Extending the photoelectrocatalytic detection range of KHP by eliminating self-inhibition at TiO2 nanoparticle electrodes

Highly efficient photoelectrochemical removal of atrazine and the mechanism investigation: Bias potential effect and reactive species

In this work, efficient photoelectrochemical (PEC) removal of atrazine, one of the most widely used chemical herbicides in the world, was obtained by adjusting the bias potential applied on the photo-anode, and the optimal atrazine removal efficiency reached 96.8% at the potential of 0.2 V vs. SCE in 2 h with the reaction rate constant of 1.72 h^-1. The results indicated at the optimal potential, the separation efficiency of photo-generated holes and electrons was the highest with the lowest electron transfer resistance. Mechanism investigation revealed that superoxide radicals, hydroxyl radicals and holes all contributed to atrazine degradation, and the bias potential on the photo-anode could influence atrazine removal efficiency by changing the generation amount and distribution of the reactive oxygen species (ROS). It was presumed the nucleophilicity of superoxide radical played an important role in atrazine dechlorination, leading to the enhanced removal efficiency. However, the bias potential did not show obvious influence on the degradation intermediates of atrazine in the PEC system compared with that in photocatalytic oxidation, since it was actually an electro-assisted photocatalytic process in the potential range investigated. The work will provide fundamental basis for establishing efficient PEC system for pollutant remediation experimentally and theoretically.

Photoelectrocatalytic activity of an ordered and vertically aligned TiO2 nanorod array/BDD heterojunction electrode

Rutile TiO₂ nanorod (TiNR) arrays were fabricated on a boron-doped diamond (BDD) substrate by a simple hydrothermal synthesis method. A fluorine-doped tin oxide (FTO) electrode grown with TiNR arrays was also prepared using the same technology for comparison. Field-emission scanning electron microscopy results show that oriented TiNR arrays can grow vertically on the surface of BDD and FTO electrodes. TiNR arrays grown on both electrodes had the same length (3μm). In comparison with the TiNR/FTO electrode, the TiNR/BDD electrode demonstrated a higher photoelectrocatalytic activity for the degradation of water and organic compounds, which is mostly attributed to the formation of a p-n heterojunction between the TiNR arrays and BDD at high potential, apart from the density of TiNR. A linear relationship between the photoelectrocatalytic current and the organic concentration can be observed on both electrodes. However, the linear range between net photoelectrocatalytic current values and organic compound concentrations for the TiNR/BDD electrode are much greater than those for the TiNR/FTO electrode, which makes the TiNR/BDD electrode a versatile material for the photocatalytic degradation and sensing of organic compounds.

Phosphate Changes Effect of Humic Acids on TiO2 Photocatalysis: From Inhibition to Mitigation of Electron-Hole Recombination

A major challenge for photocatalytic water purification with TiO₂ is the strong inhibitory effect of natural organic matter (NOM), which can scavenge photogenerated holes and radicals and occlude ROS generation sites upon adsorption. This study shows that phosphate counteracts the inhibitory effect of humic acids (HA) by decreasing HA adsorption and mitigating electron-hole recombination. As a measure of the inhibitory effect of HA, the ratios of first-order reaction rate constants between photocatalytic phenol degradation in the absence versus presence of HA were calculated. This ratio was very high, up to 5.72 at 30 mg/L HA and pH 4.8 without phosphate, but was decreased to 0.76 (5 mg/L HA, pH 8.4) with 2 mM phosphate. The latter ratio indicates a surprising favorable effect of HA on TiO₂ photocatalysis. FTIR analyses suggest that this favorable effect is likely due to a change in the conformation of adsorbed HA, from a multiligand exchange arrangement to a complexation predominantly between COOH groups in HA and the TiO₂ surface in the presence of phosphate. This configuration can reduce hole consumption and facilitate electron transfer to O₂ by the adsorbed HA (indicated by linear sweep voltammetry), which mitigates electron-hole recombination and enhances contaminant degradation. A decrease in HA surface adsorption and hole scavenging (the predominant inhibitory mechanisms of HA) by phosphate (2 mM) was indicated by a 50% decrease in the photocatalytic degradation rate of HA and 80% decrease in the decay rate coefficient of interfacial-related photooxidation in photocurrent transients. These results, which were validated with other compounds (FFA and cimetidine), indicate that anchoring phosphate - or anions that exert similar effects on the TiO₂ surface - might be a feasible strategy to counteract the inhibitory effect of NOM during photocatalytic water treatment.