In situ formation of metal Cd_xZn_1−xS nanocrystals on graphene surface: a novel method to synthesise sulfide–graphene nanocomposites

Abstract

Herein, Cd_xZn_1−xS–graphene composites were prepared via an in situ growth solvothermal process using sulfur–graphene composites as precursor for the first time. The as-prepared samples exhibit efficient photocatalytic activities, including the production of H₂ as well as the photo-oxidation of methylene blue dye under visible light irradiation.

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Owing to its exceptional physical properties, it is highly desirable to take graphene as a two-dimensional (2D) supporting platform for the growth of nanocrystals on it to form graphene-based materials. Graphene-based materials have been intensively explored in a wide range of applications over the past years.^1–3 However, the controllable synthesis of graphene-based materials remains a challenge. To date, various methods including hydro/solvothermal method,⁴ physical vapor deposition (PVD) method⁵ and chemical vapor deposition (CVD) method⁶ etc. have been developed for the preparation of graphene-based materials. However, limited researches focused on the exact factors during the preparation process to achieve the goal of in situ growth of nanocrystals on the surface of graphene.^7,8 Although both PVD and CVD could make the nanocrystals in situ grow on the surface of graphene, harsh conditions such as high temperature is also needed during the preparation progress. Hydro/solvothermal method is a widely used method to prepare graphene-based materials owing to its outstanding advantages in preparing nanomaterials.⁹ However, the nucleation and crystallization sites of the nanocrystals are uncontrollable under the hydro/solvothermal condition so that it is hard to realize the in situ growth of nanocrystals on the surface of graphene. Thus, controlling the nucleation and crystallization sites to certain place on the surface of graphene is the key factor for the in situ growth of nanocrystals on the surface of graphene during the hydro/solvothermal preparation process.

Metal sulphide–graphene nanocomposites have been intensively studied in application of photocatalytic application.^10,11 Herein, sulfur–graphene composites are taken as sulfur source as well as graphene source for the preparation of sulfide nanocrystals–graphene composites for the first time. The nucleation and crystallization sites are controlled to the surface of graphene when the sulfur which is deposited firstly on the surface of graphene is reduced and reacted with cations in the solvothermal process. In this work, the Cd_xZn_1−xS–graphene (ω) (CZS(x)–G(ω), where ω is the mass percentage of graphene in sulfur–graphene composite) is taken as a typical model and was prepared through this solvothermal route. The as-prepared samples showed excellent visible light photocatalytic H₂ production and photo-oxidation methylene blue dye activity in the absence of any cocatalysts.

The fabrication process of the CZS(0.4)–G(ω) composites is illustrated in Fig. 1. Graphene is prepared by sonication assisted liquid-phase exfoliation of graphite flakes which is known as an efficient method to prepare high-yield and defect-free graphene (Fig. 1a and b) as reported previously.^12,13 N-Methylpyrrolidone (NMP) is chosen as solvent for the effective exfoliation of graphite.¹⁴ Sulfur is deposited onto the graphene surface when formic acid is added into the mixture of Na₂S_x aqueous solution (Fig. 1c) containing graphene sheets as suggested by the following reactions:

Na₂S + (x − 1)S → Na₂S_x (1)

Graphene + S_x²⁻ + 2H⁺ → (x − 1)S–graphene + H₂S (2)

Fig. 1 Schematic illustration of the fabrication process of CZS(0.4)–G(ω) composites. Graphite flakes are exfoliated by ultrasonic in NMP (a) to form a homogeneous graphene suspension (b). Sodium polysulfide solution is added in the graphene suspension (c) and elemental sulfur forms on the surface of graphene after addition of formic acid (d). Finally, the nanoparticle-decorated graphene is obtained by using the S–graphene composite as sulfur and graphene source in the presence of other salts during the solvothermal progress (e).

The amount of graphene in the as-prepared samples is controlled by adjusting the weight ratio of sulfur and graphene in sulfur–graphene (S–G) composites. The obtained S–G composite is taken as raw material for the next progress (Fig. 1d). The sulfur is reduced and the target nanocrystals formed and grew in situ on the surface of graphene at the same time (Fig. 1e).

Quality of the graphene obtained in this work is investigated. A large graphene sheet about 30 μm in length and 20 μm in width is shown in Fig. 2a, indicating the advantages in preparation of large-area graphene sheets using the liquid-phase exfoliation method. Raman spectrum in Fig. 2b gives a well-defined G band and a broad 2D band, characteristic of few layer flakes,¹⁵ which is also confirmed by the atomic force microscopy (AFM) analysis (Fig. S1†). D band is not observed in the Raman spectrum indicating the defect-free graphene was obtained. Results from all of these analyses imply the high quality of the graphene, which is essential to maintain the unique physical properties of graphene.

Fig. 2 (a) SEM image of graphene sheet obtained by liquid-phase exfoliation and its Raman spectrum (b). The inset image in (b) shows a magnification view of the 2D band in the Raman spectrum. (c) SEM image and EDX analysis of S–G (d).

The spherical structures with highly developed porous structures are clearly illustrated in the scanning electron microscope (SEM) image for the as-prepared S–G nanocomposites (Fig. 2c). The energy-dispersive X-ray (EDX) microanalysis in Fig. 2d confirms the as-prepared composite is only composed of C and S. No impure elements are detected indicating that no impurities remained in the as-prepared S–G composites.

To investigate the effects of S–G composites on the morphology of the final products, the as-synthesized CZS(0.4)–G(0.5) sample was characterized by SEM and transmission electron microscopy (TEM). As a comparison, pure CZS(0.4) was also prepared through the same route using elemental sulfur as sulfur source without the addition of graphene. In sample pure CZS(0.4), a significant aggregation of the CZS(0.4) nanocrystals as well as some big polyhedron particles with nonuniform particle size was observed (Fig. S2†). However, Fig. 3a shows that much smaller particles covered entirely and tightly on the graphene sheets in sample CZS(0.4)–G(0.5), implying that graphene prevent the nanocrystals from aggregation. EDX microanalysis reveals that the existence of C, Zn, S and Cd for the sheets. In addition, the atomic ratio of Cd and Zn was determined to be 4 : 5.7 from inductively coupled plasma (ICP) elemental analysis, which is close to that from the raw materials of 4 : 6 (Table S1†). Hexagonal wurtzite phase CZS(0.4)–G(0.5) was obtained in this work which was revealed by X-ray diffractometer (XRD) analysis (Fig. S3†). The TEM image of CZS(0.4)–G(0.5) (Fig. 3c) shows that the graphene sheet is well decorated by CZS(0.4) nanoparticles. Magnification view of the corner of the sheet reveals the thin graphene sheet in the composites (Fig. 3d). The lattice fringes with interplanar distance of 0.337 nm can be assigned to the (0 1 3) plane of the hexagonal phase CZS(0.4) (Fig. S4†). Result from the selected-area electron diffraction pattern (SAED) clearly indicates the polycrystalline characteristic of these nanocrystals (Fig. S4†). As a comparison, elemental sulfur and graphene dispersion were taken as precursor for the preparation process. However, a mixture of aggregated particles and graphene sheets was obtained rather than well composited CZS(0.4)–G(0.5) (Fig. S5†). Furthermore, almost the similar result was obtained when CZS(0.4)–G(0.5) was prepared through the method that being used for the preparation of well composited CdS–graphene nanocomposites (Fig. S6†).¹⁶ These results demonstrate that the deposition of element sulfur on the surface of graphene to form the S–G composites plays a key role in the in situ formation of CZS(0.4) nanocrystals on the surface of graphene.

Fig. 3 SEM images (a), EDX analysis (b) and TEM images (c), (d) of CZS(0.4)–G(0.5). TEM image in (d) gives a magnification view of the selected section of (c).

To demonstrate the effects of graphene on the optical properties of CZS(0.4), UV-vis diffuse reflectance spectra for CZS(0.4)–G(ω) with different weight ratio of graphene were recorded (Fig. 4a). Compared with pure CZS(0.4), the CZS(0.4)–G(ω) photocatalysts show a capability to adsorb visible light in a wide range. In addition, continuous absorption band in the visible light region is also observed while increasing the amount of graphene.

Fig. 4 Optical properties and photocatalytic performance of the as-prepared samples. (a) UV-vis absorption spectra of samples with different content of graphene. (b) Photocatalytic H₂ evolution by the former samples and graphene under visible light illumination (λ > 400 nm). (c) Dynamic curves of methylene blue dye degradation and kinetic study (inset) over CZS(0.4)–G(0.5) and CZS(0.4) samples. (d) UV-vis spectral changes of methylene blue dye as a function of irradiation time over CZS(0.4)–G(0.5).

Graphene-based semiconductor photocatalysts have been widely studied in photocatalytic water-splitting and degradation of pollutants. In this work, the photocatalytic activities of all samples are evaluated on H₂ evolution from photocatalytic water splitting and degradation of methylene blue dye under visible light irradiation (λ > 400 nm). Fig. 4b shows the varied amount of H₂ evolution from an aqueous solution containing Na₂SO₃ and Na₂S for pure CZS(0.4) and CZS(0.4)–G(ω). CZS(0.4)–G(0.5) shows the highest H₂ evolution rate up to 421 μmol h⁻¹ 0.015 g⁻¹ of photocatalysts without loading of any noble metal, which is about 1.5 times higher than that of pure CZS(0.4) (280 μmol h⁻¹ 0.015 g⁻¹ of photocatalysts). Furthermore, apparent quantum yields (AQY) for water reduction were also investigated at 420 nm to be 23.1% and 14.3% for CZS(0.4)–G(0.5) respectively, corresponding to recent study.¹⁷ Simultaneously, CZS(0.4)–G(0.5) also exhibits enhanced photocatalytic degradation of methylene blue dye in a certain time (60 min) compared to pure CZS(0.4). The reaction rate constant of CZS(0.4)–G(0.5) is 0.0267 min⁻¹, which is 1.7 times of that of pure CZS(0.4) (0.0155 min⁻¹) (Fig. 4c). No shift corresponding to the maximum absorption wavelengths of methylene blue dye solutions are observed, which implies benzene/heterocyclic rings are decomposed rather than the simple decolorization process of the removal of organic groups (Fig. 4d).¹⁸ A smaller arc radius in EIS Nyquist plots for CZS(0.4)–G(0.5) corresponds to more effective separation of photogenerated electron–hole pairs in compared to that of CZS(0.4), indicating that graphene act as an efficient electron acceptor to promote the separation and transfer of photoinduced charge transfers for improved photocatalytic activity due to its superior electron mobility (Fig. S7†).^19,20 However, pure graphene showed no performance in water-splitting and decomposition of methylene blue dye under the same experimental condition. This implies the potential application of this graphene-based composite in energy conversion.

In summary, in situ formation of CZS(0.4) nanocrystals on the surface of defect-free graphene using S–G composite as raw material via a facile solvothermal method is reported for the first time. The preparation process includes the depositing of elemental sulfur on the graphene surface and in situ conversion of the elemental sulfur into sulfide nanocrystals during the solvothermal process. The as-prepared composites act as efficient photocatalysts for photocatalytic water splitting and degradation of methylene blue dye under visible-light irradiation without any co-catalysts. It's expected that this idea could also be applied for the preparation of not only sulfides but selenides and tellurides by depositing elemental selenium or elemental tellurium on the surface of graphene and in situ conversion of these elemental chalcogen into chalcogenide nanocrystals for further application.

Acknowledgements

This work was financially supported by projects of Natural Science Foundation of China (21271055) and Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (no. QAK201304).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and supplementary figures. See DOI: 10.1039/c4ra05361c

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