CdS Microspheres Composed of Nanocrystals and Their Photocatalytic Activity

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Copyright © 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 11, 1–10, 2011

CdS Microspheres Composed of Nanocrystals andTheir Photocatalytic Activity

Selvaraj Rengaraj1�∗, Sun Hee Jee2, Selvaraj Venkataraj3, Younghun Kim2�∗,Selvaraj Vijayalakshmi3, Eveliina Repo1, Arto Koistinen4, and Mika Sillanpää1�5

1Laboratory of Applied Environmental Chemistry (LAEC), University of Eastern Finland,Patteristonkatu 1, FI-50100 Mikkeli, Finland

2Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Korea3Crystal Growth Center, Anna University, Chennai 600025, India

4BioMater Center, Department of Physics and Mathematics, University of Eastern Finland, FI-70211 Kuopio, Finland5LUT Faculty of Technology, Lappeenranta University of Technology, Pateristonkatu 1, FI 50100 Mikkeli, Finland

A simple and template-free solution phase synthesis method has been developed for the prepara-tion of novel CdS hollow microspheres using cadmium nitrate and thioacetamide precursors. In thismanuscript, we demonstrate that process parameters such as the reaction time, precursor ratio,and reaction temperature strongly influence the morphology of the final product. The synthesizedproducts have been characterized by a variety of methods, including X-ray powder diffraction (XRD),Raman spectroscopy, high-resolution scanning electron microscopy (HR-SEM), high-resolutiontransmission electron microscopy (HRTEM), energy-dispersive X-ray diffraction (EDX) analysis,X-ray photoelectron spectroscopy (XPS), and UV-visible diffused reflectance spectroscopy (UV-DRS). XRD analysis confirmed the cubic structure of the CdS microspheres, which has also beenfurther supported by Raman spectroscopy. The HR-SEM measurements revealed the sphericalmorphology of the CdS microspheres which has been evolved by the oriented aggregation of theprimary CdS nanocrystals. The TEM measurements confirmed the hollow shell-like structure of thespheres; the formation of their hollow interiors can be explained by the Ostwald ripening mecha-nism. UV-DRS studies showed that the band gap of the CdS microspheres increased with increas-ing cadmium-nitrate-to-thioacetamide ratio. Furthermore, studies of photocatalytic activity revealedthat the synthesized CdS hollow microspheres exhibit an excellent photocatalytic performance inrapidly degrading methyl tert-butyl ether (MTBE) in aqueous solution under visible-light illumination.These results suggest that CdS microspheres will be an interesting candidate for photocatalyticdetoxification studies under visible light radiation.

Keywords: CdS Microsphere, Characterization, Photocatalysis, MTBE Degradation, VisibleLight.

1. INTRODUCTION

In recent years, hollow-structured micro- and nanostruc-tured materials have attracted special interest because oftheir higher specific surface area, lower density, better per-meability, and special optical/electrical/magnetic proper-ties, which are distinctively different from those of theirsolid counterparts. Materials of this kind can enable manytechnologically important applications, such as catalysis,nanoscale chemical reactors, encapsulation and controlledrelease of bioactive agents, sensors, drug delivery, chem-ical/biological separation, photonic devices, lightweight

∗Authors to whom correspondence should be addressed.

fillers, and various new application fields.1–7 Until now,much effort has been devoted to generating inorganic hol-low structures, including conventional templating meth-ods as well as newly emerging template-free methods.Template-assisted syntheses have proven to be a versa-tile approach to fabricating hollow structures. To preparehollow structures, a range of templates have been exten-sively employed, including hard templates such as polymerlatex particles,1 silica spheres,8 and carbon spheres,9 andsoft templates such as emulsion droplets,10 micelles,11 andgas bubbles.12 However, template-assisted methods usuallyrequire tedious procedures, including surface modification,precursor attachment, and core removal. It is still a greatchallenge in material science to understand the factors

J. Nanosci. Nanotechnol. 2011, Vol. 11, No. xx 1533-4880/2011/11/001/010 doi:10.1166/jnn.2011.3760 1

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CdS Microspheres Composed of Nanocrystals and Their Photocatalytic Activity Rengaraj et al.

governing the creation of nanocrystal assemblies and todevelop simple, reliable methods of fabricating variousmaterials using simple chemical components to achievecontrolled morphologies.As an important II–IV compound semiconductor, CdS,

with a direct band gap energy of 2.4 eV at roomtemperature, has attracted much attention because ofits unique properties and potential application in light-emitting diodes, flat panel displays, solar cells, photo-catalysis, and thin-film transistors.13–15 Since a material’sproperties depend greatly on its morphological features,nanostructured CdS with different sizes, shapes, anddimensionalities has been fabricated and characterized.16�17

Extensive studies have prepared CdS nanostructures-with various shapes, such as nanowires, nanobelts,and self-organized spheres on the scale of micro/nano-meters.18–22 The size, shape, and crystalline structureof semiconductor nanocrystallites, which lead to consid-erable changes in the recombination of electrons andholes trapped at spatially separated donors and accep-tors, are important in determining their optical proper-ties. For instance, Xu et al. reported the synthesis ofnanostructured flower-like CdS by a solvothermal methodusing ethylenediamine as a structure-directing template.23

Qing Xia et al. successfully synthesized a wide range ofcadmium sulfide (CdS) three-dimensional (3D) polycrys-talline walnut-like nanocrystals by a solvothermal methodwith polyvinylpyrrolidone (PVP) as stabilizer.24 Lin et al.synthesized uniform-size CdS hollow nanospheres viahydrothermal treatment of aqueous solutions of cadmiumacetate and thiourea and reported that the spheres couldbe made solid or hollow by manipulating the precursorCd/S molar ratio in the synthesis system.25 Li et al. syn-thesized CdS microspheres assembled from high-qualitynanorods by a hydrothermal method using PVP as asurfactant.26 Danjun Wang et al. synthesized novel 3Ddendritic CdS nanoarchitectures via a facile template-freehydrothermal process using CdSO4 and thiourea as pre-cursors and cetylpyridinium chloride (CPC) as a cappingreagent.27 Zhang et al. prepared CdS submicro- and micro-spheres by a convenient hydrothermal process throughthe reactions of CdCl2 and Na2S2O3 in aqueous solutionat a relatively low temperature (100 �C).28 The solutionphase or hydrothermal or solvothermal technique has beendemonstrated as an effective method for preparing low-dimensional nanomaterials from sulfides.29

To the best of our knowledge, a simple, solutionphase template-free method for synthesizing hollow, pearl-shaped CdS microspheres constructed of nanocrystals hasnot been reported in the literature. In this study, wepresent a facile one-pot template-free route to preparingmonodisperse CdS hollow spheres assembled from nano-crystals. To best understand the physical properties of theCdS microspheres, the structure and morphology of theprepared products were characterized by X-ray powder

diffraction (XRD), Raman spectroscopy, scanning elec-tron microscopy (SEM), transmission electron microscopy(TEM), energy-dispersive X-ray diffraction (EDX), andX-ray photoelectron spectroscopy (XPS). Methyl tert-butyl ether (MTBE) degradation experiments have alsobeen done to evaluate the catalytic properties of the CdSmicrospheres.

2. EXPERIMENTAL DETAILS

2.1. Synthesis of CdS Microspheres

The CdS microspheres were synthesized by usinganalytical-grade cadmium nitrate [Cd(NO3)2 · 4H2O](Sigma-Aldrich 99%) and thioacetamide (TAA, C2H5NS)(Sigma-Aldrich 99%) without further purification. In atypical synthesis, 3.00 g of Cd(NO3)2 · 4H2O and 1.00–4.00 g of TAA were dissolved in 100 mL of deionizedwater and continuously stirred for 30 minutes to form aclear solution. The TAA serve as an S2− source to formCdS. The solution was then transferred to a flask andrefluxed at ∼105 �C for 30 minutes, yielding a yellowprecipitate. The precipitate was harvested by centrifuga-tion and washed several times using deionized water andethanol to remove possible remaining cations and anionsand then dried in an oven at 70 �C for 24 h. Finally,the products were calcined at 350 �C for 2 h for furthercharacterization.

2.2. Characterization of CdS Microspheres

The crystalline properties of the synthesized CdS micro-spheres were studied by XRD using a Bruker (D5005)X-ray diffractometer equipped with graphite monochrom-atized CuK� radiation (� = 1�54056 Å). An acceleratingvoltage of 40 kV and emission current of 30 mA wereadopted for the measurements. In addition to XRD, Ramanspectroscopy measurements were also performed to con-firm the structure of the CdS microspheres. The dispersiveRaman spectrometer (Bruker SENTERA 200 LX model)employs a solid-state laser with an excitation wavelengthof 532 nm. Typical Raman measurements were carried outin the spectral range between wavenumbers 200 and 1100.The morphology of the microspheres was characterized bySEM and TEM. The SEM measurements were performedby a Hitachi S-4800 high-resolution (HR) field emissionscanning electron microscope. The HR-SEM equipmentwas also furnished with an EDX spectrometer that wasused for elemental analysis. The morphology of the micro-spheres was also studied by TEM (JEOL JEM-3010). Thechemical states and relative compositions of the sampleswere studied by XPS, a highly surface-sensitive techniquewith a typical information depth of a few nanometers. TheX-ray source used Al K� radiation (1.486.7 eV) from an Alanode. Photoemitted electrons from the sample were ana-lyzed in a hemispherical energy analyzer at a pass energy

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Rengaraj et al. CdS Microspheres Composed of Nanocrystals and Their Photocatalytic Activity

of Ep = 20 eV. All spectra were obtained with an energystep of 0.1 eV and a dwell time of 50 ms. A softwarepackage (Avantage Thermo VG) was used to analyze theXPS data and fit the curves. Absorption spectra of thesamples in the diffused reflectance spectrum (DRS) modewere recorded in the wavelength range of 200–1000 nmusing a spectrophotometer (Jasco V670), with BaSO4 as areference. From the absorption edge, the band gap valueswere calculated by extrapolation.To understand the specific surface area and calculate

the pore size diameter, N2 adsorption/desorption experi-ments were performed using a Quantachrome InstrumentsAutosorb 1. The pore size distributions were also calcu-lated using the Barrett-Joyner-Halenda (BJH) model onthe desorption branch. All photoreaction experiments wereperformed in a photocatalytic reactor system consisting ofa cylindrical borosilicate glass reactor vessel with an effec-tive volume of 250 ml, a cooling water jacket, and a 8-Wdaylight visible lamp (OSRAM L 8W/954, Lumilux deLuxe Daylight, Italy) positioned axially at the center as avisible light source. The reaction temperature was kept at20 �C by cooling water. A special glass fitted as an air dif-fuser was fixed at the bottom of the reactor to uniformlydisperse air into the solution.30�31

Visible light photocatalytic activity studies were per-formed to study MTBE degradation in an aqueous solu-tion. For each run, reaction suspensions were freshlyprepared by adding 0.25 g of catalyst to 250 ml ofaqueous MTBE solution with an initial concentrationof 25 mg/L. Prior to photoreaction, the suspension wasmagnetically stirred in the dark for 30 min to attainadsorption/desorption equilibrium. The aqueous suspen-sion containing MTBE and photocatalyst was then irra-diated by visible light with constant aeration. At specifictime intervals, the analytical samples were taken from thesuspension and immediately centrifuged at 4000 rpm for15 min and then filtered to remove the catalyst. The filtratewas analyzed by a gas chromatography (GC) instrumentto examine the MTBE degradation. The degree of degra-dation was derived from the GC peak intensity, which wasintegrated at a retention time of 8.77 min.

2.3. Analytical Methods

The MTBE concentration was analyzed by a GC instru-ment (Agilent Technologies, 6890 N) equipped witha headspace sampler (Agilent Technologies G1888), inwhich a DB-624 column (L: 60 m, IC: 0.250 mm narrow-bore; film: 1.40 �m) was employed and a mobile phase ofhelium was used at a flow rate of 1.2 ml/min. The amountof MTBE was determined by an inert mass selective detec-tor (Detector 5975). The total organic carbon (TOC) con-centration was determined by a TOC analyzer (Shimadzu5000 A) equipped with an auto sampler (ASI 5000).

3. RESULTS AND DISCUSSION

3.1. Crystal Structure Analysis

Figure 1 shows the XRD patterns of CdS microspheresprepared with different ratios of cadmium nitrate toTAA. The observed intense diffraction peaks are perfectlymatched to the cubic phase of cadmium sulfide (JCPDScard no. 80-0019). Sample CdS 3:1 showed diffractionpeaks corresponding to the pure cubic crystalline phase.For samples CdS 3:2 to CdS 3:4, two weak diffractionspeaks at 24.9� and 28.3� were observed, indicating theexistence of some hexagonal cadmium sulfide crystals inthe final product. The grain sizes of the synthesized CdSmicrospheres have been calculated from the full width athalf maximum (FWHM) of the diffraction peaks using theDebye-Scherrer formula,32

D = ��/� cos� (1)

where D is the mean grain size, � is a geometric factor(here, �= 1�, � is the wavelength of X-rays used for thediffraction measurements (here, � = 1�54056 Å), � is theFWHM of the diffraction peaks, which can be measuredfrom the XRD peaks, and � is the diffraction angle. Thevariation in grain size as a function of the variation in the

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CdS: Cubic phase

JCPDS card No: 80-0019

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Fig. 1. XRD pattern of CdS microspeheres prepared with different cad-mium nitrate and thioacetamide ratio, (a) CdS 3:1, (b) CdS 3:2, (c) CdS3:3, and (d) CdS 3:4.

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ratio of cadmium nitrate to TAA during CdS microspheresynthesis is plotted in Figure 2. The grain size values cal-culated from XRD analysis reveal that the average grainsize of the CdS microspheres was initially ∼20 nm (forsample CdS 3:1) and increased to ∼35 nm (for sampleCdS 3:4). That is, grain size increased with the ratio ofcadmium nitrate to TAA.Raman spectroscopy, which is useful for investigat-

ing the microstructure of crystalline materials, was alsoemployed to investigate the CdS microspheres. II–VI com-pound semiconductors are well known to crystallize fre-quently in the hexagonal wurtzite as well as the cubiczinc-blende structure.33 The Raman vibrational modes ofthe hexagonal wurtzite and cubic structures coincide veryclosely, and in the case of CdS, Raman spectroscopy can-not be used to distinguish structural differences. How-ever, the number of observed vibrational modes wouldbe sufficient for understanding the material properties. Inwurtzite CdS, there should be six observable Raman activemodes,33�34 i.e., E2 at 43 cm−1, A1 (TO) at 234 cm−1, E2

at 256 cm−1, E1 (TO) at 243 cm−1, A1 (LO) at 305 cm−1,and E1 (LO) at 307 cm−1. In cubic CdS, the TO and LOmodes appear close to the E1 (TO) and E1 (LO) modes ofthe hexagonal wurtzite structure. However, in most casesthe E1 (TO) mode cannot be observed easily, and only theE1 (LO) mode and its overtones are easily observed undernormal conditions.The Raman spectrum of the CdS microspheres is shown

in Figure 3. The first-order LO Raman peak clearlyappears at 299.3 cm−1. The peaks appearing at 600.3 and903.5 cm−1 are the second- and third-order overtones ofLO optical phonons, respectively. The observed LO Ramanpeak position agrees very well with the LO phonon peakposition reported for cubic CdS.33�35 The observed peaksobey the selection rule for zinc-blende-type CdS. Also,the peaks that could be expected in hexagonal CdS werenot detected in our observations, confirming that the CdS

3:1 3:2 3:3 3:416

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Fig. 2. Variation of CdS grain size with respect to the cadmium nitrateto thioacetamide ratio variation during CdS microsphere synthesis.

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Fig. 3. Raman spectra of CdS microspeheres. The intensity ratio for1LO to 2LO increased while increasing the Cadmium nitrate to Thioac-etamide ratio is an indication for increasing crystallinity and grain size.The overtones of LO mode occur due to the cubic structure of CdS.

microspheres possess the zinc-blende cubic structure. Fur-ther, Figure 3 also shows that the intensity ratio for 1LO to2LO increased with increasing ratio of cadmium nitrate toTAA during CdS microsphere preparation. This increasingintensity ratio can be attributed to the increased grain sizeof CdS,22 and these observations agree well with the XRDmeasurements (see Fig. 2).

3.2. Morphology

SEM and TEM were employed to examine the detailedmicrostructure and morphology of the CdS microspheres.Figure 4 depicts a typical SEM image recorded for sam-ple CdS 3:2. Similar images were also recorded for othersamples (not shown here). From the SEM images, it isclear that the synthesized CdS products are composed ofmicro- and submicrospheres varying in diameter between2 and 3 �m. A high-magnification image (Fig. 4(b)) of themicrospheres reveals that the maximum diameter is around2.5 �m. A further magnified image of these microspheresreveals that a microsphere is constructed of thousands ofsmall nanoparticles (Fig. 4(c)), the size of which tends tobe very uniform at approximately 30 to 40 nm. This valueagrees very closely with the grain size values calculatedfrom XRD measurements. Similar observations have alsobeen reported by Zhang et al.28 for CdS microspheres pre-pared by a self-assembly method.HR-TEM measurements were made to confirm the

morphology of the CdS microspheres. Typical HR-TEM images recorded for sample CdS 3:1 are shownin Figure 5. Figure 5(a) and the inset show high-magnification TEM images of the CdS microspheres.A high-resolution bright-field TEM image of a CdS sphere(sample CdS 3:1) is shown in Figure 5(b). The CdS micro-sphere clearly possesses a hollow shell structure with atypical diameter of around 2.5 �m. A magnified image


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Fig. 4. SEM images of CdS microspheres (sample-CdS 3:2). The highmagnification images of the microsphere reveal that a microsphere isconstructed by several nanocrystals.

of the shell wall revealed that it is constructed of sev-eral nanoparticles and is around 300 nm wide (Fig. 5(c)).Similar hollow structures have been observed in all sam-ples prepared with different ratios of cadmium nitrate toTAA. The morphological studies clearly showed that vari-ations in this ratio did not change the hollow structure orwall thickness of the microspheres. An HR-TEM imageof a selected area is shown in Figure 5(d); more detailedstructural information can be derived from such images.The image clearly shows lattice fringes, indicating goodcrystallinity. The corresponding fast Fourier transforma-tion (FFT) of selected area electron diffraction (SAED)is given in Figure 5(e), which exhibits relatively sharpdiffraction spots, implying a long-range ordering of thenanocrystals in the samples. Furthermore, the HR-TEM

image (Fig. 5(d)) also reveals an amorphous layer betweenthe crystallized CdS nanoparticles. EDX analysis was usedto explore the components of both the nanoparticles andthe amorphous layer, as discussed in the following section.The formation of CdS microspheres with a hollow inte-

rior could be explained by the Ostwald ripening pro-cess. Ripening is generally observed in crystal growthand involves the “growth of larger crystals from those ofsmaller size which have a higher solubility than the largerones.”36 Recently, several hollow structures such as TiO2,

37

Cu2O,38 and ZnS39 have been prepared on the basis of

this well-established phenomenon. The probable reactionsequence for the formation of CdS microspheres can besummarized as follows:

CH3CSNH2+H2O→ CH3CONH2+H2S (2)

H2S→ H++HS− → 2H++S2− (3)

Cd2++S2− → CdSNanoparticle� (4)

CdSNanoparticle� → CdSMicrosphere� (5)

As soon as reaction (4) occurs, i.e., after CdS nanocrystalsform, the nanocrystals would aggregate and form solidspheres as a result of the minimization of the system’stotal energy. At this stage in a newly formed sphere, thelarger crystallites in the exterior of the sphere are gen-erally much more loosely packed than the crystallites inthe interior. Ostwald ripening will occur during this stagebecause smaller, less crystallized, or less dense particles inan aggregate will dissolve gradually. Simultaneously, thelarger, better crystallized, or denser particles in the sameaggregate grow. Therefore, the inner small crystallites of asphere undergo a mass relocation through dissolving andre-growing, whereas the outer larger ones serve as newgrowth sites. The continuous mass transportation from theinner core to the outer surface of a single sphere wouldcreate a sphere with a hollow interior through reaction (5).SEM and TEM measurements confirm this.

3.3. Compositional Analysis

To identify the components of the synthesized micro-spheres, EDX microanalysis and elemental mappings wereperformed. The EDX spectrum and the corresponding ele-mental mappings recorded for sample CdS 3:2 are shownin Figure 6, which illustrate the actual distribution of Cdand S in the sample separately. The EDX analysis con-firmed that no elements other than Cd and S are presentin the sample. From Figure 6(b), it is clear that theEDX spectrum displays three intense peaks between 3 and3.6 KeV corresponding to cadmium L�1 (3.14 keV), L�1

(3.35 keV), and L�2 (3.54 keV); an equally strong sul-fur K�1 peak appears at 2.3 keV. These correspond to themajor constituents of the CdS microsphere. The strong Cpeak observed at 0.2 keV could be due to adsorbed C on


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Fig. 5. (a) High magnification dark field TEM image of CdS microspehers. Magnified bright filed image of (b) CdS microsphere, shows the hollowinterior of the microsphere, and (c) magnified image of the sphere wall. (d) HR-TEM image of a selected area diffraction pattern and (e) correspondingfast Fourier transformation (FFT) of the selected area electron diffraction. The sharp diffraction spots indicate the long range ordering of the nanocrystals in the samples.

the surface of the CdS microspheres due to exposure tothe atmosphere. Quantitative analysis of all samples indi-cated that the atomic ratio of Cd to S in the sampleswere approximately 1:1, which is close to the stoichiom-etry of bulk-like CdS, suggesting that the synthesized

microspheres possess nearly stoichiometric composition.The calculated stoichiometry values of all the samples arelisted in Table I. From this table it is clear that withinthe experimental error bar limits, the stoichiometry of allsamples are similar to each other and is independent to


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Fig. 6. (a) High magnification SEM image and (b) EDX pattern of a CdS microsphere. (c–d) EDX elemental mapping images of Cd and S in themicrosphere. The elemental mappings confirmed the homogeneous distribution of Cd and S elements in the CdS microsphere.

the ratio between cadmium nitrate and thioacetamide inthe starting precursor solution. Even though we changedratio of the starting materials, once after reaching thechemical equilibrium state to form CdS the increase ofthioacetamide does not have any influence to change thestoichiometry.XPS measurements were also performed to determine

the chemical composition of the microspheres. The XPSsurvey spectrum recorded for sample CdS 3:2 is depictedin Figure 7(a). Clearly, no peaks other than those ofCd, S, C, and O are observed. The XPS spectra for theCd 3d and S 2s peaks were also recorded and are shownin Figures 7(b) and (c), respectively. The binding ener-gies of Cd 3d5/2 and Cd 3d3/2 agree well with the previ-ously reported values for CdS,19�40 405.2 and 411.9 eV forCd 3d5/2 and Cd 3d3/2.

19�40 The peaks for S 2p obtained at161.5 and 162.8 eV also agree well with previous reports.

Table I. Chemical Stoichiometry of CdS microsphere calculated fromEDX.

Atomic%

Sample Name Cd S

CdS 3:1 48.51 51.49CdS 3:2 50.00 50.00CdS 3:3 50.07 49.93CdS 3:4 49.94 50.06

The Cd 3d5/2 and S 2p3/2 peak areas were determined byquantitative elemental analysis of Cd and S in the prod-ucts, yielding an atomic ratio of approximately 1:1, whichfurther confirmed that the final products are pure CdS. Theobserved C and O peaks can be attributed to the adsorp-tion of these elements on the surface of the microspheresdue to exposure to the atmosphere.

3.4. Optical Properties

UV-visible DRS was used to study the optical proper-ties of the hollow CdS microspheres. Figure 8 shows theDRS spectrum. The reflectance data were converted to theF R∝) values according to the Kubelka-Munk theory.41

The Kubelka-Munk function, F R∝� = 1− R∝�2/2R∝,was used as the equivalent of absorbance and is plottedin Figure 8. The absorption edge of the CdS microsphereswas located between 560 and 570 nm, corresponding tothe absorption edge of a semiconductor material. Also, itis interesting to note that the absorption edges for the firstthree samples are very close to each other except for sam-ple CdS 3:4, the absorption edge of which showed a slightblue shift, which can be attributed to the quantization sizeeffect42 of the CdS nanoparticles. Furthermore, the steepabsorption edge indicates a narrow size distribution anduniform crystallites in the CdS microspheres.43 The parti-cle size and uniform distribution are confirmed by SEM


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Fig. 7. XPS analysis of the CdS microsphere (sample CdS 3:2): (a) sur-vey spectrum; (b) Cd-3d binding energy spectrum; (c) S-2p bindingenergy spectrum.

(Fig. 4). By extrapolating the absorption edge by a linearfit method, the band gap values of CdS microspheres werecalculated and are listed in Table II. The band gap valuesclearly increase linearly from 2.15 to 2.22 eV.

Table II. Band gap values of CdS microspheres calculated from UV-VisDRS data.

Linear fitSample wavelength (nm) Band gap (eV)

CdS 3:1 575.114 2.156CdS 3:2 574.202 2.159CdS 3:3 573.199 2.163CdS 3:4 556.700 2.227

3.5. Photocatalytic Activity Studies

To assess the photocatalytic activity of CdS microspheres,nitrogen adsorption–desorption isotherm experiments wereperformed as described in the experimental section. TheN2 adsorption–desorption isotherm of CdS microsphereswere recorded for all the samples. The surface area analy-sis confirmed that sample CdS 3:2 possesses a larger sur-face area, and hence further experiments were performedon this sample to identify its suitability for photocatalyticstudies. The Brunauer–Emmett–Teller (BET) surface areais found to be 58 m2/g, and BJH calculations for the poresize distribution, derived from desorption data, revealedthat the distribution maximum is centered at 3.9 nm forthe sample CdS 3:2.Volatile organic compounds (VOCs) are major pollu-

tants and are considered to be one of the most importantcontaminants generated by human beings living in urbanand industrial areas.44 MTBE is a VOC that has beenwidely used as a gasoline additive to reduce VOC emis-sions from motor vehicles. However, new gasoline addi-tives such as MTBE are having negative environmentalimpacts. Recent survey reports clearly show that ground-water is often polluted due to leakage of petroleum prod-ucts from underground storage tanks. MTBE is highlysoluble in water (e.g., 0.35–0.71 M) and has beendetected at high concentrations in groundwater.45 The pres-ence of MTBE in groundwater poses a potential healthproblem.46�47 The documented effects of MTBE exposure

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are headaches, vomiting, diarrhea, fever, cough, muscleaches, sleepiness, disorientation, dizziness, and skin andeye irritation.47 To address these problems, photocatalytictreatment is the preferred treatment for polluted water.48

Figure 9 shows the degradation of MTBE as a func-tion of irradiation time in the presence of hollow CdSmicrospheres under identical reaction conditions. Two con-trol experiments were performed: (1) with visible-lightirradiation but in the absence of photocatalysts; and (2)in the presence of CdS microspheres but in the dark. Acooling water jacket at 20 �C was applied to minimizeself-degradation of MTBE due to the thermal effect, inparticular by light illumination. Although several authorshave reported that CdS itself was not an effective photo-catalyst for degrading organic pollutants,49�50 the presentinvestigation clearly demonstrates that the hollow CdSmicrospheres were active and can be efficient photocat-alysts for the degradation of MTBE under visible-lightirradiation.As shown in Figure 9, around 98% of MTBE degra-

dation was achieved at 40 min, and complete degradationwas achieved at 60 min. The curve plotted for the con-trol experiment in the presence of CdS microspheres butin the dark showed an MTBE decrease of less than 20%at 90 min, which can be attributed to the adsorption ofMTBE on CdS microspheres. Pseudo first-order kineticswas found to be obeyed for the photocatalytic degrada-tion of MTBE. On the other hand, in the absence of thephotocatalyst, MTBE degradation was much slower, andcomplete degradation could not be reached even at 2 hunder visible-light irradiation. It has been reported that theunique hollow structure of CdS microspheres would resultin special electrical properties that facilitate the transporta-tion of charge carriers, i.e., photogenerated electrons and

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0

Time (min.)

Fig. 9. Photocatalytic degradation of MTBE (25 mg/l, 250 mL, pH-5.6)under different conditions. (CdS microsphres: 250 mg of photocatalyst,visible light irradiation; CdS-Dark: 250 mg of CdS microspheres, in thedark; No catalyst: visible light irradiation in the absence of photocatalyst).

8.70 8.72 8.74 8.76 8.78 8.80 8.82 8.84 8.860.0

2.0×105

4.0×105

6.0×105

8.0×105

0 min5 min10 min15 min20 min30 min40 min50 min60 min

GC

pea

k in

tens

ity (

a.u)

Retention time (min.)

Fig. 10. The time-dependent GC peak intensity of the MTBE solution(25 mg/l, 250 mL) in the presence of CdS 3:2 microspheres under visiblelight.

holes, to the surface of the nanocrystals in the micro-spheres, making them available for reaction.51

Figure 10 displays the time-dependent GC peak inten-sity of the MTBE solution during photodegradation inthe presence of CdS microspheres. The peak at around8.77 min decreased gradually with irradiation time. At theend of the degradation, no peak could be detected, imply-ing that complete oxidation of MTBE occurred due to thepresence of hollow CdS microspheres under visible light.Finally, to study the mineralization of MTBE in the pho-

tocatalytic reaction, experiments were performed with aninitial MTBE concentration of 25 mg/l (TOC= 5�4 mg/l)in the presence of CdS microspheres. The samples werecollected at suitable time intervals for up to 60 min andthen analyzed for TOC concentration. From the photo-catalytic reaction, it has been observed that the initialTOC concentration decreased significantly in the presenceof CdS microspheres, as shown in Figure 10. The resultdemonstrated that MTBE completely degraded within 1 h,but 25% of the TOC was left after 1 h. These results con-firmed that in the presence of CdS microspheres, MTBEdegradation is faster than its mineralization.

4. CONCLUSION

In this study we demonstrated a simple, versatile solutionphase method for synthesizing high-quality CdS micro-spheres. The ratio of cadmium nitrate to TAA and thereaction time are critical parameters for tailoring the prop-erties of the microspheres. XRD analysis confirmed thecubic structure of the microspheres, which did not changewith increasing cadmium nitrate-to-TAA ratio. The crys-talline structure of the microspheres was further confirmedby Raman spectroscopy, which showed the characteris-tic Raman active modes corresponding to the CdS cubicstructure. The spherical morphology of the microsphereswas confirmed by HR-SEM and HRTEM measurements.


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Morphological studies also revealed that the diameter ofthese microspheres varied between 2 and 3 �m, and theyhad a hollow interior structure. High-magnification HR-SEM measurements showed that the hollow shell wall wasconstructed by the aggregation of several thousands ofnanoparticles 30 to 40 nm in size. The hollow interiorof these microspheres formed by Ostwald ripening. BothEDX and XPS analysis revealed that the atomic ratio of Cdand S in the sample was approximately 1:1, which is closeto the stoichiometry of bulk-like CdS. DRS measurementsshowed that the band gap of CdS microspheres increasedwith increasing cadmium nitrate-to-TAA ratio. The CdSmicrospheres exhibited good photocatalytic activity for thedegradation of MTBE aqueous solution under visible-lightillumination. Because of their facile synthesis and con-trolled morphology, the obtained CdS microspheres mayfind potential applications in catalysis.

Acknowledgments: The authors gratefully acknowl-edge research funding from the European Commis-sion (Marie Curie Transfer of Knowledge Fellowshipof the sixth Framework program under contract numberMTKD-CT 2006-042637). One of the authors (YounghunKim) is grateful for the Seoul R&BD Support Program andthe Basic Science Research Program through the NationalResearch Foundation of Korea (NRF-2010-0007050).

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Received: xx Xxxx xxxx. Accepted: xx Xxxx xxxx.


CdS Microspheres Composed of Nanocrystals and Their Photocatalytic Activity

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