Recent studies on the electrical conductivity and photocatalytic activity properties of semiconductor nanocrystals such as CuO, AgO, TiO, PbS, and AgPO exposing well-defined surfaces have revealed strong facet effects. For example, the electrical conductivity of CuO crystals can vary from highly conductive to nonconductive, and they can be highly photocatalytically active or inactive depending on the exposed faces. The crystal surfaces can even tune their light absorption wavelengths. Our understanding is that the emergence of these unusual phenomena can be explained in terms of the presence of an ultrathin surface layer having different band structures and degrees of band bending for different surfaces, which affects charge transport and photons into and out of the crystals. This review uses primarily results from our research on this frontier area of semiconductor properties to illustrate the existence of semiconductor facet effects. A simple adjustment to normal semiconductor band diagram allows good understanding of the observed phenomena. Recognizing that facet-dependent behaviors are intrinsic semiconductor properties, we should pay attention to their influence in the explanation of the measured photocatalytic properties, and consider ways to enhance photocatalytic efficiency or design electrical components utilizing the facet effects. There should be many opportunities to advance applications of semiconductor nanocrystals and nanostructures with continued research on the facet-dependent properties of various semiconductor materials.

Controlling which facets are exposed in nanocrystals is crucial to understanding different activity between ordered and disordered alloy electrocatalysts. We modify the degree of ordering of Pt3Sn nanocubes, while maintaining the shape and size, to enable a direct evaluation of the effect of the order on ORR catalytic activity. We demonstrate a 2.3-fold enhancement in specific activity by 60- and 30%-ordered Pt3Sn nanocubes compared to 95%-ordered. This was shown to be likely due to surface vacancies in the less-ordered particles. The greater order, however, results in higher stability of the electrocatalyst, with the more disordered nanoparticles showing the dissolution of tin and platinum species during electrocatalysis.

When comparing alloy catalysts with different degrees of ordering, it is important to maintain surface facets to understand the effect of different arrangements of surface atoms. This is even more important when both metals are involved in the reaction steps, which is the case of Pt 3 Sn for the methanol oxidation reaction (MOR). We have prepared 95 and 60% ordered Pt 3 Sn nanocubes with {100} facets for the MOR. We show that the Sn atoms in the 60% ordered Pt 3 Sn nanocubes can be electrochemically oxidized to Sn 4+ , whereas the Sn atoms in the 95% ordered Pt 3 Sn nanocubes are more resistant to oxidation. The Sn 4+ in the disordered catalysts makes them more active than the ordered catalysts. At low overpotentials, the electrochemically formed Sn 4+ in the 60% ordered Pt 3 Sn nanocubes bind OH, oxidizing the CO intermediate adsorbed on Pt more efficiently. At high overpotentials, Sn 4+ prevents the passivation of the Pt sites due to adsorption of OH. These effects lead to a 5.6 times higher activity of the 60% ordered nanocubes compared to the 95% ordered nanocubes. These results illustrate the importance in catalyst design of controlling the environment and especially the atoms neighboring Pt for intermetallic Pt−M electrocatalysts.

A one-pot synthesis for the growth of PbS nanocubes and octahedra has been developed by heating an aqueous mixture of cetyltrimethylammonium bromide (CTAB), thioacetamide (TAA), lead acetate, and nitric acid at 90 °C for 3 h. The method allows for direct large-scale production of small and uniform PbS nanocubes. The PbS cubes and octahedra exhibit different surface properties, as evidenced by zeta potential measurements and stability tests in methyl orange and methylene blue solutions. Examination of the reagent introduction sequence indicates that early or late addition of nitric acid to tune the initial solution pH can strongly influence crystal growth rate and result in the formation of PbS crystals with different sizes and shapes. Remarkably, the use of pre-formed PbS nanocubes for further crystal growth under low TAA concentration can transform the cubes into large octahedra through a series of particle shape evolution.

A strategy of direct growth of Pt on Ni was used to create and control Pt crystal defects on the surface of Ni–Pt core–shell nanoparticles. The control over the types of defects was easily achieved by changing the surfactant system. In this work, two types of crystal defects have been introduced into Ni–Pt core–shell nanoparticles: polycrystalline shells with multiple grain boundaries and step-edge shells with undercoordinated atoms at corners and steps. We show that the step-edge shell has a higher specific activity for the oxygen reduction reaction (ORR), while the thinner polycrystalline shell results in a higher activity per mass and stability. Our results suggest that Ni–Pt core–shell nanoparticles with a thin Pt shell that have high density of crystal defect should be targeted for high performance ORR catalysts.