Ferroelectric Fe-Cr Co-Doped BaTiO 3 Nanoparticles for the

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Ferroelectric Fe-Cr Co-Doped BaTiO Nanoparticles for the Photocatalytic Oxidation of Azo Dyes Ifeanyichukwu C. Amaechi, Azza Hadj Youssef, Diane Rawach, Jerome P. Claverie, Shuhui Sun, and Andreas Ruediger ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00336 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Applied Nano Materials

Ferroelectric Fe-Cr Co-Doped BaTiO3 Nanoparticles for the Photocatalytic Oxidation of Azo Dyes Ifeanyichukwu C. Amaechi1, Azza Hadj Youssef1, Diane Rawach2, Jérôme P. Claverie2, Shuhui Sun1, and Andreas Ruediger1* 1.

Institut National de la Recherche Scientifique, Centre Énergie, Matériaux, Télécommunications

(INRS-EMT), 1650 Boulevard Lionel-Boulet, Varennes, Québec J3X 1S2, Canada. 2.

Département of Chemistry, Université de Sherbrooke, 2500 Sherbrooke Université Boulevard,

Québec J1K 2R1, Canada. *Corresponding

author: [email protected].

ABSTRACT Ferroelectric Fe3+Cr3+co-doped BaTiO3 nanopowder has been successfully synthesized by an microwave-assisted hydrothermal method. The results indicate that by molar adjustment of the cations, the apparent optical absorption edge of the co-doped ferroelectrics can be red-shifted in comparison to undoped BaTiO3 ferroelectric. The interfacial charge transfer properties of the materials

as

evaluated

through

electrochemical

impedance

spectroscopy

(EIS)

and

chronoamperometry reveal an average charge transfer resistance. The nanopowders are then explored as photocatalysts for the degradation of methyl orange under calibrated solar irradiation. The results show that Fe3+ doping increases the photoabsorption and improves the degradation process for up to 4% iron in the precursors. For higher concentrations, the effect is reduced due to a partial loss of tetragonality of the ferroelectric material. The best photocatalytic performance achieved with the 4 1|Page ACS Paragon Plus Environment

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mol.% Fe3+ is 3.7 times higher than that of undoped BaTiO3 nanopowder under otherwise identical conditions. The improvement is ascribed to synergy between ferroelectric polarization and enhanced visible light utilization.

Keywords: Photocatalysis, Bulk photovoltaic effect, Co-doped BaTiO 3, Chronoamperometry, Ferroelectrics, Electrochemical impedance spectroscopy.

1.0 Introduction Recently, industrial wastes which constitutes major source of environmental pollution has become a global concern. This relates to the discharge of effluents in waterbodies thereby endangering lives of aquatic organisms and making water unfit for drinking. More so, the contaminated waters contain different organic and sometimes inorganic compounds such as dyes or their metabolites which are carcinogenic and/or mutagenic in some cases.1 The environmental remediation is based on conventional treatment techniques such as chemical2, electrochemical3, membrane filtration, biological (aerobic and anaerobic), sorption process, and catalytic oxidation. 4 Depending on the level of contamination, a combination of techniques can also be employed. Han et al. 5 has reported that some of these pollutants e.g. azo dyes get reduced to byproducts containing hazardous aromatic amines which are not easily photodegraded by conventional techniques. The implication is that the enumerated techniques are still inefficient. In order to address the problem, heterogeneous photocatalysis employing the principles of an advanced oxidation process is promising. The basic principle follows a series of redox chemical reactions, which are initiated by photogenerated charge carriers diffused to the surface of the semiconductor photocatalyst. In terms of electronic energy band

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structure, the energy of redox couples of the target pollutant must be enveloped in between the conduction and valence band of the semiconductor photocatalyst. 6 This guarantees the band bending at the interface and facilitates the charge transport required in the photochemistry of desired oxidative species with adsorbed pollutants into ideally carbon dioxide and water. 6, 7 Oxide-based photocatalysts are very stable and possess suitable band alignment but being wide bandgap semiconductors, they suffer from a low density of photogenerated charges and a high charge recombination, which ultimately translates into very few charge carriers available at the surface for redox catalytic reaction. In addition to the low generation rate and the low mobility for charge carrier separation, the recombination of charge carriers which requires a few picoseconds occurs on a much faster time scale than the charge transport and the often rate-limiting charge-consuming catalysis. 8 While the catalysis is entirely a surface phenomenon and despite progresses made so far in this field, the major challenges remain the charge carrier generation by light absorption, separation and transport. 9, 10 During the search for abundant, inexpensive and environmentally benign alternatives for Pt group catalysts, perovskite bimetallic oxides have emerged as a promising class of materials. 11 Some of the studies have shown the possibility of improving the efficiency of photocatalytic process by the use of materials with a spontaneous polarization.12-14 These materials, also referred to as pyroelectrics possess a unique polarization-induced internal electric field. 15 Technically speaking, there is no need for ferroelectricity for the phenomena under discussion as ferroelectricity refers to the reversibility of the spontaneous polarization under and external bias field, which is of no use for photocatalysis. The lack of inversion symmetry associated to the presence of a spontaneous polarization provides the driving force14, 16 for the charge separation and migration of light-generated electron-hole pairs in the form of the bulk photovoltaic effect represented by a third rank tensor. 17 This additional driving force promotes the separation of electrons and holes are on their way to the surface.



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Ferroelectric BaTiO3 is an archetype of ABO3 inorganic perovskite structure, sensitive only to UV light due to its relatively large band gap (3.2 eV). To improve its applicability in photocatalysis, a red shift of the absorption edge (introduction of broad absorption bands) is required and generally achieved by compositional modifications aimed at introducing metal dopants on the B-site. The crystallographic modifications have been investigated to a large extent. For example, Cr-doped BaTiO3 has been employed for the catalytic reduction of nitrobenzene to aniline with a yield of 98.2% in 6 h (i.e 63.4%, recalculated to 90 min).18 The photodegradation of Rhodamine B (RhB) over Ndoped BaTiO3 and Ag-doped BaTiO3 nanoparticles19, 20, and crystal violet (CV) using nano-cubic BaTiO321 have also been investigated. The concept also extended to BaTiO 3-based heterostructures1214

such that the enhanced photoactivity was ascribed to external screening of the surface charges in

ferroelectrics. The use of noble metal dopants is not even economically feasible and limit the scope of large scale applications. Chromium doping, as mentioned above, introduces a broad absorption band near the fundamental absorption and allows for the generation of additional charge carriers from a broader spectral range, now extending into the visible. At the same time, this aliovalent substitution (Cr3+ replaces Ti4+ introducing a charged oxygen vacancy for charge neutrality reasons) introduces new recombination sites as it also acts as a charge transfer center. 22-24 To optimize the efficiency of the photocatalytic process in which the efficiencies for all individual steps are combined, there is an expected trade-off between more doping to enhance the absorption and too much doping to eventually provide excessive recombination or other detrimental parasitic effects that we will address in the following. In this study, we report on the synthesis of ferroelectric BaTiO3 co-doped with Fe3+ and Cr3+ ions by microwave-assisted hydrothermal method. The interest on Fe3+/Cr3+ ions is due to their charge transfer centers, which are associated to strong absorption bands in the visible range. By attempting


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to preserve the ferroelectricity, a systematic study was carried out to determine the effect of spontaneous polarization as a function of nominal Fe 3+ concentration and cationic co-doping on the photocatalytic activity. For comparison, undoped ferroelectric BaTiO 3 was also prepared by the same way. Their photocatalytic activities were evaluated by the degradation of azoic methyl orange in aqueous solution under atmospheric condition.

2.0 EXPERIMENTAL SECTION 2.1 Materials All the chemical reagents: barium hydroxide (Ba(OH)2•8H2O, > 98%), titanium dioxide (TiO2, > 99.7%), iron nitrate (Fe(NO3)3•9H2O, > 98%), chromium nitrate (Cr(NO3)3•9H2O, > 98%), hydrogen peroxide (H2O2, 30%wt.), methylene orange (C14H14N3NaO3S), benzoquinone (C6H4O2, ≥ 98%), tertbutyl alcohol (C4H10O, ≥ 99.5%), and ethylene diaminetetraacetic acid (C10H16N2O8, ≥ 99%) were purchased from Sigma-Aldrich. The analytical grade reagents were used without further purification. 2.2 Synthesis of ferroelectric Fe3+Cr3+ co-doped BaTiO3 nanoparticles An microwave-assisted hydrothermal method was adopted for the synthesis of ferroelectric codoped BaTiO3 nanoparticles. The internal pressure and temperature of a typical microwave hydrothermal reactor vessel is a parametric function of volume fill factor, power level and exposure time of microwave radiation. Hence, the volume fill factor of mixture was fixed at 0.52 relative to the volume of the Teflon container, 23 mL. The final mixture composed of TiO 2, Ba(OH)2•8H2O and Cr(NO3)3•9H2O and Fe(NO3)3•9H2O in the molar ratio of 18: 29: 2: 1 was dissolved in distilled water in the presence of H2O2 (30%). The H2O2 was used as a scavenger for excess hydrogen that would otherwise introduce hydroxyl groups within the perovskite lattice that are associated to substantial leakage currents. First, an aqueous suspension of TiO2 dispersed in distilled water was stirred for



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10 min followed by an addition of Ba(OH)2•8H2O, Cr(NO3)3•9H2O and Fe(NO3)3•9H2O (Cr3+ = 4 mol.%, Fe3+ = 0, 4 and 8 mol.%). After 20 min of vigorous magnetic stirring, H2O2 was then added as a scavenger for excessive hydrogen. Chemical homogeneity of the mixture was obtained by further stirring for another 10 min. The mixture was finally transferred to a Teflon container housed by a polymer autoclave and the reactor assembly from Parr Instrument was then placed in a Panasonic Inverter Microwave Oven (2.45 GHz) at 120 W for 10 min corresponding to 1 cycle. The vessel was later allowed to cool down to room temperature after the initial microwave exposure and the process was repeated for another 2 cycles using the same microwave heating condition. After 3 complete cycles and having cooled down to ambient temperature, the final product was washed several times with distilled water, filtered, and dried in the oven at 80 oC for 15 h. Similarly, undoped BaTiO 3 nanoparticles was prepared without the addition of the transition metals. 2.3 Characterization The crystal phase of the samples was identified by X-ray diffraction (XRD) with a Bruker D8 Advance diffractometer at ambient temperature using Cu-K radiation ( = 1.5406 Å). Bright field microstructural images of the samples were acquired by transmission electron microscopy (TEM) using JOEL (Model: JEM-2100F) operating at 200 kV. The ferroelectric nanoparticles were dispersed in 2 mL of ethanol and ultrasonicated for 20 min before being deposited on carbon coated Cu grids. The crystallinity and lattice distortion were investigated by Raman scattering using an Horiba iHR320 spectrometer incorporated with a 473 nm DPSS laser. The signals were detected by a thermoelectrically cooled CCD (Synapse BIDD QE). Optical absorption studies was conducted on a UV-Visible-NIR spectrometer, PerkinElmer, Lambda 750 using quartz as reference while samples for the absorption studies were prepared by dip-coating of pretreated quartz substrates in a solution containing the dispersed nanoparticles and dried at 80 °C. The atomic concentration of the samples


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was determined by X-ray photoelectron spectroscopy (XPS) using a VG Escalab 220i XL system operated with a 1486.6 eV Al-Kα source at 15 kV and 20 mA. To calibrate the energy scale, the C 1s peak at 284.6 eV was used with ± 0.05 eV degree of uncertainty. The photoelectrochemical measurement of the samples was carried out on a reactor assembly comprising three electrodes using a Zahner Elecktik Zennium Photoelectrochemical Workstation. The counter, reference and working electrodes are Pt, Ag/AgCl reference electrode and ferroelectric films on FTO respectively. The electrolyte was a freshly prepared 0.5 M Na2SO4 aqueous solution. The working electrode was prepared as follows: firstly, 50 mg of the ferroelectric nanoparticles was dispersed in 600 µL of a composite solution of N-methyl-2-pyrrolidone (NMP) and polyvinylidene fluoride (PVDF). The solution was ultrasonicated for 10 min. Secondly, the solution was spin-coated on the pre-treated FTO substrates at 4500 rpm. Lastly, the working electrode was dried in the oven at 100 oC for 15 h. 2.4 Photoactivity measurements The setup used to investigate the photocatalytic activity of the nanopowders is as illustrated in Figure 1. Methyl orange was used as a model pollutant. The photoactivity of Fe 3+Cr3+ co-doped BaTiO3 ferroelectrics was examined by photodegradation of the methyl orange dye in a glass reactor under simulated light irradiation. 50 mg of photocatalyst was dispersed in 50 mL of 20 mg/L concentration of aqueous solution of methyl orange. The suspension was stirred at ambient temperature for 30 min in the dark to achieve adsorption-desorption equilibrium and to saturate the surface of the photocatalyst. Photoirradiation of the chemisorbed photocatalyst was done using a solar simulator (Model - #SS50AAA, PET Photoemission Tech., Inc.) equipped with a xenon (Xe) lamp. The distance between the glass reactor and light shutter was fixed at 16 cm throughout the whole experiment. Therefore, at the elapse of every irradiation time, measurement was taken from a digital



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multimeter (Keithley DMM 2000) connected to a photodetector. The photovoltage signals was processed yielding an output voltage, V0 equivalent to absorbance, A as given by Equation 1: (1)

𝑉 = 𝑙𝑜𝑔

The reactive oxygen species generated during the photocatalytic degradation of methyl orange with co-doped BaTiO3 nanoparticles was investigated by in situ capture experiments. 0.5 mM Benzoquinone (BQ), 2 mM tert-butyl alcohol (TBA), and 2 mM ethylene diaminetetraacetic acid (EDTA) was introduced into the photocatalytic system under identical experimental conditions to scavenge the following radicals; superoxide (O •), holes (h ), and hydroxyl (OH • ) respectively.

Figure 1: The setup for measurement of photocatalytic activity.

3.0 RESULTS AND DISCUSSION 3.1 Structure and Surface Morphology The crystal structure of the aliovalently co-doped BaTiO3 ferroelectrics was characterized by XRD. Figure 2a shows the XRD patterns of the undoped and co-doped BaTiO 3 ferroelectrics with different concentrations of Fe3+ ion. The patterns of all the samples can be well indexed to tetragonal 8|Page ACS Paragon Plus Environment

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BaTiO3 with space group P4mm (JCPDS card: 01-075-1606).25 Even though the distortion of the unit cell is quite small [(c/a) – 1 = 0.00773], the tetragonal splitting of the (002) and (200) diffraction lines at 2  45o is visible as displayed in the expanded view of Figure 2b. The splitting of the peak therefore confirms the existence of tetragonality, which implies polarization in the samples at room temperature. The patterns reveal that the as-synthesized ferroelectrics contain some impurities like BaCO3 and traces of TiO2 resulting from surface reactivity of CO2 with BaO and probably residual TiO2 species respectively. For the co-doped samples, the diffraction peaks are similar to the undoped BaTiO3 except for the additional peaks showing biphasic composition of the tetragonal and hexagonal (P63/mmc) symmetries. The phase transformation is unique particularly when Ti 4+ is substituted with d-block elements such as Mn, Fe, Co etc. and similar observation had been reported. 26, 27 Apart from BaCO3, which is usually a common contamination in the synthesis of barium titanate, impurities such as metallic Fe/Cr and Fe2O3/Cr2O3 are not detected. This indicates that Fe and Cr atoms replaced Ti atoms in the BaTiO3 host lattice. The peaks observed at 41.4 o and 53.8o are due to the formation of a secondary hexagonal phase28 the intensity of which increases with the Fe concentration. In other words, one effect of large amounts of oxygen vacancies formed by co-substitution of Ti 4+ by Cr3+ and Fe3+ is that the crystal structure will shift towards the hexagonal phase. Therefore, strong electrostatic repulsion between Ti4+ cations induced by disappearance of oxygen manifests in oxygen vacancies such that high amount of these vacancies are not structurally permitted in the corner-sharing structures like tetragonal phase. This explains the existence of secondary hexagonal phase in which its formation is dependent on a large amount of oxygen vacancies.29 The tetragonal lattice parameters a and c, and corresponding tetragonality i.e c/a as a function of Fe3+ ion content is determined based on the (002) and (200) Bragg diffractions. Also, the cell parameters remain within the error bar as shown in Figure 2c. The results indicate a decreasing tetragonal distortion and possible transition into a cubic structure



at high Fe3+ doping concentration. At this point, we would like to point out that all concentrations are given as weight percent in the educts and do not necessarily reflect the exact concentration in the crystal. Besides, the c/a for the highest doping concentration is 1.00562 ± 0.00012, which is still larger than unity. This confirms that the co-doped BaTiO3 nanoparticles are still tetragonal up to 8 mol.% even though the Ti/Fe/CrO6 octahedra show an ever smaller distortion due to less off-center displacement. Raman scattering results provide additional evidence that the specific phonon modes of tetragonal BaTiO3 still appear even for the highest doping level in our work.

T = Tetragonal H = Hexagonal · BaCO3  TiO2

35

40

2 (o)

45

55

60

c a c/a - 1

(c)

403.2

50

Fe4%Cr4%:BaTiO3

403.0 402.8

8.0 7.5 7.0

402.6 400.4

6.5

400.2

6.0

400.0

5.5

BaTiO3

0

4

Intensity (a.u.)

30

Expt. Fit

(c/a - 1)´10-3

25

·

(b)

Fe8%Cr4%:BaTiO3

(210)T (213)H (211)T

· ·

20

BaTiO3 Cr4%:BaTiO3 Fe4% Cr4%:BaTiO3 Fe8% Cr4%:BaTiO3

(111)T (104)H (203)H (002)T (200)T

(110)T

(102)H/(100)T

Intensity (a.u.)

(a)

Lattice parameters (pm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cr4%:BaTiO3

BaTiO3

44.0

44.5

45.0

o

2 ( )

45.5

46.0

8

Fe content (mol.%)

Figure 2: (a) X-ray diffraction patterns of undoped and Fe3+Cr3+ co-doped BaTiO3 ferroelectrics. (b) Expanded view of (002) and (200) diffraction lines at various mol.% of Fe. (c) Fe content dependence of lattice parameters and tetragonality, c/a.

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Furthermore, Raman spectroscopy was employed to investigate the effect of Fe, lattice defects and existence of ferroelectric phase in the co-doped BaTiO 3 samples. Figure 3a shows typical Raman spectra of the different nanoparticles analyzed at room temperature. It is well known that cubic BaTiO3 is Raman inactive due to centrosymmetry for each ion.30 The Raman spectra measured for all the ferroelectrics are consistent with the spectrum of tetragonal BaTiO 3 reported in literature.31, 32 All major phonon modes, which are characteristic of tetragonal BaTiO 3 are observed in the spectra. The dominant spectral features are: (i) the presence of Raman mode around 186 cm -1. This is assigned to A1(TO) and confirms the nanosize of the as-synthesized samples. In the bulk BaTiO 3 material, the peak at 186 cm-1 transforms to a dip; a behaviour indicating the effect of coupling of the A1 transverse optic mode. (ii) the two characteristic peaks at 308 cm-1 [B1, E(TO + LO)] and 716 cm-1 [A1(LO), E(LO)] caused by splitting of transverse and longitudinal optic modes respectively are evidence of induced spontaneous polarization at order-disorder transition. These peaks are known to disappear above the Curie temperature, TC as the material transforms to a high-temperature cubic phase. At this point, we would like to point out that there is a reported size-driven phase transition at room temperature for very small particle sizes.33 From the spectra of co-doped lattices, we observe a decrease in Raman intensity and broadening of the aforesaid peaks which is a clear indication of partial tetragonality.32 This shows that the ferroelectric response could vanish even when the superparaelectric limit is not approached due to high Fe content doping. In other words, the arrangements of TiO6, FeO6 and CrO6 octahedra in the lattice may possibly reduce the dipole-dipole interaction.34 Hence, the result of XRD analysis showing a downturn in c/a is in agreement with those obtained by Raman spectroscopy as the Ti/Cr off-center vibrational displacement become progressively small with Fe doping. (iii) the asymmetric mode A1(TO) around 516 cm -1 due to O-TiO symmetric stretching becomes broadened with increase in bandwidth in the case of co-doped



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BaTiO3. The observation quantitatively shows the strength of Fe at B-site of BaTiO 3. The expanded view and its deconvolution is shown in Figure 3b. Also, variation of the Raman peak shift with Fe content is depicted in Figure 3c. From the series of Lorentzian-shaped peaks, it shows that the 516 cm 1

phonon mode blue-shifts to a lower wavenumber i.e 510 cm -1 indicating tensile stress and disorder

due to mismatch in ionic radii 𝑅

= 0.61 Å, 𝑅

= 0.62 Å and 𝑅

= 0.65 Å . We therefore

remark that aliovalent co-substitution with acceptor atoms produces structural disorder and distortions which tend to increase the bandwidth of the phonon mode. Furthermore, the two asymmetric A1(TO) modes at 251 and 516 cm-1 in our case are known to be sensitive to impurities and defects. 25 Surface contamination with BaCO3 is equally detected in all the spectra around 226 cm -1 due to symmetric stretching of carbonate CO32- ion in the nanoparticles.31, 35 This is caused by surface reactivity of the powdered samples. Barium carbonate is extremely stable chemisorbate and does not thermally desorb. In addition, traces of residual TiO2 show one strong intense Raman mode at low frequency 144 cm -1 and two weak peaks around 394 cm-1 and 639 cm-1. These bands are assigned to Eg, B1g and Eg modes respectively36 and indicate an incomplete reaction of the TiO2 precursor. Also, the hexagonal phase of BaTiO3 (h-BaTiO3) is known to crystallize at room temperature by doping with 3d transition metal ions such as Fe, Co, Ni, Cr, Cu, Mn etc.37 Its fingerprint at 637 cm-1 38 makes it difficult to distinguish between TiO2 phase and h-BaTiO3 phase. However, from the result of XRD we infer that there is a small volume fraction of the hexagonal phase in the crystal. Apart from the phonon modes mentioned previously, a distinct Raman band for the doped samples appear at 354 cm -1. This mode is assigned to the stretching vibration of Cr-O39, 40 and confirms the presence of trivalent chromium.


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Fe8% Cr4%:BaTiO3 Fe4% Cr4%:BaTiO3 Cr4%:BaTiO3 BaTiO3

639 600

800

Raman shift (cm-1)

518

515

(c)

Intensity (a.u.)

400

717

398

200

516

(b)

BaTiO3

- TiO2 - BaCO3

516

186 226 263 354 308

144

Intensity (a.u.)

(a)

Phonon mode (cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cr4%:BaTiO3

510.5

Fe4%Cr4%:BaTiO3

511

Fe8%Cr4%:BaTiO3

516 514 512

50

510 508

BaTiO3

0

4

450 500 550 600 650

-1

Raman shift (cm )

750

8

Fe content (mol.%)

Figure 3: (a) Room temperature Raman spectra of undoped and Fe3+Cr3+ co-doped BaTiO3 nanoparticles. (b) Expanded view of Lorentzian deconvoluted 516 cm -1 phonon mode. (c) Variation of 516 cm-1 phonon mode with Fe content. The solid line is a guide to the eye.

The microstructural properties of the samples are also studied using TEM on carbon coated Cu grids. The bright field images are shown in Figure 4(a - d). From Figure 4a, the TEM image of the undoped BaTiO3 shows essentially irregularly-shaped nanospheres with mean particle size of 42.7 ± 3.3 nm (Figure S1a).



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Figure 4: Bright field TEM images of (a) undoped (b) 0 mol.% Fe (c) 4 mol.% Fe (d) 8 mol.% Fe. Nanocubes are denoted by open square with broken red lines.

For the topography of Cr3+:BaTiO3 (Figure 4b), the surface is composed of a mixture of spatially distributed spherical and nanorod-like grains with an average length of 123 nm. This points to the fact that there is a preferred uniaxial growth of the grains in the presence of Cr ion. Nevertheless, the addition of Fe3+ e.g. 4 mol.% (see Figure 4c and d) appears to suppress the elongated nanorods restricting their growth to spherically shaped grains and nanocubes. We attribute this to an increased lattice microstrain resulting from co-doping that will create regions of inhomogeneously distributed defects. The high surface energy from these highly disordered area could facilitate dynamic recrystallization process.25,

41

The entire growth process is illustrated in Scheme 1. From the 14 | P a g e ACS Paragon Plus Environment

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morphologies with high Fe concentration, the transformation to nanocubes suggests the existence of partial tetragonality and possible ferro-to-paraelectric transition. This corroborates the results of XRD and Raman scattering. Due to shape anisotropy for Cr3+:BaTiO3, two mean particle size distributions corresponding to the length and width are estimated and presented in Figure S1b while for 4 mol.% Fe and 8 mol.% Fe, 35.9 ± 2.0 nm (Figure S1c) and 51.6 ± 3.2 nm (Figure S1d) are estimated respectively. The high-resolution TEM shows the interatomic spacing is 0.28 nm and 0.40 nm corresponding to the (101) and (100) lattice planes of the tetragonal BaTiO 3. In order to confirm the elemental composition of co-doped nanoparticles, energy dispersive X-ray (EDX) is carried out on the undoped, Cr3+:BaTiO3 and 4 mol.% Fe3+Cr3+:BaTiO3 samples. The presence of Ba, Ti, O, Cr and Fe in the ferroelectric are also confirmed as shown in Figure S2(a– d).

Scheme 1: Schematic illustrating the synthesis of ferroelectric co-doped BaTiO 3 nanoparticles.



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3.2 Optical Properties The photocatalytic properties of a material depend strongly on its electronic structure. 42 In order to increase the photocatalytic efficiency through visible light utilization, a moderate band gap is required for maximum absorption of visible light. By co-doping, the spectral response is extended into the visible region and this is beneficial to a photocatalytic redox reaction. Figure 5 shows the optical absorption spectra of our undoped and co-doped BaTiO 3. Clearly, the co-doping with Fe3+/Cr3+ makes the ferroelectric active in visible light due to two well documented absorption bands.43 The co-doped lattices have an intense absorption band with steep edges in visible light region, which is different from the undoped lattice that has an intrinsic absorption band in UV region around ~ 3.18 eV. These steep edges exhibited by the co-doped nanoparticles are due to charge transfer centers and show a pronounced redshift. These states could be the localized electronic states of either dopants or intrinsic defects occupying the region close to the top of the valence band. Therefore, the strong absorption bands, which are below the fundamental absorption of undoped ferroelectric are consequences of charge transfer between Ti 4+ and Cr3+/Fe3+ centers. By extrapolating the ℎ𝜐 axis when (𝛼ℎ𝜐) = 0 (for inset) or the wavelength, λ when absorbance is zero and using 𝐸 = 1240⁄𝜆, a cut-off energy of ~ 2.7, 2.3 and 1.7 eV was obtained for the samples Cr3+:BaTiO3, 4 mol.% Fe and 8 mol.% Fe respectively.


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(hu)2 (eV2m-2)

1.5

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

0.5

1.5 2.0 2.5 3.0 3.5 4.0 Photon energy, hu (eV)

0.0 200

400

600

800

1000

Wavelength (nm) Figure 5: Optical absorption spectra of undoped and Fe3+Cr3+ co-doped BaTiO3 ferroelectrics. Inset is the variation of (𝛼ℎ𝜐) vs. ℎ𝜐 for the samples.

The apparent absorption edges are in agreement with those reported in literature. 34 It shows that by varying different composition of B-site cationic ordering, electronic transition states from the 3d electrons of transition metals could be generated. Hence, co-doping of BaTiO 3 with Cr/Fe not only changes the apparent absorption edge of the ferroelectric but also improves the visible light utilization, which increases the efficiency for charge carrier generation, albeit at the expense of pinning the Fermilevel at the level of the n-type charge transfer dopants. X-ray photoelectron spectroscopy (XPS) technique was further employed to study the elemental composition in order to understand the chemical states of different elements present in the material. Undoped, Cr3+:BaTiO3 and 4 mol.% Fe3+Cr3+:BaTiO3 samples are used for this purpose and the survey spectra are displayed in Figure 6a. The high resolution XPS spectrum of undoped and Cr3+:BaTiO3 for individual elements are shown in Figure S3(a - d), while that of 4 mol.% Fe3+Cr3+:BaTiO3 sample is shown in Figure 6(b - f). All the spectra are fitted with GaussianLorentzian line shape. In Figure 6b, the low binding energies of Ba2+ ion are reflected by Ba 4d and 17 | P a g e ACS Paragon Plus Environment


Page 18 of 39

Ba 4p peaks. The most intense peak corresponding to Ba 3d5/2 was resolved yielding 778.51 eV and 780.18 eV. These peaks are typical binding energies of the Ba ion in oxidation state of +2. 44 The doublet of Ti 2p state is shown in Figure 6c. In the spectrum, the peak of Ti 2p3/2 is more intense and shows a smaller FWHM than Ti 2p1/2. The peaks i.e. 458.05 eV and 463.80 eV confirm that Ti exists as Ti4+ ion in the lattice. Also, the two small peak areas in the Ti 2p core levels are attributed to the presence of Ti3+.45, 46 In all the Ti 2p spectra, the presence of reduced titanium ion is obsevrved primarily due to the formation of defects. While a spin-orbit splitting of 5.93 eV is observed for the undoped sample (see Figure S3c), a value of 5.78 eV and 5.75 eV are estimated for the mono-doped and co-doped samples respectively. 47 These values are useful in distinguishing the valence states of the atom. However, the addition of Fe/Cr ions lowers the splitting orbital energy by 0.18 eV. The spectrum of O 1s peak (Figure 6d) is deconvoluted for proper identification. The peaks at 529.39 eV and 530.74 ± 0.3 eV are due to crystalline oxygen and surface-adsorbed oxygen respectively. 44 This surface adsorbed oxygen form surface impurities by bonding to carbon (C-O) or hydrogen (H 2O). Figure 6e depicts the spectrum of the Cr 2p doublet. The peaks at 576.77 eV and 585.93 eV indicate the presence of Cr3+ for the Cr 2p3/2 and Cr 2p1/2 states respectively,48, 49 whereas 575.21 eV and 583.40 eV are associated to Cr4+ oxidation state probably triggered by charge compensation.49 Similarly, the same idea was extended to the Cr 2p core level of the mono-doped sample. The XPS analyses of the samples with Cr content revealed the dominance of Cr 3+ up to 51.2% in Cr3+:BaTiO3 and 59.3% in 4 mol.% Fe3+Cr3+:BaTiO3. The results show that co-doping with Fe increases the content of trivalent chromium. The high resolution XPS spectrum of Fe 2p is shown in Figure 6f. The difference in binding energy between the Fe 2p1/2 and Fe 2p3/2 peak are caused by a spin orbit (jj) coupling in which Fe 2p1/2 has a degeneracy of 2 states while the later has 4.50 The peaks at 710.06 eV and 724.06 eV are typical binding energies of Fe 2p3/2 and Fe 2p1/2 states respectively indicating the


Page 19 of 39

presence of Fe2+, while those at 711.76 eV and 725.76 eV are associated to Fe 3+ of the aforesaid states. The satellite peaks of 2+ and 3+ oxidation states are located at 717.06 eV and 720.06 eV respectively.51, 52

0.4

Ti 2p

0.0

784

782

780

778

Fe4%Cr4%:BaTiO3

O 1s

456

452

536

534

588

(e)

530.74

Fe4%Cr4%:BaTiO3

575.21

725.76

576.77

584

580

576

572

Binding energy (eV)

530

528

526

524

Binding energy (eV)

Cr 2p

583.40

585.93

Fe4%Cr4%:BaTiO3

532

568

730

725

720

Fe 2p 711.76

460

717.06

464

Binding energy (eV)

724.06 720.06

468

774

(d)

456.21

461.35

531.06

458.05

776

Binding energy (eV)

(c)

Fe4%Cr4%:BaTiO3 463.80

778.51

780.18

0.2

Binding energy (keV)

(b)

715

(f) 710.06

0.6

C 1s Ba 4s Ba 4p 3/2

Cr 2p O 1s Ti 2p

Fe 2p

0.8

Ba 3d

Fe4%Cr4%:BaTiO3

529.39

1.0

(a)

Ba 4d

BaTiO3 Cr4%:BaTiO3 Fe4% Cr4%:BaTiO3

Ba 3d 3/2 Ba 3d 5/2 Cr LMM Ba MNN

CPS (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


710

705

Binding energy (eV)

Figure 6: (a) XPS survey spectra of ferroelectric Fe3+Cr3+ co-doped BaTiO3 nanoparticles; High resolution spectra of 4 mol.% Fe content for (b) Ba 3d (c) Ti 2p (d) O 1s (e) Cr 2p (f) Fe 2p elements.



3.3 Photoelectrochemical Measurements In a catalytic redox reaction, one of the major determinants of accelerated photodegradation kinetics is the charge transfer property of the material. Photoelectrochemical measurements are carried out in order to understand the process of charge transfer. The photocurrent vs. time profile recorded from the chopped light amperometry (CLA) under simulated solar light is displayed in Figure 7a. It is observed that the undoped BaTiO3 ferroelectric shows photocurrent responses, which decay with time due to recombination of charge carriers in shallow traps. For the Fe 3+ co-doped samples, the photocurrents are low but stable over time. The low current density is usually a remarkable characteristic of ferroelectrics which is caused by poor bulk dc conduction 53, 54 associated with an extremely low mobility. 1.00

104


(a)

0.50

Dark

102

BaTiO3 Cr4%:BaTiO3 Fe4% Cr4%:BaTiO3 Fe8% Cr4%:BaTiO3 Fit Fit Fit Fit

101

0.25 0.00

(b)

103

ôZô(W)

0.75

0

120

240

360

480

600

100 100

101

Irradiation time (s) 105

102

103

104

105

Frequency (Hz)

(c)

BaTiO3 Cr4%:BaTiO3 Fe4% Cr4%:BaTiO3 Fe8% Cr4%:BaTiO3 Fit Fit Fit Fit

104

Light

ôZô(W)

Norm. photocurrent (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 39

103 102 101 100

10-1

100

101

102

103

104

105

Frequency (Hz) 20 | P a g e ACS Paragon Plus Environment

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Figure 7: (a) Normalized photocurrent response recorded at 0 V vs. Ag/AgCl under simulated solar light for the ferroelectric samples. EIS data presented as Bode plot for the undoped, mono and codoped ferroelectric samples measured in 0.5 M NaSO4 electrolyte under (b) dark and (c) simulated solar light.

Electrochemical impedance spectroscopic (EIS) measurements are also carried out on the samples to complement the study of charge transport properties. The plots of the absolute value of the impedance,│Z│with frequency (i.e Bode plots) measured in the dark and under simulated light irradiation are depicted in Figure 7b and 7c respectively for the samples. In all the cases, the impedance shows a sharp decrease in the frequency region i.e. 100 mHz – 1 kHz. This feature can be used to evaluate the charge transfer process at the electrolyte/semiconductor electrode interface due to the formation of a Helmholtz layer at the boundary. It is observed that there is a variation in both the charge transfer and bulk resistances, obtained under the dark and simulated solar irradiation. Hence, a ZView software is used for the complex non-linear least-squares regression. 55,

56

The

simplified Randles equivalent circuit comprises of a charge transfer resistance, (Rct) which is in series with a parallel connection of bulk resistance (Rbulk) and a bulk capacitance, otherwise known as the constant phase element (CPE). The parameters obtained from the fitting (i.e. both dark and light) show that the change in Rct is negligible for the undoped, Cr3+:BaTiO3, and 8 mol.% Fe3+Cr3+:BaTiO3; whereas it increased reasonably from 4.9 ± 0.08 Ω to 5.4 ± 0.02 Ω for the 4 mol.% Fe 3+Cr3+:BaTiO3 sample. Moreover, a decrease in the Rbulk is observed for all other samples except the Cr3+:BaTiO3 sample. The results are summarized in Table 1. The decrease in the bulk resistance when a positive bias is applied under photoirradiation relates to the dramatic change in charge transport resulting to absorption/desorption of ions across the electrolytic diffuse layer.



Page 22 of 39

Table 1: The charge transfer (Rct) and bulk (Rbulk) resistances obtained from fitting the simplified Randles equivalent circuit. Dark and light are indicated by D* and L* respectively. Sample

Rct (Ω) D*

Rbulk (kΩ) D*

Rct (Ω) L*

Rbulk (kΩ) L*

BaTiO3

5.0

22.3 ± 4.7

5.2

15.8 ± 0.2

Cr4%:BaTiO3

7.3

61.4 ± 5.9

7.6

109.0 ± 19.1

Fe4%Cr4%:BaTiO3

4.9

172.0 ± 40.1

5.4

114.0 ± 54.3

Fe8%Cr4%:BaTiO3

5.6

56.7 ± 2.5

5.4

21.4 ± 0.3

3.4 Photocatalytic Evaluation The oxidation strength of Fe3+Cr3+co-doped BaTiO3 ferroelectrics with apparent absorption edge is examined by photodegradation of a standard organic pollutant, methyl orange under simulated light irradiation. The results of adsorption of methyl orange with the samples in the dark are shown in Figure S4. It is observed that the mono-doped ferroelectric exhibited a better adsorption properties in contrast to the undoped and co-doped ferroelectrics. Methyl orange as well known is an anionic dye, which will likely introduce some surface charge effect on the adsorptivity of the nanoparticles. 57 However, with the ferroelectrics providing two different configuration of charged surfaces, electrostatic interactions are enhanced therefore between the negatively charged dye molecules and positively screened charged surface of the ferroelectric material. Surfaces bearing charges opposite to that of the dye molecules results in low adsorption due to repulsion and as a consequence manifests in retarded photocatalytic degradation. Hence, the adsorption capacity of the dye molecules can be explained in terms of the microstructural properties of the materials. In relation to Cr 3+-doped BaTiO3, the morphology provides increased anisotropic surface, which favours the electrostatic attraction of the dye molecules. For comparison, photoactivity of undoped BaTiO 3 ferroelectric is equally measured. Figure 8a shows the time-dependent photodegradation of the photocatalysts. The 22 | P a g e ACS Paragon Plus Environment

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degradation efficiency is evaluated by calculating the ratio of photovoltage equivalent of intensity at any irradiation time relative to the photovoltage of intensity of initial concentration, Co as expressed in Equation 1. From the plot, no meaningful photodegradation nor photobleaching of the organic dye is observed during the light irradiation without catalyst. In the dark, similar observation is equally noted with methyl orange in the presence of Fe3+Cr3+:BaTiO3 ferroelectric. During the light irradiation, the photodegradation efficiency for methyl orange initially increased, then decreased with increasing Fe3+ concentration in Fe3+Cr3+:BaTiO3 ferroelectrics. The best photodegradation efficiency is obtained with Fe3+Cr3+:BaTiO3 (4 mol.%) as shown in Figure 8b. After 90 min exposure of light irradiation, 52%, 73%, 94% and 88% of methyl orange are photodegraded using BaTiO 3, Cr4%:BaTiO3, Fe4%Cr4%:BaTiO3 and Fe8%Cr4%:BaTiO3 respectively. It is interesting to know that all the doped samples show strong photoactivity in the visible region, which is an indication of the transition of photogenerated electrons from charge transfer centers into the conduction band. The enhanced degradation efficiency could be due to the following factors (i) the built-in electric field in the ferroelectric which assists in charge carrier separation so that more electrons and holes can partake in the redox reaction (see inset of Figure 8b); (ii) improved visible light absorption due to apparent absorption edge of the co-doped ferroelectrics; (iii) increased adsorption due to enhanced surface-tovolume ratio and grain size effect. Hence, the photogenerated electrons and holes can easily migrate from the interior of the ferroelectrics to the surface where they can oxidize the surrounding pollutant. Despite the increased visible light utilization by the high Fe 3+ co-doped ferroelectrics, a further increase of iron content does not improve the photocatalytic performance, even a small reduction is observed. It is understood that the ferroelectric with high concentration of Fe 3+ will be disadvantageous for improving the photocatalytic activity because of low surface charges available due to a loss of tetragonality in the material and an excessive amount of recombination sites. Notably,



Page 24 of 39

the half-filled d-orbital configuration of the used transition metals offers another advantage as they transfer the trapped electrons to oxygen adsorbed on the surface of the photocatalyst. This plays an important role in the transfer and separation of charge carriers associated with shallow traps thereby increasing the degradation efficiency. Again, the existence of the bulk photovoltaic effect in ferroelectric Fe3+Cr3+:BaTiO3 is an important complementary factor. The bulk photovoltaic effect promotes the separation of charge carriers, primarily towards the charge surfaces but not limited to it as the bulk photovoltaic tensor in BaTiO3 has off-axis elements. The effect is however challenging to quantify from the experiment, however a qualitative analysis could suffice. In this regard, we rely on the spectroscopic results to provide insight on its effect on the photocatalytic activity. Also, in all the ferroelectric materials used for photocatalytic measurements, it is observed they follow pseudo-first order kinetics for liquid phase heterogeneous photocatalysis. This can be explained by Langmuir-Hinshelwood model58, 59 according to Equation 2: 𝑘

(2)

𝑡 = −𝑙𝑛

where C0 is the initial molar concentration of dye solution, Ct is the molar concentration of the dye solution at any time, t, and kobs corresponds to the observed reaction rate constant. A quantitative analysis of the kinetics is made from the fitted linear plots in Figure 8c and Fe3+Cr3+:BaTiO3 (4 mol.%) yielded a reaction rate, kobs  0.0303 min-1 which is almost 3.7 times higher than the undoped BaTiO3 ferroelectric (Figure 8d). To investigate the role of reactive oxygen species (ROS) in the degradation of methyl orange over ferroelectric Fe3+Cr3+:BaTiO3 nanoparticles, scavenger experiments are conducted by adding scavengers or inhibitors to the photodegradation system. To this effect, scavengers such as such BQ, TBA, and EDTA are used to scavenge the following radicals; O •, h , and OH • respectively. As the 4 mol.% Fe3+Cr3+:BaTiO3 nanoparticles exhibited the best photoactivity, the time evolution of the 24 | P a g e ACS Paragon Plus Environment

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photocatalytic degradation of methyl orange in the presence of the scavengers are shown in Figure 8e. When BQ is added to the photocatalytic system, a notable reduction in the degradation efficiency is observed. The discrepancy in the efficiency as compared to that without scavenger suggests the inhibiting role of BQ. This implies that a reasonable fraction of the produced superoxides are suppressed and this impacts on the overall degradation of organic dye. On the other hand, the kinetics of the photodegradation slows down slightly in the presence of TBA yielding an efficiency of 65 ± 7% after 90 min. It suffices that though holes may contribute to the photoactivity, their influence could be minimal. Upon the addition of EDTA, the kinetics retarded within the first 60 min. This could be attributed to slow generation of OH • radicals from adduct of protonated O • . Therefore, under this circumstance, the probability of hydroxyl radicals participating in the redox reaction depends on the yield culminating from dissociated H2O2. For a catalytic redox reaction to be initiated, the redox pairs’ energies of the target molecules should lie between the potential energies of the semiconductor material. The potential energies constitute the conduction band minimum (CBM) and valence band maximum (VBM). Hence, the conduction (𝑉 ) and valence (𝑉 ) band potentials are calculated from the following equations 60,

where 𝑉

𝑉

= 𝜒 − 𝑉 − 0.5𝐸

(3)

𝑉

= 𝐸 −𝑉

(4)

and 𝑉

are the conduction band potentials respectively, 𝜒 denotes the electronegativity,

𝑉 is potential of free electron on normal hydrogen energy (NHE) scale (4.50 eV) 60, and 𝐸 is the apparent band gap energy of the codoped material. The apparent band gap of the material (i.e. 2.3 eV) is extrapolated from the extra absorption introduced by charge transfer centers of the transition metals. The electronegativity of the codoped ferroelectric is determined using the geometric mean 25 | P a g e ACS Paragon Plus Environment


Page 26 of 39

electronegativity of the individual elements composing the material. By fitting the XPS data of 4 mol.% Fe3+Cr3+:BaTiO3, the molar ratio is calculated from the atomic concentration (%) as summarized in Table S1. It is important to note that surface adsorbed oxygen is not considered in the determination of molar ratio. According to Equation 3 and 4, the 𝑉

and 𝑉

are estimated to be -

0.35 eV and 1.95 eV respectively. Considering the redox potentials of O /O

•

(-0.16 eV)61, and

H O/OH • (2.32 eV)61 redox couples respectively, the results show that the photocatalytic process is favoured when cascades of superoxides are generated. This stems from the fact that the 𝑉 doped lattice is at a higher potential energy compared to the redox potential of O /O 0.16 eV). However, the conclusion does not apply to 𝑉

of the co•

species (-

as it is lower in energy scale when compared

to H O/OH • species (2.32 eV). Furthermore, the valence band potential (1.95 eV) is higher than the potential energy of H O /OH • (0.89 eV)62 redox couple suggesting that the yield of OH • radical is possibly through the dissociation of hydrogen peroxide, which is consistent with the ROS experiments. Based on this, a tentative schematic illustrating the photocatalytic mechanism is depicted in Figure 8f. Figure 8g shows the variation of degradation efficiency as a function of photocatalyst loading. The degradation efficiency converges at photocatalyst loading of 150 mg/L meaning that the optimum performance of the ferroelectric is maintained at the aforesaid catalyst dosage. Therefore, any increase in the catalyst loading would probably result to terminal reactions as presented in Equation 5 and 6.


1.2

(a)

1.0

(Ct/C0)

0.8 0.6 MO BaTiO3 Cr4%:BaTiO3 Fe4% Cr4%:BaTiO3 Fe8% Cr4%:BaTiO3

0.4 0.2 0.0

0

20

40

60

80

O e

100

2

O

60

(b)

O

52

40 20 0

100

BaTiO3

0

4

(d)

30.3

~ 4´

30

8

Fe content (mol.%)

35

1

89

73

O

O h+

(c)


94

O

80

kobs x10-3 (min-1)

ln(Co/Ct)

3

O Ti

Irradiation time (min)

25.4

25

21.5

20 15

7.8

10 5

0

0

60

Irradiation time (min)

(e)

1.0

0.6 0.4 0.2 0.0

0

20

40

60

80

100

(f)

-1

96

94

92

50

100

150

200

250

300

350

8

CO2 + H2O CB

0

Ti 3d

1 2 3

O 2p

CB

VB

VB

[i] Degradation efficiency (%)

(g)

4

Fe content (mol.%)

Irradiation time (min) 98

0

BaTiO3

100

No scavenger BQ EDTA TBA

0.8

(Ct/C0)

80

-

-

-

-

e e e e

-

e e-

-

e- e-

h+ h+ h+ h+

+ + hv + + h+ h+ + h h+

[ii]

[iii]

(2.3 eV)

40

(3.2 eV)

20

Band energy in NHE (eV)

0

Degradation efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Degradation efficiency (%)

Page 27 of 39

Dye CB

O2/O-2 ·

PS

H2O2/OH·

VB

H2O/OH·

- + Surface localized charges

100 (h) h1

h = -2.9%

h4

80 60 40 20 0

Photocatalyst loading (mg)

1

2

3

4

No. of cycles



Page 28 of 39

Figure 8. (a) Time dependent photocatalytic activities. (b) Photocatalytic degradation of methyl orange using the co-doped ferroelectrics. Inset of Figure 8b shows that the effect of screening of surface charges influence photodregradation. (c) Plot of ln(C0/Ct) versus irradiation time for determination of rate of photodegradation. (d) The kinetics of photodegradation of methyl orange over co-doped ferroelectrics. (e) The photocatalytic degradation with different scavengers. (f) The proposed charge transfer mechanism reflecting the band edge potentials of [i] undoped [ii] co-doped BaTiO3 ferroelectric (4 mol.%) [iii] photoexcited state compared with the redox potential of O /O

•

and H O/OH • . (g) Variation of degradation efficiency with photocatalyst loading. (h) Recyclability test of ferroelectric Fe3+Cr3+:BaTiO3 nanoparticles.

However, in higher concentration of the dissolved oxygen, it leads to reduction of the photodegradation rate as TiO2 surface terminated becomes highly hydroxylated and this limits the adsorption of organic waste molecules on the active sites. 59 OH ⦁ + OH ⦁ → H O

(5)

H O + OH ⦁ → H O + HO⦁

(6)

Basically, in photocatalytic reactions involving semiconductor materials, recyclability and photostability are important factors for its industrial application. To ensure that the as-synthesized ferroelectric Fe3+Cr3+:BaTiO3 nanoparticles can be recycled, four series of sample reactions are conducted by reusing the Fe3+Cr3+:BaTiO3 (4 mol.%). Between these cycles, the catalyst is rinsed with distilled water. As observed in Figure 8h, a decrease in degradation efficiency of ~ 2.9% was recorded after four complete cycles. This shows that the sample maintains a good photostability up to four series of photocatalytic reactions.


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4.0 CONCLUSION In summary, ferroelectrics Fe3+Cr3+ co-doped BaTiO3 nanoparticles with different Fe3+ concentrations are successfully prepared by an microwave-assisted hydrothermal method. We see a substantial improvement of the photocatalytic activity of 21% compared to chromium doping alone and of 42% compared to the undoped barium titanate. The effect of Fe 3+ ion on ferroelectric response and kinetics of photocatalytic activity of the co-doped ferroelectrics have been studied by depollution of methyl orange under visible light irradiation. An XRD study confirms a loss of tetragonality for iron doping above 4%. The best photocatalytic activity with about 42% enhancement compared to pure barium titanate is obtained with 4 mol.% for both iron and chromium in Fe 3+Cr3+:BaTiO3 ferroelectrics, which is attributed to an additional absorption band. It shows a recovery of degradation time compared to undoped BaTiO3, which required about 120 min or more. The photocatalyst demonstrates a very good stability after 4 consecutive, cycles, which is favourable from a commercial viewpoint. Therefore, the microwave hydrothermal method would be an efficient technology for the synthesis of ferroelectrics with better physicochemical and enhanced photocatalytic properties.

SUPPORTING INFORMATION Particle size distribution, Energy dispersive X-ray spectra, High resolution XPS spectra, Table for calculated band potentials, Adsorption capacity in the dark, and Schematic of optical absorption spectra.



Page 30 of 39

AUTHOR INFORMATION Corresponding Author [email protected]

ORCID Ifeanyichukwu C. Amaechi: 0000-0002-8569-4114 Azza Hadj Youssef: 0000-0001-6715-3923 Jérôme P. Claverie: 0000-0002-8569-4114 Shuhui Sun: 0000-0002-0508-2944 Andreas Ruediger: 0000-0003-0815-5288

ACKNOWLEDGEMENTS I.C.A. and A.H.Y. acknowledge the financial supports through MATECSS Excellence and TunisiaINRS scholarships respectively. S.S. and A.R. are grateful for the NSERC discovery grants (RGPIN2014-05024).


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