Predicted Unusual Catalytic Activity of One-Dimensional Pt-Induced

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Predicted Unusual Catalytic Activity of One Dimensional Pt-Induced Atomic Nanowires on Ge (001) Surface Yongjie Xi, Na Guo, and Chun Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10547 • Publication Date (Web): 17 Dec 2015 Downloaded from http://pubs.acs.org on December 28, 2015

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The Journal of Physical Chemistry

Predicted Unusual Catalytic Activity of One Dimensional PtInduced Atomic Nanowires on Ge (001) Surface Yongjie Xi,§,† Na Guo,†,§ and Chun Zhang*,†,§ †Department of Physics and Graphene Research Centre, National University of Singapore, Singapore 117542 §Department of Chemistry, National University of Singapore, Singapore 117542 *[email protected]

Abstract ： One dimensional (1D) metal-induced nanowires on semiconductor surface have attracted enormous interests recently due to their unique electronic properties and great potential in device applications arising from the ideal 1D nature of the systems. Via ab initio modeling, we investigate for the first time the catalytic properties of Pt-induced nanowires (Pt-INWs) on Ge (001) surface that have been successfully fabricated in experiments. We show that these 1D atomic wires can also be used as excellent catalysts for chemical reaction of CO oxidation. A new ground-state configuration of Pt-INWs on Ge (001) that is significantly more stable than previously reported ones is predicted. The origin of the low reaction barrier of the catalyzed CO oxidation is thoroughly discussed. These results pave the way for a new class of highperformance catalysts with 1D characteristic.

KEYWORDS: 1D nanowire, Surface catalysis, CO oxidation.

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1. Introduction Looking for new materials and novel phenomena at low dimensionality is a topic of everlasting interest in scientific research. Recently, one dimensional (1D) metal-induced nanowires (M-INWs) on a semiconductor (Si or Ge) surface have attracted great attention.

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These self-assembled atomic nanowires can be readily fabricated in experiments by depositing metal atoms (Au, Bi, Co, In, Ir, Pt, etc.) on Ge or Si surface followed by a nicely controlled annealing process.

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Due to the ideal 1D nature of these M-INWs, fascinating electronic

phenomena such as charge-density wave, spin charge separation and Luttinger-liquid behaviors of electrons have been observed.1-2 Current research effort has been mainly focused on improving fundamental understanding of M-INWs (atomic structures and electronic properties etc.) and exploring their potential in applications of novel 1D solid-state electronic devices. In contrast, discussions on possible applications of these exciting 1D materials on solid-state chemistry are still missing. In this paper, via state-of-art ab initio modeling, we demonstrate for the first time that M-INWs may exhibit high catalytic activity, and therefore can be used as highperformance solid-state catalysts, providing new avenues for future applications of M-INWs. Among all M-INWs, Pt-induced nanowires (Pt-INWs) on Ge (001) surface are probably the most intensively studied ones. In this paper, we chose Pt-INWs as model systems to examine the catalytic activity. One-atom-thick Pt-INWs (as solitary wires or in large arrays) have been fabricated in several different experiments under various conditions.3-4, 7 On theoretical side, two distinctly different types of atomic models of Pt-INWs have been proposed by computational studies. One corresponds to the uniform distribution of Pt atoms on Ge (001) with low concentrations (~0.25 ML), a typical example of which is the so-called tetramer-dimer chain (TDC) model.

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Another one (the best example so far is the PINW model13) assumes the non-

uniform distribution of Pt atoms with relatively high local concentrations. Both TDC and PINW models suggest that the nice 1D feature of the system originates from Pt induced one-atom thick Ge (instead of Pt) wires on the surface and agree reasonably well with STM images obtained in experiments. While, a very recent experiment provided evidences of non-uniform distribution of Pt atoms on Ge surface with a local concentration of 0.75 ML or higher.

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We therefore adopt

the same assumptions as the latter model in current studies. Via ab initio simulations, we first propose a new model which is significantly more stable than the PINW structure, and then


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theoretically examine the catalytic activity of these 1D wires for the chemical reaction of CO oxidation. 2. Computational Method The ab initio calculations were performed using the spin-polarized density functional theory (spin-DFT) as implemented in Vienna ab initio simulation package (VASP).

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Reaction

barriers of catalyzed CO oxidations are determined by the Climbing Image Nudged Elastic Band 17

(c-NEB) method.

format for exchange-correlation functional and the projector-augmented

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method of describing electron-ion interactions are employed in all

Ernzerhof (PBE) wave (PAW)

The generalized gradient approximation (GGA) in the Perdew-Burke-

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calculations. A plane wave basis with cut-off energy of 400 eV and 2×4×1 of Brillouin zone k sampling were used. The convergence criteria for energy and structure optimizations are set to 1×10-5 eV and 0.03 eV/Å, respectively. Quantum molecular dynamics (MD) simulations are performed to verify the temperature stability of the system under study. The adsorption energy of molecules on surface is calculated by the difference between the energy of the combined system and the total energy of separated ones as follows, Eads= Esurface+adsorbate - (Esurface + Eadsorbate). 3. Results and Discussions As aforementioned, we assume the non-uniform distribution with 0.75 ML of local concentration of Pt atoms on Ge (001), which seemly agree well with experimental conditions. The atomic model of Pt-INWs we propose is displayed in Figure 1a. Our model also suggests that the 1D nature of the system comes from the Pt-induced formation of one-atom thick Ge Nano Wires (PGNWs) on the surface. The atomic structure in Figure 1a is referred to as PGNW model in the rest of the paper. As a comparison, the previously suggested PINW model that has the same concentration of Pt atoms is shown in Figure 1b. The two models have the same c(4x2) symmetry on surface that leads to around 16 Å separation of two neighboring wires, in good agreement with experiments. The PGNW model (one supercell) is energetically 1.10 eV more stable than the PINW model. The lower energy of PGNW stems from the fact that compared with PINW, two Ge atoms in the 1D wire of PGNW form chemical bonds with all surrounding top-layer Pt atoms.


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Figure 1. Top and side views of (a) PGNW and (b) PINW models. The supercell with c(4×2) symmetry is indicated by dashed red line in (a). Ge (Pt) atoms are in grey (yellow). Top-layer Ge and Pt atoms (labeled as Ge1-6 and Pt1-4) are highlighted by black and darker yellow, respectively. The bottom Ge surface is saturated by H (in white) atoms. To avoid the unphysical interaction between different slabs, a vacuum layer of 15.0 Å is added into the supercell in the direction perpendicular to the surface.

We also conducted quantum molecular dynamics (MD) simulations to test the stability of two models. The MD simulation was done under a temperature of 600 K.

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In experiments,

the annealing process started from around 600 K or higher (~1000 K). As illustrated in Figure 2, the MD simulation begins with the PINW model. Then starting from around 500 fs, the Ge dimer glides along the [110] direction, forming bonds with more top-layer Pt atoms, and finally ends at the more stable PGNW model around 1300 fs. After that, the system stays as the PGNW, indicating the good temperature stability of the model. Later, we will show that such automatic structure transition from PINW to PGNW can also occur at zero temperature when O2 molecules are approaching the surface. Because of these findings, we believe that the PGNW model we suggested may be closer to experimentally fabricated systems compared with the PINW structure.


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Figure 2. (a) Energy evolution in MD simulations of the PINW model under a temperature of 600 k. After around 1300 fs, the system goes to PGNW model. Average energies of the PINW and PGNW models are indicated by blue dashed lines; (b) Snap shots of the system at different times. The MD step is set to 1 fs.

We now examine the chemical reactivity of the PGNW model using the adsorption of CO and O2 molecules. All possible inequivalent adsorption sites (Ge1-6 and Pt1-4 as shown in Figure 1a) on the surface are tested. According to our calculations, O2 can be strongly adsorbed on top-layer Ge sites next to Pt atoms (Ge1-Ge2 or Ge3-Ge4) with the adsorption energy of -1.38 eV on Ge1-Ge2 or -1.36 eV on Ge3-Ge4. The adsorption of an O2 molecule on the Pt sites however is much weaker (around -0.30 eV), making the O2 adsorption predominant at Ge sites. On contrary, CO molecule strongly adsorbs on Pt sites with the adsorption energy around -1.0 eV and binds weakly on Ge sites with the adsorption energy less than -0.20 eV, which agree with previous theoretical studies.21, 22 The optimized adsorption configurations of O2 are displayed in Figure S1 in supporting information. It worth mentioning here that when an O2 molecule is approaching the Ge nanowire in PINW model, the whole system automatically turns to the Ge1Ge2 adsorption configuration of O2 on PGNW as shown in Figure S2, indicating that PINW is not stable under O2-rich conditions.


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In Figure 3a and 3b, we presented two different configurations of co-adsorption of CO and O2 (co-1 and co-2). In both cases, all Pt sites are bonded by CO molecules with O2 on Ge1-Ge2 for co-1 and O2 on Ge3-Ge4 for co-2. Two cases, co-1 and co-2, have comparable adsorption energies (for all adsorbates), being 5.1 and 4.7 eV respectively. Two cases also have similar O-O bond lengths around 1.45 Å that leads to a non-magnetic peroxo-state of O2 molecule. In Figure 3c and 3d, we plotted the isosurfaces of charge redistribution caused by CO and O2 coadsorption. It can be clearly seen that upon adsorption, electrons are transferred from Ge atoms to 2π* orbital of O2, elongating the O-O bond and activating the molecule. The Bader charge analysis23 indicates that for both co-adsorption configurations, nearly 1.1 electrons are transferred to O2 molecule, making each O atom negatively charged by around 0.55 |e|. On the other hand, each C atom in adsorbed CO molecules is positively charged according to Bader charge analysis by around 0.6 |e|, resulting in significant electrostatic attractions between these C atoms and the O2 molecule.

Figure 3. Co-adsorption of CO and O2 on PGNW. (a) The co-1 configuration: O2 adsorbed on Ge1-Ge2. (b) The co-2 configuration: O2 adsorbed on Ge3-Ge4. Isosurfaces of charge redistribution caused by the adsorption of CO and O2 are shown in (c) for co-1 and (d) for co-2. Charge accumulation (depletion) is denoted by blue (green). C atoms are in orange and O atoms are in red.

The chemical reaction of CO oxidation has been intensively used in previous theoretical studies to probe the catalytic activity of low dimensional (zero and/or two dimensional) systems. 24-29

In our case, since both CO and O2 strongly bind on surface, we only consider the Langmuir-

Hinshelwood (LH) type of reaction. We start from the analysis of the co-1 configuration. For this case, our simulations suggest that the catalysed CO oxidation proceeds in the following way: 1) the dissociation of O2 with a low barrier of 0.34 eV; 2) the oxidation of the CO molecule adsorbed on Pt1, CO+O2→CO2+O, and 3) the oxidation of the CO on Pt2 by the leftover O atom, CO+O→CO2. The O2 dissociation in this case is found to be exothermic by 0.44 eV. After


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dissociation, two O atoms reside on top of Ge1 and Ge2, respectively. The top-site bonded O atoms then can react with the two CO molecules on Pt1 and Pt2 with a barrier around 0.2 eV for each reaction of CO oxidation. The formed CO2 molecules can desorb with a small energy less than 0.2 eV. The reaction path is summarized in Figure 4. More details for the reaction energy profiles can be found in Figure S3 in supporting information. Note that Ge5-Ge6 sites are equivalent to Ge3-Ge4, so the catalysed reaction can also happen in the same manner for O2 on Ge5-Ge6 and CO molecules on Pt3 and Pt4.

Figure 4. (a) The summarized energy profile along the CO oxidation reaction path for co-1 case. (b) Atomic structures of key species along the reaction path. IS (initial state): d(O-O)=1.45 Å; TS1 (transition state for O2 dissociation): d(O-O)=2.04 Å; Int1 (the dissociation of O2): d(O-O)=4.29 Å, d(O2-CO@Pt1)=3.33 Å; TS2 (transition state of the first CO oxidation): d(O2-CO@Pt1)=2.13 Å; Int2 (CO@Pt1 is oxidized and one CO2 is formed): d(O2-CO@Pt2)=3.42 Å; TS3 (transition state of the second CO oxidation): d(O2-CO@Pt2)=2.13 Å; FS: CO@Pt2 is oxidized and another CO2 is formed. Definitions: d(O-O) is the O-O bond length of O2 molecule; d(O2-CO@Pt1) is the distance between the C in CO@Pt1 and the nearest O in O2; d(O2-CO@Pt4) is the distance between the C in CO@Pt4 and the nearest O in O2. Color scheme is the same as Figure 3.

Low reaction barriers for co-1 case clearly suggest that the 1D Pt-INW on Ge surface can be used as an excellent catalyst for CO oxidation. One obvious advantage of this 1D catalyst is that since CO and O2 bind on different places of the surface, the CO poisoning is not an issue here. We now check the catalytic activity of co-2 case. The CO oxidation for the co-2 configuration is quite different from the co-1 configuration. For this case, we found that the catalysed CO


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oxidation prefers to follow a one-step mechanism for which two CO molecules on Pt1/Pt4 sites (that are closer to O2 on Ge3-4 than other two CO) react with O2 simultaneously, 2CO+O2→2CO2. A similar one-step mechanism has been reported previously in a different system.

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For this mechanism, our c-NEB calculations yielded low reaction barrier of 0.38 eV.

Two CO2 molecules formed at the final stage can desorb with a negligible energy penalty similar to co-1 case. Low barrier of the one-step mechanism originate from two factors. One, of course, is the activation of O2 molecule by the charge transfer from the Ge dimer. Another one is the significant electrostatic attractions between O2 and both C atoms on Pt1/Pt4 sites, which are in opposite directions and therefore helpful for breaking the O-O bond. The energy profile along the reaction path is presented in Figure 5. Considering both co-1 and co-2 configurations, 4 out of 6 Pt atoms in one super cell of the PGNW model play essential roles in the catalytic reactions, therefore the atom efficiency of the 1D Pt-INW is comparable to single-atom Pt catalysts. We also tested the usual two-step reaction mechanism of CO oxidation for the co-2 case, and found that the first step of the reaction, the oxidation of CO on Pt1 (CO+O2→CO2+O), has a high reaction barrier around 0.8 eV, so cannot happen at room temperature. The energy profile of the aforementioned first step CO oxidation is displayed in Figure S4 in supporting information.

Figure 5. The energy profiles of the one-step mechanism of CO oxidation for co-1. The initial state (IS): d(OO)=1.45 Å, d(O2-CO@Pt1)=3.16 Å, d(O2-CO@Pt4)=3.10Å; the transition state (TS): d(O-O)=2.56 Å, d(O2CO@Pt1)=2.55 Å and d(O2-CO@Pt4)=2.48 Å; the final state (FS): Two CO2 molecules are formed

4. Conclusions


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In conclusion, via ab initio modeling, we suggested a new atomic model (PGNW model) for experimentally fabricated Pt induced nanowires on Ge (001) and tested the stability as well as the catalytic activity of the model. We showed that the PGNW model is significantly more stable than the previously proposed one with the same Pt concentration. Under a temperature of 600 k or O2-rich conditions, the system automatically turns from PINW to PGNW. The adsorption of CO and O2 molecules on the PGNW structure is discussed and high catalytic activity of the system for CO oxidation is demonstrated. Four out of six Pt atoms in one unit cell of PGNW play essential roles in the catalyzed reaction, indicating the high atom efficiency of this novel Pt catalyst. Also, the CO poisoning effect that is often a serious problem for Pt catalysts is minimal for the catalyst we presented here. We believe that the predicted excellent catalytic performance for Pt-INWs on Ge (001) may also exist for other M-INWs. Our findings pave a way for discovery of a new family of high-efficiency catalysts with 1D characteristic.

Associated Content Supporting Information. The following files are available free of charge. Adsorption configurations and energies of O2 on PGNW model; structural transition of PINW model to PGNW model upon O2 adsorption; the detailed energy profiles of CO oxidation for co-2 configuration; first step CO oxidation in the usual two-step reaction mechanism for co-2 case. (file type, PDF) Author Information Notes The authors declare no competing financial interests. Acknowledgment We acknowledge the support from academic research fund of MOE Singapore and National University of Singapore (Grant number R144-000-325-112 and R144-000-344-112). Computational works were performed at the NUS Graphene Research Centre computing cluster.


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TOC


Predicted Unusual Catalytic Activity of One-Dimensional Pt-Induced

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