## 3 Results and Discussion  
### Si Micropillar Growth  
Figure 1 shows the SEM image, particle size distribution, and XRD pattern of as-received Si powders. It can be seen that the as-received Si powders had an inherently irregular morphology, with the size having a wide range from a few sub-microns to over 10 microns. As illustrated in Figure 1c, the strong X-ray diffraction peaks of the as-received waste powder were all indexed to crystal planes of silicon and no other phases were observed. In addition, since Cu K\(\alpha\)1 (\(\lambda\) = 1.5406 A) and Cu K\(\alpha\)2 (\(\lambda\) = 1.54443 A) radiation were both involved, the diffraction peaks were split at high angles.  
Figure 1: **(a)** SEM image, **(b)** particle size distribution, and **(c)** X-ray diffraction pattern of as-received Si waste powders.  
Figure 2a-c show SEM images of Si micropillars growing from 8-, 13-, and 20-\(\upmu\)m-thick slurry layers by single-line laser irradiations. Arrays of micropillars were formed along the laser irradiation path. It can be observed that the micropillars from the 20-\(\upmu\)m-thick slurry layer were the largest (Figure 2c). With the decrease in slurry layer thickness, the micropillars became smaller and were distributed more densely (Figure 2a,b). The average base diameters of micropillars were measured and presented in Figure 2d. The diameter of micropillars growing from the 20-\(\upmu\)m-thick slurry layer was the largest, with a value of 8 \(\upmu\)m. By decreasing the slurry layer thickness to 13 \(\upmu\)m, the diameter of the resultant micropillars was reduced to 6 \(\upmu\)m. The smallest base diameter (~4 \(\upmu\)m) was obtained from the thinnest slurry layer (8 \(\upmu\)m), which was only half that from the 20-\(\upmu\)m-thick slurry layer.  
The mechanism of Si micropillar growth from slurry layers of different thicknesses during the laser irradiation process is illustrated in Figure 3. As clarified by our previous research [39], Si particles on the top layers were directly heated by the laser and melted onto the Si particles under them. Due to the thermal conduction, with further laser heating, all the Si particles completely melted and spread out onto the Cu foil. In order to achieve stable equilibrium with low surface energy, the liquid Si merged to form larger Si islands on the Cu foil. During this process, carbon black, as a sacrificial agent, underwent plasma gasification and partial ignition in the air, generating a high-pressure plasma and carbon oxide gas around the Si islands. Owing to the plasma-induced pressure around the Si islands, they were stretched along the laser beam direction and eventually micropillars were formed after solidification. In the course of this process, two scenarios were considered. If the slurry layer was thin, small Si islands formed, leading to small and thin pillars (Figure 3a). On the other hand, if a thick slurry layer was applied, the Si islands were relatively large since more particles were melted from the top layers and accumulated on the Cu foil. Therefore, larger micropillars were obtained (Figure 3b). This study demonstrates the possibility of generating extremely small Si pillars on Cu foils by laser irradiation of a very thin layer of waste Si slurry. If the pillar was thin enough, the situation may approach that of Si NWs.  
Figure 2: SEM images of Si micropillars growing from slurry layers with thicknesses of (**a**) 8, (**b**) 13 and (**c**) 20 \(\upmu\)m. (**d**) Base diameters of micropillars growing from various slurry layer thicknesses.