Sol-gel synthesis of novel Li-based boron oxides nanocomposite for photodegradation of azo-dye pollutant under UV light irradiation

Highlights

• Li₂B₄O₇/LiBO₂/Li₃BO₃ nanocomposites have been successfully prepared by an improved Pechini approach.

• Effects of the various parameter on the morphology and particle size of products were examined.

• By controlling the synthetic conditions, different morphologies of the nanostructures were achieved.

• Photocatalytic efficiency for acid violet 7 azo dye under UV light irradiation was evaluated.

Abstract

In the present study, Li₂B₄O₇/LiBO₂/Li₃BO₃ (LBO) nanocomposites were synthesized by a modified Pechini sol-gel route utilizing the mixture of different polybasic acids and gelling agents. This work demonstrated that by controlling the reaction variables including kind of polybasic acids and gelling agents, pH and mole ratio of polybasic acid to total metals, the LBO nanostructures with various morphologies and particle sizes were prepared. Furthermore, the LBO nanocomposites showed excellent TOC removal (70%) and photocatalytic efficiency (91%) in order to photodegradation of acid violet 7 azo dye as water pollutant under UV light irradiation. Moreover, compared to commercial P25 photocatalyst, the LBO heterostructures exhibited higher photodegradation activity for removal of acid violet 7 dye in 90 min under UV light irradiation, even after four times cycles. The excellent photocatalytic degradation of LBO photocatalyst can be ascribed to the strong photon absorption, high-efficiency charge carrier separation, uniform particle size and appropriate band gap energy of the nanocomposite. Also, radical trapping capability test exhibited that the photoinduced superoxide radicals (^•O₂⁻) and holes (h⁺) were the prevailing active species in the photocatalytic performance.

Introduction

Synthetic textile azo dyes are one of the major categories of industrial dye families, which contain one or more azo (N=N) groups and account for over one-half of total dye production worldwide. Azo dye pollutants are hazardous to human health and environment and exhibit high stability against natural degradation. Acid violet 7 as a main commercial azo dye have been widely employed in the textile dyeing, paper printing, food and cosmetic coloring, due to their facility and cost effectiveness in synthesis compared to natural dyes. These dyes are dangerous and highly resistant and accumulate in environments, therefore requiring effective treatment prior to their release into water bodies [[1], [2], [3]].

In recent years, various treatment approaches have been expanded for the complete removal of toxic pollutants or converting them to harmless products in water. The conventional physical methods are inefficient and expensive, which makes them unsuitable for practical applications. Also, the biological processes have low-efficiency and complicated stages for the elimination of harmful compounds. Therefore, there is a necessity to develop more effective approaches for treatment of industrial wastewater. It is well known that chemical procedures have many advantages for the removal of organic and inorganic pollutants from aqueous phase including simplicity, quickness, non-selectivity, affordable, use in ambient conditions and non-toxic nature [1,[4], [5], [6]]. Recently, a lot of attention has been focused on the use of chemical processes for the elimination of pollutants in wastewater. For example, Sharma et al. used the biopolymers-based anion exchanger for the removal of phosphate anions, as well as the biopolymers-based nanohydrogel for the elimination of thiophanate methyl from aqueous environment [6,7]. Martinez-Huitle et al. reported the electrochemical degradation of acid violet 7 dye using boron-doped diamond anode and titanium cathode [3]. Li et al. suggested that the CdS–TiO₂ nanocomposite is suitable for the photocatalytic degradation of rhodamine B dye under visible light irradiation [8].

Among these technologies, photocatalysis process has attracted much attention for solving the difficult problems of environment crises, because it can employ to decompose organic pollutants for environmental purification under UV and visible-light irradiation [9]. Photocatalytic degradation has shown the promising potential to eliminate the organic pollutants from wastewater, because it is an eco-friendly, solar-driven, cost-effective, reliable and repeatable process. This process generates the powerful oxidizing agents such as hydroxyl and superoxide radicals, which completely destroys the organic pollutants to CO₂, water and mineral acids [4,10]. Semiconductor-based photocatalyst has received increasing attention for its promising application to environmental purification. An excellent photocatalyst requires efficient separation and low recombination of photo-generated electron-hole pairs to ensure that the photo-oxidation reactions can effectively occur in the photocatalyst for degradation of harmful pollutants under ultraviolet or visible light irradiation [11].

Metal borates semiconductors have generated much interest recently due to their promising properties including strong nonlinear optical effect, second harmonic generation, piezoelectricity performance, fast ionic conductivity, excellent luminescence effect, high sensitivity and storage properties and neutron absorption cross sections [[12], [13], [14], [15]]. Metal-borates compounds have been employed for various applications such as water splitting, photo-and thermoluminescent materials, lithium-ion batteries, laser technology and optical sensor protection, radiation detectors and dosimeters, CO₂ adsorbents, surface acoustic wave devices, plasma displays, neutron detection devices, ferroelectric random access memory devices, and photocatalytic fields [[15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]].

It is well known that the lithium borate compounds have a perovskite-like structure, which exhibit interesting structure-property relationships [13]. It is commonly agreed that the photocatalyst materials on the nanoscale are more efficient than bulk materials, because they can remarkably reduce the length for photo-generated excitons moving to the surface, and also provide a higher specific area and more active sites for the interactions [12]. Furthermore, heterojunction semiconductor nanocomposites can efficiently increase the separation and lifetime of the charge carriers and also interfacial charge transfer, consequently enhance the photocatalytic performance [27].

Metal-borates structures have previously been fabricated by various methods including melt-quenching technique, solid-state reaction, hydrothermal, high-pressure syntheses, precipitation, combustion, and sol-gel approaches. Pechini-type sol–gel process is a facile and effective route for the preparation of nanostructures, which possesses benefits of high purity of nanoscale product, eco-friendly, high chemical homogeneity, and low calcination temperature [12], [16], [17], [22], [25], [28], [29], [30], [31].

In the present work, Li₂B₄O₇/LiBO₂/Li₃BO₃ (LBO) heterostructure nanocomposites were fabricated via an improved sol-gel method by employing the mixtures of various gelling agents and polybasic acids. This study indicated that various morphologies of the LBO nanostructures can be obtained by controlling the sol-gel reaction conditions such as type of polybasic acids and gelling agents, pH and mole ratio of polybasic acid to total metals. It is the first time that LBO heterostructure nanocomposites have been synthesized via a Pechini-type sol–gel route in the presence of tannic acid as polybasic acid and tepa as gelling agent. In addition, the various morphologies of the LBO nanocomposites exhibited high photocatalytic activity and TOC removal for the degradation of acid violet 7 azo dye as water pollutant under UV light irradiation. Moreover, active species trapping experiment revealed the photoinduced superoxide radicals and holes were the prevailing active species in the photocatalytic process.