March 2021
Published: 29 May 2021

c©2021 Journal of the Nigerian Society of Physical Sciences. All rights reserved.
Communicated by: B. J. Falaye

1. Introduction

One of the attendant drawbacks of decades of global re-
liance on fossil energy sources is the environmental pollutions

∗Corresponding author tel. no:
Email addresses: luqman.animasahun@gmail.com (L. O. Animasahun

), afasasi@cerd.gov.ng (A. Y. Fasasi)

caused exploration processes and release of toxic wastes into
the environment. Prominent among the various techniques that
have been used in solving this problem is the use of metal ox-
ides semiconductor heterostructures in photoabsorption and pho-
tocatalytic degradation of many pollutants [1-7] and detection
of harmful gases [8-11]. Metal oxides heterostructures offer

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Animasahun et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 74–81 75

several advantages such as easy band engineering, improved
charge carrier transport, increased electromagnetic photon ab-
sorption and overall better efficiencies over single oxides [1,4-
5,12-15].

Many metal oxide heterostructures have been synthesized
as mixed oxides or layered structures [2,5,6,13,15,16]. Be-
sides, a lot of these heterostructures have been used for other
applications such as electrocatalysts for fuel cells, an electron
transporting layer in perovskite solar cells, photocatalyst for hy-
drogen productions [11,15,17,18]. However, despite these ob-
served advantages, the confusion about the actual role of the
added second oxides in photocatalysis or gas sensing in many
heterostructures is yet to be cleared. The argument has been
whether the added second oxide plays catalytic or carrier trans-
port role or aids in photon absorption [17].

One way to gain insight into this problem is to know the rel-
ative positions of each constituent in the heterostructure. For in-
stance, in gas sensing, an oxide that is not in contact with the an-
alyte gas could not have performed a catalytic role. Hence, we
have demonstrated in this work the use of Rutherford Backscat-
tering Spectroscopy to investigate the composition, and thick-
ness, and interfacial mixing of the inserted CuO layer in a SnO2
/CuO/SnO2 multi-layered film (Figure 1). RBS belongs to a
cluster of techniques called ion beam analysis. It is a quantita-
tive materials characterization technique that has been excellent
for evaluating atomic densities of thin films and compositional
depth profiling of blanket and multi-layered structures [19]. It is
also capable of measuring quantitatively the interfacial mixing
in multi-layered films. All of these parameters are central to the
understanding and optimization of the properties of materials.
Nevertheless, its application to investigate laterally inhomoge-
neous samples is chiefly unexplored.

Also, we investigated the effect of copper oxide insertion
and interfacial diffusion on the optical bandgap structure of
SnO2. This is of importance to us because the optical bandgap
determines to a very great extent the electromagnetic photon
absorption properties of most materials. It predicts effectively
whether a material will absorb only in the UV region or it will
extend its absorption edge to the visible region of the e-m spec-
trum. This is of great importance in photocatalysis, photodegra-
dation of pollutants and other light-harvesting applications. Be-
sides, we examined the variations in the Urbach tail width as
a result of thermal treatment to have an insight into the disor-
der, which could also affect optical absorption, in the multilayer
film. We noted that several methods could be used in thickness
determination of films [20]. However, most of these methods
could not give compositional stoichiometry and depth profil-
ing of the films. Our choice of SnO2 as the base material is
informed by its popularity in photodegradation and gas sens-
ing applications [3,9,10,21-26]. Besides, because of its n-type
semiconducting nature, CuO – a known p-type material was se-
lected as the heterostructure pair.

We used a chemical spray pyrolysis method to obtain uni-
form and adherent films of the oxides. The spray pyrolysis pro-
cess is a simple, robust and if properly controlled yields oxide
films of high quality at rather low costs. Besides, it has the po-
tential to produce thin layers of film on varieties of substrates.

2. Materials and Methods

Multi-layered films of SnO2 and CuO on soda-lime glass
substrate were deposited via spray pyrolysis technique. The
spray solutions were prepared from analytical grade tin II chlo-
ride dihydrate salt, copper II nitrate trihydrate salt, ethanol and
distilled water. 0.1 M clear solutions of SnCl2.2H2O and
Cu(NO3)2.3H2O were prepared by dissolving appropriate mass
of each salt in 50 ml of solvent and followed by vigorous stir-
ring using a magnetic stirrer.

The soda-lime substrates were cleaned by washing in dilute
hydrochloric acid, ethanol, acetone and distilled water in that
order before drying. The deposition of the multi-layered films
was carried out using the chemical spray pyrolysis technique.
A schematic diagram of the setup is shown in Figure 2. In a
typical deposition, the substrate temperature was maintained at
350 ± 5 oC while the nozzle-to-substrate distance and tilt angle
of the spray gun was fixed at 23 cm and 45o respectively. Each
layer was deposited by spraying constant volume of the starting
solutions for a constant number of passes (to and fro movement
of the spray gun). The sequence of the layers is depicted in Fig-
ure 1. The first/bottom layer is SnO2, followed by CuO and the
topmost/surface SnO2 layer. After deposition, the sample was
annealed at temperatures between 400 - 550 oC for six hours in
a tubular furnace.

The RBS experiment was performed using the 1.7 MeV Pel-
letron Tandem Accelerator at the Centre for Energy Research &
Development, Obafemi Awolowo University, Ile-Ife, Nigeria.
For this purpose, 4He2+ ion beam was used as the projectile
ions. The scattering angle was 165o and the resolution of the
detector was 12 keV. During the process of acquiring the spec-
tra data, the energy of the ion beam used was 2.2 MeV. All the
measurements were performed at room temperature with cur-
rent varying between 20 and 60 nA at a constant charge of 20
C. All the spectra were ?tted using Windows SIMNRA software
for the estimation of composition and thickness of the ?lms.
The surface morphology was studied using Scanning Electron
Microscope at the University KwaZulu-Nata, Durban, South
Africa. Crystal structures of the ?lms were determined through
X-ray diffraction studies carried out with an X-ray Diffractome-
ter model Bruker AXS D8 Advance. The optical properties
were studied using UV–Visible – NIR Spectranet Ultraviolet
spectrometer model EP2000.

3. Results and Discussions

3.1. Compositional Study
The simulations of RBS data were done using the Windows

SIMNRA software. The result of the simulations gave the sto-
ichiometric ratio of elements, compositions of layers and the
film thickness of each sample. The RBS spectra are shown in
figures (3) and the analysis presented in the table (1). Some im-
portant features could be inferred from the RBS analysis. The
as-deposited sample has its inner/middle layer of CuO solidly
intact with a minute amount of SnO2. This is reflected in the
Sn:Cu atomic ratio as shown in Table 1. This effect is also no-
ticeable in the surface (SnO2) and bottom (SnO2) layers where

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Animasahun et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 74–81 76

Figure 1. Spray pyrolysis set-up

Figure 2. Multilayered SnO2/CuO/SnO2 films showing inter-diffusion

a small atomic per cent of Cu atoms is also present. The pres-
ence of the Sn atoms in the middle layer and Cu atoms in the
surface and bottom layers could be attributed to a lot of fac-
tors – interfacial mixing, inhomogeneous layer thickness or sur-
face roughness. Though these factors are somehow difficult to
take into consideration in most IBA simulations, recent codes
were able to incorporate these factors in RBS analysis [27]. In
the heat-treated sample, however, the embedded middle CuO
layer seems to have disappeared leaving behind multi-layered
films which are composed of essentially solid solutions of the
SnO2 and CuO phases. This could be seen in the Sn:Cu ra-
tio of the middle layer and the increase in the number of Cu
atoms in the surface and bottom layers of the sample. It could
be argued that this could also be attributed to any of the afore-
mentioned factors - interfacial mixing, inhomogeneous layer
thickness or surface roughness - however the obvious increase
in the amounts indicated the effect of thermal treatment since
both samples (as-deposited and annealed) were synthesized un-
der the same conditions and with the same parameters, any ob-
served difference in properties should be attributed to the effect
of thermal treatment on the annealed sample. Besides, the an-
nealed sample also showed an increase in thickness compared
to the as-deposited one. This we attributed to the well-known
behaviour of SnO2 and most oxides whose grains grow in sizes
when heated [28, 29].

The effects of these changes in compositions are pronounced
in the optical absorption spectra of these samples. Also, it could

result in the variations in the number of heterojunctions formed
between the grains of the copper oxide and the tin oxide in the
structure. The significance of this is that in explaining any ob-
served effects in the applications of the layered films whether in
gas sensing, energy harvesting or photocatalysis, attention must
be paid to the possibility of compositional inhomogeneities in
layers of the films. So, for instance, improved photon absorp-
tion could not possibly be the only justification/reason for im-
proved photocatalytic behaviour for a surface that contains two
catalytically active oxides. Summarily, the power of RBS in
detecting a minute amount of interfacial diffusion and mixing
is crucial in gaining insight into the understanding of observed
effects and possible optimization of materials properties.

The surface morphology of the thin film structure of the
annealed deposited SnO2/CuO/SnO2 multilayer is investigated
using the SEM micrographs presented in Figure 4. To ade-
quately assess its surface morphology, the micrographs are rep-
resented at various magnifications. The low magnification im-
age shows networks of nanowalls that have grown with a nest-
like structure. The nest-like structures were made up of a net-
work of nanowalls that were non-uniform and interconnected.
In terms of a vapour-solid model, the formation of such a spe-
cific structure has been explained [30]. Generally, precursor
decomposition in spray pyrolysis involves four real-time impor-
tant processes - residual solvent evaporation, droplet spreading,
the formation of precipitate and vaporization, and salt decom-
position – which precede and affect the formation of the film
when aerosol droplets from the spray gun hit the surface of
the heated substrate. The elevated temperature around the sub-
strate caused the aerosol droplets to evaporate before reaching
the substrate vicinity. Hence, the precipitate formation would
occur as early as possible. As soon as the precipitates reached
the heated substrate, they were converted to the vapour phase
and then undergo a sequential heterogeneous reaction process

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Animasahun et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 74–81 77

Figure 3. RBS Spectrum of (a) as-deposited and (b) annealed (at 500 oC) layered SnO2/CuO/SnO2 sample

Table 1. Results of the analysis of RBS data showing the compositions, layer thickness and Sn/Cu atomic ratios of the as-deposited and films annealed at 500 0C
Layers Compositions

As-deposited/Annealed@500 0C
Sn/Cu Ratios
As-deposited/Annealed@500 0C

Thickness (nm)
As-deposited/Annealed

Surface Sn0.1200.84Cu0.04 / Sn0.1200.81Cu0.07 3.00/1.71 380/400
Middle Sn0.0800.40Cu0.52 / Sn0.0700.84Cu0.09 0.15/0.88 140/230
Bottom Sn0.1000.88Cu0.02 / Sn0.0800.87Cu0.05 5.00/1.60 500/550

starting with diffusion and adsorption of reactant molecules to
the surface of the substrate. These are followed by surface dif-
fusion and incorporation of molecules into the lattice and fi-
nally desorption and diffusion of the product molecules from
the surface of the heated substrate. For that reason, during the
beginning of the growth, a templating/seeding effect occurred
directly on the substrate surface due to self-nucleation, follow-
ing a high substrate temperature (360 oC) regime. After the self
– nucleation of the seeds, heterogeneous nucleation and energy
of the product molecules favoured the arrangement of product
molecules into the observed nest – like morphology. The high
density of both SnO2 and CuO nuclei on the substrate surface
easily coalesced into the growth of SnO2/CuO/SnO2 nanostruc-
tures during the SnO2/CuO/SnO2 nucleation and growth phase.
At higher magnification Figure 4(b), interaction between each
interface of the films is suspected. For our SnO2/CuO/SnO2
structure networks, the nanowall heights and widths have been
found to be in the submicron region. Our parameters led to a
classical CVD – like process that has been known to yield high
sticking probability and quality films.

3.2. X-ray Diffraction Analysis

The X-ray diffraction patterns of the samples (shown in Fig-
ure 5) were studied to confirm the crystallinity of the samples
and investigate the crystal structures of the as-deposited and
heat-treated samples. The analysis of dominant peaks using
Scherrer’s formula confirmed the presence of polycrystalline
SnO2 and CuO phases. The SnO2 is tetragonal rutile (cassi-
terite) and CuO is tenorite in nature according to crystallogra-

phy open database (COD) reference entry no: 96-100-0063 and
entry no: 96-101-1195, respectively [13,26]. Besides the in-
creased intensities in the peaks of the annealed sample which
implies improved crystallization, the XRD experiment did not
reveal any new phases of the oxides even after the heat treat-
ments. The orientation of the dominant peak remained the same
indicating that the film is textured and corroborating our asser-
tion that the annealing of the multi-layered films only led to
interlayer mixing and diffusion of the oxides rather than the
formation of a new chemical compound. Moreover, the posi-
tions of the peaks also agreed well with other reported works
[31-33].

3.3. Films’ Optical Transmittance and Band Gaps

The transmittance data obtained from the UV – Vis spec-
trophotometer at the CERD were analyzed to investigate the
samples’ optical energy band structures. The transmittance and
Tauc’s plots showing the bandgap determination and the effect
of annealing temperature are presented in Figures 5(a and b).
Figure 5(a) presents the normalized transmittance of the as-
deposited and annealed multi-layered films. The as-deposited
film was far more absorbing over the visible region than any of
the annealed films. This could be attributed to the high trans-
mittance of SnO2 and the absorbing nature of CuO. When the
film has not been annealed, the copper layer absorbed the trans-
mitted light by the SnO2 layer because the diffusion of the CuO
in SnO2 is still low. However, after the film was annealed the
films become less absorbing because of improved crystalliza-
tion and onset of diffusion of the absorbing CuO atoms in the

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Figure 4. SEM micrographs showing surface features of annealed layered SnO2/CuO/SnO2

Table 2. Estimated values of optical bandgap and sub-bandgap
Sample Optical bandgap (eV) Urbach Energy Eu (eV)
As-deposited 2.75 1.37
Annealed @ 400 oC 3.45 1.92
Annealed @ 450 oC 3.47 1.92
Annealed @ 500 oC 3.49 1.76

Figure 5. XRD pattern of layered SnO2/CuO/SnO2

SnO2 matrix. In addition, adsorbed surface impurities which
could have emanated from the deposition process, hinders the
transition of light beam and might have been wiped off as a
result of the heat treatments. This consequently reduces the
absorption effect. The absorption coefficient α was generated
from equation (1) using the transmittance data (assuming the
films’ reflectance is negligible) and film thicknesses obtained
from the RBS analyses.

α =
1
d

ln
1
T

(1)

where d is the film thickness and T is the normalized transmit-
tance. Optical energy band gaps for allowed direct and indirect
transition were evaluated using the Tauc’s relation in equation
(2) assuming that the valence band and the conduction band are
parabolic.

(αhυ)1/n = A
(
hυ− Eg

)
(2)

where hυ is the photon energy, n is a power factor that deter-
mines the nature of transition, A is a band tail constant and Eg
is the optical bandgap. n will take values of 1/2, 3/2, 2 and 3 for
direct allowed, direct forbidden, indirect allowed and indirect
forbidden transition respectively.

Tauc’s method of plotting (αhυ)1/n against hv was adopted
to evaluate our deposited films’ optical band gaps. The linear
section of the resulting curve was then extrapolated to the pho-
ton energy, hv axis which then gives the bandgap energy, Eg
value. The plots are presented in Figure 5(b) and the films’ es-
timated optical band gaps values are reported in Table 2. The
results revealed the increase in the bandgap of the annealed film
compared to the as-deposited film reflecting the observed fea-
tures of the transmittance spectra. Also, in Figure 5(c), it was
observed that there is a wide disparity (increase) between the
bandgap of the as-deposited film and the film annealed at 400
oC. However, beyond 400 oC the multi-layered films showed a
very negligible change in bandgap up to maximum annealing
temperature of 550 oC. Of particular importance to us is the be-
haviour of the layered films compared to the well-known prop-
erties of pure SnO2. For all values of annealing temperature

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Figure 6. Figure 6: Plots showing (a) transmittance versus wavelength and (b) Tauc’s plot for optical bandgap estimation

Figure 7. Plot for estimation of sub-bandgap (Urbach energy)

used, there was bandgap narrowing compared to the reported
values between 3.5 – 4.0 eV [34,35] for pure SnO2 even though
the annealed films displayed higher band energies. This is ev-
ident that our deposited samples exhibit direct allowed photon
energy transition. It also corroborates our earlier result showing
that annealing has led to increased transmittance as a result of
diffusion and/or mixing of the absorbing copper oxide layer and
better crystallization. Incidentally, the optical bandgaps of the
annealed layered films also coincide with the reported values
for the copper doped tin oxide and SnO2-CuO mixed or com-
posite oxides. This corroborated the RBS analysis that after the
heat treatment, the layers of the films essentially become mixed
or solid solutions of the two constituent oxides. Band narrow-
ing in the multi-layered film can be attributed to its increased
photon absorption making it a better candidate than single layer
SnO2 for photocatalysis, photodegradation and photodetection

applications. Our results also demonstrated the influence of
heat treatments on the material’s properties suitable for these
applications. The thermal treatment led to decreased photon
absorption compared to the as-deposited film as evidenced in
bandgap behaviour. This would have profound effect on the
photocatalysis and photodegradation ability of the multilayered
films. Hence, in designing multilayered structures for light ab-
sorption applications, optimum annealing temperature should
be chosen such that integrity of each layer would be maintained
with little or less interfacial diffusion or mixing.

3.4. Urbach Energy
While it is has been established that optical bandgap gives

fundamental pieces of information about band to band absorp-
tion properties of materials. However, beyond the lower limit
value of bandgap, the structural characteristics of a material
such as lattice distortions or disorder are capable of produc-
ing extended localized states in the energy gap of such mate-
rials leading to sub-bandgap absorption edge commonly called
Urbach energy, Eu. In this lower photon energy range, the spec-
tral dependence of the absorption edge follows the empirical
Urbach rule given by equation (3) [1,33] where αo and Ec are
material constants, E is the photon energy and Eu denotes the
width of the tail of localized states in the bandgap (Urbach en-
ergy)

∝ (E) = αo exp
(

E − Ec
Eu

)
(3)

The Urbach energy Eu was estimated by calculating the inverse
of the slope of the linear fit obtained by plotting ln(α) against
the photon energy E. The graphs are shown in Figure 7 and the
results are presented in Table 2. Urbach energy can be seen as
a measure of the energetic sharpness of the optical band edge.
Usually, lower values of Eu suggest a high structural quality
film or material which also indicates good electronic proper-
ties such as high carrier mobility [33]. As shown in Table 2,

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the sub-bandgap for all the films was small indicating the good
quality of the films. But most importantly the values of the
Urbach energy also justify the RBS results that annealing the
multi-layered films led to solid-state diffusion in their structure
and consequently lattice disorder which resulted in the slight
increase in the Urbach energies of the annealed. Though one
would expect that annealing the film should lead to improving
crystallization and better organization of the lattice and there-
fore lower Urbach energy, it seems the effect interlayer mixing
dominated that of the crystallization and hence the increase in
Urbach energy.

4. Conclusion

The thickness and composition of multi-layered SnO2 /CuO
/SnO2 composite thin films were investigated using the RBS
method. The RBS analysis of the films showed that the cop-
per oxide layer in between the two SnO2 diffused into the tin
oxide matrix after annealing with minute amount reaching the
surface layer and that the annealed sets of the same composition
increased in thickness compared with the as-deposited sample.
The bandgap for the as-deposited and annealed films was eval-
uated. It was discovered that copper oxide addition led to band
narrowing and indirect band type as against the direct band type
of tin IV oxide. This work gave the evidence of the suitability
of copper oxide in narrowing the bandgap of tin oxide which is
necessary for visible light photocatalysis and light-harvesting
applications. It also demonstrated the applicability of the RBS
in the study of solid-state diffusion in semiconductor materials.

Acknowledgments

We thank the anonymous referees for the positive enlight-
ening comments and suggestions, which have greatly helped us
in making improvements to this paper.

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