n-PbTiO3 Heterojunction

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

Stupendous Photocatalytic Activity of p-BiOI/n-PbTiO Heterojunction: The Significant Role of Oxygen Vacancies and Interface Coupling Lekha Paramanik, K. Hemalata Reddy, and Kulamani Parida J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05747 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

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The Journal of Physical Chemistry

Stupendous photocatalytic activity of p-BiOI/n-PbTiO3 heterojunction: the significant role of oxygen vacancies and interface coupling Lekha Paramanik, K. Hemalata Reddy and K.M. Parida* Centre for Nanoscience and Nanotechnology, SOA (Deemed to be University), Bhubaneswar-751030, Odisha (India)

*Corresponding

author

E-mail: [email protected] & [email protected] Tel. No. +91-674-2379425, Fax. +91-6 74-2581637

1 ACS Paragon Plus Environment

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Abstract A fusion of oxygen vacancies and p-n heterojunction is exploited to pinpoint imperative procedure for effective minimization of charge recombination and enhanced photocatalytic activity. Herein, the oxygen vacancy-rich BiOI/PbTiO3 p-n heterojunction have been architected by a facile precipitation-deposition method. The synergistic interaction between p-BiOI and nPbTiO3 with abundant oxygen vacancies was confirmed from the HRTEM and XPS analysis. Enrichment of oxygen vacancies in BiOI/PbTiO3 p-n heterojunction was further verified from the ESR test. The concept of combining effect of p-n heterojunction and oxygen vacancies was effectively modulated the physicochemical, electronic and catalytic properties of the photocatalyst simultaneously. The BiOI/PbTiO3 p-n heterojunction with rich oxygen vacancies under optimum concentration showed superior antibiotic tetracycline degradation i.e. 94% in 2h under solar harvest. Besides, the improved activity of p-n heterojunction is attributed to the presence of inner electric field and oxygen vacancies that serve a new channel for the transformation of photoelectrons from BiOI to PbTiO3, results to an effective decrement of charge recombination process, confirmed by the PL, EIS, photocurrent and bode analysis.


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1. Introduction To triumph over the issues of high charge recombination process, limited light absorption capability and constrained photocatalytic activity of a single semiconductor, the erection of a heterojunction between couple of dissimilar semiconductors has been achieved marvelous interest over one decade.1,2 In particular, the strategy to construct a p-n heterojunction between two semiconductors is considered to be one of the smart approaches to enhance the photocatalytic performance due to the efficient charge carrier separation through an internal electric field across the p-n junction.3,4 Moreover, some recent investigations evident that the development of oxygen vacancy rich p-n junctions are more fruitful to promote the photocatalytic reactions in comparison to the traditional p-n junctions.5 To date, the construction of p-n junction materials with rich oxygen vacancies are still in the babyhood stage. In this aspect, BiOI has emerged as a visible-light-active p-type semiconductor, owing to its small band gap energy (1.7~1.9 eV) and unique crystal structure.6 BiOI crystallizes in tetragonal Matlockite structure of Sillen family. It possess an open, layered crystal structure consisting of [I–Bi–O–Bi–I] sheets attached to each other by non-bonding Vander Waal forces through the I atom in the direction of c-axis. In a single [I–Bi–O–Bi–I] sheet, each Bi atom is surrounded by four O and four I atoms, to arrange in an asymmetric decahedral structure. These distinctive interleaved layer crystal lattice structures are favorable to impart an internal static electric field spontaneously that aids efficient anti-charge recombination process and broaden the photocatalytic prospective.7-9 Nonetheless, the reported photocatalytic efficiencies are disreputably low for practical purposes. Very recently, Fan.et al. reported high efficiency by the oxygen defective BiOI towards photocatalytic formaldehyde gas degradation.10 The presence of oxygen vacancy states in BiOI can dramatically trap the directly excited photoelectrons from the



conduction band leading to an improved donor density by the introduction of oxygen vacancies onto BiOI.11 However, most of the researchers have neglected the role of oxygen vacancies in the creation of a built-in electric field in BiOI based p-n heterojunctions, whenever these oxygen vacancies are playing a good impact on the improvement of their photocatalytic performance. On the other hand, lead titanate is a classical n-type visible-light-responsive (VLR) perovskite material with a narrow band gap energy of 2.75eV.12 The attractiveness of this perovskite material is its ferroelectric property with visible light harvesting capacity. The ferroelectric nature of PbTiO3 acquires a dipolar internal fields that force the photo-generated charge carriers to travel in reverse directions, that advance the charge anti-recombination rate and thus leading to improve the photocatalytic activities.13 However, this PbTiO3 participating in the heterojunction samples have shown more advanced results in comparison to the individual one. Since BiOI is a p-type semiconductor with oxygen vacancies and PbTiO3 is an n-type semiconductor, the coupling of BiOI with PbTiO3 gives a advanced type p-n heterojunction with rich oxygen vacancies to foster the interfacial charge transfer and results to significantly boost the photocatalytic performance. Moreover, this type of p-n heterojunction yet had not been reported so far for photocatalytic degradation of TC-HCl. In this work, the oxygen vacancy rich BiOI/PbTiO3 p-n junctions were fabricated through a precipitation-deposition route with a strategy to accelerate the anti-charge recombination process and achieve more accessible sites for photocatalytic reaction. The work illustrated that combining merits of oxygen vacancies and p-n junction is an advanced strategy for abundant channelization of electrons from BiOI to PbTiO3 through the oxygen vacancy states, to explore the higher degradation efficiency of tetracycline in comparison to the pristine one. The effect of promoted oxygen vacancies tailors efficient mobility of charge carriers across the heterojunction


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boundary was confirmed by the EIS, PL, bode plot and photocurrent results. Thus, this piece of work may afford a new emerging opportunity and inspiration for the design and development of p-n junctions with rich oxygen vacancies in the field of photocatalysis. 2. Experimental section Fabrication of PbTiO3 crystal Pure phase PbTiO3 was synthesized through a simple combustion process. All the chemicals used for the work was of analytical grade. Pb(CH3COO)2, TiO2 and citric acid (C6H8O7) have been used as the starting materials, where the C6H8O7 served as a fuel for the reaction system. In a synthetic procedure, the starting materials such as Pb(CH3COO)2, TiO2 and C6H8O7 were added to the deionized water maintaining a fixed molar ratio i.e.,1:1:2, respectively. The entire molar amount of metal ions was equal to the molar amount of fuel, i.e., 1:1 in the solution. The mixed solution was stirred continuously to acquire a well dispersed suspension followed by evaporation from 80°C to 130°C to get a thick viscous gel. The viscous gel was then allowed to cool down at room temperature to form dried crust type material. The dried crust material was grounded to powder and activated in a muffle furnace at 800°C for 4 h.13,14 Fabrication of BOI/PT p-n junction The BOI/PT p-n junction materials were fabricated by using a precipitation-deposition method. A certain quantity of Bi(NO3)3·5H2O was first dissolved completely in 100 mL of deionized water having glacial acetic acid (10 mL) and then stirred for 15 minutes to attain a crystal clear solution. Afterwards, the fabricated single phase PbTiO3 crystals were introduced into the above clear solution at room temperature with successive sonication and stirring for 30minutes. The content of Bi(NO3)3·5H2O was varied in a molar ratio of 10%, 20%, 30% and 40%, named as 5 ACS Paragon Plus Environment


10% BOI /PT, 20% BOI /PT, 30% BOI /PT and 40% BOI /PT p-n junction photacalysts, respectively later on. The obtained solution was added rapidly to the 30 mL of aqueous KI solution. On adding, a reddish white precipitation was formed immediately. The obtained precipitate was sonicated, stirred continuously for another 30 minutes and later it was aged for 3h at room temperature. The formed precipitate was centrifuged, washed thoroughly with deionised water and air dried at 65°C for a night. The BiOI nanosheets were synthsized following the same route without using PbTiO3 precursor. 15 Characterization The powder X-ray diffraction (PXRD) of the samples was examined through Rigaku Ultima IV diffractometer with automatic control operated under the voltage of 40kV and current of 40mA, using monochromatic Cu-Kɑ X-ray radiation source at wavelength 1.5 Å in range of 2θ=10-80°, with a scan rate of 5°min-1. The morphology of the prepared samples was examined using scanning electron microscopy (SEM) (HITACHI S-3400N instrument with an acceleration voltage of 20kV) and high-Resolution transmission electron microscopy (HRTEM) was obtained on Philips TECNAI G2 instrument with an acceleration voltage 200 kV. The Fourier Transform Infrared Spectra (FTIR) was performed on FT/IT-4600 spectrometer for the prepared samples in the range of 400-4000 cm-1 embedded in KBr pellets. The UV-Vis diffused reflectance spectra was acquired using JASCO 750 UV-visible spectrophotometer while the emission spectrum was obtained by using spectrofluorometer (JASCO FP8300) with an excitation wavelength 350 nm. XPS measurements was examined on ESCA+ spectrometer instrument equipped with monochromatised Aluminium source (Al-Ka radiation hυ =1486.7ev). The binding energy rectification was executed by considering carbon C1s peak at 284.6 eV as reference. A JESFA200, ESR spectrometer was used to detect ESR signals of the samples at room temperature. 6 ACS Paragon Plus Environment

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Photoelectrochemical Analysis Photoelectrochemical (PEC) performances were conducted in a three electrode configuration system at room temperature on IVIUM n STAT electrochemical workstation. The prepared samples was deposited on Fluorine doped tin oxide(FTO) glass substrate that served as working electrode, while the Platinum wire and the commercially available Ag/AgCl electrode were counter and reference electrodes, respectively in the electrochemical studies. The working electrode was prepared by electrophoretic deposition method. The sample coated area on the working electrode was 1cm x 1cm and 0.5 M Na2SO4 aqueous solution having pH=6.9 served as electrolyte. Newport 300W Xenon lamp equipped with a UV-light cut off filter (λ≥ 400nm) behaved as visible light source. It was noticed, the neat FTO doesn’t shows any light response. The photocurrent measurements were obtained using a 300W Xenon lamp with a cut-off filter (wavelength λ≥ 400 nm). A baising of 0.2V was applied to study the photocurrent transient responses. Electrochemical impedance spectroscopy measurements was carried out at zero bias potential in a frequency range from 100 Hz to 0.01 Hz under open circuit reaction condition. Mott-Schottky analysis was examined with an applied AC voltage of 25 mV at a frequency of 500Hz. Photocatalytic Activity Degradation of tetracycline antibiotic The photocatalytic performance of the prepared sample was tested for the degradation of organic compound (tetracycline hydrochloride) under direct solar radiation. Initially, a suspension containing photocatalyst (20 mg) and 20ppm of aqueous solution of tetracycline (20 mL) was magnetically stirred for 30 minutes in the dark condition to attain adsorption-desorption 7 ACS Paragon Plus Environment


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equilibrium between the tetracycline molecules and photocatalysts. The above suspension was then kept under solar light radiation with continuous stirring (slowly) for a time duration of 2 h. The sample solution was separated by centrifugation. The concentration of TC-HCL solution was monitored by a UV-Vis spectrophotometer. Blank experiment was performed without using any photocatalysts. Active-species trapping tests was performed by the introducing varied quenching agents. Determination of point of zero charge (pHpzc) The pHpzc determination involves the preparation NaCl solution (0.005 M) free of dissolved CO2 . Then pH of NaCl solution was adjusted to 1, 3, 5, and 7 by the addition of HCl (0.5 M) and NaOH (0.5 M). In the next step, 0.02 g of 20% BOI/PT photocatalyst was added to the 20 mL of pH adjusted solutions followed by continuous stirring for 24 h to attain sorption equilibrium. At last, the final solution pH was plotted against the initial pH. 3. Results and discussion

Bi(NO3)3.5H2O + CH3COOH(aq)

PbTiO3

Stirred,30min Ultrasonicated

Bi(NO3)3.5H2O + CH3COOH(aq) + PbTiO3

Stirried,30min Ultrasonicated

Suspension

Aging,3h

Centrifuged, Washed, Dried 65°C,12h

BOI/PT sample

Scheme1. Schematic illustration of BOI/PT p-n junction fabrication 8 ACS Paragon Plus Environment

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The oxygen vacancies rich BOI/PT p-n junctions were architected by a simple precipitation-deposition method. The detail synthetic route followed for said materials have been illustrated in Scheme 1. At first, the Bi(NO3)3 was dissolved in aqueous acidic solution to get a clear homogenous bismuth based solution. On addition of PbTiO3 and followed by KI to the bismuth based solution, leads to the in-situ formation of BOI/PT p-n junctions with rich oxygen vacancies. The oxygen vacancies rich in BOI/PT p-n junction sample was initially confirmed by the XPS measurement. Figure. S1 represents the survey spectrum of bare BiOI and BiOI/PbTiO3 heterojunction samples, confirming the presence of all elements i.e. Pb4f, Ti2p, O1s, Bi4f and I3d in the respective BiOI and BiOI/PbTiO3 samples. Each elemental peaks of BiOI and BiOI/PbTiO3 samples in the spectrum was finely fitted with the XPS software (CASA) as shown in the Figure.1. The high resolution XPS spectra of Pb4f, Ti2p, O1s, Bi4f and I3d for BiOI/PbTiO3 p-n heterojunction are shown in Figure. 1 (a-e). The p-n junction material showed a doublet peaks owing to its spin orbit coupling feature at the binding energies of 138.0 eV and 142.8 eV for Pb4f7/2 and Pb4f5/2, respectively, which characterizes the presence of +2 oxidation state of lead.16 In addition, the spin-orbit doublet spectrum of Ti2p in heterojunction sample having the binding energies at 457.9 and 465.2 eV, respectively, verifies the core levels of Ti2p3/2 and Ti2p1/2 states of Ti4+.16 The characteristics doublet peaks corresponding to the binding energies for Bi4f5/2 and Bi4f7/2 of Bi3+; and for I3d5/2 and I3d3/2 of I- were detected at 159.2 and 164.5 eV; and 618.8 and 630.2 eV, respectively in BOI/PT material.17 Furthermore, the O1s spectrum was deconvoluted into two asymmetric peaks. The peak residing at lower binding energy attributes to the lattice oxygen related to Ti-O, Pb-O and Bi-O in [BiO2]2+ slabs.18 It is well known that the higher binding energy peak was attributed to the O atoms in vicinity of oxygen vacancies in the BOI/PT materials. The specific ratios of each element of 20% BOI/PT



material was found out to be 8.92 : 19.23 : 55.15 : 5.79 : 10.91:: Pb : Ti : O : I : Bi respectively. For comparison study, the XPS spectra of pure BiOI were also taken. In general, the intensity of surface active oxygen species raises with induced more number of oxygen vacancies, which is quite significant for the photocatalysis.19 Accordingly, the peak intensity/area of surface active oxygen in the synthesized BOI/ PT material have been increased compared to the neat BiOI, clearly suggesting that some oxygen vacancies have been successfully introduced in the p-n junction material by the in-situ growth of OVs-BiOI nanosheets on PbTiO3.19 Moreover, the shifting behavior of Bi4f, I3d and O1s (0.2eV) to higher binding energy in BOI/PT compared to BiOI, suggested that there is a strong interaction taking place between BiOI and PbTiO3 due to formation of p-n junction and enrichment of oxygen vacancies.


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4f 7/2 138.0 eV

(a)

Pb4f BOI/PT

Intensity(a.u.)

140

145

455

Binding energy(eV)

(c)

Ti2p

2p 3/2 457.9eV

(b)

2p 1/2 465.2eV

BOI/PT

Intensity(a.u.)

4f 5/2 142.8eV

135

Bi4f BOI/PT

4f 5/2 159.2eV

4f 7/2 164.5eV

460

465

470

Binding energy(eV)

(d)

I 3d

3d 5/2 618.8eV

BOI/PT

(e)

O1S BOI/PT

529.7 eV

3d 3/2 630.2eV

BiOI 164.3eV

618.6eV

BiOI 630.0eV

Intensity (a.u.)

159.0eV

Intensity (a.u.)

531.8 eV

Intensity(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


529.5 eV

BiOI

531.6 eV

160

165

615

Binding energy(ev)

620

625

630

Binding energy(eV)

635

525

530

535

Binding energy(eV)

Figure 1 High resolution XPS spectra of 20% BOI/PT in comparision of BiOI in the region of: (a) Pb4f, (b) Ti2p, (c) Bi4f, (d) I3d and (e) O1s Furthermore, the electron spin resonance (ESR) spectroscopy was adopted to provide confirmative evidence for the occurrence of oxygen vacancies in the materials. ESR analysis not only identify deficiencies through a direct and insightful way, but also authenticated the free radicals or paramagnetic centers generated under various environmental conditions.5 As shown in Figure. 2, the symmetrical peaks at about g~2.001, ascribed to the formation of oxygen vacancies, have been augmented in the BOI/PT p-n junction in comparision to pristine BiOI.19 This is because, the coupling of BiOI with PbTiO3 through the strong interfacial interaction



results in the redistribution of electrons and damage the surface structures, thereby could promote the generation and separation of electron-hole pairs through the interfaces between BiOI and PbTiO3 which causes creation of more oxygen vacancies in the heterojunction material. 20-22 These ESR results further provides the prerequisite for investigating the effect of rich oxygen vacancies on the photocatalytic performance.23,5 20% BOI/PT BiOI

Intensity (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

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g  2.001

200

400

600

Magnetic field (mT)

Figure 2 ESR spectra of 20% BOI/PT and BiOI The morphology of the prepared samples was demonstrated by scanning electron microscope (SEM) and transmission electron microscopy (TEM). Pristine PbTiO3 exhibited irregular, aggregated non-uniform particles (Figure. S2 (a)). In contrast, a dozen of thin plates with flaky like nanosheets that have a tendency to get assembled into microflower like morphology have been visualized for BiOI, Figure. S2(b). The SEM micrograph of 20% BOI/PT is shown in Figure. S2 (c). The figure displayed that the irregular grain particles of PbTiO3 staked with thin nanosheets of BiOI. A further insight into the interior structure of pristine BiOI, PbTiO3 and oxygen vacancy rich BOI/PT p-n junction, the TEM analysis was performed. As illustrated in Figure. S3, the PbTiO3 appeared as irregular polyhedron shaped particles with 0.28 12 ACS Paragon Plus Environment

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nm lattice spacing corresponding to the (101) crystallographic plane. The SAED pattern demonstrate the (100) and (110) planes of single crystalline PbTiO3. On the other hand, the thin nanosheets of BiOI having lattice spacing 0.28 nm was matched with its (110) plane in the micrograph. The SAED pattern illustrates the single crystalline nature with indexing (110) and (200) planes of BiOI (Figure. S4). On coupling, the TEM image of p-n junction sample showed smooth edge nanosheets of BiOI with transparent feature overlapped with dark polyhedron patches of PbTiO3(Figure. 3(a)). Besides, the fringes with interplanar spacing of BiOI and PbTiO3 were also observed as shown in Figure. 3(b). The lattice spacing observed at 0.30 nm and 0.28 nm are indexed according to the (102) and (101) planes of BiOI and PbTiO3, respectively. The overlapping of lattice fringes (circled in yellow dash line) was again verified the formation of BOI/PT p-n junction. However, the interface region appeared very dim and disordered illustrating the surface structures have been damaged and the oxygen vacancies disorder are probably formed by the in-situ growth of OVs-BiOI nanosheets on PbTiO3.5 The SAED pattern of p-n junction shows polycrystalline in nature as shown in Figure. 3(c). The figure consists of two sets of diffraction spots i.e. (100) and (101) planes of PbTiO3 and (110) and (103) planes of BiOI, respectively. It again reveals the coexistence of both crystal phases in the sample. This result is well coincides with the XRD data (discussed in the later section). Further, the dark field (Figure. 3(d)) and HAADF image (Figure. S5 (a-c)) analysis verified that the BiOI nanosheets are well assembled on the PbTiO3. The glowing particles in the dark field image were ascribed to PbTiO3 over which nanosheets of BiOI (dark shaded portion) have grown-up to architect a p-n junction between them. The formed p-n junction structure could have the advantage for the separation and migration of photo-induced charge carriers for a better photocatalytic performance. The result from HAADF images corresponding to elemental mapping evaluated the



co-existence of BiOI and PbTiO3 in the p-n junction sample. The colorful image represented the uniform distribution of Pb, Ti, O, Bi and I elements in 20% BOI/PT material, following the sketch of TEM result concluded that BiOI nanosheets were successfully combined with the PbTiO3.

(a)


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(b) PbTiO3 d(101)= 0.28 nm

BiOI d(102)= 0.30 nm

BiOI d(102)= 0.30 nm

(c)



(d)

Figure 3 (a) TEM, (b) HRTEM, (c) SAED pattern and (d) dark field image of 20% BOI/PT p-n junction. To achieve an insight molecular organization in terms of phase purity and crystal structure, the XRD analysis was performed. Figure. 4 shows the X-ray diffraction pattern of pristine PbTiO3, BiOI and BiOI/PbTiO3 heterojunction with varying BiOI content. The peak position and relative intensity of all the diffraction patterns of PbTiO3 and BiOI were perfectly indexed to the tetragonal phase of both PbTiO3 and BiOI. The obtained results are well consistent with the JCPDS data such as file no.01-075-0438(space group P4/mmm) and 00-0100445(space group P4/nmm), respectively. The diffraction patterns of BOI/PT p-n junction samples were indexed to those of BiOI and PbTiO3 crystals without any other detectable impurity peaks. This indicates that the fabricated materials have in pure form. More importantly, 16 ACS Paragon Plus Environment

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the diffraction peaks intensities belonging to the tetragonal BiOI phase gradually tend to intensified and sharpen whereas the diffraction peak intensities of tetragonal phase PbTiO3 becomes weaker and wider with increasing the loading amount of BiOI in BOI/PT materials from 10% to 40%.15 As a result, the signature diffraction peak intensity ratio of I PbTiO3(101)

BiOI(102)/I

between the two materials steadily enhances with enhancing the BiOI content in the

BOI/PT and vice versa (Figure. S5(b)). The shifting of signature diffraction peaks in BOI/PT materials, i.e., (101) plane of PbTiO3 and (102) plane of BOI in opposite direction was observed in Figure. S5(a), reflecting that a proper synergistic interaction have ensue between PbTiO3 and BiOI nanosheets with promoted oxygen vacncies.19,15



Relative Intensity (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


 

 

10





20





     



 

30

40



 

BiOI 40% BOI/PT 30% BOI/PT 20% BOI/PT 10% BOI/PT PbTiO3

      

   

50

   

60

70

80

2 (degree)

Figure 4 XRD patterns of PbTiO3 (PT), 10% BOI/PT, 20% BOI/PT, 30% BOI/PT, 40% BOI/PT and BiOI (BOI).



In addition, the unchanged backbones of PbTiO3 and BiOI in BOI/PT p-n junction were further verified by FTIR spectra (Figure. 5). The PbTiO3 shows its characteristic peaks in the range of 400-900cm-1. The absorption peaks located at 580cm-1 and 717cm-1 were assigned to metaloxide, υ(M-O) bonds in the perovskite matrix which was ascribed to [TiO6]2- octahedral unit.24,25 With regards to the pristine BiOI nanosheets, absorption band range in 400-700cm-1 was indexed to the various stretching vibration of Bi-O-Bi, Bi-O-I and Bi-O bonds in BiOI lattice.26 The peak located at 497cm-1 originates from symmetric A2u-type vibration of Bi-O bond, whereas the absorption peak at 737cm-1 belongs to the asymmetric stretching vibration of Bi-O bond along with the presence of another characteristic peak at 1345cm-1, consistent with published results.17,18,27 After fabrication of BiOI nanosheets with PbTiO3 crystal lattice, the characteristic peak emerged from the combination of Ti-O vibration (717cm-1) and Bi-O asymmetric vibration (737cm-1) has been slightly blue shifted indicating BiOI with oxygen vacancies has been coupled with the PbTiO3 matrix in p-n junction materials. 24,28 In addition, the absorption peaks located at 1610cm-1 and broad peak around 3100-3416cm-1were assigned to the δ(O−H) bending and ν(O−H) stretching vibration, respectively that aroused because of free water molecules adsorbed on the photocatalyst surface. 29


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497

737

1025

1610

2915 2848

1345 BiOI 40%BOI/PT 30%BOI/PT 20%BOI/PT 10%BOI/PT PbTiO3

717 580

Transmittance (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


3416

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4000 3500 3000 2500 2000 1500 1000

500

Wavenumber(cm -1)

Figure 5 FTIR spectra of PbTiO3 (PT), 10% BOI/PT, 20% BOI/PT, 30% BOI/PT, 40% BOI/PT and BiOI (BOI). To explore the effect of rich oxygen vacancies in BOI/PT p-n junction on the photocatalytic performance, its physicochemical features were speculated under various photoabsorption processes, such as; (i) the light absorption capability to produce photogenerated charge carriers, (ii) efficient charge carriers separation and migration, and (iii) the catalytic reaction on the photocatalyst surface. First and foremost, the light absorption potential of all the as-synthesized materials was investigated from UV-visible diffuse reflectance spectroscopy measurements. The PbTiO3 19 ACS Paragon Plus Environment


exhibited a steeped absorption edge nearly around 469 nm indicating that it absorbs solar light in the visible region to a certain extent (shown in Figure. 6). In contrast, the assemble of BiOI nanosheets into PbTiO3 demonstrated a regular red shifting of absorption edge nearly from 469 to 680 nm, concurrent with color of the p-n junction materials were turned from whitish red to pitch red. The enhanced optical response attributed to the intrinsic band gap absorption of oxygen defective BiOI nanosheets.30 The absorption edge shifting on interaction of PbTiO3 with thin BiOI nanosheets with oxygen vacancies can promote the enormous charge carriers generation on the photocatalyst surface to utilize the visible light of solar spectrum.31 Furthermore, the band gap of bare materials was outlined by following the Tauc equation.32 Since PbTiO3 and BiOI exhibited direct and indirect interband transition, the approximate optical band gap energy (Eg) was evaluated to be 2.93 eV and 1.72 eV, respectively (Figure. S6). Moreover, the narrowing band gap of BiOI than the reported value were attributed to the presence of oxygen vacancies in BiOI nanosheets.10,33 Consequently, the Mott–Schottky analysis was performed to evaluate the flat band potential, conductivity type and oxygen vacancies of the prepared materials. The band-edge potential values were also very crucial for evaluating the flowchart of photo-generated charge carriers in a p-n junction material. As shown in Figure. 7, the apparent positive and negative slope of the curves described the typical n-type and p-type electronic characteristics behavior of PbTiO3 and BiOI, respectively. The flat band potential for pristine PbTiO3 and BiOI were identified approximately at -0.63 V and +1.94 V vs Ag/AgCl after extrapolation of the curve. Thereafter, the rule of thumb dictates that the conduction band and valence band position is about -0.1 V unit apart compared to the flat band potential retrieved from the Mott–Schottky analysis. Therefore, the conduction band position (ECB) of PbTiO3 and valence band position (EVB) of BiOI was -0.12 V and +2.64V vs. RHE at pH 6.9 after the


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conversion using equation: ERHE= EAg/AgCl + 0.197 V+ 0.059pH. Thereafter, the valence band (EVB) and conduction band (ECB) position of PbTiO3 and BiOI was further determined through ECB = EVB – Eg (Eg = energy band gap value determined from the UV-Vis DRS) as shown in Scheme. 2. The combined Mott-Schottky plot analysis for PbTiO3 and 20% BOI/PT primarily demonstrated positive slopes featuring n-type behavior of the semiconductor. Further, it was observed that the 20% BOI/PT had a gentler slope compared to PbTiO3, revealing an increased charge carrier density. The enhancement of donor density in p-n junction sample also provided evince for the presence of oxygen vacancies in the material.10 Thus, the defective oxygen vacancies of p- type BiOI would not only initiate a new donor level below the conduction band, that probably contributes the photogenerated electrons to absorb more visible light but also transfer the photogenerated electrons to the PbTiO3 surface with increasing their lifetime for the photocatalytic reaction.34,35 1.0 0.8 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


0.6 0.4 0.2 0.0 200


300

400

500

600

700

800

Wavelength(nm)

Figure 6 UV−vis DRS absorption spectra of PbTiO3 (PT), BiOI (BOI) and BOI/PT p-n junction materials. 21 ACS Paragon Plus Environment


(a)

10

10

1.6x10

6x10

PT 20% BOI/PT

10

1.2x10

(b)

BiOI 10

5x10

1/C2(cm 4F-2)

10

1/C2(cm 4F-2)

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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9

8.0x10

Efb = -0.63

9

4.0x10

4x10

10

3x10

10

2x10

Efb=1.94

10

1x10 0.0

-0.6

-0.4

-0.2

0

Potential(V vs Ag/AgCl)

1.2

1.4

1.6

1.8

2.0

2.2

Potential(V vs Ag/AgCl)

Figure 7 Mott-Schottky plot of (a) PbTiO3 and 20% BOI/PT, and (b) BiOI Beyond that, the charge carrier separation and transfer efficiency associated with the oxygen vacancy rich p-n junction was investigated by using photoluminescence (PL) emission spectra and photoelectrochemical (PEC) measurements. As illustrated in Figure. 8(a), on exciting at 350 nm, the strong emission peak of PbTiO3 at 438nm arises due to near band edge emission whereas pristine BiOI displayed an emission peak nearly 420 nm, consistent with the emission characteristic of bismuth oxyiodide.36 In contrast to pristine materials, all the p-n junctions showed lower intensity emission peaks and the least intense peak was exhibited by 20% BOI/PT photocatalyst. The quenched photoluminescence intensity of 20% BOI/PT was considered to be around 82.5% of PbTiO3 and 71.5 % of BiOI, respectively. This significant declined in intensity was referred to the better charge carrier separation efficiency property of junction samples and also provided auxiliary evidence that the induced oxygen vacancies BiOI into a perovskite matrix can effectively separate the charge carriers due to predominant proviso of surface charge transfer in p-n junction materials.19,32,37,38 Thereafter, the electrochemical impedance spectroscopy (EIS) was acquired under visible light illumination to gain in-depth knowledge regarding the transport kinetics of electrodes and interface reaction in p-n junction material.39 In 22 ACS Paragon Plus Environment

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Figure. 8 (b), the 20% BOI/PT shows quite smaller charge transfer semicircle arc in comparision to pristine PbTiO3 and BiOI, implying a good suppression of photogenerated charge carriers for photocatalysis reaction. The result can be related to the gentler M-S slope or higher carrier density of 20% BOI/PT due to simultaneous introduction of oxygen vacancies (commonly addressed as electron donor) and intimate interface contact at p-n junction boundary that force to reduce the electrical resistance and accelerate the electron transfer property .19,39-41 Additionally, the Bode-phase spectra further proved the eminently prolonged lifetime of injected electrons (τ) of the prepared samples. The characteristic frequency peak of 20%BOI/PT has notably shifted to lower frequency region compared with bare photocatalysts as shown in Figure. 8 (c). The peak shifting from higher to lower frequency suggests that a quick electrons transfer taking place in the material.36,42 This is because the frequency peak is related to injected electrons lifetime in the prepared photocatalysts and is expressed by using the equation 𝜏 = 1 2𝛱𝑓 where, f indicates the inverse minimum frequency.43 The electron lifetime of 20% BOI/PT (40.66ms) were substantially larger than that of PbTiO3 (21.42ms) and BiOI (28.69ms). This improvement again identified the oxygen defect states corroborate the high separation and ultrafast charge carrier migration efficiency by inducing more oxygen vacancies in the BOI/PT p-n junction photocatalyst.19 Collectively through the photoabsorption and electronic contribution, it was undoubtedly verified that inclusion of BiOI nanosheetes into the perovskite matrix reinforces an increased photo-activated charge carriers, efficient interfacial charge dynamics and a greater intrinsic charge carrier density originated from the rich oxygen vacancies and p-n junction property.32




Intensity (a.u.)

(a)

390

420

450

480

510

Wavelength (nm) 0

(c)-10

(b) 4x10

4

4

Phase (degree)

3x10

Z'()

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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BiOI 20% BOI/PT PbTiO3

4

2x10

4

1x10

BOI 20%BOI/PT PbTiO3

-20 -30 -40 -50

0 0

4

1x10

4

2x10

4

3x10

4

4x10

4

5x10

-60

0

20

40

60

80

100

Frequency (Hz)

Z'()

Figure 8 (a) Photoluminescence Spectra of PbTiO3 (PT), BiOI (BOI) and BOI/PT p-n junction, (b) EIS Nyquist curve and (c) Bode plot of PbTiO3 (PT), BiOI (BOI) and 20% BOI/PT p-n junction material. Further to assess the separation and migration efficiency of photo-generated charge carriers on the p-n junction surface, the photoelectrochemical measurements were investigated under visible light condition.34,44 As shown in Figure. 9, the photocurrent spectra of BOI/PT p-n junction materials along with bare materials reflected that the samples were sensitive to light illumination and quite stable, focusing most of the photo-generated electrons were drift swiftly across the interface of the semiconductor through the defect states of BiOI into the PbTiO3 to produce photocurrent under visible light irradiation.45 Bare PbTiO3 and BiOI showed its 24 ACS Paragon Plus Environment

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characteristics anodic and cathodic polarization curves suggesting an n- and p- type behavior of the said materials. Figure. 9(a), displays the polarization curve of PbTiO3 under both dark and visible light condition. Under dark condition, PbTiO3 generated minute quantity of anodic current with an applied potential range of -0.5 to +1.0 V. However, on irradiation with light, the photocurrent intensity increases and reaches a maximum value of 1.119 μA cm-2 at +1.0 V. In contrast, the cathodic photocurrent generated by BiOI in the potential range of +0.4to -1.0 V showed an increment with the applied bias and reached maxima of -22.974 μA cm-2 at -1.0 V under light irradiation condition, Figure. 9(b). On the other hand, to obtain a strong confirmation of dual photocurrent responses of designed oxygen vacancies rich p-n junction materials, the I-V polarization curve of BOI/PT materials under light irradiation were taken into account (shown in Figure. 9(c)).The figure showed an improved asymmetric photocurrent intensity measurement in both forward and reverse directions. The photocurrent intensity of the samples in reverse directions is following the order i.e. 20%BOI/PT>10% BOI/ PT >30% BOI/ PT in the range of 1.0 to +1.0 V. In addition, the transient photocurrent density (J-t) of PbTiO3 and 20% BOI/PT was further investigated under chopped illumination (Figure. S8). Compared with the bare PbTiO3 material, a significant enhancement was achieved for heterojunction material. This improved photocurrent response can be verified by several factors, one of which is the increased number of excited charge carriers by absorbing light energy higher than the energy band gap.16 The inclusion of BiOI with oxygen vacancies into the perovskite matrix fabricated a p-n junction that administered electrons to freely transfer through the interface. The merit of oxygen vacancies also justified as it grant more sites for trapping electrons, thus suppressing detrimental recombination effects.19,40,46 These subsequent processes was revealed to accomplish the



passivation of surface states and trigger the various catalytic reaction on the photocatalyst surfaces. 41

PbTiO3(Light) PbTiO3(Dark)

(b)

0.8

0.4

0.0 -0.4

0.0

0.4

5

Photocurrent density (A cm -2)

(a)

1.2


BiOI (Light) BiOI (Dark)

0 -5 -10 -15 -20 -1.0

0.8

(c)

-0.8

-0.6

-0.4

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Potential (V vs Ag/AgCl)



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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10% BOI/PT 20% BOI/PT 30% BOI/PT

20

0

-20

-40

-60 -1.0

-0.5

0.0

0.5

1.0


Figure 9 Photocurrent responses of (a) PbTiO3 (PT), (b) BiOI (BOI) under dark and light irradiation; and (c) BOI/PT p-n junction samples under light irradiation. Undeniably, the subsequent route was surface catalytic reactions. The photodegradation of antibiotic TC-HCl under open sunlight was selected as a model reaction to evaluate the efficiency of oxygen vacancy rich BOI/PT p-n junction materials. The electronic coupling of visible active PbTiO3 perovskite with slimmed BiOI nanosheets having massive active reactive 26 ACS Paragon Plus Environment

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centers was anticipating greatly in accelerating the surface photocatalytic reactions. We have also performed the photocatalytic studies of TiO2 and P25 in comparative with the prepared materials. As shown in Figure. 10, the rate of degradation was calculated according to the 𝐶

formula 𝑙𝑛𝐶0 = - 𝑘𝑡 where C0 is the initial concentration, C is the concentration at time t and k refers to the apparent pseudo-first order rate constant, respectively. The kinetic study was carried out by plotting ln C/C0 against time and the curves were fitted to the corresponding linear equations, suggesting TC-HCl degradation follows the typical pseudo-first-order kinetics. The yielding half-life time, t1/2 (in min) have been expressed by following the equation: t1/2= 0.693/k. The apparent rate constant (k), half-life time of TC-HCl and the linearization coefficient (r2) were summarized in the Table.1 (supporting information). The degradation rate of TC-HCl was following the order: TiO2 < P25 < PbTiO3 < BiOI < 40%BOI/PT < 10%BOI/PT < 30%BOI/PT < 20%BOI/PT. With the incorparation of oxygen defective BiOI, the p-n junctions showed the highest degradation rate (d%) in comparision to the bare PbTiO3 and BiOI; whereas, the commercial TiO2 and P25 were degraded only 22 % and 29.5 % under direct solar radiation. The enhanced photocatalytic performance of hybrid materials was explained by the formation of p-n junction with enhanced oxygen vacancies. Among all the heterojunctions, the 20% BOI/PT showed the highest degradation rate for TC-HCl degradation i.e., 94% in 2h which is around 2.1 and 1.6 times greater than the bare PbTiO3 and BiOI, respectively. This indicates that the optimum cooperative effect of nanosheets structure of BiOI and perovskite crystal lattice structure of PbTiO3 are more effective for the TC-HCl degradation.47 Moreover, the degradation rate of heterojunction samples doesn’t increases linearly with the BiOI content. This is because the excessive BiOI with smaller energy band gap and oxygen vacancies may provide more recombination centers for the photo-generated charge carriers.48 Overall, we can say that, in the 27 ACS Paragon Plus Environment


photocatalytic reaction processes, the oxygen vacancies were not only broadening the solar light harvest, but also becomes the shallow center to capture photo-generated electrons in heterojunction samples, so that the recombination of photo-generated electrons and holes gets effectively inhibited. In contrast, when the oxygen vacancies inducing dopant content is more than its optimal value, the oxygen vacancies defect states behaves as a recombination center.49 Accordingly to the above discussion, the presence of a suitable amount photocatalyst and photoactive reaction centers are the vital parameters for rising the photocatalytic activities. 45,50

(a)

(b)

0.8

Blank TiO2 P25 PbTiO3

H3C

10 % BOI/PT 20 % BOI/PT 30 % BOI/PT 40 % BOI/PT BiOI

0

20

HO

lnC/C0

C/C0

-0.4

0

-1

0.4

0.0

CH3

-2

N

CH3

H

H

TiO2 P25 PbTiO3

OH

10% BOI/PT 20% BOI/PT 30% BOI/PT 40% BOI/PT BiOI

HCl NH2

OH OH

40

O

OH

O

60

O

80

100

-3

120

0

20

Irradiation Time (min)

100

100 20%BOI/PT BOI PT

PL Intensity (a.u.)

BQ

60

NO QUENCHER

80

80

(d)

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20%BOI/PT

80

60 40 20 0 400

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Wavelength (nm)

MeOH

IPA

40 20

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Degradation rate (d%)

(c) 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

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60 40 20 0

0 PHOTOCATALYST

pH =1

pH = 3

pH = 5

pH = 7

pH of TC-HCl solution

Figure 10 (a) Photocatalytic TC-HCl degradation, (b) Pseudo first-order kinetics, (c) Histograms showing the effect of different scavengers on 20% BOI/PT p-n junction and the inset represent


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the fluorescence spectra of PbTiO3 (PT), BiOI (BOI) and 20% BOI/PT p-n junction in a basic solution (5 × 10-3 M) of terephthalic acid (d) Degradation rate at different pH values. The pH of reaction solution is also considered to be one of the important parameter for the removal of tetracycline. The photocatalytic performance of optimal 20% BOI/ PT p-n junction was analyzed at different pH (1, 3, 5 and 7) values. All the aqueous solutions were adjusted to the desired pH using 0.1M hydrochloride or 0.1M ammonical solution. Figure. 10 (d), displays the outcome result of tetracycline degradation under different pH values. The efficiency towards antibiotic removal was achieved maximum at pH 3 and its efficiency decreased from 95.2 % to71.9%, as the solution pH varied from acidic towards basic medium. Thus, in acidic environment the 20% BOI/PT photocatalyst played an efficient role in tetracycline removal. Safari et al and his co-worker

had also reported, in acidic reaction

condition the positive holes proved to be leading species which is consistent with our trapping experiment result (discussed in the later section).51 In addition, it has been widely reported the surface electrical charge characteristics of a photocatalyst plays a significant role towards the degradation of organic compounds. The ionization state of photocatalyst surface dictates the adsorption capacity of tetracycline molecules. In order to evaluate this parameter, the pHpzc result of a photocatalyst adsorbent was determined. In simple words, the term pHpzc elaborately at the point of zero charge; is defined as the pH upon which the net charge of a photocatalyst surface is zero.42 The pHpzc value also depends on the nature and amount of functional groups present on the surface of a photocatalyst. Figure. S9, shows the pHpzc value of 20% BOI/PT determined by following Drift method and it was found out to be 4.9.52,53 The effect of solution pH on TC-HCl adsorption can be put in plain words on the basis of TCHCl speciation and adsorbent surface charge with different solution pH values. Initially, at 29 ACS Paragon Plus Environment


different solution pH, the tetracycline subsists in various ionic species. Antibiotic tetracycline is an amphoteric compound with different kinds of ionizable functional groups. Dissociation of tetracycline predominantly results in the formation of a series of species, i.e. cationic form in acidic solution (pH< 3.32), zwitter ions in moderately acidic to neutral solution (3.32

n-PbTiO3 Heterojunction

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