Molybdenum disulfide (MoS₂) is a promising nonprecious catalyst for the hydrogen evolution reaction (HER) that has been extensively studied due to its excellent performance, but the lack of understanding of the factors that impact its catalytic activity hinders further design and enhancement of MoS₂-based electrocatalysts. Here, by using novel porous (holey) metallic 1T phase MoS₂ nanosheets synthesized by a liquid-ammonia-assisted lithiation route, we systematically investigated the contributions of crystal structure (phase), edges, and sulfur vacancies (S-vacancies) to the catalytic activity toward HER from five representative MoS₂ nanosheet samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated porous 2H phase. Superior HER catalytic activity was achieved in the porous 1T phase MoS₂ nanosheets that have even more edges and S-vacancies than conventional 1T phase MoS₂. A comparative study revealed that the phase serves as the key role in determining the HER performance, as 1T phase MoS₂ always outperforms the corresponding 2H phase MoS₂ samples, and that both edges and S-vacancies also contribute significantly to the catalytic activity in porous MoS₂ samples. Then, using combined defect characterization techniques of electron spin resonance spectroscopy and positron annihilation lifetime spectroscopy to quantify the S-vacancies, the contributions of each factor were individually elucidated. This study presents new insights and opens up new avenues for designing electrocatalysts based on MoS₂ or other layered materials with enhanced HER performance.

The sole effect of surface/bulk defects of TiO₂ samples on their photocatalytic activity was investigated. Nano-sized anatase and rutile TiO₂ were prepared by hydrothermal method and their surface/bulk defects were adjusted simply by calcination at different temperatures, i.e. 400–700 °C. High temperature calcinations induced the growth of crystalline sizes and a decrease in the surface areas, while the crystalline phase and the exposed facets were kept unchanged during calcination, as indicated by the characterization results from XRD, Raman, nitrogen adsorption–desorption, TEM and UV-Vis spectra. The existence of surface/bulk defects in calcined TiO₂ samples was confirmed by photoluminescence and XPS spectra, and the surface/bulk defect ratio was quantitatively analyzed according to positron annihilation results. The photocatalytic activity of calcined TiO₂ samples was evaluated in the photocatalytic reforming of methanol and the photocatalytic oxidation of α-phenethyl alcohol. Based on the characterization and catalytic results, a direct correlation between the surface specific photocatalytic activity and the surface/bulk defect density ratio could be drawn for both anatase TiO₂ and rutile TiO₂. The surface defects of TiO₂, i.e. oxygen vacancy clusters, could promote the separation of electron–hole pairs under irradiation, and therefore, enhance the activity during photocatalytic reaction.