Well-controlled layer-by-layer assembly of carbon dot/CdS heterojunctions for efficient visible-light-driven photocatalysis

Article abstract of DOI:10.1039/C5TA03649F

Fluorescent carbon dots have attracted great attention, but their application in photocatalysis has not been well explored. Herein we report a facile layer-by-layer method to fabricate uniform C dot/CdS heterojunction films via an electrophoretic and sequ

Full text of DOI:10.1039/C5TA03649F

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Well-controlled Layer-by-Layer Assembly of Carbon Dots/CdS
Heterojunction for Efficient Visible-Light-Driven Photocatalysis
Na-Na Chai,a Hang-Xing Wang,a Chen-Xia Hu,a Qiang Wang*b and Hao-Li Zhang*a
Received 00th January 20xx,
Accepted 00th January 20xx
Abstract: Fluorescent carbon dots have attracted great attention, but the application in potocatalysis has not been well
explored. Herein we report a facile layer-by-layer method to fabricate uniform C dots/CdS heterojunction films via
electrophoretic and sequential chemical bath deposition method. Because no ligands are used, the strategy facilitates the
formation of intimate interfacial contact beneficial for charge separation and transfer, which can lead to a high
photocurrent density of 2.6 mA∙cm-2. In addition, the electron donor-acceptor heterojunction can expedite charge
separation and effectively suppress electron-hole pair recombination, eventually contributing to enhanced
photoelectrochemical and/or photocatalytic efficiency of the system. As a proof-of-concept, the hybrid films manifested
themselves as efficient visible-light-driven photocatalyst when applied for reduction of nitro-benzene derivatives in
aqueous phase under low power irradiation. Our findings thus establish a new frontier on the rational design and
fabrication of well-controlled hybrid films with built-in heterojunctions for solar light conversion.
DOI: 10.1039/x0xx00000x
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photocatalysis.16-24 With a bandgap of 2.4 eV, the absorption
of CdS matches well with the visible regime of solar irradiation.
Although they have been studied for decades, most CdS
Introduction
Fluorescent carbon-based nanodots have recently attracted
mounting interest because of their facile preparation,1
excellent photo2 and chemical stability and versatile
applications in opto-electronic field3, 4 and biological imaging.2,
materials still suffer from poor photostability and low
conversion efficiency stemming from rapid electron-hole
recombination, which severely limit their further applications.
Recently, researchers are making efforts in combining CdS
quantum dots with various 2D nano materials such as
graphene, MoS2 and TiO2 sheets, in the purpose of achieving
3, 5-7
Compared with two-dimensional (2D) zero bandgap
graphene, 0-dimensional (0D) carbon dots (C dots) has intrinsic
bandgap and is more promising for photochemical reactions
due to their high optical activities associated with quantum
confinement and edge effects.4, 8-11 Lately, C dots have been
employed for photocatalytic applications owing to their
adjustable size, composition and visible light response.12-14 For
enhanced photoconversion efficiency and improved stability.19,
21, 23-31
These facts prompted us to assemble CdS with 0D C dots in
order to obtain synergistic effects and hence to achieve even
better opto-electronic response. Our rationale is that
integration of C dots into CdS films could form a novel
heterojunction structure, which would facilitate spatial
separation of the photoinduced generated electrons and holes
and prohibit exciton recombination.15, 24 Hence in this work,
we employed a facile LBL method to manufacture uniform C
dots/CdS nano-hybrid films by electrophoretic and sequential
chemical bath deposition (S-CBD).2 In comparison with
conventional methods such as Langmuir-Blodgett deposition,32
vacuum filtration,33 drop casting,34 dip coating,35 Our method
enable the formation of a well-controlled assembly of C
dots/CdS multilayers with intimate interfacial contact, which is
beneficial for charge separation and transfer. The well-
instance, C dots/semiconductors composites were explored for
8
photocatalytic pollutants degradation4,
and hydrogen
evolution.15 It was suggested that a strong interfacial contact
between the C dots and inorganic semiconductors facilitated
efficient charge transfer and thus beneficial for high
photocatalytic activity.15
Cadmium sulfide (CdS) is a well-established semiconductor
materials employed for a wide range of applications, including
optoelectronics,
photovoltaics,
photodetectors,
and
controlled assembly of
C dots/CdS multilayers herein
demonstrated stable and superior photoelectrochemical
and/or photocatalytic performance upon visible light
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irradiation, indicative of promising applications in the solar again with cool ethanol. The resulting L-capped-CdS
light conversion field.
nanoparticles were dried overnight under vacuum at 40
℃ for
further experiments.
Experiment section
Self-Assembly of the C dots/CdS Multilayered Films
Materials and Characterization
Electrophoretic deposition was performed using a custom-built
reaction equipment with three electrodes: a ITO-coated glass
electrode (deposition area: 1.0 cm×0.75 cm, the actual
photocatalyst quality: 0.33 mg), platinum electrode and the
saturated calomel electrode. The ITO glasses was pre-cleaned
by detergent, and then rinsed with plenty of water, followed
by sonication with pure water, ethanol, acetone and
isopropanol for 15 min, respectively. The C dots were then
deposited onto the ITO substrate at a constant voltage of 10 V
in 1 mg∙ml-1 C dots aqueous solution for a desired time. After
electrophoretic deposition, the C dots-coated ITO glass was
withdrawn from the solutions and washed with ultrapure
water, then dried in air at room temperature.
All of the reagents and materials were obtained commercially,
and were of analytical grade. Ultrapure water was produced by
Milipore, Billerica, MA, U.S.
UV-Vis absorption spectra were recorded using a T6 UV-Vis
spectrometer (Purkinje General, China). Photoluminescent
spectra were operated on
a FLS920 steady/transient
fluorescence spectrometer (Edinburgh Instruments, U.K.).
Photoelectrochemical measurements were carried out on a
CHI 660b (CHI USA) electrochemical station. The Fourier
transform infrared (FTIR) spectra were obtained on a Vertex
70v FTIR spectrophotometer (BRUKER USA). X-ray powder
diffraction (XRD) patterns were obtained on a Panalypical X’
Pert PRO. X-Ray photoelectron spectroscopy (XPS) data were
measured by an Axis Ultra DLD system having an Al K source.
Scanning electron microscopy (SEM) studies were performed
using Hitachi S-4800 operated at 5kV, 10μA. Transmission
electron microscopy (TEM) studies were performed using a
TECNAI G2 F30 (FEI) operated at 300 kV. The TEM samples
were prepared by dropping solution onto ultra-thin carbon
membrane and dried in air. Atomic force microscopy (AFM)
measurements were conducted in air by tapping-mode on an
Agilent 5500 SPM. The samples were prepared by drop-casting
1mL suspension on freshly cleaved mica surface and dried in
air.
To directly synthesize CdS QDs from aqueous solution
without any ligand, sequential chemical bath deposition (S-
CBD)7 method was applied. The C dots-coated ITO glass was
successively immersed in four different beakers for about 30
seconds in each beaker. One beaker contained 0.05 M CdCl2
aqueous solution, and another contained 0.05
M Na2S
aqueous solution, and the other two contained ultrapure
water to remove excess precursor solution. Such a cycle was
repeated 20 times, which receive a yellow film with a thickness
of 180 nm. When the bath deposition cycle was further
increased, the CdS nanosheets appeared aggregation on the
ITO glass. Layered C dots/CdS photoelectrodes were prepared
by repeating these two steps of electrophorectic deposition
and S-CBD.
Synthesis of C dots
The synthesis of C dots adopted a pyrolysis method similar to
Photocurrent response
that reported by liu et al.4 L-cysteine (1 g
，
8.25 mmol) were
added into 25ml beaker, then the beaker was transferred into
electric oven and heated at 280 for 5 min (Fig. S3 ).
Photoelectrochemical measurements were carried out in a
custom-made three-electrode quartz cell in which platinum
wire was used as a counter electrode, saturated calomel
electrode as a reference electrode, and the ITO samples as the
working electrodes. An aqueous Na2S solution (0.05 M) was
chosen as the electrolyte and a 500 W xenon arc lamp (in a
distance of 30 cm, with power density of 43 mW∙cm-2) was
applied as the light source.
℃
†
Thereafter, the beaker was cooled to room temperature, and
the final product was collected by adding ultrapure water into
the beaker and centrifuged at 8000 rpm for 5 min. The
supernatant can be collected by rotary evaporation.
Synthesis of L-cysteine-capped-CdS nanoparticles
Water-solution CdS nanoparticles were synthesized according
to the method reported previously.36 110 mg L-cysteine was
added to 25 ml of 1 mol∙l−1Tris buffer that was argon-saturated
and thoroughly degassed, to the mixture 15 ml of 0.01 mol∙l−1
of CdCl2 aqueous solution was added, resulting in an L-
cysteine/Cd molar ratio of 3:1. Then, 95 μl of 0.4 mol∙l−1
Na2S•9H2O solution was slowly added into the solution to
afford an S/Cd molar ratio of 0.25:1. The mixed solution was
stirred for 1 h at room temperature, followed by 10 min of
flushing with argon to remove the unreacted sulfide. To
remove L-cysteine-Cd complexes, the L-cysteine-capped-CdS
(L-capped-CdS) nanoparticles were precipitated by adding cool
ethanol to the reaction mixture of room temperature. The
nanoparticles were dissolved in pure water, and precipitated
Photocatalytic experiment
The photocatalytic experiment, a 3W LED lamp equipped with
a UV cutoff filter (λ > 420 nm) was used as a low energy-
consuming visible light source. In a round bottom flask (25 ml),
the ITO samples with active area of around 75 mm×100 mm
were immersed into 15 mL aqueous solution (10 mg∙L−1) of
aromatic nitro compounds along with 20 mg of HCOONH4.
Before irradiation, the solution was stirred in dark for 30 min
to ensure an adsorption-desorption equilibrium between the
photocatalyst and reactant. The reaction was monitored by
UV-Vis spectra. The whole experimental process was
conducted under N2 atmosphere at ambient conditions.
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was electrodeposited on the surface of ITO at 10V, and the
positively charged Cd2+ ions subsequently adsorbed onto the C
dots film. As a result, the negatively charged S2- ions could
quickly adsorb Cd2+ ions via electrostatic interactions to form
CdS QDs thin film. By repeating the above steps, the well-
controlled uniform C dots/CdS QDs hybrid multilayered film
was constructed.
Results and discussion
N-S-doped C dot is used in this work, as previous research
has suggested N-S-doping could significantly enhance the
photoemission properties of carbon dots.2 Herein the C dot is
prepared by direct carbonization of organic precursors (L-
cysteine) in solid state based on a modified procedure from
previous reports.38 Transmission electron microscope (TEM)
images of the as-synthesized C dots are shown in Fig. 1a, which
are well-dispersed, and exhibit near-spherical morphology
with uniform size distribution. According to the histogram
(inset Fig. 1a), the average size of the C dots is measured to be
about 3.6 nm. Meanwhile, the C dot displays lattice spacing of
0.21 nm in high resolution TEM images (Fig. 1b), consistent
with the (100) lattice fringes of graphene. The AFM images
further confirm the uniform of the C dots, and indicate that
the average thickness of the C dots is below 2 nm (Fig. 1c). X-
ray photoelectron spectroscopy (XPS) was employed to
determine the composition of the C dots. As shown in Fig. 1d,
three peaks at 533, 400, and 284 eV, correspond to O 1s, N 1s
and C 1s, respectively. Additional peaks at 227 and 164 eV are
found in the full survey of S, which were attributed to S 2s and
S 2p peak, respectively. The observation verifies that the C
dots are successfully doped with N and S elements. In addition,
GO was synthesized from natural graphite using a modified
Hummers method as we reported before,39, 40 which has also
been characterized by TEM, AFM and XPS data (Fig. 1).The
TEM image of GO (Fig. S2†) reveals that the graphene oxide
sheets mostly exist as large thin sheets with the width of
several microns. Slight corrugations are also observed due to
the extremely large aspect ratio of the sheet. Typical AFM
topography image (Fig. S2b†) indicates that the contrast of
platelets is very uniform except for the isolated wrinkles and
protrusions, with the corresponding line profile showing a
monolayer thickness of ~ 1.0 nm. XPS spectra of GO in Fig.
S2c† only exhibit two peaks at 533 and 284 eV, corresponding
to O 1s and C 1s, respectively. No S and N signals are observed
for the GO sample.
Scheme 1. Schematic illustration for the layer-by-layer
assembly of C dots/CdS, GO/CdS, and pure C dots multilayered
films.
The layer-by-layer (LBL) assembly process for preparing the
heterojunction films is illustrated in Scheme 1. Taking
advantage of the excellent controllability of the strategy, C
dots/CdS multilayers with various layer numbers and thickness
can be conveniently constructed. In order to reveal the role of
the C dots in photocatalysis, we have fabricated two other
types of photoactive electrodes, including graphene oxide
(GO)/CdS multilayer and CdS thin film. The classic
photocatalytic electrode of CdS is used as a reference here.
Meanwhile, GO has been widely studied for various photo-
electrochemical applications.37 A comparison between the
GO/CdS and C dots/CdS heterojunctions fabricated under
similar condition shall help to identify which carbon-based
nanomaterial is more efficient for photocatalysis applications.
Electrostatic interaction is a key factor for manufacturing
the heterojunction multilayers. The ITO substrate was
hydrophilic in nature. Initially, the negatively charged C dots
Fig. 2a shows the FT-IR spectra of C dots in comparison with
that of GO. Similar to GO, the distinct peaks at around 1730,
1645, 1397, 1075 cm-1 are attributed to C=O, C=C stretching, C-
H bending and C-O stretching modes, respectively, indicative
of concomitant formation of these bonds during C dots growth.
The digital image and UV-Vis absorption of the C dots and GO
suspension in water are shown in Fig. 2b. The UV-Vis
absorption of C dots bears certain resemblance to that of GO,
extending to the visible range up to ~600 nm, a property
favourable for visible-light-driven photoactivity.
Fig. 1. TEM images of (a, b) C-dots, AFM images of (c) C-dots
and (d) XPS spectra of C dots.
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Fig. 2. (a) FTIR spectra of C dots and GO. (b) UV-vis absorption
spectra of C dots and GO. (The inset images are C dots and GO
solution under visible light irradiation) (c) Photoluminescent
spectra of C dots and (d) Photoluminescence of CdS QDs and
“CdS QDs + C dots” with increased C dots concentration under
the excitation of 365nm. (Inset image is CdS QDs solution
under UV light irradiation).
Fig. 3. AFM images and 3D images of (a, b) CdS QDs, (c, d)
(GO/CdS)6 composite films and (e, f) (C dots/CdS)6 assembled
on ITO substrate.
Fig. 2c exhibits the photoluminescence (PL) of the C dots.
Clearly, when the excitation wavelength varies from 340 nm to
450 nm, the PL peaks also red shift and the intensity reaches a
maximum when excited at 340 nm. The excitation-dependent
emission phenomena is typical for C dots, as their surface state
and the size effect induce complexity of the excited states.6
The construction of an effective heterojunction in the C
dots/ CdS hybrid requires the occurrence of efficient charge
transfer process. We thus examined the interplay of the
individual components via steady-state fluorescence
quenching in aqueous solution. The fluorescence spectra of
the as-synthesized L-capped-CdS were shown in Fig. 2d. With
continuous addition of C dots in the solution, the emission
intensity of CdS was gradually quenched, implying possible
electron transfer process from CdS QDs to C dots. This result
was also confirmed by time-resolved fluorescence spectra (Fig.
S1†). The decay profiles of CdS QDs and “CdS QDs + C dots”
were fitted with multiexponential curves to derive three
lifetime components for each. The averaged fluorescence
lifetime was found to decrease from ~4 ns for CdS to ~3 ns of
“CdS QDs + C dots”, further verifying the charge transfer
process between them (Table S1†). Indeed, Mott-Schottky
plots41, 42 estimated the conduction band (CB) edge of -0.37 V
and -0.04 V (vs. NHE) for CdS QDs and C dots, respectively (Fig.
S4†). The much lower-lying CB edge of C dots would render the
electron transfer from CdS QDs to C dots and thus an efficient
charge separated state.
uniformly distributed surface with a tightly stacked
morphology (Fig. 3c and e), reflecting that grafting of C dots or
GO 2D planes on ITO substrate renders smoother CdS QDs
deposition. The interconnected structure between carbon
layer and CdS QDs may account for the close packing of these
multilayered films. 3D AFM images of the three films illustrate
even sharper contrast. As displayed in Fig. 3d and f, CdS QDs
were closely and compactly assembled on the GO or C dots top
layer, distinct from the morphology of pure CdS QDs film (Fig.
3b). Hence, AFM images strongly evidence the intimate
The film morphology of the CdS QDs, GO/CdS QDs and C
dots/CdS multilayer films are characterized by AFM (Fig. 3).
The surface of CdS thin films was lumpy as a result of too much
agglomerated CdS QDs on the surface (Fig. 3a). By contrast,
the two hybrid films of C dots/CdS and GO/CdS showed
Fig. 4. Top view SEM images and cross-sectional SEM
micrographs for hybrid films of (a, b, c) CdS QDs, (d, e, f)
(GO/CdS)6 and (g, h, i) (C dots/CdS)6 assembled on ITO
substrate.
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Fig. 5. UV-Vis diffuse reflectance spectra of (a) multilayers of
pure CdS QDs, C dots/CdS hybrid films, and GO/CdS hybrid
films (b) Absorption of multilayered films at wavelength of 500
nm. (c) Plot of [F(R)hν]2 versus hν for these hybrid multilayered
thin films.
integration of CdS QDs and C dots or GO via the LBL-assembly
build-up.
As shown in Fig. 4, the SEM images of pure CdS QDs thin film
(Fig. 4a-c) exhibit obvious surface defects, which would not be
beneficial for transporting of photogenerated electron-hole
pairs. By contrast, over a large region of the substrate, the
GO/CdS QDs and C dots/CdS QDs multilayered films with a
distribution over 50 micrometres are uniformly spread on ITO
substrate by electrodeposition and S-CBD method, and no
obvious cracks are observed. Fig. 4c, f, i display cross section
images of CdS QDs, GO/CdS QDs film and C dots/CdS QDs film,
C dots/CdS QDs film being the thinnest whereas CdS QDs film
the thickest. Further SEM measurements indicated these
hybrid films became thicker with increasing deposition cycles
(Fig. S5†). Among these three hybrid films, C dots/CdS QDs and
GO/CdS QDs films exhibit rather smooth surfaces while those
of pure CdS QDs film are clearly uneven as the QDs
agglomerated on the ITO surface, which might potentially
cause recombination of photogenerated electron-hole pairs of
CdS and thus reduce photocatalytic activity.
Fig.6. (a) Photocurrent-time curves of (C dots/CdS)3 with
different C dots deposition time at the same three cycles. (b)
Photocurrent-time curves of pure CdS QDs films, GO/CdS
multilayered films and different deposition cycles of
C
dots/CdS multilayered film on glass ITO substrates in 0.05M
Na2S aqueous solution under Xe light irradiation (43mW∙cm-2).
Applied potential: 0V vs SCE. (c) Transient photocurrent
responses of these hybrid films in 0.05 M Na2S aqueous
solution at zero bias versus Pt counter electrode under Xe light
irradiation. (d) Photocurrent of multilayered films versus the
number of deposition cycles. (e) Images of C dots/CdS
multilayered films on glass ITO substrates. (f) Electrochemical
impedance spectroscopy (EIS) Nyquist plots of CdS QDs film,
GO/CdS and C dots/CdS multi-layered film with the same
number of deposition cycles in 0.1 M Na2S aqueous solution at
zero bias versus Pt counter electrode under 3W LED light
irradiation; the amplitude of the sinusoidal wave was set at 5
mV, and the frequency varied from 100 kHz to 0.05 Hz.
The layered structure of C dots/ CdS was probed by SEM.
The optimal time of 7 seconds for the C dots layer deposition
corresponds to a thickness of only 3 nm, thus it is difficult to
clearly identify a single C dots layer. However, the interface
between every thicker layer of (C dots/ CdS) is distinguishable.
For instance, the cross-sectional image of (C dots/CdS)4 shows
discernable multilayer structure, which is distinct from that of
pure (CdS)4 layer exhibiting no obvious interfaces within the
structure (Fig. S6†).
Table S2†. This result indicates a bandgap narrowing with the
increase of deposition cycles for the C dots/CdS hybrid film,
which can be ascribed to increased interfacial interaction
between CdS QDs and C dots that enhance electronic coupling.
We also tested the UV-vis diffuse reflectance spectra of (C
dots)6 and (GO)6, the absorbance intensity of (C dots)6 is higher
than (GO)6 at visible light range, this means that C dots can
make full use of visible light (Fig. S7†). Fig. 6a shows transient
photocurrent response for C dots/CdS hybrid films with three
deposition cycles. The only difference is the deposition time of
C dots. Clearly, the film with medium deposition time of 7s
generates the largest photocurrent. Too long deposition time
leads to thicker C dots layer, which increase photon absorption
but may hinder the electron-hole pair separation from
neighboured CdS layer and hence reduce the photocurrent of
UV-vis diffuse reflectance spectra were used to test the
optical properties of the prepared hybrid films. It can be
clearly seen from Fig. 5a that pure CdS QDs, multilayered films
of GO/CdS and C dots/CdS all exhibit strong absorbance in
visible light range, with GO/CdS even exhibiting a broader
absorption extending to the infrared region. For C dots/CdS
films with different deposition cycles (Fig. 5b), the absorbance
intensity at 500 nm rises accordingly with the cycles increase.
Tauc plots obtained via the transformation based on
Kubelka−Munk funcꢀon versus energy of light is shown in Fig.
5c.23 The estimated bandgaps of the composite films C
dots/CdS hybrid films with assembly cycles of 3, 5, 6, 7, and 6
layers of CdS, GO/CdS films are shown to be around 2.43 eV,
2.25 eV, 2.14 eV, 2.05 eV, 2.33 eV and 1.30 eV, respectively, in
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hybrid films. While too short deposition time simply
deteriorates light absorption of the layer and charge transfer
with CdS. Thus it is of importance to balance photon
absorption and separation of interfacial photogenerated
electron-hole pairs. Specifically, the layer thickness can be
roughly estimated by comparing with C dots layer deposited
on ITO with a much longer deposition time of 120 second,
which showed a thickness about 52 nm (Fig. S8†). Meanwhile,
different stacking patterns between CdS QDs and C dots could
also influence the photocurrent of these hybrid films (Fig. 6b).
Consequently,
a directly alternating deposition pattern
between CdS and C dots, i.e., (C dots/CdS)n, is adopted in
order to maximize the photocurrent output. Fig. 6c and d show
transient photocurrent response for CdS QDs, C dots, and (C
dots/CdS QDs)n (n= 3, 5, 6, 7) multilayered films under Xe lamp
light irradiation (43 mW∙cm-2) at zero bias condition. Obviously,
the photocurrent of C dots/CdS hybrid film was enhanced
significantly compared to that of pure CdS QDs, C dots films
and GO/CdS with the same number of deposition cycles. Note
that the largest photocurrent density of 2.6 mA∙cm-2 for (C
dots/CdS)6 is much higher than the values reported in relevant
literatures.16, 20, 22, 23, 43 In addition, the photocurrent density of
(C dots/CdS)6 can still reach a high value of 1.25 mA∙cm-2 even
under sole visible light irradiation (3W LED irradiation,
15.4mW/cm2, λ>420nm, Fig. S9†). The observaꢀon can be
attributed to the directly alternating deposition between CdS
QDs and C dots, which leads to the formation of closely
interfacial contact and renders the effective transfer of the
photoexcited electrons in CdS QDs to the adjacent C dots film.
Hence, C dots serve as efficient electron collector and
transporter, and thus suppress recombination of
photogenerated electron-hole pairs of CdS, in agreement with
the above spectroscopic and Mott-Schottky plots evidence. In
addition, the photocurrent of the C dots/CdS multilayered
films increases gradually with the number of deposition cycles,
reaches a maximum value of 2.6 mA∙cm-2 at 6 cycles, and then
decreases with an even larger cycle number. This trend is
Fig. 7. Photocatalytic performances of the C dots/CdS QDs
hybrid films were evaluated by photoreduction of nitro-
aromatic compounds to corresponding amino organics in
aqueous phase under visible light irradiation.
photocorrosion of CdS may be curbed and the stability of (C
dots/CdS)n layers is thus enhanced. The findings herein are in
consistent with previous reports on the suppression of
photocorrosion of photosensitizers via hybrid architectures.45,
46
To further understand the superior performance of C
dots/CdS QDs multilayered film over GO/CdS QDs and pure
CdS QDs in enhancing the transfer of photogenerated
electron-hole charge carriers, electrochemical impedance
23, 47
as a method to monitor charge
spectroscopy (EIS)22,
transfer process on the electrode and at the contact interface
between electrode and electrolyte, was utilized to probe the
frequency response of the electrode and electrolyte system. In
Fig. 6f, the Nyquist plots show that
C dots/CdS QDs
multilayered films has smaller semicircle than that of GO/CdS
QDs films with the same number of deposition cycles, which
reveals more efficient interfacial electron transfer and faster
electron transport process at the electrode-electrolyte
interface for the former. The result explains although GO/CdS
films exhibit broader absorption range, they generate a
smaller photocurrent density than C dots/CdS QDs.
possibly caused by the competition of photon absorption
between CdS QDs and C dots,23,
44
as is confirmed by
measuring the photocurrent of composite films with different
stacking methods illustrated in Fig. 6b. Fig. 6e is the digital
images of the multilayered films with different deposition
cycles. With increasing layer numbers, the color gradually
deepens accordingly, indicative of mounting augment in visible
light absorption.
In summary, the enhanced photocurrent for the C dots/CdS
hybrid film corroborates the role of C dots in promoting
separation of photogenerated electron-hole pairs. Additionally,
integration of C dots with CdS QDs based on layer-by-layer
alternating deposition are beneficial for charge separation and
transfer due to the formation of intimate interfacial contact,
which also contribute to the photocurrent improvement.
Moreover, more efficient charge transfer of the C dots/CdS
films, as disclosed by EIS, implying they are of great potential
for photocatalytic applications.
We made supplemental photoelectrochemical experiments
to test the photostability of the hybrid films up to 8 days (Fig.
S10†). The photocurrent of pure CdS QDs film fell significantly
to about 42% of its original values after 8 days, whereas the C
dots/CdS hybrid film exhibited much more stable photocurrent
output with 1.7% decrease. The result indicated that the C
dots layer could effectively serve as a protective coating for
the CdS QDs as well. We speculate that as C dots can act as
electron acceptors, the electrons from conduct band of CdS
upon photoexcitation can be efficiently immigrated to the
surface of C dots. Through this electron transfer process, the
As a proof-of-concept, the C dots/CdS hybrid film was
applied for photocatalytic reduction of nitro-benzene
derivatives in aqueous phase under low power visible light
irradiation. Using LED lamps as light sources offer several
advantages such as high photon efficiency, low electrical
power, high power stability, emission in broader spectral
6 | J. Name., 2012, 00, 1-3
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reactions. Meanwhile, sacrificial reagent replenishes electrons
to make the photocatalytic cycle sustainable.
Conclusions
To sum up, in this work a facile LBL method is adopted to
manufacture uniform C dots/CdS nano-hybrid films, which
demonstrate stable and superior photoelectrochemical and/or
photocatalytic performance upon visible light irradiation. It is
found the photocurrent density of the C dots/CdS hybrid film
could reach 2.6 mA∙cm-2, which is higher than ever reported in
relevant literatures. Meanwhile, the films also showed
efficient photocatalytic reduction of nitro-aromatic
compounds to corresponding amino organics in aqueous
phase under low power visible light irradiation. The
Scheme 2. Proposed schematic mechanism for C dots/CdS
involved photocatalytic process.
wavelengths, and no need for cooling during long time
operation.48 Photocatalytic reduction of p-nitroaniline on the C
dots/CdS QDs film under a 3W LED light was presented as a
typical example (Fig. S11†), which was monitored by the UV-vis
light absorption spectra. Under the irradiation for 3 hours, the
380 nm absorption peak corresponding to p-nitroaniline
vanished and new peaks at 300 and 240 nm appeared,
indicating the formation of p-phenylenediamine. The products
of this reduction reaction have also been confirmed by high
performance liquid chromatography (HPLC). With prolonged
reaction time to 6 hours, the reduction completed with a
conversion rate as high as 99% (Fig. S11b†). Note the C
dots/CdS QDs hybrid film demonstrated the best performance
in comparison with GO/CdS and CdS QDs film with the same
number of deposition cycles, in the order of C dots/CdS>
GO/CdS> CdS QDs. For example, with the same reaction time
of 2.5 hour, the conversion of p-nitroaniline using
photocatalyst C dots/CdS is 70%, while that of GO/CdS is 49%
and for CdS QDs the number is only 24%.The result from blank
experiment under the same condition without photocatalyst is
supplied in Fig. S12†. To test the photocatalytic activity of the
films on various compounds, we studied three other different
reactions (Fig. 7b, c, d) and all obtained similar trends, i.e., C
dots/CdS films exhibited the best photocatalytic performance.
All of these observation point to the crucial roles of C dots in
the hybrid films for photocatalysis. To study the photostability
of C dots/CdS QDs, three continuous recycling measurements
were performed, where C dots/CdS QDs composite film
exhibited high photoactivity and good photostability (Fig.
S13†). In addition, it is noteworthy that the use of C dots/CdS
QDs hybrid film as photocatalyst could avoid the tedious
procedure of powder catalyst separation after photoreaction.
Based on the above results, a schematic mechanism for C
dots/CdS involved photocatalytic application is provided in
Scheme 2. Upon visible light irradiation, both C dots and CdS
are excited and separated electrons and holes are thus
generated within each entity. The lower-lying CB edge of C
dots enable the electron transfer from CdS QDs to C dots, and
concomitant movement of holes in the VB of C dots to that of
CdS QDs. Consequently, efficient charge separated state is
formed and the electrons thereafter lead to reduction
significantly
enhanced
photoelectrochemical
and/or
photocatalytic efficiency of C dots/CdS were attributed to the
following reasons: First, the combination of C dots with CdS
form intimate interfacial contact and facilitate charge
separation and transfer; Secondly, C dots herein act as
efficient electron sinks, accepting photoinduced electrons
from CdS and thus supress electron-hole pair recombination.
Last, the presence of C dots augments visible light absorption
of the composites, leading to more photogenerated carriers
and ultimate photocatalytic efficiency improvement. Our
findings herein thus establish a new frontier on the rational
design and fabrication of well-controlled hybrid films with
built-in heterojunctions for solar light conversion. The
conclusions section should come at the end of article, before
the acknowledgements.
Acknowledgements
This work is supported by National Basic Research Program of
China (973 Program) No.2012CB933102, National Natural
Science Foundation of China (NSFC. 21233001, 21190034
，
21073079), Specialized Research Fund for the Doctoral
Program of Higher Education (SRFDP. 20110211130001), the
Fundamental Research Funds for the Central Universities and
111 Project.
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