Core-crown Quantum Nanoplatelets with Favorable Type-II Heterojunctions Boost Charge Separation and Photocatalytic NO Oxidation on TiO2

Article abstract of DOI:10.1002/cctc.202000749

Functionalization of TiO₂ (P25) with oleic acid-capped CdSe(core)/CdSeTe(crown) quantum-well nanoplatelets (NPL) yielded remarkable activity and selectivity toward nitrate formation in photocatalytic NO_x oxidation and storage (PHONOS) under both ultraviolet (UV-A) and visible (VIS) light irradiation. In the NPL/P25 photocatalytic system, photocatalytic active sites responsible for the NO(g) photo-oxidation and NO₂ formation reside mostly on titania, while the main function of the NPL is associated with the photocatalytic conversion of the generated NO₂ into the adsorbed NO₃^? species, significantly boosting selectivity toward NO_x storage. Photocatalytic improvement in NO_x oxidation and storage upon NPL functionalization of titania can also be associated with enhanced electron-hole separation due to a favorable Type-II heterojunction formation and photo-induced electron transfer from the CdSeTe crown to the CdSe core of the quantum well system, where the trapped electrons in the CdSe core can later be transferred to titania. Re-usability of NPL/P25 system was also demonstrated upon prolonged use of the photocatalyst, where NPL/P25 catalyst surpassed P25 benchmark in all tests.

Full text of DOI:10.1002/cctc.202000749

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Core-crown Quantum Nanoplatelets with Favorable Type-II
Heterojunctions Boost Charge Separation and Photocatalytic NO
Oxidation on TiO2
Authors: Elnaz Ebrahimi, Muhammad Irfan, Farzan Shabani, Yusuf
Koçak, Bartu Karakurt, Emre Erdem, Yusuf Kocak, Hilmi
Volkan Demir, and Emrah Ozensoy
This manuscript has been accepted after peer review and appears as an
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to ensure accuracy of information. The authors are responsible for the
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To be cited as: ChemCatChem 10.1002/cctc.202000749
Link to VoR: https://doi.org/10.1002/cctc.202000749
01/2020

10.1002/cctc.202000749
ChemCatChem
FULL PAPER
Core-crown Quantum Nanoplatelets with Favorable Type-II
Heterojunctions Boost Charge Separation and Photocatalytic NO
Oxidation on TiO2
Elnaz Ebrahimi,[a] Muhammad Irfan, [a,c] Farzan Shabani,[b] Yusuf Kocak,[a] Bartu Karakurt,[a] Emre
Erdem,[d,e] Hilmi Volkan Demir,[b,f,g] and Emrah Ozensoy,*[a,b]
[a]
E. Ebrahimi, Dr. M. Irfan, Dr. Y. Kocak, B. Karakurt, Assoc. Prof. E. Ozensoy
Chemistry Department, Bilkent University
06800, Ankara, Turkey
E-mail: ozensoy@fen.bilkent.edu.tr
[b]
[c]
[d]
[e]
[f]
Assoc. Prof. E. Ozensoy, F.Shabani, Prof. H.V. Demir
UNAM-National Nanotechnology Center, Bilkent University
06800, Ankara, Turkey
Dr. M. Irfan
Nanoscience and Catalysis Department, National Centre of Physics
44000, Islamabad, Pakistan
Dr. Emre Erdem
SUNUM Nanotechnology Research Center, Sabanci University
34956, Istanbul, Turkey
Dr. Emre Erdem
Faculty of Engineering and Natural Sciences, Sabanci University
34956, Istanbul, Turkey
Prof. H.V. Demir
Department of Electrical and Electronics Engineering and Department of Physics, Bilkent University
06800, Ankara, Turkey
[g]
Prof. H.V. Demir
School of Electrical and Electronic Engineering, School of Physical and Mathematical Sciences, and School of Materials Science and Engineering,
Nanyang Technological University
639798, Singapore
Supporting information for this article is given via a link at the end of the document.
Abstract: Functionalization of TiO2 (P25) with oleic acid-capped
CdSe(core)/CdSeTe(crown) quantum-well nanoplatelets (NPLs)
yielded remarkable activity and selectivity toward nitrate formation in
photocatalytic NOx oxidation and storage (PHONOS) under both
ultraviolet (UV-A) and visible (VIS) light irradiation. In the NPL/P25
photocatalytic system, photocatalytic active sites responsible for the
NO(g) photo-oxidation and NO2 formation reside mostly on titania,
while the main function of the NPLs is associated with the
photocatalytic conversion of the generated NO2 into the adsorbed
Introduction
Atmospheric pollution is one of the major environmental
challenges faced by modern human society. Various
anthropogenic air pollutants such as COx, SOx, particulate matter
(PM), volatile organic compounds (VOC), and nitrogen oxides
(NOx) can contribute to acid rain and/or induce ozone (O3)
production in troposphere (i.e. unwanted ground-level ozone
formation). In addition, human respiratory and immune systems
are severely affected by nitric oxide (NO) and nitrogen dioxide
(NO2).[1] Environmental protection agencies have set a value of ≤
0.2 ppm for NOx emissions but it is often exceeded in urban
settings.[2] Hence, it is crucial to find improved protocols for the
removal of NOx species from the atmosphere. Conventionally,
NOx abatement is typically performed at elevated temperatures
(e.g. 200 - 550 °C) using thermal catalytic technologies (e.g. via
-
NO3 species, significantly boosting selectivity toward NOx storage.
Photocatalytic improvement in NOx oxidation and storage upon NPL
functionalization of titania can also be associated with enhanced
electron-hole separation due to a favorable Type-II heterojunction
formation and photo-induced electron transfer from the CdSeTe
crown to the CdSe core of the quantum well system, where the
trapped electrons in the CdSe core can later be transferred to titania.
Re-usability of NPL/P25 system was also demonstrated upon
prolonged use of the photocatalyst, where NPL/P25 catalyst
surpassed P25 benchmark in all tests.
selective catalytic reduction/SCR,
NOx storage
and
reduction/NSR, or three way catalysis/TWC) [3-10] with an intend to
curtail the toxic NOx emissions at the source of generation.
Frequent violations of transportation-based NOx emission
regulation [11] necessitates the reduction of gaseous NOx species
1
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10.1002/cctc.202000749
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FULL PAPER
after their point of origin under ambient conditions (i.e. at room
temperature and under atmospheric pressure). It is important to
note that, while all of these conventional catalytic DeNOx
technologies operate at the point of NOx production (i.e. at the
tailpipe or in the chimney etc.), there is no conventional catalytic
DeNOx technology that can be utilized after toxic NOx species are
released into the atmosphere. Hence, it is essential to develop
novel catalytic technologies that can also function after the point
of NOx emission to the atmosphere.
is necessary to develop TiO2-based photocatalysts that can
harvest light across a broad spectral window with high selectivity,
efficiency and stability.
To achieve these challenging objectives, in the current work,
two-dimensional (2D) rectangular CdSe (core)/CdSeTe (crown)
quantum-well nanoplatelets (NPLs) were synthesized and utilized
in the modification of TiO2 toward PHONOS applications.
Semiconductor NPLs (also known as colloidal quantum wells)
provide novel opportunities in photocatalytic applications as their
thicknesses, diameters, shapes, electronic and optical properties
can be fine-tuned with high precision via colloidal synthesis and
hetero-structure growth strategies.[43-44] In addition, 2D
semiconductor NPLs can also be utilized to enhance light
absorption/harvesting and photon-induced charge (electron
and/or hole) transfer properties of the overall photocatalytic
system. Quantum NPLs systems can be more advantageous as
compared to conventional quantum dot (QD) systems due to the
stronger light absorbance capability of NPLs, which can be
attributed to the NPL's extraordinary large absorption cross-
section per particle, associated with their very tight quantum
confinement and the relatively larger volume.[45-46] The faster
charge transfer/charge separation capability of the NPLs could
originate from the extended surface area of the NPLs and/ or their
continuous density of electronic acceptor states.[47] Additionally,
unlike conventional QDs, 2D quantum NPLs have large linear and
non-linear absorption cross sections and preferable excitonic
features, rendering them promising alternatives for zero-
dimensional QDs.[48] NPL heterostructures with different chemical
compositions, vertical thicknesses and ,architectures such as
core-only,[49] core/crown,[50] core/shell,[51] and core/crown/shell [52]
can be synthesized. Furthermore, particle size distribution and
shape uniformity can also be controlled to achieve desired
electronic structures and optical properties. Different band
In this regard, photocatalytic oxidative NOx storage (PHONOS)
[12-18]
presents an appealing alternative as it can store airborne
NOx in the solid state under ambient conditions after the point of
NOx discharge to the atmosphere. This approach requires the
presence of only sunlight, water and oxygen (all of which are
abundantly present in the atmosphere) and a photocatalytically
active material.[19] This methodology has been implemented in
advanced construction materials to combat urban NOx pollution,
using mainly titanium dioxide (TiO2) based photocatalysts.[20-22]
TiO2 is an abundant, cost-efficient, chemically and thermally
stable semiconductor, which is extensively used in the
photocatalytic decomposition of both gas and liquid- phase
pollutants.[23] Photocatalytic activity of TiO2 is governed by a
multitude of complex chemical, physical, optical and electronic
properties, including (but not limited to) the phase/crystal
structure, specific surface area (SSA), crystallite size and
electronic band gap.[24-26] Due to the relatively large band gap of
TiO2 (3.0-3.2 eV), it is commonly reported in the literature that
TiO2 enables the harvesting of mostly ultraviolet (UV-A) light,
which constitutes only 4-5% of the solar spectrum.[27] Therefore,
development of visible (VIS) light-responsive photocatalysts with
higher stability, selectivity and activity is highly desired. For this
purpose, different strategies have been adopted to enhance the
photocatalytic efficiency of TiO2 in the VIS light region such as
non-metal [28-30] and precious/non-precious metal [15-16, 31-36] doping, alignments between the core, crown, and shell materials may
[37-39]
functionalization with quantum dots (QDs)
modifications with polymers.[40-41]
and surface
yield either Type-I or Type-II semiconductor heterojunctions,[53]
which can be fine-tuned for particular operational functionalities.
In the current study, NPL systems (i.e. CdSe/CdSeTe) with Type-
II heterojunctions are selected, since in these NPLs, photo-
excited electrons are expected to be localized in the CdSe core,
while holes may be trapped in the CdSeTe crown, forming
spatially indirect excitons with presumably much longer lifetimes
(as compared to core/crown and core/shell NPLs with a Type-I
electronic structure, where both electron and hole wave functions
are typically confined in the same region of the NPLs).[54-55] Apart
from exhibiting strongly red-shifted emission along with
significantly increased radiative fluoresce lifetimes, charge
separation at the core/crown interface of Type-II heterostructures
An important requirement in PHONOS applications is the high
-
selectivity of the designed photocatalyst toward nitrate (NO3 )
formation, which is the preferred final storage product of the
photocatalytic NOx abatement process. In one of our recent
reports,[18] we have demonstrated the formation of nitrates on TiO2
(P25) with different adsorption geometries (e.g. monodentate,
bidentate and bridging nitrates) upon interaction of TiO2 (P25)
with NO2(g) via in-situ FTIR spectroscopy. Unfortunately, TiO2 has
an extremely low selectivity towards NOx storage in solid state
and tends to oxidize NO(g) into a more toxic product, NO2(g),
which is ultimately released into the atmosphere.[42] Therefore, it
2
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10.1002/cctc.202000749
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can also promote the suppression of non-radiative
recombination/relaxation channels owing to their large in-plane
exciton mobility.[56] Our hypothesis here is that in the proposed
CdSe/CdSeTe/TiO2 composite system, photogenerated electrons
from the CdSe/CdSeTe NPLs can be injected to the conduction
band of TiO2, whereas the corresponding holes in the valence
band of TiO2 can be transferred to the valence band of
CdSe/CdSeTe NPLs.[14, 57-58] Such functionalization of TiO2 with
NPLs is also expected to enhance the photon absorption in the
VIS region of the solar spectrum. Hence, an improvement in the
photocatalytic DeNOx performance of TiO2 is anticipated upon
incorporation of NPLs.
dimensions of ca. 90 nm × 30 nm. The stacks of NPLs in these
images clearly reveal the uniformity of these quasi-2D
nanostructures both in terms of lateral dimensions as well as
thickness.
A
(b)
A
6.0 NPL/P25
A
R
A A
A
A
A
R
A
R
A
R
2.0 NPL/P25
1.0 NPL/P25
0.5 NPL/ P25
0.1 NPL/ P25
0.05 NPL/ P25
P25
t
In the present study, CdSe/CdSeTe NPLs were synthesized by
using CdSe core as the seed. Then, core/alloyed crown of
CdSe/CdSe0.75Te0.25 was synthesized, where the crown layer was
grown laterally while the vertical thickness was kept fixed. The
structure of as-synthesized NPLs was characterized and verified
through a multitude of spectroscopic, diffraction and imaging
techniques. Next, photocatalytic performance of NPL/TiO2
composites were tested in NO photo-oxidation under
t
(111)
(a)
(220)
(311)
50
(200)
CdSe/CdSeTe NPL
20
30
40
60
70
2θ / degree
Figure 1. XRD patterns of (a) CdSe/CdSeTe NPLs (without TiO2), and (b) pure
P25, and CdSe/CdSeTe/P25 composite materials with various NPL loadings. A:
anatase, R: rutile.
environmentally relevant reaction conditions.
A noteworthy
enhancement in the photocatalytic activity and selectivity of TiO2
(P25) was observed upon functionalization with NPLs.
Results and Discussion
Structural analysis by XRD
Crystal structures of CdSe/CdSeTe NPLs, P25 (TiO2), as well as
CdSe/CdSeTe/P25 with different NPL loadings were investigated
via XRD. The peaks at 2θ values of 24.69ᵒ, 29.02ᵒ, 41.23ᵒ and
48.85ᵒ were attributed to (111), (200), (220) and (311) facets of
the CdSe core of NPLs (Figure 1a). As mentioned in the former
reports,[55] addition of a CdSeTe crown to the CdSe core did not
lead to major changes in the XRD pattern of the CdSe core. XRD
profile of P25 in Figure 1b demonstrates characteristic diffraction
signals of TiO2, confirming the presence of anatase (ICDD card
no. 00-021-1272) and rutile (ICDD card no. 00-021-1276)
domains. Diffraction patterns of the CdSe/CdSeTe NPL/P25
composites are similar to that of P25 due to the miniscule loadings
of NPLs used in the functionalization of P25 and the small NPL
particle sizes. Currently used minute loadings of CdSe/CdSeTe
NPLs were chosen in order to minimize material cost and toxicity.
200 nm
500 nm
(b)
(a)
100 nm
50 nm
(c)
(d)
Figure 2. (a-d) Representative TEM images of CdSe/CdSeTe NPLs with
different magnifications.
Surface structural analysis via XPS
In order to examine the surface chemistry, electronic structure
and elemental composition of the core/crown NPLs, XPS
measurements (Figure 3) were carried out (additional data and
discussion about the XPS results are provided in the SI section,
Figures S1 and S2). In the XPS measurements, fresh
CdSe/CdSeTe NPLs in the absence of TiO2 (Figures 3a-3e),
CdSe/CdSeTe/TiO2 (NPL/P25) composite system before and
Electron microscopy analysis via TEM
Several representative TEM images of the CdSe/CdSeTe
core/crown NPLs (in the absence of TiO2) are shown in Figure 2.
Synthesized NPLs exhibited rectangular and uniform shapes with
3
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10.1002/cctc.202000749
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after 5 h PHONOS reaction under UV-A (Figure 3f) or VIS
irradiation were (Figure 3g) investigated.
dot systems, containing heterojunctions including both CdSe and
CdTe domains.[61-63] Hence, Cd3d5/2 feature at 405.1 eV in Figure
3c is consistent with the Cd species interacting with Te and/or Se
species [61-63] and lack of significant amounts of CdOx species. The
Te3d5/2 feature located at 572.5 eV in Figure 3d can be assigned
to Te species that are interacting with Se and/or Cd [60-64] Likewise,
the broad and complex Se3d signal in Figure 3e shows a Se3d5/2
B. E. of ca. 53.9 eV in accordance with the existence of CdSe and
CdSeTe species. Thus, XPS results given in Figure 3 are in line
with the presence of closely interacting Cd, Se and Te species as
would be expected in the CdSe/CdSeTe NPLs. Post-reaction
XPS analysis of the CdSe/CdSeTe/TiO2(P25) composites
indicate blueshifts in the Cd3d5/2 B. E. after PHONOS reaction
under UV-A or VIS illumination suggesting that detectable
electronic structural changes occur in the CdSe/CdSeTe/TiO2
composite system upon extensive UV-A or VIS aging under
PHONOS reaction conditions, resulting in partial oxidation of Cd
sites (apparent by the blue shift in Cd3d B.E.). Presumably, these
electronic structural changes could be due to the photo-oxidation
of Cd species, as well as due to the changes in the chemical
environment of Cd and Te surface species upon partial photo-
degradation of the oleic acid capping agent. Note that electronic
structural alterations of the Te and Se sites of the
CdSe/CdSeTe/TiO2 system could not be accurately analyzed via
XPS due to the Ti2s loss feature of TiO2 overwhelming Te3d
region and the small XP cross-section of Se falling below the
detection limit.
O1s spectrum of the fresh NPLs without TiO2 revealed a broad
and a convoluted signal (Figure 3a) associated with the C-O, C=O
and O-H functionalities of the oleic acid capping of the NPLs (see
experimental section for the synthesis details of NPLs).[59] Figure
3b presents the corresponding C1s spectrum of fresh NPLs
without TiO2, where the shoulder visible at 288.1 eV can be
ascribed to C-O and C=O functionalities of the oleic acid capping
and the larger signal at 284.8 eV can be attributed to C-C linkages
of oleic acid as well as the surface (adventitious) carbon species.
Note that binding energy (B. E.) positions of the spectra in Figures
3a-3e for the fresh NPLs without TiO2 were calibrated using the
adventitious of carbon signal at 284.8 eV while the spectra in
Figures 3f-3g were calibrated using the Ti4+ features of P25 (TiO2)
(i.e. Ti2p3/2 and Ti2p1/2 features at 458.5 eV and 464.1 eV,
respectively [14, 60]). Figures S1 and S2 show that in the case of
NPL/P25 composite system, O1s and C1s signal intensities
diminish after PHONOS reaction under UV-A or VIS illumination
due to decomposition of the oleic acid capping functionalities and
possibly photo-desorption of COx/HxCyOz species. This is also in
line with the growth of the Ti2p signal intensities (Figures S1 and
S2) after PHONOS reaction on NPL/P25 composite system under
UV-A or VIS illumination indicating increasing exposure of TiO2 to
incoming X-rays after the decomposition of the oleic acid capping
of the NPLs. XPS data for the Cd, Te and Se regions (Figures 3c-
3e), reveal close resemblance to the multicomponent quantum
Cd 3d
Te 3d
284.8 eV
531.6eV
C 1s
O 1s
405.1 eV
3d5/2
(a)
(b)
(c)
(d)
572.5 eV
582.8 eV
3d5/2
411.8 eV
3d3/2
y
3d3/2
y
y
y
288.1eV
288
416
412
408
404
588
584
580
576
572
568
296
292
284
280
540
536
532
528
Binding Energy / eV
Binding Energy / eV
Binding Energy / eV
Binding Energy / eV
3d
Cd 3d
5/2
Cd 3d
Se 3d
(g)
(e)
(f)
3d
5/2
405.1
405.2
fresh
VIS
fresh
UVA
53.9 eV
3d
404.9
404.9
3d
3/2
3d3/2
5/2
54.7 eV
3d
y
3/2
y
y
t
i
s
n
e
t
n
I
420
416
412
408
404
400
420
416
412
408
404
400
60
56
52
Binding Energy / eV
Binding Energy / eV
Binding Energy / eV
Figure 3. XPS spectra of fresh (as prepared) CdSe/CdSeTe NPLs (in the absence of TiO2): (a) O1s, (b) C1s, (c) Cd3d, (d) Te3d, and (e) Se3d regions. Cd3d XPS
spectra for CdSe/CdSeTe/TiO2(P25) composites before (green) and after (blue) 5 h PHONOS reaction under (f) UV-A, and (g) VIS illumination.
4
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Specific surface area (SSA) analysis with BET
SSA of pure P25 was found to be 50 m2/g, while SSA of
0.05NPL/P25, 0.1NPL/P25, 0.5NPL/P25, 1.0NPL/P25 and
2.0NPL/P25 were determined to be 44, 45, 41, 40 and 39 m2/g,
respectively. This minor decrease in SSA with increasing NPL
loading can be attributed to the partial blocking of the P25 pores
by NPLs.
Absorbance
Photoluminescence
(a)
CdSe (hh)
512
/
CdSe (lh)
482
CdSeTe (lh)
563
Electronic structural analysis via UV-VIS and
Photoluminescence spectroscopy
r
Absorption and PL spectra of 4 ML CdSe/CdSeTe NPLs (in the
absence of TiO2) is presented in Figure 4a. Absorption spectrum
in Figure 4a reveals light-hole (lh) and heavy-hole (hh) transition
of the CdSe core of CdSe/CdSeTe NPLs at 482 and 512 nm,
respectively.[65] It was demonstrated in one of our former reports
that,[65] these sharp excitonic transitions in the absorption
spectrum of core-only CdSe remain almost intact during the
lateral growth of CdSeTe crown, irrespective of CdSeTe crown
size. When CdSeTe-crown layers are laterally grown on CdSe
core seed, a new absorption peak at 563 nm is observed for
CdSe/CdSeTe NPLs due to the light-hole electron transition in the
CdSeTe crown.[65] It is apparent from Figure 4a that both CdSe
core and CdSeTe crown regions contribute to the absorption
spectrum in agreement with the formation of CdSe/CdSeTe
core/crown NPLs having Type-II band alignment. PL spectrum in
Figure 4a also presents an emission peak at 591 nm with a full
width at half maximum (FWHM) of 45 nm. PL spectra of P25 and
CdSe/CdSeTe NPLs given in Figure S3 also show that the
presence of NPLs suppresses the photoluminescence of P25
(titania) which is consistent with the enhanced electron-hole
separation and extended exciton lifetime. Figure 4b presents the
Tauc plot for the CdSe/CdSeTe NPLs (without P25) revealing a
direct band gap of 2.45 eV for the CdSe core and 2.14 eV for the
CdSeTe crown. These values further support the formation of a
VIS-light-driven Type II heterojunction in CdSe/CdSeTe NPLs,
where the outer crown/shell (which can also include Te-doped
CdSe domains) can have a stronger VIS-light absorption as
compared to the inner core material.[66]
A
450
500
550
600
650
700
Wavelength / nm
2.5
2.0
1.5
1.0
0.5
0.0
(b)
CdSe (hh)
CdSe (lh)
5
)
CdSeTe (lh)
2.0
2.2
2.4
2.6
2.8
3.0
Energy / eV
Figure 4. (a) Absorbance and photoluminescence spectra of 4 ML-thick bare
CdSe/CdSeTe NPLs (i.e. without TiO2) dispersed in toluene; (b) Tauc plot for
bare CdSe/CdSeTe NPLs.
Since the conduction band edge of CdSe core lies lower in
energy than that of CdSeTe crown,[65] during the photoexcitation
of the CdSe/CdSeTe/P25 system, excited photoelectrons in the
conduction band of the CdSeTe crown can be transferred to the
conduction band of the CdSe core which can ultimately be
transported to the conduction band of the TiO2 (Figure 5). In turn,
generated holes can be confined in the CdSeTe crown valence
band.[67]
Figure 5. Electron energy diagram for core/crown CdSe/CdSeTe NPLs on TiO2.
Equations (1-2) [68-69] and the band gap energies (determined
from Figure 4) of various structural components in the NPLs and
NPL/TiO2 system (e.g. CdSe, CdSeTe, and TiO2) can be used to
estimate the valence band and conduction band positions of these
components:
5
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Ec0 = Ee - XAB + 0.5 Eg
(1)
(2)
that most of the photo-induced charge carriers originated from
NPLs (Figure S4). Thus, current EPR data supports the
synergistic electronic effect of the CdSe/CdSeTe NPLs in the
CdSe/CdSeTe/TiO2 composite system extending the electron-
hole recombination lifetime which presumably has a positive
influence on the photocatalytic activity, as will be described in the
further sections. Note that EPR signals for the experiments
performed on CdSe/CdSeTe/TiO2 were below the detection limit,
due to the small concentration of NPLs present in the NPL/P25
system and thus not shown here.
y
1/x+y
X AxBy = (XA x . XB
)
Here Ec is the conduction band potential, XAB is the
electronegativity of the semiconductor expressed as the
geometric mean of the absolute electronegativity of the
constituent atoms, x and y are stoichiometric coefficients in the
empirical formula, Ee is the energy of free electrons on the
[69]
hydrogen scale (i.e. 4.5 eV)
and Eg is the bandgap. Electron
energy diagram for the CdSe/CdSeTe/TiO2 system, based on the
estimated energy positions are summarized in Figure 5. Electron
transfer cascade shown in Figure 5 may yield efficient electron-
hole separation, extended exciton lifetime and suppression of the
electron-hole recombination processes, which we believe are
associated to the origins of the photocatalytic activity boost of the
CdSe/CdSeTe/TiO2 composite system studied in the current work.
Electron and hole trapping analysis of CdSe/CdSeTe
NPLs via EPR Spectroscopy
In an attempt to understand the influence of UV irradiation on the
electronic structure of the CdSe/CdSeTe NPLs, we performed in-
situ EPR experiments under dark as well as under 2 h-long UV
irradiation (Figure 6). In the former in-situ EPR studies, trapping
of electrons in the colloidal InP system upon optical excitation was
reported.[70] Since the typical life times of such electron trapping
events are rather short (in the order of 10-9-10-3 s [71-72]), long-life
time (101-102 s) radical spin traps such as 2,2,6,6-
tetramethylpiperidin-1-yl) oxidanyl (TEMPO) or 5,5-dimethyl-
pyrroline N-oxide (DMPO) or metal cation dopants such as Mn2+
were externally added to the analyzed samples to allow longer
data acquisition times with a higher signal to noise ratio (S/N).[73-
Figure 6. Room temperature X-band EPR spectra of CdSe/CdSeTe NPLs
(without TiO2) under dark and UV illumination conditions.
Photocatalytic NOx(g) Oxidation and Storage (PHONOS)
Performance Tests
Figure 7a pictorially summarizes the PHONOS process while,
Figure 7b shows the typical time-dependent NO(g), NO2(g) and
total NOx(g) (i.e. NO(g) + NO2(g)) concentration profiles as a
function of the VIS-light irradiation time during the NO photo-
oxidation over 0.1NPL/P25 photocatalyst. In the first stage of the
photocatalytic activity tests, a synthetic polluted air gas mixture
containing ca. 1 ppm NO(g) was fed to the photocatalyst surface
under dark conditions. During this initial phase (i.e. 0-5 min), a
minor transitory fall in the total NOx(g) and NO(g) concentrations
was observed due to adsorption of NOx species on the reactor
lines, expansion of the gas in the reactor as well as non-
photocatalytic adsorption of NOx on the photocatalyst surface. In
addition, a tiny amount of NO2(g) was produced due to thermal
catalytic processes occurring on the catalyst surface. Following
the saturation of the reactor system and photocatalyst surface,
NOx(g) and NO(g) levels quickly returned to the original inlet
values and reached a steady state in dark conditions.
75]
In the current in-situ X-band EPR measurements, no external
spin-trap agents were added to the measured samples. Figure 6
reveals no detectable EPR signals for CdSe/CdSeTe NPLs in
dark. However, UV irradiation leads to the emergence of a new
EPR signal (Figure 6) suggesting the generation of paramagnetic
states in NPLs. Due to the corresponding g-factor of the detected
EPR signal, it is likely that this is new signal which is occurring
only in the presence of UV irradiation is due to photo-induced
electron transfer from the CdSeTe crown to the CdSe core of the
quantum well system, where electron-trapping occurs in the CdSe
core. While trapped electrons and trapped holes yield similar g-
values (ge = 2.0023[76]) in EPR, detection of trapped hole states in
EPR is difficult due to the significantly smaller EPR intensities of
hole states.[77] It should be noted that control EPR experiments
performed on P25 under UV irradiation and under dark conditions
did not yield any evidence for such charge carriers, suggesting
6
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10.1002/cctc.202000749
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1.2
1.0
0.8
0.6
0.4
0.2
0.0
(a)
(b)
hv
NOx
NO
n
ait
Surface
Products
NPL
t
cne
Light off
Light on
NO2
0
10
20
30
40
50
60
70
80
90
Time / min
Figure 7. (a) Schematic representation of PHONOS process. (b) Concentration vs. time profiles for a typical PHONOS activity test performed with 0.1NPL/P25
under VIS irradiation. Red (bottom), black (middle) and blue (top) traces correspond to NO2(g), NO(g) and total NOx (i.e. NO(g) + NO2(g)) concentrations, respectively.
Feed composition: N2(g) 0.750 SLM, O2(g) 0.250 SLM, and NO(g) 0.010 SLM (100 ppm NO(g) diluted in balance N2(g), 50% relative humidity (RH) at 23 °C.
the UV-A PHONOS performance tests were prepared by
Next, UV-A or VIS-light irradiation was turned on after the first
exposing the NPL/P25 catalysts to an UV-A light source for 18 h
ca. 15 min (Figure 7b) and a drastic fall in the NO(g) and total NOx
under ambient conditions prior to the photocatalytic activity
(g) concentrations was detected along with a small increase in the
measurements. It is apparent from Figure 8 and Figure S5 that
NO2(g) level. While the latter observation clearly shows the
P25 reveals a rather high NO conversion % (33%) while yielding
photocatalytic oxidation of NO(g) into NO2(g), decrease in the
a very low NOx storage selectivity % (23%) due to the production
NO(g) and total NOx(g) concentrations indicates that in addition to
of large quantities of unwanted NO2(g). Although TiO2 has a much
this process, solid state storage of NO(g) and NO2(g) in the form
higher adsorption capacity for NO2(g) than that for NO(g),[81]
of chemisorbed NO2, nitrites and/or nitrates also occur on the
photocatalytic NO2(g) production rate and the total amount of NO2
photocatalyst surface.[6, 78-79] In principle, N2(g) and/or N2O(g) can
generated readily overwhelms the NOx adsorption capacity of
also be produced as
a
result of direct photocatalytic
titania leading to unwanted NO2(g) release into the atmosphere.
As NO2(g) is a much more toxic pollutant than NO(g), P25 does
not qualify as an efficient photocatalyst for NOx abatement under
UV-A light irradiation, evident by the extremely negative DeNOx
index values of fresh (-0.42) and aged (-0.45) P25 given in Figure
8. Incorporation of different loadings of CdSe/CdSeTe NPLs to
P25 results in enhanced PHONOS performance under UV-A
irradiation (Figure 8a). For the fresh 0.05 NPL/P25 photocatalyst,
NO conversion % and NOx storage selectivity % values reach
41 % and 81 % under UV-A illumination, respectively (Figure 8a).
For the fresh NPL/P25 photocatalysts increasing NPL loadings
within 0.1-2.0 mL monotonically enhance the PHONOS
decomposition and photo-reduction of NO(g).[80] However, this is
known to be a relatively inefficient reaction pathway, particularly
in the presence of O2 and H2O. Thus, N2 and/or N2O formation
can readily be ruled out in the current study.[17] It is apparent in
Figure 7b that photocatalytic NOx abatement action continues
after this initial stage during the entire duration of the activity test.
Thus, total NOx abatement effect can be calculated by duration of
the PHONOS test.
Photocatalytic performance of fresh and aged
CdSe/CdSeTe/P25 catalysts under UV-A light
PHONOS activity tests were performed for the fresh and aged
NPL/P25 photocatalysts with different loadings and compared
with that of a commercial benchmark P25 (titania) photocatalyst
under identical experimental conditions. Details of the
photocatalytic performance parameters and Figures of merits
used for performance comparison are described in detail in the
Supporting Information section along with the supplementary
photonic efficiency data (Figures S5-S8). Aged catalysts used in
performance, where NO conversion
% and NOx storage
selectivity % values of 44% and 94% can be achieved for
2.0NPL/P25 sample, respectively. In contrast, a further increase
in NPL loading (i.e. for 6.0NPL/P25) results in a deteriorated
performance. We believe that the attenuation of PHONOS
performance at high NPL loadings is due to the blocking of the
photocatalytic active sites of titania or substitution of the surface
-OH functionalities of titania with oleic acid capped-NPLs.
7
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UV-A
(a)
(b)
94
100
94
0.35
84
86
86
81
0.27
0.30
0.15
0.00
-0.15
-0.30
-0.45
0.24
0.24
0.20
80
60
40
20
0
80 82
0.15
0.20
69
0.10
0.03
55
34
44
55
41
23
61
42
44
42
33
37
43
24
38
33
31
19
-0.01
-0.14
-0.12
-0.42
8
-0.45
Fresh
Aged
Fresh
Aged
Figure 8. NO(g) conversion %, NOx storage selectivity % and DeNOx index values for different NPL/P25 composite photocatalysts obtained under UV-A irradiation.
[85-86]
Ohko et al.
proposed an alternative photocatalytic NO
It is apparent that fresh 2.0NPL/P25 photocatalyst is capable
of storing significant amounts of NOx in the solid state and
suppresses NO2(g) release significantly under UV-A irradiation.
These results clearly demonstrate that proper incorporation of
NPLs to titania (P25) alters the unfavorably negative DeNOx index
of P25 (−0.42) to a significantly positive value (+0.35) under UV-
A illumination (Figure 8b). The mechanism for the photocatalytic
oxidation of NO on TiO2 has been studied in detail in former
reports and the previously proposed mechanisms consist of
multiple pathways which yield nitrites and/or nitrates (along with
their protonated acidic forms) as the primary products.[82-85]
Formation of nitrates in similar processes was also demonstrated
via in-situ FTIR spectroscopy in one of our former reports.[18] NO
photooxidation on TiO2 starts with the absorption of photons by
TiO2 which leads to the generation of electron and hole (i.e. e- - h+)
charge carriers and proceeds with the trapping of these charge
carriers to form various surface species (eq. 3-5). Next, adsorbed
NO on the TiO2 surface is photo-oxidized via its interaction with
surface hydroxyl radicals (●OHads) or holes (h+) to form NO2.
Alternatively, hydroxyl radicals (eq. 6-8) or holes (eq. 9-11) can
also attack adsorbed NO2 species to form HNO3.
oxidation mechanism on TiO2 involving lattice oxygen species. In
this mechanism, NO oxidation can be carried out either via
superoxide radical anions (eq. 12-13) or via an electron deficient
surface oxygen species (eq. 14-16). Furthermore, adsorbed water
can also play an important role in the photocatalytic oxidation of
NO, as indicated in eq. 3-16.[82]
NO2ads + •OH
•-
(12)
O2
ads
NOads
H+
HNO3ads
TiO2 (Os•)
(13)
(14)
TiO2 (h+) + TiO (Os)
2
TiO2 (Os•)
H2O
TiO2 + 2H+
(15)
(16)
NOads
NO2ads + TiO2 (V)
TiO2 (Os•)
H2O
-
TiO2 + NO3 ads + 2H+
NO2ads
Aging of the NPL/P25 photocatalysts under UV-A irradiation for
18 h under ambient conditions leads to interesting variations in
the photocatalytic performance trends (Figure 8). For the NPL
loadings within 0.05 - 2.0 mL, although, NO conversion is not
significantly affected by UV-A aging, selectivity severely
decreases (Figure 8a). Nevertheless, despite the loss in
PHONOS performance, some of these aged photocatalysts are
able to yield significantly positive DeNOx index values. This
decrease in the photocatalytic performance of the NPL/P25
system upon UV-A aging can be ascribed to the
degradation/decomposition of the oleic acid capping of the NPLs
[87] and/or electronic changes in the CdSe and CdSeTe domains
(such as oxidation of Cd sites), as suggested by the current XPS
data (Figure 3). It is interesting to note that NO conversion
capability of the NPL/P25 system is only slightly affected by the
aging process (Figure 8), suggesting that a majority of the
photocatalytic NO oxidation active sites on titania remained intact
TiO2 (h+ + e-)
TiO2 + hν
(3)
(4)
(5)
(6)
(7)
TiO2 (h+) + H2Oads
TiO2 + •OHads + H+
•-
TiO2 (e-) + O2ads
NOads + •OHads
TiO2 + O2
HNO2ads
ads
HNO2ads + •OHads
NO2ads + •OHads
NO2ads + H2O
HNO3ads
(8)
(9)
HNO2ads + H+
NO2ads + H+
TiO2 (h+) + NOads + H2Oads
TiO2 (h+) + HNO2ads
(10)
(11)
TiO2 (h+) + NO2ads + H2Oads
HNO3ads + H+
8
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VIS
(a)
(b)
93
95
89
0.30
100
80
60
40
20
0
88
99
0.27
88
0.30
0.15
0.00
-0.15
-0.30
99
94
90
0.22
81
0.24
0.15
0.18
0.19
0.07
0.11
37
37
0.10
38
33
33
26
25
26
18
27
25
28
19
-0.29
-0.27
10
Fresh
Aged
Fresh
Aged
Figure 9. NO(g) conversion %, NO2 storage selectivity %, and DeNOx index values for different NPL/P25 photocatalysts obtained under VIS-light irradiation.
does not lead to significant detrimental changes in the PHONOS
and could still function efficiently even after aging under UV-A. On
performance of P25. Furthermore, fresh and aged P25 samples
the other hand, aging of 6.0NPL/P25 system with the highest NPL
lead to significantly negative and unfavorable DeNOx index values
loading leads to a drastic loss in both NO conversion activity as
under VIS-light illumination (Figure 9b), suggesting that without
well as in NOx storage selectivity (Figure 8a). This is probably due
NPLs, P25 cannot store photogenerated NO2 effectively and
to the extended obstruction of the photocatalytic active sites and
instead, releases toxic NO2(g) into the atmosphere. Incorporation
NOx storage sites of titania and CdSe/CdSeTe domains of the
of NPLs to P25 titania resulted in a minor increase in NO
6.0NPL/P25 system with the abundant oleic acid degradation
conversion % along with a drastic boost in NOx storage
products; evident by the very low DeNOx index of aged
selectivity % under VIS irradiation (Figure 9a). For the fresh
6.0NPL/P25 (-0.01), as shown in Figure 8b.
NPL/P25 photocatalysts with different NPL loadings, a volcano
plot in performance is visible, both in terms of NO conversion %,
NOx storage selectivity % (Figure 9a), as well as in terms of
DeNOx index values (Figure 9b). It is seen that fresh 0.1NPL/P25
provided the best NO conversion % along with the most positive
DeNOx index value (+0.30) under VIS illumination. The best
PHONOS performance under VIS irradiation was achieved with a
comparatively lower loading of NPL on P25 (0.1NPL/P25) as
compared to the UV-A case, where in the latter case the best
performance was observed for the 2.0NPL/P25 sample. This
important difference highlights the dissimilarities in the
corresponding photocatalytic reaction mechanisms driving the
PHONOS process under UV-A vs. VIS illumination. Some of
these important differences in the photocatalytic reaction
mechanisms as well as variations in the nature and coverage of
various surface species/intermediates formed during the
PHONOS reaction under UV-A vs. VIS will be elucidated in our
forthcoming reports utilizing a dedicated photocatalytic flow
reactor equipped with in-situ vibrational spectroscopic capabilities.
As discussed in the previous section, current results suggest that
the primary photocatalytic site for NO oxidation on the NPL/P25
system resides on titania, while a synergistic effect between
titania and NPLs results in an additional increase in NO
conversion capability. On the other hand, the major function of
oleic acid-capped NPLs is the photocatalytic conversion and
Photocatalytic performance of fresh and aged catalysts
under VIS light
Figure 9a indicates that fresh P25 titania photocatalyst has a
reasonably good NO conversion and NOx storage selectivity
under VIS irradiation (as in the case of UV-A irradiation). It is
interesting to note that even though P25 titania is commonly
considered to be an UV-active photocatalyst, current results
demonstrate that it also reveals significant PHONOS activity
under VIS illumination. Commercially available P25 titania used
in our measurements typically contains 78-85 wt.% anatase, 14-
17 wt.% rutile and 0-13 wt.% amorphous titania phases
constituting a variety of heterojunctions which can also reveal
significant photocatalytic NO conversion under VIS-light. In
addition, currently used photon flux values utilized in the VIS
experiments were greater than that of the UV-A experiments (see
section 2.4). Hence, photonic efficiency % results presented in
Figure S6, suggest significantly lower NOx storage photonic
efficiency % for P25 under VIS illumination compared to that of
UV-A (due to the normalization of conversion values with photon
flux in the photonic efficiency % definition), despite the fact that
both illumination sources reveal rather comparable NO
conversion % and NOx storage Selectivity % values (Figures 8
and 9). It is important to note that as in the UV-A case, VIS- aging
9
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10.1002/cctc.202000749
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storage of photogenerated NO2 species into other oxidized
Thus, the best overall PHONOS performance among VIS-aged
samples in terms of DeNOx index is achieved for the 0.5NPL/P25
catalyst (Figure 9b). Hence, in order to circumvent VIS-aging
effects, increasing NPL loading from 0.1 to 0.5 could be an
efficient approach that can extend the catalytic lifetime of
NPL/P25 photocatalytic architecture with a minor sacrifice in the
performance of the fresh NPL/P25 catalytic system.
−
−
surface species, such as HONO, HONO2, NO2 and NO3 . It is
apparent that for extremely high loadings of NPLs on P25,
PHONOS performance gradually decreases due to the blocking
of the titania active sites with NPLs. Aging of the NPL/P25
photocatalysts under VIS irradiation for 18 h decreases the NO
conversion % and thus leads to lower rates of NO2(g) generation
enabling a more efficient NOx capture by the photocatalyst
resulting in a higher NOx storage selectivity %. It is important to
mention here that VIS-aging has a comparatively less drastic
effect on the overall performance of NPL/P25 (Figure 9) as
compared to that of UV-A-aging (Figure 8), probably due to the
higher stability of NPLs and oleic acid capping agent under VIS
irradiation as demonstrated by the current C1s XPS data given in
Figure S2. Additional aging experiments (data not shown) carried
out on the 0.1NPL/P25 photocatalyst under UV irradiation for 18
h followed by PHONOS tests under VIS illumination showed that
this catalyst was able to maintain 79% of its original NO
conversion capability, %90 of its original NOx storage selectivity
under VIS irradiation after UV aging.
Effect of NPLs dispersing medium (toluene) on the
photocatalytic performance
In the current work, before the incorporation of P25 with NPLs,
NPLs were initially dispersed in toluene (C6H5CH3(l)) and then
transferred onto the P25 surface (see experimental section).
Hence, it is important to know whether toluene adsorption on P25
influences PHONOS performance. In an attempt to address this
issue, additional control experiments were performed by dosing
various amounts of C6H5CH3(l) on P25 at room temperature
followed by evaporation and drying at 70 ᵒC for 18 h.
UV-A
(a)
(b)
80
63
0.00
51
60
40
20
0
-0.04
41
-0.15
41
33
39
23
32
35
36
-0.19
31
19
25
-0.30
34
25
32
-0.42
-0.45
-0.35
-0.42
-0.36
-0.45
-0.43
VIS
Fresh
Aged
Aged
Fresh
(c)
(d)
0.15
80
60
40
20
0
77
0.12
63
63
-0.03
37
0.00
-0.15
-0.30
40
39
33
40
44
-0.04
33
38
34
30
27
30
28
-0.29
-0.19
-0.27
-0.26
-0.29
Figure 10. NO(g) conversion %, NO2 storage selectivity % results and the corresponding DeNOx index values obtained under UV-A (a, b), and VIS illumination (c,
d) for P25 photocatalysts dosed with various amounts of toluene (C6H5CH3(l)).
10
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UV-A
(b)
(a)
93
100
80
71
0.33
59
0.30
53
60
42
47
42
0.15
0.00
-0.15
-0.30
0.06
42
42
40
20
0
44
-0.10
-0.17
-0.27
VIS
(c)
(d)
93
0.29
0.30
80
100
80
60
40
20
0
79
80
78
0.20
0.10
0.00
0.14
37
0.14
0.14
36
37
37
0.12
36
Figure 11. Long-term NO conversion % and NOx storage selectivity % results and the corresponding DeNOx index values obtained for five consecutive 1 h-long
PHONOS tests under UV-A (a, b), and VIS illumination (c, d). UV-A experiments were performed with 2.0NPL/P25, while VIS experiments were performed with
0.1NPL/P25.
various amounts of C6H5CH3(l) on P25 at room temperature
followed by evaporation and drying at 70 ᵒC for 18 h. Along these
lines, fresh and UV-A or VIS-aged T/P25 samples were prepared
using the same volumes chosen for the preparation of the
NPL/P25 composite systems. Then, PHONOS performances of
these samples were tested under UV-A (Figures 10a and 10b) or
VIS illumination (Figures 10c and 10d). It is interesting to observe
that addition of increasing loadings of toluene leads to an increase
in NO conversion % and NOx storage selectivity % of fresh P25
under both UV-A and VIS light. While the enhancement in the
PHONOS performance of fresh P25 due to toluene dosing cannot
account for the entire photocatalytic enhancement observed for
the NPL/P25 system, it clearly reveals some contribution to the
observed performance boost. It is likely that in the presence of
UV-A or VIS illumination, fresh P25 surface can carry out partial
oxidation (as well as total oxidation) of toluene, resulting in the
formation of various surface species (such as aromatic
oxygenates with -OH, -COH, -COOH, functionalities etc.),[88]
which may expedite the NO and NO2 adsorption. Thus, facilitated
adsorption and increased surface residence times of NO(ads) and
NO2(ads) species on the P25 surface, may enhance the oxidative
storage of such species and improve PHONOS performance. On
the other hand, extensive (18 h) irradiation of the T/P25 samples
with UV-A or VIS light seems to eliminate the
Favorable influence of toluene on P25 PHONOS performance
due to total oxidation of toluene and its partial oxidation products
to CO2(ads,g) and H2O(ads,g). Hence, it is likely that while toluene
may have a small but measurable positive influence on the
PHONOS performance of the fresh NPL/P25 samples
investigated in Figures 8 and 9, its effect on the (18 h UV-A or
VIS) aged samples seems to be rather insignificant.
Photochemical stability of NPL/P25 composites and
reusability
To demonstrate the photocatalytic reusability of the NPL/P25
materials, a series of experiments were performed on the best
performing catalyst for each illumination type, where five
successive 1 h-long PHONOS tests were carried out under UV-A
or VIS-light on 2.0NPL/P25 or 0.1NPL/P25, respectively (Figure
11). Under UV-A illumination, initial NO conversion % of 2.0
NPL/P25 remained invariant, while its initially high NOx Storage
Selectivity % (i.e. 93 %) monotonically decreased to 47 % after
five runs (Figure 11a). In addition, under UV-A irradiation, DeNOx
index values of 2.0NPL/P25 also rapidly declined and became
negative after the second cycle (Figure 11b). This monotonic
attenuation in NOx abatement capability could be probably due to
the continuous accumulation of nitrites/nitrates on the
photocatalyst surface diminishing the NOx storage capacity and
11
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facilitation of NO2(g) desorption/release via nitrate reduction. It is
important to note here that identical stability experiments
performed with titania P25 benchmark catalyst (data not shown)
also revealed a monotonically decreasing DeNOx index values
where the values ranged between -0.32 and -0.37. Similarly,
under VIS illumination, NO conversion of 0.1NPL/P25 remained
relatively constant after five runs, while initial NOx Storage
Selectivity %(93%) decreased slightly (to 78%), maintaining a
positive DeNOx index value after five runs (Figures 11c and 11d).
This monotonic attenuation in NOx abatement capability could be
probably due to the continuous accumulation of nitrites/nitrates on
the photocatalyst surface diminishing the NOx storage capacity
and facilitation of NO2(g) desorption/release via nitrate reduction.
It is important to note here that identical stability experiments
performed with titania P25 benchmark catalyst (data not shown)
also revealed a monotonically decreasing DeNOx index values
where the values ranged between -0.32 and -0.37. Similarly,
under VIS illumination, NO conversion of 0.1NPL/P25 remained
relatively constant after five runs, while initial NOx Storage
Selectivity % (93%) decreased slightly (to 78%), maintaining a
positive DeNOx index value after five runs (Figures 11c and 11d).
understand the contribution of the temperature and validate that
the observed catalytic enhancement under VIS light illumination
originates from photocatalytic processes and not due to thermal
catalytic routes. In order to address this issue, temperature of the
photocatalytic reactor was varied by changing the distance
between the VIS-light source and the reactor. VIS photon flux was
decreased by increasing the distance between the VIS-light
source and the reactor, which ultimately led to lowering of the
reactor temperature during PHONOS tests. Next, photocatalytic
performance of fresh P25 and 0.1NPL/P25 samples were
measured under VIS-light illumination at different reactor
temperatures, as shown in Figure 12. It should be emphasized
that since absolute NO conversion
% and NOx storage
selectivity % values directly depend on the utilized VIS photon
flux, these values cannot be used to compare PHONOS
performance at different temperatures employing different photon
flux values. Thus, for the evaluation of the photocatalytic activities
at different VIS flux values (i.e. at different temperatures),
photocatalytic NOx storage and NO2 generation were normalized
with the incident VIS photon flux values and presented in terms of
corresponding photonic efficiency % values, as shown in Figure
12a. As a comparison, corresponding NO Conversion % and NOx
Storage % data are also provided in Figure 12b.
Effect of temperature on photocatalytic performance
during VIS irradiation
These results clearly demonstrate that increasing temperature
during VIS-light illumination tends to decline the NOx storage
photonic efficiency % of pure P25 possibly due to accelerated
electron-hole recombination or increased water dissociation and
poisoning of the titania surface with an extensive amount of
adsorbed hydroxyl/hydroxide species.
During the UV-A activated PHONOS tests, temperature of the
photocatalytic reactor stayed within 20−25 °C, while it reached up
to 46 °C during the VIS-light experiments. This is due to the
differences in the emission spectra of the UV-A vs. VIS light
sources employed in these tests. Therefore, it is important to
VIS
(a)
(b)
97
0.117
0.12
100
93
58
80
60
40
20
0
0.076
0.09
0.06
37
0.008
0.023
0.004
0.03
0.00
18
33
38
0.041
0.003
17
0.014
P25
0.1 NPL/P25
P25
0.1 NPL/P25
P25
P25
0.1 NPL/P25
0.1 NPL/P25
20 3 ⁰C
Low flux (~90 µmol/m2s)
44 3 ⁰C
High flux (~ 480 µmol/m2s)
20 3 ⁰C
Low flux (~90 µmol/m2s)
44 3 ⁰C
High flux (~ 480 µmol/m2s)
Figure 12. Temperature-dependent PHONOS data for pure P25 and 0.1NPL/P25 under VIS-light illumination (a) NOx storage photonic efficiency % and NO2 release
photonic efficiency %, (b) NO conversion % and NOx storage selectivity % values.
12
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In the case of 0.1NPL/P25, while increasing temperature has
almost no effect on NO2 release photonic efficiency %, it
drastically diminishes the NOx Storage photonic efficiency %,
again suggesting a decrease in overall PHONOS performance
with increasing temperature. Therefore, both of the control
experiments presented in Figure 12a indicate that an increase in
the reactor temperature during the VIS-light illumination cannot
be responsible for the observed boost in photocatalytic activity,
depicted in Figure 9 and the photocatalytic enhancement
observed for the NPL/P25 composite system is a result of
photocatalytic routes rather than conventional thermal catalytic
routes.
illumination, while reusability was limited under UV-A illumination.
Attenuation of the PHONOS performance upon photochemical
aging and extended operation can be linked to electronic changes
in the Cd species as well as degradation of the oleic acid capping
of the NPL system. Influence of temperature was also studied for
the PHONOS performance of NPL/P25 composites under VIS-
light suggesting a decrease in photocatalytic efficiency with
increasing temperature due to the possible increase in electron-
hole recombination rate and the increased formation of
hydroxide/hydroxyl surface functionalities poisoning the
photocatalyst surface. Similar NPL/P25-based composite
systems can be used as a new family of active photocatalytic
materials with a significant potential in various environmental
remediation and solar energy conversion applications.
Conclusions
In this work, oleic acid functionalized CdSe/CdSeTe core/crown
nanoplatelets were incorporated to titania (P25) and the
synthesized composites were successfully utilized in
photocatalytic NOx oxidation and storage (PHONOS) under both
UV-A and VIS-light illumination. The proposed NPL/P25
composite photocatalysts with optimized NPL loadings displayed
a relatively high NO(g) conversion and extremely high selectivity
toward NOx storage as compared to the commercial P25 titania
benchmark photocatalyst both under UV-A and VIS-illumination.
Through a series of comprehensive control experiments, roles of
the individual components of the NPL/P25 composite were
identified. These experiments demonstrated that NPL/P25
composite outperformed the individual functionalities in their
separated forms. It was shown that while primary photocatalytic
active sites for NO oxidation reside on titania, NPLs provided a
synergistic boost both in NO conversion as well as NOx storage
selectivity. The origin of the enhanced NO conversion can be
attributed to the presumably improved electron-hole separation
capability of the utilized colloidal quantum wells. Having Type-II
band alignment, the electron and hole wave functions are possibly
positioned in different parts of the NPL heterostructure, resulting
in spatial separation of charge carriers and longer fluorescence
lifetimes. Furthermore, oleic acid functionalities as well as
CdSe/CdSeTe core/crown nanoplatelets also facilitated NO2
adsorption, extended the surface residence time of adsorbed NO2
species and assisted their conversion to nitrates. Control
experiments revealed that toluene (C6H5CH3(l)) used as the
dispersing medium for NPLs also provided a relatively small boost
in NO conversion, however this effect disappeared upon
Experimental Section
Cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O; 99% trace metal
basis), cadmium acetate dihydrate (Cd(OAc)2.2H2O; 98%),
sodium myristate (CH3(CH2)12COONa, ≥ 99%), titanium (IV) oxide
(P25, ≥ 99.5% trace metal basis), 1-octadecene (ODE, technical
grade), tellurium (Te), selenium (Se), oleic acid (OA, 90%
technical grade), trioctylphoshine (TOP), hexane, ethanol,
methanol and toluene were purchased from Sigma Aldrich and
used as received without further purification.
Synthesis of CdSe(core)/CdSeTe (crown) nanoplatelets
Synthesis of Cadmium Myristate
Cadmium myristate (C28H54CdO4, Cd(II) tetradecanoate) was
prepared according to our previously published protocol with
slight modifications.[89] In a typical synthesis, 6.26 g of sodium
myristate and 2.46 g of cadmium nitrate tetrahydrate were
separately dissolved in 500 and 80 mL of methanol, respectively.
Upon complete dissolution, the solutions were mixed and stirred
vigorously for 3 h. Consequently, the white cadmium myristate
powder was precipitated by centrifugation and washed with
methanol three times to remove any unwanted impurities and/or
side products. Lastly, cadmium myristate was dried overnight
under vacuum at room temperature (RT).
Synthesis of CdSe core of NPLs
CdSe core of NPLs having a four-monolayer (4 ML, i.e. consisting
of four Se atomic layers and five Cd atomic layers) thickness were
synthesized by using a former procedure discussed elsewhere.[50,
90] Along these lines, 20 mg of Se, 340 mg of cadmium myristate
extended
operational
durations
due
to
the
photocatalytic/photochemical degradation of adsorbed toluene.
NPL/P25 systems revealed reasonable reusability under VIS
13
This article is protected by copyright. All rights reserved.

10.1002/cctc.202000749
ChemCatChem
FULL PAPER
(synthesized in previous step) and 30 mL of 1-octadecene were
placed in a 100 mL three-neck flask and degassed at 95 °C for 1
h to remove oxygen, water and any other volatile species. Next,
the sample temperature was increased under the Ar(g) flow. At
ca. 195 °C, the color of the reaction solution turned golden yellow,
after which 120 mg of Cd (OAc)2•2H2O was promptly added to the
reaction system. After having kept the reaction solution at 240 °C
for 8 min for further growth of NPLs, 1 mL of oleic acid was
injected. At this point, reaction was stopped, and the temperature
of the solution was brought to room temperature. Subsequently,
hexane (10 mL) and ethanol (10 mL) were added to the solution
to precipitate the synthesized NPLs. Finally, NPLs were
centrifuged and stored in hexane for the crown coating step.
in the left-hand side of the acronyms represent the corresponding
volume (mL) of NPL loading on P25. Furthermore, additional
samples were also prepared in order to elucidate the effect of
toluene (i.e. NPLs dispersing medium) on the photocatalytic
performance of P25. For this purpose, various amounts of toluene
were drop-cast on P25 at RT followed by evaporation and drying
ᵒ
at 70 C for 18 h. These samples are designated as XT/P25,
where X represents the volume of toluene dosed on P25 in mL
(i.e. X = 0.1, 0.5, or 2.0 mL).
Structural characterization Methods
Crystallographic structures of the synthesized materials were
determined by using a PANalytical Empyrean XRD diffractometer
equipped with Cu Kα irradiation source. (40 kV, 45 mA, λ=1.5405
Å). Transmission electron microscopy (TEM), scanning
transmission electron microscopy (STEM), high-angle annular
dark field (HAADF) imaging and energy dispersive X-ray (EDX)
analysis experiments were carried out at 120 kV using a Hitachi
High-tech HT7700 TEM equipped with BF-/DF-STEM-EDX
modules at DAYTAM user facility (Ataturk University, Erzurum,
Turkey). X-ray photoelectron spectroscopy (XPS) experiments
were performed with a SPECS PHOIBOS hemispherical energy
analyzer. A monochromatic Al-Kα X-ray excitation source (14 kV,
350 W) was employed during the XPS data acquisition. BET
specific surface area (SSA) measurements of the synthesized
catalysts were determined by using nitrogen adsorption–
desorption isotherms obtained with a Micromeritics 3Flex surface
area and pore size analyzer. Prior to SSA analysis, all samples
were outgassed in vacuum for 2 h at 150 °C. UV-VIS absorption
spectra were obtained using an UV-VIS-NIR Spectrometer-Cary
5000-and toluene (i.e. dispersing medium) was used as a
reference. The photoluminescence (PL) spectra were acquired at
room temperature using a Jobin-Yvon Horiba Fluorolog-3
spectrometer equipped with a Hamamatsu R928 P detector and
a 450 W ozone-free Osram XBO xenon arc lamp. The excitation
wavelength was 400 nm (i.e. 3.1 eV), which is well above the band
gap of NPLs. The fluorescence was monitored at a right angle
relative to the excitation. Electron paramagnetic resonance (EPR)
measurements were performed with a Bruker EMX Nano
spectrometer with an integrated referencing for g-factor
calculation and also integrated spin counting units. The
microwave frequency of the cavity was 9.41 GHz (X-band) and all
spectra were measured at RT with 0.1 mT modulation amplitude,
2 mW microwave power and 120 scans (sweep time 60 s/scan,
time constant 81.92 ms). Before the measurements, EPR
samples were inserted into a spin-free 25 cm long quartz tube
(Qsil®, Germany). In-situ EPR experiments were also carried out
Synthesis of the 4 ML-Thick CdSe/CdSe0.75Te0.25 Core/Alloyed
Crown
For a typical synthesis,[55] 2 mL of 4 ML-thick CdSe core
suspension in hexane (where 0.1 mL of this suspension in 3 mL
of hexane had an optical density of 0.9 at 512 nm), 10 mL of 1-
octadecene, 100 μL of oleic acid and 50 mg of cadmium acetate
dihydrate were combined in a 25 mL three-neck round bottom
flask. The reaction mixture was degassed at RT for 1 h and then
was set to 215 °C under Ar(g) flow for the growth of the CdTe
crown region. At 185 °C, 0.75 mL of ODE−TOP-Se-Te solution
was injected to the reaction medium at a rate of 2.5 mL/h (where
ODE−TOP-Se-Te solution was prepared by mixing 79 mg Se and
127.6 mg Te with 2 mL of TOP in an inert atmosphere overnight
to ensure complete dissolution. Next, 60 μL of this mixture was
diluted with 2 mL of ODE to achieve the final ODE−TOP-Se-Te
solution). It is important to note here that, if needed, the size of
the crown region could be controlled by varying the amount of the
injected ODE−TOP-Se-Te solution. Immediately after the
injection of ODE-TOP-Se-Te solution, 1 mL of oleic acid was
swiftly added to the reaction medium and the solution was
quenched to RT. CdSe/CdSe0.75Te0.25 core/alloyed crown NPL
were precipitated by the addition of ethanol, centrifuged and
finally dispersed in toluene for further studies.
Preparation of CdSe/CdSe0.75Te0.25/P25 composite Materials
To synthesize CdSe/CdSe0.75Te0.25/P25 composite systems,
different volumes of CdSe/CdSe0.75Te0.25 NPLs colloidal
suspensions in toluene were drop cast on 250 mg of P25 (i.e.
TiO2). After physical mixing of the NPLs with P25 for 2 min,
ᵒ
samples were dried in an oven at 70 C for 18 h. The samples
were labeled as 0.05NPL/P25, 0.1NPL/P25, 0.5NPL/P25,
1.0NPL/P25, 2.0NPL/P25, and 6.0NPL/P25, where the numbers
14
This article is protected by copyright. All rights reserved.

10.1002/cctc.202000749
ChemCatChem
FULL PAPER
in the presence of UV-light irradiation using an ER 203 UV
radiation system containing a short-arc mercury lamp (LSB 610
100 W Hg, LOT-Quantum Design).
flux of the VIS light source was about 15 times greater than that
of the UV-A light source. Relative percentile of VIS photon flux in
typical solar radiation is also significantly greater than that of UV-
A photon flux (i.e., ca. 42% VIS vs. 6% UV-A). Reactor
Photocatalytic activity measurements
temperature remained within 23
±
2
°C during UV-A
measurements, whereas during VIS light experiments, the reactor
temperature reached up to 46 °C after a typical 60 min
photocatalytic activity test. In each performance analysis test, 250
mg of photocatalyst was packed in a 2 mm × 40 mm × 40 mm
polymethyl methacrylate (PMMA) sample holder and placed into
the flow reactor. To quantify the photocatalytic activity and
selectivity, at the end of a typical 1 h photocatalytic activity test,
obtained raw data was processed in order to obtain various
figures of merit (such as % photonic efficiency towards NOx
storage, % photonic efficiency for NO2 generation, % NO
conversion , % selectivity towards NOx storage, and DeNOx index).
Numerical definitions of these figures of merit are provided in the
Supporting Information (SI) section.
The photocatalytic NOx oxidation and storage performance
measurements were executed at RT in a custom-designed flow
reactor by considering the experimental requirements reported in
the ISO 22197-1 standard.[91] Inlet gas mixture introduced to the
reactor (Figure S9) contained 0.750 standard liters per minute
(SLM) N2(g) (purity: 99.99%, Linde GmbH), 0.250 SLM O2(g)
(purity: 99%, Linde GmbH) and 0.01 SLM NO(g) (100 ppm NO (g)
diluted in balance N2(g), Linde GmbH). To obtain gas flow values
mentioned above, mass flow controllers (MKS 1479A for N2 (g)
and Teledyne HFC-202 for NO(g) diluted in N2(g)) were utilized
so that the total gas flow over the photocatalyst was kept at 1.010
SLM ± 0.05 SLM, where the NO(g) content of the inlet gas mixture
was fixed at 1.00 ppm. The pressure inside the reactor was kept
at ca. 1 bar and measured with an MKS Baratron 622B
capacitance manometer. Humidity of the inlet gas mixture was
also carefully controlled by dosing varying amounts of water vapor
into the inlet gas mixture (i.e. before the reactor entrance) with the
help of PermSelect (PDMSXA-2500) semipermeable membrane
module attached to an external variable-temperature water
chiller/recycler. Typical relative humidity of the reactor was kept
within 50±3% at 23±2 ᵒC, measured at the sample position using
a Hanna HI 9565 humidity analyzer. The changes in the total NOx,
NO and NO2 concentrations at the reactor outlet were monitored
via a chemiluminescent NOx analyzer (Horiba APNA-370) with a
0.1 ppb sensitivity and 1 Hz data acquisition speed. For the
experiments performed with UV-A irradiation, an 8 W UV-A lamp
(F8W/T5/BL350, Sylvania, Germany) was used, while for the
experiments carried out with VIS light illumination, a 35 W metal
halide lamp (HCI-TC 35 W/942 NDL PB 400−700 nm range,
OSRAM) was utilized. Since the utilized VIS light source also
emitted a limited but detectable flux of UV-A light, a commercial
VIS transparent UV-A-blocker/filtering film (LLumar window film
UV-A CL SR PS (clear)) was placed on top of the reactor during
the VIS-light experiments. This was crucial for ruling out any
contribution from UV-A photons during the VIS- light illumination.
The incoming light flux was measured carefully before and after
each UV-A and VIS-light measurement with a photo radiometer
(HD2302.0, Delta Ohm/Italy) using a UV-A probe (315−400 nm)
and a PAR VIS probe (400−700 nm), respectively. Typical VIS-
light photon flux values used in the current experiments were
within 450−500 μmol/(m2.s), while typical UV-A-light power
density values were within 7.7−8.3 W/m2. Note that the photon
Acknowledgements
EO, EE, MI acknowledge the financial support from the Scientific
and Technological Research Council of Turkey (TUBITAK)
(Project Code: 116M435). EO acknowledges the scientific
collaboration with TARLA project founded by the Ministry of
Development of Turkey (project code: DPT2006K–120470). HVD
gratefully acknowledge support from TÜBA. Authors also
acknowledge Mr. Mete Duman (UNAM- National Nanotechnology
Center) for the design of technical graphics.
Keywords: Photocatalytic NOx abatement, PHONOS, DeNOx,
titanium dioxide, CdSe/CdSeTe nanoplatelets, quantum-wells.
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hv
NPLs
Decoration of TiO2 (P25) with two-dimensional (2D) oleic acid-capped CdSe(core)/CdSeTe(crown) quantum-well nanoplatelets (NPLs)
with Type-II heterojunctions boost activity and selectivity toward nitrate formation in photocatalytic NOx oxidation and storage
(PHONOS) under both ultraviolet (UV-A) and visible (VIS) light irradiation.
18
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