Materials Chemistry and Physics 296 (2023) 127275
Contents lists available at ScienceDirect 
Materials Chemistry and Physics 
journal homepage: www.elsevier.com/locate/matchemphys 
Photocatalytic enhancement mechanisms for novel g-C3N4/ 
PVK nanoheterojunction 
V.W. Elloh a,b,*, E. Okoampa Boadu b, D. Abbeyquaye b, D.E. Anderson b, A. Yaya c 
a Department of Physics, University of Petroleum and Energy Studies (UPES), Dehradun, India 
b Department of Biomedical Engineering, Koforidua Technical University, Koforidua, Ghana 
c Department of Materials Science and Engineering, University of Ghana, Legon, Ghana   
H I G H L I G H T S  
• Band alignment ensures photogenerated electrons migrate from g-C3N4 to PVK monomer. 
• High hydrogen-evolution reaction activity. 
• Charge transfer between g-C3N4 monolayer and PVK results. 
• Polarized field within interface region separation of photogenerated carriers. 
• PVK is a non-noble metal co-catalyst for g-C3N4 photocatalysis.  
A R T I C L E  I N F O   A B S T R A C T   
Keywords: The interactions between monolayer graphitic carbon nitride (g-C3N4) and conjugated polymer poly(9- 
DFT vinylcarbazole) (PVK) have been explored. We investigated the enhanced photocatalytic mechanisms for the 
GGA novel g-C3N4/PVK nanoheterojunction covering the state-of-the-art of DFT by performing rigorous DFT calcu-
PBE lations combined with van der Waals corrections (GGA + vdW). The calculated band alignment between g-C N
LDA 3 4 
Monolayer g-C N monolayer and PVK monomer clearly reveals that the conduction band minimum and the valence band 3 4 
PVK maximum of g-C3N4 monolayer are higher than those of the conjugated polymer PVK. This predicted band 
Photocatalytic semiconductor alignment ensures the photogenerated electrons easily migrate from the g-C3N4 monolayer to the PVK monomer, 
and will lead to high hydrogen-evolution reaction activity. The charge transfer between g-C3N4 monolayer and 
PVK results in a polarized field within the interface region, which will benefit the separation of photogenerated 
carriers. The calculated density of electronic states, Lowdin charge transfer and charge density difference certify 
that this proposed layered nanoheterojunction is an excellent light-harvesting semiconductor. These findings 
indicate that the conjugated polymer PVK is a promising candidate as a non-noble metal co-catalyst for g-C3N4 
photocatalysis. It also provides useful information for understanding the observed enhanced photocatalytic 
mechanisms in experiments.   
1. Introduction Semiconductor photocatalysis is a novel and efficient technology. It has 
attracted tremendous attention of researchers and industrialists due to 
The world has witnessed a great deal of increase in environmental its versatile applications in solar energy utilization and pollutant elim-
challenges and energy demands as a consequence of the depletion of ination [3,10–12]. Semiconductor photocatalysts like TiO2, ZnO and 
fossil fuels for some number of decades now. By way of addressing the SrTiO3 are known to exhibit high photocatalytic performance [13]. 
above challenges, various technologies have been proposed. One of However, they can only use a few per cent of the solar energy because of 
them is semiconductor photocatalysis. This is a potential technology their wide energy band gaps. Photocatalysts like CdS has suitable band 
proposed to address the environmental problems. This technology in- gap to absorb visible light. Concurrently, photogenerated electrons of 
volves converting the raw material CO2 into chemical solar fuels [1–9]. conduction bands have sufficient reducing ability for photocatalytic 
* Corresponding author. Department of Physics, University of Petroleum and Energy Studies (UPES), Dehradun, India. 
E-mail address: vanw.elloh@ktu.edu.gh (V.W. Elloh).  
https://doi.org/10.1016/j.matchemphys.2022.127275 
Received 6 September 2022; Received in revised form 10 December 2022; Accepted 24 December 2022   
Available online 27 December 2022
0254-0584/© 2022 Elsevier B.V. All rights reserved.
V.W. Elloh et al.                                                                                                                                                                                  M   a  t e  r i a  ls   C  h  e m   i s t r y   a  n  d  P  h  y  s i c s 296 (2023) 127275
hydrogen evolution. However, CdS lacks stability because of the and a supercell approximation with lattice parameters a = 6.511 Å, b =
self-oxidation of photogenerated holes. For the surpassing reasons, it is 6.511 Å and c = 4.745 Å. The sampling of the Brillouin zone was done 
prudent to search for novel high-efficient photocatalytic materials. using a 9 × 9 × 1 grid and tested to give convergent results for the total 
g-C3N4 was reported by Wang et al. [14] to be able to generate energy. In this work, the Grimme’s D3 correction term [47] is applied to 
hydrogen from water under visible light. Two-dimensional layered include the van der Waals (vdW) interaction which is found to be sub-
g-C3N4 has the appropriate band gap. It has unique physicochemical stantial in carbon nano systems [48–50]. The vacuum layer in the 
properties such as high thermal and chemical stability, unique electronic simulation cell is set to 12 Å thick in both y- and z-directions to ensure 
properties, low density and extreme hardness [15–18]. These properties negligible interactions between the supercell and its periodic images. 
make g-C3N4 an excellent potential material for application in a wide The structural optimizations were performed using a conjugated 
range of areas such as photocatalytic CO2 reduction, fuel cells, pollutant gradient procedure. For the nanoheterostructure, we have chosen the 
removal, photochemical water splitting for hydrogen production, envi- position and the orientation in which the whole system has minimum 
ronmental remediation [1,3,10,19–22]. Therefore, g-C3N4 is considered energy. For this position and orientation, the structure is fully relaxed 
a promising candidate for visible-light photocatalytic hydrogen gener- until all the force components were smaller than 0.4 × 10− 2 Ry bohr− 1 
ation, organic pollutant decomposition and carbon dioxide reduction. − 1
(∼ 0.1 eV Å ), in order to obtain the exact position and orientation for 
However, g-C3N4 suffers from a high recombination rate of photo- the nanoheterojunction. 
generated electrons and holes. This leads to low quantum efficiency 
[23]. The possibility to restrain the high recombination rate of photo-
generated electron− hole pairs is crucial to improve the poor quantum 3. Results and discussions 
efficiency of g-C3N4. 
Various modification techniques have been explored in recent times 3.1. Geometry and structure of monolayer g-C3N4 and PVK unit cells 
to optimize the performance of g-C3N4 [24–26]. Band-gap engineering, 
micro-nanostructuring, surface modifications, cocatalyst combination, Fig. 1 is a schematic illustration of (a) monolayer g-C3N4 and (b) PVK 
semiconductor coupling, heterojunction formation are just but a few to monomer, N-vinylcarbazole structures. Graphitic carbon nitride, g- 
mention. For example, band-gap engineering through metal and C3N4, is a metal-free, highly efficient, durable, low-cost and earth- 
nonmetal elements doping has been studied. It inhibits photogenerated abundant semiconductor [1]. g-C3N4 is a polymeric semiconductor 
electron-hole pair recombination and greatly increases the with a band-gap of approximately 2.7 eV [3]. It is easily synthesized 
light-harvesting ability in the visible region in pristine g-C N [27–29]. through thermal condensation of simple and cheap precursors like urea, 3 4 
In order to increase the surface area of g-C3N4, various modification 
cyanamide, dicyandiamide, melamine, etc. [10]. Also, it. 
strategies have been explored and observed to be effective [2,3,23, has unique physicochemical properties such as high thermal and 
30–34]. A study conducted by Zhu et al. based on DFT calculations chemical stability, unique electronic properties, low density and 
shows that halogen-doped monolayer g-C N systems tend to have extreme hardness [12]. These properties make g-C3N4 an excellent po-3 4 
narrow band-gaps and increase light absorption rate which are required tential material for application in a wide range of areas such as photo-
for high photocatalytic activity [30]. catalytic CO2 reduction, fuel cells, pollutant removal, photochemical 
Recently, construction of g-C3N4/semiconductor heterostructures 
water splitting for hydrogen production, environmental remediation, 
demonstrated an enormous potential to promote the photocatalytic etc. [13–15]. Nevertheless, serious limitations of pristine g-C3N4 are its 
performance of g-C3N4 mediated by efficient separation of the electro-
poor visible light utilization ability, fast recombination of photo-
n− hole pairs, thereby restraining the recombination rate of photo- generated electron-hole pairs, i.e., low mobility of charge carriers and 
generated carriers [35–40]. g-C3N4/semiconductor heterojunctions 
low surface area [10–12,16]. Poly(9-vinylcarbazole) (PVK) is a conju-
have exhibited enhanced photocatalytic activity. In particular, the gated polymer with hydrophobic properties. PVK films are mainly used 
monolayer g-C3N4/semiconductor heterostructure exhibits significantly 
as holes. (PVK) is a temperature-resistant [51] thermoplastic polymer 
enhanced photocatalytic hydrogen production and dye degradation produced by radical polymerization from the monomer, N-vinyl-
under visible-light irradiation [41,42]. The mechanisms of photo- carbazole. It is photoconductive and thus the basis for photorefractive 
catalysis enhancements in g-C3N4/semiconductor nanoheterojunctions, 
polymers and organic light-emitting diodes [52]. Poly 
however, remain unclear. The band structure, charge transfer and (9-vinylcarbazole), a conjugated polymer has been explored as an 
interface interactions of the g-C3N4/semiconductor nanoheterojunctions 
efficient hole transport material for the preparation of p-i-n type, 
have not been fully investigated. To this end, we seek to understand the inverted planar heterojunction perovskite solar cells. Poly(N-vinyl 
mechanisms of enhanced photocatalysis in the g-C N /PVK nano- carbazole) as a hole transport organic semiconducting polymer has 3 4
heterojunction and to further design a novel g-C N -based nano- been widely used as an electronic and optical material [53]. PVK is used 3 4
heterostructures for visible-light harvesting photocatalysis. In this to synthesize titanium oxide (TiO2) quantum dots fillers that find po-
paper, we perform DFT calculations combined with van der Waals cor- tential applications as donor-acceptor layers in polymeric solar cells 
rections (GGA vdW) to investigate the structural and electronic [54]. PVK is used to functionalize carbon nanotubes (CNTs) and form +
properties of pristine g-C N and g-C N /PVK nanoheterojunction. conductive reinforcements on epoxy groups [55]. Iridium composites 3 4 3 4 are normally surface modified by PVK for the fabrication of highly 
2. Computational methods efficient organic light emitting devices. The electrical conductivity of 
PVK changes according to intensity of illumination. As a consequence, 
All calculations in this study are based on the methods of DFT and the PVK is classified as a semiconductor or photoconductor. Poly 
ab-initio pseudo-potential plane wave basis set as implemented in the (9-vinylcarbazole) is extremely brittle, but its brittleness can be 
PWSCF code of the Quantum ESPRESSO [43]. We performed the reduced by copolymerization with a small amount of isoprene [56]. 
calculation within the local density approximation (LDA), parameter-
ized by Perdew and Zunger [44], the generalized gradient approxima- 3.2. Structural properties 
tion (GGA) of Perdew, Burke and Ernzerhof (PBE) [45]. The Kohn-Sham 
orbitals are expanded in a plane-wave basis set. The electronic wave The structural geometry and stability of PVK interacting with 
functions and the charge density are expanded in plane waves up to 30 monolayer g-C3N4 as a nanoheterojunction model in a supercell struc-
Ry and 180Ry respectively. The core and valence electrons are treated ture was studied. We examined different structural geometries and sta-
with Vanderbilt ultrasoft pseudopotentials [46]. The g-C3N4/PVK bility of PVK polymer chain interacting with monolayer g-C3N4. The 
nanoheterostructure was simulated using periodic boundary conditions PVK polymer chain was aligned parallel to the monolayer g-C3N4 along 
2
V.W. Elloh et al.                                                                                                                                                                                  M   a  t e  r i a  ls   C  h  e m   i s t r y   a  n  d  P  h  y  s i c s 296 (2023) 127275
of formation for this composite g-C3N4/PVK is low compare to similar 
structures in literature, therefore, energetically stable. Additionally, the 
negative formation energy implies composite is thermodynamically 
favorable for studies. A separation distance of ~3.383 Å was kept be-
tween monolayer g-C3N4 and PVK monomer for all the different nano-
heterojunction orientations examined. This interlayer distance is 
comparable to the interlayer separations in graphite (ca. 3.35 Å). We 
computed the structural stability of the nanoheterojunction, g-C3N4/ 
PVK, studied by using equations (1) and (2). Relative to this adsorption 
distance of 3.383 Å, we calculated the adsorption energy, Ead, for the g- 
C3N4/PVK nanoheterojunction as recorded in Table 2. The adsorption 
energy, Ead, was determined from the relation [1] below [57]. 
( )
Ead = EPVK/g− C3N4 − EPVK − Eg− C3N4 (1)  
where EPVK/g− C3N4 , EPVK, Eg− C3N4 represent the total ground-state energy 
of the g-C3N4/PVK composite, the total energy of PVK, the total energy 
of g-C3N4 respectively predicted by DFT calculation. Quantum Espresso 
code reports the value of dispersion contribution to total energy for 
every step during the geometry optimization process. Accordingly, the 
dispersion contribution to adsorption energy was determined using 
Equation (1) above, by replacing the terms on the right-hand side with 
the dispersion contribution quantities for the converged geometry of 
each structure. 
Whenever the Ead > 0, then it suggests that the surface adsorption is 
thermodynamically feasible. The greater the Ead value, the tendency of 
the adsorbate molecule to bind on the g-C3N4/PVK nanoheterostructure 
surface becomes stronger. From Table 2, a clear distinctive tendency of 
the monolayer g-C3N4 to adsorb favorably on PVK monomer surface is 
seen. 
From DFT optimized configuration of the system considered, we 
calculated Ead ∼ 2.236 eV for GGA + vdW exchange-correlation func-
tional and Ead ∼ 2.129 eV for LDA exchange-correlation functional. The 
adsorption energies, Ead ∼ 2.236,2.129 eV, for the respective exchange- 
correlation functionals, are attributed to the orientation and proximity 
of the nitride functional group of the conjugated polymer and to the 
π-electrons of the g-C3N4 and also via strong π-π interactions between 
the carbazole unit and the g-C3N4 to chemisorb at the g-C3N4/PVK 
surface. 
For detail insight into the new structure, we calculated the formation 
energy per atom, Epa, for monolayer g-C3N4 and the formation energy 
per atom for PVK separately and found their values to be − 6.486 eV and 
− 5.263 eV respectively. The formation energies were determined from 
the relation:  
ΔE = ECHN – NCEC – NHEH – NNEN                                                 (2) 
where ECHN is the total energy of the ground state of the corresponding 
nanoheterojunction modelled. EC is the total energy of carbon in its 
ground state (graphite), NC is the number of carbon atoms in the 
nanoheterojunction. EH is the total energy of ground state of H, NH is the 
number of hydrogen atoms in the nanoheterojunction, EN is the total 
energy of ground state of N and NN is the number of nitrogen atoms in 
the nanoheterojunction modelled. The calculated formation energies per 
Fig. 1. Schematic illustrations of (a) monolayer g-C3N4 and (b) PVK monomer atom for each of the systems considered is tabulated in Table 2. It is 
unit cell used in this work. observed that the new nanoheterojunction modelled has large negative 
formation energy value per atom; an indication that this new structure is 
with its side groups towards and/or away from the monolayer g-C3N4. thermodynamically stable. It can be seen that the formation energy per 
Other structural combinations and orientations notably with the PVK atom values for g-C3N4 and PVK individually are much larger than the 
curved around the monolayer g-C3N4 and by flipping the PVK through formation energy per atom value for g-C3N4/PVK as depicted in Table 2. 
180◦ about its centre were considered in order to determine the This shows that the new nanoheterojunction is more stable and ther-
preferred orientation of the g-C3N4/PVK geometry. It was found that the modynamically favorable compare to the original molecules. Further-
PVK polymer chain preferred to align with its side groups towards the g- more, our calculations reveal that the new nanoheterojunction modelled 
C3N4. We noted that the enthalpy of formation for this particular is stable in both GGA + vdW and LDA model types. The value of the 
structural arrangement was lower than that of all other orientations energy difference E0(eV) for the simulated nanoheterojunction exhibits 
examined. This is therefore energetically more stable and favorable. This structural stability over g-C3N4 and PVK as shown in Table 1. The 
orientation was therefore used for this study. We noted that the enthalpy calculated band gap values along with the optimized lattice parameters 
3
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Table 1 3.3. Electronic properties 
Optimized Lattice Parameters a(Å), b(Å), c(Å), Energy gap Eg(eV) and Energy 
Difference E0(eV) of PVK, g-C3N4 and g-C3N4/PVK nano structures.   The calculated band structures for PVK monomer, monolayer g-C3N4 
a(Å) b(Å) c(Å) Eg(eV) E0(eV) and g-C3N4/PVK nanoheterojunction are respectively illustrated in 
PVK 4.451 4.451 2.423 3.751 7.660 Fig. 2(a, b & c). For the PVK monomer, the minimum energy gap of 
g-C N 4.277 4.505 2.293 2.692 6.021 about 3.751 eV is an indirect bandgap transition from the top of the 3 4 
g-C3N4/PVK 6.511 6.511 4.745 2.143 0.00  valence band maximum (VBM) located at the Γ point and the bottom of 
the conduction band minimum (CBM) located at the midpoint in- 
between the X and the Γ high symmetry points of the Brillouin zone. 
Table 2 In the case of the monolayer g-C3N4, with a minimum energy gap of 
Adsorption energy, Ead, and formation energy per atom, Epa, calculated using about 2.692 eV, an indirect bandgap transition from the top of the 
Equations (1) and (2).  valence band maximum (VBM) is located at the Γ point and the bottom 
(GGA + vdW) E /eV E /eV of the conduction band minimum (CBM) is located at the midpoint in- ad pa
between the K and the X high symmetry points. The g-C N
PVK  3 4
/PVK 
− 5.263 
g-C N 6.486 nanoheterojunction has a direct energy gap of approximately 2.143 eV, 3 4  −
g-C3N4/PVK 2.236 − 9.021 with both CBM and VBM located at the Γ-point as seen in Fig. 2(c). The 
(LDA) Γ-to-Γ direct band gap guarantees the high efficiency energy conversion 
PVK  − 4.327 by solar energy without any lattice dynamic behaviour. 
g-C3N4  − 5.746 The band gap value 2.143 eV satisfies the required minimum band 
g-C3N4/PVK 2.129 − 8.331  gap, Eg > 1.23 eV, showing the potential application of g-C3N4/PVK 
vdW nanoheterostructure for photocatalytic processes. Furthermore, the 
and other structural values of all the systems are presented in Table 1. band edge alignments with respect to water reduction and oxidation 
The calculated bond lengths of C–N are 1.33 and 1.46 Å, which is in potential levels of the g-C3N4/PVK nanoheterojunction satisfy photo-
agreement with experimental bond length of C–N 1.32 and 1.44 Å. catalytic water splitting. As is seen, PVK monomer has a favorable band 
The negative formation energy indicates that this heterostructure is position for water splitting, which is in accordance with other works 
energetically favorable and could be easily obtained. Furthermore, the [59,60]. For g-C3N4/PVK vdW nanoheterostructure, the VBM locates 
binding energy, Eb, between g-C3N4 monolayer and PVK monomer in the more positive than the water oxidation potential and the CBM is more 
nanoheterojunction is calculated to be ∼ 21.14 meV/Å2 which is close to negative than the hydrogen reduction potential, the feature of which is 
the typical van der Waals binding energy, E , of ∼ 20 meV/Å2 b by other necessary for water splitting. The realignment of both VBM and CBM of 
advanced DFT calculations [58]. Hence, the g-C3N4/PVK nano- the g-C3N4/PVK nanoheterostructure is attributed to the weak vdW in-
heterojunction belongs to van der Waals (vdW) heterostructures. teractions between monolayer g-C3N4 and PVK monomer. Further, since 
the VBM position with respect to water oxidation potential is much more 
Fig. 2. Calculated band structures at high-symmetry k-points for (a) PVK, (b) monolayer g-C3N4 and (c) g-C3N4/PVK nanoheterojunction respectively. The Fermi 
energy level, EF, is fixed as the reference of zero energy. 
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V.W. Elloh et al.                                                                                                                                                                                  M   a  t e  r i a  ls   C  h  e m   i s t r y   a  n  d  P  h  y  s i c s 296 (2023) 127275
positive for g-C3N4/PVK, it indicates that better photocatalytic perfor- due to contributions from C atoms. The partial electron density of states 
mance will be expected in g-C3N4/PVK vdW nanoheterostructure. (p-DOS) plot, Fig. 4(c), the occupied states are due mainly to p-states of 
N and the bottom of the conduction band minimum are entirely s-states 
3.3.1. Analysis of the electronic density of states of N. 
Analysis of the nature of interactions and origin of energy gaps in the 
new nanoheterojunction is carried out by way of plotting the projected 3.3.2. Photocatalytic properties analysis 
electronic density of states (DOS), partial electronic density of states (p- Further analysis of the band state characterizations demonstrates 
DOS), projected electronic density of states (DOS)-total electronic den- that CBM and VBM are localized in different monolayers of the g-C3N4/ 
sity of states (t-DOS) superimposed and total electronic density of states PVK vdW heterostructures. The CBM originates from the N-s states in the 
(t-DOS) for PVK, g-C3N4 and g-C3N4/PVK nanoheterojunction in PVK monomer, while the VBM is occupied by the N-p states in the g- 
Figs. 3–6 respectively. For the sake of comparison, the Fermi energy C3N4 monolayer. In order to understand the role of the constituent layers 
level, EF, was set as zero on the energy scale of the energy band-gaps, Eg, in photocatalytic water splitting for g-C3N4/PVK vdW heterostructure, 
between the occupied atomic orbitals and the empty atomic bands and the band-decomposed charge density was calculated for the lowest un-
the same for the DOS, which are shown in Figures (3-6). For PVK: the occupied molecular orbital (LUMO, CBM at the Γ point) and the highest 
projected electronic density of states (DOS) plot, Fig. 3(a), shows that occupied molecular orbital (HOMO, VBM at the Γ point). As seen from 
the entire portion of the DOS between − 4 eV and 4 eV is due to H. The the partial density of states (p-DOS) in Fig. 4(c), the LUMO is mainly 
partial electron density of states (p-DOS) plot, Fig. 4(a), the occupied contributed by the N-s orbitals, and the HOMO mainly consists of the N- 
states are due largely to p-states of N and the bottom of the conduction p orbitals for the g-C3N4/PVK vdW heterostructures. The nature of the 
band minimum are entirely p-states of C. band states implies that the photoexcited electrons will transfer from the 
For monolayer g-C3N4: the projected electronic density of states states localized in g-C3N4 to the states localized in PVK during the 
(DOS) plot, Fig. 3(b), the portion of DOS below -1eV is due to N atoms photocatalysis process, where PVK behaves as the electron acceptor and 
and the portion which is above 1eV is also due to the contributions from g-C3N4 behaves as the electron donor. 
N atoms. The partial electron density of states (p-DOS) plot, Fig. 4(b), Consequently, during the photocatalytic water splitting the 
the occupied states are due largely to p-states of N and the bottom of the hydrogen production process takes place in the PVK monomer, while the 
conduction band minimum are entirely p-states of N. oxygen production locates at the g-C3N4 monolayer. On the basis of the 
For the g-C3N4/PVK nanoheterojunction: the projected electronic above analysis, the mechanism of the solar energy driven g-C3N4/PVK 
density of states (DOS) plot, Fig. 3(c), the portion of DOS below -1eV is vdW heterostructure for the water splitting process is illustrated in 
due to N atoms and in similar manner, the portion which is above 1eV is Ref. [61]. When the incoming solar light is absorbed by the g-C3N4/PVK 
Fig. 3. Projected electronic density of states (DOS) for (a) PVK, (b) monolayer g-C3N4 and (c) g-C3N4/PVK nanoheterojunction respectively. The Fermi energy level, 
EF, is set as zero on the energy scales. 
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V.W. Elloh et al.                                                                                                                                                                                  M   a  t e  r i a  ls   C  h  e m   i s t r y   a  n  d  P  h  y  s i c s 296 (2023) 127275
Fig. 4. Partial density of states (p-DOS) for (a) PVK, (b) monolayer g-C3N4 and (c) g-C3N4/PVK nanoheterojunction respectively. The Fermi energy level, EF, is fixed 
as the reference of zero energy. 
vdW heterostructure, photogenerated electrons will transfer from VBM acceptor or as a donor. The charge transfer from the monolayer g-C3N4 
to CBM, and hence hydrogen and oxygen will be separately produced at to the polymer PVK monomer in the g-C3N4/PVK nanoheterojunction 
the PVK monomer and the g-C3N4 monolayer during the photocatalytic turns out to be 0.39 electrons. Here we would like to point out that it is 
water splitting. This is because the vdW heterostructure shows direct important to note that the size of the charge transfer slightly depends on 
and suitable band gap at the Γ point. The orbital overlap modifies the the method chosen for calculations. 
orbital and enhances the optical absorption. We have illustrated the 
density of states (DOS) of the g-C3N4/PVK vdW heterostructures in Fig. 3 3.3.4. Charge density 
(c) to further understand the optical absorption mechanism of the The surface distribution of the electronic charge density gives the 
nanoheterojunction. The partial DOS in Fig. 4(c) shows the overlap of variation in the electronic density induced by the internal charges. Fig. 7 
N-s and N-p electrons in the valence bands of the g-C3N4/PVK vdW shows for g-C3N4/PVK nanoheterojunction the electronic charge den-
nanoheterojunction. On the other hand, the g-C3N4/PVK vdW nano- sities distribution in response to internal electric field obtained by using 
heterojunction exhibits the requisite absorption spectra in the Xcrysden software, version 1.5.60 [62]. The contributions to the charge 
visible-light range, and thus the photocatalysis performance of the density from different atoms are clearly visible from Fig. 7. It is apparent 
g-C3N4/PVK vdW nanoheterojunction is expected to be optimal. that the polarization of the surface charges is dependent on the atomic 
Nevertheless, the advantage of g-C3N4/PVK vdW nanoheterojunction environment. Notable differences can be found in the surface charge 
over monolayer g-C3N4 is clear. The vdW nanoheterojunction not only distribution densities for the pristine aromatic polymers such as the 
has significantly improved photocatalysis properties but also can sepa- benzene rings and for the carbon atoms and the π-conjugate plane of 
rately produce hydrogen and oxygen at the opposite monolayers of the PVK, especially in the vicinity of the heteroatom nitrogen. Furthermore, 
vdW heterostructures. This is the mostly desired photocatalysis perfor- from the figure, the polarizabilities are strongly anisotropic with a 
mance in practice, which however has been achieved in the current relatively large response for the surface charge distributions towards the 
work. π-conjugate plane of PVK. 
The charge density plot in Fig. 7 shows a homogenous charge dis-
3.3.3. Lowdin charge transfer tribution among the nitrogen, hydrogen and carbon atoms signifying 
The amount of charge transfer from g-C3N4 to the PVK was estimated that a significant interaction took place. In terms of charge distribution, 
by projecting the charge density onto the atomic orbitals. We calculated there is a uniform sharing between the nitrogen and carbon atoms with 
the charge transfer as the difference between the Lowdin charges for respective to electronegativity values of 3.04 and 2.55. There is also an 
pristine PVK and g-C3N4/PVK with the adsorbate molecules. From this excellent charge distribution between the hydrogen and carbon atoms 
result, we determined whether the adsorbate molecule acts as an because of similar electronegativity values of 2.20 and 2.55 respectively. 
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V.W. Elloh et al.                                                                                                                                                                                  M   a  t e  r i a  ls   C  h  e m   i s t r y   a  n  d  P  h  y  s i c s 296 (2023) 127275
Fig. 5. Projected electronic density of states (DOS) and total DOS (t-DOS) superimposed for (a) PVK, (b) monolayer g-C3N4 and (c) g-C3N4/PVK nanoheterojunction 
respectively. The Fermi energy level, EF, is set as zero on the energy scales. 
The PDOS in Fig. 4 indicates that the p-orbitals of carbon and nitrogen 4. Conclusions 
atoms contributed most significantly to the total density of states. The 
reason for this can be traced back to the charge density plot in Fig. 7. The We have performed detailed DFT calculations to explore the 
plot shows a higher density of charges around the carbon and nitrogen enhanced photocatalytic mechanism for the novel hybrid g-C3N4/PVK 
atoms. This is apparently due to nitrogen having comparable electro- nanoheterojunction. The calculated band alignment between the g-C3N4 
negativity value to that of carbon. As a result, the carbon and nitrogen monolayer and the PVK monomer reveals that the CBM (VBM) of g-C3N4 
atoms draw more electrons to themselves. is higher than that of the CBM (VBM) of the PVK monomer. This pre-
dicted band alignment ensures the photogenerated electrons can easily 
3.3.5. Optical properties migrate from the g-C3N4 layer to the PVK monomer, and leads to high 
hydrogen-evolution reaction activity. The charge transfer between PVK 
3.3.5.1. Calculated ultra violet optical absorption. Fig. 8(a, b & c) show and g-C3N4 results in a polarized field within the interface region, which 
calculated optical spectral absorption for pristine PVK, pristine mono- can effectively improve the separation efficiency of these photo-
layer g-C3N4 and g-C3N4/PVK nanoheterojunction respectively. Fig. 8(a) generated carriers. In addition, this new hybrid layered junction has 
shows the absorption spectra for PVK without the monolayer g-C3N4. high-light absorption ability. The g-C3N4/PVK vdW nanoheterojunction 
Fig. 8(b) shows the absorption spectra for the monolayer g-C3N4 without exhibits very good optical absorption characteristics in the visible-light 
PVK. Fig. 8(c) is the calculated absorption spectra for the nano- wavelengths where g-C3N4 monolayer and PVK monomer act as electron 
heterojunction when the monolayer g-C3N4 was added to PVK. In Fig. 8 donor and electron acceptor respectively. This character will facilitate 
(a), it can be observed that there are five intensity peaks located at 0.41, the efficient separation and transportation of the photogenerated 
0.45, 0.49, 0.51 and 0.53 Ry. The most intense peak is located at 0.41 charges and thus increase the efficiency of the photocatalytic processes. 
Ry. In Fig. 8(b), we observed that there are three intensity peaks located These theoretical predictions provide insight to understand the related 
at 0.41, 0.45 and 0.67 Ry with the most intense peak located at 0.67 Ry. experimental observations, and verify that PVK monomer is a promising 
Comparing Fig. 8(c), it is observed that the addition of monolayer g- candidate as a non-noble metal co-catalyst for g-C3N4 photocatalysis. 
C3N4 to PVK has resulted in the quenching of the intensity peaks located 
at the various energies in the pristine monolayer g-C3N4 and PVK optical Funding 
absorption spectra. This observation is an indication of charge transfer 
processes occurring between the monolayer g-C3N4 and PVK. This research received no external funding. 
7
V.W. Elloh et al.                                                                                                                                                                                  M   a  t e  r i a  ls   C  h  e m   i s t r y   a  n  d  P  h  y  s i c s 296 (2023) 127275
Fig. 6. Total electronic density of states (t-DOS) for (a) PVK, (b) monolayer g-C3N4 and (c) g-C3N4/PVK nanoheterojunction respectively. The Fermi energy level, EF, 
is fixed as the reference of zero energy. 
Fig. 7. Calculated self-consistent charge density isolines and isosurfaces of g-C3N4/PVK nanoheterojunction. The rainbow type color-coding refers to violet as regions 
of maximum charge density which decreases gradually finally to red as regions of minimum charge density. (For interpretation of the references to color in this figure 
legend, the reader is referred to the Web version of this article.) 
CRediT authorship contribution statement Data curation, repetition, Writing – review & editing. D. Abbeyquaye: 
Conceptualization, Data curation, Formal analysis, Funding acquisition, 
V.W. Elloh: Visualization, Investigation, Formal analysis, Writing – Writing – review & editing. D.E. Anderson: Methodology, Project 
original draft. E. Okoampa Boadu: Visualization, Resources, Software, administration, Resources, Software, Supervision, Validation. A. Yaya: 
8
V.W. Elloh et al.                                                                                                                                                                                  M   a  t e  r i a  ls   C  h  e m   i s t r y   a  n  d  P  h  y  s i c s 296 (2023) 127275
Fig. 8. Calculated (a) optical absorption spectra of PVK, (b) optical absorption spectra of monolayer g-C3N4 and (c) optical absorption spectra of g-C3N4/PVK 
nanoheterojunction respectively. 
Supervision, Project administration, Funding acquisition, Visualization. [5] S. Cao, J. Yu, Carbon-based H2-production photocatalytic materials, J. Photochem. 
Photobiol., C 27 (2016) 72–99, https://doi.org/10.1016/j. 
jphotochemrev.2016.04.002. 
Declaration of competing interest [6] J. Low, B. Cheng, J. Yu, Surface modification and enhanced photocatalytic CO2 
reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658–686, 
The authors declare that they have no known competing financial https://doi.org/10.1016/j.apsusc.2016.09.093. 
[7] J. Lin, W. Tian, H. Zhang, X. Duan, H. Sun, S. Wang, Graphitic Carbon Nitride- 
interests or personal relationships that could have appeared to influence Based Z-Scheme Structure for Photocatalytic CO2 Reduction, Energy & Fuels, 2020, 
the work reported in this paper. https://doi.org/10.1021/acs.energyfuels.0c03048. 
[8] G. Zhang, G. Li, T. Heil, S. Zafeiratos, F. Lai, A. Savateev, M. Antonietti, X. Wang, 
Data availability Tailoring the grain boundary chemistry of polymeric carbon nitride for enhanced solar hydrogen production and CO2 reduction, Angew. Chem. Int. Ed. 58 (2019) 
3433–3437, https://doi.org/10.1002/anie.201811938. 
Data will be made available on request. [9] M.S. Akple, S.P. Chimmikuttanda, G.K.S. Takyi, V.W. Elloh, Fabrication and 
density functional theory calculations of bromine doped carbon nitride nanosheets 
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