Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazole-containing conjugated polymers

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Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazole-containing conjugated polymers Muhammad Mansha a,b, Ibrahim Khan a,b, Nisar Ullah a,*, Ahsanulhaq Qurashi a,b a

Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia

b

article info

abstract

Article history:

Hydrogen (H2) is one of the most important fuel candidates and its low-cost production

Received 10 December 2016

would necessitate the development of efficient electrocatalysts. In this study, we report the

Received in revised form

synthesis and evaluation of two new carbazole-containing polymers as organic photo-

6 February 2017

electrochemical (PEC) catalysts for hydrogen evolution reaction (HER). The synthesis of

Accepted 8 February 2017

these new conjugated polymers, poly(N-(2-ethylhexyl)-3,6-carbazole-p-bisdodecyloxy-

Available online xxx

phenylene vinylene) (P1) and poly(N-(2-ethylhexyl)-3,6-carbazole-p-bis(2-ethylhexyloxy)phenylene vinylene) (P2), was accomplished by the HornereEmmons polymerization re-

Keywords:

action and subsequently characterized by 1H NMR, FTIR, diffuse reflectance UVevis spec-

Hydrogen evolution reaction (HER)

troscopy (DR UVevis), scanning electron microscope (SEM) and thermogravimetric analysis

Photoelectrocatalysis

(TGA). The optical band gaps of P1 and P2, derived from the onset absorption edge, were

Water splitting

found to be 2.10 and 2.14 eV, respectively. The chronoamperometric (CA) measurements

Low band gap polymer

revealed that the photo-current density generated at ~0 V by P1 and P2, without the use of additional noble metal based cocatalysts or sacrificial electron donors, was
1.8 and
2.1 mA/cm2, respectively. The enhanced PEC performance of P2 was attributed due to its narrow band gap that enhanced light harvesting ability and the larger surface area which helped in minimizing charge recombination. The experimental observations were well supported by the drastic quenching of PL emission intensity of P2. The linear sweep voltammetry (LSV) measurements showed the onset potential at around
0.3 V for both polymers. The photocurrent density difference for P2 at
1.2 V reached to maximum value of 0.37 mA/cm2, amounting to ~25% current enhancement under illumination. Long-term stability testing via CA measurements revealed that P2 was relatively more stable than

* Corresponding author. Fax: þ966 13 860 4277. E-mail address: [email protected] (N. Ullah). http://dx.doi.org/10.1016/j.ijhydene.2017.02.053 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Mansha M, et al., Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazole-containing conjugated polymers, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.02.053

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P1, which warranted its potential as photocatalyst for solar water splitting. In addition, P1 and P2 are readily soluble in common organic solvents which make them potential candidates for photovoltaic devices application. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The reliance of world economy on fossil fuel based technologies has after-effects of anthropogenic climate change. On the other hand, the intermittent nature of the sustainable energy technologies such as wind, solar and geothermal results in low energy delivery efficiencies which in turn make them not ideal for daily applications [1,2]. Among the prospective solution, H2 is one of the most important fuel candidates due to its relatively high abundance and clean burning during consumption, with only H2O as a by-product [3,4]. The production of H2 from water splitting using solar light and semiconductor photoelectrodes is an important technology since it provides high purity H2 from abundant resource of water and operates at lower electrolysis voltage of water compared to the theoretical electrolysis value (1.23 V) [5e8]. Over the past decades, substantial efforts have been devoted to developing photocatalytic systems [9,10] which contain semiconducting materials including inorganic metal oxides, (oxy) sulfides, (oxy) nitrides and molecular catalysts including iron hydrogenase enzymes and ruthenium(II)-tris-bipyridine. However, the higher photocatalytic activity of these photocatalysts requires the use of co-catalysts which help in the optimized light harvesting, improved charge separation and surface catalytic kinetics [11,12]. However, these photocatalytic systems suffer from the use of rare and expensive noble metals based co-catalysts, relatively limited variety in the properties and unsatisfactory efficiency of H2 production or lifetime [13]. Conjugated polymers possess a delocalized p electron and, consequently, have been implicated in the development of photovoltaic devices and solar cells [14]. The discovery of poly(p-phenylene)s as a photocatalyst for H2 evolution in 1985 [15] spurred substantial research interests to design and synthesize new conjugated photocatalysts. As a result, conjugated microporous network polymers (CMPs), polymeric carbon nitride based materials and organic dyes based on perylene diimides or porphyrins for solar water splitting were developed [16]. It is fascinating to see that the performance of some organic photocatalyst for photocatalytic H2 evolution is comparable to inorganic photocatalysts. For instance, the optical band gaps of CMPs were tunable (from 1.94 to 2.95 eV) to enhance the visible light absorption and CP-CMP10 in the presence of diethylamine as a sacrificial agent generated H2 evolution rate of 17.4 mmol h
1 [17]. Recently, planarized fluorene-type conjugated polymers for photocatalytic H2 evolution were reported. The performance of optimal P7 polymer reached to 92 mmol h
1, which was increased to 116 mmol h
1 upon deposition of Pt as cocatalyst [18,19]. In another report, crystalline polyimide (PI) photocatalyst for H2 evolution under visible light irradiation was developed. The

optical band gap of PI was flexibly decreasing from 3.39 eV to 2.56 eV, depending on the variation in the reaction temperature of polymerization from 250 to 350  C [20]. It is known that in the case of inorganic crystalline solids, the systematic adjustment of the structure and properties at molecular level is difficult. The organic photocatalysts, on the other hand, can be produced over a continuous range of monomer compositions, which, in turn, would allow systematic control over physical properties, tunable band gap, accessibility, and chemical versatility [16,21e24]. Furthermore, the ease of deposition of organic semiconductors on low-cost substrates through high-throughput solution process [25] and, moreover, their high absorption coefficient make them ideal to completely absorb the incident photons [26,27]. Ng et al. has utilized polybithiophene (PBTh) film as an efficient photo-electrocatalytic electrode. Upon illumination of visible light, PBTh film generated an onset potential of
0.03 V vs SCE for the HER. However, long-term stability and catalytic activity of PBTh were unsatisfactory [28]. On the basis of earlier studies, organic conjugated semiconductors have been strongly recommended as visible light photocatalysts for solar to chemical conversion [18,19]. However, the harsh conditions of PEC water oxidation and the strong tendency of organic dyes based materials to aggregate, making their solution-processing problematic, are some of the limitations of these photocatalytic systems [29]. Therefore, economically competitive PEC H2 production requires the development of new robust semiconducting materials in a cost-effective fashion. It is essential that the polymeric photocatalyst should contain a narrow band gap, which would improve light harvesting ability to produce sufficient charge carriers for proton reduction and sacrificial oxidation and, subsequently, improved H2 evolution [30,31]. In addition, an extended conjugation length would improve exciton/polaron migration along the polymer chain. Finally, photocatalyst should be stable towards water corrosion and the band edge is located at the redox window of water splitting potentials [32]. In an earlier study, linear conjugated phenylenes fused with methylene bridge or other bridging functionalities resulted in increased planarity or degree of conjugation, which resulted in improved H2 evolution compared to their unfused counterparts [32]. The H2 evolution kinetics was promoted by depositions of cocatalysts such as Pt [14] or Ru [15]. Moreover, H2 evolution activity was also enhanced by the use sacrificial electron donors including triethanolamine, methanol, ethanol [33] or mixture of methanol and triethylamine or diethylamine (80 vol.%) [14]. However, the use of noble metals as cocatalysts for water splitting adds up to the overall cost of the catalyst system and thus become unviable for its large scale production from the viewpoint of sustainable development.

Please cite this article in press as: Mansha M, et al., Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazole-containing conjugated polymers, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.02.053

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In the light of above, we have synthesized two new carbazole-containing conjugated polymers P1 and P2. Herein, we wish to disclose the synthesis, characterization, HER and stability testing of these polymers. The photo-current density generated at ~0 V by P1 and P2, without the use of additional noble metal based cocatalysts or sacrificial electron donors, was
1.8 and
2.1 mA/cm2, respectively. An attempt is made to link the enhanced performance of P2 to its narrow band gap and larger surface area. These experimental observations are supported by the drastic quenching of PL emission intensity of P2.

Experimental Materials and instruments Melting points were determined on a Bu¨chi apparatus (Bu¨chi Labortechnik AG, Switzerland) and are uncorrected. Elemental analysis was carried out on a Perkin Elmer Elemental Analyzer Series 11 Model 2400 (PerkinElmer Inc. USA). IR spectra were recorded on a Perkin Elmer 16F PC FTIR spectrophotometer (PerkinElmer Inc. USA). 1H and 13C NMR spectra were measured on a JEOL JNM-LA 500 MHz spectrometer (JEOL USA Inc.). Analytical thin layer chromatography (TLC) was carried out on silica gel 60 F254 plates (E. Merck); column chromatography was carried out on silica gel (200e400 mesh, E. Merck). UVevis absorption spectra were measured on Cary 5000 UV-vis-NIR (Agilent Technologies) spectrophotometer by dropping 250 mL of copolymers dissolved in THF (2.5 mg/mL) on the surface of fluorinated tin oxide (FTO). Fluorescence emission spectra were recorded using a FL3-2iHR (Horiba Jobin YVON, USA) spectrofluorometer. Cyclic voltammograms were recorded using a threeelectrode cell system, consisting of a working electrode (polymer film drop casted over gold disk electrode), Hg/Hg2Cl2/ 1 M KCl reference electrode and a platinum sheet as an auxiliary electrode. A scan rate of 100 mV/s was employed. Gel permeation chromatography (GPC) was employed to obtain the molecular weight of the polymers using PL-GPC 220 High Temperature GPC/SEC System (Agilent Technologies) with reference to polystyrene standards and THF as eluent. TGA was carried out using SDT Q600 (V20.9 Build 20) Thermal Analyser. Samples were heated in oxygen atmosphere with a purge rate of 50 mL/min at the rate of 10  C per minute rise in temperature from room temperature to 700  C.

Synthesis of polymers Synthesis of 4,40 -(9-(2-ethylhexyl)-9H-carbazole-3,6-diyl) dibenzaldehyde (4) To a suspension of carbazole 2 (0.30 g, 0.69 mmol), boronic ester 3 (0.40 g, 1.7 mmol), tetrabutylammonium bromide (0.44 g, 1.36 mmol) and Na2CO3 (0.28 g, 2.64 mmol) in deionized water (4 mL) was added toluene (3 mL). The nitrogen gas was bubbled through the reaction mixture gently for 3 min followed by the addition of [PdCl2(dppf)] (0.082 g, 0.1 mmol) and the mixture was heated for 12 h at 45  C. After cooling to room temperature, the reaction mixture was filtered through a pad of Celite and the pad was then washed with ethyl acetate

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(20 mL). The filtrate was transferred into a separatory funnel and washed with water (10 mL). The organic layer was separated and washed with brine (10 mL), dried over Na2SO4 and evaporated under reduced pressure. Column chromatography of the dark yellow oily material, eluting with ethyl acetateehexane (15:85) to get the title compound 4 as a light yellow solid (0.27 g, 80%). IR (KBr): 3056, 2985, 2866, 2733, 1691, 1595, 1480, 1376, 1220, 1164 cm
1. 1H NMR (500 MHz, CDCl3): d 0.86 (3H, t, J ¼ 7.0 Hz, CH3), 0.94 (3H, t, J ¼ 7.3 Hz, CH3), 1.28e1.41 (8H, m, Aliphatic-H), 2.04 (1H, m, Aliphatic-H), 4.25 (2H, dd, J ¼ 4.5, 7.9 Hz, NCH2), 7.51 (2H, d, J ¼ 8.5 Hz, AreH), 7.79 (2H, dd, J ¼ 1.8, 8.3 Hz, AreH), 7.90 (4H, d, J ¼ 8.3 Hz, AreH), 7.98e8.00 (4H, m, AreH), 7.51 (2H, d, J ¼ 1.8 Hz, AreH), 10.07 (1H, s, CHO), 10.09 (1H, s, CHO). 13C NMR (125.7 MHz, CDCl3): d 10.92 (Aliphatic-C), 14.05 (Aliphatic-C), 23.06 (Aliphatic-C), 24.40 (Aliphatic-C), 28.82 (Aliphatic-C), 31.01 (Aliphatic-C), 39.51 (Aliphatic-C), 47.76 (NCH2), 109.83 (AreC), 119.36 (AreC), 123.50 (AreC), 125.64 (AreC), 127.56 (AreC), 128.04 (AreC), 130.41 (AreC), 130.98 (AreC), 134.56 (AreC), 141.58 (AreC), 147.96 (AreC), 192.0 (CHO), 191.73 (CHO). Anal. Calcd for C34H33NO2: C, 83.74; H, 6.82; N, 2.87. Found: C, 83.66; H, 6.89; N, 2.80.

Synthesis of P1 To a solution of dialdehyde 4 (0.20 g, 0.42 mmol) and diphosphonate 5 (0.31 g, 0.42 mmol) in anhydrous DMF (20 mL) was added sodium tert-butoxide (0.20 g, 2.12 mmol) under nitrogen atmosphere and the mixture was then stirred for 24 h at 100  C. The reaction mixture was cooled to room temperature and poured over 100 mL of methanol and the product was then centrifuged. The solvent was decanted and the residue was re-dissolved in THF and successively reprecipitated from methanol, isopropanol, and hexane to get the final product as a brownish red solid (0.21 g, 53%). IR (KBr): 3050, 3026, 2958, 2923, 1597, 1480, 1416, 1379, 1196, 1030, 962 cm
1. 1H NMR (500 MHz, CDCl3): d 0.84e0.96 (m, 12H), 1.20e1.43 (m, 40H), 1.58 (bs, 4H), 1.92 (bs, 4H), 2.12 (bs, 1H), 4.10 (bs, 4H), 4.22 (bs, 2H), 7.19 (bs, 2H), 7.48e776 (m, 14H), 8.41 (bs, 2H). GPC analysis: Mn ¼ 3345, Mw ¼ 5725, PDI ¼ 1.71. Anal. Calcd for (C66H87NO2)n: C, 85.57; H, 9.47; N, 1.51. Found: C, 85.04; H, 9.67; N, 1.20.

Synthesis of P2 Following the same procedure adopted for the synthesis of P1, P2 was synthesized from the reaction of monomers 4 and 6 as a light yellow solid (0.24 g, 55%). FTIR (KBr): 3049, 3024, 2922, 1598, 1480, 1378, 1201, 1028 cm
1. 1H NMR (500 MHz, CDCl3): d 0.90e1.04 (m, 18H), 1.41e1.86 (m, 27H), 2.12 (bs, 1H), 4.01 (bs, 4H), 4.22 (bs, 2H), 7.19 (bs, 2H), 7.46e7.76 (m, 14H), 8.42 (bs, 2H). GPC analysis: Mn ¼ 3622, Mw ¼ 5935, PDI ¼ 1.63. Anal. Calcd for (C58H71NO2)n: C, 85.56; H, 8.79; N, 1.72. Found: C, 85.24; H, 8.99; N, 1.42.

Results and discussion Synthesis and characterization The synthesis of P1 and P2 required the synthesis of monomer 4, which was synthesized as outlined in Scheme 1. The Nalkylation of carbazole 1 with 2-ethyl hexylbromide produced

Please cite this article in press as: Mansha M, et al., Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazole-containing conjugated polymers, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.02.053

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Scheme 1 e Synthesis of P1 and P2.

the known intermediate 2 [34] which in turn was reacted with boronic ester 3 [35] under SuzukieMiyaura conditions to generate monomer 4 (Fig. S-1) in good yield (80%). The synthesis of diphosphonates 5 (Fig. S-2) and 6 (Fig. S-3), on the other hand, was achieved from hydroquinone by employing literature known procedures (Scheme 1) [36,37]. The reaction of monomer 4 with monomers 5 and 6 under HornereEmmons reaction conditions led us to the synthesis of copolymers P1 and P2 with excellent E olefin stereochemistry. The polymerization was accomplished by slow addition of t-BuONa (5 equiv) to the solution of monomers 4 with 5 or 6 in DMF followed by stirring the mixture for 24 h at 100  C. The reaction was quenched with aqueous ammonium chloride and then poured into excess methanol and centrifuged followed by successive reprecipitation in methanol, isopropanol, and hexane to produce P1 and P2 in moderate yields (Scheme 1). Both polymers were soluble in common organic solvents such as chloroform, methylene chloride and tetrahydrofuran. The structures of these polymers were ascertained with aid of 1H NMR and IR spectra. The characteristic aldehyde peaks at 2866, 2733, and 1691 cm
1 in the IR spectrum of monomer 4 were absent in the IR spectra of polymers. In addition, in the 1 H NMR spectra of polymers, the end groups of phosphonate monomers 5 and 6 were not visible. Moreover, the 1H NMR of P1 in CDCl3 (Fig. S-4) showed broadened peaks for eNCH2e and eOCH2e groups at d 4.22 and 4.10, respectively. The spectrum displayed peaks for eOCH2CH2e and eOCH2CH2CH2e at d 1.92 and 1.58, respectively, and another peak for methine proton of N-alkyl chain at d 2.12. Furthermore, resonances for terminal methyl groups of alkoxy and N-alkyl side chains appeared at d 0.96e0.84 whereas other alkoxy and N-

alkyl side chain proton signals were present at d 1.43e1.26. Likewise downfield peaks for carbazole ring protons (4, 5positions) appeared at d 8.41. In a similar fashion, 1H NMR data was consistent with the proposed structure of P2 (Fig. S5). GPC (polystyrene standards, THF as mobile phase) was used to determine the number-average molecular weights which are summarized in the Experimental section. The thermal properties of polymers were evaluated by TGA under oxygen atmosphere at a heating rate of 10  C min
1 (Fig. S-6). Both polymers were found to be stable even at high temperatures with no appreciable loss of mass up to 300  C. The onset decomposition temperatures (Td) of these polymers were at 350  C, followed by the abrupt loss of weight upon increase in temperature above Td, which indicated the decomposition of the backbone of the polymers. The assessment of morphology of polymers was performed by SEM analysis (Fig. 1). The SEM images revealed that P1 and P2 have well defined average microspheres of the size 7 and 2 mm, respectively. It has been well established that conjugated polymers tend to form microspheres through their amorphous aggregation [38] and the size of the microspheres depends mainly on the average number molecular weight of the polymer (Mn) i.e., polymers with higher Mn give smaller microspheres [38]. Moreover, the micrographs further displayed that P2 has larger surface area than P1 (Fig. 1).

Photophysical properties The polymers layers were deposited over fluorine-doped tin oxide (FTO) conducting substrate by dip deposition technique. The FTO substrate (FTOs) was dipped in a solution of polymer in THF at a concentration of 10 mg/2.5 mL for 15 min followed

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Fig. 1 e SEM images of P1 and P2.

by gentle removal of electrodes from the solution and placing them over hot plate for 1 h at 80  C. The resultant formed yellowish thin films were used for DR-UVevis spectrometric measurements to obtain %R vs nm and eV vs F(R) spectra. The optical band gaps of the polymers were determined by the onset of UVevis absorption spectra of polymers thin films. The band gaps of P1 and P2 were determined by the onset absorption edge at 580 and 590 nm (Fig. 2a), corresponding to the band gaps of 2.14 and 2.10 eV, respectively (Table 1). The onset absorptions of P1 and P2 at 554 and 566 nm (Fig. 2b) corresponded to the optical band gaps of 2.24 and 2.19 eV, respectively (Table 1).

The observed low band gaps of P1 and P2 could be attributed to the strong intramolecular charge transfer (ICT) between electron donor segments such as carbazole and/or dialkoxybenzene and strong electron acceptor moiety like phenylvinylene spacer. Moreover, the direct and indirect band gaps of polymers were also measured, as outlined in Fig. S-7. The enhanced photoexcited charge separation efficiency was verified by photoluminescence (PL) spectroscopy. PL emission spectra of polymers in THF solution were recorded with the excitation wavelength of 390 nm (Fig. 3). P1 showed a higher intensity emission maximum at 535 nm, whereas P2 displayed a lower intensity emission maximum at 542 nm,

Fig. 2 e (a) % Reflectance (R) and (b) UVevis absorption (KM) of P1 and P2 on FTO support. Please cite this article in press as: Mansha M, et al., Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazole-containing conjugated polymers, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.02.053

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Table 1 e Band gap measurements of P1 and P2 by DR-UVevis spectrophotometry. Polymers

eV vs F(R)

KM vs nm

%R vs nm

Indirect band gap

Direct band gap

Onset wavelength

Band gap

Absorption edge wavelength

Band gap

2.41 eV 2.38 eV

2.74 eV 2.51 eV

554 nm 566 nm

2.24 eV 2.19 eV

580 nm 590 nm

2.14 eV 2.10 eV

P1 P2

Likewise, HOMO and LUMO values of P2 were found to be 5.28 and 2.92 eV, respectively (Fig. S-8).

CA measurements

Fig. 3 e PL emission spectra of P1 and P2 in THF solution at 390 nm excitation.

showing a bathochromic shift of 7 nm. Moreover, emission intensity of P2 was much lower than P1. The drastic quenching of the emission intensity suggested that the recombination of charges was effectively suppressed in P2 [39].

Computational study The geometries of both polymers were optimized using GaussView 5.0 and Gaussian 09 software package (Fig. S-8) [40]. Density functional theory (DFT) calculations were performed with the B3LYP methods [41e43] and the optimized structures were used to calculate the band gaps of P1 and P2 as 2.57 and 2.41 eV, respectively (Table 2). These theoretical band gaps values were comparable with the experimental values, which were determined by cyclic voltammetry (CV) and DR-UVevis methods. The cyclic voltammogram of P1 (Fig. S-8) revealed the oxidation onset intercepts at Eox ¼ 0.99 V, whereas the reduction onset intercept was observed at Ered ¼
1.53 V. These values were incorporated into CV based Bredas equation (EHOMO ¼ [(Eox
Eferrocene) þ 4.8] eV and ELUMO ¼ [(Ered
Eferrocene) þ 4.8] eV) for P1 to deduce HOMO and LUMO values as 5.36 and 2.84 eV, respectively [43].

Table 2 e Band gap measurement by CV and computational methods. Polymer

CV

B3LYP/6-311*

HOMO (EH) LUMO (EL) Eg ¼ EH
EL P1 P2

5.36 5.28

2.84 2.92

2.52 eV 2.36 eV

2.57 eV 2.41 eV

A standard three electrode photocell system supported by an artificial solar simulator (Oriel Sol-3A), calibrated with a silicon diode to 1 SUN power (100 mW cm
2), and potentiostat (AutoLab) was used for photoelectrochemical (PEC) measurements. The FTOs was dipped in a solution of polymer in THF at concentration of 10 mg/2.5 mL for 15 min followed by gentle removal and heating them over hot plate for 1 h at 80  C. The resultant formed yellowish thin films (polymereFTO) were used for CA measurements. The polymereFTO (photoanodes) served as working electrode, a platinum (Pt) wire and standard calomel electrode (SCE) were used as auxiliary and reference electrodes, respectively. All these electrodes were immersed in 0.5 M Na2SO4 solution (pH 7.0), which served as an electrolyte. To measure photo-current in the deoxygenated solution, argon was bubbled through the electrolyte for 5 min to remove oxygen from the solution prior to measurement. A gentle flow of argon was then maintained during the course of photo-current measurements. To isolate the contribution of visible light in the solar simulator, solar simulator with the UV light filter (UV cut off filter <420 nm) was used to realise the visible light induced PEC water splitting performance of polymers. In CA measurements, the photocurrent response by PolymeFTOs was studied at zero open circuit potential (OCP) ~0 V while maintaining the on/off illumination with regular intervals of time (~15 s). In stacked CA measurements, photocurrent density is plotted as a function of time, which revealed that the photo-current density generated by PolymeFTOs at ~0 V was
1.8 and
2.1 mA/cm2, respectively (Fig. 4). The negative current densities suggested that the HER was the dominant process in the water splitting at bias 0 V vs SCE [44]. Furthermore, shifting of the Jpet photocurrents to its normal baseline under dark (no illumination) suggested a reversible response. These observations also commended that photocurrent generation from PolymeFTOs was exclusively due to solar driven water splitting reaction. The enhanced PEC performance of P2 is attributed due to its narrow band gap, which in turn improved light harvesting ability to produce sufficient charge carriers [30,31] and the larger surface area, as evident from SEM analysis (Fig. 2), that helped in minimizing the charge recombination and hold longer transportation pathway for photo-generated excitons [45]. Moreover, the drastic quenching of the PL emission intensity of P2 suggested that the recombination of charges was effectively suppressed in P2 (Fig. 3), which was in excellent agreement with our experimental observations [39]. It is known that introduction of large specific surface area in

Please cite this article in press as: Mansha M, et al., Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazole-containing conjugated polymers, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.02.053

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Fig. 4 e CA measurements of P1 and P2 coated on FTO glass showing photocurrent densities recorded at 0 V vs SCE bias, under dark and simulated solar light (1 Sun) in a standard three electrode system.

graphitic carbon nitride (g-C3N4) polymers leads to the quenching of the fluorescence [46]. Moreover, the higher PEC performance of P2 is in line with the report of Zou et al. [20], which revealed that increase in particle size of the polymer would result in a higher probability of charge carrier recombination. In addition, a decrease in specific surface area of the polymer would lead to less available surface sites for the photocatalytic reaction. In another study, large surface area and optimized charge separation in g-C3N4 polymers have been linked to improved H2 production from the photochemical reduction of water [47]. Similarly, a comparative study of TiC/TiO2NShts/Ti, TiC/TiO2NTbs/Ti and TiC/TiO2NWrs/Ti based photoelectrodes (where NShts ¼ nanosheets; NTbs ¼ nanotubes and NWrs ¼ nanowires) has been reported [48]. The study indicated that TiC/TiO2NWrs/Ti exhibited enhanced PEC performance for the HER. The high performance of TiC/TiO2NWrs/Ti was linked to high specific surface area of TiO2NWrs, which provided large number of adsorption sites. The significance of high surface area of organic semiconductors in the enhanced PEC performance to produce H2 from water has been delineated and reviewed [23]. It is noteworthy, however, that in most of the previous studies, HER by organic semiconductors necessitated the use of cocatalysts such as Pt [14] and Ru [15] or the use of sacrificial agents including diethylamine [17], triethanolamine, methanol, ethanol [33] and mixture of methanol and triethylamine or diethylamine [14]. The PBTh film based electrode has been used under aqueous conditions at neutral pH for the HER without the use of cocatalysts or sacrificial agent. However, the produced H2 was only to the detectable level, coupled with the generation of a small amount of current [28]. Our results are significant for a cocatalyst and sacrificial agent free HER under aqueous conditions at neutral pH, with the generation of appreciable amount of photo-current density. The electrolyte (0.5 M Na2SO4 solution) wettability of P1 and P2 revealed that these polymers were almost equally suspended in the

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electrolyte solution (Fig. S-9). Both the nitrogen of carbazole and oxygens of alkoxy moiety can act as hydrogen bond acceptor and may involving hydrogen-bonding interactions with water to attain a certain degree of wetting. On the other hand, the nature of alkyl side chain (dodecyl or 2-EtHex) of alkoxy groups on the phenyl ring does not play a significant role in the electrolyte wettability of P1 and P2. The influence of wettability on the relative PEC performance of these polymers does not seem to be significant [45]. The oxygen evolution reaction and the effect of dissolved oxygen content on the generation of photo-current by P1 and P2 were also studied. These polymers evolved oxygen as a dominant reaction at positive applied potential where the photo-current density was first increased at þ0.3 V and then decreased at þ0.6 V (Fig. S-10). To measure the photo-current in the deoxygenated solution, argon was bubbled through the electrolyte to remove oxygen from the solution. In argonpurged solution, both polymers at positive applied potential did not produce any significant photo-current compared to that generated by the oxygenated solution at 0 V, as illustrated in Fig. 4. However, upon irradiation of 1 Sun solar light at applied voltage
0.3 V, a significant enhancement in the photo-current density was observed (Fig. S-11). Thus, a bias voltage of
0.3 V was needed to generate similar amount of photo-current from the deoxygenated solution, which was produced by the oxygenated solution at 0 V. The impact of electrochemical oxidation or reduction on P1 and P2 was examined by CV analysis under irradiation of 1 Sun solar light. The CV results indicated the oxidation and reduction peaks at þ0.75 V and
0.4 V, respectively (Fig. S-12). These potential values are far away from the applied potential 0 V used to study the PEC performance of P1 and P2. This concluded that photo-current generation both in the oxygenated and deoxygenated solutions by P1 and P2 was due to water splitting reaction. The catalytic current as a function of potential was measured by LSV measurement, which was performed at a scan rate of 2.45 mV/s under the dark and light conditions. The SCE was calibrated against the reversible hydrogen potential (RHE) at room temperature. As illustrated in Fig. 5, negative

Fig. 5 e LSV of P1 and P2 coated on FTO glass under dark and simulated solar light (1 Sun) with scan rate of ¡2.45 mV/s in a standard three electrode system.

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0

voltage scanning from 0 to
1.2 V dramatically enhanced the HER. Under the illumination of solar light, the current density was reached to a maximum value of
1.5 mA/cm2 for both materials. The onset potential for these polymers was observed at around
0.3 V, which was followed by a strong reduction dip in the voltammograms. The LSV results revealed a clear difference between the dark and light current densities. In case of P2, the current density difference at
1.2 V reached to a maximum value of 0.37 mA/cm2, which amounted to ~25% of current enhancement under illumination (Fig. 5). The measured current densities during the water splitting at different potentials manifested that the reaction dominantly occurred at relatively higher positive voltages (Fig. S10). The HER occurred at LUMO levels of the polymer, where Hþ accepted the available photogenerated negative charges and produce H2 (Scheme 2). A previous study has revealed that for HER and OER (oxygen evolution reaction), the ideal HOMO and LUMO levels should be at 4.44 and 5.67, respectively [49]. Thus, water splitting by P1 and P2 would require essential energy to adjust HOMOeLUMO levels, which is achieved from electromagnetic radiations by keeping the bias voltage at 0 V.

Fig. 6 e Long-term testing of P1 and P2 coated on FTO glass over a period of 360 min at an applied potential of 0 V vs SCE bias in 0.5 M Na2SO4 (pH 7.0).

the higher charge recombination in P1 may be attributed to its significant photocorrosion.

Stability testing The long-term stability is vital for the performance and commercialization of a catalyst. Thus, the stability of the polymers was tested via CA at 0 V bias vs SCE under illumination at pH 7.0 (Fig. 6). These results revealed that the current density of P2 was relatively stable than P1. After 360 min of time lapse, a decrease in the current density from 2.1 to 2.0 mA/ cm2 (3%) was observed for P2, whereas a relatively higher decrease in the current density, from 1.18 to 1.12 mA/cm2 (33%), was realized for P1. The higher stability of P2 could be associated to its larger surface area that would help minimize the charge recombination [45], as was evident by the drastic quenching of the PL emission intensity of P2. Consequently,

Conclusions The synthesis of two new carbazole-containing conjugated polymers, P1 and P2, has been accomplished by the HornereEmmons polymerization reactions. The chemical structures of these polymers were established with the aid of 1H NMR and FTIR spectroscopic methods. The optical band gaps, determined on the basis of onset absorption edge, were found to be 2.10 and 2.14 eV for P1 and P2, respectively. In the CA measurements, the photo-current density generated by P1 and P2, without the use of additional noble metal based

Scheme 2 e Plausible mechanism of water splitting. Please cite this article in press as: Mansha M, et al., Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazole-containing conjugated polymers, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.02.053

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0

cocatalysts or sacrificial electron donors, at ~0 V was
1.8 and
2.1 mA/cm2, respectively. The enhanced PEC performance of P2 was due to the narrow band gap that improved its light harvesting ability and the larger surface area which helped minimizing the charge recombination. The drastic quenching of the PL emission intensity of P2 suggested that the recombination of charges was effectively suppressed, which was in excellent agreement with the experimental observations. The long-term stability testing via CA demonstrated that P2 was relatively more stable than P1, which warranted its potential as photocatalyst for solar water splitting. In addition, P1 and P2 are readily soluble in common organic solvents, which make them suitable for photovoltaic devices application.

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Acknowledgements [16]

The financial support from KFUPM project # NUS15103/4 and research facilities provided by KFUPM are gratefully acknowledged.

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Appendix A. Supplementary data [18]

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.02.053. [19]

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Please cite this article in press as: Mansha M, et al., Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazole-containing conjugated polymers, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.02.053