General self-assembly of metal/metal chalcogenide heterostructures initiated by a surface linker: Modulating tunable charge flow toward versatile photoredox catalysis

Article abstract of DOI:10.1039/c9ta07569k

Interfacial charge separation and transfer enduringly constitutes a core issue, which dictates the efficiency of a photocatalytic reaction. However, the exquisite modulation of directional charge transfer to ideal reaction sites remains challenging in terms of the fast recombination rates of photoinduced charge carriers and the difficulty in constructing spatially separated high-speed charge transfer channels. Herein, we demonstrate the general construction of metal/metal chalcogenide heterostructures by a facile, green, universal, scalable and simple yet efficient electrostatic self-assembly strategy, wherein tailor-made positively charged 4-dimethylaminopyridine (DMAP)-capped metal nanocrystals (NCs, Au, Pd) were spontaneously and intimately tethered on negatively charged one-dimensional (1D) transition metal chalcogenides (TMC: CdS, CdIn₂S₄, ZnIn₂S₄, and Zn_0.5Cd_0.5S) initiated by a surface linker (DMAP) to fabricate well-defined heterostructured photocatalysts. Moreover, the interface configurations between the metal NCs and TMC matrices were finely designed by ligand engineering to afford controllable charge transfer pathways. Intriguingly, this tunable charge flow endows the metal NC/TMC heterostructures with markedly enhanced and versatile photoredox performance toward the anaerobic-selective photoreduction of aromatic nitro compounds, photocatalytic hydrogen generation, and photocatalytic selective oxidation of aromatic alcohols under visible light irradiation, in which metal NCs play imperative roles in efficiently capturing Schottky-junction-driven electrons from the TMC substrates without the interference of the hierarchically branched ligand capped on the metal NCs. Our studysheds light on the rational modulation of interfacial charge flow in photoredox catalysis for substantial solar energy conversion.

Full text of DOI:10.1039/c9ta07569k

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ARTICLE
General Self-assembly of Metal/Metal Chalcogenide
Heterostructures Initiated by Surface Linker: Modulating Tunable
Charge Flow Toward Versatile Photoredox Catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Tao Li,a Ming-Hui Huang,a Yu-Bing Li,a Xiao-Cheng Dai,a Yunhui He,b Guangcan Xiao,b Fang-Xing
Xiaoa*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Interfacial charge separation and transfer enduringly constitutes a core issue which dictates the photocatalytic reaction
efficiency. However, exquisite modulation of directional charge transfer to the ideal reaction sites remains challenging in
terms of the fast recombination rate of photoinduced charge carriers and the difficulty in constructing spatially separated
high-speed charge transfer channels. Herein, we demonstrate the general construction of metal/metal chalcogenide
heterostructures by a facile, green, universal, scalable and simple yet rather efficient electrostatic self-assembly strategy,
wherein tailor-made positively charged 4-dimethylaminopyridine (DMAP)-capped metal nanocrystals (NCs, Au, Pd) were
spontaneously and intimately tethered on the negatively charged one-dimensional (1D) transition metal chalcogenides
(TMC: CdS, CdIn2S4, ZnIn2S4, Zn0.5Cd0.5S) initiated by surface linker (DMAP) for fabricating well-defined heterostructured
photocatalysts. Moreover, interface configuration between metal NCs and TMC matrixes was finely designed by ligand
engineering, which affords controllable charge transfer pathways. Intriguingly, such tunable charge flow endows metal
NCs/TMC heterostructures with markedly enhanced and versatile photoredox performances toward anaerobic selective
photoreduction of aromatic nitro compounds, photocatalytic hydrogen generation, and photocatalytic selective oxidation
of aromatic alcohols under visible light irradiation, in which metal NCs play imperative roles in efficiently capturing Schottky-
junction-driven electrons from the TMC substrates without the interference of the hierarchically branched ligand capped on
the metal NCs. Our work would shed light on rationally modulating the interfacial charge flow in photoredox catalysis for
substantial solar energy conversion.
hydrogen and oxygen, photocatalytic CO2 reduction to value-
added chemicals and photocatalytic mineralization of various
contaminants under sunlight irradiation.10-14 Besides the
electron-withdrawing capability of metal NCs (e.g., Au, Ag, Pd,
Pt, etc.), surface plasmon resonance (SPR) effect of metal NCs
makes them excellent light-harvesting antennas for triggering
plasmon-induced photocatalytic reactions under visible or even
near-infrared light irradiation.15-17
1. Introduction
Construction of metal-semiconductor heterostructures has been
deemed as an effective strategy to retard the fast recombination
of photoinduced charge carriers due primarily to the capability
of metal nanocrystals (NCs) for boosting the interfacial charge
transfer kinetic.1-3 It has been well-established that space-charge
separation region (i.e., Schottky barrier) can be readily generated
at the interface once semiconductors and metal NCs are closely
contacted, which intrinsically causes the spontaneous and
directional electrons flow from the material with higher Fermi
level (i.e., semiconductor) to the material with lower Fermi level
(i.e., metal NCs) to align the balance of Fermi energy levels.4-6
More significantly, Schottky barrier renders the noble metal NCs
efficacious electron traps to efficiently capture electrons for
retarding the electron-hole recombination in photocatalysis,7-9
which ultimately results in the remarkably improved
photoactivities including photocatalytic water splitting to
Normally,
photoactivities
of
metal-semiconductor
heterostructures are closely associated with the interfacial charge
transfer rate which predominantly dictates the solar energy
conversion efficiency.18,19 A better understanding on the
interfacial charge transfer kinetic between metal NCs and
semiconductor and simultaneously, finding applicable way to
finely modulate the interfacial charge transfer pathways are of
great significance for designing highly efficient photocatalysts.
Nonetheless, most of the previous works are mainly focused on
over-emphasizing the net photoactivity enhancement, deep
understanding on the interfacial charge transfer kinetic at the
nanoscale level remains elusive. Noteworthily, previous works
have pointed out that organic ligands capped on the surface of
tailor-made metal NCs are normally detrimental to the intimate
interfacial contact between metal NCs and semiconductors,
which inevitably increases the charge migration distance and
a. College of Materials Science and Engineering, Fuzhou University, New Campus,
Minhou, Fujian Province 350108, China. E-mail: fxxiao@fzu.edu.cn
b. Instrumental Measurement and Analysis Center, Fuzhou University, Fuzhou,
350002, People’s Republic of China.
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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thus leads to the unfavorable photoactivities.20-22 Besides, it is pollutant outperforming single counterpart. Based VoinewwArhticiclehO,nwlinee
generally accepted that high-temperature annealing is required to verified that hierarchically branched liganDdOcI:a1p0p.1e0d39o/nC9tThAe0m75e6t9aKl
completely remove the ligands which seriously affects the NCs surface did not affect the directional interfacial charge
interfacial charge transfer. However, calcination is not always transfer, which is distinct from conventional opinion that ligand
beneficial for charge transfer since it may cause agglomeration normally restrains the exposure of active sites on the metal NCs
of catalyst, reduced specific surface area, and decreased number surface, thus influencing the formation of intimate interfacial
of reactive sites, inevitably reducing the photoactivities. As such, contact and retarding the charge separation/transfer. More
it is of paramount importance to unveil the correlation of significantly, photoactivities of metal NCs/TMC along with the
interface configuration with the interfacial charge transfer interfacial charge transfer efficiency were finely tuned by ligand-
features in metal-semiconductor heterostructures without triggered interface configuration modulation. Based on which,
definitely removing the ligand and meanwhile, to achieve detailed photoredox catalytic mechanism in terms of the
tunable modulation on the interfacial charge transport via ligand- Schottky-junction-driven charge flow was elucidated and the
triggered interface engineering.
roles of metal NCs and surface ligand were clarified.
On the other hand, in the past one decade, one-dimensional
(1D) semiconductors (e.g., nanorods, nanowires, nanotubes, etc.)
have been overwhelmingly explored by virtue of their peculiar
2. Experimental section
structural
advantages
compared
with
conventional
2.1. Preparation
nanoparticulate and bulk counterparts, including larger specific
surface area and pore volume, more enhanced light absorption
and propagation as well as the more markedly fast and efficient
charge transport along the vectorial direction of 1D scaffold,23-25
considerably benefiting the spatial separation of photogenerated
electron-hole pairs.26,27 Hence, it is not surprised to see the wide-
spread photocatalytic applications of 1D semiconductors in a
myriad of fields, such as environmental remediation, hydrogen
production, and selective organic transformation.28-30 Among the
fruitful 1D semiconductors, transition metal chalcogenides
(TMC) have garnered enormous attention due to their favorable
bandgaps for substantial solar light harvesting in the visible or
even near-infrared light region and suitable energy level
alignment for triggering multifarious photoredox catalytic
reactions, which maximumly favors the solar energy
conversion.31-33 In this regard, high-efficiency metal/TMC
heterostructures with tunable charge transfer capability can be
exquisitely designed by judiciously utilizing the tailor-made
metal NCs and TMC as the building blocks in terms of their
respective structural merits, in which ligand capped on the metal
NCs surface functions as the bridging medium to enable the
interface engineering.
Sparked by these motivations, in this contribution, 4-
dimethylaminopyridine (DMAP)-capped gold nanoparticles
(Au@DMAP NPs) and mercaptoacetic acid (MAA)-modified
CdS nanowires (CdS@MAA NWs) were selected as the
quintessential metal NCs and TMC for constructing metal
NCs/TMC composite photocatalysts with variable interface
configuration. Attracted by the pronounced electrostatic
interaction, intrinsically positively charged Au@DMAP
(Pd@DMAP) NPs were spontaneously and intimately self-
assembled on the negatively charged CdS NWs (CdIn2S4,
ZnIn2S4, Zn0.5Cd0.5S) framework resulting in the well-defined
Au-CdS NWs heterostructures. The ligand-triggered
Material.
Sodium
diethyldithiocarbamate
trihydrate
(C5H10NNaS2·3H2O), cadmium chloride (CdCl2·2.5H2O),
ethylenediamine (C2H8N2), mercaptoacetic acid (MAA,
C2H4O2S), tetraoctylammonium bromide (TOAB, C32H68NBr),
toluene (C7H8), sodium borohydride (NaBH4), 4-
dimethylaminopyridine (DMAP, C7H10N2), ammonium formate
(NH4HCO2) and RhB were obtained from Sinopharm Chemical
Reagent Co., Ltd. (Shanghai China). Gold chloride trihydrate
(HAuCl4·3H2O), 2-nitroaniline (2-NA), 3-nitroaniline (3-NA),
4-nitroaniline (4-NA), 2-nitrophenol (2-NP), 3-nitrophenol (3-
NP), 4-nitrophenol (4-NP), 4-nitrotoluene (4-NT), 1-chloro-4-
nitrobenzene and 1-bromo-4-nitrobenzene were purchased from
Aladdin. Poly(sodium 4-styrenesulfonate), benzyl alcohol (BA),
p-methylbenzyl alcohol (p-MBA), p-methoxybenzyl alcohol (p-
MOBA), p-fluorobenzyl alcohol (p-FBA), p-chlorobenzyl
alcohol (p-CBA), p-nitrobenzyl alcohol (p-NBA), cinnamyl
alcohol (CA), and 3-methyl-2-buten-1-ol were purchased from
Alfa Aesar (China) Chemicals Co., Ltd.
Synthesis. (Ⅰ) Fabrication of CdS NWs. CdS NWs were
prepared by a hydrothermal method. Details of the synthesis
were provided in Supporting Information. (Ⅱ) Preparation of
CdS@MAA NWs. Detail of the synthesis were provided in
Supporting Information. (Ⅲ) Preparation of Au@DMAP NPs.
Au@DMAP NPs were prepared by a previously published
work.34 Details of the synthesis were provided in Supporting
Information. (Ⅳ) Synthesis of Au-CdS NWs heterostructures.
Au-CdS NWs heterostructures were fabricated by a facile yet
efficient electrostatic self-assembly strategy at ambient
conditions. Specifically, positively charged Au@DMAP NPs
aqueous solution (2.2 mg mL−1) was added dropwise to the
negatively charged CdS@MAA NWs aqueous dispersion at the
weight addition ratio of 0.005:1, 0.01:1 and 0.02:1 under
vigorous stirring. After mixing for 2 h, the mixture was
centrifuged and dried in an oven at 333 K. Alternatively,
preparation of Pd-CdS NWs, Au-Zn0.5Cd0.5S, Au-CdIn2S4, Au-
ZnIn2S4 heterostructures is similar to that of Au-CdS NWs and
detailed information was provided in the Supporting
information.
electrostatic
self-assembly
endows
Au-CdS
NWs
heterostructures with markedly enhanced and versatile
photoredox catalytic performances under visible light irradiation
toward anaerobic photoreduction of aromatic nitro compounds,
photocatalytic hydrogen generation, photocatalytic selective
oxidation of aromatic alcohols and mineralization of organic
2.2. Characterization
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Zeta potential (ξ) measurements were carried out on dynamic (BTF) (1.5 mL) solvent, which was saturated with pure
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DOI: 10.1039/C9TA07569K
light scattering analysis instrument (ZetasizerNano ZS-90). molecular oxygen. The reason for selecting BTF as solvent is
Field-emission scanning electron microscopy (FESEM, Supra55, ascribed to its inertness to oxidation and high solubility for
Carl Zeiss, Germany) was used to characterize the morphologies molecular oxygen. The above mixture was transferred into a 10
of the samples. Transmission electron microscopy (TEM), high- mL Pyrex glass bottle filled with molecular oxygen at a pressure
resolution (HR) TEM and energy-dispersive spectroscopy (EDS) of 0.1 MPa and stirred for half an hour to make the catalyst blend
images were obtained using a JEOL-2010 instrument at an evenly in the solution. The suspension was then irradiated by a
accelerating voltage of 200 kV. Crystal phases of the samples 300 W Xe arc lamp (PLS-SXE300D, Beijing Perfect Light co.
were determined by X-ray diffraction (XRD, X’Pert Pro MPD, LTD, China) with a 420 nm cut-off filter. After the reaction, the
Philips, Holland) using Cu Kα radiation at 40 kV and 40 mA. mixture was centrifuged at 12000 rpm for 5 mins to completely
Fourier transform infrared spectroscopy (FTIR) measurements remove the catalyst. The remaining solution was analyzed with
were performed on a TJ270-30A spectrophotometer at a a Gas Chromatograph (Shimadzu GC2140).
resolution of 4 cm-1. Raman spectra were measured on a
(b) Photocatalytic mineralization of organic pollutant.
Renishaw inVia spectrometer. Optical properties of the samples Photocatalytic mineralization of organic pollutant over Au-CdS
were probed by a Cary 500 UV-vis diffuse reflectance NWs heterostructure was evaluated by degradation of RhB at
spectroscopy (DRS), in which BaSO4 was employed as the ambient conditions. Specifically, 20 mg of catalyst was dispersed
internal reflectance standard. X-ray photoelectron spectroscopy in 40 mL of RhB aqueous solution (5 ppm). Prior to irradiation,
(XPS) measurements were carried out on a Thermo Scientific the reaction system was stirred in the dark for 0.5 h to achieve
ESCALAB250 spectrometer using Al Kα radiation source and the adsorption-desorption equilibrium between catalyst and
binding energies (B.E.) of elements were calibrated by the B.E. reactant. Then, the suspension was irradiated with visible light
of carbon at 284.6 eV. Thermogravimetric analysis (TGA) was (300 W Xe lamp equipped with a 420 nm cut-off filter). During
carried out on a TG209F1 thermal analyzer with a heating rate the photocatalytic reaction, 3 mL of the sample solution was
of 10 ℃ min-1 under N2. Brunauer-Emmett-Teller (BET) specific withdrawn at every 30 mins interval and centrifuged to remove
surface areas of the samples were measured on an ASAP 2460 the catalyst. The residual RhB solution was analyzed by
instrument. Photoluminescence (PL) spectra were acquired on a monitoring the variation of its characteristic light absorption at
Varian Cary Eclipse spectrometer with an excitation wavelength 554 nm using a UV-Vis spectrophotometer (Thermal Fisher,
of 350 nm.
GENESYS, 10S).
2.3. Photocatalytic reduction performances
3. Results and discussion
(a) Photocatalytic selective reduction. In
a typical
photoreduction reaction, a 300 W Xe lamp (PLS-SXE300D,
Beijing Perfect Light co. LTD, China) equipped with a 420 nm
cut-off filter (λ>420 nm) was used as the irradiation source. 10
mg of catalyst and 40 mg of ammonium formate (NH4HCO2)
were added into 30 mL of 4-NA aqueous solution (20 mg L-1) in
a glass reactor (80 mL). Prior to irradiation, the suspension was
magnetically stirred in the dark for 1 h to establish the
adsorption-desorption equilibrium between reactants and
catalyst. After the regular time interval of 1 min, 3 mL of the
solution was withdrawn, centrifuged to separate the catalyst, and
then analyzed on a UV-Vis spectrophotometer (Thermal Fisher,
GENESYS, 10S). The whole experiments were conducted under
N2 bubbling with a flow rate of 80 mL min-1.
(b) Photocatalytic hydrogen evolution. A 300 W Xe lamp
(PLS-SXE300D, Beijing Perfect Light co. LTD, China)
equipped with a 420 nm cut-off filter was used as the visible light
source to trigger the photocatalytic reaction. Typically, 10 mg of
catalyst was dispersed in 5 mL of aqueous solution containing
0.5 mL of lactic acid. The amount of H2 produced was
determined by gas chromatograph (Shimadzu GC-8A, MS-5A
column and argon was utilized as the carrier gas).
Scheme 1 vividly illustrates the flowchart for ligand-triggered
electrostatic self-assembly of Au-CdS NWs heterostructures at
ambient conditions. Specifically, tailor-made Au NPs were
uniformly and closely capped by hierarchically branched ligand
(Fig. S1) which endows Au@DMAP NPs with intrinsically
positively charged surface, whereas CdS@MAA NWs substrate
is featured by the adsorption of a large amount of MAA
molecules on the surface with plentiful deprotonated carboxyl
groups exposing on the outmost layer, thus affording negatively
charged surface. As such, oppositely charged Au@DMAP NPs
and CdS@MAA NWs render them suitable building blocks for
electrostatic self-assembly buildup. In this regard, attracted by
the substantial electrostatic attractive force, Au@DMAP NPs
can be spontaneously and uniformly deposited on the negatively
charged CdS@MAA NWs framework resulting in the well-
defined Au-CdS NWs heterostructures. The loading amount of
2.4. Photocatalytic oxidation performances
(a) Photocatalytic selective oxidation. Photocatalytic
selective oxidation of a series of aromatic alcohols was
performed as follows. Specifically, a mixture of alcohol (0.1
mmol) and 8 mg of catalyst was dissolved in benzotrifluoride
Scheme 1 Schematic illustration for electrostatic self-assembly of Au-CdS NWs
heterostructures.
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Au@DMAP NPs on the CdS NWs substrate can be readily tuned into positively charged (3-aminopropyl)trimethoxysilane
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DOI: 10.1039/C9TA07569K
by changing the Au@DMAP concentration. The accessibility of modified-CdS NWs (CdS@APTES NWs) aqueous dispersion.
this ligand-initiated self-assembly process can be validated by
Our research begins with the systematic characterizations of
the zeta potentials (ξ) of the assembly units. As shown in Fig. Au-CdS NWs heterostructures. Field-emission scanning electron
S2a, CdS@MAA NWs aqueous solution demonstrates microscopy (FESEM) and transmission electron microscopy
pronounced negatively charged surface over a large pH profile (TEM) are utilized to directly analyze the microscopic
(2-12), attributable to the deprotonation of carboxyl groups morphologies of the samples. As shown in Fig. 1a, blank CdS
anchored on the surface.35 Fig. S2b shows the Au@DMAP NPs NWs matrix is featured by the typical 1D morphology with
colloidal aqueous solution exhibits substantially positively smooth surface, which is favorable to provide excellent charge
charged surface within the same pH profile (2-12), owing to the separation/transfer efficiency in terms of the excellent structural
partial protonation of exocyclic nitrogen atoms in the DMAP merits of 1D semiconductors such as enhanced light
linker,34 which is in line with the phenomenon that Au@DMAP scattering/absorption and efficient vectorial charge transfer
NPs aqueous solution can be well stored for several months along the 1D framework.24 Fig. 1 (b & c) reveal that 1D
without agglomeration due to the strong electrostatic repulsion nanoarchitecture of CdS NWs matrix was retained when
among Au@DMAP NPs. Apparently, surface ligand plays a Au@DMAP NPs were evenly deposited on the CdS@MAA
crucial role in triggering the spontaneous and monodispersed NWs via electrostatic self-assembly. As marked by the red
attachment of Au@DMAP NPs on the CdS@MAA NWs circles, sporadic Au@DMAP NPs were uniformly distributed on
scaffold leading to Au-CdS NWs heterostructures. To the CdS@MAA NWs without agglomeration, indicating the
corroborate this speculation, negatively charged citrate- electrostatic self-assembly strategy developed in our work is
stabilized Au NPs (Au@citrate NPs) were utilized as the efficacious in fabricating Au-CdS NWs heterostructures.
building block for construction of Au-CdS NWs and the result Furthermore, TEM was further utilized to probe the microscopic
demonstrates that negatively charged Au@citrate NPs cannot be structures of the samples. As displayed in Fig. 1d, pristine CdS
deposited on the CdS@MAA NWs substrate by virtue of the NWs substrate is well-crystallized (inset image) and possesses
substantial electrostatic repulsion between them, as shown in uniform size and shape with a mean width of ca. 70 nm and
Fig. S3 and Fig. S4. Similar phenomenon was also observed length of several micrometer. HRTEM image in Fig. 1d (inset)
when positively charged Au@DMAP NPs were dropwise added unravels the adjacent lattice fringe of ca. 0.34 nm along the
Fig. 1 Panoramic FESEM image of (a) pristine CdS NWs with high-magnified image in the inset, (b) low and (c) high-magnified FESEM images of 1.0% Au-CdS NWs
heterostructure; TEM images of (d) pristine CdS NWs with corresponding HRTEM image in the inset, (e) TEM and (f) HRTEM images of 1.0% Au-CdS NWs heterostructure
with low-magnified image in the inset accompanied by the (g-j) elemental mapping results.
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growth direction and it corresponds well to the (002) 1049),38
ARTICLE
respectively.
Noteworthily,
Au-VCiedwSArticleNOnWlines
crystallographic plane of greenockite CdS.36 Fig. 1e displays the heterostructure with Au deposition percentage of 0.5%, 1.0%
low-magnified TEM image of self-assembled Au-CdS NWs and 2.0% exhibit similar diffraction peaks to blank CdS NWs
heterostructure, from which it can be clearly seen that and no diffraction peak corresponding to Au can be easily
Au@DMAP NPs with a mean diameter of ca. 8 nm were discerned, which is mainly attributed to the ultra-low loading
uniformly distributed on the CdS@MAA NWs substrate without amount of Au@DMAP NPs compared with the predominant
agglomeration, as marked by the small red circles, consistent CdS@MAA NWs matrix. Nevertheless, a new peak at 38.2° was
with the FESEM results. Noteworthily, particle size of observed in the XRD pattern of 2.0% Au-CdS NWs (inset, Fig.
Au@DMAP NPs in the nanocomposite is consistent with that of 2a) and it matches well with the (111) crystal plane of cubic Au
pure Au@DMAP NPs (Fig. S1b and Fig. S5), indicating the size phase (JCPDS card No. 65-2870),39 which once again
of Au@DMAP NPs was not changed during the self-assembly corroborates electrostatic attachment of Au@DMAP NPs on the
process. This speculation can be confirmed by the HRTEM CdS NWs, consistent with the FESEM and TEM results. Raman
image in Fig. 1f, wherein lattice fringe of 0.235 nm spectra were utilized to further analyze the structure of these
corresponding to the (111) crystal plane of Au is apparently samples. As unveiled in Fig. S7, two distinct vibration peaks
observed at the interface in addition to the lattice spacing of CdS located at 306 and 610 cm-1 were apparently observed in the
(0.34 nm).37 Consistently, as reflected by Fig. 1 (g-j), element Raman spectra of blank CdS NWs and Au-CdS NWs and they
mapping results of Au-CdS NWs heterostructure in the dark field are assigned to the 1 LO (longitudinal optical) and 2 LO phonon
reflect the uniform distribution of Cd, S, and Au signals and the modes of hexagonal CdS.40 Note that no peak shift was observed
results together with the energy dispersive spectroscopy (EDS) in the Raman result of Au-CdS NWs relative to CdS NWs and
results (Fig. S6) concurrently corroborate the monodispersed this indicates structure integrity of CdS NWs framework was
dispersion of Au@DMAP NPs on the CdS@MAA NWs. retained.
DOI: 10.1039/C9TA07569K
Considering the interfacial interaction exerts profound influence
Fourier transform infrared spectroscopy (FTIR) was harnessed
on the charge separation/transfer efficiency of metal- to uncover the structural features of Au-CdS NWs
semiconductor heterostructures, it can be expected that such heterostructure. As displayed in Fig. 2b, two broad peaks located
intimate interfacial contact in Au-CdS NWs heterostructure at 3440 cm-1 and 1631 cm-1 in the FTIR spectrum of pristine CdS
would considerably benefit the directional charge transfer NWs are assignable to the stretching and bending vibration
process.
modes of O-H group,41 respectively. Notably, no new peak was
X-ray diffraction (XRD) patterns of pristine CdS NWs and observed in the FTIR spectrum of CdS@MAA NWs with respect
Au-CdS NWs heterostructures with different weight loading to blank CdS NWs and this is primarily due to the low loading
percentage of Au@DMAP NPs are displayed in Fig. 2a, from amount MAA molecules on the CdS NWs surface. Nevertheless,
which it is clearly seen that the peaks at 24.8, 26.6, 28.2, 36.6, it should be stressed that peak intensity at 1631 cm-1 for blank
43.7, 47.9, 50.9, 51.8, 52.8, 66.8, 69.2, 70.9, 72.3 and 75.4° in CdS NWs increases to some extent after MAA modification,
the XRD patterns of all these samples are accurately indexed to which results from the contribution of C=O stretching vibration
the (100), (002), (101), (102), (110), (103), (200), (112), (201), mode (-COOH) from MAA molecules, thus substantiating the
(203), (210), (211), (114) and (115) crystal planes of greenockite adsorption of MAA molecules on the CdS NWs before self-
CdS (a=4.132 Å, b=4.132 Å, c=6.734 Å, JCPDS card No. 41- assembly with Au@DMAP NPs. Note that two pronounced
peaks at 2923 and 2852 cm-1 were obviously seen in the FTIR
spectrum of Au-CdS NWs and they are collectively assigned to
the stretching vibration mode of -CH2 groups from DMAP
ligands capped on the Au NPs surface. In addition, another peak
at 1465 cm-1 belonging to the stretching vibration mode of C=N
was also observed in the FTIR spectrum of Au-CdS NWs and
this agrees well with the FTIR spectrum of pure Au@DMAP
NPs,42 thus strongly verifying successful deposition of
Au@DMAP NPs on the CdS@MAA NWs assisted by the
surface ligand (DMAP).
Optical properties of the samples were probed by UV-vis
diffuse reflectance spectroscopy (DRS). As shown in Fig. 2c, all
the samples demonstrate substantial light absorption in the
wavelength region below ca. 500 nm and this is stemmed from
the inherent band-gap-photoexcitation of CdS NWs matrix.
Notably, Au-CdS NWs heterostructures exhibit a slight red shift
in the absorption band edge together with increased absorption
Fig. 2 (a) XRD patterns of (I) blank CdS NWs, (II) 0.5% Au-CdS NWs, (III) 1.0% Au-
intensity in the visible region ranging from 500-800 nm as
compared with pristine CdS NWs. This is mainly due to the
background absorption of Au@DMAP NPs in the visible region.
Note that no obvious SPR peak can be discerned in the DRS
CdS NWs and (IV) 2.0% Au-CdS NWs. (b) FTIR spectra of pristine CdS NWs,
CdS@MAA NWs, Au@DMAP NPs and 1.0% Au-CdS NWs, and (c) DRS spectra of
blank CdS NWs and Au-CdS NWs with corresponding (d) transformed plots
calculated based on the Kubelka-Munk function vs. the energy of light for
bandgap determination.
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results of Au-CdS NWs heterostructures, which may be ascribed Au@DMAP NPs and CdS@MAA NWs once they were
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to the relative low loading percentage of Au@DMAP NPs.21 The intimately integrated through electrostatDicOI:se10lf.1-0a3s9s/eCm9TbAly0.75F69oKr
transformed plots obtained based on the Kubelka-Munk function comparison, detailed chemical bond species vs. B.E. for blank
vs. the photon energy are provided in Fig. 2d, by which bandgap CdS NWs and Au-CdS NWs heterostructure were summarized
values of blank CdS NWs and 1.0% Au-CdS NWs were roughly in Tables S1.
determined to be 2.395 and 2.405 eV, respectively. The result
Photoactivities of Au-CdS NWs heterostructures were initially
implies that electrostatic self-assembly of Au@DMAP NPs on evaluated by photoreduction of 4-nitroaniline (4-NA) to 4-
the CdS@MAA NWs did not exert influence on the light phenylenediamine (4-PDA) in an aqueous phase under visible
absorption band edge of Au-CdS NWs heterostructure, but light irradiation (λ>420 nm) with the addition of ammonium
enhance the visible light absorption.
formate as quencher for photogenerated holes and N2 purge
X-ray photoelectron spectroscopy (XPS) was employed to under ambient conditions. Blank experiments (without light or
unveil the surface chemical composition and elemental chemical catalyst) under the identical experimental conditions
states of the samples. As unlocked in Fig. 3a, survey spectrum demonstrate negligible photoactivities (Fig. S8), suggesting the
of Au-CdS NWs heterostructure reflects the co-existence of C, reaction was truly driven by a photocatalytic process. Fig. 4a
O, Cd, S and Au elements arising from CdS NWs substrate and shows the photoactivities of pristine CdS NWs and Au-CdS
Au@DMAP NPs. Fig. 3b shows the high-resolution Au 4f NWs heterostructures with different weight addition percentage
spectrum of Au-CdS NWs, for which two characteristic peaks at of Au@DMAP NPs (0.5~2%), from which it is clear to see that
binding energy (B.E.) of 83.98 and 87.33 eV corresponding to Au-CdS NWs heterostructures always exhibit remarkably
Au 4f7/2 and Au 4f5/2 point to the metallic Au specie (i.e., Au0).43 enhanced photoactivities compared with blank CdS NWs under
As unveiled by the high-resolution Cd 3d spectrum of Au-CdS the same experimental conditions, implying Au@DMAP NP
NWs (Fig. 3cII), two pronounced peaks at 404.9 and 411.7 eV plays an indispensable role in boosting the photoreduction
corresponding to Cd 3d5/2 and Cd 3d3/2 signify Cd is in the +2 performances of Au-CdS NWs heterostructures for efficiently
oxidation state and this is in line with the high-resolution Cd 3d capturing electrons photoexcited from CdS NWs. Photoactivities
spectrum of blank CdS NWs (Fig. 3cI).44 Consistently, as shown of the samples toward 4-NA reduction follow the order of 1.0%
in Fig. 3d, high-resolution S 2p spectrum of Au-CdS NWs with Au-CdS
NWs>2.0%
Au-CdS
NWs>0.5%
Au-CdS
two featured peaks at 161.3 eV (S 2p3/2) and 162.6 eV (S 2p1/2 NWs>pristine CdS NWs, by which the optimal loading
)
evidence the S element is in -2 valance state.45 Based on the percentage of Au@DMAP NPs was determined to be 1.0%. As
above analysis, it can be deduced that elemental chemical states displayed in Fig. S9, conversion of 4-NA to 4-PDA can be
of CdS@MAA NWs substrate were not altered with the monitored by UV-visible absorption spectrum, wherein only one
deposition of Au@DMAP NPs via electrostatic self-assembly. absorption peak at 380 nm corresponding to 4-NA was observed
Nonetheless, it is worth noting that substantial B.E. redshift was prior to light irradiation and two new peaks at ca. 300 and 240
observed in the high-resolution Cd 3d spectrum of Au-CdS NWs nm gradually arose with the progress of reaction, which are
as compared with pristine CdS NWs, i.e., 0.7 and 0.8 eV for Cd attributed to the formation of 4-PDA, indicating successful
3d5/2 and Cd 3d3/2, which might be caused by the electronic reduction of 4-NA to 4-PDA.48,49 Based on the first-order-kinetic
interaction-induced charge transfer from CdS NWs to model, reaction rate constants of different samples (Fig. 4b)
Au@DMAP NPs.46,47 Similar B.E. redshift was also seen in the were calculated to be 0.034, 0.177, 0.399 and 0.243 min-1
high-resolution S 2p spectrum of Au-CdS NWs relative to blank corresponding to blank CdS NWs, 0.5% Au-CdS NWs, 1.0%
CdS NWs, i.e., 0.7 and 0.9 eV for S 2p3/2 and S 2p1/2, once again Au-CdS NWs and 2.0% Au-CdS NWs, respectively (Table S2).
substantiating the pronounced electronic interaction between Obviously, 1.0% Au-CdS NWs demonstrates the largest reaction
rate constant as compared to other counterparts, hence
highlighting the loading amount of Au@DMAP NPs exerts
profound influence on the photoactivities of Au-CdS NWs
heterostructure. Note that 2.0% Au-CdS NWs exhibits decreased
photoactivity compared with the optimal 1.0% Au-CdS NWs,
which is mainly ascribed to the excess Au@DMAP NPs
deposition that causes the agglomeration of Au@DMAP NPs
(Fig. S10), thereby leading to reduced number of active sites on
the Au surface and reduced photoactivities.50 Moreover, as
revealed in Fig. 4c, cyclic experiments imply that 1.0% Au-CdS
NWs demonstrates good photostability with negligible decay in
photoactivity even for successive five cyclic reactions under
visible light irradiation. Consistently, as reflected by the XPS
results in Fig. S11, elemental chemical states of 1.0% Au-CdS
NWs after cyclic reactions were not altered and this is in line
with its favorable photostability. It should be particularly noted
that such a similar photoactivity enhancement was also observed
in other 1.0% metal NCs (Au, Pd, Ag, Pt)/TMC (CdS,
Fig. 3 (a) Survey spectra and high-resolution XPS spectra of (b) Au 4f, (c) Cd 3d, (d)
S 2p for (I) pristine CdS NWs and (II) 1.0% Au-CdS NWs heterostructure.
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DOI: 10.1039/C9TA07569K
Fig. 4 (a) Photoactivities of blank CdS NWs and Au-CdS NWs heterostructures with different loading percentage (0.5, 1.0 and 2.0%) of Au@DMAP NPs toward selective reduction of
4-NA under visible light irradiation (λ>420 nm) with the addition of ammonium formate as hole scavenger and N2 bubbling at ambient conditions together with the corresponding
(b) kinetic curves; (c) cyclic photoreduction performances of 1.0% Au-CdS NWs heterostructure, (d) control experiments with and without adding K2S2O8 as electron scavenger and
photoreduction reaction performed in an O2 atmosphere over 1.0% Au-CdS NWs, (e) photoreduction performances of 1.0% Au-CdS NWs calcined at different temperature in N2
atmosphere, (f) photoreduction performance of 1.0% Au-CdS NWs heterostructure by replacing MAA with PSS, and (g-i) corresponding electron transfer models of pristine 1.0%
Au-CdS NWs, calcined 1.0% Au-CdS NWs and 1.0% Au-CdS NWs (PSS).
Zn0.5Cd0.5S, CdIn2S4, ZnIn2S4) heterostructures toward S14), verifying the ligand-triggered electrostatic self-assembly
photoreduction of 4-NA under visible light irradiation (Fig. 5, method developed in our work is universal and can be utilized to
Fig. S12 and Fig. S13) and that the metal NCs/TMC fabricate a large variety of metal/TMC heterostructures.
heterostructures prepared by electrostatic self-assembly strategy
To deeply understand the decisive role of electrons in
show the optimal photoactivity in comparison with the triggering the photoreduction of aromatic nitro compounds over
counterparts prepared by traditional methods (Photoreduction Au-CdS NWs heterostructure, control experiment with the
method, NaBH4 reduction method, impregnation method) (Fig. addition of K2S2O8 as electron scavenger for quenching
photoelectrons was carried out. It can be clearly seen from Fig.
4d that photocatalytic reduction of 4-NA over 1.0% Au-CdS
NWs was remarkably retarded when K2S2O8 was added into the
reaction system. Consistently, photoactivity of 1.0% Au-CdS
NWs in the O2 atmosphere was considerably inferior to that in
N2 atmosphere with other experimental conditions unchanged,
which once again highlights the crucial role of photoelectrons in
driving the photoreduction of aromatic nitro compounds.
Based on the remarkably enhanced photoactivity of Au-CdS
NWs compared with pristine CdS NWs, it can be concluded that
hierarchically branched DMAP ligands capped on the Au NPs
surface did not influence the directional charge transfer from
CdS NWs to Au NPs (Fig. 4g). In this regard, more efficient
charge transfer can be expected if the interfacial distance
between Au NPs and CdS NWs building blocks can be finely
modulated. To validate this speculation, photoactivities of Au-
Fig.
5 Photoactivities of metal NCs (Au@DMAP, Pd@DMAP)/TMC (CdS,
CdS NWs calcined in N2 atmosphere at different temperature
was probed to clarify the correlation of molecular distance
Zn0.5Cd0.5S, CdIn2S4, ZnIn2S4) heterostructures toward photoreduction of 4-NA
under identical conditions.
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between DMAP and MAA with the interfacial charge transfer electron transfer rate across the contacting interface, ultimately
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efficiency. As displayed in Fig. 4e, photoactivity of calcined 1.0% leading to the significantly improveDdOI:p1h0o.1t0o3a9c/Cti9vTiAty0.756T9oK
Au-CdS NWs was gradually improved as calcination rationalize this speculation, MAA grafted on the CdS NWs
temperature increases, which is mainly ascribed to the gradual surface was replaced by the negatively charged linear
removal of DMAP ligands capped on the Au NPs surface upon polyelectrolyte of poly (sodium 4-styrenesulfonate) (PSS) with a
elevating the annealing temperature, thereby reducing the view to revealing the influence of interfacial molecular distance
interfacial contact distance, reinforcing the interfacial interaction on the photoactivity of Au-CdS NWs heterostructure.51 The
between Au NPs and CdS NWs, and boosting the photoactivity accessibility of this assembly buildup is validated by the FESEM
of heterostructure (Fig. 4h). Notably, as unveiled by the FESEM images and FTIR results, which confirm successful self-
images and FTIR spectra (Fig. S15), morphology of calcined 1.0% assembly of Au@DMAP NPs on the CdS@PSS framework via
Au-CdS NWs is analogous to pristine 1.0% Au-CdS NWs, electrostatic interaction (Fig. S16). As shown in Fig. 4f,
implying low-temperature calcination treatment is not photoactivity of 1.0% Au-CdS NWs (PSS) heterostructure
detrimental to the monodispersivity of Au NPs but rather remove wherein MAA was replaced with PSS was considerably reduced
the ligands (i.e., DMAP) to afford markedly reduced charge in comparison with the pristine Au-CdS NWs counterpart under
transfer distance and intimate interfacial integration, thus the same conditions. The inferior photoactivity of 1.0% Au-CdS
rendering more enhanced photoactivity compared with pristine NWs (PSS) is attributed to the steric effect caused by ultra-long
counterpart. Besides, it should be emphasized that the higher molecular chain of PSS which closely and tightly encapsulates
calcination temperature the more complete removal of ligands the whole CdS NWs framework, hence conspicuously increasing
capped on the Au NPs surface, which accelerates the directional the diffusion distance of electrons and retarding the smooth
Fig. 6 Photoactivities of blank CdS NWs, 1.0% Au-CdS NWs, calcined 1.0% Au-CdS NWs and 1.0% Au-CdS NWs (PSS) toward selective reduction of aromatic nitro
compounds including (a) 2-NA, (b) 3-NA, (c) 2-NP, (d) 3-NP, (e) 4-NP, (f) 1-chloro-4-nitrobenzene, (g) 1-bromo-4-nitrobenzene and (h) 4-NT with the addition of
ammonium formate as hole scavenger and N2 bubbling at ambient conditions under visible light irradiation (λ>420 nm), and (i) photocatalytic hydrogen evolution over
the same four samples under visible light irradiation (λ>420 nm). (j) Photoreduction reaction model under the current experimental conditions.
8 | J. Name., 2012, 00, 1-3
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electrons flow from CdS NWs to Au NPs (Fig. 4i). Consistently, peak at ca. 530 nm was observed in the action specVtireuwmArtoicfle1O.n0li%ne
inferior photoactivity was achieved when the interfacial Au-CdS NWs within the wavelength profDilOeIo: 1f05.100039-6/C090TAn0m75a6n9dK
molecular distance between Au NPs and CdS NWs was the result indicates that the SPR effect of Au@DMAP NPs did
increased by replacing the DMAP ligands capped on the Au NPs not contribute to the photoactivity enhancement of Au-CdS NWs
with sodium citrate and simultaneously, replacing MAA heterostructure. In addition, based on the nitrogen adsorption-
molecules capped on the CdS with MEA molecules (Fig. S17). desorption isotherms (Fig. S19), specific surface area and pore
These controlled experiments strongly corroborate the influence volume of 1.0% Au-CdS NWs were not substantially changed
of ligand-triggered interface configuration on the interfacial compared with blank CdS NWs (Table S3), implying analogous
charge transfer efficiency. That is, the shorter interfacial distance adsorption capability of 1.0% Au-CdS NWs to pristine CdS NWs
the higher charge transfer efficiency from CdS NWs to Au NPs. and the influence of specific surface areas on the photoactivity
Analogous results were also observed in selective can be ignored.
photoreduction of other aromatic nitro compounds including 2-
With a view to disclosing the origin accounting for the
nitroaniline (2-NA), 3-nitroaniline (3-NA), 2-nitrophenol (2-NP), significantly enhanced photocatalytic performances of Au-CdS
3-nitrophenol (3-NP), 4-nitrophenol (4-NP), 1-chloro-4- NWs heterostructures and simultaneously, to uncover the
nitrobenzene, 1-bromo-4-nitrobenzene, 4-nitrotoluene (4-NT) influence of interface configuration on the interfacial charge
and o-nitroacetophenone over the same samples, as shown in Fig. transfer efficiency during the photocatalytic reaction,
6(a-h) and Fig. S18. That is, calcined 1.0% Au-CdS NWs always photoelectrochemical (PEC) measurements were performed. It
exhibits the optimal photoreduction performances, followed by has been well-established that photocurrent originates from the
pristine 1.0% Au-CdS NWs, 1.0% Au-CdS NWs (PSS) and pure diffusion of photoelectrons to the back contact and, meanwhile,
CdS NWs under the same conditions. More intriguingly, similar photoinduced holes were consumed by the electron donor in the
trend was also seen in photocatalytic hydrogen generation over electrolyte.52,53 As displayed in Fig. 7b, 1.0% Au-CdS NWs
the same samples (Fig. 6i).
demonstrates the markedly enhanced photocurrent in
To determine whether the SPR effect of Au@DMAP NPs in comparison with blank CdS NWs under the same conditions,
heterostructure exerts influence on the photoreduction of strongly revealing more efficient separation and longer lifetime
aromatic nitro compounds, action spectrum of 1.0% Au-CdS of photogenerated electron-hole pairs over 1.0% Au-CdS NWs
NWs heterostructure under different monochromatic light wherein Au@DMAP NPs servs as electron reservoirs for
irradiation was probed. As exhibited in Fig. 7a, no alike SPR efficaciously trapping electrons from CdS NWs. It is worthwhile
that photocurrent of 1.0% Au-CdS NWs was improved to some
extent after calcination, which is ascribed to the reduced
interfacial charge transfer distance and more intimate interfacial
integration between CdS NWs and Au NPs upon removing the
surface ligand by annealing, thereby accelerating the interfacial
electron transfer rate from CdS NWs to Au NPs. Conversely,
inferior photocurrent was observed when the interfacial distance
between CdS NWs and Au@DMAP in the nanocomposite was
increased by replacing MAA with long-chain PSS, persuasively
suggesting interface configuration indeed plays a pivotal role in
tuning the interfacial electron transfer between metal NCs and
semiconductors. Electrochemical impedance spectroscopy (EIS),
as a powerful tool to study the charge transfer process on the
electrode and at the contact interface between electrode and
electrolyte, was utilized to probe the separation efficiency of
charge carrier.54,55 Normally, semicircle arc radius in the Nyquist
plots of photoelectrodes conveys enriched information on the
charge transfer resistance. It has been well-established that the
smaller semicircle arc radius the more efficient interfacial charge
transfer efficiency. Fig. 7c shows that calcined 1.0% Au-CdS
NWs exhibits the smallest semicircular arc radius among all
these samples under visible light irradiation followed by pristine
1.0% Au-CdS NWs, 1.0% Au-CdS NWs (PSS) and blank CdS
NWs. The result evidences the calcined 1.0% Au-CdS NWs
Fig. 7 (a) Photoreduction of 4-NA over 1.0% Au-CdS NWs under different
monochromatic light irradiation with other experimental conditions unchanged,
charge separation efficiency compared with other counterparts.
(b) on-off transient photocurrents, (c) Electrochemical impedance spectroscopy
exhibits the smallest charge transfer resistance and the optimal
Alternatively, note that only one arc or semicircle was observed
in the EIS Nyquist plots of all these samples, indicating surface
charge transfer process is the dominant step that dictates the
photocatalytic reaction.29 Furthermore, electron lifetimes of
Nyquist plots, (d) open circuit potential decay and (e) electron lifetimes of blank
CdS NWs, pristine 1.0% Au-CdS NWs, calcined 1.0% Au-CdS NWs and 1.0% Au-CdS
NWs (PSS) under visible light irradiation (λ>420 nm) in an aqueous Na2SO4
solution (0.5 M, pH=7). (f) PL spectra of different samples with an excitation
wavelength of 350 nm.
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different photoanodes were determined by monitoring the
variation of open-circuit photovoltage with time upon turning off
the light irradiation (Fig. 7d). It is worth noting that the electron
lifetime increases with increasing the photovoltage and calcined
1.0% Au-CdS NWs shows the longest electron lifetime (Fig. 7e),
which is in faithful agreement with the photocurrent, EIS
Nyquist plots and photoreduction performances.
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DOI: 10.1039/C9TA07569K
PL measurement was conducted to additionally explore the
charge separation and transport efficiency over different samples.
It has been ascertained that PL emission intensity is inversely
proportional to the charge separation efficiency of
photocatalyst.56 As mirrored in Fig. 7f, PL intensity of the
samples follow the order of blank CdS NWs>1.0% Au-CdS
NWs (PSS)>1.0% Au-CdS NWs>calcined 1.0% Au-CdS NWs,
which corresponds to the charge separation efficiency order of
calcined 1.0% Au-CdS NWs>1.0% Au-CdS NWs>1.0% Au-
CdS NWs (PSS)> blank CdS NWs. The result implies that
electrons photoexcited from CdS@MAA NWs can
spontaneously flow to Au@DMAP NPs which serve as electron
traps, hence prolonging the lifetime of charge carriers and
resulting in reduced PL intensity of Au-CdS NWs compared with
blank CdS NWs. When MAA and DMAP were removed from
the interfacial region of Au-CdS NWs heterostructure via
calcination, charge separation efficiency was boosted as a result
of reduced interfacial charge transfer distance, therefore leading
to additional decrease in the PL intensity of calcined 1.0% Au-
CdS NWs with respect to pristine 1.0% Au-CdS NWs and blank
CdS NWs. Contrarily, involvement of PSS in the interfacial
domain increases the charge transfer distance and lowers the
charge transfer efficiency, thereby giving rise to much stronger
PL intensity of 1.0% Au-CdS NWs (PSS) than pristine 1.0% Au-
CdS NWs. Consequently, interface configuration between metal
and semiconductor should be carefully and exquisitely designed
for developing high-efficiency photocatalysts.
Apart from the photoactivities toward reduction of aromatic
nitro compounds and photocatalytic hydrogen evolution,
photocatalytic selective oxidation of aromatic alcohols to
corresponding aldehydes was also carried out to gain more deep
understanding on the role of Au@DMAP NPs in modulating the
interfacial charge transfer. Blank experiments in the absence of
catalysts or light irradiation suggest no conversion of alcohols
(Fig. S20a), strongly confirming the reaction is driven by a
photocatalytic process. As reflected by Fig. 8, 1.0% Au-CdS
NWs exhibits substantially enhanced visible-light-driven
photoactivity toward selective oxidation of aromatic alcohols to
aldehydes with high conversion and selectivity (85-100%). In
contrast, blank CdS NWs shows very poor photoactivity in terms
of conversion and yield, which is reasonable considering the
ultra-fast charge recombination rate over blank CdS NWs.
Apparently, reducing the coverage of organic surfactants and
increasing the interfacial contact, i.e., enhancing exposure of the
surface area on Au NPs and reducing the interfacial charge
transfer distance between Au NPs and CdS NWs in
heterostructure by calcination are beneficial for enhancing the
conversion of alcohols and yield of aldehydes. Hence, once again,
calcined 1.0% Au-CdS NWs always demonstrates the optimal
photoactivity toward selective oxidation of aromatic alcohols,
Fig. 8 Photoactivities of (I) blank CdS NWs, (II) 1.0% Au-CdS NWs, (III) calcined 1.0% Au-
CdS NWs and (IV) 1.0% Au-CdS NWs (PSS) toward selective oxidation of aromatic
alcohols including (a) benzyl alcohol, (b) p-methylbenzyl alcohol, (c) p-methoxybenzyl
alcohol, (d) p-fluorobenzyl alcohol, (e) p-chlorobenzyl alcohol, (f) p-nitrobenzyl alcohol,
(g) cinnamyl alcohol, and (h) 3-methyl-2-buten-1-ol to corresponding aldehydes under
visible light irradiation (λ>420 nm) for 4 h. (i) Photocatalytic selective oxidation of benzyl
alcohol over 1.0% Au-CdS NWs for 4 h by adding BQ, TBA, TEOA and AgNO3 as scavengers
for quenching superoxide radical (O2•–), hydroxyl radical (OH•), hole (h+) and electron (e–
), respectively. (j) Photooxidation reaction model under the current experimental
conditions.
far surpassing pristine 1.0% Au-CdS NWs and blank CdS NWs
counterparts under the same conditions. As expected, decreased
photoactivity was observed once PSS was introduced in the
interfacial domain of 1.0% Au-CdS NWs heterostructure.
Besides, as displayed in Fig. S20b, SPR effect of Au@DMAP
NPs on the selective photocatalytic oxidation capability of 1.0%
Au-CdS NWs was ruled out by probing the photoactivity under
monochromatic light irradiation (550 nm). Interestingly,
photocatalytic selective oxidation of aromatic alcohols over
different samples are in line with photoreduction and PEC
performances. Consequently, the above analysis verifies the
same conclusion as drawn by photoreduction performances, that
is, DMAP ligand did not refrain the interfacial electron transfer
from CdS NWs to Au NPs and more importantly, interfacial
charge transfer between Au NPs and CdS NWs can be finely
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Scheme 2 Schematic illustration depicting the photoredox mechanism of Au-CdS NWs heterostructure.
tuned by interface configuration and the more intimate interfacial restrains the recombination and prolongs the lifetime of
contact the more efficient charge transfer efficiency.
photogenerated electron-hole pairs.57,58 That is, DMAP ligands
The dominant active species responsible for the photocatalytic and MAA molecules did not retard the directional electron
selective oxidation of aromatic alcohols to aldehydes over Au- transfer from CdS NWs to Au NPs. Regarding the photocatalytic
CdS NWs heterostructure were determined by quenching reduction of aromatic nitro compounds, the whole reaction was
experiments with adding different scavengers. Fig. 8i shows the performed under N2 bubbling along with the addition of
photoactivities of 1.0% Au-CdS NWs with the addition of ammonium formate as hole scavenger, electrons trapped by
benzoquinone (BQ), tert-butyl alcohol (TBA), triethanolamine Au@DMAP NPs enable rapid reduction of nitroaromatics, thus
(TEOA) and silver nitrate (AgNO3), which are used as the fulfilling the photoreduction process under visible light
scavengers for trapping superoxide radical (O2•–), hydroxyl irradiation. Besides, electrons trapped by Au@DMAP NPs can
radical (OH•), hole (h+) and electron (e–), respectively. It is also reduce water to produce hydrogen leading to efficient
clearly seen that photooxidation activities of 1.0% Au-CdS NWs photocatalytic water splitting.
are concurrently inhibited upon adding these four scavengers,
On the other hand, with regard to the photocatalytic selective
among which TBA causes only a slight change in the conversion, oxidation of aromatic alcohols to corresponding aldehydes, a
•–
indicating OH• radical exerts negligible influence on the large amount of O2 radicals and holes are produced in the
photooxidation performance and it is not the main active specie. reaction system and both of them are involved in the
However, photoactivities of 1.0% Au-CdS NWs decrease photocatalytic selective oxidation reaction. It should be stressed
dramatically with the addition of BQ, TEOA and AgNO3, which that O2•– radical is mainly stemmed from the reaction of electrons
substantiates O2•– radical, hole and electron are the predominant trapped by Au@DMAP NPs with the dissolved O2 and it serves
active species responsible for the photocatalytic selective as the predominant active specie to facilitate conversion of
•–
oxidation of alcohols. Noteworthily, O2 radical is mainly alcohols to aldehydes, hence fulfilling the direct oxidation of
originated from the trapping of photogenerated electron by aromatic alcohols to corresponding aldehydes. Simultaneously,
oxygen dissolved in the reaction system.48 On the other hand, hole can also directly oxidize the alcohols by virtue of its high
analogous results were also observed in photocatalytic oxidation capability. Similar photocatalytic mechanism was
mineralization of organic pollutant over the same samples (Fig. observed in photocatalytic mineralization of organic pollutant.
S21).
Notably, tunable charge transfer in the interfacial region can be
Based on the above systematic investigation, photocatalytic achieved by rationally designing the interfacial integration
mechanism of self-assembled Au-CdS NWs heterostructure was modes between metals and semiconductors through ligand-
tentatively proposed and correlation of interface configuration involved interface configuration engineering.
with the interfacial charge transfer characteristics was clarified.
As vividly depicted in Scheme 2, when CdS NWs was irradiated
by visible light, electrons are instantly bandgap-photoexcited
4. Conclusions
from the valence band (VB) to the conduction band (CB), thus
In summary, tunable charge transfer was achieved by rational
leaving holes in the VB and creating photoinduced electron-hole
interface configuration engineering based on a facile, green and
pairs. Due to the band bending in conjunction with the formation
general surface ligand-triggered electrostatic self-assembly
of Schottky junction in the interfacial region, electrons in the CB
strategy. For conceptual study, tailor-made intrinsically
of CdS NWs are able to transfer to the Au@DMAP NPs across
positively charged Au@DMAP NPs and negatively charged
the interfacial region consisting of spatially hierarchically
CdS@MAA NWs building blocks were selected as the
branched DMAP ligands and MAA molecules, which effectively
quintessential metal NCs and TMC ingredients for constructing
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10 K. Wu, H. Zhu, Z. Liu, W. Rodríguez-Córdoba and T. Lian, J.
well-defined Au-CdS NWs heterostructures. It was undoubtedly
unveiled that smooth, spontaneous, and directional electron
transfer in the interfacial domain from CdS@MAA NW to
Au@DMAP NPs was not refrained by the spatially
hierarchically branched ligands capped on the metal surface, thus
demonstrating versatile and markedly enhanced photoredox
catalytic performances under visible light irradiation compared
with blank counterparts toward anaerobic photoreduction of
aromatic nitro compounds, photocatalytic hydrogen generation,
photocatalytic selective oxidation of aromatic alcohols to
aldehydes, and photocatalytic mineralization of organic
pollutant, due predominantly to the salient role of Au@DMAP
NPs in efficiently capturing electrons from CdS@MAA NWs
without the inhibition of surface hierarchical DMAP ligand and
interfacial architecture. More intriguingly, we have also
demonstrated that interfacial electron transfer between
Au@DMAP and CdS NWs can be finely modulated via
controllably increasing or decreasing the interface contacting
distance via suitable interface configuration engineering. Based
on which, intrinsic correlation of interface configuration with the
interfacial charge transfer efficiency in multifarious photoredox
catalysis was unambiguously disclosed. Consequently, it can be
concluded that interfacial charge transfer between metal and
semiconductor can be finely tuned by judicious interface
configuration design and more importantly, ligands capped on
the metal NCs are not definitely affect the charge separation and
transport.
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DOI: 10.1039/C9TA07569K
Table of Contents Graphic
Interfacial charge transfer over metal/metal chalcogenide heterostructures was finely tuned by a
facile, green, scalable, simple yet efficient surface ligand-triggered self-assembly strategy for
multifarious photoredox catalysis.