Mesocrystalline anatase nanoparticles synthesized using a simple hydrothermal approach with enhanced light harvesting for gas-phase reaction

Article abstract of DOI:10.1039/c8dt00721g

Mesocrystalline TiO₂ nanoparticles were synthesized using a hydrothermal approach. A simple two-step procedure at low temperature (<140 °C) allowed the nucleation of primary particles sized 2-4 nm and their subsequent assembly as almost spherical aggregates sized ≈20 nm. X-ray powder diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) studies, and HRTEM studies confirmed anatase as the unique TiO₂ crystalline phase. The mesocrystalline structure of the anatase aggregates was clearly evidenced by HRTEM and SAED results. The mesocrystalline nanopowders exhibit a mesoporous structure with a surface area and pore volume of 63.5 m² g^-1 and 0.22 cm³ g^-1, respectively. Ultraviolet (UV) and visible light (Vis) absorption ability were recorded. The combined high effectiveness and selectivity for the NO_x abatement of the new mesocrystalline photocatalyst are reported. It is worth remarking that the maximised selectivity values reached for the NO_x process are reported for the first time and could be associated with the mesoporous nature of the anatase photocatalyst.

Full text of DOI:10.1039/c8dt00721g

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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ARTICLE
Mesocrystalline anatase nanoparticles synthesized by a simple
hydrothermal approach with enhanced light harvesting for gas‐
phase reaction
Received 00th January 20xx,
Accepted 00th January 20xx
b
c
José Balbuena,a José Miguel Calatayud,b Manuel Cruz‐Yusta,a Pablo Pardo, Francisco Martín,
Javier Alarcón,*b and Luis Sánchez*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Mesocrystalline TiO2 nanoparticles are synthesized by a hydrothermal approach. A simple two‐step procedure at low
temperature (< 140 C) let the nucleation of primary particles sized 2‐4 nm and their subsequent assembly as almost
spherical aggregates sized  20 nm. X‐ray powder diffraction (XRD), Raman and X‐ray photoelectron (XPS) spectroscopies,
and HRTEM studies confirm the anatase as unique TiO2 crystalline phase. The mesocrystalline structure of the anatase
aggregates is clearly evidenced by HRTEM and SAED. The mesocrystalline nanopowders exhibit a mesopore structure with
surface area and pore volume of 63.5 m2∙g‐1 and 0.22 cm3∙g‐1, respectively. Apart from ultraviolet (UV), visible light (Vis)
absorption ability was recorded. The combined high effectiveness and selectivity on the NOx abatement of the new
mesocrystalline photocatalyst is reported. It is to remark that the reached maximized selectivity through the NOx process is
reported by the first time and could be associated with the mesopore nature of the anatase photocatalyst.
According to Cölfen and Antonietti a mesocrystal is defined as a
superstructure of crystalline nanoparticles with extended
Introduction
crystal faces on the scale of some hundred nanometres to
micrometres 6. However, it is interesting to point out that have
been reported mesocrystalline structures made up by
nanoparticle assemblies but without a well faceted external
morphologies 7, 8. The specific characteristics of mesocrystals, i.
e. nanoparticle size and shape, order and porosity allow
developing unusual mechanical, magnetic, chemical and optical
properties. A main potential application of mesocrystals is
photocatalyst due to their inner surface area and porosity,
particle orientation and exposure of catalytically active crystal
The arrangement of nanoparticle building blocks into well‐
defined assemblies and superstructures has been a crucial
target in materials chemistry because these hierarchical
structures lead to novel and unique properties that are not
found in the individual building blocks. Those highly ordered
nanoparticle superstructures in which the nanocrystals are
organized in a crystallographic register are called mesocrystals,
which is the abbreviation mesoscopically structured crystals 1‐5
.
9
a. Departamento de Química Inorgánica, Instituto Universitario de Investigación en
Química Fina y Nanoquímica IUIQFN, Universidad de Córdoba, Campus de
Rabanales, E‐14071 Córdoba, Spain. E‐mail: luis‐sanchez@uco.es
faces . Among semiconductors advanced titania compounds
have been used as photocatalysts for photodegradation of
pollutants. They have not been chosen only by their chemical
stability, low cost and non‐toxicity but the improved
photocatalytic activity of mesoporous TiO2 consisting of
compactly packed nanoparticles due to the efficient
interparticle electron transfer between nanocrystals 10. In
addition, experimental and theoretical studies have suggested
that the ordering of a nanocrystalline structure significantly
b. Inorganic Chemistry Department, University of Valencia, Calle Dr. Moliner 50,
Burjasot (Valencia), 46100, Spain. E‐mail: javier.alarcon@uv.es
c. Chemical Engineering Department, Campus de Teatinos, Universidad de Málaga,
Málaga, 29071, Spain. E‐mail: marjim@uma.es
* Corresponding Authors:
Prof. Luis Sánchez
Inorganic Chemistry Department, Instituto Universitario de Investigación en Química
Fina y Nanoquímica IUIQFN, Universidad de Córdoba, Campus de Rabanales, E‐
14071 Córdoba, Spain. E‐mail: luis‐sanchez@uco.es
E‐mail: luis‐sanchez@uco.es
influences charge transport and recombination 11
.
Tel: +00‐34‐957‐218634
Prof. Javier Alarcón
Nitrogen oxides, commonly referred as NOX ‐ the nitric oxide
(NO) and nitrogen dioxide (NO2) ‐, are a group of different
compounds that play a major role in atmospheric chemistry and
air pollution. In spite of several procedures have been
developed to reduce the NOx emission of combustion
processes, air purification in large cities and industrial areas is
still a great challenge. Even though the selective catalytic or
non‐catalytic reduction processes are well‐known, they aren´t
Inorganic Chemistry Department, University of Valencia, Calle Dr. Moliner 50,
Burjasot (Valencia), 46100, Spain.
E‐mail: javier.alarcon@uv.es
Tel: +00‐34‐96‐354‐4584
Electronic Supplementary Information (ESI) available: [Supplementary Information
accompanying this paper includes more information about: the characterization of
the samples; reported De‐NOX values of different titanium based photocatalysts;
crystallographic data, chemical analysis and textural properties; X‐ray diffraction;
XPS spectroscopy; SEM‐TEM micrographs; P25 characterization; photocatalytic
activity.]. See DOI: 10.1039/x0xx00000x
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applicable to directly remove the NOX gases in the urban alkoxide/TEOA molar ratio of 1/2. This solution was
atmosphere (De‐NOX action), which inlets are mostly coming hydrothermally aged in two steps. Through the first a gel was
from the road traffic. Anatase TiO2 has been already used formed after heating at 100
mainly as integrated additive in building materials for an at 140 C for 72 h. After the hydrothermal aging the sample was
effective NOx depollution in urban areas 12. In the last few years centrifuged and washed three times with water. The product
C for 24 h followed by the heating

new titania‐based materials with enhanced De‐NOx was finally dried overnight in a drying oven at 100 C and
13‐20
photocatalytic performance have been developed
.
labelled as A. In order to remove TEOA, the as‐prepared sample
C for 30 min and named as A350.
However, by those modified titania photocatalysts the NO2 gas A was annealed at 350
emissions intermediate specie appearing during the
photochemical oxidation of nitrogen oxide gases to nitrate 21



Instrumentation
are not fully inhibited, reaching only low levels for either a very
short period of light irradiation or when negligible NO
Chemical, structural and microstructural characteristics of
photochemical oxidation accounts (Electronic Supplementary nanoparticles were monitored by different experimental
Information (ESI), Table S1). At this point it seems to be techniques.
worthwhile to remember that nitrogen dioxide (NO2) is
Elemental analysis was carried out in a CE Instruments
undesired because is 8 to 25 times more dangerous than NO. EA1110 Elemental Analyzer, set for the detection of Nitrogen,
Therefore, in the De‐NOX photocatalytic process the target is Carbon and Hydrogen, using atropine as standard.
not only the highest NO conversion to NO2 but the highest
X‐ray diffraction (XRD) analysis (model D‐8 Advance, Bruker)
conversion to nitrate or nitric acid. In our viewpoint, the was performed using CuKα radiation. The diffractometer had 1
selectivity of the De‐NOx photocatalytic reaction, i.e. the and 3 mm divergence and antiscattering slits respectively, and
complete transformation of NO to nitrate or nitric acid, a 3
deserves much attention and our objective is to prepare and identification, the diffractogram was run with a step size of 0.02
characterize an anatase photocalyst with the suitable 2θ and a counting time of 0.2 s. Raman spectra of samples
nanostructure, porosity and absorption of visible light to reach were obtained by means of a Raman spectrometer (Model
 2θ range Lynxeye linear detector. For the phase

enhanced selectivity.
XploRA, Horiba) with 785 nm laser. X‐ray photoelectron spectra
(XPS) were recorded using non‐monochromated MgK radiation
In this work, a mesocrystalline and mesoporous anatase (hν= 1253.6 eV) and a hemi‐spherical analyser operating at a
structure is prepared by using simple hydrothermal constant pass energy of 29.35 eV (Physical Electronics PHI 5700
a
methodology, and characterized by several suitable techniques. spectrometer) with the X‐ray generator operating at 15 kV and
As afore‐mentioned the ordering of nanocrystalline structures 300 W.
in self‐assembled nanomaterials considerably influences charge
The microstructure of samples was examined by field
transport and recombination, which are directly related to the emission scanning electron microscopy (FESEM) at 20 kV
performance of photocatalysts. Moreover, the presence of (Model S‐4800, Hitachi Ltd., Tokyo, Japan). The samples were
mesoporous on these structures increases the surface area of prepared by dropping a dispersion of the powders in water
the samples and its accessibility towards the reactant directly onto the holder sample. Before the examination all
molecules, given rise to high efficiency degradation processes specimens were coated with gold/palladium in an ion beam
22‐24. The mesocrystalline TiO2 is studied as photocatalyst for the coater. The morphology of A and A350 titania particles was also
NOX abatement. As a novelty, here is presented a very high examined using transmission electron microscopy (TEM; Model
selective De‐NOX titania photocatalyst combining high NO 1010, Jeol Ltd., Tokyo, Japan) at an accelerating voltage of 100
conversion values also, being both characteristics maintained kV. Samples were dispersed in doubly distilled water and drops
for a large irradiation period. To achieve this successfully we of the dispersion were transferred to a specimen copper grid
have improved the photochemical performance of titania by carrying a lacey carbon film. High resolution transmission
expanding its activity towards Vis light and developing a electron microscopy (HRTEM) images and selected area
mesoporous structure.
electron diffraction (SAED) were collected using a Tecnai G2 F20
field emission electron microscope equipped with a Gatan CCD
camera. Data treatment was performed with Digital Micrograph
software. The samples were prepared in a similar way to the
ones for TEM.
Materials and methods
Preparation of mesoporous anatase
Nitrogen absorption isotherms were obtained at 77.4 K on
an ASAP 2020 instrument from Micromeritics. Specific surface
areas were calculated from N2 sorption isotherms, using the
multipoint Brunauer–Emmett–Teller (BET) method and
microporosity and size distributions were estimated using the t‐
plot, Barret‐Joyner‐Halenda (BJH).
UV‐Vis diffuse reflectance (DR) spectra of the specimens
(Model V‐670, Jasco) were obtained using the diffuse
reflectance technique in the range of 200 to 2500 nm.
Samples were prepared by a hydrothermal approach in two
steps. The starting materials were tetraisopropyl orthotitanate
(C12H28O4Ti) and triethanolamine (C6H15NO3, TEOA) as
assembler agent. The nominal concentration of TiO2 in the PTFE
stainless steel autoclave was 0.25 M. The synthetic procedure
can be described as follows. First, an amount of TEOA was
solved in doubly distilled water to reach
a titanium
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Probe NO (>99.9%, from Abello Linde S.A.) was carried out Samples preparation and characterization
in an IR cell connected with a conventional outgassing/gas
manipulation apparatus and spectra were recorded in a Nicolet
Nexus 6700 spectrometer with MCT detector.
By the hydrothermal synthesis approach discrete, size‐
controlled and well‐defined morphologically mesocrystalline
Additional information is provided in Electronic nanoparticles were obtained. According to the literature
25, 26
Supplementary Information.
different assembler
and polymeric additives have been
used 27‐30. TEOA was used as additive some years before to
prepare uniform spindle‐type titania by a sol‐gel via 31. The
product was identified by XRD as anatase particles elongated in
Photocatalysis
The photocatalytic activity of the materials towards the the direction of the c‐axis, being both the size and the aspect
oxidation of NO was studied by using a 50 x 50 mm sampler ratio of the resulting particles dependent on the pH. The
holder placed in a laminar flow reactor. The reactor was placed method of preparation named “the gel‐sol method” was
inside a light sealed irradiation box (Solarbox 3000e RH) proposed for the preparation of monodisperse particles in large
equipped with a Xe lamp with controlled irradiance. A 200 mg quantities. Polleux et al. added small amounts of low‐molecular‐
sample was used in each photocatalytic test. Artificial sunlight weight functional ligands as the ethanolamine as assembler to
(25 and 550 W∙m‐2 for UV and visible irradiances, respectively) control the assembly behaviour of preformed titania
was used as irradiation. A mixture of air/NO was sent to the nanocrystals 26. This synthesis was performed in benzyl alcohol
photoreactor. Air and NO gas streams were mixed to obtain the as solvent. The results were not successful and the
desired NO concentration: 500 ppb. Previously, air was redispersions in water of surface‐functionalized anatase
conveyed by a gas‐washing bottle, filled with demineralised crystals lead to uncontrolled agglomeration. In our synthetic
water in order to keep the relative humidity of the supplied gas procedure we have used titanium tetraisopropyl orthotitanate
fixed at 50 ± 5 %. A flow rate, Q, of 0.75 L∙min‐1 was employed. as titanium source, triethanolamine amine as assembler and
The accurate measurement of the concentration of NO, NOX water as solvent. We are going to prove that by the present
and NO2 was carried out using a chemiluminescence analyser synthetic procedure oriented superstructures of nanocrystals
(model Environnement AC32M). For each test, the air/NO gas formed from crystalline building‐block, although without a
stream ran over the sample in the dark for a period of 10 min faceting of the nanocrystals aggregates.
without changes in the NOX concentration profiles were
observed, discarding the NOX adsorption on the sample surface hydrothermal aging at 140
The XRD pattern of the as‐prepared sample after
C for 72 h, named sample A, is

or its direct photolysis. Subsequently, the photoreactor was shown in Fig. 1(a). All detected peaks can be attributed to a
irradiated for 300 min. The tests were repeated three times to crystalline phase with anatase structure (ICDD 00‐021‐1272).
obtain average concentration values. The calculated standard The lattice parameters of sample A obtained by refinement of
deviation is ± 0.3 ppb for NO concentration and ± 1.0 ppb for the XRD pattern by the Rietveld method (Table S2) and the
NO2 and NOX concentration.
corresponding Rietveld plot (Fig. S1) confirmed anatase as the
only crystalline phase formed through the hydrothermal aging.
NO conversion, NO2 released and NOX conversion were
defined as follows:
NO removal rate (%) =

([NO]in

[NO]out)/[NO]in

x 100 (1)
(2)
NO2 released (%) = ([NO2]out /[NO]in) × 100
NOX conversion (%) = {([NOX]in ‐ [NOX]out)/[NOX]in } × 100 (3)
where [NO]in, [NOX]in and [NO]out, [NO2]out, [NOX]out denote the
measured inlet and outlet concentrations, respectively, while
[NOX] = [NO] + [NO2]. The used values were the average of the
measured one under steady state period
Conversely, the selectivity towards the removal of efficient
removal of NOX gas molecules, S, here is derived according to
equation (4):
ꢄ
ꢈꢉꢊ
ꢄ
ꢈ
ꢅꢆꢇ
ꢌꢍꢎ
ꢋ
ꢂ/ꢄꢅꢆꢇꢈꢉꢊ
ꢂ/ꢄꢅꢆꢈꢉꢊ
ꢀ ꢁ%ꢂ ꢃ ꢁ ꢅꢆꢇ
ꢏ 100
(4)
Fig. 1. (a) XRD patterns and (b) Raman spectra of samples: A (as‐
prepared by hydrothermal aging) and A350 (after further
ꢄ
ꢈ
ꢄ
ꢈ
ꢅꢆ
ꢌꢍꢎ
ꢁ ꢅꢆ
ꢋ
ꢉꢊ
annealing at 350 C for 0.5 h). [ = anatase phase].
The Raman spectrum of the as‐prepared sample A is shown
in Fig. 1b. Bands at around 142, 195, 389, 510 and 635 cm‐1 are
Results and discussion
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detected, confirming the presence of a phase with anatase contain non‐aggregated particles with sizes around 20 nm. It is
structure32. The amount of carbon and nitrogen determined by to note that the almost spherical particles displayed in sample
elemental analysis in the as‐prepared sample A is shown in A become more elongated after annealing at 350 C (sample
Table S3. The C/N ratio near to the theoretical one, 6.0, reveals A350). Further features related with their mesocrystallinity can
the presence of TEOA attached to anatase nanoparticles, being also be found in the HRTEM images of these nanoparticles.
estimated in 3.4 TEOA wt % content. Through the calcination of Thus, the HRTEM micrographs of single particles of samples A
sample A a rapid decreases of C/N ratio and C and N content is and A350, in Figs. 2c and 2d, reveal that these particles are built
found (Table S3). Therefore, TEOA is easily eliminated at from very small primary nanocrystals sized in the range 2‐4 nm.
relatively low annealing temperatures. For sample A350 Likewise, SEM micrographs of both samples A and A350 (Fig. S5)
(obtained after annealing the pristine sample A at 350 C during confirm the large scale‐formation of particles sized around 20
30 min) can be assumed that no TEOA amine remains attached nm and that the roughness of their surface indicates that both
to the surface so it can be considered to be pure anatase. Thus, prepared anatase samples are constructed from smaller
as it could be expected for that sample A350 no structural primary nanocrystals. Interestingly, the clear lattice fringes
changes were detected (Fig 1; Fig S2; Table S2).
observed in HRTEM images of both single particles is in
agreement with the single‐crystal characteristic of them. This
The valence states of titanium cations were investigated by fact means that the resulting anatase particles A and A350 were
XPS measurements on the surface. Fig. S3 (left) shows built by aggregation of the primary anatase nanocrystals along
comparatively the Ti 2p XPS regions of the samples A and A350. specific directions. Anyway, it is to note that some dislocations
The binding energies at 458.8 eV and 464 eV are attributed to and edge defects can be distinguished in the areas of primary
Ti4+ 2p3/2 and Ti 2p1/2 33. The XPS spectra for the O 1s region for nanocrystals interfaces.
samples A and A350 are also displayed in Fig. S3 (right). The O
1s peak can be de‐convoluted in two peaks for both samples, A350 shown in Fig.
HRTEM images and SAED patterns of anatase samples A and
provide further structural and
3
one main peak appearing at 530 eV which is associated with microstructural features. Figs. 3a and 3d show ordered fringes
lattice oxygen (Ti–O–Ti) in TiO2 33, being the signal at 531 eV with 0.35 nm of spacing in samples A and A350. Those fringes
attributed to Ti‐OH hydroxyl groups 34. No nitrogen signal was can be assigned to the (101) planes of anatase with the lattice
found in the region of N 1s for both samples, Fig. S4. This could parameters shown in Table S2. SAED of both samples, Figs. 3c
be attributed to the fact that the adsorbed residual amine could and 3f, confirm the high crystallinity of the nanoparticles. All
have been desorbed during the overnight high vacuum rings in the patterns can be unambiguously attributed to a
treatment carried out before XPS measurement. This fact crystalline phase with the structure of anatase. These results
indicates that no substitutional and/or interstitial nitrogen is clearly indicate the formation of mesocrystalline anatase built
inside the anatase nanoparticles.
by anatase nanocrystals sized around 2‐4 nm which aggregate
all oriented in the same direction giving rise to uniform
assemblies (nanoparticles) with almost spherical shape.
Fig. 3. HRTEM micrographs of samples A and A350: a) and d)
lattice fringes of samples A and A350 associated with planes
(101); b) and e) set of nanoparticles of sample A and A350; c)
and f) SAED of the previous set of nanoparticles showing the
indexed rings to anatase phase.
Fig. 2. TEM micrograph of samples: (a) A; (b) A350. HRTEM
micrographs of samples: (c) A; (d) A350.
TEM micrographs of samples A and A350 are displayed in
Figs 2a and 2b, respectively. As can be seen, both samples
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strong absorption extended over the UV radiation up to around
400 nm for the both samples. Sample A exhibits also a strong
absorption on 400 – 800 nm Vis range, which could be
associated with the presence of TEOA attached to the anatase
nanoparticle surface 35. Interestingly, even though the TEOA is
removed, a weak absorption tail remains at wavelengths higher
than 400 nm even for the sample A350. A similar behaviour was
reported by Elbanna et al. 22 on preparing anatase mesocrystals,
being the absorption associated to both interstitial and
molecularly chemisorbed N in N‐doped TiO2 or oxygen
vacancies. Since our results from XPS did not indicate the
presence of structural or interstitial nitrogen it could be
associated with some chemisorbed nitrogen and/or even
oxygen vacancies in the lattice of mesocrystalline anatase. For
comparison purposes the commercial Aeroxide® TiO2 P25
(Evonik) was also studied, a material broadly used worldwide as
reference in photocatalysis. The corresponding chemical and
physical characterizations are provided in ESI, Figs. S3 and S6.
0,010
0,008
0,006
0,004
0,002
a)
Sample A
140
120
100
80
0
20
40
60
80
100
120
60
Pore size / nm
40
Sample A350
20
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0
Relative pressure / P/P0
1,0
b)
A
A350
P25
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
Gas‐phase photocatalytic activity
The surface area values, mesopore microstructure and UV‐
Vis light absorption ability observed bring together the
characteristics for a potential photocatalyst of high interest. In
this sense, it was studied the ability of the studied samples
towards the NOX abatement through photocatalytic reactions.
The De‐NOX behaviour is evaluated from the evolution of
nitrogen oxides concentration profiles recorded for the
mesocrystalline A350 titania sample as a function of UV‐Vis
irradiation time, Fig. 5a. The NO concentration only decreases
under irradiation, evidencing the occurrence of a light‐induced
chemical reaction. Previously, during the first 10 min the gas
concentration remains constant avoiding the existence of
adsorption phenomena. When illumination is shutdown, the
gas concentration returned almost to its initial value. The NO
conversion values recorded for sample A350 were quite similar
to that of reference P25 (Fig. 5b): 59 and 70 % NO conversion,
respectively. In both cases, the constant values achieved after
five hours indicated that a steady state condition of maximum
300
400
500
600
700
800
900
Wavelenght / nm
Fig. 4. (a) N2 adsorption‐desorption isotherm and (b) UV‐Vis DR
spectra for samples A (black) and A350 (red). The spectra of the
reference P25 is also included (blue).
Owing to the well‐known characteristics of titania as
photocatalyst, the samples were subjected to subsequent
characterization concerning to their light activation and the
surface area exposed to the reactant molecules. The surface
area and porous structure were elucidated from the
corresponding N2 adsorption‐desorption isotherm of samples A
and A350, Fig. 4a. As can be seen the isotherms of both
prepared samples exhibit hysteresis loop at high relative
pressure between around 0.08 and 1.0, which indicates the
presence of mesopores. Table S4 summarizes the main
microstructural characteristics of the prepared samples A and
A350. The pore size distribution for both prepared anatase
samples is similar with maximum pore size around 13 nm (Fig
4a inset), being the surface area 74.9 and 63.6 m2 g‐1 for the
samples A and A350, respectively. It is to note that the
mesoporosity within these almost spherical assemblies
(nanoparticles) comes from the arrangement of the primary
nanocrystals. However, this porosity is not shown in HRTEM
images because is disordered. The set of structural
characteristics together with the results of XRD, SEM and TEM
indicate that the annealing at low temperature of the as‐
prepared sample A has only small effects on the morphology
and crystal structure (sample A350). The corresponding UV‐Vis
spectra of the samples are shown in Fig. 4b. As expected, a
36, 37
activity was reached
.
However, the photocatalytic
behaviour of sample A resulted radically different and NO
concentration increases continuously under light irradiation,
Fig. S7. This is consequence of the presence of TEOA molecules
anchored to the surface of the titania particles, being this
organic compound degraded by photocatalysis. Therefore, the
pristine sample is not suitable as De‐NOX photocatalyst, being
necessary to work with calcined samples on which TEOA was
previously eliminated.
Anyway, the effective air quality improvement reached by
photocatalyst must take into account the presence of NO2
undesired intermediates 19, 38. The complete photocatalytic NOX
removal from air proceeds through the complete oxidation of
NO (the main nitrogen gas emitted by vehicles) to nitrate or
‐
nitric acid (NO3 /HNO3). This is a complex process, that involves
several intermediate species21, 39, from which the nitrogen
dioxide (NO2) appears as highly toxic intermediate specie 40
.
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time that this significant feature is reported for long time
irradiated samples.
With regard to this, it is worth to mention that some of the
samples listed on Table S1 exhibited high selectivity values (low
emission of NO2). However, it must be noted that those studies
were performed at short time of irradiation (30 to 120 min),
gives rise to uncertainty about those values because the NOX
removal capability of many TiO2 based phototacalysts
36, 37
diminishes as the illumination proceeds in a large period
.
This feature has been related to the photochemical
renoxification pathway leading to the release of NO and NO2
from surface NO3‐ molecules 37, 41. This is corroborated for the
36
P25 titania, as previously reported
. From its NOX
concentration profiles, Fig 5b, it is observed that the theoretical
full oxidation of NO towards nitrites/nitrates lowers with the
irradiation time and very high amounts of NO2 molecules are
increasingly released during the experiment. In fact, after five
hours of irradiation, the P25 oxidized around 70 % of molecules,
but one third of them are converted in the highly toxic NO2.
Consequently, in terms of NOx removal capability the prepared
titania appear as a desired photocatalyst, as summarized in Fig.
5c. Thus, it combines the high efficiency on the photochemical
NO conversion (grey bar) and selectivity (green bar), resulting
the highest NOX conversion (orange bar). Moreover, the
stability of sample A350 was investigated. After the
photocatalytic reaction, the sample was subjected to a washing
process with 5 mL of MilliQ water for 1.0 h, subsequently the
product was collected by centrifugation and dried. The washed
product exhibited identical chemical and physical
characteristics to that of pristine sample, Figs. S8 and S9.
Identical nitrogen oxides concentration profiles were recorded
(Fig. S10) over the course of a new 5 hours photocatalytic cycle,
supporting the excellent reusability of this catalyst in
comparison with similar titania compounds 42
.
Fig. 5. Nitrogen oxides concentration profiles obtained during
the photo‐degradation of gaseous NO under UV‐Vis irradiation
on Samples (a) A350 and (b) P25. (c) NO conversion (%, grey),
NO2 released (%, pink), NOX conversion (%, orange) and
selectivity values (%, green) for A350 and P25 TiO2 samples after
five hours of light irradiation.
It is worthwhile to note that the sample A350 also exhibits
De‐NOX activity under Vis light. This functionality is of high
interest for practical applications because the 4‐5% of UV
energy incoming from sunlight rapidly decreases to 0‐2% in the
cities of Central/Northern Europe or North America, owing to
geography and gloomy weather condition 43. Fig. S11 shows the
NO concentration profile recorded for the A350 and P25
samples under Vis light ( > 510 nm), obtained as a short
demonstrative test. The NO conversion efficiency is
proportional to the Vis absorption ability showed by each
sample, Fig 4b 35. It is outstanding that the conversion efficiency
for sample A350 is 2.5 times greater than that of P25. It must be
highlighted that other advanced De‐NOX titania photocatalysts
and P25 also, with similar or lower particle size or higher active
surface area did not shown such high selectivity (Table S1).
Therefore, the singular mesopore structure seems to play an
important role in the photochemical NO oxidation process
inhibiting the NO2 emission. During the photochemical
oxidation of NO towards nitrites/nitrates, following the already
proposed mechanisms 21, NO2 molecules could be released to
the atmosphere because: i) their weak adsorption on the TiO2
surface 41; ii) or the existence of a renoxification pathway
Therefore, the excellence for a titania De‐NOX photocatalyst
won´t be only focused into obtaining the highest NO conversion
values but also the highest De‐NOx selectivity. This means that
no photo‐oxidized NO molecules were released again to the
atmosphere as NO2. However, as it was referred in the
Introduction section, it is to note that in the recent literature,
the reported modified titania photocatalysts do not fully inhibit
the emission of NO2 gas. It is to remark in this sense the
excellent De‐NOX activity of the A350 prepared titania. As
shown in Fig. 5a, very low NO2 concentrations were measured
for sample A350. Therefore, the annealed at very low
temperature titania sample A350 is able to photo‐chemically
oxidize the NO molecules minimizing as maximum the release
of harmful nitrogen oxide gases to the atmosphere, with a very
high selectivity value of 89 %. As far as we know this is the first
involving the photochemistry of the NO3 .
radical 44, 45
6 | J. Name., 2012, 00, 1‐3
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though mesopores still exist, the volume of pores and surface area
are reduced more than half (Fig S12 a), this had a direct drastic
influence on its photochemical activity also for a short irradiation
period as shown in Fig S12 b, mainly concerning to the selectivity.
The NO conversion efficiency wasn´t highly affected by the decrease
of surface area. However, the loss of mesopore volume seems to be
crucial because most of the NO converted molecules are now
released as NO2 and the selectivity value decays to 71 %. Therefore,
the adsorption of NO2 molecules seems to be favoured on
mesostructured sample A350, facilitating the completion of the NO
1620
1284
1582
1237
1504
(c)
P25
1375
1.351
‐
 NO2  NO3 photo‐oxidation process to reach the highest
selective De‐NOX values.
A350
(b)
(a)
Conclusions
In summary, discrete, size‐controlled and well‐defined
morphologically mesocrystalline anatase nanoparticles were
synthesized by a hydrothermal approach. On the basis of the
experimental results, it can be concluded that the as‐prepared
mesocrystals were constructed through assembly of
nanocrystals formed in the aqueous solution at the first reaction
stage. The assembler agent TEOA allowed both controlling the
hydrolysis of the titanium alkoxide and stabilizing the primary
anatase nanocrystals against uncontrolled aggregation. The
1700
1600
1500
1400
1300
1200
Wavenumber / cm-1
Fig. 6. (a) IR spectra of Sample A350 after 8 hours of outgassing
at 50 C in vacuum. IR spectra of samples (b) A350 and (c) P25

after exposure to 5.0 Torr NO and evacuation at 50
min.
C for 10
In order to gain a deeper knowledge, two additional
experiences were performed. Firstly, NO adsorption studies
were carried out onto sample A350 surface (experimental
details are provided in ESI). Prior to the NO admission into the
IR cell, the sample was outgassed during 8 h at 50 C in vacuum,
Fig 6a. Subsequently, a pulse of NO (5 Torr) was introduced at
the cell. The IR spectrum was recorded after gas phase
assembler was removed by short annealing at 350 C without
significant structural and microstructural changes. The titania
samples exhibited mesoporosity and photoactivity towards Vis
light. Both singular characteristics allow obtaining not only high
NO conversion values but also a more selective De‐NOX process.
The mesoporous network facilitates an almost complete NO
photochemical oxidation for long irradiation periods, being this
reported by the first time. The high reproducibility of this facile
method of preparation of nanoparticles, it is easy scaled up to
industrial level, and the high efficiency for the abatement of
NOX gases provide optimism in the preparation of new
advanced applications to remove the NOX breathable by the
residents that are located in the polluted urban centres.
evacuation at 50 C during 10 min, Fig. 6b. For comparison, the
IR spectrum for P25 sample was also obtained, Fig 6c. The
spectrum of samples A350 and P25 were rather different
indicating a remarkably difference for the reactive NO
adsorption. For sample A350, two main IR bands located at
1.351 and 1375 cm‐1, characteristic of nitro/nitrite compounds
(MNO2; M
ONO;) were found 46, 47. Conversely, a set of bands
at 1620, 1582, 1504, 1284 and 1237 cm‐1 are identified on the
P25 IR spectrum. According with similar experiences of NO
adsorption on P25, these bands are assigned to bidentate and
Conflicts of interest
There are no conflicts to declare.
bridged nitrate (M
NO3) 48. The elucidation of the specific
mechanism for the reactive NO adsorption in both samples is
beyond to this study. However it is interesting to point out that,
‐
in contrast for sample A350, the presence of NO3 molecules on
Acknowledgements
P25 after NO adsorption let to suggest that a renoxification
mechanism would be easily initiated when the surface is
irradiated with UV light44, 45, in agreement with the NOX
concentration profiles of Fig 5b.
On the other hand, it is known that mesoporous networks make
easy the accessibility of active reactive sites to the reactant
molecules 23, 24. These pores would be a sink on which the nitrogen
oxide gases and water vapour molecules are trapped, which serve to
restrain the in situ formed NO2 molecules. With the aim to obtain
indirect evidence about the influence of the mesoporous network
onto the photochemical activity, the NOx removal capability of A600
sample (Table S2) was preliminary studied. The microstructure of this
titania A600 was slightly different to that of sample A350. Even
This work was financially supported by the Spanish Government
(CSD2010‐00065 and TEC2014‐53906‐R projects), Generalitat
Valenciana (Prometeo II/2015/002), Junta de Andalucía (Group
FQM‐175) and Córdoba University (XX PP. Modalidad 4.1). We
thank to Marina Cortes Reyes for her collaboration in the NO
adsorption measurements.
This journal is © The Royal Society of Chemistry 20xx
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