Liquid-phase Exfoliated Two-dimensional Bi Nanosheet as a Durable Electrocatalyst for Hydrogen Evolution Reaction

Electrocatalytic hydrogen evolution is an exercisable way to achieve large-scale application of hydrogen energy. It is of great signicance to develop an effect, stable and cost-effective electrocatalyst. Here, we applied the two-dimensional (2D) bismuth (Bi) to the electrocatalytic hydrogen evolution, and proposed the strategies to enhance the catalytic performance of the catalyst. The exfoliated Bi nanosheets via sonication assisted liquid-phase exfoliation display higher electrocatalytic activity (overpotential of -958 mV vs RHE at 10 mA cm − 2 ) compared to the bulk counterpart. Theoretical calculations about Gibbs free energy from the hydrogen adsorption for 1 layer, 2 layers and 5 layers Bi also manifest the decrease of thickness is favorable for hydrogen evolution reaction (HER). To further evaluate the electrocatalytic performance of Bi nanosheets, the typical parameters measured in different H + concentration (C[H + ]) are carried out. The improved catalytic activity obtained in 0.5 M H 2 SO 4 is attributed to enhancing the hydrogen adsorption and accelerating the charge transport on the surface of catalyst. Moreover, the durability of Bi nanosheets electrode has been texted, where the current is not evident uctuation during the 40000 s electrolysis measurement indicating its excellent stability. The present work expands the application of Bi in the catalysis and provides the simple strategies to improve its hydrogen evolution performance.


Introduction
Hydrogen with high energy density and no carbon contaminant is considered as one of the most important energy carriers for humanity in the future [1][2][3]. Electrocatalytic hydrogen evolution, being a promising method for the large-scale production of hydrogen [4,5], has been extensively studied in recent decades [6]. However, developing the effective, stable and cost-effective catalysts to meet the needs of hydrogen economy is still a huge challenge [7,8]. Some signi cant advances have shown that 2D layered materials have great potential in the eld of catalysis due to their unique structures and properties [9][10][11][12][13].
Black phosphorus (BP), a newly emerging 2D pnictogens [14,15], has been widely used in catalysis due to high carrier mobility (1000 cm 2 V − 1 s − 1 )[16], a large number of exposed active sites [17] and non-noble element [18,19]. It has been reported that the application of BP to photocatalytic hydrogen production achieves a quantum e ciency of 42.55 % under the light of 430 nm and an energy conversion e ciency of 5.4 % at 353 K [20]. However, the inherently poor stability of BP under ambient conditions hinders its further development [21]. Recently, 2D Bi, with the similar structure and properties to BP has attracted a lot of attention due to its good environmental stability [22][23][24][25]. In addition, the advantages of low cost, low toxicity and environmental friendliness[26] make bismuth-based catalyst show great potential in the eld of catalysis.
2D Bi have the buckled honeycomb structure similar to BP, which can provide large number of active sites for catalytic reactions [27,28]. Kim et al. grew the Bi nanosheets on the Cu substrate using the pulse electrodeposition method, nding the prepared nanosheets with large number of edge and corner sites had better catalytic performance than the Bi lm [29]. Furthermore, the Bi as a natural semi-metallic material has high intrinsic electron mobility (5.7 × 10 6 cm 2 V − 1 s − 1 ) [30][31][32], which is believed to facilitate the catalytic reaction [33,34]. Aktürk et al. used the rst-principles phonon and nite temperature molecular dynamics calculation to demonstrate that 2D Bi has the excellent durability even at high temperatures [35]. The attractive qualities of 2D Bi presented above make it show great potential in the eld of electrocatalysis. Most recently, Li et al. demonstrated 2D Bi nanosheets with effective p-orbital electron delocalization and su ciently exposed active sites prepared by situ electrochemical reduction could signi cantly promote electrocatalytic NRR (N 2 reduction reaction) [36]. Simultaneously, Su et al.
reported that the ultrathin Bi nanosheet has an enhanced activity of CO 2 electrocatalytic reduction due to its much higher electron state density around Fermi level than the bulk counterpart [37]. At the same time, the high catalytic activity of 2D Bi is also expected to be effective in electrocatalytic water splitting. Pillai et al. reported the adsorption energy of the hydrogen atom and oxygen atom on the surface of Bi were − 1.418 eV and − 3.963 eV, respectively. The small adsorption energy of the atomic hydrogen indicates that hydrogen evolution reaction is more likely to occur on the surface of 2D Bi [38,39]. Therefore, exploring the application of bismuth in electrocatalytic hydrogen evolution has great signi cance due to the promising features of 2D Bi nanosheets.
In this work, the 2D Bi nanosheets were prepared by the simple sonication assisted liquid-phase exfoliation method. Bene tting from the exfoliation, 2D Bi has more active sites and shorter charge diffusion distance as compared to bulk counterpart [40,41], which is the remarkable feature of e cient

Synthesis of Materials
Bismuth nanosheets were prepared by the sonication assisted liquid-phase exfoliation method. The granular Bi (400 mg) were ground in an agate mortar for 20 min to make the particles small. Adding the Bi powders into the wide-mouth glass bottle with 90 mL isopropyl alcohol, which was tip sonicated in ice bath using probe sonication (BILON-1800Y, with the power of 30% of 1800 W) for 12 h. Subsequently, the dispersions were centrifuged at 1000 rpm for 20 min to remove the large aggregates. The collected supernatant was further centrifuged at 7000 rpm for 30 min, and the required Bi nanosheets were acquired. Finally, the Bi nanosheets was obtained after washing and centrifugation for two times using ethanol and deionized water.

Preparation of Working Electrodes
The Bi nanosheets (2 mg) and 20 µL of Na on solution (5 wt%) were mixed in 2 mL deionized water to prepare the homogeneous dispersion by the sonication (about 10 min). Drop 25 µL of the mixed solution (1 mg/ mL) on the glassy carbon electrode with the area of 0.0706 cm 2 in batches and then the electrocatalytic performance of working electrode with 25 µg Bi nanosheets would be tested after drying in a vacuum environment for 5 hours.

Characterization of Electrocatalyst
The structural of Bi nanosheets can be characterized by X-ray diffraction and Raman spectra. Here, we obtain the information of X-ray diffraction by Ultima IV with Cu/Ka radiation and Raman spectra by HORIBA JY Raman microscope with excitation laser wavelength of 532 nm at ambient temperature.
Besides, scanning electron microscopy (VEGA3 SBH, Tescan) is used to characterize the morphology and structure of Bi nanosheets.

Electrochemical Measurements of Bi nanosheets
The electrochemical performance of as-prepared materials is tested by the electrochemical workstation (CHI660E, CH Instruments, Inc., Shanghai) equipped with a standard three-electrode (Working electrode, Reference electrode, Counter electrode). The glassy carbon electrode modi ed with Bi, Pt foil and saturated calomel electrode are used as the working electrode, counter electrode and reference electrode, respectively.

Results And Discussion
The typical process of sonication assisted liquid-phase exfoliation for preparing Bi nanosheets has been illustrated in the Fig. 1. The advantage of large-scale production of Bi nanosheets can meet the potential demand of industry. Figure 2a is the SEM image of Bi before exfoliation. It is clear that the surface of pristine bulk Bi is compact and at. In contrast, the exfoliated materials, as shown in Fig. 2b, exhibit irregularly shaped straight, smooth edges, as well as the apparent lateral diameters are not uniform, indicating that the 2D Bi nanosheets are successfully exfoliated from the bulk counterparts by the liquidphase exfoliation. In order to further study the crystal structure, the Raman spectra of bulk Bi and exfoliated Bi nanosheets are carried out in the Fig. 2c. It is known that E g and A 1g peak are corresponding to an in-plane vibrational mode at low wavenumbers (v) and out-of-plane vibrational mode at high v, respectively, which are consistent with previous reports [42]. This proves that the structure of Bi nanosheets is not destroyed in the process of liquid-phase exfoliation. Compared with the Raman peaks of the bulk Bi, the peak intensities of exfoliated Bi reduced, indicating the reduced number of layers [43]. In addition, the slight blue shift toward a higher wavenumber is attributed to the decrease of lateral dimensions and the reduction of thickness of Bi [44]. Figure 2d shows the XRD patterns of Bi  Figure 3a displays the schematic illustration of three-electrode electrochemical cell. The polarization curves of the current density plotted against potential in Fig. 3b presents the electrocatalytic activity of the Bi nanosheets and the bulk Bi. The exfoliated Bi nanosheets require a lower potential of -0.96 V (versus RHE, reversible hydrogen electrode) to reach the current density of 10 mA/cm 2 than the bulk Bi (-1.06 V vs. RHE). The enhanced electrochemical activity of Bi nanosheets is attributed to the su cient exposed active sites and shortened charge diffusion distance after the exfoliation [45].  Fig. S3. Bi nanosheets as the working electrode possess a much smaller resistance, which is believed that the nanostructure is bene cial for electronic transport and reduction of parasitic Ohmic losses. Furthermore, the rst-principles calculation is used to explore the dependence of electrocatalytic activities on the thickness of Bi. The Gibbs free energy (ΔG) of Bi with 1, 2 and 5 layers toward HER has been presented in the Fig. 3d (the details about the theoretical calculations are presented in Fig. S4). It is observed that the ΔG decreases as the reduction of layer number, which suggests the reducing the thickness of Bi is bene cial to hydrogen evolution reaction.

Conclusion
In this work, the 2D Bi nanosheets had been exfoliated successfully from the Bi powders by the sonication assisted liquid-phase exfoliation. The exfoliated Bi nanosheets have much higher electrocatalytic activity, which is attributed to the su cient exposure of active sites and short charge diffusion distance. Theoretical calculation also shows that the reduction of thickness of Bi can improve its electrocatalytic performance due to the enhanced hydrogen adsorption on catalyst surface.  SO 4 ] solutions has been carried out, which demonstrates excellent durability and e cient electrocatalytic activity of Bi nanosheets. This work explores the application of the 2D Bi in electrocatalytic hydrogen evolution, and proposes the strategies to enhance the electrocatalytic performance of the catalyst. Figure 1 Schematic illustration of preparation for Bi nanosheets via sonication assisted liquid-phase exfoliation.