Synthesis of ZnO Nanorods-Based Photoanode and Electrochemical Characterization for Azoic Dyes Degradation: Reactive Red 239 Study Case

Abstract

The use of solar energy as an energy vector has been a subject of great interest and study in recent years, showing vast improvements in the efficiencies associated with the conversion to electrical energy. This interest has extended to various applications, among which the production of green hydrogen from the decomposition of water and in environmental remediation processes stand out. The main motivation in focusing the use of solar energy in photoelectrocatalytic degradation processes is the negative impact that the disposal of colored water has on the different effluents, causing a deterioration of the quality of these ecosystems and the death of many forms of life. In this sense, the application of semiconductors based on zinc oxide (ZnO) supported on FTO conductive glass was studied for the treatment of wastewater from the textile industry, contaminated with reactive red azo dye 239. The main motivation in focusing the use of solar energy in photoelectrocatalytic degradation processes is the negative impact that the disposal of colored water effluents has on the different ecosystems, causing a deterioration of the quality of these ecosystems and the death of many forms of life. In this sense, the application of semiconductors based on zinc oxide (ZnO) supported on FTO was studied for the treatment of wastewater from the textile industry, contaminated with reactive red azo dye 239. In the present writing, a process for the synthesis of a ZnO-based semiconductor was defined. This precedence was performed directly on the substrate of 15x15x1.1mm FTO conductive glass plates. Prior to the synthesis process, a pretreatment of the glass is performed. The pretreatment of the plates consists of washing with neutral soap, then immersion in 0.01 M nitric acid (HNO3) for 10 minutes, and finally, cleaning with deionized water in ultrasound for 10 minutes. The synthesis of the nanostructured ZnO was performed in two steps. First, a chronoamperometric electrodeposition of the ZnO seeds on the substrate at a fixed potential of -1.38 V vs. Ag/AgCl as reference, and a platinum electrode as counter electrode, was performed at 70ºC for 200 seconds with a Gamry® Interface 1000 potentiostat, then subjected to a heating period at 400ºC for 2 hours. After electronucleation of the seeds and heat treatment, an additional step is carried out, which is the chemical growth of ZnO, waiting for the formation of ZnO nanorods in the (100), (002) and (101) planes typical of the wurtzite structure, this is done in a zinc nitrate solution at 90ºC, pH > 13.5, for 75 minutes. In the semiconductor characterization, different electrochemical techniques such as cyclic voltammetry, electrochemical impedance spectroscopy and anodic linear scanning were evaluated. At the same time, the response of the photoanode to illumination with a 350 W xenon arc lamp was studied, showing the remarkable effect of light on the generation of charge carriers, decreasing recombination, and increasing photocurrents. On the other hand, the density of charge carriers in the semiconductor was evaluated and defined, as well as the position of the valence and conduction bands and their respective band gap. The charge carrier density was determined from Mott-Schottky, finding densities of 6.25x1021 and 3.72x1021 carriers/cm3, for frequencies of 500 and 1000 Hz, respectively. In parallel, the flat band potential, which under certain circumstances of charge carrier density can approximate the conduction band potential, was evaluated. This was estimated from three methods, Mott-Schottky (500 and 1000 Hz), photocurrent onset potential and open circuit potential, finding values of -0.086 and -0.124 (at 500 and 1000 Hz), -0.121 and -0.161 V vs. Ag/AgCl, respectively. The determination of the bad gap was performed from UV-VIS, obtaining a value of 3.13 eV. Finally, the discoloration of the solution was evaluated from UV-VIS measurements. This analysis was performed on a problem solution that resembles the concentration of dye and salts of a textile industry effluent. A ZnO electrode was used, with a defined working area of 1.30 cm2, and a dye volume of 25 mL. The experiment was carried out for a time period of 4 hours, and an applied potential of 1 V vs. Ag/AgCl/Cl- 3M, reaching a decolorization of 23.2% of the initial solution.