Elsevier

Catalysis Today

Volume 360, 15 January 2021, Pages 90-98
Catalysis Today

Ultrafast microwave-assisted hydrothermal synthesis and photocatalytic behaviour of ferroelectric Fe3+-doped BaTiO3 nanoparticles under simulated sunlight

https://doi.org/10.1016/j.cattod.2019.07.021Get rights and content

Abstract

We report on the synthesis of polycrystalline Fe-doped BaTiO3 ferroelectric nanopowder that exhibits high visible light photocatalytic activity. The ferroelectric Fe-doped BaTiO3 samples with nominal Fe contents ranging from 2 to 8 at% are prepared by fast microwave-assisted hydrothermal synthesis. All the doped ferroelectrics exhibit enhanced light harvesting properties due to the charge transfer center of Fe being associated to strong absorption in the visible region. Also, the bulk photovoltaic effect arising from net shift current in the non-centrosymmetric crystal, provides an additional driving force to charge carrier separation. While the best photocatalytic activity is obtained with the 2 at% Fe3+-doped BaTiO3 ferroelectric, the more heavily doped systems show a gradual decrease in photoactivity. In comparison to the undoped BaTiO3, all the Fe3+-doped systems show improved photocatalytic activity under similar experimental conditions. We invoke the iron-associated absorption for an enhanced charge carrier generation and the bulk photovoltaic effect, a property of non-centrosymmetric crystals, for an enhanced charge carrier separation and migration.

Introduction

Nowadays, with the textile and dyeing industries still expanding, water pollution has become a global challenge [1]. More than ever, the elimination of toxic chemicals from contaminated water has become a necessity. Among the different types of treatment technologies, heterogeneous photocatalysis is a simple and viable technology that offers several advantages such as a low cost, a high stability and an enhanced photodegradation efficiency [2]. However, the photocatalytic performance is known to be restrained by charge carrier generation due to poor light absorption, charge carrier separation and transport [3,4], redox potential of charge carriers on the surface, low surface-to-volume ratio, and challenges with photocorrosion.

The principle of photocatalysis is based on the degradation of organic molecules by photo-irradiation with either a visible or a UV source. For this purpose, a semiconductor material is introduced in the contaminated water. The electron-hole pair resulting from photon-semiconductor interaction reacts with water to produce hydroxyl and superoxide radicals that in turn further react with certain contaminants [5]. The efficiency of water purification thus depends on the semiconductor material’s properties [6]. In particular, a chemically stable surface termination is required to resist degradation and poisoning. Ferroelectric materials are ideal candidates for such applications. In both aqueous and non-aqueous solutions, ferroelectrics are most commonly insulating, and present a high thermal and chemical stability over a wide range of temperature, as well as a high resistance to dissolution [7]. An additional, substantial benefit arises from the bulk photovoltaic effect (BPVE), a property represented by a third rank tensor and thus only present in materials without inversion symmetry. Microscopically, the irradiation of ferroelectrics with light of any polarization leads to an asymmetric propagation of electron and hole wavefunctions [8]. Yet, two commonly encountered features of ferroelectrics limit their potential in photocatalytic (and photovoltaic) applications: the large intrinsic band gap and a very low charge carrier mobility.

Barium titanate (BaTiO3) is an important, non-toxic member of the perovskite family with a cut-off wavelength of ˜ 390 nm (3.2 eV) [9]. It is non-centrosymmetric, has a unique polar axis, and can be considered as alternating stacked layers of BaO and TiO2 with lattice parameters a =0.3995 nm and c =0.4024 nm along the main crystallographic axis. At room temperature BaTiO3 is tetragonal (P4mm) and ferroelectric along the c-axis. Ferroelectric BaTiO3 exhibits decent charge carrier separation properties, [10] holding the promise for applications in photocatalysis, where the rate limiting step is typically related to surface kinetics of ad- and desorption, which is orders of magnitude slower than the intrinsic processes. To enhance the light absorption necessary for the redox activity, aliovalent substitution of Ti4+ cations with Fe3+ cations in BaTiO3 introduce a strong absorption band in the visible range (broad absorption band in the green, i.e. around 2.3 eV) [6,11]. Fe3+ is a transition metal with half-filled electronic configuration (i.e d5-orbital). It acts as an acceptor dopant and exhibits high chemical stability with the host lattice. Fe3+ has a charge transfer center that can be readily reduced to Fe2+ (or even photo-oxidized to Fe4+) [5]. The combination of defect doping and the bulk photovoltaic effect results in an enhanced photoinduced charge carrier generation and a better separation of the electrons and holes, together promoting the transport of charges to the catalytic surfaces [12]. We would like to point out that defect doping does not affect the band gap, it rather introduces a phonon-broadened absorption band that, if overlapping with the fundamental absorption causes a reduction of the detectable absorption edge.

While Co, Fe, Mn and Ni mono- and co-doping of BaTiO3 have been performed for piezoelectric, electric, magnetic applications [[13], [14], [15]], and organic synthesis [16], there has been little or no report on the photocatalytic evaluation of ferroelectric Fe3+-doped BaTiO3 for environmental remediation. In view of this, we report the visible light activated photocatalytic activity of environmentally benign Fe3+-doped BaTiO3 ferroelectric. We synthesize this material by ultrafast microwave-assisted hydrothermal synthesis. The advantage of this approach arises from an accelerated growth due to microwave surface activation at the liquid or solid interface [17]. The photocatalytic activity of these ferroelectrics is evaluated by time dependent absorption measurements of photodegraded methyl orange. It reveals that the doped system absorbs visible light and improves the charge carrier separation due to internal spontaneous polarization induced in the non-centrosymmetric material. This study highlights both the role of transition metal ions in extending the absorption edge of the host lattice towards the visible range, and the benefits of the additional charge carrier separation mechanism through the bulk photovoltaic effect. We will also discuss that both effects are coupled to a certain extent through the detrimental effect of substantial iron doping on the tetragonality of the host lattice.

Section snippets

Materials

All the chemicals were purchased from Sigma Aldrich. High purity barium hydroxide (Ba(OH)2•8H2O > 98%), titanium oxide (Anatase TiO2 > 99.7%), hydrogen peroxide (H2O2, 30%wt.) and iron nitrate (Fe(NO3)3•9H2O > 98%) were used for the synthesis of ferroelectric Fe3+-doped BaTiO3 nanoparticles while methyl orange ([4-[[4-dimethylaminophenyl]-azo] benzene sulfonic acid sodium salt) (C14H14N3NaO3S) was used as the model organic dye.

Synthesis of ferroelectric Fe3+-doped BaTiO3 nanoparticles

Fe-doped BaTiO3 nanoparticles were prepared by an ultrafast

Structural studies

Fig. 2a presents the XRD pattern of the ferroelectrics with different nominal Fe3+ ion contents. The diffraction peaks of all the samples correspond to the tetragonal phase of BaTiO3 with space group, P4mm [18]. This agrees with the standard JCPDS card #: 01-075-1606 and confirms B-site substitution for the acceptor atom (Fe3+). Also, secondary phases indexed to cubic symmetry of BaCO3 (JCPDS card #: 00-011-0697) and tetragonal TiO2 with space group P63/mmc and I41/amd, respectively, were also

Conclusion

In summary, ferroelectric Fe3+-doped BaTiO3 nanoparticles with high photocatalytic activity were successfully synthesized by the ultrafast microwave-assisted hydrothermal route. We demonstrate that the polarity of nanoparticles creates microscopic electric fields across the domains which aide in the charge carrier separation of excitons and their transport. The optimum photoactivity was obtained with the 2 at% Fe-doped BaTiO3 under illumination of simulated sunlight at 100 mW/cm2. UV–vis

Acknowledgements

I.C.A. is grateful for the financial support through MATECSS Excellence scholarship. G.K. is thankful for an FRQNT Postdoctoral scholarship. S.S. and A.R. are grateful for NSERC discovery (RGPIN-2014-05024) and strategic partnership (STPGP 506953-17) grants.

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