ZnO Synthesized Using Bipolar Electrochemistry: Structure and Activity.

Arya Hakimian,Anna Ignaszak,Steven Mcwilliams

doi:10.3390/ma12030535

Abstract

The photoactive materials broadly applied in catalysis and energy conversion are generally composed of metal oxides. Among these oxides, ZnO showed a promising photocatalytic activity; however, traditional synthetic routes generated by-products and large amounts of secondary waste. Herein, we report the use of bipolar electrochemistry to generate ZnO nanoparticles using deionized water and a zinc metal to conform to green chemistry practices. TEM imaging demonstrated that the sizes of the bipolar-made ZnO particles were smaller than the commercial sample. The presence of structural defects in ZnO was correlated with the chemical shifts analyzed by X-ray photoelectron spectroscopy (XPS) and by different concentrations of O2− ions in stoichiometric and defected lattice. Further, the diffuse reflectance UV–Vis studies demonstrated a blue-shift in the reflectance spectrum for the bipolar-made oxide. This was also an indication of defects in the ZnO lattice, which related to the formation of shallow levels in the bandgap of the material. The structural and morphological differences influenced the photocatalytic characteristics, revealing a higher photocurrent for the bipolar-made ZnO when compared to the reference sample. This was further manifested in lower total resistivity for all anodes made from the non-stoichiometric ZnO, and also in their shorter diffusion length for charge exchange and electron lifetimes.

Highlights

Metal oxides are main components of inorganic photovoltaics (IPVs) and photocatalysts employed in a light-driven water splitting, waste water disinfection or oxidation of organic matter
Transmission electron microscopy (TEM) and Scanning electron microscope (SEM) observations of the synthesized oxide indicated significant differences in morphology depending on synthesis time (Figure 2)
The TEM image of ZnO generated after 2 h of polarization at ~5 ◦ C and collected via filtration was composed of connected spheres of varying sizes (20–40 nm) arranged in a chaplet-like manner

Summary

Introduction

Metal oxides are main components of inorganic photovoltaics (IPVs) and photocatalysts employed in a light-driven water splitting, waste water disinfection or oxidation of organic matter. They are classified as broad bandgap semiconductors and generally have large excitation binding energies (BE) [1]. A metal oxide that has gained attention is ZnO in part due to its wide bandgap in the range of 3.37–3.44 eV [3]. ZnO (and metal oxides in general) have shown excellent photocatalytic and photovoltaic properties, typical synthetic routes can incur large amounts of secondary waste from the use of excessive solvents, generation of by-products, including halogenated species, and in some cases require extreme temperatures [2]. ZnO has been recently synthesized in a bipolar electrochemical reactor using only water and Zn metal, reacting with each other in a constant electric field [5]

Methods

Results

Conclusion