Optical, photocatalytic properties of novel CuS nanoplate-based architectures synthesised by a solvothermal route

Cryst. Res. Technol. 44, No. 7, 729 – 735 (2009) / DOI 10.1002/crat.200800618

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Optical, photocatalytic properties of novel CuS nanoplate-based

architectures synthesised by a solvothermal route

Fei Li*, Wentuan Bi, Tao Kong, and Qinghua Qin

Faculty of Materials Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, P. R. China

Received 21 December 2008, revised 14 April 2009, accepted 21 April 2009 Published online 4 May 2009

Key words CuS, solvothermal, nanoplate, optical properties, photocatalytic. PACS 81.07.Bc, 81.10.Dn, 81.16.Dn

CuS architectures were successfully prepared by a simple solvothermal route without any surfactant, in which copper nitrate trihydrate and element sulfur were used as reactants. The products were characterized by X-ray diffraction, field emission scanning electron microscopy, and transmission electron microscopy. The optical properties of CuS architectres were investigated by Raman spectrometer, ultraviolet-visible spectroscopy, and fluorescence spectrophotometer. The results showed that the CuS architectures were hexagonal-structured phase and composed of intersectional nanoplates. UV-Vis absorption peaks of CuS architectures showed large blue shifts and PL spectrum exhibited a strong blue emission and a weak green emission. Photocatalytic activity of the CuS architectures was evaluated by measuring the decomposition rate of methylene blue solution under solar light. The CuS architectures show good photocatalytic activity. The effects of the molar ratio of Cu:S and the growth time on the synthesis of CuS crystalline were discussed and the growth mechanism of CuS nanoplate-based architectures was also proposed.

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

The control over size and morphology of nanometer- and micrometer-sized semiconductor materials is a great challenge in realizing the design of novel functional devices due to their size or shaped-dependent properties [1,2]. Recently, complex nano/micro crystals with well-defined shape and inner structure have attracted great interests due to their properties and application convenience [3,4]. These architectures should facilitate a deeper understanding of the "bottom-up" approaches, offer opportunities in searching for exciting new properties of materials, and be useful for fabricating functional nanodevices [5,6]. However, the development of facile, mild, and effective methods for creating novel architectures based on semiconductor nanocrystals remains a key scientific challenge.

As a well-known p-type semiconductor, CuS is one of the most intensively studied materials owing to its technologically applications in the fields of solar cell devices [7], nonlinear optical material [8], lithium-ion batteries [9], nanometer-scale switches [10], and gas sensors [11]. To date, CuS nano- and microcrystals have been synthesized as rods [12], wires [13], belts [14], platelets [15], nanoribbons [16], and flower-like structures [17] via different approaches, for example, sonochemical, template-assisted growth, hydrothermal and solvothermal methods. Although various architectures with nano- and microscale dimensions have been observed for the above-mentioned nanostructures in solution-based processes, the selectively controllable synthesis of CuS architectures has rarely been reported. Therefore, designing and developing new and simple solution-based methods to synthesize CuS architectures and other similar semiconductors is still a challenging task at present. ____________________

* Corresponding author: e-mail: [email protected]

730 Fei Li et al.: Properties of novel CuS nanoplate-based architectures

© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

Herein, we report a novel, template-free, solvothermal approach to fabricating CuS nanoplate-based architectures by using copper nitrate trihydrate and element sulfur without the use of any surfactant and template. The effects of molar ratio of Cu:S and growth time on the synthesis of CuS crystallines were studied. The crystal structure, morphologies, optical and photocatalytic properties of the CuS architectures were investegated in detail. In addition, the growth mechanism of the CuS architectures was also proposed and discussed. We expect these new morphologies will enrich the variety of CuS nanostructures and enhance its currently existing applications.

2 Experimental

All chemicals were of analytical grade and used without further purification. In a typical synthesis, 1 mmol Cu(NO3)2.3H2O was dissolved in 40 mL ethylene glycol (EG) and a green solution was formed. Then 2 mmol element sulfur was added into above-mentioned solution under vigorous stirring for 30min. Afterwards, the solution was transferred into a 60 mL Teflon-lined stainless steel autoclave, sealed, and maintained at 150 °C for 24 h and then cooled naturally to room temperature. Finally, the precipitates were centrifuged and washed with distilled water and ethanol several times and dried under vacuum at 60 °C for 3 h. To explore the effect of molar ratio of Cu:S on the synthesis of CuS crystallines, we conducted similar experiments with different molar ratio of Cu:S=1:3, 1:1, 2:1. In order to investigate the effect of growth time, the above procedure was followed at constant molar ratio (Cu:S=1:2), but the reaction time was varied at 2 h, 4 h, 10 h, 18 h and 36 h.

The as-synthesized products were characterized by X-ray diffraction (XRD) using a Dmax-3β diffractometer with nickel-filtered Cu Kα radiation (λ= 1.54178 Å). SEM images were taken with a field-emission scanning electron microscope (FESEM, JEOL-6300F, 15 kV). Transmission electron micrographs were performed on a Tecnai F20 transmission electron microscope (TEM) operated at 200 kV. Raman spectrum was recorded on a Renishow Raman Imaging System 1000 with a 514.5 nm Ar+ laser as an exciting source. UV-Vis absorption spectrum was recorded on Lambda 35 UV-vis spectrometer. PL spectrum was measured on a fluorescence spectrophotometer (F-4500) using Xe lamp with excitation wavelength of 210 nm. Photocatalytic activity of CuS achitectures was evaluated by the degradation of methylene blue (MB) at room temperature. The original solution was prepared by adding 1.3 mL H2O2 (30%) to 40ml MB solution (20 mg/L), then 20 mg CuS crystalline was added into the solution to form the aqueous photocatalyst dispersion. At once, the dispersion was magnetically stirred in dark condition for 30 min to establish an adsorption/desorption equilibrium condition. Afterwards, the dispersion was irradiated by solar light. At given irradiation time intervals, the dispersion was sampled (1 mL), diluted (10 mL), centrifuged, and subsequently filtered through a Millipore filter (pore size, 0.22 mm) to separate the catalyst particles. The filtrates were analyzed using UV-Vis spectra to determine the concentration of MB. The tests of optical and photocatalytic properties were all performed on the CuS samples prepared at the molar ratio of Cu:S=1:2.

3 Results and discussion

Figure 1 shows the XRD patterns of CuS samples at the molar ratio of Cu:S=1:2, 1:3, 2:1 and 1:1, respectively. Seen form figure 1 a and b, all peaks in the patterns can be indexed as the hexagonal CuS phase with lattice parameters a =3.792 Å and c =16.34 Å, which are in good agreement with the reported data for CuS (JCPDS Card. No. 06-0464). No characteristic peaks are observed for impurities, indicating pure CuS synthesized at the molar ratio of Cu:S=1:2 and 1:3. The strong and sharp diffraction peaks suggest that the CuS samples are well crystalline. However, the samples are mixture of Cu2S (JCPDS Card. No. 26-1116) and Cu1.96S (JCPDS Card. No. 29-0578) at the molar ratio of Cu:S=2:1 (Fig. 1c), and mixture of CuS (JCPDS Card. No. 06-0464) and Cu7.2S4 (JCPDS Card. No. 24-0061) at the molar ratio of Cu:S=1:1 (Fig. 1d). It can be concluded that superfluous element sulfur is necessary for the formation of hexagonal and pure CuS crystalline corresponding to its stoichiometric proportion.

Cryst. Res. Technol. 44, No. 7 (2009) 731

www.crt-journal.org © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 1 XRD patterns of CuS prepared at various molar ratio of Cu:S: (a) 1:2, (b) 1:3, (c) 2:1, (d) 1:1. (Online color at www.crt-journal.org)

Fig. 2 SEM images of CuS architectures. (A, B, C, and D) CuS samples prepared at the molar ratio of Cu:S=1:2; (E and F) CuS samples prepared at the molar ratio of Cu:S=1:3.

The morphologies of CuS samples are investigated by FESEM. A low-magnification image of CuS samples prepared at the molar ratio of Cu:S=1:2 is shown in figure 2A, clearly exhibiting that the samples are well-dispersed CuS architectures. For clarifying the structures, high-magnification images are shown in figure 2 B, C, and D, which indicate the CuS architectures are composed of intersectional hexagonal nanoplates with a mean edge length of ca. 1μm and an average thickness of ca. 100 nm. The hexagonal nanoplates may be related to the hexagonal phase of CuS crystalline. Among these CuS architectures, some are constructed by only two perpendicular hexagonal nanoplates (Fig. 2B), and some are made up of several similar hexagonal plates (Fig. 2C). Especially, there are a few well-defined concaved cuboctahedrons of CuS architectures with high symmetry, which are constructed by four identical hexagonal nanoplates (Fig. 2D). Each CuS concaved cuboctahedrons are apparently caved with 14 high symmetric cavities, among which, there are eight caved regular pyrometric cone and six caved regular quadrangular pyramid. The morphology and size of CuS architectures prepared at the molar ratio of Cu:S=1:3 (Fig. 2 E and F) is very similar to those prepared at the molar ratio of Cu:S=1:2.



To further reveal the microstructure of the CuS architectures, TEM and HRTEM images are recorded (Fig. 3). It can be clearly seen that the CuS architectures consist of intersectional nanoplates. Figure 3B shows a HRTEM image taken from the labeled area in figure 3A. The HRTEM image clearly shows the well-resolved 2D lattice fringes of the hexagonal CuS nanoplate, and the 3.29 Å and 3.04 Å lattice spacings correspond to the {100} and {102} plane spacing, respectively, of hexagonal CuS (covellite). The HRTEM result indicates the single crystallinity of the CuS nanoplate.

Fig. 3 TEM and HRTEM images of the CuS architectures. (A) TEM image; (B) HRTEM image of a single CuS nanoplate.

Fig. 4 SEM images of CuS samples recorded at various growth time: (A) 2 h, (B) 4 h, (C) 10 h, (D) 18 h, (E) 36 h.

Figure 4 presents the morphology development of CuS products grown for different period of times. SEM images in figure 4 A and B reveal that most of the CuS are nanoplate-based architectures at the early stage of the reaction (2 h and 4 h). A small quantity of CuS nanoplates can also be observed. As time develops (10 h and 18 h, Fig. 4 C and D), almost all CuS samples are in the morphology of nanoplate-based architectures with the size unchanged. When the growth time was further increased to 36h (Fig. 4E), the nanoplate-based architectures interlace with each other to form assembled architectures.

Although the exact mechanism for the formation of CuS nanoplate-based architectures is still under investigation, EG plays an important role without any doubt because there are no products obtained when we conduct a similar experiment in aqueous solutions. The formation of CuS can be described by the following three steps based on the reduction mechanism of polyols [18]:

HOCH2CH2OH → CH3CHO + H2O (1) S + 2CH3CHO → CH3CO-OCCH3 + S2- + 2H+ (2) Cu2+ + S2- → CuS(s) (3)



Acetaldehyde is produced by the dehydration of EG at elevated temperatures, where the acetaldehyde can donate a hydrogen atom and act as a reducing agent to convert sulfur powder to S2-, as shown in reactions 1 and 2. Then S2- reacts with Cu2+ released from copper nitrate trihydrate to form CuS nuclei, as shown in reaction 3.

The formation of CuS architectures with various symmetries and shapes is determined by the nucleation and the subsequent growth stages. The growth process develops according to the crystal habit and to the branching process [19]. The initial factor responsible for the final shape of the structures is the crystallographic phase of the seed formed during the nucleation process. In general, this factor depends on the nature of the material and on the environmental conditions. It is well known that the anisotropic structure of CuS is beneficial for the formation of platelike CuS [20].On the basis of the above analyses and the experiment results, we propose the following growth mechanism for the nanoplate-based architectures. At the initial stage, CuS nuclei are formed through the reaction of Cu2+ and S2- in EG. In the subsequent step, these nuclei preferentially are grown in the same direction and further transferred into CuS nanoplates owing to the different surface energies of the hexagonal crystal structure. These nanoplates will meet with each other in such modes as one perpendicular to another, one intersecting with two, or three, et al. The purpose of this intersection is to obtain the most stable morphology. Accordingly, CuS nanoplate-based architectures are formed. With the further extention of growth (36 h), the nanoplate-based architectures will meet with each other to form assembled CuS architectures. The growth process of CuS architectures is schematically illustrated in figure 5.

Fig. 5 Illustration for the growth process of CuS architectures.

Fig. 6 Raman spectrum of CuS architectures.

Fig. 7 (a) UV–Vis absorption spectrum of the CuS architectures, (b) PL spectrum of the CuS architectures.

The Raman spectrum (Fig. 6) of CuS samples exhibits a very sharp peak at 462 cm−1, which can be assigned to the lattice vibrations of CuS. The present result shows that the lattice atoms are aligned in the periodic array. Comparing to refs [21,22], the peak shows some blue shift of about 10nm, which might result from the special shape of the CuS samples. Figure 7 a and b show the UV-Vis absorption spectrum and PL spectrum of the CuS architectures, respectively. The UV-Vis absorption spectrum includes a strong absorption peak at 623nm and a



weak peak at 314 nm. Similar emission peaks are also given out in the researches by Xue and Wang [23,24]. Compared with bulk covellite CuS, which has a characteristic absorption band in the near-IR region [25], the absorption peaks of nanoplate-based CuS architectures obtained by us exhibit a large and distinct blue-shift, which is possibly attributed to the quantum confinement of the CuS nanoplates. The PL spectrum has a strong and broad blue emission band (464 nm) and a weak green emission band (550 nm). The blue emission is in good agreement with the PL result of spherical CuS hierarchical structures [26] and the green emission band shows a red shift compared to that of CuS nanorods reported by Srivastava [27]. The size and morphology may be responsible for the red shift of green emission peak.

UV-Vis spectra were applied to demonstrate the photocatalytic degradation activity of MB. The characteristic absorption peak at 662 nm of MB was used as a monitored parameter during the photocatalytic degradation process. Figure 8a shows the absorption spectra of aqueous solutions of MB tested at different intervals in the presence of CuS architectures. The intensity absorption peak at 662 nm of MB decreased gradually with the irradiation time prolonged, indicating the photocatalytic degradation of MB. Figure 8b shows the degradation rate of MB at different intervals, about 80% of the MB was degraded after 2.5 h solar light irradiation. For comparison, we also carried out similar test without the addition of CuS samples under solar light. We found that the the intensity absorption peak at 662 nm of MB almost had no changes with the irradiation time prolonged. In our work, the obtained CuS architectures have unfolded nanoplates that could absorb more photons to produce electron-hole pairs [28], and the deep caved pores enable the architectures to be exposed to the MB solution sufficiently. Furthermore, the nano-size of plates could reduce the radiationless recombination of electron-hole pairs [29], which is also in favour of the photocatalysis of MB.

Fig. 8 (a) Absorption spectra of MB aqueous solutions in the presence of CuS architectures, (b) Degradation rate of MB at different intervals in the presence of photocatalysts. (Online color at www.crt-journal.org)

4 Conclusions

In summary, we have demonstrated a facile solvothermal route to synthesize CuS architectures without the use of any surfactant and template. The CuS architectures are constructed by nanoplates and the CuS nanoplates are of hexagonal phase and single crystalline in nature. The Raman spectrum shows that the lattice atoms of CuS are aligned in the periodic array.The UV-Vis spectrum shows that the absorption peaks have significant blue shifts and the PL spectrum exhibits a strong blue emission band and a weak green emission band. The CuS architectures have good photocatalytic activity on the degradation of MB under the irradiation of solar light. The CuS architectures may have potential applications in solar cell devices and luminescent materials. A possible synthesis mechanism has been proposed and our work may provide a new method to fabrication of other metal sulfide nanostructures.

References

[1] S. Erokhina, V. Erokhin, and C. Nicolini, Langmuir 19, 766 (2003). [2] L. Reijnen, B. Meester, A. Goossens, and J. Schoonman, J. Chem. Vapor. Depos. 9, 15 (2003). [3] H. T. Shi, L. M. Qi, J. M. Ma, and H. M. Cheng, J. Am. Chem. Soc. 125, 3450 (2003). [4] X. Y. Chen, X. Wang, Z. H. Wang, X. G. Yang, and Y. T. Qian, Cryst. Growth. Des. 5, 347 (2005). [5] S. Mann, Angew. Chem. Int. Ed. 39, 3392 (2000).



[6] Y. Huang, X. F. Duan, and C. M. Lieber, Small 1, 142 (2005). [7] R. S. Mane and C. D. Lokhande, Mater. Chem. Phys. 65, 1 (2000). [8] A. M. Malyarevich, K. V. Yumashev, N. N. Posnov, and V. P. Mikhailov, J. Appl. Phys. 87, 212 (2000). [9] J. S. Chung and H. J. Sohn, J. Power Sources 108, 226 (2002).

[10] T. Sakamoto, H. Sunamura, and H. Kawaura, Appl. Phys. Lett. 82, 3032 (2003). [11] A. Galdikas, A. Mironas, V. Strazdiene, A. Šetkus, I. Ancutiene, and V. Janickis, Sens. Actuat. B 67, 76 (2000). [12] K.P. Kalyanikutty, M. Nikhila, U. Maitra, and C. N. R. Rao, Chem. Phys. Lett. 432, 190 (2006). [13] C. E. Wu, J. B. Shi, C. J. Chen, Y. C. Chen, Y. T. Lin, P. F. Wu, and S. Y. Wei, Mater. Lett. 62, 1074 (2008). [14] C. L. Jiang, W. Q. Zhang, G. F. Zou, L. Q. Xu, W. C. Yu, and Y. T. Qian, Mater. Lett. 59, 1008 (2005). [15] A. P. Gonçalves, E. B. Lopes, A. Casaca, M. Dias, and M. Almeida, J Cryst. Growth 310, 2742 (2008). [16] C. H. Tan, R. Lu, P. C. Xue, C. Y. Bao, and Y. Y. Zhao, Mater. Chem. Phys. 112, 500 (2008). [17] B. Li, Y. Xie, and Y. Xue, J. Phys. Chem. C 111, 12181 (2007). [18] S. H. Im, Y. T. Lee, B. Wiely, and Y. N. Xia, Angew. Chem. Int. Ed. 44, 2154 (2005). [19] M. Bashouti and E. Lifshitz, Inorg. Chem. 47, 678 (2008). [20] W. M. Du, X. F. Qian, X. D. Ma, Q. Gong, H. L. Cao, and J. Yin, Chem. Eur. J. 13, 3241 (2007). [21] K. J. Xu and W. P. Ding, Mater. Lett. 62, 4437 (2008). [22] T. Thongtem, A. Phuruangrat, and S. Thongtem, Curr. Appl. Phys. 9, 195 (2009). [23] J. Liu and D. F Xue, J Cryst. Growth 311, 500 (2009). [24] H. L Xu, W. Z Wang, and W. Zhu, Mater. Lett. 60, 2203 (2006). [25] S. K. Haram, A. R. Mahadeshwar, and S. G. Dixit, J. Phys. Chem. 100, 5868 (1996). [26] K. J. Xu and W. P. Ding, Mater. Lett. 62, 4437 (2008). [27] P. Roy, K. Mondal, and. K. Srivastava, Cryst. Growth Des. 8, 1530 (2008). [28] M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann, Chem. Rev. 95, 69 (1995). [29] Z. Zhang, C. C. Wang, R. Zakaria, and J. Y. Ying, J. Phys. Chem. B 102, 10871 (1998).

Documents

Optical, photocatalytic properties of novel CuS nanoplate-based architectures synthesised by a solvothermal route