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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Combining Dithienosilole-Based Organic Dyes with a Brookite/ Platinum Photocatalyst toward Enhanced Visible-Light-Driven Hydrogen Production Ottavia Bettucci,†,‡,⊥ Theodosis Skaltsas,§,⊥ Massimo Calamante,‡,△ Alessio Dessì,‡ Matteo Bartolini,†,‡ Adalgisa Sinicropi,†,‡,∥ Jonathan Filippi,‡ Gianna Reginato,‡ Alessandro Mordini,‡,△ Paolo Fornasiero,*,§ and Lorenzo Zani*,‡ Downloaded via 193.56.73.125 on August 16, 2019 at 04:10:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Biotechnology, Chemistry and Pharmacy, University of Siena, Via A. Moro 2, 53100 Siena, Italy Institute for the Chemistry of Organometallic Compounds (ICCOM), Consiglio Nazionale delle Ricerche (CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy § Department of Chemical and Pharmaceutical Sciences, ICCOM-CNR Trieste Research Unit and INSTM Research Unit, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy ∥ CSGI, Consorzio per lo Sviluppo dei Sistemi a Grande Interfase, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy △ Department of Chemistry “U. Schiff”, University of Florence, Via della Lastruccia 13, 50019 Sesto Fiorentino, Italy ‡

S Supporting Information *

ABSTRACT: Dye-sensitized photocatalytic hydrogen generation is emerging as a promising process to produce fuels using a clean and abundant energy source such as sunlight. In the first part of this work, three organic dyes featuring a dithieno[3,2-b:2′,3′-d]silole heterocyclic unit (OB1−OB3), bearing different substituents on various parts of the molecular scaffold, were synthesized, characterized, and used as sensitizers for the commercially available benchmark TiO2 (P25), first in dye-sensitized solar cells and then for the photocatalyzed production of hydrogen with triethanolamine as a sacrificial electron donor. In the second part of the study, aiming to improve the efficiency of the photocatalytic system, P25 was replaced with the less investigated brookite TiO2 polymorph. The photocatalyst obtained upon sensitization with the best performing dye, OB2, still in the presence of Pt as co-catalyst, displayed an enhanced performance in hydrogen production compared to that based on P25 at a lower dye loading. Extended time experiments confirmed that the catalyst was still significantly active after 1 week under continuous illumination, providing a maximum TON of 4201. The higher efficiency of the brookite-based catalytic system and its prolonged stability are especially significant in the perspective of the practical application of the dye-sensitized photocatalytic H2 production technology. KEYWORDS: hydrogen, brookite, dithienosilole, organic dyes, photocatalysis

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INTRODUCTION

reason, research on efficient systems for the conversion of sunlight into fuels has become crucially important. In this context, molecular hydrogen obtained from water appears an ideal energy carrier since it has a high energy content and no carbon footprint and its reaction with oxygen in a fuel cell produces only water as a byproduct.8 Although photocatalytic water splitting represents the ideal reaction for the sustainable production of hydrogen, it is still characterized by a limited efficiency that hinders its practical application.9 Therefore, photocatalytic H2 production in an aqueous environment by employment of a sacrificial electron donor (SED), bypassing the water oxidation reaction, can represent a

The transition in energy production from fossil fuels to renewable sources is one of the most important technological challenges currently faced by our society.1 Among renewable energy sources, sunlight is considered particularly attractive since it is abundant, widely distributed, free, and practically inexhaustible.2 Accordingly, over time, massive research efforts have been devoted to the discovery of efficient methods for the conversion of sunlight into electricity, including the recent development of next-generation photovoltaic devices such as organic (OPV),3 dye-sensitized (DSSC),4,5 and perovskite (PSC)6 solar cells. Nevertheless, electricity demand does not constitute the major portion of global energy needs, since almost 80% of world energy consumption is represented by fuels, chiefly used for transportation and heating.7 For this © XXXX American Chemical Society

Received: April 19, 2019 Accepted: July 9, 2019 Published: July 9, 2019 A

DOI: 10.1021/acsaem.9b00782 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials

those of 2,2′-bisthiophene and cyclopentadithiophene (CPDT) due to its lower lying LUMO. This is the result of the interaction of silicon σ*-orbital and bithiophene π*-orbital, namely, σ*−π* conjugation.43 (2) The tetrahedral silicon center hinders the π−π stacking of the dye molecules on the semiconductor surface without affecting the planarity of the πconjugated spacer, thereby limiting the adverse effects of dissipative intermolecular charge transfer.34 Despite such interesting properties, DTS-based dyes have never been employed for photosensitized hydrogen production, prompting us to investigate their application in this emerging field of research. Besides the sensitizer structure, it should be noted that the possibility to combine organic sensitizers with different crystalline forms of TiO2 to obtain optimized photocatalysts has not been tested yet, and no comparison between materials having a different phase composition has been described to date. Indeed, TiO2 exists in three main crystalline polymorphs: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic), whose different lattice arrangements result in different mass densities and electronic band structures. Rutile is the stable form, whereas anatase and brookite are metastable and are readily transformed to rutile when heated.44 Anatase has been traditionally the most employed TiO2 polymorph in photocatalytic studies, either in pure form or as a mixture with rutile (the commonly employed P25 material is a mixture of 80% anatase and 20% rutile).45 Brookite, on the other hand, despite having been synthesized in pure form since the 1960s,46,47 until recently has been scarcely investigated in photocatalysis, mostly due to the difficulty in preparing it with high phase purity and large surface area.48−50 In the past decade, however, advances in synthetic procedures to produce pure brookite and well-defined brookite-containing composites, together with the development of methods for the thorough characterization of the materials, made it possible to study its photocatalytic properties in much more detail.51 Recently, it has been shown that the exposed facets of Pt/ brookite, in combination with its favorable electronic properties, work more efficiently toward proton reduction compared to anatase,52 which suggested to us its use to replace P25 in our experiments. Despite that, to date, studies on photosensitized brookite NPs have been confined to the field of DSSCs. As widely known, most of the TiO2-based DSSC investigations have focused on anatase particles because of their good electrical conductivity4,53 and lowest crystal surface energy (compared to other TiO2 polymorphs) efficiently supporting dye adsorption.54,55 In general, brookite-based DSSCs showed worse performances than their anatase-based counterparts due to their smaller surface area (which means lower dye absorption on the semiconductor surface) and lower electric conductibility.53,56,57 Notwithstanding these two drawbacks, an interesting study of Kusumawati et al., investigating chemical capacitance and charge-transfer resistance of DSSCs built with both anatase and brookite, showed a lower reactivity of the brookite surface, which was translated in a reduced charge recombination of injected electrons, giving higher values of Voc.54 On the basis of the above considerations, in this work we will first present the design, synthesis, and application of three novel organic photosensitizers (OB1−OB3, Figure 2b) incorporating the dithieno[3,2-b:2′,3′-d]silole heterocyclic unit. These dyes were first used to build the corresponding

valid alternative, especially if the SED is a renewable feedstock10 or derives from waste materials that need to be reused.11 Since the pioneering experiments of Fujishima and Honda in 1972,12 TiO2 has been the most used material in the field of photocatalytic H2 production, being cheap, nontoxic, chemically and biologically inert, and photostable. In particular, when coupled with appropriate metal co-catalysts (such as Au, Ni, Pt, or Ru), it performs well in various photocatalytic reactions,13 among which is photo-re-forming of several different SEDs.14−17 However, TiO2 does not absorb visible light because of its large band gap (3.0−3.2 eV, depending on the crystal form), so that suitable strategies have to be devised to enhance light harvesting in the visible region. Among them, sensitization with appropriate dyes is a well-established approach, which has been largely employed in the field of dye-sensitized solar cells (DSSCs).4,5 The dye acts as an antenna, efficiently absorbing visible light and triggering the subsequent steps of the hydrogen production process by electron injection in the TiO2 conduction band (Figure 1).18−20

Figure 1. General mechanism of dye-sensitized hydrogen production with a SED (energy levels of Ru-based dye Ru(dcbpy)3Cl219 were taken as an example).

Thanks to their easy synthesis, low cost, tunable photophysical properties, and good stability,21 metal-free organic sensitizers are gaining a prominent role for this application; whereas early studies focused on emissive dyes such as eosin Y and rhodamine B, 22 more recently very efficient D (donor)−π−A (acceptor) sensitizers have been reported, incorporating various different heterocyclic moieties as main chromophores,23−28 as well as triphenylamine,29 carbazole,30 or dithiafulvalene31,32 as donor groups. Despite that, there is still ample room to test new heterocyclic units in the search for enhanced spectroscopic, electrochemical, and geometrical properties. One such structural motif is represented by the dithieno[3,2-b:2′,3′-d]silole (DTS) tricyclic unit (Figure 2a). Conjugated heteroaromatic compounds containing the DTS moiety have found extensive application in organic electronics and photovoltaics33 and, starting from 2010, also as photosensitizers for TiO2-based dye-sensitized solar cells, providing good to excellent results in terms of power conversion efficiencies (PCEs).34−42 Compared to similar bisthiophene units, the DTS system presents some peculiar properties that enhance both its light-harvesting ability and charge injection efficiency: (1) the HOMO−LUMO gap of DTS is smaller than B


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Figure 2. (a) Structure of the DTS heterocyclic unit; (b) three DTS-based sensitizers prepared in this work; (c) ProDOT-based sensitizers MB25 and AD418 previously reported by some of us.58 irradiating the sample at a wavelength close to that of maximum absorption in the corresponding UV spectrum. Cyclic voltammetry experiments were conducted in dichloromethane solution with a PARSTAT 2273 electrochemical workstation (Princeton Applied Research) employing a three-electrode cell having a 3 mm glassy carbon working electrode, a platinum counter electrode, and an aqueous Ag/AgCl (sat. KCl) reference electrode. The supporting electrolyte was electrochemical-grade 0.1 M [N(Bu)4]PF6; the dye concentration was 10−3 M. Under these experimental conditions, the one-electron oxidation of ferrocene occurs at E0′ = 0.52 V. Dye-Sensitized Solar Cells Fabrication and Characterization. An 8 Ω/sq conductive glass substrate (TCO30-8, Solaronix) was washed with a detergent and water in an ultrasonic bath at 150 W for 10 min (10 s work, 2 s rest) and then rinsed with water, ethanol, and acetone. To prepare the TiO2 photoanodes, a commercial transparent titania paste (18NR-T, Greatcell Solar) was screenprinted on the substrates (0.25 cm2 active area) and the resulting electrodes were heated for 6 min at 125 °C. The screen-printing and heating procedure was repeated one more time with the same paste and then a third time with a scattering paste (WER2-O, Greatcell Solar), to obtain films with a final thickness of 13 μm (measured with KLA Tencor-P10 surface profilometer). Finally, the electrodes were sintered for 5 min at 325 °C, then 5 min at 375 °C, 15 min at 450 °C, and finally 15 min at 500 °C. Titania photoelectrodes were sensitized by overnight immersion at room temperature (RT) into the appropriate dye solution (2.0 × 10−4 M in 1:10 THF/EtOH (v/ v)), then rinsed with ethanol, and dried. Pt counter electrodes (CE) were prepared by screen-printing a commercial Pt paste (PT1, Greatcell Solar) followed by activation by means of thermal treatment (500 °C, 30 min). Cells were obtained by pressing the two electrodes against each other in a heat press, separated by a thermoplastic frame (TPS 065093-30, Greatcell Solar). The interstitial space was filled with a commercial I−/I3−-based liquid electrolyte (EL-HPE, Greatcell Solar). Current density−voltage (J/V) characteristics of all solar cell devices were measured using an AM 1.5G Class A ABET SUN2000 solar simulator. The measurement was carried out at a light intensity of 1000 W/m2, which was calibrated using a Si reference cell. J/V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with an Agilent B2901A digital source meter. The density of adsorbed dyes on TiO2 was determined using the following procedure: 1 mL of a solution of the dye in THF/EtOH

DSSCs and measure their power conversion efficiencies. The same dyes were subsequently tested also in hydrogen production experiments, using a P25-based TiO2/Pt catalyst and triethanolamine (TEOA) as a sacrificial electron donor, showing that all of them were active in the gas evolution reaction. Interestingly, in both applications their performances were strongly affected by the position of the hydrophobic substituents present on their scaffold but followed different trends. Subsequently, we will describe the use of the best performing dye (OB2) for H2 production in combination with a brookite/Pt TiO2 catalyst, comparing its performances with those of the corresponding P25/Pt material. Finally, we will also present the results of a prolonged (170 h) hydrogen evolution experiment, aiming to assess the stability of the optimized photocatalytic system.

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EXPERIMENTAL SECTION

Computational Details. DFT and TD-DFT calculations on compounds OB1−OB3 have been performed using the Gaussian 16, Revision B.01 suite of programs.59 Geometry optimization was carried out in vacuo using the B3LYP functional60,61 and the standard 631G* basis set for all atoms. The absorption maximum (λamax), vertical excitation energy (Eexc), and oscillator strength ( f) in solution were calculated on the optimized structures via time-dependent DFT (TD-DFT) at the CAM-B3LYP62/6-311G(d,p) and MPW1K63/6311G(d,p) levels of theory. Solvent effects have been included by using the polarizable continuum model (PCM).64 The UV−vis spectra have been simulated considering a Gaussian distribution and an arbitrary line width of 0.03 eV using GaussSum 3.0.65 Spectroscopic and Electrochemical Measurements. UV/vis spectra in different solvents were recorded on diluted solutions of the analyte (approximately 10−5 M) with a Shimadzu UV-2600 spectrometer. UV/vis spectra of the compounds adsorbed on TiO2 were recorded in transmission mode after sensitization of thin, transparent semiconductor films (thickness, approximately 5 μm). Diffuse reflectance spectra of the compounds adsorbed on P25/Pt or brookite/Pt were recorded with the same instrument in the reflectance mode using an integrating sphere (with barium sulfate as a reference material) and were converted to the corresponding absorption spectra by using the Kubelka−Munk equation. Photoluminescence spectra were recorded with a Cary Eclipse instrument, C


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ACS Applied Energy Materials (1:10 (v/v); concentrations, (OB1) 5.0 × 10−4 M, (OB2) 2.5 × 10−4 M, and (OB3) 2.0 × 10−4 M) was inserted in a small sealed chamber containing a TiO2 electrode identical to those employed for solar cell characterization (0.25 cm2). After 6 h staining, a 0.1 mL aliquot of the solution was removed from the chamber and diluted 10 times. The absorbance of the resulting colored solution was measured by UV/vis spectroscopy and compared to that of a standard (2.0−5.0) × 10−5 M solution of the same dye in the same solvent. The amount of dye present in the unknown solution was calculated, and by difference the amount of dye adsorbed on the TiO2 film could thus be estimated. Electrochemical impedance spectroscopy (EIS) measurements were performed in the dark using a PARSTAT 2273 electrochemical workstation (Princeton Applied Research), working at Voc (forward bias). The spectra were recorded over a frequency range of 10−1 to 105 Hz with an amplitude of 10 mV. Data fitting was carried out using the EC-Lab software (V9.46). Preparation of Pt/P25-TiO2 Nanopowder. Pt was photodeposited on TiO2 P25 (Evonik/Degussa) following a previously reported procedure.66,67 Briefly, 2 g of TiO2 P25 was suspended in a Pt(NO3)2 aqueous solution (400 mL, EtOH 50% (v/v)), in order to reach a final Pt loading of 1.0 wt %. After stirring for 1 h in the dark, the suspension was irradiated with a 450 W medium pressure Hg lamp for 4 h. Nanopowders were recovered through centrifugation, washed with EtOH three times, and dried under vacuum at 50 °C overnight. Preparation of Pt/Brookite-TiO2 Nanopowder. To prepare the brookite-TiO2 nanopowder, commercial titanium(IV) bis(ammonium lactate) dihydroxide [Ti(NH4C3H4O3)2(OH)2] aqueous solution (50 wt %, Sigma-Aldrich) was subjected to hydrothermal treatment in the presence of urea, adapting the procedure reported by Zhao et al.68 A 40 mL Teflon lined autoclave was charged with 1.5 mL of the Tibased precursor and 13.5 mL of urea solution. The concentration of urea solution was 7.0 M, in order to reach a urea/Ti molar ratio of 44.5, for which pure brookite is obtained.52 After careful mixing, the autoclave was heated at 160 °C for 24 h in a convection oven. The product is obtained as a white precipitate. The precipitate was collected by centrifugation, washed several times with bidistilled water, and finally dried at 80 °C overnight. The materials were subjected to calcination at 400 °C for 3 h in order to remove any organic contaminants deriving from partial decomposition of the lactate precursor, obtaining white solids. Pt was then photodeposited on the surface of brookite nanoparticles in the same way as described above for P25/Pt. Dyes Adsorption on Pt/TiO2. To achieve a dye loading of 10 μmol g−1, 200 mg of Pt/TiO2 nanopowder (either P25 or brookite) was suspended in 20 mL of dye solution (0.1 mM in ethanol, in turn obtained by diluting a concentrated stock solution in THF) for 24 h in the dark. Then, the nanopowder was separated through centrifugation, washed twice with ethanol, and dried under vacuum at 60 °C overnight. After staining, the concentration of the dyes in the supernatant solution was measured by UV−vis spectroscopy. More than 95% of the dye was adsorbed on the Pt/TiO2 material in all cases, as testified by the almost colorless supernatant. To obtain the other dye loadings, the appropriate amount (5−15 mL) of the initial 0.1 mM dye solution was measured and diluted to 20 mL with ethanol and then the staining procedure was repeated as described above. Hydrogen Production Experiments. The dye-sensitized Pt/ TiO2 nanomaterials have been tested for H2 production using TEOA as sacrificial electron donor, following or slightly modifying our previously described procedure.58 A 60 mg amount of of the dyesensitized Pt/TiO2 catalyst was suspended into 60 mL of 10% (v/v) aqueous solution of TEOA, whose pH was previously adjusted to 7 with aqueous HCl. After purging with Ar (15 mL min−1) for 30 min, the suspension was irradiated using a 150 W Xe lamp with a cutoff filter at 420 nm (to exclude contribution from TiO2). Irradiance was ∼6 × 10−3 W m−2 in the UV-A range and ∼1080 W m−2 in the visible and near-IR range (400−1000 nm). The concentration of H2 in the gas stream coming from the reactor has been quantified using an Agilent 7890 gas chromatograph equipped with a TCD detector, connected to a Carboxen 1010 column (Supelco, 30 m × 0.53 mm

i.d., 30 μm film) using Ar as carrier. Blank experiments on Pt/TiO2 in the absence of dyes showed no H2 evolution under any of the experimental conditions used in this work. The performances of the sensitized photocatalysts have been reported in terms of overall H2 productivity. Turnover numbers (TONs) were calculated from the total amount of H2 produced during the entire experiments as TON =

2 × overall H 2 amount/(μmol g −1) dye loading/(μmol g −1)

(1)

The light-to-fuel efficiency (LFE) was calculated as LFE =

FH2 × ΔHH02 S × A irr

(2)

where FH2 is the flow of H2 produced (expressed in mol s−1), ΔH0H2 is the enthalpy associated with H2 combustion (285.8 kJ mol−1), S is the total incident light irradiance, as measured by adequate radiometers in the 400−1000 nm range (expressed in W cm−2), and Airr is the irradiated area (expressed in cm2). UV/vis spectra of the aqueous solutions recovered at the end of the photocatalytic runs highlighted that no desorption of the dyes took place during the experiments.

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RESULTS AND DISCUSSION Design and Computational Analysis of DTS-Based Organic Sensitizers. In a recent work, we have shown that D−π−A sensitizers having long alkyl chains on their central section provided better performances in the photo-re-forming of TEOA compared to their analogues devoid of hydrophobic chains, or bearing them on the donor portion of the molecule.58 Such effect was attributed to the better shielding of TiO2 surface, limiting the absorption of the hydrophilic sacrificial donor and thus hindering the recombination of injected electrons.69 In particular, the best results were obtained with dyes MB25 and AD418 (Figure 2c),58 bearing n-pentyl chains on a central propylenedioxythiophene (ProDOT) unit. Interestingly, the tridimensional arrangement of alkyl chains on the quaternary carbon of the ProDOT moiety resembles that around the silicon center in a DTS spacer. Due to such structural similarity, the DTS unit then appeared to be an ideal platform to extend our studies on the effect of the position and number of hydrophobic substituents on the performances of organic sensitizers for photocatalytic H2 production. With this aim, inspired by the structure of the best performing dye in DSSCs,34 we designed three DTS-based organic sensitizers, OB1−OB3, differing only for the presence and the placement of hydrophobic alkyl chains on their molecular structures (Figure 2b). Before preparing compounds OB1−OB3, we carried out a computational analysis to obtain a first assessment of their structural and electronic properties. First, structures were optimized in vacuo using DFT calculations at the B3LYP/631G* level.60,61 All compounds presented a fairly linear conjugated scaffold with small dihedral angles between the cyanoacrylic, DTS, and thiophene units (Supporting Information Figure S1), and a slightly more pronounced torsion angle between the thiophene and triphenylamine groups (22.9°− 24.4°). As expected, the long alkyl chains bound to silicon in dyes OB1 and OB2 were almost perpendicular to the plane of the conjugated system, likely providing an efficient way to minimize dye aggregation upon adsorption on the TiO2 surface. Then, the energy and shape of the frontier molecular orbitals (FMOs) of the new dyes were calculated (Figure 3). In D


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Figure 3. Computed (B3LYP/6-31G*) HOMO−LUMO energy gaps of compounds OB1−OB3 and frontier orbitals electron density distributions.

Scheme 1. Preparation of Sensitizers OB1−OB3 from starting materials 1a,b and 3

the HOMO−LUMO gap. The partial FMO superposition observed for all dyes was anticipated to improve their light absorption intensity by facilitating intramolecular charge transfer. Subsequently, the absorption maxima (λamax), oscillator strengths (f), and vertical excitation energies (Eexc) of the new sensitizers were assessed by TD-DFT calculations in CH2Cl2 at the CAM-B3LYP62/6-311G(d,p) and MPW1K63/6-

all cases, the HOMO was mostly localized on the donor triarylamine unit, with a significant contribution of the thiophene−DTS system, while the LUMO was preferentially located on the acceptor group, albeit still with a sizable contribution of the conjugated scaffold. Introduction of electron-rich alkoxy moieties in the donor group of OB3 was found to induce a significant HOMO destabilization, while the effect on LUMO was much smaller, resulting in a decrease of E


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Figure 4. UV−vis absorption spectra of sensitizers OB1−OB3 in CH2Cl2 solution (a) and adsorbed on transparent TiO2 films (b). OB1, green dotted line; OB2, red solid line; OB3, blue dashed line; blank TiO2, black dashed−dotted line.

Table 1. Spectroscopic and Electrochemical Properties of Dyes OB1−OB3 compd

λmax(CH2Cl2) (nm)a

E0−0 (eV)b

λmax(TiO2) (nm)

Eox (V)c

EHOMO (eV)d

ELUMO (eV)d,e

OB1 OB2 OB3

495 (2.72) 506 (3.37) 532 (4.00)

2.18 2.11 2.04

420 485 485

+0.90 +0.90 +0.73

−5.50 −5.50 −5.33

−3.32 −3.39 −3.29

Values in parentheses refer to molar absorption coefficients (×104 M−1 cm−1). bOptical band gap determined from the corresponding Tauc plots (Figure S5). cValues reported vs NHE. dValues relative to a vacuum, using a potential value of 4.6 ± 0.2 eV for NHE vs vacuum.77 eCalculated using the expression ELUMO = EHOMO − E0−0. a

demethylation of compound 4a, giving rise to compounds 5a,b in good yields. Brominated compounds 6a,b were then accessed from aldehydes 5a,b by means of halogenation under standard conditions (treatment with NBS in CH2Cl2 solution). Subsequently, bromides 6a,b were coupled with 2thienyltributylstannane (7) under Pd catalysis in refluxing toluene to provide intermediates 8a,b in high yields; then, the latter compounds were reacted once again with NBS in CH2Cl2, giving the desired bromine-containing building blocks 9a,b. The key Stille−Migita cross-coupling reaction between building blocks 2a,b and 9a,b proceeded with good efficiency, providing the desired products 10a−c in 58−70% yield, accompanied by a small amount of the homodimerization products of 2a,b, which could nevertheless be easily separated by flash chromatography. Conversion of the advanced aldehyde intermediates 10a−c to the final sensitizers took place using the commonly adopted Knoevenagel condensation with cyanoacetic acid (11), allowing isolation of cyanoacrylic derivatives OB1−OB3 in high yields (Scheme 1). Although similar structures can be found in the literature,34−38 the three sensitizers OB1−OB3 were previously unreported and thus have been fully characterized via NMR spectroscopy and mass spectrometry. Their thermal stability was assessed by means of thermogravimetric analysis, revealing that all samples retained 92−95% of their weight up to 200 °C, with OB1 displaying a slightly accelerated weight loss compared to those of OB2 and OB3 (Figure S3). Spectroscopic and Electrochemical Characterization of Dyes OB1−OB3. To confirm that dyes OB1−OB3 could be employed as visible light harvesters in the photocatalytic production of hydrogen, their spectroscopic and electrochemical characterization was carried out. As expected on the basis of their structures, when dissolved in CH2Cl2, all dyes displayed an intense absorption band in the visible region, attributed to the intramolecular donor-to-acceptor chargetransfer transition (Figure 4a). The maximum absorption wavelength increased in the order OB1 < OB2 < OB3,

311G(d,p) levels of theory (Table S1). All dyes exhibited computed absorption maxima well above 500 nm, as can be seen by the simulated absorption spectra shown in Figure S2 (obtained from the CAM-B3LYP data). With both functionals, OB3 was predicted to show a red-shifted absorption compared to the other dyes, corresponding to a smaller vertical excitation energy, while oscillator strengths were found to be similar in all cases. For all three dyes, the light absorption process in the visible region results mostly from HOMO−LUMO transitions, supporting the hypothesis of a strong degree of intramolecular charge transfer upon photoexcitation. In general, all dyes were predicted to have structural and electronic features compatible with their employment in DSSCs and photocatalytic hydrogen production and were therefore prepared in useful quantities. Synthesis of DTS-Based Organic Sensitizers. After having explored different disconnection approaches, the synthesis of dyes OB1−OB3 was carried out by preparing two separated building blocks (donor section 2a,b and πspacer/acceptor section 9a,b, respectively) and then joining them by means of a Stille−Migita cross-coupling reaction (see Scheme 1). In our hands, such strategy provided a higher yield of the final product compared to those previously reported in the literature.34 Detailed experimental procedures for the preparation of compounds OB1−OB3 are reported in the Supporting Information. Stannanes 2a,b were easily prepared from the corresponding bromides 1a,b following literature procedures, by means of metal−halogen exchange with n-BuLi at −78 °C followed by reaction with Bu3SnCl.70,71 Compounds 9a,b were instead obtained starting from 3,3′-dibromo-2,2′-bisthiophene (3) using a five steps sequence. In the first step, dibromide 3 was converted to dithienosiloles 4a,b by modification of a literature procedure, treating the substrate with n-BuLi and the appropriate dialkyldichlorosilane and stirring at −78 °C for 2.5−5 h; in this way both products were obtained in 75% yield.72 Subsequently, the aldehyde function was installed via a Vilsmeier−Haack reaction which, differently from previous reports,35 was performed at RT for 24 h to avoid partial F


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ACS Applied Energy Materials Table 2. Characterization Data of DSSCs Built with Dyes OB1−OB3a compd

Jsc (mA cm−2)

OB1 OB2 OB3 N719c

11.54 11.13 13.70 14.49

± ± ± ±

0.31 0.29 0.26 0.53

Voc (mV) 0.661 0.620 0.694 0.730

± ± ± ±

0.004 0.006 0.005 0.031

ff (%) 69.6 69.4 63.3 65.7

± ± ± ±

0.9 1.1 1.2 3.3

Γ × 10−7 (mol cm−2)b

η (%) 5.31 4.79 6.03 6.94

± ± ± ±

0.12 0.18 0.15 0.31

(5.39) (5.00) (6.18) (7.35)

1.37 0.44 0.66

a

Average values of at least four devices (best cell efficiencies are written in parentheses). bDensity of adsorbed dye on TiO2. cRu-containing dye N719 employed as a reference.

solar cells. Following well-established preparation procedures,81 we built small-area cells (0.25 cm2) having the classic sandwich architecture and featuring a 13 μm thick P25 photoanode, a Pt cathode, and a liquid electrolyte based on the I−/I3− redox couple. Additional details on the cells’ fabrication and characterization are given in the Experimental Section. The results obtained are reported in Table 2 and Figure S7, showing that all three dyes were able to convert solar energy into electric current. Although the cell fabrication procedure was not specifically optimized toward obtainment of high power conversion efficiencies, our results were comparable to those reported for most DTS-containing sensitizers having similar structures and employed in combination with I−/I3−based electrolytes (PCEs between 4.80% and 7.60%),35−38 despite being still inferior to the record values obtained by Wang et al. under fully optimized conditions (maximum PCE, 10.0%).34 The best power conversion efficiency was provided by compound OB3, which yielded both the highest photocurrent and photovoltage values, likely due to its better lightharvesting properties (higher molar extinction coefficient and red-shifted absorption spectrum; see Table 1) coupled with the effect of the hydrophobic alkyl substituents, efficiently preventing dye aggregation and shielding the semiconductor surface to minimize charge recombination.34 The better performance recorded with dye OB1 compared to OB2, on the other hand, could be due to its much higher adsorption density on TiO2 (consistent with its smaller size). The efficiency data were supported by electrochemical impedance spectroscopy analysis (EIS, Table S3 and Figure S8), showing that cells built with OB3 were characterized by the highest charge recombination resistance and longest electron lifetime, mainly related to the contribution of a larger chemical capacitance. We also manufactured and characterized DSSCs using the well-known disaggregating co-adsorbent chenodeoxycholic acid (CDCA) in the sensitizing bath, at a concentration of 1 mM. In all cases, however, lower power conversion efficiencies were observed compared to those of the original cells (Table S4 and Figure S9), indicating that dye aggregation was not the major limiting factor for the performances of the photovoltaic devices. By looking at the dyes’ geometries, in the case of OB2 and OB3 such a result can be easily understood considering that the long n-octyl chains bound to silicon stick outside the plane of the DTS ring system (Figure S1), hindering close contact between dye molecules. On the other hand, in the case of dye OB1, having a fairly linear molecular structure with simple methyl groups bound to silicon, co-adsorption with CDCA probably led to a decrease in the amount of adsorbed sensitizer, counterbalancing the disaggregating effect of the additive. Photocatalytic H2 Production with a P25/Pt Catalyst. For the photocatalytic experiments, the sensitizers were first adsorbed on a TiO2 (P25)/Pt catalyst66 to assess their

together with the molar extinction coefficient (Table 1). The large red shift observed for dye OB3 can be explained considering the higher energy of its HOMO level, reducing the HOMO−LUMO gap (see the computational analysis above and the electrochemical characterization below). Spectra recorded in EtOH showed the same trend as those in CH2Cl2 but had maxima at shorter wavelengths, probably due to partial dye deprotonation in the more polar and protic medium (Figure S4 and Table S2, Supporting Information).73 Optical band gaps (E0−0) for the three dyes were estimated by means of the corresponding Tauc plots (Figure S5):74,75 each dye showed an E0−0 value slightly higher than 2.0 eV, confirming a good light-harvesting ability in the visible region. In general, experimental UV−vis spectra were in good agreement with computed TD-DFT data, especially those generated at the CAMB3LYP/6-311G(d,p) level (Table S1); in particular, experimental vertical excitation energies (Eexc) were comprised in the 2.33−2.50 eV range, which was within 0.1 eV from the calculated values (2.41−2.45 eV). When adsorbed on transparent TiO2 films (P25 thickness, approximately 5 μm), all compounds showed spectra of similar width having clearly blue-shifted absorption maxima compared to those recorded in solution and a dramatic broadening of the absorption bands (Figure 4b), which could be due both to deprotonation following adsorption on the semiconductor surface and formation of blue-shifting H-aggregates.76 In this respect, the most blue-shifted absorption, around 60 nm compared with the other two acids, was that of compound OB1. Indeed, being OB1 the only sensitizer devoid of long alkyl chains either on the DTS core or on the donor moiety, this result suggested that aggregation is more significant for this compound. Cyclic voltammetries of the three acids in CH2Cl2 solution were then recorded to determine their ground-state oxidation potentials Eox (Figure S6, Table 1). In agreement with the computational and spectroscopic analysis, dye OB3 displayed a less positive Eox value compared to those of OB1 and OB2 (+0.73 V vs NHE compared to +0.90 V), indicative of a higher HOMO energy due to the presence of the electron-rich alkoxy substituents on the donor group. Nevertheless, Eox values for all of the sensitizers were generally compatible with regeneration by the I−/I3− redox shuttle used in DSSCs (E° = +0.35 V vs NHE),78 as well as the most commonly employed SEDs (such as TEOA, E° = +0.82 V vs NHE, in H2O at pH = 7).79 Finally, LUMO energies for all dyes were found to be far more negative than the conduction band edge of both anatase and brookite TiO2,80 ensuring facile electron injection upon photoexcitation of the sensitizers. Photoelectrochemical Experiments. Fabrication and Characterization of Dye-Sensitized Solar Cells. To evaluate the relative light-harvesting and charge-transfer capabilities of dyes OB1−OB3 under illumination conditions, we first fabricated and characterized the corresponding dye-sensitized G


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ACS Applied Energy Materials capability to promote H2 evolution upon simulated solar light irradiation. Details of catalyst preparation and dye sensitization can be found in the Experimental Section. After sensitization, almost transparent supernatants were obtained in all cases, as determined with UV−vis spectroscopy (Figure S10), proving the near-complete adsorption of the dyes on the TiO2/Pt surface. The spectroscopic characteristics of the adsorbed dyes were probed via diffuse reflectance spectroscopy (DRS), showing that all sensitized photocatalysts absorbed light in the visible region (Figure S11a) with a progressive red shift going from OB1 (λmax = 423 nm) to OB3 (λmax = 473 nm), in good agreement with the previously discussed UV−vis analysis (Figure 4). In addition, their good electron injection capability was confirmed by recording their photoluminescence spectra both in CH2Cl2 solution and adsorbed on P25 TiO2/Pt (Figure S12): in the latter case, emission was completely quenched in the 600−800 nm range, indicating that efficient charge transfer took place following photoexcitation. Initially, the dye loading was set to 10 μmol g−1 catalyst, a value that was found optimal in our previous study.58 All three dye-sensitized catalysts under examination were active in the gas evolution reaction and showed an initial hydrogen production rate increase, followed by stabilization after 2 h of solar irradiation, which may be attributed to chromatographic effects and/or the activation of passivated Pt under light exposure. As evidenced in Table 3 and by observing the

Figure 5. Hydrogen production per catalyst surface area of N719 (gray rhombs), OB1 (green squares), OB2 (red circles), and OB3 (blue triangles) sensitized P25/Pt catalysts with TEOA as SED under irradiation with visible light (λ > 420 nm).

Remarkably, all DTS-based dyes yielded an equal or superior performance compared to that of standard Ru-based sensitizer N719, which was the best performing dye in DSSC experiments; furthermore, the LFE of N719 decreased substantially during the 20 h irradiation time, suggesting that the latter dye was less stable than OB1−OB3 in the photocatalytic experiment conditions. With use of the best performing sensitizer OB2, we carried out experiments at three different dye loading values (2.5, 5, and 10 μmol g−1): although at lower dye loading a slight increase of the TON was observed up to a value of 622, a much smaller quantity of gas was produced, indicating that a reduction of dye amount under these conditions was unproductive (Figure S14 and Table S5). Finally, it should be mentioned that when a blank experiment was set with neat P25/Pt as catalyst under visible light (λ > 420 nm), no activity was observed. Interestingly, the relative order of performances observed in the hydrogen evolution experiments with dyes OB1−OB3 was different from that of the dye-sensitized solar cells built with the same sensitizers (Table 2). Considering that in this case a much lower (and equal) amount of each dye was adsorbed on the catalyst, the superior performance of OB2 compared to OB1 could be explained by the protecting effect of the long alkyl chains placed in the middle part of the molecule, which might reduce charge recombination by preventing an approach to TiO2 surface by potential quenchers present in the aqueous environment, in agreement with our previous results.58 On the other hand, the relatively poor performance of dye OB3 could be related to a more difficult regeneration, caused on one hand by its higher lying HOMO (Table 1), and on the other by the presence of hydrophobic alkoxy substituents on its donor group, hindering the approach by the relatively large and hydrophilic TEOA molecule. Such effects would cancel out the positive influence of enhanced light harvesting (which seemed very important in DSSCs). These results highlight that although dye design principles are similar for employment in DSSCs and photocatalytic hydrogen production, a careful structural optimization must be carried out separately for each application in order to identify the best sensitizer. Photocatalytic H2 Production with a Brookite/Pt Catalyst. To begin this series of experiments, pure brookite nanorods were synthesized and characterized according to our previously reported procedure52 and then Pt was photodeposited on their surface in the same way as on P25 (see Figure S13 for a representative TEM micrograph of the brookite/Pt nano-

Table 3. Comparison of the Photocatalytic Performances of OB-Series-Sensitized (10 μmol g−1) P25/Pt Catalysts in H2 Production with TEOA as SED under Irradiation with Visible Light (λ > 420 nm) light-to-fuel efficiency (%)

dye

overall H2 production rate (μmol/m2)a

overall H2 production (μmol/g)b

TONc

3h

20 h

OB1 OB2 OB3 N719

19 47 23 19

1035 2550 1254 1038

207 510 251 208

0.037 0.074 0.027 0.051

0.050 0.066 0.032 0.017

a

Overall H2 amount produced after 20 h of irradiation per unit surface area of catalyst. bOverall H2 amount produced after 20 h of irradiation per unit mass of catalyst. cTON = (2 × H2 total amount after 20 h of irradiation)/(dye loading).

H2 production graph reported in Figure 5, OB2-sensitized P25/Pt was found to be twice more active than the catalysts sensitized with the other two dyes (OB1 and OB3). Specifically, when OB2-sensitized P25/Pt was used as photocatalyst, 47 μmol of H2 was produced per m2 of catalyst surface (the specific surface area of P25/Pt was 54 m2/g according to BET measurements), whereas gas production was significantly lower at 19 and 23 μmol m−2 with OB1- and OB3sensitized catalysts, respectively. Hydrogen generation was found to be quite stable along the entire experiment, as evident by looking at the almost constant slope of the curves in Figure 5 and, more precisely, at the light-to-fuel efficiency of the three dyes, which is used to quantitatively assess the fraction of solar energy stored in the form of H2 (see Experimental Section).82,83 For all compounds, LFE values after 20 h (LFE20) were almost the same, or even slightly higher, than those after 3 h (LFE03), indicating that performances were relatively constant during the experiment time (Table 3). H


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Table 4. Comparison of the Photocatalytic Performances of the OB2-Sensitized P25/Pt and Brookite/Pt Catalysts in H2 Production with TEOA as SED under Irradiation with Visible Light (λ > 420 nm) light-to-fuel efficiency (%) catalyst

dye loading (μmol/g)

overall H2 production (μmol/m2)a

overall H2 production (μmol/g)b

TONc

3h

20 h

OB2@P25/Pt OB2@brookite/Pt

10 10 7.5 5 2.5

48 75 79 61 39

2550 2745 2841 2199 1404

510 549 758 880 1123

0.074 0.088 0.091 0.056 0.054

0.066 0.088 0.072 0.045 0.031

a

Overall H2 amount produced after 20 h of irradiation per unit surface area of catalyst. bOverall H2 amount produced after 20 h of irradiation per unit mass of catalyst. cTON = (2 × H2 total amount after 20 h of irradiation)/(dye loading).

obtained using different dye amounts (10, 5, or 2.5 μmol g−1; Table 4 and Figure S15) of the dye. From these results, it appears that the optimal dye loading for brookite is lower than that for P25, presumably due to its smaller surface area. This feature might lead to an enhanced agglomeration of dye molecules on the TiO2 surface, reducing the penetration depth of incident light and causing a decrease of the photocatalytic activity.31,32,84,85 Although the turnover number was found to increase progressively when decreasing the dye loading, with a maximum value of 1123 for the 2.5 μmol g−1 catalyst, the overall performance of the 7.5 μmol g−1 OB2 loading on brookite/Pt was clearly the best one. On the other hand, the 10 μmol g−1 dye loading catalyst showed the best stability upon a closer look at the LFE values, while the 2.5 μmol g−1 catalyst appeared to be less stable under the experimental conditions (Table 4). Thus, in contrast with what was previously found for brookite-based DSSCs,53−56 the dye-sensitized brookite/Pt photocatalyst seems to be more effective in H2 production compared to its P25/Pt counterpart, especially at lower dye loading (Table 4 vs Table S5, consider in particular the values for 2.5 μmol g−1 dye loading). Indeed, the two systems present some differences which might be relevant: for instance, compared to DSSCs, in our photocatalytic system dye adsorption is not maximized but rather fixed to a precise amount for both catalysts (supernatant solutions were always almost colorless after sensitization) and is therefore not depending on the catalyst surface area. Furthermore, since Pt is directly deposited on TiO2 nanoparticles, electron transfer to the metal is not limited by charge transport through a micrometer-thick semiconductor layer (as in DSSCs), thus not suffering from the lower conductivity of brookite. On the other hand, the lower reactivity of conduction band electrons of brookite compared to anatase54,86 should mean that a higher fraction of injected electrons can reach Pt and drive a sustained H2 production even in the presence of a smaller amount of dye, in agreement with our results. Finally, to evaluate the stability over time of the best performing photocatalyst (OB2@brookite/Pt, 7.5 μmol g−1), a one week (i.e., 170 h) long experiment was set up (Figure 7). After the first 2 h of irradiation, where the H2 production increases and reaches a maximum, a continuous gas evolution was observed for the entire duration of the experiment. A certain decrease of the reaction rate was observed over time, as can be seen from the variation of the slope of the curve in Figure 7; however, such loss of activity does not seem related to dye desorption from TiO2, since the reaction medium (10% (v/v) TEOA in H2O) was found to be still colorless after 170 h

catalyst). OB2 adsorption on brookite/Pt surface was achieved by following the same protocol applied previously for all dyes. Also in this case a DRS spectrum of adsorbed OB2 was recorded (λmax = 461 nm, Figure S11b), which was in good agreement with that obtained for the same compound adsorbed on P25 (λmax = 458 nm, Figure S11a). Initially, and for comparison reasons, photocatalytic experiments were conducted with a 10 μmol g−1 dye loading, under conditions otherwise identical to those previously employed with P25. We observed that after 20 h of visible light irradiation the OB2@ brookite/Pt system produced 8% more H2 than OB2@P25/Pt, referring the gas amount to the catalyst mass (μmol g−1, Table 4). However, the increase corresponded to 58% when the catalyst surface area was taken into account (Table 4 and Figure 6), owing to the smaller surface area of brookite

Figure 6. Hydrogen production relative to surface area of the OB2sensitized P25/Pt (red circles) and brookite/Pt (black hollow circles) catalysts with TEOA as SED over 20 h visible light irradiation. Dye loading was 10 μmol g−1.

compared to P25 (SSAbrookite = 36.1 m2/g; SSAP25 = 54 m2/g). In addition, we calculated again the light-to-fuel efficiency values after 3 and 20 h of visible light irradiation: whereas a slight decrease of the value from 0.074% to 0.066% at 3−20 h of irradiation had been observed when P25/Pt was the photocatalyst, in the case of brookite/Pt the value remained constant at 0.088%. Considering that the surface morphology and area of brookite is different from that of P25,44 the optimal dye loading for the new catalytic system was also investigated. The best result was obtained using the photocatalyst with a 7.5 μmol g−1 loading of OB2, which showed a higher H2 production (79 μmol m−2, 2841 μmol g−1) compared to that I


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sensitized solar cells, indicating that, despite the similarities existing between the two technologies, fine-tuning of the dyes’ structures must follow different principles to optimize the photoconversion efficiency. The best performing dye, OB2, was also used to sensitize Ptdecorated brookite nanoparticles. Remarkably, the dyesensitized brookite/Pt catalyst provided a superior performance compared to P25/Pt, especially if the amount of evolved gas per unit surface area is considered (+58%). Furthermore, the catalyst showed a good stability over a period of 20 h and the dye loading on brookite/Pt could be lowered from 10 to 7.5 μmol g−1 without any loss of performances, resulting in a significant TON increase. The efficiency increase shown by the brookite-based catalyst was tentatively attributed to a reduced rate of recombination of injected electrons, in agreement with previous studies on dye-sensitized solar cells. Finally, the longterm stability of the best photocatalytic system was tested by carrying out a week-long (i.e., 170 h) experiment under constant simulated solar light irradiation: although the hydrogen evolution rate was found to decrease from the initial value, the catalyst was still significantly active at the end of the experiment, allowing one to achieve a notable TON value of 4201. The efficiency enhancement and the prolonged stability displayed by the brookite-containing catalyst should be particularly relevant in the perspective of the practical application of the dye-sensitized hydrogen production process.

Figure 7. Hydrogen production relative to surface area of OB2@ brookite/Pt (loading, 7.5 μmol·g−1) with TEOA as SED over 170 h of visible light irradiation.

and could therefore be due to a partial photobleaching of the dye.87 Despite that, the photocatalyst was still remarkably active at the end of the experiment, leading to the formation of a significant amount of H2 (437.6 μmol m−2), corresponding to a TON value of 4201. Considering that experiments of this duration are uncommon, this constitutes a relevant result, as long-term stability is an essential feature in the perspective of the practical application of the dye-sensitized hydrogen production process.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00782. Practical procedures for the synthesis of compounds 4− 6a,b, 8, 9a,b, 10a−c, and OB1−OB3, and copies of the corresponding 1H and 13C NMR spectra (Figures S16− S42); additional spectroscopic and electrochemical characterization of compounds OB1−OB3; additional data on the performances and characterization of the corresponding dye-sensitized solar cells and photocatalytic H2 production systems (PDF)

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CONCLUSIONS In this work, we have presented our results on the photocatalytic hydrogen production using different TiO2/Pt catalysts sensitized with dithieno[3,2-b:2′,3′-d]silole (DTS)containing organic dyes. We designed D−π−A organic photosensitizers bearing different alkyl chains on the donor or π-bridge units, with the aim to assess the influence of the number and position of hydrophobic substituents on the hydrogen production rates of the corresponding photocatalysts. The dyes were prepared following a convergent synthetic strategy based on a Stille−Migita cross-coupling as the key step and featured appropriate spectroscopic and electrochemical properties for employment both in dyesensitized solar cells (DSSCs) and photocatalysis. The light-harvesting and charge-transfer properties of the dyes were first assessed by fabrication of the corresponding P25-based dye-sensitized solar cells, with the best power conversion efficiency provided by the fully substituted dye OB3. Subsequently, hydrogen evolution experiments employing a dye-sensitized P25/Pt catalyst confirmed that the presence of long alkyl chains on the central part of the molecules (DTS bridge) had a positive influence on H2 production efficiency, leading to a superior performance of dye OB2. Conversely, incorporation of alkoxy substituents also on the donor part of the compounds (as in dye OB3) had a detrimental effect on the gas evolution reaction, which was explained considering both the higher lying HOMO of OB3 as well as the bulky and hydrophobic nature of its donor substituents, hindering the approach by the relatively large and hydrophilic SED molecule (TEOA). Remarkably, the performances of the dyes in photocatalytic experiments displayed a different trend from that observed for the corresponding dye-

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AUTHOR INFORMATION

Corresponding Authors

*(L.Z.) E-mail: [email protected];. *(P.F.) E-mail: [email protected];. ORCID

Theodosis Skaltsas: 0000-0003-3728-8207 Alessio Dessì: 0000-0003-2358-227X Gianna Reginato: 0000-0002-7712-3426 Paolo Fornasiero: 0000-0003-1082-9157 Lorenzo Zani: 0000-0003-0621-2648 Author Contributions ⊥

O.B. and T.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Fondazione Cassa di Risparmio di Firenze (ENERGYLAB project) for financial support. T.S. acknowledges the TALENTS3 Fellowship Programme, Incoming and Outgoing mobility of Researchers funded from the European Social Fund (Operational Programme 2007−2013, within Axis J


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3, Educational and Training, specific programme no. 26). Dr. Giulia Tuci (CNR-ICCOM) is acknowledged for her help in conducting the TGA experiments.

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