Orientation Selection during Heterogeneous Nucleation: Implications

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Orientation Selection during Heterogeneous Nucleation: Implications for Heterogeneous Catalysis Dipanwita Chatterjee,† Akash R,‡ K. Kamalnath,† Rafia Ahmad,† Abhishek Kumar Singh,† and N. Ravishankar*,† †

Materials Research Centre and ‡Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India S Supporting Information *

ABSTRACT: Hybrids based on supported nanoparticles are used for many applications such as catalysis and sensing. It is wellknown that the size and shape of the particles and their interaction with the substrate play a key role in controlling the properties of the hybrid. Here, we show that in addition to these commonly considered effects, the orientation of the particle on the substrate could play a critical role that could dominate over the other effects. For the same nominal size of the particle, changes in its orientation on the substrate lead to dramatic changes in the accessible surface area and the electronic structure, thus profoundly affecting the properties. Using analytical calculations, we show that the orientation of the crystal, heterogeneously nucleating on a substrate, is determined by the barrier for nucleation and experimentally demonstrate that it is possible to tune this orientation by tuning the interfacial energy between the nucleus and substrate. Our study provides some fundamentally new insights into the process of heterogeneous nucleation that can be exploited for practical applications.

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INTRODUCTION Supported metal catalysts are conventionally used in industrially important reactions including heterogeneous gas-phase reactions,1−6 electrochemical redox reactions,4,7−11 photocatalysis,12−16 and photoelectrocatalysis.17−20 Extensive theoretical and experimental studies on the effect of morphology,21−24 size,21,25 surface structure of the catalyst nanoparticles,26−29 and the support6,30,31 on stability and efficiency32 have been carried out. In addition to the shape and size of the catalyst, the support could significantly alter the catalytic activity of the particles. There are broadly two types of metal−support interactions discussed in the literature: the Strong Metal−Support Interaction (SMSI)33,34 in which extreme wetting of the metal particle on the oxide substrate enables selective adsorption of certain molecules and hinders adsorption of other molecules, and the Electronic Metal Support Interaction (EMSI)35 in which charge transfer from the particle to the support alters the electronic structure of the catalyst, changing its activity remarkably.36−39 The pioneering work of Haruta and co-workers demonstrated the exceptional CO oxidation catalytic properties of highly dispersed gold nanoparticles on a metal oxide support,40 in striking contrast to their mesoscopic and bulk counterparts that are chemically inert.41 Since then the catalytic applications of gold have been explored extensively in several processes such as chemical processing, hydrotreatment, purification of hydrogen for fuel cell applications, and reactions of environmental importance such as catalytic treatment of vehicle exhausts and removal of volatile organic compounds.42−44 It is ubiquitously © XXXX American Chemical Society

believed that the under-coordinated surface atoms located at the particle corners and edges, whose abundance increases significantly as the particle size shrinks down to the sub-5 nm size regime, provide a key contribution to the remarkable catalytic activities of small Au NPs. To understand the fundamental mechanism of the high catalytic activity, extensive experimental and computational efforts have been made to explore the most active sites on oxide-supported gold nanoparticles as well as the size and shape dependence. Besides the shape and the size dependence, the role of the oxide support has been intensively investigated since the gold clusters on a different support can exhibit different catalytic activities.36,45 To date, several factors have been proposed to explain high catalytic activity of the supported Au clusters, such as the chemical structure of the support, charge transfer between Au clusters and the support, and the perimeter sites at the interface between Au clusters and the support.46 While the heterogeneous catalysis by nanosize gold aggregates supported on oxides is of great significance in current and future environmental, sensor, and chemical technologies, there is still much debate on the processes underlying the catalytic activity of gold in reduced dimensions and the reaction mechanism.47 In this direction it is important to understand the catalytic activity of subnanometer gold clusters with precisely controlled size and shape. Although catalytic Received: March 9, 2017 Revised: April 15, 2017 Published: April 21, 2017 A

DOI: 10.1021/acs.jpcc.7b02237 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Heterogeneous Nucleation of Metal (M) on the Grids. Au was nucleated on both amine-functionalized carbon and thiolfunctionalized carbon and Ag was nucleated on amine-functionalized carbon by microwave reduction of the respective precursor salts. The schematic in Figure 6a explains the sample preparation procedure. Synthesis Conditions. In a typical microwave assisted synthesis, 10 mL of 10−5 mM solution of HAuCl4·xH2O in ethylene glycol was taken in a microwave vial and an oleyl amine functionalized grid was dipped in the solution. This reaction was carried out at 180 °C for 15 min at microwave power of 100 W. For nucleation of Au on butanedithiol functionalized grid, 10 mL of 1.142 × 10−5 mM solution of HAuCl4·xH2O in ethylene glycol was taken, and correspondingly, butanedithiol functionalized grid was dipped in the solution and reaction was carried out at 140 °C for 15 min at microwave power of 100 W. For nucleation of Ag on an oleyl amine functionalized grid, 10 mL of 0.88 × 10−5 mM solution of AgNO3 was taken and an oleyl amine functionalized grid was inserted into the solution and the reaction was carried out at 150 °C for 15 min at microwave power of 100 W. After each reaction the grid was taken out and cleaned by dipping in DI water and subsequently in ethanol and dried. The cleaned and dried grid was used for TEM experiments. Reactions were carried out at a series of temperature and concentration conditions and finally optimized to obtain the maximum number of single crystalline particles and an optimum number density of particles on the grid. The optimum reaction conditions are summarized in the Supporting Information (SI). The range of concentration and temperature conditions at which the reactions were done is summarized in SI (S3.5, Figures S9− S11, Tables S5−S8) along with the results. Nucleation of Particles on Carbon Nanospheres. The centrifuged and dried carbon nanospheres were collected and transferred to ethylene glycol (10 mL) in which the calculated amount of HAuCl4·xH2O was added and microwave assisted synthesis was done following the previously tabulated conditions for the corresponding systems. Conditions Used for Microscopy and Analysis. All the microscopy work was carried out in Tecnai 300 kV FEG source (F 30). For PED technique C2 aperture of 50 μm and beam in microprobe mode was used to obtain a parallel illumination of ∼2 nm probe diameter, spot size 11, and acquire diffraction patterns from individual particles. Digistar Nanomegas precession system was used to precess the beam at an angle of 0.6° to minimize the dynamical effects and also to obtain higher-order diffraction spots to ensure accurate indexing of the obtained diffraction patterns. The diffraction patterns were captured using a high speed external camera and later on indexed using Nanomegas indexing software. The precession electron diffraction technique has been discussed in details in SI S3.6 and Figure S12. The orientations were plotted using mtex 3.5.0 software in an inverse pole figure (ipf) plot.

activities of subnanometer gold clusters in the gas phase have been investigated, few studies have addressed orientationdependent catalytic properties of supported small gold clusters.48 Here, we show that in addition to the aforementioned effects, the crystallographic orientation of the particle on the substrate has a significant effect; this effect has usually been neglected in the literature. We present the general principles of heterogeneous nucleation and orientation selection by choosing a model system involving the nucleation of an FCC metal on an amorphous planar substrate. Using analytical calculations, we show that the favored orientation for heterogeneous nucleation of FCC metal particles on amorphous substrates changes by changing the interfacial energy between the substrate and the particle. Employing firstprinciples calculations, we estimate the adsorption energies of CO on a model system of Au nanoparticles on doped MgO substrates; we have shown that the orientation of FCC Au particle on the support has a profound effect on adsorption and hence the energy barrier for CO oxidation reaction. Thus, we justify that orientation of the catalyst particle indeed has an effect on its catalytic property, and it is extremely important to be able to tune the orientation preference of the catalyst particles according to the requirement of the reaction. Experimentally we show that it is possible to change the orientation of the particle on the substrate by tuning the interfacial energy between the particle and the substrate by suitable functionalization of the substrate. Our study thus provides fundamental insights and new possibilities for tuning the efficiency of supported catalysts.

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METHODS Ab Initio Simulations. The first-principles calculations presented in this study were performed using density functional theory (DFT) as implemented in the Vienna ab initio simulation package (VASP).49 The core electrons were described using the all-electron projector-augmented wave method.50 The exchange and correlation energy were evaluated using the Perdew− Burke−Ernzerhof generalized gradient approximation.51 The periodic images are separated in vacuum along three directions for all by a distance of 15 Å. The Brillouin for the clusters supported on Li-doped MgO(100), a well converged k-mesh of 7 × 7 × 1 was employed. The energy cutoff of 400 eV was set to plane wave basis in Kohn−Sham single-electron wave functions. The structures were allowed to relax using the conjugate gradient algorithm until the force on each atom was less than 0.005 eV/ Å. Functionalization of Grid. Functionalization of grid with oleylamine/1,4 butanedithiol was done by dipping fresh grid in a solution of oleyl amine/1,4 butanedithiol (20 μLit) in hexane (1 mL) in a 1.5 mL Eppendorf vial and kept for 1 h after which it was washed by dipping in fresh hexane (1 mL) and dried. Dried grid was attached to a 1 cm × 1 cm glass slide by Teflon tape leaving only its central portion exposed for heterogeneous nucleation and immersion into the reaction mixture. Functionalization of Carbon Nanospheres. Carbon nanospheres were synthesized using the method reported in the literature.52 Twenty milligrams of carbon nanospheres was weighed and added to a solution of oleylamine (100 μLit) in hexane (1 mL), kept for an hour, and then centrifuged, dried, and used for further reaction with Au/Ag precursors. Functionalization with butanedithiol was done by adding 20 mg of carbon nanospheres to a solution of butanedithiol (120 μLit) in 1 mL hexane, holding for an hour, centrifuging, and drying; the solution was used for further reaction with Au precursor.

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BACKGROUND Homogeneous vs Heterogeneous Nucleation. The birth of a crystal in a solution phase proceeds by the chance aggregation of a small number of monomers (atoms, in the case of a simple monometallic crystal). When the size of the aggregate is small, the increase in interfacial energy on the formation of this aggregate outweighs the reduction in the volume free energy and thus the overall system energy is increased. However, beyond a critical size, the volume free energy term dominates and takes the nucleus in to the “growth”

B


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Figure 1. (a) Homogeneously nucleated cuboctahedral equilibrium shape or Wulff shape of FCC metal, (b) Winterbottom shape of the FCC metal nucleated heterogeneously on a substrate, (c) isotropic material (liquid, for instance) with spherical Wulff shape nucleated on substrate with a high interfacial energy, and (d) change in shape of the liquid when the interfacial energy is low. Both (c) and (d) are parts of the equilibrium spherical shape for the liquid. (e) Three out of the infinite possible orientations or Winterbottom shapes for a hypothetical cube-shaped nucleus nucleating on an isotropic substrate. (f) Nucleation barrier for each of the orientations in (e). It is evident that for the given situation, the [011] orientation is the most preferred one for heterogeneous nucleation.

regime where further addition of monomers leads to the reduction in the total free energy of the system. The barrier that must be surmounted is primarily overcome by the gain in thermal energy and is proportional to the interfacial energy between the growing crystal and the medium. In the presence of a foreign substrate, the barrier for the nucleation process is reduced as compared with the barrier for homogeneous nucleation leading to the formation of the crystal by heterogeneous nucleation. For the same radius (r) of the nucleus, ΔGhet(r) = ΔGhom(r)·f(θ), where ΔG is the nucleation barrier and θ is the contact angle of the nucleus with the substrate. Lower f(θ) signifies more wetting of the nucleus on the substrate (i.e., a lower nucleus/substrate interfacial energy) and hence a lower nucleation barrier. For the case of homogeneous nucleation of a crystal in an isotropic medium, the shape with the least barrier of formation is same as the equilibrium shape of the crystal. The equilibrium shape of a free crystalline particle is obtained by the Wulff construction on the polar plot of the surface energy.53 In the case of heterogeneous nucleation, the analogous shape is termed the Winterbottom shape. The equilibrium shape of a particle on a substrate is derived from its Wulff shape and is given by Winterbottom construction.54 The Winterbottom shape is constructed on the polar plot of surface energy (γ-plot for a given OR (orientation relationship between the substrate and nucleus)). To construct the Winterbottom shape, we first construct the Wulff shape of the free particle as a γ-plot and then truncate this shape parallel to the substrate with the extent of trunctation dependent on γPS, the interfacial energy between the particle and the substrate. The resulting shape is the Winterbottom shape and its surface area is termed as the exposed surface area (i.e., the surface exposed from the Wulff shape) in the remainder of this paper. Figure 1a,b, respectively, illustrates the example of the equilibrium cuboctahedral shape for an FCC crystal (Wulff shape) and the corresponding Winterbottom shape of the crystal on a substrate for a specific value of the interfacial energy between the particle and the substrate.

We have discussed directly in terms of the barrier for nucleation that implicitly includes the volume free energy term (that is the same for a given system) and the interfacial energy term (that depends on the nucleation condition, viz., homogeneous vs heterogeneous). In terms of the barriers, heterogeneous nucleation has a lower barrier as compared to homogeneous nucleation. It is important to note that r*, the critical size of the nucleus, remains the same for both homogeneous and heterogeneous nucleation while the barriers are different (as shown in Figure 1f, for instance). While r* remains the same, the volume of the critical nucleus is different for homogeneous and heterogeneous nucleation. Specifically, the volume of the critical nucleus directly relates the barrier for nucleation. For solids nucleating on a substrate the additional variable of orientation can change the barrier as discussed below. Effect of Interfacial Energy on Winterbottom Shapes. For the case of a liquid undergoing heterogeneous nucleation, the resultant equilibrium shape (Winterbottom shape) would be a truncated sphere, where the extent of truncation by the substrate plane depends on the liquid−substrate interfacial energy or the wetting. Two cases of wetting of a liquid on two different substrates have been depicted in Figure 1c,d. The exposed surface area of a liquid nucleus is dependent only on the extent of truncation and is independent of the orientation (OR) of truncation because it is a part of the sphere. However, in the case of crystals, that have anisotropic surface energy, the exposed surface area of the nucleus depends on the orientation for the same extent of truncation. The OR is defined by the direction in the crystal that is normal to the planar substrate (or equivalently the plane that is parallel to the substrate in the case of a cubic crystal). Thus, there are innumerable possibilities for the orientation of the nucleus on the substrate, each with a different nucleation barrier. The orientation that is eventually selected for nucleation will be the one with the least barrier of nucleation. Concept of the Most Preferred Winterbottom Shape for a Particular Interfacial Energy between the Substrate and the Particle. Considering the nucleation of a cube-shaped C


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Figure 2. Orientation of a heterogeneously nucleating particle depends on its interfacial energy with substrate. Results of the calculation of the favored orientation for heterogeneous nucleation of an FCC metal on an isotropic substrate under the assumptions of isotropic interfacial energy. The shape of the crystal that is nucleated is a part of the Wulff shape for the FCC crystal that is truncated to an extent dependent upon the interfacial energy with the substrate. The orientation with the least barrier for heterogeneous nucleation has been shown as a function for different values of the interfacial energy (quantified by an effective contact angle). Abrupt changes in the favored orientation are observed.

1/√2, and γ 001 = 2/3 (calculated using the bond breaking model64 and normalized such that γ111 = 1/√3). For a particular substrate−particle interaction energy, quantified by ds, where ds = (γSP − γSV), γSP is the substrate− particle interfacial energy, and γSV is the substrate−vapor interfacial energy, the cuboctahedron is truncated along all possible orientations and the resulting Winterbottom shapes at each of these orientations are obtained by minimizing the Helmholtz free energy (F) of the system.

nucleus (i.e., the Wulff shape being a cube) on an isotropic substrate, the crystal can, in principle, nucleate in any of the infinite number of possible orientations. Three out of the infinite number of possible orientations ([100], [110], or [111] perpendicular to the substrate) have been shown in Figure 1e. Each of these orientations will have a different nucleation barrier as depicted in the schematic in Figure 1f. In this case, the crystal orientation with [011] perpendicular to the substrate has the minimum nucleation barrier height (as shown in Figure 1f) as the Winterbottom shape has the lowest total surface energy and will be the preferred orientation for nucleation among these three orientations shown. While the previously mentioned example only considered 3 possible orientations, the actual determination of the preferred orientation should include the complete range of orientations in order to determine the most preferred orientation. Similar calculations have been carried out earlier for the case of hypothetical two-dimensional shapes;55 extension to 3D shapes of crystal has not been carried out so far. There has been a growing interest in the study of Wulff shapes and Winterbottom shapes of metals,56−59 metal oxides,60,61 and alloys57,62,63 for the design of better supported catalyst systems. In this paper, we calculate the orientations with the minimum barrier for the case of an FCC crystal (cuboctahedral shape) nucleating on an amorphous substrate for a wide range of interfacial energies and show that the orientation with the minimum barrier does indeed change with the interfacial energy between the particle and the substrate, thus providing a handle to control the orientation of the particle on the substrate. The implications of this change in orientation of the particle on heterogeneous catalysis are discussed in the later sections.

dF = − S dT − P dV + γ dA +

∑ μi Ni i

where S is the entropy, T is the temperature, P the pressure, V the volume, γ(n) the surface energy per unit area in the nth direction, A the area, and μi the chemical potential of the Nith object. Under conditions of constant T, V, μi, Ni dF = γ dA

F=

∫A γ(n) dA···

(1)

where A is the entire surface of the system including (i) substrate−vapor, (ii) particle−vapor, and (iii) substrate−particle interface. Minimization of F at a particular ds and orientation generates the corresponding Winterbottom shape. The activation barrier to form each of these Winterbottom shapes at different orientations is calculated and the orientation that has the minimum activation barrier is concluded to have the maximum nucleation preference for that value of ds. This routine has been used to find out the most preferred orientation of Winterbottom shape for a range of ds values. It should be noted that for a given substrate and particle, ds in principle can be function of OR as γPS can depend on the direction. However, for the lack of better approximation, ds is assumed to be independent of direction. The result of the calculations has been illustrated in Figure 2. To relate directly to the wetting efficiency, we have converted the ds values into effective wetting angles (θ) between the particle and the substrate. As the substrate−particle interfacial energy increases, θ increases, and we see abrupt changes in the favored orientation with changes in θ. The details of the calculation, minimization of

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RESULTS AND DISCUSSION Analytical Results. We first present the results of the calculation of the orientation with the least barrier for nucleation for the case of an FCC crystal nucleating on a planar, isotropic substrate under isothermal conditions, keeping the volume of the nuclei constant (as in the Wulff construction) for a range of substrate-particle interaction energies. We start off by constructing a truncated cuboctahedron having the following normalized surface energies: γ 111 = 1/√3, γ 011 = D


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The Journal of Physical Chemistry C the Helmholtz free energy, estimation of nucleation activation barrier, and conversion of ds into θ are presented in the SI (S1.1− S1.3). Discussion of the Analytical Results. If the particle wets the substrate, i.e., the particle−substrate interfacial energy is very low as compared to γSV, the surface energy of the substrate (substrate−vapor interfacial energy), then the truncation of the Wulff shape is such that only a very small part of the Wulff shape is exposed. As the wetting tendency decreases, a larger area of the Wulff shape is exposed. The key observation from the results presented here is that the preferred orientation for nucleation changes abruptly as the wetting condition is changed in accordance with the earlier calculations on hypothetical 2D square-shaped nuclei on substrates55 where abrupt bifurcations in the orientations were predicted. It is interesting to note that even though we assumed ds to be independent of the direction,55 the minimization of the nucleation barrier yields a preferred orientation. The directionality originates from the underlying anisotropy in Wulff shape. The preferred orientation is calculated based on the orientation with the minimum volume of the Winterbottom shape in the polar space (not the physical volume of the particle) for which the barrier for nucleation is the minimum. While the calculations predict specific orientations, it is obvious that experimentally all possible orientations within an energy window of kT will be accessible and hence can be observed experimentally. In the extreme case where the differences in the energies of the different orientations are less than kT, there will be no preferred orientation observed experimentally. Nucleation is intrinsically a stochastic process and hence we can expect that the orientation of the nuclei have a distribution. The higher the driving force of nucleation (higher temperature or lower interfacial energy, for instance), the lower the preference will be for a specific orientation and the broader the distribution. Geometric Effect of Orientation on the Catalytic Activity of Supported Particles. The catalytic activity of supported catalysts is affected by both geometric and electronic effects. The geometric effects include the surface area of the particle and the crystallography of the facets or specifically the distribution of the various adsorption sites on the surface of the catalyst. The electronic effects directly control the binding energies of the participating species on the catalyst surface. Here, we argue that the orientation of the particle on the substrate profoundly affects both these contributions and hence has a critical role to play on the catalytic activity of supported catalysts. As an example, we take a cube-shaped particle with two different orientations with respect to the substrate (Figure 3a,b). The size (physical volume) of the particle is the same in both cases. For the same value of the interfacial energy between the particle and the substrate, it can be analytically calculated that a particle with [001] normal to the substrate has ∼67% of the total surface area that is exposed (from the Wulff shape) and available for catalysis, while the particle with the [111] normal to the substrate has ∼72% of the total surface area that is exposed, and thus there is a difference in the exposed area with respect to the orientation of the particle even though the nominal particle size is the same in both cases. There is an additional effect for very small particles (∼2 nm) (that are usually employed/active for catalysis), that comes about due to the inclination of the facets with respect to the substrate. If we assume that facets, that are inclined with their outward surface normal at an obtuse angle with the outward surface normal of the substrate, are sterically unavailable to the adsorbing species, the accessible surface areas

Figure 3. Figure illustrates the difference in exposed area and accessible area for a cube at two different orientations [001] in (a) and [111] in (b), and a cuboctahedron at two different orientations [001] in (c) and [111] in (d) of the particle at same dS value with respect to the substrate.

for the different wetting conditions can be calculated. Of course in a real system comprising particles of >5 nm, there is the possibility of adsorption of small molecules like H2 and CO on the “unfavorable” facets via diffusion through the substrate. However, for the purpose of estimation of accessible areas directly from the vapor phase, we have neglected this possibility. Thus, even though the exposed area of a [111] oriented cubic particle is 72%, the accessible area is only ∼50%, whereas the accessible area for [001] oriented particle is still ∼67%. Similar calculations are illustrated for a cuboctahedral particle (Figure 3c,d). The values presented above are only estimates to illustrate the effect of orientation while the actual accessible area would depend upon the shape of the particle and the effective size and conformation of the molecule on the surface. Analytical estimation of change in accessible area and exposed area of 111 and 100 facets with change in dS at different orientations of a cuboctahedron is plotted in SI, Figure S1 (section S1.4). Electronic Effect of the Orientation: Results of Analytical Calculation and Density Functional Theory (DFT). The electronic effects are significantly influenced by the orientation of the particle in the case of particles having strong EMSI. Based on simple arguments of symmetry, it is observed that in Figure 4a,b, the binding sites marked a, b, c, and d on the particle that were symmetry equivalent sites on the particle with the [001] orientation become nonequivalent sites once the orientation of the particle changes to [011]. This implies that the binding energies of any species on a and b will be different from those of c and d, and hence the catalytic efficiency of these two particles with different orientations is expected to be significantly different even though they are of comparable size and shape. The effect of size on EMSI has been discussed in detail recently.35 Of course, this effect becomes lower as the size of the particle increases and/or the interaction between the substrate and the particle becomes weaker. As a specific example, the results of first-principles DFT calculations on the binding energies of CO at the same symmetry-equivalent site of a supported Au catalyst particle for different orientations of the particle with respect to the support is demonstrated. For this study, two different orientations of Au clusters are attached on a Li-doped MgO (100) surface and completely optimized. The binding energies for the two clusters on the Li-doped MgO (100) surface are 3.98 and 4.01 eV, higher than the Au bulk cohesive energy, which will favor the cluster stability on the substrate. The relaxed structures of the two systems used for this study are shown in Figure 4c and d. Figure 4c shows the geometry of the adsorbed gold cluster with 100 facets parallel to the substrate (designated as cluster “A”) and E


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particles, once again emphasizing the key role of the orientation of the particle on the substrate. To rule out the effect of doping (location of the Li ions) on the trends of CO adsorption on different facets of Au clusters, similar calculation with Au clusters attached to pristine MgO has been performed (SI, section S2, Figures S2 and S3, and Table S1) that shows a similar trend of CO adsorption energies indicating the key role of orientation on the adsorption of analytes. Effect of Orientation on the CO Oxidation Kinetics: DFT Results. We investigated CO oxidation on these systems by the Marv van Krevelan (MvK) mechanism, where the lattice oxygen interacts with the adsorbed CO on the cluster. However, on the pristine MgO(100) the vacancy formation energy of oxygen is more than 7 eV. Therefore, the reaction barrier will be very high to achieve in normal circumstances. Doping of MgO(100) surfaces has been shown to lower the vacancy formation energy.66−68 Previous work suggests that dopants can lower the vacancy formation energy of the host oxide and thus promote the oxidation reaction by the MvK mechanism.69−71 Hence, we dope the MgO(100) surface with two Li atoms to model a charge stabilized system which loses a lattice O. It was found that doping the substrate with Li promotes the MvK mechanism of CO oxidation on these systems. The electron localization function of Au doped on pristine MgO and Li-doped MgO is presented in SI S2.3 and Figure S4 to understand the effect of Li-doping on the interaction with attached Au cluster and adsorbed CO. The different CO adsorption energy on changing facets and their orientations can be better understood by looking into the electronic structure alterations for the constituent gold atoms. The d-band model, introduced by Norskov and Hammer41 is very useful in explaining the adsorption strength of adsorbents on transition metals.72,73 The lower the d-band centers (dBC) from the Fermi-energy are, the lower the empty antibonding states available for CO adsorption.72,74 There is a direct correlation between the position of the d-band center and the CO adsorption strength in this study as tabulated in Table 1.

Figure 4. Effect of orientation of catalyst particle on electronic effect. Symmetry-equivalent binding sites marked by a, b, c, and d on the cubic particle at orientation [001] in (a) become nonequivalent when the particle is oriented at [011] with respect to the substrate in (b). Completely optimized models of 1 nm gold clusters with (100) parallel (cluster “A”) in (c) and (111) parallel (cluster “B”) in (d) with respect to Li-doped MgO(100) substrate. Red, yellow, gold, and blue represent O, Mg, Au, and Li, respectively. Completely optimized geometries of CO adsorbed on (e) On Top site of 100 facet in cluster A (OT/A), (f) On Top site of 100 facet in cluster B (OT/B), (g) Interstitial site of 111 facet in cluster A (INS/A), and (h) Interstitial site of 111 facet in cluster B INS/B of Au cluster on Li-doped MgO(100) substrate.

Table 1. d-Band Center (dBC) and the Corresponding Reaction Barriers for CO Oxidation Reaction for Each of the Four Adsorption Geometries

Figure 4d shows the completely relaxed geometry of the cluster with 111 facets parallel to the substrate (designated as cluster “B”). For Au NPs with face-centered cubic (fcc) crystalline structures, the surface energies of the low-index 111 and 100 facets are significantly lower than the 110 and other high-index facets.65 Therefore, we model 111 and 100 facets for adsorption of CO as they are most likely exposed for catalysis. For the given systems it is seen that CO prefers the interstitial site between two atoms (INS) for 111 surface and the on-top site (OT) for 100 surface of Au cluster (a comparative adsorption energies on the individual sites is provided in Table S1 in SI). In the case of adsorption on 100 facets (be it parallel/inclined), it is seen that the cluster becomes distorted after the adsorption, while in the case of adsorption on the 111 facet (parallel or inclined), it retains the structure. This indicates that the 111 facet is catalytically more stable than the 100 facet. Figure 4e−h lists the adsorption energies for the four different geometries studied, viz., OT adsorption site on 100 facet of the cluster A (OT/A, Figure 4e) and OT adsorption site on 100 facet of the cluster B (OT/B, Figure 4f), INS adsorption site on 111 facet of the cluster A (INS/A, Figure 4g), and INS adsorption site on 100 facet of the cluster B (INS/B, Figure 4h). It clearly shows that there can be dramatic changes in the binding energy of the analytes on the

adsorption geometries dBC Reaction barrier

OT/A

OT/B

INS/A

INS/B

−3.98 eV 0.59 eV

−2.22 eV 0.50 eV

−2.86 eV 0.38 eV

−1.66 eV 0.43 eV

To gain insight into the kinetics of the CO oxidation on these differently oriented facets we perform nudged elastic band (NEB) calculations to get the minimum energy path and reaction barriers for the different systems. The plot shown in Figure 5 depicts the reaction barriers calculated by the NEB method for the four different systems in our study. The reaction barriers are also tabulated in Table 1 as marked in the plot. The plot shows that the best kinetics in terms of low reaction barrier of 0.38 eV is for an optimal dBC position of −2.86 eV which is for INS/A. The optimized dBC ensures a CO adsorption strength which is appropriate for good CO oxidation catalysis. Hence the effect of orientation of the Au cluster on the Li-doped MgO is evident in the CO oxidation reaction kinetics study. The structures of the initial (IS), the transition (TS), and the final state (FS) of CO oxidation reaction on 111-inclined Au cluster on MgO are presented in Figure 5b. F


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Experimental Demonstration of Changing Orientation. Having demonstrated that the orientation of the particle could play a key role in the catalytic activity of the particle, we turn our attention to an experimental demonstration of the changes in orientation that could be induced by changing the interfacial energy between the particle and the substrate. Based on the results of our analytical calculations, the orientation of the crystal can be altered by changing the nature of the substrate (and hence the interfacial energy). To experimentally demonstrate the possibility of changing the orientation of nucleation (nucleation texture) of the particles, heterogeneous nucleation of Au or Ag on functionalized amorphous carbon substrates (TEM grids) was chosen as a model system. Functionalization of the carbon substrate was done with either an amine or a thiol (X-ray photoelectron spectroscopy data in SI section S3.1, Figure S5) to vary the interfacial energy between the nucleating crystal and the substrate. Depending on the combination of the metal and the

Figure 5. Reaction barrier plotted against the d-band center positions with respect to the Fermi energy for the four systems of OT/A, OT/B, INS/B, and INS/A. The structures shown are for the most promising kinetics of CO oxidation on 111-inclined facet on Li-doped MgO(100) surface by MvK mechanism. IS, TS, and FS depict the initial, transition, and final states of the CO oxidation reaction, respectively. Red, crossed red, yellow, gold, blue, and dark brown represent O, lattice O taking part in CO oxidation, and Mg, Au, Li, and C atoms, respectively.

Figure 6. Model system and the nucleation texture of the particles on substrates due to changes in the interfacial energy. Schematic illustration of the variation of the amorphous carbon surface by functionalization with either oleylamine and butanedithiol and its effect on the orientation of the heterogeneously nucleating metal is shown in (a). Experimentally obtained transmission electron micrographs (TEM) showing low magnification images of Au nucleated on butanedithiol functionalized carbon-coated TEM grid, Au nucleated on oleylamine functionalized carbon-coated TEM grid, and Ag nucleated on oleyl amine functionalized carbon-coated grid in (b), (c), and (d), respectively. Corresponding high resolution images of typical single crystalline particles of each system are shown in (e), (f), and (g), respectively. FFTs in the insets in the high-resolution images show the d-spacings corresponding to Au/Ag. Inverse pole figure showing maximum orientation preference for Ag-oleylamine around [011] in (h), around [111] for Auoleylmine in (i), and around [111] for Au-butanedithiol in (j). G


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The Journal of Physical Chemistry C Table 2. Correlation between Experimentally Observed Orientation and Theoretical Predictionsa

a

The experimental contact angle is estimated based on the measurement of the particle breadth to height from particles that are oriented edge-on to the substrate.

A detailed analysis of the ipf plot with the statistics of orientation distribution is presented in SI S3.2, Figure S6, Tables S2−S3. Discussion of the Experimental Results. The preferred orientation and the distribution in orientation changes with the changes in the functionalization of the substrate as predicted by the analytical calculations. Uncertainties in the interfacial energy (and hence the contact angle75) make it difficult to obtain a direct correlation between the calculations and the experimental observations. However, the experimental observations are qualitatively consistent with the expected trends. Based on the calculations (Figure 2), a contact angle around 106° leads to nucleation with a [110] preferred orientation. For higher values of the contact angle, around 118°, an orientation close to [111] becomes favored (Figure 2). This trend is seen experimentally also; in the case of Au-oleylamine, for which the contact angle estimated experimentally is 133°, has [111] as the most preferred orientation; in the case of Au-thiol for which the experimentally obtained contact angle is 122°, the most preferred orientation is [111], and also in the case of Ag-oleylamine, for which the contact angle is estimated to be 110°, the [110] orientation is the most favored. The experimentally obtained and analytically obtained contact angles and the orientations are summarized in

functional group on the surface, the samples are denoted as Authiol, Au-amine, and Ag-amine. The scheme of sample synthesis and low magnification images and high-resolution TEM images of Ag and Au particles nucleated on functionalized grids are shown in Figure 6. Orientation Analysis Using Precession Electron Diffraction. The crystallographic orientation of individual particles on the substrates was obtained by analyzing diffraction patterns collected using the precession electron diffraction (PED) technique as described in the Methods section. To obtain a statistical estimate of orientation, diffraction patterns were collected from at least 250 particles from each sample set and analyzed. The inverse pole figures (ipf) for the individual sample sets are presented in Figure 6h−j. In Figure 6h, the ipf for Agamine shows orientations close to [110] to be most frequently occurring. However, for both Au-amine and Au-thiol, the maximum number of particles nucleate with a [111] orientation, as evident in Figure 6i,j. In both these cases, orientations around [011] are also populated. Comparing Au-amine and Au-thiol, the spread in orientation is observed to be higher for Au-amine than for Au-thiol. It is evident that experimentally there is a distribution of orientations for each system as predicted before. H


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The Journal of Physical Chemistry C Table 2 and there is a reasonably good agreement with the predictions of the analytical calculations and the experimental results. The error in the contact angle is given in SI Table S4. The contact angle is estimated experimentally by measuring the ratio of the lateral breadth (b) of the particle to its height (h)32 as illustrated in SI Figures S7 and S8. For measurement of b/h and hence contact angle, the particle should be imaged edgeon on the amorphous substrate. This was not possible from the existing design of the system where we nucleated particles on the functionalized polymer coating of the grid. So a new design of the system was engineered, keeping the particle and substrate the same. Carbon nanospheres of ∼50 nm were functionalized with oleylamine and butanedithiol and nucleation of Au/Ag was done maintaining the same reaction conditions. During imaging in TEM the holder was tilted to get an edge-on configuration of the particle on the substrate. b/h was measured from about 20 such images of particles of each system and the mean value of b/h ratio and the standard deviation is presented in SI Table S4. The higher the ratio (b/h), the stronger the substrate−particle interaction is, and the lower the contact angle. The trends shown in the ipf are also consistent with the expected distribution in the orientations. When the substrate− particle interaction is stronger, the distribution in orientation is expected to be sharper. The distribution in orientation is thus in accordance with the wetting of the particle and substrate, viz., Agamine has the narrowest distribution and Au-amine has the broadest distribution. The temperature for nucleation of single crystalline Au on amine-functionalized carbon was higher compared to that for nucleation of single crystalline Au on thiol-functionalized carbon (Methods section). The Gibbs’ free energy (ΔG) for the formation of Au is estimated to be −270 kJ/mol on the aminefunctionalized substrate and −135 kJ/mol on the thiolfunctionalized substrate (SI S3.4). The higher driving force in the Au-amine case leads to a lower orientation selectivity which explains the higher spread in orientation for the Au-amine system as compared to the Au-thiol system. The interfacial energy of Authiol is lower as compared to Au-oleylamine,76 which also supports the lower spread in this system.

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CONCLUSION

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ASSOCIATED CONTENT

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Methodology of analytical calculation, supporting DFT calculations, supporting experimental results, detailed analysis of precession electron diffraction data, and experimental contact angle measurement (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Abhishek Kumar Singh: 0000-0002-7631-6744 N. Ravishankar: 0000-0003-0012-046X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge DST for funding and AFMM, IISc for the microscopy facility.

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REFERENCES

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The fundamental question about orientation preference during nucleation has been addressed for the first time. It has been both theoretically as well as experimentally demonstrated that FCC metals indeed show nucleation texture during heterogeneous nucleation and the texture depends on the interfacial energy between the nucleus and the substrate. The large difference between the observed catalytic activities in systems with seemingly similar catalyst size/distribution can possibly be explained by the differences in the nucleation texture. Larger driving forces and lower interfacial energies leads to a larger spread in the orientation of the particles, and thus a better control of the orientation may be obtained by a suitable choice of synthesis condition and substrate. We believe that the insights provided here are general and widely applicable for nanoscale heterostructures for different applications.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02237. I


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Orientation Selection during Heterogeneous Nucleation: Implications

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