Synthesis and Photocatalytic Performance of ZnIn2S4 Nanotubes and

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Synthesis and photocatalytic performance of ZnIn2S4 nanotubes and nanowires Liang Shi, Peiqun Yin, and Yumei Dai Langmuir, Just Accepted Manuscript • DOI: 10.1021/la402473k • Publication Date (Web): 23 Sep 2013 Downloaded from http://pubs.acs.org on September 28, 2013

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Synthesis and photocatalytic performance of ZnIn2S4 nanotubes and nanowires Liang Shi*, Peiqun Yin, Yumei Dai Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China Abstract

ZnIn2S4 nanotubes and nanowires have been selectively fabricated via a convenient one-step wet-chemical approach by using porous polycarbonate membrane as a hard template. The wall of nanotubes is as thin as 5 nm and the diameter of them is 200 nm. Formation mechanism of ZnIn2S4 nanotubes and nanowires is also discussed according to the experimental results. The structure, morphology, composition properties of the as-prepared samples were characterized using X-ray powder diffraction, UV–Vis spectrophotometer, transmission electron microscopy, energy dispersive X-ray spectrometry and scanning electron microscopy.

* To whom correspondence should be addressed. Phone: 86-551-3607234; Fax: 86-551-3607402; Email: [email protected]

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1. Introduction As an important II–III2–VI4 semiconductor, ZnIn2S4 has attracted increasing attention due to its excellent optical and electrical properties. 1-5 ZnIn2S4 has a layered structure with high chemical stability. The band gap of ZnIn2S4 is within the visible range, leaving it a good candidate for ecofriendly, visible-light-driven photocatalyst.6 ZnIn2S4 has also been found with great potential applications in different fields, such as thermoelectricity, photoconduction, charge storage.7-9 Synthesis and functional property studies of nanostructured semiconductors including ZnIn2S4 have been actively studied because the shape and size of these nanomaterials may exert a significant influence on their optoelectronic function and device performance, and sometimes induce much improved physical and chemical properties by comparison with their bulk counterparts. For example, in the case of photocatalytic process, the reaction usually occurs at the interface between the catalyst surfaces and organic pollutants. The surface atomic arrangements and coordinations are expected to be modified by the variation on shape and size of nanostructured photocatalyst, as a consequence, the corresponding catalytic performance is affected.

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Therefore, great

effort has been made to prepare novel ZnIn2S4 nanostructures and investigate their properties and potential applications. Up to now, various shape of ZnIn2S4 nanostructures, such as nanocrystals, nanosheet, and microsphere consisted of flakes, have been successfully fabricated by a variety of chemical or physical approaches. 11-22

Most studies of ZnIn2S4 focused on its two dimensional morphology type because

ZnIn2S4 tends to grow into this type shape (e.g. nanosheets, nanoribbons) or related

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shapes (e.g. microsphere consisted of flakes) due to its layered crystal structure. The research on ZnIn2S4 one dimensional nanostructures has been lagging far behind. To the best of our knowledge, only two reports have been devoted to the preparation of ZnIn2S4 one dimensional nanostructures, such as nanotubes and nanoribbons, in which the organic solvents and surfactants were used.

23, 24

Up to date, the controlled

synthesis of ZnIn2S4 one dimensional nanostructures is still much less developed. Here we developed a facile template-aided solution approach for preparation of ZnIn2S4 one dimensional nanostructures without the using of organic solvents or surfactants. ZnIn2S4 nanotubes and nanowires can be selectively fabricated respectively by simply modifying reaction condition. Moreover, the as-prepared ZnIn2S4 nanotubes and nanowires were found to have high efficient photocatalytic activity to degrade organic pollutants in aqueous solutions.

2. Experimental section All reagents are analytical grade and used without further purification. Commercially available polycarbonate (PC) membranes (Millipore Co.) used in this study contained pore sizes of 200 nm in diameter. In a typical procedure, a precursor solution was made by introducing 0.17 g anhydrous InCl3 (Aldrich, 99.9%), 0.05 g ZnCl2 and 0.23 g thioacetamide (TAA) into 20 mL distilled water with stirring in a 25 mL flask, the solution pH was adjusted to be 2.5 by adding appropriate hydrochloric acid. Then, a PC template was added into the flask and immersed in the liquid. The above mixture in the flask was treated by sonication for 5 min to remove air in the

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pore of PC template and fill the pore with precursor solution. The flask was attached to a Schlenk line and purged of air by pulling vacuum for 10 min, followed by nitrogen bubbling for 10 min with mild magnetic stirring. The evacuation and N2 bubbling process was cycled for 3 times at room temperature. The sonication and multi-evacuation and bubbling process was important to enhance the filling efficiency of the PC template pores with precursor solution. As a result, a uniform and high density of nanotubes can be obtained. For fabrication of nanotubes, the above solution including the PC template in the flask was then heated at 95 °C for 12 h before being cooled down to room temperature. For the fabrication of nanowires, the 15ml above solution including the PC template was transferred into a 20 ml stainless steel teflon-lined autoclave. The autoclave was sealed and the temperature was maintained at 110 °C for 4 days before being cooled down to room temperature. The PC template containing the product was taken out, thoroughly washed with ethanol and distilled water, air-dried for characterization. The overall crystallinity of the product is examined by X-ray diffraction (XRD, Rigakau RU-300 with CuΚα radiation). The UV–Vis spectrum of the product was recorded in a UV–Vis spectrophotometer (UV-1601PC, Shimadzu Corporation). The general morphology of the products was characterized using scanning electron microscopy (FESEM QF400). Detailed microstructure analysis was carried using transmission electron microscopy (TEM Tecnai 20ST). The chemical composition analysis was obtained by energy dispersive X-ray spectrometry (EDX) using an EDX spectrometer attached to the same microscope. For the SEM, TEM and HRTEM

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measurements, the PC template was completely removed by immersion in methylene chloride for 10 min. Isolated nanoscale products were then centrifuged, washed with ethanol and distilled water, and finally air-dried for 24 h. For photocatalytic experiment, Methyl orange (MO) was used as the organic contaminant in the waste water and the as-prepared ZnIn2S4 nanotubes or nanowires were used as the catalyst. A TU-1901 UV-Vis spectrometer was used to monitor the MO remaining in the solutions. Briefly, a 50 mg ZnIn2S4 nanotubes or nanowires was suspended in a 100 mL aqueous solution containing 20 ppm MO. Prior to UV light treatment, the suspensions including MO and ZnIn2S4 product were stirred magnetically in a dark condition for 30 min to establish adsorption/desorption equilibrium, and then exposed to visible light irradiation with a 500 W Xe lamp and a 420 nm cutoff filter.

3. Results and Discussion

Figure 1a shows a typical SEM image of the ZnIn2S4 product with PC template completely removed. A large amount of nanotubes with several micrometers length can be seen. The size distribution of the as prepared nanotubes is uniform and the average diameter of the nanowires is about 200 nm, which is consistent with the pore size dimension of the PC template. The open ends of some nanotubes can be observed, shown by arrows, disclosing their hollow tube structure feature. The product was characterized by X-ray diffraction (XRD) to obtain information on crystal structure 5


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and phase composition. A typical XRD pattern of the product is shown in Figure 1(b), which can be indexed to be hexagonal phase ZnIn2S4. The refined lattice constants are a = 3.84 Å and c =24.66 Å, in accord with the reported value for ZnIn2S4 crystal (JCPDS card, No. 652303). A phase-pure ZnIn2S4 is determined and no signals of impurities or side products were found.

Figure 1

A SEM image (a) and a representative XRD pattern (b) of the as-prepared

ZnIn2S4 nanotubes.

The microstructure and chemical composition of the ZnIn2S4 nanotubes are investigated with TEM accompanied by selected area electron diffraction (SAED) and energy dispersive X-ray spectrometry (EDX). Figure 2a shows a TEM image of a typical individual straight ZnIn2S4 nanotube, in which the strong contrast between the dark edges and the bright center gives evidence for its hollow feature. The diameter of these nanotubes is about 200 nm in accordance with the SEM observation. The TEM

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image also shows that the wall of the nanotubes is very thin. A high resolution TEM (HRTEM) image of the ZnIn2S4 nanotubes in Figure 2b reveals clearly the wall of nanotube is as thin as 5 nm. A clear lattice spacing of 0.41 nm is obtained from the line profile in the right upper inset of Figure 2b, which corresponds well to the d spacing of the (006) planes in hexagonal structured ZnIn2S4, confirming the well-crystallized ZnIn2S4 nanotubes. It can be seen clearly the (006) planes are parallel to the nanotube’s axis. A two-dimensional Fourier transform pattern of the lattice resolved image (as shown in the right lower inset of Figure 2b) reveals a bright spot line perpendicular to the lattice fringes in Figure 2b. The HRTEM results give evidence that the radial growth direction of ZnIn2S4 nanotube is [001]. The EDX spectrum (Figure 2c) taken from the ZnIn2S4 sample shows intense peaks of Zn, In and S, disclosing the chemical composition of Zn, In and S only. The Cu and carbon signals come from the supporting TEM grid. EDX quantitative analysis gives an average Zn/In/S composition ratio of 1:2:4, corresponding well with the stoichiometry of ZnIn2S4. The spatial distribution of the compositional elements within the nanotube is obtained using scanning transmission electron microscope (STEM)-EDX line scans along the nanotube’s radial direction (marked by the red arrow in Figure 2a). In the intensity profile of the compositional elements shown in Figure 2d, a higher intensity of Zn, In and S is found in the wall region. This is consistent with the hollow tube configuration observed in the TEM image.

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Figure 2

TEM (a), HRTEM (b) images and EDX spectrum(c) of the ZnIn2S4

nanotubes. (d) elemental profile extract from a STEM-EDX showing the distribution of the compositional elements (Zn, In and S) along the radial direction of the nanotube (indicated by the red arrow in Figure 2 (a); The right upper inset of Figure 2 (b) is a line profile from the area marked with the rectangular frame, the right lower inset of Figure 2 (b) shows a two-dimensional Fourier transform pattern of the lattice resolved image.

If the reaction was conducted in a sealed autoclave at 110 °C and heating time was extended to 4 days, the final ZnIn2S4 product became nanowires, instead of nanotubes. Figure 3a illustrates a SEM image of the as-prepared sample. A large area

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of uniform nanowires can be found in the sample. Figure 3b gives a typical XRD pattern of the product, which can be indexed to be pure phase hexagonal ZnIn2S4. No impurities were detected from the XRD analysis. The relative strength of the (110) peak is enhanced compared with that of bulk ZnIn2S4 (JCPDS card, No. 652303), suggesting the nanowires have a preferential growth of (110) planes. Further evidence about this can be found in the later TEM result.

Figure 3 A SEM image (a) and a representative XRD pattern (b) of the as-prepared ZnIn2S4 nanowires.

Figure 4a gives a TEM image, indicating that the as-prepared solid nanowires have a diameter of about 200 nm. A high HRTEM image in Figure 4b displays a clear lattice spacing of 0.33 nm corresponding to the d spacing of the (100) planes in hexagonal crystal structured ZnIn2S4, confirming the high crystallnity of the as prepared nanowires. The inset of Figure 4b shows a two-dimensional Fourier transform (FFT) pattern of the lattice resolved image, which can be indexed to the [00-1] zone of hexagonal structured ZnIn2S4. The HRTEM results showed that 9


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nanowires have an axial growth plane of (110), which is consistent with the XRD result. The EDX spectrum (Figure 4c) taken from the sample shows intense peaks of Zn, In and S, suggesting the chemical composition of Cu, In and S, only. EDX elemental maps (as shown in inset of Figure 4 c ) of Zn, In, S give information on the spatial distribution of the compositional elements. The uniform spatial distribution of different compositional elements is illustrated evidently in the elemental maps.

Figure 4

TEM (a), HRTEM (b) images and EDX spectrum (c) of the ZnIn2S4

nanowires. The inset of Figure 4 (b) is a two-dimensional Fourier transform pattern of the lattice resolved image, the inset of Figure 4 (c) shows a dark -field TEM image of part of a ZnIn2S4 nanowire and the corresponding EDX elemental maps of Zn, In, S.

Based on above experimental results, a possible formation mechanism of ZnIn2S4 nanotubes and nanowires could be proposed as follows. During the experimental process, PC templates were immersed in the reaction solution. The sonication and multi-cycles of evacuation and bubbling process ensure removing gas

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from the template pores and filling them with precursor solution by negative pressure and capillary action. Here we believe that TAA plays an important role in the formation of ZnIn2S4 nanostructure based on a solution coordination model, which has been proposed to explain the synthesis of ZnIn2S4. 7, 20, 23 In our present case, TAA acted as a chelating agent. Once InCl3, ZnCl2 and TAA were dissolved in the distilled water, Zn2+ and In3+ ions were released and coordinated with TAA, existing as Zn(TAA)n2+ and In(TAA)n3+ in the solution (equation 1 and 2). Zn2+ + nTAA ↔ Zn(TAA)n2+

(1)

In3+ + nTAA ↔ In(TAA)n3+

(2)

On the other hand, TAA was also a sulfur source in the reaction. With the increase of temperature to above 95 °C, TAA decomposed and released S2- ions gradually, according to the follow reactions: CH3CSNH2 + H2O ↔H2S + CH3CONH2

(3)

H2S ↔HS- + H+

(4)

HS- ↔ H+ + S2-

(5)

The newly generated S2- ions combined with Zn(TAA)n2+ and In(TAA)n3+ and produced thermodynamically stable hexagonal ZnIn2S4 phase, which subsequently undergoes nucleation and crystal growth process. The whole synthetic reaction can be expressed as follows: ZnCl2 + 2InCl3 + 4CH3CSNH2 + 4H2O → ZnIn2S4 + 4CH3CONH2 + 8HCl

(6)

As for template aided synthesis, the nucleation and crystal growth are all

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confined in the pores of template. In the nucleation stage, either homogeneous nucleation or heterogeneous nucleation is possible to occur.

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Heterogeneous

nucleation and crystal growth on pore walls will happen if the interfacial energy between crystal nuclei and pore wall is smaller than that between crystal nuclei and solution. In heterogeneous nucleation case, the product will be tube-like at initial stage of crystal growth. Then, if the reaction is continuously conducted, the tube wall will be gradually thickening since the crystal growth started from the inner surface of pore walls and extend to the center. As an extreme case, solid nanowires could be obtained due to completely filling of the PC pores. In our present ZnIn2S4 case, nanotubes have been formed. Therefore, the heterogeneous nucleation on PC template pore walls should happen at initial stage and subsequent on-site crystal growth leads to the formation of ZnIn2S4 nanotubes. Furthermore, long-time reaction and a little higher temperature in a sealed autoclave could produce high pressure and promote the lateral crystal growth from the inner pore walls. The PC template pores were filled completely eventually and ZnIn2S4 solid nanowires were formed as a result of the continuous lateral thickening. Figure 5 shows the room temperature UV-Vis light absorption spectrum for the as prepared ZnIn2S4 nanotubes and nanowires. A strong light absorption rise at wavelengths shorter than 500 nm can be observed for both samples，which can be assigned to intrinsic band gap absorption. The steep shape of the spectra gives evidence that the light absorption originates not from a transition from an impurity level but from a band gap transition. 26 Estimation on the optical band gap (Eg) of the

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sample can be obtained by plotting (αhν)2 as a function of the photon energy (in the inset of Figure 5), with α being the absorption coefficient, h Planck’s constant, and ν the frequency. The Eg values are estimated as 2.32 eV for ZnIn2S4 nanowires and 2.85 eV for nanotubes according to the intersection of the extrapolated linear portion with the x axis (photon energy), being close to the reported values of ZnIn2S4 in literature. 18,19 Here, the band gap for the ZnIn2S4 nanotubes has an obvious blue shift compared to that of ZnIn2S4 nanowires. This blue shift may be resulted from quantum size effects associated with the ultrathin wall thickness of the nanotubes.

2

b a

2

(αhv) (eV)

Absorbance (a.u.)

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b

1 2 3 Photo energy(eV)

a

400

600

800

1000

1200

Wavelength (nm) Figure 5

The typical room-temperature UV-visible absorbance spectra and the

band gap values (the inset, estimated by a related curve of (αhν)2 versus photon energy plotted) of the ZnIn2S4 nanotubes (a) and nanowires (b). The photocatalytic activity of the as-prepared ZnIn2S4 nanotubes and nanowires in the wastewater treatment was investigated. Methyl orange (MO) was chosen as a typical organic contaminant in the waste water because it is often used as dye in

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textile industry. UV-Vis absorption spectroscopy was used to record the photodegradation behavior of ZnIn2S4 nanomaterials in the MO solution and the characteristic absorption peak of MO at 464 nm was selected to monitor the temporal changes of MO concentration. Prior to visible light irradiation, the solution was kept in the dark for 30 min to obtain adsorption equilibrium states. Figure 6 shows the concentration changes of MO versus visible light treatment time. Here, C0 and C are concentrations of MO before and after treatment, respectively.

It discloses clearly

that negligible decrease in the concentration of MO was detected without any catalyst, as shown in the blank curve. The degradation of MO was promoted significantly once the catalysts were added. However, the ZnIn2S4 nanotubes show an obviously higher efficiency than that of ZnIn2S4 nanowires. The photocatalytic degradation rate of the Zn2SnO4 nanotubes is faster and the value of C/C0 was about zero after 210 min, suggesting a complete decomposition of MO. Meanwhile, 76 % of MO was degraded by ZnIn2S4 nanowires. The specific surface areas of ZnIn2S4 nanotubes and nanowire were evaluated to be 78.3 m2 g-1 and 55.9 m2 g-1 respectively based on the BET result. It is known that catalyst powders with very small particle size usually offer high efficient catalytic activity due to their large surface-to-volume ratio, which can increase largely the number of active surface sites.

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In this case, more chances are

produced for the photogenerated charge carriers to degrade the absorbed MO molecules to hydroxyl and superoxide radicals. Here, the

ZnIn2S4 nanotubes has

much higher specific surface areas due to their hollow nanotube structure and ultra thin tube wall, which may result into its obviously enhanced photocatalytic efficiency

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by comparison to that of ZnIn2S4 nanowires.

1.0 0.8

C/C0

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0.6 0.4 blank nanowires nanotubes

0.2 0.0 0

60

120

180

240

Time (min)

Figure 6 Visible-light photocatalytic activities of MO for the ZnIn2S4 nanotubes and nanowires

4. Conclusions

A mild low temperature solution method has been successfully developed for controlled fabrication of ZnIn2S4 nanotubes or nanowires respectively without the using of organic solvents or surfactants. During the reaction process, porous polycarbonate membrane was used as a morphology directing template. The average thickness of nanotube wall is as thin as 5 nm and the nanotube diameter is 200 nm. The band gap for the ZnIn2S4 nanotubes was found to have an obvious blue shift compared to that of ZnIn2S4 nanowires, possibly resulted from quantum size effect. The higher efficient photocatalytic effect of ZnIn2S4 nanotubes may be induced by their hollow tube structure and ultrathin tube wall. The as-prepared ZnIn2S4 nanotubes 15


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and nanowires may find their applications in the waste water treatment for the environmental protection and the present synthesis strategy may be extended to prepare other one dimensional nanostructured ternary sulfides.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21071135).

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Photocatalyst for H2 Production under Visible-Light Irradiation. J. Phys. Chem. C 2011, 115, 6149-6155. 20 Chen, Z. X.; Li, D. Z.; Zhang, W. J.; Chen, C.; Li, W. J.; Sun, M.; He, Y. H.; Fu, X. Z. Low-Temperature and Template-Free Synthesis of ZnIn2S4 Microspheres. Inorg. Chem. 2008, 47, 9766-9772. 21 Xu, Z. D.; Li, Y. X.; Peng, S. Q.; Lu, G. X.; Li, S. B. NaCl-assisted low temperature synthesis of layered Zn-In-S photocatalyst with high visible-light activity for hydrogen evolution. RSC Advances 2012, 2, 3458-3466. 22 Peng, S. J.; Li, L. L.;Wu, Y. Z.; Jia, L.; Tian, L. L.; Srinivasan, M.; Ramakrishna, S.; Yan, Q. Y.; Mhaisalkarab, S. G. Size- and shape-controlled synthesis of ZnIn2S4 nanocrystals with high photocatalytic performance. CrystEngComm 2013, 15, 1922-1930. 23 Gou, X. L.; Cheng, F. Y.; Shi, Y. H.; Zhang, L.; Peng, S. J.; Chen, J.; Shen, P. W. Shape-controlled synthesis of ternary chalcogenide ZnIn2S4 and CuIn (S, Se)2 nano-/microstructures via facile solution route. J. Am. Chem. Soc. 2006, 128, 7222-7229. 24 Wei, Q. L.; Mu, S.; Yan, Y.; Lu, Y.; Kang, S. Z.; Mu, J. Preparation and Surfactant-Assisted Morphology-Controllable Growth of ZnIn2S4. Chinese. J. Inorg. Chem. 2010, 26, 269-273. 25 Mao, Y.; Wong, S. S. General, room-temperature method for the synthesis of isolated as well as arrays of single-crystalline ABO4-type nanorods. J. Am. Chem. Soc. 2004, 126, 15245-15252.

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26 Kudo, A.; Tsuji, I.; Kato, H. AgInZn7S9 solid solution photocatalyst for H2 evolution from aqueous solutions under visible light irradiation. Chem. Commun. 2002, 1958-1959. 27 Xu, N. P.; Shi, Z. F.; Fan,Y. Q.; Dong, J. H.; Shi, J.; Hu, Z. C. Effects of particle size of TiO2 on photocatalytic degradation of methylene blue in aqueous suspensions. Ind. Eng. Chem. Res. 1999, 38, 373-379.

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Table of Contents Graphic and Synopsis.

ZnIn2S4 nanotubes and nanowires have been selectively fabricated via a convenient one-step wet-chemical approach by using porous polycarbonate membrane as a hard template and the wall of nanotubes is as thin as 5 nm.

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Synthesis and Photocatalytic Performance of ZnIn2S4 Nanotubes and

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