Synergistic effect of iodine doped TiO2 nanoparticle/g-C3N4 nanosheets with upgraded visible-light-sensitive performance toward highly efficient and selective photocatalytic oxidation of aromatic alcohols under blue LED irradiation

Article abstract of DOI:10.1016/j.mcat.2021.111527

Selective oxidation of alcohols via photocatalytic processes by O₂ molecules as a green oxidant and visible light has a great potential to dissolve the environmental crisis and worldwide energy. Herein, I-TiO₂ Nanoparticle/g-C₃N₄ Nanosheets (I-TiO₂/CNNSs) heterostructured composite with different contents of graphitic carbon nitride nanosheets has been successfully prepared via a facile and simple procedure. The structure of the I-TiO₂/CNNSs nanocomposite was analyzed by FT-IR, XRD, EDX, BET, TGA, FESEM, DRS, and PL techniques. The resultant nanocomposite (I-TiO₂/(20 wt%)CNNSs) exhibits superior photocatalytic efficiency than its pristine g-C₃N₄ and iodine doped TiO₂ for the oxidation of benzyl alcohols under the illumination of visible light (λ ≥ 400 nm). Introduction of the I-TiO₂ on the surface of CNNSs can improve the absorption ability in the visible region and separation efficiency of charge carriers during the photocatalytic oxidation. Hence, these obtained results show that the I-TiO₂/(20 wt%)CNNSs nanocomposite possess high photocatalytic performance and excellent reusability in oxidation reactions and other sustainable energy-related applications.

Full text of DOI:10.1016/j.mcat.2021.111527

Molecular Catalysis 506 (2021) 111527
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Synergistic effect of iodine doped TiO2 nanoparticle/g-C3N4 nanosheets
with upgraded visible-light-sensitive performance toward highly efficient
and selective photocatalytic oxidation of aromatic alcohols under blue
LED irradiation
Behrooz Akhtar, Hossein Ghafuri*, Afsaneh Rashidizadeh
Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, Tehran, 16846-13114, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
Selective oxidation of alcohols via photocatalytic processes by O2 molecules as a green oxidant and visible light
has a great potential to dissolve the environmental crisis and worldwide energy. Herein, I-TiO2 Nanoparticle/g-
C3N4 Nanosheets (I-TiO2/CNNSs) heterostructured composite with different contents of graphitic carbon nitride
nanosheets has been successfully prepared via a facile and simple procedure. The structure of the I-TiO2/CNNSs
nanocomposite was analyzed by FT-IR, XRD, EDX, BET, TGA, FESEM, DRS, and PL techniques. The resultant
nanocomposite (I-TiO2/(20 wt%)CNNSs) exhibits superior photocatalytic efficiency than its pristine g-C3N4 and
iodine doped TiO2 for the oxidation of benzyl alcohols under the illumination of visible light (λ ≥ 400 nm).
Introduction of the I-TiO2 on the surface of CNNSs can improve the absorption ability in the visible region and
separation efficiency of charge carriers during the photocatalytic oxidation. Hence, these obtained results show
that the I-TiO2/(20 wt%)CNNSs nanocomposite possess high photocatalytic performance and excellent reus-
ability in oxidation reactions and other sustainable energy-related applications.
Graphitic carbon nitride nanosheets
I-TiO2/CNNSs
Nanocomposite
Photocatalysis
Selective oxidation
Introduction
increasing attention in the arena of solar-to-fuel conversion [23], cata-
lytic transformations [24], photodegradation of organic pollutants [25],
The most overriding and serious challenges facing the present-day
world are environmental pollution and dilapidation, as well as a lack
of sufficient clean and natural energy resources. Therefore, the devel-
opment of environmentally friendly, affordable, green and sustainable
technologies is an essential and challenging task for scientists [1–8].
Among different organic compounds, aldehydes are important in-
termediates for the synthesis of biologically active compounds, drugs,
and fine chemicals. The selective oxidation of alcohols is one of the
greatest processes to manufacture corresponding aldehydes [9–12]. In
conventional methods, the stoichiometric amounts of toxic oxidizing
agents such as chromium trioxide (CrO3), Potassium permanganate
(KMnO4), Vanadium pentoxide (V2O5), and bromine (Br2) used in the
oxidation reactions [13–18]. Thus, designing a novel eco-friendly het-
erogeneous photocatalyst has great potential to bring a green and sus-
tainable path of development into modern chemistry [19–22].
Semiconductor photocatalysts like metal oxides, metal sulfides, organic
polymers, composites, and metal-organic hybrid materials have drawn
water splitting [26], and photocatalytic conversion of the carbon diox-
ide to chemical fuels [27]. Newly, research and development on the
various semiconductor photocatalysts for the selective photocatalytic
oxidation of aromatic alcohols under simulated sunlight has been
increasing [28–35]. TiO2 is the most common and well-known semi-
conductor due to its great photochemical stability, non-toxicity, cost--
effectiveness, and high oxidizing power. In addition, TiO2 as an ideal
photocatalyst has displayed significant potential for various applications
in the field of energy conversion [4,36–39]. However, the large band gap
of TiO2 about 3.2 eV, requires irradiation of ultraviolet light [36,38].
Also, the photocatalytic activity of TiO2 has limited due to the poor solar
energy conversion efficiency, low rate of separation photo-induced
charges, and high recombination rate of photogenerated electron-hole
pairs [4,40–42]. To date, polymeric graphitic carbon nitride (g-C3N4)
with a band gap (~2.7 eV) has received remarkable interesting in the
field of photocatalysis owing to high visible-light absorption, simple
synthesis, high thermal stability, and unique electronic band structure
* Corresponding author.
E-mail address: ghafuri@iust.ac.ir (H. Ghafuri).
https://doi.org/10.1016/j.mcat.2021.111527
Received 15 November 2020; Received in revised form 28 February 2021; Accepted 9 March 2021
Available online 31 March 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

B. Akhtar et al.
Molecular Catalysis 506 (2021) 111527
Fig. 1. (a) Schematic illustration of the synthetic procedure of the I-TiO2/CNNSs, (b)Photocatalytic selective oxidation of benzyl alcohols catalyzed by I-TiO2/CNNSs.
[7,43–49]. Nevertheless, the photocatalytic performance of the pristine
g-C3N4 is restricted because of the inefficient absorbance of the solar
spectrum, low specific surface area, fast recombination rate of
electron-hole pairs, and low separation spatial of photo-induced charge
carriers [43,44,50,51]. Therefore, many synthetic strategies and effec-
tive modification procedures including the construction of hetero-
junction systems, co-catalysts incorporation, and heteroatoms doping
have been established to overcome these constraints [52,53]. Among
these reported methods, heterojunction construction is an efficient
procedure for the improvement of photocatalytic performance in
organic transformations [43,44,54]. In recent years, some hetero-
structured graphitic carbon nitride composites have been reported for
the photocatalytic oxidation of alcohols [11,55–57]. In continuation of
our researches in the field of heterogeneous catalysis and photocatalysis,
we have demonstrated a simple synthetic method for the preparation of
I-TiO2/CNNSs heterojunction nanocomposites by the solvothermal
process (Fig. 1a). The resulting I-TiO2/CNNSs heterojunction nano-
composites displayed more photocatalytic efficiency for selective
oxidation of benzyl alcohols than Degussa P25, anatase TiO2, I-TiO2 and
bulk g-C3N4 under irradiation of visible light. I-TiO2/CNNSs hetero-
junction nanocomposites were utilized as photocatalysts to selective
photo-oxidation of benzyl alcohols to the corresponding aldehydes in
the aqueous media under blue LED light irradiation (Fig. 1b). The results
show that the combination of I-TiO2 and CNNSs leads to improved
photocatalytic performance.
Experimental
Chemicals and instruments
All the materials including melamine (C3H6N6, 99.0 %), titanium
(IV) butoxide (Ti(C4H9O)4, 97.0 %), iodic acid (HIO3, ≥99.99 %), benzyl
alcohol (C7H8O, 99.8 %), p-methylbenzyl alcohol (C8H10O, 98.0 %), p-
methoxybenzyl alcohol (C8H10O2, 98.0 %), p-hydroxybenzyl alcohol
(C7H8O2, 99.0 %), p-chlorobenzyl alcohol (C7H7ClO, 99.0 %), o-chlor-
obenzyl alcohol (C7H7ClO, 99.0 %), p-nitrobenzyl alcohol (C7H7NO3,
99.0 %), p-fluorobenzyl alcohol (C7H7FO, 97.0 %), 2,4- dimethox-
ybenzyl alcohol (C9H12O3, 99.0 %), 1-phenylethanol (C8H10O, 98.0 %),
ammonium oxalate monohydrate (C2H10N2O5, ≥99.99 %), tert-Butanol
(C4H10O, ≥99.5 %), potassium persulfate (K2S2O8, 99.0 %), p-benzo-
quinone (C6H4O2, 98.0 %) and commercial anatase titanium (IV) oxide
(anatase TiO2, <25 nm particle size, 99.7 %) were purchased from
Sigma-Aldrich Co. A commercial Degussa P25 (85 % rutile TiO2 and 15
% anatase TiO2, 20 nm particle size, 99.9 %) was obtained from Nano-
shell Co. Absolute ethanol (C2H6O, 99.8 %) was purchased from Merck
Co. All of the reagents were analytical grade and used without any
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B. Akhtar et al.
Molecular Catalysis 506 (2021) 111527
Fig. 2. FT-IR spectrum of g-C3N4 bulk, I-TiO2, and I-TiO2/(X wt.%)CNNSs.
further purification. Deionized water was utilized during the experi-
ment. Powder X-ray diffraction (PXRD) pattern of the synthesized ma-
terials were carried out using a Bruker D8 Advance powder X-ray
for 4 h in a magnetic stirrer at 80 ◦C and at this stage, g-C3N4 nanosheets
are synthesized. After that, the temperature reduced to 50 ◦C and then,
the iodic acid solution (0.5 g iodic acid in 5.0 mL water) was added to
the above mixture and stirred for 30 min. Then, titanium (IV) butoxide
solution (1.0 mL titanium (IV) butoxide solution in 10.0 mL absolute
ethanol) was added dropwise to the resulting mixture and stirred for 2 h
at 50 ◦C. Afterward, the mixture was transferred in a Teflon lined
autoclave (150.0 mL) and retained at 180 ◦C for 24 h. Next, the I-TiO2/
CNNSs nanocomposite was obtained after centrifugation (8000 rpm for
10 min) and washed several times with H2O and ethanol, and dried
overnight at 80 ◦C. Various nanocomposites were obtained by changing
the amount of g-C3N4. The as-prepared samples were labeled as I-TiO2/
(20 wt.%)CNNSs, I-TiO2/(40 wt.%)CNNSs, I-TiO2/(60 wt.%)CNNSs,
respectively. I-TiO2 nanoparticles was synthesized using the same sol-
vothermal process mentioned above without adding g-C3N4.
diffractometer with Cu K
α (λ = 0.15406 nm) radiation over the
diffraction angle (2θ) of 10◦–90◦ and examined using X’pert Highscore
Plus software. Fourier transform infrared (FTIR) spectra were collected
on a Shimadzu 8400S infrared spectrometer in the range of 400–4000
cmꢀ 1 using pressed KBr pellets. TESCAN MIRA3 field emission scanning
electron microscope (FE-SEM) was applied to analyze the morphologies
of the as-prepared samples. Thermogravimetric analysis (TGA) and
differential thermal analysis (DTG) were performed on a Bahr STA504
analyzer in the range of 20ꢀ 900 ◦C with heating rate of 10 ◦C minꢀ 1
.
Ultraviolet–visible diffuse reflectance spectra (DRS) of the synthesized
samples were collected using a Shimadzu UV-2550 UV–vis spectropho-
tometer with an integrating sphere attachment within the range of
250–800 nm and with BaSO4 as the reflectance standard. Energy
dispersive X-ray spectroscopy (EDS) and Elemental mapping-EDS were
examined by using a Zeiss Sigma VP Field Emission Scanning Electron
Microscope (FE-SEM) with an Oxford Instruments energy dispersive X-
ray spectrometer (EDS). The photoluminescence spectra of the samples
were collected by an Agilent G9800A fluorescence spectrometer with an
excitation wavelength at 380 nm. To determine the surface area and
pore size of the samples, the adsorption of nitrogen was performed at 77
K using a Microtrac Bel Corp Belsorp mini II analyzer.
General procedure for selective photocatalytic oxidation of aromatic
alcohols to corresponding aldehydes (or corresponding ketones)
The photocatalytic performance of the prepared nanocomposites was
investigated through selective photo-oxidation of aromatic alcohols
with oxygen atmosphere under simulated sunlight. Photocatalytic re-
actions were carried out in transparent pyrex Round-bottom flask with
5.0 cc volume. For each examination, 10.0 mg photocatalyst in 3.0 mL
water containing aromatic alcohol (0.1 mmol) was dispersed by ultra-
sonication. To achieve the adsorption-desorption equilibrium for benzyl
alcohol as a substrate and dissolved O2 molecules on the surface of
photocatalyst, the mixture was stirred in the dark for 30 min. The
photocatalysis reaction was performed under visible light for 8 h, using a
10 W blue LED Floodlight with λ ≥ 400 nm as the light source (distance
of light source from the reaction flask is 15 cm). The suspension tem-
perature was controlled at about 25 ◦C using a cooling water system. The
progress of the reaction was examined by Thin Layer Chromatography
(TLC). After completion of the photocatalytic oxidation, the nano-
composite was separated from the reaction media by using a centrifuge
(10,000 rpm, 10 min) and the reaction solution was then filtered
Synthesis of bulk g-C3N4
The pristine g-C3N4 was prepared through the thermal poly-
condensation of C3H6N6. In an alumina crucible with cover, 10.0 g of
C3H6N6 was poured and it was transferred to a muffle furnace and then
heated at 550 ◦C for 4 h with a heating ramp of 8 ◦C minꢀ 1. The obtained
pale-yellow bulk g-C3N4 sample were grinded into powder using ball
mill, when cooled to room temperature.
Synthesis of I-TiO2/CNNSs nanocomposite
The I-TiO2/CNNSs nanocomposite was fabricated through in situ
solvothermal assisted synthesis procedure. Specifically, a certain
amount of g-C3N4 powder was initially dispersed in 100.0 mL of absolute
ethanol for 2 h in an ultrasonicator. Then, the dispersed solution mixed
through a nylon syringe filter (0.22 μm, 25 mm) to separate the pho-
tocatalyst particles prior to HPLC analysis. Concentration of the sub-
strates and desired products were evaluated by a high-performance
liquid chromatography (HPLC, Knauer model Smartline Pump 1000/
3

B. Akhtar et al.
Molecular Catalysis 506 (2021) 111527
Fig. 3. XRD patterns of bulk g-C3N4, I-TiO2, and I-TiO2/(X wt%)CNNSs.
•
•
Germany) equipped with a Smartline PDA Detector (modal S2800 UV/
OH, h+, eꢀ , and Oꢀ2 respectively. We investigated the control experi-
Vis 200–400 nm) and Eurospher 100-5 C18 (5
μ
m, 250 mm length, 4.4
ments over the I-TiO2/(20 wt.%)CNNSs nanocomposite and benzyl
alcohol as a substrate under the optimized reaction conditions. 0.1 mmol
of the scavenger, which was tantamount to the mole amount of benzyl
alcohol (0.1 mmol) was added to the suspensions to perform free radical
trapping trial under visible light (λ > 400 nm). To find out the role of the
O2, we also evaluated the model reaction under N2 atmosphere [58–61].
mm inner diameter) column. According to the following equations, we
calculated the aromatic alcohol conversions, yields of corresponding
aldehydes, and reaction selectivity.
(Co-Calcohol
)
Conversion(%) =
× 100
Co
Result and discussion
Caldehyde
(Co ꢀ Calcohol
Reaction selectivity(%) =
× 100
)
Catalyst characterizations
C
Yield(%) = aldehyde × 100
Fig. 2, presents the FT-IR spectroscopy of the g-C3N4 bulk, I-TiO2,
and I-TiO2/(X wt.%)CNNSs nanocomposites to investigate surface
functional groups. The FT-IR spectrum of the g-C3N4 bulk exhibits a
distinct absorption frequency at 809 cmꢀ 1 for the out of plane bending
mode of tri-s-triazine subunits. The bands located in the region of
Co
Where, C0 is the initial concentration of alcohol, Calcohol and Caldehyde are
the substrate alcohol concentration and the corresponding aldehyde
respectively, at a certain time interval of the photocatalytic reaction
1200ꢀ 1700 cmꢀ 1 belong to the stretching vibrations of the C N and
–
–
CN bonds in the aromatic heterocyclic rings. The stretching modes of
–
Trapping experiments
–
the uncondensed terminal amine groups ( NH2 and NH) appeared at
3100–3400 cmꢀ 1 [62–64]. The FT-IR analysis of the I-TiO2 shows the
–
significant peaks at 3000–3400 cmꢀ 1 and 1650 cmꢀ 1 can be ascribed to
the stretching and bending vibration modes of water species adsorbed
on the I-TiO2. Also, the peak in the region of 500–800 cmꢀ 1 is assigned to
To determine the key reactive species for the selective photo-
oxidation of the aromatic alcohol over I-TiO2/(20 wt.%)CNNSs nano-
composite, a series of control experiments were executed under the
optimized conditions. Hence, tert-butanol, ammonium oxalate, potas-
sium persulfate, and p-benzoquinone were applied as trapping agents for
– –
the stretching modes of the Ti OT i [63–65]. The main absorption
4

B. Akhtar et al.
Molecular Catalysis 506 (2021) 111527
Table 1
The crystallite size of the prepared samples.
Sample
Crystallite size (nm)
Bulk g-C3N4
-
Iodine doped TiO2
I-TiO2/(20%wt)CNNSs
I-TiO2/(40%wt)CNNSs
I-TiO2/(60%wt)CNNSs
6.27
5.82
5.61
5.09
Fig. 6. (a) N2 adsorption desorption isotherm of the I-TiO2/(20 wt%)CNNSs,
(b) BJH pore size distribution of the I-TiO2/(20 wt%)CNNSs.
Table 2
Porous characteristics of I-TiO2/(20 wt%)CNNSs.
Sample
SBET
Pore
Total pore
volume (P/P0
= 0.0090)
(cm3/g)
Mean pore
diameter
(nm)
Crystallite
size (nm)
Fig. 4. EDS analysis of I-TiO2/(20 wt%)CNNSs.
(m2/g)
volume
(cm3/g)
bands of I-TiO2/(Xwt.%)CNNSs resemble g-C3N4 and I-TiO2, suggesting
the coexistence both of them in the nanocomposite structure.
I-TiO2/
(20%
wt)
116.18
0.4308
0.4224
14.5
5.82
The X-ray diffraction (XRD) patterns of the graphitic carbon nitride
baulk, I-TiO2, I-TiO2/(20 wt%)CNNSs, I-TiO2/(40 wt%)CNNSs, and I-
TiO2/(60 wt%)CNNSs are presented in Fig. 3, to investigate the crys-
talline structures of the prepared samples. The graphitic-like layer
structure of the g-C3N4 bulk was confirmed via two characteristic
diffraction peaks at 2θ = 13.0◦ and 27.34◦ which indexed as (1 0 0) and
(0 0 2) diffraction planes (JCPDS Card No: 87-1526). The peaks located
at 2θ = 13.0◦ and 27.34◦ are ascribed to the in-plane diffraction of the
CNNSs
aromatic system respectively. The XRD pattern of the I-TiO2 shows the
peaks at 2θ = 25.2◦, 37.8◦, 47.9◦, 53.9◦, 55.1◦, 62.6◦, 68.7◦, 70.3◦, and
75.0◦ corresponding to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1
1 6), (2 2 0), and (2 1 5) crystal planes of the tetragonal phase of TiO2
tri-s-triazine packing units and inter-planar
π stacking of the conjugated
Fig. 5. EDS mapping of I-TiO2/(20 wt%)CNNSs.
5

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Molecular Catalysis 506 (2021) 111527
Fig. 7. Thermogravimetric analysis of (a) bulk g-C3N4, (b) I-TiO2, and (c) I-TiO2/(20 wt%)CNNSs.
respectively (JCPDS Card No: 21-1272). As demonstrated in XRD pat-
terns of the synthesized nanocomposites with different content of
graphitic carbon nitride nanosheets (CNNSs), the intensity of the peak at
2θ = 13.0◦ reduced relative to graphitic carbon nitride due to structural
defects and decreased planar size of the g-C3N4 layers after exfoliated. In
addition, (0 0 2) reflection peak in the g-C3N4 bulk is shifted from 27.34◦
to 27.64◦ (d = 0.035 nm) and (1 0 0) reflection peak is shifted from 13.0◦
to 12.5◦ (d = 0.30 nm), suggesting a reduced distance between the layers
and the composition of the nanocomposite (Fig. 4). The appeared peaks
in the EDS spectrum indicate the presence of carbon, nitrogen, titanium,
oxygen, and iodine elements in the synthesized nanocomposite (I-TiO2/
CNNSs). The weight percent of the existing elements are summarized in
Fig. 4. As illustrated in elemental mapping of I-TiO2/CNNSs (Fig. 5), the
I-TiO2 are uniformly distributed on the graphitic carbon nitride
nanosheets.
The N2 adsorption/desorption isotherm of the nanocomposite (I-
TiO2/(20 wt%)CNNSs) demonstrates a usual type IV isotherm with an
H3 hysteresis loop, which confirms mesoporous structure of the nano-
composite (Fig. 6) [66]. The features of the porosity of the I-TiO2/CNNSs
are shown in Table 2, the specific Brunauer- Emmett- Teller (BET) sur-
face area, pore volume, total pore volume, pore diameter, and crystallite
size is 116.18 m2/g, 0.4308 cm3/g, 0.4224, 14.5 nm, and 5.82 nm
respectively.
in the CNNSs and increased π stacking in the g-C3N4 structure. As can be
seen in XRD patterns of the nanocomposites (I-TiO2/(20 wt%)CNNSs, I-
TiO2/(40 wt%)CNNSs, and I-TiO2/(60 wt%)CNNSs) with increasing the
content of CNNSs, diffraction peak located at 2θ = 27.64◦ became
stronger and the intensity of the diffraction signal at 2θ = 25.2◦ related
to TiO2 declined. Moreover, the crystallite size of the prepared samples
was estimated using Scherrer’s equation. The obtained results are
summarized in Table 1.
Thermal stability of the g-C3N4, I-TiO2, and I-TiO2/CNNSs nano-
composite were investigated by using thermal gravimetric analysis
(TGA) (Fig. 7). As observed in the TGA curve, the negligible weight loss
Moreover, the energy-dispersive Xray spectroscopy (EDX) elemental
mapping of I-TiO2/(20 wt%)CNNSs was carried out to deduce the purity
6

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Molecular Catalysis 506 (2021) 111527
Fig. 8. FESEM images of I-TiO2/(20 wt%)CNNSs.
(~3.9 %) in the first step at the temperature between 80ꢀ 200 ◦C was
attributed to the evaporation of H2O molecules and other solvents
absorbed on the surface of the I-TiO2/CNNSs nanocomposite. In Next
step, the gradual weight loss (~7.3 %) at the temperature between
200ꢀ 500 ◦C corresponding to the decomposition of I-TiO2 and eventu-
ally, the greater weight loss (~13.1 %) at the temperature between
500ꢀ 700 ◦C corresponding to the decomposition of g-C3N4 with its
higher thermal stability. A comparison of TGA-DTA analysis for I-TiO2
and I-TiO2/CNNSs nanocomposite obviously shows that the thermal
stability of the I-TiO2 increased after hybridized with g-C3N4
nanosheets.
The FE-SEM images of the CNNs (carbon nitride nanosheets) and I-
TiO2/(20 wt%)CNNSs nanocomposite are indicated in Fig. 8. The
layered structure of the CNNs was confirmed by using field emission
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Molecular Catalysis 506 (2021) 111527
Fig. 9. The number-frequency histogram of I-TiO2/(20 wt%)CNNSs.
wt%)CNNSs. As can be seen in Fig. 11, the g-C3N4 bulk has absorption
ability in visible region with the absorption edge about at 475 nm. As
reported in the literature, anatase TiO2 exhibits light absorption at about
390 nm. Although the DRS of I-TiO2 displayed red-shifted and improved
visible-light absorption in comparison with TiO2. All the synthesized
nanocomposites (I-TiO2/(20 wt%)CNNSs, I-TiO2/(40 wt%)CNNSs, and
I-TiO2/(60 wt%)CNNSs) show enhanced absorption in the range of
500ꢀ 700 nm. According to the DRS results, the band gap energy (Eg) of
the synthesized samples was evaluated by using the Kubelka–Munk
2
Equation: (
αh
ν
)
= K(h
ν
-Eg)n/2 [11,62]. The direct band gap values of
the g-C3N4 bulk, I-TiO2, I-TiO2/(20 wt%)CNNSs, I-TiO2/(40 wt%)
CNNSs, and I-TiO2/(60 wt%)CNNSs were 2.73, 3.11, 2.68, 2.71, 2.74 eV
respectively. Hence, the I-TiO2/(20 wt%)CNNSs nanocomposite shows
significant photocatalytic performance under visible light due to the
narrower band gap energy.
The electrochemical impedance spectroscopy (EIS) is implemented
to further scrutiny the separation and transfer efficiency of the photo-
generated carriers. The experiments are executed, in an aqueous solu-
tion containing 0.1 M of K3[Fe(CN)6]-K4[Fe(CN)6] (1:1)) and 0.1 M of
KCl. As shown in Fig. 12, the resistance of the samples is ascertained by
the radius of the arc at high frequencies on the EIS plots. As it is known,
the resistance value of photocatalyst corresponds to radius of the arc in
EIS Nyquist plots, and the smaller the radius of the arc, the lower the
resistance value. It can be observed from the Fig. 12 that the EIS Nyquist
plots of the I-TiO2/(20 wt%)CNNSs sample has the smaller radius than
Bulk g-C3N4, reflecting that the conductive potentiality of I-TiO2/(20 wt
%)CNNSs sample is maximum, namely, the separation and transfer ef-
ficiency of photoinduced carriers for I-TiO2/(20 wt%)CNNSs sample is
the quickest. This result is compatible with the photocatalytic activities.
Fig. 10. Photoluminescence spectra of bulk g-C3N4 and I-TiO2/(20 wt
%)CNNSs.
scanning electron microscopy. Also, the FE-SEM images of the I-TiO2/
CNNSs nanocomposite displayed the layer structure of the CNNs after
hybridized with I-TiO2. As revealed in Fig. 8, the I-TiO2 attached to
CNNSs with spherical morphology. The number-frequency histogram
was applied to determine the particle size and its distribution (Fig. 9).
Histograms gained from 415 nanoparticles demonstrate the average
particle size of I-TiO2 on the surface of CNNSs about 18.10 nm.
Photoluminescence (PL) spectra analysis was applied as a beneficial
method to evaluate the recombination efficiency of charge carriers (eꢀ
and h+) in the g-C3N4 bulk and I-TiO2/(20 wt%)CNNSs nanocomposite
(Fig. 10). The PL spectra of the g-C3N4 bulk and I-TiO2/CNNSs nano-
composite were carried out with an excitation wavelength of 380 nm. As
depicted in Fig. 10, the g-C3N4 bulk exhibited the strong emission peak
at 445 nm, which was equal to the narrow bandgap energy of g-C3N4
(~2.73 eV). While the intensity of the PL emission of I-TiO2/CNNSs
decreased during the formation of the heterojunction system, suggesting
that hybridizing I-TiO2 with g-C3N4 could improve the photocatalytic
activity via an effective separation and low rate recombination of
excited carriers.
Photocatalytic properties
To assess the photocatalytic potential of the prepared nano-
composites (I-TiO2/(20 wt%)CNNSs, I-TiO2/(40 wt%)CNNSs, and I-
TiO2/(60 wt%)CNNSs), the selective oxidation of benzyl alcohol to
benzaldehyde under irradiation of blue light LED ((λ ≥ 400 nm) in water
was chosen as a model reaction. As observed in Table 3, no photo-
oxidation occurred in water without using any catalyst under visible
light illumination or not (entry 1, 2). In the next step, the photocatalytic
performance of the nanocomposites (I-TiO2/(20 wt%)CNNSs, I-TiO2/
(40 wt%)CNNSs, and I-TiO2/(60 wt%)CNNSs (10.0 mg)) was investi-
gated in the oxidation of benzyl alcohol in aqueous media, resulting in
25 %, 20 % and 17 % benzyl alcohol conversion respectively (Table 3,
entries 3–5). It should be mentioned that no over oxidation products like
The UV–vis diffuse reflectance spectra (DRS) spectrum was utilized
to discover the optical aspects of the synthesized samples g-C3N4 bulk, I-
TiO2, I-TiO2/(20 wt%)CNNSs, I-TiO2/(40 wt%)CNNSs, and I-TiO2/(60
8

B. Akhtar et al.
Molecular Catalysis 506 (2021) 111527
Fig. 11. Diffuse reflectance spectrum (DRS) of prepared samples (a). The Kubelka–Munk plot of (
α
hυ
)
2 vs h
υ
of the prepared samples (b).
benzoic acid and CO2 were afforded during the oxidation reaction.
Hence, the I-TiO2/(X wt%)CNNSs illustrated high selectivity of photo-
oxidation in aqueous media. To further evaluate the photocatalytic
process, the model reaction was carried out with I-TiO2/(20 wt%)CNNSs
in the dark (Table 3, entry 6). The obtained result proves that no
oxidation product was detected in the absence of light. In order to
determine the photocatalytic activity of the synthesized nanocomposites
(I-TiO2/(X wt%)CNNSs), other photocatalytic systems including P25,
anatase TiO2, iodine doped TiO2, and bulk g-C3N4 were applied for the
oxidation of benzyl alcohol under same conditions (Table 3, entries
7–10). According to the experimental data summarized in Table 3, in the
presence of P25, anatase TiO2, and iodine doped TiO2 the benzyl alcohol
conversion increased to 78 %, 83 %, and 87 % respectively. However,
the selectivity of the benzaldehyde significantly decreased (Table 3,
entries 7–9). In addition, the photocatalytic performance of the I-TiO2/
(X wt%)CNNSs was also compared with bulk g-C3N4 (Table 3, entry 10).
As expected, bulk g-C3N4 demonstrated low photocatalytic activity
compared with prepared nanocomposite (I-TiO2/(X%wt)CNNSs) in the
oxidation reaction with only 8 % benzyl alcohol conversion and high
selectivity. It seems that the I-TiO2/(20 wt%)CNNSs shows superior
photocatalytic activity in the selective oxidation reaction of benzyl
alcohol. Increasing the amount of the nanocomposite I-TiO2/(20 wt%)
CNNSs to 20.0 mg improved the conversion of benzyl alcohol to 29 %
(Table 3, entry 13).
To develop the photocatalytic efficiency of the I-TiO2/(20 wt%)
CNNSs nanocomposite, some diverse benzyl alcohols were examined in
the photo-oxidation reaction in aqueous media (Table 4). As illustrated
in Table 4 different substituted benzyl alcohols including electron-
donating groups (2,4- dimethoxy, p-OH, p-OCH3, and p-CH3) and
electron-withdrawing groups (p-Cl, p-NO2,o-Cl, and p-F) were converted
to corresponding aldehydes with good conversion and high selectivity
(>99 %). In general, benzyl alcohols with electron-donating substituents
display higher conversion in contrast to benzyl alcohols with electron-
withdrawing groups. In fact, a higher conversion of benzyl alcohols
with electron-donating groups could be attributed to the high electron
– –
density of CH 2O H which leads to active holes and facilitate radical
formation.
On the basis of the above results, we proposed a possible mechanism
for the photo-oxidation of benzyl alcohols with O2 over I-TiO2/(20 wt%)
CNNSs (Fig. 13). According to the band structure of iodine doped TiO2
9

B. Akhtar et al.
Molecular Catalysis 506 (2021) 111527
•
( Oꢀ2 ). On the other hand, the photoexcited holes (h+) are transferred
from the valence band (VB) of the I-TiO2 to the VB of CNNSs. In
•
continuation, the superoxide anion radical ( Oꢀ2 ) can abstract a proton
•
from benzyl alcohol to generate OOH and alkoxide anion A as an in-
termediate. The basic property of the CNNSs can facilitate the depro-
tonation of benzyl alcohol. The obtained alkoxide anion A can be
oxidized by holes (h+) from the VB of I-TiO2 to afford radical B. Then,
•
radical B quickly combines with the OOH to form benzaldehyde. As well
as, there is another possibility to progress the photooxidation reaction.
•
As can be seen in Fig. 13, alkoxide anion A can react with the OOH
radical to produce anion radical C and hydrogen peroxide (H2O2).
Finally, the holes from VB of I-TiO2 oxidize the anion radical C to
benzaldehyde [67–70].
In order to validate the proposed mechanism and determine the main
active species for the photo-oxidation of benzyl alcohols in water,
various scavengers were applied in the model reaction (Fig. 14). First,
•
benzoquinone (BQ) as a scavenger for Oꢀ2 was added to the model re-
action mixture. In the presence of BQ, the conversion of benzyl alcohol
decreased significantly to 5%. As well as, when potassium persulfate
(K2S2O8) was utilized as a trapping agent for the photoinduced electrons
(eꢀ ), the conversion of benzyl alcohol reached 7 %. These obtained re-
Fig. 12. EIS Nyquist plots of the Bulk g-C3N4 and I-TiO2/(20 wt%)CNNSs
samples in an aqueous solution containing with 0.1 M of K3[Fe(CN)6]-K4[Fe
(CN)6] (1:1) and 0.1 M of KCl.
•
sults represent the main role of the Oꢀ2 in the photo-oxidation of benzyl
alcohol. Further, to prove the importance of the O2 in the photocatalytic
process, we tested the model reaction under N2 atmosphere. As shown in
Fig. 14, the conversion of benzyl alcohol was 11 % under N2 atmosphere.
It is worth to mention that the conversion of benzyl alcohol was 14 %
and 16 % with the addition of ammonium oxalate (h+ scavenger) and
Table 3
Optimization of the reaction conditions for the selective photo-oxidation reac-
tion of benzyl alcohola.
Entry
•
Catalyst
h
ν
Conversion
(%)
Selectivity
(%)
Yield
(%)
tertiary butanol ( OH scavenger) respectively. The controlled experi-
ments reveal that O2, eꢀ , and holes are active species for photo-oxidation
•
1
2
3
–
–
Trace
Trace
25
–
–
of benzyl alcohol over I-TiO2/(20 wt%)CNNSs. While OH has a slight
–
+
–
–
influence in the photocatalytic process.
I-TiO2/(20 wt%)
CNNSs
+
>99
25
The durability and stability of the heterogeneous photocatalysts have
a critical role in practical applications. Therefore, we tested a set of
experiments for the oxidation of benzyl alcohol using I-TiO2/(20 %wt)
CNNSs as a photocatalyst. As shown in Fig. 15, it could be concluded
that the I-TiO2/(20 wt%)CNNSs nanocomposite was stable after 5 runs
of photo-oxidation processes and exhibited no remarkable decrease in
photocatalytic efficiency.
4
5
6
I-TiO2/(40 wt%)
CNNSs
+
+
20
>99
>99
–
20
17
–
I-TiO2/(60 wt%)
CNNSs
17
I-TiO2/(20 wt%)
CNNSs
In
Trace
dark
+
7
8
9
Degussa P25
Anatase TiO2
Iodine doped
TiO2
78
83
87
9
7
+
12
16
10
14
+
Conclusion
10
Bulk g-C3N4
I-TiO2/(20 wt%)
CNNSs
+
+
8
>99
>99
8
11b
14
14
In conclusion, the I-TiO2/CNNSs heterostructured photocatalyst was
synthesized via a facile solvothermal procedure to introduce I-TiO2 on
the surface of g-C3N4 nanosheets. This nanocomposite exhibited
improved photocatalytic performance for selective oxidation of benzyl
alcohols in aqueous media. Based on the photoluminescence spectrum
and radical trapping analyzes, the mechanism of photocatalytic oxida-
tion of benzyl alcohols to the corresponding benzaldehydes on I-TiO2/
CNNSs heterojunction nanocomposites was discussed. The obtained
results showed that these photocatalysts have considerable photo-
activity and great stability in selective photocatalytic oxidation of aro-
matic alcohols to the corresponding aldehydes under visible light irra-
diation due to the synergistic effects between I-TiO2 and g-C3N4
nanosheets. Also, the prepared nanocomposite has great potential in
practical applications.
12c
13d
I-TiO2/(20 wt%)
CNNSs
+
+
27
29
>99
>99
27
29
I-TiO2/(20 wt%)
CNNSs
a
Reaction condition: catalyst (10 mg), benzyl alcohol (0.1 mmol), solvent (3
mL water), room temperature, oxidizing agent (molecular oxygen in balloon),
light source (10 W blue LED Lamp), reaction time (8 h), desired product:
benzaldehyde.
b
c
Catalyst (5.0 mg).
Catalyst (15.0 mg).
d
Catalyst (20.0 mg).
(I-TiO2) and carbon nitride nanosheets (CNNSs), type II heterojunction
mechanism is possible. Under visible light illumination, CNNSs excited
to produce electrons (eꢀ ) and holes (h+). To improve charge separation,
the electrons from the conduction band (CB) of CNNSs can be trans-
ferred to the conduction band (CB) of I-TiO2. Meanwhile, the present
electrons in CB of I-TiO2 can reduce O2 to superoxide anion radical
CRediT authorship contribution statement
Behrooz Akhtar: Investigation, Methodology, Writing, Formal
analysis. Hossein Ghafuri: Supervision, Conceptualization, Writing -
10

B. Akhtar et al.
Molecular Catalysis 506 (2021) 111527
Table 4
I-TiO2/(20 wt%)CNNSs catalyzed photo-oxidation reaction of aromatic alcoholsa.
Entry
Substrate
Product
Conversion (%)
28
Selectivity (%)
>99
Yield (%)
28
1
2
31
>99
31
3
4
26
19
>99
26
19
>99
5
6
21
15
>99
>99
21
15
7
8
13
37
>99
13
>99
37
(continued on next page)
11

B. Akhtar et al.
Molecular Catalysis 506 (2021) 111527
Table 4 (continued)
Entry
Substrate
Product
Conversion (%)
Selectivity (%)
Yield (%)
9
a
24
>99
24
Reaction condition: catalyst (10 mg), alcohol (0.1 mmol), solvent (3.0 mL water), room temperature, oxidizing agent (molecular oxygen in balloon), light source
(10 W blue LED Lamp), reaction time (8 h), desired product: Corresponding aldehyde.
Fig. 14. Trapping experiment of active types of oxidants during the photo-
oxidation reaction.
Fig. 13. A plausible mechanism for the photo-oxidation of benzyl alcohols
catalyzed by I-TiO2/(20 wt%)CNNSs.
Review & Editing. Afsaneh Rashidizadeh: Methodology, Writing -
Original Draft, Project administration.
Fig. 15. Recyclability of the I-TiO2/(20 wt%)CNNSs in photo-oxidation reac-
tion of benzyl alcohol.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
12

B. Akhtar et al.
Molecular Catalysis 506 (2021) 111527
with optically activated Au nanoparticles via click chemistry for
Acknowledgment
photoelectrochemical water splitting, ACS Sustain. Chem. Eng. 4 (2016)
4511–4520.
The authors gratefully acknowledge the partial support from the
Research Council of the Iran University of Science and Technology.
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