Semiconductor Nanomaterials: Photocatalytic Characteristics of Wide Bandgap Semiconductor Nanomaterials

Abstract

The synthesis and characterization of wide bandgap II–VI (~3.6 eV) semiconductor materials has been the major area of research in the field of nanotechnology since last 10 years. The nano-sized wide bandgap semiconductor crystallites have unique chemical, physical, and electronic properties, which support their potential application in opto-electronics, data storage devices, quantum dot lasers, nanosensors, nanophosphors, biological markers, and efficient photocatalytic applications [1–6]. When the size of semiconductor nanoparticles is reduced below a critical diameter (<100 nm), the spatial confinement of the charge carrier causes them to behave quantum mechanically. As a result, the bands split into discrete electronic states in the valence and conduction bands, and nanoparticles behave more like a giant atom. Due to the quantum confinement effect [7,8], semiconductor nanomaterials also exhibit large surface area to volume ratio. The high surface-to-volume ratio of nanoclusters leads to high surface reactivity and enhance the chemical reaction dynamics and photon absorption efficiency. In case of photocatalytic processes, the electron–hole pairs generated on the surface of semiconductor materials by visible or ultraviolet (UV) photon undergo redox reactions with molecules adsorbed onto the surface, thereby breaking them into smaller fragments. The process finds its applications for the mineralization of toxic organic compounds, hazardous inorganic constituents, and bacteria disinfection. An ideal photocatalyst should be stable, inexpensive, nontoxic, and highly photoactive. Doping of pure semiconductor nanomaterials with rare earths and/or transition metals creates quasistable energy states within the bandgap, thereby affecting the electronic structure, optical, and transition probabilities. In case of doped semiconductor nanocrystals, impurity ion occupies the host atom lattice site and behaves as trap site for electron and holes. The electrons are excited from the conduction band by absorbing the energy equal or greater than their bandgap energy. Subsequent relaxation of these photoexcited electrons to some surface states or levels is followed by radiative and nonradiative decay processes. In case of photocatalysis, increased electron trapping due to higher defect sites leads to enhancement in the photocatalytic efficiency. This increase in photocatalytic efficiency is possible provided the electron– hole pair recombination rate is lower than the rate of electron transfer to adsorbed molecules. Therefore, the photocatalytic and luminescent properties of wide bandgap semiconductor nanomaterials are strongly dependent upon the type of dopant ions. Moreover, photophysical and photochemical behavior correlation and their dependence on doping concentration and nanostructure morphology will be very beneficial for making the phosphor and photocatalytic calibration curves, which will serve as the backbone for the future applications of these smart materials. CONTENTS