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Pinpointing the Cause of Platinum Tipping on CdS Nanorods Claudia Caddeo, Vasco Calzia, Luigi Bagolini, Mark T. Lusk, and Alessandro Mattoni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06264 • Publication Date (Web): 04 Sep 2015 Downloaded from http://pubs.acs.org on September 9, 2015

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The Journal of Physical Chemistry

Pinpointing the Cause of Platinum Tipping on CdS Nanorods Claudia Caddeo,†,‡ Vasco Calzia,†,‡ Luigi Bagolini,†,‡ Mark T. Lusk,¶ and Alessandro Mattoni∗,† Istituto Officina dei Materiali (CNR - IOM), Unità di Cagliari SLACS, Cittadella Universitaria, I-09042 Monserrato (Ca), Italy , Dipartimento di Fisica, Università degli Studi di di Cagliari, Cittadella Universitaria, I-09042 Monserrato (Ca), Italy , and Department of Physics, Colorado School of Mines, Golden, CO 80401, USA E-mail: [email protected]

∗

To whom correspondence should be addressed IOM-CNR SLACS ‡ UNICA ¶ MINES †

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Abstract We computationally identify the precise mechanism by which metallic platinum aggregates at the tips of cadmium sulfide (CdS) nanostructures. Large-scale atomistic simulations of physically realistic nanorods are used to quantify the chemical, dispersive and electrostatic contributions to platinum interaction with CdS. Crystallographic anisotropy as well as facet, edge and tip effects are accounted for to show that Pt aggregation, known as "tipping", is not due to the dynamics of adhesion and diffusion. Instead, efficient tipping is found to be due to long-range electrostatic interactions of metallic ions with polar tips set up by CdS surface stoichiometry. The results are used to stipulate the physical conditions by which metallic decoration of ionic nanostructures can be optimized. This is expected to be useful in the realization of nanoscale metal-semiconductor devices.

Hybrid semiconductor-metal nanoparticles have great relevance in the design of innovative devices. 1–4 Possible applications of metal/semiconductor heterostructures range from nanoscale ohmic contacts in MOSFETs, 5 light harvesting, 6 energy conversion, 7 high performance lithium ion batteries, 8 photocatalytic applications, 9,10 etc. Particularly intriguing is the case of metallic nano contacts in which, by a controlled deposition of metal quantum dots, each individual nano particle can become a fully working device at the nanoscale. The ability to selectively control the interaction of the metal with the semiconductor is accordingly very important in nanotechnology. Among the various semiconductor-metal hybrids, CdSe@CdS/Pt nanocrystals have been recently considered as a potential system for water photolysis. CdS and CdSe are probably the most studied semiconductors for the realization of quantum dots with the possibility to synthesize a great number of CdSe/CdS nanostructures including core-shell, dot-rod, tetrapods, octapods. 11 Solar water splitting for hydrogen production has been achieved with CdSe/CdS-based nanostructures decorated at the surface with Pt domains. The water splitting reaction is typically limited by large activation barriers and requires catalysts. Noble 2


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metals such as Pt are the most widely used. Efficient catalysis depends, among other factors, on the control of the metal decoration and patterning that, in turn, strongly depend on deposition conditions such as temperature, illumination and Pt concentration. 12,13 For example, it has been shown that by varying the Pt concentration in solution from ∼40 to ∼200 mg/l, the metal deposition can switch from tipping to uniform coverage 14 . The ability to deposit the catalysts nonuniformly in specific regions of the semiconductor nanostructure surface is crucial to keep photo-generated charges separated so as to carry out oxidation/reduction reactions. Nonuniform metal coverage (tipping) has been observed as a preferential deposition on the CdS tips or in the proximity of the CdSe seed (see e.g. Fig.1). The cause of this metal tipping phenomenon has not been clarified yet. For example, the role of illumination has been investigated and it has been found that it can enhance tipping but is not a necessary requirement. 15–17 Conversely, an excess of negative charge in the site where the metal nanoclusters are formed is often found in association with tipping , but such electron accumulation can originate from different mechanisms such as Cd etching, electrochemical processes and photogeneration. Some works on Au and Pt deposition on CdSe@CdS nanorods have shown that the initial deposition of Au/Pt (at low metal concentration) occurs onto sulfur-rich facets of the nanorod that are negatively charged. 12,15,16,18 Higher metal concentrations and/or longer deposition reactions ended up in a non-selective coverage of the CdS nanostructures. It has been also proposed that the selectivity could have chemical origin, with the tips of the semiconducting nanostructures being more reactive due to structural anisotropy and/or the presence of surface defects leading to a higher surface energy. 15,16,19,20 The effect of ligands has also been taken into account: amine ligands used for the deposition reaction are in fact more likely to attach to apolar surfaces hindering the metal deposition on these facets. 12,21 From a theoretical standpoint, the interface between Pt and the (10¯10) apolar surface of CdS has been studied by ab initio methods for nano, amorphous and epitaxial interfaces, and their Schottky barrier heights have been calculated. 22 CdS nanorods have been simulated

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also in presence of amine ligands and in explicit solvent in order to study the rod-rod interaction, but in absence of Pt or other metals. 23 Despite its great technological relevance, a quantitative theoretical analysis to elucidate the physical mechanisms that control Pt tipping on CdS nanostructures is still missing. In this work, we make use of large scale model potential molecular dynamics (MPMD) simulations to generate realistic models of CdS nanostructures. Adhesion and metal diffusivity are quantified on surfaces, edges and tips, and the role of dispersive Van der Waals and electrostatic driving forces on tipping is elucidated. In particular, by varying the average fraction of charged platinum atoms, we identify the optimal conditions for tipping and conclude that the major driving force for this phenomenon are the long-range electrostatic interactions between positively charged metal atoms and sulfur rich facets of the nanorod.

Figure 1: a,b: example of experimental samples of Pt-tipped CdS nanorods. Preferential Pt deposition can be observed on both tip and bottom of the nanostructures. (adapted with permission from Ref. 13,19. Copyright 2008 and 2012 Americal Chemical Society ). c: atomistic model of Pt tipped CdS nanorod.

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Methods CdS has a wurtzite crystalline structure and CdS nanorods and nanomultipods are typically grown along the c axis ([0001] direction), thus exposing the apolar (10¯10) surfaces as lateral facets. They have triangular 24,25 or hexagonal 26 cross-section and a pyramidal tip. We have chosen to represent the nanocrystals in our atomistic models by a single CdS nanorod with triangular cross-section, with a flat tip (bottom) which exhibits the polar (0001) surface, and a sharp tip (head) which instead exposes the (1011) surface (and equivalent ones, see Ref. 24). Fig. 2 shows an example of model system. The force field adopted is a two-body

Figure 2: Model system for the study of CdS nanostructures. Insets show the detail of each surface of the nanorod. interatomic potential consisting of a long range Coulomb part and a short range LennardJones (LJ) term q i qj Vij = + 4ǫij rij

"

σij rij

12

−

σij rij

6 #

(1)

Parameters were taken from Ref. 27 for CdS and from Ref. 28 for Pt. Parameters for Cd-Pt and S-Pt interactions are first calculated with usual mixing rules (geometrical and aritmetical mean for ǫ and σ, respectively) and then rescaled in order to reproduce the ab initio bond strength. Model potential molecular dynamics simulations were performed by using the NAMD 29 molecular simulations package (v. 2.9). The equations of motion of atoms were integrated by 5



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using the Velocity Verlet algorithm with a timestep as small as 1.0 fs. Multiple timestepping was used, with short-range nonbonded interactions calculated every two timesteps and full electrostatics evaluated every 4 timesteps. All the electrostatic contributions were computed by the particle mesh Ewald (PME) sum method, with PME grid spacing of 1Å. All the calculations have been performed at room temperature in the canonical ensemble. Temperature was controlled by Langevin thermostat with damping coefficient γ=1 ps−1 . The density functional theory (DFT) calculations were performed with a generalized gradient approximation (GGA), using the Perdew-Burke Ernzerhof (PBE) formula 30 for the electron exchange and correlation energy and norm-conserving pseudopotentials generated with the Troullier-Martins scheme. 31 First-principles calculations have been performed using a plane wave basis set expansion scheme with the QUANTUM-ESPRESSO package, 32 and an energy cutoff of 45 Ry was used for the wave functions. In such scheme, the Pt dimer formation energy and bond distance is found to be of 3.9796 eV and 2.34 Å, respectively, in good agreement with experimental data. 33

Results and discussion CdS rods with triangular cross section are the simplest structures for which tipping has been reported 13,19 (see Fig.1).

Our computational investigation therefore began with a

stability analysis of such geometries as a function of their size and polarity. We followed an incremental strategy in which analyses of surfaces and pyramids (representing the tip) informed an investigation of the rods of interest. The formation energy, Ex , of a nanostructure, x, is defined as the difference between the energy of the nanostructure, E(x), with respect to that of its constituents:

Ex = E(x) − Nx µbulk

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In the case of surfaces, the formation energy per unit surface defines the surface energy, Es . The relevant rod facets, 24 shown in Fig. 2, are (0001), (1011) and (10¯10). Atomistic Table 1: Surface energies for three CdS surfaces. surface γ (0001) 0.60 N/m (1011) 0.92 N/m (10¯10) 0.41 N/m models of of surfaces were obtained by cutting a thick slab from the bulk along the proper crystallographic planes and by optimizing the atomic position through standard energy minimization. The resulting surface energies are reported in Table 1. For polar surfaces, e.g. for (0001), we used an average of the two possible polarities between the Cd and S- terminated surfaces. The lowest energy was associated with the nonpolar (1010) surface. Pyramidal nanostructures were first studied as they represent the tip of rods of interest. For each pyramid size, two different models with opposite polarity are possible: sulphurterminated and cadmium-terminated. For each geometry, the atomic positions were fully relaxed and the resulting formation energies per CdS dimer are plotted in Fig. 3. As expected, the formation energy decreases as the surface-to-volume ratio decreases. Systems with less than 7 CdS bonds per side were found to be unstable: they lose their pyramidal shape and transform in compact clusters of quasi-amorphous structure during relaxation. The S-terminated structures have a formation energy that is several per cent lower than their Cd-terminated counterparts. The formation energy of the pyramids, Ep , was also compared with Γ, i.e. the formation energy of pyramids composed of perfect surface facets:

Γ=

X

γ(ˆ nf )Af

(2)

f

Here f runs over the Nf facets of area Af , n ˆ f is the crystallographic (normal) direction of the surface corresponding to facet f , and γ(ˆ nf ) its formation energy. This is shown as a continuous curve in Fig. 3. The small difference, Ep − Γ, shows that the formation energy 7



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of the pyramid Ep is mainly due to the cost of forming the surfaces while only a few percent of its value depends on other factors such as edges or strain.

The formation energies of

Figure 3: Formation energy of pyramidal CdS nanoclusters. Insets show some examples of investigated clusters. L is the side length of the pyramid. the rods, Er , are provided in Fig. 4. They are smaller for increasing size nanostructures, as expected, and are well described by the sum of formation energies of the surfaces Γ. The difference Er − Γ (which is small as for the pyramids) increases with decreasing size attributable to a concomitant reduction in crystalline order. By comparing the results on the formation energy we conclude that: (1) most of the strain and atomic rearrangement is expected at the tip; and (2) the formation energy is dominated by surface effects with edges and tips playing only a minor role. With the morphology and energetic stability of CdS rods understood, we focused on the interaction of nanorods with Pt to identify the dynamics that would induce metal accumu8


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Figure 4: Left: formation energy of CdS nanorods. Insets show some examples of investigated structures. Right: definition of head, body and bottom of nanorod. L is the side length of the nanorod. lation at the tip. We first investigated the interaction of a single Pt atom with different CdS surfaces. The energy as a function of distance for Pt on infinite (10¯10), (1011) and (0001) surfaces can be found in the SI. The choice of these surfaces is motivated by experimental indications. 24 The minimum energy (compared to the asymptotic value at large distance) gives the Pt binding on the surfaces. A large binding energy ( ∼ 3 eV) is found for all the crystallographic surfaces of the rods. This means that Pt has a strong tendency to bind all over the surface of the rod. We further analyzed the Pt-CdS interaction with the nanostructure instead of an infinite surface. In this case the binding energy was calculated for different positions of the Pt with respect to the nanostructure. No strong differences in the Pt-CdS interactions, as a function of position, were found. This implies that the local interaction of the neutral Pt atoms with the different facets, tips or edges cannot induce by itself an efficient Pt tipping. To further validate this important conclusion, we performed a comparison with ab initio calculations using computationally tractable structures. Classical and DFT binding energies

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are in reasonable agreement for both relaxed geometry and magnitude of binding energy. Both methods predict a higher binding energy for Pt at the center of the nanorod surfaces that decreases by 30% near the edges. The agreement between DFT and MP is a robust indication that any enhancement of Pt/CdS interaction cannot be attributed to morphological features such as edges or crystallography. Since Pt binding is equally probable over the rod, tipping could be the consequence of a thermally activated dynamic process in which Pt atoms move from facets with higher Pt mobility to facets (or regions) with lower mobility. Tipping would be due to a low Pt mobility at the rod tips. We performed a careful analysis of Pt mobility on CdS surfaces (of single Pt atoms, dimers and trimers), which revealed that the energy barriers for Pt hopping are so high that the atoms do not move significantly during the times explored by molecular dynamics (∼20 ns).

To bypass this computational bottleneck, we carried out a pseudo

molecular dynamics analysis based on a suitable rescaling of the hybrid interactions. More details are provided in the SI. Other methods for accelerating the dynamics exist and could be used 34,35 but these require extensive analysis that are beyond the scope of the present work. The mobility of the single Pt atom on the tip is found to be smaller with respect to the body and bottom of the nanorod, but the values become closer for larger Pt clusters such as dimers and trimers. The results suggest that the nonuniform Pt diffusion on the rod may contribute to tipping but it cannot be a strong driving force. The above analyses preclude the possibility that the observed tipping of CdS with Pt is associated with charge neutral processes. However, allowance for charged Pt ions allows electrostatic forces to come into play and results in a markedly different interaction with the nanorod. To evaluate their importance, we have calculated the forces exerted on Pt atoms of different prescribed charge at a large number of points at a fixed distance of 1 nm from the rod. The resulting forces are represented by arrows pointing to the corresponding position, with lengths proportional to the magnitude of the force as shown in Fig. 5. The force field associated with a neutral Pt atom is shown in top left panel, depicting

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Figure 5: CdS/Pt force field for Pt atoms at constant distance equal to 1 nm from the CdS nanorod. Red (blue) is used for attractive (repulsive) interactions. A magnification factor of 50 has been applied to the forces shown in the upper panels. a relatively uniform profile with reductions at the edges and tips. The forces are always attractive (red) but decrease with distance r as ∼ r−5 , the same decay as for van der Waals forces. The force field therefore has a dispersive nature. The results are qualitatively different, though, for positively charged Pt. Consistent with

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the chemical precursors used in the decoration process, 19 we considered doubly ionized atoms. The result is a strongly nonuniform force field with attractive and repulsive regions (Fig. 5, bottom left). The rod is neutral but it induces an electrostatic force on the Pt atoms that can be visualized as the rod being a macroscopic electric dipole with forces converging at the negative pole and diverging from the positive one. The transition from attractive to repulsive field is due to the presence of a negative sulfur-rich bottom of the rod. This indicates that the charged Pt is the key to activate a non-uniform attraction. It also highlights the crucial role of rod polarity resulting from the presence of sulfur atoms at the bottom of the rod. To further elucidate the role of sulfur, an alternative (non stoichiometric) S-rich rod model was also considered. This structure was obtained by shifting the crystallographic planes used to cut the rod from a perfect crystal resulting in a sulfur-terminated tip. This rod has an excess number of S atoms with respect to Cd ones (about 3%). The nanorod is kept neutral by compensating the excess negative charge with a constant counteracting charge: q → q + dq where

dq = −(qS NS + qCd NCd )/(NS + NCd )

(3)

Similar deviations from stoichiometry are also suggested by experiments indicating that both the head and bottom of colloidal CdS nanorods are sulfur-rich. 13,19 By repeating the force-field calculations for these structures we found (Fig. 5, bottom right) that the Pt ions are strongly attracted by the head and (although less strongly) by the bottom and repelled by the body of the nanorod. Furthermore, the interaction in this case has a long range electrostatic behavior ∼ r−1 compared to the neutral case ∼ r−5 . In real experimental conditions, screening effects are expected due to the presence of ligands or solvents but they would not qualitatively alter our results and conclusions. Such effects could be included by using a suitable dielectric constant ǫ/ǫ0 > 1 that rescales the electrostatic forces ∼ 1/ǫr2 . Our analysis causes us to conclude that it is the charge of Pt ions that results in the observed

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tipping. Having identified the driving force for tipping to be electrostatic, it is important to quantify the phenomenon and the optimal tipping conditions. The key parameter controlling 0 the tipping is the fraction of doubly ionized Pt atoms, ν = NP++ t /NP t , in the reference volume.

Fixing the value of ν is equivalent to setting the average charge per Pt atom, qP t = 2ν, and it is used here to describe the ionization conditions. We simulated a system containing a CdS rod embedded within a uniformly random distribution of isolated Pt atoms (see Fig. S11 in Supporting Information). The system evolution at room temperature is studied by molecular dynamics.

In order to keep the

computational cost low we used a concentration of Pt atoms corresponding to ∼8 g/l, which is at least one order of magnitude higher than the experimental one. This made it possible to accumulate large Pt statistics during a single run so avoiding very large simulation boxes and times. At the end of each simulation (when the number of Pt atoms on the rod is stationary) we analyzed the percentage of metal atoms attached to the different regions (head, bottom and body) of the rod. The choice of the borders separating each region is arbitrary but fixed once and for all while varying the charge qP t . Head and bottom as a whole are hereafter referred to as tips. In Fig. 6 we report the local density of Pt atoms (i.e. the number of Pt per unit of the area σ = NP t /A) calculated on the tips and divided by the local density on the body as a function of qP t : τ (qP t ) =

Ntip Abody σtip = · σbody Nbody Atip

(4)

the parameter, τ , measures the tipping efficiency. When τ > 1 Pt atoms preferentially go on the tips. τ = 1 means that

Ntip Nbody

=

Atip , Abody

i.e. the platinum density is the same everywhere

on the surface. If τ < 1, the adhesion on the tip is unfavorable. The results are collected in Fig.6. In the case of stoichiometric rods (green line), the left panel of Fig. 6 shows that the density of the Pt atoms on the tip is moderately higher than that of the body (τ ∼< 5 everywhere). The largest tipping is found when qP t ∼ 0.2 corresponding to 10% of 13



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Figure 6: Tipping parameter τ as a function of Pt charge qP t calculated for stoichiometric (green) and S-rich (red) nanorods. Continuous lines are a guide for the eye. Horizontal axis is in logarithmic scale to better visualize the fast growth of the curves. Pt ionization. In the case of a S-rich nanorod (red) the tipping is strongly increased. It was found that τ ∼ 5 at small qP t and a tipping value as high as τ ∼ 25 for qP t = 0.2, where the curve gives a broad maximum. If the Pt charge is further increased, the tipping starts decreasing due to the repulsion between Pt atoms. In the limit of complete ionization, qP t ∼ 2, the Pt-Pt repulsion dominates over the difference between Pt-tip and Pt-body interactions and τ goes down to one (i.e. uniform density over the rod). Our results showing the strong tipping on S-rich rods are in agreement with experiments reporting the tendency of Au and Pt to deposit on S-rich regions of CdSe@CdS nanorods. 12,13 We predict that optimal conditions for tipping requires from 5% to 20% doubly ionized Pt atoms. The above calculations were repeated by using unrelaxed rod structures, i.e. with atoms kept in the ideal crystalline structure before atomic optimization. Higher τ values are found (see SI) and can be explained by the larger local electric fields that are expected at the surfaces of the unrelaxed structures and that decrease upon structural optimization. Notably, the conclusions found for the relaxed structures are further confirmed in the unrelaxed case 14


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and in particular the dependence of tipping on qP t and the strong effect due to S-rich rods.

Conclusions Large-scale atomistic simulations on realistic models of CdS nanostructures were used to quantify the driving forces and identify which one is responsible for Pt tipping. The analysis included the metal adhesion and diffusivity on all surfaces, edges and tip as well as the study of dispersive and electrostatic forces.

This is important since the available experimental

works have not elucidated unambiguously the main driving force for metal tipping. The relevance of electrostatic interaction was suggested in some cases (for example for Au tipping of semiconducting nanorods) but many other works have attributed the tipping to the effect of ligands, dangling bonds, defects at the tip or surfaces, and structural anisotropies. Our analysis clarifies that the tipping is the result of long-range electrostatic forces between ionized Pt and the nanorod with a strong enhancement in the case of S-rich nanorods. We further predict that the most efficient tipping should occur for a fraction of doubly ionized Pt atoms in the range 5% - 20%. The other investigated physical factors (defects and anisotropies) are found to have a minor effect.

Acknowledgments We acknowledge useful discussions with Pietro Delugas (CompuNet, Istituto Italiano di Tecnologia). This work has been funded by CompuNet of Istituto Italiano di Tecnologia, by Regione Autonoma della Sardegna under L.R. 7/2007 CRP-24978 and CRP-18013, and by Consiglio Nazionale delle Ricerche (Progetto Premialità RADIUS). We acknowledge computational support by CINECA through ISCRA Initiative (Projects SPASS and TIPTAP).

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Supporting Information Available Details on nanorod construction, interaction basins and diffusion analysis can be found in the Supporting Information.

This material is available free of charge via the Internet at

http://pubs.acs.org/.

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Pinpointing the Cause of Platinum Tipping on CdS Nanorods Claudia Caddeo, Vasco Calzia, Luigi Bagolini, Mark T. Lusk and Alessandro Mattoni For Table Of Contents Use Only

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Pinpointing the Cause of Platinum Tipping on CdS Nanorods - The

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