Solvothermal synthesis of Ag/ZnO micro/nanostructures with different precursors for advanced photocatalytic applications

Highlights

• Ag/ZnO micro/nanostructures were synthesized by solvothermal method at 200 °C.

• ZnO presents prism-like rods (PR) or quasi-spherical particles morphologies (QS).

• FCC metallic Ag spherical nanoparticles are homogeneously located on ZnO surface.

• Using PVP and different zinc precursors affect to particle morphologies and sizes.

• The improvement in photocatalyic activity is shown due to the silver and PVP.

Abstract

Micro/nanostructured systems based on metallic oxide (ZnO) with noble metal (Ag) on the surface (Ag/ZnO) are synthesized by solvothermal method from zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), zinc acetate dehydrate (Zn(CH₃COO)₂·2H₂O), zinc acetylacetonate hydrate (Zn(C₅H₇O₂)₂·xH₂O) and silver nitrate (Ag(NO₃)) as precursors. In these systems, polyvinylpyrrolidone (PVP) is used as surfactant for controlling particle morphology, size and dispersion. The obtained materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), UV–vis diffuse reflectance spectroscopy (DRS), N₂ gas adsorption–desorption (BET) and Raman spectroscopy (RS). By XRD results, all major peaks are indexed to the hexagonal wurtzite-type structure of the ZnO and samples with noble metal, extra diffraction peaks are detected which correspond to the face-centered-cubic (fcc) structure of the metallic Ag. Depending on used precursor, different morphologies have been obtained. Mainly, ZnO prims-like rods – NRs (with 0.8 ⩽ aspect ratio ⩾ 3.4) – have been observed. Quasi-spherical particles of metallic Ag (with diameters between 558 ± 111 μm and 22 ± 1 nm) have been detected on the ZnO surface. Photocatalytic results (all samples studied >30% MB degradation) verify the important effect of surfactant and the viability of synthesized Ag/ZnO micro/nanocomposites for environmental applications.

Introduction

Control of size and morphology of certain materials have received an extensive interest due to the fact that they have an important influence on the functional properties of these materials [1]. Taking into account the ever-decreasing dimensions of commercial devices and since their properties can be remarkably different from those of the bulk materials (due to quantum-size effects), the research of micro- and nanostructured materials is raising its technological importance [2].

Zinc oxide is a very important basic semiconductor material owing to its large band gap (3.37 eV at room temperature (RT)), low cost and eco-friendly behavior. Due to its worthwhile properties, it is widely used in many advanced applications, such as gas sensors, transparent conductor films, varistors, piezoelectric transducers, dye-sensitized solar cell, UV-light detectors, catalyst and antimicrobial materials [1], [2], [3], [4], [5].

One of its most important applications is in photocatalysis. However, the rapid recombination of photogenerated electron-hole pair reduces its photocatalytic efficiency. It has been demonstrated that the noble metal presence avoids this recombination and increases the photocatalytic performance [6]. Among them, silver and platinum particles are very useful due to its effects on the enhancement of photo-activity. In our previous work [7], it is shown that silver has a greater improvement effect than platinum, furthermore silver presents antibacterial properties. Currently, there are several methods for synthesizing Ag/ZnO systems [8], [9], [10]. Even then, the main problem is to disperse Ag particles on the surface of ZnO. Moreover, the morphologies and functional properties of ZnO depend on the method of preparation [5]. There are many techniques reported to achieve these materials, including physical and chemical techniques [1], [2], [3], [4], [11], [12]. Regarding the latter case, there are solution chemical routes, such as sol-gel and hydro/solvo-thermal processes, which are very attractive because the experimental conditions are moderate, and the procedure is usually simple and economic. Besides, the last one (solvothermal method) achieves high dispersibility of Ag on the surface of ZnO. Therefore, the solvothermal method appears like an effective method to large-scale ZnO production. Thus, to prepare Ag/ZnO hetero-structure with required properties, we have to find a way to avoid aggregation during the synthesis process. Polyvinylpyrrolidone (PVP) is frequently used with a dual function: as a stabilizer and reductant of the noble metal in solvothermal synthesis [3], [4], [13].

There are several works where the influence of zinc precursors on morphology and particle size in shown [1], [2], [5], [14]. Zinc nitrate and zinc acetate stand out among other zinc sources commonly used in wet chemical methods because of their higher solubility in organic solvents. On the other hand, zinc acetylacetonate is barely studied as a zinc precursor in solvothermal method. However, it is a suitable option for the vapor transport processes, such as CVD and spray pyrolysis, due to its sublimation [1].

Herein, a simple solvothermal approach assisted by PVP for the synthesis of Ag/ZnO micro- and nanostructured systems from different precursors, and its photocatalytic behaviors will be studied in detail. Thus, in this paper, the effect of zinc precursor and the use of PVP as surfactant on the solvothermal treatment of nanocrystalline Ag/ZnO, have been tried for the first time. So, we report the preparation and characterization of micro and nanostructured Ag/ZnO with different morphology, particle size, high purity and improved photocatalytic behaviors. These results may contribute to encourage the large-scale production of these nanomaterials, given its environmental applications.