Modulating the photocatalytic activity of Ag nanoparticles-titanate nanotubes heterojunctions through control of microwave-assisted synthesis conditions

Highlights

• Silver nanoparticles were synthesized under simply microwave-assisted method in different solvents.

• Silver nanoparticles-titanate nanotubes heterojunctions were obtained using MPA (3-Sulfanylpropanoic acid) as bridge agent.

• Physical-chemical properties of the heterojunctions were ascribed to the solvent choice during the synthesis of the AgNP.

• Dye degradation and quantification of radical species were used to investigate the photocatalytic activity.

Abstract

Protonic titanate nanotubes (HTiNTs) have been widely used as photocatalyst for dye degradation, however, due to its high band-gap energy (∼3.2 eV), it can only absorb UV light, which is mainly disadvantageous when sunlight radiation is used as light source. The significant portion of the fallout from the Sun to the Earth corresponds to visible light, whereas the UV is only around 4 %. Thus, it is critical to develop strategies to sensitize this kind of photocatalyst to visible radiation. In this work, silver nanoparticles (AgNP)-HTiNTs heterojunctions were prepared, aiming to produce a visible light sensitive catalyst. The AgNPs were prepared by simple microwave-assisted routes, using water (Ag(H₂O)), ethanol (Ag(EtOH)) and ethylene glycol (Ag(EG))as solvents, and sodium borohydride as reducing agent. The obtained AgNPs exhibit different physical-chemical properties, such as shapes, sizes and crystalline structures. As a result, the heterojunctions have different photocatalytic efficiencies for rhodamine B degradation under simulated sunlight irradiation: 72 % using Ag(EG)-HTiNT, 46 % using Ag(EtOH)-HTiNT and 33 % using Ag(H₂O)-HTiNT as catalysts. The changes on the catalytic activities were ascribed to the plasmonic effect, lowering of surface area and more negative zeta potentials caused by the presence AgNPs. To better understand the photocatalytic response, the amount of radical species generated by the materials was determined by using spectroscopic methods.

Introduction

The optimization of material properties through its morphological control has attracted much attention since the discovery of carbon nanotubes by Iijima and co-workers in 1991 [1]. Since then, the development of similar nanostructures based on metal oxides has been growing [2,3]. Among several semiconductor oxides, titanium dioxide (TiO₂) nanostructures are of great interest for applications such as photocatalysts, sensors, solar cells and photovoltaic devices [4,5]. This is due to its electronic, optoelectronic and catalytic properties, which are intrinsically related to its high band-gap energy, conductivity, electronic mobility, surface area and chemical stability [6]. Titanium dioxide is an amphoteric oxide, since it can react either with strong acids (forming titanium salts) and bases (producing titanates) [7]. Among the TiO₂ derivatives, the titanates nanotubes (TiNTs) have been attracting much attention; [7] its origin is ascribed to Kasuga and co-workers that reported its synthesis for the first time in 1998 [8]. TiNT combine the properties of TiO₂ – open and mesoporous surface, high surface area and band-gap energy – to the properties of a lamellar material, exhibiting high ion exchange capacity, ionic conductivity, and a surface that can be easily functionalized due to −OH surface groups that act as Brönsted acids. [7] The photo-induced process of both materials is similar: when the semiconductor is irradiated with light, electrons are promoted to the valence band leaving holes at the conduction band [[4], [5], [6], [7],9,10]. For photodegradation processes the photogenerated electrons react with adsorbed O₂ molecules forming superoxide radicals, whereas OH⁻ ions are oxidized by the holes, forming hydroxyl radicals. Both radicals are capable of degrading organic species. [[4], [5], [6], [7],9,10] However, as the TiO₂, TiNTs can only absorb UV light (band-gap energy of ∼3.2 ev), which limits its application in photo-induced process carried out under sunlight irradiation, that is mainly composed by visible light. [7,9] Thus, several strategies have been investigated for sensitizing TiNT to visible light [11,12], among them the use of plasmonic nanostructures, such as silver nanoparticles (AgNP), has been growing [[13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29]]. The combination of surface plasmon effects, ascribed to the metallic nanoparticles, and the catalytic properties of the semiconductor oxide allow the acquisition of a catalyst with peculiar properties, such as higher visible light absorptivity, better charge separation and transfer, which leads to the intensification of its photochemical activity. [[30], [31], [32]]

In this work, Ag-TiNT heterojunctions were prepared aiming to increase the photocatalytic efficiency of the resultant materials under simulated sunlight irradiation. AgNPs were synthesized through a simple microwave assisted technique, using different solvents (water, ethanol and ethylene glycol). The choice of solvent is of particular interest in microwave-assisted wet chemistry, since the heating mechanism involves a dielectric selective interaction between the microwaves and the reactional species (which is mainly composed by the solvent), specially the polar ones [[33], [34], [35], [36], [37]]. Solvents that are capable of interacting strongly with the microwaves will promote higher heating rates, which will influence the crystallinity of the obtained material [[33], [34], [35], [36], [37]]. Thus, the properties of the AgNPs can be modulated by changing the solvent and [[38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52]], consequently, the properties of the resultant Ag-HTiNT heterojunction will be also influenced.

The photocatalytic activity is also influenced by the heterojunction’s properties; this activity can be evaluated by using model dyes, such as Rhodamine B (RhB). This dye has been widely used for this kind of experiment so far, [[53], [54], [55], [56], [57]] since its degradation can be easily detected using common UV–vis spectroscopy. However, the literature is controversial regarding its use, due to the fact that RhB absorbs the emission spectrum of the lamp, which can lead to the so called “indirect photocatalytic mechanism” – the light is not absorbed by the photocatalyst but by the dye which gets into an excited state, from which it injects an electron into the conduction band of the catalyst, and is thereby oxidized. [[58], [59], [60], [61], [62]] Other authors claim that changes in the catalyst, by the formation of heterojunctions for instance, can tune the potential edge of the conduction-band preventing the electron injection from the excited dye [63]. Thus, additional experiments must be conducted in order to understand the origin of the photocatalytic activity of the materials, such as the direct quantification of the radical species produced by the photogenerated charges [64,65].

Here we demonstrate that the AgNPs obtained in the three solvents exhibit different morphologies and crystalline structures, as consequence, the prepared Ag-TiNT heterojunction exhibited different physical and chemical properties that affect their photocatalytic activities. Rhodamine B photodegradation under simulated sunlight, was used as model to determine the photocatalytic efficiency of the heterojunctions. The amounts of hydroxyl and superoxide radical, formed by the photogenerated charges, were also quantified, aiming to better understand the origin of the catalytic efficiency.