Unraveling Defects Introduced on the Basal Plane of MoS₂ Monolayers by Hydrogen Peroxide for Hydrogen Evolution Reaction

Hydrogen as a fuel is promising for a more sustainable economy once it possesses high energy density and is considered a clean energy source. A prospective route for green hydrogen production is the electrochemical water splitting powered by renewable electricity. However, the best catalysts are currently based on noble metals such as platinum, which are expensive and scarce. Aiming for economically feasible alternatives, MoS₂ has been studied for such applications due to its high abundance and low cost.¹ As well-known for MoS₂, the catalytic activity relates to edge sites while the basal plane is predominantly inert to such a process. Thus, different works have proposed different defect engineering routes to generate defects on the basal plane of MoS₂ and create catalytic sites. The introduction of such defects plays an essential role in tuning the electrocatalytic activity on the basal plane of MoS₂ monolayers towards HER. Solution-based defect-engineering routes are scalable,² can effectively activate porous electrodes,¹ and do not require high-cost equipment to generate defects on MoS₂.^3,4 Among the different possibilities hydrogen peroxide (H₂O₂) is a mild, metal-free, and carbon-free, oxidant that can activate the basal plane to drive HER. It is reported that depending on the experimental conditions, H₂O₂ might lead to the formation of edge-like defects and vacancies on the basal plane.⁵ In this work, we demonstrate for the first time the insertion of defects on monolayer MoS₂ and study the impact of such defects on the HER activity. In a first approach, we prepared ultra-large supported and free-standing monolayers on gold surfaces to evaluate chemical changes that occurred only in the basal plane. Raman, photoluminescence, and X-ray photoelectron spectroscopies results confirmed the alterations on the surface of MoS₂ after the treatment with H₂O₂. HR-TEM was employed to provide in-depth information regarding the changes in the structure of MoS₂. Remarkably, we tracked the same area with nanoscale resolution before and after H₂O₂ treatment to comprehend the impact of H₂O₂ in free-standing monolayers. Our results reveal that there is lack of structural coherence on the basal plane at high exposure times. The electrocatalytic activity of the monolayers was measured without introducing unintentional defects related to microfabrication processes.⁶ The obtained results revealed a cathodic shift of the overpotential required to drive HER, achieving 372 mV at 10 mA cm^-2. Thus, our work provides new fundamental basis for the chemical transformations of MoS₂ monolayers.<br/><br/>Acknowledgments: We thank the São Paulo Research Foundation (FAPESP: 2022/00955-0), and Brazilian System of Laboratories in Nanotechnologies (SisNano).<br/><br/><br/>References<br/>1 L. H. Hasimoto, J. Bettini, E. R. Leite, R. S. Lima, J. B. Souza Junior, L. Liu and M. Santhiago, <i>ACS Appl. Eng. Mater.</i>, 2023, <b>1</b>, 708–719.<br/>2 L. Gao, Q. Liao, X. Zhang, X. Liu, L. Gu, B. Liu, J. Du, Y. Ou, J. Xiao, Z. Kang, Z. Zhang and Y. Zhang, <i>Advanced Materials</i>, 2020, <b>32</b>, 1906646.<br/>3 K. K. Amara, L. Chu, R. Kumar, M. Toh and G. Eda, <i>APL Materials</i>, 2014, <b>2</b>, 092509.<br/>4 P. Zhang, H. Xiang, L. Tao, H. Dong, Y. Zhou, T. S. Hu, X. Chen, S. Liu, S. Wang and S. Garaj, <i>Nano Energy</i>, 2019, <b>57</b>, 535–541.<br/>5 X. Wang, Y. Zhang, H. Si, Q. Zhang, J. Wu, L. Gao, X. Wei, Y. Sun, Q. Liao, Z. Zhang, K. Ammarah, L. Gu, Z. Kang and Y. Zhang, <i>Journal of the American Chemical Society</i>, 2020, <b>142</b>, 4298–4308.<br/>6 B. R. Florindo, L. H. Hasimoto, N. De Freitas, G. Candiotto, E. N. Lima, C. De Lourenço, A. B. S. De Araujo, C. Ospina, J. Bettini, E. R. Leite, R. S. Lima, A. Fazzio, R. B. Capaz and M. Santhiago, <i>J. Mater. Chem. A</i>, 2023, <b>11</b>, 19890–19899.