Spectroscopic investigations of interaction between TiO2 and newly synthesized phenothiazine derivative-PTA dye and its role as photo-sensitizer

Author’s Accepted Manuscript Spectroscopic investigations of interaction between TiO2 and newly synthesized phenothiazine derivative-PTA dye and its role as photo-sensitizer Lakkanna S. Chougala, Jagadish S. Kadadevarmath, Atulkumar A. Kamble, Anand I. Torvi, Mahantesh S. Yatnatti, J.M. Nirupama, Ravindra R. Kamble

PII: DOI: Reference:

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S0022-2313(17)31554-5 https://doi.org/10.1016/j.jlumin.2018.02.025 LUMIN15372

To appear in: Journal of Luminescence Received date: 7 September 2017 Revised date: 31 January 2018 Accepted date: 7 February 2018 Cite this article as: Lakkanna S. Chougala, Jagadish S. Kadadevarmath, Atulkumar A. Kamble, Anand I. Torvi, Mahantesh S. Yatnatti, J.M. Nirupama and Ravindra R. Kamble, Spectroscopic investigations of interaction between TiO2 and newly synthesized phenothiazine derivative-PTA dye and its role as p h o t o - s e n s i t i z e r , Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.02.025 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.

Spectroscopic investigations of interaction between TiO2 and newly synthesized phenothiazine derivative-PTA dye and its role as photo-sensitizer Lakkanna S. Chougalaa, Jagadish S. Kadadevarmatha,*, Atulkumar A. Kambleb, Anand I. Torvib, Mahantesh S. Yatnattia, Nirupama J. Ma, Ravindra R. Kambleb. a

Department of Physics, Karnatak University, Dharwad-580 003, Karnataka, India.

b

Department of Chemistry, Karnatak University, Dharwad-580003, Karnataka, India.

*Corresponding author: E-mail address: [email protected]. ABSTRACT: Interaction of TiO2 nanoparticles (NPs) on newly synthesized phenothiazine derivative(E)-2-cyano-3-(10-decyl-10H-phenothiazin-7-yl) acrylic acid (PTA dye) has been studied using absorption, fluorescence and electrochemical techniques. From the results of absorption spectra of PTA dye in the presence of TiO2 NPs and the magnitude of association constant (ka) determined from Benesi-Hildebrand theory, strong association between the PTA dye and TiO2 NPs was confirmed. Fluorescence study, under both steady and transient states, reveals the energy transfer between PTA dye and TiO2 NPs as dynamic phenomenon in ethyl acetate. A study of Rehm-Weller theory indicates the predominant electron transfer process between PTA dye and TiO2 NPs in polar solvent (Acetonitrile) than in less polar solvent (Ethyl acetate). Electron transfer process has been exploited in solar energy harvesting applications by fabricating PTA dye sensitized solar cell. Photovoltaic energy conversion efficiency (η) and fill factor (FF) of PTA dye were found to be 1.24% and 0.54 respectively under AM 1.5 irradiation (1000 W/m2).

Graphical Abstract

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Key words: (E)-2-cyano-3-(10-decyl-10H-phenothiazin-7-yl) acrylic acid (PTA dye); TiO2 NPs; Dynamic quenching; Electron transfer; Dye sensitized solar cell (DSSC). 1. Introduction Several semiconductor nanoparticles have shown greater importance in several applications [1-5]. Particularly, titanium dioxide (TiO2) reveals excellent size dependent electronic, chemical and optical properties [6-7]. Promising application areas include pharmaceutical, medical, solar energy harvesting, photo-catalysis, lithium batteries, photochromic devices, light emitting diodes (LEDs) etc., [8-19]. Indeed TiO2 is one of the most successful materials used widely in solar cells and also as a biomaterial [14-19]. Interactions of TiO2 nanoparticles (NPs) with some molecular systems have been reported. Namely, interactions with π-conjugated systems at a molecular scale to construct hetero-supramolecular assembles,

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with lactate dehydrogenase (LDH), superoxide dismutase (SOD) in vitro which caused DNA damage in vivo and with Human serum albumin (HSA) which reveals the nature of bonding and quenching behavior [19-28]. Recently, effect of TiO2 NPs on newly synthesized phenothiazine derivative-CPTA dye reveals the nature of interactions and photovoltaic property [29]. Phenothiazine contains electron-rich nitrogen and sulfur hetero-atoms, show strong electron-donating feature thus it finds its applications in electronic and optoelectronic devices [30-38]. Phenothiazines are nonhazardous compounds available abundantly with comparatively low cost [39]. Some reports deal with promising anti-cancer, antibacterial, anti-plasmid, multidrug resistance (MDR) reversal activities and potential treatment in Alzheimer’s and Creutzfeldt-Jakob diseases of classical phenothiazines [39-45]. The structural features of phenothiazine-based dye make it a promising type of sensitizers for DSSCs [30]. Recently, a various strategies have been used to extend the range of π-electron delocalization and to increase the molar absorptivity of the materials [30, 46-56]. As a result of its promising applications, effect of TiO2 nanoparticle on newly synthesized Phenothiazine derivative-(E)-2-cyano-3-(10decyl-10H-phenothiazin-7-yl) acrylic acid (PTA dye) has been investigated using spectroscopic and electrochemical methods. Molecular structure of PTA dye is shown in Fig. 1 which contains one electron acceptor functional group (cyanoacrylic acid) and cyanoacrylic acid attached at the ends of the phenothiazine moiety (donor). Fig. 1 Molecular structure of PTA dye.

2. Materials 2.1. Materials for fluorescence quenching

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(E)-2-cyano-3-(10-decyl-10H-phenothiazin-7-yl) acrylic acid (PTA dye) was synthesized in the Department of Chemistry, Karnatak university, Dharwad (Supplementary). Anatase titanium dioxide

(TiO2)

(<25

nm

sized

nanoparticles)

powder

and

tetrabutylammonium

hexafluorophosphate (TBAPF6) were purchased from Sigma Aldrich. Spectroscopic grade of ethyl acetate was obtained from SD-fine chemicals, India and was used without further purification. 2.2. Materials for preparation of dye sensitized solar cell Anatase titanium dioxide (TiO2) (<25 nm sized nanoparticles), 4-tert-butyl pyridine (TBP) and Chloroplatanic acid (H2PtCl6.H2O) were purchased from Sigma Aldrich. Iodine (I2), Triton-x100 and Lithium iodide (LI) were obtained from HiMedia laboratory and Alfa Aesar chemical limited respectively. Di methyl-3-propyl imidazolium iodide (DMPII), Di-tetrabutyl ammoniumcis-bis (isothiocyanato) bis (2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II) (N-719 dye) and Fluorine doped tin oxide (FTO) glasses were obtained from Solaronix. FTO glasses have thickness of 2.2 mm with 80% visible light-transmission and 7 Ω/sq sheet resistance. Spectroscopic grade of acetonitrile, ethanol and acetyl acetone were obtained from SD-fine chemicals, India and were used without further purification. Distilled (Milli Q) water was used. The details of fabrication of dye sensitized solar cell (DSSC) were reported in our earlier publication [29]. 3. Methods The absorption, fluorescence and fluorescence lifetimes of samples were recorded using UVVis NIR spectrophotometer (model: JASCO V-670, Japan), fluorescence spectrophotometer (model: Hitachi F-7000, Japan) and time-correlated single photon counting technique (TCSPC ISS model: 90021) respectively. Cyclic voltammetry (CV) study of PTA dye was carried out

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using an electrochemical analyzer/Work station (model: 600E series, USA). CV consists of three electrodes that are Ag/Agcl reference electrode (RE), glassy carbon working electrode (WE) and platinum counter electrode (CE) respectively. The CV measurement of PTA dye was obtained at 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte with a scan rate 100 mVS-1. The photo-current density versus photo-voltage (J-V) curves of Dye sensitized solar cell (DSSCs) were measured using a laboratory designed automated solar simulator [29, 57]. Philips halogen lamp of 100 W was used as a source of light and its illumination was calibrated to 1000 W/m2 using NREL calibrated single crystal silicon reference solar cell. The active area of the TiO2 photoelectrode film sensitized by PTA dye was approximately 0.50 cm2. 3. Results and discussion 3.1. Spectroscopic studies. Fig. 2 shows the absorption spectra of PTA dye (9 × 10-5 M) in the absence and presence of TiO2 NPs (0.305 mM to 1.862 mM) in ethyl acetate at room temperature. From the Fig. 2, it is noticed that as the TiO2 NPs concentration increase, absorption peak increase with broadening of spectra without shifting the peak position suggesting possible interaction and absence of groundstate complex formation between PTA dye and TiO2 NPs [28, 29]. Possible interaction may further be verified from the magnitudes of association constant ka as suggested by Benesi Hildebrand Eq. (1) [58] (1) Where C is concentration of PTA dye, ΔA and Δε are the difference between absorbance and absorption co-efficient respectively of PTA dye with and without TiO2 NPs. ΔA values for 5

corresponding concentrations are shown in Table 1 and Δε was obtained from the plot of C/ΔA versus 1/ [TiO2] (Fig. 3). From the slope and intercept of the plot, the association constant (ka) is 0.648 × 102 such magnitude suggests there is a possible strong association between PTA dye and TiO2 NPs [19, 29]. Fig. 2. The absorption spectra of PTA dye in ethyl acetate in the absence and presence of TiO2 NPs. Fig. 3. The Plot of C/ΔA versus 1/ [TiO2]. Table 1 Values of TiO2 concentrations, absorbance of PTA dye, difference in absorbance (ΔA) and association constant (ka). 3.2. Fluorescence quenching studies. The fluorescence steady state and lifetime spectra (Transient state) of PTA dye with 9×10-5 M concentration in ethyl acetate at excitation wavelength of 384 nm were recorded at room temperature in the absence and presence of TiO2 NPs (concentrations from 0.305 mM to 1.862 mM). Fluorescence intensities and lifetimes of PTA dye in the absence and presence of TiO2 NPs are given in Table 2. Where, F0 or (τ0) and F or (τ) are the fluorescence intensities or (lifetimes) of PTA dye in the absence and presence of quencher [TiO2] respectively, Fig. 4 and Fig. 5 shows the steady state and life time spectra of PTA dye respectively in the absence and presence of TiO2 NPs. From Fig. 4, it is observed that the fluorescence intensity of PTA dye decrease systematically with increase of TiO2 NPs concentration without affecting the shape and peak positions of fluorescence spectrum. This type of fluorescence behavior under both steady state and transient state can be understood by Stern-Volmer [S-V] theory, by plotting the graphs of F0 /F versus [TiO2] and τ0/τ versus [TiO2] using S-V Eqs. (2) (Steady state) and (3) (transient state) respectively [59].

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Where, Ksv = kqτ0 (or K sv = k'qτ0) is the S-V constant and kq (or k'q) is the bimolecular quenching rate parameter. Plots of F0/F versus [TiO2] and τ0/τ versus [TiO2] for steady and transient states respectively are shown in Fig. 6. These plots are found to be linear with intercept and correlation coefficient nearly equal to unity suggesting dynamic or collisional quenching phenomenon. From the least square fit values of slope, KSV (K’SV) and kq (k'q) were determined and are shown in Table 3. From the magnitudes kq (k'q) it is noticed that this value is approximately equal to the order of maximum collisional quenching rate parameter (1010 M-1 s-1) [59, 60]. These results suggest that short range interactions between PTA dye and TiO2 NPs is playing a role. Involvement of resonance energy transfer between PTA dye and TiO2 NPs is ruled out as there is a non-overlap of absorption spectrum of TiO2 and fluorescence spectrum of PTA dye (Fig. 7). So, the energy transfer may be understood using Rehm-Weller formula of Eq. (4) [29, 61]. (4)

Where

is the oxidation potential of PTA dye,

is the reduction potential of

TiO2, Es is the excited singlet state energy of PTA dye and Ct is the coulombic term. Oxidation potentials (

) of PTA dye were 1.533 V and 0.999 V in ethyl acetate and acetonitrile

respectively determined from cyclic voltammogram (Fig. 8(a and b)). Es of PTA dye were 3.237 eV for ethyl acetate and 3.229 eV for acetonitrile respectively determined from absorption

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spectrum. Reduction potential of TiO2 (

) is -0.5 V [29, 62-64]. Substituting the values of

Es in equation (4), ΔGet values for ethyl acetate and acetonitrile were determined as -1.704 eV and -2.23 eV respectively and coulombic term is neglected since in polar solvents coulombic term is very small [19, 29]. From the magnitudes of ΔGet it is observed that ΔGet is greater in acetonitrile than ethyl acetate indicating prominent thermodynamically favorable electron transfer process from PTA dye to TiO2 NPs in acetonitrile than in ethyl acetate (less polar) [29, 65, 66]. These observations are the preliminary indications for the transfer of photogenerated electrons from PTA dye to TiO2 NPs and allow us to fabricate DSSC. Fig. 4. The fluorescence spectra of PTA dye in ethyl acetate in the absence and presence of TiO2 NPs. Fig. 5. The fluorescence lifetime decay curve for PTA dye in ethyl acetate in the absence and presence of TiO2 NPs. Fig. 6. S-V plot of PTA dye for steady state and transient state methods. Fig. 7. Normalized absorption spectrum of TiO2 NPs and fluorescence spectrum of PTA dye. Fig. 8. Cyclic voltammogram of PTA dye (a) for ethyl acetate and (b) for acetonitrile. Table 2 Values of TiO2 concentrations, fluorescence intensity and fluorescence lifetime of PTA dye. Table 3 Values of slope, intercept and correlation co-efficients of S-V plot, quenching rate parameter (kq) for steady state and transient state methods.

3.3. Dye sensitized solar cell study. 3.3.1. Absorption, fluorescence and electrochemical studies Fig. 9 shows the absorption (black colour) and fluorescence (blue color) spectra of PTA dye (9×10-5 M) in acetonitrile at room temperature. In the same figure, absorption spectrum (Red color) of the PTA dye when attached on TiO2 is also shown. Absorption peaks appear at 291 nm 8

and 384 nm respectively with low energy edge at 510 nm in pure PTA dye [29, 62]. 291 nm corresponds to π-π* electron transition of the phenothiazine unit, and 384 nm may be assigned to an intramolecular charge transfer (ICT) from the phenothiazine (donor) to the cyanoacrylic acid (acceptor) indicating charge separation in the excited state [29, 62, 67, 68]. The absorption spectra of the PTA dye when attached on the TiO2 (red color line) appears broadened and redshifted by 62 nm with low energy edge shifting beyond 510 nm compared to absorption spectrum of PTA dye alone. That is, the absorption band of PTA dye alone does not exceed 510 nm, where as the absorption threshold of PTA dye when attached to TiO2 extends beyond 510 nm. Such a type of shift was also noticed in the literature [56, 69, 70]. Significant variation in the absorption spectra of PTA dye may be ascribed to a change in the format of aggregation on TiO2 surface as compared to the solution. Red shift (hypo-chromic shift) were assigned to the interaction of the anchoring group (-COOH) with the surface titanium ion which directly reduces the energy of the π* level and the formation of J-aggregates on the TiO2 surface [29, 62, 67]. For comparing of absorption peak of PTA dye with standard N-719, absorption spectrum of N-719 is shown at supplementary (Fig. S7). It is observed that absorption peak of PTA dye appears at lower wavelength region as compared to that of standard N719 dye. From cyclic voltammogram (Fig. 8(b)) oxidation potential

or highest occupied

molecular orbital (HOMO) level of the PTA dye was found as 0.999 V. That is HOMO level of the PTA dye was more positive than the redox potential of the iodide/tri-iodide couple (0.4V vs. NHE). This condition may be favorable for the oxidized dye to accept electrons from Ithermodynamically, for effective dye regeneration and reducing charge recombination [29, 71]. The lowest unoccupied molecular orbital (LUMO) is equal to HOMO level minus Zero-zeroth energy (ΔE00). Where Zero–zeroth energy (ΔE00=1240/λ) of the PTA dye alone is the intersection 9

of the normalized absorption and fluorescence spectra (Fig. 9) and was found as 2.707 eV. Thus, LUMO level was found to be -1.708 V which lies well above the conduction band edge of TiO2 (-0.5V vs. NHE) [29, 62-65], ensuring the required driving force for electron injection from the dye into the TiO2 semiconductor. Fig. 10 shows schematic energy level diagram of each component of dye sensitized TiO2 electrode. So, solar cell sensitized by PTA dye was thus fabricated in order to investigate the solar to electricity conversion efficiency [29]. Fig. 9. Normalized absorption (Black color line) and fluorescence spectrum (Blue color line) of the PTA dye compared with the absorption spectrum of the dye attached on TiO2 (Red color line). Fig. 10. The schematic energy level diagram of each component of dye sensitized TiO2 electrode. 3.3.2. Photovoltaic performance in dye-sensitized solar cells Using laboratory designed automatic load variable solar simulator, photo-current density (J) versus voltage (V) of fabricated DSSCs sensitized with PTA dye was measured [29, 57]. Here, N-719 dye was used as a standard reference for comparison. For a dye soaking time of 4hours, optimized photo-current density (J) and voltage (V) were obtained. Photovoltaic conversion efficiency (η) and fill factor (FF) according to Eqs. (5) and (6) are 100

(5)

(6) where Voc, Jsc, Vmax, Jmax and I0 are the open circuit voltage, short circuit current density, maximum power point voltage, maximum power point current density and total incident irradiance (Io=1000 W/m2). The plots of photo-current density (J) versus voltage (V) are shown in Fig. 11 and the values of Voc, Jsc, η and FF are shown in Table 4. Photovoltaic conversion efficiency (η) of DSSCs sensitized with PTA and N-719 dyes were 1.24% and 4.81% respectively under AM 10

1.5 irradiation (1000 W/m2). The photovoltaic conversion efficiency of PTA dye is lower than the standard N-719 dye may be due to lower value of molar extinction coefficient of PTA dye and may be due to some other regions which need further investigation. Fig. 11. Photo-current density-voltage (J-V) characteristics of the best performed DSSCs (both PTA dye and N-719 dye (standard)). Table 4 Optimized photovoltaic parameters values of PTA dye and N-719 dye devices. 4. Conclusions Spectroscopic and electrochemical techniques were employed to investigate the nature of interactions between TiO2 nanoparticles and newly synthesized phenothiazine derivative-PTA dye. From the studies of absorption spectra and Benesi–Hildebrand theory the possible strong interactions between PTA dye and TiO2 NPs was noticed based on the higher magnitude of association constant. Fluorescence quenching between PTA dye and TiO2 indicates the role of energy transfer as dynamic phenomenon in ethyl acetate. Thermodynamically favorable electron transfer process between PTA dye and TiO2 NPs was confirmed from the magnitudes of free energy as suggested by Rehm-Weller theory. Magnitude of free energy is higher in more polar solvent (Acetonitrile) than in less polar solvent (Ethyl acetate). Thus, solar energy harvesting process has been invetigated by fabricating solar cell in which TiO2 photoanode is sensitized by PTA dye in acetonitrile. Photovoltaic conversion efficiency and fill factor of fabricated DSSC was found as 1.24% and 0.54 respectively under optimized conditions of dye soaking time. Further improvement of power conversion efficiency of DSSC may be achieved with further optimization of solar cell parameters. Acknowledgements LSC, AAK and AIT are thankful, to Karnatak University, Dharwad (KUD) for UGCUPE research fellowship. Authors are thankful to Prof N M Badiger, Director, USIC, KUD for 11

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Fig. 1 Molecular structure of PTA dye. 17

Fig. 2 The absorption spectra of PTA dye in ethyl acetate in the absence and presence of TiO2 NPs.

18

Fig. 3. The Plot of C/ΔA versus 1/ [TiO2].

Fig. 4 The fluorescence spectra of PTA dye in ethyl acetate in the absence and presence of TiO2 NPs.

19

Fig. 5. The fluorescence lifetime decay curve for PTA dye in ethyl acetate in the absence and presence of TiO2 NPs.

20

Fig. 6 S-V Plot of PTA dye for steady state and transient state methods.

Fig. 7 Normalized absorption spectrum of TiO2 NPs and fluorescence spectrum of PTA dye. 21

Fig. 8. Cyclic voltammogram of PTA dye (a) for ethyl acetate and (b) for acetonitrile.

Fig. 9. Normalized absorption (Black color line) and fluorescence spectrum (Blue color line) of the PTA dye compared with the absorption spectrum of the dye attached on TiO2 (Red color line).

22

Fig. 10. The schematic energy level diagram of each component of dye sensitized TiO2 electrode.

Fig. 11. Photo-current density-voltage (J-V) characteristics of the best performed DSSCs (both PTA dye and N-719 dye (standard)). 23

Table 1. Values of TiO2 concentrations, absorbance of PTA dye, difference in absorbance SNo. TiO2 NPs Absorbance ΔA Association concentration

(a, u)

constant (ka) (M−1)

(mM/L) 1

0.00

0.493

0

2

0.305

0.562

0.069

3

0.595

0.627

0.134

4

0.872

0.684

0.191

5

1.136

0.730

0.237

6

1.389

0.787

0.294

7

1.630

0.837

0.344

8

1.862

0.879

0.386

64.86

Table 2. Values of TiO2 concentrations, fluorescence intensity and fluorescence lifetime of PTA dye with corresponding χ2 values. SNo. TiO2 NPs Fluorescence Fluorescence life χ2 concentration (mM/L)

intensity

time (ns)

1

0.00

5180.4

6.59

1.11

2

0.305

4836.09

6.56

1.11

3

0.595

4470.1

6.53

1.07

4

0.872

4190.3

6.51

1.07

5

1.136

3910.5

6.48

1.20

6

1.389

3673.84

6.46

1.06

7

1.630

3415.56

6.40

1.13

8

1.862

3200.33

6.39

1.14

24

Table 3. Values of slope, intercept and correlation co-efficients of S-V plot, quenching rate parameter (kq ) for steady state and transient state methods. Method Slope (KSV) (M−1) Intercept Correlation Quenching rate coefficient (r) Parameter (kq)(M−1s−1) Steady state

347.66

Transient state 17.701

10

0.9469

0.995

5.275×10

0.998

0.981

0.268 ×10

10

Table 4. Optimized photovoltaic parameters values of PTA dye and N-719 dye devices. Name of the dyes Jsc (mA/cm2) Voc (V)

FF

η (%)

PTA

3.86

0.598

0.54

1.24

N-719 (Standard)

10.90

0.777

0.57

4.81

25