23August2019

Nano-Micro Letters

Synthesis and photocatalytic activity of one-dimensional CdS@TiO2 core-shell heterostructures

Hongwei Wei1, Le Wang2, Zhipeng Li1, Shouqing Ni1, Quanqin Zhao1,*

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Nano-Micro Letters, , Volume 3, Issue 1, pp 6-11

Publication Date (Web): March 31, 2011 (Article)

DOI:10.3786/nml.v3i1.p6-11

*Corresponding author. E-mail:zhaoqq@sdu.edu.cn

 

Abstract

 


Figure 1 XRD patterns of a) neat CdS nanowires and of b) nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure. The stick spectral represent the standard reflections of anatase TiO2 and hexagonal CdS.

One-dimensional CdS@4TiO2 core-shell heterostructures were fabricated via the hydrolysis of tetrabutyl titanate (TBT) on the preformed CdS nanowires. The as-prepared products were characterized by X-ray diffraction, transmission electron microscopy, selected area electron diffraction and diffuse reflectance spectroscopy techniques. Results demonstrated that the hydrolysis of TBT had a great influence on the morphology of the coated TiO2 shell, resulting in the formation of TiO2 nanoparticles and nanolayer-modified CdS@4TiO2 heterostructures. Degradation of methylene blue using CdS@4TiO2 core-shell heterostructures as photocatalysts under visible light irradiation was investigated. Comparative photocatalytic tests showed that the TiO2 nanoparticles-modified heterostructure exhibited a superior activity due to the passivity of photogenerated charge carriers.


 

Keywords

One-dimensional CdS@TiO2 heterostructures; Photocatalysis; Methylene blue; Morphology; Mechanism

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Introduction

 

Photocatalysis using semiconductor has aroused growing interests since photoinduced splitting of water on TiO2 electrode was reported in 1972 [1]. For semiconductor, some disadvantages such as limited visible-light absorption, incomplete charge separation and little internal surface area remain to be pivotal problems that hinder practical applications of this high efficient, “green” technique. The general strategy to solve the above-mentioned problems have involved the ion doping [2-6], coupling of two semiconductors [7-12], metal deposition [13], or surface chemical modification etc. [14-15]. One-dimensional (1D) nanosized heterostructures, such as ZnO/ZnS [9], CdS/MoS2 [10], ZnO/CdSe [11] heterostructures, are considered as the excellent candidates because of their high surface areas, modulated band structures and promoted separation of photogenerated charge carriers.

With a relative narrow band gap of about 2.4 eV at room temperature, wurtzite CdS is one of the first discovered visible-light driven semiconductors which has promising applications in photochemical catalysis, gas sensor, detector, solar cell, nonlinear optical material, luminescence device, and optoelectronic device [16-18]. On account of this, 1D CdS nanocrystal was obtained through various routes [19-21]. TiO2 is the most widely investigated photocatalytic material owning to its favorable chemical property, high stability and low cost. With proper band structures, CdS/TiO2 nanocomposite exhibits good property in photocatalysis [22]. Therefore, it is significant to prepare CdS/TiO2 heterostructures and explore their effect of morphology and structure on the photocatalytic activity. Up to now, CdS sensitized TiO2@4CdS core-shell nanoparticles [23], nanotubes [24-26], nanowires [27-28], and CdS-embeded TiO2 heterostructures [29-31] have been successfully fabricated.

Herein, we reported the fabrication of 1D CdS@4TiO2 core-shellnanostructures via the hydrolysis of tetrabutyl titanate (TBT) on the preformed CdS nanowires. By controlling the hydrolysis of TBT on the surface of CdS nanowires, TiO2 nanoparticles and nanolayer-coated CdS@4TiO2 heterostructures were obtained. Degradation of methylene blue (MB) using CdS@4TiO2 core-shell heterostructures as photocatalysts under visible light irradiation was investigated. Photocatalytic tests demonstrated that the CdS@4TiO2 heterostructures displayed a strong correlation between their morphology and the photocatalytic activity. The TiO2 nanoparticles-modified CdS@4TiO2 core-shell heterostructures exhibited enhanced photocatalytic activity due to the promoted separation of charge carriers.

 

Experimental

 

All reagents in this study were of analytical grade and used without further purification. TiO2 nanoparticles and nanolayer-modified 1D CdS@4TiO2 core-shell nanostructures were fabricated via a two-step process as follows. The preparation of CdS nanowires was achieved as described in literature [32]. In a typical process, the as-prepared CdS nanowires (0.2 mmol) was ultrasonically dispersed in 60 mL absolute ethanol containing 5 mL TBT. Ethanol-water (20 mL, volume ratio = 6:1) solution was added dropwise to the above CdS-TBT suspension for TBT hydrolysis, and the dropping rate was controlled at 0.5 mL/min. After being stirred at room temperature for 1 h, TiO2 nanoparticles-modified CdS@4TiO2 core-shell heterostructure was collected and thoroughly washed with deionizeded water and absolute ethanol and dried in vacuum for 10 h at 70°C. When the dropping rate adjusted to 2 mL/min and the added ethanol-water solution increased to 50 mL, TiO2 nanolayer-modified 1D core-shell heterostructure was obtained. The CdS@4TiO2 core-shell products were calcined at 500 °C for 3 h in air atmosphere to eliminate the residual organisms, and cooled to room temperature naturally.

The X-ray diffraction (XRD) analysis was operated on an X-ray diffractometer (Bruker D8) with Cu-Kα radiation (λ=1.5418Å) at the scanning rate of 2°/min for 2θ ranging from 10 to 80º, and the tube voltage and electric current were 40 kV and 20 mA respectively. Transmission electron microscopic (TEM) images were performed with a JEM-100CXII machine at an accelerating voltage of 80 kV. High-resolution TEM images and selected area electron diffraction (SAED) patterns were collected on a JEOl-2000 machine at an accelerating voltage of 200 kV. UV-visible diffuse reflectance spectroscopy was measured on a TU-1901 UV-visible spectrophotometer.

Photocatalytic activity of nanoparticles and nanolayer-modified 1D CdS@4TiO2 core-shell heterostructures was evaluated by the degradation of methylene blue (MB) under visible light irradiation. This study was carried out in an XPA-Photochemical Reactor (XuJiang Electromechanical Plant, Nanjing, China), which contains several jacketed quartz tubes and a 500 W Xe lamp with optical cutoff filters to remove the light at wavelength below 420 nm. For each experiment, CdS@4TiO2 heterostructures (50 mg) were dispersed in 50 mL MB aqueous solution (15 mg/L). The resulting suspensions were stirred for 30 min in the dark to reach the adsorption-desorption equilibrium between MB solution and the solid catalysts. Some solution was taken out from the reaction system before illumination or at 60 min interval during illumination. The obtained solution was centrifuged to collect the supernatant for concentration test.

 

Results and Discussions

 

It was found that the dropping rate and volume of ethanol-water solution played an important role in the formation of 1D CdS@4TiO2 core-shell heterostructures. When the ethanol-water solution (20 mL) was added to the CdS-TBT suspension at the rate of 0.5 mL/min, rough product coated with TiO2 nanoparticles (ca. 3 nm in diameter) was fabricated. When the rate adjusted to 2 mL/min (50 mL), the coated TiO2 shell was composed of a dense TiO2 layer with a thickness of ca. 30 nm. Rapid addition of ethanol-water solution favored the hydrolysis of TBT to a certain extent and accelerated the growth of TiO2 nanocrystals, finally resulting in the formation of TiO2 nanoparticles and nanolayer coated 1D CdS@4TiO2 core-shell heterostructures. Therefore, the dropping rate and volume of ethanol-water solution is a key factor in controlling the morphology and microstructure of 1D CdS@4TiO2 heterostructures in this study.

 

Figure 1 XRD patterns of a) neat CdS nanowires and of b) nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure. The stick spectral represent the standard reflections of anatase TiO2 and hexagonal CdS.

Figure 1 XRD patterns of a) neat CdS nanowires and of b) nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure. The stick spectral represent the standard reflections of anatase TiO2 and hexagonal CdS.

 

The phase and purity of the as-prepared products were examined by XRD technique. Fig. 1 gives XRD patterns of the neat CdS nanowires and nanoparticles-modified 1D CdS@4TiO2 core-shell heterostructure, along with the standard cards of anatase TiO2 (JCPDS No. 21-1272) and hexagonal CdS (JCPDS No. 41-1049) for comparisons. As shown in Fig. 1a, all the diffraction peaks could be perfectly indexed to the hexagonal phase of CdS, with lattice constants of a = 0.414 nm and c = 0.672 nm. No characteristic peaks ascribing to other phase were observed, indicating high purity of the prepared CdS nanowires. The (002) peak of the CdS nanowires is weak, while the (100) and (110) peaks are strong compared with the standard card. The reason is that the CdS nanowires trend to grow along [001] direction preferentially, resulting in the fewer exposed (002) crystal planes during the measurement process [33]. The additional diffraction peaks at 2θ of 25.3, 37.8, 48.0, 55.1, and 62.7° in Fig. 1b matched well with tetragonal anatase TiO2, with lattice constants of a = 0.378 nm and c = 0.951 nm. Fig. 1 demonstrated that well-crystallined CdS phase and CdS@4TiO2 nanocomposite were produced in our study.

 

Figure 2 a) TEM and b) high-resolution TEM images of nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure.

Figure 2 a) TEM and b) high-resolution TEM images of nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure.

 

Further structural analysis of nanoparticles-modified 1D CdS@4TiO2 core-shell heterostructure was carried out by TEM technique. As can be seen in the TEM image (Fig. 2a), the product was several tens of micrometers in length and 20-50 nm in width. The high-resolution TEM image (Fig. 2b) of this material reveals that the heterostructure’s surface was rough and the shell was composed of TiO2 nanoparticles with a diameter of ca. 3 nm. Well-resolved lattice fringes were visible across the entire region, with the interplanar distances of 0.243 nm and 0.335 nm according with (103) d-spacing of anatase TiO2 and (002) d-spacing of wurtzite CdS respectively. From Fig. 2b, the CdS nanowires grow along [001] orientation preferentially, which was in good agreement with the XRD results.

 

Figure 3 TEM images of a) nanolayer-modified 1D CdS@TiO2 core-shell heterostructure and d) TiO2 nanotubes. High-resolution TEM images of b) TiO2 shell and c) CdS core of the rectangular region in a). e) SAED pattern of the heterostructure.

Figure 3 TEM images of a) nanolayer-modified 1D CdS@TiO2 core-shell heterostructure and d) TiO2 nanotubes. High-resolution TEM images of b) TiO2 shell and c) CdS core of the rectangular region in a). e) SAED pattern of the heterostructure.

 

Fig. 3 showed high-resolution TEM images and SAED pattern of the nanolayer-modified 1D CdS@4TiO2 core-shell heterostructure. As shown in the TEM image (Fig. 3a), the heterostructure’ s surface was smooth and the coated TiO2 layer was ca.30 nm in thick. Additionally, the product assembled to wiring harness due to the conglutination of the thick TiO2 shell. High-resolution TEM images given in Fig. 3b and 3c corresponded to marginal and inner part of the square part in Fig. 3a respectively. Well-resolved lattice fringes can be clearly observed in both regions, with the measured d-spacings of 0.351 nm and 0.357 nm ascribed to (101) crystal planes of anatase TiO2 and (100) crystal planes of hexagonal CdS phase. SAED pattern shown in Fig. 3e was mainly composed of two sets of diffraction dots. The dashed parallelogram was indexed as that of hexagonal CdS along the [010] zone axis and diffraction rings as that of TiO2 shell with polycrystalline nature. TiO2 nanotubes (Fig. 3d) can be obtained as expected when the core-shell heterostructure was dealt with 5 M HCl solution.

To confirm the thermal stability of CdS@4TiO2 core-shell structures, we calcined the pure CdS nanowires at 500oC for 3 h in air atmosphere. It was observed that the yellow CdS nanowires turned to be white. Fig. 4 showed XRD pattern of the white product, which was indexed as orthorhombic CdO2SO4 (JCPDS No. 32-0140). Thermal stability of pure CdS nanowires was enhanced due to the coated TiO2 shell.

 

Figure 4 XRD pattern of pure CdS nanowires annealed at 500 °C for 3 h in air atmosphere.

Figure 4 XRD pattern of pure CdS nanowires annealed at 500 °C for 3 h in air atmosphere.

 

Diffuse reflectance spectroscopy was used to measure optical property of nanoparticles-modified 1D CdS@4TiO2 core-shell nanostructure, together with the neat CdS nanowires for comparison. As seen from the normalized UV-visible absorption spectral (Fig. 5), both the samples had a steep absorption in visible light region, ascribing to the intrinsic absorption of CdS semiconductor. Band gap of 2.38 eV is given for pure CdS phase by the absroption edge (520 nm), which was comparable to the standard bulk CdS (2.4 eV). The heterostructure had strong absorption in the UV light region resulting from the absorption of TiO2 (Eg = 3.2 eV) shell.

 

Figure 5 UV-vis absorption spectral of (a) neat CdS nanowires, and (b) nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure.

Figure 5 UV-vis absorption spectral of (a) neat CdS nanowires, and (b) nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure.

 

To investigate photocatalytic activity of 1D CdS@4TiO2 core-shell heterostructure under visible light irradiation (λ>420 nm), MB aqueous solution was used as probe contamination [34-35]. Degradation of MB solution was evaluated by variety of the color and absorbance at the characteristic wavelength of 664 nm in optical absorption spectrum. The degradation rate is calculated based on the following equation: MB degradation rate (%) = (A0 - At) / A0 * 100%, where A0 and At are the initial MB absorbance and residual MB absorbance at t0 and t time respectively. It is noted that the sample before illumination was treated as the starting point. In the presence of nanoparticles-modified 1D CdS@4TiO2 core-shell heterostructure, MB aqueous solution gradually faded under visible-light irradiation as time of illumination increased. The characteristic absorbance decreased from 1.685 to 0.634 as depicted in the absorption spectrum (Fig. 6a). The photodegradation of MB over nanolayer-modified 1D CdS@4TiO2 core-shell heterostructure, and neat CdS nanowires were investigated under the same conditions, and the degradation of MB aqueous solution without any photocatalyst was also tested for comparison. As shown in Fig. 6b, about 63% of the MB was degraded using nanoparticles-modified 1D CdS@4TiO2 heterostructure (curve 1) as catalyst, and the degradation efficiencies were 30%, 28% and 25% under neat CdS nanowires (curve 2), nanolayer-modified 1D CdS@4TiO2 core-shell heterostructure (curve 3) as catalysts, and without any catalyst (curve 4). About 40% of MB was dagraded over TiO2 photocatalyst after 3 h visible-light irradiation as reported in the article [32]. Clearly, nanoparticles-modified 1D CdS@4TiO2 core-shellheterostructure exhibited greater photocatalytic activity than the neat CdS nanowires and nanolayer-modified 1D CdS@4TiO2 heterostructure. Based on these results, we propose and discuss a possible mechanism as follows.

 

Figure 6 a) Absorption spectrum changes of MB aqueous solution (15 mg/L) degraded by nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure with irradiation time: 0, 1, 2, and 3 h, b). Visible-light photodegradation of MB under different conditions. Curves: (1) over nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure, (2) over neat CdS nanowires, (3) over nanolayer-modified 1D CdS@TiO2 core-shell heterostructure and (4) no catalyst.

Figure 6 a) Absorption spectrum changes of MB aqueous solution (15 mg/L) degraded by nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure with irradiation time: 0, 1, 2, and 3 h, b). Visible-light photodegradation of MB under different conditions. Curves: (1) over nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure, (2) over neat CdS nanowires, (3) over nanolayer-modified 1D CdS@TiO2 core-shell heterostructure and (4) no catalyst.

 

Degradation Mechanism

 

The photocatalytic degradation attributes to the chemical reactions of active groups with contaminant molecules located on photocatalysts’ surface. The mechanism mainly involves charge carriers’ generation, charge carriers’ transfer, and chemical reaction processes. Once the semiconductor is excited by photons, charge carriers, including electron (e-) and positive hole (h+), are generated. The e- andh+ may move to the surface and react with the adsorbents in the desired process, producing active groups (such as O2-, OH·), or they undergo the undesired recombination [36]. A rate increase in the former process or a rate decrease in the latter process will lead to higher photocatalytic efficiency. Holding high oxidation capacity, the active group OH· and h+ would decompose MB molecules. As well known, photogenerated charge carriers are produced in CdS phase only when CdS@4TiO2 heterostructure is irradiated by visible light. To react with the adsorbents, the charge carriers should pass the TiO2 shell and migrate to the surface. The greater activity of nanoparticles-modified 1D CdS@4TiO2 core-shell heterostructure ascribed to the special electronic states on the interface [37], which allows for a semiconductor-semiconductor junction formation. With more negative conduction band potential and less positive valence band potential of CdS nanocrystal, photogenerated electrons prefer to transfer from CdS to TiO2 phase while the holes remain on the CdS. This shift favors the separation of photogenerated charge carriers [38-39] and the enhancement of CdS@4TiO2 heterostructures’ photocatalytic ativity further. The mechanism diagram is shown in Figure 7. For the nanolayer-modified 1D CdS@4TiO2 heterostructure, the thick TiO2 shell hinders the photo absorption of CdS phase or the shift of photogenerated charge carriers, which results in a reduction in active groups and weak photocatalytic activity finally.

 

Figure 7 Diagram of photogenerated charge carriers transmission on nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure.

Figure 7 Diagram of photogenerated charge carriers transmission on nanoparticles-modified 1D CdS@TiO2 core-shell heterostructure.

 

Conclusion

 

In summary, nanoparticles and nanolayer-modified 1D CdS@4TiO2 core-shell heterostructures were prepared successfully via the hydrolysis of TBT on the preformed CdS nanowires. By controlling the dropping rate and volumeof ethanol-water solution, the anchored TiO2 shell was composed of nanoparticles with a diameter of ca. 3 nm or a nanolayer of ca. 30 nm respectively. The optical property and photocatalytic activity of 1D CdS@4TiO2 core-shell heterostructures were separately investigated. Compared with neat CdS nanowires, the nanoparticles-modified 1D CdS@4TiO2 core-shell heterostructure exhibited enhanced photocatalytic activity, attributing to the promoted separation of charge carriers.However,the coated TiO2 nanolayer lowered photocatalytic activity of neat CdS nanowires.

 

Acknowledgement

 

We express our thanks to financial support Independent Innovation Foundation of Shandong University (IIFSDU-2009JQ011).

 

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References

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[5] X. Du, J.H. He and Y. Q. Zhao, J. Phys. Chem. C 113, 14151 (2009). http://dx.doi.org/10.1021/jp9056175

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Citation Information

Hongwei Wei, Le Wang, Zhipeng Li, Shouqing Ni and Quanqin Zhao, Synthesis and Photocatalytic Activity of One-Dimensional CdS@TiO>2 Core-Shell Heterostructures. Nano-Micro Lett. 3 (1), 6-11 (2011). http://dx.doi.org/10.3786/nml.v3i1.p6-11

History

Received 15 Dec 2010; accepted 08 Mar 2011; published online 31 March 2011.

 


Additional Info

  • Type of Publishing: JOUR - Journal
  • Title:

    Synthesis and photocatalytic activity of one-dimensional CdS@TiO2 core-shell heterostructures

  • Author: Hongwei Wei, Le Wang, Zhipeng Li, Shouqing Ni, Quanqin Zhao
  • Year: 2011
  • Volume: 3
  • Issue: 1
  • Journal Name: Nano-Micro Letters
  • Publisher: OPEN ACCESS HOUSE SCIENCE & TECHNOLOGY
  • ISSN: 2150-5551
  • URL: http://dx.doi.org/10.5101/nml.v3i1.p6-11
  • Abstract:

        One-dimensionalCdS@TiO2 core-shell heterostructures were fabricated via the hydrolysis of tetrabutyl titanate (TBT) on the preformed CdS nanowires. The as-prepared products were characterized by X-ray diffraction, transmission electron microscopy, selected area electron diffraction and diffuse reflectance spectroscopy techniques. Results demonstrated that the hydrolysis of TBT had a great influence on the morphology of the coated @TiO2 heterostructures. Degradation of methylene blue using CdS@TiO2 core-shell heterostructures as photocatalysts under visible light irradiation was investigated. Comparative photocatalytic tests showed that the TiO2 nanoparticles-modified heterostructure exhibited a superior activity due to the passivity of photogenerated charge carriers.

     

  • Publish Date: Thursday, 31 March 2011
  • Start Page: 6
  • Endpage: 11
  • DOI: 10.5101/nml.v3i1.p6-11