Extraordinarily Stable Noncubic Structures of Au: A High-Pressure and

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Extraordinarily stable non-cubic structures of Au: A high pressure and temperature study Gangaiah Mettela, Sorb A. Yesudas, Abhay Shukla, Christophe Bellin, Volodymyr Svitlyk, Mohamed Mezouar, Chandrabhas Narayana, and Giridhar U. Kulkarni Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03418 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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Chemistry of Materials

Extraordinarily stable non-cubic structures of Au: A high pressure and temperature study Gangaiah Mettela,†,‡ Sorb A. Yesudas,#,‡ Abhay Shukla,§ Christophe Bellin,§ Volodymyr Svitlyk,¶ Mohamed Mezouar,¶ Chandrabhas Narayana,# and Giridhar U. Kulkarni‡,∥,* †

Thematic Unit of Excellence on Nanochemistry and Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore 560 064, India.

#

Light Scattering Laboratory, Chemistry and Physics of Materials Unit, JNCASR, Jakkur P.O., Bangalore 560064, India.

§

IMPMC, University Pierre et Marie Curie, UMR CNRS 7590, IRD, case 115, 4 Place Jussieu, 75252 Paris cedex 05, France. ¶

ID27 High Pressure Beamline, ESRF, 38043 Grenoble, France

‡

Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India.

∥ On

lien from JNCASR, Bangalore.

ABSTRACT: Although the stability of Au in face centered cubic (FCC) phase under high temperatures and pressures is well studied, the stability in other lattice phases rarely encountered in crystallite domains in microscopy studies, has not been much explored due to their nanometric extensions. A recent report on Au microcrystallites crystallized in bodycentered tetragonal (BCT) and body-centered orthorhombic (BCO) phases prompted the present work, in which we have investigated for the first time, the structural stability of the BCT and BCO phases under high temperatures and separately under high pressures using high energy synchrotron x-ray diffraction. A reversible phase transition was observed for pressures up to ~ 40 GPa, indicating unusual stability of the non-FCC Au phases. However under high temperature treatment at ~ 700 °C, the transformation to FCC was irreversible.

Structural phase transitions in metals has been a topic of immense interest in the past.1 At ambient temperature, most metals crystallize in close packed structures- face centered cubic (FCC) and hexagonal close packed (HCP) lattices.2 A large number of them phase transform under moderate pressures (< 100 GPa). Many alkali and alkali earth metals, for instance, undergo phase transformation from body centered cubic (BCC) to FCC lattices well below 50 GPa. Similarly some transition metals (Fe, Co, Ir etc.) undergo phase transformation but at relatively high pressures.1 There are examples of temperature induced structural transformations in metals; Fe for instance, undergoes BCC to FCC transition at high temperatures.3 Unlike other metals, noble metals resist phase transition even at high pressures and temperatures. Among them, Au which is well known for its extraordinary chemical stability, is quite also distinguishable in terms of its structural stability under high pressures and

temperatures. Hence, it is not surprising that Au is used as a pressure sensor in high pressure research.4 Dubrovinsky et al. for the first time, reported the HCP phase in bulk Au foil under extreme conditions (P=250 GPa and T=860 K).5 In contrast, recently, some of us have reported the occurrence of stable phases of body centered tetragonal (BCT) and body centered orthorhombic (BCO) lattices in Au microcrystallites of unusual morphology penta-twinned bipyramid with high index nanofacets forming a corrugated surface.6 Based on detailed analysis of monochromatic X-ray diffraction (XRD) and selective area electron diffraction (SAED) data, it has been shown that the non-FCC phases coexist with FCC in large proportions in these microcrystallites. In nanosized metals on the other hand, the occurrences of nonnative crystal structures and their transformations to bulk lattice forms are relatively more common. Ag and Pd nanocrystallites exhibit metastable phases in trigonal7

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and BCT8 structures, respectively under pressures of a few tens of GPa. High energy XRD studies have revealed that even at ambient pressure, the penta-twinned Ag nanowires contain BCT phases,9 which transform back to FCC upon thermal annealing.10 Mirkin et al. have reported a chemical route to Au nanosheets made of a HCP structure, which readily convert to FCC on exposure to ebeam11 or following adsorption of organic ligands.12 Such observations on large bulk-like crystallites are, however, not quite common. In this study, we have investigated the structural stability of the BCT and BCO phases found in Au microcrystallites under high pressure (~ 40 GPa) using high energy synchrotron XRD. We have also examined the effect of annealing at high temperatures (~ 700 °C) on these structures. While annealing at such high temperatures is found to cause irreversible transformation to FCC, interestingly, applying high pressure brings about such transformation only in some non-FCC phases.

Figure 1. (a) Low magnification SEM images of Au microcrystallites obtained by the thermolysis of AuAgToABr at 220 °C. (b) High magnification images of (b) a bipyramid, (c) a tetrahexahedral and (d) a irregular hexagons. (e-f) Le Bail fitting of XRD patterns obtained from as-prepared Au microcrystallites.

Au microcrystallites were prepared by the thermal decomposition of a complex, termed as AuAgToABr, which consists of Au(III) and Ag(I) anions stabilized in toluene by ToABr (see experimental section in supplementary information).6, 13 The obtained Au crystallites, bipyramids with penta-twinned cross section are ~ 3 µm in length (Fig. 1a-b and Fig. S1), while the tetrahexahedral and irregular hexagons are of ~ 0.5 µm (Fig. 1c-d). The XRD analysis reveals that the microcrystallites host ~ 80% of non-FCC Au (Figs. 1a-f and Fig. S2) in BCT and BCO structures (detailed analysis is given in supporting information and ref. 12), which is even higher than reported earlier. From the SEM image in Fig. 1a, it is obvious that the non-FCC phases must be residing largely in the bipyramids (also see ED pattern in Fig. 2

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from ref. 6). The obtained lattice constants for various unit cells are as follows; aFCC=4.0810(2) Å; aBCT = bBCT = 2.9069(5) Å, cBCT = 4.0430(6) Å and aBCO = 2.9178 (3) Å, bBCO = 2.8930(6) Å and cBCO = 4.0314(5) Å. The estimated mole fractions of the phases are FCC:BCT:BCO = 23:51:26. In the following discussion, FCC is expressed as BCT/FCC with the lattice constants of a = b = 2.8861(3) Å and c = 4.0810 (8) Å to directly compare with BCT and BCO lattices. The volume of BCT (34.16 Å3) was found to be 0.5% and 0.38% higher than that of BCT/FCC (33.99 Å3) and BCO (34.03 Å3) respectively. It clearly shows that the BCT lattice is relatively more strained than the BCO lattice.

Figure 2. XRD patterns of Au microcrystallites after annealing at different temperatures. The corresponding SEM images of a Au bipyramid are given on left hand side. Scale bar, 1 µm. (c and d) Le Bail fitting XRD data obtained after annealing at 400 °C for 1h. Arrow in a indicates the shift in the position of the BCT(002) peak.

The non-FCC phases have been found to be stable for over two years under ambient conditions. In order to examine the effect of high temperature annealing, the Au microcrystallites containing ~ 80% non-FCC were annealed at various temperatures in the range, 300 - 700 ˚C in air for 1h and the changes in structure and morphology were monitored at ambient temperature using XRD and SEM respectively. The non-FCC peaks gradually reduced in intensity upon annealing at elevated temperatures (Fig. 2a). From Fig. 2a, it is clear that after 300 ˚C annealing, in the vicinity of the BCO(002) reflections, the BCT(002) peak emerges as a distinct peak, gradually shifting to lower angles as clearly seen after 400 °C annealing. The (101)T and (011)O reflections diminish completely, whereas the (101)O continues to be present at least up to 500 °C. The overall intensities of non-FCC


faster at higher temperatures. Figure 3b shows the variation in the non-FCC fraction as a function of the annealing time for different annealing temperatures (see Fig. S7 for Le bail fitting). After annealing at 500 ˚C for 540 min, the non-FCC fraction decreased to only ~ 65% (Fig. 3a and b). Similarly at 600 and 700 ˚C, we observe non-FCC to FCC conversion; however, the conversion occurred at much faster rate. The residual non-FCC fractions are 52% (after 100 min) and 44.6% (after 30 min) at 600 and 700 ˚C respectively (see Fig. 3a for the XRD patterns). The obtained rate constant is derived from the linear approximation of the available data, and the trend may not suit the zero order (Fig. S8). -d[BCT or BCO]/dt = k where [BCT or BCO] represents the mole fraction of BCT or BCO lattice at time (t), and k is the phase transition rate constant. The calculated rate constants of BCT and BCO lattices are provided in the Fig. S7. The effective activation energy (Ea) was estimated from the linear fitting of 1/T and k according to the Arrhenius

equation (k ), to be 110 kJ/mol (BCO) and 142 kJ/mol (BCT). This Ea value is not significantly lower compared to the self-diffusion activation of Au atoms in bulk Au (165 kJ/mol),14 which may be the reason why through the transition, the overall morphology of the crystallites is well retained without fragmentation (see Fig. 3d), although the sharp corners and edges of the corrugated facets become rounded. This observation may be compared to that in the case of Ag nanowires,10 where due to large difference in the two energies (190 and 110 kJ/mol, respectively), the wires are seen to fragment through the phase transition. Reverse to 5.5 GPa

a

40 GPa

40 GPa

36.9

36.9

30.1

30.1

24.9

24.9

20

20

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15.1

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Further, we have calculated the activation energy of non-FCC to FCC phase transition by monitoring the changes in the mole-fraction of non-FCC at different annealing temperatures (see Fig. 3). At a given temperature, the non-FCC mole-fraction decreased steadily, while this reduction in mole-fraction was much

1.1

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9.1

9.2

(002)O (110)T

10.35 10.50 10.65

(020)O (112)T

14.6

(111)C

(200)O

Figure 3. (a) XRD patterns of Au microcrystallites obtained after isothermal annealing for different durations. (b) Dependency of non-FCC on annealing temperature. (c) Linear fitting of rate constant (k) and temperature (T) according to Arrhenius equation. (d) SEM images of Au microcrystallites obtained after isothermal annealing for different durations. Scale bar, 500 nm. Black, red, blue and green color curves indicate the experimental, calculated, residual (experimental - calculated) and Bragg position respectively.

Reverse to 5.5 GPa

(112)O (200)T

14.8

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15.2 c

15.1 GPa (200)C (111)C (101)O (011)O (101)O

9.2

9.3

9.4


(002)O

9.5 10.7 10.8 10.9 11.0 2θ (°)

(200)O

reflections are reduced by a factor of 3, which indicates a rapid conversion of non-FCC to FCC. The Le bail fitting analysis revealed that the peaks are assignable to a new phase, BCT-I, with the lattice parameter of a = b = 2.9010(5) Å and c = 4.0631(8) Å (Fig. 2c, d and Fig. S3). The cell volume difference between BCT and BCT-I is 0.09%. However the difference is significant to make BCT-I distinct from BCT. Similar differences have been observed in the case of Ag nanowires,9 where the observed cell volume difference between FCC and FCT is reported to be 0.22%. Further, the new peaks shift toward the FCC reflections and appear to merge with FCC reflections upon annealing at 700 ˚C (see arrow in Fig. 2a). As the annealing temperature increased, the FCC{111} orientation became prominent and the pattern obtained at 700 ˚C was entirely indexable to FCC (supporting Fig. S4). The size of the Au crystallite reduced post annealing. The length and width of the Au bipyramids were reduced by ~ 6% and ~ 9% respectively on annealing at higher temperatures. (Fig. 2b and Fig. S5). Although the overall morphology of the bipyramids did not change with the lattice conversion, the corrugated surfaces and sharp edges got rounded. In comparison, the irregular hexagons and tetrahedral crystallites readily converted to spherical crystallites (Fig. S6).

(002)T

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(020)O

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15.2

(111)C

(112)O

15.4

15.6

Chemistry of Materials Figure 4. (a) XRD patterns of Au microcrystallites up to ~40 GPa. (b and c) Rietveld fitting of XRD data obtained at 0.4 and 15.1 GPa.

Pressure is another important parameter that quantifies the stability of the crystal structure of the material. As prepared Au microcrystallites were transferred and compressed in a diamond anvil cell (DAC) at room temperature and the structural changes were monitored in situ using XRD on the beamline at European Synchrotron Radiation Facility (ESRF). Figure 4 shows the representative XRD patterns of the Au microcrystallites subjected up to ~40 GPa pressure. Detailed high pressure XRD (HPXRD) study reveals that the ambient phases (FCC+BCO+BCT) of the Au micro crystallites are stable up to ~ 15 GPa (Figs. 4a- b and S9). Thereafter, the intensities of peaks corresponding to the BCT phase become quite weak making it difficult to distinguish them from those from the other two phases (see Figs. S10 and S11 for BCT lattice parameters and phase quantifications). The XRD pattern at ~15 GPa matches well with the combined FCC and BCO phases indicating that the BCT phase has been transformed into BCO phase on account of the close similarity between the symmetries of BCO and BCT lattices (Fig. 4c). Under high pressure, the BCT phase may release the strain by slight displacement of atoms within the a-b plane. The FCC and BCO phases are observed co-existing up to 40 GPa, which is the maximum pressure achieved in this study (Fig. 4). When the pressure is released to 5.5 GPa, interestingly, the high pressure FCC+BCO phases transform back to the ambient FCC+BCO+BCT phases as shown in the Fig.4 and Fig. S9. The recent atomistic simulations have shown that the BCO phase is also a metastable phase for various noble metals which crystallize predominantly in the FCC lattice.15 4.10 Lattice parameters (Å)

3.95 3.90

0

c

8

16 24 32 Pressure (GPa)

0.95

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e

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FCC

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a b c/1.414

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B0 = 143.5 GPa 17 B0' = 7.3

FCC

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Volume per Au atom (Å3) V/V0

Lattice parameter (Å)

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FCC phase 4.05

7

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7 130

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Volume per Au atom (Å3)

a

K'

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Figure 5. Pressure dependence of unit cell parameters and volume changes in FCC (a and c) and BCO (b and d) lattices. The calculated bulk modulus is given in (c) and (d). (e, f) The confidence limit in K0 and K’ of the FCC and BCO phases for the fit of third order Birch-Murnaghan equation of state with fixed V0. The pressure dependence of lattice parameter values of FCC and BCO phases exhibit a monotonic decrease with pressure [Fig. 5a and b]. A plot of a, b and c/√2 of BCO phase is shown in Fig. 5b for comparison. As the BCO cell parameters (a and b) differ only by ~ 0.9%, it can be considered as a pseudo tetragonal system. However, in literature examples, even such small differences have been attributed new crystal structure.9 The bulk moduli of both FCC and BCO phases were calculated by fitting the P vs V/Vo data of the respective phases to the third order Birch-Murnaghan equation of state (BM-EOS) [Figs. 5c and d].16

/ 3 / − 1 2 3 #/ + ! " − 4 − 1$ 4

where B0 the isothermal bulk modulus, B0′ its pressure derivative, V is the unit cell volume at the given pressure and V0 is the unit cell volume at zero pressure. The calculated bulk modulus and its first derivative for the FCC and BCO phases are: B0 = 143.5 GPa & B0′= 7.3 and B0 = 138.3 GPa & B0′ = 8.3 respectively (Fig. 5c and d). On comparing with bulk Au (B0 = 171 GPa and B0′ = 5),17, 18 we notice that the values are significantly lesser for the FCC phase present the microcrystallites, perhaps due to the coexistence with strained non-FCC phases. The situation with the BCO phase is understandable, since the cell volume is somewhat larger, leading to a softened phase.19, 20 Hence, the bulk modulus of BCO is significantly lesser than that of bulk Au. The strained phases BCO is much softer than the FCC bulk Au. The compressibility of the FCC and BCO is also understandable from the nearly similar volumes per atom (Figs. 5c-d, S12 and Table S1). The confidence limit in K0 and K’ of the FCC and BCO phases are plotted for the fit of third order BirchMurnaghan equation of state with fixed V0 and is shown in Fig. 5e and f. The major axis of the ellipse is elongated with negative slope indicating negative correlation between K0 and K’. It is seen from the figure that the area enclosed by the ellipse in the K0-K’ space indicates 68.3% chance of true values of K0 and K’ lie within the K0 - K’ space. In conclusion, we have studied the structural stability of BCT and BCO phases under high temperature and pressures. Annealing at high temperature causes an irreversible phase transition of non-FCC to FCC at 700 °C, while retaining the morphology. However, the transition is slow due to the higher activation energy. The intermediates stages during the BCT and BCO to FCC transition have also been captured. On the contrary, under the pressure, the BCT phase has disappeared at ~15 GPa, while BCO and FCC are coexisted up to 40 GPa. The


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BCT phase is recovered back when the pressure is released. These observations suggest that BCT and BCO are metastable structure of gold. The overall structural stability is FCC>BCO>BCT.

ASSOCIATED CONTENT Supporting Information. Synthetic conditions and X-ray diffraction analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *G. U. K. ([email protected])

Author Contributions All authors have given approval to the final version of the manuscript. ‡ These authors contributed equally.

Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT The authors thank Prof. C. N. R. Rao for his constant encouragement. G. M. thanks CSIR, India, for fellowship.

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6. Mettela, G.; Bhogra, M.; Waghmare, U. V.; Kulkarni, G. U., Ambient Stable Tetragonal and Orthorhombic Phases in PentaTwinned Bipyramidal Au Microcrystals. J. Am. Chem. Soc. 2015, 137, 3024-3030. 7. Guo, Q.; Zhao, Y.; Wang, Z.; Skrabalak, S. E.; Lin, Z.; Xia, Y., Size Dependence of Cubic to Trigonal Structural Distortion in Silver Micro- and Nanocrystals under High Pressure. J. Phys. Chem. C 2008, 112, 20135-20137. 8. Guo, Q.; Zhao, Y.; Mao, W. L.; Wang, Z.; Xiong, Y.; Xia, Y., Cubic to Tetragonal Phase Transformation in Cold-Compressed Pd Nanocubes. Nano Lett. 2008, 8, 972-975. 9. Sun, Y.; Ren, Y.; Liu, Y.; Wen, J.; Okasinski, J. S.; Miller, D. J., Ambient-stable tetragonal phase in silver nanostructures. Nat Commun 2012, 3, 971. 10. Li, Z.; Okasinski, J. S.; Almer, J. D.; Ren, Y.; Zuo, X.; Sun, Y., Quantitative determination of fragmentation kinetics and thermodynamics of colloidal silver nanowires by in situ high-energy synchrotron X-ray diffraction. Nanoscale 2014, 6, 365-370. 11. Huang, X.; Li, S.; Huang, Y.; Wu, S.; Zhou, X.; Li, S.; Gan, C. L.; Boey, F.; Mirkin, C. A.; Zhang, H., Synthesis of hexagonal closepacked gold nanostructures. Nat Commun 2011, 2, 292. 12. Fan, Z.; Huang, X.; Han, Y.; Bosman, M.; Wang, Q.; Zhu, Y.; Liu, Q.; Li, B.; Zeng, Z.; Wu, J.; Shi, W.; Li, S.; Gan, C. L.; Zhang, H., Surface modification-induced phase transformation of hexagonal close-packed gold square sheets. Nat Commun 2015, 6, 6562. 13. Mettela, G.; Boya, R.; Singh, D.; Kumar, G. V. P.; Kulkarni, G. U., Highly tapered pentagonal bipyramidal Au microcrystals with high index faceted corrugation: Synthesis and optical properties. Sci. Rep. 2013, 3, 1793. 14. Werner Martienssen, H. W., Springer Handbook of Condensed Matter and Materials Data 2005, part 3, 342, Table 3.1-153. 15. Zhou, Y.; Fichthorn, K. A., Internal Stress-Induced Orthorhombic Phase in 5-Fold-Twinned Noble Metal Nanowires. J. Phys. Chem. C 2014, 118, 18746-18755. 16. Birch, F., The Effect of Pressure Upon the Elastic Parameters of Isotropic Solids, According to Murnaghan's Theory of Finite Strain. J. Appl. Phys. 1938, 9, 279-288. 17. Gu, Q. F.; Krauss, G.; Steurer, W.; Gramm, F.; Cervellino, A., Unexpected High Stiffness of Ag and Au Nanoparticles. Phys. Rev. Lett. 2008, 100, 045502. 18. Takemura, K., Evaluation of the hydrostaticity of a heliumpressure medium with powder x-ray diffraction techniques. J. Appl. Phys. 2001, 89, 662-668. 19. Gilbert, B.; Huang, F.; Zhang, H.; Waychunas, G. A.; Banfield, J. F., Nanoparticles: Strained and Stiff. Science 2004, 305, 651-654. 20. Ouyang, G.; Zhu, W. G.; Sun, C. Q.; Zhu, Z. M.; Liao, S. Z., Atomistic origin of lattice strain on stiffness of nanoparticles. Phys. Chem. Chem. Phys. 2010, 12, 1543-1549.



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Extraordinarily Stable Noncubic Structures of Au: A High-Pressure and

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