# Synthesis and performance evaluation of zinc oxide tubes/alginate microfibre composites for photodegradation of methylene blue: a novel reporting approach

## TL;DR

This paper introduces a new method to evaluate how well zinc oxide tubes in alginate microfibers can break down a water pollutant called methylene blue under light.

## Contribution

The study introduces a novel reporting approach for photocatalytic degradation using normalized pollutant concentration and a new metric called Specific Energy Efficiency (SEE).

## Key findings

- Zinc oxide tubes in alginate microfibers achieved 88% methylene blue degradation after 5 hours of irradiation.
- The composite showed consistent performance in the first four cycles but decreased to 72% in the fifth cycle due to microfibre breakdown.
- A new metric, Specific Energy Efficiency (SEE), was introduced and reached a maximum of 1.044 μg g−1 J−1.

## Abstract

This research investigates the efficacy of zinc oxide (ZnO) tubes in decontaminating polluted water using a substrate-free hydrothermal synthesis process for ZnO tubes. The synthesized tubes are impregnated into calcium alginate microfibres, strategically chosen for their high surface area to enhance photocatalytic degradation performance and for practical handling during decontamination and subsequent collection, thereby preventing secondary contamination. Structural and morphological analyses, conducted using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD), thoroughly characterize the properties of the ZnO tubes and the composite material. The efficacy of this composite is demonstrated through the photocatalytic degradation of methylene blue (MB), as a representative organic pollutant, resulting in an 88% degradation of MB after 5 hours of irradiation by a sun simulator. Cyclic tests exhibit consistent degradation levels in the first four cycles (81–89%), followed by a subsequent decrease to 72% in the fifth cycle, coinciding with the breakdown of the microfibres into shorter fragments. Innovatively, this study introduces an approach to reporting photocatalytic degradation results, utilizing normalized pollutant concentration plotted against irradiated energy instead of time, as energy encompasses irradiated power, time, and surface area. This reveals that the 88% degradation of MB is achieved by irradiating the sample with an approximately 18 kJ. Additionally, a new metric, Specific Energy Efficiency (SEE), is introduced. It expresses the ratio of degraded pollutant mass to the mass of photocatalytic active material per unit of irradiated energy, with the maximum and cumulative SEE in this study being 1.044 μg g−1 J−1 and 326 ng g−1 J−1, respectively. This research not only contributes to the understanding of ZnO tubes' efficiency in polluted water decontamination but also introduces valuable insights for standardized reporting in photocatalytic degradation studies.

The synthesis of ZnO nanotubes-alginate microfibres for MB photocatalytic degradation with innovative results presentation.