Supplementary material

APPLIED PHYSICS LETTERS 108, 033112 (2016)

Supplementary material Multifunctional BiFeO3/TiO2 nano-heterostructure: photo-ferroelectricity, rectifying transport, and nonvolatile resistive switching property Ayan Sarkar, † Gobinda Gopal Khan, †,* Arka Chaudhuri, ‡, Avishek Das,# and Kalyan Mandal‡ †

Centre for Research in Nanoscience and Nanotechnology, University of Calcutta,

Technology Campus, Block JD2, Sector III, Salt Lake City, Kolkata 700 098, India ‡

Department of Condensed Matter Physics and Material Sciences, S. N. Bose National

Centre for Basic Sciences, Block JD, Sector III, Salt Lake City, Kolkata 700 098, India 

Department of Applied Science, Haldia Institute of Technology, Haldia 721657, Purba Medinipur,

West Bengal, India #

Department of Electronic Science, University of Calcutta, 92 APC Road, Kolkata 700009, India

Experimental Section 2.1. Reagents: Titanium foil (99.98% pure, Alfa Aesar, 0.25mm thick), ammonium fluoride (NH4F, Loba Chemie), formamide (CH3NO, Loba Chemie), Bismuth (III) Nitrate Pentahydrate

(Bi(NO3)3.

5H2O,

Sigma-Aldrich),

Iron

(III)

Nitrate

nonahydrate

(Fe(NO3)3.9H2O, Merck), 2-Methoxyethanol (C3H8O2, Merck). All the reagents used, are of analytical grade and were used without further purification. 2.2. Synthesis of TiO2 NTs: The TiO2 NTs have been grown on Ti foil by means of anodization technique. A piece of Ti foil (40600.25 mm3) was thoroughly cleaned by 10 minute ultrasonication in the mixture of acetone and ethyl alcohol, followed by rinsing in DI water. This Ti foil was used as the anode in a two electrode electrochemical cell containing a 150 ml solution of 0.2 M NH4F solvated in the mixture of 95 vol % formamide and 5 vol % DI water. A highly pure and clean copper plate, of similar dimension like the Ti foil served as the cathode. The anodization process was executed at a constant DC voltage of 25V (supplied 1

Citation: Applied Physics Letters 108, 033112 (2016); doi:10.1063/1.4940118

by a regulated DC power source) for 20 hour at 20C. The as prepared sample was ultrasonicated in ethanol for a mere 10s, followed by thorough rinsing in DI water. The sample was then dried in IR radiation and finally, was annealed in air for 90 minutes at 400C to obtain anatase phase of the TiO2 NTs. 2.3. Synthesis of BiFeO3 nanostructure anchored TiO2 NTs: The BiFeO3/TiO2 nanoheterostructure was synthesized by facile wet chemical route. The Ti plate having the grown anatase TiO2 NT array on both sides, was immersed in the 2-Methoxyethanolic solution of the mixture of 0.3M Fe(NO3)3.9H2O and 0.3M Bi(NO3)3.5H2O for 4 hours at room temperature. Afterwards, the dip-coated sample was dried under IR radiation and finally, was annealed for 2 hours at 480C. 2.4. Characterization: The morphology and structure of the as prepared BiFeO3/TiO2 nanoheterostructure were investigated by the field emission scanning electron microscope (FESEM, JEOL JSM-7600F) and high resolution transmission electron microscope (HRTEM, JEOL JEM-2100). The constituent elements of the nano-heterostructure were analyzed by energy dispersive x-ray spectroscopy (EDS, Oxford Instruments, attached to the FESEM). X-ray diffraction (XRD, Panalytical X’Pert Pro diffractometer) patterns for both pristine TiO2 NTs and BiFeO3/TiO2 nano-heterostructure were recorded using Cu Kα line ( = 1.54 Å) with step size of 0.01. The chemical states of the nano-heterostructure were analyzed by x-ray photoelectron spectroscopy (XPS, VG-SCIENTA SCIENTA MX-650) using Al K beam (1486.6 eV) with beam current equals to 20 mA. Furthermore, Raman spectra for both the samples were recorded in the range of 50-1000 cm-1 using a microRaman spectrometer (LABRAM HR, Horiba Jobin Yvon). 2.5. Photo-ferroelectric measurements: The ferroelectric property was studied by the multiferroic material analyzer (Precision Premier II, Radiant Technologies) integrated with a software controlled high voltage (10 kV) source/amplifier (Trek model 609B). An ordinary 2

Citation: Applied Physics Letters 108, 033112 (2016); doi: 10.1063/1.4940118

electric lamp (fixed intensity of 10 mW/cm2) with filter ( > 420 nm) was used as the light source for the visible light photo-ferroelectric measurements. 2.6. Photocurrent and Resistive switching measurements: The photocurrent measurements were accomplished with the help of the Keithley 2401 source meter having the in-built two probes. An ordinary electric lamp (fixed intensity of 10 mW/cm2) with filter ( > 420 nm) was used as the light source. The resistive switching phenomena of the concerned samples were studied using the Keithley Semiconductor Characterization System (4200-SCS), connected with the computer controlled probe station (Cascade Microtech, Summit 12000M) having gold probe tips (diameter ~ 19 m). Measurements were conducted with Ti grounded while the voltage sweep was applied to the nano-heterostructure.

Table S1: Summary of the variation of saturation and remanent polarization with the applied voltage under dark and light-illumination conditions for the BiFeO3/TiO2 nanoheterostructure. Applied voltage

Dark condition

Light-illumination condition

(V)

Saturation

Remanent

Saturation

Remanent

polarization

polarization

polarization

polarization

(C/cm2)

(C/cm2)

(C/cm2)

(C/cm2)

30

0.18

0.09

0.26

0.12

50

0.38

0.18

0.44

0.23

100

0.65

0.28

0.7

0.35

150

0.95

0.44

1.02

0.51

200

1.22

0.59

1.3

0.68

250

1.47

0.73

1.55

0.82

300

1.74

0.9

1.83

1

3


FESEM image:

Figure S1: The FESEM image of the specified area of the BiFeO3/TiO2 nanoheterostructures sample on which EDS analysis was performed.

XRD analysis The XRD pattern (Figure 1(f) and (g) in the manuscript) of the BiFeO3/TiO2 nanoheterostructure (NH) shows that BiFeO3 has a polycrystalline nature with orientations along (101), (012), (110), (021), (202), (113), (300) etc.(JCPDS File#20-0169) which correspond to the rhombohedral distorted perovskite structure of BiFeO3 with R3c space group. The TiO2 NTs also have a polycrystalline, anatase phase with preferential (101) orientation. It is also found that the (101) peak of anatase pristine TiO2 is located at 25.48, while the (101) peak of anatase TiO2, in the BiFeO3/TiO2 NH sample is located at 25.40. With step size of 0.01 in the XRD measurement, this difference of 0.08 (see Figure S2) is quite meaningful (the data for other anatase peak shifts are given in Table S2). The reason behind this can be attributed to the substitution of some of the Ti4+ (ionic radius for coordination VI = 0.605Å, for coordination V=0.51 Å) ions in the TiO2 host lattice by some Fe3+ (ionic radius = 0.645Å) ions. As the ionic radii of Fe3+ and Ti4+ are comparable, Fe3+ ions can effortlessly substitute Ti4+ ions and the substitutions occur at the TiO2 NT surface during the calcination treatment. The substitutions introduce a crystal expansion of TiO2 at the surface, which in turn results the left shift of the concerned XRD peaks. As Fe3+ ions replace Ti4+ ions, at the interface, additional oxygen vacancies (Vo) are created for the charge neutralization of the host crystal lattice.1

4


Figure S2: The difference of A(101) XRD peak positions in pristine TiO2 NT sample and BiFeO3/TiO2 nano-heterostructures.

Table S2: Summary of the XRD anatase peak positions in pristine TiO2 NT and BiFeO3/TiO2 nano-heterostructure. (h k l)

Anatase peak positions (2 in deg) in Pristine TiO2

Shift in 2

BiFeO3/TiO2

(1 0 1)

25.48

25.40

0.08

(1 0 3)

37.22

37.12

0.10

(0 0 4)

38.16

38.04

0.12

(2 0 0)

48.15

48.10

0.05

(1 0 5)

54.27

54.14

0.13

(2 1 1)

55.16

55.14

0.02

(2 0 4)

63.13

63.05

0.08

5


XPS analysis

Figure S3: The XPS survey spectrum of the BiFeO3/TiO2 NH, showing the core levels of the constituent elements. To have a better realization of the ionic structure and chemical composition of the NH, surface sensitive X-ray Photoelectron Spectroscopy (XPS, VG-SCIENTA SCIENTA MX-650) was done using the monochromatic x-ray, obtained from Al Kα X-ray beam (1486.6 eV) with beam current 20 mA. The typical XPS survey spectrum (Figure S3) confirms the presence of Ti, O, Bi and Fe in the NH. The high resolution curve-fitted XPS spectra for Ti 2p, O 1s, Bi 4f and Fe 2p have been presented in Figure S4. The binding energies of the Ti core levels, Ti 2p1/2 and Ti 2p3/2 spin-orbit states are located at about 464.8eV and 459eV, respectively with E = 5.8eV (Figure S4 (a)) which in turn evinces the presence of +4 oxidation state of Ti in anatase TiO2.2,3 The O1s XPS signal, shown in (Figure S4 (b)) consists of a sharp peak at binding energy 530.3eV and a shoulder, located at 532.2eV. The lower binding energy peak at 530.3eV is ascribed to the -2 oxidation state of oxygen, present in the NH. Whereas the higher binding energy peak can be assigned to the OH- species present on the surface due to the adsorption of moisture4 as well as to the oxygen vacancies.5,6 The XPS spectrum of Bi 4f (Figure S4 (c)) consists of two prominent peaks located at 164.3eV and 159eV. These respectively correspond to Bi 4f5/2 and Bi 4f7/2 spin-orbit energy levels with E=5.3eV and indicate to the presence of +3 oxidation state of Bi in BFO. No evidence of the presence of metallic Bi is found.7,8 Finally, the deconvolution of the Fe 2p core level spectrum produces four Gaussian peaks, as shown in (Figure S4 (d)). The Fe 2p1/2 is located in the vicinity of 725eV, while the Fe 2p3/2 peak is positioned in the vicinity of 711.1eV. These correspond to the presence of +3 oxidation state of Fe in BFO.

6


Moreover, two small satellite peaks with binding energies 719eV and 714.2eV have also been found which actually redefine the electronic structure of Fe3+.

Figure S4: High resolution curve-fitted XPS spectra of (a) Ti 2p (b) O 1s (c) Bi 4f and (d) Fe 2p levels. Raman Spectroscopy Study Raman spectroscopy is one of the key tools for the understanding of crystallinity, defect states, spin-phonon coupling in a material. In our present work Raman spectroscopy measurements were performed on pristine TiO2 NTs and BiFeO3/TiO2 NH in the range of 50−1000 cm−1 using a micro-Raman spectrometer (LABRAM HR, Horiba Jobin Yvon). The plot of Raman spectra for both of the samples has been depicted in Figure S5. The six active Raman Modes, Eg(1), Eg(2), B1g(1), A1g, B1g(2) and Eg(3) are assigned to the anatase TiO2. It is seen that the Eg(1) peak for the pristine TiO2 sample is located at 148.909cm-1 with full width at half maximum (FWHM) of 19.606cm-1 ,while the E1(g) peak for the BiFeO3/TiO2 NH is located at 150.145cm-1 with FWHM equals to 20.359cm-1. Similarly, the B1(g) peaks for TiO2 and BiFeO3/TiO2 NH samples are respectively positioned at 396.620cm-1 and 401.562cm-1 with the respective FWHMs of 38.838 cm-1 and 43.194 cm-1. These blue shifts and augmentations in FWHMs in the NH indicate towards the non-stoichiometry due to the presence of Vo defects in the NH.9

7


Figure S5: The Raman spectra of pristine TiO2 NT and BiFeO3/TiO2 NH. Inset: The scaled in picture of Eg(1) mode, showing the blue shift in Eg(1) mode of the NH. Photo-ferroelectric property

Figure S6: The P-E hysteresis loops of BiFeO3/TiO2 nano-heterostructure at different applied voltages, under (a) dark condition and (b) illuminated condition. References: [1] B. Santra, P.K. Giri, S. Dhara, K. Imakita, M. Fujii, J.Phys.D: Appl. Phys. 2014, 47, 235304. [2] H. Liu, W. Yang, Y. Ma, Y. Cao, J. Yao, J. Zhang, T. Hu, Langmuir 2003, 19, 30013005. [3] R. Sanjinés, H. Tang, H. Berger, F. Gozzo, G. Margaritondo, F. Lévy, J. Appl. Phys. 1994, 75, 2945. [4] B. Erdem, R.A. Hunsicker, G.W. Simmons, E.D. Sudol, V.L. Dimonie, M.S. El-Aasser, Langmuir 2001,17,2664-2669. [5] L. R. Shah, B. Ali, H. Zhu, W. G. Wang, Y. Q. Song, H. W. Zhang, S. I. Shah, J. Q. Xiao, J. Phys.: Condens. Matter 2009, 21, 486004. [6] M. Naeem, S. K. Hasanain, M. Kobayashi, Y. Ishida, A. Fujimori, S. Buzby, S. I. Shah, Nanotechnology 2006, 17, 2675. [7] R. Das, G. G. Khan, S. Varma, G. D. Mukherjee, K. Mandal, Journal of Physical Chemistry C 2013, 117, 20209. [8] W. B. Luo, J. Zhu, Y. R. Li, X. P. Wang, D. Zhao, J. Xiong, and Y. Zhang, Appl. Phys. Lett. 2007, 91, 082501. [9] B. Santra, P.K. Giri, K. Imakita, M. Fujii, Nanoscale, 2013, 5, 5476. 8
