Photoactive Heterostructures: How They Are Made and Explored

Alexei V. Emeline,Detlef W. Bahnemann,Aida V. Rudakova,Ruslan V. Mikhaylov,Kirill M. Bulanin

doi:10.3390/catal11020294

Abstract

In our review we consider the results on the development and exploration of heterostructured photoactive materials with major attention focused on what are the better ways to form this type of materials and how to explore them correctly. Regardless of what type of heterostructure, metal–semiconductor or semiconductor–semiconductor, is formed, its functionality strongly depends on the quality of heterojunction. In turn, it depends on the selection of the heterostructure components (their chemical and physical properties) and on the proper choice of the synthesis method. Several examples of the different approaches such as in situ and ex situ, bottom-up and top-down, are reviewed. At the same time, even if the synthesis of heterostructured photoactive materials seems to be successful, strong experimental physical evidence demonstrating true heterojunction formation are required. A possibility for obtaining such evidence using different physical techniques is discussed. Particularly, it is demonstrated that the ability of optical spectroscopy to study heterostructured materials is in fact very limited. At the same time, such experimental techniques as high-resolution transmission electron microscopy (HRTEM) and electrophysical methods (work function measurements and impedance spectroscopy) present a true signature of heterojunction formation. Therefore, whatever the purpose of heterostructure formation and studies is, the application of HRTEM and electrophysical methods is necessary to confirm that formation of the heterojunction was successful.

Highlights

For decades heterogeneous photocatalysis and photoelectrochemistry have attracted significant attention from researchers working in the areas of both fundamental and applied science [1,2,3,4,5,6]
It is wise to note that an application of CPE in an equivalent circuit is more physically correct than using a “true” capacitor to describe a behavior of heterojunctions, since photoexcitation of heterostructure can lead to significant decay of the barrier between heterostructure components, which is caused by interfacial charge transfer that diminishes a capacitor effect, transforming the heterojunction into a “real” resistor
It should be emphasized that in order to improve the physical properties of heterostructures, the key issues are the design and synthesis of complex heterostructures with controlled assembly of each section of materials, including the size, shape, and uniformity of the building blocks

Summary

Introduction

For decades heterogeneous photocatalysis and photoelectrochemistry have attracted significant attention from researchers working in the areas of both fundamental and applied science [1,2,3,4,5,6]. In recent years two major targets have been the focus of practical research: the activity and spectral sensitivity of photoactive materials. An illustrative example of a three-dimensional heterosystem is a conductive metal or semiconductor substrates (see Figure 1b, right image) Another possible structure of 2D heterostructure is a so called deposited heterostructure. The interface of three-dimensional heterostructures extends in all three directions and can be described as a 3D surface of rather complex shape (Figure 1c) This type of heterostructured material is the most common and typically includes powdered (nanoparticle based) components. We consider some approaches typically used to make and explore photoactive heterostructured materials

How Photoactive Heterostructures Are Made

Optical Characterization

Electrophysical Characterization

Conclusions