## 3 Results and Discussion  
Cobalt selenide (Co\({}_{3}\)Se\({}_{4}\)) was deposited electrochemically onto conductive electrodes by the electrodeposition method. The X-ray diffraction (XRD) pattern shown in Figure 1 reveals highly intense and crystalline peaks of prepared cobalt selenide sample. The peaks confirm the single-phase formation of pure monoclinic crystal system of Co\({}_{3}\)Se\({}_{4}\), which exactly corresponds with the standard JCPDS card No. 98-009-9990. The prepared Co\({}_{3}\)Se\({}_{4}\) was in well agreement with the lattice parameters of space group of P21/c (space group number 14). Field-emission scanning electron microscopy (FESEM) measurements were employed to understand the surface morphology of the Co\({}_{3}\)Se\({}_{4}\) film on a glassy carbon electrode (GC) and a fluorine-doped tin oxide (FTO) as shown in Figure 1b,c. The morphology of Co\({}_{3}\)Se\({}_{4}\) film has almost same 3D porous structures despite of the different electrodesubstrates. The electrodeposition time was increased from 5 s to 25 s and confirmed that almost same 3D porous structure of Co\({}_{3}\)Se\({}_{4}\) film among samples of 15 s, 20 s and 25 s of electrodeposition time. Representative samples on GC electrode were selected with samples of the electrodeposition time of 20 s (Figure 1b) and 25 s (the inset image in Figure 1b), respectively. Meanwhile, as shown in Figure 1b,c, the morphologies of Co\({}_{3}\)Se\({}_{4}\) films on GC electrode are almost same with the Co\({}_{3}\)Se\({}_{4}\) film on FTO with the electrodeposition time of 15 s in Figure 1d, showing the top-view and cross-sectional FESEM view images. As shown in Figure 1d, nanostructured Co\({}_{3}\)Se\({}_{4}\) film was uniformly deposited with 3D porous structure. To determine the elemental composition of the film, energy dispersive spectrometry (EDS) was performed (Figure 1d). To clarify this further, transmission electron microscopy (TEM) measurements of the deposited film were conducted as shown in Figure 1f. Information about the lattice fringe with an inter-planar spacing of 2.67 nm, revealed that the electro-deposited Co\({}_{3}\)Se\({}_{4}\) film strongly corresponds to the (111) plane of the obtained XRD pattern. Photoelectrochemical measurements indicated that the electrodeposited Co\({}_{3}\)Se\({}_{4}\) film on the photoelectrode acted as an effective layer for the hydrogen evolution reaction (HER). Figure 2a shows the resulting photoresponse of bare planar Si and Co\({}_{3}\)Se\({}_{4}\) on planar Si. In the case of Co\({}_{3}\)Se\({}_{4}\) on planar Si, the current density started to increase negatively much earlier than that of bare planar Si. This indicates that the Co\({}_{3}\)Se\({}_{4}\)/Si system has higher onset potential, defined as the specific potential at \(-\)1 mA/cm\({}^{2}\). The onset potential (V\({}_{\text{os}}\)) of Co\({}_{3}\)Se\({}_{4}\)/Si (V\({}_{\text{os}}\) = 0.216 V vs. RHE at \(-\)1 mA/cm\({}^{2}\)) is  
Figure 1: Surface characterization of the Co\({}_{3}\)Se\({}_{4}\) film: (**a**) XRD (x-ray diffraction) pattern of electrodeposited Co\({}_{3}\)Se\({}_{4}\) on FTO (fluorine doped tin oxide). (**b**) Plan-view FESEM (field-emission scanning electron microscope) images of electrodeposited Co\({}_{3}\)Se\({}_{4}\) on GC electrode. (**c**) Plan-view and cross-sectional (inset) FESEM images of electrodeposited Co\({}_{3}\)Se\({}_{4}\) on FTO electrode. Different but conducting electrode substrates result in the same 3D porous structure of Co\({}_{3}\)Se\({}_{4}\) film through the electrodeposition process. (**d**) Energy dispersive spectrometry spectrum showing the presence of cobalt and selenide elements and (**e**) High-resolution transmission electron microscopy (HR-TEM) image to identify Co\({}_{3}\)Se\({}_{4}\). (**f**) TEM (transmission electron microscopy) image showing uniformly deposited Co\({}_{3}\)Se\({}_{4}\). 303 mV higher than that of planar Si (V\({}_{\rm os}\) = \(-\)0.087 V vs. RHE at \(-\)1 mA/cm\({}^{2}\)).