Theoretical Insights into Two-Dimensional IVâV Compounds

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Theoretical Insights into Two-Dimensional IV−V Compounds: Photocatalysts for the Overall Water Splitting and Nanoelectronic Applications Xu Gao,† Yanqing Shen,*,† Yanyan Ma, Shengyao Wu, and Zhongxiang Zhou

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Department of Physics, Harbin Institute of Technology, Harbin 150001, P. R. China ABSTRACT: Two-dimensional (2D) materials have attracted enormous attention in many fields because of their appealing performances. In this contribution, we perform first-principles calculations on the photocatalytic properties of IV−V compounds, along with the design of a functional Schottky device based on a graphene/SiAs van der Waals heterostructure (vdWH). Our results indicate that eight IV−V compound materials are all excellent photocatalysts for watersplitting reactions with high efficiency of visible light, with the conduction band minimum (CBM) and valence band maximum (VBM) both involving the corresponding band-gap region. It is examined whether a weak acid environment is beneficial for the hydrogen production process. Monolayer GeAs is characterized by an excellent absorption coefficient of up to 105−2 × 105 cm−1 in the visible region. The other nanostructures also have a considerable optical absorption as high as approximately half of 105 cm−1. These illustrate fascinating application prospectives for IV−V compounds in photocatalysis for water splitting under the irradiation of visible light, predicting tremendous significance in the fields of energy conversion and hydrogen production. The graphene/SiAs vdWH nanocomposite at the equilibrium state is featured for an n-type Schottky contact. External strain and electric-field applications are employed to practically present the transition for interface contact between the n- and p-type Schottky contacts or between the Schottky and ohmic contacts, which suggests appealing applications for the graphene/SiAs vdWH as a competitive candidate for functional Schottky devices and nanoelectronic materials.

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INTRODUCTION Energy resource usage has been regarded as an increasingly prominent issue that requires great effort and is caused by the sharply promoted demands for renewable energy upon their rapid development ranging from industrial production to daily life.1−3 Solar and hydrogen energy, as representative members in the family of clean energy, have been gradually increasingly valued and are considered to be excellent candidates for the replacement of traditional resources.4,5 However, one has to face the nonignorable hindrance in the process of energy conversion and storage due to the limitations of the recent industrial technology and methods.6,7 Regarding hydrogen energy, the low content of the hydrogen element in the biosphere is noteworthy. Similar to the solar energy, considering its special physical and chemical properties, it is enormously difficult to achieve safe and efficient availability and storage.8,9 Here, one can find that when utilizing solar and hydrogen energy, practical and effective strategies must be proposed and carried out. Under this background, photocatalysts have been discovered that are devoted to visible superiority in handling the problems in both fields of solar and hydrogen energy utilization. As is well-known, the organic pollutants can be transferred into H2O and CO2 under irradiation by sunlight,10,11 leading to the conversion of solar energy into chemical energy, along with the available benefits for environmental protection. On the other hand, H2O can be © XXXX American Chemical Society

further decomposed into H2 and O2 molecules through redox reactions, considerably attributed to the hydrogen production process.12,13 Hence, the employment of photocatalysts is quite favorable to solar energy conversion and the acquirement of hydrogen energy. Therefore, a suitable photocatalyst material is so crucial that many efforts have been made in searching for good candidates. Titanium dioxide (TiO2), as a traditional photocatalyst, has been widely used for organic degradation and water splitting.14 However, its wide band gap of up to around 3.2 eV15 limits its optical absorption, mainly occurring in the ultraviolet zone (3.1−4.0 eV), and only accounts for around 7% of solar energy,16 which shrinks its utilization efficiency at a large level. Recently, two-dimensional (2D) materials have been tightly concentrated, contributing to promising potential applications for water splitting because of their novel optoelectronic performances. Up to now, the 2D photocatalytic materials that have been experimentally or theoretically studied include phosphene,17,18 III−VI compounds (e.g., GaS and InSe),19,20 IV−VI compounds (e.g., GeSe and GeTe),21,22 IV−IV compounds (e.g., hexagonal GeC),23 and transition-metal dichalcogenides (TMDs; e.g., MoS2 and WS2),24,25 etc. For instance, Gu et al.26 have suggested that monolayer GeS and GeSe own photocatalytic Received: May 1, 2019

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DOI: 10.1021/acs.inorgchem.9b01255 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Schottky contact into an ohmic contact, and varying the interlayer spacing can effectively tailor the Schottky contact (n and p types).35 Pham et al. have claimed that the n-type Schottky contact in the graphene/GaS heterostructure can be changed to p type when the interlayer spacing decreased to and exceeded 2.60 Å.39 A similar phenomenon can also be found in other graphene-based vdWHs, such as graphene/ GaSe,34 graphene/MoSSe,35 and graphene/InSe.42 In this work, we perform systematic investigations on the photocatalytic performances of IV−V compounds and design a functional Schottky device based on the graphene/SiAs vdWH. Our findings illustrate that eight types of IV−V compound materials all have excellent application prospectives as photocatalysts for the overall water splitting with high efficiency of visible light. A weak acid environment is confirmed to be favorable to the hydrogen production process. These show tremendous significance for IV−V compounds in the fields of energy conversion and hydrogen production. Besides, the graphene/SiAs vdWH at the equilibrium state is characterized by an n-type Schottky contact with p- and n-type Schottky barrier heights (SBHs) of 0.960 and 0.584 eV, respectively. By applying an external strain and electric field, we realized the transition for the interface contact between nand p-type Schottky contacts and between Schottky and ohmic contacts, which suggests appealing applications for the graphene/SiAs vdWH used in functional Schottky devices and nanoelectronic materials.

properties for water splitting and tensile strain can enhance the photocatalytic activity under ultraviolet and visible light. Zhao and coauthors27 found that intrinsic polarization of In2X3 (X = S, Se, and Te) systems leads to a spontaneous built-in electric field, significantly benefiting the spatial separation of photogenerated electrons and holes and thus improving the photocatalytic efficiency. In the case of monolayer WS2, Kumar et al.25 proposed it as an outstanding photocatalyst, and the WS2/C2N van der Waals heterostructure (vdWH) exhibits a type II band alignment, high charge-carrier mobility, and significant optical absorption in the visible region, which together contribute to enhanced photocatalytic performances. Note that similar type II band alignment can also be seen in blue phosphorene/BSe,28 g-C3N4/CdS,29 GaSe/InS,30 and so on. By virtue of these, it is clearly found that 2D materials exhibit huge application potential in the fields of optoelectronic devices and photocatalysts. Nevertheless, exploration of IV−V compound materials is currently absent, but partial research is underway in our work on the photocatalytic properties of IV−V compounds. On the other hand, since its successful experimental realization in 2004, graphene has received considerable attention because of its significantly excellent electronic, optical, and transport properties,31−33 predicting its appealing application prospective in the design of next-generation nanoelectronic and optoelectronic devices, e.g., field-effect transistors (FETs).34,35 However, the electronic band gap of 0 extensively restricts its wide application. In parallel with the massive efforts to obtain an opened band gap for graphene, it has already been evident that constructing 2D vdWHs is an efficient strategy to obtaining modified electronic properties of freestanding materials.36 This can be particularly observed in graphene-based vdWH nanocomposites. The 2D materials used in combination with graphene include, but are not limited to, TMDs, 37 III−V compounds (e.g., BN), 38 III−VI compounds (e.g., GaS),39 IV−VI compounds (e.g., GeSe),40 and phosphorene.41 The 2D vdWHs comprising graphene that have been investigated include the heterostructures graphene/ GaSe,34 graphene/Janus MoSSe,35 graphene/GaS,39 graphene/ GeSe,40 graphene/phosphorene,41 graphene/InSe,42 graphene/CeO2,43 graphene/MoS2,44 etc. The band gap is commonly opened in these nanostructures at diverse levels because of interlayer coupling, with a well-preserved Dirac point, indicating their appealing potential application in nextgeneration optoelectronic devices. For example, Phuc et al. have argued that an opened band gap of 10 meV is found for graphene/GaSe vdWH, attributing the success to the sublattice symmetry breaking.34 The Guo group has forever claimed that the SnO/graphene/SnO trilayer nanostructure possesses a sizable opened band gap of 115 meV.45 In Asl et al.’s work, a very small opening of the band gap (2.8 meV) at the K point is formed in the graphene/MoS2 vdWH.46 A Schottky or ohmic contact can be expected at the interface composed of graphene and semiconducting materials, forming a metal−semiconductor (MS) system.47 Therefore, the Schottky barriers formed at the interface play a vital role in device design and performance explorations.39 Great efforts have been devoted to implying that the application of an external strain or electric field is an efficient means to tuning the Schottky barriers in graphene-based vdWHs, further promoting the research process of functional Schottky devices. Li et al. found that a negative electric field vertical applied to the graphene/SMoSe vdWH can push transportation for the

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EXPERIMENTAL SECTION

Model Construction and Computational Details. We complete all of the calculations based on the density functional theory (DFT) in the Cambridge Serial Total Energy Package (CASTEP). To achieve the purpose of better describing the exchange-correlation energy and potential, we utilize the generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhof (PBE)48 scheme, which has been widely used because of its satisfying efficiency. The complex electron−ion interactions in the systems have been treated by an ultrasoft pseudopotential. The DFT-D2 method with Grimme vdW correction49 is adopted for the sake of more accurately describing the long-range van der Waals (vdW) force. A plane-wave kinetic energy cutoff of 500 eV is chosen for attaining fully relaxed configurations. 4 × 4 × 1 and 6 × 6 × 1 k-point grids are respectively used to sample the Brillouin zone in self-consistent-field calculations of geometric optimization and energy calculations. Dipole correction is utilized to remove the impact of asymmetry in nanostructures.50 The valence electrons for the corresponding atoms are treated as follows: C 2s22p2, Si 3s23p2, P 3s23p3, Ge 4s24p2, and As 4s24p3. The optimized geometries are considered converged when atomic forces are converged to 0.01 eV Å−1 and the convergence threshold for energy is set to 1 × 10−6 eV. A vacuum layer of 15 Å thickness51 along the Z direction is employed to avoid interactions between periodic images. Besides, to determine the dynamic stability of IV−V compounds, the phonon dispersions52 have been calculated. For calculations about phonon dispersions, the local density approximation of normconserving pseudopotential is used for both geometric optimization and phonon calculations, with an energy cutoff of 500 eV, which are also conducted in the CASTEP code. For calculations about the optical properties, the absorption coefficient I(ω) can be obtained through eq 1:53 I(ω) = B

2ω [ ε12(ω) + ε22(ω) − ε1(ω)]1/2 c

(1)


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Figure 1. Atomic structures (side and top views) of IV−V compounds (a and b), Janus Si2PAs and Ge2PAs (c and d), and SiGeP2 and SiGeAs2 (e and f).

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RESULTS AND DISCUSSION Photocatalysts for Overall Water Splitting. Parts a and b of Figure 1 display the lattice structures of IV−V compounds in the ball−stick model, in which the Si (Ge) and P (As) atoms are signed by green and pink balls, respectively. The Janus nanostructures can be regarded as analogues of the IV−V compounds by substituting the whole row of Si, Ge, P, and As atoms, as depicted in Figure 1c−f. It is considered to stay in an equilibrium pattern for the IV−V compounds when the corresponding parameters are within the standards set in computational details. After full relaxation, the corresponding bond lengths (d), lattice constants (a or b), band gaps (PBE and HSE06; EG), and work functions (Φ) of the corresponding materials are listed in Tables 1 and2.

Table 2. Corresponding Parameters of IV−V Compounds, Including the Lattice Constant (a or b), Work Function (Φ), and PBE/HSE06 Band Gap (EG) EG (eV)

Table 1. Corresponding Bond Lengths (d, Å) in IV−V Compounds after Full Relaxation SiP

SiAs

GeP

GeAs

Si−P 2.276 Si−Si 2.367 Si2PAs

Si−As 2.372 Si−Si 2.340 Ge2PAs

Ge−P 2.342 Ge−Ge 2.454 SiGeP2

Ge−As 2.431 Ge−Ge 2.440 SiGeAs2

Si−P Si−As Si−Si

Ge−P Ge−As Ge−Ge

2.289 2.356 2.341

2.360 2.424 2.448

Si−P Ge−P Si−Ge

2.283 2.328 2.404

Si−As Ge−As Si−Ge

IV−V compound

a or b (Å)

Φ (eV)

PBE

HSE06

gap type

SiP SiAs GeP GeAs Si2PAs Ge2PAs SiGeP2 SiGeAs2

3.543 3.701 3.653 3.807 3.589 3.638 3.570 3.696

5.872 5.612 5.703 5.483 5.686/5.403 5.641/5.305 5.508/5.272 5.397

1.495 1.537 1.385 1.173 1.257 1.081 1.426 1.230

2.170 2.426 2.179 1.947 2.067 1.913 2.272 2.046

indirect direct indirect direct direct direct direct direct

Besides, the electron localization functions (ELFs)54 of IV− V compounds are calculated for further insight into the bonding mechanism between the layers in the nanostructures and are displayed in Figure 3. As the scale plate shows, the values of the ELFs are localized in the range of 0−1, and the values represent the degree of electron accumulation. It can be found that obviously covalent bonding is presented between the two single layers and between the atoms in the single layers. We also find that more powerful bonding occurs between the layers contacted via Si and Si atoms and Si and Ge atoms, whereas the interactions between layers contacted by Ge and Ge atoms seem to be relatively weakened. In the single layers, the covalent intensity of the corresponding bonds follows the relative order of Si−P > Ge−P > Si−As > Ge−As. The covalent values of the Si, P, Ge, and As atoms are 1.11, 1.06, 1.22 and 1.20, respectively. From Table 1, it can be clearly observed that the bond lengths are all within the range of 2.2−2.45 Å, and the sums of the covalent radius of Si and Si

2.386 2.435 2.389

Excellent stability of the material is necessary in order to possibly fabricate it in technological applications. We, hence, adopt the phonon spectra of IV−V compounds to ensure their dynamic stability, and the results are plotted in Figure 2. Commonly, no imaginary frequency is observed in the phonon spectra of all of the nanostructures mentioned above, demonstrating that the eight nanostructures possess outstanding dynamic stability. C


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Figure 2. Phonon dispersions of IV−V compounds: (a) SiP, (b) SiAs, (c) GeP, (d) GeAs, (e) Si2PAs, (f) Ge2PAs, (g) SiGeP2, and (h) SiGeAs2.

Figure 3. ELFs of IV−V compounds: (a) SiP, (b) SiAs, (c) GeP, (d) GeAs, (e) Si2PAs, (f) Ge2PAs, (g) SiGeP2, and (h) SiGeAs2

D


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respectively.57 The CBMs and VBMs of the eight nanostructures with respect to the vacuum level, along with the work functions of the systems, can be acquired from the formulas (3), (4),58 and (5):

atoms, Ge and Ge atoms, Si and P atoms, Si and As atoms, Ge and P atoms, and Ge and As atoms are calculated to be 2.22, 2.44, 2.17, 2.31, 2.28, and 2.42 Å, respectively, which are falling into or extensively approaching the range of 2.2−2.45 Å. These results together confirm that the two single layers in the nanostructures are contacted via strong covalent bonding, implying their reliable structural stability. Further, we calculate the cohesive energy of IV−V compounds, which can be obtained from the expression (2), and the results are shown in Table 3. Ecoh = (E MaXb − aEM − bE X )/(a + b)

Table 3. Cohesive Energy (Ecoh)/eV of IV−V Compounds SiAs

GeP

GeAs

−0.756 Si2PAs

−0.701 Ge2PAs

−0.662 SiGeP2

−0.752 SiGeAs2

−0.723

−0.676

−0.708

−0.701

(3)

E VBM = −I = −χ − 0.5Eg

(4)

Φ = Evac − E F

(5)

Here, A, I, χ, and Eg represent the electron affinity, ionization energy, absolute electronegativity, and band gap of the corresponding materials, respectively. The χ value of the corresponding materials can be calculated from ref 59. The work functions of IV−V compounds are listed in Table 2, from which we can acquire the Fermi level of the materials. Notably, because of the existence of intrinsic dipoles, the two surfaces in Janus Si2PAs, Ge2PAs, and SiGeP2 nanostructures do not own the same vacuum level, leading to the difference in the electrostatic potential energy, i.e., ΔΦ. The values of ΔΦ in these three systems are calculated to be 0.28, 0.336, and 0.236 eV, respectively. According to the previous works on such materials featured by ΔΦ,55,60,61 it is found that the redox potentials of water splitting of one side have been shifted downward by ΔΦ, based on which we have aligned the band edges of these Janus materials (as shown in Figure 4j−l). Combining parts i and j−l of Figure 4, we can find that the eight IV−V compounds are all outstanding candidates for water-splitting reactions, with the CBMs and VBMs localized more positive and negative than the reduction and oxidation potentials at the same time. Moreover, it should be particularly pointed out that this ΔΦ has more significant applications in the field of photocatalysis. The Yang group55 has reported a new route to designing photocatalytic materials with high efficiency of near-IR light by adopting this huge difference in the electrostatic potential energy between the two parts in a heterostructure, and we have presented a simple illustration of this mechanism in Figure 4m, as well as the traditional photocatalytic mechanism in Figure 4n. It was found that the photocatalytic materials are traditionally required to have band gaps larger than 1.23 eV to conduct the overall water-splitting reactions. Note that this unexpected limitation of the band gaps of photocatalytic materials into EG > 1.23 eV has been practically relaxed to EG > 1.23 − ΔΦ, where ΔΦ > 0, which means that it is significantly possible that the materials with band gaps smaller than 1.23 eV are good candidates for water splitting under the irradiation of near-IR light. This appealing phenomenon can also be observed in Janus MoSSe62,63 and PtSSe16 monolayers. Furthermore, the huge ΔΦ can lead to the formation of an induced internal electric field, which is also a crucial improvement for efficient separation of the photoexcited electron−hole pairs and enhanced photocatalytic activity. In the cases of the 2D M2X3 (M = Al, Ga, In; X = S, Se, Te) materials researched by Fu et al.,60 some values of ΔΦ are calculated to be much larger than 1.23 eV, i.e., Al2S3 of 2.35, Ga2S3 of 1.65, and In2S3 of 1.68 eV. Such a huge ΔΦ plays a significant role in solely excluding the limitations of band gaps of photocatalytic materials, predicting their fascinating applications in novel optoelectronics. In addition, the acid−base properties of conditions that the materials serve have been evident providing nonignorable effects on the photocatalytic performances. Therefore, we carry out a comparison of the band alignments of IV−V compounds

(2)

SiP

ECBM = −A = −χ + 0.5Eg

Here, EMaXb is the total energy of the MaXb unit cell, and EM and EX are the energies of the single M and X under vacuum, respectively. a and b are the atom numbers of M and X in the unit cell, respectively. It can be observed that all nanostructures have a negative cohesive energy, indicating that these IV−V compounds are energetically stable. On the basis of the results of phonon spectra, ELF, and cohesive energy, it is evident that IV−V compound nanostructures own excellent dynamic, structural, and energetic stabilities, which can ensure significant feasibility of their successful fabrication in industrial applications. The energy band structures of IV−V compounds are then calculated using PBE and HSE06 functionals for the purpose of attaining deeper insight into the electronic properties, which are shown in Figure 4a−h. One can find that the calculations provide indirect band gaps for SiP and GeP, whereas the other six materials possess commonly direct band gaps. Combined with Table 1, it is seen that the band gaps of these materials are exactly falling in, or are extensively close to, range of 2.0−2.2 eV, which has already been reported as the most advantageous energy region for sufficient utilization of solar energy,28 guiding us through the choice and design of suitable materials as photocatalysts for water splitting. Further, the semiconducting materials characterized by direct band gaps have a more appealing application prospective as optoelectronic devices, particularly in the field of photocatalysis. Taking SiAs as an example, obvious semiconducting performances are presented, with a direct band gap of 2.426 eV under the HSE06 degree, leaving the conduction band minimum (CBM) and valence band maximum (VBM) together at the Γ points. To verify the photocatalytic performances of IV−V compounds, the band alignments given by the HSE06 functional under pH = 0 are calculated and displayed in Figure 4i−l. From the point of energetic view, the reduction potential energy EH+/H2 and oxidation potential energy EO2/H2O of the overall water splitting should be contained within the band-gap region of photocatalytic materials;55,56 that is to say, the energy positions of the CBMs and VBMs should be higher and lower than EH+/H2 and EO2/H2O, respectively. The previous literature has pointed out that the EH+/H2 and EO2/H2O values with regard to the vacuum level are −4.44 and −5.67 eV, E


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Inorganic Chemistry

Figure 4. continued

F


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Figure 4. continued

G


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Figure 4. Energy band structures given by the HSE06 functional of IV−V compounds: (a) SiP, (b) SiAs, (c) GeP, (d) GeAs, (e) Janus Si2PAs, (f) Ge2PAs, (g) SiGeP2, and (h) SiGeAs2. (i) Band alignments of the SiP, SiAs, GeP, GeAs, and Janus SiGeAs2 relative to the vacuum level under varied pH. (j−l) Band alignments of Janus Si2PAs, Ge2PAs, and SiGeP2 nanostructures. (m and n) Illustrations of the photocatalytic mechanism proposed by Yang et al. and the traditional mechanism. (o) Comparison of the optical absorption spectra by the HSE06 functional between IV−V compounds in the IR and visible regions. (p) Band alignments with regard to the vacuum level of bilayer IV−V compound nanostructures under the HSE06 degree. (q) Diagrammatic illustration of the photocatalytic mechanism of a bilayer SiP nanostructure.

condition (pH = 7) is then employed for the corresponding materials, and it is noteworthy that for SiP continuous transportation for the reduction potential and oxidation potential to higher energy levels leads to the shrinkage or even the vanishment of photocatalytic performances for the hydrogen production process, accompanied by the reduction potential H+/H2 localized more positively than CBM. In the cases of GeP, GeAs, and SiGeAs2, despite the reduction potential still being involved in the band-gap regions, the CBMs are extensively approaching the reduction potential H+/ H2, which demonstrates that the capacity of these materials for hydrogen production may be eliminated. One can see that only SiAs exhibits an obvious photocatalytic performance in a neutral environment. When pH = 10 (i.e., weak base), except for SiAs, the absence of a photocatalytic performance for hydrogen production is presented for other materials, with the reduction potential moving beyond the band-gap region of the materials. Therefore, it is shown that an acid environment is

to obtain their photocatalytic performances under diverse pH values. The positions of the reduction and oxidation potentials of water under tunable pH can be acquired by the expressions (6) and (7):64 E H+ /H2 red = −4.44 eV + pH × 0.059 eV

(6)

EO2 /H2Ooxd = −5.67 eV + pH × 0.059 eV

(7)

Based on these, the band edges of SiP, SiAs, GeP, GeAs, and Janus SiGeAs2 under different pH values are available (shown in Figure 4i). One can find that, with increasing pH from 0 to 4, the reduction potential and oxidation potential are commonly observed to be transported to higher energy levels at the same time and remain within the band-gap regions, with the CBMs a little closer to the reduction potential H+/H2. This suggests that an enhanced photocatalytic performance can be expected and realized for the materials by tailoring the pH value to a relatively weak acid environment. A neutral H


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Inorganic Chemistry Table 4. HSE06 Band Gaps EG (eV) and ΔΦ (eV) of Bilayer IV−V Compound Nanostructures bilayer configuration EG ΔΦ

SiP

SiAs

GeP

GeAs

Si2PAs

Ge2PAs

SiGeP2

SiGeAs2

1.549 0

1.752 0

1.577 0

1.514 0

1.579 0.590

1.641 0.659

1.472 0.361

1.563 0

lying on one SiP layer will be transported to the other SiP layer. Similarly, the photoexcited holes staying on the other SiP layer will be transported to the former SiP layer dominated by the contribution of valence band offset (VBO) of 0.620 eV, leading to the photoexcited electron−hole pairs being separated. Similar results can also be found in other bilayer nanostructures. More attractively, in the bilayers Si2PAs, Ge2PAs, and SiGeP2, the values of ΔΦ have increased to 0.590, 0.659, and 0.361, respectively, larger than those of their monolayer states. These can further relax the band gaps of these materials for the overall water splitting to EG > 0.640, 0.571, and 0.869 eV, respectively. Besides, the bilayer nanostructures all have appropriate band gaps of 1.40−1.75 eV, which are extensively approaching 1.59 eV (the optical absorption edge of near-IR light), meaning there is a significant possibility for optical absorption of the bilayer nanostructures covering the entire visible region. Therefore, a high efficiency of visible light can be achieved with efficient hindrance of electron−hole pair recombination and enhanced optical absorption. Schottky Device Based on the Graphene/SiAs Heterostructure. Optimization Results. The graphene and SiAs monolayers are chosen to construct the 2D graphene/ SiAs heterostructure by their vertical stacking. The fully optimized lattice parameters of graphene and SiAs in unit cells are respectively shown to be 2.460 and 3.701 Å, and that of the graphene monolayer is in good accordance to the reported value,65 revealing the good precision of our computational details and reliability of the theoretical models. A small lattice mismatch is quite expected between the different parts in the heterostructure nanocomposites for their practical fabrication in experiments. For this purpose, we utilize 3 × 3 × 1 and 2 × 2 × 1 supercells for the graphene and SiAs parts, respectively, in a graphene/SiAs heterostructure. That is to say, the lattice constants of SiAs and graphene are 7.400 and 7.380 Å, along with the average of 7.390 Å as the lattice constant of the graphene/SiAs nanocomposite, implying a lattice mismatch of ∼0.14%. The SiAs and graphene layers are also found within the same hexagonal lattices, together with the tiny lattice mismatch, ensuring significant feasibility for it to fabricate in technological applications. Considering the different lattice orientations, two potential configurations of the graphene/SiAs heterostruture are constructed, and the side and top views of the two configurations are plotted in Figure 5. The binding energy (Eb) is employed to verify the stability of the graphene/SiAs heterostructures, which is defined in the expression (8):

more favorable for the photocatalytic performance of IV−V compounds, especially for the hydrogen production process. For an outstanding photocatalytic material, it is strongly expected that it has enormous optical absorption from the solar energy for the sake of a high utilization efficiency. As is known, the solar energy is comprised of IR, visible, and ultraviolet light, which account for 43, 50, and 7%,16 respectively, and it thus is very crucial to make efforts to utilize the IR or visible light to conduct photocatalytic reactions for the consideration of high efficiency. Figure 4o shows comparative optical absorption spectra of IV−V compounds as a function of the photon energy. It should be noted that GeAs owns a relatively wider absorption range in comparison with others, along with the absorption edge situated at approximately 1.6 eV. Further, excellent absorption coefficients as high as 105−2 × 105 cm−1 can be obtained in the visible region (1.59−3.1 eV). Such a satisfying intensity of optical absorption provides abundant potential for ensuring the high efficiency of solar energy. In Peng et al.’s work about the PtSSe nanostructure,16 an optical absorption of 105 cm−1 for the photocatalyst material has been considered to be advantageous to the high utilization of solar energy. Gu et al.26 have also argued monolayers GeS and GeSe characterized by an optical absorption degree of 105 cm−1 as candidates in photocatalysts with high efficiency. For IV−V compounds, an optical absorption of up to half of 105 cm−1 is also presented. Herein, it can be concluded that IV−V compounds that we have researched own a fascinating application prospective as photocatalysts for the overall water splitting under visible light. This besides predicts tremendous significance in the fields of energy conversion and hydrogen production. The efficient separation of photoexcited electron−hole pairs is very important for the enhanced photocatalytic activity of photocatalytic materials. Recently, the construction of heterostructures with type II band alignment has been regarded as a practical and effective route and widely used. Herein, we take further insight into the photocatalytic properties of bilayer IV−V compounds, with the calculated HSE06 band gaps and ΔΦ of bilayer nanostructures presented in Table 4. The advantage of type II heterostructures is that the photoexcited electron−hole (e−/h+) pairs can be separated at the different parts, which is quite beneficial for extending the carrier lifetimes and improving the photocatalytic activity. The band alignments of bilayer IV−V compounds are displayed in Figure 4p. Common type II band alignments are observed for the bilayer nanostructures, and their CBMs/VBMs are well satisfied with the requirements of photocatalytic materials for the overall water splitting. Next, the bilayer SiP nanostructure is taken as an example to explore the photocatalytic mechanism for the overall water splitting, and the illustration is depicted in Figure 4q. When the bilayer SiP nanostructure is irradiated by visible light, the electrons on the two SiP layers will absorb enough energy and be excited into the conduction bands (CBs); at the same time, the photoexcited holes are formed at the valence bands (VBs). Then, promoted by the conduction band offset (CBO) of 0.620 eV, the photoexcited electrons

E b = Egraphene/SiAs − Egraphene − ESiAs

(8)

Here, Egraphene/SiAs, Egraphene, and ESiAs are respectively the total energies of the graphene/SiAs heterostructure, accompanied by separate graphene and SiAs monolayers. The graphene/ SiAs heterostructure reaches its equilibrium state with a calculated binding energy of −44.8 meV per C atom, extensively approaching those in the graphene/MoSSe (−41.5 and −32.9 meV per C atom)35 and graphene/InSe I


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its promising application in novel optoelectronic devices. In the case of the graphene/SiAs vdWH, GGA-PBE provides a direct band gap of 9 meV, indicating that the graphene/SiAs vdWH nanocomposite inherits a high carrier mobility from the graphene monolayer, which is extensively favorable in the field of high-speed nanoelectronic devices.34 Besides, the band structure of the graphene/SiAs vdWH seems to be a pure combination of those of the individual graphene and SiAs monolayers. Moreover, a linear-like dispersion relationship is presented near the Dirac point, and it is still well preserved, which further supports the existence of the weak vdW force between the graphene and SiAs components. The partial density of states (PDOS; Figure 6d) demonstrates that the states of the graphene/SiAs vdWH near the Fermi level are mainly attributed to the bands associated with carbon atoms of the graphene layer. For the SiAs part, the CBM and VBM are equally devoted to the contributions of the Si 3p and As 4p orbitals, with a direct band gap of 1.544 eV and 7 meV larger than that of the freestanding state, which illustrates that the SiAs part maintains outstanding semiconductor properties before and after being combined with the graphene monolayer. We also calculate work functions of the isolated graphene and SiAs. The work functions of the above two systems are respectively calculated to be 3.533 and 5.612 eV, indicating a spontaneous charge transfer from graphene to the SiAs part once they vertically combined, leading to the formation of a built-in electric field (Eint) pointing from graphene to SiAs. This electron-transfer process can be vividly described through a density charge difference (DCD) image, which focuses on the charge distribution and reconstruction in vdWH nanocomposites, as displayed in Figure 6e. The cyan and yellow regions highlight the electron accumulation and depletion degrees, respectively. We can see that the electrons are rapidly accumulated on the As atoms in the SiAs layer, with electron depletion occurring at the top of the graphene layer. Figure 6f shows the electrostatic potential of the graphene/SiAs vdWH at the equilibrium state along the Z direction. A deeper potential is presented for graphene than the SiAs part, together with DCD analysis, indicating that SiAs acts as an electron acceptor from the graphene layer. Mulliken atomic population is used to provide a quantificational definition for the charge transfer of 0.54 e from SiAs to graphene, in good accordance

Figure 5. (a) Side view of the structural model of the graphene/SiAs vdWH. (b and c) Top view of two potential configurations of the graphene/SiAs vdWH.

(−40 meV per C atom)42 nanocomposites, which have been evidenced as a type of vdWH. The equilibrium interlayer spacing of the graphene/SiAs heterostructure is 3.518 Å, and those of the above two vdWHs are respectively 3.340 Å and 3.240 and 3.370 Å. The binding energy of the other configuration of the graphene/SiAs heterostructure is 0.22 meV per C atom smaller than the one discussed above, implying that the stacking pattern does not affect much the binding energy of the graphene/SiAs heterostructure. These results confirm that graphene and SiAs are contacted via the weak vdW force and that graphene/SiAs is a vdWH. The electronic band structures of the freestanding graphene and SiAs monolayers, along with the graphene/SiAs vdWH at the equilibrium state, are then calculated and depicted in Figure 6a−c. For SiAs, the band gaps are calculated to be 1.537 and 2.426 eV at the GGA-PBE and HSE06 levels, respectively, with CBM and VBM both localized at the Γ points. This shows an excellent semiconducting performance for SiAs, predicting

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Figure 6. (a−c) Energy band structures of the freestanding graphene, SiAs monolayers, and graphene/SiAs vdWH at the equilibrium configuration. (d) PDOS of the graphene/SiAs vdWH at the equilibrium configuration. (e) DCD of the graphene/SiAs vdWH at the equilibrium configuration, and the isosurface refers to an isovalue of 8 × 10−4 e Å−3. (f) Electrostatic potential of the graphene/SiAs vdWH with an equilibrium interlayer spacing along the Z direction.

forms an n-type Schottky contact with p- and n-type SBHs of 0.960 and 0.584 eV, respectively. Graphene/SiAs vdWH under an External Strain and Electric Field. External strains and electric fields have been confirmed as practical methods to achieving efficient modulation of Schottky barriers in 2D vdWHs, such as graphene/GaS39 and graphene/InSe42 vdWHs. Besides, the tunable interlayer spacing between the different parts has been widely used to apply a vertical strain. We thus obtain insight into the tailoring of Schottky barriers in the graphene/SiAs vdWH by modifying the interlayer spacing, and Figure 7b shows a model of vertical strain-induced graphene/SiAs vdWH. Because of the enormous computational expenses for HSE06 calculations,39 it should be emphasized that the traditional GGA-PBE functional is used here to obtain the evaluation trend of the SBHs as a function of the changed interlayer spacing or applied electric field, where our main goal is not the availability of the precise band gaps of systems.42 Therefore, the following calculations are all conducted using GGA-PBE instead of the HSE06 functional. To examine the interactions between the different parts, we employ the binding energy per unit of graphene/SiAs vdWH as

with the discussions about DCD. Efficient separation of the photogenerated electron−hole pairs in the graphene/SiAs vdWH due to the contribution of Eint can also be expected. This finding predicts its fascinating application prospective in nanoelectronic and optoelectronic devices. It has been claimed that Schottky (or ohmic) contact can be formed between metallic graphene and its semiconducting substrates, forming a MS system.47 As shown in Figure 6b, SiAs exhibits obvious semiconducting properties with a band gap of 1.537 eV, illustrating that the interface composed of graphene and SiAs can be connected via Schottky or ohmic contact. Figure 7a depicts a diagrammatic illustration of the pand n-type Schottky (ohmic) contact at the interface region. According to the Schottky−Mott mode42 at the MS interface, the n-type (ΦBn) and p-type (ΦBp) Schottky barriers are respectively defined as the energy difference between the Fermi level (EF) and CBM and between the Fermi level (EF) and VBM, i.e., ΦBn = ECBM − EF and ΦBp = EF − EVBM. Besides, it is noteworthy that the sum of the n- and p-type Schottky barriers is approximately equal to the band gap of the semiconducting substrate SiAs, i.e., 1.544 eV. In terms of these, we find that the graphene/SiAs vdWH at the equilibrium state K


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Figure 7. (a) Diagrammatic illustration of p-type and n-type Schottky and ohmic contact at the MS interface region. (b) Structural model of the vertical strain-induced graphene/SiAs vdWH, where the arrows looking upward and downward respectively indicate stretching and compressing strains. (c) Binding energy per unit of graphene/SiAs vdWH as a function of the tailoring interlayer spacing. (d) Evolution trend of SBH of graphene/SiAs vdWH as a function of tunable interlayer spacing. (e) Electronic band structures of the graphene/SiAs vdWH with various interlayer spacing. L


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Figure 8. (a) Structural model of the graphene/SiAs vdWH under an external electric field. (b) Evolution trend of the SBH of the graphene/SiAs vdWH as a function of the electric field. (c) Electronic band structures of the graphene/SiAs vdWH under diverse electric fields. (d) Designed FET composed of the graphene/SiAs vdWH.

accordance with the stable configuration after being fully converged. With the interlayer distance increasing from 2.40 to 2.90 Å, the binding energy decreases rapidly and remains characteristic of a positive value, indicating that the vdWHs

a function of the tunable interlayer spacing, as plotted in Figure 7c. One can observe that the spacing with D = 3.518 Å corresponds to the minimum value of Eb, indicating the most stable structure at this interlayer spacing, which is in excellent M


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Inorganic Chemistry

applying an electric field. SiAs is used as the transistor channel and graphene as the metal contact.

with these interlayer spacings cannot naturally be prepared in experiments. The vdWH owns common negative binding energies when the interlayer spacing is larger than 2.90 Å, which shows an exothermic process for the formation of graphene/SiAs vdWHs with such interlayer spacings, implying stable structures for the graphene/SiAs vdWHs. Figure 7d displays the SBH of the graphene/SiAs vdWH as a function of the tunable interlayer spacing. We find that, with decreasing interlayer spacing (compared with the equilibrium state of 3.518 Å), n- and p-type SBHs are respectively increased and decreased gradually in a linear-like evolution. It should be particularly noted that when the interlayer spacing is localized between 3.30 and 3.40 Å, n- and p-type SBHs are shown to be 0.818 and 0.759 eV and 0.714, and 0.860 eV, respectively, where a prediction can be proposed about the transition between the n- and p-type Schottky contacts by limiting the interlayer spacing to 3.30−3.40 Å. Further, by a continuous narrowing of the interlayer spacing to 2.60 Å, we find that the p-type SBH nearly approaches the zero level and remains unchanged when more strengthened compressive strains are applied, demonstrating the transition for a Schottky contact into an ohmic contact (p-type) for the graphene/SiAs vdWH. Electronic band structures of the graphene/SiAs vdWH with a tunable interlayer spacing (depicted in Figure 7e) are adopted to imply in detail that, with a change of the interlayer spacing, the Fermi level has been transported upward or downward during the band-gap region of the semiconducting SiAs part, leading to an increase or a decrease in the n- or ptype SBHs. Notice that the Fermi level is nearly localized at the center of the band-gap region when the interlayer spacing is set in the range of 3.30−3.40 Å, which is consistent with the conclusions above. Based on these, it is significantly feasible that the practical transitions between n- and p-type Schottky contacts and between the Schottky and ohmic contacts can be realized by modification of the interlayer spacing (i.e., application of a vertical strain), which is very crucial to the design of new electronic Schottky devices based on graphenebased vdWH nanocomposites. Similar to the external strains, applying an external electric field also provides nonignorable effects on the SBHs of the graphene/SiAs vdWH, and Figure 8a plots an external electricfield-induced model of the graphene/SiAs vdWH. Parts b and c of Figure 8 display the evolution trend of the SBHs of the graphene/SiAs vdWH as a function of the electric field and the energy band structures of the graphene/SiAs vdWH under varied external electric fields. By exerting a positive electric field in the range of 0−0.10 V Å−1, n- and p-type SBHs are slowly increased and decreased in linear evolution, respectively. This reveals that the vdWH still keeps an n-type Schottky contact. When the electric field is strengthened to 0.10 and 0.15 V Å−1, n- and p-type SBHs are respectively 0.747 and 0.794 eV and 0.815 and 0.716 eV, which implies a transition from the n-type Schottky contact to the p-type Schottky contact when the electric field is limited to 0.10−0.15 V Å−1. Besides, it is seen that, by the application of a negative electric field to 0.45 V Å−1, the n-type SBH approaches zero degree, indicating that an ohmic contact (n-type) is formed. Thus, the application of an electric field can also be ascribed to the efficient modulation of the Schottky barriers and interface contacts in the graphene/SiAs vdWH. By virtue of these, we design a schematic model of the FET (as shown in Figure 8d), in which both the SBH and Schottky contact can be controlled by tailoring the interlayer spacing or

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CONCLUSIONS In summary, we carry out first-principles calculations on the photocatalytic properties of IV−V compounds and design a functional Schottky device based on the graphene/SiAs vdWH. The phonon spectra, ELFs, and cohesive energies are calculated to guarantee the stability of the IV−V compound materials. Our results show that the eight IV−V compound materials all have excellent application prospectives as photocatalysts for the overall water splitting. For the GeAs monolayer, high absorption coefficients of up to 105−2 × 105 cm−1 can be obtained in the visible region. Such a satisfying intensity of optical absorption provides abundant potential to ensure the high efficiency of solar energy. Other compounds also have a relatively considerable optical absorption as high as approximately half of 105 cm−1. Type II band alignments and appropriate band gaps of bilayer IV−V compound nanostructures predict their promising applications in photocatalyts with enhanced photocatalytic activity. In particular, for bilayers Si2PAs, Ge2PAs, and SiGeP2, the huge difference in the electrostatic potential between the two parts induces an internal electric field, which can effectively hinder the recombination of photoexcited electron−hole pairs. On the other hand, the Schottky barriers in the graphene/SiAs vdWH can be efficiently tailored by applying a vertical strain or an electric field, which greatly contributed to the realization of transportation for the interface between p- and n-type Schottky contacts and between Schottky and ohmic contacts. These findings denote appealing applications for IV−V compounds in the fields of energy conversion and hydrogen production and for the graphene/SiAs vdWH composite in functional Schottky devices and nanoelectronic materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.S.). ORCID

Yanqing Shen: 0000-0002-7826-2700 Zhongxiang Zhou: 0000-0003-1952-7147 Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research is supported by National Natural Science Foundation of China (Grants 11204053 and 11074059) and the China Postdoctoral Science Foundation (Grant 2013M531028).

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REFERENCES

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