Hierarchical CdS nanostructure by Lawesson's reagent and its enhanced photocatalytic hydrogen production

Abstract

Lawesson's reagent (LR) has been effectively exploited for the synthesis of hierarchical architectures of cadmium sulphide (CdS) nanostructures for the first time. The X-ray diffractograms of the as synthesised CdS nanostructures confirm the formation of hexagonal CdS. The broadness of the XRD peaks clearly indicates the nanocrystalline nature of CdS with average crystallite size of 4 nm. A FESEM study revealed the formation of hierarchical nanostructures, whereas a TEM study showed that the hierarchical arrangement is composed of nanosized CdS particles. A band-gap i.e. 2.4 eV was derived from diffuse reflectance spectroscopy. The photoluminescence spectrum showed an emission peak at 535 and 568 nm which can be attributed to band-edge emission and surface emissions or possible metal vacancies, respectively. Considering the band-gap within the visible region, the photocatalytic hydrogen evolution performance of these CdS nanostructures was performed under visible light irradiation from hydrogen sulphide and water, respectively. Utmost hydrogen evolution i.e. 14 136 μmol h⁻¹ g⁻¹ and 2065 μmol h⁻¹ g⁻¹ was observed over a naked CdS nanostructure via H₂S and water decomposition, respectively. The amount of hydrogen obtained by H₂S splitting is much higher as compared to earlier reports.

---

1. Introduction

Over the last few decades, nanostructured materials have been at the centre of attention owing to their fascinating size-dependent optical, electronic, thermal, mechanical, chemical, and physical properties.^1–3 These nanomaterial display unique properties not only from their bulk counterparts but also from those of the atomic or molecular precursors from which they are synthesized. For the fabrication of next-generation optoelectronic devices and for fulfilling energy demands the use of nanoscale semiconductor nanostructures is imperative.^4,5 Of late, lasers, sensors and field emitters using nanomaterials have been demonstrated by many researchers. Instead of all the efforts to date, there exists a quest to find better, cheaper and efficient nanomaterials.^6,7

In this context, cadmium sulphide with a band gap 2.4 eV is a promising candidate owing to its wide ranging applications in nonlinear optical devices,⁸ flat-panel displays, light-emitting diodes,⁹ lasers,^10,11 logic gates, and transistors, wide application in telecommunications, data storage, and near-field optical lithography.¹² Hence, a lot of efforts have been concentrated to study the effect of manipulation of size, shape and crystal structure of CdS materials on their optoelectronic properties.¹³ Studies have also been devoted for the synthesis and characterization of various nanostructures, such as nanotubes, nanowires, nanorods, and nanobelts.^14,15 The ideal band-gap of CdS in bulk as well as nano form makes it an attractive semiconductor for photocatalytic applications like hydrogen sulphide (H₂S) and water splitting. A large quantity of H₂S is produced in oil refineries which possess a serious environmental threat. Once produced, it remains for many years in environment thereby causing pollution.¹⁶ Till date, only few techniques have been effectively used for treating this H₂S. Of all the H₂S produced globally, a negligible amount is utilized to produce sulphur by Claus process for agricultural and pharmaceutical industries.^17–19 This process is not eco-friendly as it produces large amount of waste and is also energy intensive. Oil refineries or natural gas extraction satisfy the basic demand for fuel of the uncontrollably growing global population and hence their operations cannot be terminated. Therefore, there is an immediate demand to convert environmentally hazardous side products like H₂S into viable fuel sources like H₂.^20,21

Hydrogen is an ideal fuel for future owing to its clean and renewable nature. Hydrogen can be produced by gasification from biomass, methane reforming, through the electrolysis of H₂O or H₂S and photocatalytic cleavage of H₂S or H₂O.^16,22,23 Of all the processes, the photocatalytic processes using solar energy for hydrogen production seems to be the most environmentally safe and less energy intensive methodology as solar energy is free of cost and abundantly available.

Among all the available catalysts, CdS is a promising photocatalyst for H₂S as well as H₂O splitting due to suitable position of the conduction band edge that is more negative than the reduction potential of H⁺/H₂.²⁷ Many approaches for improving the hydrogen production rate and photo-stability of the photocatalyst have been investigated/are under investigation.²⁸ For example, CdS loaded with MoS₂ co-loaded with Pt and PdS, incorporation of CdS particles into the mesopores of Ti-MCM-41 and introduction of graphene nanosheets into CdS have been used.^29–32 In general, there are different factors that control the H₂ production rate on CdS photocatalyst, such as morphology, particle size, phase structure and crystalline nature.^33–37 A variety of approaches have been proposed to enhance the photocatalytic activity of CdS photocatalyst, including the synthesis of CdS nano material's with various morphologies (nanowires, nanorods, nanospheres, nanosheets etc.) and quantum-sized CdS.^38–46 In spite of recent advances in synthesis of nanomaterial, the production of CdS nanostructures still requires an expensive template and harsh conditions, which limit large-scale production or commercialization of the process.^47–49

With all these clues in mind and our experience^17–25 in H₂S splitting for H₂ production using UV-visible light, we decided to explore the efficacy of ultra-small CdS nanostructures produced by using an inexpensive and simple procedure with Lawesson's reagent as a sulphur source for visible-light driven photocatalytic hydrogen generation from H₂S and H₂O. The formation mechanism and optical properties have also been discussed.

2. Experimental

2.1. Synthesis of CdS flowers

In a dry 100 mL beaker, 2 g of cadmium nitrate was dissolved in small amount of water and in another 100 mL beaker 0.70 g of Lawesson's reagent (97%, Spectrochem) was dissolved in chloroform. Both the solutions were transferred to 150 mL teflon coated hydrothermal reactor and kept for 90 °C for 15 h. The yellow solids are filtered and washed with water and copious amount of chloroform to remove unreacted starting materials.

2.2. Characterization

Cadmium sulphide nanostructures thus synthesized were characterized by X-ray diffraction (Model-D8, Advance, Bruker AXS). The samples were also characterized with field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM, model Philips, EM-CM-12) to determine the morphology and particle size respectively. The optical properties were recorded using UV-visible-near infrared (UV-Vis-NIR) spectrophotometer (Perkin Elmer lambda-950 spectrometer). Photoluminescence spectra were recorded by the photoluminescence spectrometer Perkin-Elmer-LS-55. The formation of side products during the reaction was confirmed by ¹H NMR (Varian-NMR-Mercury 300). The quantification of H₂ and O₂ was carried out using Gas Chromatograph (Model Shimadzu GC-14B, MS-5 Å column, TCD, Ar carrier).

2.3. Photocatalytic activity measurement

2.3.1. Photocatalytic H₂S splitting. The hydrogen evolution was performed in a quartz photo reactor using 0.1 g CdS with the help of a Xe-lamp light source (Oriel) of intensity 450 W.^19,20 The cylindrical quartz reactor was filled with 250 mL, 0.25 M aqueous KOH and purged with argon for 30 min. Hydrogen sulphide (H₂S) was bubbled through the solution for 60 min. at the rate of 2.5 mL min⁻¹. Photocatalyst (CdS) was introduced in to the reactor and irradiated with the visible-light source at a constant stirring with continuous H₂S bubbling (2.5 mL min⁻¹). The excess H₂S was trapped in (0.5 M) NaOH solution. The amount of evolved H₂ was measured using graduated gas burette and analyzed using Gas Chromatograph (Model Shimadzu GC-14B, MS-5 Å column, TCD, Ar carrier). The schematic of H₂S splitting setup is depicted in ESI (Fig. S1†).

2.3.2. Photocatalytic hydrogen generation from water. Photocatalytic hydrogen generation experiments were conducted in a two neck 250 mL borosilicate round bottom flask. In a typical run, 0.1 g of catalyst (containing 0.5 wt% Pt loading) was suspended in an aqueous solution and irradiated under a 400 W high pressure mercury vapour lamp with continuous stirring. In this study, we used three types of scavengers/sacrificial reagents viz. methanol, Na₂S and Na₂SO₃ as well as benzyl alcohol. The Pt-loaded CdS photocatalysts were prepared by a photochemical deposition method. For this purpose, powder photocatalyst was stirred in a 25% methanol aqueous solution containing H₂PtCl₆ and irradiated under Hg lamp for 4 h.⁵⁰ The mercury vapour lamp was kept vertically in the quartz tube, provided with a water circulation arrangement in order to minimize the heating effect due to IR radiation. The evolved gases were collected in a eudiometer tube having a septum arrangement to remove the gas for GC analysis. Prior to each photocatalytic experiment; the reaction mixture was purged with ultra pure grade argon gas for 15 min to flush out dissolved gases in the reaction mixture. The eudiometer tube is filled with a saturated solution of NaCl to avoid the dissolution of evolved gas.

3. Results and discussion

3.1. Materials characterization

3.1.1. X-ray analysis. The XRD pattern of the as-synthesized CdS at 10 and 15 h are depicted in Fig. 1. The product prepared at 10 h of reaction time shows broad and noisy peaks indicating amorphous nature of product. However, the reflection peak position(s) validates the formation of hexagonal CdS.

Fig. 1 XRD pattern of cadmium sulphide nanostructures.

Furthermore, to obtain well-defined crystalline CdS, the reaction was performed at 15 h. X-ray powder diffraction (XRD) pattern (Fig. 1) for this sample displays peaks at 2θ = 24.8°, 26°, 27.8°, 44°, and 52.5° corresponding to (100), (002), (101), (110) and (112) crystal planes of hexagonal CdS (JCPDS no. 41-1049), respectively. The broad XRD peaks indicate the formation of nano-structured CdS. The average crystallite size calculated using Scherrer's formula is found to be 4 nm.

3.1.2. Morphological study by FE-SEM. To investigate the morphology of the as prepared CdS nanopowders at macroscopic scale, FESEM study was performed. Fig. 2a and b shows FESEM images of the CdS sample prepared at 10 h. The magnified image (Fig. 2b) clearly shows porous nanostructure with arrangement of thin nanosheets having petal like structures. Fig. 2c and d shows FESEM images of CdS prepared at 15 h, where magnified image shows twin puffy flowers having elongated spikes with pointed tips. The spike arranged in circular fashion, originating from the centre. The sizes of these flowers are in the range of 3–5 μm.

Fig. 2 FESEM images of cadmium sulphide nanostructure (a and b) 10 h and (c and d) 15 h.

3.1.3. TEM analysis. TEM images for as prepared CdS nanostructures along with selected area diffraction pattern (SAED) are shown in Fig. 3. From the TEM images, it can be seen that the flowers are formed via self-assembling of very thin nanopetals which are created by self-alignment of 5 to 10 nm CdS nanoparticles.¹⁷ The petals are curving in nature which was also seen in FESEM. Few more TEM images are provided in ESI (Fig. S2†). TEM images (ESI, S2†) clearly shows the petal surface with the tiny nanoparticles. The concentric fused rings can be observed in the SAED pattern and matches with the hexagonal phase of CdS thereby supporting XRD analysis (inset of Fig. 3b).

Fig. 3 TEM micrograph of CdS nanoparticles and SAED image (inset of Fig. 3b).

3.1.4. UV-Vis spectrophotometry. The optical properties of as prepared CdS nanopowders were studied using UV-visible spectrophotometer and photoluminescence spectrometry. Fig. 4 depicts diffuse reflectance spectra of as prepared CdS nanoflowers. The band edge was observed at around 530 nm which corresponds to the band gap of 2.4 eV. The absorbance maxima observed to be around 450 nm indicating the blue shift in the absorbance maxima depicts the formation of nano-sized CdS.^17,18,51

Fig. 4 DRS spectra of cadmium sulphide nanostructures.

3.1.5. Photoluminescence spectra. A PL spectrum (Fig. 5) of powdered CdS was taken at an excitation wavelength of 350 nm. A broad emission peak centred at 550 nm with slight hump at 580 nm was observed. After deconvolution of the original spectra, two separate peaks at 535 and 568 nm were seen. The PL peak at 535 nm attributed to band-edge emission whereas, the peak at 568 nm is corresponds to surface emissions or possible metal vacancies.³⁴

Fig. 5 Photoluminescence spectra of CdS.

3.1.6. Formation and growth mechanism. The probable mechanism⁵² for the formation of cadmium sulphide nanostructures by using Lawesson's reagent (LR) can be envisioned as follows.

Cadmium nitrate (aqueous), reacts with LR compound which on dissolution with chloroform, dissociate to form a monomer (B) with well-known mechanism (see ESI, Scheme S1†).^52–54 The monomer (B) forms a complex with cadmium as shown in the Scheme 1. Further, this complex upon decomposition under hydrothermal conditions forms cadmium sulphide and side products A and unreacted starting monomer B. The as formed CdS using Lawesson's reagent was exclusively characterized by XRD and TEM described above. The side products A and B formed in this reaction was analysed and confirmed by using ¹H NMR (ESI, S3 and S4† respectively) and mass spectroscopy. Both the side products are having nearly same ¹H NMR spectrum but only in case of the A, the aromatic protons are slightly more deshielded than the B due to electro negativity of oxygen which can be clearly seen in the schematic of the mechanism (Scheme 1). The replacement of sulphur by the oxygen from monomer B is a further evidence for the formation of cadmium sulphide.

Scheme 1 Mechanism of formation of CdS.

During hydrothermal reaction, the intermediate complex slowly decomposed and tiny nuclei of CdS were formed (see Scheme 2A). Initially, tiny nuclei of CdS are produced in the supersaturated solution and further growth of nanoparticles take place with time (Scheme 2b). Under prolonged hydrothermal conditions, the CdS nuclei were grown to form CdS nanocrystals. The newly formed CdS nanoparticles are spontaneously aggregated to minimize their surface energy. These nanoparticles further grow anisotropically along the 2D direction results formation of thin nanopetals like morphology (Scheme 2b and c).

Scheme 2 Growth mechanism for CdS nanostructure.

Initially, these nanopetals are transformed into curving nanopetals due to the surface strain created by vapor pressure of solvent at hydrothermal condition (Scheme 2c). At prolonged hydrothermal reaction time, further growth is hindered due to reduction in the precursor concentration. Hence, with prolong reaction time, these curving nanopetals are self-assembled and form a 3D hierarchical nanostructure i.e. flower like structure (depicted in Scheme 2d). Additionally, the use of chloroform as a solvent for this reaction is aiding the formation of CdS nanostructures. Generally, the dielectric constant of solvents governs the morphology of the product. The use of ethylenediamine gives rod like morphology which is well known for one dimensional growth due to its pristine structure whereas ethylene glycol produce spherical particles. It is quite well known that the use of different solvents with different dielectric constant produces different morphologies.⁵² The BET surface area of CdS nanostructure was observed to be 54 m² g⁻¹.

3.1.7. Photocatalytic H₂S splitting. Photocatalytic study was performed using synthesized cadmium sulphide for hydrogen generation using H₂S. Fig. 6 represents the graph of H₂ evolution as function of irradiation time. The hydrogen production rate achieved was 14 136 μmol h⁻¹ g⁻¹ which is much higher than earlier reports for the same photocatalyst (ESI, Table S1†).^17–26 It also showed good stability under the same experimental conditions.

Fig. 6 Photocatalytic hydrogen evolution from H₂S vs. time.

Fig. 7 Photocatalytic hydrogen evolution from water vs. irradiation time.

In 0.25 M KOH solution (pH 12.5), the weak diprotic acid H₂S (two pK_a values are 7.0 and 11.96) dissociates and maintains equilibrium with hydrosulphide HS⁻ ions (1). The CdS absorbs light and generates electron–hole pairs (2). The photo-generated valence band hole (h⁺VB) upon band gap excitation of CdS nanopowder oxidizes the HS⁻ ion to disulphide ion (S₂²⁻), liberating a proton from the HS⁻ ion (3). The conduction band electron (e⁻CB) from CdS photocatalyst reduces protons to produce molecular hydrogen (4).

H₂S + OH⁻ ↔ HS⁻ + H₂O (1)

Semiconductor CdS ↔ h⁺VB + e⁻CB (2)

Oxidation: 2HS⁻ + 2h⁺VB ↔ S₂²⁻ + 2H⁺ (3)

Reduction: 2H⁺ + 2e⁻CB ↔ H₂ (4)

The efficiency of our photocatalyst was confirmed by the experiment which showed that in absence of photocatalyst as well as in dark condition, evolution of hydrogen gas was not observed from 0.25 M KOH solution flushed with H₂S gas under the visible light irradiation. Also, we carried out the recycle study of CdS nanostructures and found almost same hydrogen production even after three cycles (see ESI, Table S2†).

3.1.8. Photocatalytic hydrogen generation from water. The photocatalytic hydrogen generation was carried out using prepared CdS nanostructures in presence of three different sacrificial reagents viz., (a) methanol (b) 0.1 M Na₂S and 0.04 M Na₂SO₃ and (c) 2 mL benzyl alcohol dissolved in 5 mL acetic acid (Fig. 7). For this purpose we used 100 mL DI water and 100 mg Pt loaded CdS nanostructures. The Pt acts as a co-catalyst and improves the hydrogen evolution rate. Chemical interactions between cadmium sulphide and another semiconductor or metal will affect the low energy surface states of cadmium sulphide, leading to an enhanced activity.

Hence, under the given experimental conditions, the presence of a sacrificial agent, an electron sink in the form of Pt on the surface and the size of the photocatalyst particles have perceptible influence on the photocatalytic hydrogen production. Among the studied sacrificial reagents, benzyl alcohol gave highest H₂ gas evolution around 2065 μmol h⁻¹ g⁻¹.²⁸ In presence of Na₂S and Na₂SO₃ rate of H₂ production is 1060 μmol h⁻¹ g⁻¹ whereas, in presence of methanol lower rate of H₂ generation was observed (223 μmol h⁻¹ g⁻¹).

We also performed the H₂ generation in absence of Pt as a co-catalyst and found 103 μmol h⁻¹ g⁻¹ of H₂ evolution. The TEM images of Pt-loaded CdS are also depicted in ESI (Fig. S4†). In absence of Pt the formed electron–hole pair recombines rather being getting separated which result lower H₂ evolution.

Considering the earlier reports for H₂S splitting, the flowers of CdS gave enhanced photocatalytic activity due to high aspect ratio. If we see the FESEM carefully, each petal has average size is 0.5 (W) × 1.5 (L) μm. Each petal has an average thickness around 40 to 50 nm. The flower is composed of such individual petals which ultimately have higher aspect ratio. All flowers are observed to be puffy in nature and greatly expose to the light. Hence, maximum surface is exposed to the light which enhances overall photo-absorption which ultimately increases photo-generated charge carriers, significantly.¹⁷ The photoluminescence study clearly indicates broad emission peak at 568 nm due to defects created by metal vacancies. This defects may be acting as a mid-trap vacancies which decreases electron and hole recombination by trapping the holes. Additionally, due to thin petals (plate structure), transport of electron to the surface also get enhanced. Since, it is well known that free electrons are responsible for water and H₂S splitting; the enhancement in hydrogen evolution is quite justifiable. Additionally, the surface area observed to be fairly larger which is also responsible for good activity.

The stability of the photocatalyst is already confirmed by performing recycle experiments. The chemistry of photo-corrosion is well understood in case of CdS for water splitting. However, in case of H₂S splitting reaction in presence of sacrificial reagents, the photo-corrosion process is hindered and ultimately the formation of Cd ions also negligible. There is a continuous supply of H₂S during reaction which favours the CdS formation and suppresses the formation of cadmium ion or any other cadmium compound. In case of H₂ production from H₂S, Cd ions (if formed) may be reacting with the S ions which are already available as per reaction (1) in alkaline medium. Even if the Cd ion or CdO formed we can easily convert it into CdS by treating it with Lawesson's reagent. Hence, in technological point of view, catalyst regeneration is very easy in the present system. In view of the above, there will not be any leaching and disposal of cadmium which avoids the environmental issue. The large scale production of CdS nanostructures using the Lawesson's reagent is possible. With the use of sophisticated equipment's (Parr reactor) the CdS with same physico-chemical properties can be reproducible.

4. Conclusions

In summary, we have architectured hierarchical CdS nanostructures i.e. carnation flower like morphology using simple hydrothermal method. We have used Lawesson's reagent as a sulphur source as well as a capping agent for the first time. Considering the unique morphology, the possible formation mechanism has been furnished. The CdS carnation flowers showed highest photocatalytic H₂ evolution i.e. 14 136 μmol h⁻¹ g⁻¹ for H₂S splitting which is much higher than the earlier reports on CdS nanoparticles/bulk for the same reaction. The CdS nano carnation flowers also showed good H₂ evolution i.e. 2065 μmol h⁻¹ g⁻¹ from water. The rate of H₂ generation was highly dependent on type of sacrificial reagents. The enhanced photocatalytic activity is due to puffy flower like morphology which ultimately enhances photo-absorption and photo-generated charge carriers. More significantly, the catalyst has shown very stable activity after recycles. Such kinds of nanostructures are attractive candidates for solar cell and other optoelectronic devices. The present investigation will be greatly helpful for the synthesis of other hierarchically nanostructured metal chalcogenides from Lawesson's reagent.

Acknowledgements

Authors are grateful to C-MET and DeitY, New Delhi for providing the facilities. Vikram Pandit would like to thank the Council of Scientific and Industrial Research, CSIR, New Delhi, India for financial support.

References

1. H. M. Chen, C. K. Chen, R. Liu, L. Zhang, J. Zhang and D. P. Wilkinson, Chem. Soc. Rev., 2012, 41, 5654–5671.
2. X. Chen, C. Li, M. Gratzel, R. Kosteckid and S. S. Mao, Chem. Soc. Rev., 2012, 41, 7909–7937.
3. F. E. Osterloh, Chem. Soc. Rev., 2013, 42, 2294–2320.
4. P. Zhang, J. Zhang and J. Gong, Chem. Soc. Rev., 2014, 43, 4395–4422.
5. J. Ran, J. Zhang, J. Yu, M. Jaroniecc and S. Z. Qiao, Chem. Soc. Rev., 2014, 43, 7787–7812.
6. T. Hisatomi, J. Kubota and K. Domen, Chem. Soc. Rev., 2014, 43, 7520–7535.
7. H. You, S. Yang, B. Dinga and H. Yang, Chem. Soc. Rev., 2013, 42, 2880–2904.
8. H. Kind, H. Yan, B. Messer, M. Law and P. D. Yang, Adv. Mater., 2002, 14, 158–160.
9. X. S. Fang, Y. Bando, U. K. Gautam, C. H. Ye and D. Golberg, J. Mater. Chem., 2008, 18, 509–522.
10. B. D. Liu, T. Y. Zhai, T. Sekiguchi, Y. Koide and D. Golberg, Adv. Mater., 2009, 21, 2034–2039.
11. Z. B. He, J. S. Jie, W. J. Zhang, W. F. Zhang, L. B. Luo, X. Fan, G. D. Yuan, I. Bello and S. T. Lee, Small, 2009, 5, 345–350.
12. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo and P. D. Yang, Science, 2001, 92, 1897–1899.
13. T. Y. Zhai, Z. J. Gu, H. Z. Zhong, Y. Dong, Y. Ma, H. B. Fu, Y. F. Li and J. N. Yao, Cryst. Growth Des., 2007, 7, 448–491.
14. G. Z. Shen and C. J. Lee, Cryst. Growth Des., 2005, 5, 1085–1089.
15. X. F. Duan, C. M. Niu, V. Sahi, J. Chen, J. W. Parce, S. Empedocles and J. L. Goldman, Nature, 2003, 425, 274–278.
16. M. Gratzel, Nature, 2001, 414, 338–344.
17. B. B. Kale, J. O. Baeg, S. M. Lee, H. Chang, S. J. Moon and C. W. Lee, Adv. Funct. Mater., 2006, 16, 1349–1354.
18. S. K. Apte, S. N. Garaje, G. P. Mane, A. Vinu, S. D. Naik, D. P. Amalnerkar and B. B. Kale, Small, 2011, 7(7), 957–964.
19. S. K. Apte, S. N. Garaje, S. D. Naik, R. P. Waichal, J. O. Baeg and B. B. Kale, Nanoscale, 2014, 6, 908–915.
20. V. U. Pandit, S. S. Arbuj, U. P. Mulik and B. B. Kale, Environ. Sci. Technol., 2014, 48(7), 4178–4183.
21. S. K. Apte, S. N. Garaje, S. D. Naik, R. P. Waichal and B. B. Kale, Green Chem., 2013, 15, 3459–3467.
22. S. K. Apte, S. N. Garaje, M. Valant and B. B. Kale, Green Chem., 2012, 14, 1455–1462.
23. N. S. Chaudhari, A. P. Bhirud, R. S. Sonawane, L. K. Nikam, S. S. Warule, V. H. Rane and B. B. Kale, Green Chem., 2011, 13, 2500–2506.
24. S. K. Apte, S. N. Garaje, S. S. Arbuj, B. B. Kale, J. O. Baeg, U. P. Mulik, S. D. Naik, D. P. Amalnerkar and S. W. Gosavi, J. Mater. Chem., 2011, 21, 19241–19248.
25. B. B. Kale, J. O. Baeg, S. K. Apte, R. S. Sonawane, S. D. Naik and K. R. Patil, J. Mater. Chem., 2007, 17, 4297–4303.
26. N. S. Chaudhari, S. S. Warule, S. A. Dhanmane, M. V. Kulkarni, M. Valant and B. B. Kale, Nanoscale, 2013, 5, 9383–9390.
27. T. D. Vu, F. Mighri, A. Ajji and T. Do, Ind. Eng. Chem. Res., 2014, 53, 3888–3897.
28. S. R. Lingampalli, U. K. Gautam and C. N. R. Rao, Energy Environ. Sci., 2013, 6, 3589–3594.
29. W. Li, S. Xie, M. Li, X. Ouyang, G. Cui, X. Lu and Y. Tong, J. Mater. Chem. A, 2013, 1, 4190–4193.
30. Y. Xu, W. Zhao, R. Xu, Y. Shi and B. Zhang, Chem. Commun., 2013, 49, 9803–9805.
31. T. Xuan, S. Wang, X. Wang, J. Liu, J. Chen, H. Li, L. Pana and Z. Sun, Chem. Commun., 2013, 49, 9045–9047.
32. R. Peng, C. Wu, J. Baltrusaitis, N. Dimitrijevic, T. Rajh and R. Koodali, Chem. Commun., 2013, 49, 3221–3223.
33. J. He, Z. Yan, J. Wang, J. Xie, L. Jiang, Y. Shi, F. Yuan, F. Yu and Y. Sun, Chem. Commun., 2013, 49, 6761–6763.
34. A. B. Panda, G. Glaspell and M. S. Shall, J. Am. Chem. Soc., 2006, 128, 2790–2791.
35. J. Zhang, S. Z. Qiao, L. Qi and J. Yu, Phys. Chem. Chem. Phys., 2013, 15, 12088–12094.
36. L. Mao, Y. Wang, Y. Zhong, J. Ning and Y. Hu, J. Mater. Chem. A, 2013, 1, 8101–8104.
37. Z. B. Yu, Y. P. Xie, G. Liu, G. Q. Lu, X. L. Ma and H. M. Cheng, J. Mater. Chem. A, 2013, 1, 2773–2776.
38. X. Zong, J. Han, G. Ma, H. Yan, G. Wu and C. Li, J. Phys. Chem. C, 2011, 115, 12202–12208.
39. D. Barpuzary, Z. Khan, N. Vinothkumar, M. De and M. Qureshi, J. Phys. Chem. C, 2012, 116, 150–156.
40. J. Jin, J. Yu, G. Liu and P. K. Wong, J. Mater. Chem. A, 2013, 1, 10927–10934.
41. Y. Wang, Y. Wang and R. Xu, J. Phys. Chem. C, 2013, 117, 783–790.
42. D. Lang, Q. Xiang, G. Qiu, X. Feng and F. Liu, Dalton Trans., 2014, 43, 7245–7253.
43. X. Wang, L. Yinb and G. Liu, Chem. Commun., 2014, 50, 3460–3463.
44. J. Yu, J. Jin, B. Chenga and M. Jaroniec, J. Mater. Chem. A, 2014, 2, 3407–3416.
45. Y. Min, G. Q. He, Q. J. Xu and Y. C. Chen, J. Mater. Chem. A, 2014, 2, 2578–2584.
46. Y. Peng, Z. Guo, J. Yang, D. Wang and W. Yuan, J. Mater. Chem. A, 2014, 2, 6296–6300.
47. S. Liu, M. Q. Yangab and Y. J. Xu, J. Mater. Chem. A, 2014, 2, 430–440.
48. F. Wang, Y. Wang, X. Zhan, M. Safdar, J. Gong and J. He, CrystEngComm, 2014, 16, 1389–1394.
49. Y. Shi, K. Zhou, B. Wang, S. Jiang, X. Qian, Z. Gui, R. K. K. Yuen and Y. Hu, J. Mater. Chem. A, 2014, 2, 535–544.
50. J. K. Vaishnav, S. S. Arbuj, S. B. Rane and D. P. Amalnerkar, RSC Adv., 2014, 4, 47637–47642.
51. H. Weller, Angew. Chem., Int. Ed. Engl., 1993, 32, 41–53.
52. H. Z. Lecher, R. A. Greenwood, K. C. Whitehouse and T. H. Chao, J. Am. Chem. Soc., 1956, 78(19), 5018–5022.
53. T. J. Curphey, J. Org. Chem., 2002, 67(18), 6461–6473.
54. T. Ozturk, E. Ertas and O. Mert, Chem. Rev., 2007, 107(11), 5210–5278.

---

Footnote

† Electronic supplementary information (ESI) available: (Fig. S1) Schematic representation of H₂S splitting setup, (Scheme S1) mechanism for the formation of monomer (B), (Fig. S2) TEM images of CdS nanostructures and Pt–CdS, (Fig. S3 and S4) ¹H NMR spectra of side products A and B respectively. Comparison of H₂ production with earlier reports (Table S1) and the recycle study of H₂S generation (Table S2). See DOI: 10.1039/c4ra15138k

---