Layer-by-Layer Self-Assembly of Metal/Metal Oxide Superstructures: Self-Etching Enables Boosted Photoredox Catalysis

Article abstract of DOI:10.1021/acs.inorgchem.0c00229

The capability of noble metal nanoparticles (NPs) as efficient charge transfer mediators to stimulate Schottky-junction-triggered charge flow in multifarious photocatalysis has garnered enormous attention in the past decade. Nevertheless, fine-tuning and controllable fabrication of a directional charge transport channel in metal/semiconductor heterostructures via suitable interface engineering is poorly investigated. Here, we report the progressive fabrication of a tailor-made directional charge transfer channel in Pt nanoparticles (NPs)-inlaid WO₃ (Pt-WO₃) nanocomposites via an efficient electrostatic layer-by-layer (LbL) self-assembly integrated with a thermal reduction treatment, by which oppositely charged metal precursor ions and polyelectrolyte building blocks were intimately and alternately assembled on the WO₃ nanorods (NRs) by substantial electrostatic interaction. LbL self-assembly buildup and in situ self-etching-induced structural variation of WO₃ NRs to a microsized superstructure occur simultaneously. We found that such exquisitely crafted Pt-WO₃ nanocomposites exhibit conspicuously enhanced and versatile photoactivities for nonselective mineralizing of organic dye pollution and reduction of heavy metal ions at ambient conditions under both visible and simulated sunlight irradiation, demonstrating a synergistic effect. This is attributed to the imperative contribution of Pt NPs as electron traps to accelerate the directional high-efficiency electron transport from WO₃ to Pt NPs, surpassing the confinement of electron transfer kinetics of WO₃ owing to low conduction level. More intriguingly, photoredox catalysis can also be triggered simultaneously in the same reaction system. The primary in situ produced active species in the photocatalytic reactions were specifically analyzed, and underlying photocatalytic mechanisms were determined. Our work would provide a universal synthesis strategy for constructing various metal-decorated semiconductor nanocomposites for widespread photocatalytic utilizations.

Full text of DOI:10.1021/acs.inorgchem.0c00229

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Article
Layer-by-Layer Self-Assembly of Metal/Metal Oxide Superstructures:
Self-Etching Enables Boosted Photoredox Catalysis
Hua-Jian Lin, Shuai Xu, Xiao-Yan Fu, Zhi-Quan Wei, Ming-Hui Huang, Xin Lin, Yunhui He,
Guangcan Xiao,* and Fang-Xing Xiao*
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ABSTRACT: The capability of noble metal nanoparticles (NPs)
as efficient charge transfer mediators to stimulate Schottky-
junction-triggered charge flow in multifarious photocatalysis has
garnered enormous attention in the past decade. Nevertheless, fine-
tuning and controllable fabrication of a directional charge transport
channel in metal/semiconductor heterostructures via suitable
interface engineering is poorly investigated. Here, we report the
progressive fabrication of a tailor-made directional charge transfer
channel in Pt nanoparticles (NPs)-inlaid WO3 (Pt-WO3) nano-
composites via an efficient electrostatic layer-by-layer (LbL) self-
assembly integrated with a thermal reduction treatment, by which
oppositely charged metal precursor ions and polyelectrolyte
building blocks were intimately and alternately assembled on the
WO3 nanorods (NRs) by substantial electrostatic interaction. LbL
self-assembly buildup and in situ self-etching-induced structural variation of WO3 NRs to a microsized superstructure occur
simultaneously. We found that such exquisitely crafted Pt-WO3 nanocomposites exhibit conspicuously enhanced and versatile
photoactivities for nonselective mineralizing of organic dye pollution and reduction of heavy metal ions at ambient conditions under
both visible and simulated sunlight irradiation, demonstrating a synergistic effect. This is attributed to the imperative contribution of
Pt NPs as electron traps to accelerate the directional high-efficiency electron transport from WO3 to Pt NPs, surpassing the
confinement of electron transfer kinetics of WO3 owing to low conduction level. More intriguingly, photoredox catalysis can also be
triggered simultaneously in the same reaction system. The primary in situ produced active species in the photocatalytic reactions
were specifically analyzed, and underlying photocatalytic mechanisms were determined. Our work would provide a universal
synthesis strategy for constructing various metal-decorated semiconductor nanocomposites for widespread photocatalytic
utilizations.
reduction catalysis inaccessible considering its low reduction
potential of electrons in the conduction band (CB).20−25
Construction of metal-adorned semiconductors is regarded
as an efficacious route to stimulate the directional electron
transfer from semiconductor to metal nanocrystals (NCs)
which is accompanied by the Schottky-junction-induced charge
flow, boosting the charge transport kinetics and retarding the
charge recombination.26−29 Inspired by this, the rational
combination of metal NCs with 1D WO3 in an appropriate
fashion for fabricating metal/1D WO3 composite photo-
catalytic systems would afford a feasible avenue for the fine-
1. INTRODUCTION
Photocatalysis represents a promising and green technique to
achieve solar energy conversion including photocatalytic H2
and O2 production,1−9 photoreduction of CO2 to value-added
fuels,10 and mineralization of organic contaminations.11 With
regard to the various nanostructured semiconductors utilized
for photocatalysis, one-dimensional (1D) semiconductors
demonstrate incomparable structural merits compared with
nanoparticulate and bulk counterparts including excellent light
absorption and propagation in conjunction with enhanced
charge separation/transfer efficiency along the 1D frame-
work.12−15 Tungsten oxide (WO3) has been deemed as an
attractive light-harvesting antenna owing to a favorable band
gap (2.4−2.8 eV), good physicochemical stability, and high
oxidation potential.16−19 Nevertheless, detrimental defects on
the WO3 surface result in a high charge recombination rate and
sluggish electron transfer kinetics which makes the photo-
Received: January 22, 2020
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tuning of electron transport over WO3. It has been well-
established that interfacial charge transfer efficiency is closely
associated with the integration mode between metal NCs and a
semiconductor, rendering the pursuit of suitable and feasible
synthesis strategies highly desirable. Conventional approaches
for the construction of metal/WO3 heterostructures are
predominantly centered on hydrothermal, photodeposition,
and dipping-calcination approaches.26 However, most of these
methods involve the preparation of nanocomposites in a slurry
ensemble without exquisite interface modulation and structure
engineering, which hurdles the preparation of metal/WO3
heterostructures with a well-defined interface, failing to boost
the photoactivities.
2. EXPERIMENTAL SECTION
2.1. Preparation. 2.1.1. Preparation of WO3 NRs. WO3 NRs
were prepared by a hydrothermal method.42 Detailed information is
provided in the Supporting Information.
2.1.2. LbL Assembly of Pt-WO3 Nanocomposites. PDDA (0.5 M
NaCl, pH = 5.5) and poly(styrenesulfonate) (PSS, 0.5 M NaCl, pH =
3.5) were utilized as the assembly units for LbL assembly. Specifically,
100 mg of WO3 NRs was dispersed in aqueous PDDA solution,
stirred for 10 min, and washed with deionized (DI) H2O. PDDA-
modified WO3 NRs were then distributed into aqueous PSS solution,
stirred, and washed with DI H2O, which leads to WO3 NRs@
PDDA@PSS. Another PDDA layer was deposited on the WO3 NRs@
PDDA@PSS by the same method, which produces the WO3 NRs@
PDDA@PSS@PDDA. Subsequently, 10 mg of WO3 NRs@PDDA@
PSS@PDDA was redispersed into aqueous K2PtCl4 (1 mg/mL)
solution, stirred for 2 h, and washed with DI H2O. The above process
was defined as one assembly bilayer, i.e., WO3 NRs@PDDA@
PSS@(PDDA-PtCl42−)1. The following procedures were repeated by
Layer-by-layer (LbL) self-assembly has been made evident
as a versatile platform to design various multilayered
nanostructures by rationally selecting the suitable building
blocks such as polyelectrolyte (PE) and colloidal nanoparticles,
rendering harmony integration of diverse assembly units in an
integrated ensemble accessible.19,30−37 LbL self-assembly is
driven by varying molecular interaction such as electrostatic
force, hydrogen bonding, covalent bonding, and complemen-
tary base pairing which serves as the predominant impetus.38
Among which, of particular note is the electrostatic LbL
assembly which is dominated by the surface charge properties
of assembly units.39−41 The ionic metal precursors (e.g.,
2−
alternate deposition of PDDA and PtCl4 ions on the WO3 NRs@
PDDA@PSS, engendering the WO3 NRs@PDDA@PSS@(PDDA-
PtCl42−)n which were finally annealed at 450 °C in air for 1 h to
obtain Pt-WO3 nanocomposites.
2.2. Characterization. Zeta potentials (ξ) were detected by
dynamic light scattering analysis (ZetasizerNano ZS-90). Morpholo-
gies were probed by field-emission scanning electron microscopy
(FESEM, Carl Zeiss). Transmission electron microscopy (TEM)
images were collected on a JEOL-2010 instrument. The crystal
structure was determined by X-ray diffraction (XRD). Fourier
transform infrared (FTIR) spectra were collected on an infrared
spectrometer (Tianjin, China). UV−vis diffuse reflectance spectra
(DRS) (Varian) were obtained using BaSO4 as the reflectance
background. Specific surface areas were probed on an automated gas
sorption analyzer. Raman spectra were detected on a Raman
spectroscopy (Dxr-2xi) instrument. X-ray photoelectron spectroscopy
(XPS) spectra were recorded on a photoelectron spectrometer
(ESCALAB 245). Photoluminescence (PL) spectra were tested on a
Varian spectrometer. Transient PL decay curves were obtained by
time-correlated single photocounting technique (Edinburgh Analyt-
ical Instruments F900). Photoelectrochemical (PEC) measurements
were performed utilizing a conventional three-electrode configuration,
and primary active species produced in the photocatalytic reactions
were probed. The detailed information is provided in the Supporting
Information.
2.3. Photoactivity Measurements. 2.3.1. Photooxidation
Performance. Catalyst (20 mg) was added into Rhodamine B
(RhB) aqueous solution (40 mL, 5 ppm). Before photocatalytic
reaction, the mixture was stirred in the dark for 1 h to reach the
equilibrium of adsorption−desorption at ambient conditions. After
that, a 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfect Light Co.,
Ltd.) with a UV-CUT filter (λ > 420 nm) was utilized as the light
source. A 2 mL portion of the solution was withdrawn from the
reaction system at every 0.5 h interval. After removing the catalyst,
residual solution was analyzed by a UV−vis absorption spectropho-
tometer (GENESYS, 10S).
2−
AuCl4−1, PtCl42−, and PdCl4 etc.) can be harnessed as the
building block for electrostatic LbL self-assembly with the
oppositely charged PE on the semiconductor, by which metal
precursors and PEs can be intimately and alternately deposited
on the semiconductors. In this way, metal/semiconductor
heterostructures are readily attained by calcination to remove
the PE, and meanwhile, metal precursors are thermally reduced
to metal NCs.
Herein, potassium tetrachloroplatinate (PtCl42−) ions and
poly(diallyldimethylammonium chloride) (PDDA) with oppo-
sitely charged properties were selected as the building blocks
for constructing Pt-WO3 nanocomposites by an electrostatic
LbL self-assembly. Alternate attachment of PtCl42− ions and an
ultrathin PDDA layer on the WO3 nanorods (NRs) via
electrostatic interaction followed by postannealing treatment
affords monodisperse deposition of Pt nanoparticles (NPs) on
the WO3 substrate. Simultaneously, in situ structural variation
of WO3 NRs to a spherical microsized nanostructure as a result
of in situ self-etching occurs during the progressive LbL
assembly, resulting in the hierarchical Pt-WO3 nanocompo-
sites. The results suggest that LbL-assembled Pt-WO3
nanocomposites demonstrate considerably boosted photo-
activities toward oxidation of organic dye pollution and
reduction of heavy metal ions under the irradiation of visible
and simulated sunlight, superior to pristine WO3 NRs. The Pt
NPs in Pt-WO3 nanocomposites serve as highly efficient
electron traps to accelerate the directional electron flow from
WO3 to Pt NPs and promote the charge separation over WO3
NRs. Alternatively, primary active species in the photo-
reactions were specifically analyzed, and the photoredox
catalysis mechanism along with the in situ etching mechanism
were elucidated. It is hoped that our work would offer an
effective and general approach to fabricate diverse metal/metal
oxide heterostructures at ambient conditions for solar energy
conversion.
2.3.2. Photoreduction Performance. Catalyst (20 mg) was
dispersed in Cr(VI) aqueous solution (K2Cr2O7, 40 mL, 5 ppm)
and stirred in the dark for 0.5 h to ensure the equilibrium of
adsorption−desorption between the catalysts and reactant. A 2 mL
portion of the solution was withdrawn at every 0.5 h interval and
analyzed by a UV−vis absorption spectrophotometer (GENESYS,
10S). Photocatalytic efficiency was evaluated by the formula below:32
conversion = (C0 − C)/C0 × 100%
(1)
where C0 and C are the original concentration and concentration of
reactant after reaction, respectively.
2.3.3. Simultaneous Photoredox Catalytic Performances. Simul-
taneous photocatalytic reduction of Cr (VI) (5 ppm) and
mineralization of RhB (5 ppm) were carried out under visible light
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Scheme 1. Schematic Flowchart for LbL Self-Assembly of Pt-WO3 Nanocomposites
Figure 1. (a) XRD results, (b) FTIR spectra, and (c) DRS results with (d) band gap determination for the WO3 NRs and (Pt)2-WO3
nanocomposite.
(λ > 420 nm) irradiation. The detailed analysis process is analogous
to the aforementioned elucidation.
WO3 NRs are negative, indicating that WO3 NRs are
intrinsically negatively charged in the aqueous phase. The
changes of WO3 NRs surface charge after PDDA and PSS
modifications are unveiled in Figure S2 which shows that
PDDA is responsible for the generation of a positively charged
surface, and PSS favors the formation of a negatively charged
surface (Table S1). Hence, PtCl42− ions as the precursor of Pt
NCs can be deposited on the WO3 NRs@PDDA@PSS@
PDDA via electrostatic interaction, and its amount is mediated
3. RESULTS AND DISCUSSION
Scheme 1 demonstrates the fabrication of Pt-WO3 nano-
composites via an electrostatic LbL assembly. First of all, an
ultrathin PDDA layer was deposited on the surface of WO3
NRs (WO3 NRs@PDDA) to provide a positively charged
surface considering that WO3 NRs are negatively charged.
Subsequently, an ultrathin PSS layer with a negatively charged
surface was deposited on the positively charged WO3 NRs@
PDDA, after which another PDDA layer was electrostatically
deposited to produce WO3 NRs@PDDA@PSS@PDDA. The
favorable surface charges of PEs and WO3 NRs are beneficial
for electrostatic LbL self-assembly. The variation of the WO3
NRs’ surface charge after PE modification can be monitored by
ζ potential results. As displayed in Figure S1, ζ potentials of
by the assembly number, which produces the multilayered
2−
WO3 NRs@PDDA@PSS@(PDDA/PtCl4
)
nanocompo-
n
sites. Eventually, Pt-WO3 nanocomposites were obtained by
2−
calcining WO3 NRs@PDDA@PSS@(PDDA/PtCl4
)
to
n
2−
enable the reduction of PtCl4 precursor to Pt NPs, and
meanwhile, PEs (PDDA and PSS) were removed to afford a
more intimate interface. Intriguingly, a spherical hierarchical
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WO3 superstructure was produced owing to the acidic etching
in the LbL self-assembly.
As displayed in Figure 1a, the (Pt)2-WO3 nanocomposite
exhibits a similar XRD result to blank WO3 NRs with all the
characteristic peaks corresponding to hexagonal WO3 (JCPDS
75-2187), respectively. No diffraction peak of Pt NPs is
discerned, which is due to the relatively low deposition
percentage of Pt NPs. As shown in Figure S3a, two obvious
peaks at 758 and 810 cm−1 in the Raman spectrum of blank
WO3 NRs are attributable to the vibration mode of WO
bonds [i.e., υ(OWO)];43,44 the peak at 927 cm−1
corresponds to the WO bonds,57 and the peak at 326
cm−1 arises from the bending mode of OWO bonds [i.e.,
δ(OWO)].25,55,57 A red-shift of the peak position was
visualized in the Raman spectrum of the (Pt)2-WO3 nano-
composite compared with WO3 NRs, implying that electron
transport occurs between WO3 and Pt NPs due to electronic
interaction (Figure S3b). Figure 1b shows an apparent peak at
825.2 cm−1 in the FTIR spectrum of the (Pt)2-WO3
nanocomposite, and it is assigned to the stretching vibration
mode of WOW bonds. Besides, another two peaks at
2922.7 and 2854 cm−1 (Figure 1b, inset) were also visualized,
and they stem from the stretching vibration mode of the 
CH2 group (Table S2) from the minute remaining PDDA on
the WO3 surface.
As displayed in Figure 1c, both WO3 NRs and the (Pt)2-
WO3 nanocomposite demonstrate almost the same absorption
band edge at ca. 455 nm, and it arises from the photoexcitation
of the WO3 substrate. Note that light absorption of the (Pt)2-
WO3 nanocomposite was enhanced compared with WO3 NRs,
suggesting that Pt NPs deposition reinforces the light
absorption of the (Pt)2-WO3 nanocomposite, consistent with
the sample color (Figure 1c, inset). Band gaps (Eg) of WO3
NRs and the (Pt)2-WO3 nanocomposite are identified by the
formula below:44
Figure 2. FESEM images of (a−c) blank WO3 NRs and (d−f) the
(Pt)2-WO3 nanocomposite with corresponding (g−i) elemental
mapping results. (j) HRTEM images of WO3 NRs. (k) TEM and
(l) HRTEM images of the (Pt)2-WO3 nanocomposite.
treated WO3 NRs was also probed, wherein a HCl solution
possessing the same pH as the K2PtCl4 precursor was utilized
to treat WO3 NRs under the same conditions. It is obvious that
morphologies of HCl-etched WO3 NRs without (Figure S4e)
and with (Figure S4f) calcination are similar to the (Pt)2-WO3
nanocomposite, substantiating that WO3 NRs are etched by
the acidic precursor solution in the LbL self-assembly process.
Consistently, as shown in Figure 2f−i, EDX and elemental
mapping results of the (Pt)2-WO3 nanocomposite uncover a
homogeneous distribution of W, O, and Pt elements, indicative
of the attachment of Pt NPs on the WO3 NRs. TEM images of
WO3 NRs in Figure S5a,b exhibit the 1D structure with
smooth surface, and the HRTEM image (Figure 2j) uncovers
the lattice fringe of ca. 0.159 nm corresponding to the (311)
crystal plane of WO3 NRs. TEM images of the (Pt)2-WO3
nanocomposite in Figure 2k and Figure S5c make evident that
Pt NPs are deposited on the WO3 NRs. Figure 2l and Figure
S5d show the HRTEM images of the (Pt)2-WO3 nano-
composite, wherein a lattice fringe of 0.139 nm is attributable
to the (220) crystal plane of Pt NPs.
αhγ = A(hγ − Eg)n/2
(2)
where α, hγ, and A are the absorption coefficient, light energy,
and constant, respectively. According to this formula, Eg values
of WO3 NRs and the (Pt)2-WO3 nanocomposite are estimated
as ca. 2.73 and 2.69 eV (Figure 1d), respectively. The result
suggests that in situ etching and Pt NP deposition fail to
change the Eg of WO3.
As shown in Figure 2a−c and Figure S4a, WO3 NRs
demonstrate a 1D nanostructure with an average size of ca. 100
nm and length of ca. 5 μm. Figure 2d reveals that the
morphology of the (Pt)2-WO3 nanocomposite is different from
WO3 NRs with a nanoarchitecture varying from NR to
microsized nanostructure comprising various small NPs
(Figure 2e) along with sporadic Pt NPs deposited on the
WO3 surface. A series of control experiments were performed
to unlock the reason for the structure change of WO3 NRs.
First, the morphology of WO3 NR annealed at the same
temperature was explored, and the result is exhibited in Figure
S4b, wherein WO3 NRs still exhibit a rodlike structure,
verifying that the structure change of the (Pt)2-WO3
nanocomposite did not originate from calcination. As unveiled
in Figure S4c,d, FESEM images of the (Pt)2-WO3 nano-
composite before calcination are different from WO3 NRs
without the appearance of NRs, corroborating that the
structure of the (Pt)2-WO3 nanocomposite has been altered
before calcination. Alternatively, the morphology of HCl-
Survey spectra of pristine WO3 NRs and the (Pt)2-WO3
nanocomposite (Figure 3a) reveal the presence of W and O
elements from the WO3 substrate, but no Pt signal was
visualized owing to its low loading amount. As mirrored in
Figure 3b, the high-resolution Pt 4f spectrum of the (Pt)2-WO3
nanocomposite demonstrates two peaks at 71.04 (Pt 4f7/2) and
73.15 (Pt 4f5/2) eV which point to the metallic Pt species (i.e.,
Pt0), verifying that the PtCl4 precursor has been reduced to
2−
Pt NPs after annealing.45 The high-resolution W 4f spectrum
of the (Pt)2-WO3 nanocomposite (Figure 3cI) exhibits two
peaks at 35.93 (W 4f7/2) and 38.08 (W 4f5/2) eV which
correspond to W6+, coincidental with the high-resolution W 4f
spectrum of WO3 NRs (Figure 3cII).46 Analogously, the high-
resolution O 1s spectrum (Figure 3dI) of the (Pt)2-WO3
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nanocomposite demonstrates two peaks at 530.48 and 532.38
eV which are attributable to lattice oxygen (W−O) and the
hydroxyl (−OH) group,47 which is in line with the high-
resolution O 1s spectrum of WO3 NRs (Figure 3dII).
Alternatively, high-resolution N 1s (Figure S6a) and Cl 2p
spectra (Figure S6b) of the (Pt)2-WO3 nanocomposite make
evident that most of the PEs were removed after annealing. For
reference, chemical bond species vs binding energy are
summarized in Table S3.
Photocatalytic performances of WO3 NRs and the Pt-WO3
nanocomposite were assessed by mineralization of Rhodamine
B (RhB) under the irradiation of visible (λ > 420 nm) and
simulated sunlight.48 Blank experiments (without light or
catalyst) demonstrate no photoactivity (Figure S7), confirming
that the reaction is indeed a photocatalytic process. Figure 4a
exhibits the visible-light-driven photocatalytic activities of WO3
NRs and (Pt)n-WO3 (n = 1, 2, 3, 4) nanocomposites, and the
result suggests that (Pt)n-WO3 (n = 1, 2, 3, 4) nanocomposites
exhibit substantially improved photoactivities with respect to
blank WO3 NRs with the involvement of Pt NPs, implying that
Pt NP can serve as electron traps for efficiently capturing
electrons photoexcited from WO3 NRs. Photoactivities of the
Figure 3. (a) Survey spectra and high-resolution (b) Pt 4f, (c) W 4f,
and (d) O 1s spectra of (I) the (Pt)2-WO3 nanocomposite and (II)
blank WO3 NRs.
Figure 4. (a) Photoactivities of blank WO3 NRs and (Pt)n-WO3 (n = 1, 2, 3, 4) nanocomposites toward RhB mineralization under the irradiation of
visible light (λ > 420 nm). (c) Photoactivities of blank WO3 NRs and the (Pt)2-WO3 nanocomposite under the irradiation of simulated sunlight
with kinetic curves in parts b and d, respectively. Cyclic photoreactions of the (Pt)2-WO3 nanocomposite under (e) visible (λ > 420 nm) and (f)
simulated sunlight irradiation. Photocatalytic reduction of Cr2O72− ions over blank WO3 NRs and the (Pt)2-WO3 nanocomposite under (g) visible
(λ > 420 nm) and (h) simulated solar light irradiation with kinetic curves in parts i and j, respectively.
E
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samples follow the order of (Pt)2-WO3 > (Pt)3-WO3 > (Pt)1-
WO3 > (Pt)4-WO3 > WO3 NRs, by which the optimal
assembly bilayer number is determined to be 2. As displayed in
Figure 4b, according to the first-order-kinetic model,42 kinetic
rate constants of the samples were determined as 0.0074,
0.020 25, 0.033, 0.0309, and 0.0173 min−1 (Figure S8)
corresponding to WO3 NRs, (Pt)1-WO3, (Pt)2-WO3, (Pt)3-
WO3, and (Pt)4-WO3, respectively. (Pt)n-WO3 demonstrates a
markedly enhanced reaction kinetic rate constant relative to
blank WO3 NRs, verifying that the Pt NP loading amount is
closely associated with the photoactivities of the (Pt)n-WO3
nanocomposite, among which the (Pt)2-WO3 nanocomposite
exhibits the optimal photoactivity. Besides, as mirrored in
Figure 4c, the (Pt)2-WO3 nanocomposite still exhibits a
considerably improved photoactivity and larger kinetic
constant (Figure 4d) in comparison with blank WO3 NRs
under the irradiation of simulated sunlight. The general
contributing role of Pt NPs as an electron trap was thus
ascertained regardless of utilizing visible or simulated sunlight
as the light source. Subsequently, photostability of the catalysts
was probed. Cyclic photoreactions indicate that the (Pt)2-WO3
nanocomposite demonstrates good photostability with no
obvious photoactivity decay under visible (λ > 420 nm)
(Figure 4e) and simulated sunlight (Figure 4f) irradiation.
Consistently, as reflected by the XPS results (Figure S9) of the
(Pt)2-WO3 nanocomposite after cyclic reaction, chemical
valence states of W, O, and Pt elements were not altered.
Alternatively, XRD patterns (Figure S10a) of the (Pt)2-WO3
nanocomposite before and after cyclic reaction also exhibit the
same crystalline structure. In addition, FTIR spectra of (Pt)2-
WO3 (Figure S10b) were not changed after cyclic reaction,
once again corroborating the good photostability of the (Pt)2-
WO3 nanocomposite.
Figure 5. Photocatalytic performances of blank WO3 NRs and the
(Pt)2-WO3 nanocomposite for simultaneous removal of (a) RhB and
(b) Cr (VI) ions under the irradiation of visible light (λ > 420 nm)
with kinetic rate constants in parts c and d, respectively.
blank WO3 NRs in simultaneous photocatalytic RhB
mineralization and Cr(VI) photoreduction under visible light
irradiation, highlighting the essential role of Pt NPs in boosting
the photoredox catalysis in an integrated reaction system.
Predominant active species generated in the photocatalytic
reactions were probed to reveal the underlying reaction
mechanism of the (Pt)2-WO3 nanocomposite. To this end, a
series of control experiments with adding potassium persulfate
(K2S2O8), benzoquinone (BQ), ammonium oxalate (AO), and
tert-butyl alcohol (TBA) for trapping electrons (e−), super-
Photoreduction of heavy metal ions (e.g., Cr2O72−) over
WO3 NRs and the (Pt)2-WO3 nanocomposite was also
performed. Blank experiments (Figure S11) suggest that it is
a light-driven photocatalytic process. As displayed in Figure
4g,h, the (Pt)2-WO3 nanocomposite exhibits significantly
improved photoreduction performances compared with blank
−
oxide (•O2 ) radicals, holes (h+), and hydroxyl (•OH) radicals
were carried out.23 As mirrored by Figure 6a, when AO and
BQ were added into the reaction system, visible-light-driven
photoactivity of the (Pt)2-WO3 nanocomposite was remarkably
•
−
reduced, implying that holes and O2 radicals are the main
active species. However, when TBA and K2S2O8 were added,
photoactivity of the (Pt)2-WO3 nanocomposite was not
2−
WO3 NRs toward Cr2O7 reduction under visible (λ > 420
nm) (Figure 4g) and simulated sunlight (Figure 4h)
irradiation. The results suggest that more electrons are
involved in the reaction as a result of more enhanced
electron−hole separation over the (Pt)2-WO3 nanocomposite.
This deduction can be verified by the kinetic rate constants of
WO3 NRs and the (Pt)2-WO3 nanocomposite (Figure 4i,j).
markedly influenced. Based on this, the contributing role of
the active species is •O2 > h+ > e−
>
•OH. Besides,
−
photoactivities of the (Pt)2-WO3 nanocomposite in the
presence of N2 and O2 purge were also probed. Figure 6b
shows that photoactivity of the (Pt)2-WO3 nanocomposite was
decreased in the N2-saturated reaction system but increased in
the O2-saturated reaction environment, verifying that O2 is in
favor of boosting the photoactivity of the (Pt)2-WO3
nanocomposite. This is reasonable since dissolved O2 is closely
related to the •O2− and •OH radical generation. As reflected by
Figure 6c,d, when these control experiments were carried out
under simulated solar light irradiation, analogous results were
With a view to highlighting the crucial role of electrons in the
2−
photoreduction of Cr2O7
over the (Pt)2-WO3 nano-
composite, a control experiment with adding K2S2O8 as the
electron scavenger was performed. Figure S12 implies that the
photoreduction performances of the (Pt)2-WO3 nanocompo-
site were substantially reduced under light (visible and
simulated sunlight) irradiation upon adding K2S2O8, verifying
that the electrons are the active species in the reaction.
•
observed. OH radical production in the reaction system can
also be probed by liquid-phase PL spectra.48 As shown in
Figure 6e,f, the peak intensity at 426 nm increases gradually
with prolonging the irradiation time, confirming the pro-
gressive generation of •OH radicals.42 Additionally, as revealed
in Figure 6g, the amount of OH• radicals produced over the
(Pt)2-WO3 nanocomposite under simulated solar light
irradiation is higher than that produced under visible light
irradiation because of larger light intensity. On the other hand,
H2O2 can be probed by a DPD-POD method.50 As shown in
Simultaneous photocatalytic removal of RhB and Cr(VI)
over the (Pt)2-WO3 nanocomposite under the irradiation of
visible light (λ > 420 nm) was performed.49 As reflected by
Figure 5a,b, the (Pt)2-WO3 nanocomposite shows enhanced
photoactivity with a higher removal efficiency of 99.1% for
RhB and reduction efficiency of 54.5% for Cr(VI) than those
of WO3 NRs, i.e., 94% for RhB and 38.4% for Cr (VI). As
displayed in Figure 5c,d, consistently, the (Pt)2-WO3 nano-
composite exhibits a larger kinetic rate constant relative to
F
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Figure 6. Photoactivities of the (Pt)2-WO3 nanocomposite with adding different scavengers under the irradiation of (a) visible (λ > 420 nm) and
(c) simulated sunlight. Photoactivities of the (Pt)2-WO3 nanocomposite with O2 and N2 purge under (b) visible (λ > 420 nm) and (d) simulated
solar light irradiation. Liquid-phase PL spectra of the (Pt)2-WO3 nanocomposite under (e) visible (λ > 420 nm) and (f) simulated solar light
irradiation using TA as a probe molecule. (g) PL peak intensity vs irradiation time for the (Pt)2-WO3 nanocomposite under (λ > 420 nm) visible
and simulated solar light irradiation. H2O2 production amount over blank WO3 NRs and the (Pt)2-WO3 nanocomposite under (h) visible (λ > 420
nm) and (i) simulated solar light irradiation based on a DPD-POD method.
Figure S13a, the peak intensity (512 and 552 nm) gradually
increases under visible light irradiation (λ > 420 nm) with
elongating the irradiation time, indicating the generation of
H2O2 in the reaction system. Similar results were observed for
the (Pt)2-WO3 nanocomposite under simulated solar light
irradiation (Figure S13b). It is noteworthy that the H2O2
amount produced over the (Pt)2-WO3 nanocomposite is
higher than that produced over blank WO3 NRs (Figure
S13c,d) under visible (λ > 420 nm) (Figure 6h) and simulated
sunlight irradiation (Figure 6i), confirming the important role
of Pt NPs in promoting the H2O2 production. This is
understandable considering the crucial role of Pt NPs as
electron reservoirs, which is beneficial for H2O2 production.
The PEC investigation was performed to evaluate the charge
separation efficiency.36 Figure 7a,b shows the photocurrents of
WO3 NRs and the (Pt)2-WO3 nanocomposite under both
visible (λ > 420) and simulated solar light irradiation, from
which it is obvious that the (Pt)2-WO3 nanocomposite
demonstrates a significantly enhanced photocurrent with
respect to blank WO3 NRs, implying a higher charge
separation efficiency over the (Pt)2-WO3 nanocomposite
than WO3 NRs. This is ascribed to the fact that electrons
produced in the conduction band (CB) of WO3 NRs can
smoothly transfer to the Pt NPs on account of the occurrence
of Schottky-junction-driven electron flow under light irradi-
ation, resulting in the enhanced charge separation. In this case,
Pt NPs act as efficient electron reservoirs to prolong the charge
lifetime.51 As displayed in Figure 7c,d, EIS Nyquist plots of the
(Pt)2-WO3 nanocomposite exhibit a smaller semicircle arc
radius than WO3 NRs under both visible (λ > 420 nm) and
simulated sunlight irradiation, corroborating that a combina-
tion of Pt NPs with WO3 NRs reduces the interfacial charge
transfer resistance.52,53 As reflected by the fitted EIS results in
Tables S4 and S5, the (Pt)2-WO3 nanocomposite exhibits a
much lower charge transfer resistance (Rct) under both visible
(λ > 420 nm) and simulated solar light irradiation relative to
WO3 NRs. Figure 7e shows the Mott−Schottky (M−S) plots
of the (Pt)2-WO3 nanocomposite and WO3 NRs, by which the
flat-band potentials (Ef) of blank WO3 NRs and the (Pt)2-WO3
nanocomposite are determined as 0.36 and 0.30 V vs NHE,
respectively. Apparently, Ef potential of WO3 was almost
unchanged, and this indicates that Pt NPs deposition does not
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Figure 7. (a, b) Photocurrent and (c, d) EIS Nyquist plots of blank WO3 NRs and the (Pt)2-WO3 nanocomposite under (a, c) visible (λ > 420 nm)
and (b, d) simulated sunlight irradiation, respectively. (e) M−S curves and (f) charge density (ND) of blank WO3 NRs and the (Pt)2-WO3
nanocomposite with a PEC cell diagram in part g. (h) Solid PL spectra (λex = 380 nm) and (i) time-resolved PL decay curves (λex = 375 nm) of
blank WO3 NRs and the (Pt)2-WO3 nanocomposite.
alter the energy level position of the WO3 substrate; this is in
faithful agreement with previous work.19 Besides, note that the
M−S plot slope of the (Pt)2-WO3 nanocomposite is smaller
than WO3 NRs, which indicates that the (Pt)2-WO3 nano-
composite possesses a larger charge carrier density than WO3
NRs. Charge density (ND) of photoelectrodes is calculated
based on the following formula:42
compared with the (Pt)2-WO3 nanocomposite, which suggests
that charge separation over (Pt)2-WO3 is more efficient than
that over blank WO3 NRs, wherein electrons transfer from
WO3 to Pt NPs leading to the prolonged charge lifetime. This
deduction can be confirmed by the TRPL results of WO3 NRs
and the (Pt)2-WO3 nanocomposite.54,55 As unveiled by Figure
7i, the lifetime of charge carriers produced over the (Pt)2-WO3
nanocomposite is longer than blank WO3 NRs, consistent with
the PL emission spectra (Figure 7h). The results suggest that
Pt NPs decoration can effectively inhibit the charge
recombination and elongate the lifetime, manifesting the
imperative role of Pt NPs as an efficient charge transport
medium for accelerating the unidirectional electron transfer
from WO3 NRs to Pt NPs.
Specific surface areas of WO3 NRs and the (Pt)2-WO3
nanocomposite were explored to evaluate whether they
contribute to the enhanced photoactivities. As displayed in
Figure S14, according to the IUPAC classification, WO3 NRs
and the (Pt)2-WO3 nanocomposite exhibit the type IV
isotherm which implies the formation of a mesoporous
solid.19 Specific surface area and pore volume of the (Pt)2-
WO3 nanocomposite are determined as 7.2 m2 g−1 and 0.012
cm3 g−1, which are comparable to WO3 NRs (i.e., 9.44 m2 g−1
Ä
É
−1
Å
Ñ
Å
Å
Å
Å
Å
Å
Å
Å
Å
1
C
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
d
i
y
z
z
z
z
z
2
( )
2
j
j
ND = j
j
j
εrε0e dUFL
Å
k
{
Å
Ñ
Ñ
Å
(3)
Å
Ç
Ñ
Ö
where e = 1.6 × 10−19 C, ε0 = 8.86 × 10−12 F m−1, ε = 300 for
WO3,48 and C is the capacitance. As displayed in Figure 7f,g,
ND values of WO3 NRs and the (Pt)2-WO3 nanocomposite are
1.88 × 1018 and 3.0 × 1018 cm−3, for which (Pt)2-WO3
demonstrates a larger ND than WO3 NRs, consistent with
the photoactivities.
PL and time-resolved PL (TRPL) spectroscopy were
harnessed to probe the charge separation efficiency over the
(Pt)2-WO3 nanocomposite and to further understand the
essential role of Pt NPs in facilitating electron transfer.54
Figure 7h shows that WO3 NRs possess a higher PL intensity
H
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Article
−
H2O2 + •O2 → •OH + OH− + O2
h+ + −OH/H2O → 2•OH + 2H+
Cr(VI) + e− → Cr(III)
and 0.012 cm3 g−1), as summarized in Table S6. The results
suggest that enhanced photoactivity of the (Pt)2-WO3
nanocomposite relative to WO3 NRs is attributed to the
improved charge separation as a result of the Pt NPs’
deposition.
(6)
(7)
(8)
Figure S15 depicts the energy level configuration of the Pt-
WO3 nanocomposite to elucidate the charge transfer
mechanism. CB and VB potentials of WO3 were determined
as 0.36 and 3.08 V vs NHE (Figure S15). The work function
(ΦA) of Pt is ca. 5.7 V (vs Vac) according to previous work,55
and the electron affinity (χ) of WO3 is determined by the
potential difference between Evac and ECB and is ca. 3.50 V (vs
Vac).42,48,56,57 Hence, when Pt NPs are decorated on the WO3,
electrons in the CB of WO3 can flow to Pt NPs, resulting in
negatively charged Pt NPs, while a positively charged space
layer is generated on the WO3, giving rise to a built-in electric
field accompanied by the band bending and Schottky junction
formation at the interface. Schottky barrier height (Φb)
between WO3 and Pt is calculated as 2.199 V (vs Vac)
according to the χ of WO3 and work function of Pt. Note that
the work function of WO3 is smaller than that of Pt (ΦA = 5.7
V) (Figure S16a). Therefore, the electron can spontaneously
transfer from WO3 to Pt NPs (Figure S16b).
•
−
Notably, O2 radicals are generated by reducing O2 with
electrons; OH radicals result from direct oxidation of H2O
with holes, and the H2O2 intermediate stems from reduction of
O2 with two electrons. Ultimately, these in situ formed active
species produced over Pt-WO3 nanocomposites synergistically
promote the mineralization of organic dye pollutant. With
regard to the photoreduction reaction, electrons trapped by Pt
•
2−
NPs are able to reduce Cr2O7 under light irradiation.
4. CONCLUSIONS
In summary, a directional electron transfer pathway was
constructed in the Pt-WO3 nanocomposite by an efficient
electrostatic LbL self-assembly approach, wherein negatively
charged noble metal precursor (PtCl42−) and positively
charged PDDA assembly units were alternately deposited on
the WO3 NRs surface. Etching of WO3 NRs occurs owing to
the use of the acidic metal ion precursor and then thermal
According to the above discussion, the photocatalytic
mechanism of Pt-WO3 nanocomposites was proposed
(Scheme 2). When the Pt-WO3 nanocomposite was irradiated
2−
reduction of the PtCl4 precursor to Pt NPs by annealing,
giving rise to the hierarchical Pt-WO3 superstructure with Pt
NPs evenly deposited on the surface. Significantly, (Pt)n-WO3
exhibit considerably enhanced photoredox performances
toward mineralization of organic dye pollutants and photo-
Scheme 2. Schematic Demonstration of the Photocatalytic
Mechanism of the Pt-WO3 Nanocomposite
2−
reduction of Cr2O7 with respect to WO3 NRs under both
visible and simulated solar light irradiation. This is attributed
to the imperative role of Pt NPs as electron traps to efficiently
capture the electrons for accelerating the electrons’ transport
from WO3 to Pt NPs, boosting the charge separation and
endowing the Pt-WO3 nanocomposite with considerably
improved photoactivities. In situ etching of WO3 NRs to a
hierarchical nanostructure afforded by LbL self-assembly
benefits light harvesting, which also favors the improved
photoactivities of the (Pt)n-WO3 nanocomposites. The main
active species in the photoredox catalysis were probed, and the
photocatalytic mechanism was clearly determined. Our work
would afford an efficacious and universal synthesis strategy for
the construction of diverse metal/semiconductor heterostruc-
tures for photocatalytic applications.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00229.
by visible or simulated sunlight, electrons (e−) were photo-
excited from the valence band (VB) of WO3 to the CB with
holes (h+) remaining in the VB. Immediately, electrons transfer
from the CB of WO3 to Pt NPs because of favorable energy
level alignment between the CB of WO3 and work function of
Pt, wherein Pt NPs act as electron reservoirs to trap electrons
and accelerate charge transfer, retarding the charge recombi-
nation. Subsequently, electrons captured by Pt NPs and holes
in the VB of WO3 participate in the following cascade reactions
Additional experimental details; ζ potentials; Raman
spectra; FESEM images; TEM and HRTEM images;
high-resolution N 1s and Cl 2p spectra; blank experi-
ments; kinetic rate constants; survey and high-resolution
spectra; XRD patterns and FTIR spectra; control
experiments; UV−vis absorption spectra; N2 adsorp-
tion−desorption isotherms; schematic illustration; M−S
result; DRS result; chemical structure and surface charge
properties; peak position with corresponding functional
groups; chemical bond species vs B.E. for different
samples; fitted EIS results; and a summary of specific
surface area, pore volume, and pore size (PDF)
•
(eqs 4−8) to produce OH radicals and H2O2.
WO3 + hν → e− + h+
(4)
−
−
•O2 + •O2 + H+ → H2O2 + O2
(5)
I
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AUTHOR INFORMATION
Corresponding Authors
■
Guangcan Xiao − Instrumental Measurement and Analysis
Center, Fuzhou University, Fuzhou 350002, People’s Republic
of China; Email: xgc@fzu.edu.cn
Fang-Xing Xiao − College of Materials Science and Engineering,
Fuzhou University, Minhou, Fujian Province 350108, People’s
Republic of China; orcid.org/0000-0001-5673-5362;
Email: fxxiao@fzu.edu.cn
Authors
Hua-Jian Lin − College of Materials Science and Engineering,
Fuzhou University, Minhou, Fujian Province 350108, People’s
Republic of China
Shuai Xu − College of Materials Science and Engineering, Fuzhou
University, Minhou, Fujian Province 350108, People’s Republic
of China
Xiao-Yan Fu − College of Materials Science and Engineering,
Fuzhou University, Minhou, Fujian Province 350108, People’s
Republic of China
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Xiao, F.-X. Ligand-Triggered Tunable Charge Transfer toward
Multifarious Photoreduction Catalysis. J. Phys. Chem. C 2019, 123,
4701−4714.
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Xiao, F.-X. Cascade Charge Transfer Mediated by in Situ Interface
Modulation toward Solar Hydrogen Production. J. Mater. Chem. A
2019, 7, 8938−8951.
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Guo, M.-L.; Liu, Z.-Q. Enhanced Photoelectrocatalytic Activity of
Direct Z-Scheme Porous Amorphous Carbon Nitride/Manganese
Dioxide Nanorod Arrays. J. Colloid Interface Sci. 2019, 557, 644−654.
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Liu, Z.-Q. Ultrathin CdS Shell-Sensitized Hollow S-Doped CeO2
Spheres for Efficient Visible-Light Photocatalysis. Catal. Sci. Technol.
2019, 9, 1357−1364.
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Hydrogenation of Nitrobenzene to Aniline over Tungsten Oxide-
Silver Nanowires. Mater. Lett. 2015, 142, 201−203.
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Light-Driven Pure Water Splitting by a Nature-Inspired Organic
Semiconductor-Based System. J. Am. Chem. Soc. 2014, 136, 12568−
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(18) Zhang, H.; Liu, G.; Shi, L.; Liu, H.; Wang, T.; Ye, J.
Engineering Coordination Polymers for Photocatalysis. Nano Energy
2016, 22, 149−168.
(19) Liang, Z.-Y.; Wei, J.-X.; Wang, X.; Yu, Y.; Xiao, F.-X. Elegant Z-
Scheme-Dictated g-C3N4 Enwrapped WO3 Superstructures: A
Multifarious Platform for Versatile Photoredox Catalysis. J. Mater.
Chem. A 2017, 5, 15601−15612.
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Li, Y. Hydrogen-Treated WO3 Nanoflakes Show Enhanced Photo-
stability. Energy Environ. Sci. 2012, 5, 6180−6187.
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L.; Chen, J.; Gao, Y.; Zhou, J.; Tong, Y.; Wang, Z. WO3−X@Au@
MnO2 Core−Shell Nanowires on Carbon Fabric for High-Perform-
ance Flexible Supercapacitors. Adv. Mater. 2012, 24, 938−944.
(22) Hong, S. J.; Lee, S.; Jang, J. S.; Lee, J. S. Heterojunction BiVO4/
WO3 Electrodes for Enhanced Photoactivity of Water Oxidation.
Energy Environ. Sci. 2011, 4, 1781−1787.
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Mechanisms of Photogenerated Electrons and Holes for Composite
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(24) Lu, N.; Zhang, Z.; Wang, Y.; Liu, B.; Guo, L.; Wang, L.; Huang,
J.; Liu, K.; Dong, B. Direct Evidence of Ir-Driven Hot Electron
Transfer in Metal-Free Plasmonic W18O49/Carbon Heterostructures
Zhi-Quan Wei − College of Materials Science and Engineering,
Fuzhou University, Minhou, Fujian Province 350108, People’s
Republic of China
Ming-Hui Huang − College of Materials Science and
Engineering, Fuzhou University, Minhou, Fujian Province
350108, People’s Republic of China
Xin Lin − College of Materials Science and Engineering, Fuzhou
University, Minhou, Fujian Province 350108, People’s Republic
of China
Yunhui He − Instrumental Measurement and Analysis Center,
Fuzhou University, Fuzhou 350002, People’s Republic of China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00229
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The support by the award Program for Minjiang scholar
professorship is greatly acknowledged. This work was
financially supported by the National Natural Science
Foundation of China (21703038).
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https://dx.doi.org/10.1021/acs.inorgchem.0c00229
Inorg. Chem. XXXX, XXX, XXX−XXX