Comparative Study of C3N- and Graphene-Supported Single-Atom Pt

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C: Physical Processes in Nanomaterials and Nanostructures 3

A Comparative Study of CN- and Graphene-Supported Single-Atom Pt Bowen Yang, and Zhaoming Fu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12142 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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The Journal of Physical Chemistry

A Comparative Study of C3N- and Graphene-Supported Single-Atom Pt Bowen Yang and Zhaoming Fu* College of Physics and Materials Science, Henan Normal University, Xinxiang, Henan, 453007, China *E-mail: [email protected]

ABSTRACT: Substrate-supported single atoms have been of interest in many fields, such as magnetism, semiconductors, catalysis, and others. As far as single-atom catalysts (such as Pt, Pd, and Au atoms) are concerned, the substrates play vital roles for the stability and activity of single-atom states. Carbon-based two-dimensional material (such as graphene) is one sort of potential substrates for single-atom catalysis. Recently, C3N, as a new carbon-based two-dimensional material, has been synthesized and attracted broad attention. We use single-atom Pt load as a probe to compare the differences of graphene and C3N substrates. It is found that C3N substrates have interesting physicochemical properties and significant superiorities. On graphene, the aggregation of Pt atoms easily occurs. But on C3N, Pt can remain separated to each other with a single-atom state even at a high temperature. On graphene single-atom Pt is always positively charged (Pt ＋ ). In contrast, both Pt— and Pt ＋ are observed on perfect and defective C3N respectively, which significantly affect molecule adsorption. Our calculations suggest that different gas molecules prefer to different charge states of Pt. Additionally, Pt-modified defective graphene and C3N are confirmed to have different magnetism. The former is paramagnetic; the latter, conversely, has nonzero magnetic moment. These are instructive to design single-atom nanomaterials and nanodevices.

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1. INTRODUCTION Single-atom modified two-dimensional (2D) nanomaterials exhibit different transport,1 magnetism,2-3 catalysis,4 and other behaviors,5-6 compared to pristine ones. Interactions between supports and adsorbates can change the physical and chemical characteristics of both 2D monolayer substrates and single adatoms. Therefore, single-atom modification has attracted extensive attentions for designing functional film materials in both theory and experiments. Especially for catalysis, single atoms supported by substrates can maximize the utilization of supported noble-metal catalysts by exposing single metal atom to reactants. In practical applications, the stability of single-atoms on 2D monolayer is a vital issue. Experimentally, fabrication of stable single atoms on 2D substrates remains a challenge because the isolated atoms often aggregate into cluster on substrates.7 In theory, Zhou et al. studied the self-assembly of metal atoms on graphene.8 In order to inhibit aggregation, the single atom must be anchored onto 2D substrates for being stabilized at a certain temperature.

However, the single-atom

stability often exhibit stronger selectivity for substrates, compared to nanoparticles or clusters. In recent years, graphene-supported metal single-atoms have been extensively studied, which display rich features and functionality, such as induced magnetism,3, 9 controlled quantum transport, and promising catalysis.10 On the other hand, clustering behaviors of the single atoms must be seriously considered at certain temperatures. The defect-free graphene has relatively weak bonding to single-atom metals,11-12 which lead to the aggregation of single atoms and therefore limit its use. C3N is a new carbon-based 2D material with the similar hexagonal structure, which can be regarded as one kind of ordered N-doped graphene (see Figure 1a for graphen and (b) for C3N). Therefore, C3N inherits the advantages of graphene such as the excellent flexibility and stability. Importantly, it also possesses some novel electronic properties, which lead to very different physics and chemical behaviors. Firstly, C3N is semiconductor with band gap of 0.39 eV,13 different from the zero band-gap graphene. Secondly, C3N has a smaller workfunction than graphene,14-15 which would 2


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significantly affect adsorbed single-atom. Thirdly, the non-equivalence of C and N could provide an unsmooth potential surface of single atoms compared to graphene, which may efficiently inhibit the adatom migration and then avoid the clustering. These remarkable characteristics indicate that C3N is a possible candidate of substrate materials of single atoms, and inspire us to study the relevant physical and chemical properties. In this work we first compare the bonding strength of VIII (Ni, Pd, Pt) and IB (Cu, Ag, Au) main group metals on graphene and C3N substrates, and point out that substrate effects on the stability of single-atom Pt is the most pronounced. Therefore, we further focus on the differences of physicochemical properties of graphene- and C3N-supported Pt, such as charge states, clustering behavior, defect effect, magnetism. Lastly, adsorbing activity of Pt/graphene and Pt/C3N for gas molecules is investigated. These will provide the theoretical help and instruction for really single-atom design. 2. CALCULATION METHODS First-principles calculations were carried out within the framework of density functional theory using Vienna ab initio simulation package (VASP).16-17 The exchange-correlation potential was treated with the generalized gradient approximation of Perdew-Burke-Enzerhof (PBE).18 The ion-electron interaction was described by the projector-augmented wave (PAW) method.19 The cutoff energy for plane-wave basis was chosen to be 500 eV, and the Brillouin zone was sampled by a 20 × 20 × 1 Monkhorst-Pack k-point mesh. A 2 × 2 C3N supercell containing 32 atoms was constructed, the corresponding supercell size is equivalent to that of 4×4 graphene supercell. To examine the stability of Pt supported on C3N, ab initio molecular dynamics (MD) simulation from 400 K to 800 K were performed with the VASP software, for which the larger 4×4 and 6×6 supercells containing 128 and 288 atoms were modeled. The convergence criteria for energy and force were set to 10-6 eV and 0.02 eV/Å, respectively. The vacuum layer was set to be 15 Å in the z direction to avoid the interaction between adjacent monolayers. The van der Waals (vdW) interactions are 3



considered using the DFT-D3 method.20

3. RESULTS AND DISCUSSION In order to improve the single-atom stability on substrates, the bonding between substrate and adatoms, as well as the diffusion of adatoms on surface must both be considered. The former plays a role for hindering desorption of adsorbed atoms, described by adsorption stability; the latter is responsible for inhibiting the in-plane aggregation, described by separation stability. These two kinds of stability correspond to two different freedom degrees, perpendicular and parallel to substrates respectively. Two aspects decide the single-atom stability together. We firstly investigate the bonding strength of VIII (Ni, Pd, Pt) and IB (Cu, Ag, Au) main group metals on graphene and C3N. To obtain the most stable adsorption structure, we consider various highly symmetric adsorption sites on graphene and C3N, as denoted by the serial number 1-6 in Fig. 1a and 1b.

Figure 1. Structures of unit cell (1×1) of graphene (a) and C3N (b), where 1, 2, 3, 4, 5, and 6 represent C-top, C-C bridge, C-hollow, N-top, C-N bridge, and N-hollow sites respectively; (c) Bonding energies of the atoms of VIII and IB group on 4×4 graphene (black line) and 2×2 C3N (blue line). 4


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The bonding energies (Eb) of the most stable adsorption are displayed in Figure 1c, which is defined as Eb = Eadatom/substrate ― Esubstrate ― Eadatom. Eadatom/C3N, Esubstrate, and Eadatom are the energies of the adatom/substrate, free substrate, and free atom respectively. All adsorption configurations and bondlength details can be found in the Supplementary Materials (see Figure S1). The results in Figure 1c suggest that all these single-atom metals always have stronger interaction with C3N (blue lines) than graphene (black lines), indicating the better adsorption stabilities on C3N than on graphene, especially for Pt with an energy difference of about 0.84 eV. In addition, the bonding energy curves in Figure 1c take on obvious regularity and significant dependency to main group and period. On graphene and C3N, VIII group metals have the better stabilities than IB group ones, which could be attributed to the different dorbital occupation. The former d-orbitals such as Ni_3d and Pt_5d are partiallyoccupied, but the latter ones (Cu_3d, Ag_4d, Au_5d) are filled up. The same singleatom on different substrates (C3N and on graphene) have different Eb. We denote this energy difference by ΔEb. It is found that the ΔEb for 5d metals such as Pt are significant larger than those for 3d and 4d metals. The possible reason will be discussed below. These results indicate d-orbital electrons will play an important role in bonding strength and possible stabilities. According to Figure 1c, the substrate effect on Pt adsorption is the most pronounced with the largest change of bonding energies from graphene to C3N substrate. Therefore we will focus on graphene- and C3N-supported single atom Pt next. For the adsorption of single atom Pt on C3N, the optimized structures suggest that there only exist one stable adsorption sites, i.e. C-C bridge sites, displayed in Figure S2a. The corresponding 𝐸𝑏 are -2.694 eV, much larger than that on graphene, meaning the better adsorption stability than on graphene. Just as mentioned above, the stability of single atom states also depends on the migration flexibility of adatoms, which is even more significant than the strong adsorption on substrates. Therefore, the MD simulations are performed for Pt on graphene and C3N, to investigate the thermodynamics stabilities of single-atom states. The simulation time is 150 ps for each 5



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system. The dynamic evolutions of configurations can be seen in Figure S3 (Supplementary Materials). To depict the single-atom stability, we define the rootmean-square deviations (RMSD) of adatom positions by the formula:

RMSD =

[

𝑡′

1 𝑡′ ― 𝑡0 𝜏

𝑁

∑ ∑(𝒓

𝑡,𝒊

∙𝑁

]

2

2

― 〈𝒓𝒊〉)

𝑡 = 𝑡0𝑖 = 1

1

Here 𝒓𝑡,𝒊 is the position of the ith Pt atom in the configuration at time 𝑡, 〈𝒓𝒊〉 is the mean value of positions of ith Pt over time, N is the number of Pt atoms in supercell, and 𝜏 is the time step. The calculated RMSDs at varying temperatures are given in Figure 2a based on MD results from 0ps to 150 ps. Obviously, the large RMSD corresponds to a long-range migration of single-atom Pt, indicating easier diffusion and instability of single-atom states.

6


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Figure 2. RMSD of MD simulations on graphene and C3N (a), potential energy surfaces of Pt atoms on graphene (b) and C3N (c). The color bar represents the energy values (eV), where the energy minimum corresponding to stable adsorption is calculated by structural optimization; the energy of saddle point is calculated by CINEB method;21 and the energy of unstable point is calculated by constrained dynamics method.

The black and blue lines correspond to graphene and C3N substrates. The results in Figure 2a suggest that C3N-supported single-atom Pt are very stable and do not tend to 7



form Pt-cluster even at a high temperature. In contrast, on graphene substrate the migration of single-atom Pt is very flexible even at an intermediate temperature (600 K), which reflects the remarkable property difference between two substrates. In order to understand this difference, we calculated the potential energy surface of single-atom Pt on two substrates, as shown in Figure 2b and c. It is found that, in C3N, the ordered doping of N atoms can fundamentally change the structure of potential energy surface and form very deep potential well (see Figure 2c). These deep potential wells can trap the Pt atoms and inhibit the diffusion and aggregation, compared to that of graphene (see Figure 2b). We analyzed the dynamic trajectories of a single-atom Pt at 800 K on two substrates, shown in Figure S4 (Supplementary Materials). The long-range diffusions of Pt atoms are observed clearly on graphene. However, on C3N Pt atoms only have a local motion constrained by the deep potential well. Bader charge analysis shows that graphene-supported Pt atom is positively charged with a very small Bader charge of +0.07e. In contrast, the C3N-supported Pt is negative charged with a Bader charge of -0.15e, qualitatively different from the case on graphene. The electron transfer from C3N to Pt atom is clearly displayed by charge density difference (CDD), shown in Figure S5a. This difference is understandable according to the work functions of C3N and graphene, the calculated work function of C3N (3.1 eV) is obviously smaller than that of graphene (4.5eV), i.e., introducing N reduce the work function of substrate, which is responsible for the qualitative variance of charge transfer. As known, 5d metals (Au and Pt) possess larger electronegativity, and then have stronger ability obtaining electrons than 3d and 4d metals. On the other hand, the significant decrease of work function, from graphene to C3N, means the electrons of substrate more easily flow to adatoms. Therefore, 5d metals (Au and Pt) are more sensitive to changing substrate from graphene to C3N. Correspondingly, the changes of bonding energies are more remarkable for Au and Pt with 5d electrons (see Figure 1c). In addition, the electronic structures are also calculated to help us understand the enhancement of adsorption strength on C3N. Figure 3a shows Pt on C3N have a deeper d-electron energy level than that on graphene, indicating the lower energy and better 8


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adsorption stability.

Figure 3. (a) Density of states (DOSs) of Pt on perfect graphene and C3N, (b) DOS of Pt on defective graphene and C3N, in which the blue curve is the projected DOS of Pt_5d electrons.

For 2D materials, Defects are often formed spontaneously in preparation process. The defect contents can even be controlled by the synthetic method. Therefore, studying the Pt adsorption on defective C3N is of practical significance. In present work both Cand N-defective C3N are taken into account. Our calculations suggest Pt atoms prefer to occupy the C- and N-vacancy with the Eb of -5.703 eV and -5.819 eV, much larger than that on defective-free C3N. This indicates Pt atoms would be trapped by vacancies. Therefore the defects also efficiently hinder the in-plane migration of adatoms and further inhibit clustering of single-atoms. Interestingly, Bader charge analysis suggests that Pt adsorbed on vacancies is positively charged with +0.16 e. The charge transfer can be observed by CDD (see Figure S5b). Comparing with the adsorbing on pristine C3N, the charge states are 9



reversed from the negatively charged to positively charged. According to the calculated DOS in Figure 3b, it can be seen that Pt embodied in defective C3N (or graphene) has much more empty states in 5d orbitals above Fermi level (blue curves), compared to that on perfect C3N (see Figure 3a). This accounts for the positively charged state. In addition, we use 2×2 supercells with a C or N vacancy to calculate the work functions of defective C3N. The calculated values are 3.6 and 3.8 eV for C- and N-defective C3N respectively, larger than that of perfect system (3.1 eV), which could also be responsible for the inversion of charge transfer from the negatively-charged Pt on perfect C3N to the positively-charged Pt on defective C3N. Here we highlight, for single-atom Pt on graphene and C3N, the charge states are quite different. On graphene substrates, Pt is always positively charged whenever the substrate is defective or not. In addition, it is also found that, for defective graphene and C3N substrates, the magnetic properties of adsorption systems are completely different from each other. Pt embodied in graphene is paramagnetic; however, Pt on N-deficient C3N has a nonzero magnetic moment (1 μB), which are reflected by the difference of spin-up and spin-down DOS in Figure 3b. Taking single-atom catalysts into account, an ideal substrate of single atoms often plays a double role. One is to stabilize single atoms and inhibit the aggregation, as discussed above; the other is to remain and enhance the catalytic activity of single atoms. Previous studies have pointed out that the charge states of single atom could be responsible for adsorption and catalysis activity in different reactions.22-24 In this work we focus on the effects of charge states on adsorption activity. For graphene substrate, single-atom Pt is stable only on defective graphene with a positively charged state because of the existence of vacancies. However, on C3N the positively and negatively charged Pt can both be prepared by controlling the defects of substrates, which provides an opportunity to investigate the relation of adsorption activities and charge states. The adsorption energies of five gas molecules (H2, O2, CO, NO, NH3) on both Pt- and Pt+ are calculated. The values are listed in Table 1. The adsorption activities of Pt+ and Ptare confirmed to be quite different, which could be related to the different electronic structures according to the DOS of Pt/C3N and Pt/C3N1-x in Fig. 3. All adsorption 10


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configurations details can be found in the Supplementary Materials (see Figure S6). Table 1. Adsorption Energies (eV) of Gas Molecules (H2, O2, CO, NO, NH3) on Pt/C3N, Pt/C3N1-x, and Pt/Graphene with C Defects.

Pt/C3N Pt/C3N1-x

H2

O2

CO

NO

NH3

-1.405

-1.378

-2.665

-2.268

-1.482

(Pt+) -1.882

-2.849

-2.203

-2.732

-1.306

-0.119

-1.391

-1.192

-1.504

-0.829

(Pt—)

Pt/defective graphene (Pt+)

Table 1 shows different gas molecules prefer to single-atom Pt with different charge states. For H2, O2 and NO, Pt+ presents larger adsorption activities than Pt—; for CO and NH3, Pt— has a stronger adsorption ability than Pt+. These results are very important to understand the catalytic mechanisms of single atoms. Taking CO oxidation as an example, For Pt/C3N1-x catalysts, O2 has a stronger adsorption on Pt-top site than CO, therefore single-atom Pt will be covered by O2 preferentially. The gas-phase CO molecules will directly react with adsorbed O2, corresponding to Eley-Rideal (ER) mechanism,25 For Pt/C3N catalysts, Pt will be covered by CO preferentially due to the larger adsorption energy of CO, which would lead to Langmuir-Hinshelwood (LH) mechanisms of CO oxidation,26 as well as other possible mechanisms.4 For the adsorption activity of graphene-supported single-atom Pt, only defective graphene substrate is taken into account because Pt single atoms on pristine graphene are thermally unstable. On defective graphene single atom Pt is positively-charged, similar to that on defective C3N. The calculated adsorption energies of gas molecules are given in the third line of Table 1. It is found that, for all five gas molecules, the adsorption strength is obviously weaker than that on C3N-supported Pt, suggesting C3N substrate not only strengthen the single-atom stability but also enhance their chemical 11



activity adsorbing gas molecules.

4. CONCLUSION In summary, we performed a comparative study on C3N- and graphene-supported single-atom Pt by first-principles calculations. For the stability of single-atom states, MD simulations suggest that, single-atom Pt on perfect graphene have a large migration flexibility and therefore trend to form cluster. However, on C3N the single-atom state of Pt is quite stable even at a high temperature. The corresponding mechanism is illustrated. For the charge and magnetism of single-atom Pt, the defects of C3N substrates can qualitatively change the charge states of Pt atoms. Pt atoms are negatively charged on perfect C3N, and positively charged on defective C3N, respectively. On graphene, however, Pt is always positively charged with and without substrate defects. In addition, defective graphene and C3N with single-atom Pt display different magnetic properties. The former is paramagnetic and the latter has a nonzero moment. For the adsorption activity of single-atom Pt, we investigated the adsorption of various gas molecules on single-atom Pt, and found the adsorption strength of different gas molecules depend on the charge states of Pt atoms, which would have an effect on the relevant reaction processes. Our results are helpful to design nanomaterials and nanodevices of single-atom loaded 2D materials. ACKNOWLEDGMENTS

This work was supported by the National Natural

Science Foundation of China (Grant Nos. U180411019) and the High Performance Computing Centre of Henan Normal University. Supporting Information The most stable configurations for Cu, Ag, Au, Ni, Pd and Pt adsorbed on perfect graphene; The most stable configurations of the Pt adsorbed on C-deficient and Ndeficient C3N; MD simulations of Pt atoms on the perfect graphene and C3N; Dynamic trajectories of single-atom Pt in MD simulations; Charge density difference maps of Pt12


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Comparative Study of C3N- and Graphene-Supported Single-Atom Pt

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