Stable and Efficient Homogeneous Photocatalytic H2 Evolution Based on Water Soluble Pyrenetetrasulfonic Acid Functionalized Platinum Nanocomposites

Abstract

Herein, 1,3,6,8-pyrenetetrasulfonic acid (PTSA) functionalized Pt nanocomposites were synthesized and characterized by UV/vis, X-ray photoelectron spectroscopic (XPS), FTIR, TEM, and XRD methods. Pyrenetetrasulfonic acid was not only used as the stabilizer to prevent agglomeration of Pt nanoparticles but also served as the light-harvesting photosensitizer, absorbing irradiating light and transferring photoexited electrons to the platinum nanoparticles. The occurrence of the photoinduced electron transfer process was confirmed by the combination of time-resolved fluorescence and photoelectrochemical spectral measurements. Photocatalytic results showed that PTSA functionalized Pt nanocomposites could be used as stable photocatalysts for photoinducing H₂ evolution. At the optimal reaction conditions (n_Pt:n_PTSA=100, pH 3), enhanced amounts of hydrogen were evolved from the system under UV/vis irradiation in the absence of an electron mediator. The corresponding amount of hydrogen evolution was 125.1 μmol for 12 h exposure to UV/vis irradiation, and the apparent quantum efficiency at a wavelength of λ=365 nm was 11.5 %.

Introduction

In the present era of depleting fossil fuel reserves, the development of photocatalytic hydrogen evolution along with solar thermal conversion, has become a challenging task and a research subject of high priority.1 Since Fujishima and Honda2 reported the first photocatalyst suitable for splitting water into H₂ and O₂, numerous studies on developing novel photocatalysts to convert solar energy into H₂ fuel have been made and considerable progress has been achieved.3–5 Both heterogeneous and homogeneous photocatalysts for light-driven hydrogen production have been extensively studied.5–7 Among these photocatalysts, homogeneous photocatalysts, however, are very attractive in the sense that their chemical and photochemical properties can be understood and strategically tuned on the molecular level.5b In general, a homogeneous photocatalytic system consists of a sacrificial reductant [e.g. ethylenediaminetetraacetic acid (EDTA)], a photosensitizer [e.g. tris(2,2′-bipyridyl) ruthenium(II)], an electron mediator (e.g. methyl viologen), and a catalyst (e.g. platinum).5, 7 However, the decomposition of the sensitizer during long irradiation times as well as low charge separation efficiency between the photosensitizer and the catalyst still need to be resolved.8 Thus, obtaining a highly efficient and stable photocatalyst remains a great challenge.

Recently, some dye functionalized nanocomposites composed of organic molecules anchored on noble metal nanoparticles showed high efficiency for photoinduced energy and/or electron transfer between the photosensitizer and the metal nanoparticles.9 The charge density of the metal nanoparticles was greatly altered by the electron transfer from excited dye molecules to the metal surfaces. Binding of photoactive molecules to a noble metal nanoparticle renders the organic–inorganic hybrid nanoassembly suitable for light-harvesting and optoelectronic applications.9 The porphyrin functionalized Pt nanocomposites were found to be useful as efficient photocatalysts for photocatalytic hydrogen evolution in our previous studies.10 Pyrene and its derivatives have been widely used in the fields of optical sensors, molecular electronics, and photovoltaic cells owing to their exceptional stability and photophysical properties.11, 12 Among these compounds, 1,3,6,8-pyrenetetrasulfonic acid (PTSA, Figure S1) is not only water soluble but also has a long photoexcition lifetime,11, 13 which would allow it to be used in light-harvesting devices.13, 14 In this paper, we report the synthesis, characterization, and photocatalysis of PTSA functionalized Pt nanocomposites (Pt-PTSA-x, in which x is the molar ratio of platinum to PTSA). Pyrenetetrasulfonic acid was used as a stabilizer to prevent agglomeration of the Pt nanoparticles and to serve as a light-harvesting photosensitizer in the photocatalytic system. It was found that the efficiency of the electron transfer from the pyrenetetrasulfonic acid to the platinum catalyst for the H₂ generation was high, owing to photoexcited pyrenetetrasulfonic acid directly transferring electrons to the platinum nanoparticles. Moreover, enhanced amounts of hydrogen evolved from this system under ultraviolet and visible (UV/vis) light irradiation in the absence of an electron mediator. The photocatalytic activities of the Pt-PTSA photocatalysts were stable even after a long irradiation period. Furthermore, the effects of the molar ratio of Pt to PTSA and the pH value of the photocatalytic system on the activity of hydrogen evolution were also investigated.

Results and Discussion

Characterization of pyrenetetrasulfonic acid functionalized platinum nanocomposites

Shown in Figure 1 A are the UV/vis absorption spectra of aqueous solutions of PTSA, H₂PtCl₆, and the Pt-PTSA-100 nanocomposite. The spectrum of PTSA (Figure 1 A, spectrum a) shows three distinct absorption peaks in the λ=200–500 nm region, which was attributed to a π–π* transition derived from the pyrene backbone.11, 14e, f The H₂PtCl₆ aqueous solution (Figure 1 A, spectrum b) shows an absorption band centered at λ=259 nm, which was attributed to ligand-to-metal charge transfer in the [PtCl₆]²⁻ ions.15 The absorption spectrum of the Pt-PTSA-100 nanocomposite (Figure 1 A, spectrum c) was characterized by a broad absorption at wavelengths >300 nm and an absorption peak at about λ=220 nm.16 X-ray photoelectron spectroscopic (XPS) analysis is often also used for the characterization of metallic platinum. Shown in Figure 1 B are the XPS spectra of the Pt 4f from the Pt-PTSA-100 nanocomposite. The binding energies of spectra are referenced to a C 1s value of 284.8 eV. Pt 4f displays a doublet from the spin orbital splitting of the 4f_7/2 and 4f_5/2 states, which are centered at 71.5 eV and 74.8 eV, respectively, and which agree well with literature values for Pt⁰.17 This result reveals further that the PTSA functionalized Pt nanoparticles are in metallic form.

Figure 1. A) Typical UV/vis spectra of aqueous solutions of a) PTSA, b) H₂PtCl₆, and c) the Pt-PTSA-100 nanocomposite. B) XPS spectrum of Pt 4f of the Pt-PTSA-100 nanocomposite.

Shown in Figure 2 A–D are the TEM images of the Pt nanocomposites. The average Pt nanoparticle size is 2.3±0.3 nm when the molar ratio of the metal to the stabilizer equals 50. An increase in the size of the nanoparticles is observed as the molar ratio of Pt/PTSA increases. If the molar ratio of Pt/PTSA is 100, 200, and 300, the average diameters of platinum particles are 2.6±0.3, 3.1±0.5, and 3.4±0.5 nm, respectively. Similar phenomena have also been observed when other different stabilizers were used to prepared noble metal nanoparticles.18 Elemental analysis of the Pt-PTSA-50 nanocomposite was performed with energy dispersive X-ray spectroscopy (EDS). The results showed that the molar ratio of Pt to S in the nanocomposite was about 16:1 (Figure S2), which corresponds to a 64:1 molar ratio for Pt/PTSA. The value is somewhat larger than expected for the starting materials of Pt-PTSA-50 and may be caused by a discrepancy of the measurement. As seen in Figure 2 E, the Pt nanocomposite has XRD peaks at 39.7°, 46.2°, and 67.5°, corresponding to the (111), (200), and (220) crystalline planes of the face-centered cubic metallic platinum, respectively.19 The broadening of the diffraction peaks indicates that the size of the Pt particles is in the nanometer range. The XRD results are consistent with the TEM images.

Figure 2. Typical TEM images of the nanocomposites A) Pt-PTSA-50, B) Pt-PTSA-100, C) Pt-PTSA-200, and D) Pt-PTSA-300. E) Typical XRD patterns of the nanocomposites a) Pt-PTSA-50, b) Pt-PTSA-100, c) Pt-PTSA-200, and d) Pt-PTSA-300.

Fourier transform infrared spectroscopy (FTIR) of PTSA stabilized Pt nanocomposites provided evidence of the interactions between the Pt nanoparticles and the sulphonic groups of the PTSA. As shown in Figure 3, the SO stretching vibration of PTSA salt occurred at 1132 cm⁻¹, but the SO vibrational peak in Pt-PTSA-100 was at 1122 cm⁻¹. This is a shift of about 10 cm⁻¹ to lower wave numbers for the SO vibration of PTSA in the nanocomposite. In addition, the intensity of this peak noticeably decreased. The peak shift and intensity decrease for Pt-PTSA-100 can be interpreted as the interaction between the Pt nanoparticles and the PTSA terminal sulphonic groups because the SO bond was weakened when the oxygen atom of the sulphonic group interacted with surface atoms of the platinum nanoparticle.10b, 20 Thus, we suggest that the configuration of the nanocomposite is like a bayberry with a photoreceptive dye shell enveloping the Pt nanocore.

Figure 3. Typical FTIR spectra of a) PTSA and b) the Pt-PTSA-100 nanocomposite.

Electron transfer from the excited pyrenetetrasulfonic acid to the platinum nanoparticle

When excited at λ=340 nm, the fluorescence spectrum of the free PTSA has two main emission peaks centered at λ=383 and 403 nm (Figure 4 A, spectrum a). In contrast, the fluorescence intensity of PTSA in the Pt-PTSA-100 nanocomposite decreased by about 95 % (Figure 4 A, spectrum b). These observations demonstrate the occurrence of a photoexcited electron transfer from the excited pyrenetetrasulfonic acid to the platinum in the nanocomposite.9

Figure 4. A) Fluorescence spectra and B) fluorescence decay curves of aqueous solutions (5 μM) of a) PTSA and b) Pt-PTSA-100 excited at λ=340 nm at room temperature.

The fluorescence decay spectra of PTSA and Pt-PTSA-100 nanocomposite excited at λ=340 nm are depicted in Figure 4 B. The profile of the emission over time for the PTSA entity (Figure 4 B, spectrum a) can be fitted to a mono-exponential decay function with a lifetime of 12.2 ns, but the emission profile of the Pt-PTSA-100 nanocomposite (Figure 4 B, spectrum b) can be fitted to a bi-exponential decay function with a short lifetime (4.6 ns) and a long lifetime (10.3 ns). The short lifetime and the long lifetime may be assigned to PTSA attached to the surface of Pt nanoparticles and free PTSA in solution, respectively. This fact provides additional evidence that electron transfer between the PTAS shell and the Pt nanocore occurs.10b, c, 21 The electron transfer specific rate K_et is about 1.2×10⁸ s⁻¹ as estimated by Equation (1), in which τ_{Pt−PTSA−100} and τ_PTSA are the fluorescence lifetimes of the PTSA attached to metallic Pt and free PTSA molecules in the Pt-PTSA-100 colloidal solution, respectively.10b, c, 21

• ((1))

Shown in Figure 5 is the photocurrent generation on a Pt-PTSA-100/indium tin oxide (ITO) electrode. A blank experiment conducted with the bare ITO (Figure 5, spectrum a) did not produce an obvious photocurrent under UV/vis irradiation. However, the photocurrent intensity reached 48.8 μA cm⁻² for Pt-PTSA-100/ITO (Figure 5, spectrum b) under similar illumination conditions. The photocurrent also responded to on/off cycles of illumination. The photoelectrochemical properties of Pt-PTSA-100/ITO imply that the nanocomposite deposited on ITO transfer photoexcited electrons to the electrode surface under light illumination. This study further confirms that the efficient electron transfer process occurs in the Pt-PTSA-100 system.9 From the above results, one can envisage that these kinds of dye functionalized metallic nanocomposites can be used as efficient photocatalysts, owing to the high electron transfer efficiency between photosensitizer molecules and the metallic nanoparticles (Scheme 1).

Figure 5. Photocurrent responses of a) ITO and b) Pt-PTSA-100/ITO to UV/vis irradiation in DMF (30 mL) containing LiClO₄ (0.1 M) recorded at 0.05 V. The illumination from a 150 W xenon lamp was interrupted roughly every 20 s.

Scheme 1. Schematic representation of the electron transfer from pyrenetetrasulfonic acid to the Pt nanocore and photoinduced hydrogen evolution.

Photoinduced hydrogen evolution

The turnover number (TON) is defined as the moles of H₂ evolved from the system per mole of Pt or photosensitizer after irradiation at a certain time [Eq. (2)]. The quantum yield of hydrogen (Φ) is defined by Equation (3),10a, 22 in which I₀ is the number of incident photons per second measured at λ=365 nm. I₀ was found to be 7.22×10⁻⁹ mol s⁻¹.

• ((2))

• ((3))

The photocatalytic results of Pt-PTSA-100 are shown in Figure 6 A. The total amount of H₂ evolved from the Pt-PTSA-100 system using methyl viologen (MV²⁺) as an electron mediator is about 32.2 μmol after UV/vis irradiation for 12 h. It is interesting to note that the amount of H₂ evolved from the Pt-PTSA-100 system without an electron mediator under the same reaction conditions is much higher (125.1 μmol). The TON_Pt, and TON_dye, for Pt-PTSA-100 were 63 and 6311, respectively. The apparent quantum yield at a wavelength of 365 nm calculated according to Equation (3) is 11.5 %. High photocatalytic efficiency of Pt-PTSA-100 without an electron mediator is attributed to the direct electron transfer from the excited sensitizer molecules to the platinum nanoparticles, avoiding energy loss or back reactions related to charge transfer to the mediator.10, 23 Shown in Figure 6 B is the amount of H₂ evolution as a function of the incident light wavelength. The wavelength most suitable for hydrogen evolution is λ=255 nm, which corresponds well with the major absorption wavelength of PTSA.

Figure 6. A) The amount of H₂ evolved as a function of irradiation time using Pt-PTSA-100 as a photocatalyst in the presence (▪) and absence (•) of MV²⁺ ([MV²⁺]=1×10⁻⁶ mol ml⁻¹). B) Dependence of the photocatalytic activity of Pt-PTSA-100 on the wavelength of incident light. Reaction conditions: pH 3, [Pt]=50 μmol ml⁻¹, [PTSA]=0.5 μmol ml⁻¹, [EDTA]=2 mmol ml⁻¹, T=298 K.

Shown in Figure 7 is the influence of the composition of Pt-PTSA on the amount of H₂ evolved. The optimized molar ratio of platinum to PTSA is 100; this ratio leads to the greatest production of H₂ over a given time. If the ratio is lower than 100, an overabundance of sensitizer molecules in the system shortens the distance between the dye molecules, resulting in intermolecular energy transfer and a decrease in photocatalytic efficiency. If the molar ratio of the metal to PTSA is higher than 100, the amount of light absorbed by the system is significantly reduced because there are not enough sensitizer molecules. Moreover, the platinum nanoparticles are prone to agglomerate with high molar ratios of Pt and PTSA, which is not beneficial for photocatalysis.10a

Figure 7. The effect of the molar ratio of Pt to PTSA on hydrogen evolution. Reaction conditions: [Pt]=50  μmol ml⁻¹, [EDTA]=2 mmol ml⁻¹, pH 3, T=298 K.

The effect of the reaction media pH is shown in Figure 8. The maximum efficiency was observed at pH 3. The amount of hydrogen evolution decreased as the medium became either more acidic or more basic (Figure 8). Hydrogen evolution involves the reduction of H⁺ and the donating ability of EDTA.24 As the concentration of H⁺ increases, the reduction of H⁺ occurs at a faster rate. However, the donating ability of EDTA decreases due to protonation of EDTA at low pH.25 The photosensitized hydrogen evolution process should occur at an optimized pH.

Figure 8. Influence of pH on the amount of H₂ generated from Pt-PTSA-100. Reaction conditions: [Pt]=50 μmol ml⁻¹, [PTSA]=0.5 μmol ml⁻¹, [EDTA]=2 mmol ml⁻¹, T=298 K.

Owing to the importance of the stability of a photocatalyst for its practical application, the photocatalytic stability of Pt-PTSA-100 was further investigated by cycle photocatalytic hydrogen evolution experiments. The H₂ evolution stopped when the light was switched off. The catalytic system was kept in dark overnight, and then light irradiation was restarted. The continuous H₂ evolution with no noticeable degradation of the Pt-PTSA-100 was observed in the subsequent runs, which suggests that the photocatalytic activity of Pt-PTSA-100 is stable under UV/vis irradiation (Figure 9).

Figure 9. Hydrogen evolution with light irradiation on and off. Reaction conditions: [Pt]=50 μmol ml⁻¹, [PTSA]=0.5 μmol ml⁻¹, [EDTA]=2 mmol ml⁻¹, pH 3, T=298 K.

On the basis of the above results, a possible pathway (Scheme 1) of the photoinduced hydrogen production from the photocatalytic system may be described as: 1) under UV/vis irradiation, the pyrenetetrasulfonic acid forms a excited moiety, 2) the pyrenetetrasulfonic acid transfers the photoexcited electron directly to the metallic Pt nanocore, 3) water is reduced on the metallic Pt nanocore forming H₂, and 4) the photosensitizer is regenerated by a sacrificial electron donor (EDTA).

Conclusions

We have demonstrated a novel and stable photoinduced hydrogen evolution system without an electron mediator by constructing 1,3,6,8-pyrenetetrasulfonic acid with platinum nanoparticles to form a functionalized nanocomposite. Efficient photoinduced electron transfer from the pyrenetetrasulfonic acid to the platinum nanocore was proved. The effects of parameters such as the molar ratio of Pt to PTSA on the activity of hydrogen evolution and the pH value of the photocatalytic system were optimized. In addition, the photocatalytic activity of the catalyst did not obviously decrease even after four cycles for a total of 48 h of irradiation. These findings reveal that directly binding photoactive molecules to a noble metal nanoparticle may render a novel organic–inorganic hybrid nanocomposite, which has potential applications in the fields of light harvesting and photoinduced hydrogen evolution.

Experimental Section

Preparation of pyrenetetrasulfonic acid functionalized platinum nanocomposites

1,3,6,8-Pyrenetetrasulfonic acid (PTSA) functionalized platinum nanocomposites were prepared by an ethanol reduction method.26 Typically, aqueous solutions of H₂PtCl₆ (7.7×10⁻⁶ mol) and PTSA (7.7×10⁻⁸ mol) were mixed with 30 mL of an aqueous ethanol solution (V_water:V_ethanol=1). The mixture was heated to reflux for 2 h, resulting in a dark brown Pt colloidal solution. In our experiment, the ratio of the metal and PTSA was varied from 50 to 300. The higher the molar ratio of Pt to PTSA, the darker the color of the sample became (Figure S3). The samples were labeled as Pt-PTSA-x, in which x is the molar ratio of platinum to PTSA. The colloidal solution was stable without signs of precipitation at room temperature for several months.

Characterization of pyrenetetrasulfonic acid functionalized platinum nanocomposites

UV/vis absorption spectra of aqueous solutions of PTSA and Pt-PTSA were performed on a TU1810 SPC spectrophotometer. X-ray photoelectron spectroscopy (XPS) was performed on an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al_Kα radiation. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. Fourier transform infrared (FTIR) spectra of the samples were recorded on a Nicolet Magna 550 spectrometer. Fluorescence spectra of the samples were taken on an Edinburgh FLS920 fluorospectrophotometer. Transmission electron microscopy (TEM) studies were conducted on a TECNAI-G20 electron microscope operating at an accelerating voltage of 200 kV. The samples for TEM analysis were prepared by dropping about 3 μL of the dilute colloidal solution onto a carbon covered copper grid and letting the solution dry in air at room temperature. Energy dispersive X-ray spectroscopic (EDS) analyses were performed on a Philips CM30 TEM equipped with an EDS detector. A Philips diffractometer with Ni filtered Cu_Kα radiation was used to obtain X-ray diffraction (XRD) patterns of the samples. The photoelectrochemical experiments were performed on a three-electrode system devised of an indium tin oxide (ITO) glass covered with Pt-PTSA-100 as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The electrodes were immersed in a DMF solution containing LiClO₄ (0.1 mol ml⁻¹). The working electrode was irradiated with a GY-10 xenon lamp (150 W) during the measurements. Photocurrent over time characteristics were recorded with a CHI 660B potentiostat/galvanostat electrochemical analyzer.

Photocatalytic reactions

The photocatalytic reactions were performed in a 50 mL quartz flask equipped with a flat optical entry window. To the quartz flask, distilled water (38 mL), aqueous solutions of EDTA (1×10⁻⁴ mol) as a sacrificial electron donor, and the Pt-PTSA colloidal solution (10 mL, about 2.5×10⁻³ mmol of Pt) as the photocatalyst were added. The system was deaerated by bubbling nitrogen into the solution for 30 min before the reaction took place. The solution was stirred continuously and irradiated by a GY-10 xenon lamp (150 W). The gases evolved were analyzed with an online gas chromatograph (GC102AT) equipped with a thermal conductivity cell detector. All of the photocatalytic reactions were performed at room temperature.

Acknowledgements

The authors gratefully appreciate the financial support of the National Natural Science Foundation of China (20673075, 50973077, and 51073114), Natural Science Foundation of Jiangsu Province (BK2010209), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).