Effect of metal decoration on sulfur-based gas molecules adsorption on phosphorene

Based on first-principles calculation, the adsorption of sulfur-based gas molecules (H2S, SO2, SO3) on various metal-decorated phosphorenes is researched systematically. Eleven metals (Li, Na, K, Rb, Cs, Ca, Sr, Ba, Ni, La, Tl) which can avoid the formation of clusters on the phosphorene are considered. Noticeably, all metal decorations can enhance the adsorption strength of phosphorene to sulfur-based gas molecules except for H2S on Tl-decorated phosphorene. Meanwhile, the adsorption energy (Eads) shows the trend of Eads(H2S) < Eads(SO2) < Eads(SO3) for the same metal decoration case. In addition, some metal-decorated phosphorene systems exhibit intriguing magnetic and electrical variation after sulfur-based gas molecule adsorptions, indicating that these systems are promising to be candidates for the detection and removal of sulfur-based gas molecules.


Computational methods
In this article, all the density functional theory (DFT) calculations have been carried out by the Vienna ab initio simulation software package code (VASP) 33,34 . We took advantage of the Perdew-Burke-Ernzerhof (PBE) functional of generalized gradient approximation (GGA) to describe exchange-correlation interaction 35 . The van der Waals (vdW) interactions were dealt with by adopting empirical correction scheme of Grimme (DFT + D3) 36 . In all the calculations, the kinetic energy cut-off for the plane-wave basis was 500 eV. The 3 × 4 supercells and the vacuum distances of 15 Å were utilized to reduce the interaction of mirror adsorbates and phosphorene layers, respectively, and the corresponding k-point grids were set as 3 × 3 × 1 by Monkhorst-Pack k-point scheme. The energy convergence accuracy was set to 10 −5 eV, and all the structures were fully relaxed until the forces acting upon each atom were less than 0.01 eV/Å. For sulfur-based gas molecules adsorption on pristine phosphorene or metal-decorated phosphorene, the E ads is calculated by the formula, where E tot , E sub and E gas are the energies of the adsorption system, substrate (pristine phosphorene or metal decorated phosphorene) and sulfur-based gas molecule (H 2 S, SO 2 , or SO 3 ), respectively.

Results and discussion
Sulfur-based gas molecules adsorption on pristine phosphorene. The lattice constants of the pristine phosphorene monolayer along the armchair and zigzag directions are 4.57 and 3.31 Å, respectively, with a direct bandgap of 0.88 eV, in agreement with previous studies 37,38 . For the adsorption of sulfur-based gas molecules (H 2 S, SO 2 , SO 3 ) on pristine phosphorene, various possible initial adsorption sites [e.g. hollow (H), bridge (B), top (T)] and adsorption configurations were considered [Figures S1-S3]. By comparing the total energies of the adsorption configurations after structural optimization, the configurations with the highest E ads were obtained as shown in Fig. 1. The H 2 S prefers to adsorb at the hollow site with H atoms pointing to the phosphorene surface, while the SO 2 and SO 3 prefer to adsorb at the T site lying parallel to the phosphorene surface.
(1) E ads = E tot − E sub + E gas , Figure 1. The optimized structure of (a) H 2 S, (b) SO 2 and (c) SO 3 adsorbed 3 × 4 pristine phosphorene, (d-f) the DCD corresponding to (a-c), respectively, (g-i) the LDOS corresponding to (a-c), respectively. Purple, yellow, pink, and red balls in (a-f) represent P, S, H and O atoms, respectively. Yellow and blue regions in (d-f) denote charge accumulation and charge depletion, respectively. The black, red and blue curves in (g-i) represent LDOS of P, S and H (O), respectively, with the Fermi level set to zero. To analyze the mechanism of interaction between sulfur-based gas molecules and phosphorene, the differential charge density (DCD) of the most stable adsorption configurations were calculated, as illustrated in Fig. 1d-f. It can be seen that, from H 2 S to SO 2 to SO 3 , the electron accumulations around gas molecules increase significantly, which are also confirmed by the Bader charge analysis. The Bader charge analysis shows that the electrons transferred from the phosphorene to H 2 S, SO 2 and SO 3 molecules are 0.003, 0.182 and 0.451 e, respectively, ascribing to higher electronegativity of gas molecules over the phosphorene. Generally, the more charge quantities transferred means the stronger interactions, and thus the larger E ads values 39 . Here, the variation trend of charge quantities transferred does agree with that of the E ads values (see Table 1).
To understand the effect of the adsorption of sulfur-based gas molecules on the electrical properties of phosphorene, the local density of states (LDOS) of the adsorption system were calculated, as shown in Fig. 1. In LDOSs, the DOS near the Fermi level is zero for all three adsorption systems, indicating the adsorption of sulfur-based gas don't change the phosphorene's electronic structure. The bandgaps of phosphorenes are 0.86 and 0.87 eV, respectively, after H 2 S and SO 2 adsorptions, which are marginally smaller than that of pristine one, while the bandgap of phosphorene slightly increase to 0.89 eV after SO 3 adsorption. After sulfur-based gas molecule adsorptions, the slight variation of bandgap may be ascribed to the change of channel of phosphorene. It has been pointed out that a narrower channel (3.49 Å for H 2 S and 3.44 Å for SO 2 as compared to 3.54 Å for pristine phosphorene) would result in the stronger repulsive interaction between the facing lone pairs at the ditch of phosphorene and thus the decrease of bandgap, and vice-versa 40 . For H 2 S adsorption case, the H atom is the nearest to the P atom, but the H atomic DOS is far from the Fermi level, which should be also responsible for the smaller E ads value. In the case of SO 2 and SO 3 , the S atoms are the closest to P atoms. Near Fermi levels, the S DOS peak is just above the conduction band minimum. Especially for SO 3, the S DOS distributes widely in the conduction band, which should be responsible for the largest E ads values among the three adsorption cases. The adsorption energy closed to 1 eV is an ideal binding for the efficient and reversible gas sensor. However, the E ads values of sulfur-based gas molecules on pristine phosphorene are too small for this purpose. The metal decoration can improve the adsorption of gas, so we will discuss the metal decorations and the sulfur-based gas molecule adsorptions on metal decorated phosphorenes.

Sulfur-based gas molecules adsorption on bP-Ms.
The theoretical studies have shown that metal decoration can significantly influence the electronic properties of phosphorene 41 . It is well-known if the E ads value of a metal on 2D materials is less than its bulk cohesive energy (E coh ), it is going to cluster on the 2D surfaces. Therefore, to enhance the adsorption of sulfur-based gas on phosphorenes and avert clustering of metal atoms on the surface, eleven metals (Li, Na, K, Rb, Cs, Ca, Sr, Ba, Ni, La, Tl) have been opted to decorate the phosphorene on the basis of E ads /E coh > 1 42 . Here, the electronic properties of black phosphorenes decorated with metals (bP-Ms) are discussed first to contrast before and after sulfur-based gas molecule adsorptions. Figure 2 shows the band structures and projected density of states (PDOSs) of the bP-Ms. Except for Ni, the outermost s-states of alkali metals (AMs = Li, Na, K, Rb, Cs) and those of alkaline earth metals (AEMs = Ca, Sr, Ba), the 6 p-state of Tl and 5 d-state of La are mainly distributed in the conduction band of phosphorenes, which cause valence-electron transfer from metals to phosphorenes, and thus the Fermi levels shift up in energy. For AM decorated and Tl decorated phosphorenes (bP-AMs and bP-Tl), the Fermi levels shift upward into the conduction bands of phosphorenes and the corresponding bP-AMs and bP-Tl show metal properties. The s-states of AMs and p-state of Tl are far above the lowest conduction bands of phosphorenes. Unlike the AMs and Tl, the s-states of AEMs or d-state of La are aligned to the lowest conduction band of phosphorene, which causes a strongly repulsive interaction between metal atomic states and the lowest conduction bands of phosphorenes, so www.nature.com/scientificreports/ the lowest conduction bands are pushed down. As a consequence, there are large separations between the lowest and second lowest conduction bands for AEM decorated phosphorenes (bP-AEMs) and bP-La. On the other hand, the more s-and d-electrons transfer for bP-AEMs and bP-La, the more energies shift for Fermi levels.
The Fermi levels of bP-AEMs and bP-La are located between the lowest and second lowest conduction bands. The bP-Ca, bP-Sr and bP-La show semiconductor properties, while bP-Ba shows metal property due to that the Fermi level cross slightly through the second lowest conduction band of phosphorene. Additionally, bP-La has spin-polarized LDOS (Fig. 2), with magnetic moment of 1 µ B . For Ni decorated case, the s-and d-states are distributed in valence band of phosphorene, and the semiconductor property of bP-Ni is remained. However, the bandgap of bP-Ni decreases to 0.769 eV as compared with that of pristine phosphorene, which may be ascribed to the strong hybridized interaction between Ni d-state and valence band of phosphorene, and thus the highest valence band is pushed upward.  www.nature.com/scientificreports/ shown in Fig. 3. In all the structures, the distances between the sulfur and metal atoms are apparently nearer than those between H and metal atoms. Except for bP-Li, bP-Ba and bP-La systems, H 2 S is adsorbed on the bP-Ms in parallel to the surface of phosphorene. The E ads values of S-based gases, the charge transfer amounts (∆Q) and adsorption distances (D sub-gas ) between sulfur-based gas molecules and substrates are summarized in Table 1. As compared with the adsorption energy of 0.220 eV for H 2 S on pristine phosphorene, except for Cs and Tl atoms, the decorations of the rest metals enhance the adsorption of phosphorene to H 2 S. Especially, the E ads value of H 2 S on bP-Ni is up to 1.158 eV (see Table 1), which is the largest value for all the H 2 S adsorption systems. Correspondingly, the adsorption distance between Ni and S atoms is the smallest among the adsorption of H 2 S on the metal decorated phosphorene, and the value is 2.207 Å. However, the adsorption energies of H 2 S on bP-Tl and bP-Cs are the smallest, even smaller than that on pristine phosphorene. On the one hand, it may be due to the fact that the electrons of all atoms in H2S molecule are full shell structure, resulting in less charge transfer and interaction; On the other hand, it may be attributed to the relatively large atomic radii of Tl and Cs, which cause the longer adsorption distance as well as less interaction between gas and metal decorated phosphorene.  www.nature.com/scientificreports/ The H 2 S on bP-Tl and bP-Cs have the largest adsorption distance of 3.795 and 3.844 Å, respectively, which may be due to the larger Tl atomic size.
To understand the mechanism of adsorption-energy enhancement and the effect of H 2 S on the electronic and magnetic properties of bP-AM, the LDOSs of the adsorption systems were calculated, as shown in Fig. 4. The magnetic moment of bP-La reduces slightly from 1.00 to 0.98 µ B after H 2 S adsorption, while those of the other bP-Ms systems remain zero, which are corroborated by the spin asymmetric LDOS for H 2 S adsorbed bP-La and the spin symmetric LDOSs for the other H 2 S adsorbed bP-M systems. Except for Ca, Sr and La decorated cases, the adsorptions of H 2 S have no effect on the electronic properties of the rest bP-M systems. The metal or semiconductor properties of the rest bP-M systems remained after H 2 S adsorption. The adsorption of H 2 S enhances the interaction between Ca/Sr atom and the conduction band of phosphorene, which leads to a slight downward shift of the conduction band and the Fermi energy level entering the sub-low conduction band, and further causes that bP-Ca and bP-Sr undergo the transformation from semiconductor to metal after the adsorption of H 2 S molecule. As shown in Fig. 4, there are the overlapping peaks between the sulfur and metal atoms located near − 5 or − 4 eV for AMs, AEMs and La decorated cases, which should be a reason of the improved H 2 S adsorption. The smallest E ads values for Cs and Tl decorated may be mainly attributed to the large radii of Cs and Tl atoms. For Ni decorated case, the apparent overlapping peaks of S and Ni atoms near − 6, − 4 and − 1 eV imply the strong hybridized interaction between H 2 S and bP-Ni substrate, which may be primarily responsible for the largest E ads value. Additionally, the states of S and H atoms are far from the Fermi level, implying that H 2 S adsorption almost has no effect on the band structures near it for bP-Ms. Thus, the conductivity properties of bP-Ms are not changed by H 2 S adsorption except for Sr and La decorated cases. For Sr decorated case, the state of H 2 S molecule in conduction band pushes slightly the conduction band minimum of bP-Sr down, which makes the small bandgap (0.07 eV) of bP-Sr disappear. For La decorated case, the charge transfer from substrate to H 2 S makes the Fermi level shift down in energy after H 2 S adsorption, and finally the Fermi level www.nature.com/scientificreports/ crosses through the spin-up band [see the red curve below the Fermi level in Fig. 2(j)], which leads to the halfmetal property of bP-La.

SO 2 gas molecules adsorption on bP-Ms.
For the SO 2 adsorption on bP-Ms, the most stable structures are illustrated in Fig. 5. Except for SO 2 adsorption on bP-Ni, the SO 2 preferably adsorbs on the bP-AMs with bond angles toward the metal atoms, and oxygens of SO 2 are the nearest atoms to the metal atoms. As given in Table 1, all the metal decorations can significantly improve the adsorption capacity of phosphorene to SO 2 . The corresponding E ads values vary from 0.597 to 2.822 eV, and stay larger than the pristine phosphorene case of 0.396 eV. In addition, we can see that the decoration of AEMs (Ca, Sr, Ba) is more effective than AMs (Li, Na, K, Rb, Cs) for adsorbing SO 2 due to larger E ads values ( Table 1), and that decrease with atomic number in the same group elements. As we know, the more charge transfer means the stronger interaction, i.e. larger E ads value. Consequently, the more valence electrons of AEMs and thus the more electron transfers should be in charge of their larger E ads values. On the other hand, short adsorption distance is beneficial for the charge transfer. Thus, www.nature.com/scientificreports/ an increasing adsorption distance should be responsible for decreasing E ads values in the same group elements. Adsorption distance may be attributed to the atomic radius increasing with atomic number. To understand how the SO 2 effect on the properties of the metal decorated substrates, the LDOSs of all SO 2 adsorption systems were calculated and shown in Fig. 6. The non-zero LDOSs at the Fermi level imply the metal properties for SO 2 adsorptions on bP-AM, bP-AEM, bP-La and bP-Tl systems, while the zero LDOSs at the Fermi level imply semiconductor properties for that on bP-Ni system. Except for bP-Ni, almost all the bP-Ms exhibit spin asymmetry after SO 2 adsorption. The SO 2 adsorbed bP-La has a negligible magnetic moment of 0.0002 µ B , on the spin polarization of bP-La system is suppressed by the SO 2 adsorption. Thus, the bP-AMs, bP-AEMs and bP-Tl perceive magnetization on SO 2 adsorption, while bP-La loses the magnetization. In addition, the bP-Ca, bP-Sr and bP-La become metallic, on upward movement of the lowest conduction band of phosphorene (see figure S6 in supporting materials). In fact, upward movements of the lowest conduction bands exist in all the SO 2 adsorbed bP-M systems, including the SO 2 adsorption on bP-AM, bP-Ba and bP-Tl systems (although their metal properties remained), which may be attributed to that the SO 2 adsorptions weaken the interactions between the metals and phosphorenes. For SO 2 adsorption on bP-Ni, upward movement of the lowest conduction band results in that the bandgap of bP-Ni rises from 0.769 to 0.876 eV after SO 2 adsorption. On the other hand, as compared with before SO 2 adsorption, the electron transfers from bP-M systems to SO 2 lead to downward shifts of the Fermi levels for bP-M systems, which may be ascribed to the greater electronegativity of S and O atoms. Except for Ni decorated case, the peaks of O states appear near the Fermi levels, which will introduce the flat bands into the band structures for SO 2 adsorption on bP-Ms (see figure S6). As compared with SO 2 adsorption on the AM systems, the coupling peaks between SO 2 and AEMs are more widely distributed, especially in the conduction band (Fig. 6), which may be a reason of more effectively improving SO 2 adsorption on phosphorene for the AEM decorated cases. Strong hybridized interaction between the states of SO 2 and La at − 0.8 eV in Fig. 6j can unravel a maximal E ads = 2.822 eV for SO 2 adsorption on bP-La.  (Table 1) are larger than the 0.646 eV value of SO 3 on pristine phosphorene. Especially, the adsorption configuration of SO 3 on bP-Tl is similar to that on pristine phosphorene, but the E ads value increases from above value of pristine phosphorene to 1.273 eV due to the Tl atomic decoration. Thus, all the metal decorations significantly improve the SO 3 adsorption on phosphorene. Similar to SO 2 , E ads values of SO 3 on bP-AEM are larger than that on bP-AM, indicating more efficient decorations of AEMs than AMs in improving the SO 3 adsorption on phosphorene. As mentioned above, the more charge transfer signifies the stronger interaction and in turn the larger E ads value 39 . Consequently, the more valence electrons of AEMs and thus electron transfers should be responsible for their larger E ads values. On the other hand, in the same www.nature.com/scientificreports/ group elements, the E ads values decrease with the atomic number in lieu of increasing atomic size. As mentioned before, short adsorption distance is advantageous to the charge transfer 26,27 . The larger atomic size means the larger adsorption distance and thus the more charge transfer, leading to larger interaction and larger E ads value. Additionally, we can see (Table 1) the electrons transfer from bP-Ms to SO 3 in SO 3 adsorbed bP-Ms, in accords to larger S and O electronegativities. In Fig. 8, LDOSs of SO 3 adsorbed bP-Ms display marked effects of SO 3 on the bP-Ms substrates. The LDOSs of bP-AMs and bP-Tl reveal significant spin asymmetry after SO 3 adsorptions, which are duly induced by SO 3 , indicating magnetism in the SO 3 adsorbed bP-AMs and bP-Tl. The SO 3 adsorbed bP-AEMs and bP-Ni have spin symmetrical LDOSs and no magnetism. The magnetic moment of SO 3 adsorbed bP-La is only 0.0003 µ B , which can be ignored with almost spin symmetrical LDOSs. Therefore, the bP-AMs and bP-Tl develop magnetism on SO 3 adsorption, while the bP-La adversely loses the magnetism. The SO 3 adsorbed bP-Li, bP-Na, bP-AEMs, bP-Ni and bP-Tl systems have zero DOSs at the Fermi levels of semiconductor properties. The SO 3 adsorbed bP-K, bP-Rb, bP-Cs and bP-La have non-zero DOSs at the Fermi levels, implying that they possess metal properties. In contrast to before SO 3 adsorption, the bP-Li, bP-Na, bP-Ba and bP-Tl develop metal-to-semiconductor transitions after SO 3 adsorption, while the bP-La develops the semiconductor-to-metal transition. As showed in figure S7 of supporting materials, the effect of SO 3 on the band structures of bP-M systems is the largest among the three gas molecules, likely due to the largest amount of electron transfer from the bP-Ms substrate to SO 3 .

Figure 5. The optimized structures of SO 2 adsorption on (a) bP-Li (b) bP-Na (c) bP-K (d) bP-Rb (e) bP-Cs (f) bP-Ca (g) bP-Sr (h) bP-Ba (i) bP-Ni (j) bP-La (k) bP-
In addition, the SO 3 introduces impurity states into/near bandgaps or into the valence bands. Accordingly, the Fermi levels of bP-Ms shift down differently as a result of a charge transfer from the substrate to SO 3 . As shown in Fig. 8a,b, the highest spin-up states of SO 3 for bP-Li and bP-Na systems are located in valence and bandgap, respectively, and both are below the Fermi levels. This leads to more electron transfer from bP-Li and bP-Na to SO 3 , a way in witch bP-Li and bP-Na have restored to be semiconductor, which is responsible for their metal-to-semiconductor transitions. But for bP-K, bP-Rb and bP-Cs systems, as shown in Fig. 8c-e, the highest spin-up SO 3 states cross through the lowest conduction bands of bP-Ms, leading to less electrons transfer and www.nature.com/scientificreports/ thus metal properties being remained. For bP-AEM, both the highest spin-up and spin-down bands are located near valence band maximum (VBM) and below the Fermi levels, resulting in that the Fermi levels shift down due to electrons transferring from bP-AEM to SO 3 . On the other hand, the AEM − SO 3 interaction weakens the corresponding AEM − phosphorene interactions, and thus the lowest conduction band moves upward and the energy separation between the lowest and the second lowest conduction bands restores to pristine phosphorene value. Consequently, the bP-AEMs show semiconductor properties after SO 3 adsorption, and complete the metal-to-semiconductor transitions. For SO 3 adsorbed bP-Ni system, the bandgap of bP-Ni system increases to 0.882 eV due to the AEM − SO 3 interaction. The VBM is transferred from Ŵpoint to Y point, and thus the bP-Ni exhibits indirect semiconductor properties and experiences a direct-to-indirect transition. For SO 3 adsorbed bP-La system, the metal flat band disappears and thus the lowest conduction band shifts upward owing to the La − SO 3 interaction on phosphorene. This leads to the metal → semiconductor transition of the bP-Tl system after SO 3 adsorption. This may be ascribed to the SO 3 flat bands in bandgap of bP-Tl, which push upward the lowest conduction band. Compared to the LDOS of SO 3 adsorbed bP-AM systems, the coupling peaks between SO 3 − metals are more widely distributed at − 9 eV, − 6 to − 5 eV, − 3 to 0 eV and in the conduction band, which also may be a reason of AEM decoration is more effective in improving the SO 3 adsorption on phosphorene. For SO 3 adsorbed bP-Ni, bP-La and bP-Tl systems, there are many coupling peaks between the states of metal and SO 3 near the Fermi level, which may be responsible for the large interactions between bP-Ms and SO 3 . As compared with other metal cases, more coupling peaks of La and SO 3 are located in the band, contributing to the excellent SO 3 adsorption performance on the bP-La surfaces. The values of magnetization and bandgap obtained in the various samples with H 2 S, SO 2 and SO 3 adsorption on phosphorene of various metal decorations are summarized in Table 2. As mentioned above, a larger E ads value above 1.5 eV is suitable for capturing gas molecules (or single sensing), whereas that near around 1 eV is ideal binding for highly efficient and reversible gas sensors. Therefore, from Table 1, one can see that the phosphorenes with AEM and La decorations can be served as capturing the SO 2 and SO 3 molecules, and that those with Li and Ni decorations can only be utilized as the SO 3 capture. From Table 2, it is observed that the Li, Ca, Sr and Ba decorated phosphorenes can be used as selective single sensing for SO 3 on the basis of increase of bandgap and thus decrease of conductivity after SO 3 adsorption.
The E ads values of H 2 S on K, Rb, Cs and Tl decorated phosphorenes are so small that H 2 S gases are easy to dissociate from substrates, and thus the K, Rb, Cs and Tl decorated phosphorenes are not suitable for H 2 S sensing. In addition to the moderate E ads value, the change in measurable property is necessary for reversible gas sensors. Therefore, the Tl decorated phosphorene is promising to be a selective reversible SO 3 sensor due to the metal-tosemiconductor transition after gas molecule adsorption, while the semiconductor-to-half-metal transition of the La decorated phosphorene makes it be the potential candidate as a selective reversible H 2 S sensor. Interestingly, it is found that the magnetic moment turns positive and negative, respectively, after SO 2 and SO 3 adsorptions, which may be a basis of selectively sensing these gases.

Conclusions
In summary, the adsorption of H 2 S, SO 2 and SO 3 on various metal-decorated phosphorene have been systematically investigated by using DFT. Eleven metals (Li, Na, K, Rb, Cs, Ca, Sr, Ba, Ni, La, Tl), E ads /E coh > 1, are considered. Excepting H 2 S on bP-Tl, a decoration of metals has significantly improved the adsorption of phosphorene to sulfur-based gas molecules, and the order of adsorption capacity is La > AEM > AM. In the analysis of LDOSs, it is found that the phosphorenes with Sr and La decorations undergo the transitions from semiconductor to metal and from semiconductor to half-metal after H 2 S adsorption. The AM, AEM and Tl decorated phosphorenes undergo non-magnetic-to-magnetic transitions, after SO 2 adsorption, while the La decorated phosphorene undergoes magnetic-to-non-magnetic transition. Meantime, the Ca, Sr and La decorated phosphorenes also have transition from semiconductor to metal after SO 2 adsorption. For SO 3 adsorption cases, the AM and Tl decorated phosphorenes obtain non-magnetic-to-magnetic transitions, while the La decorated phosphorene perceive the www.nature.com/scientificreports/ transition from magnetic to non-magnetic. The Li, Na and AEM decorated phosphorenes experience the transitions from metal to semiconductor, while the La decorated phosphorene experiences semiconductor-to-metal transition. On the basis of the criterion of adsorption energy around 1 eV and the changes in properties, the phosphorenes with La and Tl decorations are promising selective reversible sensors for H 2 S and SO 3 detections, respectively. According to whether the magnetic moment is positive or negative, the Cs decorated phosphorene could be a potential selective reversible sensor for SO 3 or SO 2 detection.