Assembling Sn₃O₄ nanostructures on a hydrophobic PVDF film through metal-F coordination to construct a piezotronic effect-enhanced Sn₃O₄/PVDF hybrid photocatalyst

Highlights

• Assembled Sn₃O₄ nanostructures on a hydrophobic PVDF film through metal-F coordination.

• The spontaneous electric potential of PVDF acts as a built-in electric field of Sn₃O₄/PVDF.

• Piezotronic effect enhances the activity of Sn₃O₄/PVDF photocatalyst just driven by water flow.

Abstract

Piezotronic effect has been thought as a general approach for building high performance hybrid photocatalysts by integrating semiconductor nanostructures on piezoelectric materials. Although polyvinylidene fluoride (PVDF) has been widely applied as a flexible piezoelectric material, it is thought that assembling inorganic semiconductor nanostructures on PVDF surface is promising owing to the high hydrophobic property of PVDF. In this study, we proposed that the coordination effect between a metal ion and an exposed fluorine atom on the surface of PVDF can be used to assemble a layer of photocatalytic metal oxide nanostructures on the PVDF film surface. Using Sn₃O₄ as a model oxide, a Sn₃O₄/PVDF hybrid nanostructured photocatalyst was successfully synthesized. The X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy results confirmed the existence of coordinated bonds between F ions and metal ions, which could explain the stable growth of inorganic nanostructures on the hydrophobic surface of PVDF. At the interface between Sn₃O₄ and PVDF in the hybrid nanostructured photocatalyst, a built-in electric field was constructed on the basis of the ferroelectric field of the PVDF film, which induced an enhanced photocatalytic property. Most importantly, the swing driven by flowing water can considerably enhance photocatalytic properties owing to the piezotronic effect-driven alternating reconstruction of built-in electric field induced by the deformation of PVDF film. This study provides an approach for the synthesis of metal oxide semiconductor nanostructures on hydrophobic PVDF polymer surface. The developed material will have many applications in the photodegradation of organic pollutants in waste water.

Introduction

In recent decades, photocatalysis has attracted considerable attention because of its potential in the direct utilization of solar energy without secondary pollution [[1], [2], [3], [4], [5], [6], [7]]. However, the industrial application of photocatalysis-based water treatment and hydrogen evolution still has many difficulties. One of the key issues is that the photogenerated electron-hole pairs (photogenerated carriers) easily recombine, which results in low photocatalytic efficiency [[7], [8], [9], [10], [11], [12], [13], [14]]. Up to now, considerable efforts have been made to build polycrystalline junctions or heterojunctions to promote photogenerated carrier separation and prevent their recombination [7,15,16]. However, the efficiency of these methods is still too low to completely separate photogenerated carriers in practical applications [[16], [17], [18], [19], [20], [21], [22], [23], [24]].

Therefore, a built-in electric field theory [[25], [26], [27], [28]] was proposed to solve the difficulty of separating photogenerated carriers. Wang et al. [25] have designed a heterostructure with α and β phases of Ga₂O₃ as the built-in electric field of the composite catalyst, which effectively enhanced the photocatalytic efficiency. Li et al. [7] have considerably improved the photocatalytic activity by integrating Ag₂O nanoparticles on the surface of ferroelectric BiTiO₃ nanocrystals. The built-in electric field in a photocatalyst can enhance the efficiency of photocatalysis by providing a driving force in the transportation of photogenerated carriers. However, the built-in electric field in these heterostructures is static, and is easily saturated by photogenerated electron-hole pairs. Thus, the enhancement of the photocatalysis is stopped [7]. Therefore, ferroelectric materials with spontaneous polarization have received more attention. It is known that below the Curie temperature, ferroelectric crystals can spontaneously polarize, and produce ferroelectric domains inside the crystal, and the single crystal domains has opposite charges at the different ends of the nanocrystal [29]. When ferroelectric nanocrystals form heterostructures with other semiconductor catalysts, the spontaneously polarized electric field of the nanocrystals can act as a built-in electric field, which promotes the separation of photogenerated carriers. Unfortunately, this static electric field can be also saturated by carrier migration. Fortunately, the spontaneous polarization potential of the ferroelectric crystal can be altered by applying external pressure to the nanocrystal, which produces a microscopic deformation. This type of electric field is called a piezoelectric field [30,31]. The advantage of the ferroelectric spontaneously polarized electric field over the traditional built-in electric field is that ultrasonic waves or mechanical vibration can promote the deforming of ferroelectric nanocrystal, and cause the potential change of the built-in electric field. This change leads to the rapid release of the charges adsorbed on its surface, and results in the reconstruction of the built-in electric field [29,[31], [32], [33], [34], [35]]. In our previous study [7], the continuous separation of photogenerated carriers was realized using heterostructure catalysts made of BaTiO₃ and Ag₂O, which rebuilds the built-in electric field in the presence of ultrasonic waves. Although this study provides a new idea for synthesizing high-efficiency photocatalysts, its shortcomings are clear such as high cost, high energy consumption, and recycling difficulty. Therefore, to solve the abovementioned problem, it is essential to develop a flexible polymer piezoelectric material that can be combined with semiconductor catalysts and does not need high energy acoustic wave to rebuilt built-in electric field.

It is known that polyvinylidene fluoride (PVDF) is a polymer material with unique ferroelectric and piezoelectric properties. In this study, we designed a hybrid photocatalyst made from the combination of PVDF with semiconductor materials, which can realize the rebuilding of built-in electric field by its ferroelectric and piezoelectric properties. As a flexible polymer material, PVDF can be considerably deformed by the impact of wind and water flow, which leads to the change of spontaneous polarization potential. The change of spontaneous polarization potential prevents the built-in electric field of the composite catalyst from being saturated by photogenerated carriers and improves its photocatalytic efficiency.

Because of its hydrophobicity, PVDF is considered to be unsuitable as a substrate for the growth of inorganic semiconductors. However, there are many F atoms with high electronegativity on the surface of the PVDF film. Following the coordination theory, the electron-rich property of F should create conditions for its combination with metal ions and form a layer of coordinated ions, which should facilitate the growth of inorganic semiconductors on its surface. Because tin elements have eco-friendly characteristics and possess an outstanding visible light response, tin oxides such as Sn₃O₄ have been developed as photocatalysts for organic pollution degradation [36,37]. However, the high recombination rate of carriers in Sn₃O₄ limits its practical application. In this study, Sn₃O₄/PVDF hybrid film photocatalysts were synthesized by the hydrothermal method using PVDF as the substrate and the Sn²⁺ solution as the precursor. The mechanism of Sn₃O₄ growth on the surface of PVDF was discussed in detail. The higher catalytic activity of the hybrid photocatalyst can be obtained owing to the piezotronic effect using the built-in electric field that is formed by the spontaneously polarized electric field of PVDF and the piezoelectric effect at the Sn₃O₄-PVDF interface under the impact of mechanical disturbance in water. This facile method for the synthesis of inorganic nanostructures on the surface of hydrophobic materials will open a new way for flexible organic/inorganic hybrid photocatalysts. The Sn₃O₄/PVDF hybrid film photocatalyst with enhanced photocatalytic property will have many applications in water treatment.