Sn-doped TiO2 nanotubular thin film for photocatalytic degradation of methyl orange dye

Article abstract of DOI:10.1016/j.jpcs.2020.109609

We fabricated Sn-doped TiO₂ nanotubular film via annealing of anodized TiO₂ nanotubes grown on F–SnO₂ (FTO) glass. Annealing was carried out at 500 °C in ambient air. Anatase crystal structure was achieved with no change in nanotubular morphology in respect to as-anodized amorphous TiO₂ nanotubes. The X-ray photoelectron spectroscopy analysis revealed Sn on the surface of TiO₂ film, following the thermal treatment, probably caused by the diffusion from FTO glass. Depth profile examination of the film chemical composition was conducted by elastic recoil detection analysis, which showed that in addition to the diffusion of Sn from FTO, diffusion of Ti to FTO concurrently occurred. Thus, a higher concentration of Sn was found at the bottom of the tubes, while a lower concentration was present on the tubes’ surface top. This explains the improved optical response revealed by a diffuse reflectance spectroscopy. The absorption enhancement demonstrated that Sn-doped TiO₂ film was efficient in the degradation of methyl orange dye under visible light.

Full text of DOI:10.1016/j.jpcs.2020.109609

Journal Pre-proof
Sn-doped TiO nanotubular thin film for photocatalytic degradation of methyl orange
2
dye
Andjelika Bjelajac, Rada Petrović, Jelena Vujancevic, Katerina Veltruska, Vladimir
Matolin, Zdravko Siketic, George Provatas, Milko Jaksic, George E. Stan, Gabriel
Socol, Ion N. Mihailescu, Djordje Janaćković
PII:
S0022-3697(19)32852-5
https://doi.org/10.1016/j.jpcs.2020.109609
PCS 109609
DOI:
Reference:
To appear in: Journal of Physics and Chemistry of Solids
Received Date: 24 December 2019
Revised Date: 2 June 2020
Accepted Date: 3 June 2020
Please cite this article as: A. Bjelajac, R. Petrović, J. Vujancevic, K. Veltruska, V. Matolin, Z. Siketic, G.
Provatas, M. Jaksic, G.E. Stan, G. Socol, I.N. Mihailescu, D. Janaćković, Sn-doped TiO nanotubular
2
thin film for photocatalytic degradation of methyl orange dye, Journal of Physics and Chemistry of Solids
(2020), doi: https://doi.org/10.1016/j.jpcs.2020.109609.
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Andjelika Bjelajac: Conceptualization, Formal analysis, Investigation, Writing - Original
Draft, Visualization, Funding acquisition
Rada Petrović: Writing - Review & Editing, Supervision
Jelena Vujancevic: Formal analysis, Writing - Review & Editing, Visualization
Katerina Veltruska: Formal analysis, Investigation, Writing - Review & Editing
Vladimir Matolin: Data Curation, Funding acquisition
Zdravko Siketic: Formal analysis, Investigation, Writing - Review & Editing
George Provatas: Formal analysis, Investigation
Milko Jaksic: Data Curation, Funding acquisition
George E. Stan: Formal analysis, Investigation, Writing - Review & Editing
Gabriel Socol: Investigation, Formal analysis, Writing - Review & Editing
Ion N. Mihailescu: Writing - Review & Editing, Funding acquisition
Djordje Janaćković: Project administration, Funding acquisition

Sn-doped TiO2 nanotubular thin film for photocatalytic
degradation of methyl orange dye
b
c
d
Andjelika Bjelajac a,*,1, Rada Petrović , Jelena Vujancevic , Katerina Veltruska ,
d
e
e
e
Vladimir Matolin , Zdravko Siketic , George Provatas , Milko Jaksic , George E.
Stan f, Gabriel Socol g, Ion N. Mihailescu g, Djordje Janaćković b
a University of Belgrade, Innovation center of Faculty of Technology and Metallurgy,
Karnegijeva 4, 11000 Belgrade, Serbia
b University of Belgrade, Faculty of Technology and Metallurgy, Karnegijeva 4,11000
Belgrade, Serbia
c
Institute of Technical Sciences of SASA, Knez Mihailova 35/IV, 11000 Belgrade,
Serbia
d
Charles University in Prague, Faculty of Mathematics and Physics, Department of
Surface and Plasma Science, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
e Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia
f
National Institute of Materials Physics, Laboratory of Multifunctional Materials and
Structures, Atomistilor 405A, RO-077125 Magurele, Ilfov, Romania
g
National Institute for Lasers, Plasma, and Radiation Physics, Lasers Department,
“Laser-Surface-Plasma Interactions” Laboratory, RO-77125 Magurele, Ilfov, Romania
Abstract
We fabricated Sn-doped TiO2 nanotubular film via annealing of anodized TiO2
nanotubes grown on F-SnO2 (FTO) glass. Annealing was carried out at 500 °C in
ambient air. Anatase crystal structure was achieved with no change in nanotubular
morphology in respect to as-anodized amorphous TiO2 nanotubes. The X-ray
photoelectron spectroscopy analysis revealed Sn on the surface of TiO2 film, following
the thermal treatment, probably caused by the diffusion from FTO glass. Depth profile
examination of the film chemical composition was conducted by elastic recoil detection
analysis, which showed that in addition to the diffusion of Sn from FTO, diffusion of Ti
to FTO concurrently occurred. Thus, a higher concentration of Sn was found at the
bottom of the tubes, while a lower concentration was present on the tubes’ surface top.
This explains the improved optical response revealed by a diffuse reflectance
spectroscopy. The absorption enhancement demonstrated that Sn-doped TiO2 film was
efficient in the degradation of methyl orange dye under visible light.
*Corresponding author: andelika.bjelajac@polytechnique.edu (A. Bjelajac).
1Present address: LPICM, Ecole Polytechnique, CNRS, IP Paris, 91128 Palaiseau,
France

Keywords: Thin films; Nanostructures; Photoelectron spectroscopy
1. Introduction
Titanium dioxide (TiO2) is a semiconductor with numerous applications based upon its
remarkable photoactivity [1–4]. It absorbs UV light and generates electron–hole pairs
that can be useful for degradation of organic substances in polluted water systems [5,6].
Titanium dioxide is widely used since it is cheap, abundant and easy to process [7]. One
approach to enhance its photocatalytic performance is nanostructuring of TiO2, which
provides high specific surface areas and an increased number of photoactive sites [7,8].
The widely used anodization technique for obtaining highly ordered TiO2 nanotubes
(NTs) [9] is superior to other methods, since it provides self-organization of the NTs
that are perpendicular to the substrate which makes the exploitation of the material’s
high specific surface enhanced [10]. Other explored methods for synthesis of TiO2 NTs,
such as hydrothermal or solvothermal [11], result in a random orientation on the
substrate. Moreover, specific synthesis requirements, such as high pressure, high
temperature and long reaction time, make them disadvantageous compared to the
anodization technique [12]. Differing to TiO2 thin films obtained by anodization, any
catalyst in powder form requires filtration of the system for removal of the
photocatalyst. The major benefit of anodized TiO2 NTs for photocatalytic applications
is the orthogonal separation of charge carriers due to their 1D structure, meaning that
electrons and holes are spatially separated within the tube wall. This means that
electrons are collected in the wall center and transported to the back contact, while holes
are driven to the wall–solution interface [13]. In this study, we explored the use of Ti
thin film applied on a conductive substrate, such as F-SnO2 (FTO) glass [14] for the
fabrication of photocatalytic nanotubular TiO2 thin film. Because as-synthetized NTs are
amorphous, annealing (>300 °C) is required for conversion to crystalline structure [15].
Indeed, amorphous TiO2 includes structural defects and exhibits poor photocatalytic
activity. The TiO2 has three main types of crystal structures: anatase, rutile and brookite
[16]. Among them, anatase possesses the best photocatalytic property due to long
electron lifetime [17]. When annealed at 400 °C for 2 h, the amorphous TiO2 films are
completely converted to nano-crystalline anatase phase. When increasing calcination
temperature, photocatalytic activity rises due to the formation of anatase TiO2 and
improvement of its crystalline quality [18]. With further increase of calcination
temperature from 600 to 800 °C, the photocatalytic activity rapidly decreases due to
vanishing of the anatase phase, collapse of NT structures and decrease of active surface
area [15].
In a previous study, we noted that the diffusion of Sn from FTO to TiO2 can be
promoted by annealing [19]. The aim of this work is to intensify this effect in order to
obtain Sn-doped TiO2 thin film that can then be used as an improved photocatalyst with
respect to undoped TiO2 [20]. It is well known that doping of TiO2 leads to generation of

inter-band levels in energy gap of TiO2 and/or band gap narrowing [21]. Herein, we
investigated the nature of Sn incorporation using X-ray photoelectron spectroscopy
(XPS) and surveyed the atomic content into the depth of the film using Time-of-Flight
Elastic Recoil Detection Analysis (TOF-ERDA). Scanning electron microscopy (SEM)
was used to monitor possible morphological changes due to the annealing. The phase
composition of the film was investigated by grazing incident X-ray diffraction
(GIXRD). The optical properties of the film before and after annealing were determined
by measuring their diffuse reflectance spectra (DRS). The photocatalytic activity of the
doped film was studied by assessing the degradation of methyl orange (MO) dye.
2. Experimental
Radio frequency magnetron sputtering (RF-MS) technique was employed for the
deposition of pure titanium (Ti) thin films onto FTO glass (PI-KEM Ltd, 200 nm FTO
film, 12–14 Ω/cm2). A Ti target (Alfa Aesar GmbH) was used in experiments. Prior to
deposition, the FTO substrates were successively cleaned by ultrasonication in acetone,
ethanol and deionized water. After drying they were mechanically set inside the
deposition chamber, at 40 mm from the target. Films were sputtered using a Cesar RF
Power Generator equipped with a Dressler RMC-1 Matching Controller (13.56 MHz)
deposition system with a magnetron cathode with a plasma ring of ~50 mm diameter.
The sputtering chamber was first evacuated down to a residual pressure of ~2 × 10−4 Pa.
Then pure argon was admitted into the reactor chamber at a constant gas flow rate of 5
sccm. The sputtering was carried out for 1 h at a working pressure and electrical power
of 0.5 Pa and 60 W, respectively.
The anodization of the sputtered Ti films was performed in ethylene glycol containing
0.3 wt% ammonium fluoride and 2 wt.% water. A platinum plate served as a cathode.
The electrodes were placed 20 mm apart and voltage was set at 60 V. The anodization
was stopped when reaching the transparency of the films (~10 min), indicating that all
Ti was oxidized. Next, the samples were well rinsed with water and left to dry in air for
24 h. Subsequently, the dried samples were annealed in air at 500 °C for 2 h.
Surface morphology of the samples was studied using a Tescan Mira X3 FESEM.
Transmission electron microscopy (TEM) imaging was performed using an image
corrected FEI TITAN TEM operating at 300 kV. For TEM analysis the surfaces of the
samples were scratched with a diamond wire and fragments collected by dragging the
carbon grid over the scratched surface. The DRS were recorded using a Shimadzu 2600
UV-Vis spectrophotometer with an integrating sphere attachment within wavelength
range 300–800 nm.

The crystal structure of the samples was investigated by GIXRD using a Rigaku
SmartLab 3 kW system with CuKα radiation (λ = 1.5418 Å), operated at 40 kV and 40
mA at room temperature. The incidence angle was set at 0.25° (to maximize the signal
originating from TiO2 NT arrays), and the scattered intensity was scanned in the 2θ
range of 20–50° with a step size of 0.04° and scan speed of 0.25°/min (dwell time of 9.6
s).
The XPS analyses were performed using a 1253.6-eV MgKα beam source to check the
overall chemical composition of the samples. The charging of spectra was corrected
according to adventitious carbon with BE at 284.8 eV. The surface cleaning was done
by applying 2 keV of Ar+ ionic etching for 10 min. The XPS spectra were analyzed by
fitting the Shirley-type function (i.e. non-linear background) and a sum of Voigt
functions, using KolXPD software.
The elemental depth profiles of the films were determined using TOF-ERDA.
Measurements were performed using a 23-MeV 127I6+ beam, with 20° incidence angle
toward the sample surface, and TOF-ERDA spectrometer positioned at 37.5° in regard
to the beam direction. More details about the TOF-ERDA spectrometer (and analytical
method) can be found in refs. [22,23]. Data analysis was carried using simulation code
Potku [24] and Monte Carlo (MC) code CORTEO [25].
Photocatalysis was evaluated by following the degradation of MO dye (5 mg/L) by
immersing the TiO2–FTO photocatalyst in 10 mL of dye solution and illuminating the
system with a solar simulator lamp, with the sample 100 mm away from the light
source. The first 30 min was considered as the time required for reaching adsorption–
desorption equilibrium because a prolonged period of darkness did not lead to any
change in dye concentration. The sampling was done at different time intervals and the
dye concentration was inferred by measuring the absorbance of the solution on a
Shimadzu 1800 UV-Vis spectrophotometer. The influence of photolysis was also
investigated by following the dye degradation upon illumination without the TiO2
photocatalyst.
3. Results and discussion
Surface morphology changes due to annealing were studied by comparing the FESEM
micrographs collected before and after the thermal treatment. A careful analysis of the
images showed that the annealing did not affect the NT morphology. Thus, Fig. 1a–c
presents the review of only the as-anodized TiO2 NTs, as there were no changes due to
annealing. The NTs were ~60 nm wide, ~30 nm thick and ~500 nm long. Some TEM
and HRTEM micrographs of the annealed sample are also given in Fig. 1d–f in order to
present more clearly the structure and crystallinity of NTs.

Fig. 1. FESEM and TEM micrographs of (a–c) as-anodized and (d–f) annealed TiO2
nanotubes.
Phase composition of the annealed TiO2–FTO sample vs. FTO substrate was
investigated by GIXRD (Fig. 2). In addition to the diffraction maxima of the FTO
substrate (a SnO2-like phase, e.g. ICDD: 00-041-1445), the TiO2 sample featured 101
and 200 reflections of anatase. The other anatase maxima (ICDD: 00-021-1272),
peaking in this angular region, were superimposed by the higher intensity reflections of
the substrate. Only the (103) crystal plane reflection was hinted by the slender shoulder
appearing to the left-hand side of the prominent FTO substrate peak.
Crystalline coherence length (“crystallite size”) was inferred from the full-width at half
maximum of the 101 diffraction line of anatase, using the Scherrer equation [26]. The
line width was corrected for instrumental broadening using a corundum standard
reference (NIST SRM 1976). The crystallite size for 101 anatase reflection of annealed
TiO2 was ~45 nm.

Fig. 2. Comparative GIXRD patterns of the bare FTO-glass substrate, and annealed
TiO2 film deposited onto FTO-glass substrate. For comparison, the ICDD: 00-021-1272
reference file of TiO2-anatase is presented in the bottom graph together with ICDD: 00-
041-1445 for SnO2 phase.
In order to examine in more detail the annealing influence of TiO2 film on FTO glass,
the XPS analyses were performed on the amorphous and annealed TiO2 films before
Ar+ ionic etching. The O 1s line was fitted and the results are presented in Fig. 3
together with Table 1 where the positions of the peaks are given.
Fig. 3. XPS O 1s line for (a) amorphous and (b) annealed TiO2 film, before Ar+ ionic
etching.
Peak 1 at ~530 eV was attributed to the lattice oxygen bound to Ti4+, whereas peak 2 at
~531 eV corresponded to oxygen from surface OH groups [27] and bound to Ti3+ ions.
Peak 3 at ~532 eV most probably originated from surface H2O groups [27]. We cannot
exclude, however, the contribution of oxygen bonded to C as a surface impurity. The

small shift of the peak 1 position to lower binding energy as a consequence of the
annealing indicated a change in O environment. Moreover, the intensity of peak 2
decreased after annealing, which can mean that the amount of Ti3+ ions was diminished
and consequently the number of oxygen vacancies decreased. This is supported by
spectra of Ti 2p (Fig. 4). The presence of Ti3+ ions and oxygen vacancies is
characteristic of nonstoichiometric TiO2, with nonstoichiometry increasing as the
amount of atmospheric oxygen decreased. After annealing in oxygen atmosphere, the
nonstoichiometry decreased and so did the intensity of peak 2. The decrease of peak 2
intensity is also indicative of removal of OH groups on the surface.
Table 1 The position of O 1s line obtained by XPS analyses of TiO2 films.
Sample
Position of peak 1 Position of peak 2 Position of peak 3
(eV)
(eV)
(eV)
Amorphous TiO2
Annealed TiO2 before
Ar+ ionic etching
530.08
529.97
531.08
530.86
532.36
532.01
After deconvolution of the Ti 2p line for the amorphous TiO2 film (Fig. 4a), the four
peaks were revealed: 464.39 eV (2p1/2) and 458.66 eV (2p3/2), which indicates the Ti4+
state; and 463.78 and 457.08 eV are associated with the Ti3+ state [27]. After annealing
(Fig. 4b) the two peaks at 457.08 and 463.78 eV disappeared, demonstrating the
decrease in oxygen vacancies. The two main peaks of the Ti4+ state did not change
position (464.36 eV for 2p1/2 and 458.66 eV for 2p3/2).
Fig. 4. XPS Ti 2p line for (a) amorphous and (b) annealed TiO2 film, before Ar+ ionic
etching.
Chemical composition of the annealed TiO2 film was assessed by XPS analysis. The
compositional results obtained on the as-received sample (before Ar+ ion etching) and
after 10 min of Ar+ ion etching are presented in Table 2. In both cases, Ti, O, Sn and C
were detected. There was also a small amount of N in the case of the as-received
sample. This could be N originating from surface weakly bonded species and would not
affect the intrinsic properties of the material. The presence of C can also be attributed to

surface contaminants since its amount decreased from 18.15 to 9.45 at.% after Ar+ ionic
etching.
Table 2 Chemical composition of the annealed TiO2 film determined by XPS before
and after ionic etching.
Chemical composition (at.%)
Before Ar+ ionic etching
After Ar+ ionic etching
O
C
Ti
Sn
N
1.40
-
58.05 18.15
64.9 9.45
22.26
25.35
0.15
0.30
Presence of a certain quantity of Sn both before and after Ar+ ionic etching should be
emphasized. It is likely that Sn was incorporated in the TiO2 lattice. Indeed, as already
reported, at higher temperatures (>500 °C), Sn4+ from FTO glass diffuses into TiO2 by
substituting the Ti4+ ion [29]. To check this hypothesis, XPS analyses of Sn 3d core
electron level were done (and compared) for the bare FTO-glass substrate and the
annealed TiO2 film–FTO glass (Fig. 5).
The Sn 3d line of FTO glass was fitted (Fig. 5a) as a doublet at 486.75 eV (3d5/2) and
495.13 eV (3d3/2), having the spin orbit separation of Δ = 8.38 eV, which is
characteristic of Sn4+ in pure SnO2 [30]. In the case of annealed TiO2 film, these
positions were slightly shifted to lower binding energies: 486.49 and 494.94 eV, and Δ
= 8.45 eV (Fig. 5b). This means that Sn4+ was also present in the annealed TiO2 film,
since the positions of the characteristic peaks for Sn2+ were at significantly lower
energies (484.7 eV for 3d5/2) [31]. It should be noted that for the as-anodized sample,
the content of Sn was at the detection level limit, corresponding to 0.03 at.%, and peaks
with BE of 486.83 and 495.32 eV were very close to those of FTO positions. The small
shift of Sn 3d5/2 and 3d3/2 positions in annealed TiO2 film to lower binding energies in
regard to FTO glass can be accounted for by the increase of electron cloud density
around Sn4+ ions due to higher electronegativity of Sn (1.96) vs. Ti (1.54). Indeed, the
bigger the electron density around an ion is, the larger is the shift of binding energy
positions toward lower energies. Particularly, Sn4+ ions in the TiO2 structure attract
electrons more than the Ti4+ ions do, leading to the shift of Sn 3d5/2 and 3d3/2 positions to
lower binding energies compared to the position for pure SnO2. Such shifts are
indicative of a change in the surface electronic states, which ultimately can result in
unique surface reactivity [32].

Fig. 5. XPS Sn 3d line for (a) FTO glass and (b) the annealed TiO2 film, before Ar+ ion
etching
Because the XPS technique is only surface sensitive, depth profile composition was
investigated by ToF-ERDA. The atomic contents of the elements of interest (Ti, O and
Sn) showed an overlapping of the depth profiles for Sn and Ti in the range of 750–1300
at./cm2 (Fig. 6). This indicates that potential diffusion of Sn from FTO occurred
simultaneously with potential diffusion of Ti to FTO. Similar observations were
previously reported for bilayer TiO2–SnO2 composite [29]. As expected, the Sn
concentration was higher in the vicinity of SnO2 film and gradually decreased toward
the TiO2 domain. The same trend was observed in the case of Ti diffusion to SnO2. It
was essential to combine XPS and ToF-ERDA methods to obtain a detailed insight
concerning chemical composition of the sample. The XPS provided highly precise
measurements of the surface, showing 0.15 at.% of Sn at the surface before ionic
etching, whereas ToF-ERDA showed that the Sn concentration at the surface was 0 –
thus the Sn concentration was much lower that the Ti and O concentrations, estimated
as 33 and 67%, respectively.

Fig. 6. Atomic content in depth of TiO2 annealed film. Solid line-depth profile
calculated by MC code CORTEO [22]. Dashed line-depth profile calculated using
software Potku [21] (slab analysis).
It should be noted that the analyzed TiO2 layer consisted of hollow NTs (non-
homogeneous film, Fig. 1), while the depth scale in Fig. 6 represents only the average
thickness of the TiO2 film as “seen” by the ion beam probe. Depth profile steps in Fig. 6
are equal to 190 × 1015 at./cm2, corresponding to ~20 nm (for the pure anatase TiO2 with
density of 4.23 g/cm3). Furthermore, additional energy straggling of incoming and
recoiled ions occurred, producing the broadening of measured energy spectra at the
TiO2–SnO2 interface. Unfortunately, this extra contribution in energy straggling cannot
be simulated even by MC code. Thus the depth profile in Fig. 6 is a convolution of
diffusion between the layers and energy straggling through the non-homogeneous
media.
In order to further investigate the effect of annealing on optical properties of the films,
DRS spectra were recorded and compared (Fig. 7). The absorption edges were taken
from the intersection of the tangent lines and a parallel to the x-axis that corresponded
to the reflectance. The inferred values were λ = 413 and 425 nm for the as-anodized and
annealed samples, respectively. The corresponding band gap energies, Eg, were
calculated as hc/λ, where h is Planck’s constant and c is the speed of light, and the Eg
values were 3.0 and 2.92 eV for the as-anodized and annealed samples, respectively.
Because the characteristic absorption edge for anatase phase is 387 nm, we can deduce
that in both cases there was a red shift of absorption. In the case of the non-calcinated
sample, this could be due to N incorporation from NH4F [33], but since the absorbance
red shift was larger for the annealed sample, this was likely a consequence of Sn
diffusion from FTO glass, as already evidenced by XPS analyses.
Fig. 7. DRS spectra of TiO2 film (a) before and (b) after annealing.
The photocatalytic activity of TiO2 films was studied by investigating the degradation of
MO dye over time. No photocatalytic effect was recorded for the amorphous sample,
even though a red shift of TiO2 was observed. Most probably this was due to high
recombination of the photoinduced charges, as expected in the case of amorphous

structures. We therefore present here only the results for the annealed TiO2 (Fig. 8). The
influence of photolysis was also investigated by monitoring the dye degradation upon
illumination without the TiO2 photocatalyst. Since there was no change in absorbance
even after 6 h, the possibility of photolysis should be excluded (inset of Fig. 8a).
Fig. 8. (a) Absorbance spectra of methyl orange (MO) dye in the presence of annealed
TiO2 film upon solar illumination. Inset: results of MO photolysis in the absence of the
catalyst during 6 h. (b) Efficiency of MO photodegradation as a function of time. Inset:
linear transformation of photocatalytic data in order to determine k.
The absorption spectra of MO after visible light exposure is given in Fig. 8a, and the
kinetic behavior of MO decomposition by the annealed TiO2 film is shown in Fig. 8b.
According to Fig. 8b and the straight line of function ln(A/Ao) on time t, it can be
concluded that this photocatalytic reaction followed a pseudo-first order reaction:


A




ln
= −kt
(1).
A0


where A0 is initial absorbance of MO dye after adsorption–desorption equilibrium is
reached, A is absorbance of dye at particular time t during the experiment and k is the
pseudo-first order constant; k is calculated from the slope of the linear part and is
0.0453 h−1. The reason for low value of k might be due to the clogging of the NTs with
the gas molecules produced during the photodegradation of MO (CO2 and H2O) [34],
since there was no stirring applied during the process. A well-designed agitated cell is
necessary to further address this issue.
We believe that not only the extended absorption in the visible part favored the
photocatalytic activity of the annealed TiO2, but also that the charge recombination was
reduced due to the TiO2–SnO2 interface. The presence of the TiO2–SnO2 interface is
known to be beneficial for photocatalysis compared to the two-layered independent
structure [29]. Photogenerated electrons are transferred from the conduction band of
TiO2 to that conduction band of SnO2 which is lower than that of TiO2. In other words,
SnO2 acts as a sink to collect electrons from TiO2. On the other side, holes migrate in
the opposite direction, from SnO2 to TiO2. This improves charge carrier separation and

reduces recombination [35]. Actually, Talinungsang et al. observed that the two-layer
TiO2–SnO2–glass exhibited a higher degradation rate of methyl blue than just a
monolayer deposited on a glass such as TiO2–glass or SnO2–glass [36]. Moreover,
Cheng et al. [37] reported that the SnO2–TiO2 core-shell structure grown on indium
doped tin oxide (ITO) glass had better photocatalytic performance than just the flat TiO2
film deposited on ITO. The electron–hole pair separation was significantly improved for
the SnO2–TiO2 core-shell structure due to the higher work function of SnO2 than ITO.
Duan et al. [38] investigated the influence of Sn content in TiO2 photoanodes on
efficiency of dye-sensitized solar cells. They showed that the electron lifetime and
transport time decreased with increasing Sn/Ti ratio because electron density rose and
increased the rate of recombination. Additionally, according to Liqian et al. [39], doping
of TiO2 with an appropriate amount of Sn enhanced photocatalytic activity compared to
pure TiO2. This is due to easier electron transfer from TiO2 to SnO2 due to the potential
difference in conduction bands and so improves electron–hole separation. In addition to
the valence band of SnO2 being lower than for TiO2, SnO2 has a higher oxidation
capability of photoinduced holes than TiO2. Doping TiO2 with Sn increases the ability
of surface states on TiO2 nanoparticles to bind photoinduced carriers to form more
excitons.
This dynamic diffusion of Sn from FTO to the oxide top layer at high annealing
temperature (500–550 °C) was also reported for hematite photoanodes. Shinde et al.
[40] showed that diffusion of Sn was only in the direction of the hematite layer and not
toward the glass support. Additionally, the FTO conductivity was preserved. Hence, we
assumed that the diffusion of Sn occurred with the rearrangement of the Ti and O atoms
during the transformation from amorphous to crystalline structure.
Based upon the measured dimensions of obtained NTs, we estimated that the porosity of
the film was ~32 %, calculated using the equation for close packed tubes, as in our case
[41]:
2πw(w + D)
P = 1−
(2)
2
3
(
D + 2w
)
where D is the NT’s inner diameter (60 nm) and w is the wall thickness (30 nm), as seen
in Fig. 1. Taking into account the thickness of the film (~500 nm) and the 10 mm × 10
mm film surface, that would lead to 5 × 10−5 cm3 × (1 − P) = 3.4 × 10−5 cm3 for the
volume of TiO2. As the density of anatase is known to be 3.8 mg/cm3, the amount of
photocatalyst used was estimated as ~129 μg and the photocatalyst concentration was
13 μg/L. This information was used to compare the photocatalytic activity of the sample
with previously published reports. However, direct comparison was difficult since this
is the first study about TiO2 NTs grown on FTO support that were shown to be doped
with Sn by annealing. Studies of TiO2 NTs grown on Ti support have reported longer
and wider NTs with higher kinetic constant for degradation of MO, but also had
different dye concentrations [42]. Some studies have presented the benefit of either P-

doping of TiO2 anodized NTs on Ti foil [43] or coupling with Au nanoparticles [44] or
even applying an external bias [45]. Another study of Sn-doped TiO2 fibers synthesized
by electrospinning reported almost complete MO degradation under visible light after
150 min but the concentration of the catalyst was much higher (400 mg/L) and the
catalyst was dispersed as a powder in 20 ppm MO solution [46]. A direct comparison
with our results is therefore quite difficult.
One of the methods to improve the photocatalytic activity of such a system as this,
without embedding in heterostructures (e.g. ZnO–Au–TiO2 [47]), would be Ar/O2
plasma treatment, which increases the hydrophilicity of the TiO2 films [48]. Apart from
plasma treatment, annealing in different atmospheres, such as NH3, is another way to
dope TiO2 with N and enhance the absorption properties of the film [49].
4. Conclusion
The TiO2 NT films were fabricated by anodization of a Ti film sputtered on FTO glass.
The XPS analyses were performed to study the chemical composition of the film before
and after annealing in air at 500 °C. The annealing caused the simultaneous diffusion of
Sn into TiO2 film as well as Ti diffusion to the SnO2 supporting film. By fitting the O 1s
line, a small shift to lower binding energy was observed. This indicated a modification
of the chemical environment of O due to Sn diffusion and conversion from an
amorphous to a crystal structure. The XPS analysis showed that even at the surface of
TiO2 there was 0.10 at.% of Sn before and 0.30 at.% after ionic etching, and the ToF-
ERDA investigations revealed a potential gradual increase of Sn concentration toward
the bottom of the NTs. The DRS measurements showed that the annealing induced the
absorption red shift from 413 to 425 nm and consequently a positive photocatalytic
effect for the degradation of MO dye. Obviously, the presence of the TiO2–SnO2
interface was beneficial for photocatalysis. We believe that not only the extended
absorption in the visible part favored photocatalytic activity of the annealed TiO2, but
also that the recombination of charges was reduced due to the TiO2–SnO2 interface and
the appropriate energy level arrangements. This study indicates that TiO2–SnO2
coupling is a promising direction for further research in the field of photocatalysts based
on TiO2 NTs.
Acknowledgments
The Serbian authors acknowledge with thanks the financial support of the Ministry of
Education, Science and Technological Development, Republic of Serbia (Projects III
45019 and Contracts No. 451-03-68/2020-14/200287 and 451-03-68/2020-14/200135).

The XPS and ToF-ERDA analyses were accomplished thanks to the financial support of
the CERIC-ERIC (20177018 proposal) and Structure Fund Project
CZ.02.1.01/0.0/0.0/16_013/0001788. The INFLPR authors acknowledge with thanks
the partial financial support of this work by UEFISCDI under the contract ID 304/2011.
GES thanks the NIMP Core Programme 21N.
References
[1] A. Fujishima, T.N. Rao, D.A. Tryk, Titanium dioxide photocatalysis, J.
Photochem. Photobiol. C Photochem. Rev. 1 (2000) 1–21.
https://doi.org/10.1016/S1389-5567(00)00002-2.
[2] K. Hashimoto, H. Irie, A. Fujishima, TiO2 photocatalysis: a historical overview
and future prospects, Jpn. J. Appl. Phys. 44 (2005) 8269–8285.
https://doi.org/10.1143/JJAP.44.8269.
[3] V. Etacheri, C. Di Valentin, J. Schneider, D. Bahnemann, S.C. Pillai, Visible-
light activation of TiO2 photocatalysts: advances in theory and experiments, J.
Photochem. Photobiol. C Photochem. Rev. 25 (2015) 1–29.
https://doi.org/10.1016/j.jphotochemrev.2015.08.003.
[4] P. Górska, A. Zaleska, E. Kowalska, T. Klimczuk, J.W. Sobczak, E. Skwarek, W.
Janusz, J. Hupka, TiO2 photoactivity in vis and UV light: the influence of
calcination temperature and surface properties, Appl. Catal. B Environ. 84 (2008)
440–447. https://doi.org/10.1016/j.apcatb.2008.04.028.
[5] Y. Kuwahara, J. Aoyama, K. Miyakubo, T. Eguchi, T. Kamegawa, K. Mori, H.
Yamashita, TiO2 photocatalyst for degradation of organic compounds in water
and air supported on highly hydrophobic FAU zeolite: structural, sorptive, and
photocatalytic studies, J. Catal. 285 (2012) 223–234.
https://doi.org/10.1016/j.jcat.2011.09.031.
[6] S. Javed, M. Islam, M. Mujahid, Synthesis and characterization of TiO2 quantum
dots by sol gel reflux condensation method, Ceram. Int. 45 (2019) 2676–2679.
https://doi.org/10.1016/j.ceramint.2018.10.163.
[7] S.M. Gupta, M. Tripathi, A review of TiO2 nanoparticles, Chinese Sci. Bull. 56
(2011) 1639–1657. https://doi.org/10.1007/s11434-011-4476-1.
[8] H. Lin, C.P. Huang, W. Li, C. Ni, S.I. Shah, Y.H. Tseng, Size dependency of
nanocrystalline TiO2 on its optical property and photocatalytic reactivity
exemplified by 2-chlorophenol, Appl. Catal. B Environ. 68 (2006) 1–11.
https://doi.org/10.1016/j.apcatb.2006.07.018.
[9] Q. Pang, L. Leng, L. Zhao, L. Zhou, C. Liang, Y. Lan, Dye sensitized solar cells
using freestanding TiO2 nanotube arrays on FTO substrate as photoanode, Mater.
Chem. Phys. 125 (2011) 612–616.
https://doi.org/10.1016/j.matchemphys.2010.10.009.
[10] A. Bjelajac, V. Djokić, R. Petrović, N. Bundaleski, G. Socol, I.N. Mihailescu, Z.
Rakočević, Dj. Janaćković, Absorption boost of TiO2 nanotubes by doping with
N and sensitization with CdS quantum dots, Ceram. Int. 43 (2017) 15040–15046.
https://doi.org/10.1016/j.ceramint.2017.08.029.
[11] Y. Okour, H.K. Shon, I.J. El Saliby, R. Naidu, J.B. Kim, J.H. Kim, Preparation
and characterisation of titanium dioxide (TiO2) and thiourea-doped titanate

nanotubes prepared from wastewater flocculated sludge, Bioresour. Technol. 101
(2010) 1453–1458. https://doi.org/10.1016/j.biortech.2009.06.096.
[12] Y. Suzuki, S. Yoshikawa, Synthesis and thermal analyses of TiO2-derived
nanotubes prepared by the hydrothermal method, J. Mater. Res. 19 (2004) 982–
985. https://doi.org/10.1557/JMR.2004.0128.
[13] X. Zhou, N. Liu, P. Schmuki, Photocatalysis with TiO2 nanotubes: “Colorful”
reactivity and designing site-specific photocatalytic centers into TiO2 nanotubes,
ACS Catal. 7 (2017) 3210–3235. https://doi.org/10.1021/acscatal.6b03709.
[14] Y.X. Tang, J. Tao, Y.Y. Zhang, T. Wu, H.J. Tao, Y.R. Zhu, Preparation of TiO2
nanotube on glass by anodization of Ti films at room temperature, Trans.
Nonferrous Met. Soc. China (English Ed.) 19 (2009) 192–198.
https://doi.org/10.1016/S1003-6326(08)60251-4.
[15] G. Li, Z.-Q. Liu, J. Lu, L. Wang, Z. Zhang, Effect of calcination temperature on
the morphology and surface properties of TiO2 nanotube arrays, Appl. Surf. Sci.
255 (2009) 7323–7328. https://doi.org/10.1016/j.apsusc.2009.03.097.
[16] R. Khan, S. Javed, M. Islam, Hierarchical nanostructures of titanium dioxide:
synthesis and applications, in: D. Yang (Ed), Titanium Dioxide - Material for a
Sustainable Environment, IntechOpen, 2018, pp. 1–40.
doi:10.5772/intechopen.74525.
[17] J. Wang, Q. Cai, H. Li, Y. Cui, H. Wang, A review on TiO2 nanotube film
photocatalysts prepared by liquid-phase deposition, Int. J. Photoenergy. 2012
(2012) 702940. https://doi.org/10.1155/2012/702940.
[18] S. Sreekantan, K.A. Saharudin, L.C. Wei, Formation of TiO2 nanotubes via
anodization and potential applications for photocatalysts, biomedical materials,
and photoelectrochemical cell, IOP Conf. Ser. Mater. Sci. Eng. 21 (2011)
012002. https://doi.org/10.1088/1757-899X/21/1/012002.
[19] A. Bjelajac, R. Petrović, V. Djokic, V. Matolin, M. Vondraček, K. Dembele, S.
Moldovan, O. Ersen, G. Socol, I.N. Mihailescu, Dj. Janaćković, Enhanced
absorption of TiO2 nanotubes by N-doping and CdS quantum dots sensitization:
Insight into the structure, RSC Adv. 8 (2018) 35073–35082.
https://doi.org/10.1039/c8ra06341a.
[20] A. Zaleska, Doped-TiO2: a review, Recent Patents Eng. 2 (2008) 157–164.
https://doi.org/10.2174/187221208786306289.
[21] M.K. Tariq, A. Riaz, R. Khan, A. Wajid, H.U. Haq, S. Javed, M.A. Akram, M.
Islam, Comparative study of Ag, Sn or Zn doped TiO2 thin films for
photocatalytic degradation of methylene blue and methyl orange, Mater. Res.
Express. 6 (2019) 106435. https://doi.org/10.1088/2053-1591/ab3efd.
[22] Z. Siketić, I.B. Radović, M. Jakšić, Development of a time-of-flight spectrometer
at the Ruder Bošković Institute in Zagreb, Nucl. Instruments Methods Phys. Res.
Sect. B Beam Interact. with Mater. Atoms. 266 (2008) 1328–1332.
https://doi.org/10.1016/j.nimb.2007.12.070.
[23] Z. Siketić, I.B. Radović, M. Jakšić, Quantitative analysis of hydrogen in thin
films using Time-of-Flight Elastic Recoil Detection Analysis, Thin Solid Films
518 (2010) 2617–2622. https://doi.org/10.1016/j.tsf.2009.07.196.
[24] K. Arstila, J. Julin, M.I. Laitinen, J. Aalto, T. Konu, S. Karkkainen, S. Rahkonen,
M. Raunio, J. Itkonen, J.-P. Santanen, T. Tuovinen, T. Sajavaara, Potku-new
analysis software for heavy ion elastic recoil detection analysis, Nucl. Instr.
Meth. B. 331 (2014) 34–41. https://doi.org/10.1016/j.nimb.2014.02.016

[25] F. Schiettekatte, Fast Monte Carlo for ion beam analysis simulations, Nucl. Instr.
Meth. B. 266 (2008) 1880. https://doi.org/10.1016/j.nimb.2007.11.075
[26] A.L. Patterson, The Scherrer formula for X-ray particle size determination, Phys.
Rev. 56 (1939) 978–982. https://doi.org/10.1103/PhysRev.56.978.
[27] M.J. Jackman, A.G. Thomas, C. Muryn, Photoelectron spectroscopy study of
stoichiometric and reduced anatase TiO2 (101) surfaces: The effect of subsurface
defects on water adsorption at near-ambient pressures, J. Phys. Chem. C. 119
(2015) 13682–13690. https://doi.org/10.1021/acs.jpcc.5b02732.
[28] B. Santara, P.K. Giri, K. Imakita, M. Fujii, Evidence of oxygen vacancy induced
room temperature ferromagnetism in solvothermally synthesized undoped TiO2
nanoribbons, Nanoscale 5 (2013) 5476–5488.
https://doi.org/10.1039/c3nr00799e.
[29] J. Shang, W. Yao, Y. Zhu, N. Wu, Structure and photocatalytic performances of
glass/SnO2/TiO2 interface composite film, Appl. Catal. A. Gen. 257 (2004) 25–
32. https://doi.org/10.1016/j.apcata.2003.07.001.
[30] S.G. Ansari, M.A. Dar, M.S. Dhage, Y.S. Kim, Z.A. Ansari, A. Al-Hajry, H.S.
Shin, A novel method for preparing stoichiometric SnO2 thin films at low
temperature, Rev. Sci. Instrum. 80 (2009). https://doi.org/10.1063/1.3115222.
[31] D. Toloman, O. Pana, M. Stefan, A. Popa, C. Leostean, S. Macavei, D. Silipas, I.
Perhaita, M.D. Lazar, L. Barbu-Tudoran, Photocatalytic activity of SnO2-TiO2
composite nanoparticles modified with PVP, J. Colloid Interface Sci. 542 (2019)
296–307. https://doi.org/10.1016/j.jcis.2019.02.026.
[32] L. Xu, E.M.P. Steinmiller, S.E. Skrabalak, Achieving synergy with a potential
photocatalytic Z-scheme: Synthesis and evaluation of nitrogen-doped TiO2/SnO2
composites, J. Phys. Chem. C. 116 (2012) 871–877.
https://doi.org/10.1021/jp208981h.
[33] L. Lei, Y. Su, M. Zhou, X. Zhang, X. Chen, Fabrication of multi-non-metal-
doped TiO2 nanotubes by anodization in mixed acid electrolyte, Mater. Res. Bull.
42 (2007) 2230–2236. https://doi.org/10.1016/j.materresbull.2007.01.001.
[34] A.O. Ibhadon, P. Fitzpatrick, Heterogeneous photocatalysis: Recent advances and
applications, Catalysts. 3 (2013) 189–218. https://doi.org/10.3390/catal3010189.
[35] M. Shipochka, A. Eliyas, I. Stambolova, V. Blaskov, S. Vassilev, S. Simeonova,
K. Balashev, Synthesis of TiO2 on SnO2 bicomponent system and investigation
of its structure and photocatalytic activity, Mater. Chem. Phys. 220 (2018) 249–
259. https://doi.org/10.1016/j.matchemphys.2018.08.054.
[36] Talinungsang, N. Paul, D.D. Purkayastha, M.G. Krishna, TiO2/SnO2 and
SnO2/TiO2 heterostructures as photocatalysts for degradation of stearic acid and
methylene blue under UV irradiation, Superlattices Microstruct. 129 (2019) 105–
114. https://doi.org/10.1016/j.spmi.2019.03.004.
[37] H.-E. Cheng, C.-Y. Lin, C.-M. Hsu, Fabrication of SnO2-TiO2 core-shell
nanopillar-array films for enhanced photocatalytic activity, Appl. Surf. Sci. 396
(2017) 393–399. https://doi.org/10.1016/j.apsusc.2016.10.166.
[38] Y. Duan, N. Fu, Q. Liu, Y. Fang, X. Zhou, J. Zhang, Y. Lin, Sn-doped TiO2
photoanode for dye-sensitized solar cells, J. Phys. Chem. C. 116 (2012) 8888–
8893. https://doi.org/10.1021/jp212517k.
[39] J. Liqiang, F. Honggang, W. Baiqi, W. Dejun, X. Baifu, L. Shudan, S. Jiazhong,
Effects of Sn dopant on the photoinduced charge property and photocatalytic
activity of TiO2 nanoparticles, Appl. Catal. B Environ. 62 (2006) 282–291.

https://doi.org/10.1016/j.apcatb.2005.08.012.
[40] P.S. Shinde, A. Annamalai, J.H. Kim, S.H. Choi, J.S. Lee, J.S. Jang, Exploiting
the dynamic Sn diffusion from deformation of FTO to boost the photocurrent
performance of hematite photoanodes, Sol. Energy Mater. Sol. Cells. 141 (2015)
71–79. https://doi.org/10.1016/j.solmat.2015.05.020.
[41] P. Quitério, A. Apolinário, C.T. Sousa, J.D. Costa, J. Ventura, J.P. Araújo, The
cyclic nature of porosity in anodic TiO2 nanotube arrays, J. Mater. Chem. A. 3
(2015) 3692–3698. https://doi.org/10.1039/C4TA04607B.
[42] Y.R. Smith, A. Kar, V.R. Subramanian, Investigation of physicochemical
parameters that influence photocatalytic degradation of methyl orange over TiO2
nanotubes, Ind. Eng. Chem. Res. 48 (2009) 10268–10276.
https://doi.org/10.1021/ie801851p
[43] K.A. Saharudin, S. Sreekantan, P-incorporated TiO2 nanotubes for methyl orange
degradation, Adv. Mater. Res. 620 (2013) 151–155.
https://doi.org/10.4028/www.scientific.net/AMR.620.151.
[44] C. Fu, M. Li, H. Li, C. Li, X. G. Wu, B. Yang, Fabrication of Au
nanoparticle/TiO2 hybrid films for photoelectrocatalytic degradation of methyl
orange, J. Alloys Compd. 692 (2017) 727–733.
https://doi.org/10.1016/j.jallcom.2016.09.119.
[45] Y.S. Sohn, Y.R. Smith, M. Misra, V.R. Subramanian, Electrochemically assisted
photocatalytic degradation of methyl orange using anodized titanium dioxide
nanotubes, Appl. Catal. B Environ. 84 (2008) 372–378.
https://doi.org/10.1016/j.apcatb.2008.04.021.
[46] A.K. Alves, F.A. Berutti, C.P. Bergmann, Visible and UV photocatalytic
characterization of Sn-TiO2 electrospun fibers, Catal. Today. 208 (2013) 7–10.
https://doi.org/10.1016/j.cattod.2012.10.020.
[47] F. Javed, S. Javed, M.A. Akram, M. Mujahid, M. Islam, A.S. Bhatti, Surface
plasmon mediated optical properties of ZnO/Au/TiO2 nanoheterostructure rod
arrays, Mater. Sci. Eng. B Solid-State Mater. Adv. Technol. 231 (2018) 32–39.
https://doi.org/10.1016/j.mseb.2018.08.001.
[48] A. Achour, M. Islam, S. Solaymani, S. Vizireanu, K. Saeed, G. Dinescu,
Influence of plasma functionalization treatment and gold nanoparticles on surface
chemistry and wettability of reactive-sputtered TiO2 thin films, Appl. Surf. Sci.
458 (2018) 678–685. https://doi.org/10.1016/j.apsusc.2018.07.145.
[49] A. Bjelajac, R. Petrović, M. Popović, Z. Rakočević, G. Socol, I.N. Mihailescu,
Dj. Janaćković, Doping of TiO2 nanotubes with nitrogen by annealing in
ammonia for visible light activation: Influence of pre- and post-annealing in air,
Thin Solid Films. 692 (2019). https://doi.org/10.1016/j.tsf.2019.137598.

•
•
•
•
•
Nanotubular TiO2 film was obtained by anodization of Ti sputtered on F-SnO2 glass.
XRD showed that annealing in air at 500 °C resulted in anatase phase of TiO2.
XPS and ToF-ERDA were used to investigate possible Sn diffusion from F-SnO2.
Sn-doping of TiO2 could be the reason for improved absorbance shifting to 425 nm.
Sn-doped TiO2 film was shown effective in the degradation of methyl orange dye.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.