TiO2 and nitrogen doped TiO2 prepared by different methods; on the (micro)structure and photocatalytic activity in CO2 reduction and N2O decomposition

Article abstract of DOI:10.1166/jnn.2018.13936

TiO₂ as nanostructured powders were prepared by (1) sol-gel process and (2) hydrothermal method in combination with (A) the processing by pressurized hot water and methanol or (B) calcination. The subsequent synthesis step was the modification of prepared nanostructured TiO₂ with nitrogen using commercial urea. Textural, structural, surface and optical properties of prepared TiO₂ and N/TiO₂ were characterized by nitrogen physisorption, powder X-ray diffraction, X-ray photoelectron spectroscopy and DR UV-vis spectroscopy. It was revealed that TiO₂ and N/TiO₂ processed by pressurized fluids showed the highest surface areas. Furthermore, all prepared materials were the mixtures of major anatase phase and minor brookite phase, which was in nanocrystalline or amorphous (as nuclei) form depending on the applied preparation method. All the N/TiO₂ materials exhibited enhanced crystallinity with a larger anatase crystallite-size than undoped parent TiO₂. The photocatalytic activity of the prepared TiO₂ and N/TiO₂ was tested in the photocatalytic reduction of CO2 and the photocatalytic decomposition of N₂O. The key parameters influencing the photocatalytic activity was the ratio of anatase-To-brookite and character of brookite. The optimum ratio of anataseto- brookite for the CO2 photocatalytic reduction was determined to be about 83 wt.% of anatase and 17 wt.% of brookite (amorphous-like) (TiO₂-SG-C). The presence of nitrogen decreased a bit the photocatalytic activity of tested materials. On the other hand, TiO₂-SG-C was the least active in the N₂O photocatalytic decomposition. In the case of N₂O photocatalytic decomposition, the modification of TiO₂ crystallites surface by nitrogen increased the photocatalytic activity of all investigated materials. The maximum N₂O conversion (about 63 % after 18 h of illumination) in inert gas was reached over all N/TiO₂.

Full text of DOI:10.1166/jnn.2018.13936

Article
Journal of
Nanoscience and Nanotechnology
Vol. 18, 688–698, 2018
Copyright © 2018 American Scientific Publishers
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Printed in the United States of America
TiO2 and Nitrogen Doped TiO2 Prepared by Different
Methods; on the (Micro)structure and Photocatalytic
Activity in CO2 Reduction and N2O Decomposition
1ꢀ∗
Lenka Matejová , Kamila Kocí1ꢀ2ꢀ3, Ivana Troppová1, Marcel Šihor1ꢀ2, Miroslava Edelmannová1ꢀ2,
ˇ
ˇ
1ꢀ4
5
7
1ꢀ2
ˇ 6
´
ˇ
ˇ
Jaroslav Lang , Libor Capek , Zdenek Matej , Piotr Kustrowski , and Lucie Obalová
2
1Institute of Environmental Technology, Faculty of Metallurgy and Material Engineering, 3Energy Units for Utilization of
Non Traditional Energy Sources, 4Nanotechnology Centre, VŠB - Technical University of Ostrava,
17. Listopadu 15/2172, 708 33 Ostrava–Poruba, Czech Republic
5Department of Physical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 95,
532 10 Pardubice, Czech Republic
6Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University in Prague,
Ke Karlovu 5, 121 16 Prague 2, Czech Republic
7Faculty of Chemistry, Jagiellonian University, Ingardena 3, Krakow 30 060, Poland
TiO2 as nanostructured powders were prepared by (1) sol–gel process and (2) hydrothermal method
in combination with (A) the processing by pressurized hot water and methanol or (B) calcination.
IP: 42.106.24.157 On: Fri, 15 Jun 2018 11:31:07
The subsequent synthesis step was the modification of prepared nanostructured TiO2 with nitrogen
Copyright: American Scientific Publishers
using commercial urea. Textural, structural, surface and optical properties of prepared TiO2 and
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N/TiO2 were characterized by nitrogen physisorption, powder X-ray diffraction, X-ray photoelectron
spectroscopy and DR UV-vis spectroscopy. It was revealed that TiO2 and N/TiO2 processed by pres-
surized fluids showed the highest surface areas. Furthermore, all prepared materials were the mix-
tures of major anatase phase and minor brookite phase, which was in nanocrystalline or amorphous
(as nuclei) form depending on the applied preparation method. All the N/TiO2 materials exhibited
enhanced crystallinity with a larger anatase crystallite-size than undoped parent TiO2. The photo-
catalytic activity of the prepared TiO2 and N/TiO2 was tested in the photocatalytic reduction of CO2
and the photocatalytic decomposition of N2O. The key parameters influencing the photocatalytic
activity was the ratio of anatase-to-brookite and character of brookite. The optimum ratio of anatase-
to-brookite for the CO2 photocatalytic reduction was determined to be about 83 wt.% of anatase
and 17 wt.% of brookite (amorphous-like) (TiO2-SG-C). The presence of nitrogen decreased a bit
the photocatalytic activity of tested materials. On the other hand, TiO2-SG-C was the least active in
the N2O photocatalytic decomposition. In the case of N2O photocatalytic decomposition, the modifi-
cation of TiO2 crystallites surface by nitrogen increased the photocatalytic activity of all investigated
materials. The maximum N2O conversion (about 63 % after 18 h of illumination) in inert gas was
reached over all N/TiO2.
Keywords: Titania, Pressurized Hot Fluid, Nitrogen Doped, Photocatalysis, Carbon Dioxide,
Nitrous Oxide.
1. INTRODUCTION
reductant and a visible light as the source of photon to
reusable hydrocarbons is of especially interest.1 The water
splitting is the first step of this photocatalytic reaction,
which produces hydrogen reacting subsequently with car-
bon dioxide.2
Carbon dioxide is regarded to be one of principal con-
tributors to the greenhouse effect and the limitation of
its emission is nowadays a key challenge over the world.
The photocatalytic reduction of CO2 using water as the
All types of photocatalysts, methods and photoreactors
used for the CO2 photoreduction have been reviewed in
∗Author to whom correspondence should be addressed.
688
J. Nanosci. Nanotechnol. 2018, Vol. 18, No. 1
1533-4880/2018/18/688/011
doi:10.1166/jnn.2018.13936

Mateˇjová et al.
TiO2 and Nitrogen Doped TiO2 Prepared by Different Methods
several articles.3–15 The most widely used photocatalyst is
TiO2 and generally methane is the product of reaction. The
selection of TiO2 for the CO2 photocatalytic reduction is
not astonishing due to the wide availability of this material,
its stability, lack of toxicity, and, in particular, high photo-
catalytic activity in water splitting. The efficiency of TiO2
can be increased by doping with metals3–5 or non-metallic
elements with the creation of oxygen vacancies.6–9ꢀ16 The
doping improves, in general, an electron trapping capabil-
ity of modified semiconductor, reduces the recombination
of the photogenerated pair of electron and hole (e−/h+)
and often extends the absorption capability of light until
the visible range.
N2O belongs also to the major greenhouse gases and
it plays a significant role in the destruction of the ozone
layer. The photocatalytic decomposition of N2O under UV
irradiation on semiconductor materials is one of promis-
ing methods for the N2O removal. Several different pho-
tocatalysts are able to decompose N2O into N2 and O2
at ambient temperature.1 In 2000, Sano et al.17 photocat-
alytically decomposed N2O over Ag–TiO2 with methanol
vapor. TiO2 loaded with only 0.16 wt.% of Ag was the
most active photocatalyst. Nevertheless, in these exper-
iments, methanol was used as a reducing agent. Kocˇí
et al.18 described the N2O photocatalytic reduction by pow-
dered Ag–TiO2 photocatalysts with Ag loading from 0.7
to 5.2 wt.%. The photocatalysts were prepared by the
sol–gel process and calcination. The highest N O conver-
processing beside the calcination. The prepared materials
were tested in both the photocatalytic reduction of CO2
and the photocatalytic decomposition of N2O. The photo-
catalytic activity was correlated with the (micro)structure
and optical properties of the developed materials.
2. EXPERIMENTAL DETAILS
2.1. Preparation of TiO2 and N/TiO2
In this work two sets of materials were prepared, pure
TiO2 and nitrogen doped TiO2 (N/TiO2ꢂ, where TiO2 was
prepared by three different ways:
(i) sol–gel method combined with calcination (SG-C),
(ii) hydrothermal method combined with calcination
(HT-C), and
(iii) sol–gel method combined with pressurized fluid pro-
cessing (SG-PFE).
Cyclohexane (C6H12, HPLC grade, Aldrich), non-
ionic surfactant Triton X-114 ((1,1,3,3-tetramethylbutyl)
phenyl-polyethylene glycol, C29H52O8ꢁ5, Aldrich), titanium
(IV) isopropoxide (Ti(OCH(CH3ꢂ2ꢂ4, 99.999%, Aldrich)
and distilled water were used in the sol–gel synthesis.
Titanyl sulphate (TiOSO4, Precheza a.s.), sulphur acid
(H2SO4, Penta), sodium hydroxide (NaOH, Penta) and
distilled water were used in the hydrothermal synthe-
sis. Urea (CO(NH2ꢂ2 Penta) was used for nitrogen dop-
ing. Furthermore, deionized water (electrical conductivity
∼0.06–0.08 ꢃS · cm−1) and methanol (for HPLC) were
IP: 42.106.224.157 On: Fri, 15 Jun 2018 11:31:07
sion was observed for 3.4 wt.% Ag–TiO . On the other
Copyright: American Scientific Publishers
used as media (solvents) during the pressurized fluids
processing.
2
hand, Obalová et al.19 tested Ag–TiO2 thin films with Ag
loading from 0.25 to 1 wt.% of Ag, prepared by the sol–gel
method, and did not observ the change in the N2O conver-
sion with increasing Ag amount.
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Sol–gel synthesis of TiO2. Titania gel was prepared as
follows: In the first step of preparation, cyclohexane was
mixed with Triton X-114 and distilled water, and the mix-
ture was kept under vigorous stirring for 20 min at lab-
oratory temperature. In the second step, titanium (IV)
isopropoxide was injected into the mixture. The micel-
lar titania sol was stirred for further 20 min. After that
the homogeneous transparent sol was poured into Petri’s
dishes in a thin layer (4 mm) and the dishes were left
standing on air for gelation. Generally, the titania sol was
prepared keeping the molar ratio of cyclohexane:Triton
X-114:H2O:Ti(OCH(CH3ꢂ2ꢂ4 to be 11:1:1:1. A gelation
period of titania sol was ∼24 h. After gelation period, the
titania gel was grinded to small pieces (∼3×3 mm) and
A special attention has been focused on the application
of TiO2 prepared by different methods. TiO2 processed
by pressurized hot fluids (after using the sol–gel pro-
cess controlled within reverse micelles) gave the highest
N2O conversion.20 The cerium-doped TiO2 (Ce0ꢁ05Ti0ꢁ95O2,
Ce0ꢁ30Ti0ꢁ70O2 composites), which were prepared by the
same procedure, also showed improved photocatalytic
activity compared to pure TiO2. However, the maximum
N2O conversion was reported over the Ce0ꢁ05Ti0ꢁ95O2 com-
posite. Such high activity was attributed to two factors:
(i) high disorder within the anatase crystal structure and
(ii) the absence of some cerium-containing amorphous
phase/s (e.g., CeO2 or CeTi2O6ꢂ, which can partially block
the surface active sites on the anatase crystals. Concerning
the mechanism, the photocatalytic decomposition of N2O
was accompanied with photolysis of N2O.21
ꢀ
processed either by calcination at 400 C for 4 h using
ꢀ
10 C/min temperature ramp or by pressurized hot fluids
(water and methanol) in order to produce TiO2 nanostruc-
tured powders. Processing by pressurized hot water and
subsequently by pressurized hot methanol was carried out
in a laboratory-made unit equipped with a HPLC BETA10
Plus gradient pump (Ecom s.r.o., Czech Republic), a
chromatographic oven operating in the temperature range
The technique of TiO2 preparation and the kind of
used dopant(s) have been concluded as two main fac-
tors crucially affecting the photocatalytic performance of
TiO2-based materials due to their effects on the material
(micro)structure. Therefore, the aim of this work is the
synthesis of TiO2 and nitrogen doped TiO2 by two differ-
ent chemical methods as well as by pressurized hot fluids
ꢀ
25–400 C, a capillary cooling, and a restrictor operating
at ambient temperature. For more details about the set-
up see Ref. [22] The gel sample placed in 24-ml high-
temperature stainless steel cell was processed in a flow
J. Nanosci. Nanotechnol. 18, 688–698, 2018
689

TiO2 and Nitrogen Doped TiO2 Prepared by Different Methods
Mateˇjová et al.
ꢀ
regime at 200 C and 10 MPa, using 1.5 L of deionized
nitrogen adsorption isotherm at maximum p/p0(∼0.993).
The pore-size distribution was evaluated from the adsorp-
tion branch of the nitrogen adsorption–desorption isotherm
by the Barrett–Joyner–Halenda (BJH) method.28
water, 0.25 L of methanol and 0.1 L of deionized water.
The solvent flow rate during the processing was kept at
3.5–4.5 ml·min−1. Both prepared TiO2 powders (TiO2-SG-
C, TiO2-SG-PFE) were sieved to <0.160 mm particle-size
fraction and divided to two parts: one for the photocat-
alytic tests and one for the subsequent nitrogen doping.
Hydrothermal synthesis of TiO2. Titanyl sulphate solu-
tion (160 ml, concentration of 100 g TiO2/l TiOSO4ꢂ was
mixed with sulphuric acid (840 ml, 0.5 mol · l−1 H2SO4ꢂ.
The solution was under constant mixing heated up to 80
XRD patterns were measured in the Bragg–Brentano
geometry using a Bruker D8 Advance diffractometer
equipped with a LYNXEYE position sensitive detector and
using Ni-filtered CuKꢄ radiation. The measurements were
carried out in the reflection mode with sample spinning.
Powder samples were dispersed in acetone on a low back-
ground Si-substrate. The XRD patterns were recorded in
the 2ꢅ range of 10–80ꢀ with the data step of 0.015ꢀ.
The phase composition, crystallite-size and lattice parame-
ters were evaluated from Rietveld refinement of XRD data
using MStruct software.29ꢀ30 The method was described in
more detail in Refs. [29, 31]. A special attention was paid
on the treatment of higher scattering background at posi-
tions of brookite reflections in some samples, as it can be
assumed32ꢀ33 the effect is related to presence of amorphous
TiO2. The amorphous phase was modelled by very small
(0.6 nm) particles of brookite structure.33
1
ꢀC. After this temperature was reached the 20 wt.%
NaOH solution was added dropwise until the pH was neu-
tral (the pH was checked by universal pH paper indicator).
The formed suspension was left to cool down. Then, the
suspension was filtered. The filter cake was washed with
5 l of deionized water to remove the sulphate anions, the
presence of sulphate anions was tested with 10 wt.% BaCl2
solution. The filter cake was dried in a Petri’s dish on
ꢀ
air at 40 C overnight. The prepared powder was crushed
and sieved to 0.160–0.315 mm particle-sizꢀe fraction, which
ꢀ
was calcined at 400 C for 1 h using 10 C/min tempera-
XPS spectra were recorded on a Prevac photoelectron
spectrometer equipped with a hemispherical VG SCIENTA
R3000 analyzer. The photoelectron spectra were recorded
with constant pass energy of 100 eV for the survey and
for high resolution spectra using a monochromatized alu-
minum source Al Kꢄ (E = 1486ꢁ6 eV) and a low energy
ture ramp (TiO2-HT-C). After calcination TiO2 was repeat-
edly sieved to 0.160–0.315 mm particle-size fraction and
divided to two parts: one for the photocatalytic tests and
one for the subsequent nitrogen doping.
Nitrogen doping. The prepared TiO2 powders
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(TiO2-SG-C, TiO2-HT-C, TiO2-SG-PFE) were modified
by nitrogen using commercial urea. The nitrogen amount
in TiO2 was selected to be 10 wt. % of N. The mechanical
mixtures of proper amounts of TiO2 and urea were dried
at 100 C for 2 h in an oven and then calcined at 400 C
for 2 h using the temperature increase of 2 C/min. After
that, the mixtures were sieved to <0.160 mm particle-
size fraction. This particle-size fraction was used for
all the characterizations as well as the photocatalytic
investigations. The nitrogen doped TiO2 were denoted as
N/TiO2-SG-C, N/TiO2-HT-C and N/TiO2-SG-PFE.
Copyright: American Scientific Publishers
electron flood gun (FS40A-PS) to compensate the charge
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on the surface of nonconductive samples. The base pres-
sure in the analytical chamber was 5 × 10−9 mbar. The
binding energies were referenced to the C 1s core level
(Eb = 285ꢁ0 eV). The composition and chemical surround-
ing of the sample surface were investigated on the basis of
the areas and binding energies of Ti 2p, N 1s, O 1s and
C 1s photoelectron peaks. The fitting of high resolution
spectra was provided through the Casa XPS software.
UV–Vis diffuse reflectance spectra of materials
(<0.160 mm) were measured in quartz cuvettes (thick-
ness 5 mm) by using a GBS CINTRA 303 spectrometer
(GBC Scientific Equipment, Australia) equipped with an
integrating sphere covered by a BaSO4 layer. The spectra
were recorded against a spectralon reference material in
the wavelength range of 200–900 nm (lamps switched at
350 nm). The scan rate was 100 nm/min. The scan step
was 1 nm and the width of monochromator slit was 2 nm.
The reflectances were recalculated to the absorption using
the Kubelka–Munk equation:
ꢀ
ꢀ
ꢀ
2.2. Characterization of Prepared Materials
Nitrogen physisorption, X-ray powder diffraction (XRD),
X-ray photoelectron spectroscopy (XPS) and UV-vis dif-
fuse reflectance spectroscopy were carried out to determine
the textural, structural, surface and optical properties of
the investigated materials.
Nitrogen physisorption was performed at 77 K on
an automated volumetric apparatus NOVA2000e (Quan-
tachrome Instruments, USA) after degassing of materi-
ꢀ
als at 110 C overnight under vacuum less than 1 Torr.
F ꢆR ꢂ = ꢆ1 −R ꢂ2/2 ·R
(1)
The specific surface area, SBET, was calculated accord-
ing to the classical Brunauer–Emmett–Teller (BET) the-
ory for the p/p0 range = 0.05–0.30.23ꢀ24 Additionally, the
mesopore surface area, Smeso, and the micropore volume,
Vmicro, were also evaluated based on the t-plot method.25–27
The net pore volume, Vnet, was determined from the
ꢁ
ꢁ
ꢁ
where R is the diffuse reflectance from a semi-infinite
ꢁ
layer. The obtained spectra were transformed to the depen-
dencies ꢆF ꢆR ꢂ · hꢇꢂ2 against hꢇ in order to obtain the
ꢁ
values of the energy of absorption edge.34
690
J. Nanosci. Nanotechnol. 18, 688–698, 2018

Mateˇjová et al.
TiO2 and Nitrogen Doped TiO2 Prepared by Different Methods
Table I. Surface and textural properties of investigated TiO2 and
N/TiO2.
2.3. Photocatalytic Reduction of CO2
The CO2 photocatalytic reduction was performed in a
stirred batch photoreactor with a suspended photocatalyst
(0.12 g) illuminated by a UV 8 W Hg lamp (ꢈ = 254 nm)
situated on the top of photoreactor; the shell tube was
made from stainless steel. Supercritical fluid-grade CO2
was used as a reactant and 0.2 M NaOH solution (120 mL)
was a reduction medium. The analysis of gaseous reaction
products was performed in GC/BID. The details about the
experiment setup and analytical methods were depicted in
our previous work.35
XPS
Nitrogen physisorption
Smeso Vmicro Vnet
Ti
N
SBET
Material
(at.%) (at.%) (m2/g) (m2/g) (mm3liq/g) (mml3iq/g)
TiO2-SG-C
N/TiO2-SG-C
TiO2-HT-C
N/TiO2-HT-C
TiO2-SG-PFE
n.d.
24ꢁ97 0ꢁ76
n.d. n.d.
20ꢁ48 0ꢁ00
n.d. n.d.
n.d.
105
103
115
101
180
120
88
65
26
23
26
24
31
21
140
159
190
208
301
288
71
60
133
88
N/TiO2-SG-PFE 23ꢁ37 0ꢁ49
The accuracy of measuring was verified by repeated
measurements and the relative error of products yields
of 10% was determined. Blank tests were also carried
out to ensure that the hydrocarbon production was due to
the photoreduction of CO2.35 CH4, CO and H2 were not
detected in any blank tests.
Note: n.d. not determined.
about 400 eV. The position of this peak corresponds very
well to the Eb value described in the literature as typical
of nitrogen in O–Ti–N linkages.37 So high Eb value sug-
gests the reduction of electron density in nitrogen atoms
by their introduction into the TiO2 lattice together with
highly electronegative oxygen atoms.
2.4. Photocatalytic Decomposition of N2O
The N2O photocatalytic decomposition was performed in a
batch annular photoreactor with or without a photocatalyst
(photocatalysis and photolysis, respectively). The powdery
photocatalyst was spread on the bottom of the photoreac-
tor (of 660 mL volume). The photoreactor was filled with
the N2O/N2 mixture (968 ppm). The reaction was initiated
by switching on a 8 W Hg lamp (ꢈ = 254 nm) situated in
the center of the quartz tube; the shell tube was made of
The results of nitrogen physisorption measurements are
shown in Figures 1(a–c) and summarized in Table I. All
investigated materials possess mesoporous structure with
small amounts of micropores. Each set of TiO2 and N/TiO2
materials (i.e., SG-C, HT-C and SG-PFE) show very sim-
ilar microporous-mesoporous structure. With respect to
the different preparations, TiO2 and N/TiO2 materials pro-
cessed by pressurized fluids (TiO2-SG-PFE, N/TiO2-SG-
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stainless steel. The pressure and the temperature inside the
PFE) exhibit the highest surface areas (S
of 180 m2/g
Copyright: American Scientific Publishers
BET
photoreactor were monitored constantly. A gas chromato-
graph with barrier ionization discharge detector (GC/BID)
was used for the analysis of N2O concentration at the start
of reaction and during the reaction. The reproducibility of
photocatalytic experiments was verified by repeated mea-
surements.
2
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and 120 m /g). These values are markedly higher than
that of both calcined sets (Table I). It is also evident that
in N/TiO2-SG-C undergoing re-calcination after doping of
TiO2-SG-C by nitrogen any significant sintering did not
occur, since TiO2-SG-C and N/TiO2-SG-C have very sim-
ilar surface areas (SBET of 105 m2/g and 103 m2/g, respec-
tively). This effect was, however, more significant in the
HT-C and SG-PFE sets.
3. RESULTS AND DISCUSSION
The XRD patterns (Fig. 2(a)) confirm the existence of
major anatase phase in all investigated materials. Besides
the major anatase phase, the presence of brookite phase
is also identified in all sets of materials, which is evi-
dent from the whole powder pattern fitting of XRD data
(Figs. 2(b and c)). Quantitative analysis gives a fraction
of nanocrystalline brookite around 25 wt.% in TiO2-SG-
PFE and N/TiO2-SG-PFE (Table II). Such intermixture of
nanocrystalline anatase and brookite is not surprising.38ꢀ39
Increased background between first two anatase lines in
TiO2-HT-C and N/TiO2-HT-C may indicate some amounts
of brookite-like TiO2 present rather in amorphous form
(Table II, Fig. 2). Small amounts of amorphous brookite
are also observed in TiO2-SG-C (17 wt.%) and N/TiO2-
SG-C (6 wt.%). For all N/TiO2, the peak intensities
increased compared to corresponding undoped TiO2. This
indicates that nitrogen doped TiO2 show enhanced crys-
tallinity with larger anatase crystallite-size (by ∼1 nm)
than undoped TiO2. Anatase lattice parameters and cell
3.1. Photocatalysts Characterization
The prepared materials were characterized by using several
complementary techniques in order to reveal their physico-
chemical and optical properties, which could explain their
performance in both photocatalytic reactions.
Surface chemical composition of the N-doped TiO2
materials was measured by XPS. Table I gives the con-
tent of Ti and N. All N-doped TiO2 samples exhibit the
emission of Ti 2p1/2 and Ti 2p3/2 spin-orbital splitting
photoelectrons at binding energies of 464.1–464.3 eV and
458.3–458.5 eV, respectively, which are characteristic of
Ti4+. Furthermore, the separation of peak 5.8 eV to the
Ti 2p1/2 and Ti 2p3/2 signals corresponds to the values
reported for TiO2 (NIST, 2012). Surface oxygen is present
in three chemical states–as O2− lattice ions (Eb = 529ꢁ6–
529.8 eV), hydroxyls (Eb = 530ꢁ8–531.2 eV), and strongly
adsorbed water (Eb = 535ꢁ0 eV).36 For the N/TiO2-SG-
PFE and N/TiO2-SG-C, the traces of nitrogen appear on
the surface as it is identified by very weak N 1s peak at
J. Nanosci. Nanotechnol. 18, 688–698, 2018
691

TiO2 and Nitrogen Doped TiO2 Prepared by Different Methods
Mateˇjová et al.
200
150
100
50
200
150
100
50
(a)
(b)
101
102
101
102
Pore diameter (nm)
Pore diameter (nm)
TiO2-HT-C
TiO2-HT-C
N/TiO2-HT-C
N/TiO2-HT-C
0
0
0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8 1.0
p/p0 (–)
p/p0 (–)
250
200
150
(c)
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1
2
Copyright:1A0 merican Sc1ie0ntific Publishers
100
Pore diameter (nm)
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50
TiO2-SG-PFE
N/TiO2-SG-PFE
0
0.0
0.2
0.4
0.6
0.8 1.0
p/p0 (–)
Figure 1. Measured nitrogen adsorption–desorption isotherms and evaluated pore-size distributions (in insets) of investigated TiO2 and N/TiO2.
volumes (136.27–136.41 Å3ꢂ of all TiO2 do not change
markedly and are very similar to values reported in
Ref. [40]. Moreover, it can also be seen that the anatase
cell volume of N/TiO2-SG-C and N/TiO2-SG-PFE are
comparable slightly to parent TiO2-SG-C and TiO2-SG-
PFE. These facts reveal that low nitrogen doping did not
affect the microstructure of parent TiO2 and the anatase
crystallite-size in N/TiO2 increased in a consequence of
the re-calcination during preparation.
(3.24 eV) > TiO2-SG-C (3.21 eV). Nitrogen loading did
not lead to any significant changes in the band gap ener-
gies. In correlation with structural and microstructural
results it can be said that the different band gap ener-
gies of individual TiO2 may be caused by different phase
composition (i.e., ratio of anatase-to-brookite) and crys-
tallinity (i.e., character of brookite, anatase crystallites-
size) of individual TiO2. Negligible changes in TiO2 band
gap energies after nitrogen doping can be explained by the
trace contents of nitrogen being incorporated into the TiO2
lattice.
The UV-vis reflectance spectra of all the investigated
materials from which the absorption edges were deter-
mined are shown in Figure 3. Plotting ((F ꢆR ꢂhꢇꢂ2 versus
ꢁ
3.2. Photocatalytic Reduction of CO2
hꢇ provides the extrapolated intercept corresponding to
the band gap energy (Eg) values (Table II). The absorp-
tion edge energies of TiO2 were shifted to lower val-
ues in the order: TiO2-HT-C (3.36 eV) > TiO2-SG-PFE
The different types of TiO2 and N/TiO2 were investigated
for the photoreduction of CO2 over a period of 0–24 h.
Figure 4 shows the progression of all reaction products as
692
J. Nanosci. Nanotechnol. 18, 688–698, 2018

Mateˇjová et al.
TiO2 and Nitrogen Doped TiO2 Prepared by Different Methods
Figure 2. (a) XRD patterns of all prepared TiO2 and N/TiO2, (b) the background fit of TiO2-HT-C with amorphous-like brookite and (c) the fit of
small brookite peaks in TiO2-SG-PFE. Three full black lines at the bottom of the (a) sub-plot represent simulated nanocrystalline diffraction patterns
for anatase, amorphous-like brookite and naocrystalline brookite with typical parameters used in the fits.
functions of the irradiation time for TiO2-SG-C. Two main
products were determined, methane and carbon monoxide
The yields of methane over calcined materials (TiO2-SG-
C, N/TiO2-SG-C) were significantly higher in comparison
in a gas phase. Hydrogen was also detected. The appear-
ance of other products such as formic acid, methanol
and formaldehyde was not noticed. The observed yields
(ꢃmol/gcatꢂ order was: CO > H2 > CH4.
Comparisons of CO, CH4 and H2 yields over all inves-
tigated materials are shown in Figures 5(a–c), respec-
tively. The highest yields from all detected products were
observed for carbon monoxide (Fig. 5(a)). The yields of
carbon monoxide increased evenly during the reaction.
The CO yields were comparable for all materials with the
exception of TiO2-SG-C.
with other ones.
IP: 42.106.24.157 On: Fri, 15 Jun 2018 11:31:07
Copyright: American Scientific Publishers
The yield of hydrogen (Fig. 5(c)) increased evenly dur-
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ing the reaction. The formation of hydrogen in presence
of all N/TiO2 was significantly lower in comparison with
all undoped TiO2. The substantially higher activity was
proved over TiO2-SG-C.
The highest yields of all reaction products were
observed for TiO2-SG-C. The presence of nitrogen in
N/TiO2-SG-C caused the decline of all products.
3.3. Photocatalytic Decomposition of N2O
The yields of methane were two orders of magnitude
lower than the yields of carbon monoxide (Fig. 5(b)).
The N2O photocatalytic decomposition over all prepared
materials was investigated over a period of 0–18 h. The
Table II. Structural, microstructural and optical properties of investigated TiO2 and N/TiO2.
Phase composition
XRD phase
Anatase lattice parameters
Material
Xa (wt.%)
Xb (wt.%)
character of brookite
ꢂDaꢃV (nm)
a (Å)
c (Å)
Vcell (Å3ꢂ
Absorption edge (eV)
TiO2-SG-C
N/TiO2-SG-C
TiO2-HT-C
83
94
51
17
6
49
Amorphous (n.d.)
Amorphous (n.d.)
Amorphous
9ꢁ2
10ꢁ1
10ꢁ7
3ꢁ787
3ꢁ786
3ꢁ789
9ꢁ503
9ꢁ506
9ꢁ502
136ꢁ30
136ꢁ27
136ꢁ41
3ꢁ21
3ꢁ20
3ꢁ36
(∼0.6 nm nuclei)
N/TiO2-HT-C
TiO2-SG-PFE
N/TiO2-SG-PFE
53
75
77
47
25
23
Amorphous
11ꢁ5
8ꢁ8
3ꢁ789
3ꢁ791
3ꢁ790
9ꢁ503
9ꢁ489
9ꢁ492
136ꢁ41
136ꢁ38
136ꢁ34
3ꢁ35
3ꢁ24
3ꢁ22
(∼0.6 nm nuclei)
Nanocrystalline
(∼2.5 nm)
Nanocrystalline
(∼2.5 nm)
10ꢁ3
Notes: Xa—weight fraction of anatase. Xb—weight fraction of brookite. ꢂDaꢃV —volume-weighted anatase crystallite-size.
J. Nanosci. Nanotechnol. 18, 688–698, 2018
693

TiO2 and Nitrogen Doped TiO2 Prepared by Different Methods
Mateˇjová et al.
4
the activity decreased in order: TiO2-SG-C > N/TiO2-
SG-C > TiO2-HT-C ∼ N/TiO2-HT-C ∼ TiO2-SG-PFE ∼
N/TiO2-SG-PFE. Since all investigated materials were the
mixtures of anatase and brookite, however, with different
crystallographic composition and character of the brookite
phase, it is evident that these features are more decisive
parameters influencing the photoactivity than the nitro-
gen doping or the surface area. In agreement to some
reports,41–43 the anatase–brookite mixture is more effective
than the pure anatase, brookite or anatase–rutile for the
photocatalytic reactions. However, only few reports have
been focused on the bicrystalline anatase–brookite mixture
for the CO2 photoreduction.44–46 Liu et al.46 tested the
crystal phase of TiO2, i.e., pure anatase (A), an anatase-
rich anatase/brookite mixture (AB), or a brookite-rich
anatase/brookite mixture (BA) and found out that the most
active photocatalyst was anatase-rich anatase/brookite mix-
ture. Zhao et al.46 reported that the bicrystalline anatase–
brookite was generally more active than the single-phase
anatase or brookite in the photocatalytic reduction of CO2.
Kocˇí et al.44 showed that the feature responsible for better
photocatalytic activity in the CO2 photoreduction was the
surface heterojunction of bicrystalline anatase–brookite.
It is necessary to point out that any of these works did not
discuss the effect of character of brookite in the mixture
with anatase.
TiO2-HT-C
N/TiO2-HT-C
TiO2-SG-PFE
N/TiO2-SG-PFE
TiO2-SG-C
N/TiO2-SG-C
100
2
50
0
3.0
3.1
3.2
3.3
3.4
3.5
energy, eV
0
3.0
3.5
4.0
energy, eV
Figure 3. UV-Vis DRS spectra of investigated TiO2 and N/TiO2 and
the determination of absorption edges.
time dependences of N2O conversion over all investi-
gated materials are shown in Figure 6(a). The progressive
increase in N2O conversion with the growing irradiation
time was observed. The N2O conversions reached over all
investigated materials were higher than in the case of pho-
tolysis (reaction without photocatalyst).
For TiO2, the N2O conversion decreased in the order:
TiO2-HT-C > TiO2-SG-PFE > TiO2-SG-C (Fig. 6(b)).
In contrast, the conversion of N2O for all N/TiO2 was com-
parable and it was comparable to TiO2-HT-C, which was
In our work, the different mixtures of anatase and
IP: 42.106.24.157 On: Fri, 15 Jun 2018 11:31:07
63% after 18 h (Fig. 5(c)).
Copyright: American Scientific Publishers
brookite were prepared due to the application of different
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preparation approaches (Table II). In the case of TiO2-SG-
C and N/TiO2-SG-C, the anatase phase with minimum of
brookite-like phase in rather amorphous form, 17 wt.%
and 6 wt.%, respectively, was obtained. Very similar
were TiO2-HT-C and N/TiO2-HT-C, however, with higher
content of brookite-like phase (47 wt.% and 49 wt.%,
respectively). The only nanocrystalline anatase-brookite
mixtures were TiO2-SG-PFE and N/TiO2-SG-PFE. Con-
sidering the highest surface areas of bicrystalline TiO2-SG-
PFE and N/TiO2-SG-PFE, which showed low photoactivity
in the CO2 photoreduction, and practically very similar
anatase crystallite-sizes of all investigated materials, it is
evident that the surface area and the anatase crystallite-
size, which are usually very important parameters, are not
crucial ones in this case.
3.4. Aspects Influencing the Photocatalytic Reduction
of CO2 and the Photocatalytic Decomposition of
N2O Over Different TiO2 and N/TiO2
Different photocatalytic behaviour of TiO2 and N/TiO2
revealed an important role of structure and nitrogen doping
in both reactions. In the CO2 photocatalytic reduction,
8
CH4*100
7
CO
6
H2*5
5
4
3
2
1
0
The enhanced photoactivity of anatase-brookite mixture
can be explained by three reasons:
(i) the excited electrons on brookite conduction band (CB)
can transfer to anatase conduction band (CB) due to a
slightly higher CB edge of brookite (about 0.14 eV)42ꢀ43ꢀ46
than anatase, thus enhancing charge separation;
(ii) the facilitated formation of the intrinsic defect sites in
brookite may enhance electron trapping;46ꢀ47 and
0
5
10
15
20
(iii) the distorted interfaces between anatase and brookite
crystals may facilitate interfacial electron transport and
prevent the recombination of electron–hole pairs.46ꢀ48
Time (hour)
Figure 4. The time dependences of the product yields over TiO2-SG-C.
694
J. Nanosci. Nanotechnol. 18, 688–698, 2018

Mateˇjová et al.
(a)
TiO2 and Nitrogen Doped TiO2 Prepared by Different Methods
8
7
6
5
4
3
2
1
0
(b) 0.1
TiO2-SG-C
N/TiO2-SG-C
TiO2-HT-C
TiO2-SG-C
N/TiO2-SG-C
TiO2-HT-C
0.08
0.06
0.04
0.02
0
N/TiO2-HT-C
TiO2-SG-PFE
N/TiO2-HT-C
TiO2-SG-PFE
N/TiO2-SG-PFE
N/TiO2-SG-PFE
0
5
10
15
20
0
5
10
15
20
Time (hour)
Time (hour)
1.4
(c)
TiO2-SG-C
N/TiO2-SG-C
TiO2-HT-C
1.2
N/TiO2-HT-C
TiO2-SG-PFE
1.0
0.8
N/TiO2-SG-PFE
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0.4
0.2
0.0
0
5
10
15
20
Time (hour)
Figure 5. The time dependence of (a) carbon monoxide yields (b) methane yields and (c) hydrogen yields over individual investigated materials in
the CO2 photocatalytic reduction.
Conditions: 23 h of irradiation time, 8 W Hg lamp, CO2 pressure at carbonation of 110 kPa, 120 mL of H2O, photocatalyst concentration of 1 g/L,
pH 7.
Thus, the key property affecting the photocatalytic activity
of anatase-brookite mixtures in the CO2 photoreduction is
the character of brookite, which influences the interface
arrangement of both crystal phases and thus the photoac-
tivity as mentioned in points (ii) and (iii). It can be con-
cluded that the presence of amorphous brookite in the
anatase-rich anatase-brookite mixture enhances the mate-
rial photoactivity in the CO2 photoreduction.
The surface states play a crucial role in photocatalytic
reactions. The oxygen defects on the surface of photocat-
alyst are a sort of trapped site for photoexcited electrons,
which can separate electrons from holes, which is very
significant for photocatalysis. However, too many oxygen
defects can act as recombination centers and reduce the
lifetime of photoexcited electrons and holes. The doping
by nitrogen can increase the number of oxygen defects
considerably.49ꢀ50 The nitrogen doping in TiO2 decreased
a bit its photocatalytic activity in the CO2 photocat-
alytic reduction. According to the anatase lattice parame-
ters determined by WPPM analysis (Table II), which are
similar for all investigated TiO2 and N/TiO2 it is obvi-
ous that the nitrogen doping did not influence any sig-
nificantly the anatase crystal structure. However, it can
be assumed that due to the number of valence electrons
and the atomic diameters of N and O which are various,
the amount of nitrogen doped leads to the appearance of
defect sites and non-stoichiometry in the material. The
doped nitrogen ions and oxygen defects, which can acts as
J. Nanosci. Nanotechnol. 18, 688–698, 2018
695

TiO2 and Nitrogen Doped TiO2 Prepared by Different Methods
Mateˇjová et al.
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
(a)
(b)
N/TiO2-SG-C
TiO2-HT-C
TiO2-HT-C
TiO2-SG-PFE
TiO2-SG-C
Photolysis
N/TiO2-HT-C
N/TiO2-SG-PFE
TiO2-SF-PFE
TiO2-SG-C
0
3
6
9
12
15
18
0
3
6
9
12
15
18
Time (hour)
Time (hour)
70
(c)
N/TiO2-SG-C
N/TiO2-HT-C
N/TiO2-SG-PFE
Photolysis
60
50
40
30
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10
0
0
3
6
9
12
15
18
Time (hour)
Figure 6. The time dependences of N2O conversion over (a) all investigated materials, (b) all TiO2 and (c) all N/TiO2.
recombination centers and limit the lifetime of photoex-
cited electrons and holes, thereby come down photocat-
alytic activity of materials under UV irradiation in the CO2
photoreduction, particularly water splitting and hydrogen
radicals formation which are necessary for CO2 photore-
duction. It is obvious that the production of hydrogen in
presence of all N/TiO2 is significantly lower compared to
all pure TiO2. On the other hand, an opposite effect is
evidenced in the N2O photocatalytic decomposition. The
best photoactivity was proved for all N/TiO2 and TiO2-
HT-C. As was reported, oxygen vacancies enhance the
N2O decomposition, and the lifetime of the N2O-vacancy
interaction may play the key role in this reaction.51 In
our case the nitrogen doping obviously led to creation of
more defect sites and non-stoichiometry on the TiO2 sur-
face. This feature could explain the comparable and higher
photoactivity of all N/TiO2 in the N2O photocatalytic
decomposition.
4. CONCLUSIONS
TiO2 as nanostructured powders were prepared by sol–gel
process or hydrothermal method in combination with (1)
the processing by pressurized hot water and methanol or
(2) calcination. The prepared TiO2 were modified with
nitrogen using commercial urea. The developed photo-
catalysts showed significantly different textural properties
(specific surface area) and phase composition (different
ratio of anatase-to-brookite and character of brookite).
In addition, the photocatalysts also showed different
behaviour in the CO2 photocatalytic reduction and the N2O
photocatalytic decomposition.
The modification of TiO2 by nitrogen was shown to
be beneficial in the N2O decomposition. In this case, the
nitrogen doping significantly increased the N2O decompo-
sition over N/TiO2 prepared by sol–gel method combined
with calcination (SG-C) and by sol–gel method combined
with pressurized fluid processing (SG-PFE). It was sug-
gested that the nitrogen doping led to the formation of
696
J. Nanosci. Nanotechnol. 18, 688–698, 2018

Mateˇjová et al.
TiO2 and Nitrogen Doped TiO2 Prepared by Different Methods
more defect sites and non-stoichiometry on the TiO2 sur-
face that increased the N2O decomposition in the case
of these nitrogen-doped TiO2. In contrast, the effect of
nitrogen loading was not observed in TiO2 prepared by
hydrothermal method combined with calcination (HT-C),
as hydrothermal synthesis itself represents the process
leading to a significant amount of defect sites and non-
stoichiometry on the TiO2 surface.
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Acknowledgments: The financial support of the Grant
Agency of the Czech Republic (projects Nos. 14-
23274S and 14-35327J) and the EU project “ENET”
(No. CZ.1.05/2.1.00/03.0069) is gratefully acknowledged.
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Received: 29 August 2016. Accepted: 10 October 2016.
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698
J. Nanosci. Nanotechnol. 18, 688–698, 2018