Article pubs.acs.org/JPCC
Electronic Structure and Photocatalytic Water-Splitting Properties of Ag2ZnSn(S1−xSex)4 Tao Jing,†,‡ Ying Dai,*,† Xiangchao Ma,† Wei Wei,† and Baibiao Huang† †
School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China College of Physics and Electronic Engineering, Kaili University, Kaili, Guizhou 556011, People’s Republic of China
‡
ABSTRACT: The silver-based quaternary chalcogenides have potential applications in solar cell absorbers and photocatalytic water splitting to produce hydrogen. In the present work, the electronic structures and photocatalytic properties of Ag2ZnSn(S1−xSex)4 (x = 0−1) were investigated by the combination of GGA+U and hybrid functional methods. The results indicate that Ag2ZnSnS4 (x = 0) and Ag2ZnSnSe4 (x = 1) have a dispersive conduction band and small electron effective masses, which are beneficial to the photogenerated carrier separation. The hole effective masses are remarkably direction dependent, and that along the [100] and [010] directions are sensitive to strain, which indicates easy modulation of photocatalytic properties. In addition, the band gaps of the Ag2ZnSn(S1−xSex)4 solid solution can be tuned continuously by controlling the x component, which indicates the easy manipulation of light response range. The transfer abilities of charge carriers can also be improved with increasing Se incorporation. In addition, our results strongly suggest that the proper superlattice structures can be an efficient photocatalytic material for water splitting. The present work can serve as guidance for preparing efficient photocatalytic materials.
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INTRODUCTION Photocatalytic water splitting has received much attention as a potential solution to solve the global energy shortage and environmental pollution issues.1−4 Generally, for an ideal photocatalytic material possessing the ability to split water, the conduction band minimum (CBM) should be higher in energy than the reducing potential of water splitting (−4.44 eV, which is defined with respect to the vacuum level), while the valence band maximum (VBM) should be lower than the oxidation potential (−5.67 eV). Therefore, during the past decades, great effort was devoted to exploring highly efficient, stable, inexpensive, and environmentally friendly photocatalytic materials.5−10 However, to date, the overall solar energy conversion efficiencies of current photocatalytic materials are still below a commercially viable level. Multicomponent sulfides have potential application in solar cell absorbers and photocatalytic water splitting because of their diverse structural configurations and large light absorption coefficients.11−13 Recently, the quaternary chalcogenides, such as Cu2ZnSnS4,14−16 Cu2ZnGeS4,17 and Cu2FeGeS4,18 were synthesized and exhibited excellent activities in photocatalytic water splitting. Among these compounds, Ag2ZnSnS4, which has a direct band gap at the Γ point, is a promising photocatalytic material for water splitting to produce hydrogen because the band edge positions straddle the water redox potentials. Recently, it was demonstrated that Ag2ZnSnS4 exhibits the highest activity among the tested stannite-type sulfides because of the high potential of the conduction band and the narrow band gap of 2.0 eV.19 Just opposite to the Cu© 2015 American Chemical Society
based sulfides, an n-type conductivity feature is found in the Ag2ZnSnS4 and Ag2ZnSnSe4. Surface modification using noble metals can further improve the activity of photocatalytic materials. For example, it was found that Pt-loaded Ag2ZnSnS4 nanoparticles displayed an enhanced photocatalytic activity for hydrogen evolution, which has the quantum efficiency of 15.2% at 420 nm incident light irradiation.20 The structure and phase stability of Ag2ZnSnS4 and Ag2ZnSnSe4 were investigated by first-principles calculations.21,22 However, the investigation of electronic structure and transfer ability of charge carriers for such compounds is still lacking, although they are very important for understanding and improving photocatalytic performance. Designing solid solution photocatalysts by coupling wide and narrow bandgap semiconductors provides an effective means for modifying the properties of semiconductors.23,24 Generally, the band gaps of semiconductors can be continuously tuned for specific applications. Recently, the characterization of optical absorption implied that the Ag2ZnSnSe4 has the band gap of 1.34 eV.25 It is known that the suitable band gap of semiconductors for photocatalytic water splitting should lie in the range of 1.5−2.5 eV.26 Therefore, it is hoped that increasing the light absorption range by using the solid solution method may be possible because Ag2ZnSn(S1−xSex)4 may have a large tunable bandgap energy. On the other hand, the multilayered Received: September 29, 2015 Revised: November 23, 2015 Published: November 24, 2015 27900
DOI: 10.1021/acs.jpcc.5b09522 J. Phys. Chem. C 2015, 119, 27900−27908
Article
The Journal of Physical Chemistry C
within density functional theory have the band gap underestimation problem due to the self-interaction error.31 Here, as shown in Table 1, the band gaps obtained from GGA for Ag 2 ZnSnS 4 and Ag 2 ZnSnSe 4 are 0.47 and 0.019 eV, respectively, which are much smaller than experimental values.25 To cover the shortage, the Hubbard-U correction is employed with U = 5 eV for Zn 3d and Ag 4d orbitals. From the Table 1, the GGA+U method still overestimates the lattice parameters for both Ag2ZnSnS4 and Ag2ZnSnSe4; however, it gives a value of c/2a that is almost the same as the experimental results. The band gaps of Ag2ZnSnS4 and Ag2ZnSnSe4 are improved with the value of 0.96 and 0.38 eV, respectively. The hybrid functional method (HSE) was also employed for exploring the electronic properties, as a comparison for the GGA+U results. Generally, the HSE06 method including 25% Hartree−Fock (HF) exchange can accurately reproduce the experimental band gap for many semiconductors. 32−35 However, for Ag2ZnSnS4 and Ag2ZnSnSe4, the calculated band gaps are 1.63 and 0.96 eV, respectively, which are significantly smaller than experimental values (2.01 and 1.34 eV). The similar cases have also been found in other semiconductors, such as ZnO and Ag3PO4.36,37 Therefore, the test calculations have been taken by increasing the fraction from the original value of 25% to 35% within the formalism of HSE. It is found that the use of 34% HF exchange can yield band gaps of 2.04 and 1.33 eV for Ag2ZnSnS4 and Ag2ZnSnSe4, respectively, in excellent agreement with the experimental values. Thus, our hybrid functional calculations are performed using HSE with the fraction of HF exchange at 34%.
structure, which could be prepared by thin-film deposition methods, has a tunable band gap through the control of the thickness of thin layers.27 In addition, efficient charge separation near the interface can be realized because of the formation of the built-in electric field. Thus, investigation of the electronic structures and carrier mobility for different ordering of Ag2ZnSn(S1−xSex)4 is very helpful for the rational design of high-activity photocatalytic materials. In this work, the electronic structures of Ag2ZnSn(S1−xSex)4 were investigated by first-principles density functional methods. The results indicate that the hole effective masses along [100] and [010] directions are significantly larger than that along [001] direction for both Ag2ZnSnS4 and Ag2ZnSnSe4, which can be attributed to the larger dispersion of anion pz orbitals along the [001] direction. However, the hole effective masses along the two directions are very sensitive to strain. The band gaps of solid solution Ag2ZnSn(S1−xSex)4 can be continuously reduced with increasing x. Moreover, our results show that the type C superlattice structure not only has tunable band gap but also has excellent carrier transfer abilities, which may improve the photocatalytic efficiency of Ag2ZnSnS4.
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CALCULATION METHOD The density function theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) with projector-augmented-wave (PAW) pseudopotentials.28 Generalized gradient approximation (GGA) in the formulation of Perdew−Burke−Ernzerhof (PBE) was employed for exchange−correlation functional.29 All atomic positions and the lattice constants were relaxed. The planewave basis-set expansion was used with an energy cutoff of 400 eV. The energy convergence criterion was 10−6 eV, and the Hellmann−Feynman forces were relaxed to less than 0.01 eV Å−1. Integrations over the Brillouin zone were performed using a 8 × 8 × 4 mesh for the GGA and GGA+U method,30 a smaller 4 × 4 × 2 mesh for hybrid functional method due to larger computational cost. The crystal structure of kesterite-type Ag2ZnSnS4 is shown in Figure 1, which belongs to the space group of I4.̅ It is known that both local density and generalized gradient approximations
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RESULTS AND DISCUSSION Electronic Structures. Figure 2 shows the band structures obtained from GGA+U and HSE; it is noted that both Ag2ZnSnS4 and Ag2ZnSnSe4 have a direct band gap with the VBM and CBM located at the Γ point, which is beneficial for the optical absorption and emission. The band gaps of 2.04 and 1.33 eV for Ag2ZnSnS4 and Ag2ZnSnSe4 indicate that their light responses are in the visible and near-infrared light range, respectively. In addition, the electronic structures obtained from GGA+U, in particular the shape of the energy bands, are in good agreement with the more accurate calculations using HSE, indicating that the U values we adopted are reasonable even though the band gaps are still underestimated. From the projected density of states (PDOS) of Ag2ZnSnS4 shown in Figure 2c, we find that the VBM are mainly contributed by S 3p and Ag 4d states, and the CBM are mainly derived from the S 3p and Sn 5s states. The inset figures further show that the VBM is mainly composed of S 3pz and Ag 4dxy states, which agrees with the partial charge density analysis (not shown here). According to the crystal field theory, the distortion of the AgS4 tetrahedron due to the shrinkage of c axis leads to reduced symmetry from Td to C2, which further reduces the d-orbital degeneracy.38 The orbital energy of dxy states is higher than that of dyz and dxz because of the greater repulsion interaction from the anions. From Figure 2b,d, we note that, except from the smaller band gap, Ag2ZnSnSe4 has electronic structures similar to that of Ag2ZnSnS4. Furthermore, the VBM and CBM of Ag2ZnSnSe4 have dispersion larger than that of Ag2ZnSnS4 because of the more delocalized Se 4p states, indicating the better transfer ability of charge carriers. Effective Masses of Charge Carriers. Besides the optical absorption properties, the carrier transport ability is also crucial for a high-performance solar cell absorbers or photocatalytic
Figure 1. Crystal structure of kesterite-type Ag2ZnSnS4. The S, Zn, Ag, and Sn atoms are denoted. 27901
DOI: 10.1021/acs.jpcc.5b09522 J. Phys. Chem. C 2015, 119, 27900−27908
Article
The Journal of Physical Chemistry C Table 1. Lattice Parameters and Band Gaps for Ag2ZnSnS4 and Ag2ZnSnSe4 Using GGA, GGA+U, and HSEa Ag2ZnSnS4 GGA GGA+U HSE experiment a
Ag2ZnSnSe4
a
c
c/2a
band gap
a
c
c/2a
band gap
5.854 5.904
11.067 10.950
0.945 0.927
6.125 6.162
11.645 11.582
0.951 0.940
5.81
10.76
0.92
0.47 0.96 2.04 2.01
6.03
11.30
0.93
0.019 0.38 1.33 1.34
Experimental data were taken from ref 25.
Figure 2. Band structures (a) and PDOS (c) for Ag2ZnSnS4 and the band structures (b) and PDOS (d) for Ag2ZnSnSe4.
Table 2. Calculated Electron and Hole Effective Masses for Ag2ZnSnS4 and Ag2ZnSnSe4 by GGA+U and HSE GGA+U Ag2ZnSnS4
HSE Ag2ZnSnSe4
Ag2ZnSnS4
Ag2ZnSnSe4
direction
[100]
[010]
[001]
[100]
[010]
[001]
[100]
[010]
[001]
[100]
[010]
[001]
m*e m*h
0.16 1.16
0.16 1.16
0.14 0.16
0.074 0.89
0.074 0.89
0.059 0.061
0.20 1.15
0.20 1.15
0.19 0.23
0.13 0.83
0.13 0.83
0.12 0.13
hole is anisotropic. To explore the reason why the mh* has such large differences along these directions, a detailed investigation of the electronic structures is helpful. As mentioned above, for both Ag2ZnSnS4 and Ag2ZnSnSe4, the VBM is mainly composed of S (Se) pz states and Ag dxy states. Because the pz states of anions in these compounds have large dispersion along the [001] direction, the large wave function overlap between S (Se) pz states can be expected, which further leads to the smaller effective mass and better carrier transfer ability along this direction. On the other hand, the hybridization of pz states along the [100] and [010] directions is much smaller, thereby leading to the larger effective masses. For comparison, the calculations were repeated with the HSE method, and the calculated m*e and m*h are also given in Table 2. We find that the effective masses obtained from the GGA+U are slightly different from that from HSE. However, it is reliable when we focus on only their relative change trend. Thus, the electron and hole effective masses are calculated using only GGA+U in further investigation. Based on the above analysis, the Ag2ZnSnS4 and Ag2ZnSnSe4 possess good carrier mobility and have potential applications as photocatalytic materials. Different from Cu2ZnSnS(Se)4, the length of the c axis of Ag2ZnSnS(Se)4 is significantly smaller than that of 2a. Therefore, it is important to investigate the effect of strain along the c axis on effective masses of charge carriers. Here, we only increase the c axis from the original value to 2a. The calculated effective masses with changed lattice parameter c for both Ag2ZnSnS4 and Ag2ZnSnSe4 are given in Figure 3. We note that most of effective masses of charge carriers are slightly
materials. Therefore, to be able to qualitatively compare transfer abilities of carriers along different directions, the effective masses of electrons and holes along different directions are calculated according to ⎛ d2E ⎞−1 m* = ±ℏ ⎜ 2k ⎟ ⎝ dk ⎠ 2
The region for parabolic fitting was within an energy difference of 26 meV around the CBM or VBM, which corresponds to the thermal excitation energy of carriers at room temperature.39,40 The calculated electron (me*) and hole (mh*) effective masses along different directions for Ag2ZnSnS4 and Ag2ZnSnSe4 are summarized in Table 2. We can note that the values of m*e in Ag2ZnSnS4 and Ag2ZnSnSe4 are small, reflecting rather dispersive band structures at the CBM. This is because the Sn 5s states make a considerable contribution. Additionally, both the m*e and m*h in Ag2ZnSnSe4 are much smaller than those in Ag2ZnSnS4. The reason can be assigned to larger dispersion of Se 4p compared with that of S 3p orbitals, which is consistent with band structure analysis. This feature means that the photogenerated carriers in Ag2ZnSnSe4 possess much better mobility.40 Moreover, it is found that the electron and hole effective masses along the [100] and [010] directions have the same values for both Ag2ZnSnS4 and Ag2ZnSnSe4, which can be attributed to the same symmetry along the two directions. However, m*h along the [100] and [010] directions are significantly larger than that along the [001] direction, indicating that the transfer properties of the photogenerated 27902
DOI: 10.1021/acs.jpcc.5b09522 J. Phys. Chem. C 2015, 119, 27900−27908
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The Journal of Physical Chemistry C
consisting of alternating layers of Ag2ZnSnS4 and Ag2ZnSnSe4 are also adopted to tune their band gap and improve the carrier mobility. Two representative superlattice structures are adopted, as shown in Figure 4b,c. First, to investigate the thermodynamic stability for synthesizing these compounds, we calculated the formation enthalpy using the following expression: Hf (x) = E(x) − (1 − x)E Ag ZnSnS4 − xE Ag ZnSnSe4 2
2
E(x) is the total energy of Ag2ZnSn(S1−xSex)4, EAg2ZnSnS4 the total energy of pure Ag2ZnSnS4, and EAg2ZnSnSe4 the total energy of pure Ag2ZnSnSe4. For different constitutions of Ag2ZnSn(S1−xSex)4, the corresponding formation enthalpies are shown in Figure 5a. It is noted that the formation enthalpies of the three structures are small, and the values are no more than 5 meV/atom, indicating that the synthesis of these structures should be thermodynamically feasible. In addition, the formation enthalpy of type B is significantly lower than that of the other two structures, indicating that this structure is easier to synthesize compared with the other structures. In addition, the formation enthalpy of the type C superlattice structure is almost comparable to that of the type A structure. The greater energy payment of type C compared to type B superlattice structure can be attributed to the lattice mismatch of the c axis being larger than that of the a axis (5.8% vs 4.4%), which are obtained from the theoretical lattice parameters. To examine the band gap variation with the increased Se concentration, the calculated band gap as a function of composition x are given in Figure 5b. We note that the band gap of the three structures are reduced gradually with the increased incorporation of Se into Ag2ZnSnS4, indicating that the band gap for all three ordering structures can be tunable from 2.04 to 1.33 eV. Additionally, our results show that although the GGA+U underestimates the band gap, it gives almost the same variation trends as HSE, which further indicates that our conclusion is reliable and independent of the specific functional. It is worth noting that the redox ability of Ag2ZnSn(S1−xSex)4 is likely to be reduced with the increasing incorporation of Se element, which can decrease the photocatalytic activity in water splitting. However, it should be possible to tune the band gap to appropriate values, which can increase the light absorption range and maintain considerable reduction ability. Thus, the photocatalytic activity of Ag2ZnSn(S1−xSex)4 can be expected to be improved during water splitting. For the two differently arranged superlattice structures, the inset of Ag2ZnSnSe4 layers into Ag2ZnSnS4 can also significantly reduce its band gap depending on the thickness of Ag2ZnSnSe4 layers. To further explore the band gap change trend of solid solution, as shown in Figure 5c, the band gap of Ag2ZnSn(S1−xSex)4 can be well fitted by the band gap bowing, which is defined by
Figure 3. Change of me* and mh* with the increased length of c axis for Ag2ZnSnS4 (a) and Ag2ZnSnSe4 (b). The inset in the top left corner is the enlarged figure for me* along three directions and mh* along [001] direction.
decreased with increasing the length of c axis, indicating that the lattice parameter has a small influence on the transfer ability of charge carriers along these directions. However, the hole effective masses along the [100] and [010] directions exhibit a significant enlargement. We know that the hybridization of pz states along these two directions is much smaller, the slight change of lattice parameter along c axis can significantly influence the interaction of these orbitals and further lead to the enhanced hole effective masses, while this change has no significant influence on others. This result suggests that the hole effective masses along the [100] and [010] directions are very sensitive to the change of lattice parameters. Solid Solution and Superlattice Structures. Three different atomic orderings of Ag2ZnSn(S1−xSex)4 are employed in our simulation supercell to tune the band gap and increase the light absorption range of Ag2ZnSnS4 by altering the incorporation concentration of the Se element, which are termed type A, type B, and type C structures. The hybridization of anions at different compositions (x = 0.25, 0.5, and 0.75) were taken into account. Type A is solid solution structure. To simulate the random distribution of S and Se anions in the supercell, the special quasirandom structure (SQS) approach with a 64-atom supercell are employed for the Ag2ZnSn(S1−xSex)4 solid solution calculations, which has been proven to be a good description for electronic properties by previous theoretical studies.41,42 The supercell model for x = 0.5 is shown in Figure 4a. The multilayered superlattice structures
Eg (x) = (1 − x)Eg (Ag 2ZnSnS4 ) − xEg (Ag 2ZnSnSe4) − Bx(1 − x)
E g (Ag 2 ZnSnS 4 ) is the band gap of pure Ag 2 ZnSnS 4 , Eg(Ag2ZnSnSe4) the band gap of Ag2ZnSnSe4, and B the bowing parameter. Here, the calculated bowing parameter, B, is 0.19 from both GGA+U and HSE; the small value of the bowing parameter arises from the small size and chemical difference between S and Se.42 The band gap of solid solution increases nearly linearly with the increase of the Se component.
Figure 4. Solid solution structure by the SQS approach of Ag2ZnSn(S0.5Se0.5)4, which are termed type A (a), type B (b), and type C (c) superlattice structures. 27903
DOI: 10.1021/acs.jpcc.5b09522 J. Phys. Chem. C 2015, 119, 27900−27908
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The Journal of Physical Chemistry C
Figure 5. Formation enthalpies of three atomic ordering structures (a), the calculated band gap for different composition x by GGA+U and HSE (b), and the band gap of type A fitting by the band gap bowing (c).
Table 3. Calculated Electron and Hole Effective Masses for Three Atomic Ordering Structures of Ag2ZnSn(S1−xSex)4 m*e type A
type B
type C
Ag2ZnSn(S0.75Se0.25)4 Ag2ZnSn(S0.5Se0.5)4 Ag2ZnSn(S0.25Se0.75)4 Ag2ZnSn(S0.75Se0.25)4 Ag2ZnSn(S0.5Se0.5)4 Ag2ZnSn(S0.25Se0.75)4 Ag2ZnSn(S0.75Se0.25)4 Ag2ZnSn(S0.5Se0.5)4 Ag2ZnSn(S0.25Se0.75)4
m*h
[100]
[010]
[001]
[100]
[010]
[001]
0.14 0.11 0.094 0.13 0.11 0.091 0.13 0.11 0.09
0.14 0.12 0.094 0.13 0.11 0.091 0.13 0.11 0.093
0.12 0.10 0.079 0.12 0.099 0.078 0.12 0.094 0.076
1.20 0.96 0.95 1.63 1.64 1.24 0.79 0.75 0.80
1.05 0.95 0.85 1.63 1.64 1.24 3.24 2.63 1.44
0.14 0.12 0.087 0.13 0.11 0.081 0.17 0.12 0.086
for photocatalytic activity, the type C superlattice structures can keep good transfer ability along the [001] direction. This transfer property should be considered for the design of highly efficient photocatalytic materials. To improve the solar energy conversion efficiency of photocatalytic materials, the photogenerated electrons and holes should be well-separated and easy to transfer. Thus, a high carrier separation efficiency is very advantageous for the improvement of photocatalytic activity. Generally, photocatalytic materials should meet the requirement that the value of the static dielectric constant is more than 10 and the exciton binding energy is lower than 25 meV to obtain a good exciton dissociation into free charge carriers.43 Here, the electronic (ε∞) and ionic (εvib) dielectric contributions are obtained by density functional perturbation theory using the linear response method implemented in VASP. For Ag2ZnSnSe4, it is found that the component ε∞(zz) of the static dielectric constant of the electronic contribution is so large (about 2906) by the GGA method, which is unphysical and can be attributed to its small theoretical band gap by this method (0.019 eV). Therefore, the static dielectric constants of electronic and ionic contributions were calculated by the GGA+U method. The macroscopic static dielectric tensor (εr) can be defined as the sum of electronic and ionic contributions43,44
Thus, our results indicate that the superlattice structures of Ag2ZnSn(S1−xSex)4 can also tune the band gap and enhance photocatalytic activity. Previous investigations suggested that the hole effective masses are anisotropic; thus, we can utilize this property to design materials for optimizing the mobility of charge carriers. To investigate the influence of the incorporation of the Se component on their transfer properties, the effective masses of the three ordering structures are calculated and given in Table 3. For Ag2ZnSn(S1−xSex)4 solid solution, it is noted that, compared with pure Ag2ZnSnS4, the electron and hole effective masses gradually decrease with the increase of Se concentration. This means that Se incorporation enhances the transfer ability of charge carriers. Thus, the construction of solid solution structure is a rather feasible way to improve the photocatalytic activity of materials. In contrast to the pure system, the carrier effective masses along [100] and [010] are slightly different because of the destructed symmetry in this random alloy model. For the type B and C superlattice structures, m*e values are also reduced with increasing x. However, m*h values along the [100] and [010] directions are much larger than that of pure Ag2ZnSnS4. Because the hole effective mass is very sensitive to the strain, the mismatch of lattice parameter for a or b axis between Ag2ZnSnS4 and Ag2ZnSnSe4 (about 4.37%) can significantly affect the hole effective masses along the [100] and [010] directions. However, both electron and hole effective masses along the two directions are still kept equal for the type B structure because the symmetry remains in this plane. For the type C structure, the variation trend is very similar to that of type B, but the hole effective masses along [010] reach to 3.24 for Ag2ZnSn(S0.75Se0.25)4, which is significantly larger than that along the [100] direction. This can be attributed to the larger lattice mismatch of c axis between Ag2ZnSnS4 and Ag2ZnSnSe4. Based on the analysis above, although hole effective masses have large values in some directions, which is disadvantageous
εr = ε∞ + εvib The exciton binding energy (Eb) can be estimated from the hydrogenic model μ Eb = RH m0εr 2 where RH is the Rydberg constant of the hydrogen atom (13.6 eV), m0 the free electron mass, and μ the effective reduced mass (1/μ = 1/me* + 1/mh*).45 The electron and the hole effective masses are obtained by calculating the geometric mean of the 27904
DOI: 10.1021/acs.jpcc.5b09522 J. Phys. Chem. C 2015, 119, 27900−27908
Article
The Journal of Physical Chemistry C Table 4. Calculated Static Dielectric Constant and Exciton Binding Energies for Ag2ZnSnS4, Ag2ZnSnSe4, and Ag2ZnSn(S1−xSex)4 Solid Solution ε∞ Ag2ZnSnS4 Ag2ZnSnSe4 Ag2ZnSn(S0.75Se0.25)4 Ag2ZnSn(S0.5Se0.5)4 Ag2ZnSn(S0.25Se0.75)4
εr
[100]
[010]
[001]
[100]
[010]
[001]
Eb (meV)
6.69 9.64 7.21 7.79 8.56
6.69 9.64 7.21 7.83 8.56
6.76 12.08 7.42 8.24 9.55
9.73 12.58 10.28 10.88 11.60
9.73 12.58 10.27 10.90 11.62
10.09 15.51 10.96 11.73 12.87
17 4 13 10 7
components in the three crystallographic directions, while the dielectric constants are obtained by calculating the arithmetic mean of the three nonzero components. The calculated static dielectric constant and exciton binding energies for Ag2ZnSnS4, Ag2ZnSnSe4, and Ag2ZnSn(S1−xSex)4 solid solution are given in Table 4. For Ag2ZnSn(S1−xSex)4 superlattice structures, the two parameters have not been calculated because we focus on only the transfer properties along the layer directions for these system. For Ag2ZnSnS4, the values of εr are 9.73, 9.73, and 10.09 along the [100], [010], and [001] directions, respectively, which are close to 10, indicating that this material has good dielectric properties. The calculated exciton binding energy is 17 meV, which is significantly lower than the thermal excitation energy of carriers at room temperature. This means that the exciton can be easily dissociated into free charge carriers. For Ag2ZnSnSe4, the values of εr are 12.58, 12.58, and 15.51 along the [100], [010], and [001] directions, respectively, which are significantly larger than those of Ag2ZnSnSe4. In addition, the exciton binding energy of 4 meV for Ag2ZnSnSe4 is significantly smaller than that of Ag2ZnSnS4. It suggests that Ag2ZnSnSe4 has better dielectric properties and lower exciton binding energy than that of Ag2ZnSnS4. For Ag2ZnSn(S1−xSex)4 solid solution, it is noted that the static dielectric constant will be enhanced, while the exciton binding energy can be reduced with the increasing incorporation of Se element. Based on the above analysis, our results show that Ag2ZnSnS4 and Ag2ZnSnSe4 can be promising photocatalytic materials in water splitting because of the high static dielectric constant and low exciton binding energies. In addition, the incorporation of Se in Ag 2 ZnSnS 4 can continuously tune the two parameters and improve the photocatalytic efficiency. To estimate the band edge positions of Ag2ZnSnSe4 with respect to Ag2ZnSnS4, the type B superlattice structure is used to align band edges. This process is given in the following:
Figure 6. PDOS for type B (a) and type C (b) superlattice structures, in which Ag1 and Sn1 refer to the Ag and Sn atoms from the Ag2ZnSnS4 layer, while Ag2 and Sn2 refer to the Ag and Sn atoms from the Ag2ZnSnSe4 layer.
noted that the VBM of the type B superlattice structure is mainly composed of Se 4p and Ag 4d states from Ag2ZnSnSe4 layers. However, the contribution by S 3p and Ag 4d states from Ag2ZnSnS4 layers cannot be neglected. This feature can also be observed from the CBM. For the type C superlattice structure, the smaller contribution by the atoms from Ag2ZnSnS4 layers can be found. From the partial charge density shown in Figure 7b−e, the band edges of type B superlattice structure are mainly contributed by the atoms from the Ag2ZnSnSe4 layers, and the S atoms also make considerable contribution to them. This can be assigned to the strong hybridization between S 3p and Se 4p states arising from the large dispersion of these states along the [001] direction. In contrast, for the type C superlattice structure, the VBM and CBM are mainly contributed by the atoms from the Ag2ZnSnSe4 layers, while the contribution from the Ag2ZnSnS4 layer atoms can be neglected, which is consistent with the PDOS analysis. This suggests that the stronger binding ability of type C compared to that of type B superlattice structure can be realized for the same thickness of layers. Therefore, the carrier mobility in the type C structure is higher than that of others because of much smaller hole effective masses of Ag2ZnSnSe4 layers along the [001] direction. It is thus especially advantageous for the improvement of photocatalytic activity of Ag2ZnSnS4. Finally, we will suggest a strategy for further improving the overall efficiency of photocatalytic water splitting of this system. The electron and hole possess good transfer ability in type C superlattice because of the small effective masses. The light
ΔṼ = Ṽ (B) − Ṽ (A)
where ΔṼ is the average electrostatic potential difference between Ag2ZnSnSe4 and Ag2ZnSnS4. Therefore, the valence band offset can be written as ΔE V = E V (B) − E V (A) + ΔṼ
where EV(A) is the VBM of Ag2ZnSnS4, which is given with respect to its average electrostatic potential Ṽ (A), while EV(B) is that for Ag2ZnSnSe4. Our result is reliable when charge transfer across the interface is not significant in this superlattice.42 The calculated results are shown in Figure 7a; the VBM of Ag2ZnSnSe4 is 0.17 eV higher than that of Ag2ZnSnS4, while the CBM is 0.54 eV lower. This result indicates that both electron and hole will be confined within the Ag2ZnSnSe4 layer in these superlattice structures. To further confirm this result, the PDOS are plotted in Figure 6. It is 27905
DOI: 10.1021/acs.jpcc.5b09522 J. Phys. Chem. C 2015, 119, 27900−27908
Article
The Journal of Physical Chemistry C
Figure 7. Calculated band edge alignment of Ag2ZnSnS4 and Ag2ZnSnSe4 (a), the partial charge density for VBM (b) and CBM (c) for type B superlattice, and VBM (d) and CBM (e) for type C superlattice with a 0.0002 eÅ−3 isosurface value.
effective masses for charge carriers compared to Ag2ZnSnS4, indicating its higher mobility. For both Ag2ZnSnS4 and Ag2ZnSnSe4, the hole effective mass along the [100] and [010] directions is significantly larger than that along the [001] direction, indicating the anisotropic hole migration properties. Additionally, the hole effective masses along these two directions are very sensitive to strain. For a Ag2ZnSn(S1−xSex)4 solid solution, the band gap can be continually tuned, which indicates the light absorption range can be modulated. The effective mass can also be reduced by increasing the concentration of Se incorporation. Finally, we suggested that the type C superlattice structure can improve photocatalytic activity of Ag2ZnSnS4 by utilization of the synergistic effect between increased light absorption range and improved carrier mobility.
absorption range can also be tuned to an optimized value by controlling the Se concentration. However, both electron and hole are confined in the Ag2ZnSnSe4 layers to transfer, which may increase the probability of their recombination. Therefore, we will design a multilayer structure, which is schematically illustrated in Figure 8a. Layers 1 and 3 are wide band gap
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Figure 8. Schematic diagram of the multilayer structure (a) and the transfer of charge carriers (b).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (973 program, 2013CB632401); National Science Foundation of China under Grants 11374190, 21333006, and 11404187; and 111 Project B13029. We also thank the National Supercomputer Center in Jinan for providing high-performance computation.
chalcogenides, which can absorb photons with shorter wavelengths. Moreover, they possess suitable band edges that can align with Ag2ZnSn(S1−xSex)4 solid solution, as shown in Figure 8b. The photogenerated electrons can transfer to the wide band gap chalcogenide because of its lower conduction band minimum, while the holes can transfer to the valence band of Ag2ZnSn(S1−xSex)4 solid solution. Therefore, photogenerated electrons and holes will be separated and transferred along the layer directions. In addition, the small effective mass of photogenerated holes along the [001] direction is extremely advantageous for the transfer. It is hoped that this chalcogenide is obtained because the Ag2ZnSnS4 has a high position of CBM and the quaternary chalcogenides have diverse structural and atomic configurations.19 Thus, the Ag2ZnSn(S1−xSex)4 solid solution in the middle layer can absorb photons with long wavelengths, which can increase the utilization efficiency of solar energy. Moreover, the photogenerated electrons and holes separated in different layers can transfer quickly in the layer because of their small effective masses. Therefore, higher solar energy utilization efficiency can be expected for this material.
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CONCLUSION We have investigated the electronic structures and photocatalytic properties of Ag2ZnSn(S1−xSex)4 by density functional theory combining GGA+U and HSE. Ag2ZnSnSe4 has smaller 27906
DOI: 10.1021/acs.jpcc.5b09522 J. Phys. Chem. C 2015, 119, 27900−27908
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DOI: 10.1021/acs.jpcc.5b09522 J. Phys. Chem. C 2015, 119, 27900−27908
