ZnO/NiWO₄/Ag₂CrO₄ nanocomposites with p-n-n heterojunctions: highly improved activity for degradations of water contaminants under visible light

Highlights

• ZnO/NiWO₄/Ag₂CrO₄ nanocomposites as efficient photocatalysts are reported.

• The ZnO/NiWO₄/Ag₂CrO₄ (30%) nanocomposite exhibited the highest activity.

• Activity was 43.7-folds greater than ZnO in RhB degradation under visible light.

• The enhanced activity was attributed to p-n-n heterojunctions.

Abstract

To develop an efficient photocatalyst under visible light for remediation of environmental pollutants, novel ZnO/NiWO₄/Ag₂CrO₄ nanocomposites were synthesized by a simple ultrasonic-assisted method followed by a calcination step. Physiochemical properties of the photocatalysts were characterized by XRD, EDX, SEM, TEM, HRTEM, UV–vis DRS, FT-IR, and PL studies. It was found that the ternary nanocomposite with 30 wt% of Ag₂CrO₄ displays the highest activity in removal of RhB, in terms of rate constants, which are nearly 43.7, 7.60, and 6.36 folds greater than those of the ZnO, ZnO/NiWO₄ (20%), and ZnO/Ag₂CrO₄ (30%) photocatalysts, respectively. This greatly enhanced photocatalytic performance was related to the p-n-n heterojunctions between the counterparts and strong visible-light absorption by NiWO₄ and Ag₂CrO₄ semiconductors. Additionally, the ternary nanocomposite exhibited the superior activity towards degradations of MB, MO, and fuchsine. A plausible mechanism was also discussed via active species trapping experiments. This work displayed that the rational design and construction of p-n-n heterojunctions could be powerful for developing highly efficient visible-light-active photocatalysts for environmental and energy applications.

Introduction

Based on extensive researches in the three decades ago, it is well known that heterogeneous photocatalysis could play significant role to address challenging problems of the present century including environment pollution, depletion of fossil energy, and global warming [1], [2], [3]. Up to know, a wide variety of inorganic/organic semiconductors have been applied in photocatalytic degradation of pollutants, hydrogen generation through water splitting, and reduction of CO₂ to different fuels [4], [5], [6], [7], [8], [9], [10], [11]. Among them, ZnO is the second most utilized semiconductor applied in photocatalytic processes, owing to low toxicity, straightforward preparation method, and low price [12]. Nonetheless, ZnO has some inherent disadvantages such as rapid recombination of e^-/h⁺ pairs and limitation in the solar energy absorption, resulting in poor photocatalytic efficiency under visible light [13]. Thus, improving photocatalytic performance of ZnO using different strategies is one of the most active research areas in the field of photocatalytic processes and considerable researches have been recently conducted to address the above-mentioned issues [13], [14]. Based on these researches, different heterojunction-based photocatalysts have been introduced to increase photodegradation performance of ZnO under visible light [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. In the fabricated photocatalysts, one or more semiconductors with matching band potentials are combined with ZnO to promote production and separation of e^-/h⁺ pairs by construction of binary or multicomponent nanocomposites. Based on this strategy, different binary photocatalysts with p-n and n-n heterojunctions have been fabricated through combination of an p-type with an n-type and an n-type with an n-type semiconductors, respectively [28], [29], [30], [31], [32], [33]. More importantly, very recently, it was found that by combination of three semiconductors, one can prepared much more active photocatalysts through formation of n-n-n and p-n-n heterojunctions [34], [35], [36].

We know that nickel tungstate (NiWO₄) is a small band gap semiconductor (∼2.20 eV) with some appealing properties [35], [36]. This p-type semiconductor not only can increase absorption ability of ZnO in visible range, but also can form p-n heterojunction to enhance separation of the photoinduced charge carriers [34]. In the other hand, silver chromate (Ag₂CrO₄) is known as an n-type semiconductor and it can be a good photosensitizer to more extend absorption of ZnO to visible range due to its small band gap (∼1.8 eV) [19], [33]. Hence, it seems that Ag₂CrO₄ might be a good candidate to further promote photocatalytic activity of ZnO/NiWO₄ nanocomposites. Consequently, by combining ZnO and NiWO₄ semiconductors, p-n heterojunction will be formed in one side and n-n heterojunction will be formed between ZnO with Ag₂CrO₄ in the other side. The formed p-n-n heterojunctions between constituents of the ZnO/NiWO₄/Ag₂CrO₄ nanocomposites can induce internal electric fields between these semiconductors, which helps separation of the photogenerated e⁻/h⁺ pairs, leading to highly increased photocatalytic performance.

In this work, we firstly fabricated binary ZnO/NiWO₄ nanocomposite by ultrasonic-calcination method. Then, Ag₂CrO₄ was decorated over the ZnO/NiWO₄ nanocomposite to prepare novel ZnO/NiWO₄/Ag₂CrO₄ nanocomposites. The fabricated photocatalysts were characterized by XRD, EDX, SEM, TEM, HRTEM, UV–vis DRS, FT-IR, and PL spectroscopy. For evaluation photocatalytic performance of the prepared nanocomposites under visible-light irradiation, rhodamine B (RhB), methylene blue (MB), methyl orange (MO), and fuchsine were used. It was found that the ternary nanocomposites exhibited excellent photocatalytic performance under visible light. Finally, the photodegradation mechanism of pollutants over the ZnO/NiWO₄/Ag₂CrO₄ nanocomposites was also discussed.