Improvement of electrical and photocatalytic properties of boron-doped ZnO nanorods and synthesis design optimization by taguchi approach

Abstract

Today, increasing environmental pollution affects the whole world. One of the most important pollutions among environmental pollution is water pollution. Water pollution seriously threatens living life. With the developing technology, clean water resources are decreasing, and water resources are polluted with industrial and domestic wastes every day. Today, traditional methods are used for water treatment. However, these methods are not able to provide adequate response to water treatment due to low efficiency for small-sized pollution and secondary pollution. Therefore, applications of photocatalysis offer an effective opportunity among advanced oxidative methods; in this study, innovative photocatalyst structures have been developed. Boron doped ZnO nanorods were successfully synthesised by hydrothermal method and Boron doping has led to enhancements in its photocatalytic and electrical properties. Boron doped ZnO nanorods were grown in two stages. In the first stage, since dipole forces will be effective for growth, the thin film called seed layer layer on the glass surface was coated with the surface spin coating method. Then, with zinc nitrate dehydrate and hexamethylenetetetraamine added in equal molar amounts, nanorods were grown on the glass with seed layer at 90 °C for 3 hours. X-ray diffaraction (XRD), UV-Vis spectrometer, FT-IR, DC electrical analysis (I-V), AC electrical analysis, characterisation by Scanning electron microscopy (SEM) and photocatalytic analysis by UV-A 366 nm light were also performed. Then, Taguchi experimental design method was used to determine the best conditions for selected parameters such as pH, pollutant concentration, time, additive amount and to calculate the effects between each other. The XRD results indicate that boron has successfully integrated into the structure. Also, pure and boron doped ZnO nanorods grew in wurtzite structure and the crystal sizes were confirmed by SEM images. As a result of FT-IR analysis, it was shown that the peak belonging to B-O, B-O-B bonds increased with the increase in doping. It was proved that the band gap intervals calculated as a result of UV-Vis spectrometry analysis decreased with boron doping. Electrical analyses showed that the electrical conductivity increased with increasing boron doping. As a result of all these analyses, electrical conductivity increased from 0.03 μA for pure ZnO to 1.9 μA for 10% boron doped ZnO. With the increase in surface defect, the band gap decreased with the increase in conductivity, boron doping was proven to increase the number of electrons and it was thought that their photocatalytic activity should increase. As a result of photocatalytic tests, it was shown that the efficiency increased with boron doping. When we compare the numerical values of the rate constants, under the same conditions, the 1% B doped sample has a 94% higher rate than the pure ZnO sample for pH 4. For pH 7, the 3% B-doped sample has a 36% higher rate than pure ZnO, and finally for pH 10, the 7% B-doped sample has a 194% higher rate constant. Moreover, the effect of pH was discussed. It was observed that boron doped ZnO nanorods had better photocatalytic efficiency for each pH range and concentration. As a result of the calculated rate constants, 3% boron doping for 2x10-6 M concentration, pH 7 showed the best result with a rate constant value of 0.00856 min-1. Finally, optimized parameters for pH 4, concentration 2x10-6 M, time 90 min and doping amount 7% were determined by Taguchi method. As a result of ANOVA analysis, the model was proved to be 85% fit.