Promoting Visible-Light Photocatalysis with Palladium Species as Cocatalyst

Article abstract of DOI:10.1002/cctc.201500009

Conventionally, Pd catalysis has been widely used in either heterogeneous or homogeneous thermocatalytic organic synthesis. Herein, we demonstrate a case study on the important, cocatalyst role of Pd nanoparticles in ternary In₂S₃-(reduced graphene oxide-palladium) [In₂S₃-(RGO-Pd)] composites for the selective oxidation of alcohols under visible-light irradiation. Pd acts as dual cocatalyst along with RGO to promote the more efficient separation of photogenerated electron-hole pairs and facilitate their spatial transfer across the interface in ternary In₂S₃-(RGO-Pd) composites, which leads to higher activity than that of binary In₂S₃-RGO composites. This work highlights the promising scope of using Pd nanoparticles as a cocatalyst for promoting visible-light photocatalysis towards selective organic transformations under ambient conditions.

Full text of DOI:10.1002/cctc.201500009

DOI: 10.1002/cctc.201500009
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Promoting Visible-Light Photocatalysis with Palladium
Species as Cocatalyst
Xiuzhen Li,[a, b] Nan Zhang,[a, b] and Yi-Jun Xu*[a, b]
Conventionally, Pd catalysis has been widely used in either het-
erogeneous or homogeneous thermocatalytic organic synthe-
sis. Herein, we demonstrate a case study on the important, co-
catalyst role of Pd nanoparticles in ternary In2S3–(reduced gra-
phene oxide–palladium) [In2S3–(RGO–Pd)] composites for the
selective oxidation of alcohols under visible-light irradiation.
Pd acts as dual cocatalyst along with RGO to promote the
more efficient separation of photogenerated electron–hole
pairs and facilitate their spatial transfer across the interface in
ternary In2S3–(RGO–Pd) composites, which leads to higher ac-
tivity than that of binary In2S3–RGO composites. This work
highlights the promising scope of using Pd nanoparticles as
a cocatalyst for promoting visible-light photocatalysis towards
selective organic transformations under ambient conditions.
Introduction
Since the 1960s, Pd catalysis has garnered much attention in
the field of organic synthesis owing to the compatibility of Pd
with an extensive range of functionalities and the feasibility to
selectively functionalize cyclopalladated intermediates.[1–10]
Thus far, Pd-based catalysts (Pd0 and PdII complexes) have
been used for a plethora of organic reactions, such as Heck re-
actions, Negishi reactions, Suzuki reactions, CꢀH functionaliza-
tion reactions, and cross-coupling reactions, which provide
access to agrochemicals, pharmaceutical intermediates, poly-
mer precursors, fragrances, hormones, and many other valued
chemical products.[2,4–24]
pairs and the wide bandgap of TiO2 limit their practical photo-
catalytic application on a large scale.[27,34–40]
Graphene (GR), an electron conductive 2D platform, has
been used as a cocatalyst to improve the lifetime and transfer
efficiency of charge carriers photogenerated from semiconduc-
tors upon light irradiation, thereby enhancing the photoactivi-
ty of semiconductors.[41–48] In view of the electron conductive
property of the 2D platform of GR and the intrinsically low-
lying Fermi energy level of noble metal Pd nanoparticles,[36,49]
integrating the respective advantages of these two compo-
nents in a photocatalytic system used as dual cocatalysts is ex-
pected to enhance the photocatalytic activity of semiconduc-
tors by facilitating the spatial charge carriers transfer across
the interface in the composites.
In addition to the versatility of Pd catalysts in conventional
thermocatalysis, Pd nanoparticles have been found to act as
electron reservoirs in the photocatalytic systems for solar
energy conversion.[25–28] Solar-driven photocatalytic energy con-
version particularly based on heterogeneous semiconductors is
becoming a hot topic on account of its potential for solving
environmental and energy problems in a sustainable way,
which are the biggest challenges of the 21st century.[29–34] De-
spite the notable advances in the application of common TiO2-
based photocatalysts, the low quantum efficiency resulting
from the high recombination of photogenerated electron–hole
Herein, we choose In2S3, a typical III–VI group sulfide with
desirable visible-light response,[50–53] to fabricate ternary In2S3–
(RGO–Pd) composites with Pd nanoparticles and reduced gra-
phene oxide (RGO), the most common type of GR used to con-
struct photocatalysts, as cocatalysts through a wet chemistry
method. Taking selective oxidation of alcohols as a probing re-
action, which is a significant transformation of great industrial
importance owing to the wide use of the products (e.g., alde-
hyde and ketone derivatives) in fragrance, confectionery, and
pharmaceutical industries, the In2S3–(RGO–Pd) composites ex-
hibit enhanced activity upon visible-light irradiation as com-
pared with the optimal binary In2S3–RGO composite and blank
In2S3. This result is ascribed to the fact that Pd nanoparticles,
located in the interfacial layer between In2S3 and RGO, act as
dual cocatalyst with RGO to facilitate spatial charge carriers
transfer across the interface in ternary In2S3–(RGO–Pd) compo-
sites. This case study highlights the promising scope of using
Pd nanoparticles as a cocatalyst for promoting visible-light
photocatalysis towards selective organic transformations under
ambient conditions.
[a] X. Li, N. Zhang, Prof. Y.-J. Xu
State Key Laboratory of Photocatalysis on Energy and Environment
College of Chemistry
Fuzhou University
Fuzhou 350002 (P.R. China)
E-mail: yjxu@fzu.edu.cn
[b] X. Li, N. Zhang, Prof. Y.-J. Xu
College of Chemistry
New Campus
Fuzhou University
Fuzhou 350108 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500009.
Part of a Special Issue on Palladium Catalysis. A link to the Table of Con-
tents will appear here once the Special Issue is assembled.
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Results and Discussion
Ternary In2S3–(RGO–Pd) composites with different weight addi-
tion ratios of RGO–Pd have been prepared by using a facile so-
lution approach (Scheme 1). First, we anchor Pd nanoparticles
Scheme 1. Schematic illustration of the preparation of ternary In2S3–(RGO–
Pd) composites by using a facile, wet chemistry method.
onto the surface of the graphene oxide (GO) sheet via a one-
step clean strategy without the addition of the surfactant,
2ꢀ
during which the redox reaction between PdCl4 and GO
occurs.[62,63] After this step, GO–Pd still maintains abundant sur-
face oxygen-containing functional groups (Figure S1), which
provides a flexible platform for the introduction of a third com-
ponent to fabricate ternary composites. Thus, In2S3 can be as-
sembled on the GO–Pd surface by using the “structure-direct-
ing” property of GO through a facile hydrothermal process
during which GO is sufficiently reduced to RGO to form the
final ternary In2S3–(RGO–Pd) composites.
Figure 2. A and B) TEM images, C) high-resolution TEM image, and D) select-
ed area electron diffraction pattern of the as-synthesized ternary In2S3–
1%(RGO–Pd) composite.
(Figure S2). This observation is in accordance with the SEM re-
sults elaborated above. Lattice fringe can be clearly seen in the
high-resolution TEM image shown in Figure 2C. The spacing is
measured to be 0.324 nm, which can be indexed to the (311)
crystal face of the cubic b-In2S3 phase. And the lattice spacing
of 0.226 nm corresponds to the (111) face of cubic Pd, which
indicates the successful introduction of Pd nanoparticles into
ternary In2S3–(RGO–Pd) composites. Meanwhile, the existence
of Pd nanoparticles can be indirectly confirmed by the TEM
image and X-ray photoelectron spectrum (XPS) of GO–Pd. The
TEM image of GO–Pd (Figure S3) shows that Pd nanoparticles
are well dispersed on the GO surface with a narrow size distri-
bution at approximately 0.5–3 nm. As shown in Figure S4, the
Pd 3d XPS spectrum of GO–Pd consists of two typical peaks at
approximately 335.4 and 340.6 eV, which can be, respectively,
indexed to Pd3d5/2 and Pd3d3/2 core levels of Pd. The selected
area electron diffraction image (Figure 2D) shows the polycrys-
talline structure of the In2S3–1%(RGO–Pd) composite. The dif-
fraction rings (pointed by the arrow) can be, respectively, in-
dexed to (311), (400), and (400) planes of the cubic b-In2S3
phase. These results indicate that the addition of Pd nanoparti-
cles does not have a significant effect on the morphological
structure of the ternary In2S3–(RGO–Pd) composite as com-
pared with the binary In2S3–1%RGO composite.
Field-emission SEM (FESEM) is used to provide the informa-
tion on the morphology of blank In2S3, binary In2S3–1%RGO
composite, and ternary In2S3–1%(RGO–Pd) composite, which
exhibit the optimal photoactivity (see below). Without the ad-
dition of GO or GO–Pd, blank In2S3 has a flowerlike structure
with a mean diameter of 1 mm (Figure 1A). As for the optimal
Figure 1. Typical FESEM images of A) blank In2S3, B) In2S3–1%RGO composite,
and C) ternary In2S3–1%(RGO–Pd) composite.
binary In2S3–1%RGO composite, irregular petal-like In2S3 grows
compactly along the RGO mat platform (Figure 1B). Notably,
the ternary In2S3–1%(RGO-Pd) composite shows a morphology
similar to that of the optimal binary In2S3–1%RGO composite
(Figure 1C). This suggests that the addition of a small amount
of Pd nanoparticles to the ternary In2S3–(RGO–Pd) composite
almost has no effect on its morphology as compared with the
morphology of the binary In2S3–RGO composite.
The XRD patterns of ternary In2S3–(RGO–Pd) composites with
different weight addition ratios of RGO–Pd and blank In2S3 are
shown in Figure 3. All the samples show similar XRD patterns
that can be indexed to the cubic b-In2S3 phase (JCPDS card no.
32-0456). The peaks located at approximately 14.2, 20.9, 27.4,
28.7, 33.2, and 47.78 can be, respectively, ascribed to the (111),
(211), (311), (222), (400), and (440) crystal planes of the
cubic b-In2S3 phase.[50,51,53] The XRD patterns of ternary In2S3–
(RGO–Pd) composites are similar to those of binary In2S3–RGO
To further verify the microscopic structure of the ternary
In2S3–(RGO–Pd) composite, TEM analysis has been performed
(Figure 2). As shown in Figure 2A and B, irregular petal-like
In2S3 has grown compactly onto the RGO sheet, which is simi-
lar to the morphology of the binary In2S3–1%RGO composite
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show analogous morphology, crystalline phase, and optical
properties, which provides a basic framework to study the pos-
sible role of Pd nanoparticles in affecting the photocatalytic
performance of ternary In2S3–(RGO–Pd) and binary In2S3–RGO
composites.
The catalytic activity of ternary In2S3–(RGO–Pd) composites is
initially estimated by the selective oxidation of benzyl alcohol
to benzaldehyde under visible-light irradiation (l>420 nm).
Blank In2S3 and In2S3–1%RGO composite with optimal activity
for the selective oxidation of benzyl alcohol among binary
In2S3–RGO composites (Figure S7) have been used as reference
samples. With the addition of a small amount of Pd nanoparti-
cles, the conversion of benzyl alcohol and yield for the target
product benzaldehyde over ternary In2S3–(RGO–Pd) composites
are significantly increased as compared with the optimal
binary In2S3–1%RGO composite and blank In2S3, and the In2S3–
1%(RGO–Pd) composite shows the optimal photocatalytic ac-
tivity among all ternary In2S3–(RGO–Pd) composites (Figure 5).
Figure 3. XRD spectra of the as-synthesized ternary In2S3–(RGO–Pd) compo-
sites with different weight addition ratios of RGO–Pd and blank In2S3.
composites with different weight addition ratios of RGO–Pd
(Figure S5); this finding indicates that the addition of Pd nano-
particles has little effect on the crystalline structure of In2S3. No
apparent peaks for Pd nanoparticles are detected, which can
be probably ascribed to the relatively low amount of Pd nano-
particles added to the ternary composites. No obvious diffrac-
tion peaks of RGO can be found because of two possible rea-
sons: 1) the disappearance of the layer stacking regularity after
the redox reaction of graphite and 2) the relatively low diffrac-
tion intensity of RGO at 26.08 might be shielded by the main
peak of In2S3 at 27.48.[45,55,60,61,66]
The UV/Vis diffuse reflectance spectra (DRS) for characteriza-
tion of the optical properties of the samples are displayed in
Figure 4. It can be found that the absorption intensity in the
Figure 5. Photocatalytic selective oxidation of benzyl alcohol to benzalde-
hyde over ternary In2S3–(RGO–Pd) composites with different weight addition
ratios of RGO–Pd, In2S3–1%RGO composite, and blank In2S3 under visible-
light irradiation (l>420 nm) for 2 h. C&Y=Conversion and yield.
Notably, as the weight addition ratio of RGO–Pd reaches 5%,
both activity and selectivity decrease, which could be due to
the lowered contact of the surface of semiconductor In2S3 with
light irradiation in the ternary In2S3–5%(RGO–Pd) composite;
this has also been observed in previous works.[43,61,66,68] Anoth-
er reason for the decrease in selectivity is that the target prod-
uct benzaldehyde may not be timely and easily desorbed from
the surface of photocatalysts owing to the strong adsorption
ability of the carbon material RGO with relatively higher addi-
tion in ternary In2S3–(RGO–Pd) composites. Under this circum-
stance, deep oxidation of benzaldehyde may occur, thereby re-
sulting in decrease in selectivity.[43,66,68]
Figure 4. UV/Vis diffuse reflectance spectra of ternary In2S3–(RGO–Pd) com-
posites with different weight addition ratios of RGO–Pd and blank In2S3.
visible-light region (500–800 nm) strengthens with the increase
in the amount of RGO–Pd, which is possibly due to the intrin-
sic absorption of black RGO.[45,55] Such a phenomenon is also
observed for binary In2S3–RGO composites (Figure S6), which
indicates that the introduction of Pd nanoparticles does not
have a great effect on the optical properties of ternary In2S3–
(RGO–Pd) composites as compared with binary In2S3–RGO
composites. From the above results, it can be concluded that
the ternary In2S3–(RGO–Pd) and binary In2S3–RGO composites
To exclude the possible contribution of RGO and/or Pd
nanoparticles to the enhanced photoactivity of the In2S3–
(RGO–Pd) composite as compared with blank In2S3 and the op-
timal binary In2S3–RGO composite, the activity of RGO–Pd for
the selective oxidation of benzyl alcohol has also been evaluat-
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ed with or without visible-light irradiation. The results present-
ed in Figure S8 indicate that RGO–Pd exhibits quite poor activi-
ty at ambient conditions under both dark and visible-light illu-
mination, which suggests that in ternary In2S3–(RGO–Pd) com-
posites, the active component is In2S3 and the roles of Pd
nanoparticles and RGO serve as cocatalysts.
photocatalytic activity of In2S3–(RGO–Pd) composites for the
selective oxidation of alcohols as compared with the optimal
binary In2S3–1%RGO composite and blank In2S3, relevant char-
acterizations have been performed. The photoluminescence
(PL) spectra provide the information about surface processes
concerning the fate of charge carriers photogenerated from
semiconductors.[40,55,69–71] As displayed in Figure 7, the PL inten-
Time-online profiles for the selective oxidation of benzyl al-
cohol further confirm the enhanced activity of the In2S3–
1%(RGO–Pd) photocatalyst as compared with both the optimal
binary In2S3–1%RGO composite and blank In2S3. As shown in
Figure 6, the conversion of benzyl alcohol and yield of benzal-
Figure 7. PL spectra of the ternary In2S3–1%(RGO–Pd) composite, In2S3–
1%RGO composite, and blank In2S3 at an excitation wavelength of 420 nm.
sity observed for the In2S3–1%(RGO–Pd) composite is weaker
than that observed for both the optimal binary In2S3–1%RGO
composite and blank In2S3 at the excitation wavelength of
420 nm, which indicates the more efficient inhibition of the re-
combination of electron–hole pairs photogenerated in the
In2S3–1%(RGO-Pd) composite. This increased lifetime of photo-
generated charge carriers plays a crucial role in improving the
photocatalytic performance of the In2S3–1%(RGO–Pd) compo-
site as compared with that of both In2S3–1%RGO composite
and blank In2S3. Thus, we can assume that the introduction of
Pd nanoparticles facilitates the transfer of electrons and sup-
presses the recombination of charge carriers, which thus leads
to the enhancement of photoactivity of ternary In2S3–(RGO–Pd)
composites.
Figure 6. Time-online photocatalytic selective oxidation of benzyl alcohol to
benzaldehyde over the as-prepared ternary In2S3–1%(RGO–Pd) composite,
In2S3–1%RGO composite, and blank In2S3 under visible-light irradiation
(l>420 nm).
To further identify the role of Pd nanoparticles in ternary
In2S3–(RGO–Pd) composites in enhancing the photoactivity, the
transient photocurrent density for In2S3–1%(RGO–Pd) compo-
site, In2S3–1%RGO composite, and blank In2S3 electrodes was
measured in the electrolyte of 0.2m Na2SO4 aqueous solution
without bias versus Ag/AgCl under visible-light irradiation (l>
420 nm). As shown in Figure 8A, the photocurrent density of
the In2S3–1%(RGO–Pd) composite under visible-light irradiation
(l>420 nm) is higher than that of both In2S3–1%RGO compo-
site and blank In2S3, which implies that the electron transfer is
more efficient in the ternary In2S3–1%(RGO–Pd) composite.
Moreover, electrochemical impedance spectroscopy (EIS) has
been performed to study the migration of charge carriers in
electrode materials.[72] From the EIS Nyquist plots of the In2S3–
1%(RGO–Pd) composite, In2S3–1%RGO composite, and blank
In2S3 electrode materials cycled in 0.5m KCl aqueous solution
containing 0.01m K3[Fe(CN)6]/K4[Fe(CN)6] (Figure 8B), we can
dehyde obtained over the samples increase gradually with the
increase in irradiation time and the enhanced photoactivity of
the In2S3–1%(RGO–Pd) composite compared with the In2S3–
1%RGO composite and blank In2S3 is obvious. To confirm this
superiority of the In2S3–1%(RGO–Pd) composite, the selective
oxidation of other benzylic alcohols and allylic alcohols to cor-
responding aldehydes over the samples has also been investi-
gated under visible-light irradiation. The photocatalytic per-
formance of the In2S3–1%(RGO-Pd) composite evidently out-
performs that of the In2S3–1%RGO composite and blank In2S3
in all selected reaction systems (TableS1).
The above results indicate that the introduction of Pd nano-
particles into the interlayer matrix of In2S3 and RGO can im-
prove the visible-light photocatalytic performance of In2S3–
(RGO–Pd) composites for the selective aerobic oxidation of al-
cohols. To understand and explore the origin of the enhanced
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(Table S2). These results indicate that the introduction of Pd
nanoparticles does not have a significant effect on the surface
area and porosity of ternary In2S3–(RGO–Pd) composites as
compared with binary In2S3–RGO composites. Besides, adsorp-
tion experiments of alcohols in the dark have been performed.
There is almost no difference in adsorptivity between the
In2S3–1%RGO composite and the In2S3–1%(RGO–Pd) compo-
site, although they show higher adsorptivity than did blank
In2S3 (Figure S10). Therefore, the enhanced photoactivity of the
In2S3–1%(RGO–Pd) composite compared with the optimal
binary In2S3–1%RGO composite cannot be attributed to the
difference in surface area and porosity.
Furthermore, to understand the role of Pd nanoparticles in
the reaction mechanism for the photocatalytic performance of
In2S3–(RGO–Pd) composites, controlled experiments with addi-
tion of various radical scavengers for the photocatalytic selec-
tive oxidation of benzyl alcohol in the benzotrifluoride (BTF)
solvent have been performed.[45,74,75] When the radical scaven-
gers of ammonium oxalate (AO) for photogenerated holes, po-
tassium persulfate (K2S2O8) for electrons, and benzoquinone
(BQ) for superoxide radicals (O2·ꢀ) are added respectively, the
conversion of benzyl alcohol over the optimal ternary In2S3–
1%(RGO-Pd) composite, optimal binary In2S3–1%RGO compo-
site, and blank In2S3 is inhibited (Figure 9). The controlled ex-
Figure 8. A) Transient photocurrent density without bias versus Ag/AgCl and
B) EIS Nyquist plots of the as-prepared ternary In2S3–1%(RGO–Pd) compo-
site, In2S3–1%RGO composite, and blank In2S3 electrodes on the FTO sub-
strate.
see that all the samples show semicycles at high frequencies
that are related to the resistance of the electrodes.[72,73] The
plot of the ternary In2S3–1%(RGO–Pd) composite shows more
depressed semicircles at high frequencies corresponding to
the more efficient charge carrier transfer as compared with the
plots of In2S3–1%RGO composite and blank In2S3. These results
indicate that Pd nanoparticles introduced into the interfacial
layer matrix of In2S3 and RGO act as dual cocatalyst with RGO
to improve the lifetime of photogenerated charge carriers and
the transfer efficiency of charge carriers across the interface
between semiconductor In2S3 and RGO sheets, which leads to
the enhanced photoactivity of ternary In2S3–(RGO–Pd) compo-
sites.
Figure 9. Controlled experiments with different radical scavengers for the
photocatalytic selective oxidation of benzyl alcohol over the ternary In2S3–
1%(RGO–Pd) composite, In2S3–1%RGO composite, and blank In2S3 in the
BTF solvent under visible-light irradiation (l>420 nm) for 2 h; AO is a scav-
enger for photogenerated holes, K2S2O8 for photogenerated electrons, BQ
for superoxide radicals, and TBA for hydroxyl radicals.
periment with the addition of tert-butyl alcohol (TBA) as the
radical scavenger for hydroxyl radicals (·OH) shows that there
is almost no change in the conversion of benzyl alcohol. This
result is in agreement with the previous reports that no hy-
droxyl radicals are formed in the BTF solvent.[61,64,66,76] Hence,
for the visible-light photocatalytic selective oxidation of benzyl
alcohol over the ternary In2S3–1%(RGO–Pd) composite, optimal
binary In2S3–1%RGO composite, and blank In2S3, the primary
active radical species are photogenerated holes, electrons, and
activated oxygen (e.g., O2·ꢀ).
In addition, the surface area and porosity have been investi-
gated for the samples. The N2 adsorption–desorption iso-
therms of the In2S3–1%(RGO–Pd) composite, In2S3–1%RGO
composite, and blank In2S3 exhibit type IV isotherm with H3-
type hysteresis loop, which is characteristic of mesoporous
solids according to the IUPAC classification (Figure S9).[67] The
specific surface area and pore volume for the In2S3–1%(RGO–
Pd) composite are measured to be approximately 32 m2 gꢀ1
and 0.19 cm3 gꢀ1, respectively, which are larger than those of
blank In2S3 (ꢁ18 m2 gꢀ1 and 0.11 cm3 gꢀ1) and similar to those
of the In2S3–1%RGO composite (ꢁ37 m2 gꢀ1 and 0.23 cm3 gꢀ1)
The photocatalytic stability of the optimal In2S3–1%(RGO–
Pd) composite has also been evaluated. No apparent activity
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loss is observed for the selective aerobic oxidation of benzyl al-
cohol in the BTF solvent under visible-light irradiation over the
In2S3–1%(RGO–Pd) composite after four successive recycles,
which indicates that the In2S3–1%(RGO–Pd) composite is able
to serve as a stable and reusable visible-light photocatalyst for
the selective oxidation of a series of alcohols under controlled
reaction conditions (Figure S11).
ty of In2S3–(RGO–Pd) composites for the selective aerobic oxi-
dation of alcohols upon visible-light irradiation as compared
with that of the optimal binary In2S3–RGO composite and
blank In2S3. This work highlights the promising scope of using
Pd nanoparticles as a cocatalyst for promoting visible-light
photocatalysis towards selective organic transformations under
ambient conditions.
According to the above discussion, the enhancement of the
photoactivity of ternary In2S3–(RGO–Pd) composites can be pri-
marily attributed to the introduction of Pd nanoparticles into
the interlayer matrix of In2S3 and RGO that act as dual cocata-
lyst with RGO, which thus leads to the optimization of the sep-
aration and spatial transfer of charge carriers across the inter-
face in ternary In2S3–(RGO–Pd) composites. A possible reaction
mechanism has been proposed for the photocatalytic oxida-
tion of alcohols over ternary In2S3–(RGO–Pd) composites
(Scheme 2). There are two thermodynamically possible routes
Experimental Section
Materials
Indium(III) nitrate hydrate (In(NO3)3·4.5H2O), L-cysteine (C3H7NO2S),
graphite powder, sulfuric acid (H2SO4), nitric acid (HNO3), hydro-
chloric acid (HCl), sodium hydroxide (NaOH), potassium persulfate
(K2S2O8), phosphorus pentoxide (P2O5), potassium permanganate
(KMnO4), hydrogen peroxide 30% (H2O2), ethanol (C2H6O), palladi-
um chloride (PdCl2), benzyl alcohol (C7H8O), ammonium oxalate
(N2H8C2O4, AO), benzoquinone (C6H4O2, BQ), and tert-butyl alcohol
(C4H10O, TBA) were supplied by Sinopharm chemical reagent Co.,
Ltd. (Shanghai, China). Benzotrifluoride (C7H5F3, BTF), 4-methylben-
zyl alcohol (C8H10O), 4-methoxybenzyl alcohol (C8H10O2), 4-nitroben-
zyl alcohol (C7H7NO3), 4-chlorobenzyl alcohol (C7H7OCl), 4-fluoro-
benzyl alcohol (C7H7OF), cinnamyl alcohol (C9H10O), and 3-methyl-
2-buten-1-ol (C5H10O) were purchased from Alfa Aesar Co., Ltd.
(Tianjin, China). All the chemicals were analytical grade and used
without further purification. The deionized water was from local
sources.
Synthesis
Synthesis of GO
Scheme 2. Schematic diagram of the charge carrier transfer and proposed
mechanism for the selective oxidation of alcohols to aldehydes over ternary
In2S3–(RGO–Pd) composites under visible-light irradiation.
Natural graphite powder was used to synthesize GO by using
a modified Hummers method that was presented in our previous
works as well.[45,54–61] The details of the preparation procedure are
given in the Supporting Information.
of electron transfer in ternary In2S3–(RGO–Pd) composites. One
is that the electrons can transfer from the conduction band
(CB) of semiconductor In2S3 (ꢀ0.8 V vs. normal hydrogen elec-
trode, NHE) to Pd nanoparticles with large work function
(+0.62 V vs. NHE)[77] as an electron reservoir. The other is that
the electrons photogenerated from the conduction band of
In2S3 can transfer to RGO sheets (its work function is ꢀ0.08 V
vs. NHE) and then to Pd nanoparticles for their matchable
band structure and close interfacial contact. In this way, the re-
spective advantages of both RGO and Pd in a photocatalytic
system used as dual cocatalysts is expected to enhance the
photocatalytic activity of semiconductors by spatially boosting
the charge carriers relay among the composites system as
compared to their binary In2S3–RGO counterparts.
Preparation of GO–Pd composite
The GO–Pd composite was prepared by using a facile, one-step
clean strategy.[62,63] Typically, GO (30 mg) was fully dispersed in de-
ionized water (60 mL). The H2PdCl4 solution (10 mm, 3.13 mL) was
added dropwise to the homogeneous GO solution. The mixture
was stirred in ice bath for 30 min. Then, the resulting mixture was
centrifuged at 12000 rpm for 30 min. The collected solid was
washed with deionized water until the ion concentration of the su-
pernatant was less than 10 ppm. After drying the supernatant
overnight at 608C in an electric oven, the GO–Pd composite was
obtained.
Fabrication of In2S3–(RGO–Pd) composites with different
weight addition ratios of RGO–Pd
Conclusions
The given amount of the as-prepared GO–Pd composite was dis-
persed ultrasonically in distilled water (40 mL) and mixed with in-
dium(III) nitrate hydrate (0.5 mmol). After stirring the mixture for
0.5 h, l-cysteine (2 mmol) was added to it under vigorous stirring.
Then, pH was adjusted to 8 by adding sodium hydroxide (1m)
dropwise. After stirring for 1 h, the mixture was transferred into
a 50 mL Teflon-lined stainless steel autoclave, which was sealed
and treated at 1808C for 16 h. Subsequently, the as-synthesized
In summary, we have synthesized ternary In2S3–(RGO–Pd) com-
posites by using a facile wet chemistry method. Metallic Pd
nanoparticles, embedded in the interfacial layer between In2S3
and RGO, can act as dual cocatalyst with RGO to facilitate spa-
tial charge carriers transfer across the interface in ternary In2S3–
(RGO–Pd) composites, which thus leads to the enhanced activi-
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products were separated by centrifugation and washed thrice and
twice with distilled water and absolute ethanol, respectively. After
drying the products at 608C for 6 h, the target products were ob-
tained.
sorption–desorption equilibrium. The mixture was irradiated by
a 300 W Xe arc lamp (PLS-SXE-300, Beijing Perfect light Co., Ltd.)
with a UV-cut filter to cut off light with a wavelength less than
420 nm. Then, the mixture was centrifuged at 12000 rmp for
10 min to remove the catalyst particles. The remaining solution
was analyzed with an Agilent 7820A gas chromatograph. Conver-
sion of alcohol, yield, and selectivity of the aldehyde were calculat-
ed by using Equations (1)–(3):
Preparation of In2S3–RGO composite and blank In2S3
For comparison, the In2S3–RGO composite and blank In2S3 were
also fabricated by using the same hydrothermal process as that of
In2S3–(RGO–Pd) composites with GO instead of GO–Pd and without
GO, respectively.
C0ꢀCr
ð1Þ
ð2Þ
ð3Þ
Conversion ð%Þ ¼
ꢂ 100
C0
Cp
C0
Yield ð%Þ ¼
ꢂ 100
Characterization
Cp
C0ꢀCr
Selectivity ð%Þ ¼
ꢂ 100
The XRD patterns of the samples were recorded on a Bruker D8
Advance X-ray diffractometer with CuKa radiation at 40 kV and
40 mA and 2q=10–808 (scan rate: 0.028sꢀ1). UV/Vis diffuse reflec-
tance spectroscopy (DRS) was used to characterize the optical
properties of the samples with a UV/Vis spectrophotometer (Varian
Cary 500) with BaSO4 as a reference material. FESEM was used to
determine the morphology of the samples on a FEI Nova NanoSEM
230 spectrophotometer. TEM analysis was performed with a JEOL
JEM 2010 EX instrument operating at an accelerating voltage of
200 kV. X-ray photoelectron spectroscopy (XPS) analysis was per-
formed with a Thermo Scientific ESCALAB 250 X-ray photoelectron
spectrometer with monochromatic AlKa as the X-ray source, a hemi-
spherical analyzer, and a sample stage with multiaxial adjustability
to obtain the surface composition of the as-obtained samples. All
the binding energies were calibrated by the C1s peak at 284.6 eV.
The PL spectra were recorded with an Edinburgh FL/FS900 spectro-
photometer at an excitation wavelength of 420 nm. Nitrogen ad-
sorption–desorption isotherms and BET specific surface areas were
obtained with Micromeritics ASAP 2010 system. The photoelectro-
chemical analysis was performed with a homemade three-elec-
trode cell. A Pt plate was used as the counterelectrode, and an Ag/
AgCl electrode was used as the reference electrode. The electrolyte
was the Na2SO4 aqueous solution (0.2m) without an additive
(pH 6.8). The working electrode was prepared on fluorine-doped
tin oxide (FTO) glass that was sonicated in ethanol and dried at
808C before use. The boundary of FTO glass was protected with
the Scotch tape. The sample (2 mg) was adequately dispersed in
DMF (0.5 mL, Sinopharm Chemical Reagent Co., Ltd.) through soni-
cation. The as-obtained slurry was spread onto the pretreated FTO
glass. After air drying, the working electrode was further dried at
393 K for 2 h to improve adhesion. Then, the Scotch tape was re-
moved, and the uncoated part of the electrode was isolated with
epoxy resin. The visible-light irradiation source was a 300 W Xe arc
lamp system equipped with a UV-cut filter (l>420 nm). The EIS ex-
periments were performed with a CHI-660D workstation (CH Instru-
ments) in the electrolyte of KCl aqueous solution (0.5m) containing
K3[Fe (CN)6]/K4[Fe(CN)6] (0.01m, 1:1) under open circuit potential
conditions.
in which C0 is the initial concentration of alcohol and Cr and Cp are
the concentrations of the substrate alcohol and the corresponding
product aldehyde, respectively, at a certain time after the photo-
catalytic reaction.
Acknowledgements
We gratefully acknowledge support from the Key Project of
National Natural Science Foundation of China (U1463204), the
National Natural Science Foundation of China (20903023 and
21173045), the Award Program for Minjiang Scholar Professor-
ship, the Natural Science Foundation of Fujian Province for Dis-
tinguished Young Investigator Grant (2012J06003), and the Pro-
gram for Returned High-Level Overseas Chinese Scholars of
Fujian province.
Keywords: alcohols · graphene · oxidation · palladium ·
photocatalysis
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Published online on && &&, 0000
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Duality is part of life: Ternary In2S3–
(reduced graphene oxide–palladium)
composites show enhanced visible-light
photoactivity with the introduction of
palladium nanoparticles, which act as
dual cocatalyst with reduced graphene
oxide.
X. Li, N. Zhang, Y.-J. Xu*
&& – &&
Promoting Visible-Light Photocatalysis
with Palladium Species as Cocatalyst
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