RESEARCH ARTICLE

 

Synthesis and photocatalytic activity of Zn_xCd_1-xS/TiO₂ heterostructures nanofibre prepared by combining electrospinning and hydrothermal method

 

 

Wei Changa^{I, II*}; Xiaosai Ren^I; Guorui Yang^II; Wei Yan^II*; Yanrong Guo^I

^ISchool of Environmental and Chemical Engineering, Xi'an Polytechnic University, Xi'an 710048, China
^IIDepartment of Environmental Science & Engineering, Xi'an Jiaotong University, Xi'an 710049, China

 

 

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ABSTRACT

The Zn_xCd_1-xS/TiO₂ hierarchical heterostructures were successfully fabricated by combining electrospinning technology and hydrothermal methods. The morphology, crystallinity, composition and band gap of Zn_xCd_1-xS/TiO₂ were characterized by SEM, XRD, EDS, and UV-vis diffuse reflectance spectroscopy. The results indicated the sulphur concentration had a significant influence on the composition and morphology of the products. The values of the band gap energy for Zn_xCd_1-xS/TiO₂ varied from 2.33 to 2.66 eV with the change of sulphur concentration. When the molar ratio of S/Ti was 0.96, the Zn_xCd_1xS/TiO₂ hierarchical heterostructures exhibited enhanced visible light photocatalytic behaviour for the decomposition of Rhodamine B (RhB) under visible light irradiation. The photocatalytic degradation of RhB followed first-order reaction kinetics.

Keywords: Electrospinning, heterostructures, photocatalysis, dye degradition

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1. Introduction

The wastewater generated by the textile dying and paper-printing industries are rated as one of the most polluting among all industrial sectors. Traditional techniques can generally be used efficiently for the removal of dyes. Nevertheless, they are non-destructive since they just transfer organic compounds from water to a solid phase, which can cause secondary pollution. Photocatalytic degradation of organic contaminants has received more attention due to its high efficiency, nonselective degradation and low cost. TiO₂, which is one of the most widely studied metal oxides, has attracted significant attention in the application of the degradation of organic contaminants.^1,2 However, the relatively wide band gap (Eg = 3.0-3.2 eV) restricts the TiO₂ photo-response, which only falls into the ultraviolet region, and this only takes up 5 % of the total solar spectrum reaching the earth's surface. In order to improve the solar energy utilization of TiO₂, all kinds of modification methods have been reported, such as (1) metal ions and nonmetallic elements doping with Fe³⁺,W⁶⁺,S,orN;^3-6, (2) photosensitization with organic dyes or mixed dyes such as N3⁷, or N719;⁸ (3) use of semiconductor composites such as CdS,⁹ SnO,¹⁰ PbS¹¹. Recently, many researchers have focused on the photocatalytic property of Zn_xCd_1-xS semiconductor composite.^12-15 By adjusting the components ratio (x value), the band gap of Zn_xCd_1-xS can be changed. The change renders Zn_xCd_1-xS to exhibit both high visible light photocatalytic behaviour and adjustable conduction band (CB) potential.

Electrospinning is a widely used technique, which utilizes electrical forces to produce continuous fibres with diameters down to a few nanometres. With smaller pores, higher surface area and stronger adsorption properties than regular fibres, electrospun fibres have been successfully applied in various fields, such as nanocatalysis, protective clothing and filtration^16,17 In the field of photocatalysis, the nanofibres cannot only overcome the drawback of inconvenient recycling, but also keep the higher specific surface area than film-type photocatalysts. Meanwhile, nanofibres with complex architecture can be produced by combing electrospinning and other methods.¹⁸ Therefore, loading Zn_xCd_1-xSonTiO₂ nanofibres will produce heterostructured nanofibres, and thus the photo-generated electron is supposed to gain greater migration power with the increase of the CB potential. As a consequence, the recombination of photogenerated electron (e^-)/hole (h+) pairs can be greatly suppressed, and the photocatalytic activity will be enhanced.

In the present work, a novel Zn_xCd_1-xS/TiO₂ hierarchical heterostructure was fabricated by combining the electro-spinning technology and hydrothermal method. The effect of sulphur concentration on the characteristic and photocatalytic activity of Zn_xCd_1-xS/TiO₂ were also investigated. Rhodamine B, an important representative dye, was used as a probe contaminant to evaluate the activity of Zn_xCd_1-xS/TiO₂ photocatalyst under visible light irradiation.

 

2. Materials and Methods

2.1. Chemicals and Materials

Poly(vinyl pyrrolidone) (PVP Mw »1.3x10⁶) was obtained from BASF chemical company in Germany. Zinc acetate (Zn(CH₃COO)₂-2H₂O), cadmium acetate (Cd(CH₃COO)₂-2H₂O),tetrabutyl titanate (Ti(OC₄H₉)₄), acetic acid (CH₃COOH),thioacetamide (TAA) and methyl alcohol (CH₃OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Millipore water (R>18M Ω cm) was used in all experiments. All reagents were used as received.

2.2. Preparation of Cd_1-xZn_xS/TiO₂ Heterostructures

2.2.1. TiO₂ Nanofibres

Firstly, TiO₂ precursor solution was prepared by dissolving 2.125 g of Ti(OC₄H₉)₄ in an acetic acid solution of methyl alcohol (37:3 mass ratio) coupled with vigorous stirring for 30 min. Then 1.5 g PVP powder was added to the precursor solution, and the mixture was continuously stirred for another5htomake a homogeneous precursor solution. 5 mL of the precursor solution was loaded into a 20 mL syringe with a stainless steel needle which was connected to a high-voltage supply (BGG Bmei Co., Ltd.). A sheet of aluminium foil was used as the collector. The distance between the needle tip and the collector was fixed at 12 cm, and the voltage was set at 12 kV. The solution was pumped continuously using a syringe pump (LSP01-1A, Baoding Longer Precision Pump Co., Ltd.) at a rate of 1.0 mL h^-1. The obtained composite nanofibres were calcined at 500 °C for 3 h to induce the formation of crystallized TiO₂ nanofibres.

2.2.2. Zn_xCd_1-xS/TiO₂ Heterostructures

Typically, 0.5 mmol Cd(CH₃COO)₂-2H₂O, 0.75 mmol Zn(CH₃COO)₂-2H₂O and calculated amount of TAA (which was varied according to the molar ratio with TiO₂ as 0.5, 1, 2, 3, 4 ) were dissolved in 50 mL of deionized water, followed by stirring for 5 min, then 0.1 g TiO₂ nanofibres were added into the homogeneous solution. The resulting mixture was transferred into a 100 mL teflon-lined stainless steel autoclave. Then the autoclave was kept at 200 °C for 24 h. The autoclave was cooled down to room temperature after the reaction, and the obtained composite samples were washed thoroughly with deionized water and ethanol for several times, and then finally dried in a vacuum oven at 100 °C for 12 h. The obtained samples were denoted as S0.5, S1, S2, S3, S4, respectively.

2.3. Characterization

The morphology of all the samples was observed by SEM performed on a JEOL JSM 6700F field emission instrument (Japan Electronics Co. Ltd) operated at an accelerating voltage of 5.0 kV. EDS was performed on a JEOL JSM 6460 scanning electron microscope(Japan Electronics Co., Ltd). XRD patterns of the samples were collected on X'pertMPD Pro (PANalytical Co. Ltd) diffractometer using Cu Ka radiation (40 kV, 40 mA). UV-vis diffused spectroscopy of the samples were recorded using a UV4100 spectrometer (Hitachi) operating between 800 nm to 240 nm wavelength, using BaSO₄ as a reference.

2.4. Photocatalytic Tests

Photocatalytic performance of the samples were studied by degrading RhB simulated wastewater under visible light (l > 420 nm) irradiation. The photocatalytic experiments were conducted in a homemade Pyrex photochemical reactor with a quartz jacket and cooled off by circulating water. In the experiment, a 500 W xenon lamp with a cut-off filter (l > 420 nm) was employed as the visible light source; the distance between the xenon lamp and the front surface of the reactor was fixed at 8 cm. Prior to illumination, 50 mL of RhB aqueous solutions (10 mg L^-1) containing 0.05 g sample were magnetically stirred in the dark for 30 min to achieve adsorption/desorption equilibrium and a good dispersion. During irradiation, about 3.0 mL of the reaction solution was withdrawn from the reactor at an interval of 10 min and centrifuged to separate the photocatalyst. The supernatant solution was analyzed by an Agilent 8430 UV-visible spectro-photometer at 554 nm, which is the maximum absorption wavelength of RhB. The remaining RhB concentration (%) after various intervals of time could be estimated using the following equation:

% RhB concentration=C/C0 x 100 %

where C0 is initial concentrations of RhB aqueous solution (that is, after the dark adsorption equilibrium), while C is the concentration at different intervals of time.

 

3. Results and Discussion

3.1. X-Ray Diffraction (XRD) Patterns

The XRD pattern for the as-prepared samples is shown in Fig. 1. The diffraction peaks at about 25.5 °, 37.9 °, 48.2 °, 54.1 °, 55.0 °, 62.7°, 75.1 °were perfectlyindexed to the (101), (004), (200),(105), (211), (204), (215) crystal faces of anatase TiO₂ (JCPDS21-1272), respectively, and diffraction peaks at about 27.5 °, 36.2 °, 41.4 °, 54.4 °, 69.0°were perfectly indexed to the (110), (101), (111),(211), (301) crystal faces of rutile TiO₂ (JCPDS 21-1276),¹⁶respectively. The mixed crystal phase of TiO₂ results in an improved photocatalytic activity. The other diffraction peaks of S0.5, S1, S2, S3, S4 were in good agreement with the Zn_xCd_1-xS, showing that the Zn_xCd_1-xS/TiO₂ composite samples were successfully synthesized. The diffraction peak of 26.8 ° appears in the spectra of sample S0.5 but disappears in the spectra of S1, S2, S3 and S4. It is speculated that the Zn_xCd_1-xS diffraction peak of 26.8 ° has changed and overlapped with the rutile diffraction peak of 27.4 °. It was observed that peak height of rutile TiO₂ at 27.4 ° was significantly enhanced.

 

 

3.2 SEM and EDS of the As-prepared Hierarchical Nanofibre

Figure 2a shows an SEM image of the pure TiO₂ nanofibres, which were fabricated by electrospinning followed by calcination at 500 °C for 3 h. It was clearly seen that the TiO₂ nanofibres have a relatively smooth surface and uniform thickness without secondary structures. The average fibre diameter is about 120 nm. However, as observed in of Fig. 2b-f, the Zn_xCd_1-xS nanostructures grew on the surface of TiO₂ nanofibres after solvothermal reaction at 200 °C for 24 h. That makes the fibre diameter larger and the average fibre diameter was about 280 nm. Zn_xCd_1-xS nanoparticles grew on the surface of TiO₂ with uniform size and uniform distribution in S0.5 and S1 (Fig. 2b-c). With the increase of the TAA concentration, the size of the Zn_xCd_1-xS nanoparticles became different. From Fig. 2f, it was observed that the Zn_xCd_1-xS nanoparticles became smaller, and grew on the TiO₂ nanofibres sparsely. This maybe due to the fact that excessive concentration of TAA made it release too much S^2-. Thus S^2- will react with Zn²+ and Cd²+ in solution firstly to form Zn_xCd_1-xS nanoparticles, so that the concentration of Zn²+ and Cd²+ in the solution system decreased, eventually reducing the adhesion amount of Zn_xCd_1xS nanoparticles on the TiO₂ nanofibres.

Meanwhile, the components of each of the prepared samples were studies with X-ray energy dispersive spectroscopy (EDS). As shown in Fig. 2, pure TiO₂ nanofibres are only composed of two elements which are Ti and O. The other five samples are composed of five elements which are Ti, O, Cd, Zn and S. There is no other impurity element observed.

3.3. UV-vis Diffuse Reflectance Spectra

Figure 3 shows UV-vis diffuse reflectance spectra of the as-prepared TiO₂ nanofibres and Zn_xCd_1x/TiO₂ hierarchical heterostructures. The spectrum of the pure TiO₂ nanofibres exhibited the typical absorption behaviour of a wide-band-gap oxide semiconductor, having an intense absorption band with a steep edge at about 400 nm. S0.5, S1, S2, S3 and S4 had new absorption bands between 500 nm and 600 nm. This indicated that the spectral response range of the TiO₂ composite extended to the visible region due to the Zn_xCd_1-xS nanoparticles. For the Zn_xCd_1-xS/TiO₂ heterostructures, the absorption edge of sample S1 was about 530 nm. Compared with S1, the absorption edge of samples S0.5, S2, S3 and S4 all showed a significant blue-shift. For a crystalline semiconductor, the band gap energy can be calculated by the Kubelka-Munk equation:¹⁹

 

 

 

where α= (1-R)²/2R,R = 10^-A, A is an optical absorption, h is Planck constant, u is photon frequency, K is constant. The results are shown in Table 1. The band gap energy of the pure TiO₂ was about 3.14 eV and the band gap of ZnxCdi-xS/TiO2 heterostructures ranges from 2.33-2.60 eV. S1 has a minimum band gap energy of 2.33 eV

3.4 Photocatalytic Activity

The temporal spectral changes of aqueous RhB solutions under visible light irradiation in the presence of the as-prepared samples are presented in Fig. 4. As expected, with 60 min irradation, all the samples of Zn_xCd_1-xS/TiO₂ heterostructures showed an improved photocatalytic activity in comparison to the pure TiO₂ nanofibres. This may be attributed to the cooperative roles of Zn_xCd_1-xS loading on TiO₂, i.e. a photosensitizing effect of Zn_xCd_1-xS. Zn_xCd_1-xS has a narrow band gap energy. This can be easily excited by visible light to induce the generation of photoelectrons and holes. In the case of TiO₂ nanofibre, it could not be excited by visible light irradiation with energy less than 2.95 eV due to its wide band gap energy of about 3.14 eV, as observed in this work. When Zn_xCd_1-xS/TiO₂ heterostructures were used as the photocatalysts, Zn_xCd_1-xS appears to act as a sensitizer to absorb the visible light. Under visible light irradition, electrons in the valence band of Zn_xCd_1-xS are excited to a higher potential edge. The CB edge potential of Zn_xCd_1-xSis more active than that of TiO₂, in such a way, the photoinduced electron hole pairs are effectively separated. Moreover, the formed junction between Zn_xCd_1-xS and TiO₂ in the hetero-structured photocatalysts will further prevent the recombination between photoelectrons and holes. These well-separated photoelectrons and holes will further contribute to the degradation of the dye molecules. Meanwhile, the generated conduction band electrons (e^-)ofTiO₂ probably reacted with dissolved oxygen molecules to yield superoxide radical anions (O₂^-), with protonation which generated the hydroperoxy radicals (HO₂^-), producing the hydroxyl radical OH, which is known to be a strong oxidizing agent,²⁰ to decompose the organic dye. Therefore, the Zn_xCd_1-xS/TiO₂ heterostructures exhibits better photo-catalytic properties than that of TiO₂ on the degradation of RhB under visible light irradiation. Besides, it can be seen from Table 1 that S1 has a larger loading of Zn_xCd_1-xS and a narrow band gap. The larger loading of Zn_xCd_1-xS appears to improve the optical absorption efficiency and the narrow band gap could increase the visible light absorption. Thus it will result in more stimulated electronic transfer to the conduction band and increase the carrier concentration and promote the photocatalytic degradation reaction. Therefore, S1 exhibits the best performance on the photodegradation of RhB among the five prepared samples. These results are presented in Table 1. After visible light irradiation of 60 min, 92.42 % of RhB was degraded with the S1 photo-catalyst, which is much higher than other prepared photocata-lysts.

 

 

The kinetic profiles for the degradation of RhB in aqueous solution under visible light were also investigated. Figure 5 depicts the In(C₀/C) of RhB versus irradiation time for the different catalysts. The approximate linear relationship of In(C₀/C) versus time indicates that the photodegradation process of RhB tend to follow Langmuir-Hinshelwood first-order kinetics model in the presence of the as-prepared catalysts. The kinetics can be expressed as follows: In(C₀/C) = kt, where k is the apparent reaction rate constant, C₀ is the concentration of RhB at adsorption equilibrium, and C is the residual concentration of RhB at different time intervals. The apparent reaction rate constant k of the different catalyst was identified in Fig. 5. The order of rate constants is summarized as follows: S1>S2>S3>S4>S0.5>TiO₂, which coincides well with the conclusion of the photocatalytic degradation curves presented in Fig. 4.

 

 

4. Conclusions

The Zn_xCd_1xS/TiO₂ hierarchical heterostructures with novel architectures were successfully fabricated by using electrospinning technology and hydrothermal processes. The results indicated that the composition and morphology of Zn_xCd_1-xS/TiO₂ are controlled by changing the sulphur concentration. The fastest photocatalytic decomposition rate of aqueous RhB solution was observed when S/Ti was 0.96, the photocatalytic degradation of RhB followed first-order reaction kinetics and the first-order constant was 0.04274 min^-1. Thus the Zn_xCd_1-xS/TiO₂ visible light system can be regarded as a promising candidate for the photodegradation of dyes from wastewaters.

 

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities of China under Grant 2011JDGZ15 and by the Research and Development Program of Beilin Distric, Xi'an under Grant JX1419.

 

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Received 22 October 2014
Revised 30 April 2014
Accepted 11 May 2015

 

 

* To whom correspondence should be addressed. E-mail: changwei72@163.com