ARTICLE pubs.acs.org/JPCC
Epitaxial Growth of CdS Nanoparticle on Bi2S3 Nanowire and Photocatalytic Application of the Heterostructure Zhen Fang,*,† Yufeng Liu,† Yueting Fan,† Yonghong Ni,† Xianwen Wei,† Kaibin Tang,‡ Jianmin Shen,*,§ and Yuan Chen§ †
Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, China Division of Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, China § School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore ‡
bS Supporting Information ABSTRACT: Bi2S3 nanowire/CdS nanoparticle heterostructure has been designed and constructed through an easy wetchemistry approach at 140 °C for 8 h. The product is mainly composed of Bi2S3 nanowires, several hundred nanometers long and 10 nm wide, and epitaxially grown triangle-like CdS nanoparticles with size of 20 nm at their surfaces. A possible sequential deposition growth mechanism is proposed on the basis of experimental results to reveal the formation of the nanoscale heterostructure. Under the irradiation of UV light, the as-prepared nanoscale Bi2S3/CdS heterostructure exhibits enhanced photochemical efficiency that can be mainly attributed to the microstructure of the product. In the nanoscale heterostructure, the CdS nanoparticle not only determines the overall band gap energy, but also controls the charge carrier transition, recombination, and separation, while the Bi2S3 nanowire serves as support for the CdS nanoparticle, defines the specific surface area of the product and thus influences the photocatalytic activity. The effects of reaction parameters on the structure and photocatalytic activity of the final product are also discussed.
1. INTRODUCTION In recent years, researchers have focused intently on semiconducting photocatalysts to meet increasing demand in areas such as photochemical water splitting and degradation of organic pollutants.13 However, the most popular wide band gap semiconducting photocatalyst, titanium oxide (TiO2), is only able to respond to the ultraviolet (UV) region of the whole solar light spectrum. There have been numerous efforts to enhance the photoresponse and improve the photoactivity of TiO2, such as decreasing crystal size, doping, and sensitizing TiO2 with organic dyes.48 Recently, photogenerated charge carriers can be effectively separated inside semiconducting composite materials according to different band gap structures of their components.911 Thus narrow band gap semiconductor nanoparticles, such as cadmium sulfide (CdS) and bismuth sulfide (Bi2S3) quantum dots, have been used as sensitizers to facilitate the photoresponse of TiO2 in visible light region.1219 Moreover, different migration rates of photogenerated charge carriers inside multiphase, single-component photocatalyst, such as one-dimensional TiO2(B) (monoclinic, space group C2/m)/anatase core/shell nanostructures, can also contribute to charge separation and improved photocatalytic activity.11 Although the charge separation mechanisms should be applicable to other non-TiO2 semiconductors with similar band gap configurations, r 2011 American Chemical Society
the design and realization of highly efficient mixed-phase photocatalyst still remains a challenge. Metal sulfides are proven to be a group of highly efficient catalysts for photochemical reactions, since photogenerated charge carriers can rapidly move to the surface of the catalysts, reducing or oxidizing organic molecules. For example, CdS nanostructures were demonstrated as an effective photocatalyst to degrade rhomdamine B, methyl orange, and acid fuchsine under UV irradiation conditions.20,21 Bi2S3 nanoparticles with different sizes show high efficiency in photodegradation of rhomdamine B.22 CuS and N, C codoped ZnS has also been used as photocatalysts in environmental remediation.23,24 In our study, we have found that the band edges of CdS and Bi2S3 (CdS, 0.951.95 eV; Bi2S3, 0.760.54 eV) are advantageous for charge separation, provided a defect-free interface is constructed. It is expected that the combined Bi2S3/CdS heterostructure should also possess improved photochemical response. In previous reports, preparation of nanoscale heterostructures through high-temperature methods, such as chemical vapor deposition, have been well-documented,2527 while wet-chemistry Received: October 25, 2010 Revised: June 20, 2011 Published: June 21, 2011 13968
dx.doi.org/10.1021/jp112259p | J. Phys. Chem. C 2011, 115, 13968–13976
The Journal of Physical Chemistry C synthetic route is another important strategy for its low cost and convenience for large-scale production.28,29 In this article, we report the synthesis of Bi2S3/CdS nanowire/nanoparticle heterostructures through an epitaxial growth pathway in liquid phase and explore their photocatalytic application.
2. EXPERIMENTS SECTION 2.1. Synthesis Procedure. 2.1.1. Materials and Reagents. All chemical reagents in our experiments were purchased from Sigma-Aldrich Co. and used without further purification. 2.1.2. Synthesis of Bi2S3/CdS Nanoscale Heterostructure via One-Pot Reaction. In a typical experiment, 1.75 mmol thiourea, 0.5 mmol Cd(NO3)2 3 4H2O (Warning: cadmiferous compounds are carcinogenic), 0.5 mmol anhydrous BiCl3 and 3.5 mmol polyvinylpyrrolidone (PVP) were added into a 50 mL Teflonlined stainless steel autoclave, then 35 mL of ethylene glycol was transferred into the autoclave and the solution was stirred for 30 min at room temperature. The whole mixture was then sealed and maintained at 140 °C for 8 h. The final black solid product was centrifuged, washed with acetone and distilled water several times, and finally vacuum-dried at 60 °C for 8 h. 2.1.3. Synthesis of Bi2S3 Nanowires. Bi2S3 nanowires were synthesized through a similar procedure in which 0.75 mmol thiourea, 0.5 mmol anhydrous BiCl3 and 3.5 mmol polyvinylpyrrolidone were added into a 50 mL Teflon-lined stainless steel autoclave. Then, 35 mL of ethylene glycol was transferred into the autoclave and the solution was stirred for 30 min at room temperature. The whole mixture was then sealed and maintained at 140 °C for 8 h. The final black solid product was centrifuged, washed with acetone and distilled water several times, and finally vacuum-dried at 60 °C for 8 h. 2.1.4. Synthesis of CdS Nanoparticles. CdS nanoparticles were synthesized according to the method described in the literature.30 Briefly, oleylamine (20 mL), Cd(Ac)2 3 4H2O (1 mmol), sulfur powder (1 mmol), and triphenylphosphine (2 mmol) were added to toluene (15 mL). After about 10 min of stirring, the solution turned clear and was transferred into a Teflon-lined stainless steel autoclave with a capacity of about 50 mL. The autoclave was sealed, heated, and maintained at 180 °C for 24 h. Then, it was allowed to cool down to room temperature naturally. The resulting bottom precipitates were alternately washed with toluene and ethanol several times, then dispersed in hexane followed by centrifugation to remove undissolved precipitate. 2.1.5. Synthesis of Bi2S3/CdS Nanoscale Heterostructure via Two Steps Reaction. The as-prepared Bi2S3 nanowires were redispersed in 35 mL of ethylene glycol by ultrasonication and then stirred in a 50 mL Teflon-lined stainless steel autoclave. Then, 0.5 mmol thiourea, 0.5 mmol Cd(NO3)2 3 4H2O and 3.5 mmol polyvinylpyrrolidone (PVP) were added into the mixture. The mixture was stirred for 30 min at room temperature. The whole mixture was sealed and maintained at 140 °C for 8 h. The final black solid product was centrifuged, washed with acetone and distilled water for several times, and finally dried in vacuum at 60 °C for 8 h. 2.1.6. Photocatalytic Properties Study. The photocatalytic activity of the as-prepared Bi2S3/CdS heterostructures was evaluated by degrading methyl red (MR) aqueous solution under UV irradiation. Before the reaction, 10 mg photocatalyst was added into 50 mL MR solution (0.01 g/L), and stirred in the dark for 6 h. Then the system was irradiated for 80 min with a 300 W UV lamp with wavelengths at 365 nm. Every 10 min, 5 mL solution
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was taken out of the system and centrifuged at 9 000 rpm for 3 min and the concentration of MR in the supernatants was determined with UVvis absorption spectroscopy. The photocatalytic activities of Bi2S3 nanowires (average width, 10 nm), CdS nanoparticles (average diameter, 20 nm), their equimolar mixture, and commercial TiO2 (P25) powder were also investigated according to the same procedure described above. 2.2. Characterization. The resulting products were characterized with X-ray powder diffraction (XRD) using a Shimadzu X-ray diffractometer (XRD-6000) with Cu KR irradiation (λ = 1.54187 Å). The morphologies of the as-prepared samples were examined by field-emission scanning electron microscopy (FESEM, Hitachi S-4800 at an accelerating voltage of 5 kV). High-resolution transmission electron microscopy (HRTEM) images, selected area electron diffraction (SAED), and energy-dispersive spectrometer (EDS) of the samples were obtained from a JEOL-2011 transmission electron microscope working at an accelerating voltage of 200 kV. Before HRTEM observation, the sample was first dispersed in alcohol through an ultrasonication process, and then a drop of the dispersion was placed on a copper grid coated with carbon film, which was dried naturally at room temperature. X-ray photoelectron spectra (XPS) were recorded on an ESCALab MKII X-ray photoelectron spectrometer, using Al KR radiation as the exciting source. UV spectra of the supernatants after centrifugation and diffuse reflection spectra of the solid powder products were collected on a Varian Cary-5000 UVvis-NIR spectrophotometer at room temperature. Photoluminescence (PL) spectra of the as-obtained samples were measured on a FLUOROLOG-3-TAU fluorescence spectrophotometer using 325 nm excitation at room temperature and 950 nm excitation at liquid nitrogen temperature to prevent environmental perturbance. Specific surface areas and pore-size distributions of various Bi2S3/ CdS heterostructures were analyzed on an Autosorb-6B (Quanta Chrome) instrument at 196 °C. Prior to each measurement, the sample was degassed at 200 °C for 12 h under high vacuum (
