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Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX
Band Edge Optical Excitation of Pyridine-Adsorbed CuAg Nanoparticles Junais Habeeb Mokkath* Department of Physics, Kuwait College of Science and Technology, Doha Area, Seventh Ring Road, P.O. Box 27235, Kuwait
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S Supporting Information *
ABSTRACT: Understanding the structure−property relationship of multielement nanoparticles is vital for developing novel nanodevices. In the present paper, via a combination of a basin hopping global sampling method, a symmetry-orbit shell optimization technique, and density functional theory reoptimizations, we determine the energetically most stable CuAg face-centered cubic nanoparticles. The calculated structures show a clear tendency toward CucoreAgshell chemical ordering by populating the more cohesive Cu in the core region and of Ag in the shell region. Further, using time-dependent density functional theory (TDDFT) calculations, we analyze the band edge optical excitations of the nanoparticles with pyridine molecule on top. With the help of charge difference density plots, we found dramatic modifications in the electron density distribution of the nanoparticles. We believe that the present theoretical findings will be useful for the development of novel nanosensors.
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INTRODUCTION Unraveling the physical and chemical properties of alloy nanoparticles with atomically precise composition (commonly known as nanoalloys) is of fundamental importance for a plethora of applications in optics,1,2 magnetism,3−5 and catalysis.6−8 Nanoalloys show remarkable tunability in the properties as a result of a great variety of chemical ordering patterns (for example, core−shell,9−15 multishell,16−18 ordered phases,19 ball-and-cup,20 and Janus21,22) that they can assume. The nanoalloys composed of weakly miscible metals favor a core−shell chemical ordering pattern in a wide range of sizes and compositions confirmed by both experimental and computational studies by several research groups.9−15,23 For example, small Cu−Ag clusters exhibit core−shell structures with Ag segregating at the cluster surface, as theoretically predicted and experimentally observed.9,13,15,17,24,25 This structural motif is favored by the lower surface energy of Ag (1210 compared to 2130 mJ m−2 for Cu)26 as well as by its larger atomic size (first neighbor distances are 2.89 and 2.55 Å for Ag and Cu, respectively27) and modest but non-negligible difference in cohesive energy (2.96 vs 3.54 eV, respectively27). Besides the core−shell structures, Janus-type chemical arrangement was also found in the case of bigger Cu−Ag nanoparticles having 586 atoms28 and 12 nm29 nanoparticle for some Cu:Ag compositions. Computer simulations using the state-of-the-art supercomputers can be of great help in sampling the complex and vast energy landscapes of the nanoalloys; however, the important task of finding the energetically most favorable chemical ordering pattern for a given size and composition is far from trivial.30,31 © XXXX American Chemical Society
The goal of this study is two-fold. First, we will elucidate the energetically most favorable chemical ordering patterns (the equilibrium structures at sufficiently low temperatures) of CuAg face-centered cubic (fcc) nanoparticles for different Cu:Ag concentrations. Second, we show the modifications in the band edge optical excitation due to pyridine adsorption on the nanoparticle surface. To this aim, we use the charge difference density (CDD) plots because they provide direct visual evidence of the nature of the excitation. We compare our results with previous experimental and theoretical results, wherever possible. It is important to recall that there are multiple factors affecting the interaction of light with molecules in the close proximity of nanoparticles.32−37 It is noteworthy that molecules that can be chemisorbed to the nanoparticle surface such as pyridine show the largest field enhancements in comparison to those that can only be physisorbed (weakly bind) to the nanoparticle surface such as benzene. Pyridine chemisorbed on Ag is of huge interest when studying chargetransfer and electromagnetic enhancements. For example, using electron energy loss measurements, Demuth and Sanda studied pyridine adsorbed on Ag(111)38 and found pyridine states below 1.4 eV and new excitations above 1.4 eV, identified as charge-transfer excitations between pyridine and Ag(111). Using TDDFT/CDD calculations, Sun and coworkers investigated electromagnetic and chemical enhancements for pyridine adsorbed on a Ag20 nanoparticle.39 In Received: March 31, 2018 Revised: June 27, 2018
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DOI: 10.1021/acs.jpca.8b03058 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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method are further checked at a higher level of theory using density functional calculations. The exchange−correlation functional is treated in the Perdew−Burke−Ernzerhof flavor45 of the generalized gradient approximation. Optical response of metal nanostructures can be calculated precisely using classical Mie theory46,47 or using the discretedipole approximation48 and the finite-difference time domain techniques.49 However, as the system size shrinks to a few nanometers, the classical methods fail due to emergence of the quantum mechanical effects.50,51 This problem can be solved via a fully quantum mechanical method such as TDDFT.52−58 All DFT/TDDFT calculations presented in this work were carried out using the ORCA software (version 4.0).59 We used the def2-TZVP basis set60 (further information regarding the DFT calculation parameters and/or the exchange−correlation functional used is available in the ORCA manual). The SCF convergence was set to 10−8. A gradient convergence criterion of 10−6 and an energy convergence criterion of 10−6 were used in order to obtain well-converged geometries. It should be mentioned that the DFT reoptimized nanoparticles with pyridine adsorbed on the surface were arranged in such a way that the pyridine molecule oriented perpendicular to the surface (where the Ag or Cu was bound to the pyridine nitrogen). This result is in agreement with the low-temperature scanning tunneling microscopy and Raman scattering experiments where pyridine binds to the Ag nanoparticle through the nitrogen atom.61 The band edge optical excitations of the nanoparticles are computed using the TDDFT technique in frequency space using the wB97X long-range corrected hybrid exchange−correlation functional62 (more information is available in the online ORCA manual).
addition, using TDDFT calculations, Jensen and co-workers estimated chemical enhancements to 103 and electromagnetic enhancements to 105 in the pyridine−Ag20 nanoparticle hybrid.40 Although the interaction of light with the molecules in the close proximity of the pristine nanoparticles has been widely explored, similar studies in the case of nanoalloys are still missing to a great extent.
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COMPUTATIONAL DETAILS Our computational methodology consists of two steps. First, we employ a basin hopping global sampling technique41 in combination with a symmetry-orbit shell optimization technique and density functional reoptimizations to find the energetically most stable structures. This methodology has been used extensively with success to identify the most stable structures of a variety of nanoalloys.11,42 For each nanoalloy composition, we run at least ∼50000 basin hopping steps with a quite low temperature of 150 K. This gives us a localized search of deep regions of chosen structural funnel. We maintain an acceptance ratio of 50% throughout the basin hopping sampling runs. The symmetry-orbit shell optimization technique exploits the full point group symmetry of a nanoparticle to partition the atoms into shells and is thus able to identify the high-symmetry chemical ordering patterns for a given structural motif with greatly reduced computational effort. For a given bimetallic nanoparticle, the number of homotops (isomers having the same structure and composition but different atomic arrangements) is significantly reduced from 2N to 2S, where N is the number of atoms and S is the number of atomic shells. In this work, we consider the high symmetric fcc nanoparticles having 201 atoms because this is a geometric magic size for a fcc nanoparticle. For N = 201, there are 11 shells (1, 12, 6, 24, 12, 24, 8, 48, 6, 36, and 24 atoms) and 211 = 2048 structures. We use the Gupta potential43 because it is widely used against density functional theory (DFT) calculations with acceptable agreement13 and it has also been compared favorably to experimental results.15 In the Gupta potential, the potential energy of the system depends on the relative distances between atoms rij and it is written as the sum of single-atom contributions Ei Eib
l o o o = −m ∑ ξ2 o o o o n j ≠ i , rij < rc
ÄÅ É|1/2 ÅÅ o ij rij yzÑÑÑÑo o Å j z expÅÅÅ−2qjj − 1zzÑÑÑ} o j z ÅÅ Ñ o r Ñ k 0 {ÑÖo ÅÇ o ~
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RESULTS AND DISCUSSION Before analyzing the obtained results, it is instructive to recall the properties of the bulk CuAg alloys. They exhibit a large miscibility gap, positive heat of formation for all Cu:Ag compositions, and relatively high lattice mismatch (aAg/aCu = 1.13).63 It is therefore expected that CuAg nanoalloys would show some kind of phase separation. One easily concludes from Figure 1 that the energetically most stable structures for different Cu:Ag concentrations (Cu3Ag, Cu2Ag2 and CuAg3) favor a core−shell arrangement having Cu (Ag) atoms prefer to populate the core (shell) region. This result is in agreement with the heat of formation calculations demonstrating that CuAg nanoalloys generally tend to segregate.64−66 Now let us begin our discussion starting from the Cu3Ag nanoparticle. One notices that Cu atoms preferentially populate all of the core sites, whereas most of the surface sites are populated by the Ag atoms; see Figure 1. Apparently, some Cu atoms present on the surface region due to the fact that the number of Ag atoms is insufficient to occupy all of the surface sites. Interestingly, the surface is characterized by a combination of two chemically ordered units. The first is a (111) facet with double hexagonal rings. Note that a Ag atom occupies the central site and is surrounded by six Cu atoms in the first hexagonal ring. The second is a (100) facet having nine sites fully populated by Ag atoms. Interesting surface ordering patterns emerge in the case of the 50−50 (Cu2Ag2) composition. Notice that the surface region is again characterized by a combination of two chemically ordered units. On the (111) facets, the central sites are occupied by Cu atoms and are decorated by Ag atoms. On the (100) facets, the central sites are occupied by Ag atoms, but the middle sites of
(1)
ÄÅ É ÅÅ i rij yzÑÑÑÑ j Å = ∑ A expÅÅÅ−pjjj − 1zzzÑÑÑ zÑÑ ÅÅ j r0 {ÑÖ ÅÇ k j ≠ i , rij < rc
and repulsive Eir
(2)
where r0 the nearest-neighbor distance. (A, ξ, p, q) is a set of parameters fitted to reproduce the experimental properties of the bulk,27 see Table 1. The same parameter set was used previously for the characterization of CuAg nanoparticles.44 The low-energy structures obtained from the above-mentioned Table 1. Parameters of the Empirical Gupta Potential interaction
A (eV)
ξ (eV)
p
q
r0 (Å)
Cu−Cu Cu−Ag Ag−Ag
0.0894 0.0980 0.1031
1.2799 1.2274 1.1895
10.55 10.70 10.85
2.43 2.8050 3.18
2.556 2.72405 2.8921 B
DOI: 10.1021/acs.jpca.8b03058 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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distance analysis shows that Ag atoms occupying (111) facet central sites feel significant compressive strain. Naively, this means that its optimal size at that position would be smaller than what it actually is. Thus, substituting Ag atoms occupying (111) facet central sites by smaller Cu atoms can be used to create optimal bond lengths and release the compressive strain. It is noteworthy that this finding is supported by experiments67 on CuAg fcc nanoparticles reporting some intermixing of Cu with Ag at the outermost surface layer. It is also important to mention that substituting Ag atoms occupying (111) facet central sites by Cu atoms has an energy cost due to the fact that Cu−Cu and Ag−Ag bonds are stronger than mixed Cu− Ag bonds. In general, the formation of core−shell chemical ordering of CuAg nanoparticles was confirmed in experiments63 and in several theoretical studies.9,68 Having obtained the energetically most stable structures, we glean some insight into their band edge optical excitation with and without pyridine molecule adsorption. In this context, Figure 2 illustrates dramatic modifications of the CDD plots due to pyridine molecule adsorption. By examining the contributions of the electronic transitions to the band edge excitation (see Table 2), we gain further insight into the nature of excitation. It should be stressed that each band edge optical excitation is composed of many single-particle transitions; see Table 2. First we focus on the Cu nanoparticle; the band edge optical excitation can be assigned to a linear combination of HOMO → LUMO (85%) transition, HOMO−1 → LUMO+1 (10%) transition, and HOMO−2 → LUMO+3 (5%) transition. In the case of Cu nanoparticles with pyridine molecules, the band edge optical excitation can be assigned to a linear combination of 12 transitions, HOMO → LUMO (10%), HOMO−1 → LUMO (15%), HOMO−2 → LUMO (15%), HOMO−3 → LUMO+1 (10%), HOMO−5 → LUMO+2 (10%), HOMO−7 → LUMO+2 (5%), HOMO− 9 → LUMO+6 (5%), HOMO−6 → LUMO+1 (10%), HOMO−8 → LUMO+10 (5%), HOMO−10 → LUMO+8 (5%), HOMO−12 → LUMO+9 (5%), and HOMO−15 → LUMO+3 (5%). In the case of a Ag nanoparticle, the band edge optical excitation can be assigned to a linear combination of four transitions, HOMO → LUMO (80%), HOMO → LUMO+1 (10%), HOMO−1 → LUMO (5%), and HOMO−
Figure 1. Most stable chemical ordering patterns of 201 atom CuAg fcc nanoparticles for different Cu:Ag concentrations. Nanoalloy structures were sliced in the middle for easy visualization of the core region.
the (100) edges are occupied by Cu atoms. Finally, in the case of the CuAg3 nanoparticle, one finds an almost perfect core− shell chemical arrangement but with some Cu atoms present on the surface, in particular, the central sites of (111) facets. The occupation of (111) facet central sites by Cu atoms is an interesting effect for further analysis and can be related to the nanoparticle strain release mechanism. Our interatomic
Figure 2. Band edge optical excitation of CuAg nanoparticles via CDD plots. Red (blue) spheres depicts Cu (Ag) atoms. C
DOI: 10.1021/acs.jpca.8b03058 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Table 2. Nanoparticles and the Single-Particle Excitations Contributing to the Optical Band Edge Transition nanoparticle Cu
Cu−pyridine
Ag
Ag−pyridine
Table 3. Nanoparticles and the Single-Particle Excitations Contributing to the Optical Band Edge Transition
transition from occupied orbital to unoccupied orbital
nanoparticle
85% HOMO → LUMO 10% HOMO−1 → LUMO+1 5% HOMO−2 → LUMO+3 10% HOMO → LUMO 15% HOMO−1 → LUMO 15% HOMO−2 → LUMO 10% HOMO−3 → LUMO+5 10% HOMO−5 → LUMO+2 5% HOMO−7 → LUMO+2 5% HOMO−9 → LUMO+6 10% HOMO−6 → LUMO+1 5% HOMO−8 → LUMO+10 5% HOMO−10 → LUMO+8 5% HOMO−12 → LUMO+9 5% HOMO−15 → LUMO+3 80% HOMO → LUMO 10% HOMO → LUMO+1 5% HOMO−1 → LUMO 5% HOMO−3 → LUMO+2 10% HOMO → LUMO 15% HOMO → LUMO+2 15% HOMO−1 → LUMO 10% HOMO−3 → LUMO+1 20% HOMO−4 → LUMO 20% HOMO−5 → LUMO+2 10% HOMO−6 → LUMO+4
Cu3Ag
Cu3Ag−pyridine
Cu2Ag2 Cu2Ag2−pyridine
3 → LUMO+2 (5%). In the case of Ag nanoparticles with pyridine molecules, the band edge optical excitation can be assigned to a linear combination of seven transitions, HOMO → LUMO (10%), HOMO → LUMO+2 (15%), HOMO−1 → LUMO (15%), HOMO−3 → LUMO+1 (10%), HOMO− 4 → LUMO (20%), HOMO−5 → LUMO+2 (20%), and HOMO−6 → LUMO+4 (10%). It is noteworthy in the case of pristine nanoparticles that the dominating transition is from HOMO to LUMO. In contrast, in the case of a pyridine molecule attached to the nanoparticle, the band edge optical excitation is found to be contributed to by many transitions; see Table 2. Having analyzed the band edge optical excitation of Cu and Ag nanoparticles with and without a pyridine molecule on top, we now turn our attention to the band edge optical excitation of the Cu3Ag, Cu2Ag2, and CuAg3 nanoalloys (see Figure 2). In the case of the Cu3Ag nanoparticle, we found that the band edge optical excitation is a linear combination of five transitions having HOMO → LUMO (80%), whereas in the case of the Cu3Ag nanoparticle with a pyridine molecule on top, the band edge optical excitation is a linear combination of 13 transitions having no individual transition appearing as dominant (see Table 3). Interesting changes in the band edge optical excitation emerges in the case of the Cu2Ag2 nanoparticle; see Figure 2. We observe an almost symmetric electron density distribution in the pristine Cu2Ag2 nanoparticle (the transition is dominated by HOMO → LUMO), whereas this symmetric distribution is somehow completely destroyed after the pyridine adsorption (the transition is a linear combination of nine different transitions). Finally, similar results are also found in the case of the CuAg3 nanoparticle. We remark that in the real experimental conditions the the actual orientation of the
CuAg3
CuAg3−pyridine
transition from occupied orbital to unoccupied orbital 80% HOMO → LUMO 5% HOMO → LUMO+2 5% HOMO−2 → LUMO+1 5% HOMO−5 → LUMO+4 5% HOMO−7 → LUMO+8 15% HOMO → LUMO 15% HOMO−1 → LUMO 15% HOMO−4 → LUMO 10% HOMO−6 → LUMO 5% HOMO−7 → LUMO+3 5% HOMO−8 → LUMO+4 5% HOMO−11 → LUMO+5 5% HOMO−12 → LUMO+9 5% HOMO−14 → LUMO 5% HOMO−17 → LUMO+5 5% HOMO−20 → LUMO+2 5% HOMO−22 → LUMO 5% HOMO−25 → LUMO+3 90% HOMO → LUMO 10% HOMO → LUMO+2 20% HOMO → LUMO 15% HOMO → LUMO+2 10% HOMO−1 → LUMO 10% HOMO → LUMO+5 10% HOMO−3 → LUMO+4 10% HOMO−5 → LUMO+2 10% HOMO−6 → LUMO+3 10% HOMO−9 → LUMO+7 5% HOMO−14 → LUMO+4 80% HOMO → LUMO 20% HOMO → LUMO+10 10% HOMO−1 → LUMO+11 30% HOMO → LUMO+1 30% HOMO−2 → LUMO+7 10% HOMO−3 → LUMO+8 10% HOMO−4 → LUMO+5 10% HOMO−5 → LUMO+2 10% HOMO−6 → LUMO+3
pyridine molecules is strongly dependent on both the method used to deposit them on the nanoparticle surface and the adhesion properties for each combination of molecule and nanoparticle material.
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CONCLUSIONS In conclusion, we have elucidated the energetically most stable structures of CuAg fcc nanoparticles using a combination of a basin hopping global sampling method, a symmetry-orbit shell optimization technique, and DFT reoptimizations. We found that CuAg nanoparticles prefer the CucoreAgshell chemical arrangement for all Cu:Ag compositions, in agreement with earlier theoretical and experimental works. We have also investigated the band edge optical transitions of the nanoparticles with pyridine adsorbed on top using TDDFT calculations. With the help of CDD plots and individual transition contributions, we found reasons for the dramatic modifications in the electron density distribution of the nanoparticles. We believe that our findings can be beneficial for the emerging field of ultrasmall nanosensors. D
DOI: 10.1021/acs.jpca.8b03058 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b03058.
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Atomic coordinates of the Cu3Ag, Cu2Ag2, and CuAg3 nanoalloys (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Junais Habeeb Mokkath: 0000-0001-8889-5889 Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS The research reported in this publication was supported by funding from Kuwait College of Science and Technology (KCST).
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DOI: 10.1021/acs.jpca.8b03058 J. Phys. Chem. A XXXX, XXX, XXX−XXX
