Engineering an N-doped TiO2@N-doped C butterfly-like nanostructure with long-lived photo-generated carriers for efficient photocatalytic selective amine oxidation

Article abstract of DOI:10.1039/c7ta09166d

The photocatalytic efficiency of titanium oxide (TiO₂) has been limited by the degree of visible-light absorption, electron/hole separation and surface reactivity. In this article, N-doped porous carbon incorporating N-doped TiO₂ wit

Full text of DOI:10.1039/c7ta09166d

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Engineering N-doped TiO2@N-doped C butterfly-like
Nanostructure with Long-lived Photo-generated Carriers for
Efficient Photocatalytic Selective Amine Oxidation
Received 00th January 20xx,
Accepted 00th January 20xx
Fan Wang,a† Xiaoxiao He,b† Liming Sun,a† Jinquan Chen,b,* Xiaojun Wang,a Jianhua Xu,b Xiguang Han
DOI: 10.1039/x0xx00000x
a,*
www.rsc.org/
The photocatalytic efficiency of Titanium Oxide (TiO2) has been limited by the degree of visible-light absorption,
electron/hole separation and surface reactivity. In this article, N-doped porous carbon incorporating N-doped TiO2 with
butterfly-like structure has been synthesized by using NH2-MIL-125 (Ti), a Ti-base metal-organic framework, as a hard
template. The SEM, TEM and AFM observation indicate that the as-synthesized nanocomposites preserve MOFs template
butterfly-like morphology, which are assembled by two-dimensional (2D) corrugated nanosheets. The EDX and XPS results
prove that the composites of product are N doped TiO2 small particles distributing N doped C matrix. The N2 adsorption
and desorption isotherms, UV-Vis spectra and photocurrent density measurements indicate that the N-doped porous
carbon incorporating N-doped TiO2 nanoparticles provide several favorable features for photocatalysis: Large BET surface
area, visible-light absorption and high charge separation efficiency. As a result, the nanocomposites exhibits excellent
photocatalytic activity toward selective amine oxidation to imines. Density functional calculations, electron spin resonance
(ESR) spectroscopy and transient absorption spectroscopy were used to reveal the photocatalytic process mechanism.
Unfortunately, low visible-light absorption efficiency, fast
electron/hole recombination and small specific surface area
limit their widespread application in the near further.
Introduction
The global energy and environmental crisis have stimulated
tremendous research efforts to develop sustainable and
renewable energy sources.1-2 Solar driven reactions, which can
convert solar energy into fuel and/or electricity, hold great
promise to substantially reduce the energy consumption in
modern chemical industry.3-5 In particular, metal oxide
semiconductor photocatalysis, which can harvest sunlight to
initialize chemical reactions on the catalyst surface, has
attracted considerable attention.6-8 Due to low cost, high
chemical stability, nontoxicity and high reactivity, titanium
oxide (TiO2) has shown great potential as ideal and powerful
photocatalyst for various significant reactions,9 such as
photocatalytic degrade and mineralize toxic organic
pollutants,10-12 water splitting,13-15 organic synthesis,16,17 and
reduction of carbon dioxide to carbon-based chemicals.18,19
To solve the visible-light absorption problem, various
strategies have been reported to develop of visible light-
responsive TiO2, including dye sensitization20,21 and impurity
doping by chemical or physical ion-implantation methods.22-24
Among the various approaches, N-dopping TiO2 has attracted
much attention and has been widely studied.25,26 The partial
substitution of nitrogen element for lattice oxygen can narrow
the bandgap of TiO2 by introducing sub-bandgap states, thus
improve the visible light absorption and photocatalytic activity.
However, the new energy bonds increase the photo generated
electrons and holes recombination rate because they can
introduce additional recombination centers. To overcome the
problem, modifying TiO2 with some electron acceptors have
been reported (e.g., noble NPs,27,28 semiconductor
heterojunctions,29,30
and
carbon-based
materials31).
Considering the complicated synthesis of semiconductor
heterojunctions and high prices of loading noble metals,
carbon materials (including activated carbon,32 carbon
nanotube,33 grapheme34 and fullerene35) with high mobility of
charge carriers have been demonstrated a promising co-
catalysis to improve the charge separation efficiency. However,
it is found that there is still much room left to improve their
performance. Recently, improving the surface functionalities
of pure carbon via nitrogen doping was found to remarkably
enhance electron mobility, which is identified as an effective
way to further improve the charge separation efficiency.36,37
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Therefore, the ability to control TiO2 with both doping N and N details can be referred to ESI†. The X-ray power diffraction
doped carbon coating can be beneficial to improve the light- (XRD) pattern (Fig. S1) matches well with simulated and
harvesting capacity and to reduce the number of published XRD patterns,42 confirming the product as pure
recombination centres. Nevertheless, it is still
challenge to combine all-mentioned multiple modifications in was further indicated the precursor was NH2-MIL-125(Ti) (Fig.
TiO2. S2). Scanning electron microscopy (SEM) is used to
a great phase NH2-MIL-125(Ti) with good crystallinity. FTIR spectrum
Metal organic frameworks (MOFs), constructed by metal characterize the morphology of the synthesized NH2-MIL-
ions or clysters and organic linker, behave as ideal sacrificial 125(Ti) particles. These precursor particles show high
templates to prepare metal oxide nanoparticles, distributing monodisperse and regular butterfly-like shape (Fig. 1a). The
throughout the porous carbonaceous matrix with preserved high-magnification SEM image clearly displays that each
MOFs template morphology.38-42 Furthermore, through the particle is assembled by two-dimensional (2D) corrugated
rational selection or design of organic linkers, different nanosheets with a smooth surface (Fig. S3a). EDX mapping is
heteroatoms can be introduced to doped metal oxide and also performed on the as-obtained NH2-MIL-125(Ti) precursor
carbon via thermal decomposition under different to investigate the element distributions. The results show that
atmospheres.43-47 Therefore, using suitable MOFs as template the Ti, O, C and N are well-dispersed in precursor (Fig. S4).
provides a potential avenue to synthesis of N doped TiO2 According to thermogravimetric analysis (TGA) performed on
distributing N doped C matrix with unique structure.
In this work, we firstly synthesized Ti-based frameworks, carbonization of NH2-MIL-125 (Ti) is set to 550 °C for 60 min in
NH2-MIL-125 (Ti) with butterfly-like structure as a hard a Ar gas atmosphere with a ramping rate of 2 °C
min−1 to
NH2-MIL-125 (Ti) (Fig. S5), the annealing condition for the
˙
template. The template was thermally converted to N-doped convert the resulting NH2-MIL-125 (Ti) phase into the anatase
porous carbon incorporating N-doped TiO2 with preserved TiO2 phase. As revealed by XRD analysis (Fig. S6), the NH2-MIL-
MOFs template morphology, which was assembled by two- 125 (Ti) precursors are transformed to pure anatase TiO2 (PDF
dimensional (2D) corrugated nanosheets. The primary No. 00-001-0562) during the annealing process. The FESEM
architecture provides highly exposed active sites and large image (Fig. 1b) indicates the overall butterfly-like morphology
specific surface area (122.8 m2/g), while it also effectively of the NH2-MIL-125 (Ti) precursors are well preserved with a
avoided aggregation of the active small TiO2 nanoparticles. uniform sizes distribution. The magnified observations (Fig.
Thus, the nanocomposite exhibits excellent photocatalytic S3b) further reveal the butterfly-like morphology is assembled
activity toward selective amine oxidation to imines. Electron by ultrathin nanosheets with a porous structure. The thickness
spin resonance (ESR) spectroscopy showed that superoxide of nanosheet is further confirmed by atomic force microscopy
radical is the intermediate product of the photocatalytic (AFM) analysis (Fig. 1c). The results indicate that the thickness
reaction. The photocatalytic mechanism was investigated by of ultrathin nanosheet is about 1.4 nm. The Brunauer Emmett
using density functional theory (DFT) calculations and transient Teller (BET) measurement shows that the particles have the
absorption (TA) spectroscopy. We reveal that effectively large surface area of 122.8 m2/g, and the Barrett-Joyner-
charge separation in TNPC could be the key to enhancing the Halenda (BJH) pore sizes distribution is about 2 nm and 11 nm
photocatalysis performance.
(Fig. 1d).
Results and discussion
Scheme 1. Schematic showing the synthetic process of monodisperse
TiO2@N-doped porous carbon (TNPC).
Scheme 1 illustrates the detail formation process of the
monodisperse TNPC nanoparticles. Firstly, monodisperse NH2-
MIL-125(Ti) template with butterfly-like structure is
synthesized using tetraisopropyl titanate (TIPT) as Ti salt and 2-
Fig. 1 (a) FE-SEM images of NH2-MIL-125 (Ti) precursor, (b) SEM image of TNPC, (c) AFM
image of the single ultrathin N-doped TiO2@N-doped C nanosheet, the height profile of
corresponding line in Fig. 1c, (d) N2 adsorption/desorption isotherms and BJH pore size
distribution of TNPC.
aminoterephthalic acid as organic linker by
a simple
hydrothermal method at 150 oC for 24 h. The experimental
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The butterfly-like particles are characterized by transmission
Moreover, the X-ray photoelectron spectroscopy is used to
electron microscopy (TEM) to further understand the inside characterize the chemical state of elements in TNPC
structural information. From Fig. 2a, we can see that the nanoparticles. The survey spectrum indicates that the
projection appearance of particles keeps butterfly-like existence of Ti, O, C, and N element in the TNPC nanoparticles
structures assembled with 2D ultrathin porous nanosheets, (Fig. S7). The high-resolution XPS spectrum of Ti (Fig. 3a) shows
consisting well with the shape of the SEM image at a low– peaks centred at 458.6 and 464.4 eV, corresponding to Ti 2p3/2
magnification. A high–magnification survey (Fig. 2b) on an and Ti 2p1/2, respectively. The high-resolution spectra of O 1s
individual porous nanosheet indicates that these small TiO2 displays a broad asymmetric curve, which can be resolved into
nanoparticles with a diameter of 5-10 nm are well dispersed in three components centred at 529.7, 530.1 and 531.8 eV
the carbon matrix. The corresponding selected-area electron corresponding to the lattice oxygen, C=O, and O–H bonds,
diffraction pattern (inset Fig. 2b) shows a set of concentric respectively (Fig. 3b). The C 1s XPS spectrum (Fig. 3c) can be
rings, which match well with anatase TiO2. In addition, the split into three centred Gaussian components centred at 284.7,
high-resolution TEM (HRTEM) image (Fig. 2c) gives very clear 285.2, 288.6 eV, which are in agreement with the C-C, C-N and
crystalline planes with a d-spacing of 0.37 nm, which can be C=O bonds, respectively. The existence of C-N bond confirms
ascribed to the (100) plane of anatase TiO2, further confirming the nitrogen-doping in carbon matrix, which originates from
that the phase of small particles is anatase TiO2.
the N atoms in the 2-aminoterephthalic acid ligands of the
A typical HAADF-STEM image of nanocomposite shows NH2-MIL-125 (Ti) MOFs. The convolution of N 1s peak is
uniform dispersion of the Ti, O, C, N elements in the whole displayed in the Fig. 3d. Three component peaks at the binding
butterfly-like structure (Fig. 2d-h), suggesting the distribution energy of 398.5, 399.6 and 400.6 eV are ascribed to the
of TiO2 nanoparticles in the nitrogen(N)-doped porous carbon existence of N-O, N-Ti-O and N-C bonds, which further
matrix (TNPC). The presence of these elements can be further confirms the existence of N-doped titanium and carbon.
confirmed by the elemental profiles (Fig. 2i). Therefore, during Overall, the TNPC nanoparticles is comprised N-doped TiO2
the annealing process, the titanium ion in the NH2-MIL-125 (Ti) nanoparticles embedded in an N-doped
C matrix. The
butterfly-like structure has been converted to TiO2 introduction of nitrogen and carbon into the TiO2 may modify
nanoparticles, while the nitrogen containing organic linker (2- the electronic band structure, which leading to increase the
aminoterephthalic Acid), which possesses both carbon and adsorption of visible light. As contrast, pure TiO2 and N doped
nitrogen elements, are in situ carbonized to form the N-doped TiO2 with the same particles size of TiO2 in TNPC (about 10 nm)
porous carbon matrix. Because ligands are linked by titanium have prepared (Fig. S8, S9). Form the UV-vis
paddlewheel metal nodes and titanium ions are uniformly spectrophotometer of three samples (Fig. S10), the pure TiO2
distributed in the NH2-MIL-125 (Ti) MOFs, then a uniform displays a weak absorption in the visible light region, which
dispersion of TiO2 nanoparticles in the nitrogen-doped carbon caused by the existence of Ti3+ defect and oxygen vacancies on
matrix with strong adsorption is obtained after the thermal the surface of synthesized TiO2 (Fig. S11). The introduction of
annealing the MOFs.
nitrogen into the crystalline lattice of TiO2 (N-TiO2) obviously
enhances the visible light absorption. Furthermore, the TNPC
particles further enhance the absorption of visible light from
400 to 600 nm, which may attribute to the synergic
contributions from both the N doped and the existence of
carbon matrix.
Fig. 3 High-resolution (a) Ti 2p, (b) O 1s, (c) C 1s, (d) N 1s XPS spectra of TNPC
Fig. 2 (a) TEM image; (b, c) HRTEM image of TiO2, inset: the corresponding SAED
pattern; (d-h) STEM image and EDX elemental mapping of Ti, O, C, N; (i) Elemental
profiles of TNPC nanoparticles corresponding to Fig. 2d.
nanoparticles.
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The uniformly dispersed N-doped TiO2 nanoparticles in the the easy aggregation of small TiO2 nanoparticles (Fig. S14). For
N-doped C matrix provides a large surface area, porous TNPC nanoparticles, the matrix substrate effectively
C
channels, widely visible absorption, highly approachable active prevents the aggregation of small TiO2 nanoparticles, thus
catalytic sites, which make the TNPC nanoparticles promising increased the cycle stability. To demonstrate the scope of this
photocatalysts. To test the photocatalytic activity of the TNPC system, the photocatalytic oxidation of a variety of amine
nanoparticles, the photocatalytic oxidation of a variety of substrates is investigated (Table 1). The oxidation of
amine substrates is investigated. The contrast experiments benzylamine and its derivatives proceed efficiently under
indicate that the catalysts and light are the two essential visible-light irradiation with high yield and selectivity of their
conditions for this reaction. No oxidation occurs in the absence correspond imines. Overall, the TNPC nanoparticles exhibit
of light or catalyst (Table S1). Therefore, the photocatalytic high photocatalytic activity and good stability for the
oxidation of amines is performed in a Schlenk tubes in the photocatalytic oxidation amine reaction.
CH3CN solvent using blue Light Emitting Diodes (LEDs, λ=460
nm, 3 W) as light source and 1 atm air as the terminal oxidant
(Fig. 4a). To systematically correlate catalytic performance of
TNPC nanoparticles, pure TiO2 and N doped TiO2 are used as
reference samples. Fig. 4b shows several plots of conversion
versus reacted time using different photo-catalysts in the
oxidative coupling of benzylamine. Compared with pure TiO2
and N doped TiO2, TNPC nanoparticles exhibited the highest
conversion rate of amine oxidation, with the yield of imines
reaching 99% after 17 h. However, the catalyst is less active
under irradiation with green and yellow light, with the yield of
amine being 20% and 12%, respectively. The lower activity of
TNPC nanopartilces under irradiation with light of longer
wavelengths could be due to the lower ability of the catalyst to
absorb light in this region (Fig. S10). The performances are
further quantified with the kinetic rate plots (Fig. 4c), which
regards the light-driven catalytic oxidative coupling of
benzylamine as
a pseudo-first order reaction. The rate
constants are in order of TNPC (0.14 h-1) > N-TiO2 (0.049 h-1) >
TiO2 (0.033 h-1), indicating that the N-TiO2 embedded in the
carbon matrix has a higher catalytic activity under the same
reacted condition. In an attempt to determine the parameters
relates to the reaction rate, the oxidation of benzylamine and
its derivatives with both electron-donating and –withdrawing
substituents at the para-position are examined to obtain a
Hammett plot (Fig. 4d). It turns out that the lg(kX/kH) value is
largely maintained as the Brown-Okamoto constants (σ+) of
side groups (Cl, F, H, Me and MeO), which indicate that the
reaction is not sensitive to substituents and electronic
perturbation don’t occur in the Cα-H activation step in our
light-driven catalytic process. The O2 reduction maybe the
rate-determining step in this reaction. By altering the catalytic
atmosphere from air to oxygen, the catalytic efficiency is
obviously improved (Fig. S12a). When the air atmosphere is
replaced by nitrogen, a relatively small amount of product is
generated in the catalytic process, however, the amount
product has been obviously increased by changing nitrogen to
oxygen (Fig. S12b). Therefore, the amount of adsorbed oxygen
plays an important role in determined the catalytic rate. Cycle
stability is another important parameter for an excellent
catalyst. Impressively, TNPC nanoparticles exhibit high
performance stability in 5 catalytic cycles up to 85 h (Fig. 4e),
and well maintained their original morphology and crystal
structure (Fig. S13). However, the imines yield of pure TiO2 and
N-TiO2 gradually decreases with the increase of cycle times.
The low durability for pure TiO2 and N-TiO2 may be caused by
Fig.
4 (a) Photocatalytic equation and reaction conditions of transformation of
benzylamine catalyzed by TiO2; (b) Reactant conversion with respect to reaction time in
the oxidative coupling of benzylamine; (c) Kinetic rate plots for light-driven catalytic
oxidative coupling of benzylamine; (d) Hammet plot for the oxidative coupling of
benzaylamine, the lg(kX/KH) was obtained from the ratio of conversion with reaction
time of 5 h; (e) catalytic durability of three different samples.
Table 1. Catalyzed oxidation coupling of various amines using TNPC under visible-light
irradiation.
Entry
Substrate
Product
Time
(h)
Conv.
(%)
1
2
3
R=рꢀCl
R=рꢀF
R=рꢀCl
R=рꢀF
15
15
15
99
99
99
R=рꢀBr
R=рꢀBr
4
5
R=рꢀMe
R=рꢀMe
15
15
99
99
R=рꢀOMe
R=рꢀOMe
6
7
R=рꢀCF3
R=mꢀMe
R=рꢀCF3
R=mꢀMe
15
15
70
99
8
R=oꢀMe
R=oꢀMe
15
85
All catalytic experiments were performed as follows: photocatalyst (10 mg) and
substrate (0.2 mmol) were added to 3 mL CH3CN. The mixture was stirred and
irradiated by blue LEDs (450 nm, 3 W) for 15 h under atmosphere at room temperature.
1H NMR was employed to determine the yield.
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with the free amine molecule to furnish the coupling products
and release the ammonia molecule.
Fig. 5 (a) Proposed reaction pathways for catalyzed oxidative coupling of amine to
imine using TNPC as catalyst; (b) photocurrent density of three TiO2 samples measured
at 0.75 V versus Hg/Hg2Cl2 under non-illuminated (i.e. dark) and illuminated (i. e. light)
conditions; (c) ESR detection of superoxide radicals using a DMPO spin-trapping agent.
On the basis of the above results, we propose a possible
reaction mechanism as shown in Fig. 5a. Similar to inorganic
semiconductor photocatalysts that promote organic
photosynthesis, at first, photogenerated electrons and holes
are produced by the irradiation of TNPC nanoparticles. The
electron can be easily transferred to the carbon matrix, thus,
effective separation of electrons and holes can be achieved. To
prove this, the photocurrent densities of three TiO2 samples
are obtained by applying a 0.4 V potential to the samples in
cyclic 20 second dark (i.e. non-illuminated) and light (i.e.
illuminated) intervals. As shown in Fig. 5b, the photocurrent
density of the TNPC is more than 0.07 mA/cm2, where those of
the pure TiO2 and N-TiO2 are less than 0.035 and 0.015
mA/cm2, respectively. The photocurrent density of the TNPC
nanoparticles is higher than those of the pure TiO2 and N-TiO2
by a factor of 4.6 and 2.0, respectively (Fig. 5b). This result
demonstrated that the separation rate of photo-generated
electron-hole pairs increases due to the existence of the
carbon matrix. The separated electrons reduce molecular
oxygen to produce superoxide radical (O2·-), which is supported
by electron spin resonance (ESR) and DMSO (5,5-dimethyl-1-
pyrroline N-oxide) spin trap technique in methanol (Fig. 5c). In
Fig.
6 (a) Projected density of states (PDOS) for the graphite-anatase TiO2 (101)
interface; (b-e) The HOMO-1, HOMO, LUMO, and LUMO-1 for graphite-anatase TiO2
(101) interface. The dark gray, gray and red balls represent C, Ti and O, respectively.
To further understand the mechanism of electron transfer
of the graphite-anatase interface, it is insightful to further
investigate the interface electronic structures. Fig. 6 shows the
projected density of states (PDOS) and orbitals for graphite-
anatase (101) interface. From Fig. 6a, both the valence band
maximum (VBM) and the conduction band minimum (CBM)
are mainly originated from the C 2p states, because graphite is
a conductor. The next highest occupied molecular orbital
(HOMO-1), the highest occupied molecular orbital (HOMO),
the lowest unoccupied molecular orbital (LUMO), and the next
unoccupied molecular orbital (LUMO-1) of graphite-anatase
(101) interface are shown in Fig. 6 b, c, d, and e, respectively.
The electronic densities of the HOMO and LUMO are almost
localized around the graphite with the character of the C 2p
orbital. The HOMO-1 orbital is composed of C 2p and O 2p. The
LUMO-1 orbital is comprised largely of the Ti 3d orbitals and
partly of C 2p orbitals. The above results of orbital agree well
with DOS analysis. The normal photoexcitation of TiO2 is from
the O 2p states (HOMO-1) to the Ti 3d stated (LUMO-1). While,
after interfacing with graphite, electrons in TiO2 are likely to be
directly excited from O 2p into C 2p orbits, constituting the
major electron density component of the low-lying CB states
(LUMO), as shown in Fig. 6. This suggests that the nascent
electron-hole pairs are then well-separated, potentially leading
to significantly reduced charge recombination.
·-
the dark, there is no ESR signal, indicating no O2 is produced.
However, a gradual evolution of clear ESR signal (nearly 1:1:1:1
·-
quartet pattern) for DMPO- O2 adducts is observed with
visible light irradiation. The results show that the amine
oxidation in our catalytic system proceeds via producing the
superoxide radical is the intermediate product. At the same
time, the amine is oxidized by the positive charge of the hole
to form the amine radial cation. The O2·- can abstract hydrogen
atom from the cation radical complex to produce imines
intermediates and H2O2, which have been detected by
KI/AcOH system (Fig. S15). The imines intermediates can react
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Fig. 8 Kinetic traces of TiO2, N-doped TiO2 and TNPC at 800 nm.
Fig. 8 demonstrates the kinetic curves at 800 nm and the
data are fitted using multi-exponential decay components as
listed in Table S2. As discuss above, all three nanoparticles
show an initial positive TA signal. The positive signals should
stand for the initial amount of electrons in TiO2 shallow
trapped defects according to literatures.51,54,55 It is clear that
after N-doping we suppress the amount of shallow trapped
defects compared that in pure TiO2 while we induce more
active electrons when TiO2 are dispersed on a carbon matrix.
The positive TA signals quickly switch into bleach signals, which
present filling of deep trap defects from shallow trap sites in
pure and N-doped TiO2 as well as electron transfer process
from trap defects to the carbon matrix in TNPC. The filling or
electron transfer lifetimes are 790 ps, 774 ps and 222 ps for
TiO2, N-doped TiO2, TNPC, respectively. This results indicate
that the carbon matrix can effectively remove electrons from
both the conduction band edge and shallow trapped sites in
TiO2 as reported in previous studies.31,57 When the filling or
electron transfer process finishes, the bleach signals reach to
maximum and remain almost constant at 8 ns (instrument
time limit), suggesting long-lived charge separation exists in all
three TiO2 nanoparticles. Since we keep the three samples to
have identical absorbance at the excitation wavelength, we
have compared the population of electrons in the charge
separation states and the results show that the electron
population follows the order of TiO2<N-doped TiO2<TNPC. It
means that electrons are transferred in to carbon matrix with
efficiency and it should decrease the possibility of electron
recombine with hole in TiO2 valance band. As a result, the
number of actively long-lived holes can also follow the order of
TiO2<N-doped TiO2<TNPC and it is the reason that TNPC shows
enhanced photocatalysis performance. This result is consistent
with our photocurrent measurements discussed above.
Fig. 7 Transient absorption spectra of (a) TiO2, (b) N-doped TiO2, (c) TNPC
In order to study electron-hole dynamics in TNPC
nanoparticles, femtosecond transient absorption (TA)
measurements are carried out on pure TiO2, N-TiO2 and TNPC.
The above three nanoparticles all have broad and featureless
TA band in 765-825 nm as shown in Fig. 7. TNPC displays
highest amplitude of the initial TA signal while N-doped TiO2
shows the smallest signal. The positive TA signals quickly turns
into negative bleach signals after photo excitation. Then the
bleach signals reach maximum and remained about the same
amplitude in the first 8 ns. Electron-hole dynamics in TiO2
nanocrystal have been extensively studied and reviewed in
1990s.3,48-50 In general, photo-generated electrons first reach
the edge of the conduction band and then they can either be
trapped in surface defects in TiO2 or transfer to external
electron acceptors. The trapping or electron transfer process
may occur from
a few picoseconds to hundreds of
picoseconds. After that, electrons could take part in chemical
reactions or recombine with holes in the valence band.51,52 The
transient absorption signals of TiO2 are sensitive to crystal
phase, particle size and surface trap state.3,48-50,53. Thus, clear
assignment of the signal is crucial to understand the photo-
induced charge transfer process in TiO2. Yoshihara and co-
workers identified electron and hole signals in photo-excited
TiO2 from 400 to 2500 nm.54 They found that the hole in the
valence band mainly absorbs in the visible range (400-700 nm)
and trapped electrons have strong absorption peak in the NIR
range (800-1000 nm). The lifetime of these holes and electron
can be extended to microseconds.54,55 Meanwhile, free
electron with milliseconds lifetime was reported to absorb in
the mid-IR range.56 In this case, we extracted kinetic trace at
800 nm to analyze the electron dynamic in our three TiO2
nanoparticles.
Conclusions
In summary, N-doped porous carbon incorporating N-doped
TiO2 with butterfly-like structure has been synthesized by using
a Ti-base metal-organic framework, NH2-MIL-125 (Ti) as a hard
template. The N2 adsorption and desorption isotherms, UV-Vis
spectrum and photocurrent density measurements indicate
that the TNPC nanoparticles provides several favourable
features for photocatalysis: large BET surface area, visible-light
absorption and high separated efficiency of electrons and hole.
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7TA09166D
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ARTICLE
Applying the TNPC catalyst in the photocatalytic oxidation Photocatalysis of selective amine oxidation to imines
coupling of amine. It afforded the corresponding imines in
The photocatalyst (10 mg) and benzylamine (0.2 mmol) were
good to excellent yield and cycle stability with molecular O2 in
added to 3 mL acetonitrile (CH3CN). The mixture was stirred
air as oxidant. The photoactivation process and reaction
and irradiated by blue LEDs (450 nm, 3 W) for 15 h under air at
room temperature. 1H NMR determined the yield. To carry out
mechanism of amine photooxidation over TNPC are
investigated by Hammet plot and Electron spin resonance (ESR)
the recycle reactions, the photocatalyst was recovered by
spectroscopy in detail. Density Functional calculations and
centrifuging at 10,000 rpm for 10 min, and washed with fresh
transient absorption spectroscopy are used to trace the charge
CH2Cl2 three times. Finally, the recycled catalyst was dried in a
carrier dynamics in the N-doped TNPC nanoparticles, revealing
vacuum at 50 °C overnight.
the high photocatalytic performance relies on long-lived holes.
Transient Absorption (TA) Measurements
The obtained TNPC product in this study is applicable to other
photocatalytic systems and inspires further studies to develop TA spectra and kinetics were acquired on
a
transient
visible light-driven green synthesis.
absorption spectrometer (Helios-EOS fire, Ultrafast System)
which performed with a 1 kHz Ti:sapphire amplifier system
(Coherent Astrella, 800 nm, 7 mJ/pulse). The 315 nm pump
beam was generated from an optical parametric amplifier
(Coherent OPERAsolo). The polarization between the pump
and probe beams was set to be the magic angle (54.7°). The
instrument response function (IRF) of this system was
determined to be ~110 fs by measuring solvent responses
under the same experimental conditions. All three TiO2
nanoparticles were dispersed in acetonitrile and saturated
supernatant solutions (~0.2 OD at pump wavelength) were
used in TA experiments. The UV/Vis of all samples showed only
little scattering effect after each TA measurement (Fig. S16),
suggesting that the supernatant did not precipitate during our
TA experiments. Samples were held in a fused-silica cuvette
with 2 mm path length. The kinetic signals were fit to sums of
exponentials. All fits were carried out using IGOR Pro program
version 6.37 (Wavemetrics Inc., Portland, OR).
Methods
Materials
Titanium
tetraisopropanolate
(C12H28O4Ti,
95%),
2-
Aminoterephthalic acid (C9H9NO4, 98%) was purchased from
Chinese Sinopharm Chemical Reagent Co, Ltd.
Preparation of butterfly-like NH2-MIL-125(Ti) template
In a typical synthesis, 2-Aminoterephthalic acid (0.07g, 0.39
mmol) was dissolved in DMF and ethanol mixed solvent
(VDMF/Vethanol = 5:5). Then titanium tetraisopropanolate
(0.075 mL, 0.25 mmol) was added to the solution. After stirring
by ultrasonic concussion for 10 minutes, the mixture was
transferred to a 25 mL Teflon-lined steel autoclave and placed
in an oven at 150 °C for 24 hours under static conditions. After
reaction, the product was collected via centrifugation and
washed several times with DMF and ethanol, then dried at 60
°C overnight.
Density Functional calculations
All of the density functional calculations were performed using
plane-wave pseudopotential method, as implemented in the
Cambridge Sequential Total Energy Package (CASTEP) code.58
The local density approximation (LDA)59 and ultrasoft
pseudopotential60 were used to describe the exchange-
correlation effects and electron-ion interactions, respectively.
A Morkhost-Pack mesh61 of k-points, Γ and 1×2×1 points, was
used, respectively, to sample the two-dimensional Brillouin
zone for geometry optimization and for calculating electronic
properties of TiO2 (101). The self-consistent convergence
accuracy was set at 2×10-6 eV/atom. The convergence criterion
for the force between atoms was 5.5×10-2 eV/Å, and the
maximum displacement was 2×10-3 Å.
Synthesis of TiO2@N-doped porous carbon (TNPC)
TNPC nanoaprticles was synthesized via calcination of the
obtained butterfly-like NH2-MIL-125(Ti) at 470°C for 1 h in Ar
gas atmosphere with heating rate of 2 °C·min-1.
Characterization
The composition and phase of the as-prepared products were
acquired by the powder X-ray diffraction (XRD) pattern using a
Panalytical X-pert diffractometer with CuKα radiation. The
morphology and crystal structure of as-prepared products
were observed by scanning electron microscopy (SEM,
SU8100), and high-resolution transmission electron
microscopy (HRTEM, FEI Tecnai-F20) with an acceleration
voltage of 200 kV. All TEM samples were prepared from
depositing a drop of diluted suspension in ethanol on a carbon
film coated copper grid. PHI QUANTUM2000 photoelectron
spectrometer (XPS) was using to characterize the surface
compositions of the product. The surface areas of these
samples were measured by the Brunauer-Emmett-Teller (BET)
method using nitrogen adsorption and desorption isotherms
on a Micrometrics ASAP 2020 system. Electron paramagnetic
resonance (EPR) spectra were measured at room temperature
using a Bruker ESP-300E spectrometer at 9.8 GHz, X-band, with
100 Hz field modulation.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 21671085, 11674101,
21333008), the Jiangsu Province Science Foundation for
Youths (BK20150237), the Natural Science Foundation of
Jiangsu Province (BK20161160), and the Project Funded by the
Priority Academic Program Development of Jiangsu Higher
Education Institutions.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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DOI: 10.1039/C7TA09166D
ARTICLE
Journal Name
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Page 9 of 9
Journal of Materials Chemistry A
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DOI: 10.1039/C7TA09166D
N-doped porous carbon incorporating N-doped TiO2 with butterfly-like structure has been synthesized by using a Ti-base metal-
organic framework as a hard template. The nanocomposites exhibit excellent photocatalytic activity toward selective amine
oxidation to imines.