Anisotropic strain variations during the confined growth of Au nanowires

The electrochemical deposition of metal in hard templates of nanoporous anodic aluminum oxide (NP-AAO) represents a facile and versatile bottom-up method for the fabrication of ordered arrays of metal nanowires.1–71. W. Lee and S.-J. Park, “ Porous anodic aluminum oxide: Anodization and templated synthesis of functional nanostructures,” Chem. Rev. 114, 7487–7556 (2014). https://doi.org/10.1021/cr500002z2. G. Sauer, G. Brehm, S. Schneider, K. Nielsch, R. B. Wehrspohn, J. Choi, H. Hofmeister, and U. Gösele, “ Highly ordered monocrystalline silver nanowire arrays,” J. Appl. Phys. 91, 3243–3247 (2002). https://doi.org/10.1063/1.14358303. K. Nielsch, F. Müller, A.-P. Li, and U. Gösele, “ Uniform nickel deposition into ordered alumina pores by pulsed electrodeposition,” Adv. Mater. 12, 582–586 (2000). https://doi.org/10.1002/(SICI)1521-4095(200004)12:8<582::AID-ADMA582>3.0.CO;2-34. W. Linpé, G. S. Harlow, J. Evertsson, U. Hejral, G. Abbondanza, F. Lenrick, S. Seifert, R. Felici, N. A. Vinogradov, and E. Lundgren, “ The state of electrodeposited Sn nanopillars within porous anodic alumina from in situ x-ray observations,” ACS Appl. Nano Mater. 2, 3031–3038 (2019). https://doi.org/10.1021/acsanm.9b004085. G. S. Harlow, J. Drnec, T. Wiegmann, W. Lipé, J. Evertsson, A. R. Persson, R. Wallenberg, E. Lundgren, and N. A. Vinogradov, “ Observing growth under confinement: Sn nanopillars in porous alumina templates,” Nanoscale Adv. 1, 4764–4771 (2019). https://doi.org/10.1039/C9NA00473D6. A. Larsson, G. Abbondanza, W. Linpé, F. Carlà, P. Mousley, C. Hetherington, E. Lundgren, and G. S. Harlow, “ Electrochemical fabrication and characterization of palladium nanowires in nanoporous alumina template,” J. Electrochem. Soc. 167, 122514 (2020). https://doi.org/10.1149/1945-7111/abb37e7. G. Abbondanza, A. Larsson, W. Linpé, C. Hetherington, F. Carlá, E. Lundgren, and G. S. Harlow, “ Templated electrodeposition as a scalable and surfactant-free approach to the synthesis of Au nanoparticles with tunable aspect ratios,” Nanoscale Adv. 4, 2452–2467 (2022). https://doi.org/10.1039/D2NA00188H NP-AAO is a material synthesized in the anodization of Al, and, through its self-assembling honeycomb structure, large areas of material can be fabricated by simply exposing Al to the electrolyte solution under anodizing conditions, making the template method an easily scalable procedure for industrial purposes.8,98. H. Masuda and K. Fukuda, “ Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina,” Science 268, 1466–1468 (1995). https://doi.org/10.1126/science.268.5216.14669. H. Masuda, K. Yada, and A. Osaka, “ Self-ordering of cell configuration of anodic porous alumina with large-size pores in phosphoric acid solution,” Jpn. J. Appl. Phys., Part 2 37, L1340–L1342 (1998). https://doi.org/10.1143/JJAP.37.L1340 In previous research, we presented a facile template-assisted route for the fabrication of Au nanowires in NP-AAO, and we characterized them by ex situ electron microscopy and x-ray diffraction.77. G. Abbondanza, A. Larsson, W. Linpé, C. Hetherington, F. Carlá, E. Lundgren, and G. S. Harlow, “ Templated electrodeposition as a scalable and surfactant-free approach to the synthesis of Au nanoparticles with tunable aspect ratios,” Nanoscale Adv. 4, 2452–2467 (2022). https://doi.org/10.1039/D2NA00188HThe hard template method (as opposed to the soft template method, which relies on the assistance of colloidal aggregates1010. Y. Xie, D. Kocaefe, C. Chen, and Y. Kocaefe, “ Review of research on template methods in preparation of nanomaterials,” J. Nanomater. 2016, 2302595.) consists in electrodepositing metal in an inorganic nanoporous medium. Since the electrodeposited metal adopts the shape of the pores, and the length is proportional to the deposition duration, it is possible to fabricate nanomaterials with controlled morphology and aspect ratio. Furthermore, the electrodeposited nanostructures can be released by selectively dissolving the NP-AAO template to obtain nanostructures dispersed in solution or ordered arrays of up-standing nanowire forests on a substrate.1111. R. Dou and B. Derby, “ The strength of gold nanowire forests,” Scr. Mater. 59, 151–154 (2008). https://doi.org/10.1016/j.scriptamat.2008.02.046 The release from NP-AAO, by the action of a selective etchant like NaOH, exposes the large surface area per unit volume, which gives nanomaterials their extraordinary properties.Another advantage of the hard template method is that the surface of the electrodeposited metal is surfactant-free, which means that de-capping stages are not necessary for applications where the availability of surface sites is relevant, such as in catalysis.12–1412. J. Quinson, “ Surfactant-free precious metal colloidal nanoparticles for catalysis,” Front. Nanotechnol. 3, 770281 (2021). https://doi.org/10.3389/fnano.2021.77028113. J. Quinson, S. B. Simonsen, L. Theil Kuhn, and M. Arenz, “ Commercial spirits for surfactant-free syntheses of electro-active platinum nanoparticles,” Sustainable Chem. 2, 1–7 (2021). https://doi.org/10.3390/suschem201000114. J. Quinson, S. Kunz, and M. Arenz, “ Beyond active site design: A surfactant-free toolbox approach for optimized supported nanoparticle catalysts,” ChemCatChem 13, 1692–1705 (2021). https://doi.org/10.1002/cctc.202001858 Au nanowires in NP-AAO might also be employed as nanoelectrode arrays, which enhance the mass transport and lead to steady-state voltammograms with sigmoidal shape, unaffected by the diffusion limit.1515. D. W. M. Arrigan, “ Nanoelectrodes, nanoelectrode arrays and their applications,” Analyst 129, 1157 (2004). https://doi.org/10.1039/b415395mIt has been previously demonstrated that Au nanowires deposited in NP-AAO templates have an anisotropic size-dependent strain distribution:77. G. Abbondanza, A. Larsson, W. Linpé, C. Hetherington, F. Carlá, E. Lundgren, and G. S. Harlow, “ Templated electrodeposition as a scalable and surfactant-free approach to the synthesis of Au nanoparticles with tunable aspect ratios,” Nanoscale Adv. 4, 2452–2467 (2022). https://doi.org/10.1039/D2NA00188H the lattice parameter is larger in the direction of growth than in the direction of confinement, and the strain is inversely proportional to the pore radius, which suggests that the interatomic distances of the electrodeposited nanomaterials can be fine-tuned by selecting an appropriate template pore radius (see Figs. S1 and S2 of the supplementary material). Similar findings have been previously reported about Pd66. A. Larsson, G. Abbondanza, W. Linpé, F. Carlà, P. Mousley, C. Hetherington, E. Lundgren, and G. S. Harlow, “ Electrochemical fabrication and characterization of palladium nanowires in nanoporous alumina template,” J. Electrochem. Soc. 167, 122514 (2020). https://doi.org/10.1149/1945-7111/abb37e and Sn16,1716. H. S. Shin, J. Yu, J. Y. Song, and H. M. Park, “ Size dependence of lattice deformation induced by growth stress in Sn nanowires,” Appl. Phys. Lett. 94, 011906 (2009). https://doi.org/10.1063/1.306416717. H. S. Shin, J. Yu, J. Y. Song, H. M. Park, and Y.-S. Kim, “ Origins of size-dependent lattice dilatation in tetragonal Sn nanowires: Surface stress and growth stress,” Appl. Phys. Lett. 97, 131903 (2010). https://doi.org/10.1063/1.3493179 nanowires. The potential applications of a tunable size-dependent strain range from catalysis, where it has been shown that small variations of the interatomic distances can alter the catalytic activity of materials,18–2318. N. Muralidharan, R. Carter, L. Oakes, A. P. Cohn, and C. L. Pint, “ Strain engineering to modify the electrochemistry of energy storage electrodes,” Sci. Rep. 6, 27542 (2016). https://doi.org/10.1038/srep2754219. V. Celorrio, P. M. Quaino, E. Santos, J. Flórez-Montaño, J. J. L. Humphrey, O. Guillén-Villafuerte, D. Plana, M. J. Lázaro, E. Pastor, and D. J. Fermín, “ Strain effects on the oxidation of CO and HCOOH on Au–Pd core–shell nanoparticles,” ACS Catal. 7, 1673–1680 (2017). https://doi.org/10.1021/acscatal.6b0323720. M. Luo and S. Guo, “ Strain-controlled electrocatalysis on multimetallic nanomaterials,” Nat. Rev. Mater. 2, 17059 (2017). https://doi.org/10.1038/natrevmats.2017.5921. T. Nilsson Pingel, M. Jørgensen, A. B. Yankovich, H. Grönbeck, and E. Olsson, “ Influence of atomic site-specific strain on catalytic activity of supported nanoparticles,” Nat. Commun. 9, 2722 (2018). https://doi.org/10.1038/s41467-018-05055-122. E. Westsson, S. Picken, and G. Koper, “ The effect of lattice strain on catalytic activity,” Chem. Commun. 55, 1338–1341 (2019). https://doi.org/10.1039/C8CC09063G23. R. P. Jansonius, P. A. Schauer, D. J. Dvorak, B. P. MacLeod, D. K. Fork, and C. P. Berlinguette, “ Strain influences the hydrogen evolution activity and absorption capacity of palladium,” Angew. Chem. 132, 12290–12296 (2020). https://doi.org/10.1002/ange.202005248 to hydrogen solubility in the crystal lattice of solids2424. H.-B. Zhou, S. Jin, Y. Zhang, G.-H. Lu, and F. Liu, “ Anisotropic strain enhanced hydrogen solubility in bcc metals: The independence on the sign of strain,” Phys. Rev. Lett. 109, 135502 (2012). https://doi.org/10.1103/PhysRevLett.109.135502 and strain-engineering of functional nanowires for devices.2525. X. Ben and H. S. Park, “ Strain engineering enhancement of surface plasmon polariton propagation lengths for gold nanowires,” Appl. Phys. Lett. 102, 041909 (2013). https://doi.org/10.1063/1.4790293The anisotropic strain effect has been attributed to the sum of two contributions: surface stress and growth stress.1717. H. S. Shin, J. Yu, J. Y. Song, H. M. Park, and Y.-S. Kim, “ Origins of size-dependent lattice dilatation in tetragonal Sn nanowires: Surface stress and growth stress,” Appl. Phys. Lett. 97, 131903 (2010). https://doi.org/10.1063/1.3493179 However, the origin of the growth stress and its interplay with the electrochemical conditions are not fully understood. For instance, in situ stress measurements of electrochemically grown thin films revealed stress relaxation caused by the interruption of growth,2626. T. Luo, “ In situ stress measurement of thin film growth by electrochemical deposition: the correlations among stress, morphology, and microstructures,” Ph.D. thesis ( The Johns Hopkins University, 2008). which suggested that the operating electrochemical conditions have an influence on the strain state of the deposited material. Although this is known for thin films, there is a lack of similar studies applied to the template-assisted growth of nanowires in NP-AAO.For this reason, we investigated the electrochemical growth of Au nanowires by means of grazing-incidence transmission wide-angle x-ray scattering (GTWAXS), to learn about the structure–function relation regarding the interatomic distances in the direction of growth and in the direction of confinement, and, simultaneously, we monitored the progress of the growth by means of grazing-incidence transmission small-angle x-ray scattering (GTSAXS), x-ray fluorescence (XRF), and two-dimensional surface optical reflectance (2D-SOR), which is a technique that has been proved useful in bridging macroscopic and atomic-scale observations.27–3027. S. Albertin, J. Gustafson, J. Zhou, S. Pfaff, M. Shipilin, S. Blomberg, L. R. Merte, O. Gutowski, A.-C. Dippel, J. Zetterberg, E. Lundgren, and U. Hejral, “ Surface optical reflectance combined with x-ray techniques during gas-surface interactions,” J. Phys. D 53, 224001 (2020). https://doi.org/10.1088/1361-6463/ab77df28. W. Linpé, L. Rämisch, G. Abbondanza, A. Larsson, S. Pfaff, L. Jacobse, J. Zetterberg, L. Merte, A. Stierle, Z. Hegedues, U. Lienert, E. Lundgren, and G. S. Harlow, “ Revisiting optical reflectance from Au(111) electrode surfaces with combined high-energy surface x-ray diffraction,” J. Electrochem. Soc. 168, 096511 (2021). https://doi.org/10.1149/1945-7111/ac270229. W. Linpé, G. S. Harlow, A. Larsson, G. Abbondanza, L. Rämisch, S. Pfaff, J. Zetterberg, J. Evertsson, and E. Lundgren, “ An electrochemical cell for 2-dimensional surface optical reflectance during anodization and cyclic voltammetry,” Rev. Sci. Instrum. 91, 044101 (2020). https://doi.org/10.1063/1.513390530. S. Pfaff, A. Larsson, D. Orlov, G. S. Harlow, G. Abbondanza, W. Linpé, L. Rämisch, S. M. Gericke, J. Zetterberg, and E. Lundgren, “ Operando Reflectance microscopy on polycrystalline surfaces in thermal catalysis, electrocatalysis, and corrosion,” ACS Appl. Mater. Interfaces 13, 19530–19540 (2021). https://doi.org/10.1021/acsami.1c04961 High-energy synchrotron x-rays were necessary to study the encapsulated growth of the Au nanowires with a time resolution compatible with the growth rate.A detailed description of the template preparation, the electrodeposition method, and the Au nanowire synthesis protocol are available in previous research.77. G. Abbondanza, A. Larsson, W. Linpé, C. Hetherington, F. Carlá, E. Lundgren, and G. S. Harlow, “ Templated electrodeposition as a scalable and surfactant-free approach to the synthesis of Au nanoparticles with tunable aspect ratios,” Nanoscale Adv. 4, 2452–2467 (2022). https://doi.org/10.1039/D2NA00188H In brief, the NP-AAO template was fabricated prior to the synchrotron experiment by anodizing a top-hat shaped Al sample (99.999%, Surface Preparation Laboratory, Netherlands) with a diameter of 6 mm, using an electrochemical holder to expose only the top surface of the specimen. The insulating barrier layer at the pore bottom was thinned by decreasing the anodizing potential.33. K. Nielsch, F. Müller, A.-P. Li, and U. Gösele, “ Uniform nickel deposition into ordered alumina pores by pulsed electrodeposition,” Adv. Mater. 12, 582–586 (2000). https://doi.org/10.1002/(SICI)1521-4095(200004)12:8<582::AID-ADMA582>3.0.CO;2-3The electrochemical growth was conducted using the pulse electrodeposition method in an electrolyte solution containing HAuCl4 in a phosphate buffer with neutral pH, pumped through an electrochemical flow-cell specifically designed for in situ x-ray measurements.2929. W. Linpé, G. S. Harlow, A. Larsson, G. Abbondanza, L. Rämisch, S. Pfaff, J. Zetterberg, J. Evertsson, and E. Lundgren, “ An electrochemical cell for 2-dimensional surface optical reflectance during anodization and cyclic voltammetry,” Rev. Sci. Instrum. 91, 044101 (2020). https://doi.org/10.1063/1.5133905 We alternated sequences of electrodeposition lasting 2 min with sequences of rest lasting 3 min, where the electrochemical cell was left at the open-circuit potential, aiming to study any strain variation caused by the absence of an external electric field. During the electrodeposition sequences, we measured the potential across a shunt resistor with a resistance of 25 Ω, connected in series with the electrochemical cell. The measured potential was used to calculate the current flowing through the cell using Ohm's law.The synchrotron measurements were conducted at the Swedish Material Science beamline P21.2 (Petra III, DESY), using an x-ray beam with an energy of 68.5 keV and a size of 13 × 100 m2 (vertical horizontal). The experiment was performed in grazing-incidence transmission geometry,3131. X. Lu, K. G. Yager, D. Johnston, C. T. Black, and B. M. Ocko, “ Grazing-incidence transmission x-ray scattering: Surface scattering in the Born approximation,” J. Appl. Crystallogr. 46, 165–172 (2013). https://doi.org/10.1107/S0021889812047887 using an incidence angle of 0.15 °. For a detailed description of the experimental setup, its calibration, and the data processing procedure, we refer the reader to a recent manuscript about the electrochemical growth of Pd nanowires, where we employed the same setup and a similar template.3232. G. Abbondanza, A. Grespi, A. Larsson, L. Glatthaar, T. Weber, M. Blankenburg, Z. Hegedüs, U. Lienert, H. Over, G. S. Harlow, and E. Lundgren, “ In situ hydride breathing during the template-assisted electrodeposition of Pd nanowires,” arXiv:2211.09007 (2022).A representative GTWAXS pattern collected at the end of the electrodeposition is shown in Fig. 1(a). The Bragg reflections originating from the polycrystalline Al substrate were screened using tungsten beam-stops and rectangular lead foils. The less intense Al reflections, which did not need screening, were masked out from the azimuthal integration during the data processing. The intensity distribution along the Au diffraction rings in Fig. 1(a) is homogeneous throughout the course of the deposition, which reflects the isotropic random orientation of the Au crystallites. Figure 1(b) is a waterfall plot where every horizontal line is a powder diffraction pattern obtained by integrating the GTWAXS images over the whole azimuthal range available using the “Multi-geometry” module in pyFAI.33,3433. J. Kieffer and D. Karkoulis, “ PyFAI, A versatile library for azimuthal regrouping,” J. Phys.: Conf. Ser. 425, 202012 (2013). https://doi.org/10.1088/1742-6596/425/20/20201234. G. Ashiotis, A. Deschildre, Z. Nawaz, J. P. Wright, D. Karkoulis, F. E. Picca, and J. Kieffer, “ The fast azimuthal integration Python library: PyFAI,” J. Appl. Crystallogr. 48, 510–519 (2015). https://doi.org/10.1107/S1600576715004306 Figure 1(b) gives an overview on the progressive appearance of the Au face-centered cubic (fcc) phase during the course of the electrodeposition.To study the anisotropy of the system, we integrated the GTWAXS patterns over azimuthal slices oriented along the vertical and the horizontal axis, shown as the blue and red shaded areas in Fig. 1(a), respectively, to obtain one-dimensional powder diffraction patterns. The width of the azimuthal slices was 15 °. We found that such azimuthal range was wide enough to provide a good signal-to-noise ratio and sufficiently thin to study the anisotropy effects. In the patterns obtained by integrating in the horizontal direction, the scattering vector always lies in the horizontal plane, i.e., it always lies in the direction of confinement. On the other hand, in the patterns obtained by integrating over the vertical direction, the angle between the direction of growth and the scattering vector is never zero, but it is sufficiently small. For instance, the Au(111) reflection appears at a Bragg angle θ of 2.20 ° (with an x-ray beam energy of 68.5 keV). The growth proceeds in the direction of the pores, which are vertically aligned with the specimen surface normal. This means that the angle between the growth direction and the scattering vector is θ−α=2.05 °, where α is the incidence angle (which was 0.15 ° in this setup). This angle is sufficiently small to allow an investigation of the interatomic distances in the direction of growth with good approximation.Figure 2(a) is a plot of the XRF signal, arising from the Au Lα emission line, the 2D-SOR signal integrated over the whole reflecting sample surface, the total cathodic charge that has passed through the working electrode, and the scale factor of the Au fcc phase, obtained by the sequential Rietveld refinement (performed using GSAS-II3535. B. H. Toby and R. B. Von Dreele, “ GSAS-II: The genesis of a modern open-source all purpose crystallography software package,” J. Appl. Crystallogr. 46, 544–549 (2013). https://doi.org/10.1107/S0021889813003531) of the powder diffraction patterns in the vertical direction. The white and gray regions of the plot represent the periods of time while the deposition was ongoing and interrupted, respectively. While the scale factor is proportional to the amount of crystalline material deposited, the XRF intensity is sensitive to both crystalline and amorphous phases. The fact that the XRF and the scale factor increase similarly suggests that there is no amorphous Au forming during the growth and that only crystalline Au is deposited.To illustrate the nanowire size evolution over time, Fig. 2(b) depicts the deposited Au nanostructures at two instants in time: close to the beginning and close to the end of the deposition. The size estimations are based on previous ex situ observations by scanning electron microscopy77. G. Abbondanza, A. Larsson, W. Linpé, C. Hetherington, F. Carlá, E. Lundgren, and G. S. Harlow, “ Templated electrodeposition as a scalable and surfactant-free approach to the synthesis of Au nanoparticles with tunable aspect ratios,” Nanoscale Adv. 4, 2452–2467 (2022). https://doi.org/10.1039/D2NA00188H and supported by FIBSEM images shown in Fig. S3 of the supplementary material. The linear increase in the XRF and the scale factor suggest that the growth rate is constant through the whole electrodeposition.The cathodic charge as a function of time in Fig. 2(a) was obtained by integrating the total negative current flowing through the working electrode from the beginning of the deposition to any instant in time. The cathodic charge is also a measure of the total amount of material in the pores as a function of time, and the fact that it scales with the XRF and GTWAXS is an indication that the rate of side reactions, occurring at the same time with the electrodeposition, is constant as a function of time.The 2D-SOR signal shows a rapid decrease in the first 420 s of the deposition, followed by a more gradual decrease until the end of the deposition. As the effective dielectric constant of Au/NP-AAO composite (in the optical wavelength regime) is higher than that of NP-AAO (as predicted by the Maxwell Garnett effective medium approximation3636. V. A. Markel, “ Introduction to the Maxwell Garnett approximation: Tutorial,” J. Opt. Soc. Am. A 33, 1244–1256 (2016). https://doi.org/10.1364/JOSAA.33.001244), one would expect the reflectance to increase, as predicted by the Fresnel laws for the case of unpolarized light with normal incidence to a reflecting surface. The decrease in reflectance shown in Fig. 2(a), however, evidence that reflection is not the only light–matter phenomenon involved. We surmise that the reflectance decrease is caused by light absorption by plasmon resonance and by light scattering. To support this hypothesis, UV/Vis absorbance spectra of Au nanowires in NP-AAO, found in the scientific literature, show variations as a function of the aspect ratio.3737. C. A. Foss, G. L. Hornyak, J. A. Stockert, and C. R. Martin, “ Optical properties of composite membranes containing arrays of nanoscopic gold cylinders,” J. Phys. Chem. 96, 7497–7499 (1992). https://doi.org/10.1021/j100198a004 It has also been shown that scattering events participate to the extinction of the incident light.3838. A. V. Alekseeva, V. A. Bogatyrev, B. N. Khlebtsov, A. G. Mel'nikov, L. A. Dykman, and N. G. Khlebtsov, “ Gold nanorods: Synthesis and optical properties,” Colloid J. 68, 661–678 (2006). https://doi.org/10.1134/S1061933X06060019 However, our 2D-SOR setup uses a monochromatic red LED as a source, and the measurement of absorbance spectra is beyond the purpose of the technique.From the powder diffraction patterns integrated in the horizontal and vertical direction, we estimated the lattice parameter by means of sequential Rietveld refinement, shown in Fig. 3(a). Quite rapidly from the beginning of the electrodeposition, the lattice constant in the vertical direction increases and becomes higher than the lattice constant of bulk Au. Conversely, the lattice constant in the horizontal direction decreases and becomes smaller than bulk Au. This anisotropic effect is analogous to what was reported in the literature about the size-dependent strain in electrochemically fabricated Sn nanowires in NP-AAO.5,16,395. G. S. Harlow, J. Drnec, T. Wiegmann, W. Lipé, J. Evertsson, A. R. Persson, R. Wallenberg, E. Lundgren, and N. A. Vinogradov, “ Observing growth under confinement: Sn nanopillars in porous alumina templates,” Nanoscale Adv. 1, 4764–4771 (2019). https://doi.org/10.1039/C9NA00473D16. H. S. Shin, J. Yu, J. Y. Song, and H. M. Park, “ Size dependence of lattice deformation induced by growth stress in Sn nanowires,” Appl. Phys. Lett. 94, 011906 (2009). https://doi.org/10.1063/1.306416739. H. S. Shin, J. Y. Song, and J. Yu, “ Lattice deformation of Sn nanowires for the application to nano-interconnection technology,” in Proceedings 60th Electronic Components and Technology Conference (ECTC) ( IEEE, Las Vegas, NV, 2010), pp. 1861–1865. In a similar way to the case of Sn nanowires, the anisotropic strain can be explained by a combination of surface stress and growth stress, where the latter was attributed to the presence of defects but it can also be due to deviations from the natural pathways of growth due to confined environment.40,4140. Y. Huang, P. Yan, J. Wang, L. Dong, and S. N. Atluri, “ Eshelby tensors and overall properties of nano-composites considering both interface stretching and bending effects,” J. Micromech. Mol. Phys. 7, 49–59 (2022). https://doi.org/10.1142/S242491302142009141. J. Li, M. Kothari, S. Chockalingam, T. Henzel, Q. Zhang, X. Li, J. Yan, and T. Cohen, “ Nonlinear inclusion theory with application to the growth and morphogenesis of a confined body,” J. Mech. Phys. Solids 159, 104709 (2022). https://doi.org/10.1016/j.jmps.2021.104709 As the deposition proceeds, the anisotropic effect becomes more pronounced, which suggests that the strain state depends not only on the pore radius but also on the nanowire length.Figure 3(b) shows a magnified view of the blue highlighted area in Fig. 3(a). Here, small fluctuations of the lattice parameter were observed, which have the same periodicity as the on/off state of the electrodeposition, represented by the white and gray areas of the plot. Although the origin of the stress that causes this deformation is unclear, a possible explanation is the electromigration of Au, which is the thermally assisted mass transport of ions under the influence of an electric field. This phenomenon is explained by the unbalance of electrostatic and electron-wind forces exerted on metal ions.4242. R. Landauer and J. W. F. Woo, “ Driving force in electromigration,” Phys. Rev. B 10, 1266–1271 (1974). https://doi.org/10.1103/PhysRevB.10.1266 This mechanism has proved to cause stress gradients and mechanical failure in Au nanowires.4343. C. Durkan, M. A. Schneider, and M. E. Welland, “ Analysis of failure mechanisms in electrically stressed Au nanowires,” J. Appl. Phys. 86, 1280–1286 (1999). https://doi.org/10.1063/1.370882 In situ x-ray diffraction studies on bulk Cu,4444. S.-k. Lin, Y.-c. Liu, S.-J. Chiu, Y.-T. Liu, and H.-Y. Lee, “ The electromigration effect revisited: Non-uniform local tensile stress-driven diffusion,” Sci. Rep. 7, 3082 (2017). https://doi.org/10.1038/s41598-017-03324-5 Sn,4545. Y.-H. Liao, C.-H. Chen, C.-L. Liang, K.-L. Lin, and A. T. Wu, “ A comprehensive study of electromigration in pure Sn: Effects on crystallinity, microstructure, and electrical property,” Acta Mater. 200, 200–210 (2020). https://doi.org/10.1016/j.actamat.2020.09.010 and ferroelectric thin films4646. O. Sakata, S. Yasui, T. Yamada, M. Yabashi, S. Kimura, H. Funakubo, R. Garrett, I. Gentle, K. Nugent, and S. Wilkins, “ In-situ lattice-strain analysis of a ferroelectric thin film under an applied pulse electric field,” in 10th International Conference on Radiation Instrumentation, Melbourne, Australia, 2010. revealed similar variations of the lattice parameter driven by the presence of an electric field. Figure 3(c) is a magnified view of the lattice constant in the horizontal direction, in the same time window as in Fig. 3(b). Here, the lattice parameter fluctuates less than in the vertical direction. The fact that the strain fluctuations are more pronounced along the direction of growth, which is parallel to the electric field lines, strengthens the electromigration hypothesis.4747. B. Stahlmecke, F.-J. Meyer zu Heringdorf, L. I. Chelaru, M. Horn-von Hoegen, G. Dumpich, and K. R. Roos, “ Electromigration in self-organized single-crystalline silver nanowires,” Appl. Phys. Lett. 88, 053122 (2006). https://doi.org/10.1063/1.2172012The crystallite size, extracted from the powder diffraction patterns in the vertical and horizontal direction, is reported in Fig. 3(d). At any point in time, the crystallite size is larger in the direction of growth than in the direction of confinement, which is due likely to a constrain in the horizontal plane by the confinement of the pores. In fact, the crystallite size in the horizontal plane never exceeds 25 nm, which is the pore diameter of the template anodized in sulfuric acid. The rapid increase in the crystallite size during the first sequence (120 s) of deposition can be attributed to coalescence phenomena of the grains nucleating in the pores' bottom.48,4948. A. Schröder, J. Fleig, D. Gryaznov, J. Maier, and W. Sitte, “ Quantitative model of electrochemical Ostwald ripening and its application to the time-dependent electrode potential of nanocrystalline metals,” J. Phys. Chem. B 110, 12274–12280 (2006). https://doi.org/10.1021/jp060788t49. B. Ingham, T. H. Lim, C. J. Dotzler, A. Henning, M. F. Toney, and R. D. Tilley, “ How nanoparticles coalesce: An in situ study of Au nanoparticle aggregation and grain growth,” Chem. Mater. 23, 3312–3317 (2011). https://doi.org/10.1021/cm200354d The slower kinetics of grain growth at longer deposition times can be explained by the “generalized parabolic grain growth model,” which describes the curvature of the grain boundaries as the driving force of the grain expansion.5050. H. Natter, M. Schmelzer, M.-S. Löffler, C. E. Krill, A. Fitch, and R. Hempelmann, “ Grain-growth kinetics of nanocrystalline iron studied in situ by synchrotron real-time x-ray diffraction,” J. Phys. Chem. B 104, 2467–2476 (2000). https://doi.org/10.1021/jp991622d The grain growth rate predicted by this model approaches zero for t→∞, as the curvature radii of the grains decrease over time.5151. A. Michels, C. Krill, H. Ehrhardt, R. Birringer, and D. Wu, “ Modelling the influence of grain-size-dependent solute drag on the kinetics of grain growth in nanocrystalline materials,” Acta Mater. 47, 2143–2152 (1999). https://doi.org/10.1016/S1359-6454(99)00079-8GTSAXS provided useful complementary information on the progress of the electrodeposition, and it has been used in previous research to investigate the size of the NP-AAO porous domain in situ.52,5352. N. A. Vinogradov, G. S. Harlow, F. Carlà, J. Evertsson, L. Rullik, W. Linpé, R. Felici, and E. Lundgren, “ Observation of pore growth and self-organization in anodic alumina by time-resolved x-ray scattering,” ACS Appl. Nano Mater. 1, 1265–1271 (2018). https://doi.org/10.1021/acsanm.7b0030353. J. Evertsson, N. A. Vinogradov, G. S. Harlow, F. Carlà, S. R. McKibbin, L. Rullik, W. Linpé, R. Felici, and E. Lundgren, “ Self-organization of porous anodic alumina films studied in situ by grazing-incidence transmission small-angle x-ray scattering,” RSC Adv. 8, 18980–18991 (2018). https://doi.org/10.1039/C8RA02913J The peaks of the GTSAXS patterns collected were each fitted by a Voigt profile, and the background was fitted by a third-order Chebyshev polynomial. The sum of the peak amplitudes was used as a scale factor of the overall intensity of the GTSAXS patterns, while the peak widths were used to determine the porous domain size, using the Scherrer's equation. Figure 4 is a plot of the porous domain size and the scale factor as a function of time. The domain size at the time zero is a measure of the porous order of the empty template. While the scale factor increases monotonically as a function of time, the domain size decreases to a local minimum after approximately 240 s from the beginning of the electrodeposition. This decrease can be attributed to the inhomogeneous pore filling by the electrodeposited Au. This observation is coherent with previous research about the electrodeposition of Sn.44. W. Linpé, G. S. Harlow, J. Evertsson, U. Hejral, G. Abbondanza, F. Lenrick, S. Seifert, R. Felici, N. A. Vinogradov, and E. Lundgren, “ The state of electrodeposited Sn nanopillars within porous anodic alumina from in situ x-ray observations,” ACS Appl. Nano Mater. 2, 3031–3038 (2019). https://doi.org/10.1021/acsanm.9b00408 The reason for the inhomogeneous filling is attributed to the way the template was prepared: in the early stages of deposition in templates treated by barrier layer thinning (such as the one used in this work), a heterogeneous nucleation of nanocrystals occurs at the pore bottoms, preferentially in those with a thinner barrier layer.33. K. Nielsch, F. Müller, A.-P. Li, and U. Gösele, “ Uniform nickel deposition into ordered alumina pores by pulsed electrodeposition,” Adv. Mater. 12, 582–586 (2000). https://doi.org/10.1002/(SICI)1521-4095(200004)12:8<582::AID-ADMA582>3.0.CO;2-3

However, there is an increase in the domain size after 240 s from the beginning of the electrodeposition. This can be explained by the fact that, as the pores are being filled and the scale factor increases, the GTSAXS patterns are dominated by scattering amplitude originating from the Au nanowires rather than from the empty pores. Ideally, if the Au homogeneously filled the pores to the top, the domain size would increase back to the original value of 235 nm. Instead, it plateaus around 170 nm, which is a consequence of the heterogeneous pore filling. This finding is supported by previous ex situ electron microscopy observations of heterogeneous nanowire height, where, at the end of the deposition, the mean nanowire height was 1049.205 nm (relative standard deviation of 20%).

In conclusion, we have followed the electrodeposition of Au into NP-AAO in situ using GTWAXS, GTSAXS, XRF, and 2D-SOR and gained time-resolved information on the structural progress of the deposition. The lattice parameter anisotropy evolves rapidly after the start of the electrodeposition, and subtle variations in the direction of growth were observed, possibly linked to the action of the electric field during growth.

In previous ex situ studies, we found that the anisotropic strain of confined Au nanowires depends on the template pore radius, and, in the present work, we showed that it is also possible to artificially select the strain state of the Au nanowires by an appropriate choice of deposition duration (and thus of nanowire length), which might be particularly useful in the strain-engineering of nanoparticles for catalysis. For instance, it has recently been found that Au nanoparticles under compressive strain exhibit an enhanced selectivity toward electrochemical CO2 reduction.5454. C. Zhang, W. Zhang, F. Karadas, J. Low, R. Long, C. Liang, J. Wang, Z. Li, and Y. Xiong, “ Laser-ablation assisted strain engineering of gold nanoparticles for selective electrochemical CO2 reduction,” Nanoscale 14, 7702–7710 (2022). https://doi.org/10.1039/D2NR01400AIn future studies, we could selectively dissolve the template and, at the same time, monitor the lattice parameter of Au in situ to observe any strain relaxation induced by the release from the template and determine if the deformations induced by growth under confinement are plastic or elastic. In addition, we could investigate the Au nanowires growth by combining our electrochemical flow-cell with fiber optics-based in situ UV/Vis specular reflection spectroscopy5555. B. Wickman, M. Fredriksson, L. Feng, N. Lindahl, J. Hagberg, and C. Langhammer, “ Depth probing of the hydride formation process in thin Pd films by combined electrochemistry and fiber optics-based in situ UV/vis spectroscopy,” Phys. Chem. Chem. Phys. 17, 18953–18960 (2015). https://doi.org/10.1039/C5CP01339A to validate the hypothesis of plasmon absorption introduced in this work.See the supplementary material for details regarding ex situ x-ray diffraction data and electron microscopy cross-sectional images.

We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III at the Swedish Materials Science beamline P21.2. Beamtime was allocated for proposal I-20211146 EC. We would like to thank Sven Gutschmidt for assistance in setting up the experiment. This work was financially supported by the Swedish Research Council through the Röntgen-Ångström-Cluster (Project No. 2015-06092), by the “Atomic Resolution Cluster,” a Research Infrastructure Fellow program of the Swedish Foundation for Strategic Research, and project grant (Project No. 2018-03434) by the Swedish research council. We acknowledge financial support by NanoLund.

Conflict of Interest

The authors have no conflicts to disclose.

Author Contributions

Giuseppe Abbondanza: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Visualization (equal); Writing – original draft (lead). Edvin Lundgren: Conceptualization (equal); Funding acquisition (equal); Supervision (equal); Writing – review & editing (equal). Andrea Grespi: Investigation (equal); Writing – review & editing (equal). Alfred Larsson: Formal analysis (equal); Writing – review & editing (equal). Lorena Glatthaar: Investigation (equal); Writing – review & editing (equal). Tim Weber: Investigation (equal); Writing – review & editing (equal). Malte Blankenburg: Data curation (equal); Investigation (equal); Software (equal); Writing – review & editing (equal). Zoltan Hegedüs: Data curation (equal); Investigation (equal); Software (equal); Writing – review & editing (equal). Ulrich Lienert: Data curation (equal); Investigation (equal); Supervision (equal); Writing – review & editing (equal). Herbert Over: Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corre

留言 (0)

沒有登入
gif