A porphyrin-conjugated TiO2/CoFe2O4nanostructure photocatalyst for the selective production of aldehydes under visible light

Article abstract of DOI:10.1039/d0nj06272c

We investigated the photocatalytic performance of a magnetic nanohybrid of CoFe2O4 and TiO2 hetero-nanostructures (TiO2/CoFe2O4) conjugated with zinc tetrakis carboxyphenyl porphyrin (ZnTCPP) for controlled oxidation of alcohols to aldehydes under visible

Full text of DOI:10.1039/d0nj06272c

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A porphyrin-conjugated TiO2/CoFe2O4
nanostructure photocatalyst for the selective
production of aldehydes under visible light†
Cite this: New J. Chem., 2021,
45, 8032
Mahdieh Ghobadifard,abc Elham Safaei,a Pavle V. Radovanovicb and
ac
Sajjad Mohebbi
*
We investigated the photocatalytic performance of a magnetic nanohybrid of CoFe2O4 and TiO2 hetero-
nanostructures (TiO2/CoFe2O4) conjugated with zinc tetrakis carboxyphenyl porphyrin (ZnTCPP) for
controlled oxidation of alcohols to aldehydes under visible light. The photocatalyst was prepared by
nucleating titania on pre-formed CoFe2O4 nanoparticles, generating anatase TiO2/CoFe2O4 hetero-
nanostructures upon annealing at 450 1C. Then, they were conjugated with ZnTCPP resulting in
ZnTCPP-TiO2/CoFe2O4 nanohybrid materials, which were characterized in detail by different structural
and spectroscopic methods. The ZnTCPP-TiO2/CoFe2O4 nanostructures have an average size of 21 nm
and show ferromagnetic behavior with a magnetization saturation of 47 emu gÀ1, a remanence of
22 emu gÀ1, and a coercivity of ca. 1000 Oe. The photocatalytic conversion of alcohols up to 87% under
visible light was achieved by using this hybrid nanomaterial. Such a high catalytic performance can be
related to the low charge recombination rate of the ZnTCPP-TiO2/CoFe2O4 nanostructures. The
magnetic hybrid nanostructures reported in this work have excellent potential as visible light
photocatalysts with advantages of high efficiency, selectivity, stability, and easy separation.
Received 27th December 2020,
Accepted 30th March 2021
DOI: 10.1039/d0nj06272c
rsc.li/njc
some of the wide bandgap semiconductors such as ZnO and
TiO2 has been shown to form composite photocatalysts with
1. Introduction
Semiconductor photocatalysis has attracted much attention in enhanced photocatalytic activities.12,13 As the photocatalytic
environmental remediation applications due to the possibility performance of TiO2 is still rather poor owing to the fast charge
of degrading different types of organic pollutants using recombination of exciton pairs h+–eÀ, these kinds of composite
only solar energy.1 A semiconductor photocatalyst absorbs photocatalysts could speed up the rate of charge carrier separation
solar-radiation photons having energy equal to or larger than in photocatalytic processes,14 particularly for a TiO2 semiconductor,
its bandgap energy to generate electron–hole pairs, which can due to its fine anti-photocorrosion attributes.15
then take part in the redox processes.2 Recently, non-magnetic
Similar to chlorophyll in plant photosynthesis, metallopor-
and magnetic particles as photocatalysts have been synthesized phyrin molecules anchored to the surface of TiO2/CoFe2O4
for use in photocatalysis applications.3,4 Magnetic materials could act as effective sensitizers to enhance the absorption of
having specific properties have indeed been used in a range of visible light.16 In normal photosynthesis, light absorption and
related applications.5–10 Specifically, CoFe2O4 with its spinel excitation first occur in porphyrins. Because of the vast delocaliza-
structure has attracted considerable attention for several appli- tion of p electrons, metal complexes of porphyrins have powerful
cations such as water splitting and photodegradation of organic absorption of light in the visible region. For dye-sensitized solar
pollutants due to its photocatalytic activity under visible light, cells, among different metal porphyrin complexes, zinc tetrakis
nontoxicity, chemical stability, low bandgap, corrosion resistance, carboxyphenyl porphyrin (ZnTCPP) has been thoroughly investi-
and magnetic properties.11 Besides, coupling CoFe2O4 with gated as a potential photosensitizer.17–19 We envisaged that the
modification of TiO2/CoFe2O4 with ZnTCPP could lead to an
a Department of Chemistry, University of Kurdistan, Sanandaj, 66177-15175, Iran.
E-mail: smohebbi@uok.ac.ir; Fax: +98-87-336660075; Tel: +98-87-33624133
b Department of Chemistry, University of Waterloo, 200 University Ave. W.,
Waterloo, ON, Canada
increase in the absorption in the visible region and enhancement
in the photophysical properties that could be suitable for the
photo-oxidation of alcohols.20
In this work, the fabrication of a new three-component
hybrid nanosystem composed of CoFe2O4, TiO2, and porphyrin
c Research Center for Nanotechnology, University of Kurdistan, Sanandaj,
66177-15175, Iran
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj06272c (Scheme 1) is reported, which allows for efficient oxidation of
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washed with water/ethanol several times and dried in a vacuum
oven at 60 1C for 14 h. Finally, the dried product was annealed
at 450 1C for 2 hours to obtain the nanostructures.
2.3. Synthesis of the ZnTCPP complex
As reported previously,24 the desired porphyrin (H2TCPP) was pre-
pared by pyrrole and aromatic aldehyde. Briefly, 10 mmol of pyrrole
was added dropwise to the mixture of 35 mL of propionic acid,
15 mL of nitrobenzene, and 10 mmol of benzaldehyde. After refluxing
for 2 h, the mixture was solidified overnight. Finally, the purple
sediment was separated by filtration and rinsed with water 5 times.
The ZnTCPP complex was prepared in a two-step process. In
the first step, 0.15 mmol of zinc acetate dissolved in 5 mL of
methanol was added to the solution of 100 mmol of H2TCPP in
25 mL of CH2Cl2 and refluxed overnight. After the elimination
of the solvent, a purple solid was obtained. In the next step, for
generating porphyrin acid by the hydrolysis of porphyrin ester,
100 mmol of the purple solid in a mixture of 10 mL of
tetrahydrofuran and 10 mL of ethanol (1 : 1 ratio), and 5 mL
of KOH (2 M) were refluxed for 15 h. After evaporation of the
solvents, the sediment was dissolved in 10 mL of water before
filtering. Using hydrochloric acid (1 N), the porphyrin dipotassium
salt was acidified to attain pH 2. After another filtration, the purple
solid was obtained and washed with DI water.
Scheme 1 Treatment of carboxyl groups of ZnTCPP and hydroxyl groups
of TiO2/CoFe2O4 nanocomposites.
alcohols using incident visible light. Furthermore, a plausible
mechanism is discussed for photooxidation of alcohols using
the ZnTCPP-TiO2/CoFe2O4 nanohybrid.
2. Materials and measurements
2.1. Materials
Fe(NO3)3Á9H2O (99%), Co(NO3)2Á6H2O (99%), Zn(CH3COO)2Á2H2O
(99%), CH3COONa (99%), (NH2CH2CH2)2NH (DETA; 98%),
Ti[OCH(CH3)2]4 (98%), methyl 4-formyl benzoate, pyrrole,
C6H5NO2, propionic acid, hydrochloric acid, ammonium oxalate
(AO) and benzoquinone (BQ) were all received from Sigma-Aldrich.
Dichloromethane, t-butyl hydroperoxide (TBHP), deionized water,
absolute ethanol, and methanol were used as received. The
reacting alcohols such as benzyl alcohol, 3-chlorobenzyl alcohol
98%, 4-chlorobenzyl alcohol 99%, 4-fluorobenzyl alcohol 97%,
anysil alcohol (4-methoxybenzyl alcohol), and 4-nitrobenzyl
alcohol with synthesis grade were purchased from Sigma-
Aldrich and Merck & Co. Inc.
2.4. Synthesis of the ZnTCPP-TiO2/CoFe2O4 nanohybrid
To prepare TiO2/CoFe2O4 nanostructures modified by ZnTCPP,
10 mL of the solution of ZnTCPP (1 mg) in methanol was added
to 100 mg of suspension of TiO2/CoFe2O4 nanostructures. The
mixture was refluxed for 8 h to enable the reaction between the
carboxyl groups of ZnTCPP and the hydroxyl groups on the surface
of TiO2 and link ZnTCPP on the surfaces of the nanostructures.
2.5. Measurements
The Fourier-transformed infrared (FTIR) absorption measurements
were performed with a Bruker Vertex 80v spectrophotometer on the
samples prepared as KBr pellets. A Mira3 Tescan field emission
2.2. Preparation of CoFe2O4/TiO2 nanostructures
The CoFe2O4 nanoparticles were synthesized as described else- scanning electron microscope (FE-SEM) with energy-dispersive X-ray
where.21–23 First, an aqueous solution of C2H3NaO2 (16 g or spectroscopy (EDS) capability was employed for large area sample
190 mmol in 50 mL of deionized water) was added to the imaging and elemental analysis. The powder X-ray diffraction
mixture of Co (NO3)2Á6H2O (29 g, 100 mmol) and Fe (NO3)3Á patterns of the nanostructures were recorded using a Holland-
9H2O (8 g, 20 mmol) in 50 mL of deionized water. After that, Philips diffractometer with a Cu Ka incident radiation source
the brown color precipitate was collected using a paper filter (l = 0.1542 nm) and scattering angles 2y up to 801. The magnetic
and washed with deionized water and ethanol. The brown solid properties of CoFe2O4, TiO2/CoFe2O4, and ZnTCPP-TiO2/
was dried in air. Finally, the CoFe2O4 nanoparticles were CoFe2O4 were investigated using a vibrating sample magnet-
collected by placing the precipitate in a microwave oven (900 w) ometer (VSM). The electronic absorption spectra of the nano-
for 10 min.
particles were recorded using a Varian Cary 5000 UV-Vis-NIR
The CoFe2O4/TiO2 nanostructures were also obtained spectrophotometer, and the photoluminescence (PL) spectra
through the hydrothermal process. At first, 80 mg of CoFe2O4 were recorded with a Varian Cary Eclipse fluorescence spectro-
nanoparticles were dispersed in 20 mL of ethanol under meter. The X-ray photoelectron spectra (XPS) were obtained
sonication. Then, 400 mL of (NH2CH2CH2)2NH (DETA) and using an ESCALAB 250 spectrometer model made by VG Scientific
22 mL of titanium(IV) isopropoxide were added to the solution. with an Al Ka X-ray source (Ephoton = 1486.7 eV). The transmission
After vigorous stirring, the mixture was transferred to an electron microscopy (TEM) images were obtained using a Phillips
autoclave and treated at a temperature of 200 1C for 20 h. After CM-10 microscope operating at 60 kV. The differential reflec-
cooling down the autoclave to room temperature, the solid was tance spectra (DRS) were recorded with an Avaspec-2048-TEC
separated from the solution using a magnet. The product was spectrometer.
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2.6. Photocatalytic procedure
In a typical procedure, the photocatalytic reaction was carried
out in a quartz cell. To control the vessel temperature during
irradiation, the cell was outfitted with a water circulation
system. In every run, 10 mg of the photocatalyst was added to
1.5 mL of the solvent (acetonitrile). The alcohols (referred to as
a reaction substrate) and oxidant (H2O2) in a 1 : 15 molar ratio were
then added to the cell. The mixture was placed in front of a halogen
lamp at ca. 15 cm distance. At the end of the reaction, the
photocatalyst powder was isolated using a magnet, washed with
CH3CN five times, and dried for reuse. The progress of the photo-
reaction was monitored by thin-layer chromatography (TLC),
whereas the product was identified by gas chromatography-mass
spectrometry (GC-Mass spectrophotometer).
3. Results and discussion
The absorption and photoluminescence (PL) spectra of TCPP
and ZnTCPP are shown in Fig. 1. The absorption spectrum of
TCPP exhibits a strong Soret band at 412 nm and four weak
split Q bands at 507, 541, 584, and 640 nm. On the other hand,
the absorption spectrum of ZnTCPP exhibits a strong Soret
band at 421 nm and two weak split Q bands at 554 and 593 nm.
The PL spectrum of TCPP has two peaks at 654 and 716 nm,
while ZnTCPP exhibits two peaks at 606 and 657 nm which are
similar to the literature reports for zinc porphyrin complexes.25
The FTIR vibration bands for CoFe2O4, TiO2/CoFe2O4, and
ZnTCPP-TiO2/CoFe2O4 are shown in Fig. 2a–c, respectively. The
band at around 470 cmÀ1 is related to the Fe(III)–O2À stretching
vibration of the tetrahedral metal site and the band at about
579 cmÀ1 is related to the stretching vibration in the octahedral
Co(II)–O2À group in CoFe2O4 (Fig. 2a).26 The Ti–O–Ti bending or
M–O stretching vibrations are associated with the peaks in the
range of 800–1100 cmÀ1 (Fig. 2b).27,28 The O–H bending mode
Fig. 2 FT-IR spectra of (a) CoFe2O4, (b) TiO2/CoFe2O4, and (c) ZnTCPP-
TiO2/CoFe2O4 nanohybrids.
stretching vibrations displayed at 950–1000 cmÀ1 indicate that the
ZnTCPP complex was successfully synthesized.30 The CQO and
C–O stretching modes vanished, while a strong band at 1376 cmÀ1
was observed which is related to the symmetric COO stretching,
suggesting that the ZnTCPP was immobilized on the surface of
TiO2/CoFe2O4 through the carboxylic acid groups.31,32 The pyrrole
stretching vibration bands appear at about 1650 cmÀ1 for CQN
bonds (Fig. 2c). The bands at 2800–2900 cmÀ1 correspond to the
symmetric C–H stretching mode of the CH2. The bridge bonding
between TiO2/CoFe2O4 and ZnTCPP provides a path for effective
transmission of electrons from the TiO2/CoFe2O4 to the HOMO
level of ZnTCPP, which can be used effectively for photoreactions.
vibrations of water may be associated with the band at 1625 cmÀ1
,
and another at 3000–3600 cmÀ1 is related to the OH stretching
vibration29 (Fig. 2b). In the FT-IR spectra of ZnTCPP-TiO2/CoFe2O4,
the disappearance of N–H vibrations and the appearance of Zn–N
Fig. 1 The absorption and PL spectra of (a) TCPP and (b) ZnTCPP at room temperature.
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Fig. 3 SEM images of (a) CoFe2O4, (b) TiO2/CoFe2O4, and (c) ZnTCPP-TiO2/CoFe2O4 nanohybrids.
Due to the low loading of the porphyrin complex on the surfaces of average size (about 1 nm) is due to the use of the microwave method
the nanostructures, these peaks have very low intensities. In other for obtaining CoFe2O4 instead of the sol–gel method. The CoFe2O4
words, the intensity of the peaks reflects the amount of organic was prepared under microwave irradiation using a heterometallic
materials on the surfaces of inorganic materials.
oxo-centered trinuclear [CoFe2O(CH3COO)6(H2O)3]Á2H2O complex
The FESEM images of CoFe2O4, TiO2@CoFe2O4, and ZnTCPP- as a precursor with an average size of 17 nm, while TiO2/CoFe2O4
TiO2/CoFe2O4 particles are shown in Fig. 3. The CoFe2O4 particles was prepared by the sol–gel method. This method makes it fully
adopt a flower-like morphology through the junction of nanosheets, dissolved and the size decreased.
while TiO2 is deposited on the surface of the CoFe2O4 nanoparticles
The elemental composition of CoFe2O4, TiO2/CoFe2O4, and
to form hybrid TiO2/CoFe2O4 nanostructures. The SEM image of ZnTCPP-TiO2/CoFe2O4 nanostructures was confirmed by EDS
ZnTCPP-TiO2/CoFe2O4 in Fig. 3c demonstrates that the sample analysis (Fig. 4). Co, Fe, and O were found in all samples.
consists of relatively uniform microparticles. The average sizes of Additionally, Ti was present in the TiO2/CoFe2O4 nanoparticles,
CoFe2O4, TiO2/CoFe2O4, and ZnTCPP-TiO2/CoFe2O4 particles are and Ti, N, Zn, and C in the ZnTCPP-TiO2/CoFe2O4 nanostructures.
ca. 17, 16, and 21 nm, respectively. Herein, this small decrease of The molar ratio of Co/Fe was 1 : 1.96 for the CoFe2O4
Fig. 4 EDS spectra of (a) CoFe2O4, (b) TiO2/CoFe2O4, and (c) ZnTCPP-TiO2/CoFe2O4 nanohybrids.
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nanoparticles, 1 : 1.71 for the TiO2/CoFe2O4 nanoparticles, and
1:1.62 for the ZnTCPP-TiO2/CoFe2O4 hybrid nanostructures
(Fig. 4). This result affirms that the core components of CoFe2O4,
TiO2/CoFe2O4, and ZnTCPP-TiO2/CoFe2O4 remain constant.
The morphologies of cobalt ferrite nanoparticles, cobalt
ferrite nanoparticles modified with TiO2, and ZnTCPP-TiO2/
CoFe2O4 hybrid nanostructures were characterized by TEM and
show a reasonably uniform size distribution. The CoFe2O4
nanoparticles consist of stacked nanosheets (Fig. 5a), while
the TiO2/CoFe2O4 nanoparticles have irregular morphologies,
for which the darker areas are due to CoFe2O4 and the lighter
areas correspond to TiO2, suggesting the interface formation
between TiO2 and CoFe2O4 (Fig. 5b and c). No change in the
morphology of the TiO2/CoFe2O4 nanoparticles was observed
from the TEM image after combining with ZnTCPP (Fig. 5d).
The XRD patterns of the CoFe2O4, TiO2/CoFe2O4, and
ZnTCPP-TiO2/CoFe2O4 nanostructures confirm their crystal
structures, as shown in Fig. 6. In the XRD pattern of the
CoFe2O4 nanoparticles (Fig. 6a), the reflection plains (111),
(220), (311), (222), (400), (422), (511), (440), (531) and (620)
match that of JCPDS No. 22-1086, as expected. The XRD pattern
of the TiO2/CoFe2O4 sample is shown in Fig. 6b. These samples
were prepared by a hydrothermal method and subsequently
annealed at 450 1C. The transformation of the anatase to rutile
phase occurs at an annealing temperature T 4 600 1C. The
diffraction peaks in Fig. 6b correspond to the (101), (004), (200),
Fig. 6 The powder XRD patterns of (a) CoFe2O4 nanoparticles, (b) TiO2/
CoFe2O4 hetero-nanostructures, and (c) ZnTCPP-TiO2/CoFe2O4 nanohybrid.
(105), and (204) planes of anatase TiO2 and are ascribed to the temperature (Fig. 7). Owing to the presence of the CoFe2O4
presence of anatase TiO2 in the samples (JCPDS No. 21-1272).33 nanoparticles, the samples behave as a soft ferromagnet, with
Due to the larger amount of CoFe2O4 in the sample, its peak saturation magnetization (Ms) values of 56.99, 51.77, and
intensity is higher than that of TiO2. Fig. 6c shows the XRD 47.02 emu gÀ1 for CoFe2O4, TiO2/CoFe2O4, and ZnTCPP-TiO2/
pattern of the ZnTCPP-TiO2/CoFe2O4 hybrid nanostructures, CoFe2O4, respectively. The saturation magnetization of the
which is very similar to that of the TiO2/CoFe2O4 hetero- CoFe2O4 particles is higher than that of the TiO2/CoFe2O4 and
nanostructures. Thus, the TiO2/CoFe2O4 particles have a robust ZnTCPP-TiO2/CoFe2O4 particles, showing the possibility of
structure that remained stable during sample processing.
ZnTCPP-TiO2/CoFe2O4 having a Mr value less than that in pure
The magnetic hysteresis loops for the CoFe2O4, TiO2/CoFe2O4, CoFe2O4.34 When CoFe2O4 was combined with TiO2 and
and ZnTCPP-TiO2/CoFe2O4 samples were measured at room ZnTCPP, the magnetic moments in TiO2 and ZnTCPP may be
caused owing to the magnetic proximity effect and high ferro-
magnetic properties.35 The corresponding values of remnant
Fig. 7 The room temperature field-dependent magnetization curves of
Fig. 5 The TEM images of nanoparticles: (a) CoFe2O4, (b and c) TiO2/
CoFe2O4, and (d) ZnTCPP-TiO2/CoFe2O4 nanohybrids.
the (a) CoFe2O4 nanoparticles, (b) TiO2/CoFe2O4 hetero-nanostructures,
and (c) ZnTCPP-TiO2/CoFe2O4 nanohybrids.
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Fig. 8 The UV-vis DRS analysis curves of the (a) CoFe2O4, (b) TiO2/CoFe2O4, and (c) ZnTCPP-TiO2/CoFe2O4 nanohybrids.
magnetization, Mr, are 25.44, 23.01, and 22.32 emu gÀ1, with a corresponding to the C–C and CQC bonds of the TCPP ring,
coercive field (Hc) of ca. 1500 Oe. Finally, the ferromagnetic and the carbonyl of the –COO group, respectively (Fig. 10b).36
properties of the as-prepared ZnTCPP-TiO2/CoFe2O4 nanostruc- The O 1s spectrum displays spectral features at 530.4, 531.8,
tures imply that this hybrid is easily recovered from the solution and 533.5 eV that can be assigned to the metal–oxygen–metal
mixture using a permanent magnet after a photocatalytic reaction. (M–O–M) bridge in TiO2 and/or CoFe2O4, oxygen in the term-
So, the overall photocatalytic performance improved due to easy inal –OH bonds (Ti–OH), and oxygen defects in TiO2, respec-
separation, washing, and reusing of the catalyst.
tively (Fig. 10c).37,38 This oxygen deficiency should be attributed
The DRS measurements and analyses were performed to inves- to Ti3+ which is formed during synthesis and may not con-
tigate the absorption of the CoFe2O4, TiO2/CoFe2O4, and ZnTCPP- tribute to free carbon. The Ti 2p spectrum exhibits two peaks
TiO2/CoFe2O4 powder nanostructures (Fig. 8). The CoFe2O4 nano- for Ti4+ in TiO2 at 458.9 and 464.8 eV due to the 2p3/2 and 2p1/2
particles have relatively small bandgap energy (1.24 eV), while the states, respectively (Fig. 10d).39 The peak appearing at 460.3 eV
absorption onset for the ZnTCPP-TiO2/CoFe2O4 nanostructures is corresponds to Ti3+ in Ti2O3.40 Both Ti4+ and Ti3+ on the surface
1.41 eV (Fig. 1S, ESI†). As a result, the ZnTCPP-TiO2/CoFe2O4 of TiO2 demonstrate that during the solvothermal reactions,
nanohybrid has potential to exhibit considerable photocatalytic numerous oxygen vacancies are formed.41 So, the Ti3+ with a
activity under visible light by readily generating electron–hole lower oxidation state confirms the oxygen deficiency which is
pairs. Also, Fig. 9 shows the electronic absorption spectra of the required for charge balance.42 The existing Ti3+ species increase
TiO2/CoFe2O4 and ZnTCPP-TiO2/CoFe2O4 powder nanostructures.
the photocatalytic activity through the creation of new mid-gap
The chemical states and elemental composition of the states. These states have two functions: (i) redshift the absorption
ZnTCPP-TiO2/CoFe2O4 hybrid nanostructures were determined and (ii) function as electron traps that prevent electronÀhole pair
by XPS analysis. A set of characteristic XPS peaks for Zn, Co, Fe, recombination. Under visible light irradiation, electrons can be
O, Ti, N, and C are observed in the survey scan of the ZnTCPP- excited to Ti3+ and VO impurity levels with a longer lifetime than
TiO2/CoFe2O4 nanohybrid (Fig. 10a). The XPS signal for C 1s the lifetime of the photogenerated electrons in the CB43 which
consists of three asymmetric peaks at 285.4, 286.5, and 289.2 eV could be confirmed by a jump in PL at 585 nm (Fig. 11). The mid-
gap states originating from Ti3+ greatly suppress the charge carrier
recombination rate that provides feasibility for electron transfer.44
The photoluminescence spectra of the TiO2/CoFe2O4 and
ZnTCPP-TiO2/CoFe2O4 nanostructures are shown in Fig. 11.
The higher intensity signal of TiO2/CoFe2O4 indicates higher
charge recombination relative to the ZnTCPP-TiO2/CoFe2O4
hybrid nanostructures. By loading ZnTCPP on the surfaces of
the TiO2/CoFe2O4 nanoparticles, the electron transfer could be
improved along with the reduction in charge recombination.
The lower PL intensity suggests a lower charge recombination
rate, owing to the efficient charge carrier separation, which can,
in turn, stimulate the photocatalytic activity.21 The PL intensity of
the ZnTCPP-TiO2/CoFe2O4 sample was negligible, suggesting that
the charge recombination is significantly decreased. As shown in
Fig. 11, the PL line of Zn-TCPP-TiO2/CoFe2O4 has not jumped at
about 558 nm while this small jump or namely one weak peak
was observed for TiO2/CoFe2O4 that could be attributed to the
existence of a mid-state energy level.
Fig. 9 The electronic absorption spectra of the TiO2/CoFe2O4 and
ZnTCPP-TiO2/CoFe2O4 nanohybrids.
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Fig. 10 (a) XPS spectrum of the ZnTCPP-TiO2/CoFe2O4 nanohybrid, and (b–d) characteristic high-resolution XPS peaks: (b) C 1s, (c) O 1s, and (d) Ti 2p.
reactions (Table 1). In a pyrex cell, different amounts of the
photocatalyst and 100 mmol of benzyl alcohol, used as a sub-
strate, were mixed. For photo-oxidation of alcohols, the catalyst
dosage under visible light irradiation was optimized. The oxidation
efficiency depends on the catalyst amount. The oxidation yield was
enhanced by increasing the catalyst amount up to 10 mg, but a
higher amount of catalyst from 10 to 15 mg leads to a decline in
the oxidation performance (Table 1). This occurrence might be
related to the aggregation of particles and loss of active areas
on the surface of the catalyst.45 The oxidation reaction was
negligible in the dark or in the absence of a catalyst (Table 1,
entries 13 and 14). It was corroborated that the reaction does
not occur with a ZnTCPP-TiO2/CoFe2O4 nanohybrid without
irradiation. Likewise, the reaction did not proceed without a
Fig. 11 PL spectra of the as-prepared TiO2/CoFe2O4 hetero-nano-
structures and ZnTCPP-TiO2/CoFe2O4 hybrid nanostructures. The spectra
ZnTCPP-TiO2/CoFe2O4 nanohybrid. Therefore, both the ZnTCPP-
TiO2/CoFe2O4 nanohybrid and light irradiation are essential for
catalyzing the oxidation reaction.
were collected for the excitation wavelength of 350 nm.
4.1. Effect of different solvents on the catalytic reaction
4. Photocatalytic oxidation of primary
alcohols
We used the optimized conditions to study the efficacy of solvent
polarity on the efficacy of the oxidation reaction. For this reaction,
As revealed by different analyses, the ZnTCPP-TiO2/CoFe2O4
nanohybrid is a promising candidate for controlled photocatalytic
oxidation of alcohols to aldehydes in the visible region of electro-
magnetic irradiation. The optimized conditions for the photo-
catalytic reaction were identified by performing initial photocatalytic
100 mmol of benzyl alcohol and 10 mg of the photocatalyst were
mixed in different solvents, including CH3CN, CH2Cl2, C3H7NO,
CHCl3, and H2O. The solvents have a significant effect on the
photocatalytic efficiency of hybrid nanostructures via energy trans-
fer, particle polarization, and scattering.46 Acetonitrile showed the
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Table 1 Optimization of photocatalytic oxidation conditions for benzyl alcohol to aldehyde under visible light irradiationa
Entry
Photocatalyst
Photocatalyst (mg)
Product
Selectivity%
Conversion%
1
2
3
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
5
5
5
Benzaldehyde
Benzaldehyde
Benzaldehyde
499
499
499
14
30
53
4
5
6
7
8
9
CoFe2O4
8
8
8
10
10
10
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
499
499
499
499
499
499
32
53
79
39
61
87
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
10
11
12
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
15
15
15
Benzaldehyde
Benzaldehyde
Benzaldehyde
499
499
499
33
58
82
13
14
No Catalyst
ZnTCPP-TiO2/CoFe2O4(no light irradiation)
0
10
Benzaldehyde
Benzaldehyde
—
—
o1
o1
a
H2O2, 1.5 mmol and reaction time, 120 min. Conversion percentage on the basis of benzyl alcohol consumption (DC/C0) Â 100.
Table 2 The efficacy of solvents in the photo-oxidation of benzyl alcohol
under optimized conditionsa
nanohybrid and the powerful absorption of the porphyrin
complex in the visible region due to the significant delocalization
of p electrons. This processing could be generalized for various
Time
Entry Photocatalyst
(min) Solvent Selectivity% Conversion%
aromatic alcohols containing both electron-withdrawing and
electron-donating groups. The rates of reactions of substituted
benzyl alcohol depend on their electronic character which
is higher for electron-withdrawing substituents compared to
electron-donating ones.
1
2
3
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4 120 CH3CN 499
120 CH3CN 499
120 CH3CN 499
39
61
87
4
5
6
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4 120 CH2Cl2 499
120 CH2Cl2 499
120 CH2Cl2 499
27
38
62
To achieve the optimal reaction conditions, various factors
which affect the progress of the reaction, including the amount
of catalyst, solvent polarity, and oxidants, were considered.
According to the test results, the optimized conditions for the
photocatalytic reaction are 10 mg of the catalyst and acetonitrile
as the solvent. As seen in Table 4, tert-butyl hydroperoxide
(TBHP) shows the highest conversion percentage compared to
other oxidants, such as H2O2 or O2. TBHP, as most hydroper-
oxides, is completely reactive with most polymers, metals, bases,
and acids. TBHP forms a more stable radical species than the
related H2O2 homolog. However, TBHP is a more ‘‘huge’’
substrate (due to the tert-butyl side chain) than H2O2. It displays
some differences in the substrate reactivity to certain types of
catalysts. Despite higher catalytic performance in the presence
7
8
9
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4 120 C3H7NO 499
120 C3H7NO —
120 C3H7NO —
—
—
o4
10
11
12
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4 120 CHCl3 499
120 CHCl3 499
120 CHCl3 499
32
40
73
13
14
15
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4 120 H2O
120 H2O
120 H2O
—
—
499
—
—
o6
a
H2O2 1.5 mmol.
highest conversion percentage of the oxidation of benzyl alcohol, of TBHP, H2O2 was used as an oxidant, due to several advan-
while water displayed the minimum yield (Table 2). The selectivity tages such as being inexpensive, ecofriendly, safer, and milder.
generally remained more than 99% with aldehyde as the only
4.2. Plausible photocatalytic mechanisms
product.
The photocatalytic activity of the CoFe2O4, TiO2/CoFe2O4, The active species and/or intermediates in alcohol oxidation
and ZnTCPP-TiO2/CoFe2O4 nanostructures for the oxidation of reactions using ZnTCPP-TiO2/CoFe2O4 hybrid nanostructures as
various alcohol derivatives is shown in Table 3. The oxidation of the catalyst were investigated by using sacrificial trapping reagents
alcohol was achieved with 499% selectivity. The CoFe2O4 under optimized conditions (Fig. 12). In the trapping experiment,
nanoparticles show poor photoactivity as reflected through a radical scavenger TBHP was chosen for the detection of hydroxyl
the lower yield of alcohol conversion compared to the TiO2/ radicals, AO for the detection of holes, and BQ for superoxide
CoFe2O4 hetero-nanostructures and ZnTCPP-TiO2/CoFe2O4 radicals.47,48 In the presence of TBHP, 94% of benzyl alcohol was
nanohybrids, which is sensible given their insignificant visible oxidized under visible light and optimized conditions, while in the
light absorptivity. On the other hand, the ZnTCPP-TiO2/CoFe2O4 presence of AO and BQ, only 17% and o5% of benzyl alcohol were
nanostructures exhibited higher activity than the TiO2/CoFe2O4 oxidized, respectively. Therefore, superoxide radicals are not the
nanoparticles due to the decreased charge recombination in the active species for the ZnTCPP-TiO2/CoFe2O4 nanohybrid as the
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Table
3
The selective photooxidation of substituted alcohols using
Table 4 The efficacy of various oxidants on the photo-oxidation reaction
of benzyl alcohola
CoFe2O4, TiO2/CoFe2O4, and ZnTCPP-TiO2/CoFe2O4 under visible lighta
Ingress Catalyst
Substrate
Product
Y% S%
Ingress Catalyst
Solvent Oxidant Selectivity% Yield%
1
2
3
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
39 499
61 499
87 499
1
2
3
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4 CH3CN TBHP
CH3CN TBHP
CH3CN TBHP
499
499
499
40
63
94
4
5
6
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4 CH3CN H2O2
CH3CN H2O2
CH3CN H2O2
499
499
499
39
61
87
4
5
6
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
28 499
43 499
65 499
7
8
9
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4 CH3CN O2
CH3CN O2
CH3CN O2
—
—
499
—
—
o5
a
H2O2, 1.5 mmol and reaction time, 120 min.
7
8
9
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
19 499
34 499
43 499
10
11
12
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
2
5
499
499
26 499
13
14
15
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
36 499
56 499
68 499
Fig. 12 Effects of the trapping agents on the photocatalytic oxidation of
alcohols with the ZnTCPP-TiO2/CoFe2O4 nanohybrid under incident
visible light using t-butyl hydroperoxide (TBHP), ammonium oxalate
(AO), and benzoquinone (BQ).
16
17
18
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
8
499
19 499
45 499
feasibility for electron transfer.44 Conversely, CoFe2O4 is a
p-type semiconductor.52 Thus, these two components could
form a p–n junction. The TiO2/CoFe2O4 hetero-nanostructure is
further functionalized with ZnTCPP, which acts as an excellent
sensitizer and improves the photocatalytic performance of the
ZnTCPP-TiO2/CoFe2O4 nanohybrid in the visible light region. The
CB and VB positions of the semiconductors were calculated using
the following equations:53,54
19
20
21
CoFe2O4
TiO2/CoFe2O4
ZnTCPP-TiO2/CoFe2O4
33 499
45 499
81 499
a
H2O2 1.5 mmol, reaction time 120 min.
EVB = w À Ee + 0.5Eg
ECB = EVB À Eg
(1)
(2)
photocatalyst, while h+ and 1OH appear to be the main active
species for this reaction.
where EVB and ECB are the potential energy levels of the valence
The schematics of the mechanism of the photocatalytic band and conduction band, respectively, Eg is the bandgap
oxidation of alcohols using the ZnTCPP-TiO2/CoFe2O4 hybrid energy, Ee is the hydrogen-scale free electron energy, and w is
nanosystem are demonstrated in Fig. 13. The increased photo- the electronegativity of CoFe2O4 or TiO2. Based on the values
activity of this nanohybrid can be ascribed to the fast charge of Eg for CoFe2O4 and TiO2 which are 1.24 eV (Fig. 8b) and
transfer processes.49,50 Upon irradiation with visible light, the 3.20 eV,22,55 respectively, the EVB and ECB for both of them were
electron–hole pairs are generated in CoFe2O4 and TiO2 due to calculated by eqn (1) and (2) (Table 5).
+
electron excitation from the VB to CB. Although TiO2 is a wide
The oxidation potential (ES /S1) of the excited dye is À1.00 eV,
bandgap n-type semiconductor, a redshift of the absorbed light which is more negative than the CB of TiO2, and the oxidized
+
to the visible area is observed. This redshift is due to the ZnTCPP dye shows a positive oxidation potential (ES /S),
creation of Ti3+-derived mid-gap states as a result of oxygen +1.23 eV.20 In the p-type CoFe2O4, the Fermi level (EF) is above
deficiency.51 These mid-gap states originating from Ti3+ greatly EVB, while in the n-type TiO2, it is below ECB
.
Before contact,
57
suppress the charge carrier recombination rate that provides the ECB and EF levels of CoFe2O4 are lower than those of TiO2,
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Fig. 13 A plausible mechanism of the photoactivity of the ZnTCPP-TiO2/CoFe2O4 nanohybrid in the visible region.
Table 5 The values of w,56 Eg, ECB, and EVB parameters of the semicon-
from Ti3+ greatly suppress the charge carrier recombination rate
that provides feasibility for electron transfer.44 Electrons in the
CB of TiO2 and defect sites (Ti3+ and VO) are transferred to the
oxidized levels of the ZnTCPP dye; these electrons are then excited
from the HOMO to the LUMO orbital of ZnTCPP to reduce H2O2 to
produce ꢀOH under visible light, as shown in Fig. 13b.62,63 The
holes consequently move from the VB of TiO2 to the VB of
CoFe2O4 to react with H2O/OHÀ and produce 1OH.64,65 This
betterment in the photoactivity is related to the p–n junction,
and the presence of Ti3+, VO, CoFe2O4, and ZnTCPP dye, which
increase the photoactivity of TiO2 in the visible region.
ductors in eVa
Semiconductors
w
Eg
ECB
EVB
CoFe2O4
TiO2
5.81
5.81
1.24
3.20
+0.69
À0.29
+1.93
+2.91
a
Hydrogen-scale free electron energy of 4.5 eV is considered.
as illustrated in Fig. 13a. After the two materials come into
contact, the EF level of TiO2 decreases while the EF level of
CoFe2O4 increases. The electron transfer from TiO2 to CoFe2O4
across the p–n junction continues until equilibrium is reached
at the interface, which results in the formation of the space-
charge area. All energy levels of TiO2 descend, while those of
CoFe2O4 rise.58,59 Therefore, the CB of TiO2 is lower than the
CB of CoFe2O4, and upon excitation, under visible light, the
electrons move from the CB of CoFe2O4 to the CB of TiO2.
According to previous reports,60,61 below the CB of TiO2, the
mid-state energy levels correspond to Ti3+ and VO, which causes
a decrease in the apparent optical Eg of TiO2. Under visible light
irradiation, electrons can be excited to Ti3+ and VO impurity
levels with a longer lifetime than the lifetime of the photo-
generated electrons in the CB.43 The mid-gap states originating
4.3. Comparison with other reported catalysts
The results obtained from the oxidation of alcohols with the
ZnTCPP-TiO2/CoFe2O4 hybrid were compared with some
reported catalysts in other literature studies (Table 6). Consid-
ering the used catalyst and alcohol amounts, oxidants, reaction
times, and yield percentages, the present method is more
suitable. Mainly, catalysts used in most studies took more time
for the reaction and a high amount of them was needed. Also, it
is noteworthy that due to the powerful magnetic properties of
the CoFe2O4, the ZnTCPP-TiO2/CoFe2O4 can be separated from
the reaction using a magnet.
Table 6 Comparison of the catalytic efficiency of ZnTCPP-TiO2/CoFe2O4 with those of several reported catalysts
No. Catalyst Activation method Oxidant Yield% Time (min) of reaction [Alcohol] (mmol) [Catalyst] (mg) Ref.
1
2
3
4
5
6
7
8
9
TiO2/Ti3C2
Visible light
Visible light
Visible light
UV
O2
O2
97
88
300
120
1140
720
120
1440
150
600
120
120
20
30
10
50
40
10
20
10
80
10
10
66
67
68
69
70
71
72
73
FeOx@N-C/Pd
n-TiO2-P25@TDI@DES
TiO2
200
150
250
200
200
1000
250
1000
100
Nitrate 58
O2
O2
O2
Air
O2
H2O2
H2O2
52
Ag2S-CdS
Visible light
499
94.7
89
90.6
95
NH2-MIL-125@TAPB-PDA Visible light
WO3ZnO/Fe3O4
0.5 wt% Au-Pd/ZnIn2S4
CQD@IL/WO4
HP mercury
Visible light
—
2À
74
10
Zn-TCPP-TiO2/CoFe2O4
Visible light
87
This work
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Fig. 14 (a) Recycling of the ZnTCPP-TiO2/CoFe2O4 catalyst in the oxidation reaction and (b) XRD patterns of the ZnTCPP-TiO2/CoFe2O4 nanohybrid
before (blue) and after (pink) recycling.
4.4. Reusability of the ZnTCPP-TiO2/CoFe2O4 nanohybrid
one-step separation/recycling using a permanent magnet, and
stability toward surface site poisoning, without losing efficiency or
activity. More generally, the results of this work provide useful
guideline principles for the design of complex photocatalysts.
The stability of the ZnTCPP-TiO2/CoFe2O4 nanohybrid for photo-
catalytic oxidation was studied via the continuous recycling
process. Upon completion of the reaction, the photocatalyst
was recycled by separating it from the reaction solution using a
permanent magnet. The separated catalyst was washed with
CH3CN and dried for the next run. The recovered and reused
catalyst shows good activity after 5 recycles (Fig. 14a) with
comparable efficiency. As shown in Fig. 14, the photocatalytic
performance reduced by 8 percent after 5 cycles which is due to
the inevitable loss of the catalyst during the recovery and washing
processes for reuse. Only ignorable changes in the yield could be
observed which correspond to the XRD patterns of the photo-
catalyst before and after the reaction in Fig. 14b (blue and pink
traces, respectively). Both XRD patterns are the same which
confirms the stability of the catalyst owing to the consistency of
the ZnTCPP bonding to the surface of the TiO2/CoFe2O4. It can be
noted that the phase and structure of the ZnTCPP-TiO2/CoFe2O4
remained unchanged after the fifth cycle, suggesting that the
photocatalyst is robust and stable in the photo-oxidation process.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge the University of Kurdistan,
the University of Waterloo, and the Natural Sciences and
Engineering Research Council of Canada (RGPIN-2015-04032)
for the support provided to this work. The authors thank
Dr Paul Stanish for his help with PL measurements, and also
Dr Yufeng Zhou and I-Hsuan Yeh for their help with TEM
measurements.
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