To prepare SiO2 templates, 200 ml of NH4OH, 140 ml of deionized (DI) water, and 500 ml of dehydrated ethanol were mixed together by stirring at a speed of 200 rpm to make solution (a). Sixty milliliters of tetraethoxysilane (TEOS) was mixed with 120 ml of dehydrated ethanol manually to make solution (b). Solution (b) was then quickly poured into solution (a). After stirring for 8 hours, 10 ml of DI water and 60 ml of TEOS were added successively into the above solution and stirring was kept for another 8 hours. Finally, SiO2 particles with a size of around 645 nm were collected through centrifugation and washed alternatively with ethanol and DI water for a total of six times. To prepare C/SiO2, 1 g of SiO2 nanoparticles was dispersed in 300 ml of DI water under ultrasonication for 10 to 30 min. Next, 10 ml of cetyl trimethylammonium bromide (CTAB; Sigma-Aldrich, 10 mM) and 1 ml of ammonia (NH3·H2O; Sigma-Aldrich, 28%) were added to the SiO2 solution and vigorously stirred for 20 min to ensure the adsorption of CTAB on the cluster surface. Then, 400 mg of resorcinol (Sigma-Aldrich) and 0.56 ml of formaldehyde solution [Sigma-Aldrich, 37 weight % (wt %) in H2O] were added and stirred overnight. The RF resin–coated particles were collected by centrifugation and washed with ethanol three times. Finally, the RF shell was carbonized under Ar at 900°C for 2 hours with a temperature ramping rate of 5°C min−1.