Possible Formation of Graphyne on Transition Metal Surfaces: A

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Possible Formation of Graphyne on Transition Metal Surfaces: A Competition with Graphene from the Chemical Potential Point of View Nannan Han, Hongsheng Liu, Si Zhou, and Jijun Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04384 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 13, 2016

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The Journal of Physical Chemistry

Abstract Graphyne (GY), a two-dimensional (2D) allotrope of carbon with mixed sp/sp2 hybridization, is predicted to exist in many stable phases and has recently received great attentions. However, it is energetically less stable than graphene and remains difficult to be synthesized in experiment to date. In this report, the possible environments for synthesis of graphyne on Ru(0001), Rh(111) and Pd(111) substrates are investigated by considering three typical phases of GY (α, β and γ). Their structures, interactions with metal substrates, as well as thermodynamic stability are calculated using first-principles calculations. Chemical potential phase diagram of GYs and graphene on metal substrates are constructed. For all these substrates, the α phase of GY can form in the carbon-poor environment, while formation of graphene dominates in the carbon-rich condition.

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Introduction The boom of graphene stimulates extensive efforts on exploring novel two-dimensional (2D) materials as well as other 2D carbon allotropes. Graphyne (GY), first proposed theoretically by Baughman et al. in 19871, can be viewed as a planar carbon sheet composed of hexagonal rings joint together by acetylenic carbon linkages (−C≡C−). This 2D network consists of both sp and sp2 hybridized carbon atoms, and hence possesses intriguing properties distinct from those of graphene, re-catching great attentions in recent years. GY is predicted to exist in many stable phases, e.g., three typical forms of α-, β- and γ-GYs2−4 with hexagonal symmetry, and 6,6,12-GY5 with rectangular symmetry. Moreover, due to more active sp hybridization, GY can be easily decorated by adatoms and molecules6,7. So far, only GY fragments (i.e., trigonally expanded dehydrobenzo annulenes) have been synthesized by chemical catalyst process in experiment8, while production of extended monolayer of GY remains difficult. Only one group until now, to my certain knowledge, has synthesized large-area nanofilms of 2D graphyne-like carbon allotrope – graphdiyne on Cu foil and ZnO substrates9,10. On the theoretical side, the physical and chemical properties of GY have been intensively investigated. Very excitingly, the electronic band structures of α-, β- and 6,6,12-GY systems resemble graphene with characteristic the Dirac cone2,5. Meanwhile, γ-GY is a semiconductor with direct band gap of 0.5~2.2 eV predicted by different approaches4,11. These carbon allotropes have high carrier mobilities of 104~105 cm2V-1s-1, comparable to that of graphene (~105 cm2V-1s-1)3,12. Furthermore, 3


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the band structure of GY is tunable under different loading conditions. For example, a band gap can be achieved in 6,6,12-GYs by applying in-plane tensile strain13, while the stretched γ-GY opens a band gap increased with biaxial strain but decreased with uniaxial strain14. Due to the presence of acetylenic linkages, the lattice stiffness of GY is weaker than that of graphene, with Young’s modulus in the range of 0.1~0.5 TPa15. The optical adsorption of γ-GY exhibits high anisotropy for the in-plane and out-of-plane polarizations in the low-energy region16. Therefore, GY shows great potentials for electronics, optoelectronics, and sensors. Besides, compared to graphene, GY shows enhanced binding with metal ions, and is promising for hydrogen storage and electrodes in Li ion battery7,17,18. The porous structure of GY also makes it a candidate molecular sieve for gas separation and water desalination19,20. Despite of the versatile potential applications, the fabrication of large area of GY remains an urgent and unsolved issue, which hinders the experimental study of graphyne. Chemical vapor deposition (CVD) is one of the most commonly used methods for large-scale production of 2D materials. Graphene sheets have been successfully synthesized by CVD on various metal substrates such as Cu21, Ni22, Ru23, Rh24, Pd25, Ir26 and Pt27. These metal surfaces can catalyze desorption of carbon precursor, facilitate adsorption and nucleation of carbon atoms, and stabilize the graphene sheets through interlayer binding. Both as 2D carbon allotropes, the structural and chemical properties of GY resemble those of graphene, and thus these transition metal substrates may offer excellent platforms for GY synthesis. 4



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To date, a few theoretical efforts have been made to elucidate the effect of substrates on the growth, stability and physical properties of GY. Lazic and co-workers studied γ-GY on Cu(111), Ni(111) and Co(0001) surfaces, and showed strong interaction between GY and metals with binding energies of 0.1~0.4 eV per carbon atom, a few times larger than that of graphene28. Moreover, the band gap of γ-GY can be modulated by charge transfer from the metal substrates. Ding’s group explored the formation mechanism of carbyne on selected transition metal surfaces, revealing that small carbyne chains on non-active metals like Cu have a curved polyynic (−C≡C−)n structure, while on active metals like Ni they favor a linear polycumulenic (=C=C=)n configuration, and that the sp2 carbon network becomes dominant in the carbon-rich environment29. Hence, they suggested synthesizing GY by self-assembly of the carbyne chains on suitable metals at low temperature. Apparently, choosing proper metal substrates and well controlling the supply of carbon sources are the key for GY synthesis. To further address this crucial issue and give experimentalists a direct guidance, here we for the first time investigate the growth of the three typical GY phases (α, β and γ) on transition metal surfaces: Ru(0001), Rh(111) and Pd(111). Our first-principles calculations show that the GY sheets strongly interact with metal substrates due to the presence of sp C≡C triple bonds, which significantly enhance their energetic stability. According to the chemical potential phase diagram, α-GY becomes more stable than graphene and the other two GY phases in the carbon-poor environment, while formation of graphene is dominant when the carbon source is sufficient. These theoretical results would motivate 5


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experimental fabrication of GY and other 2D allotropes of carbon.

Computational Details Density functional theory (DFT)30 calculations were performed by Vienna ab initio simulation package (VASP) using the planewave basis set with an energy cutoff of 400 eV, the projector augmented wave (PAW)31 potentials, and the generalized gradient approximation (GGA)32 parameterized by Perdew, Burke and Ernzerhof (PBE) for the exchange-correlation functional. To describe the van der Waals (vdW) interactions between GY/graphene and metal substrates, we adopted the semiempirical dispersion-corrected DFT-D3 scheme proposed by Grimme33. For the substrates, we chose Ru(0001) in the hexagonal-close-packing (hcp) structure, Rh(111) and Pd(111) in the face-centered-cubic (fcc) structure, respectively, which are widely used to synthesize graphene in experiment. The in-plane lattice parameters of Ru(0001), Rh(111) and Pd(111) solids are 2.70 Å, 2.68 Å and 2.79 Å, respectively, given by our calculations. To represent the metal surface, we used a three-layer slab model periodic along the two in-plane directions, and a vacuum region of 12 Å for the out-of-plane direction. The Brillouin zones of the supercells were sampled by uniform k-point meshes with spacing of about 0.03/Å. The model structures were optimized by only ionic and electronic degrees of freedom using thresholds for the total energy of 10-4 eV and force of 0.02 eV/Å. During relaxation, the bottom layer of metal atoms was fixed to mimic a semi-infinite metal solid. Based on the optimized structures, the scanning tunneling microscopy (STM) 6



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images of the GYs on metals were simulated based on the Tersoff-Hamann approximation34 with a constant distance of 2 Å above the uppermost carbon atom of the GY layer.

Results and Discussion Structure and interaction of GYs on metals. The atomic structures of α-, β- and γ-GYs on Rh(111) surface are shown in Figure 1. With sp C≡C triple bonds and sp2 C−C bonds alternately arranged in different fashions, α-, β- and γ-GYs present the ratio of sp/sp2 bonds of 0.5, 0.4 and 0.25, respectively. The atomic structures of GYs/graphene on Ru(0001) and Pd(111) surfaces are shown in Figure S1 and Figure S2 of Supporting Information, respectively. For freestanding GY sheets, we adopted the theoretical lattice parameters of the primitive unit cells from literature4, i.e., 6.98 Å, 9.50 Å and 6.88 Å for α, β and γ phases, respectively. Accordingly, the average lengths of C≡C and C−C bonds in these GYs are about 1.23 Å and 1.41 Å, respectively. For comparison, graphene has a C−C bond length of 1.42 Å and a lattice constant of 2.46 Å. The 2D lattice structures of these GY layers can be well matched with the lattices of metal surfaces with largest deviation less than 3%, as summarized by Table 1. For instance, the primitive unit cells of α- and β-GYs are matched with √7×√7 and √13×√13 unit cells of Rh(111), respectively; the 2×2 superlattice of γ-GY fit well with 5×5 unit cells of Ru, Rh, and Pd substrates. The exact lattice constants are given in the Supporting Information.

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Figure 1. The top views and side views of the atomic structures of (a) α-GY, (b) β-GY, (c) γ-GY, and (d) graphene adsorbed on Rh(111) substrate. The C and Rh atoms are indicated in silver and green, respectively. The blue boxes show the primitive unit cells of the freestanding GY/graphene sheets. The orange boxes show the supercells of GYs/graphene on the metal substrates used for calculations. For α-GY and β-GY, their primitive unit cells (shown by the orange boxes) are used for calculations.

Table 1. The lattice compensations of GYs/graphene on the metal substrates. The repeated unit cells of the superlattice of GYs/graphene (the first set numbers) and metals (the second set of numbers) are listed, and the lattice mismatch is indicated in the bracket. Ru (0001)

Rh (111)

Pd (111)

α-GY

1×1, √7×√7 (2.42%)

1×1, √7×√7 (1.65%)

2×2, 5×5 (0.02%)

β-GY

1×1, 2√3×2√3 (1.47%)

1×1, √13×√13 (1.79%)

1×1, 2√3×2√3 (1.80%)

γ-GY

2×2, 5×5 (1.87%)

2×2, 5×5 (2.65%)

2×2, 5×5 (1.41%)

graphene

1×1, √7×√7 (0.99%)

4×4, √13×√13 (1.76%)

3×3, √7×√7 (0.08%)

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Upon relaxation, GYs and graphene on the metal substrates basically maintain the planar structures, but exhibit different degrees of corrugation due to the lattice mismatch and the carbon-metal interaction. Hence, we define an average out-of-plane distortion ξ as

=

∑ ( )

,

(1)

where N is the number of carbon atoms in the supercell, di is the distance of ith carbon atom in GYs/graphene to the metal surface, and ̅ is the average distance of all carbon atoms to the metal surface. As illustrated by Table 2, the three GY sheets show average distance to the metal substrates of 2.0~2.5 Å, which is substantially shorter than that of graphene on the metals surfaces (2.7~3.1 Å). Moreover, the metal-supported GY layers exhibit large out-of-plane distortion of 0.1~0.6 Å. In comparison, graphene is fairly flat on Ru and Pd surfaces with ξ < 0.1 Å, while it shows larger distortion on Rh (ξ ~ 0.3 Å) due to the large compression of the graphene lattice in order to compensate the lattice mismatch with Rh (~1.8%). All these observations suggest stronger interaction between GY sheets and the metal surfaces with regard to the case of graphene.

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Table 2. The structural and energetic information of GYs/graphene on the metal substrates. The average out-of-plane distortion ξ, interlayer distance d between GYs/graphene and the metal surfaces, formation energy ∆H (per carbon atom), adsorption energy Eads (per carbon atom) between the carbon layer and the metal substrate, and the surface density ρ of carbon atoms are listed. Substrates

Ru(0001)

Rh(111)

Pd(111)

Phase

ξ (Å)

d (Å)

∆H (eV)

Eads (eV)

ρ (1/Å2)

graphene

0.03

3.41

-0.04

0.07

0.34

α-GY

0.09

2.23

0.70

0.27

0.16

β-GY

0.57

2.45

0.61

0.26

0.21

γ-GY

0.39

2.38

0.45

0.22

0.26

graphene

0.31

2.73

-0.01

0.03

0.34

α-GY

0.25

2.03

0.63

0.28

0.16

β-GY

0.21

2.14

0.59

0.26

0.19

γ-GY

0.42

2.53

0.49

0.18

0.27

graphene

0.01

3.20

-0.06

0.09

0.33

α-GY

0.43

2.43

0.77

0.20

0.16

β-GY

0.28

2.19

0.68

0.20

0.19

γ-GY

0.19

2.38

0.51

0.16

0.25

To directly characterize the interaction between GYs/graphene and the metal substrates, we define the adsorption energy Eads as

Eads = (EG + Esub − E)/N,

(2)

where EG and Esub are the energies of the GY/graphene sheet and the metal substrate, 10



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respectively; E is the total energy of GY/graphene on a metal substrate; N is the number of carbon atoms in the supercell. By definition, the larger positive Eads means stronger attractive interaction between the GY/graphene layer and the metal surface. As displayed in Table 2, GY sheets show moderate interactions with the metal substrates ranging from 0.16 to 0.28 eV per carbon atom, whereas the attraction between graphene and the metal surfaces is much weaker, i.e., only 0.03~0.09 eV per carbon atom. The strength of Eads increases with the fraction of sp bonds in the GY system, as illustrated in Figure 2a. Thus, α-GY with 50% of sp bonds exhibits strongest attraction with the metal substrates. On the other hand, Ru and Rh substrates interact more strongly than Pd with GYs, attributed to the “pseudo-noble-gas” 4d10 valence electron configuration of Pd. Compared to the adsorption energies on the other metal substrates calculated by Lazic et al., the adsorption energies of γ-GY on Ru, Rh and Pd are larger than that on Cu(111), and lower than that on Ni(111) and Co(0001)28. Moreover, the adsorption energies of GYs in this report lie between that of borophene on Cu(111) (0.19 eV per boron atom)35, and silicene on Ag(111) (0.37 eV per silicon atom)36 from our previous calculations. Note that both borophene and silicene do not quite favor sp2 hybridization in their freestanding form, but they have been successfully synthesized on metal surfaces like Ag(111) and Ir(111) due to the stabilization effect of metal passivation37−39. The same effect should be applicable for GYs on metal surfaces. Overall, these three metal substrates chosen here have moderate activity to stabilize the GY sheets; meanwhile exfoliation of the GYs sheets from these metal surfaces would not be too difficult. 11


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Figure 2. (a) Adsorption energies of GYs/graphene on the metal substrates as a function of the ratio of sp/sp2 bonds. (b) Formation energies of freestanding GYs and graphene sheets, and GYs/graphene on the metal substrates.

To gain further insight into the carbon-metal interactions, the differential charge density between GYs/graphene and Rh substrates are calculated and shown in Figure 3. Pronounced charge transfer occurs in the GY/metal systems − large amount of electrons from the top-layer metal atoms and the carbon atoms accumulate in the GY-metal interfacial region. Barder charge analysis40 shows that the average amount of electrons transferred from the metal surfaces to a carbon atom is 0.06, 0.05, 0.04 and 0.03 for α-, β-, γ-GYs and graphene, respectively, in line with the sequence of 12



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adsorption energies for GYs on Rh(111) presented in Table 2. The charge transfer is ascribed to the presence of acetylenic carbon linkages in GYs. The π bonds in C≡C triple bonds are vulnerable when the GY sheets are adsorbed on the metal surfaces, leading to strong binding between carbon and metal atoms. As the fraction of sp bonds in GY decreases, the charge transfer becomes weaker, and is almost eliminated in graphene with sp2 bonds only (see Figure 3d). Similar situations occur in GYs/graphene on Ru and Pd substrates, as shown in Figure S3 and Figure S4 of Supporting Information, respectively.

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Figure 3. Spatial distributions of differential charge of (a) α-GY, (b) β-GY, (c) γ-GY, and (d) graphene adsorbed on Rh(111) substrate (upper: top view, lower: side view). The C and Rh atoms are indicated in silver and green, respectively. The purple and orange colors represent the regions of electron accumulation and depletion, respectively. The isosurface is 0.007 e/Å3.

Thermodynamic stability and phase diagram of GYs on metals. To describe the energetic stability of the GY/graphene sheets adsorbed on the metal substrates, we define the formation energy ∆H as ∆H = (E − N×EC – Esub) / N,

(3)

where E is the energy of GYs/graphene on the metal substrate, EC is the energy of a carbon atom in graphite, Esub is the energy of the metal substrate, and N is the number of carbon atoms in the GY/graphene sheets. The freestanding α-, β- and γ-GY layers 14



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have ∆H of 0.97 eV, 0.89 eV and 0.67 eV per carbon atom, respectively, much larger than that of graphene of 0.03 eV. After adsorbed on the metal substrates, the energetic stability of the GYs is significantly enhanced with ∆H of 0.45~0.77 eV per carbon atom, which is reduced by over 20% relative to the freestanding sheets, as shown in Table 2 and Figure 2b. For comparison, the change of formation energy of graphene on the metal substrates is very small (i.e., below 0.1 eV per carbon atom) with respect to that of the freestanding sheet. Among the three phases, γ-GY presents lowest ∆H values on various metal surfaces, and it favors adsorption on Ru, while α- and β-GYs prefer adsorption on Rh. To explore the growth competition between GYs and graphene on the metal surfaces, we evaluate their Gibbs free energy of formation ∆G as a function of the chemical potential µC of carbon as41-44 ∆G = (G − Gsub – N × µC) / A,

(4)

where G and Gsub are the Gibbs free energies of GYs/graphene on the metal surface, and the metal substrate, respectively; N is the number of carbon atoms; A is the area of the metal surface within the simulation supercell. Here we use the DFT energies (E − Esub) to approximate (G − Gsub), since the entropy and enthalpy contributions to ∆G are negligible for solids as reported by Reuter45. The synthesis conditions in experiment, such as the type and concentration of carbon source, temperature and pressure, are all reflected in the variation of µC. At a given chemical potential, the system having the lowest ∆G is thermodynamically most stable and will show up in the phase diagram. 15


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Figure 4 plots the ∆G curves of the 2D carbon allotropes on various metal substrates. Only two systems can form on the metal surfaces: graphene under the carbon-rich condition, and α-GY in the carbon-poor environment. The intersection of the ∆G curves of graphene and α-GY gives a critical value of µC, under which condition the two carbon allotropes have the same probability to grow. In Figure 4, the critical µC is –0.65, −0.59 and –0.86 eV for the growth on Ru, Rh and Pd substrates, respectively. Among them, α-GY adsorbed on Rh is energetically more stable than those on the other two metal substrates, and hence shows higher critical µC value that is more accessible in experiment. While α-GY adsorbed on Ru seems also promising since its µC is only slightly lower than that on Rh. Note that, although α-GY on the metal substrates presents higher formation energy than those of β and γ phases (see Table 2), the carbon density of α-GY is much lower — only 0.16/Å2, compared to 0.19~0.27/Å2 for the β and γ phases. As a consequence, on the same area of metal surfaces and exposed to the same condition of carbon source, α-GY shows lower Gibbs free energy for most ranges of µC in Figure 4, and hence its growth is more competitive than the other two phases. Furthermore, an allotrope of graphyne, i.e., γ-graphdiyne that was synthesized on Cu and ZnO surfaces by Zhu9,10, is also considered for comparison. The phase diagrams of γ-graphdiyne on the present metal substrates are displayed in Figure S5 of Supporting Information. The ∆H of γ-graphdiyne of 0.540 eV, 0.658 eV and 0.641 eV on Ru, Rh and Pd surfaces are similar to those of α-GY (in Table 2). However, due to the higher carbon density of graphdiyne of 0.2/Å2 (compared to 0.16/Å2 of α-GY), the formation of grapdiyne on 16



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Ru(0001), Rh(111) and Pd(111) surfaces is more difficult according to the chemical potential phase diagram (Figure S5).

Figure 4. Chemical potential phase diagrams of GYs/graphene on the metal substrates. The colored lines show the Gibbs free energy of formation as a function of the carbon chemical potential for GYs and graphene. The zero point of µC is referred to the chemical potential of a carbon atom in graphite. The regions filled by pink and light blue colors indicate the ranges of µC, where α-GY and graphene have lowest ∆G, respectively.

To gain further insights into the possible growth environment in experiments, we estimate the critical pressures at the critical µC corresponding to temperature range of T = 800~1500 K, which are the typical temperatures for CVD growth of graphene46−48. To correlate the chemical potential of carbon source with a specific gas pressure, here 17


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we assume methane as a representative for carbon source since it is commonly used for graphene synthesis. Then the chemical potential of carbon as defined by Zhang et al.49 is given by µC = µCH4 − 2µH2,

(5)

where µCH4 is the chemical potential of a gaseous CH4 molecule; µH2 is the chemical potential of a gaseous H2 molecule as a by-product of the dehydrogenation process during synthesis. The chemical potential for an ideal gas can be written as

)

(, ) = + ! (, " ) + # $% &' () +,

(6)

*

where T and p are the temperature and pressure of the gas, respectively; Egas is the DFT energy of the gaseous molecule; ! is the change of Gibbs free energy of the gaseous molecule from 0 K to T at a constant pressure p0 (taking p0 = 1 bar), and can be obtained from the NIST-JANAF thermodynamics table50; kB is the Boltzmann factor. Combining Eq. (5) and (6) yields

, (, ) = (,-. − 2-# )123 + (!,-. − 2!-# ) 3,)* + # $% &' (

)456 )* )5

+.

(7)

Based on the critical value of µC from the phase diagram in Figure 4, the dependency of pCH4 on pH2 can be extracted by Eq. (7). According to our calculations, formation of α-GY on the metal substrates requires a fairly low chemical potential of carbon, which can be realized by using a sparse CH4 environment, and meanwhile maintaining a relatively high partial pressure of H2. Among the systems considered, the growth of α-GY on Rh has highest critical value of µC, thus its pH2 vs. pCH4 curves are plotted in Figure 5. The detailed derivation of critical pressure curve is given in the Supporting Information. Considering the 18



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typical partial pressure of CH4 used in experiment for graphene synthesis ~ 2×10−3 bar51, the corresponding partial pressures of H2 are 0.08 bar at T = 800 K, 0.55 bar at T = 1000 K, and so on. In other words, α-GY could be obtained on Rh substrates under a low partial pressure of the CH4 source and a H2-rich environment.

Figure 5. The partial pressure of CH4 vs. the partial pressure of H2 at various temperatures for the critical condition of the growth of α-GY on Rh(111). The dashed black lines indicate the typical pressure of CH4 (2×10-3 bar) used for experimental synthesis of graphene, and the corresponding pressure of H2 (A: 0.08 bar, B: 0.55 bar).

STM images of α-GY on metals. To help identify possible GYs in future experiments, the STM images of GY on the three substrates are simulated. As the α phase of GY is more favorable than the β and γ phases, and its growth can be realized under accessible conditions, here we only simulate the STM of α-GY, as shown in Figure 6. On Ru(0001), honeycomb lattice similar to graphene can be seen in the STM image (Figure 6a) due to the small out-of-plane distortion of about 0.25 Å. 19


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However, the side length of the hexagonal ring (~ 4.13 Å) is much larger than that of graphene (~1.42 Å). Thus, it would be easy to distinguish GY from graphene on Ru substrate from their STM images. In comparison, for α-GY on Rh(111), the STM image shows series of zigzag lines from the original hexagonal lattice, since the carbon atoms along these zigzag lines (highlighted in Figure 6b) are about 0.64 Å higher than other carbon atoms. As for α-GY on Pd(111), only two bright triangles per 2×2 unit cell can be seen in the STM image due to the large buckled height (1.12Å) of these carbon atoms (highlighted in Figure 6c). Overall speaking, α-GY on various metal substrates can be well distinguished from graphene according to their STM images.

Figure 6. The simulated STM images of α-GY on (a) Ru(0001), (b) Rh(111), and (c) Pd(111), respectively, using a bias voltage of −2 V. The top views of the atomic structures of α-GY on the three metal substrates are shown below each STM image. The C, Ru, Rh and Pd atoms are indicated in silver, blue, green, and wine, respectively. The yellow balls indicate the C atoms, which are buckled up in the 20



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out-of-plane direction and hence show larger brightness in the STM images.

Conclusions To summarize, we investigate the structure, adsorption, and thermodynamic stability of α, β and γ phases of graphyne on Ru(0001), Rh(111) and Pd(111) surfaces using first-principles calculations. Freestanding GY sheets are less stable compared to monolayer graphene, while adsorption of GYs on the transition metal surfaces significantly enhances their thermodynamic stability. These GY sheets show strong interactions with the metal substrates due to the presence of sp hybridized acetylenic linkages, with adsorption energies a few times larger than that of graphene. The interlayer binding between GYs and the metal substrates reduces the formation energies of GYs by 0.16~0.34 eV, such that the growth of GYs is competitive to that of graphene on the metal surfaces. Chemical potential phase diagrams of GY (α, β and γ) and graphene on these metal substrates are constructed to explore the possible synthesis conditions of GY. The α phase of GY is thermodynamically most favorable in the carbon-poor environment, while formation of graphene is dominant in the carbon-rich condition. These theoretical results provide vital guidance for experimental synthesis of GYs as well as other 2D carbon allotropes, and would advance the fabrication of GY-based devices and their technological applications.

Supporting Information Available. The configurations of GYs/graphene on Rh(111) and Pd(111) surfaces (S1 and S2)， 21


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the charge difference between GYs/graphene and the surfaces (S3 and S4), the chemical potential phase diagrams of α-GY/graphdiyne/graphene (S5), the exact lattice constants of GYs/graphene and the derivation of critical pressure curve. The material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was supported by the National Natural Science Foundation of China (11504041, 11574040), the China Postdoctoral Science Foundation (2015M570243), the Fundamental Research Funds for the Central Universities of China (DUT15RC(3)014, DUT16LAB01), and the Scientific Research Fund of Department of Education of Liaoning Province (L2015124). The authors thank Dr. Wenbo Li for his helpful suggestion and comments.

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References (1) Baughman, R.; Eckhardt, H.; Kertesz, M. Structure property predictions for new planar forms of carbon: Layered phases containing sp2 and sp atoms. J. Chem. Phys. 1987, 87, 6687-6699. (2) Malko, D.; Neiss, C.; Viñes, F.; Görling, A. Competition for graphene: graphynes with direction-dependent dirac cones. Phys. Rev. Lett. 2012, 108, 086804. (3) Xi, J.; Wang, D.; Shuai, Z. Electronic properties and charge carrier mobilities of graphynes and graphdiynes from first principles. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2015, 5, 215-227. (4) Kim, B. G.; Choi, H. J. Graphyne: Hexagonal network of carbon with versatile Dirac cones. Phys. Rev. B 2012, 86, 115435. (5) Cao, J.; Tang, C.; Xiong, S-J. Analytical dispersion relations of three graphynes. Physica B 2012, 407, 4387-4390. (6) Ma, D.; Li, T.; Wang, Q.; Yang, G.; He, C.; Ma, B.; Lu, Z. Graphyne as a promising substrate for the noble-metal single-atom catalysts. Carbon 2015, 95, 756-765. (7) Zhang, H.; Zhao, M.; He, X.; Wang, Z.; Zhang, X.; Liu, X. High mobility and high storage capacity of lithium in sp–sp2 hybridized carbon network: the case of graphyne. J. Phys. Chem. C 2011, 115, 8845-8850. (8) Tahara, K.; Yamamoto, Y.; Gross, D. E.; Kozuma, H.; Arikuma, Y.; Ohta, K. et al. Syntheses and properties of graphyne fragments: trigonally expanded dehydrobenzo annulenes. Chem. A. Eur. J. 2013, 19, 11251-11260. (9) Li, G.; Li, Y.; Liu, H.; Guo, Y.; Li, Y.; Zhu, D. Architecture of graphdiyne nanoscale films. Chem. Comm. 2010, 46, 3256-3258. (10) Qian, X.; Liu, H.; Huang, C.; Chen, S.; Zhang, L.; Li, Y.; Wang, J.; Li, Y. Self-catalyzed growth of large-area nanofilms of two-dimensional carbon. Sci. Rep. 2015, 5, 7756. (11) Narita, N.; Nagai, S.; Suzuki, S.; Nakao, K. Optimized geometries and electronic structures of graphyne and its family. Phys. Rev. B 1998, 58, 11009. 23


Page 22 of 28

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Chen, J.; Xi, J.; Wang, D.; Shuai, Z. Carrier mobility in graphyne should be even larger than that in graphene: a theoretical prediction. J. Phys. Chem. Lett. 2013, 4, 1443-1448. (13) Wang, G.; Si, M.; Kumar, A.; Pandey, R. Strain engineering of Dirac cones in graphyne. Appl. Phys. Lett. 2014, 104, 213107. (14) Yue, Q.; Chang, S.; Kang, J.; Qin, S.; Li, J. Mechanical and electronic properties of graphyne and its family under elastic strain: theoretical predictions. J. Phys. Chem. C 2013, 117, 14804-14811. (15) Zhang, Y.; Pei, Q.; Wang, C. Mechanical properties of graphynes under tension: a molecular dynamics study. Appl. Phys. Lett. 2012, 101, 081909. (16) Kang, J.; Li, J.; Wu, F.; Li, S.; Xia, J. Elastic, electronic, and optical properties of two-dimensional graphyne sheet. J. Phys. Chem. C 2011, 115, 20466-20470. (17) Zhang, L.; Wang, N.; Zhang, S.; Huang, S. The effect of electric field on hydrogen storage for B/N-codoped graphyne. RSC Adv. 2014, 4, 54879-54884. (18) Li, C.; Li, J.; Wu, F.; Li, S.; Xia, J.; Wang, L. High capacity hydrogen storage in Ca decorated graphyne: a first-principles study. J. Phys. Chem. C 2011, 115, 23221-23225. (19) Zhang, H.; He, X.; Zhao, M.; Zhang, M.; Zhao, L.; Feng, X.; Luo, Y. Tunable Hydrogen

Separation

in

sp–sp2

Hybridized

Carbon

Membranes:

A

First-Principles Prediction. J. Phys. Chem. C 2012, 116, 16634-16638. (20) Zhu, C.; Li, H.; Zeng, X.; Wang, E.; Meng, S. Quantized water transport: ideal desalination through graphyne-4 membrane. Sci. Rep. 2013, 3, 3163. (21) Hao, Y.; Bharathi, M.; Wang, L.; Liu, Y.; Chen, H.; Nie, S. et al. The role of surface oxygen in the growth of large single-crystal graphene on copper. Science 2013, 342, 720-723. (22) Zhang, Y.; Gomez, L.; Ishikawa, F. N.; Madaria, A.; Ryu, K.; Wang, C.; Badmaev, A.; Zhou, C. Comparison of graphene growth on single-crystalline and polycrystalline Ni by chemical vapor deposition. J. Phys. Chem. Lett. 2010, 1, 3101-3107. (23) Zhang, H.; Fu, Q.; Cui, Y.; Tan, D.; Bao, X. Growth mechanism of graphene on 24



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ru (0001) and O2 adsorption on the graphene/Ru (0001) surface. J. Phys. Chem. C 2009, 113, 8296-8301. (24) Sicot, M.; Bouvron, S.; Zander, O.; Rüdiger, U.; Dedkov, Y. S.; Fonin, M. Nucleation and growth of nickel nanoclusters on graphene Moiré on Rh (111). Appl. Phys. Lett. 2010, 96, 093115. (25) Murata, Y.; Starodub, E.; Kappes, B.; Ciobanu, C.; Bartelt, N.; McCarty, K.; Kodambaka, S. Orientation-dependent work function of graphene on Pd (111). Appl. Phys. Lett. 2010, 97, 143114. (26) Coraux, J.; Plasa, T. N.; Busse, C.; Michely, T. Structure of epitaxial graphene on Ir (111). New J. Phys. 2008, 10, 043033. (27) Feng, X.; Wu, J.; Bell, A. T.; Salmeron, M. An Atomic-Scale View of the Nucleation and Growth of Graphene Islands on Pt Surfaces. J. Phys. Chem. C 2015, 119, 7124-7129. (28) Lazić, P.; Crljen, Ž. Graphyne on metallic surfaces: A density functional theory study. Phys. Rev. B 2015, 91, 125423. (29) Yuan, Q.; Ding, F. Formation of carbyne and graphyne on transition metal surfaces. Nanoscale 2014, 6, 12727-12731. (30) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169. (31) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (33) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104. (34) Tersoff, J.; Hamann, D. Theory and application for the scanning tunneling microscope. Phys. Rev. Lett. 1983, 50, 1998. (35) Liu, H.; Gao, J.; Zhao, J. From Boron Cluster to Two-Dimensional Boron Sheet on Cu (111) Surface: Growth mechanism and hole formation. Sci. Rep. 2013, 3, 25


Page 24 of 28

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3238. (36) Liu, H.; Gao, J.; Zhao, J. In Silicene on substrates: interaction mechanism and growth behavior. J. Phys. Conf. Ser. 2014, 494, 012007. (37) Vogt, P.; Padova, P.; Quaresima, C.; Avila, J.; Frantzeskakis, E.; Asensio, M.; Resta, A.; Ealet, B.; Lay, G. L. Silicene: Compelling experimental evidence for graphenelike

two-dimensional

silicon.

Phys.

Rev.

Lett.

2012,

108,

155501-155505. (38) Meng, L.; Wang, Y.; Zhang, L.; Du, S.; Wu, R.; Li, L.; Zhang, Y.; Li, G.; Zhou, H.; Hofer, W. A.; Gao, H. Buckled silicene formation on Ir(111). Nano. Lett. 2013, 13, 685-690. (39) Mannix, A.; Zhou, X.; Kiraly, B.; Wood, J.; Alducin, D.; Myers, B. et al. Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs. Science 2015, 350, 1513-1516. (40) Henkelman, G.; Arnaldsson, A.; Jónsson, H. A fast and robust algorithm for Bader decomposition of charge density. Comp. Mate. Sci. 2006, 36, 354-360. (41) Duan, X.; Warschkow, O.; Soon, A.; Delley, B.; Stampfl, C. Density functional study of oxygen on Cu (100) and Cu (110) surfaces. Phys. Rev. B 2010, 81, 075430. (42) Geisler, B.; Kratzer, P.; Suzuki, T.; Lutz, T.; Costantini, G.; Kern, K. Growth mode and atomic structure of MnSi thin films on Si(111). Phys. Rev. B 2012, 86, 115428. (43) Pflugradt, P.; Matthes, L.; Bechstedt, F. Silicene-derived phases on Ag(111) substrate versus coverage: Ab initio studies. Phys. Rev. B 2014, 89, 035403. (44) Wei, W.; Dai, Y.; Huang, B.; Whangbo, M.; Jacob, T. Loss of linear band dispersion and trigonal structure in silicene on Ir(111). J. Phys. Chem. C 2015, 6, 1065-1070. (45) Reuter, K.; Scheffler, M. Composition, structure and stability of RuO_2 (110) as a function of oxygen pressure. Phys. Rev. B 2001, 65, 035406. (46) Kondo, D.; Stato, S.; Yagi, K.; Harada, N.; Stato, M.; Nihei, M.; Yokoyama, N. Low-temperature synthesis of graphene and fabrication of top-gated field dffect 26



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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transistors without using transfer processes. App. Phys. Express. 2010, 3, 0251021-0251023. (47) Juang, Z.; Wu, C.; Lo, C.; Chen, W.; Huang, C.; Hwang, J.; Chen, F.; Leou, K.; Tsai, C. Synthesis of graphene on silicon carbide substrates at low temperature. Carbon 2009, 47, 2026-2031. (48) Zhang, Y.; Zhang, L.; Zhou, C. Review of chemical vapor deposition of graphene and related applications. Accounts. Chem. Res. 2012, 46, 2329-2339. (49) Zhang, W.; Wu, P.; Li, Z.; Yang, J. First-principles thermodynamics of graphene growth on Cu surfaces. J. Phys. Chem. C 2011, 115, 17782-17787. (50) Chase, M. NIST-JANAF Thermochemical Tables; American Institute of Physics; New York, 1998. (51) Wang, B.; Zhang, Y.; Zhang, H.; Chen, Z.; Xie, X.; Sui, Y.; Li, X.; Yu, G.; Hu, L.; Liu,

X.

Wrinkle-dependent

hydrogen

etching

of

chemical

deposition-grown graphene domains. Carbon 2014, 70, 7075-7080.

27


vapor

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Possible Formation of Graphyne on Transition Metal Surfaces: A

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