Revealing Seed‐Mediated Structural Evolution of Copper‐Silicide Nanostructures: Generating Structured Current Collectors for Rechargeable Batteries

Metal silicide thin films and nanostructures typically employed in electronics have recently gained significant attention in battery technology, where they are used as active or inactive materials. However, unlike thin films, the science behind the evolution of silicide nanostructures, especially 1D nanowires (NWs), is a key missing aspect. CuxSiy nanostructures synthesized by solvent vapor growth technique are studied as a model system to gain insights into metal silicide formation. The temperature‐dependent phase evolution of CuxSiy structures proceeds from Cu>Cu0.83Si0.17>Cu5Si>Cu15Si4. The role of Cu diffusion kinetics on the morphological progression of Cu silicides is studied, revealing that the growth of 1D metal silicide NWs proceeds through an in situ formed, Cu seed‐mediated, self‐catalytic process. The different CuxSiy morphologies synthesized are utilized as structured current collectors for K‐ion battery anodes. Sb deposited by thermal evaporation upon Cu15Si4 tripod NWs and cube architectures exhibit reversible alloying capacities of 477.3 and 477.6 mAh g−1 at a C/5 rate. Furthermore, Sb deposited Cu15Si4 tripod NWs anode tested in Li‐ion and Na‐ion batteries demonstrate reversible capacities of ≈518 and 495 mAh g−1.


Introduction
3][4][5][6] An extensive range of binary transitional metallic silicides (M x Si y where M x = Cu, Ni, Co, Fe, Mn, Ti, Mg) have demonstrated remarkable properties such as metal-like low resistivity, rechargeable batteries with the advantage of redox potential K + /K = −2.93close to Li + /Li = −3.04V, abundance, and lower cost; [38,[43][44][45] Sb is the promising alloying anode material for Li, Na, and K-ion batteries, exhibiting a high specific capacity of 660 mAh g −1 across all three battery chemistries by forming Li 3 Sb, Na 3 Sb, and K 3 Sb upon alloying. [37,46,47]Our previous study demonstrated that employing electrochemically inactive Cu 15 Si 4 NW mesh architecture as a host for the alloying mode anode material Sb within PIBs helps attain stable cycling performance. [48]owever, the difficulty in achieving a controlled synthesis of the NWs and lack of clarity behind the growth mechanism is the bottleneck for further development of these silicide systems.Further, a broader understanding of the role of tunable silicide nanostructure morphologies in tackling the pulverization and delamination of active material due to huge volume expansion (407%) in PIB is still lacking.
][60][61][62][63][64][65][66][67][68][69] However, the controlled synthesis and reproducibility of metal silicide NWs is challenging due to the complex metal-Si phase diagrams, with the possibility of numerous metal silicide stoichiometries. [1,2,9,13,50,70,71]Furthermore, the mechanisms behind anisotropic metal silicide NWs growth remain unclear. [1,2,9,50,72,73]These drawbacks hinder the implementation of silicide nanostructures in practical scenarios. [2]The presence of an excess metal source and active limitation of vapor phase Si reactant are essential requirements for synthesizing metal-rich silicides. [1,2,49]As a result, the decomposition of Si on metal substrates enables the formation of large surface area metal-rich NWs grown at lower temperatures (320-600 °C). [13,72,74]Typically, diffusion-limited anisotropic 1D growth at lower vapor supersaturation is the main reason for silicide NW growth. [2,72]or example, Liu et al. carried out Ni 2 Si NW growth by delivery of Si vapor to Ni foil.In this method, Ni 2 Si columns were observed to nucleate first, prior to 1D NW growth.This study proposed that the Ni 2 Si NWs elongated from the columns due to the limited number of nucleation sites, coupled with a degree of low supersaturation in the vapor phase, ultimately favoring 1D morphologies. [10]][77] For example, Higgins et al. reported the synthesis of manganeserich silicides by delivering metal to Si substrates, where NWs grew from a 2 μm thick metal silicide film. [74]][89] However, it is still elusive to determine if an analogous catalyst-assisted mechanism for metal silicide NW growth exists, as no visible catalyst seeds have been observed at the tip of NWs in any reported studies. [1,2,9,49,50,77]Thus, a detailed investigation into the temporal evolution of interfacial crystal phases and morphology is vital for unraveling the growth mechanism of metal silicide NWs.
Herein, we report a complete analysis of the crystal phase and morphological progression of nanostructures formed by the delivery of Si to Cu metal foil.To fully elucidate the growth mechanism, we systematically studied each stage of growth occurring at the reaction interface as a function of temperature and time.Transmission Electron Microscopy (TEM) analysis of focused ion beam (FIB)-TEM lamella prepared from each structure allowed us to generate a complete understanding of the evolution of different Cu silicide nanostructures.As a result, the temperature-dependent phase formation sequence of this (Cu-Si) system can be explained in detail.The study reveals the formation of different Cu silicide structures, including Cu 0.83 Si 0.17 nanofoam, Cu 15 Si 4 cubes, and Cu 15 Si 4 NWs, depending on the reaction temperatures and times.The results develop a comprehensive understanding of the Cu 15 Si 4 NW growth process, where we demonstrate for the first time that silicide NW growth occurs via a self-catalyzed growth mechanism.The outward Cu diffusion through the silicide layers leads to the in situ formation of Cu nanoparticles at elevated temperatures, which act as seeds for the growth of the anisotropic silicide NWs.The detailed study of the NW and Cu foil interfacial structures represents a pivotal step forward in understanding the NW growth process and silicide nanostructure formation in general.The synthesized silicide nanostructured substrates (Cu 0.83 Si 0.17 nanofoam, Cu 15 Si 4 cubes, and Cu 15 Si 4 tripodlike nanowires structures) were then examined as hosts for Sb, which can be used as high-performance PIB anodes.We discuss the effect of copper silicide morphology in accommodating the volume expansion during alloying and de-alloying of the Sb.The anode configurations with Cu 15 Si 4 tripod NWs and cube architectures exhibit highly stable reversible discharge capacities of ≈477.3 and ≈477.6 mAh g −1 at a C/5 rate after 100 cycles.Furthermore, we demonstrate that the tripod silicide NWs are compatible across all alkali metal-ion battery chemistries, exhibiting a reversible discharge capacity of ≈518 mAh g −1 in Liion battery chemistry and 495 mAh g −1 in Na-ion battery chemistry.

Results and Discussion
Metal-rich copper silicide structures were synthesized by supplying Si (formed upon decomposition of phenylsilane [PS]) to Cu foil in a solvent-based vapor growth (SVG) system, as shown in Figure S1, Supporting Information.The high boiling point solvent (HBS) octadecene is used as a carrier to control the Si flux to perform low-temperature vapor phase growth of the metal-rich Cu x Si y silicide nanostructures.Here, the sequential crystal phase and morphological evolution in the Cu-Si system as a function of reaction temperature (in 10 °C increments from 355 to 395 °C) and time (10-40 min) are examined.The consecutive transformations occurring at each stage of growth were studied using a scanning electron microscope (SEM) and X-ray diffraction (XRD) at regular intervals.A thorough understanding of each stage of Cu x Si y nanostructure formation was gained by preparing lamella of thickness around 50-80 nm using Focused ion beam-scanning electron microscope(FIB-SEM) and studying them under TEM.

Cu Silicide Nanofoam (355 °C)
Figure 1a-c shows high magnification SEM images of Cu x Si y nanofoam formed on the surface of pristine Cu foil (Figure S2a, Supporting Information) at growth times of 10, 20, and 30 min, respectively, at 355 °C.A distinct 3D nanofoam morphology begins to grow on the Cu surface after 10 min (Figure 1a).Initially, due to the interdiffusion of Cu and Si atoms, a uniform Cu x Si y film formation occurred within 5 min (Figure S2b, Supporting Information).Cu, the most mobile species (dominant diffusion species) on the reaction surface, [1,90] diffuses out of the silicide film formed and interacts with the incoming Si to randomly form Cu x Si y nanoparticles (Figure 1a and Figure S2b, Supporting Information).With increased time (after 20 min), these nanoparticles grow and fuse, forming a 3D network (Figure 1b).After the complete growth of the foam (Figure S2c, Supporting Information), the substrate color changed from reddish brown (Figure S2a, Supporting Information (inset)) to metallic silver (Figure S2c, Supporting Information (inset)).These networks further grow into a layered structure with increasing time (30 min, Figure 1c).Figure 1d shows the Scanning transmission electron microscope Annular darkfield (STEM ADF) image of a FIB-TEM lamella from the Cu silicide nanofoam formed after 20 min.The EDS elemental maps (Figure 1e,f) show the Cu and Si-rich Cu x Si y foam and film formed on the Cu foil. Figure 1h shows the boundaries between the Cu (bottom), Cu x Si y film (top), and Cu x Si y foam.The high-resolution TEM (HRTEM) image (Figure 1g) of the bottom segment shows a (002) plane with a d-spacing of 2 Å, matched with cubic (Fm-3m) Cu.The HRTEM of the top segment (Figure 1g) displayed d-spacings of 1.9 and 2.2 Å for ( 311) and (2 20) planes, respectively, confirmed the formation of the cubic (P4132) Cu 0.83 Si 0.17 phase.Similarly, the top segment of the foam formed above the film displayed a d-spacing of 2.1 Å matching well with {221} planes of the same cubic (P4132) Cu 0.83 Si 0.17 phase.
The XRD analysis of the foam substrate (Figure S2d, Supporting Information) also supports the above observation, showing that the silicide film and foam phase are crystalline Cu 0.83 Si 0.17 (JCPDS 04-014-4308).93] The effective concentration of reactants (Cu and Si) diffusing into each other at the reaction interface could cause the Cu 0.83 Si 0.17 phase to form first in this system. [94]Due to homogenous Cu outward diffusion from the starting Cu foil, we see a uniform Cu 0.83 Si 0.17 film nucleated initially.However, the outward diffusion of Cu is non-homogenous once the Cu 0.83 Si 0.17 film is formed, leading to random Cu 0.83 Si 0.17 nanoparticle formation on the surface upon increased reaction time (10 min).This non-homogenous metal diffusion generally happens due to the different types of defects in the crystal structure. [53,55]The Cu 0.83 Si 0.17 nanoparticles grow and fuse upon constant Cu and Si interdiffusion, forming a 3D Cu 0.83 Si 0.17 foam network (20 min), which grows into a silicide layer over time.

Secondary Nucleation on the Cu 0.83 Si 0.17 Layer (365 °C)
When performing reactions at a higher temperature of 365 °C, the Cu 0.83 Si 0.17 nanofoam grew within 10 min (Figure 2a) of phenylsilane introduction, indicating the higher growth rate of the Cu 0.83 Si 0.17 phase at elevated temperatures.Figure 2b shows the transformation of Cu 0.83 Si 0.17 foam into a layer over time (20 min), facilitated by the continuous Cu outward diffusion and Si inward diffusion at the reaction interface.This shows that the growth rate of silicide formation increases upon performing the reaction at elevated temperatures.After the Cu 0.83 Si 0.17 layer reached a critical thickness of ≈1-1.5 μm, the growth of an-other Cu x Si y phase started with a foam morphology after 30 min (Figure 2c).With increased time (40 min), a new morphology of Cu x Si y islands was noted on the Cu x Si y film (Figure 2d).Additional images showing this formation sequence of the secondary Cu x Si y film and Cu x Si y islands are presented in Figure S3a-d, Supporting Information.Cu x Si y nanoparticles forming on the Cu 0.83 Si 0.17 layer amalgamate into a Cu x Si y foam structure, transforming into a Cu x Si y film over time.
A FIB lamella of the sample (365 °C, 40 min, Figure 2e) revealed the presence of two layers of different nanostructures formed on the Cu 0.83 Si 0.17 layer.The proximal material is a Cu x Si y film, which shows the presence of nanoparticles, and above that, secondary Cu x Si y islands were present (Figure 2f).HRTEM images from the various interfaces (marked in the TEM image, Figure 2f) were used to study the phase(s) of these nanostructures.The d-spacing of 1.5 Å for the (411) plane and 2.1 Å for the (211) plane confirmed the proximal film on the Cu 0.83 Si 0.17 layer to be a cubic Cu 5 Si phase (Figure 2g).The d-spacing values of 2.1 and 1.8 Å calculated from the HRTEM (Figure 2h) of the nanoparticle match with (111) and (002) planes of the cubic (Fm-3m) Cu phase.Besides the formation of Cu nanoparticles, another phase formed on the topmost layer.The d-spacing values of 2.4 and 2.1 Å calculated from the HRTEM of the topmost islands corresponded to (400) and (332) planes of cubic (I-43d) Cu 15 Si 4 phase.
At this reaction temperature, a secondary phase Cu 5 Si film forms on the Cu 0.83 Si 0.17 layer.The growth process of this secondary film is similar to the formation process of the Cu 0.83 Si 0.17 layer.The Cu 5 Si segments are formed randomly on the surface of the non-uniform Cu 0.83 Si 0.17 layer (365 °C, 30 min), which fuse to form a foam structure, and this foam grows into a layer upon constant interdiffusion over time.The third growth stage happens between 30 and 40 min, where Cu atoms segregate from the Cu 5 Si film, forming Cu nanoparticles on the surface.After these Cu nanoparticles form, the fourth stage of growth happens where these nanoparticles interact with the incoming Si flux, forming Cu 15 Si 4 islands at 365 °C, 40 min.We also observe that the Cu 0.83 Si 0.17 phase co-exists with the newly formed other secondary phases, which is a common phenomenon of bulkdiffusion couple systems. [1,2]

Growth of Cu Silicide Cubes (375 °C)
Cu 0.83 Si 0.17 foam formed within 10 min at 375 °C (Figure 3a), with the growth of Cu 15 Si 4 islands noted at 20 min (Figure 3b) formed via a reaction between Cu nanoparticles and the incoming Si.These Cu 15 Si 4 islands grew over time, transforming into cubic morphologies of size 200-500 nm (Figure 3c) after 30 min.Figure 3d shows the STEM-ADF images of the Cu x Si y cubes (375 °C, 30 min) above the Cu 0.83 Si 0.17 layer.The EDS elemental mapping of the prepared lamella (Figure 3e,f) showed the Cu and Si signals from the Cu x Si y cubes and layer formed on the Cu foil.The TEM image of the prepared lamella (Figure 3h) shows the interface between the cube and the Cu 0.83 Si 0.17 layer (marked in Figure 3g).The heterointerface formed between the two layers was further confirmed from the HRTEM analysis in Figure 3g, showing the bottom segment is Cu 0.83 Si 0.17 with the d-spacings of 4.4 and 2 Å corresponding to the (0 11) and ( 301) planes, whereas the d-spacings of 3.1 and 1.5 Å matched well with (013) and (541) planes of the cubic Cu 15 Si 4 phase (bottom segment).Similarly, the d-spacing of 2.1 Å corresponding to {332} sets of planes confirmed the cubes (Figure 3i) on the top region (marked in the TEM image shown in Figure 3h) were also of cubic Cu 15 Si 4 crystal phase.The XRD analysis of the mentioned substrate (Figure S4a, Supporting Information) also supports the above observation, showing the presence of crystalline Cu 15 Si 4 (JCPDS 04-014-4307) along with Cu and Cu 0.83 Si 0.17 (JCPDS 04-014-4308) phases.After the complete growth of the cubes, we observe black spots (Cu 15 Si 4 ) appearing on the metallic silver color Cu 0.83 Si 0.17 substrate (Figure S4b, Supporting Information (inset)).
The Cu silicide structure formation at 375 °C follows a similar phase and morphology evolution sequence observed at 355 and 365 °C but over a shorter time.The Cu 0.83 Si 0.17 foam is transformed into a layer in 10 min, and the Cu 15 Si 4 islands form in °C 30 min.The phase conversion of metal (M) to M x Si y proceeds with a sequence of silicides with increasing Si content.[92][93] We also notice the Cu 0.83 Si 0.17 co-exists with the Cu 15 Si 4 cubes.

Formation of Cu Nanoparticle on the Cu 15 Si 4 Cubes (385 °C)
We observed that the growth of the Cu 15 Si 4 cubes ceased within 10 min of the reaction at 385 °C (Figure 4a).After 20 min (Figure 4b), multiple nanoparticles formed along the edges and the surfaces of the Cu 15 Si 4 cubes.From these sites, elongation of the cube edges was observed, which eventually evolved into NWlike morphologies after 30 min of growth, as shown in Figure 4c.Thus, these nanoparticles act as seeds, leading to the NW forma-tion after 30 min.Figure 4d,f shows the high magnification SEM image of the cubes with the nanoparticle nucleated on the edge and the surface.Figure 4e shows the STEM BF images of the Cu silicide cube with the nanoparticle nucleation in the FIB lamella.HRTEM analysis of a cube from the lamella shown in Figure 4h revealed the nanoparticles nucleated on edge with d-spacing values of 2.1 and 1.8 Å matching well with {111} and {200} sets of planes of cubic (Fm-3m) Cu crystal phase (Figure 4g,i).This shows that the Cu nanoparticles diffuse from the cube structure and act as an in situ formed catalyst for Cu x Si y NW growth.

Growth of Cu Silicide NWs (395 °C)
Cu x Si y NWs nucleate preferentially at the Cu seed sites within 10 min of the reaction at 395 °C (Figure 5a).The non-vertical NWs grew from the Cu seeds nucleated from the edge of the Cu 15 Si 4 cubes, whereas the vertical NWs grew from the Cu seeds nucleated on the surfaces of the Cu 15 Si 4 cubes.Increased growth time resulted in NW elongation (Figure 5b,c).Figure S5a, Supporting Information, shows the STEM-BF image of the Cu silicide NW (395 °C, 30 min) substrate in the as-prepared FIB-TEM lamella.Figure S5b, Supporting Information, shows the TEM image focused on the root of the NW, where we observe that the NW elongated from the edge and surface of Cu 15 Si 4 cubes.This confirms that the cubes were not consumed for the NW formation but acted as an interface for the NW growth.
Figure S5c, Supporting Information, shows TEM images of the NW with the seed at the tip, which confirms the growth of the Cu x Si y NW occurred via the Cu seed-mediated self-catalyzed NW process on the Cu 15 Si 4 cube.Figure 5e shows the TEM image of the Cu silicide NW in the FIB-TEM lamella (395 °C, 30 min).The HRTEM analysis at the tip (Figure 5d

Visibility of Seed and Solvent Effect
In our previous study showing the growth of Cu 15 Si 4 NWs using the HBS squalane at 450 °C, we did not observe any metal seed at the tip of the NWs. [52]Additional reactions were carried out with i) squalene, ii) no solvent at 395 °C, and iii) squalane at 450 °C (which has been previously published by us) [52] (Figure S6, Supporting Information).We have already discussed the presence of Cu seeds on the tips of the Cu 15 Si 4 NWs using octadecene (Figure 5).For conditions (i) and (ii), we observe the formation of Cu 15 Si 4 NWs with visible Cu seeds (Figure S6a,b, Supporting Information).Thus, the reaction performed at a temperature of 395 °C in the presence of other solvents or without solvents does not affect the visibility of Cu seeds at the tip of NW.In contrast, from the reaction at an elevated temperature of 450 °C after 40 min, using squalane, we observe no Cu seeds on the tips of the NWs (Figure S6c, Supporting Information).
These results confirm that the effect of temperature solely determines the visibility of the in situ formed seeds at the tips of NWs.Self-diffusion of catalysts into NWs is a commonly speculated reason for the absence of discernible tips post-synthesis for silicide NWs. [1,2,20,50,73,74,76,77]We believe the absence of seeds on the tip of NW at high-temperature synthesis could be due to the variation in i) growth rate at elevated temperature, ii) diffusion of metal at elevated temperature, and iii) self-seed diffusion into the NWs.Further understanding using in situ experiments explaining the growth process at the growth interface, that is, between the metal seed and the Cu 15 Si 4 NW at various temperatures, will likely explain the absence of visible catalysts in this study and previous studies reported. [2,20,50,52,58,61]he link between solvent and NW growth rate was also examined (Figure S7, Supporting Information).Without solvent, the NWs grew to lengths of ≈5 μm within 20 min (Figure S7a, Supporting Information), with no further elongation on increasing the reaction.Carrier-mediated (octadecene) reaction conditions helped to slow the reaction kinetics and attain better control over the silicide NW growth (Figure S7b,c, Supporting Information), with gradual elongation over time up to 6 μm (in 40 min) (Figure S7d, Supporting Information).Due to the high rate of Cu diffusion rate in Si, employing a carrier (octadecene) in the system allows the rate of silicide formation to be tuned for precise control over the silicide structures formed. [83]

The Simultaneous Growth of Multiple Phases
The co-existence of different silicide phases as an interfacial layer between the metal foil and grown nanowire phase has been reported previously in a few studies. [74,77,95]A detailed investigation of these phases is vital for controlling NW growth.In this study, the Cu 0.83 Si 0.17 grows initially and acts as an interfacial layer for the later formation of Cu 15 Si 4 NWs.The Cu 0.83 Si 0.17 phase nucleates initially and forms faster with large grains observed in reactions above ≈365 °C (Figures 2 and 3).
In our previous studies at higher temperatures (450 °C, squalane) and in solvent-free synthesis, we observed a faster growth rate of Cu 0.83 Si 0.17 affecting the secondary nucleation of the Cu 15 Si 4 phase, which causes non-uniformity in the substrate (Figure S8a,b, Supporting Information).From the top view, identifying the growth interface of Cu 15 Si 4 NW is challenging as they are covered with monoliths of the Cu 0.83 Si 0.17 phase (Figure S8c, Supporting Information).The growth interface (Cu 15 Si 4 cube) of NWs is only visible in cross-sectional analysis (Figure S8d, Supporting Information).The high temperature and faster diffusion rate cause the first nucleated phase (Cu 0.83 Si 0.17 ) to grow above a critical thickness (in this case, above 3.5 μm), inhibiting the secondary phase nucleation. [1]

Discussion on the NW Growth Mechanism
The Cu silicide phase and morphological evolution as a function of temperature and time culminated in the formation of Cu 15 Si 4 NWs at temperatures ≥385 °C. Figure 6a-d schematically shows this NW growth progression on Cu foil, where Cu 0.83 Si 0.17 is the first nucleated phase (355 °C, 10 min).The Cu 0.83 Si 0.17 layer at elevated temperature (365 °C, 40 min) leads to secondary phase formation, that is, Cu 5 Si.The Cu 5 Si is a transient film from where the Cu atom constantly segregates, forming Cu nanoparticles on the surface.The interaction of the nucleated Cu nanoparticles with incoming Si flux leads to a Cu 15 Si 4 cube (375 °C, 30 min), on which Cu seeds nucleate, leading to Cu 15 Si 4 NW formation (385 °C, 30 min). Figure 6e shows a complete cross-sectional image of the NW substrate, where we observe the sequential layering (from bottom to top) of planar Cu, the Cu 0.83 Si 0.17 layer, Cu 15 Si 4 cubes, and Cu 15 Si 4 NWs.The formation of NWs followed this sequence, irrespective of reaction temperature.The XRD analysis of the mentioned 6f) also supports the above observation, the presence of crystalline Cu 15 Si 4 (JCPDS 04-014-4307) along with Cu and Cu 0.83 Si 0.17 (JCPDS 04-014-4308) phases.
This study demonstrates the formation mechanism for Cu silicide NWs as a multistep process with complex silicide phase interactions.Cu 15 Si 4 cubes serve as the base for NW growth, with Cu nanoparticles forming on their edges and surfaces acting as the seeds for NW formation.The constant outward diffusion of Cu through this nanoparticle and interaction with Si vapor at this growth interface leads to Cu 15 Si 4 NW formation.As the Cu 15 Si 4 NW elongates, we observe the Cu 15 Si 4 cube/Cu nanoparticle hetero-interface become a Cu 15 Si 4 NW/Cu nanoparticle hetero-interface due to the seed remaining at the tip of the NW (Figure 6d).Thus, the growth interface (Cu 15 Si 4 cube) and the NW (Cu 15 Si 4 NW) are of the same phase where one of its elements, Cu (dominant diffusing species), acts as a seed.This evidence shows that the silicide NWs are grown by a self-catalytic growth mechanism. [88,89,96]tudies have previously observed the growth of NWs from an interfacial layer of a similar phase. [75,77]Our analysis could be instrumental in explaining the self-catalytic growth mechanism for those NWs.Control over the semiconductor NWs' phase, length, and growth direction, making them workable for batteries and nanoelectronics applications. [5,80,85,97]Here, we show that at 395 °C, the NW begins to elongate within 10 min, with the length increasing over time.The growth orientation of the NWs here depends on the Cu nanoparticle orientation on the substrate (i.e., the cubes).Most Cu seeds nucleate on the cube's edge (≈85%), with only ≈15% on the cube surfaces.The seeds nucleate on all the cube edges, and the NWs elongate at the same angle as the seed nucleates.2]61,[73][74][75][76][77]98] Overall, these results fully demonstrate the self-catalytic growth mechanism of these Cu silicide NWs, which likely carry through to other metal silicides.The detailed analysis allows us to synthesize stable 3D silicide morphologies (Table S2, Supporting Information), which can be employed in various applications.

Electrochemical Studies
The use of copper silicides as current collectors for rechargeable batteries is an emerging and important application of intermetallics, which to date has been largely confined to the interest of the semiconductor industry. [35]The combination of excellent charge transfer characteristics combined with a robust hierarchical structure is particularly relevant to addressing the challenges of alloying type anodes, which offer superior energy densities to intercalation anodes but require a 3D geometry that can mitigate volume expansion. [48,99]The in-depth understanding of the growth mechanism for the first time in this system has allowed an understanding of the conditions required to produce different copper silicide structures directly from copper foil.In showing the different performances of these structures as current collectors for a rechargeable battery, we demonstrate how understanding and controlling these structures directly impact properties.
In recent works, we have shown that Sb grown on Cu x Si y NWs dramatically improves performance compared to planar structures.Further, we extend this study across the different Cu silicide nanostructures that have emerged from this growth study.
The nanostructures on Cu foil were coated with Sb by thermal evaporation compared to planar copper foil (Figure S9, Supporting Information, and Figure 7a).The conformal Sb coating on Cu silicide structured foils leads to the formation of 3D Sb@Cu 0.83 Si 0.17 foam electrode (Figure 7b), 3D Sb@Cu 15 Si 4 cube electrode (Figure 7c), and 3D Sb@Cu 15 Si 4 tripod NWs (Figure 7d) with increased surface area.The electrochemical performance of the structured anodes was evaluated in a halfcell PIB configuration.102][103][104] We observed that Sb-deposited Cu 15 Si 4 cube and Cu 15 Si 4 tripod NW architectures exhibited superior cycling stability with reversible alloying capacities of 477.6 and 477.3 mAh g −1 , respectively, compared to Sb-coated Cu 0.83 Si 0.17 foam with 96 mAh g −1 and Cu foil with 12.7 mAh g −1 .The charge-discharge voltage profiles of the Sb-coated Cu 15 Si 4 tripod NWs for selected cycles show the prominent discharge plateaus at ≈0.23 V corresponding to the formation of K 3 Sb following a step-wise potassiation reaction of Sb with amorphous K x Sb intermediate formation (with increasing K concentration).The charge plateaus at ≈1.12 V correspond to the complete depotassiation reaction of K 3 Sb to Sb, consistent with previous reports. [43,105,106]The tripod NW architecture delivers a first-cycle potassiation and depotassiation capacity of 962 and 541 mAh g −1 with a relatively low initial Columbic efficiency of ≈57%.This irreversible capacity loss is due to the SEI layer formation on the surface of the anode, corresponding to the plateau at ≈0.95 V, consistent with previous reports on Sb. [48] The Columbic efficiency increased to ≈98%, showing a reversible de-alloying capacity of 450.9 mAh g −1 after 100 cycles.Figure 7g illustrates the differential capacity plot for the Sb-coated Cu 15 Si 4 tripod NW structure.During alloying, the peak at ≈0.6 and ≈0.3 V can be attributed to K x Sb (step-wise formation of KSb 2 /KSb and K 5 Sb 4 ) intermediates formation, leading to K 3 Sb formation, with a corresponding peak at ≈0.23 V.During de-alloying, the broad peak at ≈0.6 and ≈1.12 V shows the step-wise de-alloying of K from K 3 Sb forming Sb. [48,105,106] The peak voltages during the cathodic scan are generally lower than the thermodynamic equilibrium potentials caused by overpotential.Similarly, the peak voltages are higher than theoretical values during the anodic sweep due to overpotential.
Huge volume expansion (up to 407% for complete potassiation) [38] of alloying anode during larger ionic radius K + insertion during alloying forming K 3 Sb presents significant challenges of alloying material pulverization and electrical isolation from the CC.The morphological variation of the anodes after 100 cycles was studied using SEM. Figure 7h,i shows the Sb layer deposited upon pristine Cu foil and Cu 0.83 Si 0.17 foam, which was fragmented and delaminated from CC, which explains the steady capacity loss observed during cycling.The Sb-coated 3D Cu 0.83 Si 0.17 foam architecture exhibited increased void space for accommodating volume expansion in a pristine state.Yet, it is not a suitable architecture to withstand the high mechanical stress during alloying, causing fragmentation and delamination. [38]owever, even after cycling, the Sb-coated 3D Cu 15 S i4 tripod and cube electrode maintained their respective architectures (Figure 7j,k).A porous Sb network formed from the alloying and de-alloying process firmly adhered to the cube and NW current collector structures, improving cycling stability in the PIB chemistry.Notably, the Sb@Cu 15 Si 4 tripod NWs with high surface area (Figure S9, Supporting Information) demonstrated excellent cycling stability with a high reversible capacity of 477.47 mAh g −1 after 400 cycles at a C/5 rate (Figure S10, Supporting Information).The compatibility of these anodes across other alkali metalion batteries was also demonstrated.We observe that Sb-coated 3D Cu 15 Si 4 tripod NW anodes delivered a reversible alloying capacity of 518 mAh g −1 in Li-ion battery chemistry and 495 mAh g −1 in Na-ion battery chemistry (Figures S11 and S12, Supporting Information).

Conclusion
The formation mechanism of intermetallic Cu silicide structures from copper foil through the delivery of Si is presented in this work.The temperature-dependent phase nucleation order of this Cu-Si diffusion couple is explained in detail.The study reveals that the diffusion kinetics of the dominant diffusing species (Cu) is a significant factor in the morphological evolution of the silicide structures.For the first time, we demonstrate that Cu silicide NW (Cu 15 Si 4 ) growth follows a self-catalytic growth mechanism through in situ-generated Cu NPs.This study lays the groundwork for a comprehensive understanding of metal silicide NW formation more broadly, which has previously remained elusive.The study reveals an understanding of how nucleation, reaction rate, and diffusion kinetics influence the growth of silicide nanostructures.Further, the Cu x Si y structures show promising electrochemical performance with improved cycling stability (compared to pristine Cu foil and even textured mesh) for alkali metal-ion batteries when employed as a host for Sb alloying anode.Overall, these substrate-grown anisotropic Cu-Si structures show immense promise to be used as host materials for alloying-type anodes to relieve the issue of volume expansion-related electrical contact loss during long battery cycling in Alkali metal-ion batteries.
Substrate Preparation and Post-Synthetic Treatment: Cu foil (99.9%) of 0.05 mm thickness was purchased from Polly store, China.The substrates were pre-cleaned using 0.1 m hydrochloric acid for 5 min and were repeatedly cleaned with deionized water followed by IPA and then dried completely.After the reaction, the nanostructured substrates were removed, rinsed with toluene, then by acetone to remove residual HBS, and dried under an N 2 line before characterization.
Growth Mechanism: Reactions were carried out in a custom-made long neck round bottom flask from Cambridge Glassblowing.The precleaned Cu foil was positioned just above the round bottom (at the opening of the long neck) in the flask, along with 5 mL of HBS octadecene.This setup was placed in a three-zone furnace attached to the Schlenk line setup via a water condenser.Initially, the temperature was ramped to 125 °C in a vacuum environment of at least 100 mTorr for 45 min to remove moisture in the system.Following this, the setup was switched to an argon (Ar) environment under reflux conditions, and the reaction temperature was raised to the reaction temperature under constant Ar circulation.A water condenser controlled the HBS vapor reflux and ensured the reaction was controlled.After the reaction temperature was reached, 0.5 mL of the PS precursor was injected into the system through a septum cap, and the reaction proceeded for 20 min.After 20 min of reaction, an additional 0.5 mL of PS was injected the second time, and the reaction proceeded for another 20 min.After 40 min, the reaction was stopped, the furnace was opened, and the setup was allowed to cool to 150 °C before removing the silicide-covered substrates.The nanostructured silicide substrates were rinsed with toluene and acetone to remove any residual octadecene and dried.To study the growth process while keeping the other conditions constant, different reaction temperatures of 355, 365, 375, 385, and 395 °C were examined, along with various reaction times of 10, 20, 30, and 40 min.A similar reaction was performed using other HBS, like squalane and squalene (5 mL), at 395 °C for 40 min.Reaction without solvent was performed at 395 °C for 20 min for comparison.
X-ray Diffraction: XRD analysis of nanostructured substrates placed on the Si zero background was conducted using a PANalytical Empyrean instrument equipped with a Cu K radiation source ( = 1.5418Å) and a 1-D X'celerator strip detector with the diffractometer operating at 40 kV and 40 mA.
Electron Microscopy: SEM analysis was conducted on an FEI Helios G4 CX microscope with an electron dispersive X-ray (EDX) analyzer operated at 5-10 kV.Cross-sectional analysis, FIB-TEM lamella preparation, and EDX analysis of lamella were performed using a FIB milling technique carried out on an FEI Helios G4 CX microscope equipped with an EDX analyzer operating at 5-10 kV.The low-resolution and HRTEM analysis of the prepared FIB-TEM lamella was conducted using a 200 kV JEOL JEM-2100F field emission microscope equipped with a Gatan Ultrascan CCD camera and EDAX Genesis EDS detector.EDX spectroscopy of the individual NWs was conducted on Ni TEM grids.
Sb Deposition on CuSi and Cu Foil Substrates: A vacuum-based glove box thermal evaporator operated at 6 × 10 −6 bar was used for Sb deposition on substrates.Mass loadings between 0.10 and 0.13 mg cm −2 were achieved on the substrates by tuning the deposition time and deposition rate.A Sartorius Microbalance (Sartorius SE2, ± 0.25 μg repeatability) was used to determine the active material loading.Before cell assembly, the as-prepared electrodes were stored in the Ar-filled glovebox (<0.1 ppm O 2 /H 2 O).
Electrochemical Measurements: The electrochemical characterization of the as-deposited Sb on Cu foil, Sb on Cu 0.83 Si 0.17 foam, Sb on Cu 15 Si 4 cube, and Sb on Cu 15 Si 4 tripod NW anodes was performed using a Biologic MPG-2 in the potential range of 0.01-1.5 V (vs K/K+)/(vs Li/Li+)/(vs Na/Na+).The CR2032 coin cells were assembled using the synthesized anodes as working electrode, glass fiber (GF/D, Whatman) as a separator, and potassium metal (99.95%) as the counter/reference electrode in an Ar-filled glove box (<0.1 ppm O 2 /H 2 O).The electrolyte employed was a 100 μL solution of 4 m KFSI in DME.Lithium and sodium metal were used as Li-ion and Na-ion batteries' counter/reference electrodes. 1 m NaClO 4 in DEC+5% FEC was used as an electrolyte for SIB chemistry, and 1 m LiPF 6 in EC/DMC with 3% VC was used as an electrolyte for LIB chemistry. [107]

Figure 1 .
Figure 1.Structural transformation of Cu foil over time from reactions carried out at 355 °C.SEM images after a) 10 min, b) 20 min, and c) 30 min.d) STEM ADF image of the Cu silicide nanofoam (355 °C, 20 min) from FIB-TEM lamella and corresponding EDX mapping e,f) of Cu (yellow) and Si (red).g) HRTEM image of the interface between the Cu foil and Cu silicide layer (Cu 0.83 Si 0.17 ) with the corresponding FFT pattern collected from the area shown in low mag TEM image h) of the Cu silicide nanofoam (Cu 0.83 Si 0.17 ) lamella.i) HRTEM image of the top segment of Cu silicide foam (Cu 0.83 Si 0.17 ) with the corresponding FFT.

Figure 2 .
Figure 2. Structural transformation of Cu foil at 365 °C.SEM images after a) 10 min, b) 20 min, c) 30 min, and d) 40 min reaction.e) Cross-sectional SEM image of FIB-TEM lamella (365 °C, 40 min).f) TEM image of the secondary Cu silicides (Cu 5 Si, Cu, and Cu 15 Si 4 ) nucleated on the Cu 0.83 Si 0.17 layer.g) HRTEM images of the interface between the Cu 0.83 Si 0.17 layer and Cu 5 Si layer, h) HRTEM image of Cu nanoparticles and Cu 5 Si segment nucleated on the Cu 0.83 Si 0.17 foam, and i) HRTEM images of the Cu 15 Si 4 islands.

Figure 3 .
Figure 3. Structural transformation of Cu foil at 375 °C.SEM images after a) 10 min, b) 20 min, and c) 30 min reaction time.d) STEM DF images of the Cu silicide cubes (375 °C, 30 min) FIB-TEM lamella and corresponding STEM-EDX mapping of Cu (yellow) and Si (red) (e,f).g) HRTEM image of the interface between the Cu silicide cube (Cu 15 Si 4 ) and Cu silicide layer (Cu 0.83 Si 0.17 ).h) TEM image of the Cu silicide cube (Cu 15 Si 4 ) lamella.i) HRTEM image of the Cu silicide cube (Cu 0.83 Si 0.17 ) with indexed FFTs in (g) and (h).

Figure 4 .
Figure 4. Structural transformation of Cu foil at 385 °C.SEM image after a) 10 min, b) 20 min, c) 30 min reaction time.d,f) SEM of the Cu nanoparticle nucleation on the cube used for FIB-TEM lamella preparation; e) STEM bright field image of the Cu silicide cubes with Cu seed nucleation (385 °C, 20 min) from FIB-TEM lamella.h) TEM image of the Cu silicide cube with Cu seed (Cu 15 Si 4 ) lamella, g) HRTEM images of the edge of Cu silicide cube showing the interface between Cu nanoparticle nucleated on the cube (Cu 15 Si 4 ), i) HRTEM images of the Cu nanoparticle nucleated from the surface of the cube.

Figure 5 .
Figure 5. Structural transformation of Cu foil at 395 °C.SEM image after a) 10 min, b) 20 min, and c) 30 min at 395 °C.d) HRTEM image of the interface between Cu seed and Cu 15 Si 4 NW, e) TEM image of Cu 15 Si 4 NW growing from Cu silicide (Cu 15 Si 4 ) cube from FIB-TEM lamella, and f) HRTEM image captured from the root of the Cu 15 Si 4 NW.g) STEM-ADF micrographs of Cu 15 Si 4 NW with Cu seed STEM-EDX mapping of h) Cu (yellow) and i) Si (red).
) of the NWs showing the interface between the seed and NW grown exhibited the presence of a Cu seed corroborated by d-spacing values of 2.1 and 1.8 Å for {111} and {200} sets of planes.The d-spacing values of 2.6 and 2.1 Å for the (400) plane and {332} sets of planes calculated from the tip and the base area (Figure 5d,f) of the NW indicate that the NWs are of cubic Cu 15 Si 4 crystal phase.Figure 5g shows the STEM-ADF images of the Cu 15 Si 4 NWs (395 °C, 30 min).The EDS elemental mapping of the NWs (Figure 5h,i) showed the Cu and Si-rich Cu 15 Si 4 NWs with a Cu-rich seed on the tip.

Figure 7 .
Figure 7. image Sb deposited on Cu foil, b) Cu 0.83c Si 0.17 architecture, c) Cu 15 Si 4 cube architecture, and d) Cu 15 Si 4 tripod architecture, respectively.e) Comparison of cycling performance of Sb deposited on Cu foil, Cu 0.83 Si 0.17 foam, Cu 15 Si 4 cube, and Cu 15 Si 4 tripod and their respective columbic efficiencies.f) Galvanostatic charge-discharge voltage profiles of Sb deposited Cu 15 Si 4 tripod at C/5 rate and corresponding differential capacity plots g) of 1st, 25th, 50th, 75th, and 100th cycle.SEM image of Sb deposited anodes after 100 cycles h) pristine Cu foil, i) Cu 0.83c Si 0.17 foam architecture, j) Cu 15 Si 4 cube architecture, and k) Cu 15 Si 4 nanowire architecture.