PSi was prepared in a homemade etch cell by anodizing the highly antimony doped Si (100) wafers (~0.02 Ω cm-1) using a HF (~48%)-ethanol mixture as the electrolyte at a various current density (50-150 mA/cm) for 3 min, and then, PSi was rinsed with ethanol and dried under vacuum for 2-3 h.
The carbonization of PSi was carried out in a tube furnace with a quartz tube under a 1 L/min of Ar and 200 mL H2 flow while the furnace was heated to 650 ˚C When the furnace reached 650 ˚C, 10 ml/min of C2H2 was added to the gas mixture, and the temperature was increased to 750 ˚C for 10 minutes, and increased to 850 ˚C for another 10 minutes. The C2H2 was then turned off, and the P-Si was cooled in the presence of Ar and H2 until the sample reached a room temperature. Analysis of PSi and carbonized PSi was performed with a FESEM and Raman spectroscopy. Electrochemical measurement was carried out utilizing PSi materials in a two-electrode homemade electrochemical test cell. Cyclic voltammetry and electrochemical impedance spectroscopy were conducted in a trimethylpropylammonium bis(trifluoromethanesulfonyl)amide (TMPATFSA) ionic liquid. The scan range was -3.0 to 3.0 V. Impedance measurements were performed around zero bias with a signal amplitude of 10 mV.
SEM images of PSi and carbonized PSi indicated that PSi’s was obtained with the pore diameter of 10, 20 and, 80 nm. Also it is confirmed that there is little change during the carbonization of PSi.
Raman spectroscopy was carried out to confirm graphene coating on PSi, and after the carbonization of PSi. We observed Si Raman peak near 520 cm-1 from the pre-coated PSi, and the D (1350 cm-1) and G (1580 cm-1) peaks corresponding to sp3 and sp2-hybridized carbons from the carbonized PSi, respectively. EIS shows drastic change between before or after graphene coating on PSi. This is because the contact resistance decreased by graphene coating and good wettability of graphene. For observation of dependence of double layer capacitance on pore diameter of carbon coated porous Si, we performed cyclic voltammetry of carbonized PSi as shown in Fig.1. The largest electrochemical capacitance was observed from the carbonized PSi with pore diameter of 20 nm. The contact resistance of graphene coated PSi decreased in inverse proportion to the size of the electric potential window. It is suggested that the decrease of the adsorption site of the ion by the decrease in surface area for 10 nm of pore size by hindrance of ionic mobility or decrease of electronic double layer.4
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