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Inorganic and Nano-Metal Chemistry

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Preparation of polystyrene@CdS core-shellnanocomposite materials with different cadmiumsources for photocatalysis

Guanhua Sun, Lu Zhang, Huacheng Yin, Jingtang Zheng, Chaosheng Zhu, BoJiang, Han Wang, Shi Qiu, Jianjun Yuan & Mingbo Wu

To cite this article: Guanhua Sun, Lu Zhang, Huacheng Yin, Jingtang Zheng, Chaosheng Zhu, BoJiang, Han Wang, Shi Qiu, Jianjun Yuan & Mingbo Wu (2017) Preparation of polystyrene@CdScore-shell nanocomposite materials with different cadmium sources for photocatalysis, Inorganicand Nano-Metal Chemistry, 47:5, 737-743, DOI: 10.1080/15533174.2016.1212229

To link to this article: http://dx.doi.org/10.1080/15533174.2016.1212229

Accepted author version posted online: 07Oct 2016.Published online: 07 Oct 2016.

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Preparation of polystyrene@CdS core-shell nanocomposite materials with differentcadmium sources for photocatalysis

Guanhua Suna, Lu Zhangb, Huacheng Yinb, Jingtang Zhengb, Chaosheng Zhub, Bo Jiangb, Han Wangb, Shi Qiub,Jianjun Yuana, and Mingbo Wub

aChangzheng Engineering Company Limited Lanzhou Branch, Lanzhou, China; bState Key Laboratory of Heavy Oil Processing, China University ofPetroleum, Qingdao, China

ARTICLE HISTORYReceived 26 November 2015Accepted 9 July 2016

ABSTRACTThe preparation of polystyrene@CdS core-shell nanoparticles of several hundreds nanometers wasperformed by hydrothermal method with different cadmium sources including CdCl2¢2.5H2O, Cd(Ac)2¢2H2O, and CdSO4¢8H2O. The morphology, composition, size, and crystal structure of the synthesizedpolystyrene@CdS were characterized by scanning electron microscopy, transmission electron microscopy,energy-dispersive X-ray analysis, X-ray diffraction, and Fourier transform infrared spectra. Thepolystyrene@CdS nanocomposites prepared by different Cd sources exhibited different morphologiesand thickness of CdS layer. Their photocatalytic properties were tested via a photocatalytic degradation ofRhB experiment. The polystyrene@CdS nanocomposites prepared with Cd(Ac)2¢2H2O as cadmium sourceshowed the most effective photocatalytic performance.

KEYWORDSCore-shell; nanocompositematerials; surfacemorphology; cadmiumsource; photocatalyst

Introduction

In recent years, semiconductor photocatalytic oxidation tech-nology has been widely researched as a green technology forwastewater treatment due to its high efficiency, low toxicity,and low energy consumption. Among various semiconductormaterials, CdS is a well-known semiconductor with a band gapof 2.42 eV at room temperature.[1,2] Its valence electron can beeasily evoked to conduction band, while the wavelength ofevoking light is no more than 495 nm.[3,4] This has attractedconsiderable interest in light-emitting diodes,[5] photocataly-sis,[6] solar cells,[7] and some other optoelectronic applica-tions.[8,9] There are many ways to prepare the composite core-shell materials, such as two-stage emulsion polymerization,[10]

microwave-assisted method,[11] layer-by-layer (LBL)[12,13] tech-nologies, etc. Yong qiang et al.[14] fabricated PS@CdS core-shellnanostructures by microwave irradiation at 70�C from theaqueous solution of CdCl2, thioacetamide (TAA), and polyvi-nylpyrrolidone (PVP) with PS microspheres without surfacemodification. The author discussed the effects of differentmolar mass of CdCl2 and reaction time; then, the optimumreaction conditions were obtained. Yoon et al.[15] synthesizedCdS/PS hybrid particles. The as-prepared CdS/PS compositeparticles exhibited electrorheological properties, which indi-cated wide applications in both scientific researches and correl-ative industry.

One of the most adaptable methods for fabrication of multi-level shell with highly defined structural order, as well as thick-ness and composition, is the hydrothermal method. Guoet al.[16] synthesized titania microspheres with hollow interiors

using hydrothermal technology, and this material would bepossibly used in separation technology, catalysis, and optoelec-tronic field. Jia et al.[17] prepared TiO2@CdS core-shell nano-rods films electrodes using chemical bath deposition. Afterapplying these materials to photovoltaic cells, it proved that thephotocurrent was dramatically enhanced, and better cellperformance was showed. Li et al.[18] synthesized urchin-likecore-shell CuO by hydrothermal method in assistance withpoly(ethylene glycol) (PEG) at 100�C for 10 h. The biosensorcapabilities of the as-prepared core-shell CuO targeting atH2O2 were improved.

However, most of these studies were devoted to the surfacemodification of materials. The dispersivity of these nanoparticleswas wide, and the thickness of coating was uncontrollable, whilethe mechanical strength was also poor. These would bring difficul-ties for later experiments. In this article, we report a high efficientmethod called one-step hydrothermal reaction for fabrication ofuniform PS@CdS core-shell nanocomposite materials with differ-ent cadmium sources. The advantages and disadvantages ofdifferent materials were described by comparing their surface mor-phological structure and photocatalytic performance.

Experimental

Synthesis of monodisperse Polystyrene microspheres

Monodisperse polystyrene microspheres were synthesized using aconventional chemical approach called emulsion polymeriza-tion.[19] In brief, a 500-mL flask with round bottom and long neckwas heated in a water bath at 70�C. Deionized water, NaHCO3,

CONTACT Jingtang Zheng jtzheng03@163.com State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, China.

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsrt.© 2017 Taylor & Francis Group, LLC

INORGANIC AND NANO-METAL CHEMISTRY2017, VOL. 47, NO. 5, 737–743http://dx.doi.org/10.1080/15533174.2016.1212229

sodium p-styrene sulfonate (SSS), and styrene (St) monomers weresuccessively poured into the reactor and stirred constantly for10 min. Then, potassium persulfate (KPS) was introduced into thesolution under N2 atmosphere. The whole polymerization systemwas stirred at 200 r/min speed constantly and reacted for 15 h toform PS nanospheres. The reaction product was collected aftercooling down to room temperature.

Synthesis of PS@CdS core-shell nanoparticles

The PS@CdS core-shell nanoparticles were prepared as illus-trated in Figure 1. Three sets of samples were synthesized usingdifferent cadmium sources. In concrete terms, 1.2 mL as-pre-pared PS emulsion was dissolved in 20-mL ethanol and then dis-persed for 10 min under ultrasonic effect. Finally, 0.01 mol ofCdCl2¢2.5H2O (2.28 g), Cd(Ac)2¢2H2O (2.66 g) andCdSO4¢8H2O (3.52 g) were dissolved, respectively, in 0.2 g ofPVP and 1.12 g of TAA, which were dissolved in 100 mL ofdeionized water and then added to PS emulsion. The obtainedmixture was then poured into hydrothermal autoclave at 80�Cfor 2 h. The precipitate was separated by centrifugation(10000 r/min for 10 min) and washed with deionized water andethanol several times. Finally, PS@CdS core-shell nanoparticleswere obtained after drying in a vacuum oven at 50�C for 8 h.

Results and discussion

All measurements were carried out at room temperature. Scan-ning electron microscopy (FESEM, Hitachi: S-4800) was usedto characterize the morphology of the films. Transmission elec-tron microscopy (TEM) images were operated at an accelerat-ing voltage of 200 kV (TEM, JEOL: JEM-2100UHR).Compositional analyses were performed by energy-dispersiveX-ray analysis (EDS) using the FESEM. The X-ray diffraction(XRD) patterns of the three samples were recorded on a PANa-lytical: X’Pert Pro MPD diffractometer with monochromatized

Cu Ka radiation (k D 1.54056 A�) at 40 kV and 40 mA in the

2u range of 20–80�. Fourier transform infrared (FT-IR) spectraof the samples were recorded in the wavenumber range of4000–400 cm¡1 with an FT-IR spectra (Thermo Nicole:TNEXUS). The UV–vis diffuse reflectance spectra of the threesamples were measured on a UV-2401 Shimadzu spectropho-tometer in the wavelength range of 300–700 nm.

Figure 2 presents the SEM images of polystyrene micro-spheres and PS@CdS core-shell nanoparticles obtained fromdifferent cadmium sources. Figure 2a shows the PS nanostruc-tures, from which one can see that PS particles have smoothhides, favorable dispersibility, and uniform grain size with280 nm. Figure 2b indicates one special strawberry-like coatingof 60 nm, gathering on the surface of the polystyrene micro-spheres while using CdCl2¢2.5H2O as cadmium source. Thisstructure has much potential application value. However, theincomplete shell leads to poor mechanical strength and causestrouble for the preparation of a stable hollow materials. Mean-while, its rough surface increases the difficulty of re-coating.Figure 2c shows some unique core-shell nanoparticles materialwhose shell thickness is about 90 nm by calculation. It wasshown that the PS@CdS using Cd(Ac)2¢2H2O as cadmiumsource had a fine and smooth surface and was smaller in sizecompared with Figure 2b. As it can be seen from Figure 2c, thesurface of CdS shell is glossy. The SEM image of usingCdSO4¢8H2O as cadmium source to prepare PS@CdS core-shellnanoparticles is shown in Figure 2d. The size of nanoparticles isabout 400 nm. In other words, the shell has a thickness ofaround 60 nm. It has a small grain size as well, but the surfacesmoothness is inferior since a small amount of CdS agglomer-ates on the surface of the shell. By comparing the three sets ofsamples, it can be concluded that PS@CdS core-shell nanopar-ticles derived from different cadmium sources under the samereaction conditions have disparate characteristics, and the sam-ple shown in Figure 2c possesses appropriate thickness of theshell and high mechanical strength because of its high CdS

Figure 1. Illustration of preparation of PS@CdS core-shell composite nanoparticles with different cadmium sources.

738 G. SUN ET AL.

content. In summary, the surface profiles of three samples aredifferent, and each sample has its own advantages. Therefore,we should prepare the object materials according to the specificneeds.

Figure 3 shows the TEM and EDS images of the samples.Figure 3a indicates a strawberry-like core-shell structure. Theposition and size of the CdS nanoparticles can be easily deter-mined by comparing the colors of the center and the edge.Figure 3b reveals a more denser and thicker coating on thecore. Its good mechanical strength would have extensive

applications. In addition, the CdS hollow microspheresobtained from these materials have a larger surface area, show-ing a better performance in photocatalysis, light quantum, andother areas as well. Figure 3c presents the core-shell structurederived from CdSO4¢8H2O, which would have practical appli-cations in medicine and slow-release fertilizer owing to its thinshell and favorable dispersibility. EDS images are used to deter-mine the composition of the nanostructure in Figure 3d. Thepeaks in the images of three samples are consistent, whichreveals that the shell layers contain a comparable amount of

Figure 2. SEM images of PS microspheres and PS@CdS core-shell composite nanoparticles with different Cd sources. (a) PS microspheres; (b) CdCl2¢2.5H2O; (c) Cd(Ac)2¢2H2O; and (d) CdSO4¢8H2O.

Figure 3. TEM images of PS@CdS core-shell composite nanoparticles with different Cd sources (a,b,c) and EDS image of PS@CdS (d). (a) CdCl2¢2.5H2O; (b) Cd(Ac)2¢2H2O;(c) CdSO4¢8H2O; and (d) EDS image of PS@CdS.

INORGANIC AND NANO-METAL CHEMISTRY 739

pure Cd and S elements. Therefore, it can be further confirmedthat the form of the two elements is CdS.

The XRD measurement was used to identify the crystalphase of PS@CdS nanoparticles prepared with different cad-mium sources. The results are shown in Figure 4. The crystalplanes of hexagonal CdS phase, in which the major peaks at24.8�, 26.5�, 28.2�, 43.7�, 47.9�, 51.8�, and 70.9� were assignedto the (100), (002), (101), (110), (103), (112) and (114) planes(JCPDS No. 41–1049), respectively. The crystal planes of cubicCdS phase, in which the major peaks at 26.5�, 30.7�, 43.8�,52.0� and 70.5� were assigned to the (111), (200), (220), (311),and (331) planes (JCPDS No. 42–1411), respectively. Curve ashows a pure hexagonal phase of CdS derived fromCdCl2¢2.5H2O. The peaks at 2u values of 24.8�, 28.2�, 47.9�,and 70.9� can be attributed to the crystal planes of hexagonalstructure CdS. Curves b and c that use Cd(Ac)2¢2H2O andCdSO4¢8H2O as cadmium sources, respectively, indicate thatcrystal form of CdS nanoparticles coated on the core is cubic-based phase, while a small amount of hexagonal phase due tothe (100), (111), (220), and (311) planes appeared. Comparingcurve b with c, (100) and (200) planes of curve b are more obvi-ous, which suggested a better crystallinity. Generally, CdS can-not be formed into a high-orientation crystalline when thereaction temperature is 80�C, so the CdS nanoparticles shownin curves b and c belong to the mixed crystal. When the tem-perature reaches 130�C, CdS nanoparticles are transformedfrom amorphous into hexagonal phase, and in order to fullygenerate the cubic phase, we need a higher temperature.Through the aforementioned analysis, we can safely draw theconclusion that the crystal phase of curve c consists chiefly ofhexagonal phase. One reason is that the closed high-pressuresystem leads to the result that the actual reaction temperatureis higher than 80�C under hydrothermal condition, which hasbeen confirmed by curves a and b. Another important reason isthat CdCl2¢2.5H2O can quickly release Cd2C and generate cubicCdS easily under the same condition compared with the othercadmium sources.

Figure 5 shows the FT-IR spectra of three samples. The mainabsorption peaks may be briefly summed up as follows: The

absorption peaks in the vicinity of 1164 and 1028 cm¡1 corre-spond to the Cd-S stretching. The absorption peaks at 3025and 3057 cm¡1 are attributed to the stretching vibration of C-H of benzene ring. The absorption peaks within the range of2800–3000 cm¡1 are due to the stretching vibration of satu-rated C-H and CH2. The peak at 1600 cm¡1 is due to thestretching vibration of the skeleton of benzene ring. The twoapparent peaks at around 697 and 755 cm¡1 are contributed bybending vibration of hydrogen on benzene ring. Besides, thepeak at 538 cm¡1 in the curve of PS@CdS reveals that there areintense interactions between CdS nanoparticles and PS micro-spheres. The existence of the CdS particles and PS@CdS core-shell composites can be confirmed by aforementioned analyses.Comparing the three curves, we find that there is no notabledifference in their absorption peaks because the elements andgroups of the three kinds of samples are the same. However,the absorption peaks of curves a and c as shown are moreintense than that of curve b; this may be due to a thin layer ofCdS and the partial bare PS core.

Formation mechanism of PS@CdS

As an amphiphilic and nonionic surfactant, the PVP has polaramine and carbonyl groups on one side and the nonpolarmethylene groups on the other side. PVP lacks acidic protonsbut contains two electron-donating centers (the –CHO groupand the N atom of the pyrrole ring). The –CHO group wasconsidered to be the most favorable site for interaction due tothe steric constraints on the N atom.[20] Thus, the nonpolarside of PVP was directly bound to the polymer PS, whereas the–CHO group of polar side can interact with S from TAA duringultrasonic, that is to say, PVP-functionalized PS microspheresoffered anchor sites for S. Therefore, the reaction of Cd and Sonly occurred on the surface of PS microspheres. As shown inFigure 6, once CdS nanoparticles were formed, it could act as anucleating site for the further growth of CdS until uniform CdScoating is formed. Compared with the original PS spheres,rougher surfaces were observed for all PS@CdS as inFigure 2b–d; the shell layer that seemed as a strawberry was

Figure 4. XRD images of PS@CdS core-shell composite nanoparticles with differentCd sources. (a) CdCl2¢2.5H2O; (b)Cd(Ac)2¢2H2O; and (c) CdSO4¢8H2O.

Figure 5. FT-IR images of PS@CdS core-shell composite nanoparticles with differ-ent Cd sources. (a) CdCl2¢2.5H2O; (b)Cd(Ac)2¢2H2O; and (c)CdSO4¢8H2O.

740 G. SUN ET AL.

composed of a large number of small and closely arranged CdSnanocrystallites.

The Cd[Cl]42¡ complex has a higher stability constant than the

Cd[CH3COO]2 or the Cd[SO4]34¡complexes;[21] that means much

slower release of Cd ions and consequently causes the shell thick-ness to be less than that of the Cd(CH3COO)2 or the CdSO4 as Cdresource. That is consistent with the Figure 3a–c.

As is known to us, the organic long-chain cadmium carbox-ylates such as Cd(OA)2 have the distinctly lower reactivity thansome inorganic cadmium salts such as CdSO4 and CdCl2.

[21]

The rate of releasing cadmium ions is slower than that of twoother resources. As a result, the CdS nanoparticles were muchsmaller and more difficult to agglomerate.

Photocatalyst performance analyses

The core-shell composite materials PS@CdS obtained from differ-ent cadmium sources, respectively, were used to degrade 5 mg/L ofRhB. A 500-W Xenon lamp fixed in a cylindrical reaction vesselwith a cutoff filter (λ> 450 nm) was used as the visible source. Cir-culating cooling water was provided in order to maintain the tem-perature stable at 20–30�C. 0.2 g of as-prepared samples wasdispersed in 200 mL of deionized water containing 5 mg/L RhB.Before each irradiation, the solutions were placed in the dark for30min withmagnetic stirring in order to reach adsorption-desorp-tion equilibrium. Providing lights and oxygen, 6.5£ 104 lux of lightintensity was measured by digital illuminance meter at the sametime, taking 3-mL reaction mixture every 15 min before measuringits absorbance. Based on Lambert-Beer law and standard curveequation, we can get the relationship between the degradation per-centage and time as shown in Figure 7.

It is obvious that the core-shell materials PS@CdS preparedfrom Cd(Ac)2¢2H2O as cadmium source exhibit best perfor-mance on photocatalytic degradation of RhB dye. The reasoncan be summed up as follows: First, the shell is thick with ahigh content of CdS; thus, it increases the amount of effectivesubstance of its catalyst. Second, the XRD characterizationmanifests that this stuff is cubic phase-based mix-phase, so itcan be proved that its catalytic performance is higher than thatof a pure phase. Finally, the good monodispersity of the stuffeffectively improves its light absorption efficiency and adhesionamount of contaminant. Conversely, CdCl2¢2.5H2O, as the

cadmium source, preparing core-shell materials that does notpossess a high content of CdS and effective crystal form. Inaddition, its photocatalytic effect is also the least prominentamong the three samples.

The decolorization of RhB in aqueous suspension is initiatedby the photoexcitation of the CdS nanostructure. First, the pho-tons with the energy higher than band gap are absorbed ontothe surface of CdS. This results in the excitation of the electronsfrom valence band (vb) to conduction band (cb), which canproduce the holes (hCvb) at the valence band edge and the elec-trons (e¡cb) in the conduction band of the QDs [Eq. (1)]. Sub-sequently, (hCvb) can react with water or hydroxyl groups andgenerate the highly active ¢OH [Eqs. (2) and (3)]. Finally, ¢OHcan react with RhB molecules absorbed on the surface of CdSto exert the degradation action [Eq. (4)]:

CdS nanostructureC hv!CdS.e¡cb C hCvb / (1)

HCvb CH2O!HC C �OH (2)

HCvb COH¡ ! �OH (3)

�OHC dye! degradation of the dye (4)

Figure 6. Illustration of formation mechanism of PS@CdS core-shell composite nanoparticles with different cadmium sources.

Figure 7. RhB reduction as a function of illumination time for PS@CdS core-shellcomposite nanoparticles with different Cd sources.

INORGANIC AND NANO-METAL CHEMISTRY 741

Finally, to study the mineralization of RhB in the pho-tocatalytic reaction, TOC has been measured under visi-ble light irradiation and shown in Figure 8. At 1.5 h,about 10% of TOC is removed with CdS as RhB hasdegraded completely with Cd(Ac)2¢2H2O as Cd source.Overall, the CdS with different Cd sources had the simi-lar degradation of TOC in the range of 0.3–0.4. The deg-radation rate of TOC in the first 8 h is more rapid thanthe next 12 h. The results demonstrated that RhB couldbe completely degraded within 2 h, but 60–70% of TOCwas left after 20 h. The results confirmed that in thepresence of CdS, RhB degradation is much faster than itsmineralization.

Conclusion

1. CdCl2¢2.5H2O, Cd(Ac)2¢2H2O, and CdSO4¢8H2O wereused as cadmium sources to prepare strawberry-like,thick annular, thin annular PS@CdS core-shell nanopar-ticles by hydrothermal method, respectively.

2. The SEM, TEM, XRD, and FT-IR characterizations showthat three samples as-prepared all have obvious core-shell structure with CdS shell and PS kernel.

3. Based on the characterizations of samples, this articlediscussed the characteristics and differences among thethree sets of materials and analyzed the causes of the dif-ferences, which make us more targeted to choose theright source of cadmium.

4. This study provides maneuverability for the followingmanufactures of hollow CdS core-shell materials andmulti-level core-shell structure, and thus, it furtherexpands the potential application value of CdS core-shellnanocomposite materials.

5. By degrading the RhB dye, the photocatalytic effects ofthe three composite particles are analyzed. It is foundthat the PS@CdS core-shell stuff manufactured usingCdCl2¢2.5H2O as core-shell materials for cadmiumsource preparation has superior performance in the pho-tocatalytic properties.

Funding

This work is financially supported by the National Natural Science Foun-dation of China (Grant No. 21176260 and 21376268), Taishan ScholarFoundation (No. ts20130929), and National Basic Research Program ofChina (No. 2011CB605703).

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