.

1. Contents lists available at ScienceDirect
Applied Catalysis B: Environmental
journal homepage: www.elsevier.com/locate/apcatb
Energy level tuning of CdSe colloidal quantum dots in ternary 0D-2D-2D
CdSe QD/B-rGO/O-gC3N4 as photocatalysts for enhanced hydrogen
generation
Lutfi K. Putria,b
, Boon-Junn Nga
, Wee-Jun Ongc,d
, Hing Wah Leee
, Wei Sea Changf
,
Abdul Rahman Mohamedb
, Siang-Piao Chaia,
*
a
Multidisciplinary Platform of Advanced Engineering, Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway,
47500, Selangor, Malaysia
b
School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300, Nibong Tebal, Pulau Pinang, Malaysia
c
School of Energy and Chemical Engineering, Xiamen University Malaysia, Selangor Darul Ehsan, 43900, Malaysia
d
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China
e
Nanoelectronics Lab, MIMOS Berhad, Technology Park Malaysia, Kuala Lumpur, 57000, Malaysia
f
Multidisciplinary Platform of Advanced Engineering, Mechanical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway,
47500, Selangor, Malaysia
A R T I C L E I N F O
Keywords:
0-Dimensional
Quantum dots
CdSe
2-Dimensional
g-C3N4
Graphene
Doping
Photocatalyst
Hydrogen
A B S T R A C T
Colloidal quantum dots (QD) electronic properties are tailorable via modifications of its quantum confinement
environment. Herein, surface-chemistry-mediated approach through the application of unique surface thiol li-
gands (thioglycolic acid, glutathione, 3-mercaptopropioninc acid and N-Acetylcysteine) exhibited energy level
and gap shifts in aqueous CdSe QDs. Trends in the photocatalytic performance employing ligand-specific CdSe
QDs are consistent with their respective measured energy and gap level. Results underscore that a still under-
utilized mean of surface-chemistry-mediated modification of colloidal CdSe QDs can be employed as a versatile
parameter in the performance optimizations of QD photocatalysts for photocatalytic hydrogen (H2) reactions.
This optimized CdSe QD is further utilized as sensitizers to bolster and facilitate effective charge transfer across
the ternary, multi-level heterointerfaces of 0D-2D-2D CdSe QD/B-rGO/O-gC3N4. Upon loading, the ternary
composite achieved a maximum H2 evolution of 1435 μmol h−1
g−1
even without the help of precious metal co-
catalyst which is otherwise required when used as individual unit. This augmented photoactivity is attributed to
the synergetic effect of CdSe sensitization and p-n junction administered by the p-type B-rGO and n-type O-
gC3N4.
1. Introduction
In the recent years, colloidal QDs have garnered immense attention
as photocatalysts due to their numerous advantages, deeming them
superior to organic dyes as sensitizers. This stems from their remarkable
properties as they exhibit a strong and broad absorption, high stability
under the presence of photo and chemical radiation, as well as a large
surface-volume ratio [1]. This interest towards colloidal QD has erupted
all the more as they possess characteristically tunable set of electronic
properties, a trait which is affiliated with the confinement effect of their
charge carriers (electrons and holes) inside their unique 1-Dimensional
(1D) structure. Features of a QD such as nanoparticle size and structure
are therefore strongly correlated to its optical and electrical properties,
giving them the capacity for customization, and could be put to good
advantage for application such as photocatalysis [2,3].
Particularly, CdSe QDs are a notable class of QDs photocatalysts.
Traditionally, these CdSe QDs are synthesized via the organometallic
route, using alkyl cadmium as the Cd precursor and an organometallic
compound, trioctylphosphine selenide (TOPSe) as the Se precursor, in
an organic medium consisting of coordinating solvents typically trioc-
tylphosphine oxide (TOPO), oleylamine, hexadecylamine (HDA), etc.
[4,5]. This approach however is unfavorable due to the toxicity of the
reagents used and the strenuous requirement of synthesis conditions,
which strictly requires inert atmosphere and high temperatures above
https://doi.org/10.1016/j.apcatb.2020.118592
Received 29 August 2019; Received in revised form 17 October 2019; Accepted 3 January 2020
Abbreviations: 2D, 2-dimensional; QD, quantum dots; NAC, N-acetyl-L-cysteine; GSH, L-glutathione reduced; MPA, 3-mercaptopropionic acid; TGA, thioglycolic acid
⁎
Corresponding author.
E-mail address: chai.siang.piao@monash.edu (S.-P. Chai).
Applied Catalysis B: Environmental 265 (2020) 118592
Available online 07 January 2020
0926-3373/ © 2020 Elsevier B.V. All rights reserved.
T

2. 300 °C. Furthermore, being hydrophobic in nature, CdSe QDs synthe-
sized from this route cannot be used directly for processes in aqueous
medium such as the case for photocatalytic H2 evolution, and thus re-
quires additional ligand exchange step to render them hydrophilic.
Therefore, in this work, an aqueous-based synthesis of CdSe QD was
employed using a variety of thiols as the capping ligands and water-
soluble Cd(NO)3 and NaHSeO3 as the precursors. Compared to the or-
ganometallic route, this alternate approach via reflux has low toxicity,
mild reaction conditions and is highly desirable as it is stable in water
under various pH values [6].
As of late, QDs have been emerging as efficient light harvesting
materials for H2 generation [7–9]. Many progresses have been made for
cadmium-based QDs via optimization of synthesis procedures and li-
gands, as well as addition of co-catalysts mostly in the form of Fe-, Co-,
or Ni-based complexes [10–12]. Particularly, the size-dependent band
gap of semiconductor QDs is a widely studied quantum confinement
effect [4,5]. Zhong et al. has demonstrated this band gap energy tun-
ability of CdSe QD by tailoring the size of the QDs and how this has
affected the band levels compatibility in the composite and conse-
quently its implication to H2 generation. The specific size-range of QDs
was attained by tuning reaction parameters such as the reaction pH and
aging duration [13]. Alternately, this work aims to modulate the
properties of CdSe through modification of the surface chemistry of
QDs. Complementary to this control of the QD band gap by modifica-
tion of the nanocrystal size, previous studies have also reported that the
energy levels of semiconductor QDs can be tuned up to a magnitude 2
eV through surface chemistry modification [14–16]. Particularly, this
work achieved this remarkable control through the different use of
capping ligands assigned to the CdSe semiconductor nanocrystals, al-
lowing for a well-defined, straightforward and highly-tunable chemical
system. The variety of thiol chapping ligands used in this work were
thioglycolic acid (TGA), glutathione (GSH), 3-mercaptopropioninc acid
(MPA) and N-Acetylcysteine (NAC) while reaction conditions (e.g. pH,
time, temperature) were maintained for each reaction set. By changing
this identity of the chemical binding group, the dipole moment of in-
dividual ligands changes the overall strength of the QD-ligand surface
dipole, hence shifting the vacuum energy and, in turn, the valence band
maximum (VBM) and conduction band minimum (CBM) of colloidal QD
semiconductors [17,18]. This form of modulation in CdSe QDs prop-
erties is investigated as band edge positions of QD semiconductors are
vital to the functionality of a photocatalyst system.
Meanwhile, 2D-2D graphitic carbon nitride (gC3N4) heterostructure
nanocomposites have evoked interdisciplinary research fascination due
to the unprecedented properties of gC3N4 and its resultant face-to-face
interfacial benefits [19–21]. The layered heterojunction of dissimilar
2D materials is envisaged to give rise to positive impacts on charge
transfer and separation as a result of its atomically well-defined ultra-
thin-interface [22,23]. Finally, this work will combine both of these
elements by incorporating CdSe QDs to a previously developed photo-
catalyst comprising of n-type O-gC3N4/p-type B-rGO [24]. This allows
the engineering of a cascading heterojunction architecture, whereby
CdSe QDs, functions as a sensitizer capable of absorbing longer wave-
length light to generate extra electrons in the photocatalyst system.
These electrons will consequently be utilized in the p-n junction at the
B-rGO/O-gC3N4 heterointerface. Essentially, this sequence of electron
flow permits an effective multi-level charge transfer which synergisti-
cally resulted in the remarkably augmented H2 photoactivity.
2. Experimental section
2.1. Materials for the synthesis of CdSe QDs
Cadmium nitrate tetrahydrate (Cd(NO3)2.4H2O, 99.99 %), N-Acetyl-
L-cysteine (NAC, ≥ 99.0 %), L-cysteine (LC, ≥ 98.0 %), L-glutathione
reduced (GSH, ≥ 98.0 %), sodium hydroxide (NaOH, ≥ 98.0 %), so-
dium hydrogen selenite (NaHSeO3, ≥ 99.0 %), sodium borohydride
(NaBH4, ≥ 98.0 %), and 5 % Nafion 117 solution which were pur-
chased from Sigma Aldrich. Furthermore, 3-mercaptopropionic acid
(MPA, ≥ 98.0 %) and thioglycolic acid (TGA ≥ 98.0 %) were pur-
chased from Necalai Tesque. All chemicals were analytical reagent
grade and used without further modification and purification.
Deionized water (DI) water was used (> 18.2 MΩ cm resistivity) in
cases where water was mentioned.
2.2. Synthesis of different ligand-capped CdSe QD
Typically, 0.5 mmol of Cd(NO3)2.4H2O was dissolved in 20 mL of
water. After 10 min, 1 mmol of NAC, MPA, GSH or TGA was added and
white precipitate immediately formed, giving a turbid solution. The pH
value of the resultant solution at this stage was in the range 2-3. The pH
value was then adjusted to 11 with 0.1 M of NaOH solution, and the
solution changed from cloudy to clear in the process. The aqueous
mixture was then transferred to a three-necked flask where the solution
was consequently subjected to N2 purging for 30 min, to remove O2 in
the mixture. Consequently, 0.1 mmol of NaHSeO3 and excess NaBH4
were then mixed and immediately added to the above solution. Here,
NaHSeO3 was reduced by NaBH4 to generate Se2−
ions which could be
readily used in the reaction to form CdSe. The reaction mixture was
heated to 100 °C and then refluxed for 4 h under continuous N2 flow
and with an attached condenser at the outlet. In order to eliminate the
unreacted species and unwanted byproducts, the ligand-capped CdSe
QD was precipitated with cold, refrigerated isopropanol and separated
by centrifugation at 11,000 rpm for 15 min. The precipitates were then
dried in a vacuum oven at 60 °C overnight. The collected product was
dissolved in DI water (50 mg/mL) for storage. To evaluate the effect of
precursor concentration to the H2 photocatalytic activity, the Cd2+
:
Se2−
ratio was varied from 1 : 0.005–1 : 0.4 by keeping 0.5 mmol of Cd
(NO3)2.4H2O fixed and adjusting the appropriate molar amount of
NaHSeO3.
2.3. Synthesis of 0D–2D–2D TGA-capped CdSe QD/B-rGO/O-gC3N4
nanocomposite
In the development of the ternary composite, a two-step method
was employed. First, the implantation of TGA-capped CdSe on to B-rGO
sheets was promoted by adding an appropriate mass of B-rGO in the
starting solution. The subsequent synthesis and extraction of CdSe QD/
B-rGO nanocomposite follows suit as per section 2.2. After drying, the
CdSe QD/B-rGO was then redispersed in 20 mL of water. A prescribed
mass O-gC3N4 was then added to the solution followed by sonication of
the mixture for 30 min. Afterwards, to foster the sound adhesion of the
ternary 0D–2D–2D TGA-capped CdSe QD/B-rGO/O-gC3N4 nano-
composite, the aqueous mixture was subjected to vigorous stirring for
another 2 h. After its completion, the solution was lyophilized for
drying and finally redispersed back in DI water. For optimization, dif-
ferent weight percentages of O-gC3N4/2BrGO to CdSe QD were syn-
thesized which were termed as CdSe/xBGCN whereby x = 1, 2, 5, 10
and 20.
2.4. Materials characterization
Transmission electron microscopy (TEM) and high-resolution TEM
(HR-TEM) were obtained using FEI TECNAI G2 S-Twin. X-ray diffrac-
tion (XRD) patterns were collected on a Bruker D8 Discover X-Ray
Diffractometer in the diffraction angle range (2θ) 15 – 60° at a scan rate
of 0.02°s−1
operated at 40 kV and 40 mA with Ni-filtered Cu Kα ra-
diation (λ =0.154056 Å). XRD samples were prepared by depositing a
thick layer of samples on a silicon wafer. Fourier Transform Infrared
(FTIR) spectra were measured using Thermo-Nicolet iS10 FTIR
Spectroscopy in the range of 400 – 4000 cm−1
with a resolution of 4
cm−1
using the KBr pellet method. The X-ray photon spectroscopy
(XPS) spectra were acquired using a scanning X-ray microprobe PHI
L.K. Putri, et al. Applied Catalysis B: Environmental 265 (2020) 118592
2

3. Quantera II (Ulvac-PHI) with a monochromatic Al Kα X-ray source (hv
= 1,486.6 eV) which operated at 25.6 W with a beam diameter of 100
μm. The wide scan analysis was performed using a pass energy of 280
eV with 1 eV per step for elemental screening. Narrow scan analysis was
performed at a pass energy of 112 eV with 0.1 eV per step for chemical
state analysis. Valence band XPS was performed using a pass energy of
69 eV with 0.125 eV per step for valence band analysis. The optical
properties and absorbance spectra of the sample were determined by an
Ultraviolet-visible (UV–vis) spectrophotometer (Agilent Cary 100) at
ambient temperature in the wavelength range of 200–800 nm.
Photoluminescence (PL) spectra was recorded using a fluorescence
spectrometer (Perkin Elmer, LS55) at ambient temperature. An excita-
tion wavelength of 350 nm and 450 nm was used to measure the
photoluminescence intensity of carbon nitride and CdSe, respectively.
Photoelectrochemical measurements were conducted using a CHI
6005E electrochemical workstation. A standard three-electrode cell
setup was employed using Ag/AgCl and Pt rod as the reference and
counter electrode, respectively. A 0.5 M of aqueous Na2SO4 solution
was used as the electrolyte. The working electrode was assembled by a
drop casting method within an area of 1 cm2
. In brief, 2 mg of CdSe or
CdSe/xBGCN was dispersed in 1 mL of water and 1 mL of 1 % of Nafion
solution. Then 20 μL solution mixture was dropped and dried in a va-
cuum oven. Mott-Schottky plot was evaluated at a potential range of -2
– 0.2 V vs. Ag/AgCl at a frequency of 1.2 kHz. In addition, transient
photocurrent results are acquired using a bias voltage of 0.1 V and
Nyquist plots was obtained in the frequency range 1 – 106
Hz using an
AC perturbation signal of 5 mV. For the acquisition of photocurrent and
Nyquist plots, a Xe arc lamp (CHF-XM-500 W) with a visible light filter
(λ > 400 nm) was used as the light source.
2.5. Photocatalytic H2 activity test
The photocatalytic H2 half reaction test was carried out at ambient
temperature and pressure under continuous N2 flow. In brief, 30 mg of
photocatalyst sample was added in 120 mL of aqueous solution con-
taining 0.1 M ascorbic acid (AA) with an adjusted pH of 3.5. The so-
lution mixture was initially sonicated for homogeneous mixing before
being transferred into the reactor vessel. Prior to the photoreaction, the
solution mixture was purged with high velocity N2 gas to eliminate air
in the system. A Xe arc lamp (CHF-XM-500 W) fitted with an optical
filter (λ > 400 nm) was used as the visible light source. The gaseous
reaction products were carried by N2 gas to an integrated gas chro-
matography (Agilent 7829A, Hayesep Q and mol sieve column, Argon
gas as carrier) downstream from the reaction vessel. This gaseous
product was sampled at every interval of 30 min until a full reaction
duration of 6 h has been completed. When triethanolamine (TEOA) was
used as the sacrificial reagent instead of AA, the solution preparation
was slightly different with the inclusion of 20 vol% of TEOA in 120 mL
of aqueous solution.
3. Results and discussion
3.1. Synthesis mechanism
In this work, an aqueous synthesis route for CdSe QD by simple
refluxing was performed to develop colloidal CdSe QDs. Selected sur-
face ligands which included NAC, MPA, GSH and TGA were employed
in the construction of CdSe QD and its consequences on the photo-
catalytic activity were studied. The molecular structure of each of these
ligands are depicted in Fig. 1 and all of these ligands belong to the thiol
family (R-S-H).
In the synthesis of all CdSe QD using different capping agents, the
reaction conditions were kept constant i.e. the molar ratio of [Cd2+
]:
[HSe−
]: thiols = 1.0:0.2:2.0, aging duration of 4 h, temperature of 100
°C and at a pH of 11. The pH value was tuned to pH 11 due to QDs being
generally better dispersed and more stable in an alkaline medium.
When Cd(NO3)2.4H2O precursor was added to the thiol suspension, the
metal ions (Cd2+
) were primitively capped to form Cd(R-S-H)2+
complex due to the chelating mechanism of the thiol group. Upon the
addition of Se2-
precursor, it is thermodynamically favorable for Se2-
ions to integrate into the Cd(R-S-H)2+
complex, in preference to dis-
placing the thiols (R-S-H) to form larger CdSe particles [25]. Subse-
quently, the nucleation of CdSe QD and formation of small quantum
dots particles occurred when the two precursors are combined together
at an aging temperature of 100 °C and in the span of 4 h. It is worth
mentioning, occurrences of agglomeration of CdSe QD during the
synthesis was impeded by the steric hindrance derived from the capping
agent.
In the fabrication of CdSe/xBGCN nanocomposites, the synthesis
protocol was carried out in two steps, firstly in the formation of binary
CdSe QD/B-rGO which is followed by the successive integration of O-
gC3N4 to assemble ternary 0D-2D-2D CdSe QD/B-rGO/O-gC3N4 nano-
composites. In this respect, B-rGO was added in the initial reaction
mixture and under these circumstances, B-rGO could serve as a sub-
strate for the anchoring of TGA-capped CdSe on to its sheet. This ap-
proach afforded the sufficient and intimate contact between the two
components and, at the same time, selectively tethered CdSe QD on to
B-rGO. After precipitation and vacuum-drying of the TGA-capped/B-
rGO, a defined amount of O-gC3N4 was added in the solution mixture
and sonicated for 30 min.. The attraction and therefore the assembly of
these dissimilar 2D materials is preferred due to the oppositely charged
O-gC3N4 and B-rGO which is revealed through zeta potential result (Fig.
S1). To further consolidate the strong contact of the ternary composite,
the synthesis process is concluded by room-temperature stirring for 2 h.
This entire synthesis protocol is illustrated in Fig. 2.
Fig. 1. Molecular structure of the five types of capping ligands used in the CdSe
QD synthesis.
L.K. Putri, et al. Applied Catalysis B: Environmental 265 (2020) 118592
3

4. 3.2. Structural characterization of different ligand-capped CdSe and
ternary TGA-capped CdSe QD/B-rGO/O-gC3N4 nanocomposite samples
The morphology of the as-synthesized samples was analyzed using
TEM and the images are provided in Fig. 3. From Fig. 3a, the as-pre-
pared TGA-capped CdSe QDs exhibit a spherical structure with a rela-
tively wide size distribution. From this TEM image, the prepared CdSe
QDs were calculated to have an average particle size of 5.98 nm. In
addition, it is observed that these CdSe QDs possess good dispersity on
the B-rGO sheets, thereby alluding the good affinity between the two
components. HR-TEM images further confirms the successful synthesis
of CdSe QD, which displays a lattice fringe with a spacing of 0.22 nm,
belonging to its (111) facet (Fig. 3b). Finally, Fig. 3c justifies the
construction of the ternary CdSe/B-rGO/O-gC3N4 architecture. The
spherical entities outlined in yellow are the CdSe QD and it can be
observed that these CdSe QDs are anchored on the B-rGO sheets (blue
outline), which embody a sheer, transparent sheet-like form. The B-rGO
sheet is also in direct contact with O-gC3N4 (red outline) which man-
ifests its characteristic cloud-like, porous feature, thereby confirming
the unity of the three components into a ternary nanocomposite. The
CdSe/B-rGO/O-gC3N4 nanocomposite, from here on are addressed as
CdSe/BGCN, and the schematic of the ternary structure is also provided
in Fig. 3d.
XRD patterns in Fig. 4 revealed that all NAC, MPA, GSH and TGA-
capped CdSe exhibited broad peaks, suggesting that the CdSe QD par-
ticles formed are small in size according to the Scherrer’s relationship,
Fig. 2. Schematic illustration on the synthesis protocol of 0D-2D-2D CdSe QD/B-rGO/O-gC3N4 nanocomposite.
Fig. 3. (a) TEM image of TGA-capped CdSe/B-rGO obtained from the in-situ reflux reaction. (b) HR-TEM image of CdSe QDs with lattice fringes of 0.22 nm. (c) HR-
TEM image of ternary CdSe/B-rGO/O-gC3N4 (CdSe/10BGCN) nanocomposite. (d) Schematic of CdSe/B-rGO/O-gC3N4 nanocomposite.
L.K. Putri, et al. Applied Catalysis B: Environmental 265 (2020) 118592
4

5. befitting of the nature of quantum dots. More importantly, they dis-
played the three typical distinct features of a zinc-blend structure [26].
This can be distinguished by the three peaks positioned at 2θ = 26.6°,
43.5° and 51.0° which resulted from the reflections of (111), (220) and
(311) planes, respectively (JCPDS file no. 19-0191). These are the
commonly observed crystal phases for CdSe formulated from the aqu-
eous route. Interestingly, despite the different capping ligand used on
CdSe, the crystal phases remain the same and no traces of wurtzite
structure are present due to the clear absence of peaks at 2θ = 35° and
46° [27]. For the case of CdSe/xBGCN nanocomposites, their XRD
patterns remain relatively the same to CdSe, except that with the in-
crease of BGCN in the composite, the (111) peak at 2θ = 26.6° gra-
dually increases in intensity and becomes more defined. This can be
attributed to the superimposition with O-gC3N4 distinctive (002) peak
at 2θ = 27.1° which overlaps with the broad peak of the CdSe (111)
plane.
The structural information of the thiol-capped CdSe and its com-
posites was examined by FTIR spectroscopy. The interaction between
the surface ligand TGA and the CdSe surface can be recovered from the
spectra in Fig. 5a. Apparent absorption bands in pure TGA ligand
comprise of (1) OeH vibration of adsorbed H2O at 3000–3600 cm−1
,
(2) SHe vibration at 2570–2670 cm−1
and (3) CO] stretching at 1700
cm−1
[28]. For TGA-capped CdSe, the characteristic absorption band
assigned to SeH disappeared, implying that the thiol group of TGA
molecules are attached to the surface atoms of CdSe via thiol groups.
Meanwhile, the characteristic band of C]O remains strong and slightly
shifts to reveal a carboxylate anion COOe band at 1620 cm−1
. This
presence of COOe in the CdSe QDs possibly resulted from the synthesis
protocol, which utilized NaOH and thus deprotonating the COOH
group. Overall, this result strongly suggest that TGA orients in a way
through which its thiols covalently bonds to Cd2+
ions on the surface of
QD while its hydrophilic carboxyl groups face outwards, making TGA-
capped CdSe QDs water-soluble [29]. The same interaction is observed
with the other thiol ligands (NAC, MPA and GSH) as determined by Fig.
S2. For illustration purpose, schemes are provided which represents
how each surface ligands bind to the CdSe QDs. Fig. 5b displays the
spectra by ternary CdSe/BGCN nanocomposite, the COOe bands from
TGA coordinating ligand remain evident. Moreover, in samples with
higher BGCN weightage, in the case of CdSe/2BGCN and CdSe/
10BGCN, peaks in the range 1200–1600 cm−1
began to take form,
which are ascribed to the stretching mode of CNe heterocycles from O-
gC3N4.
The XPS spectra recorded for TGA-capped CdSe and CdSe/10BGCN
are provided in Fig. 6. In the magnified survey spectra of Fig. 6a (Fig.
S3), compared to solitary CdSe, CdSe/10BGCN has an additional N peak
at the shoulder next to the Cd peaks, which proves the existence of
additional O-gC3N4 in the composite. Aside from Cd and Se, extra ele-
ments of C, O and S which are detected arise from the chemical com-
position of the TGA ligand which encapsulated the CdSe nanoparticle.
Based on the C 1s narrow scan (Fig. 6b), CdSe/10BGCN composite
displayed a broad peak which can be deconvoluted into three peaks
centered at ca. 284.42, 285.30 and 287.75 eV. The lowest binding en-
ergy peak at 284.42 eV can be accredited to the CeC bonds present in
B-rGO and in the carbon chain in TGA. The second and third peak are
accredited to the partially condensed amino groups of C–NH2 and the
sp2
aromatic C]NCe bonds present in the framework of O-gC3N4, re-
spectively [30]. This affirmed the additional presence of BGCN in the
composite. On the other hand, TGA-capped CdSe only exhibited a
narrow peak at 284.88 eV assigned to the carbon chain of the TGA and
a peak at higher binding energy 287.77 eV assigned to OCe=O tail
present in TGA.
For both cases, the Cd 1 s spectra consist of two peaks (Fig. 6c),
corresponding to Cd 3d5/2 and Cd 3d3/2 due to spin-orbital splitting. It
is apparent that CdSe/10BGCN displayed a broader peak than CdSe,
which can be due to additional chemical states ascribed to the renewed
interaction between CdSe and BGCN. The Cd 3d doublet in both has a
separation of about 6.7 eV which clearly indicates the existence of Cd
species from CdSe [31]. As for the Se 3d spectra (Fig. 6d), both de-
convolutions show the presence of two decoupled components between
53 and 55 eV as a result of 3d5/2 and 3d3/2 spins, which is a typical
occurrence of CdSe [32]. Again, the peaks exhibited by CdSe/10BGCN
are broader than that of CdSe. Furthermore, all these Cd and Se peaks
revealed a gradual shift to lower binding energy, thereby assuring the
assemblage of CdSe QDs and BGCN units. In addition to that, the XPS
spectra of bare B-rGO and O-gC3N4 are provided in Fig. S4 and their
constituent atomic composition are summarized in Table S1. In B-rGO,
a boron doping of 2.74 at% was introduced and are manifested in the
hexagonal lattice as BeO bonds from BC2O and BCO2 at lattice termi-
nations (vacancies and edges) and planar BCe bonds from graphitic BC3
[33]. Meanwhile, O-gC3N4 demonstrated an increase of O at % from
2.29 to 3.60 due to extraneous O as a result of doping manipulation.
These oxygen atoms are either attached to a nitrogen edge atom in the
Fig. 4. XRD peaks of (a) different ligand-capped CdSe and (b) a series of CdSe/BGCN nanocomposites.
L.K. Putri, et al. Applied Catalysis B: Environmental 265 (2020) 118592
5

6. triangular cavity or substituted in the same nitrogen atom to form
NeCOe bonds in the tri-s-triazine unit [34]. Overall, the results sub-
stantiated the occurrence of doping in the individual B-rGO and O-
gC3N4 samples.
The UV–vis spectra of different thiol capped CdSe are shown in
Fig. 7a. The spectra revealed that the absorption increased and shifted
towards lower energy in the order NAC > MPA > TGA > GSH, which
correlates to an energy gap of 3.6, 3.0. 2.4 and 2.3 eV respectively.
Especially for TGA and GSH, absorption shoulders at higher wavelength
at ca. ∼500 nm are formed, hence broadening the absorption range and
lowering the energy gap of the QDs. It is accepted that due to the
quantum effect, the size of the QD strongly influences the band gap
(Fig. 7b). More specifically, the larger the size of QD, the smaller the
band gap [35], thereby concluding that under identical reaction con-
ditions the QD size are in the order NAC < MPA < TGA < GSH from
smallest to largest. In other words, GSH as a capping agent provided the
highest nanoparticle growth rate as compared with other capping
agents. This may be originated from GSH higher steric hindrance and
Fig. 5. FTIR spectra of (a) TGA and TGA-capped CdSe (inset shows a schematic on the binding orientation of TGA to CdSe nanoparticle) and (b) a series of CdSe/
BGCN nanocomposites.
Fig. 6. (a) XPS survey spectra and narrow scans of (b) C 1s states (c) Cd 3d states and (d) Se 3d states.
L.K. Putri, et al. Applied Catalysis B: Environmental 265 (2020) 118592
6

7. larger susceptibility to thermal decomposition as compared to the other
ligands [36]. As a result, GSH was more turned off from the surface of
the QD.
On top of that, the Cd2+
: Se2+
ratio of TGA-capped CdSe was also
adjusted to test the effect of these variables on the photoactivity. Cd2+
:
Se2+
ratio plays as an important parameter since it has the ability to
tune the average particle size, number of density and size distribution
by “focusing” and “defocusing” the particle growth in the reaction so-
lution [37]. Based on the UV–vis results from Fig. 7c-d, as the amount of
Se precursor increased, there is an increase in the nucleation and hence
the density of the CdSe particles, thus resulting in the darkening of the
CdSe QD solutions. This is translated in the UV–vis by the increased in
the absorbance from Cd2+
: Se2+
ratio of 1 : 0.05–1 : 0.40. Further-
more, it was also speculated that by increasing the Se precursor, the
growth process of CdSe particle is also accelerated, leading to larger
size of CdSe particles which is responsible for the red shift observed in
the absorption spectra from 1 : 0.05–1 : 0.40. Overall, this transfor-
mation in the optical properties elucidates the light harvesting char-
acteristic of the CdSe samples which is influential to photocatalytic
activity and therefore helps to interpret the consequential trends in
photoactivity. Furthermore, the optical properties of the ternary na-
nocomposites CdSe/BGCN were also investigated. As shown in Fig. S5,
the absorption edge onset of lower loading CdSe/1BGCN begins at
∼530 nm, and with the increase of BGCN, this absorption edge red
gradually shifted to higher wavelength of approximately ∼545 nm for
the final sample CdSe/20BGCN. Furthermore, with this introduction of
BGCN, there is an increased background absorption in the higher wa-
velength region 500−800 nm. This is a common observation when
graphene-based materials such as B-rGO are added. Overall, these re-
sults alluded that BGCN adhered intimately to the CdSe QDs and it is
anticipated that the ternary composite CdSe/BGCN will improve the
photo-excitation efficiency and thereby contributing to the overall
enhancement in the photoactivity.
3.3. Photocatalytic activity enhancement and mechanism
The photocatalytic H2 generation performance was investigated for
the as-synthesized samples. In the case of QD, organic sacrificial re-
agents are ordinarily used to facilitate holes harvesting from QDs. In
particular, ascorbic acid is environmentally benign as it is non-toxic and
has reported to yield high H2 efficiency on QD when used as a sacrificial
electron donor and is thereby selected in this study. For different thiol-
capped CdSe, each demonstrated different photoactivity, whereby the
highest catalytic activity was exhibited by CdSe-TGA with a hydrogen
generation rate of 258.69 μmol−1
h-1
g−1
(Fig. 8a). The discrepancy in
the photocatalytic activity of different thiol-capped CdSe can be largely
contributed by (1) the band gap which is controlled by its nanocrystal
size and the (2) band edge positions administered by the surface
chemistry of the ligand/QD hybrid system. These deviations in the
electronic band properties of differently-capped CdSe QDs can be as-
cribed to the variation of ligand functionalization, ligand coordination
environment and/or ligand denotating and withdrawing properties
[38,39]. This successively shift the ionization energy and work function
of the QD, thereby allowing the engineering of the electronic band
structure of CdSe QD nanoparticles [40].
In detail, the valence band (VB) XPS results obtained in this work
has affirmed and shed light on this phenomenon, which is presented in
Fig. 9a. The valence band relativity of specific thiol-capped CdSe can be
retrieved from the VB XPS data. In complement to VB XPS, results from
Mott-Schottky plot of TGA-capped CdSe and UV–vis allow the overall
postulation of band energy diagrams of all thiol-capped CdSe. The flat-
band potential, Efb, of CdSe-TGA was first retrieved from the Mott-
Schottky (Fig. 9b) plot to be -1.8 V vs. Ag/AgCl or correspondingly
-1.19 V (vs. NHE, pH = 0) by applying the conversion equation (Efb vs.
Fig. 7. (a) UV–vis spectra and (b) Kubelka-Munk plots of different thiol-capped CdSe (c) UV–vis spectra and (d) Kubelka-Munk plots of increasing ratio of Cd : Se in
CdSe QDs. All spectra are recorded at a concentration of 0.25 mg mL−1
.
L.K. Putri, et al. Applied Catalysis B: Environmental 265 (2020) 118592
7

8. NHE = Efb vs. Ag/AgCl + EAgCl + 0.059 pH). Since it is routinely accepted
to estimate the CB minimum as 0.3 V more negative than Efb, the CB
minimum value of CdSe-TGA was calculated to be -1.49 V (vs. NHE, pH
= 0) [41]. The VB maximum of CdSe-TGA can then be obtained from
the summation with the band gap energy found from UV–vis (Eg =2.4
eV), which subsequently positioned VB maximum of CdSe-TGA at 0.9 V
(vs. NHE, pH = 0). The VB maximum of other thiol-capped CdSe QDs
are measured based on the relative displacement of the VB edge with
respect to this reference point. A shift to lower binding energy trans-
lates to the uplift of VB maximum while a shift to higher binding energy
translates to the lowering of the VB maximum [42,43]. The final band
structure for individual thiol-capped CdSe QDs can be compared from
Fig. 9c. From these calculations, the VBM position of NAC-capped CdSe
concurs well with what has been previously reported [13,44]. Parti-
cularly, the VBM position from the highest to lowest are in the fol-
lowing order CdSe-MPA > CdSe-TGA > CdSe-NAC > CdSe-GSH. A
total of 1.03 eV shift of VBM between the highest positioned MPA- and
lowest-positioned GSH-capped CdSe can be witnessed, indicating that
surface chemistry modification can induce an effect bearing to this
scale.
The CBM was found to be in the order CdSe-NAC > CdSe-
MPA > CdSe-TGA > CdSe-GSH from the highest to lowest reduction
potential. This sequence alluded that CdSe-NAC QDs have the highest
reducing power, therefore highest overpotential and capability to relay
electrons. However, regardless of this fact, CdSe-NAC QDs displayed the
worst photoactivity at 7.30 μmolg−1
h−1
. This is since this
overpotential is, at the same time, heavily compromised by its high
band gap energy of 3.6 eV. It is worth noting that all as-synthesized
colloidal CdSe QDs do not exhibit complete uniformity in size dis-
tribution due to variables in their nucleation and growth kinetics. Band
gap energy in colloidal QDs are highly susceptible to changes in their
size due to the quantum confinement effect. The marginal H2 photo-
activity that occurred under visible light excitation (λ > 400 nm) can
therefore be ascribed to the small fraction of larger sized population of
CdSe-NAC QDs that carry lower energy gaps. Next, CdSe-MPA QDs
which hold the highest VBM, by right, is the most auspicious, as they
have the highest foundation to generate higher reductive potential by
its CBM. Again, this is jeopardized by the relatively large band gap
energy of 3.0 eV which narrows its light absorption range, thus limiting
its photoactivity to third place after CdSe-GSH. Unlike CdSe-NAC, CdSe-
MPA exhibited an appreciable evolution of H2 since they possess a band
gap of 3.0 eV which is adequate to harness visible light (λ > 400 nm)
irradiation. From these results, it can be concluded that there is trade-
off between having higher reduction potential and owning a large band
gap. In all these cases, the dominating factor ultimately rests more to-
wards the light absorbing properties of the photocatalyst, whereby the
lower the band gap energy, the better the photocatalytic activity, as
observed in the case of CdSe-MPA.
In the case of CdSe-GSH, it has the least CBM value due to the low
position of its VBM. Nevertheless, its CBM still much surpasses the re-
quired potential for water reduction (H+
/H2 = 0 V vs. NHE) thereby
justifying the observation of H2 production. Although on one hand,
Fig. 8. H2 evolution rates of (a) different thiol-capped CdSe and (b) different Cd2+
: Se2+
molar ratio of CdSe-TGA using ascorbic acid as the sacrificial reagent. H2
evolution rates of a series of TGA-capped CdSe/BGCN nanocomposites under (c) ascorbic acid and (d) TEOA as sacrificial reagent.
L.K. Putri, et al. Applied Catalysis B: Environmental 265 (2020) 118592
8

9. CdSe-GSH has the lowest CBM value, on the other hand, CdSe-GSH
accommodates the lowest band gap energy of 2.3 eV and therefore has
the upper hand in its light harvesting ability compared to other thiol-
capped CdSe. Thus, this warrants its second place for the best photo-
catalytic H2 performance. Last but not least, CdSe-TGA emerged as the
victor and showed the best photoactivity since CdSe-TGA has the
overall finest share of properties. For one, they have a relatively small
band gap of 2.4 eV which is only 0.1 eV larger than CdSe-GSH. Second,
they have the highest VBM position after CdSe-MPA, which puts their
CBM 0.94 eV higher than CdSe-GSH after taking into account their band
gap energy. As a whole, this has resulted in an enhanced photoactivity
by a factor of 1.76 from CdSe-GSH to CdSe-TGA which yielded a H2
evolution rate of 154.33 and 258.69 μmolg−1
h−1
, respectively. In
addition to surface manipulation by different thiol ligands, the ratios of
Cd2+
: Se2-
of the initial precursor of CdSe-TGA used were also in-
vestigated, and results are presented in Fig. 8b. The results showed a
strong correlation of this ratio towards to the photocatalytic H2 activity
due to the transformation in the number of density and size distribution
of the resultant CdSe QDs. As expected, the photocatalytic activity in-
creases with increasing ratio from 1 : 0.05–1 : 0.4, which is in agree-
ment to the increased absorption and the shift towards higher wave-
length from their UV–vis absorbance spectra. This can be attributed to
more nucleation and faster growth rate of CdSe QDs due to the in-
creased concentration of the precursor. As per the quantum confine-
ment theory, CdSe QDs with larger particle sizes host smaller band gap
energy, thereby benefitting the photocatalytic activity. Hence, the best
Cd2+
: Se2-
ratio of 1 : 0.4 was used in the consequent coupling with
BGCN and their photocatalytic H2 results are depicted in Fig. 8c-d.
To reap the benefits of TGA-capped CdSe QD, it is further coupled
with BGCN composite to form a ternary heterostructure. First, it can
assist by having a relatively low band gap energy which enable its role
as photosensitizers. This presence of CdSe QDs will grant light of lower
energy and higher wavelength to be utilized by the CdSe/BGCN na-
nocomposite. Furthermore, TGA-capped CdSe QDs band edge position
are allocated high in the potential scale, which therefore assure the
transfer of electrons from their CB to the CB of the next component that
is in contact, therefore allowing the effective migration of its photo-
generated electrons and enriching the electron density available for
water reduction to H2. This increased photoactivity displayed by the
CdSe/xBGCN nanocomposite was demonstrated in Fig. 8c. After the
addition of BGCN, the H2 evolution rates are enhanced. Specifically,
when the amount of BGCN is 10 wt% for CdSe/10BGCN, the peak
photoactivity was achieved at 1435.85 μmolg−1
h−1
. This is three-fold
higher than the bare CdSe QD counterparts which demonstrate a H2
evolution rate of 454.96 μmolg−1
h−1
. However, further increasing the
amount to CdSe/20BGCN distinctly reduced the photoactivity. This
may be because the light scattering effect and the shading effects of the
2D materials sheet which can seriously block the absorption of incident
light by the CdSe QDs. This escalation in the photoactivity exhibited by
the composite CdSe/BGCN compared to CdSe substantiate to the charge
flow and separation occurring in the heterointerface of the ternary
structure.
Additionally, photocatalytic H2 activity was also conducted using
TEOA as the sacrificial reagent provided in Fig. 8d. On its own, CdSe
exhibited negligible photoactivity. Similarly, BGCN also showed no H2
activity without the addition of Pt under these reaction conditions.
Interestingly, when CdSe is loaded in the CdSe/BGCN composites,
photocatalytic H2 generation occurred unassisted with Pt and a similar
trend featuring CdSe/10BGCN as the optimum loading condition can be
seen, except at a lower H2 yield. Since no photoactivity is exhibited in
the form of individual component, this indicated that H2 generation is
realized when the three components work in alliance. In these cases,
charge separation and vectorial charge transfer transpired rapidly over
all three components for a very effective spatial separation of electrons
and holes. This consequently resulted in the systemized consummation
of electron to generate H2, thus leading to the observation. It is worth
noting that the photocatalytic H2 activity in acidic condition (∼ pH 3)
by ascorbic acid as the sacrificial reagent is much superior than in basic
Fig. 9. (a) VB XPS of different thiol-capped CdSe (b) Mott-Schottky plot for TGA-capped CdSe QD (c) Schematic energy level diagrams of different thiol-capped CdSe
QDs.
L.K. Putri, et al. Applied Catalysis B: Environmental 265 (2020) 118592
9

10. condition (∼ pH 11) by TEOA. This can be associated to the strong
impact of pH value on the photocatalytic H2 activity of the composite
[45]. Moreover, the acidic reaction medium (∼ pH 3) could also help to
reduce the reduction potential of water resulting in the enhanced
photocatalytic H2 activity [46].
Characterizations results that also point to these conclusions are the
transient photocurrent, electrochemical impedance spectroscopy and
photoluminescence spectra [47,48]. In Fig. 10a, BGCN and CdSe QDs
individually showed a low photocurrent density because of the low
light utilization efficiency and rapid recombination of its photo-
generated electron and holes, respectively. In CdSe/10BGCN, the pho-
tocurrent density is strongly enhanced, indicating the synergistic im-
provement in light utilization and suppressed recombination of
electron-hole pairs due to the construction of ternary heterostructure,
thereby allowing higher electrons density to contribute to higher pho-
tocurrent values. Furthermore, the subdued arc radius in the Nyquist
plots in Fig. 10b implies that the CdSe/10BGCN composite has lesser
charge transfer resistance and therefore alluding to an overall improved
separation efficiency and accelerated charge dynamics of the composite
as compared to bare CdSe and BGCN [49]. Last but not least, PL spectra
helps to investigate the efficiency of charge carrier migration, as well as
understand the fate of electron-hole pairs in a photocatalyst system
[50]. The PL spectrum of composite CdSe/10BGCN is significantly
quenched compared to BGCN (Fig. 10c) and CdSe (Fig. S6), which can
be attributed to the highly efficient electron and holes transfer at the
staggered heterojunction. In the three component systems, electron and
holes migrate in opposite direction, leading to the opportune spatial
separation of electrons and holes. Such dramatic decrease in PL in-
tensity underscores the intimate cohesion of the ternary 0D-2D-2D
systems, which could therefore prevent the direct recombination of
electrons and holes as translated in the PL spectra.
On the basis of above results, the following photocatalytic me-
chanism is proposed. Since TGA-CdSe band positions are located high in
the potential scale, the overpotential enables CdSe QD to behave as a
sensitizer and a source of electrons. This elevated band edge position
was engineered by the suitable surface manipulation of CdSe QD
through proper ligand selection. Furthermore, the chosen TGA-capped
CdSe QDs sustain a relatively low band gap of 2.3 eV, permitting the
absorbance of higher wavelength or, equivalently, lower energy frac-
tion of photons in the visible light spectrum. This therefore improves
the light harvesting capacity of the ternary CdSe/BGCN nanocompo-
sites. In detail, the photocatalytic process progresses as follows. Under
visible light irradiation, photogeneration of electron and holes takes
place. Due to the high CB of CdSe QD, it is very likely that the photo-
induced electron in CdSe QDs are energetically transferred to BGCN.
The dual doping of B-rGO and O-gC3N4 in BGCN induced a p-n
junction at their 2D-2D heterointerface. gC3N4 displays an intrinsic n-
type conductivity owing to the extra electrons from its nitrogen
network [51]. The deliberate inclusion of oxygen dopant in gC3N4 is
conducive to the photocatalytic activity as it broadened light absorption
window via introduction of intraband states and extended light ameli-
oration channels via its improved porosity (Fig. S7). Despite this
doping, Mott-Schottky plot of O-gC3N4 (Fig. S8) exhibited a positive
slope which is indicative of the preservation of the n-type conductivity
in O-gC3N4 even at 3.60 O at% doping. On the contrary to the n-type
nature of O-gC3N4, B-rGO featured a p-type conductivity as a result of
the substitutional and surface transfer doping of its adhering functional
groups [52]. Since boron has fewer valence electrons than carbon, the
substitution of carbon atoms with electron-deficient boron atoms will
result to a hole doping or equivalently a p-doping effect in the graphene
layer. Moreover, oxygen atoms which are present in many boron con-
figurations such as BC2O and BCO2 are more electronegative than
carbon atoms, therefore covalently bonded oxygen moieties in B-rGO
withdraws electrons via a surface transfer mechanism, leaving holes in
the graphene sheets and further rendering a p-type doping [53].
On that account, the union of p-type B-rGO and n-type O-gC3N4
renders a p-n junction upon contact. This phenomenon was discernable
from the Mott–Schottky plot by the featuring of an inverted V shape
exhibited by the O-gC3N4/B-rGO composite (Fig. S8). The difference in
Fermi levels, Ef, in a p-type B-rGO, which edge nearer to the VB, and a
n-type O-gC3N4, which edge nearer to the CB, led to the synchronized
rise and descent in the energy bands of B-rGO and O-gC3N4, respec-
tively, during the Fermi equilibration process (Fig. 11). At equilibrium,
the p-type B-rGO has a final conduction band which is positioned higher
than n-type O-gC3N4 [54]. Under these circumstances, the photo-
induced electrons from CdSe QDs were first transferred to B-rGO where
they are successively subjected to the p-n heterojunction. In the p-n
heterostructure, an inner electric field existed in which the p-type B-
rGO is negatively charged while the n-type O-gC3N4 is positively
charged [55]. This opposing polarity at close proximity generates an
internal electric field at the heterointerface which steered and ac-
celerated the unidirectional flow of electrons from p-type B-rGO to n-
type O-gC3N4 and the opposite flow of holes from n-type O-gC3N4 to p-
type B-rGO. This resulted in the subsequent spatial separation of the
photoinduced charge carriers and thereby hindered their recombination
probability. Overall, this forms the basis of an effective charge transfer
within the ternary CdSe/BGCN nanocomposite, which consequently
result in the overall improvement of the photocatalytic H2 activity.
Another plausible architecture is that CdSe QD could also, in addi-
tion, become anchored directly on the O-gC3N4 sheets. In this case,
electron transfer still occurred, however, from CdSe QD immediately to
O-gC3N4. In this case, electron-hole separation is still accomplished but,
however, at a slower pace due to the unavailability of p-n junction to
expedite the diffusion of electron and holes. To verify this, a CdSe/O-
gC3N4 composite at 10 wt%, and devoid of B-rGO, has been assessed for
its photocatalytic activity. This binary arrangement produced a
Fig. 10. (a) Transient photocurrent and (b) Nyquist plot of CdSe and CdSe/10BGCN and (c) PL spectra of BGCN and CdSe/10BGCN.
L.K. Putri, et al. Applied Catalysis B: Environmental 265 (2020) 118592
10

11. hydrogen evolution rate of 716.23 μmolg−1
h−1
, which was a sizeable
drop from its ternary counterpart performing at 1435.85 μmolg−1
h−1
(Fig. S9). These findings indicated that the ternary composite is su-
perior due to the faster electron flow at the staggered band structures.
The overall schematic illustration is presented in Fig. 11. The photo-
generated electrons in CdSe QDs are transferred to B-rGO due to po-
tential gradient, and finally to O-gC3N4 due to the internal electric field
of the p-n junction. This cascade of electrons flow develops an electron
rich site at O-gC3N4. Concurrently, in the binary CdSe/O-gC3N4 het-
erostructure, electron transfer to O-gC3N4 also arises but at a slower
rate. The accumulation of these electrons in O-gC3N4 is then employed
by H+
for reduction to generate H2. At the same time, the photo-
generated holes also travels, but in the reverse direction from O-gC3N4
to B-rGO or CdSe QDs. These holes are scavenged by the sacrificial
reagent ascorbic acid for the oxidation half of the reaction. All in all, the
spatial separation of electron-hole pairs can be accelerated for three
components system due to the formation p-n junction. Moreover, the
introduction of CdSe QDs in the system brings more heterojunction
spots and utilizes more visible light radiation, which effectively hinders
the charge recombination and therefore promoting the H2 evolution
performance.
4. Conclusions
In conclusion, CdSe/B-rGO/O-gC3N4 ternary heterostructures with
enhanced visible light absorption are fabricated successfully. For CdSe
alone, the modification in the band edge alignment was achieved by
surface chemistry manipulation via the use of different thiol-based
capping ligands. This work features the general tunability of QDs and
highlights an important mechanism of control over the electronic
properties of colloidal QDs. This convenient tuning of band structure of
TGA-capped CdSe, as well as the construction of intimate p-n junction
between B-rGO and O-gC3N4, aided in the accelerated transfer of pho-
togenerated electron-hole pairs. In turn, the photocatalytic H2 genera-
tion activity of the ternary structure CdSe/B-rGO/O-gC3N4 is dramati-
cally enhanced and the highest rate of H2 evolution was found to be
1435 μmolg−1
h−1
. Lastly, this systematic design synthesis of ternary
heterostructures with broadened light absorption and accelerated cas-
cade charge migration and separation paves new avenues for further
boosting future photocatalytic research endeavors.
Author contributions
The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgement
This work was funded by the Ministry of Education Malaysia under
the Malaysia Research Star Award (MRSA)-Fundamental Research
Grant Scheme (FRGS) with the project code of FRGS-MRSA/1/2018/
TK02/MUSM/01/1.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcatb.2020.118592.
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