# Grain Boundary--A Route to Enhance Electrocatalytic Activity for Hydrogen Evolution Reaction
## 3 Results and Discussion
### Experimental Verification of GB-Assisted HER Activity  
#### 3.2.1 Exposed Surface Characterization  
To determine the accessible exposed facet of the as-prepared Au electrodes, we used the lead underpotential deposition (Pb-upd) technique. The Pb-upd process is very sensitive to the surface structure of the gold electrode with a distinct voltammetric profile, where the main peaks appear at different characteristic potentials for different exposed facets of Au electrodes [28; 29]. As a result, the main peaks can be used as indicators to characterize the surface structure of the electrode. In Figure 4, it can be clearly observed that the pristine Au/CFP electrode shows only one significant deposition peak in the Pb-upd voltammogram profile, which is located at \(\sim\)\(-\)0.47 V (vs. Ag/AgCl) and corresponds to the (110) facet of Au. Upon scan reversal, the Pb-upd layer is oxidized to dissolved Pb\({}^{2+}\) in the form of Pb(OH)\({}^{-}\) in alkaline solution in a sharp peak at \(\sim\)\(-\)0.38 V (vs. Ag/AgCl), which is in good agreement with the results of a previous study [30]. Additionally, the CFP electrode exhibits no Pb-upd peak, suggesting that Pb-upd is attributed to the Au itself. Therefore, Au(110) is the dominant exposed surface faceting of the as-deposited Au/CFP electrodes, which provides the experimental basis for the verification of our GB-assisted HER on Au(110).  
We performed a detailed characterization of the morphology and structure of the as-prepared and annealed Au/CFP. From SEM and compositional maps (Figure S2), it can be seen that Au is deposited on the CFP electrode with a uniform distribution and a small particle size. The as-deposited Au/CFP was characterized by X-ray diffraction (XRD) (Figure S3). When the sputtering time is short, such as 30 s or 2 min, the intensity of the Au diffraction peaks is weak, because the content of Au is small and the particle size is small.  
The morphological structures of the as-deposited and annealed Au/CFP were investigated via transmission electron microscopy (TEM) observation (Figure 5). The high-resolution TEM (HR-TEM) images show a lattice fringe spacing of 2.35 A corresponding to the (111) facet direction of the Au nanoparticles (NPs). It can be clearly seen that the as-deposited Au NPs feature a representative surface rich in GBs, showing a polycrystalline character to some extent (Figure 5a). Notably, the grain boundary angle in Figure 5a is similar to that in our calculated GBs surface model (Figure 1).  
Figure 4: Pd\(-\)upd voltammetric profile of as\(-\)deposited Au/CFP and clean CFP electrodes (0.5 \(\times\) 1 cm\({}^{2}\)). Scan rate: 50 mV/s.  
To control the density of the GBs on Au(110), we annealed the pristine Au/CFP under an Ar atmosphere at different temperatures, namely, 300 \({}^{\circ}\)C and 400 (producing Au/CFP\({}_{300}\) and Au/CFP\({}_{400}\), respectively). By comparing the change in the morphology before and after annealing, it is interesting to note that the (110) facet is still dominant on various Au/CFP electrodes. In addition, the density of the GBs in the Au NPs dramatically decreases as the annealing temperature is increased (Figure 5b,c). At 300 \({}^{\circ}\)C, only a few Au NPs with GBs are observed, and the GB density in the Au NPs becomes much lower, suggesting that most Au NPs are transformed into a monocrystalline form. Nevertheless, Au NPs are completely monocrystalline after annealing at 400 \({}^{\circ}\)C.