Photocatalytic reduction for graphene oxide by PbTiO3 with high polarizability and its electrocatalytic application in pyrrole detection

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Abstract

Quick recombination of photogenerated electrons and holes in photocatalytic process remains a huge challenge. And the routine efforts are concentrated on heterojunction, metal decoration and surface defect strategies. PbTiO3 as a typical perovskite ferroelectrics is with a strong built-in electric field as self-junction caused by internal spontaneous polarization, facilitating the charge separation in the photocatalytic process. Here, under UV irradiation, L-shaped PbTiO3 with active (1 1 0) facet as a photocatalyst was applied to photo-reduce graphene oxide (GO), where a specific reduced graphene oxide (rGO)/PbTiO3 composite was synthesized in presence of isopropanol, a hole-trapping agent. According to the linear optical properties, the polarizability of PbTiO3 is calculated to 1.01 × 10−23 cm3 (2.68 times that of P25 (TiO2)), inducing the photo-excited charge separation by PbTiO3. Based on XPS characterization, a Tisingle bondOsingle bondC chemical bond is identified on the interface between rGO and PbTiO3. The response peak current for an electrochemical sensor based on rGO/PbTiO3 was proportional to the concentration of pyrrole (6.6 × 10−9–3.1 × 10−7 M, R2 = 0.999), and an extremely low limit reaches to 2.38 × 10−9 M. In addition, polypyrrole during the pyrrole detection was realized by the multi-cycle oxidation process. And also, the electrochemical detection has been successfully applied for the pyrrole quantification in real samples.

Introduction

Light is the greenest, most renewable and inexpensive reagent. The photocatalytic synthesis has grabbed extreme attention owing to its mild condition, flexible and controllable procedure. So far, it has been applied to organic reactions, especially in the development of drugs, fine chemicals and advanced materials. Quint’s group [1] successfully prepared benzophosphoric acid oxide using ruthenium Y as a photocatalyst through visible photoredox approach, realizing synthesis of various phosphorus–heterocycles under very mild conditions. In Panwar’s report [2], a ternary hybrid consisting of gold nanoparticles decorated on titania-polyaniline (TiO2-PANI-AuNPs) was used as photocatalyst for photocatalytic A3-coupling of aldehydes, amines and alkynes owing to electron-hole recombination suppression and better electron mobility. Accordingly, the photocatalytic approach can realize a redox reaction and catalytic synthesis for a target product based on photocatalysts which generate electrons and holes under light irradiation. So photocatalytic synthesis is believed to have great potential in the preparation of nanocomposite with advantages of avoiding high temperature sintering, clean surface, convenient operation and specific interfaces. But there are few relative applications and papers.

In recent years, the use of ‘functional’ photocatalysts such as ferroelectrics from the perovskite ABO3 family has gained a growing interest [3]. As a prototypical ferroelectric perovskite oxide [4], PbTiO3 is with internal polarization and built-in electric field caused by spontaneous dipole moment, which promotes the separation of photogenerated electron and holes during photocatalytic process [5]. Especially, the big difference in electronegativity between element Pb (2.33) and Ti (1.54) in PbTiO3 contributes to large dipole moment, inducing that a self-junction by PbTiO3 is formed similar as a heterojunction performance in photo-excited charge separation. Besides, PbTiO3 has high redox capability in terms of wide band gap (Eg = 2.95 eV) [6]. Kakekhani et al. [7] revealed the polarization-dependent surface physics and chemistry of PbTiO3 using Density Functional Theory (DFT) and electronic structure analysis, predicting that oxygen activity on its surfaces effectively drive reduction or oxidation reactions. Based on these advantages, PbTiO3 was here chosen as a deal photocatalyst in photocatalytic reduced synthesis.

Graphene is one of the most popular two-dimensional (2D) materials with large surface areas (2630 m2/g) and highest carrier mobility (exceeding 200,000 cm2 V−1 s−1) [8], [9]. Hence, graphene can be used as a filling material [10], [11] to increase contact among nanoparticles and to promote the charge transfer [12]. Owing to the low electrical conductivity of graphene oxide (GO), it is necessary to reduce GO into reduced graphene oxide (rGO) via the chemical and hydrothermal procedures [13], [14], [15], [16], [17]. Wherein, the reductants such as hydrazine or sodium borohydride were usually utilized, and it readily causes reductant residuals in rGO and environment pollution. Hence, an alternative photocatalytic synthesis is expected to reduce GO by using PbTiO3 as high-efficiency photocatalyst without reductant.

Herein, based on built-in electric field generated by spontaneous polarization, the as-prepared PbTiO3 is proposed to be an efficient photocatalyst to reduce GO on its clean surface under UV irradiation, finally a rGO/PbTiO3 nanocomposite was prepared. For the high photocatalytic efficiency, a hole-trapping agent, isopropanol [18], was used, where only the photoexcited electrons were applied to reduce GO into rGO. The polarizability of PbTiO3 was investigated through linear optical calculation compared with that of P25 (TiO2), a commonly used photocatalyst (Evonik Degussa Corporation, Germany). UV–vis absorption and Raman measurement were carried out to indicate the distinct photocatalytic performance by PbTiO3 with high polarizability in reducing GO. Furthermore, the interface nature between PbTiO3 and rGO was researched by XPS spectra. According to electrocatalytic performance of PbTiO3 and auxiliary enhancement of rGO, a rGO/PbTiO3 modified electrochemical sensor was fabricated to detect pyrrole, a poisonous substance. As to the electrocatalytic mechanism towards pyrrole, the specific electrocatalytic detail is presumed according to cyclic voltammetry (CV) and UV–vis absorption analysis. With satisfactory results of experiment in real samples (milk), the sensor is estimated to be an accurate and reliable method of pyrrole detection.

Section snippets

Synthesis reagents

Lead nitrate (Pb(NO3)2, AR) was obtained from Macklin Reagent Co., Ltd (Shanghai, China). Butyl titanate (Ti(C4H9O)4, AR) and graphene dispersion were purchased from Aladdin Reagent Co., Ltd (Shanghai, China). Isopropanol (C3H8O, HPLC) and pyrrole (C4H5N, AR) were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and Sigma–Aldrich (St. Louis, MO, USA), respectively. P25 were gained from Evonik Degussa Corporation. In addition, a working solution, i.e. pyrrole (C4H5N, AR) as

Characterization and polarizability analysis of PbTiO3

The structure and morphology of as-obtained PbTiO3 were characterized by XRD, SEM, and TEM. Fig. 1a presents the XRD patterns of as-obtained PbTiO3. All the recorded diffraction peaks are well consistent with the standard pattern (JCPDS No.06-0452), indicating the high pure phases for the sample. In its morphology (Fig. 1b), a L-shaped cube with uniform morphology is distinctly observed. The cube has a length of 800 nm, a width of 400 nm, and a height of 100 nm. Further, the detail HRTEM images

Conclusions

The special L-shaped PbTiO3 nanomaterial with active (1 1 0) facet was prepared by two-step hydrothermal synthesis, and used as a photocatalyst for GO reduction by a novel photocatalytic route under UV irradiation. The as-prepared PbTiO3 was calculated to be with high polarizability, which exhibits a strong built-in electric field, facilitating the charge separation in the photocatalytic process. And its excellent photocatalytic performance in photocatalytic reducing GO was verified by UV–vis

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (No. 51872048), Scientific and Technological Foundation of Fujian Province of China (No. 2016J01222, 2018J01518), and Open Project of Wuyi University (WYKF2017-1).

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