M. LE[NIK et al.: HYDROTHERMAL SYNTHESIS OF Mn-DOPED TiO2 WITH A STRONGLY SUPPRESSED ...
411–416
HYDROTHERMAL SYNTHESIS OF Mn-DOPED TiO
2
WITH A
STRONGLY SUPPRESSED PHOTOCATALYTIC ACTIVITY
HIDROTERMALNO SINTENTIZIRAN Z Mn DOPIRAN TiO
2
Z MO^NO INHIBIRANO FOTOKATALITSKO AKTIVNOSTJO
Maja Le{nik
1
, Dejan Verhov{ek
1
, Nika Veronovski
1
, Mitja Gra~ner
1
, Goran Dra`i}
2
,
Kristina @agar Soder`nik
3
, Mihael Drofenik
4
1
Cinkarna Celje, d.d. Inc., Kidri~eva 26, SI-3001 Celje, Slovenia
2
National Institute of Chemistry Slovenia, Hajdrihova 19, 1001 Ljubljana, Slovenia
3
Jo`ef Stefan Institute, Department for Nanostructured Materials, Jamova cesta 39, 1000 Ljubljana, Slovenia
4
University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova ulica 17, 2000 Maribor, Slovenia
maja.lesnik@cinkarna.si
Prejem rokopisa – received: 2017-01-26; sprejem za objavo – accepted for publication: 2018-01-16
doi:10.17222/mit.2017.012
Nanocrystalline rutile TiO2 doped with various amounts of manganese (Mn) atoms was prepared using a hydrothermal synthesis
route and a polycrystalline TiO2 precursor. The diffuse reflectance spectra of the highly doped rutile nanocrystallites display a
notable red shift in the band-gap transition. The absorbing band edge moved to the visible range when the Mn content in the
rutile nanocrystallite exceeded 0.4 %. The doped rutile nanocrystallites were analysed using X-ray powder diffraction (XRD),
transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and UV-Vis spectro-
scopy. The Kubelka–Munk band-gap approximation was used to examine the narrowing of the band gap in comparison with the
pure rutile TiO2 nanocrystallites. The doped rutile nanocrystallites displayed a remarkable decrease in the photocatalytic activity
due to the formation of recombination sites induced by Mn ions incorporated into the crystal structure of the TiO2.
Keywords: suspension, TiO 2
, rutile, doped with inorganic metal elements, UV absorber
Z uporabo hidrotermalne sinteze in polikristalinini~ne rutilne oblike nanodelcev TiO2, smo pripravili nanokristalini~ne rutilne
delce, dopirane z Mn. S pomo~jo UV-VIS difuzivno refleksijske spektroskopije smo zaznali izrazit premik absorpcijskega roba
k valovnim dol`inam vidne svetlobe. Energijska vrzel z Mn dopiranega TiO2 se je z nara{~anjem koncentracije dopanta o`ala.
Za analizo dopiranih rutilnih TiO2 nanodelcev smo uporabili rentgensko pra{kovno difrakcijo (XRD), elektronsko mikroskopijo
(TEM, HRTEM), UV-Vis spektroskopijo, kalkulacijo energijskih vrzeli s Kubelka Munk pribli`kom. Pri merjenju fotokatalitske
aktivnosti, z metodo degradacije izopropanola v alkohol, smo ugotovili, da dopiranje rutilne oblike TiO2 z Mn povzro~i
inhibicijo fotokatalitske aktivnosti, kar smo pripisali tvorbi rekombinacijskih centrov, ki nastanejo pri dopiranju z Mn ioni v
kristalno re{etko TiO2.
Klju~ne besede: suspenzija, TiO2, s kovinami dopiran rutil, UV absorber
1 INTRODUCTION
Titanium dioxide (TiO
2
) crystallites are very efficient
at scattering incident light and are therefore applied in
some of the finest white pigments. In addition, TiO
2
has
a wide range of photocatalytic applications in the protec-
tion of the environment, where an enhanced photo-
catalytic activity is desired. On the other hand, TiO
2
is an
excellent UV absorber and shows no UV-induced
degradation over time.
1
For this reason it is also used in a
variety of applications where it acts as an UV-protective
material, including various coatings, sunscreens and
plastics. Using TiO
2
as a UV-protective material requires
that the material is modified in such a way as to prevent
the formation of free radicals upon the absorption of UV
photons, which are otherwise preferred in environment-
protection applications.
2
Thus, the free radicals that are
involved in a number of potential health concerns, such
as skin aging, must be reduced as much as possible. One
possibility for reducing the photocatalytic activity of
TiO
2
is a nanocrystallite surface coating with inorganic
oxides that are transparent with respect to UV photons,
such as SiO
2
and Al
2
O
3
, so isolating the surface from
adsorbents, particularly moisture.
3
However, a thin layer
on the crystallite surfaces has not been shown to be
efficient in terms of significantly reducing the photocata-
lytic activity of the material.
Another option to suppress the photocatalytic activity
of TiO
2
is doping the material with transition-metal ions
(TMIs), which induced defects, and that function mainly
as carrier-recombination centres when migrating from
the inside to the surface of the photocatalyst and
represent the key to a low photocatalytic activity, for
instance, in the case of manganese doping.
4–7
Recently, some works were published that report on
the enhanced photocatalytic activity of Mn-doped
TiO
2
.
8,9
Here, the superior reactivity might be ascribed to
the anatase crystallites’ morphology and structure as one
of the main reasons for the improved photocatalytic
activity in these samples. To the best of our knowledge
the inhibition of photocatalytic activity in pure rutile
Mn-doped samples prepared by hydrothermal synthesis,
Materiali in tehnologije / Materials and technology 52 (2018) 4, 411–416 411
UDK 54.057:549.514.6:546.711:544.526.5 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(4)411(2018)

from prefabricated TiO
2
, has not been reported yet. Here,
we must emphasize that the hydrothermal method was
crucial in synthesizing the Mn-doped rutile phase, which
strongly inhibits the photocatalytic activity.
2 EXPERIMENTAL PART
2.1 Preparation method
The hydrothermal synthesis of Mn-doped rutile TiO
2
nanocrystallites was performed in a Teflon-lined, stain-
less-steel autoclave with a volume of 80 mL. To prepare
the TiO
2
-doped sample the reactor was loaded with a
50-mL aqueous suspension of polycrystalline rutile TiO
2
nanocrystallites provided by Cinkarna Celje, Inc. having
a mass concentration of 60–150 g/L (calculated as TiO
2
)
and 1 % of mass fractions (based on TiO
2
content) of
manganese chloride (MnCl
2
,9 6%w/w, Merck). The
mixture of polycrystalline rutile TiO
2
nanocrystallites
and dopant was then stirred for at least 15 min. The
autoclave was put into a preheated oven and was hydro-
thermally treated at 180 °C for 24 h. At the end of the
heating process the autoclave was taken out of the oven
and left to cool at room temperature. The as-prepared
product was diluted with distilled water, washed on a
laboratory centrifuge (MPW 350 – Med. Instruments,
High brushless centrifuge, 4000 min
–1
, 20 min). The
washing was performed until the conductivity of the
material was less than 900 μS/cm. The final product was
an aqueous suspension of doped rutile having a 10 % of
mass fraction of monosized TiO
2
nanocrystallites with a
light-brown colouration. Samples with anticipated con-
centrations of Mn (0.4 %, 0.8 % and 2 %) of mass
fractions, labelled as samples: B, C and D, were also
prepared by the same process by varying the content of
added MnCl
2
. The sample A was prepared using the
same process, but without the addition of MnCl
2
as a
dopant.
2.2 Characterization of samples
The morphology and size of the particles were
examined with a transmission electron microscope
(TEM, Jeol JEM-2100, Jeol Ltd., Tokyo, Japan). To
prepare the TEM specimens, the samples were ultrasoni-
cally dispersed and the suspension was collected using
carbon-supported copper grids. The presence of the
manganese was verified using energy-dispersive X-ray
spectroscopy (EDXS). The crystal structure, the concen-
tration of Mn and the valence state of the titanium in
sample B (nominally 0.8 % of mass fractions manga-
nese-doped TiO
2
) were characterized with probe-
corrected scanning transmission electron microscopy
(STEM, Jeol ARM 200 CF) and electron-energy-loss
spectroscopy (EELS, Gatan Quantum ER dual).
The crystallinity of the particles was examined using
X-ray diffraction (XRD) performed on a Cubi X PRO
PW 3800 instrument (PANanalytical) (Cu-K
 radiation
( = 0.15418 nm)). In order to acquire the TiO
2
powders
for the X-ray powder diffraction (XRD) investigation,
the suspensions were dried at 80 °C, ground and the
powder was pressed into pellets that were used to
perform the measurements.
The UV–Vis diffuse reflectance spectra were
collected on an Agilent-Cary 300 UV-Vis spectrophoto-
meter equipped with an integrating sphere (Varian Inc.,
USA).
The measurements of the photocatalytic activity were
performed in a sealed gas-solid reactor at room tempe-
rature and a relative humidity of 60 %, utilizing FT-IR
spectroscopy (Spectrum BX model Perkin Elmer spec-
trometer). The model pollutant was isopropanol in the
gas phase. During the photocatalytic reaction the
isopropanol oxidizes into acetone and subsequently into
carbon dioxide and water under UV irradiation (Xe
lamp, 300 W). The light imitates the solar spectrum and
emits both ultraviolet (UV) and visible (VIS) light. The
reactor is at a distance of 4 cm from the lamp. The sam-
ples were dried under ambient conditions and prepared
by milling 50 mg of the material. To perform the
measurements, 20 μL of isopropanol was injected into
the system. This volume represents around 2000 mg/L of
gas phase of the isopropanol in the system. The amount
and ratio of isopropanol and the formed acetone were
monitored in real time. The evaluation of the photo-
catalytic activity is based on the acetone-formation
kinetics and is given in mg/L h.
10
3 RESULTS AND DISCUSSION
3.1 Hydrothermal synthesis
Hydrothermal synthesis is one of the best methods
for producing pure oxide nanocrystalline materials.
Among the various hydrothermal methods, hydrothermal
precipitation is suitable for the synthesis of nanocrys-
tallite material using a polycrystalline precursor and
transforming it into a nanocrystalline material with a
controlled morphology and composition. During this
investigation the dissolution of the precursor, polycrys-
talline rutile, and afterwards a precipitation in the pre-
sence of the dopant, was applied in order to transform it
into nanocrystalline material with a controlled morphol-
ogy and composition.
After the dissolution and subsequent diffusion of
TiO
2
in the presence of hydrated Mn ions to the pre-
cipitation locations, the precipitation from the solution
on the doped rutile nanocrystallites is a relatively
sluggish process that determines the final morphology of
the crystallites and the concentration of the dopant. The
amount of dopant that was not incorporated during the
TiO
2
precipitation was washed away at the end of the
procedure.
In solution the TiO
2
exist as six-fold coordinated
structural units (TiO
6
) that undergo condensation to
become the octahedra that bond via corner- and
M. LE[NIK et al.: HYDROTHERMAL SYNTHESIS OF Mn-DOPED TiO2 WITH A STRONGLY SUPPRESSED ...
412 Materiali in tehnologije / Materials and technology 52 (2018) 4, 411–416

edge-sharing to form the final crystal structure. Hence,
the fundamental structural units are (TiO
6
) octahedrons,
having different modes of arrangement and links,
depending on the particular crystal form of the TiO
2
.In
the rutile form the (TiO
6
) octahedra link by sharing an
edge along the c-axis to form chains. During the precipi-
tation process oxygen coordinated Mn units incorporate
into the rutile by replacing the (TiO
6
) octahedrons in a
doped rutile structure. However, the incorporation of
coordinated Mn complex ions covering various oxidation
states and coordination aggravate the incorporation of
the manganese. Thus, the incorporation of Mn at higher
nominal concentrations is not straight forward, but
exhibits a lag with regards to the nominal concentration
of the manganese, Table 1.
3.2 The crystalline phase of particles
The XRD patterns of Mn-doped TiO
2
with different
nominal concentrations (0, 0.2, 0.8, 2.0) % of Mn in the
TiO
2
are shown in Figure 1. The XRD peaks confirm the
presence of a rutile crystal structure in all the samples.
No manganese-rich phase was identified in the XRD
patterns, even when the doping level concentration was
as high as 2.0 % of mass fractions.
The average crystallite size was determined using
diffraction-peak (100) broadening and Scherrer’s formu-
la based on the FWHM (Full Width at Half Maximum)
of the XRD peaks. The determined crystallite size is
shown in Table 1. For up to 0.8 % of mass fractions of
dopant the crystallite size is more or less around 30 nm.
After that, at 2.0 % of mass fractions of dopant, the
diffraction-peak broadening indicates a decrease in the
crystallite size accompanied by an increase of the
specific surface, Table 1. Thus, with up to 2.0 % of mass
fractions of manganese addition in the nanocrystallites
the morphology of the crystallites did not change
significantly.
3.3 The morphology of the doped TiO
2
particles
TEM images of the undoped TiO
2
and 0.8 % of mass
fractions of Mn-doped TiO
2
nanocrystallites are shown
in Figure 2. The hydrothermally synthesized, doped,
rutile TiO
2
nanocrystallites have an oval/spherical mor-
phology and are uniform in size. From the TEM images
no significant difference in the morphology between the
synthesized products, i.e., 0.8 % of mass fractions of
Mn-doped TiO
2
and undoped TiO
2
, can be seen, in agree-
ment with the XRD patterns, which show a comparable
crystallite size up to 0.8 % of mass fractions of Mn,
Table 1.
M. LE[NIK et al.: HYDROTHERMAL SYNTHESIS OF Mn-DOPED TiO2 WITH A STRONGLY SUPPRESSED ...
Materiali in tehnologije / Materials and technology 52 (2018) 4, 411–416 413
Table 1: The nominal concentration vs. real concentration determined by EDXS, BET specific surface area, Scherrer’s crystallite size of the
prepared samples and the measured band gap
Sample*
Nominal Mn
concentration (w/%)
Real Mn
concentration (w/%)
Specific surface area
(m
2
/g)
Crystallite size
(nm)
Band gap
(eV)
A - - 70.1 27.8 3.01
B 0.4 0.30±0.1 69.5 29.9 2.98
C 0.8 0.90±0.1 78.3 31.7 2.90
D 2.0 1.50±0.1 93.1 21.8 2.87
Figure 2: TEM image of: a) undoped sample A and b) sample C with
a nominal concentration of 0.8 % of mass fractions of Mn
Figure 1: XRD patterns of undoped and Mn-doped TiO
2
samples with
different Mn contents

The average crystallite size and the specific surface
data in Table 1 show that the morphology of samples D
doped with a nominal concentration of 2.0 % of mass
fractions of Mn are different than the other samples in
the sequence. They exhibit a smaller crystallite size and
a higher specific surface area. An increase in the nominal
concentration of Mn in the samples up to 0.8 % of mass
fractions is accompanied by a notable increase in the
reflectance from 20 % to about 50 %, as well as a band-
gap decrease from 3.01 eV to 2.90 eV in sample C.
The STEM-EELS analyses of sample C is presented
in Figure 3. The doped crystallites have an oval-sphe-
rical morphology and are uniform in size (a). No
amorphous Mn-rich oxide phase was found on the
surface of the particles, which indirectly confirms the
incorporation of the Mn phase into the TiO
2
crystal
lattice. The presence of a precipitated dopant on the
particle surface is a relatively common phenomenon.
Evidently, the concentration of 0.8 % Mn is the limit for
fully incorporating the dopant into the crystal lattice,
which is confirmed by the UV-Vis reflectance spectra,
where the absorption of the higher amount of dopant
(2 % of mass fractions of Mn) is negligible. At the edges
of the rutile nanocrystallites the reconstruction of the
surface structure took place. In these areas the presence
of Ti
3+
was observed using EELS. The observation is
quite interesting, because it is not usual to find such an
arranged structure containing Ti
3+
on the surface of a
particle. Further investigations are necessary in order to
find an appropriate explanation. The excellent match of
the experimental and the simulated electron-diffraction
patterns shown in Figure 3b confirms the presence of
individual rutile nanocrystallites and no anatase or
brookite crystal modifications were detected. The EELS
analysis identified the presence of Mn, but due to the
very low concentration it was not possible to determine
the valence state of this Mn. Finally, the Mn phase is not
uniformly incorporated inside the particles as not all the
EELS spectra from the different nanoparticles contain
Mn edges.
3.4 UV–Vis diffuse reflectance spectra
The influence of doping on the UV/Vis spectra
properties of the rutile TiO
2
is clear from Figure 4. The
Mn-doped samples exhibit a notable difference in their
colour, which depends on the dopant concentration and
extends from light-to-dark yellow to a rose (skin-like)
colour. As can be seen in Figure 4, the reflectivity
dependence of the wavelength of the undoped TiO
2
has a
typical sharp edge of reflection at around 420–400 nm,
while the reflection capacity of the visible-light region is
relatively poor at that wavelength. Using the relation
E(eV) = hc/ (nm) = 1236// (nm) we obtain, for the
measured band gaps of the samples, values of
2.94–3.06 eV, which corresponds to an electron exci-
tation from the VB to the CB of the rutile. Besides, for
the determination of the band-gap energy, the
Kubelka–Munk procedure was used.
11,12
The extra-
polated values are shown in Table 1.
In comparison, from the reflectivity spectra of the
Mn-doped samples, Figure 4, it is clear that there is a
significant reflection of visible light.
13–16
In addition, two
absorption edges are present in the reflective spectra.
The primary absorption edge contributes to the reflect-
ance at short wavelengths of the visible spectra, while
the secondary absorption edge extends into the lowest
wavelengths of the visible light spectra, in agreement
with previous reports.
7–9
The intensity of both absorption
edges changes when increasing the Mn-doping con-
centration. Moreover, it is also possible that the Mn
doping even induces the reflection of IR light, which is
in agreement with Shao’s report.
8,16,17
When the doping
level reached a nominal concentration of 0.8 %, the
optical reflection was shifted into the infrared spectral
range. In addition, samples doped with a nominal con-
centration of 0.8 % of mass fractions and 2.0 % of mass
M. LE[NIK et al.: HYDROTHERMAL SYNTHESIS OF Mn-DOPED TiO2 WITH A STRONGLY SUPPRESSED ...
414 Materiali in tehnologije / Materials and technology 52 (2018) 4, 411–416
Figure 4: UV/Vis reflectance spectra of the undoped TiO
2
and the
TiO
2
doped with different % of mass fractions of Mn ions, indicated in
the figure
Figure 3: BF-STEM image: a) of hydrothermally treated TiO
2
nano-
crystallites with 0.8 % of mass fractions of Mn, b) experimental and
simulated selected-area electron-diffraction pattern (SAED) for rutile,
c) EELS spectrum where traces of Mn can be seen, d) HAADF-STEM
micrograph of rutile nanocrystallites where an approximately
1-nm-thick phase can be seen at the edge of the particle, the edge con-
tains a large amount of Ti
3+
, based on the EELS spectra (insets)

fractions exhibited a reflectivity of 50 % at a wavelength
of 800 nm. As we can see in Figure 4, the reflectivities
of the samples C and D were actually the same. Similar
results were described by Deng for a nominal concen-
tration of 0.8 % of mass fractions and 2.0 % of mass
fractions doped samples, respectively, where the samples
were synthesized using the sol-gel method and achieved
a limiting dopant concentration of 12 % with a 75 %
absorbance in the Vis absorption spectra.
8
3.5 Photocatalytic activity measurements
To evaluate the photocatalytic activity of the undoped
and Mn-doped TiO
2
, the photocatalytic degradation of
isopropanol under UV+VIS and Vis irradiation was
studied. The results of the photocatalytic activity are
presented in Figure 5, based on the acetone-formation
kinetics, and are given in μL/L h. It is clear that under
UV+Vis irradiation the photocatalytic activity of the
Mn-doped TiO
2
drastically decreased to near-zero
values. The reduced photocatalytic activity of the
Mn-doped TiO
2
particles under UV light was also re-
ported by Chauhan.
18
When the Mn-doped TiO
2
particles
were exposed to visible light, the same results were
obtained. The pure, undoped TiO
2
acts as a visible-light
photocatalyst, but when the TiO
2
particles were doped
with Mn ions, the inhibition of the photocatalytic activity
occurs.
The photocatalytic-activity inhibition of the
Mn-doped TiO
2
was also established in Chang’s work
and in the research of Wakefield.
4,7
Manganese ions are
p-type dopants that are incorporated into the Ti
4+
lattice
sites and have a crucial effect on the activity, because
they serve as Mn
3+/4+
doping trap centres (recombination
centres), which have both occupied and unoccupied
energy levels within the band gap, trap electrons and
holes on the same sites and rapidly annihilate the trapped
carriers through intra-atomic relaxation.
4,8
Since the Mn
ions serve as recombination centres it should be pointed
out that the photocatalytic activity of doped samples
decreases when the concentration level of the dopant is
increased.
5
In addition, the electronic structure, the coordination
number, the electronegativity, and the size of the dopant
ions are all major influencing parameters that affect the
formation of free radicals, having an impact on the
effectiveness of Mn doping with respect to blocking the
photocatalytic activity.
It is interesting to note that the manganese is a very
effective inhibitor of the photocatalytic activity and even
a relatively small dopant level, of only 0.8 % of mass
fractions of Mn, nearly completely blocks the photocata-
lytic activity of rutile nanocrystallites in the UV region
and to a relatively large extent also in the VIS part of the
spectra. Less prone to photocatalytic reactions is also the
relatively high dopant concentration level of 2.0 % of
mass fractions of Mn where the smaller crystallites are
formed (sample D, Table 1), in spite of a higher specific
surface area. The manganese ions have the appropriate
characteristics to successfully inhibit the photocatalytic
activity, particularly when they are incorporated into the
interior of the matrix and the doping-induced defects are
operating as carrier-recombination centres when they
migrate from the inside of the photocatalyst to the
surface.
5
4 CONCLUSIONS
Mn-doped rutile TiO
2
nanocrystallites that demon-
strate a strong reduction in photocatalytic activity were
successfully prepared using the hydrothermal method.
The average crystallite particle size is around 30 nm;
however, this decreases at the 2 % of mass fractions of
dopant concentration level. This could indicate the maxi-
mum concentration level of the dopant ions incorporated
into the crystal structure. On the surface of the particle
no amorphous Mn oxide phase is detected, which
indirectly confirms the incorporation of the Mn phase
into the TiO
2
crystal structure. The diffuse reflectance
spectrum of the Mn-doped TiO
2
has two absorption
edges and displays a red shift in the optical absorption
range, which increases with the Mn content and narrows
the band gap. The Mn-ions-doped TiO
2
nanocrystallites,
which can display a red shift in the band-gap transition,
reduce strongly the photocatalytic activity.
Acknowledgment
This work is partially financially supported by the
Ministry of Education, Science and Sport of the
Republic of Slovenia under the contract 34/2013-SOF.
M. LE[NIK et al.: HYDROTHERMAL SYNTHESIS OF Mn-DOPED TiO2 WITH A STRONGLY SUPPRESSED ...
Materiali in tehnologije / Materials and technology 52 (2018) 4, 411–416 415
Figure 5: photocatalytic activity of undoped TiO
2
and TiO
2
doped
with different amounts of Mn ions (A – undoped TiO
2
,B–0.2%of
mass fractions of Mn doped TiO
2
, C – 0.8 % of mass fractions of Mn
doped TiO
2
,D–2%ofmass fractions of Mn doped TiO
2
) under a)
UV+Vis irradiation and b) Vis irradiation

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