A simple carbazole based sensitizer attached to a Nafion-coated-TiO₂ photocatalyst: the impact of controlling parameters towards visible light driven H₂ production

Abstract

Here we report a new composite material consisting of a simple and cost effective carbazole based photosensitizer attached onto a Nafion/Pt/TiO₂ (NPT) hybrid as a high-performance photocatalyst for H₂ evolution under visible light. The engineered composite of dye with NPT is shown to render improvement in its photocatalytic activity. We have spectroscopically observed that the spacer free simple carbazole based dye was perfectly attached onto TiO₂ surfaces, which also remained unaffected in alkaline medium. The development of the photocatalyst was confirmed by different morphological characterization procedures. Under optimum conditions (pH 10, 1.0 × 10⁻⁴ mol/10 mg AM dye, 0.5 wt% Pt), the maximal apparent quantum yield (AQY%) of D1@PT and D1@NPT for hydrogen evolution increases up to 16.5% and 19.16% under ∼2-sun-intensity of visible light irradiation with triethanolamine (TEOA) as a sacrificial agent. The combined effects of several factors may contribute to the remarkably enhanced photocatalytic activity for the D1@NPT sample including dye concentration, pH and catalyst amount variation, and the spacer effect of the sensitizers (with and without oligothiophene moieties).

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1. Introduction

Photo-assisted water splitting is a promising method for H₂ production which is one of the clean renewable energy sources. Since the publication of a report on photo induced water splitting on a TiO₂ electrode under UV irradiation by Fujishima and Honda,¹ enormous attention has been focused on photocatalytic H₂ evolution over semiconductors.² Owing to their many desirable properties,³ a large number of TiO₂ based photocatalysts have been developed to date for effective H₂ production via water splitting.⁴ However, the only drawback of the TiO₂ semiconductor is that it absorbs a small portion of the solar spectrum in the UV region because of its wide band gap of 3.2 eV. Therefore, extensive efforts have been made to develop dye sensitized photocatalysts, capable of utilizing the less energetic but more abundant visible light.⁵ Dye modification has been used to make TiO₂ active to visible-light excitation, and such modifications are usually performed by chemical bonding of a functional group (carboxylate or phosphonate) or through hydrogen bonding. In this particular process, electron injection occurs by excitation of the dye and consequent water reduction occurs at the co-catalyst site of the semiconductor which is usually Pt.⁶

Metal free dyes have profoundly higher extent of light-harvesting capability than metal complexes because of their (i) high molar extinction coefficients, (ii) donor–pi–bridge–acceptor (D–π–A) and (iii) availability of versatile functional molecules associated with fine tuning of the electronic and chemical properties.⁷ Some organic dye sensitized TiO₂ photocatalysts such as phenothiazine,^5b xanthene,⁸ merocyanine⁹ and coumarin¹⁰ are also efficient for water splitting. However, their apparent quantum yields are not high enough in the visible region.

Recently, many researchers have been paying a good deal of attention to the carbazole based dyes for use in DSSCs due to their facile molecular tuning and high molar extinction coefficients.¹¹ To the best of our knowledge, there is a single report, where Abe et al. have well established the two step photoexcitation in water splitting over a niobate semiconductor using a commercial coumarin and carbazole based sensitizer.^2a However, an inexpensive method of photocatalytic H₂ generation using organic dyes is still under sporadic investigation.

Due to the unique chemical structure and electronic properties, Nafion is widely used as a proton exchange membrane in the application of a fuel cell.¹² Furthermore H. Park et al. have properly explained the immobilization of a component through Nafion in homogeneous photo-assisted H₂ generation without using any semiconductor.¹³ Nafion was also used as the binding site for ruthenium based cationic dyes in sensitized H₂ generation.¹⁴ However, as far as we are aware, the application of Nafion in stabilization of a sensitizer for enhanced photocatalytic H₂ evolution is not yet reported.

Here, we report for the first time a simple carbazole based organic dye (Fig. 1) sensitized Nafion coated Pt/TiO₂ system (D1@NPT) for photocatalytic H₂ production under visible light irradiation. The as-prepared sensitizer molecule was developed using inexpensive synthesis methodology. This study also explores the significance of Nafion incorporation towards improved photocatalytic efficiency as D1@NPT shows a remarkably high TON of 7843. Moreover important operational parameters like dye concentration, photocatalyst dosage, effect of the activation temperature of the TiO₂ and pH of the medium etc. were screened and H₂ generation was optimized. Furthermore, the possible photocatalytic H₂ production mechanism and the role of Nafion are also discussed.

Fig. 1 Molecular structure of (a) AM, (b) MK2, (c) eosin Y, (d) N719 dyes and (e) the Nafion polymer.

2. Experimental section

9-Ethyl-9H-carbazol-3-carbaldehyde, anatase TiO₂, chloroauric acid, chloroplatinic acid and MK2 dye were purchased from Sigma Aldrich. Nafion solution (D1020) was purchased from DuPont. Piperidine was purchased from Spectrochem. All the chemicals were of AR grade and used without further purification unless otherwise stated. Organic solvents were purified and dried before synthesis of the sensitizer.

2.1. Synthesis

2.1.1. Preparation of 2-cyano-3-(9-ethyl-9H-carbazol-6-yl) acrylic acid (AM). (E)-2-Cyano-3-(9-ethyl-9H-carbazol-6-yl) acrylic acid was prepared by Knoevenagel condensation with cyanoacetic acid in the presence of a catalytic amount of piperidine in acetonitrile as shown in Scheme 1. Piperidine of 0.1 mL (0.086 mmol) was added to a solution of 9-ethyl-9H-carbazol-3-carbaldehyde-1 (250 mg, 1.119 mmol) and cyanoacetic acid (150 mg, 1.76 mmol) in acetonitrile (10 mL). The reaction mixture was refluxed for 7 h under an argon atmosphere. After completion of the reaction (monitored by TLC), the organic mixture was separated by ethyl acetate (3 × 20 mL) and washed with water, then dried over Na₂SO₄. The solvent was removed under reduced pressure and the residue was purified by column chromatography (Hex : EA, 95 : 5) on silica gel to obtain a pure compound as a yellowish green solid with 90% yield. ¹H NMR (300 MHz, methanol-d4) δ 8.39 (1H, s), (1H, d, J = 7.7 Hz), 7.56–7.39 (4H, m), 7.17 (1H, d, J = 7.7 Hz), 7.34 (1H, t), 8.2 (1H, t), 4.42 (2H, q, J = 6.9 Hz), 1.40 (3H, t, J = 6.9 Hz), ESI-MS: MS (ESI†), m/z [M˙⁺ Na]: calcd: 313.10; found: 313.00 and FTIR (details in ESI†).

Scheme 1 Synthesis of AM dye.

2.1.2. Preparation of Pt/TiO₂. To improve the better photocatalytic activity of H₂ production, 0.5 wt% Pt metal was deposited onto the surface of commercial TiO₂ nanoparticles by using a reported method.^5b In 100 mL round-bottomed flask, TiO₂ (1.0 g, Aldrich anatase) was dispersed in 20 mL of methanol. An aqueous solution of H₂PtCl₆ (0.25 mL, 8 wt% aqueous solution) was then added into the methanolic suspension of TiO₂ and the reaction mixture was irradiated with a 400 W xenon lamp for 30 minutes. The resultant Pt/TiO₂ composite of light gray colour was retrieved by centrifugation, washed five times with excess methanol and dried under vacuum.

2.1.3. Preparation of Au/TiO₂. A gold (1.5 wt%) nanoparticle supported on TiO₂ was synthesized by using a simple chemical deposition method as reported previously.¹⁵ Chloroauric acid (HAuCl₄) was added into deionised water at pH 9 and stirred at room temperature for 30 min. The colour of the resulting solution was initially yellow. After adding the commercial TiO₂, the mixture was stirred till it became transparent and colourless. Then it was refluxed for 12 h at 100 °C. After refluxing, the solution was kept for cooling at room temperature, then filtered and washed with deionised water and dried at 100 °C overnight. Finally, Au/TiO₂ powder was calcined at 400 °C in air for 4 h. The colour of the compound changed from light yellow to light gray.

2.1.4. Nafion coating on TiO₂ (Nf/TiO₂). To improve the photocatalytic activity of hydrogen production, surface modification of TiO₂ with Nafion was accomplished by using a previously reported synthesis protocol.¹⁶ In a 100 mL round-bottomed flask, Pt/TiO₂ (500 mg, Aldrich anatase) was dispersed in 5 mL methanol. Nafion (DuPont D1020) (5 mL, aqueous solution) was then added into the methanolic suspension. Thereafter the reaction mixture was stirred overnight at room temperature. The resultant NPT composite was retrieved by centrifugation and washed three times with excess methanol. The grey colored precipitate was dried at 60–80 °C.

2.1.5. Adsorption of dye molecules on the semiconductor surface. AM and MK2 dyes were separately dissolved in acetonitrile–ethanol solution (1 : 1 v/v, 10 mL).⁸ Then commercial TiO₂ (cTiO₂) was dispersed into the resulting solution and the mixture was kept under stirring conditions at room temperature in the dark for 24 hours. After completion of the reaction, it was filtered, washed with ethanol and then dried in the air. These final composites were labelled as AM@TiO₂ (D1@T) and MK2@TiO₂ (D2@T). The same method has been applied for preparation of dye anchored Au/TiO₂, Pt/TiO₂ and Nafion coated Pt/TiO₂ composites and are labelled as D1@AT, D2@AT, D1@PT, D2@PT, D1@NPT and D2@NPT.

2.2. Photocatalytic experiment

The photocatalytic H₂ generation experiments were carried out in a doubly jacketed Pyrex glass reactor with a flat optical window and an external cooling jacket. All experiments were carried out in 20 mL aqueous suspension containing 10 mg of the photocatalyst and 10 vol% of TEOA as SED. It was then air sealed with a rubber septum. Before light irradiation, the dissolved air was removed by keeping it for 20 min under high vacuum followed by purging of Ar gas. The Xenon arc lamp (400 W) was used as a light source. The reaction mixture was kept under constant stirring conditions during the course of irradiation and the resulting evolved gases were analyzed by gas chromatography using a Perkin Elmer Clarus 580 GC equipped with a molecular sieve 5 Å column, a thermal conductivity detector (TCD) and argon as carrier gas.

3. Results and discussion

3.1. Characterization

Optical properties of the dye and dye@TiO₂ composites have been clarified on the basis of the UV-vis diffuse reflectance spectra (DRS). The electrochemical properties of the dyes have been studied by cyclic voltammetry (CV). To study the phase transformation, powder X-ray diffraction (XRD) of cTiO₂ was carried out at different temperatures. FTIR spectra were recorded for dye and dye@TiO₂ composites. The morphology of the sample was investigated by field emission scanning electron microscopy (FESEM); Nafion and gold deposition was confirmed by a high resolution transmission electron microscopy (HRTEM) study. Moreover, the as-synthesized dye (AM) was fully characterized by NMR, ESI-MS, FTIR and results have been incorporated in the ESI.†

3.1.1. Optical properties of dyes, dye@TiO₂ and dye@Au/TiO₂ composites. As shown in Fig. 2a the absorption spectrum of the dyes significantly resembles the photophysical properties and the spacer effect. The optical characteristics of the sensitizers have been summarized in Table 1. In dilute chloroform solutions, absorption maxima were observed at 409 and 510 nm for AM and MK2 respectively, which is assigned to a π–π* transition with a high molar extinction coefficient. Though AM has the higher molar extinction coefficient than MK2, the latter with four oligothiophene spacers shows a broad absorption with red shifted absorption maxima^6a up to 96 nm with respect to the AM dye. DRS of bare TiO₂, D1@T and D2@T are shown in Fig. 3a. An absorption maximum of bare TiO₂ was observed at 387 nm. After being sensitized with dyes, composites showed very little blue shift with respect to absorption maxima of dyes (395 and 490 nm for D1@T and D2@T respectively) though overall spectral properties resemble the optical properties of the dyes.

Fig. 2 (a) UV-Vis spectra of AM and MK2 dye in chloroform (5 × 10⁻⁵ M), (b) cyclic voltammogram of AM dye traces at a scan rate of 50 mV s⁻¹ measured in a 0.1 M [TBA][BF₅] CH₃CN solution.

Table 1 Optical and electrochemical properties of dye molecules

Dyes	λ max (nm)/εmax^b (M^−1 cm^−1)	E 0–0 (eV)	E ox (V)	HOMO^e (eV)	LUMO^f (eV)
a Absorption. b Emission spectra of dyes. c E 0–0 was estimated from the transition energy measured at the onset of absorption spectra. d Oxidation potential of the dyes in CH3CN with 0.1 M tetrabutylammonium pentafluoroborate (TBABF5) as electrolyte. e The highest occupied molecular orbital (HOMO) value calculated using the potential value of oxidative waves. f The lowest unoccupied molecular orbital (LUMO) value was calculated by HOMO + E0–0.
AM	409/42 700	3.03	1.95	−6.45	−3.42
MK2	505/40 203	2.07	0.96	−5.05	−2.98

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Fig. 3 DRS of (a) cTiO₂, D1@T and D2@T and (b) cTiO₂, Au/TiO₂(AT) and D2@AT composites.

The DRS of bare TiO₂, Au/TiO₂ and D2@AT were taken to study the changes in optical properties which have been incorporated in Fig. 3b. In Au/TiO₂, a broad absorption band at around 575 nm was observed in the visible region, which was due to surface plasmon resonance (SPR) effects of the Au nanoparticle on the TiO₂ surface.¹⁷ The plasmonic band came around 575 nm, attributed to the size of spherical Au nanoparticles of ∼15 nm, which creates an induced electric field onto the TiO₂ surface.¹⁸ It is noteworthy that this dye sensitized system also takes advantage of the SPR effect towards hydrogen production which cannot be found in the other reported photocatalytic systems. After being sensitized with the dye, the whole composite shows a broad absorption over a wide spectral range from 400–700 nm which is attributed to the improved light harvesting nature of the photocatalyst composites.

3.1.2. Electrochemical properties. The redox behaviour of the as-synthesized dye was analyzed by CV (Fig. 2b). The measurements were performed in anhydrous acetonitrile with 0.1 M TBABF₅ as the supporting electrolyte. The reference electrode was a silver wire calibrated with ferrocene (Fc/Fc⁺) as an internal reference.¹⁹ In this study, the oxidation and reduction potentials were measured to be 1.951 V and −1.32 V respectively which correspond to their oxidation and reduction peaks. This indicates that the sensitizer is more prone to oxidation with rapid electron transfer to the CB of TiO₂.

3.1.3. XRD analysis. The TiO₂ sample was activated at different temperatures over the range of 400–700 °C and their XRD patterns are shown in Fig. 4. The peak at 25.2° corresponds to the (101) plane, which becomes more intense due to increasing crystallinity with temperature. At 400 °C TiO₂ showed little amorphous nature while at 500 °C a pure crystalline anatase phase was obtained. The diffraction peaks (2θ) at 25.2°, 37.9°, 48.3°, 53.8° and 55.0° represent the corresponding indices of (101), (103), (200), (105) and (211) planes respectively.²¹ When the temperature was increased from 500–600 °C, two diffraction peaks at around 39.0° and 65.0° were observed, ascribed to (200) and (002) planes of the rutile phase.^4f A combine anatase–rutile phase completely transforms into rutile at 700 °C.²⁰ The occurrence of the prevalent peaks at 2θ of about 27.5°, 36.0°, 39.0°, 41.2°, 44.1°, 54.2° and 56.7° correspond to the indices of (110), (101), (200), (111), (210), (211) and (220) planes, respectively, indicating the presence of the rutile phase.²¹

Fig. 4 PXRD patterns of commercial TiO₂ at different temperatures.

3.1.4. Morphology of TiO₂ with Nafion, platinum and gold. The TEM images of Nf/Pt/TiO₂ and Au/TiO₂ are shown in Fig. 5. Black spot of Pt and the shrink layer of Nafion are visible on the TiO₂ surface (Fig. 5a). The formation of the Nafion layer was further proved by the presence of elemental (C, Ti, O, F and S) abundance in the composition using an energy dispersive X-ray microanalysis (EDX) study (Fig. S4, ESI†). As shown in Fig. 5b, lattice distances of 0.341 nm and 0.228 nm correspond to the (101) and (111) planes of anatase TiO₂ and Pt respectively.²² In Fig. 5c, surface deposition of gold in Au/TiO₂ was affirmed by clearly visible black spots which were dispersedly located onto the TiO₂ surface. The lattice fringes of Au with 5d spacing of 0.2 nm are well matched with the (200) planes of Au.²³

Fig. 5 (a) TEM images of Nf/Pt/TiO₂, (b) HRTEM of Nf/Pt/TiO₂, (c) TEM images of AuTiO₂ and (d) HRTEM of Au deposited on TiO₂.

Fig. 6 shows the FESEM images of Pt/TiO₂ and Nf/Pt/TiO₂ composites. Pt/TiO₂ (Fig. 6a–c) exhibited mesoporous microspheres morphology with unsymmetrical interior. Similar morphology was found in the Nf/Pt/TiO₂ composite (Fig. 6d–f) though a Nafion layer was not visible in the image.

Fig. 6 FESEM images of Pt/TiO₂ (a–c) and Nf/TiO₂ (d–f) composites.

3.1.5. FTIR analysis. The surface interactions between dye and TiO₂ have been clarified on the basis of FTIR studies. Significant changes in peak intensity and the peak position indicate the strong attachment of carboxylate groups of the dyes onto the TiO₂ surface. The AM and MK2 dye molecules have an affinity to bond with the TiO₂ surface through the carboxylate group, as evidenced by attenuated total reflection FTIR data in Table S1 (Fig. S1, ESI†). The peak at 1677 cm⁻¹ for the –C=O is present in both AM and MK2 dye and disappeared after being sensitized on the TiO₂ surface. It is interesting to note that peaks at 1380 and 1381 cm⁻¹ for D1@T and D2@T composites respectively appeared due to the –COO⁻ group, which corroborates the formation of ester-like linkage of the carboxyl group between dye and TiO₂.²⁴

3.2. Photocatalytic H₂ evolution

The photocatalytic activity of the catalyst was analyzed by the H₂ evolution over the prepared spacer free and oligothiophene spacer dye sensitized under visible light irradiation. The experiments were carried out over 20 mL of aqueous suspension containing 10 mg of the photocatalyst and 10 vol% of TEOA as SED. In the whole experiment, important operational parameters like dye concentration, the catalyst amount, the pH effect, loading of different co-catalysts, activation temperature of TiO₂ were thoroughly screened. It is important to note that all the experiments were carried out at room temperature (rt) and pH was adjusted by addition of 1 (M) HCl. During irradiation, the colour of aqueous suspension changed from red to yellow. For comparison, sensitizing effects of two commercial dyes EY and N719 have also been experimented under the specified conditions. These are presented in Fig. 7 and results have been shown in Table 2.

Fig. 7 (a) and (b) Photocatalytic activities of the dyes with TiO₂, Au/TiO₂, Pt/TiO₂ and Nf/Pt/TiO₂ composites after 6 h irradiation under visible light (λ ≥ 400 nm), conditions: 10 mg photocatalyst in 20 mL of 10 vol% aqueous TEOA (2 mL) solution, pH-7.

Table 2 Photocatalytic activity and TON of a dye@TiO₂ composite^a

Catalyst	H2 yield (μmol)	Rate (μmol h^−1)	TON	AQY (%)
a Dye sensitized TiO2 composites after 6 h of irradiation under visible light (λ ≥ 400 nm), conditions: 10 mg of the photocatalyst (0.1 μmol dye) in 20 mL of 10 vol% neutral aqueous TEOA (2 mL) solution.
D1@T	39.6	6.6	761	1.86
D2@T	42.4	7.0	815	1.99
D1@AT	127.3	21.2	2448	5.98
D2@AT	176.1	29.3	3386	8.27
D1@PT	351.1	58.5	6751	16.5
D2@PT	470.7	78.4	9051	22.12
D1@NPT	407.87	67.9	7843	19.16
D2@NPT	566.9	94.4	10901	26.64
EY@PT	20.49	3.41	419	0.96
N719@PT	8.15	1.35	163	0.38

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The apparent quantum yield was estimated by the following equation:

The average photon flux of the incident light was determined on an optical power/energy meter 842-PE.

3.2.1. Effect of concentration of dye. The effect of concentration on the photocatalytic activity was examined and the results are depicted in Fig. S4 (ESI†). The photocatalytic activities increase with increasing concentration of dyes to show a maximum at a dye concentration of 0.1 μmol/10 mg Pt/TiO₂ and thereafter a slight decrease. H₂ production yield has reached up to 351.1 and 470.7 μmol for D1@PT and D2@PT respectively after 6 h of irradiation. It is noteworthy that upon higher loading of dye, all the photocatalytic sites are adhered to by the dye molecules and no surface active sites are available to be accommodated further, causing no enhancement of the rate with the increase in the dye concentration. The as-prepared photocatalyst exhibited more efficient activity than a previously reported dye sensitized TiO₂ composite.^6a

3.2.2. Effect of the catalyst amount. The maximum yield of hydrogen production has been found with 15 mg catalyst (Fig. S5, ESI†). Greater amount of catalyst charging causes a sudden fall of rate of H₂ generation which could be ascribed to the reduction of the penetration depth of the incident light which may well increase the outcome of losing scattered light to the exterior.²⁵

3.2.3. Effect of calcination temperature on TiO₂. Further insights were obtained by looking at the effect of sintering temperature which has significant influence on the photocatalytic activity over dye sensitized Pt/TiO₂ (Fig. S6, ESI†). The best efficiency of photocatalytic activity was obtained as 470.7 μmol when TiO₂ was activated at 500 °C. At that temperature TiO₂ exists in the pure anatase phase (Fig. 4). With increase in temperature from 500 °C, anatase to rutile transformation starts and at 700 °C TiO₂ exists mostly with the rutile phase which has a negative impact on the H₂ generation.²⁶

3.2.4. Effect of the co-catalyst. The difference in the photocatalytic activity was examined with surface deposited Pt and Au. The time profile of light-driven H₂ generation is shown in Fig. 7 whilst Table 2 summarizes the quantity of H₂ evolved and the TON with respect to the sensitizer. The AQY of the respective photocatalyst (Table 2) is calculated based on the eqn (i). D1@T without a co-catalyst showed poor activity of H₂ evolution with a TON 761. While screening the co-catalysts activity, it was envisioned from the result that Pt has the larger effect as a co-catalyst than gold on H₂ generation as D1@PT and D2@PT showed almost three times higher efficiency compared to D1@AT and D2@AT and ten times higher than D1@T and D2@T (Fig. 7). Surface modification of TiO₂ by platinum is an effective method to increase its photoactivity as platinum metal consists of a Fermi levels lower than a TiO₂ conduction band and could function as an electron trap centre to accelerate the discharge of photogenerated electrons from TiO₂²⁷. In the Au/TiO₂ based composite, the SPR effect contributes to high H₂ generation.^5b It is noteworthy here that the efficacy of visible light hydrogen production is higher compared to many reported dye sensitized TiO₂ composites.^5a,b,28 To establish the superiority of our catalysts, we have investigated the photocatalytic activities of reference dyes EY and N719 (419 and 163 TON) which exhibited about fifteen times less H₂ evolution (Table 2) under the same reaction conditions. It is worth mentioning that the reported TON of H₂ generation for EY^5b and N719²⁹ are quite similar to our reported results.

3.2.5. Effect of pH. The analyzed results showed that the photocatalytic activity increases with increasing solution pH range from 4 to 10 (Fig. 8). These results imply that the H₂ production activity of the investigated photosensitized system favours nearly moderate to neutral medium but at pH 10, and highest photocatalytic activity was obtained over D1@NPT which is again efficient than the reported photocatalyst.³⁰ One plausible explanation is that the pH of the solution mainly bears upon the electron-donating ability of the TEOA. It has been reported that hydrogen production increases as the solution pH is increased, especially at pH > 6.³¹ This is reasonable because TEOA (an amine) can abruptly donate electrons to the oxidized sensitizer in an alkaline solution since the electron-donating ability of the amine is lower in a more acidic solution because the amine is more protonated.³² Consequently, the rate of sensitizer regeneration would be enhanced in a more alkaline solution. Hence, unfavourable back reactions of the excited sensitizer would be suppressed and the efficiency of the excited sensitizer utilization would be improved. Finally, the H₂ production rate is greatly enhanced. However, when the initial solution pH is beyond the optimum value (too alkaline), the complexity of coulombic repulsion/interaction among the OH⁻, TEOA, the sensitizer and the photocatalyst surface may play a negative role in the reduction of the photocatalytic H₂ production activity. Under the specified reaction conditions, a significant yield of H₂ evolution with 655 and 631 μmol was obtained after 6 h of irradiation at pH 10, for D1@NPT and D2@NPT respectively (Table 2). The lower photocatalytic activity observed in the D2@NPT composite is probably due to the steric hindrance in MK2 dyes which blocked the active site surface of the NPT composite and consequently suppressed H₂ production under alkaline medium.

Fig. 8 Photocatalytic activities of the D1@PT, D2@PT, D1@NPT and D2@NPT composites (at different pH) after 6 h irradiation under visible light (λ ≥ 400 nm), conditions: 10 mg photocatalyst in 20 mL of 10 vol% aqueous TEOA (2 mL) solution.

3.2.6. Role of Nafion on the TiO₂ surface. From the present results it can be anticipated that sensitizers are able to diffuse through the Nafion layer and inject electrons into TiO₂, and subsequently during photo-irradiation, the Nafion layer is extending the lifetime of excited sensitizers which contributes to enhanced H₂ generation.¹⁴ Moreover in a strongly acidic environment of the Nafion pores, a homologue of perfluorosulfonic superacid provides an ideal condition for hydrogen production.¹³ Thus a close neighbourhood of TiO₂, Nafion and sensitizer components developed by easy fabrication is believed to favour synergistic photon-to-hydrogen efficiency. The result of extensive investigations depicted in Table 2 is that the Nafion substantially enhances H₂ evolution when compared to a non-Nafion system.^2b,c Indeed, bare TiO₂ showed only marginal H₂ evolution rates (22 μmol) under similar experimental conditions as compared to our researched photocatalysts.

3.2.7. Stability of AM dye. The stability of AM dye sensitized NPT composite under optimal experimental conditions has been shown in Fig. S8 (ESI†). The H₂ generation yield was 407.87 μmol after 6 h, which reached a maximum amount of 470 μmol after 12 h of irradiation and then some declining trend in the consecutive run. The decrease rate of hydrogen evolution in the consecutive run may be probably due to consumption of SED and the partial decomposition of dye. The as-prepared photocatalyst exhibited better photocatalytic activity than previously reported carbazole dye.^2a

3.3. Mechanistic pathway

We have proposed a mechanism based on the above results of photocatalytic H₂ production in D1@PT and D1@NPT systems in Scheme 2. The excitation of sensitizer was initiated upon visible light irradiation, where the photo-excited sensitizer transfers an electron from the HOMO of the dye to its LUMO which leads to the excitation of the dye molecule as D* (process 1). The oxidized dye species forms after electron injection into the TiO₂ CB and is subsequently stabilized by the Nafion layer (processes 2 and 3). Processes 4 and 5 describe the trapping of electrons by Pt. Thereafter the injected electrons reduce the water molecule at the Pt site (process 6). The dye regenerates via taking up of electrons from SED and drives the photocatalytic process in a repeated manner (process 7).

Scheme 2

4. Conclusions

In conclusion, a new highly efficient photocatalyst containing a carbazole based photosensitizer and Nafion coated Pt/TiO₂ is synthesized and its efficacy for visible-light photocatalytic water splitting is reported. Following a systematic investigation, the hybrid photocatalyst D1@NPT showed a highest turnover number of 7843 and the apparent quantum yield reached 19.16%. The sensitizer used in the hybrid composite is simple and also cost effective compared to many reported dyes (MK2, N719 etc.) for hydrogen production from water. Further, the results of the present study showed that the spacer group with an extended π-spacer linker to the carbazole sensitizer increases the light-harvesting ability and photocatalytic activity of the Nf/Pt/TiO₂ composite at neutral pH. According to comprehensive analysis, it can be concluded that the improved photocatalytic activity is due to the combined effects of several factors, including the synergistic effect of Nafion-TiO₂, strong visible light absorption of hybrid systems D1@NPT and D2@NPT and an anticipated low recombination rate. This work provides a favourable approach with a desired synergy to efficiently harness solar energy for renewable hydrogen production.

Acknowledgements

The authors acknowledge the solar energy network project TAPSUN, NWP-56 for the financial support and Prof. Dr P. Pal Roy Director of CSIR-CMERI, for his endless encouragement in research. Amritanjali Tiwari and Indranil Mondal acknowledge AcSIR for PhD enrollment.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nj01436g

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