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Article

Manipulatable Interface Electric Field and Charge Transfer in a 2D/2D Heterojunction Photocatalyst via Oxygen Intercalation

1
Theoretical Materials & Chemistry Group, Institute of Inorganic Chemistry, University of Cologne, Greinstr. 6, 50939 Cologne, Germany
2
Division of Materials Science and Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Korea
*
Authors to whom correspondence should be addressed.
Catalysts 2020, 10(5), 469; https://doi.org/10.3390/catal10050469
Received: 26 March 2020 / Revised: 21 April 2020 / Accepted: 22 April 2020 / Published: 25 April 2020

Abstract

:
Charge separation is the most important factor in determining the photocatalytic activity of a 2D/2D heterostructure. Despite the exclusive advantages of 2D/2D heterostructure semiconductor systems such as large surface/volume ratios, their use in photocatalysis is limited due to the low efficiency of charge separation and high recombination rates. As a remedy for the weak interlayer binding and low carrier transport efficiency in 2D/2D heterojunctioned semiconductors, we suggested an impurity intercalation method for the 2D/2D interface. PtS2/C3N4, as a prototype heterojunction material, was employed to investigate the effect of anion intercalation on the charge separation efficiency in a 2D/2D system using density functional theory. With oxygen intercalation at the PtS2/C3N4 interface, a reversed and stronger localized dipole moment and a built-in electric field were induced in the vertical direction of the PtS2/C3N4 interface. This theoretical work suggests that the anion intercalation method can be a way to control built-in electric fields and charge separation in designs of 2D/2D heterostructures that have high photocatalytic activity.

1. Introduction

With the increasing energy demand, energy conversion and storage are important key factors to meet the demands of humans. At the same time, the energy crisis and environmental pollution problems come one after another because the burning of fossil fuels produces a huge amount of CO2. Therefore, renewable energy sources have received intensive attention as an alternative to non-renewable fossil fuels [1,2,3]. Solar energy conversion to hydrogen and hydrocarbon fuels using photocatalysts are considered to be a very promising renewable energy resources [4,5], however, eco-friendly hydrogen production from solar energy is only 5% of the total commercial hydrogen production [6]. To increase hydrogen production by renewable energy sources, the semiconductor-based photocatalyst has received considerable interest in water splitting and pollutant degradation, such as CO2 reduction [7,8]. However, the most widely used photocatalysts such as metal oxide and precious metals are limited because of the high cost and the band gap within the ultraviolet region [9,10,11]. Therefore, semiconductor photocatalysts require a suitable photocatalytic system for efficient charge separation for electron-hole pairs [12,13,14].
Graphitic carbon nitrides (C3N4) have attracted attention for hydrogen and oxygen evolution as a water splitting photocatalyst due to their low cost, easy fabrication, and high chemical and thermal stability [11,15,16]. In particular, the band gap of C3N4 (2.70 eV) is appropriate for a photocatalyst because it covers both water reduction and oxidation [17]. However, C3N4 shows poor photocatalytic efficiency induced by the fast recombination of the electron-hole pairs. Thus, to solve these problems, many studies have been conducted using a wide variety of modification strategies such as co-catalyst incorporation, 2D/2D heterostructures, and impurity doping [18,19,20,21].
Among the various modification strategies in the photocatalyst, the fabrication of 2D/2D heterojunctions based on monolayer 2D materials has been demonstrated as an effective approach toward high performance photocatalysts. However, due to the large interface region, 2D/2D heterojunctions often exhibit a high charge mobility and low charge recombination rates [22]. To solve this problem, many researchers are trying to find a 2D/2D heterostructure with the appropriate photocatalytic system. In the case of C3N4/2D heterostructures, it was found that the photocatalytic systems such as Z-scheme and type-II vary according to the types of materials [23,24]. Therefore, using density functional theory (DFT), systematic research focusing on the rational design and construction of C3N4/2D interface is necessary for readers to better understand the principles of 2D photocatalysts.
The charge separation is a very important factor for photocatalytic water splitting [25,26]. In particular, in 2D/2D heterostructures for photocatalyst systems, the driving force for charge separation is the energy level difference of the electrons and holes built by the band alignment of the heterostructure [27,28,29]. In other words, if the exchange of electrons and holes between the two layers is more active, it is more effective to separate the charge at the interface. However, charge flow across the two layers is restricted in 2D/2D heterostructures because the nature of its interface is commonly a van der Waals (vdW) interaction [30]. This indicates that 2D/2D heterostructures still suffer from low efficiency in charge separating and transferring oriented charges [31,32]. According to previous studies, it is reported that charge separation at interfaces of vdW heterostructures can be promoted through impurity intercalation [30,33]. This means that impurity intercalation will play an important role when a 2D heterostructure strategy is adopted in PtS2/C3N4 heterostructures for increasing photogeneration of electron-hole pairs throughout its lifetime. Among the various impurity atoms, O as the impurity for intercalation was chosen because it does not influence the structure of g–C–3N4, but improves the charge separation [34,35]. Therefore, it must be confirmed that it is possible to use a PtS2/O/C3N4 heterostructure as a photocatalyst through a theoretical approach.
Among the various 2D materials, transition metal dichalcogenide (TMD) is widely used as a co-catalyst in a photocatalyst due to its hydrogen evolution reaction (HER) ability and the ease of controlling its electronic properties [36,37,38,39]. C3N4/TMD heterostructures such as C3N4/MoS2 and C3N4/SnS2 afford a high photocatalytic activity [40,41,42]. Group-10 TMD has been reported as a potentially promising photocatalyst, electrocatalyst, and opto-electronic device because of its unique properties [43,44,45]. Lattice mismatches must be considered in the heterostructure because they are associated with compressive or tensile stress, affecting the electronic properties [46,47]. Among the Group-10 TMDs, platinum disulfides (PtS2) showed a low lattice mismatch between 2 × 2 C3N4 and PtS2. In addition, PtS2 has recently been reported to have shown good HER performance and to have the appropriate band gap of 1.8 eV for a photocatalyst [44]. Thus, PtS2 was chosen to avoid other effects such as corrugation, and to see only the effects of the O-intercalation.
Herein, the effect of O-intercalation in the PtS2/C3N4 heterostructure was investigated through density functional theory (DFT) calculations. After O-intercalation at the interface, the charge redistribution trend changed. Due to the changed dipole moment by the electron acceptance between the surface and the O, the direction of the built-in electric field was reversed. The amount of charge transfer increased due to the increase in the strength of the built-in electric field at the interface. In addition, charge separation efficiency was improved as O-intercalation increased. PtS2/C3N4 is a suitable photocatalyst for water splitting because the reduction and oxidation stability is strong. However, PtS2/O/C3N4 is not suitable for photocatalytic water splitting since its reduction stability is weak. From these results, it is expected that the desired direction of the built-in electric field and the charge separation efficiency can be obtained if the ratio of anion intercalation can be properly controlled.

2. Results and Discussion

As the impurity intercalation of the heterostructure, O atoms were chosen because previous studies have reported that the photocatalytic performance of monolayer C3N4 is enhanced by O doping [48]. To theoretically examine the stability of the PtS2/C3N4 heterostructure after doping, the intercalation energy ( E i n t ) of the PtS2/Ox/C3N4 heterostructure was calculated using Equation (1):
E i n t = 1 n [ E ( P t S 2 / O x / C 3 N 4 ) E ( P t S 2 C 3 N 4 ) n 2 E O 2 ]     ( x = 1.8 ,   3.6 ,   5.4 ,   7.1   % )    
where E P t S 2 / O x / C 3 N 4 , E P t S 2 / C 3 N 4 , and E O 2 are the total energy of the PtS2/Ox/C3N4, the PtS2/C3N4 heterostructure, and the O2, respectively. The n is the number of O atoms in the heterostructure. The E i n t of the PtS2/O1.8%/C3N4 heterostructure was −1.78 eV. The optimized configuration of the PtS2/Ox/C3N4 heterostructure is shown Figure S1. The Eint of all considered PtS2/Ox/C3N4 heterostructures is shown in Table S1. Based on these results, we can infer that the PtS2/O1.8%/C3N4 heterostructure is energetically stable. This means that the O atoms remain captured between the C3N4 and PtS2 layers, rather than diffusing between the layers and recombining to yield O2 or H2O gas. The calculated interface binding energy (Eb) of the PtS2/C3N4 and the PtS2/O1.8%/C3N4 heterostructures was 12.5 and 10 meV/Å2, which was close to the typical vdW-binding energy (13–21 meV/Å2) in previous works [49,50]. Hence, the PtS2/C3N4 and PtS2/O1.8%/C3N4 heterostructures belong to a vdW heterostructure.
To explore the charge separation after the O-intercalation in the PtS2/C3N4 heterostructure, the charge density difference and planar-averaged charge density difference of the PtS2/C3N4 and PtS2/OX/C3N4 (X = 1.8, 3.6, 5.4, 7.1 %) heterostructures were calculated. The charge density difference iso-surface can be evaluated with Equation (2):
Δ ρ ( z ) = ρ t o t a l ( ρ C 3 N 4 + ρ P t S 2 ) ρ O
where ρ t o t a l , ρ C 3 N 4 , ρ P t S 2 , and ρ X denote the charge densities of the PtS2/C3N4 or PtS2/OX/C3N4 heterostructure, the monolayer C3N4, the monolayer PtS2, and the O atom, respectively. It is defined as Equation (3):
Δ Q ( z ) = z Δ ρ ( z ) d z
The charge density difference and planar charge density difference of the PtS2/C3N4 and the PtS2/O1.8%/C3N4 heterostructures is shown in Figure 1a. The charge density difference iso-surface plots are shown in the inset in Figure 1a. The yellow and sky blue colors indicate the electron accumulation and depletion regions, respectively. The red line of Figure 1a indicates that the planar-averaged charge density difference of the C3N4/PtS2 (left) and the PtS2/O1.8%/C3N4 (right) heterostructures along the Z direction exhibited charge redistribution. In both the PtS2/C3N4 and the PtS2/O1.8%/C3N4 heterostructures, the charge redistribution mainly occurred around the PtS2/C3N4 heterostructure interface and the charge transferred from the C3N4 to the PtS2 layer in the heterostructure. However, in the case of the PtS2/C3N4 heterostructure, little charge transfer showed between the C3N4 and PtS2. (Figure 1a) This indicates that the PtS2/C3N4 heterostructure shows a low efficiency of separation of photogenerated electron-hole pairs, like other 2D/2D heterostructures [31,51]. Due to the O-intercalation, the PtS2/O1.8%/C3N4 heterostructure effectively boosts the charge separation, better than that of the PtS2/C3N4 heterostructure. The charge transferred from the PtS2 to the C3N4 layer at the interface of the PtS2/O1.8%/C3N4 heterostructure, unlike in the PtS2/C3N4 heterostructure. In addition, in the other O concentration, the direction of charge transfer was the same, and the amount of charge transfer increased (Figure S2). From these results, this study confirmed the impurity intercalation effect on the amount and direction of charge transfer.
As the built-in electric field at the interface makes an important contribution to charge separation (drifted charge separation) and other charge separation pathways in semiconductors, it must be conceivable [52,53]. In general, the direction of a built-in electric field is induced by the dipole moment [54,55]. Thus, the dipole moment and built-in electric field of the PtS2/C3N4 and the PtS2/OX/C3N4 heterostructures were calculated, as shown in Figure 1b. Using Equation (3), the dipole moment ( μ ) is given by Equation (4)
μ = z Δ Q ( z ) d z
where z indicates the z-axis distance. In Figure 1b, the red line denotes the dipole moment. The μ value decreases as O-intercalation increases and the direction of μ is reversed. As the oxygen is inserted, the direction and strength of the μ changes because the electrons on the surface of the PtS2 move to the O, resulting in the surface of the PtS2 having a positive charge. This result indicates that the change in dipole moment is caused by the acceptance of electrons occurring between the PtS2 layer and the O, regardless of the interaction between the O and the C3N4 layer. From these results, it can be predicted that a similar dipole moment change will occur due to electron acceptance between the surface and the anion if the other anion as well as the O are inserted into the 2D/2D heterostructure. Next, a built-in electric field was generated at the PtS2/C3N4 and the PtS2/OX/C3N4 heterostructure. Thus, the built-in electric field was calculated as Equation (5):
E = z Δ ρ ( z ) ε 0 d z
where E and ε 0 are the built-in electric field, the charge density, and the vacuum permittivity, respectively. ∆ρ is the charge density difference obtained in Equation (2). The built-in electric field of the PtS2/C3N4 heterostructure ( 1.11 × 10 6 V/cm) was smaller than that of the PtS2/O1.8%/C3N4 heterostructure ( 1.64 × 10 6 V/cm). In addition, the direction of the built-in electric field was reversed after O-intercalation in the C3N4/PtS2 heterostructure. As the amount of O-intercalation in the PtS2/C3N4 heterostructure increased, the strength of the built-in electric field increased (Figure 2b). As shown in Figure 2b, as O-intercalation increased, the intensity of the dipole moment and the built-in electric field increased linearly. From these results, it can be inferred that the direction of the built-in electric field can be controlled by adjusting the O-intercalation. This increased built-in electric field indicates that a powerful electrostatic field existed perpendicular to the interface direction. Thus, it can be predicted that the PtS2/OX/C3N4 heterostructure will show increased photocatalytic activity because the driving force for charge separation provided by the built-in electric field is consequently enhanced, thus leading to significantly increased photocatalytic activity.
The optimized band structure of the PtS2/C3N4 and the PtS2/O1.8%/C3N4 with various O concentration heterostructures was calculated to confirm the photocatalytic activity (Figure 2). Considering that, in general, the PBE approach underestimates the band gaps (Eg), this study used a band structure calculated using the HSE06 method to increase the reliability in the following analyses. The contribution of each layer in the conduction band minimum (CBM) and valence band maximum (VBM) of the PtS2/C3N4 and the PtS2/O1.8%/C3N4 heterostructure was investigated as shown in Figure 3. Since it is generally known that the trends of hybrid functional calculations and normal DFT calculations are not very different, the contribution of each layer in the CBM and the VBM was verified using the band structure of the PBE approach [56]. The contributions of each layer in the PtS2/O3.6%/C3N4, PtS2/O5.4%/C3N4, and PtS2/O7.1%/C3N4 heterostructures are shown in Figure S3. The C3N4 layer in the PtS2/C3N4 heterostructure largely contributed to the VBM, whereas the CBM was dominated by the PtS2 layer in the C3N4/PtS2 heterostructure (Figure 2a). In addition, this phenomenon was maintained despite the amount the O-intercalation increased in the PtS2/C3N4 heterostructure (Figure 2b and Figure S3). The PtS2/C3N4 heterostructure with and without O-intercalation had staggered gap structures, resulting in Z-scheme or type-II systems. The reason for the difference of heterostructure type is due to the difference in the direction and strength of the built-in electric field caused by the difference in charge redistribution occurring by O-intercalation. From these results, it can be inferred that the electrons and holes will move differently in the PtS2/C3N4 and the PtS2/OX/C3N4 heterostructures when visible-light irradiation is absorbed.
The band edge position of the PtS2/C3N4 and the PtS2/Ox/C3N4 heterostructures, which is another important factor that determines the performance of photocatalytic water splitting of a semiconductor, was calculated to confirm the redox capability of the PtS2/C3N4 and the PtS2/Ox/C3N4 heterostructures. The potential level vs. the normal hydrogen electrode (NHE) was converted from the vacuum potential level ( E v a c u u m   p o t e n t i a l   l e v e l ) using Equation (6) [57]:
E p o t e n t i a l   l e v e l   v s   N H E = E e E v a c u u m   p o t e n t i a l   l e v e l  
where E e is the energy of the free electrons at the hydrogen scale ( E e = 4.44   e V ) and E v a c u u m   p o t e n t i a l   l e v e l is obtained by the planar-averaged electrostatic potential. To achieve water splitting on the semiconductor, the CBM potential is more negative than the H+/H2 reduction potential (0.0 eV), and the VBM potential is more positive than the H2O/O2 oxidation potential (1.23 eV). Figure 3 shows the charge transfer and band edge position of the PtS2/C3N4 and the PtS2/Ox/C3N4 heterostructures. The band edge position of the CBM (VBM) potential in the C3N4/PtS2 heterostructure was −0.56 (1.66 eV), which was more negative (positive) than the H+/H2 reduction (O2/H2O oxidation) potential. Based on the direction of the built-in electric field, in the case of the PtS2/C3N4 heterostructure, the electrons in the CBM of the PtS2 could transfer to the VBM of the C3N4 to combine with holes. In addition, the electrons in the CBM of the C3N4 were used to reduce H+ to H2, and the holes in the VBM of the C3N4 were used to reduce H+ to H2. Through this result, it can be inferred that the PtS2/C3N4 heterostructure showed a Z-scheme. In the case of the PtS2/OX/C3N4 heterostructure, the electrons transferred from the CBM of the C3N4 to the CBM of the PtS2 and the VBM holes of the PtS2 transferred into the VBM of the C3N4 structure. Thus, H+/H2 reduction and O2/H2O oxidation occurred in the CBM of the PtS2 and the VBM of the C3N4. In the density of states, there was no defect level caused by O in the energy gap of the PtS2 and the C3N4 (Figure S4). In the case of the C3N4 layer, the energy gap increased as O was inserted at the interface of the PtS2/C3N4 heterostructure. On the other hand, the energy gap of the PtS2 layer decreased. Due to the decreased energy gap of the PtS2, the CBM of the PtS2 was lower than the O2/H2O oxidation potential. Given the decreased energy gap of the PtS2, the CBM of the PtS2 was lower than the O2/H2O oxidation potential. As the monolayer C3N4 and monolayer PtS2 had an indirect band gap, C3N4 and PtS2 exhibited similar photo absorption efficiency. However, after the intercalation of O at the interface of the heterostructure, the strength of photo absorption in the PtS2 with a decreased energy gap will increase. From these results, it can be inferred that the strength of photo absorption in the PtS2 will be stronger than that of the C3N4 when visible-light irradiation is absorbed. The CBM level was located about 0.09 eV under the H+/H2 reduction potential. Using the thermal energy at room temperature (0.026 eV), the possibility of H+/H2 reduction occurring in the PtS2/O1.8%/C3N4 heterostructure was approximately 3%. Thus, it was found that the O-intercalation in the PtS2/C3N4 heterostructures not only enhanced the separation of photogenerated electron-hole pairs, but also changed the direction of the built-in electric field, resulting in the determination of the heterostructure type.
Considering the generally used synthesis conditions of C3N4, the possibility of O intercalation in the PtS2/C3N4 heterostructure was examined theoretically. The relationship between the equilibrium constants ( K p ) for reactions and temperatures was considered based on the thermodynamic energy of intercalation formation. The main reactions occurring in the PtS2/O/C3N4 heterostructure can be written as follows:
(1) O2 gas
P t S 2 / C 3 N 4 + 1 2 O 2   ( g )   P t S 2 / O / C 3 N 4
(2) H2O gas
P t S 2 / C 3 N 4 + H 2 O   ( g )   P t S 2 / O / C 3 N 4 + H 2   ( g )
The Gibbs energy for interaction is described in Equation (7):
Δ G f = G P t S 2 / O / C 3 N 4 G P t S 2 / C 3 N 4 + n O μ O
where G P t S 2 / O / C 3 N 4 , G P t S 2 / O / C 3 N 4 , n O , and μ O denote the Gibbs energy of the PtS2/O/C3N4 heterostructure, the Gibbs energy of the PtS2/C3N4 heterostructure, the coefficient atoms in the chemical equation, and the chemical potential of atoms, respectively. In the case of μ O , it can be written as Equation (8):
μ i = μ 0 ( T ) + k B T l n ( p i p 0 )
where μ 0 , k B , T , p 0 , and p i are the standard chemical potential (p0 = 1 bar), Boltzmann constant, temperature, standard partial pressure, and partial pressure, respectively. The standard chemical potential values were taken from the JANAF-NIST thermochemical table [53]. Using Equation (2) and Equation (6), the Gibbs energy for intercalation can be expressed as Equation (9):
Δ G f = Δ E D F T + Δ μ 0 ( T , P 0 ) + k B T l n ( K p )
where Δ E D F T and K p are the change in total energies of the DFT and the equilibrium constant, respectively. Hence, K p is defined as Equation (10):
K p = e ( Δ G f Δ E D F T Δ μ 0 ( T , P 0 ) k B T )
To consider the concentration of O-intercalation, the Boltzmann distribution is given by Equation (11)
x p x i = e ( G i G p k B T ) = e ( Δ G f k B T )
where xp, xi, Gp, and Gi denote the molar fraction of pristine and intercalation, and Gibbs free energy of pristine and intercalation in the PtS2/C3N4 heterostructure, respectively.
Using Equations (10) and (11), K p is given by Equation (12)
K p = x p x i · e ( Δ E D F T Δ μ 0 ( T , P 0 ) k B T )
The calculated Kp was plotted for varying temperatures (Figure 4). It is known that C3N4 exhibits good crystallinity under the synthesis conditions between 873 and 973 K [58]. Therefore, to exclude other factors that affect the photocatalytic activity, other than O-intercalation, the temperature range for KP was set from 800 K to 1000 K. When O2 gas is used as the reactant gas, the partial pressure of the O2 gas must be 2.83 × 10 21 atm at 800 K and 5.60 × 10 18 atm at 1000 K in order to insert 1 at% O. However, these partial pressures are much higher than the practical processing capabilities of synthesis devices. When H2O is the reactant gas for O-intercalation in the PtS2/C3N4 heterostructure, the partial pressure of the H2 should be controlled between 6.3 × 10−7 atm and 3.9 × 10−6 atm, which is a feasible range in synthesis devices. Through this result, it is expected that the PtS2/O/C3N4 heterostructure can be synthesized using H2O gas as the reactant gas.

3. Calculation Method

All DFT calculations were performed with the Vienna Ab initio package (VASP) [59,60]. The interactions between the valence and core electrons were described using the projector augmented wave (PAW) method [61]. A generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was employed for the plane-wave expansion with a cutoff energy of 500 eV [62,63]. Brillouin zones were sampled with a Monkhorst–Pack k-point grid of 2 × 2 × 1 in the 4 × 4 PtS2/C3N4 heterostructure with O-intercalation [64]. The energy convergence criteria in the self-consistent field were set to 10−5 eV. All geometry structures were fully relaxed until the Hellman–Feynman forces achieved a range of 0.02 eV 1 . The Methfessel–Paxton smearing scheme was applied with a smearing width of 0.1 eV [65]. To avoid the artifact of a neighboring periodic image, the size of the vacuum region in the z direction was set to a value larger than 20 . The vdW correction was considered with Grimme’s DFT-D3 (BJ) scheme to accurately describe the interaction between the C3N4 and PtS2 layers [66]. The band gap of the PtS2/C3N4 and PtS2/OX/C3N4 (X = 1.8, 3.6, 5.4, 7.1%) heterostructures were determined by the hybrid functional (HSE06) with α = 0.2 [67].

4. Conclusions

This study demonstrated that anion intercalation in a 2D/2D heterostructure is an effective way to manipulate the direction and strength of the built-in electric fields and dipole moments. The direction of the vertical built-in electric field in the PtS2/C3N4 heterostructure can be reversed with O-intercalation at the interface, as the O-atom bound to the PtS2 accepts electrons and induces a local dipole moment in the opposite direction. When the O-intercalation density increased, the intensities of the electric field and dipole moment increased nearly linearly. It was also noticeable that the band edge levels in the PtS2 and the C3N4 significantly shifted with different O-intercalation amounts. Hence, this anion intercalation method can be widely used for 2D/2D heterojunction photocatalysts for effective control of charge separation aspects and redox reactivities.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/10/5/469/s1, Figure S1: The stable configuration of the PtS2/Ox/C3N4 heterostructure (x = (a) 1.8, (b) 3.6, (c) 5.4, and (d) 7.1%). The purple, yellow, blue, gray, and red color denote the Pt, S, N, C, and O atoms, respectively, Figure S2: The charge density difference and planar-averaged electron density difference for the PtS2/O3.6%/C3N4, PtS2/O5.4%/C3N4, PtS2/O7.1%/C3N4 heterostructures. The purple, yellow, blue, gray, and red atoms indicate Pt, S, N, C, and O, respectively. The yellow and cyan areas indicate charge accumulation and depletion, respectively. The iso-values are 0.005 eV/Å3, Figure S3: The band structure of the PtS2/O3.6%/C3N4, PtS2/O5.4%/C3N4, and PtS2/O7.1%/C3N4 heterostructures, respectively. The contribution of each monolayer in the band structure of the PtS2/O3.6%/C3N4, PtS2/O5.4%/C3N4, and PtS2/O7.1%/C3N4 heterostructures. The percentages of the contributions of each layer are marked in a different color. The Fermi level was set to zero, Figure S4: The projected density of states in the PtS2/Ox/C3N4 (x = 0, 1.8, 3.6, 5.4, 7.1%) heterostructure. The black, blue, and red line indicate the PtS2 layer, C3N4 layer, and O atom in the PtS2/OX/C3N4 heterostructure. The Fermi level (vertical black dotted line) was set to zero, Table S1: The intercalation energy of the PtS2/Ox/C3N4 heterostructure (x = 1.8, 3.6, 5.4, 7.1%).

Author Contributions

Data curation, J.W.; Funding acquisition, H.C. and Y.-C.C.; Investigation, H.C. and Y.-C.C.; Software, E.S.S.; Visualization, M.J.; Writing – original draft, M.J.; Writing – review & editing, H.C. and Y.-C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the German Academic Exchange Service Deutscher Akademischer Austauschdienst (DAAD), grant number 57429784. This research was funded by the Ministry of Science, ICT, and Future Planning, grant number 2019R1A2B5B01070215.

Acknowledgments

M.J. and H.C. acknowledge the University of Cologne and the financial support of the Federal Ministry of Education and Research (BMBF) under the “Make Our Planet Great Again – German Research Initiative” (MOPGA-GRI), 57429784, implemented by the German Academic Exchange Service Deutscher Akademischer Austauschdienst (DAAD). E.S.S., J.W., and Y.-C.C. were supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science and ICT) (2019R1A2B5B01070215).

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

  1. Toroker, M.C.; Kanan, D.K.; Alidoust, N.; Isseroff, L.Y.; Liao, P.; Carter, E.A. First principles scheme to evaluate band edge positions in potential transition metal oxide photocatalysts and photoelectrodes. Phys. Chem. Chem. Phys. 2011, 13, 16644–16654. [Google Scholar] [CrossRef] [PubMed]
  2. Li, H.; Hu, H.; Bao, C.; Guo, F.; Zhang, X.; Liu, X.; Hua, J.; Tan, J.; Wang, A.; Zhou, H. Forming heterojunction: An effective strategy to enhance the photocatalytic efficiency of a new metal-free organic photocatalyst for water splitting. Sci. Rep. 2016, 6, 29327. [Google Scholar] [CrossRef] [PubMed][Green Version]
  3. Hisatomi, T.; Kubota, J.; Domen, K. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem. Soc. Rev. 2014, 43, 7520–7535. [Google Scholar] [CrossRef] [PubMed]
  4. Esswein, A.J.; Nocera, D.G. Hydrogen Production by Molecular Photocatalysis. Chem. Rev. 2007, 107, 4022–4047. [Google Scholar] [CrossRef]
  5. Ngoh, S.K.; Njomo, D. An overview of hydrogen gas production from solar energy. Renew. Sustain. Energy Rev. 2012, 16, 6782–6792. [Google Scholar] [CrossRef]
  6. Saraf, S.; Giraldo, M.; Paudel, H.P.; Sakthivel, T.S.; Shepard, C.; Gupta, A.; Leuenberger, M.N.; Seal, S. Photoelectrochemical analysis of band gap modulated TiO2 for photocatalytic water splitting. Int. J. Hydrog. Energy 2017, 42, 9938–9944. [Google Scholar] [CrossRef]
  7. Ikeda, S.; Fujikawa, S.; Harada, T.; Nguyen, T.H.; Nakanishi, S.; Takayama, T.; Iwase, A.; Kudo, A. Photocathode Characteristics of a Spray-Deposited Cu2ZnGeS4 Thin Film for CO2 Reduction in a CO2-Saturated Aqueous Solution. ACS Appl. Energy Mater. 2019, 2, 6911–6918. [Google Scholar] [CrossRef]
  8. Zhou, R.; Guzman, M.I. CO2 Reduction under Periodic Illumination of ZnS. J. Phys. Chem. C 2014, 118, 11649–11656. [Google Scholar] [CrossRef]
  9. Zhu, Z.; Kao, C.T.; Tang, B.H.; Chang, W.C.; Wu, R.J. Efficient hydrogen production by photocatalytic water-splitting using Pt-doped TiO2 hollow spheres under visible light. Ceram. Int. 2016, 42, 6749–6754. [Google Scholar] [CrossRef]
  10. Zhang, J.; Zhou, P.; Liu, J.; Yu, J. New understanding of the difference of photocatalytic activity among anatase, rutile and brookite TiO2. Phys. Chem. Chem. Phys. 2014, 16, 20382–20386. [Google Scholar] [CrossRef]
  11. Yan, J.; Wu, H.; Chen, H.; Pang, L.; Zhang, Y.; Jiang, R.; Li, L.; Liu, S.F. One-pot hydrothermal fabrication of layered β-Ni(OH)2/g-C3N4 nanohybrids for enhanced photocatalytic water splitting. Appl. Catal. B 2016, 194, 74–83. [Google Scholar] [CrossRef]
  12. Hoque, M.D.A.; Guzman, M.I. Photocatalytic Activity: Experimental Features to Report in Heterogeneous Photocatalysis. Materials 2018, 11, 1990. [Google Scholar] [CrossRef] [PubMed][Green Version]
  13. Opoku, F.; Govender, K.K.; Sittert, C.G.C.E.V.; Govender, P.P. Understanding the mechanism of enhanced charge separation and visible light photocatalytic activity of modified wurtzite ZnO with nanoclusters of ZnS and graphene oxide: From a hybrid density functional study. New. J. Chem. 2017, 41, 8140–8155. [Google Scholar] [CrossRef]
  14. Lahiri, J.; Batzill, M. Surface Functionalization of ZnO Photocatalysts with Monolayer ZnS. J. Phys. Chem. C 2008, 112, 4304–4307. [Google Scholar] [CrossRef]
  15. Tahir, M.; Cao, C.; Mahmood, N.; Butt, F.K.; Mahmood, A.; Idrees, F.; Hussain, S.; Tanveer, M.; Ali, Z.; Aslam, I. Multifunctional g-C3N4 Nanofibers: A Template-Free Fabrication and Enhanced Optical, Electrochemical, and Photocatalyst Properties. ACS Appl. Mater. Interfaces 2014, 6, 1258–1265. [Google Scholar] [CrossRef]
  16. Xu, Y.; Fu, Z.C.; Cao, S.; Chen, Y.; Fu, W.F. Highly selective oxidation of sulfides on a CdS/C3N4 catalyst with dioxygen under visible-light irradiation. Catal. Sci. Technol. 2017, 7, 587–595. [Google Scholar] [CrossRef]
  17. Wang, X.C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J.M.; Domen, K.; Antonietti, M. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8, 76–80. [Google Scholar] [CrossRef]
  18. Zhang, W.; Sun, Y.; Dong, F.; Zhang, W.; Duan, S.; Zhang, Q. Facile synthesis of organic–inorganic layered nanojunctions of g-C3N4/(BiO)2CO3 as efficient visible light photocatalyst. Dalton Trans. 2014, 43, 12026–12036. [Google Scholar] [CrossRef]
  19. Zhang, G.; Lan, Z.A.; Lin, L.; Lin, S.; Wang, X. Overall water splitting by Pt/g-C3N4 photocatalysts without using sacrificial agents. Chem. Sci. 2016, 7, 3062–3066. [Google Scholar] [CrossRef][Green Version]
  20. Liu, J.; Cheng, B.; Yu, J. A new understanding of the photocatalytic mechanism of the direct Z-scheme g-C3N4/TiO2 heterostructure. Phys. Chem. Chem. Phys. 2018, 18, 31175–31183. [Google Scholar] [CrossRef]
  21. Liu, J.; Jia, Q.; Long, J.; Wang, X.; Gao, Z.; Gu, Q. Amorphous NiO as co-catalyst for enhanced visible-light-driven hydrogen generation over g-C3N4 photocatalyst. Appl. Catal. B 2018, 222, 35–43. [Google Scholar] [CrossRef]
  22. Su, T.; Shao, Q.; Qin, Z.; Guo, Z.; Wu, Z. Role of Interfaces in Two-Dimensional Photocatalyst for Water Splitting. ACS Catal. 2018, 8, 2253–2276. [Google Scholar] [CrossRef]
  23. Li, H.; Yu, H.; Quan, X.; Chen, S.; Zhang, Y. Uncovering the Key Role of the Fermi Level of the Electron Mediator in a Z-Scheme Photocatalyst by Detecting the Charge Transfer Process of WO3-metal-gC3N4 (Metal = Cu, Ag, Au). ACS Appl. Mater. Interfaces 2016, 8, 2111–2119. [Google Scholar] [CrossRef] [PubMed]
  24. Cai, X.; Zhang, J.; Fujitsuka, M.; Majima, T. Graphitic-C3N4 hybridized N-doped La2Ti2O7 two-dimensional layered composites as efficient visible-light-driven photocatalyst. Appl. Catal. B 2017, 202, 191–198. [Google Scholar] [CrossRef][Green Version]
  25. Scanlon, D.O.; Dunnill, C.W.; Buckeridge, J.; Shevlin, S.A.; Logsdail, A.J.; Woodley, S.M.; Catlow, C.R.A.; Powell, M.J.; Palgrave, R.G.; Parkin, I.; et al. Band alignment of rutile and anatase TiO₂. Nat. Mater. 2013, 12, 798–801. [Google Scholar] [CrossRef]
  26. Khan, S.; Cho, H.; Kim, D.; Han, S.S.; Lee, K.H.; Cho, S.H.; Song, T.; Choi, H. Defect engineering toward strong photocatalysis of Nb-doped anatase TiO2: Computational predictions and experimental verifications. Appl. Catal. B 2017, 206, 520–530. [Google Scholar] [CrossRef]
  27. Zhang, J.; Zhang, M.; Sun, R.Q.; Wang, X. A facile band alignment of polymeric carbon nitride semiconductors to construct isotype heterojunctions. Angew. Chem. 2012, 51, 10145–10149. [Google Scholar] [CrossRef]
  28. Wang, X.; Xu, Q.; Li, M.; Shen, S.; Wang, X.; Wang, Y.; Feng, Z.; Shi, J.; Han, H.; Li, C. Photocatalytic overall water splitting promoted by an α-β phase junction on Ga2O3. Angew. Chem. 2012, 51, 13089–13092. [Google Scholar] [CrossRef]
  29. Hu, S.; Ma, L.; Li, F.; Fan, Z.; Wang, Q.; Bai, J.; Kang, X.; Wu, G. Construction of g-C3N4/S-g-C3N4 metal-free isotype heterojunctions with an enhanced charge driving force and their photocatalytic performance under anoxic conditions. RSC Adv. 2015, 5, 90750–90756. [Google Scholar] [CrossRef]
  30. Li, J.; Zhang, Z.; Cui, W.; Wang, H.; Cen, W.; Johnson, G.; Jiang, G.; Zhang, S.; Dong, F. The Spatially Oriented Charge Flow and Photocatalysis Mechanism on Internal van der Waals Heterostructures Enhanced g-C3N4. ACS Catal. 2018, 8, 8376–8385. [Google Scholar] [CrossRef]
  31. Chen, L.; Shi, G.; Shen, J.; Peng, B.; Zhang, B.; Wang, Y.; Bian, F.; Wang, J.; Li, D.; Qian, Z.; et al. Ion sieving in graphene oxide membranes via cationic control of interlayer spacing. Nature 2017, 550, 380–383. [Google Scholar] [CrossRef] [PubMed]
  32. Ditto, J.; Merrill, D.R.; Mitchson, G.; Gabriel, J.J.; Mathew, K.; Hennig, R.G.; Medlin, D.L.; Browning, N.D.; Johnson, D.C. Interface-Driven Structural Distortions and Composition Segregation in Two-Dimensional Heterostructure. Angew. Chem. 2017, 56, 14448–14452. [Google Scholar] [CrossRef] [PubMed]
  33. Xiong, T.; Cen, W.; Zhang, Y.; Dong, F. Bridging the g-C3N4 Interlayers for Enhanced Photocatalysis. ACS Catal. 2016, 6, 2462–2472. [Google Scholar] [CrossRef]
  34. Qu, X.; Hu, S.; Bai, J.; Lu, G.; Kang, X. A facile approach to synthesize oxygen doped g-C3N4 with enhanced visible light activity under anoxic conditions via oxygen-plasma treatment. New J. Chem. 2018, 42, 4998–5004. [Google Scholar] [CrossRef]
  35. Li, J.H.; Shen, B.; Hong, Z.; Lin, B.; Gao, B.; Chen, Y. A facile approach to synthesize novel oxygen-doped g-C3N4 with superior visible-light photoreactivity. Chem. Commum. 2012, 48, 12017–12019. [Google Scholar] [CrossRef] [PubMed]
  36. Han, B.; Hu, Y.H. MoS2 as a co-catalyst for photocatalytic hydrogen production from water. Energy Sci. Eng. 2016, 4, 285–304. [Google Scholar] [CrossRef][Green Version]
  37. Wei, L.; Chen, Y.; Lin, Y.; Wu, H.; Yuan, R.; Li, Z. MoS2 as non-noble-metal co-catalyst for photocatalytic hydrogen evolution over hexagonal ZnIn2S4 under visible light irradiations. Appl. Catal. B 2014, 144, 521–527. [Google Scholar] [CrossRef]
  38. Zhang, W.; Xiao, X.; Zeng, X.; Li, Y.; Zheng, L.; Wan, C. Enhanced photocatalytic activity of TiO2 nanoparticles using SnS2/RGO hybrid as co-catalyst: DFT study and photocatalytic mechanism. J. Alloys Comd. 2016, 685, 774–783. [Google Scholar] [CrossRef]
  39. Akple, M.S.; Low, J.; Wageh, S.; Al-Ghamdi, A.A.; Yu, J.; Zhang, J. Enhanced visible light photocatalytic H2-production of g-C3N4/WS2 composite heterostructures. Appl. Surf. Sci. 2015, 358, 196–203. [Google Scholar] [CrossRef]
  40. Yan, J.; Chen, Z.; Ji, H.; Liu, Z.; Wang, X.; Xu, Y.; She, X.; Huang, L.; Xu, L.; Xu, H.; et al. Construction of a 2D Graphene-Like MoS2/C3N4 Heterojunction with Enhanced Visible-Light Photocatalytic Activity and Photoelectrochemical Activity. Chemistry 2016, 22, 4764–4773. [Google Scholar] [CrossRef]
  41. Wang, J.; Guan, Z.; Huang, J.; Li, Q.; Yang, J. Enhanced photocatalytic mechanism for the hybrid g-C3N4/MoS2 nanocomposite. J. Mater. Chem. A 2014, 2, 7960–7966. [Google Scholar] [CrossRef]
  42. Liu, J.; Hua, E. High Photocatalytic Activity of Heptazine-Based g-C3N4/SnS2 Heterojunction and Its Origin: Insights from Hybrid DFT. J. Phys. Chem. C 2017, 121, 25827–25835. [Google Scholar] [CrossRef]
  43. Mir, S.H.; Chakraborty, S.; Wärnå, J.; Narayan, S.; Jha, P.C.; Jha, P.K.; Ahuja, R. A comparative study of hydrogen evolution reaction on pseudo-monolayer WS2 and PtS2: Insights based on the density functional theory. Catal. Sci. Technol. 2017, 7, 687–692. [Google Scholar] [CrossRef]
  44. Chia, X.; Adriano, A.; Lazar, P.; Sofer, Z.; Luxa, J.; Pumera, M. Layered Platinum Dichalcogenides (PtS2, PtSe2, and PtTe2) Electrocatalysis: Monotonic Dependence on the Chalcogen Size. Adv. Funct. Mater. 2016, 26, 4306–4318. [Google Scholar] [CrossRef]
  45. Li, L.; Wang, W.; Chai, Y.; Li, H.; Tian, M.; Zhai, T. Few-Layered PtS2 Phototransistor on h-BN with High Gain. Adv. Funct. Mater. 2017, 27, 1701011. [Google Scholar] [CrossRef]
  46. Zhang, K.; Hu, S.; Zhang, Y.; Zhang, T.; Zhou, X.; Sun, Y.; Li, T.X.; Fan, H.J.; Shen, G.; Chen, X.; et al. Self-Induced Uniaxial Strain in MoS2 Monolayers with Local van der Waals-Stacked Interlayer Interactions. ACS Nano 2015, 9, 2704–2710. [Google Scholar] [CrossRef]
  47. Jiang, J.W.; Park, H. Mechanical properties of MoS2/graphene heterostructure. Appl. Phys. Lett. 2014, 105, 033108. [Google Scholar] [CrossRef][Green Version]
  48. Huang, Z.F.; Song, J.; Pan, L.; Wang, Z.; Zhang, X.; Zou, J.J.; Mi, W.; Zhang, X.; Wang, L. Carbon nitride with simultaneous porous network and O-doping for efficient solar-energy-driven hydrogen evolution. Nano Energy 2015, 12, 646–656. [Google Scholar] [CrossRef]
  49. Björkman, T.; Gulans, A.; Krasheninnikov, A.V.; Nieminen, R.M. van der Waals Bonding in Layered Compounds from Advanced Density-Functional First-Principles Calculations. Phys. Rev. Lett. 2012, 108, 235502. [Google Scholar] [CrossRef][Green Version]
  50. Liao, J.; Sa, B.; Zhou, J.; Ahuja, R.; Sun, Z. Design of High-Efficiency Visible-Light Photocatalysts for Water Splitting: MoS2/AlN(GaN) Heterostructures. J. Phys. Chem. C 2014, 118, 17594–17599. [Google Scholar] [CrossRef]
  51. Huang, H.; Song, Z.; Wei, N.; Shi, L.; Mao, Y.; Ying, Y.; Sun, L.; Xu, Z.; Peng, X. Ultrafast viscous water flow through nanostrand-channelled graphene oxide membranes. Nat. Commun. 2013, 4, 2979. [Google Scholar] [CrossRef] [PubMed][Green Version]
  52. Baxter, J.B.; Richter, C.; Schmuttenmaer, C. Ultrafast Carrier Dynamics in Nanostructures for Solar Fuels. Annu. Rev. Phys. Chem. 2014, 65, 423–447. [Google Scholar] [CrossRef] [PubMed]
  53. Takanabe, K. Solar Water Splitting Using Semiconductor Photocatalyst Powders. In Solar Energy for Fuels; Springer: Cham, Switzerland, 2015; pp. 73–103. [Google Scholar]
  54. Li, M.; Dai, Y.; Ma, X.; Li, Z.; Huang, B. The synergistic effect between effective mass and built-in electric field for the transfer of carriers in nonlinear optical materials. Phys. Chem. Chem. Phys. 2015, 17, 17710–17717. [Google Scholar] [CrossRef] [PubMed]
  55. Fang, Q.; Zhao, X.; Huang, Y.; Xu, K.; Min, T.; Chu, P.K.; Ma, F. Interfacial electronic states and self-formed p–n junctions in hydrogenated MoS2/SiC heterostructure. J. Mater. Chem. C 2018, 6, 4523–4530. [Google Scholar] [CrossRef]
  56. Zhang, X.; Shen, J.X.; Wang, W.; Van de Walle, C.G. First-Principles Analysis of Radiative Recombination in Lead-Halide Perovskites. ACS Eng. Lett. 2018, 3, 2329–2334. [Google Scholar] [CrossRef]
  57. Gao, G.; Jiao, Y.; Ma, F.; Jiao, Y.; Waclawik, E.; Du, A. Carbon nanodot decorated graphitic carbon nitride: New insights into the enhanced photocatalytic water splitting from ab initio studies. Phys. Chem. Chem. Phys. 2015, 17, 31140–31144. [Google Scholar] [CrossRef]
  58. Mo, Z.; She, X.; Li, Y.; Liu, L.; Huang, L.; Chen, Z.; Zhang, Q.; Xu, H.; Li, H. Synthesis of g-C3N4 at different temperatures for superior visible/UV photocatalytic performance and photoelectrochemical sensing of MB solution. RSC. Adv. 2015, 5, 101552–101562. [Google Scholar] [CrossRef]
  59. Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15–50. [Google Scholar] [CrossRef]
  60. Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169. [Google Scholar] [CrossRef]
  61. Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758. [Google Scholar] [CrossRef]
  62. Perdew, J.P.; Chevary, J.A.; Vosko, S.H.; Jackson, K.A.; Pederson, M.R.; Singh, D.J.; Fiolhais, C. Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B 1992, 46, 6671. [Google Scholar] [CrossRef] [PubMed]
  63. Perdew, J.P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. [Google Scholar] [CrossRef][Green Version]
  64. Monkhorst, H.J.; Pack, J.D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188. [Google Scholar] [CrossRef]
  65. Methfessel, M.; Paxton, A. High-precision sampling for Brillouin-zone integration in metals. Phys. Rev. B 1989, 40, 3616. [Google Scholar] [CrossRef] [PubMed][Green Version]
  66. Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the damping function in dispersion corrected density functional theory. J. Comput. Chem. 2011, 32, 1456–1465. [Google Scholar] [CrossRef]
  67. Heyd, J.; Scuseria, G.E.; Ernzerhof, M. Hybrid functionals based on a screened Coulomb potential. J. Chem. Phys. 2003, 118, 8207–8215. [Google Scholar] [CrossRef][Green Version]
Figure 1. The charge density difference and planar-averaged electron density difference for the (a) PtS2/C3N4 (left) and PtS2/O1.8%/C3N4 (right) heterostructures. The yellow and cyan color indicate charge accumulation and depletion, respectively. The iso-values are 0.005 eV/Å3. The purple, yellow, blue, gray, and red colors denote the Pt, S, N, C, and O atoms, respectively. (b) The built-in electric field and dipole moment of the PtS2/Ox/C3N4 (X = 0, 1.8, 3.6, 5.4, 7.1%) heterostructure.
Figure 1. The charge density difference and planar-averaged electron density difference for the (a) PtS2/C3N4 (left) and PtS2/O1.8%/C3N4 (right) heterostructures. The yellow and cyan color indicate charge accumulation and depletion, respectively. The iso-values are 0.005 eV/Å3. The purple, yellow, blue, gray, and red colors denote the Pt, S, N, C, and O atoms, respectively. (b) The built-in electric field and dipole moment of the PtS2/Ox/C3N4 (X = 0, 1.8, 3.6, 5.4, 7.1%) heterostructure.
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Figure 2. The band structure of (a) the PtS2/C3N4 and (b) the PtS2/O1.8%/C3N4 heterostructure, respectively. The contribution of each layer in the band structure of the PtS2/C3N4 heterostructure with or without O-intercalation. The percentages of the contributions of each layer are marked in a different color. The Fermi level was set to zero.
Figure 2. The band structure of (a) the PtS2/C3N4 and (b) the PtS2/O1.8%/C3N4 heterostructure, respectively. The contribution of each layer in the band structure of the PtS2/C3N4 heterostructure with or without O-intercalation. The percentages of the contributions of each layer are marked in a different color. The Fermi level was set to zero.
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Figure 3. The CBM and VBM potential versus the normal hydrogen electrode of the PtS2/C3N4 and the PtS2/OX/C3N4 (X = 1.8, 3.6, 5.4, 7.1%) heterostructures using the HSE06 method. E means the built-in electric field.
Figure 3. The CBM and VBM potential versus the normal hydrogen electrode of the PtS2/C3N4 and the PtS2/OX/C3N4 (X = 1.8, 3.6, 5.4, 7.1%) heterostructures using the HSE06 method. E means the built-in electric field.
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Figure 4. The Kps of the PtS2/OX/C3N4 (X = 1, 5, 10, 50%) heterostructure expressed depending on the temperature for fabrication.
Figure 4. The Kps of the PtS2/OX/C3N4 (X = 1, 5, 10, 50%) heterostructure expressed depending on the temperature for fabrication.
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Je, M.; Sim, E.S.; Woo, J.; Choi, H.; Chung, Y.-C. Manipulatable Interface Electric Field and Charge Transfer in a 2D/2D Heterojunction Photocatalyst via Oxygen Intercalation. Catalysts 2020, 10, 469. https://doi.org/10.3390/catal10050469

AMA Style

Je M, Sim ES, Woo J, Choi H, Chung Y-C. Manipulatable Interface Electric Field and Charge Transfer in a 2D/2D Heterojunction Photocatalyst via Oxygen Intercalation. Catalysts. 2020; 10(5):469. https://doi.org/10.3390/catal10050469

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Je, Minyeong, Eun Seob Sim, Jungwook Woo, Heechae Choi, and Yong-Chae Chung. 2020. "Manipulatable Interface Electric Field and Charge Transfer in a 2D/2D Heterojunction Photocatalyst via Oxygen Intercalation" Catalysts 10, no. 5: 469. https://doi.org/10.3390/catal10050469

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