Energy band engineering of multi-component heteronanostructures by polarization charge design

Abstract

Catalysis is the key to improve efficiency of materials transformation, which is crucial for sustainable development. For example, photocatalytic water splitting can use clean solar energy to produce hydrogen from water, which is clean and renewable. Unfortunately, efficiencies of photocatalysts are typically low. One of main bottlenecks lies in substantial charge recombination. Consequently, much effort has been made to promote effective charge separation with new types of photocatalytic materials. Among all available means of promoting charge separation, regulation of charge flow through heterojunction is a well-accepted and effective approach in photocatalytic materials design. This review will summarize three of representative work of heterojunction design for photocatalyst contributed by our group: A ternary semiconductor-(semiconductor/metal) model (e.g., ZnS-(CdS/Au), ZnS-(CdS/Pd), and ZnS-(CdS/Pt)). In this design, a metallic nanoparticle (M) is interfaced with CdS. The difference of work function between M and CdS will drive the polarization charges flow from M to CdS until reaching equilibrium of Fermi levels. And then the Fermi level of CdS has a upshift. Therefore, the band alignment transforms from the straddling gap (type I) of ZnS-CdS heterojunction to the staggered gap (type II) of ZnS-(CdS/metal). As a result, photogenerated electrons are kept on the surface of ZnS and metal, and holes in the CdS. Improvement of photocatalysis performance in water splitting is attained through this architecture: a Ag2S-Ag-TiO2 hybrid nanostructure. We use the wide-bandgap semiconductor TiO2 and the narrow-bandgap semiconductor Ag2S to design the Z-scheme structure, and integrate an interfacial Ag between TiO2 and Ag2S. The interfacial Ag and Ag2S have strong hybridizations at the Ag-Ag2S interface, and form Schottky junction. The polarization charge at the interface of the Schottky junction flows from Ag to Ag2S until reaching the equilibrium. This causes a upshift in the Fermi level of Ag2S. But the electronic structure coupling of the TiO2-Ag interface is relatively weak. As a result, the polarization charge in Ag can hardly flows to the TiO2. Taken together, the energy band upshift of Ag2S enables the formation of the Z-scheme structure in the Ag2S-Ag-TiO2 system. Therefore, under the full-spectrum light illumination, photogenerated electrons in TiO2 flow towards Ag2S through the interfacial Ag, and then recombine with the photogenerated holes in Ag2S. This results in the accumulation of photogenerated electrons and holes in Ag2S and TiO2, respectively. Therefore, the ternary system forms a Z-scheme structure that allows for full-spectrum sunlight absorption; A ternary ZnS-CdS-Cu2S heteronanostructure. Through combination of CdS and ZnS, a bianry ZnS-CdS heteronanostructure exhibits the wide absorption range of vis and UV. We choose the NIR absorption Cu2S to incorporate into ZnS-CdS, so as to further absorb solar energy to reach the full-spectrum. To demonstrate the ZnS-CdS-Cu2S system for the separation of photogenerated electrons and holes, we utilize density functional theory to study the bandgap alignment. Both of ZnS and CdS are n-type semiconductor, and the work function of them are almost the same. So that the interaction between ZnS and CdS is very weak. The first-principles simulations demonstrate that the construction of p-n junction between CdS and Cu2S forms staggered gaps. Such band structure enables the ZnS-CdS-Cu2S system to separate photogenerated electrons and holes. In summary, steering charge flow in hetero-nanostructures via charge polarization can effectively modify the energy band alignment to promote the separation of photogenerated electron-hole pairs. It provides new ideas for the development of nano-semiconductor composites materials in photoelectric conversion and photocatalysis. At the same time, we also believe that a better photocatalytic material with its performance significantly imporved will be designed by adjusting the band structure.