Modulation of interfacial charge transfer by self-assembly of single-layer graphene enwrapped one-dimensional semiconductors toward photoredox catalysis

Article abstract of DOI:10.1039/c7ta08415c

In recent years, the exquisite modulation of the transport of photogenerated electron-hole charge carriers has constituted a long-standing challenge. To this end, herein, a spatially hierarchical single-layer graphene (GR)-wrapped and WO₃ nanorods (NRs)-coupled TiO₂ nanobelts (TNBs) ternary nano-architecture (TNBs/WO₃ NRs/GR), as a conceptual platform, has been progressively designed via a facile and green layer-by-layer assembly strategy based on pronounced electrostatic interaction. It was remarkable to find that the interfacial charge transfer of a TNBs/WO₃ NRs/GR ternary heterostructure can be finely modulated by interfacial architectural engineering, thus contributing to its significantly improved photoredox performance, including photocatalytic oxidation of organic pollutants and reduction of heavy metal ions, in comparison with single and binary counterparts. The construction of highly efficient cascade electron transfer pathways at the interface is responsible for the enhancement in photoactivities of a ternary heterostructure, which is afforded by intimately intercalating WO₃ NRs in the interfacial domains of TNBs and GR. In this unique ternary nano-architecture, the WO₃ NRs ingredient serves as an efficient interfacial charge transfer mediator and GR serves as an electron transporter and collector to conspicuously trigger a cascade electron relay from TNBs to GR, thereby expediting the efficacious charge transfer, prolonging the lifetime of photogenerated electron-hole pairs, and resulting in the significantly enhanced photoredox activities of the TNBs/WO₃ NRs/GR heterostructure. In addition, the predominant active species responsible for the photoredox process were determined and the underlying photocatalytic mechanism was delineated.

Full text of DOI:10.1039/c7ta08415c

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

Page 1 of 37
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Modulation of Interfacial Charge Transfer by
Self-assembly of Single-layer Graphene Enwrapped
One-dimensional Semiconductors Toward
Photoredox Catalysis
Junyu Zhang, Fang-Xing Xiao*
College of Materials Science and Engineering, Fuzhou University, Fuzhou, 350002, People’s
Republic of China.*Email: fangxing2010@gmail.com
Abstract
In recent years, exquisite modulation on the photogenerated electronꢀhole charge carriers transport
constitutes a longꢀstanding challenge. To this end, herein, spatially hierarchical singleꢀlayer graphene
3 2
(GR) wrapped and WO nanorods (NRs) coupled TiO nanobelts (TNBs) ternary nanoꢀarchitecture
(
TNBs/WO NRs/GR), as a conceptual platform, has been progressively designed via a facile and
3
green layerꢀbyꢀlayer assembly strategy based on the pronounced electrostatic interaction. It was
remarkable to find that interfacial charge transfer of TNBs/WO NRs/GR ternary heterostructure can
3
be finely modulated by interfacial architectural engineering, thus contributing to the significantly
improved photoredox performances including photocatalytic oxidation of organic pollutant and
reduction of heavy metal ions in comparison with single and binary counterparts. The construction of
1

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highly efficient cascade electron transfer pathways at the interface is responsible for the
photoactivities enhancements of ternary heterostructure, which is afforded by intimately intercalating
3 3
WO NRs in the interfacial domains of TNBs and GR. In this unique ternary nanoꢀarchitecture, WO
NRs ingredient serves as the efficient interfacial charge transfer mediator and GR as electron
transporter and collector to conspicuously trigger the cascade electron relay from TNBs to GR,
thereby expediting the efficacious charge transfer, prolonging the lifetime of photogenerated
electronꢀhole pairs, and resulting in significantly enhanced photoredox activities of TNBs/WO
3
NRs/GR heterostructure. Besides, predominant active species responsible for the photoredox process
were determined and underlying photocatalytic mechanism was delineated.
2 3
Keywords: TiO nanobelts, graphene, WO nanorods, layerꢀbyꢀlayer assembly, interfacial mediator.
1
. Introduction
Oneꢀdimensional (1D) semiconductorꢀbased photocatalysis has been gaining tremendous attention
owing to their unprecedented advantages compared with bulk counterparts including fast and
longꢀdistance electron transport, large specific surface area and pore volume as well as enhanced
1
,2
light absorption and scattering properties. More significantly, charge carriers transfer efficiency of
1
D semiconductors can be markedly improved along the vertical 1D framework, which substantially
boost the separation of photogenerated electronꢀhole pairs. Recent years have witnessed a cornucopia
2

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toward the exploration of 1D semiconductors such as nanowires, nanotubes and nanobelts for
wideꢀspread photocatalytic applications such as selective organic transformation, CO reduction,
2
3
ꢀ6
hydrogen production and environmental remediation. Among the diverse 1D semiconductors,
titanium oxide (TiO ), as a quintessential wideꢀbandꢀgap semiconductor, has spawned the most
2
considerable attention owing to its low toxicity, rich abundance, lowꢀcost and excellent
7
,8
2
physicochemical stability. Especially, 1D TiO nanostructures growing directly on the conducting
substrates (e.g., Ti foil) demonstrate unique merits because they guarantee vertical charge transfer
along the 1D nanostructures to the conducting substrate, which remarkably reduces the loss incurred
9
,10
by chargeꢀhopping across the grain boundaries of bulk nanoparticles.
applications of 1D TiO nanostructures are still limited by high recombination rate of photogenerated
charge carriers and scant surface active sites, which generally results in low quantum efficiency of
Nevertheless, photocatalytic
2
1
1,12
photocatalytic reactions.
In this regard, it is highly desirable to solve these obstacles to extend
nanostructures in the field of photocatalysis.
the wideꢀspread applications of 1D TiO
2
To this end, various strategies have been developed including doping with metal or nonꢀmetal
elements, deposition with precious metals (e.g., Au, Ag, Cu, Pt etc.) and hybridization with second
1
3ꢀ15
semiconductors or carbon materials (e.g., carbon nanotubes, graphene etc.).
Of particular note,
1
6
construction of TiO
2
2
ꢀbased heterostructures by rational integration of metal oxides (e.g., Ag O,
1
7
18
19
SnO
2
,
ZnO, WO
3
,
2
etc.) with TiO in an appropriate fashion could improve the photocatalytic
2
0
performances of TiO based on their cooperative synergy. Among the plentiful semiconductors,
2
considerable attention has been devoted to tungsten oxide (WO
3
) due to its relatively narrow
bandgap (2.4ꢀ2.8 eV), nontoxicity, stable physicochemical properties, resistance to photocorrosion
2
1 , 22
and high oxidation power of valence band holes.
However, rapid recombination of
3

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photogenerated charge carriers and low conduction band edge of WO
3
are two main obstacles
demonstrate favorable
2
3
inhibiting its photocatalytic efficiency. Noteworthily, TiO
2
and WO
3
electronic band structure and are able to form a type II band alignment, which implies that
photogenerated electrons and holes from both of them can be effectively separated for boosted
photocatalytic performances. Hitherto, various TiO
2
/WO
Notwithstanding the progress being made in previous
heterostructures via a facile, green, easily accessible and
bottomꢀup synthetic strategy by which WO and TiO are intimately and controllably integrated at
3
nanocomposites have been fabricated for a
2
4ꢀ26
wide range of photocatalytic applications.
works, rational construction of TiO /WO
2
3
3
2
nanoscale to combine their respective advantages has not yet been achieved. Moreover, it is worth
noting that most of the previous works are principally confined to conventional nanoparticles,
nanotubes, or hollow fibers, tunable and exquisite assembly of TiO
conditions has been scarcely reported. Furthermore, there has been no report on rational construction
of spatially hierarchical TiO nanobelts/WO NRs heterostructures by merely tuning the intrinsic
2 3
/WO nanocomposites at ambient
2
3
surface charge properties of components for creating highly efficient photocatalysts.
On the other hand, successful isolation of singleꢀlayer carbon atoms which are tightly packed into
a twoꢀdimensional (2D) honeycomb crystal structure (i.e., graphene, GR) triggered tremendous
2
7ꢀ29
excitement in the scientific communities.
Given the integrative properties of GR, including
superior electronic conductivity and mobility, theoretically high specific surface area, excellent
optical transmittance, and chemical stability, it is not surprising to see the entry of GR into the field
3
0ꢀ34
of photocatalysis.
The combination of GR with semiconductors is beneficial for highlighting the
crucial role of GR in boosting the separation and transfer of photogenerated charge carriers, which is
3
5 ꢀ 37
the critical step for photocatalytic reactions.
Till now, a myriad of GRꢀsemiconductor
4

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nanocomposites have been prepared over the past few years based on the versatile platform of
3
8 ꢀ 40
GR,
photocatalytic performances of TiO
and simultaneously, increasing the light absorption for bandgap photoexcitation of ingredients (TiO2
and WO ) to play their cooperative synergy. Thus far, there has been very few reports on the
preparation of singleꢀlayer GR wrapped TiO for all of which the
which inspires us to speculate that GR might further contribute to improving the
2
/WO nanocomposites by expediting the photoelectrons transfer
3
3
4
1,42
2
/WO
3
nanocomposites,
components were merely randomly integrated with no controllable modulation on the interfacial
architectures. On the other hand, selfꢀassembly techniques on the basis of electrostatic interaction
have attracted enormous attention during the past few years and can serve as efficacious alternatives
to conventional wetꢀchemistry approaches in terms of their high efficiency, environmental benign
and reproducibility. Nevertheless, how to harness the progressive selfꢀassembly technique to
judiciously design wellꢀdefined 1D semiconductorꢀGR hybridized nanoarchitectures for
photocatalysis still constitutes a longꢀterm challenge.
With these motivations, herein, TiO nanobelts (TNBs) intimately coꢀdecorated with singleꢀlayer
2
GR and WO
simple, facile and green electrostatic layerꢀbyꢀlayer (LbL) assembly approach under ambient
conditions. It was remarkable to find that charge transfer at the interface of TNBs/WO NRs/GR
3 3
nanorods (NRs) (i.e., TNBs/WO NRs/GR) have been progressively designed via a
3
ternary heterostructure can be finely modulated by rational interfacial architectural engineering with
WO NRs closely intercalated in the interfacial region of TNBs and singleꢀlayer GR, thereby
3
favoring the construction of an efficient cascade electron transport pathway, improving the
separation of photogenerated charge carriers, and leading to significantly enhanced photocatalytic
performances in comparison with single and binary counterparts. The underlying origin accounting
5

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for the significantly enhanced photocatalytic performances of TNBs/WO
3
NRs/GR ternary
heterostructure was systematically probed and predominant active species responsible were
unambiguously determined. Based on which, possible photocatalytic mechanism was presented.
2
．Experimental section
2
.1 Materials
Titanium sheets (50 mm×20 mm×0.1 mm, 99.9%), sodium hydroxide (NaOH), acetone (C H O),
3
6
ethanol (C
hydrochloric acid (HCl), hydrogen peroxide (H
PDDA, 1 mg/mL, 0.5 M NaCl, M ≈200 000ꢀ350 000), sodium tungstate dihydrate (Na
sodium chloride (NaCl), potassium permanganate (KMnO ), potassium persulfate (K
ammonium oxalate [(NH ) C O ], benzoquinone (C H O ), isopropanol (C H O), 5,
2
H
6
O), natural graphite sheets (Purity 99.5 %), sulfuric acid (H
, 30%), poly(diallyldimethylammonium chloride)
WO ·2H O),
),
2 4 3
SO ), nitric acid (HNO ),
2 2
O
(
w
2
4
2
4
2 2 8
S O
4
2
2
4
6
4
2
3
8
5
ꢀdimethylꢀ1ꢀpyrroline Nꢀoxide (DMPO) were obtained from Sinopharm Chemical Reagent Co., Ltd.
Shanghai, China). Deionized water (DI H O, Millipore, 18.2 Mꢁ·cm resistivity) was from local
sources. All materials were used as received without further purification.
(
2
2
.2 Synthesis of TNBs
9
,10
Synthesis of TNBs on Ti foil was referred to a previous work with a slight modification. Firstly,
Ti foil was cut into small pieces (20 mm×50 mm) and ultraꢀsonicated in acetone, ethanol, and DI
H O for 10 min, respectively. Subsequently, the treated Ti foils were dipped into 40 mL of 5 M
2
o
NaOH aqueous solution in a 50 mL Teflonꢀlined autoclave which was kept at 200 C for 24 h. After
cooling down naturally to room temperature, the samples were washed with DI H
2
O several times
and dried in a dry cabinet. The samples were then immersed in 1 M HCl aqueous solution for 1 h to
+
+
replace the Na with H . Finally, thusꢀobtained hydrogen titanate nanobelts (i.e., Hꢀtitanate
6

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o
nanobelts) were annealed in air at 600 C for 2 h and TNBs were thus obtained.
2
.3 Synthesis of WO
3
NRs
3
6
WO NRs were prepared via a oneꢀstep hydrothermal method. Typically, 1.06 g of Na WO ·2H O
3
2 4 2
and 0.94 g of NaCl were dissolved in 30 mL of DI water and kept stirring for 20 min. The pH of the
solution was adjusted to be ca. 2 using HCl (3.0 M) aqueous solution and stirred for 3 h. Then, the
solution was transferred into a Teflonꢀlined stainless steel autoclave and maintained at 180 °C for 24
h. Subsequently, the products were cooled to room temperature, centrifuged and wash with DI water.
3
Eventually, the sediment was dried at 60 °C and WO NRs were obtained.
2
.4 Preparation of TNBs/WO NRs and TNBs/GR binary nanocomposites
3
TNBs/WO
3
NRs binary nanocomposite was prepared by a simple electrostatic approach. Firstly,
TNBs substrate was vertically dipped into PDDA aqueous solution (1 mg/mL, 100 mL) for 10 min
and then rinsed with DI water for 2 min, followed by drying with a gentle stream of N
2
, resulting in
+
+
the positively charged TNBs (TNBs ). Subsequently, thusꢀobtained positively charged TNBs
substrate was vertically added into WO
3
NRs aqueous solution (0.5 g/L) and stirred for 2 h. The
o
product was finally washed and dried at 60 C for 12 h. TNBs/GR binary nanocomposite was
prepared in a similar way other than the WO NRs aqueous solution was replaced with graphene
3
oxide (GO) aqueous solution. Afterwards, TNBs/GO precursor nanocomposites were transferred to a
Teflonꢀlined stainless steel vessel filled with 40 mL of DI H
to TNBs/GR binary nanocomposite.
2
O and kept at 120 °C for 12 h, giving rise
2
.5 LbL assembly of TNBs/WO
3
NRs/GR ternary nanocomposite
2
8
GO was prepared from natural graphite sheets by a modified Hummers’ method, which was
7

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provided in supporting information. To prepare the TNBs/WO
TNBs/WO
solution (1 mg/mL, 100 mL) and then vertically dipped into 100 mL of GO aqueous solution (0.5
3
NRs/GR ternary heterostructure,
3
NRs binary nanocomposite was subjected to the same modification with PDDA aqueous
–
1
mg·mL ) for 5 h followed by the same washing and drying treatments. The thusꢀassembled
TNBs/WO NRs/GO ternary nanocomposite was completely washed with DI water for several times
and finally transferred to a Teflonꢀlined stainless steel vessel filled with 40 mL of DI H O and kept at
20 °C for 12 h. The asꢀprepared TNBs/WO NRs/GR nanocomposite was washed thoroughly with DI
O and dried in an oven at 60 °C.
3
2
1
3
H
2
2
.6 Characterization
Zeta potentials (ξ) were measured using dynamic light scattering analysis (Zeta sizer 3000HSA) at
room temperature. The crystal phases of the samples were determined by a Bruker D8 Advance
Xꢀray diffractometer (XRD) using Niꢀfiltered Cu Kα radiation at 40 kV and 40 mA with a scan rate
of 0.02° per second. Raman spectra were collected on a Renishaw inVia Raman System 1000 with a
5
32 nm Nd: YAG excitation source at room temperature. Fieldꢀemission scanning electron
microscopy (FESEM) was used to explore the morphologies of the samples on a FEI Nova
NANOSEM 230 spectrophotometer. Transmission electron microscopy (TEM), energy dispersive
Xꢀray (EDX) spectrometer and HRTEM images were acquired using a JEOL model JEM 2010 EX
instrument at an accelerating voltage of 200 kV. Xꢀray photoelectron spectroscopy (XPS)
measurements were performed on a Thermo Scientific ESCA Lab250 spectrometer with all binding
energies (BEs) calibrated to the BE of C 1s (284.6 eV). The optical properties of the samples were
probed by a Cary 500 UVꢀvisible diffuse reflectance spectrophotometer (DRS), in which BaSO was
4
employed as the internal reflectance standard. Electrochemical analysis was carried out in a
8

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conventional threeꢀelectrode cell in which Pt plate and Ag/AgCl electrode were used as counter and
reference electrode, respectively. The asꢀsynthesized samples were directly used as the working
2 4
electrodes. The electrolyte was composed of 0.1 M Na SO aqueous solution (pH=6.8). The
electrochemical impedance spectroscopy experiments were conducted on a Precision PARC
workstation, amplitude of the sinusoidal wave was set at 5 mV and frequency varies from 100 kHZ
to 0.05 HZ. Fourier transform infrared spectroscopy (FTIR) was probed on a Nicolet Nexus 670
−
1
FTIR spectrophotometer at a resolution of 4 cm . Electron spin resonance (ESR) signals of the
radicals trapped by 5, 5ꢀdimethylꢀ1ꢀpyrroline Nꢀoxide (DMPO) were recorded with a Bruker ESP
3
00E spectrometer. The photoluminescence (PL) spectra for solid samples were investigated on an
Edinburgh FL/FS900 spectrophotometer.
2
.7 Photocatalytic activity measurements
(a) Photocatalytic oxidation performances
Photocatalytic performances of the samples were evaluated by photocatalytic degradation of organic
dye pollutant, e.g., Rhodamine B (RhB) in a liquidꢀphase under UV irradiation (16 W, λ=254 nm) at
ambient conditions. The sample films with the same area (10 mm×25 mm) were soaked into 3.0 mL
of RhB aqueous solution (5 mg/L, pH=7) in a quartz cuvette (10 mm×10 mm×30 mm). To reach the
equilibrium of adsorptionꢀdesorption, the mixtures were kept in the dark for 1 h before irradiation.
Afterwards, the samples were placed 3 cm away from the light source. At each irradiation time
interval of 20 min, light absorbance of the reaction solution was measured by a Cary 500 scan
UVꢀVis spectrophotometer. The concentration of RhB was determined by monitoring the peak
absorption at 553 nm and the degradation ratio of RhB at each time interval was calculated from the
ratio of the light absorbance of irradiated to the nonꢀirradiated solution. Here, C/C
9
0
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the percentage of degradation during the photocatalytic reaction, where C is the concentration of
RhB at each irradiation time interval, while C is the initial concentration when the
0
adsorptionꢀdesorption equilibrium was achieved. In addition, a series of control experiments with
adding different radical scavengers (i.e. ammonium oxalate as scavenger for photogenerated holes,
2 2 8
K S O for photoelectrons, isopropanol for hydroxyl radicals and benzoquinone for superoxide
3
5,36
radicals)
were performed in an analogous way.
b) Photocatalytic reduction performances
Photoreduction performances were evaluated by reduction of Cr(VI) at ambient conditions. In a
typical reaction, 20 mg of the sample was dispersed in 30 mL of Cr(VI) aqueous solution (K Cr
ppm). The mixed suspensions were stirred in the dark for 1 h to ensure the adsorption–desorption
for photocatalytic degradation of aqueous RhB solution under UV irradiation (λ=254 nm)
(
2
2 7
O ,
5
equilibrium of catalysts and reactant. The mixed suspension was irradiated with simulated solar (16
W, λ=254 nm), during which 3 mL of the sample solution was withdrawn at every 15 min interval
and centrifuged to remove the catalyst. The filtrates were analyzed by recording the UV–vis spectra
of Cr(VI) using a UVꢀvis/NIR spectrophotometer (UVꢀ3600, Shimadzu). Conversion of the reaction
was determined by the following formula:
Where C was the original concentration of the reactant and C is the concentration of the reactant at
0
every 30 min interval after reaction.
3
Results and discussion
10

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Scheme 1. Schematic illustration for electrostatic LbL assembly of TNBs/WO
3
NRs/GR ternary
heterostructure.
Scheme 1 depicts the detailed flowchart for LbL assembly of TNBs/WO NRs/GR ternary
3
heterostructure. Specifically, Hꢀtitanate nanobelts were firstly grown on Ti foil via a facile
+
hydrothermal method and TiO
2
nanobelts (TNBs) were attained by Na exchange followed by
calcination. To rapidly and efficiently modify the surface charge of TNBs substrate at ambient
conditions, TNBs was treated with PDDA which has been frequently utilized as a positively charged
polyelectrolyte, thus affording the formation of positively charged surface on the TNBs substrate. In
this regard, attracted by the substantial electrostatic attractive force, intrinsically negatively charged
WO
3
NRs (Fig. S1b) are able to spontaneously and homogeneously selfꢀassemble on the
PDDAꢀmodified positively charged TNBs framework to produce TNBs/WO3 NRs binary
nanocomposite. Thereafter, in a similar way, intrinsically negatively charged GO (Fig. S1a)
precursor was deposited on the PDDAꢀtreated positively charged TNBs/WO NRs scaffold giving
3
3 3
rise to TNBs/WO NRs/GO ternary nanocomposite. Finally, TNBs/WO NRs/GR was acquired by a
facile hydrothermal reduction treatment. In such a successive LbL assembly strategy, tailorꢀmade
WO NRs and GO precursor are utilized as the building blocks and ultraꢀthin PDDA layer as a glue
3
11

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molecule.
Fig. 1. XRD patterns of (a) pristine TNBs, (b) TNBs/WO NRs binary, (c) TNBs/GR binary and (d)
3
TNBs/WO NRs/GR ternary heterostructures. (A: anatase, W: WO , T: Ti foil)
3
3
Our work begins with the structural characterizations of the samples. Crystal structures were
determined by Xꢀray diffraction (XRD). As displayed in Fig. 1a, the dominant peaks at 2θ values of
2
5.3, 36.9, 37.8, 38.6, 48.0, 53.9 and 62.7 can be accurately indexed to the (101), (103), (004), (112),
4
3,44
(
200), (105) and (204) crystallographic planes of anatase TiO
2
(JCPDS No. 21ꢀ1272).
NRs binary nanocomposite (Fig. 1b), the peaks at 22.7 and
(JCPDS
no. 33ꢀ1387, Fig. S2) were clearly observed, indicative of successful assembly of WO NRs on the
With
regard to the XRD pattern of TNBs/WO
3
2
8.2 corresponding to the (001) and (200) crystallographic planes of hexagonalꢀphase WO
3
3
TNBs matrix. However, these two peaks were not observed in the XRD pattern of TNBs/WO
NRs/GR ternary nanocomposite (Fig. 1d), which is predominantly due to the low loading amount of
WO in the interfacial region of ternary heterostructure and, probably, these diffraction peaks are
shielded by the pronounced diffraction peaks of TNBs substrate. Yet, attachment of WO NRs in the
3
3
3
ternary nanocomposite will be verified by the following XPS result (Fig. 3d). Moreover, it should be
emphasized that no characteristic peaks of GR can be seen in the XRD patterns of TNBs/GR binary
12

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3
and TNBs/WO NRs/GR ternary heterostructures and this can also be attributable to the relatively
weak peak intensity of GR in comparison with the dominant peaks of TNBs and, probably, due to the
31,32
disappearance of layerꢀstacking regularity after GO reduction.
However, successful integration of
GR in the nanocomposites will be evidenced by the following Raman result (Fig. 2bIV). For specific
comparison, XRD patterns of pristine WO
respectively.
3
and GO & GR were provided in Fig. S2 and Fig. S3,
3
Fig. 2. (a) Raman spectra of (I) pristine TNBs, (II) TNBs/WO NRs binary, (III) TNBs/GR binary,
and (IV) TNBs/WO NRs/GR ternary heterostructures; (b) magnified Raman spectra of (I) graphite
3
ꢀ1
(
GP), (II) GO, (III) TNBs/GR and (IV) TNBs/WO NRs/GR within the scope of 1200ꢀ1800 cm .
3
Raman spectra were utilized to analyze the ordered/disordered crystalline structures of
carbonꢀcontaining materials. It has been wellꢀestablished that G band is assigned to the firstꢀordered
scattering of E2g mode for sp2 carbon domains and D band is associated with disordered structural
4
5
defects, e.g., amorphous carbon or edges that can break the symmetry and selection rule. As
ꢀ1
displayed in Fig. 2a, three distinct Raman peaks at 399 (B ), 517 (A + B ) and 639 cm (E ) were
1
g
1g
1g
g
4
6
apparently observed for all samples, which can be assigned to anatase TiO
2
substrate. More
specifically, Raman spectra of TNBs/GR binary and TNBs/WO NRs/GR ternary heterostructures
3
ꢀ1
concurrently exhibit three peaks at ca. 1350, 1600 and 2700 cm , which correspond to the D, G and
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39
2
D bands, indicating GR nanosheets (NS) have been successfully integrated in the binary and
ternary heterostructures. Alternatively, it has been wellꢀestablished that peak intensity ratio of the D
and G bands, i.e., I
/I
D G
, is a measure of the relative concentration of local defects or disorders
3
2
(
particularly the sp hybridized defects) in comparison with sp hybridized GR domains in
31,47,48
carbonꢀcontaining nanomaterials.
As shown in Fig. 2b, compared with graphite (GP), the
NRs/GR
ternary nanocomposites ascertains the complete exfoliation of GP in GO and hybridized
nanocomposites. Note that I /I ratio of GO was 1.04 but it decreases to 0.94 and 1.00 for TNBs/ GR
presence of D and G bands in the Raman spectra of GO, TNBs/GR binary and TNBs/WO
3
D
G
and TNBs/WO /GR, indicating the relatively lower defect density of GR in binary and ternary
3
nanocomposites as compared to GO.
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Fig. 3. (a) Survey and highꢀresolution XPS spectra of (b) N 1s, (c) O 1s, (d) W 4f and (e) Ti 2p for
TNBs/WO NRs/GR ternary heterostructure.
3
Surface composition and elemental chemical states of TNBs/WO NRs/GR ternary heterostructure
3
were analyzed by Xꢀray photoelectron spectroscopy (XPS). As displayed in Fig. 3a, survey spectrum
of TNBs/WO
heterostructure, for which the Ti & W elements arise from TNBs & WO
Fig. 3b shows the highꢀresolution N 1s spectrum which can be deconvoluted to three peaks at 399.3,
00.2, and 402.2eV, attributable to the ꢀNH functional group, NꢀC, and NꢀO bond from PDDA
3
NRs/GR reflects the coꢀexistence of C, O, N, Ti, and W coreꢀelements in the ternary
3
NRs and N from PDDA.
4
2
intercalated in the interfacial region, respectively. Note that peak intensity of N 1s (Fig. 3b) is very
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weak in comparison with other elements, indicative of only a rather thin PDDA layer was
4
9
electrostatically assembled on the TNBs matrix. As displayed in Fig. 3c, highꢀresolution O 1s
spectrum can be deconvoluted to two peaks with binding energy (BE) of 530 and 531.85 eV
5
0
corresponding to the lattice oxygens from (TiꢀO) TNBs & (WꢀO) WO NRs and hydroxyl groups,
3
respectively. As unveiled by the highꢀresolution spectrum of W 4f (Fig. 3d), two pronounced peaks
at 35.53 and 37.46 eV corresponding to W 4f7/2 and W 4f5/2 signify that W is in the +6 oxidation
36
state. Noteworthily, a small doublet at 36.34 and 38.40 eV for W 4f7/2 and W 4f5/2 is ascribed to the
3
6
formation of some defects on the surface. Highꢀresolution spectrum of Ti 2p (Fig. 3e) exhibits two
4+
51
typical peaks at 458.86 and 464.59 eV, pointing to the oxidation state of Ti in TiO scaffold.
2
Compared with the highꢀresolution W 4f and Ti 2p spectra of pristine WO
3
NRs (Fig. S4a) and
NRs/GR
TNBs (Fig. S4b), it is apparent that chemical states of Ti and W elements in the TNBs/WO
3
ternary heterostructure were not altered during the LbL assembly. Notably, before reduction,
highꢀresolution C 1s spectrum of GO (Fig. S5a) can be fitted to four peaks at 284.4, 286.4, 287.9,
and 288.9 eV corresponding to the oxygenꢀcontaining functional groups including C=C/CꢀC
(aromatic ring), CꢀOꢀC/CꢀOH (epoxy and hydroxyl), C=O (carbonyl), and O=CꢀOH (carboxyl)
5
2,53
groups,
respectively. After chemical reduction via hydrothermal treatment, peak intensity of
these oxygenꢀcontaining functional groups conspicuously decreases and simultaneously, peak
2
intensity of sp hybridized carbon increases (Fig. S5b), implying GO assembled in the ternary
heterostructure has been completely reduced to GR. Hydrothermal treatment is an essential step to
restore the excellent electronic conductivity and mobility of GR for transferring photoelectrons.
Fourier transform infrared spectroscopy (FTIR) was employed to uncover the specific structural
features of selfꢀassembled heterostructures. Fig. S6c shows the presence of stretching vibration
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ꢀ1
ꢀ1
ꢀ1
ꢀ1
modes of C=O (1730 cm ), C=C (1633 cm ), CꢀOꢀH (1350 cm ), CꢀOH (1226 cm ), and CꢀOꢀC
ꢀ1
(
1058 cm ) species in GO, which stem from the oxygenꢀcontaining functional groups on the planar
33
surface of GO. Most of these characteristic peaks disappear in the FTIR spectrum of TNBs/WO
3
NRs/GR heterostructure (Fig. S6b), which indicates successful reduction of GO to GR after
hydrothermal reduction treatment. This observation is in agreement with the highꢀresolution C 1s
spectra of GO and TNBs/WO
3
NRs/GR heterostructure (Fig. S5). In addition, two peaks at 2976 and
884 cm were clearly seen and can be attributable to the stretching vibration mode of ꢀCH
stemming from PDDA, strongly evidencing the progressive assembly with the aid of PDDA layer in
ꢀ
1
2
2
3
7
the ternary heterostructure during the LbL selfꢀassembly. Besides, typical peaks at 3382 and 1633
ꢀ1
cm corresponding to the stretching vibration mode of surface ꢀOH (TiꢀOH) and asymmetric
vibration mode of TiꢀO from TiO scaffold were also identified in the FTIR spectra of pristine TNBs
and TNBs/WO NRs/GR (Fig. S6 a & b). Alternatively, the peaks at 828 and 1044 cm
correspond to the stretching vibration mode of WꢀOꢀW from WO . Based on the above analysis,
2
18
−1
3
3
FTIR results substantiate the successful incorporation of tailorꢀmade assembly units (i.e., GR, WO3
NRs and TNBs) in the final LbLꢀassembled nanoꢀarchitecture.
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Fig. 4. FESEM images of (a) TNBs/WO
3
NRs binary, (b) TNBs/GR binary, and (c) TNBs/WO
3
NRs/GR ternary heterostructures; TEM images of (d) pristine TNBs, (e) pristine WO
3
NRs and (f &
g) TNBs/WO
3 3
NRs/GR ternary heterostructure; (h) HRTEM image and (i) EDX result of TNBs/WO
NRs/GR ternary heterostructure.
Fieldꢀemission scanning electron microscopy (FESEM) was harnessed to intuitively probe the
morphologies of the samples. As shown in Fig. S8a, TNBs matrix exhibits uniform nanobelt
structure with smooth surface and possesses thicknesses of 20ꢀ40 nm, width of 200ꢀ400 nm and
length up to dozens of micrometers. FESEM images in Fig. S7 show that WO
mean size of ca. 50 nm with length of several micrometers. As displayed in the FESEM image of
TNBs/WO NRs binary nanocomposite (Fig. 4a), WO NRs were closely deposited on the TNBs
3
NRs demonstrate
3
3
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3
surface as marked by the white arrows, indicative of electrostatic assembly of WO NRs on the
TNBs. Fig. 4b demonstrates the FESEM image of TNBs/GR binary nanocomposite in which TNBs
framework was uniformly and intimately wrapped by GR NS in an appropriate “faceꢀtoꢀface”
stacking way as marked by the white arrows, suggesting efficacious assembly of GR NS on the
surface modifiedꢀTNBs under pronounced electrostatic interaction. For comparison, highꢀmagnified
FESEM images of TNBs matrix, TNBs/WO
3
NRs and TNBs/GR binary heterostructures were
NRs/GR ternary
displayed in Fig. S8. Fig. 4c manifests the morphology of TNBs/WO
3
heterostructure, from which it is clearly seen that GR NS were tightly integrated with TNBs and
WO NRs forming wellꢀdefined ternary nanoarchitecture. As a result, FESEM results suggest that
3
3
WO NRs and GR building blocks were progressively selfꢀassembled on the TNBs based on
electrostatic interaction. The characterizations of GO precursor were provided in Fig. S9.
Transmission electron microscope (TEM) analysis was carried out to gain more detailed
microscopic morphologies and structural information of the nanomaterials. Fig. 4d shows the TEM
image of TNBs substrate which exhibits flat surface with horizontal size of ca. 300 nm and this is in
accordance with FESEM result (Fig. S8a). As displayed in Fig. 4e, WO
bundle of nanorod with width of 30 nm and length of 250 nm. As revealed by the TEM images of
TNBs/WO NRs/GR ternary heterostructure in Fig. 4(f & g) and Fig. S10, TNBs substrate was
closely enwrapped by GR and WO NRs forming GR wrapped and WO NRs coupled ternary
3
NRs is composed of large
3
3
3
heterostructure. This can be further disclosed by the HRTEM image (Fig. 4h), in which assembly
units form good interfacial contact and the lattice fringes in the designated area are determined to be
ca. 0.35 and 0.39 nm, corresponding to the (101) and (111) crystal facets of TiO
2 3
and WO ,
respectively. Energy dispersive Xꢀray (EDX) analysis (Fig. 4i) verified the coꢀexistence of Ti, O, W,
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3
N and C elements in the TNBs/WO NRs/GR ternary heterostructure which is consistent with XPS
results (Fig. 3).
Fig. 5. (a) UVꢀvis diffuse reflectance spectra (DRS) of pristine TNBs, TNBs/WO
TNBs/WO NRs/GR ternary heterostructures and (b) the plots of transformed KubelkaꢀMunk
function vs. the energy of light.
3
NRs binary and
3
UVꢀvis diffuse reflectance spectroscopy (DRS) was utilized to probe the optical properties of
different samples. As shown in Fig. 5a, an obvious absorption peak is observed in the wavelength
region ranging from 200 to 400 nm for all samples, which is attributed to the bandgap
photoexcitation of electrons from the valence band (VB) to the conduction band (CB) of TiO
2
5
4
matrix. The plots obtained based on the transformed KubelkaꢀMunk function vs. the energy of
light were provided in Fig. 5b, from which the bandgap values (E ) of TNBs, TNBs/WO NRs, and
g
3
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TNBs/WO
suggests that incorporation of WO
the absorption band edge of nanocomposites, all of which are confined in the UV region.
3
NRs/GR were roughly determined to be ca. 3.34, 3.32, and 3.26 eV, respectively. This
3
NRs and GR with TNBs did not exert pronounced influence on
Fig. 6. (a) Photocatalytic performances of TNBs, TNBs/WO
NRs/GR heterostructures toward degradation of RhB under UV light irradiation (λ=254 nm) at
ambient conditions, (b) cyclic photocatalytic reactions of TNBs/WO NRs/GR nanocomposite under
3 3
NRs, TNBs/GR and TNBs/WO
3
the same conditions, and highꢀresolution (c) Ti 2p and (d) W 4f spectra of TNBs/WO NRs/GR
3
nanocomposite (e) XRD patterns and (f) Raman results before and after cyclic reactions.
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Photocatalytic performances were evaluated by liquidꢀphase degradation of Rhodamine B (RhB)
aqueous solution, a typical organic dye pollutant, under UV light irradiation at ambient conditions.
Blank experiments in dark and without catalyst showed the limited change of RhB concentration,
thus verifying the degradation reaction was truly driven by a photocatalytic process. Fig. 6a shows
the photocatalytic performances of different samples which follow the order of:
TNBs<TNBs/GR<TNBs/WO
3
NRs<TNBs/WO
3
NRs/GR.
Notably,
TNBs/GR
binary
nanocomposites demonstrated improved photoactivity as compared with pristine TNBs, which is
principally due to the vital role of GR as electron transporter and collector in improving the
separation of photogenerated electrons and holes over TNBs under light irradiation. Similarly,
photocatalytic performance of pristine TNBs was also enhanced when it was combined with WO
NRs, which is attributable to the formation of favorable type II band alignment between WO and
TiO , thereby boosting the separation efficiency of photogenerated charge carriers and thus
enhancing the photocatalytic performance of TNBs/WO NRs binary nanocomposite. It should be
3
3
2
3
noted that TNBs/WO NRs/GR ternary heterostructure demonstrated the optimal photoactivity
3
among the samples. The substantial photoactivity enhancement is mainly ascribed to the unique
integration mode of GR and WO
charge carriers transfer. More specifically, WO
charge transfer mediator to promote the cascade bandgapꢀphotoexcited electrons flow from TiO
GR, hence contributing to the substantially improved separation of photogenerated electronꢀhole
pairs over TNBs/WO NRs/GR ternary nanocomposite. In this way, cooperative synergy between
GR acting as efficient electron transporter & collector and WO NRs serving as an interfacial charge
transfer mediator concurrently prolong the lifetime of photogenerated electronꢀhole pairs, leading to
3
NRs with TNBs framework in an intimate fashion which facilitates
ingredient plays a pivotal role as an interfacial
to
3
2
3
3
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the significantly enhanced photocatalytic performances of ternary heterostructure. The complete
mineralization of dye pollutant over ternary heterostructure can be strongly evidenced by the change
of total organic carbon (TOC) percentage of different catalysts during the photocatalytic process (Fig.
S11), which decreases gradually with irradiation time and agrees well with the photocatalytic
performances. Photostability has been deemed as an important sector of photocatalyst for practical
application. In this regard, cyclic photodegradation experiments were carried out to evaluate the
3 3
stability of TNBs/WO NRs/GR ternary heterostructure. As shown in Fig. 6b, TNBs/WO NRs/GR
ternary heterostructure demonstrated relatively good photostability with no obvious decay in
photoactivity even after four successive cyclic reactions. Highꢀresolution Ti 2p (Fig. 6c), W 3d (Fig.
6
d) and N 1s (Fig. S13) spectra of TNBs/WO
chemical states of Ti and W elements were retained, verifying the favorable photostability of the
TNBs/WO NRs/GR ternary heterostructure. Moreover, consistently, XRD (Fig. 6e), Raman (Fig. 6f)
and survey XPS results (Fig. S12) of TNBs/WO NRs/GR ternary heterostructure before and after
3
NRs/GR before and after cyclic reactions suggest the
3
3
cyclic reactions revealed intact crystal structures of TNBs and WO NRs components in the ternary
3
heterostructure.
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6+
3+
Fig. 7. (a) Photocatalytic performances of different samples toward reduction of Cr to Cr under
UV light irradiation (λ=254 nm) at ambient conditions, (b) cyclic photoreduction performance of
TNBs/WO NRs/GR ternary heterostructure, and (c) control experiments with and without adding
3
K S O as the electrons scavenger.
2
2
8
Apart from photocatalytic oxidation reaction, photoreduction performances of different samples
were also investigated. As shown in Fig. 7a, consistently, TNBs/WO3 NRs/GR ternary
6+
heterostructure demonstrated the optimal photocatalytic performance toward the reduction of Cr to
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3
+
Cr under UV light irradiation at ambient conditions in comparison with corresponding single and
binary counterparts under the same conditions and they follow the order of: TNBs/WO NRs/GR>
TNBs/WO NRs>TNBs/GR>TNBs, which agrees well with the photoꢀoxidation performances.
3
3
Moreover, it is worth noting that photoreduction performances of TNBs/WO NRs/GR ternary
3
heterostructure were not decayed even after five successive reactions (Fig. 7b), strongly suggesting
its favorable photostability. Control experiments with and without utilizing K
2
S
2
O
8
as the electrons
scavenger confirmed that photoreduction reaction of Cr was truly driven by photoelectrons (Fig.
c). Consequently, the results indicated that TNBs/WO NRs/GR ternary heterostructure
6+
7
3
demonstrated versatile and promising photoredox catalytic performances toward the oxidation of
organic dye pollutant and reduction of heavy metal ions under ambient conditions.
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Fig. 8. (a) Onꢀoff transient photocurrent responses, (b) electrochemical impedance spectroscopy (EIS)
Nynquist plots and (c) linear sweep photovoltammetry (LSV) results of TNBs, TNBs/WO NRs,
TNBs/GR and TNBs/WO NRs/GR heterostructures in a 0.1 M Na SO (pH=6.8) aqueous solution
3
3
2
4
without bias vs. Pt counter electrode under UV light irradiation (λ=254 nm). The amplitude of the
sinusoidal wave was set at 10 mV and the frequency varies from 100 kHz to 0.05 Hz for EIS
measurements.
To probe the fate of photogenerated electronꢀhole charge carriers, photoelectrochemical (PEC)
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measurements were conducted. It has been wellꢀestablished that photocurrent is normally generated
from the diffusion of photogenerated electrons to the back contact and simultaneously, photoinduced
5
5,56
holes are captured by the electron donor in the electrolyte.
onꢀoff transient photocurrent responses of all samples under UV light irradiation showed no decay of
photocurrent and the result manifests good stability of the catalysts. Apparently, TNBs/WO
As shown in Fig. 8a, obviously,
3
NRs/GR ternary heterostructure demonstrated the most significantly enhanced photocurrent response
in comparison with single and binary counterparts under the same conditions, strongly unraveling the
most efficient separation and longest lifetime of photoexcited electronꢀhole pairs over ternary
heterostructure. Specifically, photocurrents of the different samples follow the order of:
TNBs<TNBs/GR<TNBs/WO
3
NRs<TNBs/WO
3
NRs/GR, which is consistent with the
photoactivities results (Fig. 6a). Fig. 8c demonstrates the photocurrent density vs. applied voltage
plots recorded under UV light irradiation. It was obvious that photocurrent density of the samples
5
7
increases with the forward bias voltage, indicative of the typical nꢀtype semiconductor.
Consistently, TNBs/WO NRs/GR ternary heterostructure exhibited the optimal photocurrent within
3
the same designated voltage profile. In addition, electrochemical impedance spectroscopy (EIS)
Nyquist measurements under UV light irradiation, as an efficacious tool to probe the photoꢀinduced
charge carriers transfer efficiency in the interfacial region between working electrode and electrolyte,
5
8,59
were performed.
Normally, the smaller the semicircular arc diameter of Nyquist plots the more
56,57
efficient separation of photogenerated electronꢀhole pairs.
Fig. 8b shows that TNBs/WO /GR
3
ternary heterostructure displayed the smallest semicircular arc radius among all samples under UV
light irradiation, evidencing it possesses the smallest charge transfer resistance in the interfacial
domain and the optimal separation efficiency of photoexcited electronꢀhole pairs. Furthermore, it
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should be emphasized that only one arc or semicircle was observed in the EIS Nyquist plots of all
samples, indicating surface charge transfer process is the dominant step that dictates the
photocatalytic reaction.
Fig. 9. (a) PL spectra of TNBs, TNBs/WO
heterostructures with an exciting wavelength of 365 nm. (b) Control experiments using different
radical scavengers toward photocatalytic degradation of RhB over TNBs/WO NRs/GR ternary
heterostructure including (I) without adding any radical scavengers (viz., Initial), adding (II)
ammonium oxalate (AO), (III) benzoquinone (BQ), (IV) isopropanol (Iso) and (V) K as
3 3
NRs, TNBs/GR and TNBs/WO NRs/GR
3
2 2 8
S O
scavengers for capturing photogenerated holes, superoxide radicals, hydroxyl radicals and
photogenerated electrons under UV light irradiation (λ=254 nm) for 60 min. ESR spectra for
ꢀ.
.
detecting (c) DMPOꢀO
2
and (d) DMPOꢀOH radicals inꢀsitu formed in the methanol and aqueous
solutions of TNBs/WO NRs/GR with and without UV light irradiation (λ=254 nm).
3
3
To understand why the TNBs/WO NRs/GR ternary heterostructure showed such a significantly
enhanced photoactivity enhancement compared with corresponding single and binary counterparts,
photoluminescence (PL) spectra were explored. PL spectrum has been deemed as an efficient tool to
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4
0
explore the fate of photogenerated electronꢀhole charge carriers. As displayed in Fig. 9a, PL
intensity of TNBs decreases when it was separately integrated with GR or WO
3
NRs in the binary
NRs. The results
nanocomposites and they follow the order of: TNBs>TNBs/GR>TNBs/WO
3
suggest that, photoexcited electrons produced over TNBs in binary heterostructures can
spontaneously flow to GR and WO NRs which serve as electron transporter and collector, thereby
benefiting the separation and prolonging the lifeꢀspan of photogenerated electronꢀhole pairs. More
significantly, when TNBs was coꢀintegrated with WO NRs and GR via LbL assembly, PL intensity
of TNBs/WO NRs/GR was additionally decreased and demonstrated the lowest PL intensity, i.e.,
TNBs/WO NRs/GR<TNBs/WO NRs<TNBs/GR<TNBs, indicative of the most efficient separation
3
3
3
3
3
of photogenerated electrons and holes over ternary heterostructure in comparison with single and
binary counterparts. Considering the lower work function of GR and the PL results, it is reasonably
3
speculated that intimately combining TNBs, WO NRs and GR building blocks in the ternary
heterostructure enables the construction of an efficacious cascade electron transfer pathway which
favors the transport of photoelectrons from TNBs to GR via WO NRs. PL results agree well with the
3
photocatalytic and PEC performances.
On the other hand, with a view to determining the predominant active species responsible for the
significantly enhanced photocatalytic performances and revealing the underlying photocatalytic
3
mechanism of TNBs/WO NRs/GR ternary heterostructure, a series of control experiments with
adding different scavengers for quenching various radicals (e.g., hydroxyl radicals, superoxide
radicals, holes, and electrons) were performed. As shown in Fig. 9b, when ammonium oxalate (AO)
ꢀ.
scavenger for capturing holes and benzoquinone (BQ) scavenger for superoxide radicals (O
2
) were
added into the reaction system, photoactivities of TNBs/WO NRs/GR were substantially prohibited;
3
29