Ultrathin Au-Alloy Nanowires at the Liquid–Liquid Interface - Nano

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Ultrathin Au-alloy Nanowires at the Liquid-Liquid Interface Dipanwita Chatterjee, Shwetha Shetty, Knut Müller-Caspary, Tim Grieb, Florian F. Krause, Marco Schowalter, Andreas Rosenauer, and N. Ravishankar Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05217 • Publication Date (Web): 04 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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Ultrathin Au-alloy Nanowires at the Liquid-Liquid Interface

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Dipanwita Chatterjee1, Shwetha Shetty A.1, Knut Müller-Caspary2, Tim Grieb2, F.F. Krause2, Marco Schowalter2, Andreas Rosenauer2 and N. Ravishankar1*

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Materials Research Centre, Indian Institute of Science, Bangalore 560012

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University of Bremen, Otto-Hahn-Allee NW1, D-28359 Bremen

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ABSTRACT: Ultrathin bimetallic nanowires are of importance and interest for applications in electronic devices, as sensors and heterogeneous catalysts. In this work, we have designed a new, highly reproducible and generalized wet chemical method to synthesise uniform and monodisperse Au based alloy (AuCu, AuPd and AuPt) nanowires with tunable composition using microwave-assisted reduction at liquid-liquid interface. These ultrathin alloy nanowires are sub 4-nm in diameter and a couple of microns long. Detailed microstructural characterization shows that the wires have FCC crystal structure and they have low-energy twin boundary and stacking fault defects along the growth direction. The wires exhibit remarkable thermal and mechanical stability that is critical for important applications. The alloy wires exhibit excellent electrocatalytic activity for methanol oxidation in alkaline medium.

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Keywords: Alloy nanowires, ultrathin nanowires, liquid-liquid interface reaction, stacking faults, twin boundary, electrocatalysis, methanol oxidation

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Engineering the size and morphology of crystals leads to dramatic changes in their properties that could lead to different applications. In particular, onedimensional (1-D) nanostructures find applications in various fields ranging from photovoltaics, electrical and electronic devices, sensors, thermoelectrics to catalysis. When the diameter of such 1-D nanostructures is reduced to a few atoms thick, quantum confinement effects dominate and a host of unexpected transport, mechanical and structural properties result. While template-based synthesis methods are widely exploited for 1D structure, template-less synthesis methods offer distinct advantages. A novel symmetry-breaking mechanism was proposed and used to synthesize ultrathin, single crystalline Au nanowires by template-less wet chemical method1. Since then there have been numerous studies on synthesis and understanding the defects in the nanowires2-5, their structure relaxation 6, 7 and novel electrical behaviour8-10. These wires have been used in the fabrication of nanoscale electronic devices11, 12, solar cells, fuel cells, photoelectrocatalysts13, 14, electrocatalysts15, biological and chemical sensors16. While ultrathin Au nanowires 1 ACS Paragon Plus Environment

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have been extensively studied, their fragile nature makes handling of the wires critical and limits its application and versatility.

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Alloy nanowires are interesting, not only from a fundamental point of view as a model system to study phase stability, but also for potentially dramatic enhancement of properties and stability as compared to Au nanowires. However, an extension of the method used for synthesis of ultrathin Au wires to alloy nanowires is challenging. Physical methods like CVD, lithography and electrodeposition in artificial template have been used to synthesize alloy nanowires useful for electrocatalysis but fabricating sub 5 nm ultrathin nanowires is not possible by physical methods. Most of the wet chemical methods used in literature result in high angle grain boundaries in the wires (polycrystalline nanowires) demonstrating excellent activity in electrocatalysis17. Pd-based alloy nanowires synthesis using sacrificial Te template are facile but result in thicker (> 10 nm) and polycrystalline wires18. Ultrathin nanowires with low energy defects like twin boundary or stacking fault defects and without the presence of extended high energy defects are significantly more stable against electromigration19-21 as compared to their polycrystalline counterparts. Planar defects like twin boundary (TB) or stacking fault (SF) are low energy defects. For example in Cu the TB energy is 0.02-0.16 mJ/m2 and SF energy is twice the TB energy22. These low energy defects in fact improve mechanical strength of the material23 and makes it suitable for transport property studies8, 9. High angle grain boundaries and high energy defects are the ones that contribute adversely to wire stability and electromigration24, 25. These wires containing the low-energy defects like twins and stacking faults (but free of any high-energy defects) have been designated as single crystalline in literature1, 4, 6-9, 26. Till now there has been no report on the growth of ultrathin alloy nanowires that serve as model systems to study stability and properties of 1D systems at the molecular length scale. Thus there is a pressing need to develop a general method for the synthesis of such ultrathin alloy wires.

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Synthesis of bimetallic alloy nanowires is challenging because of the difficulties in co-reducing two metal precursors with very different reduction potentials with simultaneous control over the morphology. Recently, in a report on synthesis of ultrathin, helical AuCu nanowires by Ruben et. al27 correduction of Au and Cu precursors in presence of suitable reducing agent and capping agent formed facetted AuCu alloy nanoparticles which underwent oriented attachment to form the helical alloy nanowires having Boerdijk−Coxeter−Bernal structure. However, developing a general approach towards the synthesis of a variety of ultrathin alloy nanowires by wet chemical method remains a challenge. In this letter, we report for the first time the development of a facile synthesis method of ultrathin Au based alloy nanowires at the liquid-liquid interface using ultrathin Au nanowires2 as a template. The synthesis method is explored for synthesis of AuM (M= Cu, Pd, Pt) alloy nanowires with tunability in composition of Au and M with atomic percentage of M varying between 25% and 75%. The composition tunability is precise from about 3 atomic percent for AuCu to about 10 atomic percent for AuPt. The alloy nanowires 2 ACS Paragon Plus Environment

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are sub 4 nm in diameter and a couple of microns long. Structural and microstructural features of the wires are presented along with the possibility of using these wires as excellent electrocatalysts for alcohol oxidation.

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Figure 1. Schematic explaining the alloying mechanism and the liquid-liquid interface reaction design (a) Illustrates conversion of a 2 nm Au nanowire grown along [111] direction into a ~4 nm AuM (M= Cu/Pd/Pt) wires with the same orientation. (b) Illustrates the design of the microwave assisted reaction at the liquidliquid interface (polar-nonpolar interface, Figure S1).

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We have synthesized AuM alloy nanowires by microwave assisted reduction of the second metal (M) on the surface of ultrathin Au nanowires2 and concomitant diffusion during the reaction to form the alloy wires as illustrated in figure 1(a). The microwave method relies on the use of highly polar solvents like polyols having high loss tangent value28, 29 for effective microwave radiation absorption. Ethylene glycol, having a loss tangent value of 1@3 GHz is regarded as an excellent solvent for 3 ACS Paragon Plus Environment

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microwave absorption30 with the added advantage that it acts as a reducing agent for the metal precursors. However, the oleylamine capped Au nanowires, are unstable in polar media and tend to fragment into particles (Figure S3 c). Hence, we engineer a simple reaction design to exploit the advantages of the microwave-assisted reduction while maintaining the stability of the Au nanowires at a relatively high temperature of 150oC.

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A two-layer reaction system has been designed where the Au wires reside in the non-polar hexane medium that is immiscible with the polar ethylene glycol (EG) layer. The polar layer contains the precursor of the second metal with an appropriate surfactant and serves as an effective absorber for microwave radiation. As illustrated in figure 1(b), the solvents in the microwave vial splits into a heavier polar (EG) layer that settles at the bottom and the lighter hexane (non-polar) layer that floats on the top. The surfactant, being immiscible in EG, complexes with Mn+ ions and transports Mn+ ions to the interface between the polar and non-polar layers (Stage 1 of figure 1(b)). The reduction of Mn+ and alloying of M with Au takes place at the liquid-liquid interface (Stage 2 of figure 1(b)). The EG layer is heated by the microwave and transfers heat to the non-polar hexane layer (Figure S2 a) and causes a mixing of the metal precursor layer and the Au-nanowire layer. The metal precursor is reduced on the surface of the Au nanowires at the interface between the polar and the nonpolar layer. Convection current provides continuous supply of the precursor and Au nanowires to the interface where EG reduces M on Au wire forming AuM alloy wires at the liquid-liquid interface. Optical images of the reaction (Fig S2b) shows continuous darkening of colour in the EG layer with increase of temperature showing that EG layer takes part in the reduction of the second metal precursor. Even after the reaction, these two layers remain separate with the alloy AuM wires settled at the interface between EG and remaining hexane (Stage 3 of figure 1(b)). The AuM nanowires being capped with surfactant (oleyl amine and/or oleic acid depending on type of M) which is readily miscible in hexane but immiscible in ethylene glycol (EG), unless forcefully mixed to form a cloudy suspension at room temperature, like to float atop the EG layer and settle at the bottom of the hexane layer, thus remaining trapped at the interface. Reaction only in EG, without the liquid-liquid interface with hexane gives rise to broken wires (Figure S3 b), demonstrating the necessity of the reaction design.

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Figure 2. Microstructural characterisation of the alloy wires (a)-(d) Low magnification bright field transmission electron microscopy (TEM) images of array of Au, AuCu, AuPd and AuPt nanowires respectively. (e)-(h) High resolution TEM images of Au, AuCu, AuPd and AuPt nanowires respectively. All the wires have growth direction [111]. All the scale bars in the high resolution images measure 2 nm.

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Reduction of the second metal (M) without the Au nanowire template, keeping all other reaction conditions same, results in the formation of ~3 nm particles of Pd and Pt (Figure S4) while the Cu precursor (reduction potential +0.34) was left unreduced as was evident from the colour of the solution after reduction. This confirms that the second metal precursor does not independently form wires under identical condition. However, in the presence of the Au wires, it rather reduces on the 5 ACS Paragon Plus Environment

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wire template and forms alloy nanowires by diffusion. In the case of Cu precursor, the reduction is in fact enabled at the surface of the Au wires leading to selective deposition of Cu on the wire surface and alloy formation by diffusion.

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Alloy formation in the wires can be confirmed from the shift of the X-ray diffraction peaks with respect to the peaks of Au nanowires (Figure S5a). The crystal structure of the alloy nanowires is FCC like the pristine Au nanowires. UV-visible spectrum (Figure S5b) of AuCu shows an absorbance band at 513 nm, characteristic of Au and at ~600 nm, characteristic of AuCu alloy, indicating presence of both AuCu and unconverted Au wires. The spectra for both AuPd and AuPt shows damping of the characteristic absorbance band of Au in pure Au nanowires confirming alloying of Au with Pt/Pd31, 32. XPS analysis also indicates the presence of Au and the second metal in zero oxidation state (FigureS6, Table S1).

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Representative low-magnification images and high resolution (HR) transmission electron micrographs (TEM) of the pristine Au and alloy nanowires are shown in Figure 2. Low magnification images show array of the alloy nanowires. More representative low magnification images shown in Figure S7 reveal that the wires are about 2 microns long, the average diameter, as measured from over 20 wires of each of AuCu, AuPd and AuPt wire samples, is 3.6 nm. The increase in the diameter of the alloy nanowires with respect to ~2 nm diameter Au nanowires (Figure S8 (a-d)) indicates incorporation of additional volume in the Au nanowires (calculation of composition in supplementary information). HR images (Figure 2e-h) confirm the single crystalline nature of both Au and the alloy wires. The alloy wires retain [111] growth direction of the Au nanowire template and exhibit a large number of twin boundaries like the pristine Au nanowires (Figure S9). A twin boundary was found every 7.4, 23.7 and 7.7 [111] lattice plane on an average for the AuCu, AuPd and AuPt systems respectively. In the case of AuPd wires, the density of twin boundaries is significantly reduced at the cost of a larger number of stacking faults.

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Energy dispersive spectra (EDS) in the TEM mode and point EDS in scanning transmission electron microscopy (STEM) mode have been acquired from 15-20 different regions of each sample. The average compositions are analysed to be Au23.9(±2.6) at%Cu, Au-32.9(±5.5) at%Pd and Au-30.2(±10.2) at%Pt. STEM-EDS elemental mapping (Figure 3 a-c) confirms the presence of Au and M (Cu/Pd/Pt) along the length of the respective alloy nanowires. Due to lower amount of signal from M with respect to Au, the corresponding elemental map of M appears noisier compared to those of Au. While imaging AuCu nanowires in STEM mode some of the nanowires break under the beam during scanning of the beam across the sample region. Point EDS analysis from the broken wires shows that these wires do not contain Cu while the intact wires show AuCu composition along the length of the wires. Thus, the sample consists of Au nanowires along with AuCu nanowires as also inferred from UV-visible absorbance data.

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Figure 3. Chemical characterisation of the alloy wires (a)-(c) EDS mapping of AuCu, AuPd and AuPt nanowires in HAADF-STEM mode indicating alloying of Au and M along the length of the wire. (d) Schematic showing twin boundary, stacking along the growth direction [111] and distribution of atoms along [11-2] direction, (e)-

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(g) Distribution of local atomic distances along the [112] direction of the AuCu, AuPd and AuPt system. 7 ACS Paragon Plus Environment

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AuCu, AuPd and AuPt wires of different compositions were synthesized by varying the M precursor amount (Fig S10). AuCu, AuPd and AuPt systems are three thermodynamically different systems with ∆Hmix 0 for AuPt. Phase diagram for bulk AuCu shows ordered intermetallics of compositions Au3Cu, AuCu and AuCu3 at a lower temperature. At high temperature AuCu becomes a random solid solution for all the compositions. Bulk AuPd on the other hand forms a solid solution for all the compositions and all the temperatures. Bulk AuPt remains phase separated. In the nano dimension many of the rules for construction of a bulk phase diagram are violated33, 34. In case of the ultrathin alloy wires, all of them are found to form a solid solution as shown in fig 3 e-g.

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The AuPt and AuPd wires were found to have higher mechanical and thermal stability with respect to the pristine, fragile Au nanowires (Figure S3 b-d). They survive in polar medium and higher temperatures up to 200oC (Figure S3 e-f) and also can withstand sonication for more than 1hr (Figure S3 g-h). Inspired by the stability and immense robustness of the AuPt and AuPd wires as compared to the pristine Au wires, their electrocatalytic activity for methanol oxidation reaction (MOR) in alkaline medium was investigated by cyclic voltammetry (CV). The pristine Au nanowires showed negligible current towards MOR even after cycling to an upper potential limit of 1 V13 (Figure S12). However, an enhanced catalytic activity in AuPt and AuPd is observed. In Ar-saturated 0.5 M KOH solution, both AuPt and AuPd displayed CV curves typical of bimetallic nature35, 36 (Figure S11 a-b). For AuPt wires, anodic current gradually increased between -0.5V and 0.5V due to the formation of Au and Pt-oxides and the corresponding reduction peaks were seen prominently during the cathodic sweep at -0.01 V (Au-O reduction) and -0.5 V (Pt-O reduction) along with hydrogen adsorption/ desorption between -0.7V in the cathodic sweep to -0.5V in the anodic sweep (Figure S11a and b). For AuPd wires Au-O and Pd-O reduction peaks are observed during cathodic sweep at 0.05 V and -0.3 V respectively. Pt-O reduction peak potential is almost same for both AuPt and commercial Pt/C (Figure. S11a) whereas that of Pd-O (Figure. S11b) shows a significant shift towards right which suggests that Au and Pd atoms are probably homogeneously distributed on the surface of the alloy nanowires37.

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AuPt and AuPd exhibits exceptional MOR behavior as compared to standard Pt/C (40%) or Pd/C (10%) (Figure 4 a-d). Specific activity j for AuPt and AuPd are 1.6 mA/cm2 and 1.35 mA/cm2 respectively which are comparable to the values of poly bimetallic nanostructures38. Both AuPt and AuPd exhibit mass activities of 375 mA/mgPt and 335 mA/mgPd respectively which are about 4.5 and 8 times higher as compared to the activities of standard Pt/C and Pd/C electrodes respectively. The electrochemical stability of alloy nanowires for MOR is studied using chronoamperometry scans at 0.4 V (Figure 4d). Both AuPt and AuPd show excellent stability in the measured region. Degradation of current density is more pronounced in the case of AuPt where a 10% loss of current density is seen at the end of 2000 s.

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Figure 4. Electrocatalytic activity and stability (a) and (b) CV curves for the specific activity of AuPt and AuPd towards MOR in 0.5 M KOH + 1M methanol solution. (c) Mass activity plots for AuPt, AuPd, Pt/C and Pd/C with respect to the loading of the active metal Pt/Pd in mg in 0.5M KOH + 1M methanol solution. (d) Chronoamperometry curves of AuPt, AuPd nanowire electrodes and standard Pt/C and Pd/C electrodes in 0.5(M) KOH + 1(M) methanol solution at a potential of 0.4V. Scan rate is 40mV/sec for all the studies.

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In the case of AuPd this loss is 4% demonstrating that AuPd alloy nanowires have better stability in alkaline medium. This is also evident in MOR cyclic scans performed for 50 times as shown in Figure. S11(c-d). At the end of 50th cycle AuPt shows a 50% loss in current whereas AuPd shows only 10% loss. Excellent MOR activity of AuPt is due to more feasible Pt-OHads formation as compared to Pd-OHads formation, which facilitates methanol oxidation39. However, the electrocatalytic instability in AuPt wires is due to its tendency to break up as the number of cycle is increased and simultaneous agglomeration which results in loss of active surface 9 ACS Paragon Plus Environment

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area (Figure S13 b). The evolution of the morphology of AuPt wires with increased number of cycles is studied. As shown in Figure S13 c-e, the wires tend to aggregate along the length more with increase in the number of cycles. The composition distribution in different regions of AuPt wires is also found to be much wider after MOR scans (Table S2, Figure S15) compared to the as synthesized AuPt wires. As noted from the table S2, the standard deviation in the composition of AuPt is 23.5 at % after the 50 cycles of electrocatalysis compared to 10.2 at % standard deviation in the as synthesized wires. AuPd wires, on the other hand are found to be intact in terms of morphology and composition distribution after MOR scans as compared to the as synthesized wires(Figure S13 a, S14, Table S2).

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In summary, we demonstrate a robust method for synthesizing ultrathin AuM alloy nanowires using an innovative reaction design at the liquid-liquid interface. We have explored the reaction design to synthesise AuCu, AuPd and AuPt alloy nanowires, the three of which belong to three different thermodynamic systems. We have also explored composition tunability in the wires by varying the M precursor concentration. The alloy wires are much more stable as compared to the Au wires and show excellent electrocatalytic activity for methanol oxidation. This class of material is extremely interesting for fundamental studies like surface segregation and atomic modulations and to investigate phase stability in ultrathin systems.

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Supporting Information

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Experimental Section

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Electron Microscope Condition

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Electrochemical Measurements

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Figure S1-S15

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Table S1-S2

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AUTHOR INFORMATION

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Corresponding Author

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*Email: [email protected]

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ACKNOWLEDGEMENTS

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The authors acknowledge DST for funding and AFMM, IISc for microscopy facilities.

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The authors declare no competing financial interest. 10 ACS Paragon Plus Environment

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