Article pubs.acs.org/cm
Effect of Ferroelectricity on Solar-Light-Driven Photocatalytic Activity of BaTiO3Influence on the Carrier Separation and Stern Layer Formation Yongfei Cui, Joe Briscoe, and Steve Dunn* Materials Research Institute, School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, U. K. S Supporting Information *
ABSTRACT: BaTiO3 is used as a target catalyst to probe the influence of ferroelectricity on the decolorization of a typical dye moleculeRhodamine Bunder simulated solar light. We show that there is a 3-fold increase in the decolorization rate using BaTiO3 with a high tetragonal content compared to predominantly cubic material. This is ascribed to the ferroelectricity of the tetragonal phase. The influence of ferroelectricity ensures a tightly bound layer of dye molecule and also acts to separate the photoexcited carriers due to the internal space charge layer. Both of these features act to enhance the catalytic performance. When nanostructured Ag is photochemically deposited on the surface of the BaTiO3, we find a further increase in the reaction rate that gives complete decolorization of the dye in around 45 min. KEYWORDS: ferroelectrics, photocatalysis, barium titanate
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INTRODUCTION The discharge of industrial effluent containing organic pollutants, such as dye, fertilizer, or surfactant molecules, is becoming a serious challenge for society as they damage water courses, endangering both wildlife and human life. Significant efforts are being made to remove these pollutants, including filtration, sorption processes, biological treatment, catalytic oxidation, and combination treatments.1 Among the techniques that are being investigated, heterogeneous photocatalysis is regarded as an effective way to degrade and remove hazardous compounds in the air and water.2 A semiconductor photocatalyst can generate electrons and holes under super-band-gap irradiation. These photoexcited electrons and holes are able to participate in redox reactions with pollutants. Over 50 semiconductor systems have been investigated in an effort to find a system that is suitable for efficient visible light photocatalysis. These include a variety of oxides (TiO2, ZnO, ZrO2, etc.) and sulphides (ZnS and CdS).3 However, the prospect of efficient (ca. 10% efficiency) photocatalysis has not been reached due primarily to the wide band gap associated with photostable catalysts that precludes significant activity under visible light. A number of factors limit the efficiency of the photocatalyst. As mentioned, the first issue is the low utilization of visible light. A typical semiconductor can only generate charge carriers under super-band irradiation. Taking TiO2 as an example, only 3−5% of the overall solar energy is available to produce photoexcited carriers.3,4 Additionally, the rate of recombination of photoexcited electrons and holes influences the amount © 2013 American Chemical Society
carriers that are available to do photochemistry: a high rate of recombination between photoexcited electrons and holes reduces the photoefficiency of the system.5 Finally, redox reactions occur in close proximity on the surface of the catalyst. This enables back reactions to proceed where products react to form the original starting species, and so the equilibrium is not pushed toward final products.6 A number of methods to improve the efficiency of heterogeneous photocatalysis systems have been developed, including: loading noble metals, such as Ag,7,8 Au,9,10 Pt,11,12 and Pd,11 onto the surface of photocatalysts utilizing the Schottky barrier formed between the interface and/or local surface plasmon resonance; combining two semiconductors with appropriate band gaps to facilitate charge separation, light absorption, and improve photostability;13−15 utilizing organic dyes to sensitize the semiconductor;16,17 and using a z-scheme system to mimic natural photosynthesis.18,19 A good review of the current state-of-the-art has recently been published that indicates the variety of approaches that have been investigated.20 The lack of progress with the existing materials and combinations of materials systems means that there is potential for new materials to make an impact in the area of photocatalysis; ferroelectric materials may be this new candidate. Received: June 27, 2013 Revised: September 16, 2013 Published: October 17, 2013 4215
dx.doi.org/10.1021/cm402092f | Chem. Mater. 2013, 25, 4215−4223
Chemistry of Materials
Article
helps to inhibit the recombination of holes and electrons. This is akin to the p−n junction of a typical photovoltaic or other diode structure. This has the effect of increasing the lifetime of photoinduced charge carriers; it has been reported that the decay time of photoluminescence in ferroelectric lithium niobate is 9 μs,34 whereas it was reported to be 0.1 μs in TiO2 thin film.35 Ferroelectric materials screen the surface depolarization field by developing strong Stern layers with chemisorbed molecules.36,37 It has been proposed that the dipole moment of a polar molecule (or induced dipole moment) interacts with the polarization of ferroelectric domains at the surface. This reduces the energy required to break bonds and enhances the photochemical activity.38−40 BaTiO 3 is a widely used ferroelectric material, its ferroelectricity arising from the tetragonal crystal structure, which is stable up to its Curie temperature (about 120 °C) in bulk samples.41 When the temperature is above the Curie temperature of BaTiO3, the crystalline structure changes from the ferroelectric tetragonal to the paraelectric cubic structure.42 However, it has been shown that surface-related strain can stabilize the cubic form at room temperature in small crystallites.43−45 Therefore, it can exist in both the nonferroelectric cubic phase and the ferroelectric tetragonal phase at room temperature.46With a band gap of 3.18 eV,47 BaTiO3 has been reported to degrade organic dyes like methyl red48 and methyl orange49 under super-band-gap irradiation. However, in these publications, BaTiO3 is in a cubic phase structure and only acts as a traditional wide-band-gap semiconductor under light. Recently, Hong et al. showed a new mechanism for dye decolorization that makes use of the so-called piezoelectrochemical effect in BaTiO3 dendrites.50 However, in a closely related paper, Hong et al.51 report piezoelectrical water splitting, but closer examination of the XRD data shows that the BaTiO3 was in the cubic phase, which does not have piezoelectric properties. These results clearly indicate that BaTiO3 is a very interesting material for the catalytic interaction with target pollutant molecules. However, there must be careful consideration of the phase and, therefore, extent of the ferroelectric and piezoelectric nature of the BaTiO3 that has been used. For example, it may be possible to confuse sonocatalysis52or flexoelectric behavior (potentially significant in high dielectric constant materials)53 in nanostructured hierarchical materials for a direct piezo- or ferroelectric effect. In this work, a range of BaTiO3 compositions were obtained through thermal or photochemical treatment. Half of the BaTiO3 samples were coated with nanostructured Ag using a previously reported process.54 The phase structure was investigated using X-ray diffraction, and the morphology was investigated using scanning and transmission electron microscopy. The rate of photodecolorization of a target molecule, Rhodamine B (RhB), was used to compare the activity of the photocatalysts. By obtaining both cubic, nonferroelectric and tetragonal, ferroelectric phases of the same material, we are able to directly ascertain the effect of ferroelectricity on the photocatalytic activity of BaTiO3, while keeping the chemical composition of the material unchanged.
Piezo- and ferroelectric materials have been regarded as wideband-gap semiconductors for some time21 and as such have been used in a variety of opto and optoelectronic devices, such as photovoltaics,22 LED,23 and detectors.24 A ferroelectric material possesses a spontaneous polarization arising from the displacement of the center of the positive and negative charges in a unit cell.25 The spontaneous polarization induces macroscopic charges on the surface of ferroelectrics.26 This induced bound charge is compensated by free charge carriers and defects in the ferroelectric bulk (internal screening) and/or by the adsorbed charged molecules from the environment (external screening) (see Figure 1a).27−30 Any region with an
Figure 1. Schematic of a ferroelectric material showing (a) internal polarization and screening mechanisms and (b) the effect of free carrier reorganization on band structure and photoexcited carriers. In (a), the spontaneous polarization with polarization vector P can be screened by free electrons and holes in the conduction and valence bands, respectively, and/or by ions or molecules adsorbed on the surface from the solution forming a Stern layer. In (b), the accumulation of free electrons on the C+ surface and holes on the C− surface leads to downward and upward band bending, respectively. The generation of a photoexcited electron−hole pair is shown, which are separated toward opposite surfaces of the material by the internal electric field arising from the polarization. This then leads to the spatial separation of oxidation and reduction on opposite surfaces as shown.
aligned spontaneous polarization direction is termed a ferroelectric domain. The spontaneous polarization with directions pointing from the bulk to the surface will produce a positive charge on the surface (C+ domain), and the polarization pointing away from the surface to the bulk will generate a negative charge (C− domain) (Figure 1a).31 The internal depolarization field will drive free electrons to the surface of a C+ domain, resulting in downward band bending. In a C− domain, it is the opposite, with electrons flowing away from the surface, leading to upward band bending (Figure 1b).32 Ferroelectricity of a material has a significant influence on the surface photochemistry.27,33 The separation of charge carriers due to the influence of the ferroelectric nature on band bending
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EXPERIMENTAL METHODS
BaTiO3 powder (99.9% trace metal basis,
