Simulated solar light driven performance of nanosized ZnIn₂S₄/dye system: decolourization vs. photodegradation

Highlights

• Hexagonal ZnIn₂S₄ was synthesized by hydrothermal method.

• In ZnIn₂S₄/Rhodamine B system semiconductor acts as adsorber, catalyst and photocatalyst.

• Rhodamine B is degraded through synergic action of photogenerated holes and molecular oxygen.

• In ZnIn₂S₄/Methylene blue system semiconductor acts as adsorber.

• In ZnIn₂S₄/Methyl orange system semiconductor acts as photocatalyst.

Abstract

Hexagonal ZnIn₂S₄ is synthesized by simple and low-cost hydrothermal route. TEM images revealed formation of nanosheets. The estimated band-gap energy of synthesized sample is in the visible spectral region and has a value of about 2.25 eV. Photocatalytic properties of synthesized sample are probed in photocatalytic degradation of three selected dyes different in nature and structure, Rhodamine B (RhB), Methylene blue (MB) and Methyl orange (MO), under the illumination with simulated Solar light. Obtained results revealed that photocatalytic degradation of RhB and MO is mainly result of synergic effect of photogenerated holes and oxygen, while primary effect in ZnIn₂S₄/MB system is bleaching of the dye, but no photodegradation was observed.

Introduction

Synthetic organic dyes are used in many branches of industry: leather tanning, paper production, food technology, in light-harvesting area etc. [[1], [2], [3]]. However, textile industry is the largest consumer of these dyes and, consequently, the main creator of water polluting effluents [2]. For example, 30–60 g and 70–150 l of water are required for dyeing of 1 kg of cotton [4]. About 20% of reactive dyes don’t fix to textile and is released in wastewater [4]. Environmental pollution caused in this way is remarkable, since usually less than 1 ppm is enough to produce coloration of water [5,6], which reduce Solar light penetration leading to reduced photosynthetic activities of aquatic flora and decreases the amount of dissolved oxygen, thereby affecting aquatic fauna [1]. Lately, because of shortage of drinkable water, search for ways to produce reusable water for industries are also accelerated.

One of the methods to remove dyes from water is photocatalysis. The most studied and used photocatalyst is certainly TiO₂, but its great disadvantage is that UV light (less than 5% of Solar light) must be used for its photoactivation. Usage of UV light greatly increases price of photocatalytic process and, concerning amounts of water that need to be recycled, definitely can discard photocatalysis as universal solution of dye pollution problem. However, price calculation becomes more relaxed if Solar light is used for photocatalyst activation. Chalcogenide semiconductors from II – III₂ – VI₄ family have suitable band-gap energies for Solar light activation, and contrary to their binary counterparts, they are more resistant to photocorrosion. Among them ZnIn₂S₄ (ZIS) is recognized as promising material not just in photocatalysis, but also in charge storage, electrochemical recording, and thermoelectricity [[7], [8], [9]].

ZIS is indirect n-type semiconductor with band-gap energy of 2.34–2.48 eV [10], well suited in visible region of Solar spectrum. It crystalizes in hexagonal and cubic crystal phase, whereby hexagonal phase is thermodynamically more stable [11]. In numerous reports, claims were made that both crystalline phases can efficiently be used in water and H₂S splitting for hydrogen production [[12], [13], [14], [15], [16], for example], as well as in photocatalytic degradation of dyes under visible light irradiation [7,9,11,[17], [18], [19], [20]]. However, Chen et al. [11] have shown that hexagonal phase stays stable upon recycling, while cubic phase gradually degrade and In(OH)₃ is formed. Additional advantage of hexagonal phase is that in all of its polytypes, ions are arranged in S-Zn-S-In-S-In layers that grow in 2D sheets [11], attractive morphologies as higher surface/volume ratio of sheets have larger specific surface area and are more reactive than spheres or rods.

Generally, hexagonal ZIS can be easily synthesized by hydrothermal/solvothermal method, where variation of synthetic parameters (temperature, time, reaction media, cationic precursor salts, etc.) determines size, shape, morphology and photocatalytic activity of the final product. Shen et al. [14] have shown that shorter times of hydrothermal treatment lead to higher distortion of ZIS structure that consequently results in higher photodegradation activity in water splitting. Also, Shen et al. [16] have demonstrated how exposed crystal plane of ZIS can influence photocatalytic water splitting. Namely, (006) crystal plane is “cation rich” and more suitable for photocatalytic water splitting than (112) and (102) crystal planes, that are rich in sulphur ions. Concerning dyes, because of diversity of their chemical structures, it is a challenge to design semiconductor that will successfully degrade them, since in real systems, there is always some mixture of dyes different in nature.

In study presented here, the idea was to investigate and understand in detail interaction of hexagonal ZIS with three dyes, different in structure and nature: Rhodamine B (RhB, xantene, cationic dye), Methylen blue (MB, thiazine, cationic dye) and Methyl orange (MO, azo, anionic dye) under simulated and natural Solar light irradiation. It will be shown that ZIS is not acting only as photocatalyst; its sulfur-rich crystal planes are strong adsorption area for cationic dyes (ZIS acts as adsorber) and due to convenient band potential value, it also can act as electron – transfer layer in ZIS/RhB interaction. Especially, difference that stands for decolourization and degradation (photodegradation) of studied dyes will be emphasized.