Tetrathiafulvalene Scaffold-Based Sensitizer on Hierarchical Porous

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C: Energy Conversion and Storage; Energy and Charge Transport

Tetrathiafulvalene Scaffolds Based Sensitizer on Hierarchical Porous TiO: Efficient Light Harvesting Material for Hydrogen Production 2

Amritanjali Tiwari, Naresh Duvva, Vempuluru Navakoteswara Rao, Muthukonda Venkatakrishnan Shankar, Lingamallu Giribabu, and Ujjwal Pal J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08787 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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The Journal of Physical Chemistry

Tetrathiafulvalene Scaffolds Based Sensitizer on Hierarchical Porous TiO2: Efficient Light Harvesting Material for Hydrogen Production Amritanjali Tiwari,†‡ Naresh Duvva,Ϯ Vempuluru Navakoteswara Rao,ǂ Shankar Muthukonda Venkatakrishnan,ǂ Lingamallu Giribabu,* †Ϯ Ujjwal Pal*†‡

†

Academy of Scientific and Innovative Research (AcSIR), New Delhi-201002, India

‡

Centre for Environmental Engineering & Fossil Fuels, CSIR-Indian Institute of Chemical

Technology, Hyderabad-500007, India Ϯ

Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology,

Hyderabad-500007, India ǂNanocatalysis

and Solar Fuels Research Laboratory, Department of Materials Science &

Nanotechnology, Yogi Vemana University, Kadapa-516005, India.

Abstract: In this work, a photochemical device that contains thioalkyl substituted tetrathiafulvalene dyes and hierarchical porous TiO2, has been designed and successfully employed in visible lightdriven hydrogen production. The design strategy boost up the desirable photophysical properties of the catalysts and well supported from the optical, electrochemical and computational studies. The introduction of thioalkyl substituted tetrathiafulvalene dyes as light harvesting sensitizers onto the porous TiO2 triggers unprecedented high rate of hydrogen evolution. This study focuses on the role of thiafulvalene scaffold which can promote ultrafast interfacial electron injection from excited state dye into the hierarchical porous TiO2 conduction band. The purposeful construction of this integrated composite G3T3 (dye content 1.0 mol in 10 mg Pt-TiO2 composite) significantly increases the hydrogen production rate of 24560 mol.h-1g-1cat with high apparent quantum yield (AQY) ~ 41%. In the study, both sensitizers absorption onset extends up to 500 nm in solution and 600 nm on hierarchical porous TiO2. Density functional theory (DFT) in the present study described that the HOMO levels are delocalized on anthracene as well as tetrathiafulvalene donor units, and LUMO covers on to the carboxylate anchoring group in both

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dyes. This study unveiled first time that a tetrathiafulvalene scaffolds in porous TiO2 attributes to positive synergistic effects in hydrogen production. Introduction: The necessity for developing clean energy technology to address the challenges of climatic change and environmental sustainability has led to the surge in utilizing solar energy.1–4 Hydrogen (H2) is predicted to be an ideal source of clean energy for the future. So, there is the utmost urgency of employing solar energy to mimicking the nature, and an artificial photosynthetic photocatalytic device would be alternative but challenging solution.5– 7

Empowered by the functional energy materials and hybrid organic-inorganic materials, this has

undoubtedly opened new frontiers in tailoring nanoparticles with the novel functionalities of organic photosensitizers which can be tuned at molecular level.5 Photocatalytic materials and their light harvesting capabilities under a matching solar spectrum are crucial for effective solar to chemical conversion. Generally, sensitizers for photocatalytic H2 generation can be sorted out into three main categories: organometallic complexes, natural or bioinspired dyes, and metal-free organic dyes.8,9 Up till now, visible light absorbing photosensitizers such as metal complexes of Ru,10 Pt,11 and Ni,12 porphyrine derivatives,13,14 and phthalocyanines15 have been extensively used in the photocatalytic H2 production and dye sensitized solar cell applications.5,16 Among the D--A framework of organic dyes, numerous reports such as coumarin,17,18 indoline,19,20, triarylamine,21–23 carbazole,24,25 diketopyrrolopyrrole (DPP)26 and thioalkyl decorated27–29 scaffolds have been investigated. However, most of the research in dye sensitized system is focused on the design and synthesis of the molecule due to limitation of their light harvesting capability and stability in the photo-electrochemical environment. Further, compared to the metal complexes, organic photosensitizers have advantages like: flexible, and multiple ways to conceptualize donor--bridge-acceptor (D--A), facile synthesis and environmentally benign. Considering the recent development, tetrathiafulvalene and its derivatives show excellent light harvesting properties in combination with good charge separations and impressive life times. Moreover, TTF is a readily available molecule which exhibits strong electron-donor power, these on integrating with the suitable supports, like oligomers, polymers, dendrimers or inorganic materials display efficient intermolecular interactions, optoelectronic properties and stability in the composite materials. Recently, we have reported high efficiency tetrathiafulvalene based


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sensitizers with different -spacers and anchoring groups in dye sensitized solar cell (DSSC) applications. Such dye molecule has strong π-donor property and thermal stability that can be widely used in solar cell applications.26,30 For example, Liu’s group reported the quinoxalinefused tetrathiafulvalene based sensitizer in solar cell applications.31 In view of solar energy utilization, the electrochemical and optical properties of tetrathiafulvalene scaffolds can easily be fine-tuned by their molecular engineering. On the other hand, further improvement of the photocatalytic reduction of water requires suitable morphology and photophysical property in the semiconductor particles. Literature study in retrospect reveals that the hierarchically ordered macro or mesoporous TiO2 network with accessible and open macro channels pores possesses significant advantages over light driven water reduction. The presence of interior macro-channels in the mesoporous TiO2 materials with limited crystal defects grabs extreme attention due to their low recombination process of the photogenerated electrons and holes in the photocatalytic cycle.32 As a result, it enhances the efficiency of photo-absorption and improves the mass transfer process.33,34 Moreover, there were few reports that the macro channels of porous hierarchical structured TiO2 materials exhibit superior activity over conventional bulk materials due to the presence of charge carrier transportation channel which suppresses recombination process and promotes more accessible active sites for water decomposition to H2 formation.35,36 The introduction of long alkyl chains and  liners in the ex-TTF based sensitizer molecule exhibits impressive performance in DSSC with an overall efficiency of 7.15%, reported from our group.30,37 Continuation of our success in exploiting TTF scaffolds in DSSC, the present work judiciously use the tetrathiafulvalene scaffolds in combination with hierarchical porous and fibrous mixed morphology of TiO2 nanostructures. We extend our effort to develop new highly efficient materials in photocatalytic renewable clean energy conversion which is not yet explored to best our knowledge. Herein, we report a novel approach to develop active photocatalysts by designing metal-free thioalkyl substituted tetrathiafulvalene sensitizers and hierarchical porous TiO2. These composites were employed for the first time effective photocatalytic water reduction and the generation of H2 in presence of sacrificial electron donor and Pt cocatalyst. The hierarchical porous TiO2 was obtained through facile hydrothermal route. Strong donor property and extended  conjugation of TTF (ex-TTF) shows substantial improvement in H2 evolution. The integrated dye-TiO2 composite exhibits enhanced light absorption, charge transport and



photocatalytic activity by regulating the dye structure and assembly of materials. Optical, electrochemical and DFT studies further established the fine tuned dye structure in H2 evolution over dye sensitized TiO2 photocatalysts. The operational parameters such as different dye concentration, activation temperature of HPT, and electron donor have also been studied. G3T3 (24560 µmol.h-1g-1cat H2 Yield and AQY ~41%) has shown superior photocatalytic activity compared to G1T3 (12278 µmol.h-1g-1cat H2 Yield and AQY ~20%) under visible light irradiation using glycerol aqueous mixture.

Scheme 1: Molecular structure of G1 and G3 dye. Experimental: Materials In the present work, the chemicals are commercially available and were employed in the fabrication of uniform structured porous TiO2 nanoparticles. Titanium (IV) isopropoxide was procured from Sigma-Aldrich, USA. Liquid ammonia and deionized (DI) water were purchased from the Merck, Germany. All chemicals were used without further purification. The synthesis procedures of thioalkyl substituted tetrathiafulvalene sensitizers as G1 and G3 were described in our previous publication.37 Hierarchical Porous TiO2 (HPT) Synthesis The hierarchical porous anatase TiO2 was prepared by using the modified process according to the earlier reports.38,39 Briefly, 15 mL of titanium (IV) isopropoxide (TTIP) was added drop-wise into 150 mL of 15%, 20% and 25% liquid ammonia separately without stirring under ambient


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conditions. White color precipitate was observed when TTIP droplet got in contact with the ammonia solution. After one hour of completion of reaction, the obtained precipitate was washed several times with deionized water (at pH 7) and then the samples were kept for drying at room temperature for 48 h. The as-prepared materials were labeled as HPT-15, HPT-20 and HPT-25. Later on, the prepared materials were activated at variable temperatures (i.e., rt, 400, 500, and 800 °C) at low heating rate (5 °C/min) for 4 h to explore changes in physical characteristics and photocatalytic performance. The obtained catalysts were denoted as HPT-15-rt, HPT-15-400, HPT-15-500, HPT-15-800, HPT-20-rt, HPT-20-400, HPT-20-500, HPT-20-800, HPT-25-rt, HPT-25-400, HPT-25-500 and HPT-25-800 according to their ammonia concentration and calcination temperatures. Pt Deposition onto the Hierarchical Porous TiO2 (Pt-HPT) Metallic Pt was deposited on the surface of HPT by photoreduction process that has been described elsewhere.40 Briefly, 1.0 g of the each catalyst (HPT-15-400, HPT-20-400 and HPT25-400) was dispersed in 30 mL of methanol and kept for stirring at room temperature (rt) for 30 min to form a homogeneous solutions. 1 wt% H2PtCl6 aqueous mixture added into HPT of methanolic suspension and then the reaction mixture was kept under irradiation (400 W Hg lamp) for an hour. Finally, the product was centrifuged and washed with deionized water up-till chloride ion (Cl-) was not detected in the rinse water. After being dried in vacuum at 70 °C, the grey colour resultant composite was obtained and sample was designated as PHPT (Pt-HPT). Pt deposition on HPT has been confirmed by TEM analysis (Figures S4 and S5). The dyes, G1 and G3 were embedded on PHPT surface in acetonitrile-ethanol solution (1:1 v/v, 20 ml).40 Then PHPT-20-400 was mixed into the resulting solution and the reaction mixture was kept on stirring in the dark at rt for 24 h. The resultant dye-PHPT composite was retrieved by centrifugation, subjected to ethanol washing and vacuum drying at 55°C. The dye loading amounts on the PHPT surface was studied by using UV-vis spectroscopy. It has been observed that the filtrate after dye adsorption was found almost transparent (Figure S6). The final photocatalysts were denoted as G1T3 and G3T3 according to dye concentration (1.0 µmol/10 mg PHPT). Similarly, G3T1, G3T2, G3T4 and G3T5 for dye concentrations (0.1, 0.5, 1.5, and 2.0) µmol/10 mg PHPT.



Photocatalytic Experiments The photocatalytic experiments for H2 production were performed in a Pyrex glass made photoreactor fitted with air tight rubber septum at the top loading port. Sampling and degassing were carried by this end port. In typical experiment, the sample was loaded with 50 mL of aqueous solution containing 10 vol% glycerol (Gly) or triethanolamine (TEOA, i.e. previously neutralised with 0.1 M hydrochloric acid and sodium hydroxide solution), magnetic bar and 10 mg of chosen photocatalyst was dispersed under magnetic stirring. The reactor tightly wrapped with aluminium foil and stirring continued for 30 min to achieve adsorption-desorption equilibrium under dark condition. Further, reactor port is sealed with air tight rubber septum and Teflon tape to ensure no gas leakage. In order to ensure the oxygen free environment inside the reactor, vacuum was applied followed by nitrogen gas purging for 30 min. A 450 W xenon arc lamp (New Port, USA) with AM 1.5 using cutoff filter (λ > 400 nm) was employed as irradiation source. The photoreactor with aqueous suspended reaction mixture was kept on irradiation under constant stirring i.e. 20 cm apart from the light source. The evolved H2 gas was analyzed at periodic interval (every hour) using offline gas chromatograph with thermal conductivity detector (Perkin Elmer Clarus 590 GC with Molecular Sieve/ 5Å column) using N2 as carrier Gas. The quantification of hydrogen production calculation was demonstrated in the supporting information, Table S6 and S7. Results & Discussion: XRD Characterization: The phase structure and crystallinity of the developed catalysts were studied by powdered X-ray diffraction (XRD) analysis. Figure 1 and S1, demonstrates the XRD patterns of the prepared samples without and with calcination at 400, 500 and 800 °C for 4 h. As shown in Figure 1(a & b), HPT-20-rt do not have characteristic peak at room temperature. Further, the as-prepared samples (calcined at 400 °C in Figure 1a and 500 °C in Figure 1b) showed strong intensity diffraction peaks at 2θ value of 25.5°, 38.0°, 48.2°, 54.0°, 55.2° and 64.7°, representing the (101), (004), (200), (105), (211) and (204) phase structures of the anatase TiO2 nanoparticles (JCPDS 21-1272) respectively. While the characteristic peaks at 2θ value of 27.5°, 36.1°, 41.33° and 44.1° corresponded to the indices of (110), (101), (200) and (111) crystal phase of rutile (JCPDS file no. 21‐1276) respectively. The XRD analysis was used to investigate anatase to


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rutile phase transformation that occurred after 500 to 800 °C. According to the above results, the calcination treatment played crucial role in photocatalytic H2 production due to improved crystallinity of photocatalysts through elimination defect which leads to enhance the photocatalytic activity. However, calcination process at higher temperature could be the certain reason in the reduced surface area, change in morphology has induced the phase transformation that has remarkable decline in the photocatalytic performances.

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Figure 1. XRD pattern of (a) HPT-20-rt, HPT-20-400, HPT-20-800 (b) HPT-15-500, HPT-20500 and HPT-25-500 catalysts. Morphology of HPT It is concerned that the changes in the concentration of ammonia could inevitably affect the morphology of the as-prepared catalysts (seen in Figure 2). In this study, we believe that the change in concentration of ammonia may lead to crucial photophysical properties. To examine the role of ammonia, we have tuned the composite morphology using concentration variation at 15, 20 and 25% and subsequent heat treatment at 400, 500 and 800 ºC. It is interesting to observe that at 500 ºC the morphologies turned into uniform hierarchical pattern. The efficiency of the optimum experimental condition of the as-prepared photocatalysts was established from the Raman spectroscopy (Figure 2).41,42



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Figure 2. FESEM images of HPT-15, HPT-20 and HPT-25 at different activation temperature. Raman Spectroscopy The phase crystallinity of HPT-20-rt, HPT-15-500, HPT-20-500 and HPT-20-500 was further confirmed by Raman spectroscopy. Consistent with the previous reports, Raman frequencies were observed for the as-selected catalysts at 141, 397, 514 and 638 cm-1, which attributed to the Eg(1), Blg(1), Alg + Blg(2), and Eg(2) symmetries of the vibrational mode representing the TiO2 anatase phase (Figure 3).43,44 Moreover, the intensity of HPT-20-500 catalyst is higher than other samples. Based on literature studies report it is found that the sharp Raman intensity led to the higher crystalline phase. Moreover, higher the phase crystallinity enhanced the photocatalytic activity.45


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Figure 3. Raman spectra of HPT-20-rt, HPT-15-500, HPT-20-500 and HPT-25-500 catalysts. The porous nature of the prepared samples HPT-20-400, HPT-20-500 and HPT-20-800 were analyzed by BET (Brunauer-Emmett-Teller) theory. From the results (Figure 4, and Table S1) we observed that the specific surface area measured by adsorption–desorption isotherms was affected with the increase in activation of temperature of the catalyst. The surface areas of HPT20-500 (67.68 m2g-1) is higher than HPT-20-400 (45.56 m2g-1) and HPT-20-800 (28.47 m2g-1), with the pore size 12.53, 18.12 and 20.23 nm respectively. In view of the BET results at optimal activation temperature (500 °C), the specific surface area and pore size of the catalysts exhibits higher performance. Further, the developed materials are found to be mesoporous that confirmed their pore size and pore volume (Table S1).



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Figure 4. Adsorption/desorption isotherms measured at 77 K, Inset: pore size distributions of the HPT-20-400, HPT-20-500 and HPT-20-800 catalysts. Further elucidating the surface composition and chemical state of the best performing photocatalyst (HPT-20-500), X-ray photoelectron spectroscopy (XPS) studies were carried out. The obtained XPS peaks presented in Figure 5 were corrected by referencing C1s to 286.8 eV. The XPS survey spectrum of HPT-20-500 catalyst Figure 5a demonstrated that it contained two major peaks and can be assigned to Ti 2p and O 1s spectra. The high-resolution XPS spectra of Ti 2p (Figure 5b) at 458.3 and 465.3 eV are ascribed to Ti 2p3/2 and Ti 2p1/2 respectively. The O 1s peak appears at a binding energy of 531.5 eV (Figure 5c). The obtained results are compared with that of the previous literature reports.46


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Figure 5. XPS survey scan (a), high resolution XPS spectra of Ti 2p (b) and O 1s (c) of HPT-20500 catalyst. Table 1 Physico-chemical properties of dye Dye

b

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(M−1 cm−1)

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Eox(v)

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NHE G1

416 (18665)

602

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-2.74

608

445

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0.48 0.72

-5.22

-2.74

361 (18355) G3

417 (17860) 375 (18226)

a

Absorption spectra were recorded in toluene,. bEmission spectra were recorded in toluene,

c

Absorption spectra of dye on optimized catalyst HPT-20-500, dLifetime decay of dyes,

e

Oxidation potential was estimated from the intersect of the absorption and the emission spectra

(int: E0-0 –1240/int). fOxidation potential of dyes in dichloromethane solution using tetrabutylammonium perchlorate (TBAP) as electrolyte.

g

HOMO value calculated using

potential value of oxidative wave, HOMO= - (4.5 + Eox),47. hLUMO = HOMO + E0–0.37



UV-visible Spectroscopy The opto-electrochemical characterizations are crucial parameters for estimation of electron density in HOMO and LUMO of the sensitizers, subsequently electron injection from the photoexcited dyes to the TiO2 conduction band and dye regeneration are evaluated. The absorption and emission spectra of the sensitizers in toluene and dye adsorbed TiO2 film has shown in Figure 6a, and the respective data are listed in Table 1. The intense absorption band of sensitizer G1 at 416 nm (molar absorption coefficient  = 18665 M-1cm-1) and sensitizer G3 at 417 nm ( = 17860 M-1cm-1) are ascribed to the π−π* transitions from the donor group of thioalkyl substituted tetrathiafulvalene to the acceptor carboxylate group.37,48 The UV-vis DRS results (Figure 6b and S2) fully complements the absorption spectra.40 The intense absorption band of the dye-TiO2 composites are red shifted by 8 nm and 28 nm for G1T3 and G3T3 respectively compared to those in solution and in addition onset of absorption extends up to 700 nm.37 The red shift of the absorption band causes wide spectral response of the photosensitizers which attributed to the electronic interactions between the carboxylate moiety and TiO2. It provides suitable light harvesting efficiency towards solar energy conversion. Further the estimated band gap energy of the photocatalysts has been measured by Kubelka-Munk function and extrapolations of the Tauc plot, is shown in Figure S2. Photoluminescence (PL) spectral analysis has been broadly employed technique to investigate the recombination of the charge carriers in photocatalysts.49–51 The PL spectra obtained from the sensitizers were recorded at room temperature and toluene as solvent which are incorporated in the Figure 6c. The luminescence maxima resulted at 602 and 608 nm for G1 and G3 sensitizers respectively. From absorption and emission data, we have calculated E0-0 (singlet excited state) of G1 and G3 sensitizers and were found to be 2.50±0.05, and 2.48±0.05 eV, respectively. The emission spectra are completely quenched when both sensitizers adsorbed on HPT (Figure 6c) when excited at ~416 nm which confirms electron injection from excited state of sensitizer to TiO2 conduction band.37,48


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hv )

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Figure 6 (a) UV-visible absorption spectra of G1 and G3 dye in toluene, (b) UV-vis DRS and band gap energy of G1T3 and G3T3 and (c) Emission spectra of G1 and G3 dye in toluene, and G1 and G3 adsorbed on TiO2 as G1T3 and G3T3. The emission life time profile of the dye molecules were obtained from the time resolved PL study as shown in Figure 7, where a clear correlation of the life time and the photoactivity is outlined. Further, the lifetime measurement (tau) in G1 and G3 attributed to the extent of charge recombination in the process of electron transfer towards photocatalytic H2 production. The life time 1 for G1and G3 are 0.11 ns and 0.12 ns, almost same but slightly higher for G3 in the protonated state (2 = 1.75 ns) compared to G1 (2 = 1.24 ns). A good corroboration is found in the longer life time of G3 which exhibited low charge recombination as well as higher activities in H2 production.37



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Time (ns)

1250 1300

Figure 7. PL spectra of G1 and G3 sensitizers Cyclic voltammetry of G1 and G3 dye We employed cyclic voltammetry to investigate the reduction potentials of the G1 and G3 sensitizers (Figure S3). It was observed in both the sensitizers that G1 showed reversible oneelectron oxidations at 0.50 V and 0.58 V and G3 showed 0.48 V and 0.58 V due to their respective formation of the radical cations and dications in TTF molecule. It was noted that similar reversible one-electron oxidation reactions was also observed in other TTF molecules. Each sensitizer also under goes one electron reduction at -1.57 and -1.70 V of G1 and G3 sensitizers, respectively. The reduction potential (Ered) of both dyes showing their LUMO values are considerably more negative than conduction band of TiO2, resulting ample driving force for electron transfer. Computational Study We have adopted density functional theory (DFT) for geometry optimization and time-dependent density functional theory (TDDFT) for excited state calculations at the B3LYP/6-31 G (d, p) level in dichloromethane solvent, by means of a C-PCM salvation model implemented in the Gaussian03 package of the considered tetrathiafulvalene scaffold based sensitizers. The energy optimized structure and electronic distribution computed in acetonitrile of the first occupied/unoccupied molecular orbitals of the studied species are shown in Table S2. In both sensitizers, HOMO is delocalized over anthracene and dithiol units whereas LUMO is delocalized over π-spacer and anchoring group that facilitates electron transfer from excited state


Page 15 of 29

of sensitizer to the conduction band of semiconductor TiO2. Energy optimized structures of both sensitizers are hexy-substituted dithiafulvalene at 9th and 10th position of anthracene and anchoring group at 2nd position. The hexythio groups forms an alkylthio wrapping over anthracene moiety and may retard the recombination of electrons in TiO2 conduction band with oxidized sensitizer. In order to understand the excited-state transitions, we have carried out TDDFT calculations and results are illustrated in Figure 8. These results are in reasonable agreement with the experimental values and the corresponding singlet state properties of both dyes are presented in Table S3 and Table S4. G 1 (Experimental)

1.0

G 3 (Experimental) G 3 (Theoretical) 1.0

0.8

0.8

0.8

0.8

0.6

0.6

0.6

0.6

0.4

0.4

0.4

0.4

0.2

0.2

0.2

0.2

0.0

G 1 (Theoritical)

300

400

500

Wavelength (nm)

600

0.0

Abs. Int. (nm)

1.0

1.0

Abs. Int. (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

300

400

500

600

0.0

Wavelength (nm)

Figure 8. Simulated absorption bands are shown as vertical bars, theoretical and experimental UV-Visible spectra of G1and G3 in dichloromethane solvent. Photocatalytic H2 activity The hydrogen production performance of nanocomposite photocatalysts were evaluated in pure water and aqueous solution of 10 vol.% Gly or TEOA as sacrificial electron donor (SED) under visible light irradiation. In pure water system, hydrogen and oxygen was not detected under light irradiation. To optimize dye loading for H2 production performance, a series of experiments were employed with variable concentrations of G3 dye (0.1-2.0 µmol) on to the Pt-HPT-20-500 surface (Figure 9 and S7). The H2 production rate has increased with increasing the G3 dye content and reaches a maximum when the loading amount of the G3 dye 1.0 µmol/10 mg catalysts. Further increases the dye concentration of 1.5 and 2 µmol on to the catalyst composite led to a gradual decline in H2 production rate. It may be due to the fraction of the incident light absorbed by the dye increases over dye concentration changes and reached to saturation level at certain concentration. Moreover, at optimal condition the adsorption of reactants, charge carrier generation and its utilization for hydrogen production is matching with each other, whereas



Page 16 of 29

catalyst with below and above dye concentration on TiO2, generation of charge carrier is either low or high that facilitates poor efficiency.52 The effect of photosensitizers further examined with two dyes namely G1 and G3 adsorbed PtTiO2 composite (G1T3 & G3T3) for photocatalytic hydrogen production in aqueous Gly solution. Both sensitizers exhibited hydrogen production volume increased with light irradiation time over 5h. During irradiation it has been observed that the G3T3 photocatalyst depicted the highest H2 production yield, the amount of H2 evolution of 122998 µmolg-1cat which is ~2 and ~139 folds greater H2 yield compared to G1T3 (61391 µmolg-1cat) and PHPT (892 µmolg-1cat) in aqueous glycerol solution (Figure 9a). Similarly to extent experimental results with aqueous TEOA as SED is displayed in Figure 9b, found that G3T3 showed higher efficiency but nearly 2.5 folds lower than Gly H2 production rate. The Figure 9c compares the rate of hydrogen production of the prepared photocatalysts using glycerol as SED. Figure 9c showed the H2 production

rate

of

the

prepared

photocatalysts

in

the

following

order:

G3T3>G1T3>G3T4>G3T5>G3T2>G3T1> PHPT. The photocatalyst G3T3 represents the superior activity for H2 evolution up-to 122998 µmolg-1cat along with 2460 TON and ~ 41% AQY compared to the other photocatalysts under visible light irradiation after 5h. To understand the sacrificial electron donor’s role over the photocatalyst for H2 production, two SED as TEOA and Gly was systematically studied. Introducing suitable SED into the reaction system will increase the efficiency of hydrogen production from both thermodynamic and kinetic view points. In the present study, it is found that the photocatalyst exhibits better H2 production activity with Gly compared to TEOA (Figure 9 and Table 2) under similar reaction condition. It may be due to Gly oxidized to glyceraldehyde and several intermediate compounds during hydrogen production reaction.53 It is well known fact the Gly oxidized to glyceraldehyde through two electron transfer process. It acted as photogenerated holes trap on TiO2 in nanosecond timescale which further reiterated by the transient absorption spectroscopy study.54,55 The other factors Gly may have high polarity, more solubility in aqueous mixture, better suspension and adsorption-desorption could trigger high rate of H2 production. Therefore, it is suggested that in this photocatalytic reaction, the occurrence of two concomitant reaction pathways involving adsorbed Gly and liquid-phase reactions between Gly and free OH• which is well reported. Further, we observed from our previous report that effectiveness of various sacrificial electron donors for H2 production is not simply governed by the reduction potential of the sacrificial


Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electron donor but also by the kinetic barrier of the electron transfer process where poor yield of H2 production was not anticipated even though ascorbic acid is a strong electron donor.56 Finally, stability of two different dyes in photocatalyst namely G1T3 and G3T3 were tested in aqueous glycerol under visible light irradiation for five cycles. Figure 9d shows volume of H2 production increases with time and found to be reproducible for five recycles under similar conditions. The H2 production rate was increased from the first cycle (122998 µmolg-1cat) and remains same at the second cycle (122798 µmolg-1cat). Thereafter, it retains similar in the successive runs in 3rd, 4th and 5th cycles. Further, by comparing the reaction mixture of G3T3 before and after the photocatalytic experiment (25 h), the dye–HPT material composite still remained the original colour. The light pale yellowish colour originated in the solution after prolong irradiation (25 h) might be due to decomposed fragments of Gly as sacrificial electron donor (Figure S8). The H2 generation curve is further reiterated that the photocatalytic system with G3T3 is still highly active even under a prolong irradiation.

(d)



Page 18 of 29

Figure 9. Photocatalytic activities for H2 production of the dye-Pt-TiO2 composites using 10 vol% of SED (a) Glycerol and (b) TEOA. Reaction conditions: 10 mg photocatalyst in 50 ml of 10 vol% neutral aqueous TEOA and Gly solution under visible light. (c) Histogram of photocatalytic H2 production in aqueous glycerol solution. (d) Recycled H2 production of G1T3 and G3T3 photocatalysts using Gly (10 vol%) under visible light irradiation. Table 2 Physicochemical Properties and Photocatalytic Activity of the Photocatalysts Catalyst

Band

H2

TON

AQY (%)

H2

TON

AQY (%)

code

gap

Production

(Gly)

(Gly)

Production

(TEOA)

(TEOA)

(eV)

(Gly)

(TEOA)

µmol.g-1cat

µmol.g-1cat

G3T1

2.92

4122

825

1.37

2204

440

0.73

G3T2

3.00

22423

897

7.45

13359

534

4.44

G3T3

2.95

122998

2460

40.87

47331

947

15.73

G3T4

3.01

58391

778

19.40

29390

392

9.76

G3T5

2.88

46949

470

15.60

11908

119

3.95

G1T3

2.80

61391

1228

20.40

27331

547

9.08

PHPT

3.01

892

--

0.29

204

---

0.06

The surface linkage and structural stability between dye and HPT has also been elucidated on the basis of UV-vis spectra and FTIR studies. As shown in the Figure S9, the DRS study of G1T3 and G3T3 photocatalysts before and after irradiation are almost identical. Apart from that, slight shifting of the absorption edge towards shorter wavelength is observed (Figure S9). This shift is attributed to well adsorption of the dye molecules on to the Pt-TiO2 surface and structural stability during the light irradiation.57 Further, FTIR analysis of G3 dye and G3T3 photocatalyst has been carried out to reaffirm the dye adsorption on PHPT and stability of before and after irradiation, the corresponding results are shown in the Figure S10 and Table S5. It is interesting to observe that the characteristic stretching frequency of C=O in –COOH moiety for G3 appeared at ~1617 cm-1 and increased to ~1629 cm-1 upon dye sensitization on PHPT surface. Moreover, most of the peaks have been disappeared after dye loading on to the PHPT surface which attributed to chemical adsorption of


Page 19 of 29

dyes onto the TiO2 particles via ester-like binding between the carboxylic acid groups and the OH moieties on TiO2. The FTIR peaks of photocatalyst after irradiation remain intact but in low intensity. It may be due to the SED consumption and partial decomposition of dye. To gain further insight into the dye molecules anchored on TiO2 surface through the COOH linker have adverse affect on the environmental pH. In this study, pH effect has been tested with the superior photocatalyst (G3T3) in aqueous TEOA/GLY medium where the desired pH was adjusted by using 1(M) of respective hydrochloric acid and sodium hydroxide solution. The G3T3 photocatalyst showed highest photocatalytic activity at pH 7 and poor results was observed at pH 4 and pH 10. This may be due to protonation of surface titanols in acidic medium and the dye carboxyl group deprotonated in basic condition. Thus, the esteric bond was not formed effectively. Subsequently, the pH value can also influence the existing state of the sacrificial electron donor as in acidic solution protonation of TEOA occurs which led to slow regeneration of the oxidized dye molecule.58 (a)

60000 -1

100000

50000

45000

1

2

3

4

5

(b)

TEOA

30000

15000

0

0 0

pH 4 pH 7 pH 10

cat

GLY

H2 Production (molg

cat

-1

)

pH 4 pH 7 pH 10

)

150000

H2 Production (molg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

Irradiation time (h)

1

2

3

4

5

Irradiation time (h)

Figure 10. H2 production activities of G3T3 photocatalyst at different pH; Reaction conditions: 10 mg G3T3 photocatalyst in 50 ml of TEOA (10 vol%) and Gly (10 vol%) aqueous solution under visible light. The photocatalytic performance of G3T3 has been determined under various monochromatic light ( = 400, 420, 450, 550 and 600 nm) using corresponding narrow band-pass filters.39,59 Then the wavelength-dependent AQY values are calculated based on the amount of H2 production and corresponding incident monochromatic light intensity.40,42 The obtained AQY values for G3T3 photocatalyst are closely matching with DRS spectrum (Figure 11). It is noteworthy to mention that due to photosensitisation, G3T3 gives impressive AQY values of



23.15, 24.70, 41.01, 7.10 and 3.90 % compared to significantly lower values for HPT-500 catalyst under  = 400, 420, 450, 550 and 600 nm incident light irradiation.60

Abs (a. u.)

1.0

HPT-500 G3T3

50

0.8

40

0.6

30

0.4

20

0.2

10

0.0

AQY (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

0

400

500

600

700

800

Wavelength (nm)

Figure 11. Comparison of DRS spectra and AQY values of G3T3 photocatalyst under optimal photoreaction conditions and different monochromatic light irradiation Proposed Mechanism: Thus, it is envisioned that based on the results and discussion, the possible mechanism for H2 generation over dye (G1 and G3) sensitized hierarchical porous anatase TiO2 has been proposed in Figure 12. The molecular orbital populations of these G1 and G3 sensitizers, the delocalization of electron densities in HOMO level occurred in anthracene and dithiole units where as the LUMO electron cloud centered over the π-spacer having anchoring group. The conceptual representation exhibits the feasible electron transfer from the G1 and G3 dyes excited state to the conduction band of TiO2 via the linkage of carboxylate group. Thus, photo-excitation can induce the intramolecular charge transfer from the anthracene donor to carboxylate acceptor groups of the sensitizers. Later on the photo induced electrons are entrapped by the cocatalyst Pt for taking part in H2 evolution reaction. The oxidized G1 and G3 have regenerated by taking electrons from SED for catalytic recycle process.


Page 21 of 29

e-

Potential /V vs NHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.57

- 1.5

LUMO

-1 - 0.5 0 0.5 1

2.57

G1

e-

e-

eeCB

-1.70

Pt

G3

TiO2 HOMO

0.50

0.48

VB

SED

3.5

1.22

SED+

Figure 12. Schematic illustration of the H2 production process over the tetrathiafulvalene based dye-sensitized TiO2 photocatalyst. In this account, we have employed TTF scaffolds in mesoporous channel of hierarchical titania particles, where a good corroboration is found with regard to their thermal stability and fast regeneration. Additionally, their hexy-substituted thiafulvalene moieties and least aggregation properties exhibited higher H2 production activity (see Supporting Information, Table S8). The results suggest that in cyanoacrylic acid based G1 and G3 sensitizers, the dyes are formed in blocking layers along with regular arrayed structures on the HPT surface. Thus, such arrangement could favour the suppression of electron recombination and enhance H2 production. Currently, we are developing various TTF scaffolds with optically tuned structures to further improve the H2 evolution efficiency. Compared to other photocatalytic systems, the present work exhibited impressively high AQY (~ 41%). The obtained results represent a major breakthrough in the design and development of robust thiafulvalene-based sensitizers in solar hydrogen production application. The report further highlights that the worm-hole fibrous morphology of hierarchical porous TiO2 structure in the sensitized photocatalyst led to multiple reflection and effective charge transfer that drive towards efficient hydrogen production. The enhanced H2 production activity is also attributed to the synergistic effect of thermally stable thioalkyl substituted thiafulvalene dye and tunable porosity of hierarchical porous TiO2 nanostructure. To the best of our knowledge, sunlight driven hydrogen production via dye-hierarchical porous TiO2 has been rarely reported and the use of



thiafulvalene is the first example in this study. These findings may open up a broad scope of commercial viability in solar energy application.

Conclusion: In summary, tetrathiafulvalene (TTF) scaffold based metal-free organic sensitizers are molecularly anchored on to hierarchical porous TiO2 particles have been reported for the first time. Employing it as a H2 evolution catalyst, we achieved 122998 µmolg-1cat H2 yield, TON 2460 and high AQY ~ 41% for G3T3 after 5h light irradiation. In this context, a donor-acceptor strategy has been implemented in which the scaffolds, thioalkyl in tetrathiafulvalene acted as a donor, anthracene moiety as a -spacer and the anchoring group as cyanoacrylic acid. The optical, electrochemical and density functional studies affirmed the molecular integrity of G1 and G3 sensitizers, resulting in strong absorption, optimized lowest unoccupied molecular orbital aid to electron injection from the sensitizers excited state into the conduction band of TiO2. In this account, we reported facile and scalable approach for fabricating mesoporous hierarchical titania network, preparing at room temperature through self formation approach using ammonia solution and titanium (IV) isopropoxide followed by annealing at 400−800 °C. The resulting TTF-HPT composite exhibits enhanced H2 evolution, is mainly ascribed to three reasons: i) hierarchical porous channel network of titania with increase surface area; ii) enhanced charge separation due to reduced defect density of the titania iii) thioalkyl scaffolds wrapping over the anthracene to suppress the charge recombination of electrons in the TiO2 conduction band with the oxidized sensitizer. In conclusion, the first report of TTF based hierarchical porous TiO2 catalyst represents a major breakthrough in solar to H2 conversion application, and that the present work will trigger further studies of research. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional information including structural characterization, apparent quantum yield and turnover number calculation, TEM and XRD images, DRS spectra, Differential pulse


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π-Extended

Thioalkyl

Substituted

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Graphic Tetrathiafulvalene Scaffolds Based Sensitizer on Hierarchical Porous TiO2: Efficient Light Harvesting Material for Hydrogen Production

- 1.5

-1.57 LUMO

-1 - 0.5 0 0.5

G1

e-

e-

-1.70

e-

e-

CB

2.57

TiO2

Pt

G3 1.22

HOMO

0.50

0.48

1

-1 -1 H2 Production (molh g cat)

e-

Potential /V vs NHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30000

20000

PHPT G3T1 G3T2 G3T3 G3T4 G3T5 G1T3

10000

0 PHPT G3T1 G3T2 G3T3 G3T4

VB

SED

3.5

SED+


Photocatalysts

G3T5 G1T3

Tetrathiafulvalene Scaffold-Based Sensitizer on Hierarchical Porous

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