Band gap tuning in nanocrystalline SrTi0.9Fe0.1O2.968 perovskite type for photocatalytic and photovoltaic applications

Author’s Accepted Manuscript Band Gap Tuning in NanoCrystalline SrTi0.9Fe0.1O2.968 Perovskite Type for Photocatalytic and Photovoltaic Applications K. Sedeek, Sh.A. Said, T.Z. Amer, N. Makram, H. Hantour www.elsevier.com/locate/ceri

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S0272-8842(18)32785-8 https://doi.org/10.1016/j.ceramint.2018.09.305 CERI19709

To appear in: Ceramics International Received date: 28 July 2018 Revised date: 29 September 2018 Accepted date: 29 September 2018 Cite this article as: K. Sedeek, Sh.A. Said, T.Z. Amer, N. Makram and H. Hantour, Band Gap Tuning in NanoCrystalline SrTi 0.9Fe0.1O2.968 Perovskite Type for Photocatalytic and Photovoltaic Applications, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.305 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Band Gap Tuning in NanoCrystalline SrTi0.9Fe0.1O2.968 Perovskite Type for Photocatalytic and Photovoltaic Applications K.Sedeeka, Sh.A.Saida,*, T.Z.Amerb, N.Makrama, H.Hantoura a

Nano-Materials Lab., Physics Depart., Faculty of Science, Al-Azhar University, Cairo,

Egypt b

Phys.Dept., Faculty of Science, Al-Azhar University, Cairo, Egypt

*

Corresponding author : [email protected]

Abstract In the present work and for the first time, tuning of the band gap width of the SrTiO3 (STO) perovskite to a value suitable for photocatalytic (PC) and photovoltaic (PV) applications is accomplished by the incorporation of Fe cation. Nanocrystalline SrTi0.9Fe0.1O2.968 (STFO) was prepared by a modified solid state reaction process including successive sequences of milling and calcinations at high temperature. The X-ray diffraction (XRD) pattern revealed the formation of a single cubic perovskite phase of STFO with average crystallite size equaling ⁓30 nm. The local lattice strain on (h00) and (hh0) planes was found to decrease by Fe doping. The absorption spectrum deduced from diffused reflectance showed high intense broad structure extending over the range ⁓0.5 - ⁓6 eV, whereas pure STO gave strong absorption only at the UV region (λ < 400 nm). The deduced band gap width of the STFO sample was 1.43 eV; an ideal value for PC and PV applications. Deconvolution of the broad absorption band revealed the presence of four absorption structures attributed to Fe defect centers. The narrowing of the band gap was also confirmed through the photoluminescence study where many emission lines covering the violetblue region were detected. The type of the Fe species and the relative abundance 1

of Fe3+ and Fe4+ were determined by Mössbauer spectroscopy. The presence of oxygen vacancies and Fe-OV complexes were also supposed as lattice defects located above the O2p and below the Ti-3dt2g states. The novel electronic structure researched in this study offers a new avenue in the field of band gap engineering for future application in the field of photocatalytic materials. Keywords: Cubic perovskite structure and microstructure; Mössbauer investigation; Photoluminescence emission; Diffused reflectance; Band gap tuning. 1. Introduction Several studies have been performed on perovskite ceramics due to their phase homogeneity and sinter-ability leading to unexpected giant properties [15]. Numerous perovskite type ABO3 compounds such as SrTiO3 [1], NaTaO3 [2, 4] and K2La2Ti3O10 [3] have been proven as efficient photocatalysts for solving water into H2 and O2 in the absence of an electric field to weed some environmental problems. However, their practical applications were restricted to the UV range of light because these photocatalysts are generally wide band gap semiconductors [6]. To overcome this limitation, many works have been carried out to extend the optical absorption and conversion capacities into visible range of solar spectrum [7- 9]. The wide band gap of SrTiO3 (⁓3.3 eV) made it ill suited for visible light absorption. The ideal optical gap for photocatalysis and photovoltaic equals 1.5 eV to produce a strong visible light absorption [10, 11]. Jiang et al. [12] observed the formation of defect states at the top of the valence band while doping both La3+ and Fe3+ in STO, with non-zero optical absorption at energies below 0.8 eV. Meanwhile, the interesting photovoltaic and photocatalystic materials needed somewhat wider optical gap. Also, Comes et al. [13], by using Tauc formula for a dipole forbidden direct gap of Fe-3d → Ti-3d transition, determined a direct forbidden gap of 0.83(5) eV for Fe doped 2

STO. The authors suggested that, certain technique was needed to raise the bottom of the conduction band in STO. Other workers [14] indicated that this may be achieved in a solid solution of STO (Eg= 3.2 eV) and SrZrO3 (Eg= 5.6 eV). Hence, this may produce a tunable range of band gap by removing lowlying Ti-3d states. Similar efforts were done by one of the authors to increase the optical band gap width of a-GaAs prepared by R F sputtering to 1.43 eV. This value matches well the absorption range of photovoltaic applications [15]. Herein for the first time, without any need for complicated preparation techniques, we were able to synthesize nanocrystalline Fe doped STO having an intense absorption spectrum extended to near IR region (⁓ 0.5 eV). The estimated optical gap equaled 1.43 eV. The synthesis process was achieved through successive sequences of milling and sintering at high temperature. Such method gave only cubic perovskite phase. Structure and microstructure characterization was carried out by XRD using the Rietveld method. Optical absorption was detected through diffused reflectance technique. The observed broad absorption band represents an efficient optical transition that is rarely, if ever, was observed in Fe doped STO. The presence of Fe defect levels inside the forbidden gap was confirmed through photoluminescence study, while the types of these centers as Fe3+, Fe3+-Ov and Fe4+, Fe4+-Ov were achieved using Mössbauer investigation. 2. Experimental Work For the preparation of SrTi0.9Fe0.1O3 (STFO), powder precursors containing each element: SrCO3 (AR), TiO2 (AR) and Fe2O3 (AR) were weighed minutely. The powders were first milled for 12 hs before being calcined at 1100°C for a period of 3 days. Thereafter successive cycling of sintering at 850°C for 12 hs with intermittent regrinding for 7 hs was carried out to ensure homogeneity and to complete the solid state reaction. XRD of the final

3

product confirmed the formation of a pure –one phase– cubic STFO perovskite. The chemical reaction involved was as follows: SrCO3 + 0.9TiO2 + 0.1Fe2O3 -----→ SrTi0.9Fe0.1O3-δ + CO2 ↑ Milling The oxygen deficiency (δ) calculated from Mössbauer investigation was 0.032. X-ray diffraction using X'PERT – PRO – PANalytical diffractometer with Cu-Kα radiation (λ= 1.5406Å) was elaborated. Structural and microstructural data were refined applying WinFit program and the Rietveld profile method using MAUD program. A Vibrating Sample Magnetometer (VSM) – model Lake Shore7410– was used to measure the basic magnetic properties of the material at room temperature as a function of magnetic field up to 20 KG. The Mössbauer spectroscopy measurements were performed in transmission geometry at room temperature using a 512 – channel analyzer operating as a scalar in combination with a constant acceleration electro-mechanical velocity driver. 100–mCi approximately 57Co source in chromium matrix was used and the 14.4 KeV γ-rays were detected with a proportional counter. The UV-Vis absorption and diffuse reflectance spectra were recorded at room temperature using UV-VIS-NIR spectrophotometer (Jasco-V-570) fitted with integrating sphere reflectance unit (ISN) at the wavelength range 200-2000 nm. PL spectra were elaborated using a Shimadzu RF-5301PC fluorescence spectrophotometer (Shimadzu, Kyoto, Japan). The high-throughput optical system employed a blazed holographic grating, photomultiplier and digital circuit with noise level (S/N ratio) and light source Xenon lamp.

3. Results and Discussion 3.1 Structural and Micro-Structural Analyses The XRD pattern of pure and Fe-doped samples are shown in Fig.(1). Table (1) reveals the refined structure parameters calculated using Rietveld method [16, 17]. It is clear that partial substitution of Fe for B (Ti) site 4

preserves the Pm-3m space group for the cubic perovskite structure. All peaks were indexed by the standard STO card number 04-006-0821 indicating accordingly complete and continuum solid solution of Fe in the STO lattice. No diffraction peaks related to iron oxide phases were observed. A careful comparison of the (110) diffraction peaks over the range 20 ≤ 2Ө ≤ 80 of the two samples indicates a decrease of the relative peak intensity (Ir). There is also a reduction of crystallite size by ⁓20% with Fe doping. This decrease can be attributed to the incorporation of some structural disorder into the doped lattice.

Fig.(1): XRD refinements for (a) STO and (b) STFO samples.

5

Table (1): Refined structure parameters: lattice parameters a,c, apparent crystallite size (Dβ, DF), root mean square strain and reliability factor (Rwb , Rb) calculated using Rietveld method for STO and STFO samples. Structure Parameters

STO

STFO

Structure

Cubic

Cubic

Space group

Pm-3m

Pm-3m

a (A˚) S.L.A Dβ(nm) DF(nm) M.L.A Dβ(nm) DF(nm) x10-3%

3.895

3.885

36.6 37.5

29.6 32.3

Rwb%

37 37.5 0.166 (h00) 0.197 (hh0) 7.615

29.6 31.4 0.148 (h00) 0.188 (hh0) 7.499

Rb%

6.914

6.123

A slight decrease of “a” from 3.895 Å to 3.885Å, accompanied by a reduction of local lattice strain along the (h00) and (hh0), probably due to the induced vacancies and smaller Fe ionic radius, were also detected. The “a” values calculated for pure and Fe doped samples are comparable to those reported elsewhere [18- 20]. 3.2 Vibrating sample magnetometer (VSM) investigation The magnetization versus magnetic field (M-H) curve for SrTi0.9Fe0.1O3 sample is shown in Fig.(2).

6

Moment/mass (emu/g)

0.2 0.15 0.1 0.05 20000

15000

10000

-0.05

5000

0

-5000

-10000

-15000

-20000

4E-16

Field (G)

-0.1 -0.15 -0.2

Fig.(2): The magnetization versus magnetic field (M-H) curve for STFO sample.

As shown in Fig.(2), the M-H curve of the Fe doped STO shows nonsaturated very narrow hysteresis loop. The saturation magnetization (Ms) is inferred to be 0.2 emu/gm, while the coercive field (Hc) and the remnant magnetic field (Mr) respectively equals 516.04 G and 17×10-3 emu/gm. To verify the type of magnetism, Arrott relation was applied [21, 22]:

-

,

(1)

Where M is the magnetization, H is the applied magnetic field, a and b are constants related to the material and Tc is the Curie temperature. Plot (M2 vs. H/M) is given in Fig.(3). Extrapolation of the high field data to the M2 axis yield negative intercept reflecting thus the antiferromagnetic character of the STFO sample under study. Probably, with increasing Fe content higher than x=0.1, the STFO under study may acquire ferromagnetic character.

7

Fig.(3): Arrott-Plot for STFO sample.

Our results are in good agreement with other studies as no ferromagnetism was detected for Fe doped STO with values of x ≤ 0.1 [23- 26]. 3.3 Mössbauer analysis Mössbauer spectroscopy is a powerful tool to study the electronic structure and the local environment at the different iron sites in the structure. The Mossbauer parameters, isomer shift (IS), quadrupole splitting (QS), magnetic hyperfine field (Hhf), line width (Γ) can provide direct information on the electronic density at the Fe57 nucleus, the asymmetry of the charge distribution around it, the magnetic field,… etc, through the hyperfine interactions. Fig.(4), shows the Mössbauer spectrum for STFO sample measured at room temperature. As shown, no hyperfine structure was detected. The absence of the magnetic phases argued that the magnetic moment of Fe ions order anti-ferromagnetically.

8

Fig.(4): Mössbauer spectrum for STFO measured at room temperature.

Room temperature Mössbauer spectrum of STFO sample was fitted considering three paramagnetic doublets. The obtained parameters are listed in table (2). Table (2): Mossbauer parameters for STFO perovskite sample. Subspectra A B C

IS, mm/s 0.193 0.179 -0.091

Γ, mm/s 0.38 0.38 0.40

QS, mm/s 0.585 1.12 0.390

R.A., % 42 22 36

Fe valence state Fe3+ Fe3+ Fe4+

Mössbauer data suggest that, both A and B species are assigned to Fe3+ in different crystallographic sites. Phase A corresponds to Fe 3+ ions in a symmetric local field. In contrary, the observed high value of the quadrupolar splitting (1.12 mm/s) of phase B is characteristic of Fe3+ions in a highly distorted site (lower site symmetry) which results in a large electric field gradient acting on Fe3+ ions at this site. In fact, large distortion in symmetry at surface sites, due to incomplete coordination and the discontinuity of lattice, was expected to cause 9

large quadrupole splitting [27]. The C species shown in the Mössbauer spectrum as a third doublet has an isomer shift equals -0.091 mm/s (table (2)). This negative value is consistent with the presence of higher oxidation state of iron (Fe4+). This assignment is in full agreement with previous works [28– 30]. In Mössbauer spectroscopy, the relative area of a sub spectrum is related to the relative abundance (R.A.) of iron on the corresponding crystallographic site. The relative area fractions can be therefore used to calculate the relative abundance of Fe3+ and Fe4+ species. The calculated values are given in table (2). A similar Mössbauer study by C. T. Luiskutty [31] was carried out on SrTiO3 crystals doped using 0.1 wt% of Fe2O3 during growth. No hyperfine spectrum was detected at room temperatures, only two doublets with different intensities were observed. Another study of Fe doped SrTiO3 single crystal was carried out at room temperature by Bhide and Bhasin [32]. It was reported that the two observed peaks (IS = -0.4 and +0.6 mm/sec) are of the same intensity whatever the Fe concentration or the sample temperature. The equal intensities ruled out the existence of two Fe valence states. The authors suggested that the quadruple split spectrum of Fe3+ arising out of vacancy association. It is evident that, our data of Fe doped STO differ from those of Luiskutty et al. [31] and Bhide et al. [32]. The samples used in their studies were single crystals where our sample has a nanocrystalline structure prepared by milling – combustion method. The electrical neutrality of perovskites was conserved through the change of the oxidation state of iron from Fe3+ to Fe4+ accompanied by the formation of oxygen vacancies [30, 33]. Consequently, the chemical formula of the prepared STFO can be written as SrTi0.9Fe0.1O3-δ. The oxygen deficiency (δ) in the solid solution can be calculated using the relative abundance (R.A.) of Fe4+ (table 2) according to the relation (2) [33- 35]: δ = (x-y)/2

(2)

10

where x and y represent respectively the Fe content in the sample and the estimated fractions of Fe4+ ions. Accordingly, the value of δ was found to equal 0.032 for SrTi0.9Fe0.1O2.968. 3.4 Optical Characterization by Diffused Reflectance It is known that pure STO is ordinarily transparent in the visible region when it contains a stoichiometric amount of oxygen. When doped with Fe or other transition metal (TM) impurities, a certain amount of visible absorption occurs with a smearing out of the band edge. Iron is one of the major electrically active background impurities. In general, It enters the lattice as Fe 3+ substituting for Ti4+ and brings with it oxygen-ion vacancies preserving lattice neutrality [32]. Evidently, Iron plays a complex role because, in addition to associating with vacancies, it can exist in several valence states: 2+, 3+, 4+ and 5+ as have already been indicated by ESR and optical absorption [36- 39]. The reflectivity R can be transformed into a value proportional to the absorption (A) using Kubelka-Munk function F(R∞) [40- 42]: F(R ) =

=

(3)

where (k) is the absorption coefficient and (s) is the scattering factor nearly independent of wavelength. The diffused reflectance spectra of pure and Fedoped samples together with the transformed absorption spectra over the range 200 – 2000 nm are given in Fig.(5).

11

STO STFO

Absorbance

0.8 0.6

80

Reflectance (%)

1.0

60 40 20 0

0.4

400

800 1200 1600 2000 Wavelength (nm)

0.2 0.0 0

400

800 1200 1600 2000 Wavelength (nm)

Fig.(5): Comparison of the optical absorption spectra of pure and Fe doped STO samples. Inset is a comparison of the diffused reflectance spectra.

The absorption spectrum in Fig.(6) shows a high intense broad structure extending from ⁓ 0.5 eV to ⁓ 6 eV i.e. from the UV region to near IR. Pure STO, in contrary, shows only UV absorption. The deconvolution of the absorption spectrum yielded four identified absorption features situated at 1.17 eV (∆E1), 2.61 eV (∆E2), 3.79 eV (∆E3) and 5.52 eV (∆E4). Fig.(7) gives a schematic representation of the estimated transitions and table (3) summarizes these data.

12

1.0 0.9 0.8

Absorbance

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

1

2

3

4

5

6

7

Energy (ev)

Fig.(6): Deconvolution of the absorption spectrum of STFO sample.

Fig.(7): A schematic representation of the estimated transitions and the corresponding energy difference between the final and initial states (Ef – Ei) for STFO.

Table (3): Data for the estimated transitions and the corresponding energy difference between the final and initial states (Ef – Ei) for STFO system. ΔE (Ef – Ei) (eV)

Transition type

ΔE1

1.17

Defect level → Ti3d (t2g)

ΔE2

2.61

Defect level → Ti3d (t2g)

ΔE3

3.79

O2p → Ti3d (t2g)

ΔE4

5.52

O2p → Ti3d (t2g)

13

According to Mössbauer investigation, the absorption peak observed at 1.17 eV (Fig.(6)) is probably due to Fe3+ species situated below the Ti-3dt2g band edge, while the second absorption peak at 2.61 eV can be assigned to Fe4+ situated above the O-2p. No other studies on Fe doped STO have mentioned the presence of the two absorption bands observed at 3.79 eV and 5.52 eV. These two absorption structures can be attributed to charge transfer excitation (CTE) transitions between O2p and Ti3d states. Two absorption peaks located at 2.1 eV and 2.9 eV in the Fe doped STO single crystal were reported [36, 43]. Wild et al. [36] identified the peak at 2.9 eV as being due to absorption of electrons trapped at Fe4+ ions placed in a cubic site about 0.3 eV above the valence band. The 2.1 eV peak was suggested to be a Fe3+ (axial) with an oxygen vacancy. The associated transition is between this level and the Ti-3d level inside the conduction band. Morin et al. [44] reported for STO crystal, two absorption centers at 1.06 eV and 0.7 eV above the valence band assigned respectively to Fe4+ and Fe5+. To investigate the effect of the metal atoms at the Ti site on the electronic properties of STO, the total density of states (DOS) and the projected density of states (PDOS) of doped STO structure were calculated [4]. Fe doping at Ti site was found to reduce the band gap value. Xie et al. [6] reported also that the substitution of Fe in Ti introduces some impurity states above the top of the valence band resulting in narrowing a band gap of about 1.1 eV that could be responsible for the obvious red shift of the absorption edge for Fe-doped STO whereas the conduction band edge showed no significant change. Comes et al. [13] (by using Tauc formula for a dipole forbidden direct transition) determined a value of 0.83(5) eV for Fe doped STO. The authors suggested that certain technique was needed to raise the bottom of the conduction band in order to attain an Eg value ≈ 1.5 eV available for photocatalytic and photovoltaic applications. Recently, Moghadam et al. [14] have shown that solid solution of 14

SrTiO3 (Eg ≈ 3.2 eV) and SrZrO3 (Eg ≈ 5.6 eV) produced a wider (Eg > 0.8 eV) tunable range of band gap by removing low-lying Ti-3d states. According to the authors, this may help to realize photovoltaic and photocatalytic applications. On the other hand, Xie et al. [6] reported that doping STO with Fe cation exhibited an absorption tail extending from the UV to about 650 nm (⁓1.9 eV). The authors demonstrated that the visible light excitation of Ti VI–O–FeII linkage is the cause of the enhanced degradation of RhB. The optical absorption of light in the UV−visible range was manipulated to estimate the optical band gap of the STFO sample. The values were obtained according to Eq.(3) [45], αhυ = β (hυ – Eg)n

(4)

where α is the absorption coefficient, β is a parameter determining the material quality, Eg is the optical band gap and n has different values according to the optical transition type. Accordingly, using Eq. (4), sF(R∞) hυ = β (hυ – Eg)n

(5)

s is the scattering factor that nearly independent of wavelength and was ignored. Many authors reported that diffused reflectance measurements give an approximation within a few hundredth of an electron volt for the Eg value [4648]. A plot of [hυ F(R∞)]2 vs. hυ for STFO sample is shown in Fig.(8). The inset shows the corresponding plot for the prepared STO sample.

15

120 100

[hv.F(R)]

2

120

[hv.F(R)]

2

100

80 60 40 20

80

0

3.3 eV 0

1

60

2

3

4

5

6

7

Energy (eV)

40 20 1.433 eV

0 0

1

2

3

4

Energy (eV)

5

6

7

Fig.(8): Plot of [hυ F(R∞)]2 versus photon energy for STFO sample. The inset shows the corresponding plot for STO sample.

It was interesting that the Eg value determined from the straight line intersection with the abscissa at 1.43 eV (Eg (STO) = 3.3 eV) is an ideal value to produce intense visible absorption. Similar trials to close up the band gap of SrZrO 3 by Fe doping of samples prepared by the same steps were carried out by our group. A reduction of the Eg value from 4.4 eV for pure SrZrO3 to 2.4 eV for Fe doped sample was attained. More research is in progress for more narrowing of the forbidden gap of SrZrO3 to be suitable for photocatalytic and photovoltaic applications. 3.5 Optical investigation by photoluminescence (PL) spectroscopy Fig.(9) displays the PL spectrum of Fe doped STO sample. The emission line is represented by a broad peak extending between ⁓200 nm ‫⁓ ـــ‬700 nm. Deconvolution of this peak results in five definite emission lines spreading over the green-blue emission region (table (4)). 16

PL Intensity

60 50 40 30 20 10 0 400

500

600

700

Wavelength (nm) Fig.(9): PL emission spectrum of STFO sample. The deconvoluted peaks are also illustrated. Table (4): PL emission lines of STFO sample Emission

nm

547

467

459

448

435

lines

eV

2.27

2.66

2.7

2.77

2.85

M.Ghaffari et al. [49] while analyzing the XPS results for Fe doped STO prepared by a high temperature solid state reaction method reported that the iron present in the STO sample is composed of a mixture of Fe 3+ and Fe4+. When the Fe content was increased, the density of Fe3+ and Fe4+ increased and the oxygen lattice decreased on the surface. The presence of Fe3+, Fe3+-OV and Fe4+, Fe4+OV as deduced from Mössbauer study are the main origin of the emission lines for the STFO under study. 4. Conclusion STO nanocrystalline perovskite doped with Fe has been prepared by repeated cycles of milling and sintering at high temperature. XRD analysis revealed the formation of single phase cubic STFO. High intense broad 17

absorption band extending from 0.5 eV to 6 eV has been observed, whereas pure STO gave a strong absorption only at the ultraviolet region (λ < 400 nm). For the first time, an ideal perovskite having a value of the band gap width (1.43 eV) compatible with photovoltaic and photocatalytic applications was accomplished. The close up of the optical band gap was also confirmed through the photoluminescence analysis, which revealed the spreading of many defect centers across the gap. The type of the Fe species and the relative Fe abundance were determined by Mössbauer investigation. The amount of oxygen deficient, which give rise to Fe-Ov defect complexes was also determined. This study offers a new avenue to synthesize ceramic materials with optical properties of high efficiency, allowing advances of great importance in the field of PV and PC applications. Acknowledgement The authors are grateful to the Grants Commission of Al-Azhar University–Cairo–Egypt for supporting this work. The authors are Thankful to Nanotechnology Characterization Center (NCC)-Cairo University and Central Metallurgical Research Institute (CMRDI)-El Tebeen for extending the XRD and UV-Vis. Diffused reflectance facility. References 1. K.Domen,

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