Controlling the Dynamic Instability of Capped Metal Nanoparticles on

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Controlling the Dynamic Instability of Capped Metal Nanoparticles on Metallic Surfaces Evangelina Laura Pensa, and Tim Albrecht J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02994 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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The Journal of Physical Chemistry Letters

Controlling the Dynamic Instability of Capped Metal Nanoparticles on Metallic Surfaces Evangelina Pensa a* Tim Albrecht a,b* a

Imperial College London, Department of Chemistry, Exhibition, Road, London SW7 2AZ,

UK. Present address: bUniversity of Birmingham, School of Chemistry, Edgbaston Campus, Birmingham B15 2TT, UK Corresponding Author * E-mail address: [email protected], [email protected]

1 ACS Paragon Plus Environment

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Abstract

Small metal nanoparticles (NPs) with core-sizes ranging from 1-3 nm constitute a bridge between molecules and colloids with unique electronic, catalytic and other properties. Many applications entail immobilization onto solid supports, but while NP behavior in solution is well studied, the effect of the interaction between NPs and the substrate surface is understood less. Here, we follow the structural evolution of thiolated monolayer-protected AuNPs on Au(111) substrates at the single-particle level in real-time using high-resolution in situ Scanning Tunneling Microscopy. We show how the reactivity of the substrate affects the stability of the immobilized NPs and how their structural identity can be preserved. Entropically driven redistribution of the NP's protective capping layer is an important element in the disintegration process and at the same time rather generic. Our findings may thus have wider implications on the design and optimization of functional surfaces involving NPs, made of materials other than Au.

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Immobilization of small metal nanoparticles (NPs) on macroscopic substrates is key to designing functional surfaces for electronic, catalytic and other applications.1-3 Sustained operation requires that NPs preserve the atomic arrangement, overall structure and chemical identity, while in contact with the substrate.4 This aspect has come to the fore also in nanoimpact electrochemistry, where the impact of individual NPs on an electrode surface is detected by catalytic enhancement or other redox activity, for example.5-7 To this end, questions arise as to whether the particles stick, are partially dissolved or bounce back into the solution after each collision. Each of these scenarios will have implications for the temporal evolution of the substrate surface, in terms of its structure and chemical properties, as well as the interpretation of the data. With regards to the stability of NPs, most efforts have focused on the properties of the NPs themselves,3 as well as those of the capping layer, typically used to stabilize the dispersion. Moreover, NPs in solution can dynamically exchange metal atoms and ligands in between them; a field recently explored and named as “interparticle reactions”.8-10 However, relatively little attention has been paid to the effect of the substrate surface itself in the stability and integrity of the NPs. Here, we show in a combined single-crystal electrochemistry and in situ Scanning Tunneling Microscopy (STM) study that the reactivity of the substrate surface can indeed play a key role in this context. Taking Au25(HT)18 and Au144(HT)60 as examples (HT: 1hexanethiol), we follow at the single-particle level, how these NPs evolve in real-time on bare and modified Au(111). We find that, in the former case, particle decomposition is fast on the STM timescale (< minutes), while the process is markedly slowed down once the surface reactivity is reduced. Based on high-resolution STM imaging, we find that the alkanethiol protecting layer redistributes from the NP to the substrate, resulting in domains of thiolates on the Au(111)



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surface. The NP core converts into islands on the substrate of typically mono-atomic height. Experimental evidence and theoretical considerations suggest that the redistribution process of the protecting layer is mainly entropy driven in the present case. However, the fundamental mechanism appears to be more generic and likely applies to other substrate materials, too, as long as they form stable chemical bonds with the molecules in the capping layer. This in turn may have important implications for the design of functional surfaces involving NPs, for example in sensing or catalysis.

Figure 1. (a) DPVs of Au144(HT)60 (upper, blue) and Au25(HT)18 (lower, red) dispersions recorded in 0.1 M TBAPF6 in DCM. [Au144(HT)60] = 2.5 mg·ml-1 and [Au25(HT)18] = 0.8 mg·ml-1. (b) DPVs of Au144(HT)60 samples on Au(111) obtained by two different methods: drop-casting (purple) and immersion (orange) recorded in aqueous 0.1M KClO4. All DPVs were recorded at pulse height: 0.05 V, pulse width: 50 ms, period: 200 ms and scan rate: 20 mV s-1.

First, we synthesized Au144(HT)60 and Au25(HT)18 nanoparticles according to Quian et al11 and Parker et al.12 NP samples were characterized using Atomic Force Microscopy (AFM), UVvis spectroscopy and electrochemistry (a detailed explanation of the synthesis, purification and characterization is included in sections 1-2 of the supporting information, SI). As expected based on the design, from height measurements the average diameter was found to be 3.49 ± 0.02 nm and 1.60 ± 0.02 nm for Au144(HT)60 and Au25(HT)18, respectively (fig S3). The UVvis spectra showed the expected absorption features at around 500 nm and 700 nm and no bands due to plasmon resonance, due to the small size of the particles (fig 4 ACS Paragon Plus Environment

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S4).12-13 Differential pulse voltammograms (DPV) of NP dispersions in dichloromethane (DCM) and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte, revealed multiple equidistant peaks with an average spacing (∆E) of 0.27 ± 0.06 V for Au144(HT)60, and the distinct molecule-like redox behavior with an estimated electrochemical HOMO-LUMO gap of 1.50 ± 0.08 V for the Au25(HT)18, see fig 1a.14-15 Taking all

this full characterization

data together, we can confirms that AuNPs

stoichiometry are Au25(HT)18 and Au144(HT)60. After characterization in solution, the NPs were immobilized on Au(111) substrates by immersion in NP dispersions in DCM (cNP = 2µM) for 12 h at room temperature (see section 1 in the SI). The modified surfaces were then characterized by electrochemical techniques and STM. Images were acquired using a Keysight 5100 STM (Keysight Technologies, United States), operated in constant-current mode at a setpoint current isp between 0.3 and 1 nA and a bias voltage Vbias between -0.1 and 0.7 V (actual conditions are specified in table S1). STM tips were prepared by mechanically cutting a Pt/Ir wire (80:20%, Goodfellow, UK). STM data were analyzed with the free WSxM software (Nanotec Electronica S.L., Spain).16 Somewhat

unexpectedly,

following

the

incubation

in

Au144(HT)60

dispersion,

voltammograms did not show the typical, multiple equidistant charging features observed in solution14 and in some cases on surfaces as well.17-19 Those features were visible, however, after drop-casting a sample from the same stock solution onto the substrate surface, as shown in fig. 1b (purple), implying that the NPs themselves were intact. In both cases, we observed the characteristic features from reductive desorption of the thiol and were able to exclude free, unbound thiols in solution as an explanation for the observed desorption charge (see SI, section 2.1). Taken together, that would suggest that some interaction between the substrate and the NPs must have taken place, even though the immersed and drop-cast samples displayed different electrochemical responses. Since the observation of the charge states is



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rooted in the small capacitance of the particles - usually on the order of 1aF for particle of this size - those findings could indicate that perhaps the structure of the particles was altered when in contact with the surface, or that the surface coverage might be too low for the charge features to be detected, in the case of the immersed sample. Hence, we investigated these two options by STM imaging of the substrate surfaces in air.

Figure 2. Au(111) substrates after immersion with NP solutions. (a-b) 'In air' STM images and related data for Au(111) after incubation in 2 µM Au144(HT)60 dispersion in DCM, showing Au islands and HT molecules adsorbed on the surface in a 'lying-down' configuration. (c-d) 'In air' STM images and related data of Au(111) after incubation in Au25(HT)18, showing Au islands on the substrate. Adsorbed HT molecules are randomly distributed or in a rectangular c(4x2) lattice. Scale bars: 50 nm (images a/c), 10 nm (images b/d) and 4 nm (images I, II, III and IV).

Those images show that the former is indeed the case. Rather than discrete particles, approximately round islands were observed, which were not present on the substrate before the incubation, cf. figs. 2 and S5a. Statistical analysis of the images (fig. 2a) reveals that these islands are 0.24 nm in height (or multiple times this value), corresponding to the thickness of a single (multiple) layer(s) of Au atoms.20 Close inspection of the surface reveals two different periodic patterns I and II, covering and surrounding the Au islands (fig. 2b). The structure of I is characterized by distance of 1.26 ± 0.03 nm and 0.51 ± 0.01 nm. The structure of II consists in a double row (with periodicity 2.3 ± 0.1 nm) separated by 0.5 ± 0.1


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nm, and a corrugation of 0.52 ± 0.03 nm. Similar structures have been found for alkanethiol self-assembled monolayers (SAMs) on Au(111) at low thiol coverage and have been assigned to thiolate molecules in a 'lying-down' configuration.21-22 These structures are also in line with the reductive desorption results (according to RS-Au + e- → RS-), which showed the presence of HT on the Au(111) surface at low coverage (23 ± 3 µC·cm-2 or θ = 0.10 ± 0.02 relative coverage based on the number of surface Au atoms, cf. SI-sections 3.1 and 3.4.1). For comparison, a full HT monolayer would result in a reductive desorption charge of 75 ± 3

µC·cm-2, corresponding to a relative coverage of θ = 1/3. Similar observations were made when Au(111) substrates were immersed in a dispersion of Au25(HT)18 in DCM (fig. 2c), indicating that the observed behavior is a more generic feature of the NP interaction with the surface. High-resolution STM images (fig. 2d) again show that the HT molecules are adsorbed around and on top of the Au islands. Two domains were observed, namely III and IV. In 'III', the molecules appear to be randomly distributed, whereas in 'IV' they adopt one of the most typical dense lattices: the c(4x2) structure, with characteristic distances of 0.9 ± 0.1 nm and 1.0 ± 0.1 nm . The coexistence of such mixed domains has been observed during SAM growth at high coverage.23 Accordingly, the reductive desorption data confirm that the coverage of HT molecules is high with a desorption charge of 55 ± 3 µC·cm–2 (θ = 0.24 ± 0.02), higher than for the Au144 sample (vide supra), but lower than for a full monolayer (cf. SI-sections 3.1 and 3.4.2). The average size of the Au islands is bigger for Au144(HT)60 than for Au25(HT)18 under the same preparation and imaging conditions (344 nm2 vs 89 nm2 or equivalently ≈1323 vs. ≈342 Au atoms, respectively, cf. SI-fig S8). Both trends are in line with the expected thiol/Au ratio, viz 0.41 for Au144(HT)60 and 0.72 for Au25(HT)18. Finally, we note that the surface structures shown in fig. 2 are characteristic



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for immersion times as short as 30 sec (cf. fig. S9), so restructuring must occur on timescales faster than that. Thus, both samples displayed the same qualitative behavior, in that they were unstable towards structural disintegration and island formation, even though both are rather stable in dispersion (over days). The observation prompted us to investigate the underlying reason for the observed behavior and in which way it might be controlled. Given the well-known dynamic nature of the Au/thiol bond,24 we thought it likely that thiols from the capping layer may be able to redistribute onto the bare Au(111) surface, once the NPs interact with the surface. The bonding between the thiol and the Au nanoparticle and Au(111) substrate, respectively, are presumably similar.20,

25-26

However, an unmodified

Au(111) substrate surface offers a large number of potential binding sites, hence the redistribution process should be favored from an entropic point of view. Once the protective shell around the particles at least partly disintegrates, the reactive core is exposed and restructuring into bulk Au accelerated. The above scenario would also imply that by lowering the surface reactivity of the Au(111) substrate, for example by physi- or chemisorption of a competing adsorbate, the process could be slowed down or even halted. This would also offer an explanation for the qualitatively different behavior observed for the immersed and the drop-cast sample, cf. fig. 1: since the drop-cast sample is more likely to produce an excess of NPs on the surface, potentially even multi-layers, the bare Au substrate is quickly passivated by the (partial) degradation of the first NP layer. Subsequent deposition of additional layer will then leave the NPs intact, which then dominate the electrochemical response (inc. the observation of Coulomb states). In the following, we present evidence that indeed supports this hypothesis. Accordingly, we went on to study the behavior of the NPs on the surface, for strongly and weakly passivated substrates, respectively. Starting out with the former case, Au(111)


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substrates were first immersed in a solution of thiols, and then in a dispersion of Au144(HT)60 or Au25(HT)18 in DCM for at least 12 h (cNP = 9 µM, see SI for further details inc. STM images in fig S12a). In the case of Au144(HT)60, the NPs are observed on the surface as bright spots with an average height of 3.6 ± 0.9 nm (predicted: 3.2 nm).19 Equally, for Au25(HT)18, the cross-section analysis gives a height of 1.1 ± 0.3 nm (predicted: 1.2 nm) for the NPs, fig. S12b. This is very close to the reported diameter for these NPs, indicating that NPs have retained their integrity on the surface, at least up to this point in the experiment.27 Thus, with the thiols covering the surrounding substrate surface, particles can be immobilized without significant alterations in their size or structure. However, in line with the above hypothesis, there is little driving force for the protective coating of the NPs to re-arrange, the particle size and shape remains unchanged during up to 12 hours of STM imaging. They are kinetically stabilized.

Figure 3. In situ STM images and related data of Au(111) in mesitylene after addition of (ab) Au144(HT)60 and (c-d) Au25(HT)18 in a final concentration of 3.4 µM. Scale bars in the images correspond to 60 nm. Height and area profiles show slow degradation of Au144(HT)60 at step edges (A/C) and on terraces (B). For Au25(HT)18, the same analysis shows faster degradation to Au islands. 9 ACS Paragon Plus Environment


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Then, we turned to a system where the Au(111) substrate surface is less strongly passivated, namely in the presence of mesitylene that is known to physisorb onto Au.28 The weaker interaction then means that there is more rapid exchange of mesitylene molecules between the substrate surface and the bulk solution, even though the Au substrate is in contact with a layer of mesitylene at all times. Notably, this is not expected to change the energy budget of the thiol replacement to a significant extent. When a thiol relocates from the particle to the substrate to replace a mesitylene molecule, the resulting vacancy in the protective shell is very likely going to be filled with another mesitylene molecule from solution - hence, the number (and type) of bonds broken equals the number (and type) of bonds formed. On the other hand, the entropic argument made above nevertheless remains and given that the redistribution process now also relies on mesitylene coming off the substrate surface, the overall decomposition rate should be smaller. We first imaged the Au(111) substrate in pure mesitylene, cf. fig. S5b; after 1 h, small aliquots of Au144(HT)60 and Au25(HT)18 dispersed in DCM were added to a final concentration of 3.4 µM. For the Au144(HT)60 sample, the NPs appear as bright spots in the STM images, most of them located near to step-edges (figs. 3a and S13). Their evolution over time was then tracked by analysing the height and area of the corresponding features, indicating that he surface still has not equilibrated, fig. 3. As representative examples, we take the three new features that appear from one image to the next, as indicated by the arrows in panel a. Interestingly, the initial feature heights for the just adsorbed NPs are ca. 50% lower than the expected value for single Au144(HT)60, suggesting that the first step of the decomposition is still faster than the STM imaging time scale. Moreover, based on the time evolution of A, B


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and C (fig. 3b), it appears that the process is faster on the step edges (A and C) than on the terrace (B), with degradation rates of 1.3 nm min-1 for A/C and 0.87 nm min-1 for B. After this fast initial step, the particle height for all of them steadily decreases at a rate of 0.16 nm min-1 until it reaches a plateau at 0.24 nm, the monoatomic height of the Au islands. Features A/C are incorporated into the step edges while feature B, remains as an Au island on the surface. Finally, feature B is decreasing in size. The reduction on the Au islands size have been previously observed for electrochemicalgenerated Au islands on Au(111) and Au(100) in acidic media.29-30 According to He and Borguet, if Au adatoms diffusion is rate-limited by interface-transfer process, the dissolution of individual Au island area follows the expression A(t)= A0 - k(A0)0.5t, where A(t) is the Au island area at the time t, A0 is the Au island area at t=0 and k is the dissolution constant. The fitting for particle B, gives a constant rate of 0.021 nm s-1(cf. table S5). For Au25(HT)18, STM images show that the surface is covered by islands and around them, there are regions with adsorbed HT (see dotted lines in fig 3c and fig S15). In contrast to Au144(HT)60, the appearance of new features was not detected during this set of images. The initial island height is 1.1 nm and thus only 27% of the original, expected height for the Au25(HT)18, fig 3d. After ~5 min the average height reduces to 0.25 nm, suggesting that the decomposition is almost complete and faster than Au144(HT)60. From here onwards, the evolution of the Au island size features two trends, namely growth and dissolution, fig 3d.



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Figure 4. Area histograms of Au island as a function of time obtained from degradation of Au25(HT)18 on Au(111) in mesitylene. Two populations have been found, named I and II. The dotted line shows how the area of population II increases. (b) Time-evolution of the most probable radius for population II (from Gaussian fit (black) in a). Fitting parameters: Ln(Radius) = A + n Ln(time); A= 1.43 ± 0.05, n = 0.19 ± 0.03, R2 = 0.92. The blue and red dotted lines correspond to the two slope predict by the LSW theory.

In order to shed further light into the growth Au island mechanisms, we analyze the image data in more detail below. The area of the Au islands in each STM image was determined and then plotted as a histogram, fig 4a, yielding two populations I and II. The Au islands in population I reduce in number but do not change significantly in size, while those in population II grow in size and number. Hence, we focus on population II for further analysis. Classical 2D cluster’s ripening theory based on the Lifshitz-Slyozov-Wagner (LSW) theory proposed that the time-evolution of the average particle radius, Rt, follows a power law, Rt α tn, where n is a dimensionless exponent related to the growth mechanism. When n = 1/2, the rate-limiting process is the interface transfer, i.e adsorption/desorption of an adatom at the edge of a cluster. When n= 1/3 the process is governed by surface diffusion of the adatoms from one cluster to another one.31 As shown in fig 4b, fitting of the time-evolution of the most probable radius for population II gives an exponent of n= 0.19 ± 0.03 (black dotted


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line), i.e. lower than both limits predicted by the theory (blue and red dotted lines). In other words, the average particle radius evolves slower than expected, even when compared to the surface diffusion limit. This could be related to the presence of the mesitylene or the thiol, but more refined theory may be needed, to take into account these more subtle effects. After ~ 8.37 min, the Au islands start to decrease in size at an effective rate constant of k = 0.018 nm s-1, (see SI-section 7), a similar rate than in the case of Au144(HT)60 samples. The process is most likely due to either the integration of Au atoms into the substrate surface or dissolution of Au-thiolate complexes,

21

pointing out that the initial island’s composition are

not pure gold but rather formed by a mixture of Au and thiols, as their height suggested. In our imaging data, we do not observe significant changes in the underlying terrace structure of the substrate, but at present, we are unable to rule out either of the two processes.

In summary, we found that the stability of small NPs is strongly affected by the reactivity of the substrate surface. Complete and comparatively faster disintegration takes place on bareAu(111) surfaces, where Au atoms nucleate in islands and the thiol-capping layer forms a sub-monolayer on the substrate. The rate of disintegration could however be modulated by 'passivating' the surrounding Au surface. In the presence of relatively weakly interacting, physisorbed mesitylene, the rate is markedly reduced and the decomposition process was monitored in situ at the single-particle level and in real-time by high-resolution STM imaging. When the surface was strongly passivated, i.e. by a layer of strongly bound alkanethiols, the rate was reduced even further, and the NPs remained structurally intact on the timescale of the STM imaging experiment (> 12 hours). Based on the dynamic nature of the thiol/Au bond, surface-induced disintegration of the particles appears to be driven by entropic factors, namely by the availability of a large number of binding sites on bare and weakly passivated Au(111) substrates. These rather basic pre-requisites may point to a certain



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generality of the observed effect, in that the proposed a mechanism may be at work, whenever A) the bond strength between NP core and capping layer on the one hand, and between substrate surface and capping layer on the other, are at least comparable. And B), when the substrate surface features a suitable number of binding sites, which may be either vacant or in a sufficiently labile, dynamic equilibrium (as reported here, for bare Au(111) and Au/mesitylene, respectively). This includes 'single-component/mixed SAM' systems, e.g. Au NP/dithiol/Au substrate,32 as well as 'multi-component' systems, such as Au NP/Pt substrate, Pt NP/Au substrate and core-shell particles.18-19,

33

In this context, the Au/Pt case is an

interesting one, but already significantly more complex. Namely, while thiols are known to form self-assembled monolayers on Pt, compared to Au the Pt/thiol bond is weaker and the layers are less well ordered.34 Pt also forms surface oxides more readily and is generally considered catalytically more active (albeit not for all reactions).35-36 Finally, additional factors such as the energetics of the metal bond formation (Au/Au vs. Pt/Au vs. Pt/Pt) and alloy formation on the surface, which can moreover depend on feature size.37 Taken together, one could hypothesize that in the case of 'Au NP/Pt substrate', decomposition should be slower than in the case presented here, because the driving force for the thiol to move from the Au NP to the Pt substrate is lower. This may also explain why such degradation was not observed in references 18 and 19. However, with these complexities in mind, we leave such systems for future studies, and merely note that those would allow the proposed explanation to be tested in further detail. ASSOCIATED CONTENT Supporting Information. Detailed experimental procedures, additional STM images and electrochemical data. Notes 14 ACS Paragon Plus Environment

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The authors declare no competing financial interests. ACKNOWLEDGMENT This work is supported by the Leverhulme Trust (RPG 2014-225).

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