Technion researchers reveal, in collaboration with researchers from BGU and Leibniz Institute of Photonic Technology, that the nature of a nano-junction formed between a semiconductor nanorod (photosenisitizer) and metal nanoparticle (cocatalyst) is sensitive to the size of the metal component. This is reflected in the activity towards hydrogen production, emission quantum yields, and the quantity of charge separation determined by transient absorption spectroscopy. This discovery is of significance to any application utilizing semiconductor-metal nanocomposites, and is thus expected to benefit a broad range of interdisciplinary research areas, and a wide spectrum of applications such as solar and fuel cells, photocatalysis, electronics and photonics.
The authors suggest two phenomena that contribute to the metal size effect on charge transfer probability across the semiconductor – metal nano junction, with opposing trends. The first is the so-called “Coulomb blockade”. In a nanocrystal, the presence of one charge acts to prevent the addition of another, due to the Coulomb repulsion between them. Simply put, the Coulomb blockade may be regarded as an “energy penalty” which an electron must pay to enter a confined charged domain, such as the metal nanoparticle in this system. This energy penalty is inversely proportional to the metal radius, resulting with favorable transfer of electrons from the semiconductor into larger metal domains. Competing with the Coulomb blockade is size dependent Schottky barrier, which becomes more pronounce for the larger metal size range. While the existence of a reduced Schottky barrier at the nanoscale junction is established in the literature, the origin of this interesting and technologically important behavior is still far from fully understood. The work described here might provide beneficial insights, and advance the fundamental understanding of nanoscale semiconductor-metal hybrid characteristics.
From the point of view of photocatalytic design, these two opposing trends result with an optimal metal size domain for the cocatalyst. This finding highlights the importance of careful and precise tailoring of the photocatalytic system for optimal performance.
This research was partially carried out in the framework of Russell Berrie Nanotechnology Institute (RBNI) and the Nancy and Stephen Grand Technion Energy Program (GTEP). The work was funded by the I-CORE (Israeli Centers of Research Excellence) program of the Council for Higher Education’s Planning and Budgeting Committee, and the National Science Foundation (Grant No. 152/11), the German-Israeli Foundation (GIF) for Scientific Research and Development (Grant 2307-2319.5/2011), and the COST Action CM1202 PERSPECT-H2O.
The study was published in the journal Nano Letters: https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b04210
Figure. Left: TEM micrograph of Ni tipped CdSe@CdS rods. Right: Illustration of the metal size effect on the semiconductor – metal nano junction. Schottky barrier develops with increasing metal size, while Coulomb blockade charging energy is decreasing. These two opposing trends result with an optimal metal size domain for the cocatalyst.