Piezoelectrically Enhanced Photocatalysis with BiFeO3 Nanostructures for Efficient Water Remediation

摘要

class="head no_bottom_margin" id="sec1title">IntroductionEnvironmental pollution and shortage of clean energy are among the most pressing problems that threaten sustainable development of human civilization. Water pollution caused by discharge of toxic, synthetic dyes into effluents is one of the major sources of environmental pollution (). The presence of even trace amounts of these synthetic dyes in water is extremely harmful because of their carcinogenic and mutagenic nature (, , , ). Owing to their high solubility and chemical stability, most of these synthetic dyes easily escape conventional water treatment methods and persist in the environment (). Advanced oxidation process (AOP) is a cost-effective and green approach to degrade such toxic dyes into harmless products, such as CO2 and H2O, using highly reactive species, including hydroxyl and superoxide radicals (, ).Photocatalysis is one of the most extensively researched fields of AOP, in which a semiconductor with a suitable bandgap can efficiently absorb light and form photogenerated electron-hole pairs. These electrons and holes can then migrate to the surface of the photocatalyst and initiate the oxidative/reductive processes, resulting in degradation of organic pollutants (). One of the main drawbacks that limits practical use of photocatalysts is the high electron-hole recombination rate, which ultimately lowers their photocatalytic efficiency. To overcome this problem, wide bandgap semiconductors, such as titanium dioxide (TiO2), are extensively used. However, having a wide bandgap limits the light absorption of these materials to the UV region (). Hence, researchers have proposed new strategies, including developing novel nanostructures, using co-catalysts (Pt, Pd, and RuO2), doping with rare-earth or transition metals, and fabricating heterojunctions to tune the bandgap of these materials, to lower the electron-hole recombination rate, and to increase the lifetime of the charge carriers (, , , , ). Although such approaches can improve the separation of photogenerated electrons and holes, they rely heavily on the use of expensive catalysts such as noble metals and suffer from complicated fabrication processes, which, in turn, create obstacles to their practical application (href="#bib37" rid="bib37" class=" bibr popnode">Wang et al., 2016).Apart from chemically modifying the wide bandgap, physical methods such as the application of external electric fields to an electrochemical cell were also employed to separate the electron-hole pairs and, hence, improve their photocatalytic performance (href="#bib2" rid="bib2" class=" bibr popnode">Barroso et al., 2013, href="#bib28" rid="bib28" class=" bibr popnode">Pesci et al., 2013). Even though this method has demonstrated strong potential to increase the photocatalytic efficiency, its high cost, complicated device structure, and difficult operation conditions provide significant challenges for practical use (href="#bib17" rid="bib17" class=" bibr popnode">Li et al., 2014, href="#bib11" rid="bib11" class=" bibr popnode">Hong et al., 2016, href="#bib37" rid="bib37" class=" bibr popnode">Wang et al., 2016).Generating a localized electric field directly on a photocatalyst's surface is a more practical approach than applying a macroscale electric field in a chemical cell because of cost and lower energy consumption. Combining piezoelectric materials with visible light photocatalysts is one way to achieve this. Piezoelectric materials can generate an internal electric field under strain and, hence, induce separation of photogenerated electric charges (href="#bib43" rid="bib43" class=" bibr popnode">Xue et al., 2015, href="#bib12" rid="bib12" class=" bibr popnode">Hong et al., 2010, href="#bib31" rid="bib31" class=" bibr popnode">Singh and Khare, 2017, href="#bib46" rid="bib46" class=" bibr popnode">Yun et al., 2018, href="#bib26" rid="bib26" class=" bibr popnode">Nan et al., 2017). This approach was used to fabricate primarily core-shell nanostructures, in which the core was composed of piezoelectric materials, such as ZnO, BaTiO3, and NaNbO3, whereas the shell consisted of visible light photocatalysts, including CuS, FeS, and AgO2. In this approach, it was possible to achieve enhanced reaction efficiency by using the dual stimuli of light and mechanical vibrations (href="#bib11" rid="bib11" class=" bibr popnode">Hong et al., 2016, href="#bib40" rid="bib40" class=" bibr popnode">Xiao et al., 2016, href="#bib16" rid="bib16" class=" bibr popnode">Li et al., 2015, href="#bib13" rid="bib13" class=" bibr popnode">Jia et al., 2018). However, fabrication of such dual-phase core-shell nanomaterials raises further complications, such as complex synthesis and weak mechanical coupling between the piezoelectric and photocatalytic counterparts under strain for extended periods of time.Low bandgap (i.e., visible light photocatalytic properties) and piezoelectric properties can co-exist in a single material. The use of BiFeO3 (BFO) as a promising candidate for visible light photocatalysis has been demonstrated owing to its low bandgap of ∼2.1 eV (href="#bib39" rid="bib39" class=" bibr popnode">Xian et al., 2011, href="#bib33" rid="bib33" class=" bibr popnode">Soltani and Entezari, 2013, href="#bib22" rid="bib22" class=" bibr popnode">Mocherla et al., 2013). In addition, BFO also possesses good piezo/ferroelectric performance with a large spontaneous polarization in excess of 100 μC cm⁻² and piezoelectric coefficient (d33) of about 100 pm/V (href="#bib32" rid="bib32" class=" bibr popnode">Singh et al., 2006, href="#bib15" rid="bib15" class=" bibr popnode">Jung Min et al., 2012). We assume these properties render BFO an excellent candidate for efficiently using both light and vibrational energy for catalytic degradation of organic pollutants, without the need for coupling it to other materials or using an external bias (href="#bib29" rid="bib29" class=" bibr popnode">Qi et al., 2018).In this work, we fabricated pure, single-crystalline BFO nanosheets (NSs) and nanowires (NWs), which are both visible light photoactive and piezoelectric. These BFO nanostructures were able to harness photonic as well as mechanical energy for the degradation of model organic pollutants such as rhodamine B (RhB). By coupling their photocatalytic and piezoelectric properties, degradation of RhB dye was greatly enhanced. Reactive species trapping experiments revealed the underlying mechanism of their catalytic performance. Development of these nanostructures contributes to the use of green technologies, such as harnessing natural sunlight and scavenging waste energies, such as noise and vibrations, for efficient environmental applications.