Band Gap Modification of TiO2 Nanoparticles by Ascorbic Acid-Stabilized Pd Nanoparticles for Photocatalytic Suzuki–Miyaura and Ullmann Coupling Reactions

Article abstract of DOI:10.1007/s10562-019-02749-z

In this study, synthesis, characterization and photocatalytic performance of surface-modified TiO₂ nanoparticles with ascorbic acid-stabilized Pd nanoparticles are presented. The structure, composition and morphology of as-prepared nanophotocatalyst were characterized by UV-DRS, FT-IR, ICP-AES, TEM and XPS analysis. Ascorbic acid-stabilized Pd nanoparticles induced visible light driven photocatalytic property on the surface of TiO₂ which are otherwise insensitive to visible light owing to the wide band gap. The catalytic system worked well for the Suzuki–Miyaura cross-coupling and Ullmann homocoupling under compact fluorescent light as a visible source with significant activity, selectivity and recyclability. Good to excellent yields of biaryl products were obtained for various aryl halides having different electronic demands and even aryl chlorides. Our results proposed that the improved photoactivity predominantly benefits from the synergistic effects of ascorbic acid-stabilized Pd nanoparticles on TiO₂ nanoparticles that cause efficient separation and photoexcited charge carriers and photoredox capability of nanocatalyst. Thus, tuning of band gap of TiO₂ making a visible light sensitive photocatalyst, demonstrates a significant advancement in the photocatalytic Suzuki–Miyaura and Ullmann coupling reactions. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02749-z

Catalysis Letters
https://doi.org/10.1007/s10562-019-02749-z
Band Gap Modifcation of TiO2 Nanoparticles by Ascorbic Acid-
Stabilized Pd Nanoparticles for Photocatalytic Suzuki–Miyaura
and Ullmann Coupling Reactions
Fahimeh Feizpour1 · Maasoumeh Jafarpour1 · Abdolreza Rezaeifard1
Received: 26 October 2018 / Accepted: 12 March 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
In this study, synthesis, characterization and photocatalytic performance of surface-modifed TiO2 nanoparticles with ascorbic
acid-stabilized Pd nanoparticles are presented. The structure, composition and morphology of as-prepared nanophotocata-
lyst were characterized by UV-DRS, FT-IR, ICP-AES, TEM and XPS analysis. Ascorbic acid-stabilized Pd nanoparticles
induced visible light driven photocatalytic property on the surface of TiO2 which are otherwise insensitive to visible light
owing to the wide band gap. The catalytic system worked well for the Suzuki–Miyaura cross-coupling and Ullmann homo-
coupling under compact fuorescent light as a visible source with signifcant activity, selectivity and recyclability. Good to
excellent yields of biaryl products were obtained for various aryl halides having diferent electronic demands and even aryl
chlorides. Our results proposed that the improved photoactivity predominantly benefts from the synergistic efects of ascor-
bic acid-stabilized Pd nanoparticles on TiO2 nanoparticles that cause efcient separation and photoexcited charge carriers
and photoredox capability of nanocatalyst. Thus, tuning of band gap of TiO2 making a visible light sensitive photocatalyst,
demonstrates a signifcant advancement in the photocatalytic Suzuki–Miyaura and Ullmann coupling reactions.
Graphical Abstract
X
I
I
=
X
Br
I,
O
O
Br
e
e
O
O
TiO2
O
HO
h
h
O
Base
OH
B(OH)2
B(OH)3
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02749-z) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

F. Feizpour et al.
Keywords Photo cross coupling reactions · Titanium oxide nanoparticles · Ascorbic acid complex · Visible light driven
catalyst · Suzuki–Miyaura coupling reaction
Extensive studies have been dedicated to exert of photo-
catalysts in photoredox reactions [45, 46]. However, little
attention has been developed for carbon–carbon cross and
specially homo coupling reaction like Suzuki, Heck and Ull-
mann type reaction that stay basically untouched from light
[47–53].
1 Introduction
In recent years, the search for efcient solar energy conver-
sion has grabbed considerable attention because of rapidly
growing energy and environmental concerns [1]. Up to now,
metal oxide semiconductors, has emerged as a powerful
green tool owning to their potential applications in solar
light absorption [2]. In addition, they can be deliberated
as part of “green” science, since it lets operating chemical
transformations under light irradiation as an environmentally
friendly manner [3–6]. Among them, TiO2 is a promising
alternative due to charge carrier handling properties and
attractive performance for photochemical applications such
as water splitting [7, 8], volatile organic compounds (VOCs)
degradation [9], and dye sensitization in solar cell [10, 11],
etc. Nevertheless, the wide bandgap of TiO2 (3.0–3.2 eV)
limits its photoactivity to a narrow range of ultraviolet light,
i.e. < 4% of sun light [12]. In the quest to enable TiO2 to
absorb a wider spectral range of this abundant natural gift,
several research paths have been pursued which makes it
archetypical photo catalyst [13–15]. According to limita-
tion of TiO2, it is signifcant to tune up band gap energy
of TiO2 to extend photocatalytic property for visible-light-
driven photosystem. Toward this end, possible strategy can
be done by nanosizing [16, 17], surface grafting (ion doping)
[18–20], noble metal nanoparticle deposition [21, 22], co-
sensitization [10]. Surface adsorbates including folic acid
and enzymes that do not absorb visible light themselves,
are among attractive materials for visible light activation of
wide band gap semiconductors [23, 24]. Owing to its chemi-
cal potential and quick response, ascorbic acid (AA) has
been gained attention in this regard [25, 26]. A bathochromic
shift up to 600 nm was obtained for the onset of the photore-
sponse of surface-modifed TiO2 with ascorbic acid [27–30].
Nowadays, technological advancements have led to novel
synthetic approaches that are essential to reduce the energy
consumption and ecological impact. Interestingly, C–C
bond formation using Pd-containing catalyst has garnered
attention due to its application in the synthesis of bioactive
compound, material and pharmaceuticals science [31–34].
In this direction, the studies on developing efcient catalysts
for the given reaction were mostly focused on designing the
morphology, structural texture, and catalyst supports of Pd
nanocatalyst [31, 35–41].
Further, incorporating electron transfer of photocatalysts
with heterogeneous catalysts is actually a novel and prom-
ising for chemistry. On the other hand, the design of novel
photocatalyst has seen the imperative to derive chemical
reaction under visible light irradiation that assist the cou-
pling reaction of less active compound under unexpected
conditions.
With this goal in mind as a part of our ongoing project
on surface-modifcation of metal oxides to development of
novel strategies for the organic transformation [53–58], we
modifed band gap of TiO2 nanoparticles by ascorbic acid-
stabilized Pd nanoparticles. Then, the photocatalytic activ-
ity of novel TiO2–AA–Pd was investigated in Suzuki and
Ullmann type reaction under visible light as an innocuous
mediator in solvent free condition (Scheme 1). Our experi-
ments showed that the improved photoactivity predomi-
nantly originated from the synergistic efects of ascorbic
acid-stabilized Pd nanoparticles on TiO2 nanoparticles. The
promoted separation and photogenerated charge carriers and
photoredox capability of nanocatalyst are important advan-
tages, demonstrating the tuning of the band gap of TiO2 and
its possible application in the visible light region.
2 Experimental
2.1 General Remarks
All chemicals were purchased from Merck and Fluka
Chemical Companies. Powder X-ray difraction (XRD) was
performed on a Bruker D8-advance X-ray difractometer
with Cu Kα (λ = 1.54178 Å) radiation.The FT-IR spectra
were recorded on NICOLET system. Thermogravimet-
ric analysis (TGA) of nanopowders was performed in the
air by Shimadzu 50. TEM images were obtained by TEM
instrumentation (Philips CM 10). XP spectra were recorded
using a BESTEC GMBH (10−10 mw) with Al anode. Dif-
fuse refectance UV–Vis spectra were recorded using an
Avantes spectrometer (Avaspec-2048-TEC model). The Pd
content of the catalyst was determined by OPTIMA 7300DV
ICP analyzer. Progresses of the reactions were monitored
by TLC using silica-gel SIL G/UV 254 plates and also by
GC-FID on a Shimadzu GC-16A instrument using a 25 m
Following the more efcient Pd-based catalysts which
depend on electron richness relative to the activation mecha-
nism [42–44], we are fascinated to improve the efciency of
catalysts involving palladium by increasing their electron
density through support efects.
1 3

Band Gap Modifcation of TiO2 Nanoparticles by Ascorbic Acid-Stabilized Pd Nanoparticles…
OH
O
O
O
B
O
OH
O
O
R
R1
O
O
O
O
R1
O
O
O
O
O
e
O
X
O
O
O
O
TiO2
O
O
O
R
O
h
R
R
Scheme 1 Suzuki–Miyaura and Ullmann coupling reactions in the presence of TiO2–AA–Pd nanohybrid
CBP1-S25 (0.32 mm ID, 0.5 µm coating) capillary column.
NMR spectra were recorded on a BruckerAvance DPX
400 MHz instruments. The dependence of the catalytic
performance on the light wavelength was investigated by
employing various optical flters to allow the transmission
of specifc-wavelength light.
chamber and irradiated under magnetic stirring using a CFL
lamp (philips, wavelength in the range 390–750 nm, 40 W,
1.1 W m−2) as the visible light source at 100 °C for appro-
priate time. After completion of the reaction, TiO2–AA–Pd
nanohybrid was extracted by adding of ethanol (5 ml) fol-
lowed by centrifuging and decantation (3×5 mL ethanol).
Then, desired product (liquid phase) was extracted by plate
chromatography eluted with n-hexane/EtOAc (10/2).
2.2 Preparation of TiO2–AA–Pd Nanohybrid
See supporting information.
2.5 Reusability of Catalyst
2.3 General Procedure for Photocatalytic Suzuki–
Miyaura Cross Coupling Reactions
To a mixture of iodobenzene (1 mmol), phenylboronic acid
(1.1 mmol), Et3N (2 mmol) was added TiO2-AA-Pd nanohy-
brid (0.6 mol%) and the reaction mixture transferred into a
reactor chamber and irradiated under magnetic stirring using
a CFL lamp, at 70 °C for a certain period of time. After
completion of the reaction, the TiO2-AA-Pd nanohybrid was
recycled by adding of ethanol (5 mL) followed by centrifug-
ing and decantation (3×5 mL ethanol). The isolated solid
phase (TiO2–AA–Pd nanohybrid) was dried under reduced
pressure and reused for next runs. Catalyst recovery was also
investigated in the Ullmann coupling reaction according to
the above mentioned procedures.
In a typical reaction, a mixture of aryl halid (0.2 mmol),
arylboronic acid (0.22 mmol), Et3N (0.4 mmol) and
TiO2–AA–Pd nano hybrid (0.15 mol%) was added in a
10 mL Pyrex test tube and sealed with septum cap. Then the
reaction mixture transferred into a reactor chamber and irra-
diated under magnetic stirring using a CFL lamp (philips,
wavelength in the range 390–750 nm, 40 W, 1.1 W m−2) as
the visible light source at 70 °C for appropriate time. After
completion of the reaction, TiO2–AA–Pd nanohybrid was
extracted by adding of ethanol (5 mL) followed by centrifug-
ing and decantation (3×5 mL ethanol). Then, desired prod-
uct (liquid phase) was extracted by plate chromatography
eluted with n-hexane/EtOAc (10/2).
3 Results and Discussion
3.1 Catalyst Fabrication and Characterization
2.4 General Procedure for Photocatalytic Ullmann
Coupling Reactions
TiO2 and TiO2–AA nanohybrid were prepared and puri-
fed following the literature (for experimental details see
SI). As described in Scheme S1, TiO2–AA–Pd nanohybrid
was synthesized by incorporating of Pd(OAc)2 into ascor-
bic acid decorated on TiO2 nanoparticles that has been dis-
persed in ethanol under ultrasonic agitation. The as-prepared
In a typical reaction, a mixture of aryl halid (0.25 mmol),
Et3N (0.25 mmol) and TiO2–AA–Pd nanohybrid (0.3 mol%)
was added in a 10 mL Pyrex test tube and sealed with sep-
tum cap. Then the reaction mixture transferred into a reactor
1 3

F. Feizpour et al.
TiO2–AA–Pd was analyzed by ICP-AES and revealed that
the contents of Pd was 0.2 mmol g−1 (2.12 wt%).
The morphology of catalyst was screened by TEM meas-
urement and the images are presented in Fig. 3. A uniform
distribution of spherical nanoparticles with size ranging
between 15 and 20 nm can be ascribed to TiO2–AA–Pd pre-
pared in this work.
Figure 1 shows the FT-IR spectra of (a) TiO2, (b)
TiO2–AA, and (c) TiO2–AA–Pd nanocatalyst. The major
bands at 450–775 cm−1 present in all samples are assigned
to the stretching vibrations of Ti–O groups [59, 60]. In
Fig. 1a, broad peaks at 3400 and 1629 cm−1, correspond to
the surface adsorbed water and hydroxyl groups. Figure 1b
confrms the successful fabrication of TiO2/AA composite.
Furan ring of AA with vicinal hydroxyl groups acts in a
bidentate chelating mode with Ti atoms [61]. C–O stretch-
ing vibration of Ti–O–C is appeared in the range of 927 and
1010 cm−1 [62]. The strong peak at 1320 cm−1 assigned to
the O–H enediol of free acid, disappeared after complexa-
tion indicating the coordination of the OC3 and OC2 atoms
to the Ti centers [63]. Figure 1c demonstrates signifcant
spectral changes in the FT-IR spectra of TiO2/AA/Pd com-
pared to those of TiO2 and TiO2/AA (Fig. 1a, b). A new
weak band at 498 cm−1 is appeared and other bands shift to
higher wavenumbers, evidences for Pd coordinated by AA
coated TiO2 [Pd–O bond] [64].
3.2 Catalyst Screening in Suzuki Cross‑Coupling
Reaction
Initially, to check the potential of TiO2–AA–Pd nanohybrid
for catalyzing the Suzuki–Miyaura coupling reaction, the
reaction between iodobenzene (0.2 mmol) and phenylbo-
ronic acid (0.22 mmol) was chosen as a model reaction. To
achieve the desired conditions, various factors including
the nature of solvent and base, catalyst loading, and tem-
perature were screened (Fig. S1). According to the results
presented in Fig. S1i, removing the solvent improved the
yield of biphenyl, then a solvent-free condition was cho-
sen for the reaction (Fig. S1i). Among diferent bases tested
here, Na2CO3 was a poor one for the coupling and K2CO3,
KOH, KF and NaOH gave moderate yields. Nevertheless,
the use of 0.4 mmol Et3N as an organic base led to higher
performance of this reaction and consequently, hereafter it
will be used (Fig. S1iii). Next we addressed to optimize the
catalyst loading. A quantitative yield of the desired prod-
uct was achieved at 70 °C in the presence of 0.15 mol%
TiO2–AA–Pd nanohybrid after 1 h. Decreasing the catalyst
loading to 0.1 and 0.07 mol% reduced the yields to 70 and
40%, respectively (Fig. S1iv).
To investigate the chemical state of Pd species, XPS
measurements were employed in the region from 0 to
1100 eV (Fig. 2). The peak of C 1s is assigned at 285 eV and
calibrated by taking as reference. The peaks corresponding
to oxygen [O 1s (530 eV)], carbon (C 1s), and Ti 2p3/2 and Ti
2p1/2 [459 and 464 eV] [65–67] are also distinctly elucidated
in the survey XPS spectrum of the TiO2–AA–Pd nanohy-
brid. As seen in Fig. 2c, the binding energies of Pd 3d3/2
and 3d5/2 in the nanohybrid were 340.54 eV and 335.3 eV,
respectively, [68] which revealed that Pd(0) was present.
On the basis of the above mentioned experiments,
after 1 h reaction at 70 °C, the quantitative yield of 95%
of biphenyl was obtained, in the presence of 0.15 mol%
TiO2–AA–Pd nanohybrid, 0.4 mmol Et3N under solvent-free
conditions and visible light irradiation (Table 1, entry 1). It
is noteworthy that, in the absence of iodobenzene no evi-
dence for self-coupling of phenylboronic acid was observed
under optimized conditions.
The scope of the reaction was studied with electronically
and structurally diverse aryl halides with diferent substi-
tuted arylboronic acids (Table 1). Desired activities were
observed for arylhalides even aryl chlorides as deactivated
arylhalides, giving the corresponding biphenyl compounds
in good to excellent yields within 0.5–6 h under the opti-
mized reaction conditions (Table 1).
More investigations sought to understand the impact
of the halides (I, Br, Cl) of arylhalides. When aryliodides
(entries 1–5) were replaced by bromide counterparts (entries
6–9), the trends in reactivity remained almost unchanged,
nevertheless, reactions took longer. As an example, when
iodobenzene or bromobenzene subjected to the same reac-
tion conditions, biphenyl as a Suzuki coupled product
formed in 95 and 90% yield within 1 and 4 h respectively.
Furthermore, the least reactivity for the coupling reaction
Fig. 1 FT-IR spectra of a nanostructure TiO2, b TiO2–AA, c TiO2–
AA–Pd nanohybrid
1 3

Band Gap Modifcation of TiO2 Nanoparticles by Ascorbic Acid-Stabilized Pd Nanoparticles…
Fig. 2 XPS spectra of TiO2–AA–Pd nanohybrid a wide scan, b Ti 2p, c Pd 3d, and d O 1s
Fig. 3 TEM of TiO2–AA–Pd
nanohybrid
belonged to chlorobenzene, as expected (entry 10, 80% in
6 h). This reactivity order (Cl<Br<I) refects the strength
of the C–X bond leading to the conclusion that alkyl iodides
are the most reactive members of this functional class for
C–C bond formation. According to the data in entries 3, 4,
7, 8 and 9 in Table 1, electron-rich and electron-poor aryl
halides were also appropriate substrates to give the perti-
nent unsymmetrical coupling products in 82–96% isolated
yields. Moreover, excellent activity was observed with elec-
tron-poor aryl halides with good functional group tolerance
1 3

F. Feizpour et al.
Table 1 The Suzuki–Miyaura
cross coupling using TiO2–
AA–Pd
TiO2-AA-Pd
X
B(OH)2
S. 70 o
F,
C
R1
R1
R2
R2
0.5-6 h
Base,
Visible
X=
Cl
I, Br,
light
Time
Yield
(%)b
Entry
1
Aryl halide
I
Aryl boronic acid
OH
Producta
(h)
B
OH
1
95
OH
I
B
2
OH
1
2
90
OH
I
B
OMe
NO2
3
4
5
OH
84
96
90
MeO
I
OH
B
OH
0.5
O2N
OH
I
B
OH
1.5
OH
Br
B
6
7
OH
4
5
90
82
Br
OH
B
OMe
OH
MeO
O2N
Br
Br
OH
B
NO2
8
9
OH
3
4
94
90
OH
B
COMe
OH
MeOC
OH
Cl
B
10
OH
6
80
1 3

Band Gap Modifcation of TiO2 Nanoparticles by Ascorbic Acid-Stabilized Pd Nanoparticles…
Table 1 (continued)
Time
(h)
Yield
(%)b
Entry
Aryl halide
Aryl boronic acid
OH
Producta
I
B
Br
11
OH
2
2
90
71
Br
OH
I
B
12c
OH
I
OH
B
I
OH
Cl
13
14
2
2
90
81
Cl
OH
B
I
I
Et
OH
Et
OH
B
OMe
OH
15
2.5
85
MeO
OH
B
I
I
CHO
NO2
OH
16
17
2
4
90
45
OHC
OH
B
N
O2
OH
I
OH
B
18
2
6
70
70
S
S
OH
N
N
Cl
Cl
OH
B
N
N
19d
OH
N
N
Cl
Reaction conditions: arylhalides (0.2 mmol), arylboronic acid (0.22 mmol), TiO2–AA–Pd (0.15 mol %),
Et3N (0.4 mmol) under visible light irradiation (CFL, 40 W), air and solvent free conditions at 70 °C
aThe products were identifed by comparison with authentic samples and 1H NMR spectroscopy
bIsolated yield
cSubstrate ratio 1:2
dIn DMF solvent, substrate ratio 1:3, (25% yield obtained under solvent free conditions)
1 3

F. Feizpour et al.
(entries 4, 8, 9). For example, coupling of 4-nitro derivative
aryl iodide or aryl bromide with phenylboronic acid, gave
the related 4-nitrobiphenyl with the yield of 96 and 94% in
0.5 and 3 h, respectively. Whiles, cross coupling of methoxy
derivative aryl iodide or aryl bromide with phenylboronic
acid produced 4-methoxybiphenyl in 84 and 82% yields in 2
and 5 h, respectively. Nevertheless, when phenylboronic acid
substituted with various groups, no such signifcant efect
was found in the yield and the rate of the Suzuki coupling
reaction (Table 1, entries 13–18) expect for 3-nitro phenylb-
oronic acid and 2-thiophene boronic acid.
subjected to homocoupling reactions. The reactions pro-
ceeded well under catalytic infuence of a low catalyst load-
ing of 0.3 mol% TiO2–AA–Pd to give corresponding biaryls
in 65–95% yield.
Like Suzuki–Miyaura coupling reactions aryl bromides
and chlorides (Table 2, entries 6–10) are less reactive than
those of the related aryl iodides in the Ullmann type homo-
coupling reactions. Screening the electronic demands on
the reaction performance, para-substituted halobenzenes
were tested. Regardless of halobenzene’s substitution, both
electron-poor and electron-rich substrates reacted smoothly
afording biphenyl products in good to excellent yields in
3–10 h. Apparently, the reaction with electron-rich aryl
halides was signifcantly slower with less yield under the
similar reaction conditions. Finally, chemoselectively was
remarked. In this direction, 1-bromo-4-iodobenzene as a
model substrate was coupled exclusively, at iodine position
(Table 2, entry 10).
It is notable that the bihalide containing I and Br reacted
with high efciency meanwhile, selectivity well conducted
between reactive halides (entry 11). Moreover, one-pot dou-
ble couplings of 1,4-diiodobenzene and phenylboronic acid
occurred to produce a desired yield of terphenyl (71%) in
2 h (entry 12). Aryl 1,3,5-triazine derivatives have attracted
considerable interest because of its appealing performance
in herbicide [69], suitable ligands for providing of liquid
crystals [70], material science and hydrogenation of various
compounds [71]. However, one pot multiple Suzuki cou-
pling reaction can be more challenging to such 1,3,5-triazine
derivatives. When triazine was reacted with phenylboronic
acid under optimized conditions of this work, only 25% of
2,4,6-triphenyl-1,3,5-triazine was formed. Nontheless, per-
forming the reaction in DMF at 70 °C improved the yield of
produt to 70% after 6 h (entry 19). Needless to say, reducing
the steps of reaction to access to fully aryl-substituted arenes
is a salient feature of this work which is a necessity for the
Suzuki coupling reaction.
3.4 Photocatalytic Activity
Following our investigation on photochemical properties of
surface-modifed TiO2 with cobalt(II) [or Fe(III)] ascorbic
acid complex [54, 55], herein, the infuence of Pd ascor-
bic acid complex is screened to explain the photoactivity
of prepared catalyst under visible-light irradiation. Initially,
we replaced the TiO2 core by MoO3 and γ-Fe2O3 to inves-
tigate the core dependence of the catalytic activity. Signif-
cant reduction in product yield was observed in the cross-
coupling reaction between iodobenzene (0.2 mmol) and
phenylboronic acid (0.22 mmol) under the same conditions.
(Fig. 4). Thus, the photocatalytic activity depends greatly
on TiO2 core and afected to the catalytic performance of
TiO2–AA–Pd nanohybrid (Fig. 4).
3.3 Ullmann Coupling Reactions
During the optimization of temperature in the
Suzuki–Miyaura reaction based on model reaction, we
observed a by-product. So, we decided to evaluate self-cou-
pling of phenylboronic acid and iodobenzene in the absence
of each other at higher temperature under solvent free con-
ditions. In diferent conditions, the pertinent product of
self-coupling of phenylboronic acid was not detected but at
higher temperature, self-coupling of iodobenzene occurred
under solvent-free condition.
Conducting the reaction in dark condition decreased the
product yield to 35% demonstrating that the light absorption
by the TiO2–AA–Pd photocatalyst can facilitate the Suzuki
cross-coupling reaction. Thus, the photocatalytic activity
depends greatly on TiO2 core and afected to the catalytic
performance of TiO2–AA–Pd nanohybrid (Fig. 5).
To determine the relative contribution of thermal and
photochemical processes, the Suzuki and Ullmann type
reaction, were conducted under dark as well as diferent
wavelength ranges of irradiation. Subtracting the conver-
sion in the dark (contribution of thermal efect) from the
overall conversion under light gave contribution of light
to the conversion efciency (Fig. 6). To achieve men-
tioned wavelength ranges, a series of optical low-pass
flters were employed to block light below a specifc cut-
of wavelength. For example, the 450 nm optical flter
blocks the wavelength below 450 nm and over 800 nm, in
other words, the light irradiating the reactor has a wave-
length range from 450 to 800 nm. Without any flters, the
Encouraged by these results we tried to explore a het-
erogeneous photothermal system for the Ullmann type
reaction of arylhalides. Biphenyl was achieved from homo-
coupling reaction of iodobenzene after 4 h when 0.3 mol%
TiO2–AA–Pd and 0.5 mmol Et3N was mixed under solvent
free conditions at 100 °C (Fig. S2 in SI). The good to high
yields (65–95%) as well as excellent selectivity (>99%)
were obtained in the homocoupling reaction of arylhalides
(Table 2).
As seen in Table 2, a structurally diverse aryl halides
involving electron-rich and electron-defcient groups were
1 3

Band Gap Modifcation of TiO2 Nanoparticles by Ascorbic Acid-Stabilized Pd Nanoparticles…
Table 2 The Ullmann coupling
TiO2-AA-Pd
X
reactions of aryl halides using
TiO2–AA–Pd
S. 100 oC
R
R
R
F,
NEt3
X=
Cl
I, Br,
Visible
light
Entry
Aryl halide
Product a
Time (h)
4
Yield %b
95
I
1
I
2
3
4
5
5
3
90
75
90
I
I
MeO
O2N
OMe
NO2
MeO
O2N
I
5
5
65
Br
6
7
6
7
85
74
Br
Br
MeO
O2N
OMe
NO2
MeO
8
7
82
O2N
Cl
9
10
5
65
90
I
Br
Br
10
Br
Reaction conditions: arylhalides (0.25 mmol), TiO2–AA–Pd (0.3 mol%), triethylamine (0.5 mmol) under
visible light irradiation (CFL, 40 W), air and solvent free conditions at 100 C
aThe products were identifed by comparison with authentic samples and 1H NMR spectroscopy
bIsolated yield
irradiation of the light with wavelengths ranging from
400 to 800 nm gives a 4,4-biphenyl yield of 95%. The
yield decreases to 80%, 57% and 45% when the wave-
length range of the irradiation is 450–800, 550–800, and
600–800 nm, respectively. Since the yield of 4,4-biphenyl
in the dark is 35%, the contribution of 400–450 nm light
accounts for about 25% ((60 − 45)/60 × 100%) in the total
light-induced yield. Similarly, the light in the wavelength
range of 450–550, 550–600 and 600–800 nm, respectively
accounts for 38%, 21% and 16% of the light-induced yield
(Fig. 6a). Also a similar trend was performed for Ullmann
coupling reaction.
1 3

F. Feizpour et al.
absorbance of both surface-modifed samples (TiO2–AA
and TiO2–AA–Pd) in comparison with pure TiO2. So
as-prepared samples exhibited visible light absorption,
mainly in the wavelength range of 400–600 nm which
signifes its photocatalytic activity under visible-light irra-
diation. For the TiO2–AA–Pd nanohybrid, the absorption
band was found at around 490 nm and 630 nm. Difuse
refectance UV–Vis using the Kubelka–Munk formula and
Tauc plot [74], revealed signifcant reduction of the band-
gap of TiO2–AA–Pd nanoparticles (2.96 eV) compared
with those of TiO2–AA (3.1 eV) and especially TiO2 nano-
particles (3.15 eV). It can be observed that the band gap
energies for the TiO2–AA–Pd nanohybrid is lower than
TiO2 and TiO2–AA. Therefore, ascorbic acid-stabilized
Pd nanoparticles modifed the band gap of TiO2 inducing
it to absorb the visible light.
Fig. 4 The comparison of catalytic activity of TiO2–AA–Pd with
other nanocatalyst
3.5 Mechanism Study
Control experiments were applied to realize the mechanism
of the photocatalyzed coupling reaction. On the basis of the
observations from the experiments above (“Photocatalytic
Activity” section) and previous studies, we believe the elec-
tron–hole pair is generated within the TiO2 nanoparticles
modifed with ascorbic acid-stabilized Pd nanoparticles
upon light irradiation (Fig. 9). To prove this claim, iodo-
benzene reacted with phenyl boronic acid in the presence of
ammonium oxalate (AO) as a hole scavenger and TEMPO
as a radical scavenger under light irradiation. The yields
of coupling products reduced to 22 and 31%, respectively,
much close to results obtained in dark condition regardless
the presence or lack of scavengers, attributed to the contri-
bution of thermal efect. These results demonstrated well
the light-assisted Suzuki reaction rely on the photogenerated
e− and h+. Therefore, photogenerated e− transferred onto the
Pd nanoparticles can enhance the inherent catalytic activity
of Pd and photogenerated h+ can stimulate the C–B bonds
of the arylboronic acids.
Fig. 5 The screening of photocatalytic activity of TiO2–AA–Pd,
TiO2–AA and TiO2 nanoparticle under visible light and darkness con-
dition base on optimized method
Figure 7 shows the action spectra for synthesis of
4,4-biphenyl. The action spectrum is a benefcial manner for
identifying whether an observed reaction is caused by pho-
toinduced process or a thermocatalytic process which should
represent relevance between the wavelength and the light
extinction spectrum [72, 73]. In this regard, the reaction rates
of the synthesis of 4,4-biphenyl using TiO2–AA–Pd under
irradiation with diferent wavelengths were investigated. A
good correlation between apparent quantum yield (AQY)
and the difuse refectance spectrum of the TiO2–AA–Pd
nanohybrid was observed (Fig. 7) in both the reaction of
Suzuki–Miyaura and Ullmann reaction. These results con-
vinced us that the reactions are occurring photocatalytically.
The comparative diffuse reflectance UV–Vis spec-
tra of the TiO2, TiO2–AA and TiO2–AA–Pd nanohy-
brid are shown in Fig. 8. A red shift was observed in the
Taking into account of all these inspections, we propose
a plausible mechanism on the basis of the above results and
previous reports. As displayed in Fig. 9, Pd nanoparticles
can trap the photogenerated e− due to its electron reservoir
capacity. Both the photogenerated e− and energetic Pd nano-
particles can act as active centers to attack the C–X bond
of aryl halide, resulting in facilitating the formation of oxi-
dative addition intermediate with Pd. On the other hand,
arylboronic acid can acquire an OH– in the basic reaction
medium to form negative B(OH)3− species, which is con-
tribute to the trans-metalation process. Signifcantly, the h+
can assist in cleaving the C–B bond to produce biaryl–Pd
complex. The remaining step, which is called the reductive
elimination, should not be afected by the visible light [75].
1 3

Band Gap Modifcation of TiO2 Nanoparticles by Ascorbic Acid-Stabilized Pd Nanoparticles…
Fig. 6 The dependence of the catalytic activity of TiO2–AA–Pd
nanohybrid for a Suzuki cross-coupling reaction and b Ullmann
type reaction on the irradiation wavelength. The numbers in paren-
theses represent the contribution of the light. Reaction conditions:
for a iodobenzene (0.2 mmol), phenylboronic acid (0.22 mmol),
TiO2–AA–Pd (0.15 mol%), Et3N (0.4 mmol) at 70 °C; b arylhalides
(0.25 mmol), TiO2–AA–Pd (0.3 mol %), Et3N (0.5 mmol) at 100 °C;
under visible light irradiation (CFL, 40 W), air and solvent free con-
ditions
S2). To verify the heterogeneous nature of the title nanocata-
lyst hot fltration experiments after approximately 50% con-
version was carried out, which showed no further increase
in the conversion under similar reaction conditions. Then,
the fltrate and separated nanocatalyst were analyzed by ICP-
AES and no trace of Pd metal was detected, excluding the
leaching process and an evidence for stability of catalyst
under condition used in this work. Finally, the use of innocu-
ous and abundant visible light energy source, the removing
the toxic reagents, or solvents, reusability of catalyst and
easy isolation of organic products are further advantageous
of the presented methodologies.
3.6 Stability and Reusability of Catalyst
The TiO2–AA–Pd nanocatalyst can be simply retrieved and
reused in the Suzuki–Miyaura cross coupling of model reac-
tion as well as in the Ullmann coupling of iodobenzene. For
this purpose, after completing of reactions, ethanol (5 mL)
was added to the mixture and then TiO2–AA–Pd nanocata-
lyst (solid phase) was separated by centrifuging followed
by decantation (3 ×5 mL ethanol). The recovered catalyst
was washed and reused in six consequent runs in the reac-
tions (Fig. S3). The FT-IR spectrum of the reused catalyst
indicates that the structure of catalyst almost completely
preserve after recycling (Fig. S4).
Table 3 shows the merit of this operationally photocata-
lytic protocol in comparison with those previously reported
photocatalytic methods, in terms of catalyst loading, yield,
and especially, conditions used in the reaction coupling of
To investigate of hot fltration test, we occurred the cou-
pling reactions in DMF as solvent, because product yield
under solvent free condition and DMF is the same (Figs. S1,
1 3

F. Feizpour et al.
Fig. 7 Action spectrum for synthesis of 4,4-biphenyl using TiO2–
AA–Pd photocatalyst in the Suzuki and Ullmann reaction. The AQY
versus the respective wavelengths of the reaction is plotted. AQY was
calculated with the following equation: AQY (%)=[(Yvis −Ydark)×2/
(photon number entered into the reaction vessel)]×100, where Ylight
and Ydark are the amounts of products formed under light irradiation
and dark conditions, respectively
Fig. 8 UV–Vis spectra and DRS of a bare TiO2 NPs, b TiO2–AA and c TiO2–AA–Pd nanohybrid
phenylboronic acid and iodobenzene as model substrate.
Therefore, the title methodology is environmentally benign
because of using visible light as an innocous energy, reusing of
an active catalyst with very low catalyst loading, easy isolation
of organic products, and fnally, no need for toxic reagents, or
solvents.
1 3

Band Gap Modifcation of TiO2 Nanoparticles by Ascorbic Acid-Stabilized Pd Nanoparticles…
Fig. 9 Proposed photocatalytic
mechanism of carbon–carbon
coupling reaction
Table 3 Comparison of photocatalytic Pd base catalyst for Suzuki cross coupling reaction using iodobenzene and phenylboronic acid as a sub-
strate
Entry
Catalyst (g)
Condition
Time (h)
Conv. %
Ref.
1
2
3
4
5
6
7
8
9
TiO2–AA–Pd (0.005)
Au–Pd alloy NPs (0.05)
Pd/TiO2 (0.7 mol%)
Pd@MTiO2 (0.03)
S.F, 70 °C, CFL
DMF/H2O, K2CO3, Ar, 30 °C, incident light
NMP/H2O, Na2CO3, 120 °C
1
6
4
3
4
5
8
2
2
95
98
97
99
94
98
97
98
96
This work
[72]
[75]
[76]
[52]
[39]
[77]
[78]
[49]
H2O, K2CO3, 70 °C
Pd/TiO2 (0.015)
H2O/PEG, NaOC(CH3)3, 28 °C, white LED
H2O/EtOH, K2CO3, 25 °C, blue LED
H2O/MeOH, K2CO3, 60 °C
DMF/H2O, K2CO3, rt, white LED
DMF/H2O, K2CO3, rt, white LED
Au–Pd/TiO2 (0.2 mol%)
Ni-PVPa/TiO2–ZrO2 (0.02)
Pd@B-BO3 (0.01)
Pd@PDAb-CL (0.006)
aPoly(N-vinyl-2-pyrrolidone)
bPolydopamine
in good to excellent yields. The stability of the photo-
catalyst was evaluated using different manners, indicat-
ing that there is virtually no Pd leaching into the reaction
solution thus structure of catalyst remained substantially
unchanged under reaction conditions. Furthermore, our
results represent that the catalytic system can be reused
multiple times without significant loss of its activity and
stability. It is expected that our work could assign an
impressive strategy for making better use of the potential
properties of semiconducting materials for various pho-
tocatalytic applications.
4 Conclusion
In conclusion, TiO2–AA–Pd nanohybrid is synthesized by
incorporating of Pd(OAc)2 into ascorbic acid-decorated
TiO2 nanoparticles under ultrasonic agitation. The sur-
face modification with ascorbic acid-stabilized Pd nano-
particles has been induced visible light activity in TiO2
according to DRS analysis. The title nanophotocatalyst
was illustrated to be highly active, selective, and recover-
able for the Suzuki–Miyaura cross-coupling and Ullmann
homocoupling of a structurally divers of aryl halides
including electron-releasing and electron-withdrawing
and even aryl chlorides, affording the biaryl compounds
Acknowledgements The authors are grateful for fnancial support of
this work by University of Birjand and “Iran National Science Founda-
tion” (Grant No. 96004509).
1 3

F. Feizpour et al.
21. Bingham S, Daoud WA (2011) Recent advances in making
nano-sized TiO2 visible-light active through rare-earth metal
doping. J Mater Chem 21:2041–2050
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Afliations
Fahimeh Feizpour1 · Maasoumeh Jafarpour1 · Abdolreza Rezaeifard1
1
* Maasoumeh Jafarpour
Catalysis Research Laboratory, Department
mjafarpour@birjand.ac.ir
of Chemistry, Faculty of Science, University of Birjand,
Birjand 97179-414, Iran
* Abdolreza Rezaeifard
rrezaeifard@birjand.ac.ir; rrezaeifard@gmail.com
1 3