CeO2/g-C3N4/V2O5 ternary nano hetero-structures decorated with CQDs for enhanced photo-reduction capabilities under different light sources: Dual Z-scheme mechanism

Article abstract of DOI:10.1016/j.jallcom.2020.155692

In this work, a carbon quantum dots (CQDs) decorated dual Z-scheme CeO₂/g-C₃N₄/V₂O₅ heterojunction (CCGV) was rationally synthesized by simple hydrothermal method. The crystal structure, morphology, and optical properties of the photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-Vis diffuse reflectance spectroscopy. CCGV catalyst shows high visible absorption, including near infra-red region and solar light via two-channel charge transfer (dual Z-scheme), reduced recombination, and up-conversion effect of CQDs. The heterojunction was used for visible assisted photo-reduction of carcinogenic hexavalent chromium ions (Cr(VI)) with a 99% reduction in 100 min without any sacrificial agent or hole scavenger. The reaction becomes more rapid with tartaric acid as a scavenger with a complete reduction in just 30 min under visible light. Photogenerated electrons are the dominant active species, followed by ^●O₂^?, as revealed by the scavenging experiments. The use of sacrificial agents and scavengers suggests that photogenerated electrons were the dominant active species followed by ^●O₂^? radicals. The electrochemical impedance spectroscopy and photoluminescence results confirm highly reduced recombination, high charge separation, and charge transfer capacity in the dual Z-scheme heterojunction. The conduction band edges of CeO₂ and V₂O₅ (which are protected by the dual charge transfer) have an appropriate potential for the rapid reduction of hexavalent chromium. The XPS scan of the catalyst after separation shows the presence of Cr in the +3 oxidation state, suggesting complete photo-reduction. This study is promising for developing wide spectrum active simple heterojunction photocatalysts for pollutant removal with high efficiency.

Full text of DOI:10.1016/j.jallcom.2020.155692

Journal Pre-proof
CeO /g-C N /V O ternary nano hetero-structures decorated with CQDs for
2
3 4 2 5
enhanced photo-reduction capabilities under different light sources: Dual Z-scheme
mechanism
Amit Kumar, Sunil Kumar Sharma, Gaurav Sharma, Mu. Naushad, Florian J. Stadler
PII:
S0925-8388(20)32056-9
DOI:
https://doi.org/10.1016/j.jallcom.2020.155692
JALCOM 155692
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 13 February 2020
Revised Date: 14 May 2020
Accepted Date: 16 May 2020
Please cite this article as: A. Kumar, S.K. Sharma, G. Sharma, M. Naushad, F.J. Stadler, CeO /g-C N /
2
3 4
V O ternary nano hetero-structures decorated with CQDs for enhanced photo-reduction capabilities
2 5
under different light sources: Dual Z-scheme mechanism, Journal of Alloys and Compounds (2020), doi:
https://doi.org/10.1016/j.jallcom.2020.155692.
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Credit authorship contribution statement
Amit Kumar: Conceptualization- original draft, Writing - review & editing. Sunil Kumar
Sharma: Data curation and experimental. Gaurav Sharma: Contribution to original draft,
Mu. Naushad: Reviewing, Characterization Investigation. Florian J. Stadler: Supervision,
Reviewing & Editing

CeO2/g-C3N4/V2O5 ternary nano hetero-structures decorated with CQDs for enhanced
photo-reduction capabilities under different light sources: Dual Z-scheme mechanism
Amit Kumara,b*, Sunil Kumar Sharmaa, Gaurav Sharmaa,b, Mu. Naushadc, Florian J.
Stadlera**
aCollege of Materials Science and Engineering, Shenzhen Key Laboratory of Polymer
Science and Technology, Guangdong Research Center for Interfacial Engineering of
Functional Materials, Nanshan District Key Laboratory for Biopolymers and Safety
Evaluation, Shenzhen University, Shenzhen, 518055, PR China
bSchool of Chemistry, Shoolini University, Solan, Himachal Pradesh, India-173229
cDepartment of Chemistry, College of Science, Building#5, King Saud University, Riyadh
11451, Saudi Arabia
* mittuchem83@gmail.com , ** fjstadler@szu.edu.cn
Abstract
In this work, a carbon quantum dots (CQDs) decorated dual Z-scheme CeO2/g-C3N4/V2O5
heterojunction (CCGV) was rationally synthesized by simple hydrothermal method. The
crystal structure, morphology, and optical properties of the photocatalysts were characterized
by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron
microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-Vis diffuse reflectance
spectroscopy. CCGV catalyst shows high visible absorption, including near infra-red region
and solar light via two-channel charge transfer (dual Z-scheme), reduced recombination, and
up-conversion effect of CQDs. The heterojunction was used for visible assisted photo-
reduction of carcinogenic hexavalent chromium ions (Cr(VI)) with a 99% reduction in 100
min without any sacrificial agent or hole scavenger. The reaction becomes more rapid with
tartaric acid as a scavenger with a complete reduction in just 30 min under visible light.
●
-
Photogenerated electrons are the dominant active species, followed by O2 , as revealed by
the scavenging experiments. The use of sacrificial agents and scavengers suggests that
1

●
-
photogenerated electrons were the dominant active species followed by O2 radicals. The
electrochemical impedance spectroscopy and photoluminescence results confirm highly
reduced recombination, high charge separation, and charge transfer capacity in the dual Z-
scheme heterojunction. The conduction band edges of CeO2 and V2O5 (which are protected
by the dual charge transfer) have an appropriate potential for the rapid reduction of
hexavalent chromium. The XPS scan of the catalyst after separation shows the presence of Cr
in the +3 oxidation state, suggesting complete photo-reduction. This study is promising for
developing wide spectrum active simple heterojunction photocatalysts for pollutant removal
with high efficiency.
Keywords: CeO2; Photocatalysis; Heterojunction; Cr(VI) reduction; carbon nitride; Z-
scheme
1. Introduction
The rapid industrialization and anthropogenic activities have caused detrimental effects on
the ecosystem by severe environmental contamination. With ongoing water quality
deterioration, various renewable and greener methods for efficient elimination of emerging
toxic pollutants have been explored by researchers and environmentalists [1, 2]. The world
health organization (WHO) data shows that 844 million people do not have access to basic
drinking water facilities, and these numbers will further increase tremendously in the future
[3]. Wastewater often contains numerous organic contaminants and heavy metals, which,
when entering drinking water supplies, cause health hazards [4, 5]. The increasing heavy
toxic metal contaminations due to industrial and mining discharges have created numerous
health issues [6, 7]. Among various heavy metal contaminants, carcinogenic, and mutagenic
hexavalent chromium Cr(VI) is responsible for various chronic health disorders [8, 9]. Cr(VI)
2−
has high solubility and mobility in water resources and originates from chromium Cr2O7
and CrO42− ions. Cr(VI) produces reactive oxygen species and damages the organelles of the
living cells by penetrating caused by its high solubility [10]. On the other hand, Cr(III) is
2

nearly 1000 times less toxic than Cr(VI) but has low mobility, and the former can be
precipitated as Cr(OH)3 in neutral or basic media [11]. Thus one of the most successful routes
to remove toxic Cr(VI) is the reduction to Cr(III). Here photocatalytic reduction is the most
promising choice because the process is easy and efficient. Especially, utilizing solar or
visible light energy for photocatalytic water treatment is essential for practical and large scale
utilization [12].
However, single-phase semiconductor photocatalysts suffer from severe limitations on their
performance, including limited spectrum activity, low quantum yield and high charge carrier
recombination. The catalytic degradation performance of photocatalytic materials can be
improved by suppressing the charge carrier recombination on the photocatalyst [13]. These
limitations have been most effectively addressed by forming heterojunctions using different
semiconductors with optimum bandgap and electronic band arrangement for maximum
optical absorption and electronic movement [14, 15]. The synergistic performance of the
semiconductors involved in a hetero-structure or junction involving two semiconductors is
highly beneficial for outstanding photo-oxidation and photo-reduction of pollutants [16, 17].
Z-scheme heterojunctions photocatalysts derive their charge transfer mechanism from natural
photosynthesis and accelerate the photogenerated charge carriers separation and transfer [18].
In a Z-scheme transfer pathway, not only recombination is inhibited, but also, the more
negative CB and positive VB potential is maintained for effective generation of radicals. The
introduction of a third counterpart semiconductor is encouraging, as this further elevates the
photocatalytic activity by allowing for charge transfer by a dual Z-scheme mechanism. In a
dual Z-scheme transfer mechanism, the first component is having a higher a CB with two e-
transmission channels [19]. Along with the interface, the photogenerated electrons on
component 2 are transferred to the other two components, leading to highly improved
3

separation. Thus the highly placed conduction bands of two components can generate
sufficient superoxide radicals, and more hydroxyl radicals are generated on the valence bands.
Among various semiconductors, V2O5 is widely used as a potent environmental catalyst
because of its low cost and non-toxic nature. V2O5 has a wide range of properties, such as
lower bandgap, chemical stability and lack of toxicity. It has been used in various
applications such as antifungal and antimicrobial additives to textiles and during the
wastewater treatment [20, 21]. CeO2 is a rare earth semiconductor catalyst that has grabbed
the attention because of its stability and strong oxidizing capability. Ceria has ample oxygen
vacancies and high oxygen storage capacity with the ability to utilize and release oxygen via
the Ce3+/Ce4+redox cycle [22]. Hence, CeO2 is frequently used in environmental catalysis,
water splitting, sensing applications [23-25] and particularly in photocatalysis [26].
As an organic semiconductor, graphitic carbon nitride (g-C3N4) as a metal-free conjugated
polymer has been reported as the most popular photocatalyst for environmental clean-up and
energy production [27, 28]. g-C3N4 is a tough semiconductor that has unique electrical,
thermal, and optical properties [29]. It has an energy bandgap of nearly 2.7 eV and shows
exceptional thermal and chemical stability because of tri-s-triazine based building blocks [16]
and is used as visible-light responsive photocatalyst [30]. On the other hand, various
drawbacks, including fast charge recombination and poor visible light absorption, limit its
photocatalytic performance. Structural modifications and amino group regulation have been
performed to increase the catalytic activity [31]. Various heterojunctions have been formed to
improve its photocatalytic activity [32]. Metal oxides as Fe2O3 have been incorporated to
form rGO/Fe2O3/g-C3N4 nanostructures with rapid photoexcited charge transfer via nano-
channels [33]. Various dual Z-scheme junctions utilizing g-C3N4 such as MoS2/g-
C3N4/Bi24O31Cl10 [34], Ag3PO4/AgBr/g-C3N4 [35], and AgI/LaFeO3/g-C3N4 [36] have been
prepared with enhanced photocatalytic activity. Compared with various other materials,
4

quantum dots (QDs) possess highly efficient electron transport capacity, which leads to rapid
transfer electrons and holes to the catalyst surface from inside or core. This substantially
improves the photocatalytic performance and quantum efficiency [37]. Carbon quantum dots
have been used photocatalysis as an active part of Z-scheme junctions as well as of dual Z-
scheme based ternary heterojunctions. CQDs have interesting electronic properties and also
possess fluorescence emission. Also, they show up-conversion properties and can enhance
the visible light absorption by light generated electron transfer [38] and electrons-oxygen
interactions. They also participate in the Z-scheme electron transfer in addition to extending
the light spectrum absorption. Various Z-scheme and dual Z-scheme mechanism-based
junctions have been prepared for photocatalytic reduction of hexavalent Cr, as listed in
Table1.
In light of the above, a dual Z-scheme V2O5/CeO2/g-C3N4 ternary heterojunction has been
prepared, followed by modification with CQDs for better visible and solar photocatalytic
activity. The mechanistic route to enhanced photocatalysis has been explained based on a
direct dual-Z-scheme charge carrier in comparison to a conventional type-II heterojunction
that has been elaborately discussed. This ternary hybrid heterojunction exhibits exceptional
charge separation and protection of high potential CBs via dual Z-scheme transfer and
bridging action by CQDs. It is still challenging to fabricate semiconductor composites with
coherent interfacial contact & suitable band edge placements and improving the
photocatalytic potential of g-C3N4 via the addition of suitable semiconductors. In addition,
CQDs act as bridging material for effective dual Z-scheme transfer. Their electron-accepting
property promotes the formation of more reactive oxygen species and augments the lifetime
of charge carriers for high performance wide spectral photo-reductive and photo-oxidative
applications.
2. Experimental
5

2.1 Synthesis of g-C3N4
The g-C3N4 was prepared by the thermal treatment of urea. 10 g of powdered urea was
calcined at 450°C for 3 h in a silica crucible and 10°C min-1 heating rate. After this, the
deammoniation was done at 500°C for 2h. The yellowish g-C3N4 powder was then washed
with ethanol and water repeatedly and dried at 80°C.
2.2 Synthesis of CeO2 nanoparticles
The CeO2 nanoparticles were synthesized by a hydrothermal route. In a typical method, 0.002
moles of Ce(NO3)3 and 0.004 mol of PVP was dispersed in 40 mL double distilled water. The
suspension was then transferred to a 50 mL and heated at 200°C for 2.5 h in an autoclave.
The prepared CeO2 nanoparticles were then repeatedly washed with water/ethanol and freeze-
dried under vacuum.
2.3 Synthesis of CQDs decorated CeO2/g-C3N4/V2O5 dual Z-scheme heterojunction (CCGV)
In another procedure, 2.9 g ammonium metavanadate (NH4VO3) was added to 100 mL
ethylene glycol. The resulting mixture was then heated to 110°C with continuous stirring
followed by refluxing at 160°C for 1h. Thus vanadyl glycolate (VEG) is precipitated. To
VEG precursor, 0.3 g of g-C3N4 and 0.3 g of CeO2 nanoparticles were added, followed by
sonication for 1h. The resultant mixture was then annealed at 500°C for 2 h. CeO2/g-
C3N4/V2O5 catalyst was thus formed. The vacuum dried catalyst was dipped in CQDs
(procured from Sigma Aldrich, AR) suspension and autoclaved at 180°C for 24 h. The
obtained CQDs@CeO2/g-C3N4/V2O5 catalyst was then washed repeatedly with distilled water
and vacuum dried at 80°C. The synthesis scheme is represented in Figure 1.
2.4 Characterization
A Rigaku (D/MAX-2500) X-ray diffractometer was employed to determine the crystal
structure and phase purity of single and ternary photocatalysts. The surface morphology and
6

shape of the ternary heterojunction was analyzed by scanning electron microscopy (LEO 435
VP instrument) and microstructure by TEM, ESCA transmission electron microscope. The
specific surface area was measured by the Brunauer−Emmett−Teller (BET) method with a
NOVA 1200 analyzer. The elemental and oxidation state analysis was done by X-ray
photoelectron spectroscopy measurements using a K-alpha XPS (Thermo VG Scientific)
instrument with an Al Kα X-ray monochromating source.
2.5 Optical and electrochemical experiments
The UV-visible diffuse reflectance spectra (UV-DRS) were captured using a Hitachi U-2550
instrument (BaSO4 standard method). The potential band edges for these photocatalysts were
then calculated by using following equation [39]:
EVB= X – Ee + 0.5Eg
ECB = EVB – Eg
(1)
(2)
Where ECB and EVB signify band conduction and valence band edges respectively, Ee = ~4 eV
(electron energy on hydrogen scale), X is the geometric mean of Pearson absolute
electronegativity (PAE), and Eg denotes the bandgap. The Photoluminescence (PL) spectra of
single and ternary heterojunctions were obtained using a Hitachi fluorescence
spectrophotometer for determining the extent of recombination of electrons and holes. The
electrochemical impedance spectroscopy (EIS) was also performed on synthesized
photocatalysts. The experiments were performed on CHI 660D three-electrode system based
workstation. The sample modified carbon electrode was employed as a working electrode,
Ag/AgCl working electrode, and a Pt wire counter electrode. The impedance measurements
were done in Na2SO4 (0.2ꢀmol/L, pH 7) solution as electrolyte (0.2 V; frequency 0.1 Hz - 100
kHz).
2.6 Photo-reduction of Cr(VI)
7

The photocatalytic potential of g-C3N4, V2O5, CeO2, and CCGV junction was tested for
visible light and sunlight assisted reduction of Cr(VI) into Cr(III). A 300 W xenon lamp with
visible (480 mW cm-2) and NIR filter (200 mW cm-2) was used as the light source in an
automated photo-reactor (TOPTION). The temperature inside the reactor was maintained at
30 ± 0.5 °C with the help of water circulation. A distance of 10 cm between the lamp and
reaction system was maintained. The batch reduction experiments were performed firstly
under Xe lamp (visible), and all the reaction parameters were studied. The operations
conditions are as follows: [Catalyst] = 0.3 mg/mL, [Cr (VI)] = 20 mg L-1 and pH = 2 were
fixed. In a typical methodology, 30 mg catalyst was dispersed into a 100 mL Cr6+ solution
with constant stirring. After exposure to the lamp, the fall in Cr(VI) concentration was
monitored as a function of time. For each measurement, 3 ml aliquots were extracted from
the reaction medium, followed by centrifugation and filtration using a membrane filter
(0.45μm Teflon). Diphenylcarbazide colorimetric method was used for estimating residual
Cr6+ concentration by recording the absorbance at 540 nm on Shimadzu 2202
spectrophotometer. The experiments were also carried out under NIR light and natural
sunlight.
3. Results
3.1 Material characterization
3.1.1 XRD analysis
Figure 2 shows the XRD patterns of as-synthesized CeO2, g-C3N4, V2O5, and CCGV
photocatalysts. The diffractogram for g-C3N4 shows characteristic peaks at 12.8° and 27.4°
corresponding to (100) in-plane structural repeating unit and periodic stacking of layers along
the (002) planes, respectively (JCPDS 87-1526) [40]. The diffraction pattern for CeO2 shows
peaks for (111), (220), (311), (222), (200), (331) and (400) diffraction planes for face-
centered cubic structure in agreement with standard JCPDS card No. 34-0394 [41]. For V2O5
8

the characteristic peaks for (020), (001), (110), (040), (101), (012), (102), (200), (200), (201)
and (170) planes of the orthorhombic phase of V2O5, which is in good agreement with JCPDS
card no. 89-2482 [42]. The XRD pattern for the junction shows all the peaks for CeO2, g-
C3N4, and V2O5 are observed with the same phases. Therefore, the nature of all the three
components is preserved during synthesis, although as compared to the bare semiconductors,
slight shifts in position can be observed. This can be ascribed to the minute structural
modifications due to mutual interactions. Because of amorphous structure of CQDs, low
content and good dispersion no peaks are observed [43]. The XRD pattern confirms the
successful formation of the heterojunction.
3.1.2 SEM and TEM analysis
Figure 3 (a-b) represents the SEM images of CCGV, showing the morphology. It can be seen
that V2O5 and CeO2 nanoparticles dispersed over thin g-C3N4 nanosheets. On higher
resolution, the semiconductor metal oxide nanoparticles can be seen in intimate contact with
each other and with carbon nitride.The lighter colored and cubic nanoparticles represent
CeO2 and darker spherical particles are the V2O5. The corresponding TEM images showing
the microstructure of CCGV are presented in Figure 3 (c-d), in which the V2O5 and CeO2
nanoparticles can be seen uniformly distributed over the g-C3N4 matrix confirming the
heterojunction structure. The intimate contact among all three can be seen. HRTEM image
(Figure 3d) shows the three moieties in intimate interfacial contact with lattice fringes visible.
The fringes and contact can be more clearly visible in the HRTEM image in Figure S1. The
d-spacings, as calculated from the HRTEM image, agree with XRD results. SAED pattern
(Figure 3e) shows the high crystallinity of the photocatalyst. The elemental mapping for
CCGV can be seen in Figure 3(f-k). All the constituent elements are present in the material,
and a uniform structure can be observed. Further elemental analysis is done by XPS
measurements. Surface area measurements (Figure S2) show that N2 adsorption for CCGV
9

exhibits type-IV isotherm (H4/H3 hybrid hysteresis) [44] with 48.2 m2g-1 surface area. The
pore size for CCGV is 19.23 nm and pore volume of 0.22 cm3g-1. The surface area is
sufficiently high for high adsorption of Cr(VI) ions owing to the various possible adsorption
sites.
3.1.3 XPS analysis
XPS analysis was further used for the analysis of chemical composition and surface oxidation
state of the ternary heterojunction CCGV. The survey scan (Figure 4a) shows the peaks for
all the constituent elements, i.e., V, C, N, O, and Ce confirming their presence in the material.
The Ce 3d spectrum (Figure 4b) shows the deconvoluted peak at 883.1ꢀeV, which can be
ascribed to Ce(III) 3d3/2 and at 900.7ꢀeV refers to Ce(III) 3d5/2. Also the peak at 897.2 eV
corresponds to Ce(IV) 3d5/2 [45]. Figure 4 (c) shows the V 2p spectrum for V2O5 exhibiting
two typical peaks at 514.3ꢀeV, 515.2ꢀeV referring to V4+ and V5+ oxidation states (2p3/2) and
the third peak at 524.5 eV is ascribed to the splitting of the V5+ 2p1/2 spin-orbital [46]. The
O1s spectrum (Figure 4d) exhibits two peaks at 528.6 & 530.4 eV corresponding to oxygen
from oxide and hydroxyl groups, respectively. The adsorbed OH groups on the catalyst
●
●
-
surface are beneficial for the production of OH and O2 radicals during the irradiation
process. The C 1s spectrum (Figure 4e) shows two peaks at 283.6 eV and 287.1 eV
corresponding to sp2 C and N-C=N bonds of graphitic C3N4, respectively [47]. The
deconvoluted N1s spectrum is shown in Figure 4 (f). The three peaks at 397.3, 398.2, and
399.3 eV are assigned to pyridine N (sp2-bonded N ) involved in the formation of triazine
rings, i.e. (C=N-C), N–(C)3 and N from (C-N-H) respectively [48]. The C1s peaks are shifted
to slightly lower B.E. values as per literature [49], but the 284.6 eV standard carbon peak has
not changed after junction formation with metal oxides. There have been slight changes in the
binding energies from standard values of these bare semiconductors, and these shifts are an
indicator of changes in surface charge densities, which can occur due to electron transfer
between semiconductors.
10

3.1.4 Optical and electrochemical studies
The UV-DRS spectra of V2O5, CeO2, g-C3N4 and CCGV were recorded results are shown in
Figure 5(a). The g-C3N4 shows visible absorption with a typical absorption edge near 450 nm.
On the other hand, CeO2 has low visible absorption, and V2O5 shows considerably high. With
the junction formed between these three, the absorption edge shows a considerable redshift
with the extension of absorption to the near infra-red region. The improved spectral response
is because of dual Z-scheme hetero-junction formation [50] and the up-conversion effect of
the CQDs. The bandgaps were measured from Tauc plots (Figure 5b). The bands as
calculated are 3.1 eV, 2.7 eV, and 2.6 eV for CeO2, g-C3N4, and V2O5, respectively. The
valence band edges for CeO2, g-C3N4, and V2O5 are calculated as 2.03 eV, 3.68 eV, and 2.9
eV, respectively. The corresponding CB edges in the same order are -1.07 eV, 0.98 eV and
0.5 eV. As it can be observed that the VB and CB edges are such placed that there is easy
diffusion of charge carriers along with the interfaces. To ascertain the reduced recombination
in the ternary heterojunction photoluminescence (PL) was also performed. The results in
Figure 5(c) show that the PL intensity order is: V2O5 > g-C3N4 > CeO2 > CCGV. V2O5 shows
the highest photoluminescence because its bandgap is the lowest of the three components
suggesting the highest recombination. The ternary junction CCGV shows the lowest
recombination with the highest separation capacity. The dual Z-scheme transfer
heterojunction formation among V2O5, g-C3N4 and CeO2 diminishes the recombination. The
transfer of photogenerated electrons from CB of g-C3N4 to VB of V2O5 and CeO2 reduces the
recombination in the semiconductors and increases the separation leading to poor
photoluminescence. Also, CQDs ensure the dual Z-transfer and further reduce the
recombination. Electrochemical impedance spectroscopy (EIS) was also performed on the
electrochemical work station to test the electron transfer capacity and flow. Figure 5 (d)
shows EIS Nyquist plots for single and ternary photocatalysts. The changes in
electrochemical impedance in a hetero-structure actually indicates the changes in the
11

interfacial properties [51]. When the transfer of electrons occurs at the interfaces or along
with the Fermi levels, the impedance gets reduced. This reduction is represented by a
decreased arc radius in the Nyquist plots. As can be observed, the ternary junction CCGV
shows the lowest radius, which indicates the best electron transfer and the least impedance.
3.2 Photo-reduction of Cr(VI)
To test the photocatalytic ability, several photo-reduction experiments were performed under
a Xe lamp with a visible filter at optimized pH 2. The results are presented in Figure 6.
Without any sacrificial agent (hole scavenging), the Cr(VI) to Cr(III) photo-reduction in 90
min visible light exposure the performance follows the order : CCGV (99.4%) > V2O5
(62.6%) > g-C3N4 (56.9%) > CeO2 (17.5%). As expected, the CeO2 bandgap of 3.1 eV
shows the poorest performance because of limited absorption in visible light (Figure 6a).
Followed by CeO2, g-C3N4, and V2O5 shows improvement because of the low bandgap, but
the performance is poor because of the high recombination rate. On the other hand, the
ternary heterojunction performs exceptionally exhibiting an almost complete Cr6+ removal
within 90 min. Figure S3 shows the time-dependent absorption spectra of the DPC-Cr(VI)
complex solution obtained by sampling from the reactor. It can be observed that the
absorption peak at 540 nm (for complex) falls rapidly with the exposure time and almost
disappears with time. The dual Z-scheme transfer and CQDs increase the visible absorption
and reduce the recombination dramatically. A pseudo-first-order kinetics model was applied
for reaction, and plots are shown in Figure 6(b). The rate constant order is CCGV (0.08122
min-1) > V2O5 (0.03423 min-1) > g-C3N4 (0.03122 min-1) > CeO2 (0.01012 min-1). This
shows that the photo-reduction rate is ~8 times faster than bare CeO2. The results are in
accordance with UV-DRS, PL, and EIS results. On light irradiation, the holes-electrons pairs
are generated, where electrons bring about the reduction of Cr6+ to Cr3+ (scheme 1, reactions
3-5) and the holes produce oxygen from water.
12

CCGV
CCGV(h+
)
+ CCGV(e-vb)
4H+ + O2
(3)
(4)
(5)
vb
4CCGV(h+vb) + 2H2O
Cr2O72- + 14H+ +6e-
2Cr3+ + 7H2O
In basic medium
CrO42- + 3e- + 4H2O
In acidic medium
HCrO-4 + 3e- + 7H+
Reaction with OH
Cr(OH)3 + 5OH-
(6)
Cr3+ + 4H2O
Cr6+ + 3OH-
(7)
(8)
(9)
Cr3+
+
3OH
In presence of sacrificial agent (SA)
SA + h+
SA CO2 + H2O
OH
Scheme 1: Various reactions during photo-reduction of Cr(VI) over CCGV
The electrons from CB of g-C3N4 move to VB of V2O5 and CeO2 and high potential
electrons at CB of V2O5 and CeO2 for effective photo-reduction of Cr(VI) (Figure 7). Also,
CQDs absorb visible light to enhance the capture. They may also act as bridges to facilitate
the Z-scheme transfer. As mentioned earlier, the pH was optimized for experiments and the
best results were obtained at pH=2. The effect of pH onto Cr6+ reduction by CCGV under
visible light was tested (Figure 6c). The reactions are given in scheme 2, reactions 6-7. At
-
acidic pH, the performance is better, the catalyst surface is positively charged, and HCrO4
ions are dominant, which favors the interactions with the catalyst surface, and Cr(VI) gets
rapidly reduced. However, the reduction falls at higher pH or neutral medium because of the
dominance of CrO42- and negative charge on the catalyst surface, which leads to repulsions.
Also, in alkaline medium, Cr(OH)3 precipitates and deposits on the surface of the catalyst,
thus, hampering the reduction [52].
3.2.1 Effect of operational parameters
13

In the water system, there are many common electrolytes or ions present, which may affect
the photocatalytic reaction. Figure 6(d) displays the role of the most common electrolytes as
-
Ca2+, Cu2+, Na+, Mg2+, NO3 , Cl-, and SO42- on Cr6+ reduction over CCGV with visible light.
As far as cations are concerned, no noticeable change is observed. Na+, Mg2+ & Ca2+ ions do
not affect the rate while it falls slightly in the presence of Cu2+ ions, as they can scavenge
2-
photogenerated electrons [53]. Anions as SO4 and Cl- lead to an increase in the reduction
capacity because they can scavenge the photogenerated holes and thus decrease the
-
●
recombination. On the other hand, NO3 ions produce OH radicals under visible light [32],
which can shift the equilibrium to the opposite side (scheme 1, equation 8). Further, the
photocatalytic Cr6+ reduction was also tested under distilled water (DW) and municipal tap
water (TW) to study the water matrix effect and applicability under real systems. Figure 6(d)
shows that without any catalyst, no reduction is observed both in distilled and tap water.
However, the degradation falls in tap water containing electrolytes and humic acid. However,
the performance is not reduced to a great extent which means that catalyst is effective under
natural water systems too.
3.2.2 Effect of sacrificial agents and scavengers
In the previous sections, it has been explained that the photogenerated electrons are
responsible for the reduction of Cr6+ into Cr3+. So some low weight organic acids and
ethanol were added as sacrificial agents (SA), which accelerates the reduction by undergoing
oxidation. For these experiments, 10-4 mol of ethanol (EA), maleic acid (MA), and tartaric
acid (TA) were used as SA. It is clearly visible that the rate increases manifolds with
sacrificial agents. The fastest reduction is observed with TA with 100% Cr6+ removal in 30
min. The reaction rate follows the order: TA > MA > EA > CCGV. The role of sacrificial
agents can be observed in equation 9. These acids scavenge holes and get oxidized and keep
electrons free for reduction.
14

The number of -COOH and –OH groups influences the reactivity of sacrificial agents. TA
has 2 OH groups and 2 carboxylic groups. This is followed by MA with 1 COOH group and
ethanol with just 1 OH group. Because of the higher number of polar groups of TA, the
Cr(VI) adsorption performance of CCGV is even improved. Further, KI, Ag+, and
●
-
benzoquinone (BQ) were used for scavenging h+, e- and O2 respectively, from CCGV +
visible system to assess the involved active species (Figure 8a). The photo-reduction
efficiency markedly increased on the addition of KI, implying that on scavenging holes, the
electrons are free for reduction. The addition of Ag+ scavenges electrons, which decreases
-
the rate. Hence, photogenerated electrons are primarily responsible for reduction, while ●O2
radicals are of secondary importance.
3.2.3 Photocatalytic mechanism, performance, and Reusability
The ternary junction CCGV shows remarkable photocatalytic activity, which is manifested
in the Cr(VI) reduction results and supported by EIS and PL results. The wide spectral
response, low impedance, reduced recombination, and electron flow is responsible for the
remarkable activity of CCGV. Figure 7 shows the possible conventional heterojunction and
dual Z-scheme heterojunction with the charge flow along with the bands. In the traditional
ternary heterojunction, on irradiation, all the three semiconductors show transition and the e-
from CB of V2O5 (-1.07 eV) and CeO2 (0.5 eV) migrate to CB of g-C3N4 (0.98 eV). The h+
from VB of g-C3N4 (3.68 eV) migrate to VB of V2O5 (2.03 eV) and that of CeO2 (2.9 eV).
The potential of electrons in CB of g-C3N4 (0.98 eV) is higher than reduction potential
Eo[Cr(VI)/Cr(III)] = 0.51 eV [54]. This high potential brings about a rapid reduction of
Cr(VI). The oxidation of H2O to O2 will coincide with the VB of V2O5 and CeO2 having
-
high potential than Eo(H2O/O2)= 1.23 eV. Secondly, the formation of ●O2 , the second most
active species, is not feasible by this traditional mechanism because CB of g-C3N4 has not
-
higher potential than E (O2/•O2 ) = -0.33 eV. However, hydroxyl radicals can still be
15

produced as VB potentials of both V2O5 and CeO2 are highly positive than potential E
(•OH/H2O) (2.27 eV) vs. NHE and E (•OH/-OH) (1.99 eV) [19]. By analysis of this
conventional transfer mechanism, we do not except faster or exceptional results.
Let us analyze the dual Z-scheme transfer mechanism. The photogenerated electrons in the
CB of g-C3N4 migrate to VB of V2O5 and CeO2 as they are appropriately aligned. In
addition, the presence of CQDs as bridges at each interface facilitates the migration of CB of
g-C3N4 to VB of the other two semiconductors. The route is g-C3N4 → CQDs → V2O5 and
g-C3N4 → CQDs → CeO2. This makes the electrons in CB of V2O5 and CeO2 free for
reduction. The electrons in these two bands are highly negative than the required potential
and also than g-C3N4.
Similarly, the CB potential of V2O5 (-1.07 eV) is highly negative than E (O2/•O2 ) = -0.33 eV.
-
Hence, as observed from scavenging experiments, the second active species present in the
-
-
system is ●O2 . If we consider the traditional transfer mechanism, then the generation of ●O2
is not thermodynamically feasible, and its generation can only be explained by dual Z-
scheme transfer. The rapid rate of reduction can only be explained only via dual Z-scheme
transfer. Sacrificial agents like acids or ethanol, scavenge the holes and oxidizing OH
●
radicals and speeding up the reduction process. The OH radicals tend to shift the
Cr(VI)/Cr(III) equilibrium to the opposite side. In addition, these agents with polar groups
form charge transfer complexes with the catalyst for better interaction and reduction.
The redshift in CCGV samples on account of bandgap lowering is attributed to the Z-scheme
transfer of charge carriers and oxygen vacancies or defects on the CQDs or induced defects
on catalysts by CQDs. The up-conversion properties of the carbon quantum dots help in high
visible absorption and charge separation. With Fermi level located at -0.08 eV, which is
more +ve than that of three semiconductors, a Schottky barrier can be easily formed between
two intimate contacts, i.e., g-C3N4-V2O5 and CeO2-g-C3N4 via CQDs. This leads to the
16

migration of electrons and the shifting of the Fermi level. The oxygen vacancies on the
catalyst surface induced by CQDs also contribute to the photocatalytic activity. These
surface oxygen vacancies/defects trap electrons or holes and inhibit the recombination rate
and also have a tendency to transfer the charge carriers to the catalyst as well as adsorbed
pollutant species [55].
As photocatalyst shows a broad-spectrum response, the photo-reduction tests were also
performed under natural sunlight and NIR light (Figure 8). Under sunlight, CCGV shows
78.2% Cr(VI) photo-reduction at pH=2, which increases to 90.2 % if TA is used as a
sacrificial agent. These results show that the high photo-activity of CCGV catalysts is
retained outdoors under the sun. However, the reaction is slower because of the lower
energy of sunlight. Under NIR light, a 21.1% reduction is achieved, which is possible
because of NIR absorbing capacity. The photocatalytic performance of CCGV as compared
to other Z-scheme and dual Z-scheme heterojunctions is shown in Table 1. The results
reveal that CCGV is a highly active photocatalyst active under a wide spectrum with
excellent performance for Cr(VI) reduction. Also, the CQDs aid in the facile dual Z-scheme
transfer for highly improved separation and extended absorption. Compared to similar or
related junctions, this CCGV junction not only performs exceptional for photo-reductive
performance but is also efficient under NIR light and natural solar light. The effectiveness of
Z-scheme transfer is accelerated or routed by bridging materials as quantum dots (CQDS in
this case). Thus, the heterojunction performs better than other dual Z-scheme materials
because of the correct placing of Fermi levels of these quantum dots for faster transfer from
CB to VB for maintaining high potential conduction bands.
Table 1: Comparative performance of Z-scheme and dual Z-scheme heterojunction
photocatalysts for visible light Cr(VI) reduction
17

S.No Photocatalyst
.
Cr(VI)
Catalyst
Performance
Ref
concentration, dosage
pH
20 mg L-1
1.
2.
TiO2@C/CdS
0.6 mg mL-1
98.1% reduction in 50
min under visible light
(300 W Xe lamp)
98% reduction in 60 min [57]
under visible light (300
W Xe lamp)
71% reduction within
90ꢀmin under sunlight
99.8% reduction
efficiency for Cr(VI)
within 120 min (350 W
Xe lamp)
98.3% reduction in 100
min under visible light
(300 W Xe lamp)
88% photo-reduction
under visible light in 90
min (300 W Xe lamp)
98% reduction in 120
min under visible light
(300 W Xe lamp)
100% Cr(VI) removal
within 120ꢀmin (300 W
Xe lamp)
81.7% reduction in 120
min (500 W Xe lamp)
~80% reduction in 120
min, (300 W Xe lamp)
99% reduction in 100
min (300 W Xe lamp)
[56]
Ag/Bi4O7/g-C3N4 50ꢀ mg L-1,
pHꢀ=ꢀ3
0.3 mg mL-1
3.
4.
V2O5/g-C3N4
20 mg L-1, pH 0.1 mg mL-1
= 2
20 mg L-1,
pH=2
[58]
CoS2/g-C3N4-
rGO
0.2 mg mL-1
[59]
5.
6.
7.
8.
Cu2WS4/g-C3N4
ZnO-Fe2O3
α-Fe2O3/g-C3N4
g-
10 mgL-1
0.5 mg mL-1
[60]
[61]
[62]
[63]
20 mg L-1, pH 0.4 mg mL-1
= 2
10 mg L-1,
pH=2
2 mg mL-1
5 mg L-1,
2.5 mg mL-1
C3N4/GO/BiFeO3 pH=2
9.
SnO2/Bi2O3/BiOI 50 mg L-1,
pH=3
0.5 mg mL-1
0.4 mg mL-1
0.3 mg mL-1
[64]
[65]
10.
11.
GO/g-C3N4/MoS2 10ꢀ mg L-1
CQDs decorated 20 mg L-1 ,
This
stud
y
CeO2/g-
C3N4/V2O5
(CCGV)
pH=2
After the reduction tests are over, XPS analysis of CCGV (catalyst + visible light + TA
system) was performed to confirm the photo-reduction of Cr(VI) into Cr(III). The survey
scan (Figure 8c) confirms that all the constituent elements are present with an additional
peak of adsorbed Cr(III). The structure of the catalyst is not changed as no changes are
18

observed in the survey scan. The Cr 2p spectrum (Figure 8d) is deconvoluted further into
two peaks at 576.9 and 586.5 eV, which are assigned to 2p3/2 and 2p1/2 for Cr3+, respectively
[66]. The complete conversion is confirmed as no peaks for Cr(VI) are present.
Further, the reusability studies for CCGV catalyst under the visible system are shown in
Figure 8(d). There is no abnormal fall in the catalytic activity after five cycles. Thus, the
catalyst is stable, reusable, and fully recoverable from the reaction system. The TEM image
of CCGV (Figure 8e) shows that the microstructure of the catalyst does not change after use
and recovery. Thus CQDs decorated CeO2/g-C3N4/V2O5/ternary nano hetero-structures are
an efficient and wide spectrum photocatalytic material with a dual Z-scheme mechanism for
rapid Cr(VI) reduction. CCGV works efficiently under natural sunlight, tap water and is
promising for photocatalysis based environmental remediation and energy production.
4. Conclusions
In summary, CQDs decorated CeO2/g-C3N4/V2O5 ternary heterojunction (CCGV) with a dual
Z-scheme transfer of electrons has been prepared by a simple technique. UV-DRS, PL, and
EIS results reveal that the CCGV catalyst shows a wide spectral response extending up to the
NIR region, with low recombination and high charge transfer capacity. The ternary
heterojunction displayed remarkable photocatalytic activity with 99.4% Cr6+ reduction in 90
min without any sacrificial agent. The catalyst shows an eightfold higher rate than bare CeO2.
The enhanced Cr6+ reduction with CCGV is mainly ascribed to improved visible absorption
capacity, accelerated charge transfer, and restrained recombination rate. Also, the high
protected energy bands via Z-scheme transfer and up-conversion by CQDs facilitate the
charge transfer and generation of high energy reactive oxygen species. The role of dominant
oxygen species was also examined by scavenging experiments, which reveal that
●
-
photogenerated electrons are the main species followed by O2 . The excellent performance
was explained in terms of possibilities of traditional double charge transfer mechanism and
19

dual Z-scheme transfer. By analysis of band structure and thermodynamic feasibility of
reactive oxygen species, it was found that dual Z-scheme transfer facilitated by CQDs is
responsible for high photo-reductive capabilities.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
One of the authors (M. Naushad) is grateful to the Researchers Supporting Project number
(RSP-2019/8), King Saud University, Riyadh, Saudi Arabia for funding this research.
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27

Figure 1: Synthesis Scheme