Sonochemical Synthesis of Hollow Copper Doped Zinc Sulfide

Article pubs.acs.org/IECR

Sonochemical Synthesis of Hollow Copper Doped Zinc Sulfide Nanostructures: Optical and Catalytic Properties for Visible Light Assisted Photosplitting of Water Gang-Juan Lee,† Sambandam Anandan,†,‡ Susan J. Masten,§ and Jerry J. Wu*,† †

Department of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Trichy 620 015, India § Department of Civil and Environmental Engineering, Michigan State University, East Lansing, Michigan 48824, United States ‡

S Supporting Information *

ABSTRACT: Hollow copper doped ZnS (Cu−ZnS) nanostructures were synthesized by sonochemical approach and characterized well using various analytical tools. TEM images show that the prepared copper doped ZnS crystallites have a hollow-sphere-like self-assembled nanostructure. In Fourier transform infrared (FT-IR) spectra, a noticed splitting of the ZnS peak at 1110 cm−1 into two peaks at 1124 and 998 cm−1 for copper doped ZnS indicates that the copper atom can be partially substituted in the zinc as Cu−S−Zn. Such a bonding interaction between copper and zinc ions toward sulfur was also established from XRD and XPS analysis. Tuning of the band gap from 3.65 to 2.89 eV was successfully achieved upon doping copper (0− 5%) into ZnS. The catalytic activity of hollow Cu−ZnS nanosructures was tested by photosplitting of water containing Na2S via visible light irradiation.

1. INTRODUCTION ZnS is one of the most important II−VI wide band gap semiconductors (∼3.7 eV at 300 K) showing excellent physical properties, such as size-dependent electrical and optical properties due to the quantum confinement.1 Atomic structure and chemical properties of ZnS are more comparable to those of the widely known semiconductor ZnO. However, certain properties pertaining to ZnS are unique and advantageous, and further, it may be more suitable for visible blind ultraviolet light based devices.2 Therefore, researchers are interested in designing ZnS nanoparticles that could become responsive to visible light irradiation. For that doping of foreign elements, such as Cu, Mn, and Cd, into a UV-active non-oxide semiconductor (ZnS) has shown necessary activity under visible light irradiation.3−13 Among all, doping of Cu ions into zinc chalcogenides results in a wide range of tunable emission.5−9 Furthermore, doped Cu ion into ZnS has been found to shift the absorption edge of ZnS into the visible light region.13−15 Due to such principal reasons, several synthetic strategies (thermal evaporation, sol−gel processing, coprecipitation, microemulsion, etc.) have been so far applied to obtain Cu doped ZnS nanostructures with different morphologies.3−15 In this regard, our decision is to prepare Cu doped ZnS nanostructures at room temperature, ambient pressure, and short reaction time via a sonochemical approach. The benefits of sonochemistry in creating nanostructural materials arise principally from acoustic cavitation including the formation, growth, and implosive collapse of bubbles in a liquid. Bubble collapse stimulated by cavitation produces intense local heating and high pressures.16 The structural and spectral properties of as-prepared Cu doped ZnS nanostructures were well characterized using transmission electron microscopy (TEM), © 2014 American Chemical Society

X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), absorption, and luminescence studies. In addition, we explored its catalytic performance by photocatalytic splitting of water containing Na2S via the irradiation of visible light.

2. EXPERIMENTAL DETAILS 2.1. Materials. All chemicals were of the highest purity available and were used as received without further purification. Zinc(II) acetate dihydrate [Zn(CH 3COO)2 ·2H 2O] was purchased from SHOWA Inc. Copper(II) acetate monohydrate [Cu(CH3COO)2·H2O] and thioacetamide (C2H5NS) were purchased from Merck Inc. Sodium sulfide nonahydrate (Na2S· 9H2O) was purchased from Acros Inc. Unless otherwise specified, all the reagents used were of analytical grade and the solutions were prepared using deionized water collected from a Millipore water purification system (18.2 MΩ). 2.2. Sonochemical Synthesis of Copper Doped ZnS Nanostructures. Sonochemical synthesis of copper doped ZnS nanostructures was carried out as follows: zinc acetate dihydrate and thioacetamide were used as the source material for preparation of zinc sulfide. First prepared was 0.5 M Zn(CH3COO)2·2H2O in 50 mL of deionized water. Subsequently, 3.8 g of C 2 H 5 NS (0.5 M) was added to Zn(CH3COO)2 solution by continuous stirring for 20 min (solution A). Different molar amounts of Cu(CH3COO)2·H2O (0−5 mol %) in 50 mL of deionized water (solution B) were also made. Under continuous stirring solution B was added Received: Revised: Accepted: Published: 8766

February April 26, April 30, April 30,

15, 2014 2014 2014 2014

dx.doi.org/10.1021/ie500663n | Ind. Eng. Chem. Res. 2014, 53, 8766−8772

Industrial & Engineering Chemistry Research

Article

Figure 1. (a) Typical HR-TEM image, (b) magnified HR-TEM image, (c) corresponding SAED pattern, (d) lattice fringes, and (e) EDX spectrum of ZnS nanostructures.

Figure 2. (a) Typical HR-TEM image, (b) magnified HR-TEM image, (c) corresponding SAED patterns, (d) lattice fringes, and (e) EDX spectrum of 2.0% Cu−ZnS hollow nanostructures.

irradiated using a 350 W xenon lamp having a cutoff filter that only allows visible light irradiation (≥400 nm). The amount of hydrogen generated during the photocatalytic splitting of water was determined by an online TCD gas chromatograph (GC2014, Shimadzu) with a molecular sieve column. 2.4. Characterization Techniques. The X-ray diffraction (XRD) patterns were recorded using a Rigaku Ultima III diffractometer (Japan) with Cu Kα radiation, in the scan angle 2θ ranging from 10 to 80°. High resolution transmission electron microscopic (HR-TEM) images were recorded with a JEOL JEM-2010 model. The FT-IR spectra of the prepared nanoparticles were measured at room temperature by a PerkinElmer-FTIR spectrometer ranging from 650 to 4000 cm−1. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Physical Electronics PHI 5600 XPS instrument with monochromatic Al Kα (1486.6 eV) as the

dropwise into solution A. The stirring was maintained for 1 h to achieve complete reaction. Then, the reaction mixture was irradiated with high-intensity ultrasound (600 W, 20 kHz) for 0.5 h. The temperature was maintained at 15 °C by circulating cooling water. Finally, the copper doped ZnS powders (0−5 mol %), denoted as ZnS, 0.5Cu/ZnS, 1.0Cu/ZnS, 1.5Cu/ZnS, 2.0Cu/ZnS, 2.5Cu/ZnS, 3.0Cu/ZnS, 4.0Cu/ZnS, and 5.0Cu/ ZnS, respectively, were centrifuged (10 000 rpm) and washed by water and ethanol several times, and then dried at 60 °C in a vacuum oven overnight. 2.3. Photocatalytic Activity. Photosplitting experiments were performed in a Pyrex cell assembly connected to a closed gas circulation and evacuation system (see Supporting Information, Figure S1). About 0.015 g of prepared nanostructures was dispersed in 100 mL of aqueous solution containing 0.1 M Na2S as the sacrificial agent. The cell was 8767



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excitation source. The UV−visible diffuse reflectance spectra were recorded using a Shimadzu UV-2600 spectrophotometer with integrated sphere attachment, and barium sulfate was used as a reference. Photoluminescence properties were measured at room temperature using a HORIBA HR800 spectrometer excited at 325 nm.

3. RESULTS AND DISCUSSION 3.1. Characterization of Copper Doped ZnS Hollow Nanostructures. The morphology of prepared pristine ZnS and copper doped ZnS (0−5%) were characterized by HRTEM. Parts a and b of Figure 1 are the TEM images of the assynthesized and magnified pristine ZnS, which looks like agglomerated spherical clusters. The corresponding selected area electron diffraction (SAED) pattern (Figure 1c) exhibits rings, which are related to diffraction arising from different planes of ZnS nanocrystallites. The major diffraction ring matches well with the (111), (220), and (311) planes of the cubic lattice of ZnS phase (JCPDS Card No. 05-0566, cell parameter a = 5.406 Å). Also, such planes were calculated from the difference in lattice fringes and marked in the HR-TEM image (Figure 1d). The energy dispersive X-ray (EDX) image (Figure 1e) and its percentage ratio clearly indicate that the purity of ZnS is 100% (1:1 molar ratio). However, viewing copper doped ZnS crystallites (0−5%) under TEM, it appears to have a hollow-sphere-like self-assembled nanostructure with a diameter ranging from 50 to 100 nm. A TEM image (Figure 2a) is provided here for 2% copper doped ZnS sample (see the Supporting Information for other remaining samples, Figure S2). The magnified image of Cu doped ZnS (Figure 2b) clearly shows that the spheres have a hollow structure. The corresponding selected area electron diffraction (SAED) pattern (Figure 2c) also exhibits rings instead of spots due to the random orientation of crystallites similar to pristine ZnS. In addition, no change in the major diffraction planes ((111), (220), and (311)) was noticed. Such planes were calculated from the difference in lattice fringes and marked in the HRTEM image (Figure 2d). The EDX image (Figure 2e) and its percentage ratio indicate the approximate formation of 2% Cu doped ZnS. Further, to clearly clarify the formation of such hollow-sphere-like self-assembled nanostructures, mapping the TEM image for 5% Cu doped ZnS nanoparticles (Figure 3) was provided. In order to demonstrate the formation of hollow-sphere-like self-assembled Cu−ZnS nanostructures, further characterizations were performed and the observed results are discussed below. The crystallinity and crystal phase of pristine ZnS and copper doped ZnS (0−5%) were characterized by XRD and are demonstrated in Figure 4. The observed major diffraction peaks at 28.91, 48.00, and 56.54° belong to the (111), (220), and (311) planes of pristine ZnS and match well with the cubic zinc blende phase structure (JCPDS Card No. 05-0566, cell parameter a = 5.406 Å). However, no other peaks corresponding to copper and its related phase were found in the copper doped ZnS nanostructure, revealing that the doping of copper does not affect the cubic zinc blende phase structure. However, a small shift was noticed in the peak positions toward the lower scattering angles, indicating an increase of lattice constant which can be attributed to the nonuniform substitution of copper ion into the zinc ion site as the radius of the copper ion (0.057 nm) is smaller than that of the zinc ion (0.06 nm).6,17 In addition, it was noticed that the

Figure 3. (a) TEM and EDX-mapping images of (b) Zn, (c) S, and (d) Cu of 5% Cu−ZnS hollow nanostructures.

Figure 4. XRD patterns of copper doped ZnS and bare ZnS.

broadening of diffraction peaks with the increase of copper concentrations is due to the increase in microstrain. FT-IR spectra of the pristine ZnS and copper doped ZnS are shown in Figure 5. The broad band around 3000−3600 cm−1 is due to the OH stretching frequency (existence of water absorbed on the surface of nanocrystals). The bands at 1430 and 1555 cm−1 can be assigned to CO symmetric and asymmetric stretching modes arising from the absorption of atmospheric CO2 on the surface of the nanocrystals.9,18 The characteristic ZnS vibration peaks can be noticed at 1110 and 672 cm−1.9,13 However, the FT-IR spectrum of 2% Cu doped ZnS nanostructures shows almost similar peaks with respect to the bare ZnS nanoparticles and in addition noticeable new peaks at 1124 and 998 cm−1 may be due to arising resonance interaction due to the difference in vibrational modes of zinc and copper sulfide ions. This indicates that the copper ions can be partially substituted between the zinc ions as Cu−S−Zn, which may be responsible for splitting of the original ZnS peak at 1110 cm−1 into two new peaks, i.e., 1124 and 998 cm−1. Such competitive bonding interaction among copper and zinc ions 8768



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Moreover, the binding energies of Zn(2p3/2) and Zn(2p1/2) are found at 1022.1 and 1045.1 eV, belonging to the Zn2+ oxidation state; in addition, there is no noticeable shift after the Cu doping.20 However, for the dopant copper, the binding energies of Cu(2p3/2) and Cu(2p1/2) are 932.6 and 952.5 eV, respectively, which can be assigned to either metallic or cationic Cu species based on available literature references.21,22 Nevertheless, Cu species detected in our hollow-sphere-like self-assembled nanostructure may appear to be cationic (Cu− S−Zn) based on FT-IR studies stated in the above paragraph. The absence of satellite peaks (942 and 963 eV) indirectly indicates that the cationic species present in our system may be in +1 oxidation state (Cu+) although Cu2+ was used for the synthesis. Therefore, all the characterization studies mentioned here clearly support that there may be bonding interactions occurring between copper and zinc ions toward sulfur as Cu− S−Zn, which leads to hollow-sphere-like self-assembled nanostructures. 3.2. Optical Properties of Copper Doped ZnS Hollow Nanostructures. UV−vis absorption of pristine ZnS and copper doped ZnS (0−5%) is demonstrated in Figure 7. The observed red shift of the absorption edge for the hollow Cu doped ZnS nanostructures compared to pristine ZnS revealed that copper doping can moderately benefit the optical property of bare ZnS, which is due to the substitution of Cu ions on Zn lattice sites. The calculated band gap using Tauc’s relation23 starts decreasing from 3.65 to 2.89 eV upon increasing the concentration of copper dopant from 0 to 5%, which indicates that Cu doped ZnS can be used as a visible light response catalyst for various applications, such as degradation of pollutants, photosplitting of water, etc.

Figure 5. FT-IR spectra of copper doped ZnS and bare ZnS.

toward sulfur changes initially formed agglomerated pristine ZnS spherical clusters into hollow-sphere-like self-assembled nanostructures (Cu−ZnS).9,19 To better understand the chemical states of the elements, XPS analyses were performed and the typical XPS results are shown in Figure 6. The XPS survey spectra of hollow (2%) Cu doped ZnS nanostructure shows extra peaks for the element copper upon comparison to the pristine ZnS nanostructure. The peaks of S and Zn can also be noticed in both survey spectra. The binding energies of S(2p3/2) and S(2p1/2) clearly seen at 161.9 and 163.1 eV are consistent with the binding energy values as reported in the literature for sulfur.20

Figure 6. (a) Survey spectra of ZnS and 2% Cu−ZnS nanostructures and high-resolution XPS spectra of (b) Zn(2p) and (c) S(2p) of ZnS and 2% Cu−ZnS, and (d) Cu(2p) of 2% Cu−ZnS. 8769



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CH3(NH 2)C(OH)2 + ))))) → CH3(NH 2)CO + H 2O (3)

Zn(CH3COO)2 + Cu(CH3COO)2 + 2H 2S + ))))) → Cu − S − Zn + 4CH3COOH

(4)

Upon sonication, thioacetamide is hydrolyzed to generate acetamide and hydrogen sulfide. Such in situ generated H2S has equal ease to react with copper ion (0.057 nm) and zinc ion (0.06 nm) and produce Cu−S−Zn nanostructures. Such nanostructures resemble hollow-like morphology due to cavitation phenomena. 3.3. Photocatalytic Activity of Copper Doped ZnS Hollow Nanostructures. In general, the prepared hollow spheres with porous shell walls and uniform structural parameters have manyfold advantages over porous particles due to their relatively low density.26,27 Therefore, they can be used in a wide range of potential applications such as controlled release of encapsulated agents, protection of light-sensitive components, artificial cells, photonic crystals, fillers, vehicle systems, and catalysts.26,27 In this context, the photocatalytic activities of the prepared hollow copper doped ZnS nanoparticles were evaluated here for visible-light-driven hydrogen production by photosplitting of aqueous solution containing Na2S as a sacrificial agent (see Supporting Information, Figure S3). Obviously, copper doped ZnS nanostructures cause an improvement in the activity and liberate more hydrogen (6.7 μmol h−1 g−1), which is due to the benefit in the optical property toward visible light while compared to pristine ZnS that only absorbs UV light (Figure 9). The reason behind this is

Figure 7. UV−vis absorption spectra of copper doped ZnS and bare ZnS.

The photoluminescence spectra of the pristine ZnS and copper doped ZnS (0−5%) excited at 325 nm are shown in Figure 8. Pristine ZnS shows a peak at 520 nm and a shoulder

Figure 8. Photoluminescence spectra of copper doped ZnS and bare ZnS.

at 420 nm. However, for the copper doped ZnS hollow nanostructures, broad and red-shifted peaks are noticed at 560 and 430 nm, indirectly indicating the presence of more than one metal ion, probably Cu−S−Zn. On the basis of the observed experimental results, we considered that the formation mechanism of the copper doped ZnS hollow nanostructures could be explained by the selfassembled mechanism,24 and in addition, such a chemical process can be described by the following overall chemical equations:25

Figure 9. Hydrogen evolution from an aqueous solution containing 0.1 M Na2S catalyzed by copper doped ZnS and bare ZnS.

that copper could form deep trap energy levels between the valence band and the conduction band of ZnS, and by absorbing the external energy (visible light), electrons are excited from the valence band (VB) to conduction the band (CB), which are then relaxed at shallow defect levels formed by impurity ions. The activated electrons can recombine with holes left in the CB or the holes transferred to copper trap energy levels by irradiative transitions which give out photons.14,15,28,29 Thus, the efficient interparticle charge transfer prevents the electron−hole recombination in copper doped ZnS, resulting in a superior photocatalytic activity toward catalytic photosplitting of water molecules in the

CH3CSNH 2 + H 2O + ))))) → CH3(NH 2)C(OH)− SH (1)

CH3(NH 2)C(OH)− SH + H 2O + ))))) → CH3(NH 2)C(OH)2 + H 2S

(2) 8770



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presence of Na2S as a sacrificial agent. In addition, the hole will oxidize S2− or H2O to form S22− or O2, whereas the electron reacts with H+ to generate a hydrogen atom. Then, two hydrogen atoms react with each other to produce molecular H2. A possible mechanism of charge transfer in Cu/ZnS composite system was proposed as shown in Scheme 1.

4. CONCLUSIONS In summary, hollow copper doped ZnS self-assembled nanostructures were prepared by a facile sonochemical synthesis approach and suitable reasons are provided for the observed morphology based on the chararcterization studies, i.e., the competitive bonding interaction between copper and zinc ions toward sulfur (Cu−S−Zn). The observed decrease in the energy band gap shows that the copper doped ZnS can better enhance the optical property of pristine ZnS by harvesting more visible light. The photosplitting of water in the presence of Na2S indicates as-prepared hollow Cu−ZnS self-assembled nanostructures can act as an active visible-lightresponsive photocatalyst. ASSOCIATED CONTENT

S Supporting Information *

Schematic diagram of the apparatus used in generating hydrogen evolution by photosplitting processes; TEM images of ZnS and Cu−ZnS nanostructures; GC of liberation of H2 evolution as a function of different copper doped ZnS mixtures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Scheme 1. Proposed Reaction Mechanism for Hydrogen Evolution Using the Cu/ZnS Photocatalyst and Na2S as the Electron Donor

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +886-4-24517250, ext 5206. Fax: +886-4-24517686. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was financially supported by the Department of Science and Technology, India (GITA/DST/TWN/P-50/ 2013), and National Science Council (NSC-102-2923-035001-MY3), Taiwan, under the India−Taiwan collaborative research grant. The support in providing the fabrication and measurement facilities from the Precision Instrument Support Center of Feng Chia University is also acknowledged. 8771



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(22) Herng, T. S.; Lau, S. P.; Yu, S. F.; Yang, H. Y.; Wang, L.; Tanemura, M.; Chen, J. S. Magnetic Anisotropy in the Ferromagnetic Cu-doped ZnO Nanoneedles. Appl. Phys. Lett. 2007, 90, 032509. (23) Maji, S. K.; Mukherjee, N.; Mondal, A.; Adhikary, B.; Karmakar, B.; Dutta, S. Synthesis and Characterization of Nanocrystalline Zinc Sulfide via Zinc Thiobenzoate-lutidine Single-source Precursor. Inorg. Chim. Acta 2011, 371, 20. (24) Zhang, J.; Huang, F.; Lin, Z. Progress of Nanocrystalline Growth Kinetics Based on Oriented Attachment. Nanoscale 2010, 2, 18. (25) Goharshadi, E. K.; Mehrkhah, R.; Nancarrow, P. Synthesis, Characterization, and Measurement of Structural, Optical, and Phtotoluminescent Properties of Zinc Sulfide Quantum Dots. Mater. Sci. Semicond. Process. 2013, 16, 356. (26) Agrawal, M.; Gupta, S.; Pich, A.; Zafeiropoulos, N. E.; Stamm, M. A Facile Approach to Fabrication of ZnO-TiO2 Hollow Spheres. Chem. Mater. 2009, 21, 5343. (27) Chen, X.; Wang, Z.; Wang, X.; Zhang, R.; Liu, X.; Lin, W.; Qian, Y. Synthesis of Novel Copper Sulfide Hollow Spheres Generated from Copper (II)−thiourea Complex. J. Cryst. Growth 2004, 263, 570. (28) Chauhan, R.; Kumar, A.; Chaudhary, R. P. Photocatalytic Degradation of Methylene Blue with Cu Doped ZnS Nanoparticles. J. Lumin. 2014, 145, 6. (29) Anandan, S.; Madhavan, J.; Ashokkumar, M. The Contribution of the Nanotechnology to Hydrogen. Nanotechnology for the Energy Challenge; Wiley-VCH: Weinheim, Germany, 2010; pp 111−136.

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Sonochemical Synthesis of Hollow Copper Doped Zinc Sulfide

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