solar light: Extended visible light absorption by the bulk lattice F− ions and suppression of photogenerated charge carrier recombination by the surface F− ions

Journal Pre-proofs Research paper Photocatalytic activity of Fluorine doped SrTiO3 under the irradiation of UV/

solar light: Extended visible light absorption by the bulk lattice F- ions and suppression of photogenerated charge carrier recombination by the surface Fions B.G. Anitha, L. Gomathi Devi PII: DOI: Reference:

S0009-2614(20)30053-1 https://doi.org/10.1016/j.cplett.2020.137138 CPLETT 137138

To appear in:

Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

25 October 2019 10 January 2020 20 January 2020

Please cite this article as: B.G. Anitha, L. Gomathi Devi, Photocatalytic activity of Fluorine doped SrTiO3 under

the irradiation of UV/solar light: Extended visible light absorption by the bulk lattice F- ions and suppression of photogenerated charge carrier recombination by the surface F- ions, Chemical Physics Letters (2020), doi: https:// doi.org/10.1016/j.cplett.2020.137138

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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.

© 2020 Published by Elsevier B.V.

Photocatalytic activity of Fluorine doped SrTiO3 under the irradiation of UV/solar light: Extended visible light absorption by the bulk lattice F- ions and suppression of photogenerated charge carrier recombination by the surface F- ions B. G. Anitha, L. Gomathi Devi* Department of Post Graduate Studies in Chemistry, Jnana Bharathi Campus, Sneha Bhavana, Bangalore University, Bangalore 560056, India Email: [email protected] Abstract: The SrTiO3 and F doped SrTiO3 samples retains cubic perovskite structure. XPS results confirmed the presence of F- ion on the surface and also in the bulk lattice. Energy levels like Ti3+- Vo states and oxygen vacancies (Vo) existing as neutral / singly ionized (F+ center) / doubly ionized (F center) states and the dopant energy levels are responsible for the visible light photocatalytic activity of the doped samples. Further the surface fluorine acts as an efficient electron trapping center due to the high electronegativity of F- ions and it also hinders the electron hole recombination. The existence of the above energy states are confirmed by the XPS and PL techniques. The enhanced photocatalytic activity is attributed to the synergistic effects of both surface and bulk fluorine ions in the SrTiO3 lattice. Keywords: Fluorine doped SrTiO3, F/F+ centers, Cubic perovskite structure, Substitutional lattice position, Defect energy states.

1. Introduction

The utilization of semiconductor materials in the photocatalytic waste water treatment and also in the production of hydrogen as a clean and renewable energy resource under the light irradiation has drawn the attention of researchers [1, 2]. Various semiconductors such as TiO2, ZnO and SrTiO3 with an appropriate energy band gap and with suitable band edge positions matching with the redox potential of water were used for the generation of hydrogen from water splitting reaction under the illumination of light energy. These materials were also used as photocatalysts for the degradation of organic/inorganic pollutants in air/water under UV/solar light irradiation [3-5]. Among the various metal oxides photocatalysts, perovskite SrTiO3 of ABO3 (A2+B4+O3) type structure has drawn the attention of researchers due to its attributes in the photoelectrochemical water splitting reaction and also in the photodegradation of organic pollutants [6-7]. Perovskite SrTiO3 (STO) is resistive to corrosion in aqueous solutions, possess higher potential due to its larger band gap and shows good structural stability towards the incorporation of metal/nonmetal ions into the lattice [8, 9]. STO is a wide bandgap metal oxide with bandgap energy (Eg) of 3.0-3.2 eV. Its practical use is restricted under the illumination of visible light and is found to be active only under UV light (wavelength < 387 nm). Thus for the efficient utilization of solar light, many strategies are adopted such as photosensitization by organic dyes/inorganic metal complexes, surface modifications like sulphation/fluorination/phosphation and doping of suitable metal/nonmetal ions with higher or lower valances at Sr2+ or Ti4+ or O2- lattice sites [10, 11]. Incorporation of non-metal ions like C, P, N, S and F at oxygen lattice site is found to reduce the band gap energy either by the formation of distinct energy levels above the valance band (VB) or by merging the dopant energy levels within the existing VB [12-14]. Literature shows higher photocatalytic activity for F incorporated TiO2 under both UV/visible light due to the modification of its optical properties [15]. Its higher activity was accounted to several factors like

creation of surface oxygen vacancies, efficient interfacial charge transfer dynamics, increase in surface acidic properties and number of active sites on the catalyst surface [16]. In the present research an attempt is made to incorporate fluorine (F-) ions into the STO lattice by wet impregnation method to study the structure-reactivity correlation. Efficiency of the prepared samples were evaluated for the photodegradation of the methyl orange (MO) dye under UV/solar light irradiation. 2. Materials and methods 2.1. Chemicals Tetra butyl titanate (97%) was obtained from Sigma Aldrich Limited. Citric acid, strontium nitrate, sodium fluoride and ethanol (HPLC grade) were obtained from Merck Chemicals Limited. MO was obtained from SD Fine Chemicals Limited. Double distilled water was used in all the experiments. 2.2. Preparation of catalysts 2.2.1. Preparation of pure STO STO was prepared by sol-gel method as reported earlier and the method adopted was similar to the procedure reported by Bui et al. [17, 18]. 2.2.2. Preparation of fluorine doped STO catalyst. Fluorine was incorporated into STO lattice by wet impregnation method and the method adopted is similar to the method suggested by W. Choi et al., for the TiO2 catalyst [19, 18]. 1g of STO was dispersed in 10 ml of 10 mM NaF solution. The pH value of the NaF solution was adjusted to 3 with dilute HCl. The above solution was ultra-sonicated for 30 minutes and the catalyst sample was filtered and dried in an oven to get the fluorine doped STO sample with 10mM fluorine content (FSTO-10). Two other catalyst samples were prepared by similar procedure with higher NaF

concentration. The samples containing 50 and 100 mM NaF concentration were termed as FSTO50 and FSTO-100 respectively. 10, 50 and 100 mM NaF solution contains 0.01899 g, 0.09495 g and 0.1899 g of fluorine respectively. More precisely the stoichiometric molecular formulae of the doped samples can be represented as SrTiO2.981F0.019, SrTiO2.905F0.095 and SrTiO2.81F0.19 for FSTO10, FSTO-50 and FSTO-100 respectively. 2.3. Experimental details. The catalysts were characterized by the following techniques: powder X-ray diffraction (PXRD), Uv-visible absorption spectroscopy, photoluminescence (PL) spectroscopy, scanning electron microscope (SEM) coupled with energy-dispersive X-ray spectroscopy (EDAX) (TESCAN Vega 3 LMU microscope operating at 5-30 kV), X-ray photoelectron spectroscopy (XPS), Brunner– Emmet–Teller (BET) method for determining surface area and determination of pore volume by BJH method. The details of the instruments used and the experimental conditions of these techniques are mentioned in our previous research article [1, 20-21]. The details regarding the photocatalytic experiments and the elaborate experimental procedures were also mentioned in our previous reports [18, 21]. 3. Results and discussions. 3.1. PXRD studies The PXRD pattern of STO, FSTO-10 FSTO-50 and FSTO-100 samples showed peaks at 32.35◦ (110), 39.91◦ (111), 46.42◦ (200), 57.72◦ (211), 67.74◦ (220) and 77.09◦ (310) corresponding to the standard PXRD pattern of STO with cubic unit cell (JCPDS card No: 74-1296) (Fig.1a). The incorporation of F- ion into the STO lattice had not lead to any structural change. The PXRD peak positions of FSTO samples shows a shift by 0.01◦ towards the higher 2 θ value and the magnitude of the shift increases with increase in the concentration of F- ions (Fig. 6.1 b). The dimensions of

the STO frame work can be determined by the Ti-O bond length which requires lattice parameter ‘a’ to be equal to √2 (R Ti +RO). This determines the size of dodecahedral void in which ‘A’ cation can be accommodated if it possesses permissible size for the occupation. Perovskite structure is stabilized based on the Goldschmidt’s tolerance factor‘t’ which is given by following equation [22]:

t=

RSr  RO 2(R Ti  RO )

(1)

Where RSr (0.144 nm), RTi (0.060 nm) and RO (0.138 nm) are the ionic radius of Sr2+, Ti4+ and O2ions respectively. The solid adopts perovskite structure when the t factor is in the range of 1 ≥ t ≥ 0.75. The value of t was calculated for FSTO sample by substituting the radius of F- ion (RF = 0.133 nm) instead of RO in the above equation. The calculated t values were found to be 1.0 and 1.01 for STO and FSTO samples respectively. These values confirm the perovskite structure for both STO and FSTO samples. The average crystallite size of all the samples were estimated using Scherer’s equation D = kλ / βcosθ, where λ is the wavelength of the Cu Kα source (λ=1.541 Å), β is the full width at half maximum (FWHM) of (110) diffraction peak. Crystallite size values were found to decrease with increase in the F- concentration may be due to the smaller ionic radius of fluorine compared to the ionic radius of oxygen (Table 1). The cubic unit cell parameters (a = b = c) were calculated by using the following equations: nλ 2sinθ

 2

1 h 2 k 2 l2 = + + d 2hkl a 2 b 2 c 2

 3

d hkl =

Where d hkl is the distance between crystal planes, θ is diffraction angle, λ is the wavelength of Xray used, n is an integer and a, b, c are the unit cell lattice parameters. X-ray density (ρx) values

can be calculated by using the formula ρx = ZM / N a3, where Z is the number of formula units per unit cell (Z = 1 for STO), M is the formula weight of the unit cell of STO which is equal to 183.484 g/mol, ‘a’ is the unit cell parameter and N is the Avogadro number (6.022 × 1023 mol−1). It is observed that ρx values increases with increase in dopant F- ion concentration (Table.1).

110 200

211

110

a

30

40

50

60

70

Intensity (arb. units)

FSTO-100 FSTO-50 FSTO-10 STO

20

b

310

220

Intensity (arb. units)

111

31.0

80

31.5

32.0

32.5

33.0

33.5

2 Theta (degrre)

2Theta (degree)

3.92 c

d

5.5

X-ray density value

Unit cell parameter

3.90 3.88 3.86 3.84 3.82

5.0 0

20

40 60 80 Dopant F concentartion

100

3.80 0

40 80 Dopant F concentration (mM)

120

Fig.1. (a) PXRD patterns of STO, FSTO-10, FSTO-50 and FSTO-100 samples, (b) PXRD patterns showing shift in (110) hkl plane of STO (c) plot of X-ray density versus F- ion dopant concentration and (d) plot of unit cell parameter versus F- ion dopant concentration (Vegard’s law).

The F- ion can get incorporated into the STO lattice in two different ways. It can either enter substitutional lattice position or interstitial position. It is usually assumed that substitution can occur only if the radius of the substitutional atom does not exceed the radius of the host atom by more than 15%. If the ionic size of the substituent atom is small, such an atom can also occupy interstitial position. Fluorine with higher atomic weight than oxygen should occupy substitutional lattice position. The occupation site of the dopant ion can be readily distinguished by the plot of X-ray density versus concentration of the dopant F- ion (Fig. 1c) [23]. The linearity of this plot and its positive slope confirms the occupation of F- ions at substitutional lattice position. The substitution of F- ion at O2- lattice site leads to the remarkable contraction of unit cell volume and a small decrease in the lattice parameter [24]. According to Vegard’s law the dimensions of the unit cell parameter change linearly with dopant composition (Fig. 1d) [25]. Vegard’s law is not really a law but, rather a generalization that applies to solid solutions formed by the random substitution or distribution of ions. It assumes implicitly that the change in the values of unit cell parameter with composition is governed purely by the relative sizes of atoms or ions that are substituted. The values of crystallite size, lattice parameters, unit cell volume and X-ray density (ρx) of STO, FSTO-10, FSTO-50 and FSTO-100 samples are given in the Table.1.

Table 1. The values of crystallite size (D), lattice parameter a (a = b =c for a cubic unit cell), Xray density (ρx) and unit cell volume (V) of STO, FSTO-10, FSTO-50 and FSTO-100 samples.

Samples

D (nm)

a (Å)

ρx (g cm-3)

V (Å) 3

STO FSTO-10

24.36 25.11

3.89 3.88

5.17 5.22

58.9 58.4

FSTO-50

22.8

3.87

5.26

58.0

FSTO-100

21.2

3.82

5.47

55.7

3.2 UV-Visible absorption spectra. The optical absorption properties of all the catalysts were investigated using UV-visible absorbance spectral technique (Fig.2a). The band gap energy values of all the samples were calculated by using the Kubelka-Munk plot where, [F (R∞) hʋ] 1/2 is plotted versus photon energy (eV) (Fig.2b) [18]. The band gap energy values were found to be 3.23 eV, 3.20 eV, 3.17 eV and 3.08 eV for STO, FSTO-10, FSTO-50 and FSTO-100 samples respectively. The band gap value gradually decreases with increase in the dopant concentration.

2.0 STO FSTO-10 FSTO-50 FSTO-100 a

0.4

STO FSTO-10 FSTO-50 FSTO-100

1.6

b

1.2

[ F ( R) hv ]

Absorbance (arb.units)

0.5

1 /2

0.6

0.3

0.8

0.2

0.4 0.1

0.0

0.0 300

400

Wavelength (nm)

500

2

3 Band gap in eV

4

Fig. 2 (a) UV−vis absorbance spectra and (b) plot of [F (R∞) hʋ] 1/2 versus photon energy (eV) of STO, FSTO-10, FSTO-50 and FSTO-100 samples. 3.3. SEM and EDX analysis

The texture and morphologies of STO, FSTO-10, FSTO-50 and FSTO-100 photocatalyst samples were obtained by SEM analysis (Fig. 3 a-d). The morphology of STO sample was found to be long distinct nano rods. FSTO-10 sample retains the nano rod morphology. But these nano rods were found to be thicker and shorter compared to the morphology of STO sample. This rod-like morphology was completely destroyed in the case of SEM images of FSTO-50 and FSTO-100 samples. The morphology of these samples was found to be completely different and they appear as flat plate-like structures. The nano rod morphology was completely destroyed by F- ions at higher concentrations in the FSTO-50 and FSTO-100 samples may be due to the higher electronegativity and etching characteristics of these ions. EDX technique was used for qualitative and quantitative determination of elements present in the samples (Fig.4). The elemental analysis of the prepared samples clearly indicates the expected catalyst profile and confirms the successful incorporation of F- ions by the wet impregnation method.

a) STO

b) FSTO-10

c) FSTO-50

d) FSTO-100

Fig. 3 SEM images of (a) STO, (b) FSTO-10, (c) FSTO-50 and (d) FSTO-100 samples.

Fig. 4 EDX patterns of STO and FSTO-50 samples. 3.4 BET studies. Nitrogen adsorption–desorption isotherms were recorded to determine the values of specific surface area and pore volumes of the prepared photocatalyst samples (Fig.5). The pore size

distribution curves for STO, FSTO-10, FSTO-50 and FSTO-100 samples are shown in inset of Fig.5. The obtained isotherms of all the samples can be compared to the standard pattern of type IV isotherms as per the classification of IUPAC system. The nitrogen adsorption–desorption isotherms of all the samples exhibit H3 hysteresis loop implying the capillary condensation of N2 molecules on the surface of the catalysts at relatively low partial pressure due to the mesoporous structure. The pore size distribution curves were obtained by plotting the values of volume of nitrogen desorbed versus pore diameter by the BJH method. The observed increase in the pore volume values with increase in F- content may imply etching process by F- ions and the destruction of nano rod morphology. The experimentally obtained values of BET surface area, pore volume and average pore diameter of all the samples are given in Table 2. Table 2. The values of BET surface area, pore volume and average pore diameter of STO, FSTO10, FSTO-50 and FSTO-100 catalyst samples. Photocatalyst

BET surface area (m2 g−1) 8.09 7.55 11.6 13.4

STO FSTO-10 FSTO-50 FSTO-100

STO FSTO-10 FSTO-50 FSTO-100

35 Pore Volume X 10 -3 in cc/nm /g

Volume Adsorbed @STP in cc/g

40

30 25 20 15 10

Pore volume (cm3 g−1) 0.029 0.030 0.036 0.045

0.02

0.00 0

2

4

6

8

10 12 14 16 18 20 22 24 26

Pore Diameter in nm 5 0 0.0

0.2

0.4

0.6

Relative Pressure (P/P0)

0.8

1.0

Average pore diameter (nm) 13.7 16.2 12.6 13.4

Fig. 5 The nitrogen adsorption-desorption isotherms and the pore size distribution curves (inset) of STO, FSTO-10, FSTO-50 and FSTO-100 samples.

3.5 XPS Analysis

Fig.6. Wide scan XPS of (a) STO, (b) FSTO-50 samples. XPS of (c) 3d BE peaks of Sr2+ of STO and FSTO-50, (d) 2p BE peaks of Ti4+ of STO , (e) 2p BE peaks of Ti4+ and Ti3+ of FSTO-50, (f) 1s BE peaks of O of STO and FSTO-50 and (g) 1s BE peaks of F- of FSTO-50 sample. The Sr 3d5/2 and 3d3/2 spin states were observed at the BE values of 132.4 eV and 134.2 eV for STO sample and at 132.6 eV and 134.3 eV for FSTO-50 sample confirming the presence of Sr2+ in the STO and FSTO lattice (Fig. 6c). The Ti 2p3/2 and Ti 2p1/2 BE peaks were observed at 458.76

eV and 464.6 eV with the spin-orbital splitting energy (Δ = Ti 2p1/2- Ti 2p3/2) of 5.84 eV suggests the presence of Ti4+ ions in the STO lattice (Fig.6 d). In the case of FSTO sample there were four different Ti 2p BE peaks. The slightly shifted BE peaks pertaining to Ti 2p3/2 and Ti 2p1/2 states were found at 459.29 eV and 463.56 eV confirming the presence of Ti4+ ions and also at 462.2 eV and 457.8 eV corresponding to the presence of Ti3+ ions in the FSTO-50 lattice [26, 27]. It is observed that the BE peaks pertaining to Ti4+ state has decreased in intensity, since some of the Ti4+ ions are reduced to Ti3+ state to maintain the electrical charge neutrality in the FSTO sample. The O1s BE peaks of STO and FSTO-50 samples are shown in Fig. 6f. STO sample shows O1s BE peak at 529.8 eV corresponding to the oxygen of O-Ti bond. The FSTO-50 sample show three BE peaks at 528, 530.5 and 531.7 eV. The peak at 528 eV can be assigned to O-Ti bond. The observed oxygen BE peaks at 530.5 eV and 531.7 eV can be due to the oxygen of surface hydroxyl groups and chemisorbed oxygen molecules respectively [28]. FSTO sample shows characteristic F1s BE peaks at 683.4 eV and 688.7 eV corresponding to the physisorbed fluoride ions on the surface and substituted F- ions at the oxygen lattice sites (Fig. 6 g) [14].

Table 3. Qualitative and quantitative analysis of atom % of different elements present in STO and FSTO-50 samples from EDAX and XPS techniques. Samples

EDAX

STO FSTO-50

Sr 42.57 40.69

XPS Ti 26.10 20.34

O 31.33 18.70

F 20.27

Sr 39.57 32.17

Ti 23.59 22.37

O 36.84 21.87

F 23.59

3.6. Photoluminescence (PL) studies. The PL spectroscopic technique gives the fundamental information about the electronic structure and the optical properties of the semiconductor. It gives an insight into the process of

photogenerated charge carrier recombination corresponding to the band gap and also with respect to the presence of surface states and dopant energy levels. The observed PL emission is due to the recombination of photogenerated electrons and holes. The intensity of the peaks in the PL spectra is directly proportional to the rate of recombination of photogenerated charge carriers. The PL spectra of pure STO can be compared to the FSTO samples.

160 STO

140

FSTO-10 FSTO-50

120

Intensity in (a.u.)

FSTO-100 100 80 60 40 20 0 440

460

480

500

Wavelength (nm)

Fig. 7 PL spectra of STO, FSTO-10, FSTO-50 and FSTO-100 samples. The conduction band (CB) and VB of STO were formed by the 3d orbitals of Ti4+ ion and 2p orbitals of O2- ion respectively. The 5s orbitals of Sr2+ ions were found to be energetically higher compared to the 3d orbitals of the Ti4+ ion and hence they do not contribute for the band gap energy. The emission peak observed at 440 nm is ascribed to the recombination of photogenerated electron-hole pairs corresponding to the charge transfer from the central Ti4+ ion to the neighboring O2- ions inside the TiO68- octahedron. The intensity of this peak decreases in the following way: FSTO-100 > STO > FSTO-10 > FSTO-50. The PL intensity of the peak at 440 nm was found to be lower for FSTO-50 sample compared to all the other samples suggesting the rate of recombination of photogenerated charge carriers is reduced by 22 % (Fig.7). The observed PL emission around 488 nm can be assigned to the emission occurring from the charge transfer

transitions from the VB to Ti3+-Vo defect states. Ti3+-Vo states are usually located below the CB around 0.3 eV where Vo represents the oxygen vacancies. The intensity of this peak is slightly decreased for FSTO-50 sample. The observed PL peak around 460 nm is usually assigned to the charge transfer transition of trapped electron in a doubly/singly ionized oxygen vacancies V o., V .. and they are also referred to as F centers (oxygen vacancy with two trapped electrons V ..) and o

o

F+ centers (oxygen vacancy with one trapped electron V o.) respectively [15]. All the above peaks are observed for both STO and FSTO samples since these defects can be invariantly present in the STO sample and their proportions may vary in the FSTO doped samples based on the concentration of dopant. However one can observe lower PL peak intensities for FSTO-50 and FSTO-10 samples compared to STO and FSTO-100 samples. The peak intensities have decreased by almost 20% for FSTO-50 and FSTO-10 samples. But with further increase in fluorine content the PL peak intensity increases. This suggests that if the dopant concentration increases above the optimum limit, dopant energy levels may facilitate recombination process [29]. Low trap density, high carrier mobility and efficient charge transfer process are the key parameters to reduce the photogenerated charge carrier recombination [30]. 3.7. Photocatalytic activity of STO, FSTO-10, FSTO-50 and FSTO-100 samples. 3.7.1 Effect of pH conditions. The pH condition of the reaction solution is a complex parameter and it plays a significant role for the effective mineralization of pollutants. The influence of pH on the photocatalytic degradation reaction can be summarized as follows: (i) surface charge on the catalyst depends on the pH and its isoelectric point; (ii) adsorption characteristics of substrate molecule on the catalyst surface depends on various factors like nature of the dye (cationic/anionic/neutral) and surface charge of

the catalyst; iii) band edge positions of the semiconductor can show slight variation with the change in the pH conditions and iv) aggregation of catalyst particles. The point of zero charge (PZC) of STO in aqueous solution is ~8.5 ± 0.3 as widely reported in the literature [31]. The charge density on the STO surface is positive below the PZC value and negative above it. The rate of MO degradation reaction with STO is found to be higher at pH 3 compared to pH 7 and pH 9. In the acidic condition, the MO is found to be in the quinonoid form and possess lower conjugation compared to azo form (Scheme 1) [32]. The color of MO dye depends on the pH conditions, more specifically on the concentration of H+ and OH- ions. MO is red colored below the pH 3 and changes to yellow above pH 4.5 and the pKin value is ~3.7. The adsorption characteristics of MO depends on the concentration of ionized indicator MO- and protonated indicator HMO in the acidic solution and it also depends on the concentration of ionized indicator MO+ and hydroxylated indicator MOOH in the alkaline solution [33]. According to Ostwald’s theory ionization will be reduced in the presence of excess H+ ions due to the common ion effect and the concentration of MO- will be very small and the color will therefore be that of unionized. The ionization increases in the alkaline medium and the dye shows the color of ionized form. The concentration of the ionized and unionized forms is thus directly related to the hydrogen ion concentration. However according to Hantzsch and others both the forms of MO are present at any pH condition. According to them, the adsorption may not depend on the ionization alone. Further, MO can also exist in two or more tautomeric forms like benzenoid, quinonoid and also as a non-electrolyte form which can also be referred to as pseudo acid or pseudo base [33]. MO- in acidic condition can give rise to quinine diimine, azonium and ammonium tautomer as shown in Scheme 2. The concentration ratios of MO-/HMO and MOOH/MO+ in acidic and basic solutions determine the adsorption characteristics on the surface of photocatalysts. The combined effect of surface charges of the

catalyst and the ionized concentration of dye molecules will determine the extent of adsorption. In the case of STO sample, the enhanced photodegradation activity in the acidic condition may be attributed to the quinonoid form of the MO molecule which is less stable compared to conjugated benzenoid form. The hydroxylated STO can be protonated under acidic conditions and deprotonated under alkaline conditions and hence the positively charged STO surface favors the adsorption of negatively charged molecules. At high pH conditions both the catalyst and substrate MO molecules are negatively charged leading to the columbic force of repulsion which may lead to the decrease in the degradation rate. Rate constant values and percentage of MO degraded under UV light illumination at different pH reaction conditions are given in Table 4 and Fig. 8. O CH3 N

N

N

S

ONa

CH3 O

Methyle Orange

O CH3 N+

NH

N

S

O-

CH3 Quinonoid form in acidic medium _ H+ OH

O O

CH3 N

N

N

S

O-

CH3 Benzenoid form in basic medium

O

Scheme 1: Structure of MO in acidic and basic medium

O CH3 N+

N

S

NH

O-

CH3

(a) CH3 N

N

N

O O

+

S

O-

CH3 H

O O

(b) CH3 N

+

N

N

S

(c)

O

O-

CH3 H

Scheme 2: MO- in acidic condition can give rise to (a) quinine diimine; (b) azonium and (c) ammonium tautomer. Table 4. The values of reaction rate constant (k) and percentage of MO degraded in a time period of 180 min using STO catalyst under UV light illumination at specified pH conditions. Catalyst

pH

Time (min)

STO

3

180

Percentage of k × 10-2 min-1 MO degraded 69 0.58

STO

7

180

5

0.03

STO

9

180

3

0.02

1.6

Absorbance(arb.units)

1.2 1.0

pH 3

0.8 0.6 0.4

2.5

Absorbance(arb.units)

0min 30min 60min 90min 120min 150min 180min

1.4

o min 30 min 60 min 90 min 120 min 150 min 180 min

2.0

1.5

pH 7 1.0

0.5

0.2 0.0

0.0 300

400

500

Wavelength (nm)

600

300

400

500

Wavelenght (nm)

600

o min 30 min 60 min 90 min 120 min 150 min 180 min

Absorbance(arb.units)

2.5 2.0 1.5

pH 9 1.0 0.5 0.0 300

400

500

600

Wavelenght (nm)

Fig. 8. Photocatalytic degradation of MO using STO photocatalyst sample at different pH values (pH 3, pH 7 and pH 9). 3.7.2 Comparison of photocatalytic activities. Photocatalytic degradation experiments with FSTO-10, FSTO-50 and FSTO-100 catalyst samples were performed at pH 3 under the irradiation of UV/solar light based on the results obtained for STO. Efficiency of a photocatalyst generally depends on various parameters such as the rate of photogeneration of electron-hole pairs, lifetime of the photogenerated charge carriers, surface charge/acidity, surface area, crystallinity, crystallite size, nature of the dye (cationic/ anionic / neutral) and number of surface active sites. The extent of MO degraded and the kinetics of the reaction is followed by the plot of lnC/C0 versus time (where C0 is the initial concentration of the substrate MO molecule and C is the concentration at time t). This plot implicates the reaction to be of first order (Fig. 9a and b). The rate constant values for the various reaction systems were calculated based on the obtained negative slope. Higher photocatalytic degradation efficiency was observed for FSTO-50 sample when compared to all the other photocatalysts under both UV/solar light illumination and the activity of various catalysts can be graded as: MO< FSTO-100
increase in the dopant F- ion concentration. The efficiency increases up to 50mM concentration of F- ions and further increase in the F- ion concentration results in lower efficiency [34]. Hence the optimum concentration of F- ion was found to be 50 mM. The percentage of MO degraded with FSTO-50 photocatalyst was found to be 99.7 % in 90 minutes time duration under the UV irradiation and 56 % in 180 minutes under the solar light irradiation. The higher efficiency of FSTO-50 photocatalyst sample can be attributed to the presence of highly electronegative F- ions in the optimum amount and also to the synergistic effect of surface F- ions with the bulk F- ions in the lattice. Further the higher activity can be attributed to several factors like: i) minute variations in the position of dopant energy level may take place with change in the dopant concentration; ii) the dopant energy level may facilitate electron transfer process and extend the response to the visible region; iii) above the optimum dopant concentration, the dopant energy level is shifted away from the Fermi energy level in such a way that this dopant energy level may facilitate recombination by trapping both holes and electrons. This fact is confirmed by the changes observed in the UV-visible absorption spectroscopic technique. The magnitude of the indirect band gap value decreased with increase in fluorine content; iv) Ti3+ species in an octahedral environment introduces localized states just below the CB. The concentration of Ti3+-Vo states in the catalyst will increase with increase in F- content and these states facilitate the charge transfer process up to the optimum dopant concentrations. The presence of Ti3+ states in the FSTO-50 sample was confirmed by the XPS studies; v) the charge carriers generated within the space charge region remain separated for longer time periods due to the potential experienced by them in this region. The higher number of Ti3+ states above the optimum dopant concentration may lead to the decrease in width of the space charge region leading to the decrease in the efficiency of the catalyst, since the dopant concentration is inversely proportional to the thickness of the space charge region [35,

36] ; vi) the presence of highly electronegative F- ions on the surface of STO is confirmed by the XPS technique and these ions acts as electron trapping centers reducing the photogenerated charge carrier recombination. The surface states created by the F- ions acts as mediator for the interfacial charge transfer process by trapping the CB electron and detrapping it to oxygen molecule. Efficient trapping and detrapping of electrons takes place only when the surface fluorine ions are at optimum concentration. Above this optimum concentration the trapping centers are close to one another and the possibility of trapping both electrons and holes are maximum, hence the efficiency decreases; vii) the presence of surface states created by the dopant F- ions are very essential for the exchange of charge carriers between the electronic states on the semiconductor and the appropriately located redox energy level of MO within the solution. The interfacial charge transfer between the electrode and the solution is possible only when the energy difference is not more than ± kT, where k is the Boltzmann constant and T is absolute temperature. However the position of these surface states may change with higher concentration of dopant ions and may facilitate recombination at higher dopant concentrations; viii) the NaF which is used as the precursor of F- ions may etch the STO surface in such a way that the surface becomes inhomogeneous in a complex way. Etching process breaks several bonds on the catalyst surface leading to the unsaturation. These effects may increase the reactivity of the catalyst; ix) surface etching process may lead to the oxidation and reduction reactions which are individually dominant. Reduction causes continuous shift of Ti2p1/2 and Ti2p3/2 BE peaks in the XPS of FSTO samples and hence the Ti exists in mixed valance states Ti3+/Ti4+; x) alternatively oxidized surface may retain higher content of OH- ions as reflected by the O1s XPS peaks, which are highly essential for the formation of hydroxyl free radicals for the efficient photocatalysis ; xi) the presence of negatively charged surface F- ions on the FSTO sample may change the acidic/basic properties; xii) the presence of surface fluorine increases the hydrophilicity

of STO powders resulting in large number of adsorbed water molecules for the formation of hydroxyl free radicals ; xiii) the actual stoichiometry of FSTO can be represented as Sr2+Ti4+13+ 2xTi xO 3-xF x,

with increase in the value of x, formation of Ti3+ states increases along with the

formation of oxygen vacancies which can be neutral / singly ionized / doubly ionized (Vo / Vᵒ o / Vᵒᵒo ) to maintain the charge neutrality of the solid [37-39]; xiv) the energy level corresponding to F- can be located anywhere within the band gap [27, 35]. If the F-energy level is located near the Fermi energy level inelastic capture of electron takes place. If the dopant F- energy level are near the VB it can act as recombination center by capturing the holes from the VB; xiv) the presence of lattice-bound fluoride in the form of Ti-F-Ti bonds increases the surface acidity. The Ti4+ ion adjacent to highly electronegative F- ion experiences a inductive effect and acts as Lewis acid centers; xvi) synergistic effect is observed between the surface fluorine ions and substituted F- ion in the bulk lattice; xv) FSTO samples generates highly reactive singlet oxygen (1O2) efficiently [40]; xvi) two trapping process can be recognized intrinsic trapping of photogenerated charge carriers by dopant ions or defect states taking place in a time period of 1-2 ps and trapping by the surface F- ions in a time period of 50-100 ps [41]. The experiments were conducted under the illumination of UV light and solar light separately. The values of rate constant, percentage of MO degraded and the time period are given in Table 5.

0.1

0

0.0 -0.1

-1

-0.2 -0.3

-3 -4

ln C/C0

ln C/C0

-2

MO a STO FSTO-10 FSTO-50 FSTO-100

-5 -6

-0.4 -0.5 -0.6

MO STO FSTO-10 FSTO-50 FSTO-100

-0.7 -0.8 -0.9

0

20

40

60

80

100

120

140

160

180

0

Time (min)

20

40

60

80

100

120

b 140

160

180

Time (min)

Fig. 9 Plot of lnC/C0 versus time in minutes for the degradation of MO using STO, FSTO-10, FSTO-50 and FSTO-100 photocatalyst samples under (a) UV light irradiation (b) solar light irradiation. Table. 5 The values of rate constant and percentage of MO degraded for the mentioned reaction systems carried out at pH 3 under the illumination of UV/solar light.

Under UV illumination Catalyst

Under solar illumination

MO STO FSTO-10

Time (min) 180 180 180

Percentage of MO degraded 14 69 71.83

Rate constant k × 10-2 min-1 0.086 0.577 0.604

Percentage of MO degraded 3 13 37

Rate constant k × 10-2 min-1 0.016 0.073 0.219

FSTO-50

90

99.7

5.188

66.35

0.575

56 (for min) 33

0.156

FSTO-100 180

180 0.424

3.7.3 Proposed energy level model for FSTO sample. The photocatalytic experiments indicated that FSTO samples could absorb not only the UV light like STO sample, but it can also absorb visible light photons. An energy level diagram for FSTO can be drawn based on the results obtained by PL and XPS techniques (Fig.10). The band gap of

the FSTO samples decreases from 3.23 eV to 3.08 eV with increase in F- dopant concentration and these catalysts are supposed to show photoresponse only under the UV region. But these catalysts showed good response under the irradiation of visible light. This is because of the various defect energy states which are created within the band gap of STO. The presence of Ti3+ state was confirmed by XPS studies and the presence of F and F+ centers were confirmed by the PL technique. The dopant energy level can be proposed to be situated 2 eV below the CB [15, 42-44]. The activity of FSTO sample under visible light can be accounted to the presence of energy levels pertaining to dopant and defects. The suppression of recombination of photogenerated charge carriers is due to the trapping of electrons by the surface F- ion and the activity under visible light could be accounted to the presence of the substituted F- ion in the STO lattice. The synergistic effect between the bulk lattice F- ions and surface F- ions drives the photocatalytic reactions.

Fig.10 Energy level diagram of FSTO sample depicting the various energy states that exists between VB and CB due to the incorporation of F- ions and the mechanism of various charge transfer process (EF : Fermi energy, Ti3+-Vo : Ti3+ state associated with oxygen vacancy, F and F+ centers : doubly and singly ionized oxygen vacancies). 4. Conclusions:

STO was prepared by sol-gel method and F- ions were incorporated into the STO lattice by wet impregnation method under the sonication conditions. XPS results show the presence of surface fluorine and also its occupation at substitutional position in the bulk lattice at oxygen lattice sites. The presence of highly electronegative F- ions on the surface of FSTO sample acts as electron trapping centers and the presence of lattice-bound fluoride in the form of Ti-F-Ti bonds increases the Lewis acid sites due to the strong inductive effect. Incorporation of F- ion with smaller ionic radius at oxygen lattice site leads to the decrease in the crystallite size, increase in the surface area and increase in the number of active sites. The aim of inhibiting both surface and bulk recombination of photogenerated charge carriers can be realized by the simultaneous process of surface fluorination and bulk lattice fluorine doping. This study reports the enhanced activity of fluorine doped photocatalyst and gives insight into the synergistic effect of surface fluorine with bulk lattice fluorine. STO with wide band gap has been made to show the visible light response. Acknowledgements The authors would like to acknowledge Department Science and Technology (DST) and University Grants Commission (UGC), Government of India for the financial support. References [1] L. Gomathi Devi, M.L. ArunaKumari, B.G. Anitha, R. Shyamala, G. Poornima, Photocatalytic evaluation of Hemin (chloro(protoporhyinato)iron(III)) anchored ZnO hetero-aggregate system under UV/solar light irradiation: A surface modification method, Surf. Interface 1-3 (2016) 52-58. [2] He Yu, S. Ouyang, S. Yan, Z. Li, Tao Yu and Z. Zou, Sol–gel hydrothermal synthesis of visible-light-driven Cr-doped SrTiO3 for efficient hydrogen production, J. Mater. Chem., 21 (2011) 11347-11351.

[3] Y. Yang, K. Lee, Y. Kado, P. Schmuki, Nb-doping of TiO2/SrTiO3 nanotubular heterostructure for enhanced photocatalytic water splitting, Electrochem. Commun. 17 (2012) 5659. [4] X. Wang, Gang Liu, Z.-G. Chen, Feng Li, L. Wang, G. Q. Lu, and H-M. Cheng, Enhanced photocatalytic hydrogen evolution by prolonging the lifetime of carriers in ZnO/CdS heterostructures, Chem. Commun., (2009) 3452–3454. [5] He Yu, S. Yan, Z. Li, Tao Yu, Z. Zou, Efficient visible-light-driven photocatalytic H2 production over Cr/N-codoped SrTiO3, Int. J. Hydrogen Energy 37 (2012) 12120-12127. [6] R. Niishiro, H. Kato and A. Kudo, Nickel and either tantalum or niobium codoped TiO2 and SrTiO3 photocatalysts with visible light response for H2 or O2 evolution from aqueous solutions, Phys. Chem. Chem. Phys., 7 (2005) 2241-2245. [7] Guo, S. Ouyang, Peng Li, Y. Zhang, T. Kako, Jinhua Ye, A new heterojunction Ag3PO4/CrSrTiO3 photocatalyst towards efficient elimination of gaseous organic pollutants under visible light irradiation, Appl. Catal., B Environ. 134-135 (2013) 286-292. [8] T. Ohno, T. Tsubota, Y. Nakamura, K. Sayama, Preparation of S, C cation-codoped SrTiO3 and its photocatalytic activity under visible light, Appl. Catal., A 288 (2005) 74-79. [9] N. Wang, D. Kong, H. He, Solvothermal synthesis of strontium titanate nanocrystallines from metatitanic acid and photocatalytic activities, Powder Technol. 207 (2011) 470-473. [10] H. Li, S. Yin, Y. Wang, and T. Sato, Microwave-Assisted hydrothermal synthesis of Fe2O3sensitized SrTiO3 and its luminescent photocatalytic deNOx Activity with CaAl2O4: (Eu, Nd) assistance, J. Am. Ceram. Soc., 96 [4] (2013) 1258-1262.

[11] H. Kato and A. Kudo, Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts codoped with antimony and chromium, J. Phys. Chem. B, 106 (19) (2002) 50295034. [12] W. Wei, Y. Dai, M. Guo, L. Yu, H. Jin, S. Han and B. Huang, Codoping synergistic effects in N-doped SrTiO3 for higher energy conversion efficiency, Phys. Chem. Chem. Phys., 12 (2010) 7612-7619. [13] J. Wang, S. Yin, M. Komatsu, Q. Zhang, F. Saito, T. Sato, Photo-oxidation properties of nitrogen doped SrTiO3 made by mechanical activation, Appl. Catal., B Environ. 52 (2004) 11-21. [14] S. Liu, J. Yu, B. Cheng, M. Jaroniec, Fluorinated semiconductor photocatalysts: Tunable synthesis and unique properties, Adv. Colloid Interface Sci. 173 (2012) 35-53. [15] D. Li, H. Haneda, S. Hishita, N. Ohashia, N. K. Labhsetwar, Fluorine-doped TiO2 powders prepared by spray pyrolysis and their improved photocatalytic activity for decomposition of gasphase acetaldehyde, J. Fluorine Chem. 126 (2005) 69-77. [16] X. F. Cheng, W. H. Leng, D. P. Liu, Y. M. Xu, J. Q. Zhang, and C. N. Cao, Electrochemical preparation and characterization of surface-fluorinated TiO2 nanoporous film and its enhanced photoelectrochemical and photocatalytic properties, J. Phys. Chem. C 112 (2008) 8725-8734. [17] D-N. Buia, J. Mua, L. Wangb, S-Z. Kanga, X. Li, Preparation of Cu-loaded SrTiO3 nanoparticles and their photocatalytic activity for hydrogen evolution from methanol aqueous solution, Appl. Surf. Sci. 274 (2013) 328-333. [18] L.G. Devi, B.G. Anitha, Exploration of vectorial charge transfer mechanism in TiO2/SrTiO3 composite under UV light illumination for the degradation of 4-Nitrophenol: A comparative study with TiO2 and SrTiO3, Surf. Interface 11 (2018) 48-56.

[19] J. S. Park and W. Choi, Enhanced remote photocatalytic oxidation on surface-fluorinated TiO2, Langmuir 20 (2004) 11523-11527. [20] R. Kavitha, L.G. Devi, Synergistic effect between carbon dopant in titania lattice and surface carbonaceous species for enhancing the visible light photocatalysis, J. Environ. Chem. Eng. 2 (2014) 857-867. [21] B. G. Anitha, L. Gomathi Devi, Synergistic effect between sulphur dopant in SrTiO3 lattice with surface sulphate species for enhancing visible light photo catalytic activity, REST Journal on Emerging trends in Modelling and Manufacturing 4 (1), 1-10 (2018) ISSN: 2455-4537. [22] A.S. Bhalla, R. Guo, R. Roy, The perovskite structure a review of its role in ceramic science and technology, Mater. Res. Innovations 4 (2000) 3-26. [23] Leonid V. Azroff, Introduction to solids, Tata McGraw-Hill publishing company, New Delhi, T M H Edition 10th reprint (1990). [24] Jinshu Wang, Shu Yin, Qiwu Zhang, F. Saito, T. Sato, Synthesis and photocatalytic activity of fluorine doped SrTiO3, J. mater. Sci. 39 (2004) 715-717. [25] Anthony R. West, John Wiley and sons, solid state chemistry and its applications, Singapura Edition (1989). [26] H. Liu, W. Yang, Y. Ma, Y. Cao, J. Yao, J. Zhang, and T. Hu, Synthesis and characterization of titania prepared by using a photoassited sol-gel method, Langmuir 19 (2003) 3001-3005. [27] L-B. Xiong, J-L. Li, B. Yang, Y. Yu, Ti3+ in the surface of titanium dioxide: generation, properties and photocatalytic application, J. Nanomater. 831524 (2012) 13 pages.

[28] G. Yang, W. Yan, J. Wang, Q. Zhang, H. Yang, Fabrication and photocatalytic activities of SrTiO3 nanofibers by sol–gel assisted electrospinning, J. Sol-Gel Sci. Technol. 71 (2014) 159-167. [29] C. Y. Jimmy, J. Yu, W. Ho, Z. Jiang, and L. Zhang, Effects of F- doping on the photocatalytic activity and microstructures of nanocrystalline TiO2 powders, Chem. Mater., 14 (2002) 38083816. [30] Bin Yang, Fengying Zhang, Junsheng Chen, Songqiu Yang, Xusheng Xia, Tõnu Pullerits, Weiqiao Deng, and Keli Han, Ultrasensitive and Fast All-Inorganic Perovskite-Based Photodetector via Fast Carrier Diffusion, Adv. Mater., 1703758 (2017) 1-8. [31] H. B. Ortiz-Oliveros, E. Ordoñez-Regil, S.M. Fernández-Valverde , Sorption of uranium(VI) onto strontium titanate in KNO3 medium, J. Radioanal. Nucl. Chem. 279 (2009) 601–610. [32] Yue Sun, Jiawen Liu, Zhonghua Li, Design of highly ordered Ag–SrTiO3 nanotube arrays for photocatalytic degradation of methyl orange, J. Solid State Chem. 184 (2011) 19241930. [33] Arthur I. Vogel, A Text-Book of Quantitative Inorganic Analysis, Second Edition, Longmans, Green and Co., London, 1951. [34] J. Yu, Q. Xiang, J. Rana and S. Mann, One-step hydrothermal fabrication and photocatalytic activity of surface-fluorinated TiO2 hollow microspheres and tabular anatase single micro-crystals with high-energy facets, Cryst. Eng. Comm. 12 (2010) 872-879. [35] G. Xing, L. Zhao, T. Sun, Y. Su, X. Wang, Hydrothermal derived nitrogen doped SrTiO3 for efficient visible light driven photocatalytic reduction of chromium (VI), Springer Plus 5 (2016) 1132.

[36] L. Palmisano, V. Augugliaro, A. Sclafani, and M. Schiavello, Activity of chromium-iondoped titania for the dinitrogen photoreduction to ammonia and for the phenol photodegradation, J. Phys. Chem., 92 (1988) 6710-6713. [37] J. wang, H. Quan, J. Ye, Photocatalytic properties and photoinduced hydrophilicity of surface fluorinated TiO2, Chem. Mater., 19 (2007) 116-122. [38] E.M. Samsudin, S.B. Abd Hamid, J.C. Juan, W.J. Basirun, A.E. Kandjani, S.K. Bhargava, Effective role of trifluoroacetic acid (TFA) to enhance the photocatalytic activity of F-doped TiO2 prepared by modified sol–gel method, Appl. Surf. Sci. 365 (2016) 57-68. [39] J. Yu, W. Wang, B. Cheng, and B-L. Su, Enhancement of photocatalytic activity of mesporous TiO2 powders by hydrothermal surface fluorination treatment, J. Phys. Chem. C, 113 (2009) 67436750. [40] A. Janczyk, E. Krakowska, G. Stochel, W. Macyk, Singlet oxygen photogeneration at surface modified titanium dioxide, J. Am. Chem. Soc. 128 (49) (2006) 15574-15575. [41] Bin Yang, Feng Hong, Junsheng Chen, Yuxuan Tang, Li Yang, Youbao Sang, Xusheng Xia, Jingwei Guo, Haixiang He, Songqiu Yang, Weiqiao Deng, and Keli Han, Colloidal Synthesis and Charge-Carrier Dynamics of Cs2AgSb1-yBiyX6 (X:Br,Cl; 0  y  1) Double Perovskite Nanocrystals, Angew. Chem. Int. Ed. 58 (2019) 1-7 [42] Di Li, Hajime Haneda, Shunichi Hishita, and N. Ohashi, Visible-light-driven N−F−codoped TiO2 photocatalysts optical characterization, photocatalysis, and potential application to air purification, Chem. Meter., 17 (10) (2005) 2596-2602.

[43] Xu J, Ao Y, Fu D, Yuan C, Low-temperature preparation of F-doped TiO2 film and its photocatalytic activity under solar light, Appl. Surf. Sci.254 (2008) 3033-3038. [44] C. M Wang and E.T. Mallou, Photoelectrochemistry and interfacial energetics of titanium dioxide photoelectrodes in fluoride-containing solutions, J. Phys. Chem. 94 (1990) 423-428.

Graphical abstract

Photocatalytic activity of Fluorine doped SrTiO3 under the irradiation of UV/solar light: Extended visible light absorption by the bulk lattice F- ions and suppression of photogenerated charge carrier recombination by the surface F- ions B. G. Anitha, L. Gomathi Devi*

Highlights:  Fluorine was incorporated into SrTiO3 crystal lattice  Lattice-bound fluoride in the form of Ti-F-Ti bonds increases Lewis acid sites  Synergistic effect exist between surface fluorine and bulk lattice fluorine

Credit Author Statement L. Gomathi Devi and B. G. Anitha: Conceptualization, Methodology, Writing-Original draft preparation, Investigation. L. Gomathi Devi: Supervision, Validation, Writing- Reviewing and Editing