4H-SiC shows great prospects in power devices due to its high electric breakdown strength, high bulk carrier mobility, high-temperature stability and high thermal conductivity. However, due to the technical challenges in the fabrication process, high cost of 4H-SiC power devices hinders their widespread application. Therefore, porosified 4H-SiC substrates are targeted for epitaxial growth for reducing the defect density in the epitaxy layer [1]. Even more, this technology promises the optimization of power device fabrication processes on cost-efficient, engineered substrates [2]. Besides the application in power electronics, porosified SiC has extensive applications in catalysis, sensors and MEMS [3, 4]. Photoelectrochemical etching (PECE) can effectively porosify 4H-SiC from both Si-face and C-face, and the degree of porosity into the bulk is well controllable for specific application, for instance rugate mirrors [4]. The PECE from the C-face of 4H-SiC wafers leads to columnar pores along the [0001] direction (Fig.1a). Pore diameters have been discovered to be related to the applied voltage, hydrofluoric acid (HF) concentration, etching depth and diffusion coefficients [5-6]. However, it is still not possible to precisely control the diameter profile along the columnar pores. From the crystalline structure of 4H-SiC, the columnar pores along [0001] direction should mostly expose {10-10} (Fig.1b) and {11-20} (Fig.1c) crystal planes, or some other high-index crystal planes which are composed of the {10-10} and {11-20} in a different ratio. To precisely adjust the pore diameter, the etching rates on the {10-10} and {11-20} crystal planes need to be tailored, which can be enabled by modifying the surface passivation of the specific crystal plane. Doing so, an in-depth understanding of the surface chemistry of the {10-10} and {11-20} crystal planes during PECE process in an HF electrolyte is necessary. Recently, density functional theory (DFT) calculations have successfully revealed the mechanism of platinum-assisted HF etching of SiC [7]. In our PECE of 4H-SiC from the C-face [4-6], the pore walls of the columnar pores are newly created during the etching process and they are immediately passivated in the HF electrolyte containing HF and H2O molecules as well as F− and HF2− ions. In this work, the adsorption and reaction of the molecules and ions on the 4H-SiC {10-10} and {11-20} surfaces will be simulated by DFT. Fig. 2 shows one single HF or H2O molecule adsorbed on the 4H-SiC {10-10} or {11-20} surface, whose adsorption energies (Eads) are listed in Table I. On both 4H-SiC {10-10} and {11-20}, HF can be dissociatively adsorbed with F bonding to Si and H to C with high Eads value of −4.08 and −3.55 eV, respectively. The presence of Si-F and C-H bonds have also been experimentally demonstrated by infrared spectroscopy of the porous 4H-SiC after PECE [5]. Water is molecularly adsorbed with Eads of −0.93 eV on {10-10} surface, while it also dissociates on {11-20} surface with Eads of −2.66 eV, forming Si-OH and C-H. Considering the high affinity of Si to F, the surfaces should be terminated by F at the Si sites and by H at the C sites, with a moderate probability of Si to be terminated with OH groups. These surface terminations stabilize the newly created pore walls. However, to describe the passivation in more detail, simulations of subsequent reaction paths of the terminated surfaces in the HF electrolyte must be performed, in which the effect of the applied voltage during PECE process on the pore walls needs to be included.