Visible-Light Driven Photocatalytic Degradation of Organic Dyesover Ordered Mesoporous CdxZn1−xS MaterialsYoon Yun Lee,† Jong Hun Moon,‡ Yun Seok Choi,‡ Gwi Ok Park,‡ Mingshi Jin,§ Long Yi Jin,§

Donghao Li,§ Jin Yong Lee,*,‡ Seung Uk Son,*,‡ and Ji Man Kim*,†,‡

†SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 16419, Republic of Korea‡Department of Chemistry, Sungkyunkwan University, Suwon 16419, Republic of Korea§Key Laboratory of Natural Resource of the Changbai Mountain and Functional Molecular, Ministry of Education, and Departmentof Chemistry, Yanbian University, Park Road 977, Yanji 133002, China

*S Supporting Information

ABSTRACT: Highly ordered mesoporous CdxZn1−xS materialswere obtained via a simple nanoreplication method using amesoporous silica template with a 3-D bicontinuous cubic Ia3dmesostructure. Combined analyses using X-ray diffraction, N2sorption, electron microscopy, and diffuse reflectance UV−visible spectroscopy revealed that the ordered mesoporousternary compound semiconductor materials exhibited well-developed crystalline frameworks, high surface areas of 80−120 m2g−1, uniform mesopore sizes of about 20 nm, orderedarrangement of mesopores, and outstanding visible lightabsorption properties. Photocatalytic activities were investigatedby degradation of methylene blue and rhodamine B undervisible light over the mesoporous CdxZn1−xS materials. Due to the high surface area and outstanding light absorption properties,the ordered mesoporous CdxZn1−xS exhibited excellent photocatalytic performances for the degradation of methylene blue andrhodamine B. This study indicates a potential application of the mesoporous compound semiconductors in the efficient visible-light-driven photolysis of organics that may cause environmental pollution.

1. INTRODUCTION

Since the discovery of photocatalytic water splitting underultraviolet (UV) light illumination by using TiO2, the synthesesand photocatalytic applications of TiO2 with high UVabsorption property and reasonable stability have attractedmuch attention for the purpose of environmental remediationsuch as degradation of organic pollutants, water purification,and decomposition of carbonic acid gases.1−4 In photocatalysis,anatase TiO2 is more efficient than rutile phase due to a moreopen crystal structure that is good for diffusion of reactants andlight. However, the relatively poor stability of anatase TiO2

renders crystal structure transformation into the less efficientrutile phase upon heat treatment above 500 °C.5 AlthoughTiO2 is the most widely used material in heterogeneousphotocatalysis because of its advantages (cheap, chemicallyinert, etc.), some drawbacks do exist which undermine itsphotoefficiency in practical applications. A wide band gap (e.g.,3.2 eV for anatase TiO2), necessitates UV light irradiation forphotoexcitation to generate electron−hole pairs.6 The UV lightpossesses only a small portion (about 5%) of the entire solarspectrum, and the rest is predominantly visible light. Anyobservable shift in the photoresponse of TiO2 from UV tovisible light regime would enhance much of its photocatalyticactivities.7 Researchers have tried to extend the photoresponse

of TiO2 into the visible light region, through doping withtransition metals or nonmetal ions.7,8 However, although thetransition metal doping of TiO2 decreases the photoexcitationenergy to some extent, the metal ions also act as therecombination sites for the electron−hole pairs, thus toundermine the overall efficiency of the photocatalyst.9 More-over, elemental doping to the TiO2 crystal lattice requires quitecomplex experimental procedures and involves high temper-ature treatment. These processes will likely cause the anatasephase TiO2 to undergo phase-transition into the less activerutile phase.10

Development of alternative semiconductors with capacitiesto absorb visible light (the largest portion, 44−47% of the solarspectrum reaching the earth’s surface) is required to enhancethe photocatalytic efficiency. Therefore, the exploration ofphotochemically active semiconductors, especially with highphotocatalytic activity under visible light irradiation, is one ofthe most challenging topics in the global environmental andenergy problems.6,11−14 The nanostructured compound semi-conductors with reasonable band gap energies can be regarded

Received: January 2, 2017Revised: February 19, 2017Published: February 20, 2017

Article

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© 2017 American Chemical Society 5137 DOI: 10.1021/acs.jpcc.7b00038J. Phys. Chem. C 2017, 121, 5137−5144

as good candidates for photocatalytic decomposition of organicpollutants under visible light illumination. In particular, somemetal chalcogenides have outstanding visible light absorptionproperties with band gap energies that correspond well to thesolar spectrum, and with conduction band edges that are morenegative than the H+/H2 redox potential. Owing to theirexcellent optical and physicochemical properties, there has beengreat interest in the design and application of nanostructuredmetal chalcogenides.15 Among the various metal chalcogenides,CdS and ZnS are most widely regarded as suitable materials forphotocatalysis, due to their unique optical properties andsuitable band positions for the photocatalytic degradation oforganics in aqueous solution (the lowest conduction bandpotential is more negative than the redox potential of H+/H2;the highest valence band potential is more positive than theredox potential of O2/H2O). Various nanostructures of CdSand ZnS have been investigated as a photocatalyst for H2evolution reaction (HER), alcohol oxidation to form aldehydeor ketones, and decomposition of organic dyes or pollutantsunder visible light illumination.16−20 Typically, the mesoporousmaterials show higher photocatalytic activity than othernanostructured materials such as nanoparticles, nanowires,nanorods, and so on, due to some advantages: (1) they caninteract with ions and molecules not only at their internalsurfaces but also all over the bulk of the material, (2) highinternal surface areas provide more active sites, and (3) propermesopore sizes offer ease penetration and diffusion of reactantmolecules. For example, the mesoporous CdS with 3-Dnetworks exhibits higher conversion efficiency in conversionof 1-phenylethanol to acetophenone compared with CdSnanocrystal.21,22

In this study, the band gap energy-tuned mesoporousCdxZn1−xS materials (from 2.43 to 3.64 eV) were successfullyprepared by a nanoreplication method using an orderedmesoporous silica KIT-6 (bicontinuous cubic Ia3d structure)as a hard-template. The band gap energy of the replicatedmesoporous CdxZn1−xS was chemically controlled by control ofthe molar composition of metals (Cd/Zn). Because the bandgap energies of the mesoporous CdxZn1−xS are suitable forabsorbing visible light, it can be used as visible-light-drivenphotocatalysts for decomposition of organic pollutants.Consequently, the band gap energy dependent photocatalyticactivities of the mesoporous CdxZn1−xS for the degradation oforganic dyes, methylene blue and rhodamine B, were measuredunder visible light irradiation (λ ≥ 400 nm).

2. EXPERIMENTAL SECTION2.1. Synthesis of 3-D Cubic Ia3d Mesoporous Silica

KIT-6. Ordered mesoporous silica KIT-6 was synthesized froma published procedure, and all the chemicals were used asreceived.23,24 First, 30.0 g of Pluronic P-123 block copolymers(EO20PO70EO20, Aldrich, Mw = 5800) was dissolved in 30.0 gof n-butanol (C4H9OH, Aldrich, > 99.0%), and 1085 g ofdistilled water was added to the mixture at room temperaturefollowed by aging for 6 h to achieve complete dissolution ofblock copolymers. The clear polymer solution was kept in a 35°C water bath for 6 h in order to reach thermal equilibrium, andthen, 59 g of hydrochloric acid (HCl, Samchun Chemical, 35−37%) and 64.5 g of tetraethyl orthosilicate (Si(OCH2CH3)4,Samchun Chemical, 98%) were added into this solution. Thereaction mixture was vigorously stirred for 24 h at 35 °C andsubsequently kept for another 24 h in an oven at 100 °C understatic conditions. The white precipitates were filtered, dried,

and washed with distilled water, and some of the polymers wereremoved by ethanol extraction process. To remove the allPluronic P-123 block copolymers completely, the product wascalcined in air at 550 °C for 3 h.

2.2. Preparation of Mesoporous CdxZn1−xS Photo-catalyst. The ordered mesoporous CdxZn1−xS materials wereobtained by the published method.25 The reagent forsynthesizing the ordered mesoporous CdxZn1−xS, cadmiumsulfate hydrate (3CdSO4·8H2O, Aldrich, 99.999% trace metalbasis) and zinc sulfate hydrate (ZnSO4·7H2O, Aldrich,99.999%, trace metal basis) were used as received. In orderto synthesize the ordered mesoporous CdS or ZnS, 1.06 g of3CdSO4·8H2O or 1.77 g of ZnSO4·7H2O was dissolved in 1.5 gof doubly distilled water, respectively. A solution containing theprecursors was then infiltrated into mesopore of KIT-6 silicatemplate (1.5 g) via incipient-wetness method. After drying at80 °C in an oven for 12 h, the precursor/silica templatecomposite was heated to 500 °C for 3 h under H2 atmospherefor reductive sulfurization of precursor reagents. Subsequently,the H2 flow was changed with N2 flow, and the furnace wascooled to room temperature. The silica template was almostcompletely removed by using 2 M NaOH aqueous solutionthree times. The mesoporous CdxZn1−xS materials wereprepared by the same synthetic procedures described abovewith controlled molar composition between the Cd and Znprecursors.

2.3. Characterization. Powder X-ray diffraction (XRD)patterns were obtained in reflection mode using a RigakuUltima IV instrument equipped with Cu Kα (λ = 1.54 Å)radiation operating at 40 kV and 30 mA. Scanning electronmicroscopy (SEM) images were obtained using a Hitachi UHRS 5500 FE-SEM operating at 30 kV, and transmission electronmicroscopy (TEM) images were taken using a JEOL JEM 3010instrument at an accelerating voltage of 300 kV. N2-sorptionisotherms were measured with a Micromeritics Tristar 3000 atliquid N2 temperature, and all of the samples were completelydried under vacuum at 100 °C for 24 h before themeasurement. The diffuse reflectance UV−visible spectrawere obtained with a Shimadzu UV-3600 spectrometer.

2.4. Photocatalytic Degradation of Organic Dyes. Thephotocatalytic activities of the ordered mesoporous CdxZn1−xSmaterials were evaluated by photocatalytic degradation ofmethylene blue (MB, C16H18N3SCl, Fluka, for microscopy) andrhodamine B (RhB, C28H31ClN2O3, Sigma, ≥ 95%), in anaqueous solution. A 250 mL Pyrex beaker and 200 W Xe lampequipped with a cutoff filter (λ ≥ 420 nm) were used as areaction vessel and a light source, respectively. A total of 50 mgof the photocatalyst, ordered mesoporous CdxZn1−xS materials,was suspended in a 100 mL aqueous solution of 2 × 10−5 Mmethylene blue or rhodamine B. Before the light irradiation, asuspension containing organic dyes and photocatalyst particleswas sonicated for 10 min and vigorously stirred for 30 min inthe dark to allow dispersion of photocatalyst particles insuspension and sorption equilibrium, respectively. To retainreaction temperature and absorb infrared region during thephotocatalysis, a cooling water circulation system was equippedon the outside of the reaction vessel as shown in Scheme 1. Inthe present work, the concentrations of methylene blue,rhodamine B, and their degradation products were analyzedby checking the absorbance at 660 and 553 nm, respectively, byusing a UV−visible absorption spectrometer. At given timeintervals, 2 mL aliquots were extracted and filtered by cellulose

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acetate syringe filter (pore size =0.20 μm, 13 mm in diameter,Advantec, 13CP020AS) for removal of photocatalyst particles.

3. RESULTS AND DISCUSSION3.1. Characterization of Mesoporous CdxZn1−xS

Photocatalyst. Low- and wide-angle powder XRD patternsof the mesoporous CdxZn1−xS materials with various chemicalcomposition (x = 0−1) were represented in Figure 1. As

depicted in Figure 1A, all the low-angle XRD patterns of themesoporous CdxZn1−xS materials are different from that of theKIT-6 silica template (see Figure S1 (a)). A new peak appearedin the lower angle region than 2θ of 0.5°, which corresponds tothe (110) reflection plane, and the appearance of this new peakrepresents the mesostructure transition from cubic Ia3d totetragonal I41/a or lower space group after the silica removalprocess. This phenomenon occurs due to the specific poresystem of the Ia3d mesostructured KIT-6 silica template. Themesoscopic phase transformation could be observed in thereplication of other mesoporous materials via nanoreplicationmethods using Ia3d structured silica such as KIT-6 and MCM-48.26−30 When only one of the two chiral pore channels isincorporated with precursors, large pores (typically >10 nm)generated after the silica template etching process due to thetwo walls (wall-thickness of KIT-6 silica: 3.75 nm) and onemesopore (7.3 nm) of silica template are converted intomesopore of the resulting mesoporous CdxZn1−xS material.The crystallinity of the mesoporous CdxZn1−xS was

characterized by a wide-angle XRD technique (Figure 1B).The mesoporous CdS (Figure 1B(a)) and ZnS (Figure 1B(g))show wurtzite hexagonal P63mc and zinc blende cubic F-43m,respectively. Because the mesoporous CdS has sharper

diffraction peaks than ZnS, crystallite size of CdS would belarger than ZnS. The calculated crystallite sizes of themesoporous CdS by Scherer’s formula of about 13 nm islarger than mesopore size of silica template. It is expected thatthe crystallization process of CdS occurred at the outside of thesilica mesopores; therefore, mesostructures collapsed after silicatemplate removal and there are no specific diffraction peaks inthe low-angle XRD pattern in Figure 1A(a). In contrast, themesoporous ZnS has a crystallite size of about 5 nm, smallerthan mesopore size of silica template, so it would be crystallizedin silica pores, and formed ordered mesoporous network. As aresult, the replicated mesoporous ZnS shows several diffractionpeaks in a high-angle region (Figure 1B(g)). The crystalstructure of Cd rich (x = 0.5−0.9) materials is similar to that ofwurtzite CdS, and Zn rich (x = 0.1−0.3) materials are similar tothat of zinc blende ZnS. Thus, the diffraction peaks of themesoporous CdxZn1−xS materials are gradually shifted with thechanges of Cd/Zn ratio, owing to the atomic size difference ofthem.Figure 2A shows N2 adsorption−desorption isotherms of the

mesoporous CdxZn1−xS materials. All of the mesoporous

CdxZn1−xS materials exhibit type-IV isotherms with the H1hysteresis loop in the range of p/p0 = 0.8−1.0 and a highsurface area of 81.2−131.8 m2g−1 and large total pore volume of0.32−0.93 cm3 g−1. The pore size distributions achieved fromthe adsorption branches of about 20 nm are very similar to eachother despite their chemical composition differences (Figure2B). A pore size of 20 nm is much larger than the wall-thicknessof KIT-6 silica template, and it is well-known that the largepores generated when the microporous linker channel betweenthe two chiral pore systems of KIT-6 is broken, resulting insingle gyroid structures with mesostructure transition fromcubic Ia3d of KIT-6 silica to tetragonal I41/a or lowersymmetry of replica, which matched with low-angle XRDpatterns. The details of physical properties for the mesoporousCdxZn1−xS materials are shown in Table 1.The overall particle morphologies and several micrometers

(μm) of particle sizes of the KIT-6 silica template andreplicated mesoporous CdxZn1−xS materials are similar to eachother as depicted in Figure S1 and Figure 3a−c. Thus, asconfirmed by the HR-SEM images in the insets of Figure 3a−c,all of the mesoporous CdxZn1−xS materials exhibited highly

Scheme 1. Schematic Illustration of PhotocatalyticDegradation of Organic Dyes

Figure 1. (A) Low- and (B) wide-angle XRD patterns of themesoporous CdxZn1−xS materials, where x is equal to (a) 1.0 (CdS),(b) 0.9, (c) 0.7, (d) 0.5, (e) 0.3, (f) 0.1, and (g) 0.0 (ZnS). Figure 2. (A) N2 adsorption−desorption isotherms and (B) the

corresponding BJH pore size distribution curves of the mesoporousCdxZn1−xS materials, where x is equal to (a) 1.0 (CdS), (b) 0.9, (c)0.7, (d) 0.5, (e) 0.3, (f) 0.1, and (g) 0.0 (ZnS).

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ordered mesostructures over the whole particles. Themesoporous CdS shows disordered mesostructures (Figure3d), and this is good agreement with low-angle XRD pattern inFigure 1A(a). The HR-TEM image of the mesoporous CdS inFigure 3d inset indicates the (002) plane of crystallite, and italso supports the developed crystallinity which was suggestedby the wide-angle XRD pattern in Figure 1B(a). Slightlydifferent from the mesoporous CdS, the mesoporousCd0.5Zn0.5S and ZnS show ordered mesostructures and uniformpre sizes as depicted in Figure 3e,f. The HR-TEM images of themesoporous Cd0.5Zn0.5S and ZnS in Figure 3e,f insets are (103)and (311) crystal planes, respectively.The visible light sensitivity of the mesoporous CdxZn1−xS

materials was characterized by diffuse reflectance UV−visiblespectroscopy. As shown in Figure 4A, the light absorptionedges of the mesoporous CdxZn1−xS materials are graduallyshifted to a shorter wavelength region with Cd composition xas decreases from 1.0 (CdS) to 0.0 (ZnS). Figure 4B representsthe Kubelka−Munk plots for determining the band gap energyof the mesoporous CdxZn1−xS materials. The estimated bandgap of the mesoporous CdS (2.43 eV) and ZnS (3.64 eV) areslightly larger than those of bulk CdS (wurtzite 2.42 eV) andZnS (zinc blende 3.50 eV). The band gap expansions of themesoporous CdS and ZnS are related to the quantumconfinement effects, due to their nanosized frameworks. Theband gap energies of the mixed mesoporous CdxZn1−xSmaterials are almost linearly increased from 2.47 to 3.47 eV

with decreases of Cd composition x as goes from 0.9 to 0.1, anddetailed values are presented in Table 1.

3.2. Photocatalytic Degradation of Organic Dyes. Inthe presence of air or oxygen, the light illuminated semi-conductor photocatalysts are capable of destroying manyorganics. The activation of semiconducting materials by lightenergy (hv) generates electron (e−) and hole (h+) pairs whichare reductant and oxidant, respectively. In the degradation oforganics, the hydroxyl radial (•OH) which comes from theoxidation adsorbed water or adsorbed hydroxyl (−OH) is theprimary oxidant, and the presence of oxygen could prevent therecombination of electron−hole pairs. For a complete photo-catalytic degradation of methylene blue (MB) and rhodamine B(RhB), the final products of the reaction among others are CO2and H2O.

→ + + + +

+

+ − −

−

C H N SCl CO H O NH NO SO

Cl (MB)16 18 3 2 2 4 3 4

2

→ + + +− −C H ClN O CO H O NO Cl (RhB)28 31 2 3 2 2 3

In order to determine the concentration of organic dyes at agiven time after photocatalysis initiated from the UV−visibleabsorption spectra, standard curves for methylene blue andrhodamine B (relationship between the absorbance andconcentration) were prepared (see Figure S2). Figure 5shows photocatalytic degradation of methylene blue andrhodamine B in terms of relative concentration changes as afunction of the light irradiation time in the presence ofmesoporous binary compound semiconductors, CdS and ZnS,respectively, and detailed UV−visible spectra are presented inFigure S3. Due to the higher surface area and larger porevolume of mesoporous ZnS (123.7 m2g−1, 0.82 cm3g−1) thanCdS (81.2 m2g−1, 0.38 cm3g−1), adsorption amounts of organicdyes on the surface of photocatalyst particles are also larger inthe case of mesoporous ZnS than CdS. During the sorptionequilibrium process conducted for 30 min in the dark, only2.1% and 5.7% of the initial concentrations of methylene blueand rhodamine B were adsorbed by mesoporous CdS (Figure6A(a) and B(a)), while mesoporous ZnS adsorbed 10.3% and13.4% of dyes (Figure 5A(b) and B(b)), respectively. As aresult, the initial relative concentration diminutions of themesoporous ZnS were larger than those of mesoporous CdS.The absorbance of organic dyes gradually diminished as the

exposure time increased, which is more pronounced in the caseof using mesoporous CdS as a photocatalyst. Therefore, theconcentration changes of methylene blue and rhodamine B in

Table 1. Physical properties of the mesoporous CdxZn1−xSmaterials

material SBET (m2 g−1)a Vtotal (cm3 g−1)b Dp (nm)

c Eg (eV)d

CdS 81.2 0.38 18.3 2.43Cd0.9Zn0.1S 85.1 0.38 21.7 2.47Cd0.7Zn0.3S 79.8 0.32 18.0 2.61Cd0.5Zn0.5S 102.9 0.67 19.7 2.78Cd0.3Zn0.7S 131.8 0.65 20.1 3.10Cd0.1Zn0.9S 119.8 0.93 19.7 3.47ZnS 123.7 0.82 19.6 3.64

aSurface areas were calculated by BET method. bTotal pore volumeswere estimated from the N2-sorption isotherms at p/p0 = 0.99. cPoresizes were calculated from the adsorption branches of the N2-sorptionisotherms by the BJH method. dBand gap energies were estimatedfrom the Kubelka−Munk plots.

Figure 3. (a−c) SEM and (d−f) TEM images of the mesoporous (a,d) CdS, (b, e) Cd0.5Zn0.5S, and (c, f) ZnS.

Figure 4. (A) Diffuse reflectance UV−visible spectra and (B) Kubelka-Munk plots of the mesoporous CdxZn1−xS materials, where x is equalto (a) 1.0 (CdS), (b) 0.9, (c) 0.7, (d) 0.5, (e) 0.3, (f) 0.1, and (g) 0.0(ZnS).

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the presence of mesoporous CdS particles is greater than in thepresence of mesoporous ZnS, so that for CdS photocatalyst, theconcentration of methylene blue decreased by half after 30 min,and 99.1% of methylene blue and 99.7% of rhodamine Bdecomposed after 90 min, while for the mesoporous ZnS, only8.4% of methylene blue and 10.5% of rhodamine B weredecomposed after 90 min. To show the visible-light-drivenphotocatalytic activity, a small band gap is required for aphotocatalyst material. Another important criterion for thephotocatalytic degradation of organic compounds is that theredox potential of the H2O/•OH (E° = −2.8 V) couple lieswithin the valence and conduction bands of the semiconductingphotocatalysts. The different photocatalytic activities of themesoporous CdS and ZnS might be related to their band gapenergies and band alignment with a redox potential of H2O and

O2. The larger band energy of the mesoporous ZnS (3.64 eV)gives a poor absorption of visible light range, although itsconduction band bottom potential is sufficiently more negativethan the redox potential of H+/H2O and O2/O2

−. In contrast,mesoporous CdS has a small band gap energy of 2.43 eV whichis good for the visible light absorption and shows reasonablephotocatalytic activity for degradation of organic dyes inaqueous media containing oxygen.In order to increase overall photocatalytic activities,

mesoporous compound semiconductors with controlled bandgap energies were prepared by controlled chemical compositionof nanosized frameworks. For comparison, the concentrationchanges of methylene blue and rhodamine B in the presence ofcommercial TiO2 particle (Degussa P25) under exposure to thevisible were measured and compared with those determined inthe presence of mesoporous CdxZn1−xS materials as shown inFigure 6A,C. Because of the wide band gap energy, the P25(TiO2) particle could not absorb wavelength range of irradiatedlight (λ ≥ 420 nm) and shows no photocatalytic activity asdepicted in Figure 6A(a) and C(a). Apparently, the photo-catalytic activity of the mesoporous CdxZn1−xS materials wasstrongly affected by their chemical composition. The Cd richmaterials (0.7 ≤ x ≤ 1.0) almost perfectly decomposemethylene blue and rhodamine B molecules after the lightirradiation time of 90 min as seen in Figure 6A(b−d) and B(b−d). The mesoporous Cd0.7Zn0.3S material, in particular, showsthe best photocatalytic activity in both cases. In the case ofmesoporous Cd0.5Zn0.5S, only 60% of methylene blue and 56%of rhodamine B were degraded after 90 min (Figure 6A(e) andC(e)), and the photocatalytic activity was decreased withincreasing Zn contents from 0.7 to 1.0 as depicted in Figure6A(f−h) and C(f−h). Detailed UV−visible absorption spectralchanges methylene blue and rhodamine B by using P25 and themesoporous CdxZn1−xS materials are presented in Figure S4and S5, respectively.Figure 6B,D represent the linear plots of ln(C/C0) for the

photodegradation of methylene blue and rhodamine B with themesoporous CdxZn1−xS photocatalyst under visible lightirradiation after 90 min, respectively. The slope of the plotswhich represents the apparent rate constant (kapp) ofphotocatalysis was calculated by ln(C/C0) = −kappt and listedin Table 2. It could be seen from Table 2 that the apparentreaction rate of the mesoporous CdxZn1−xS in the degradationof organic dyes increases with decreasing Cd ratio and reaches

Figure 5. Changes of relative concentration of (A) methylene blue and (B) rhodamine B by using the mesoporous (a) CdS and (b) ZnS.

Figure 6. (A, C) Changes of relative concentration and (B, D) linearplots of ln(C/C0) for the photocatalytic degradation of (A, B)methylene blue and (C, D) rhodamine B, respectively, by using (a)P25 (TiO2), the mesoporous CdxZn1−xS materials where x is equal to(b) 1.0 (CdS), (c) 0.9, (d) 0.7, (e) 0.5, (f) 0.3, (g) 0.1, and (h) 0.0(ZnS).

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its maximum at x = 0.7. The maximum apparent rate constantsfor methylene blue and rhodamine B are 7.88 × 10−2 min−1 and9.53 × 10−2 min−1, respectively.For the separate photocatalytic activities of the mesoporous

CdxZn1−xS photocatalysts, the difference of the band gapenergy would be the dominating reason that results in thedifferent absorption properties in the visible light range.6 Withthe decline of Cd composition x in mesoporous CdxZn1−xSmaterials, the position of the valence band edge hardly changes,whereas the conduction band edge becomes more negative,which lead to the noticeable blue-shift of the band gap energyof them (Figure 7). The widened band gap energy of the

mesoporous CdxZn1−xS results in reduced absorption of thevisible light, which further leads to a decrease in photochemi-cally generated electron−hole pairs. As a result, photocatalyticactivities could decrease. In contrast, the more negativeconduction band edge of the photocatalyst is advantageousfor the photoreduction, which might also affect the redoxprocesses. The manipulation of the composition of themesoporous CdxZn1−xS materials offers a powerful role inproducing a photocatalyst with controlled activities.As discussed, there are two important reasons for the visible-

light-driven photocatalytic degradation of organic dyes: (1)absorption capacity of the visible light and (2) position of theconduction band edge of the photocatalysts. The photocatalystwith smaller band gap energy, i.e., Cd rich phases in themesoporous CdxZn1−xS, is the better photocatalyst in theviewpoint of the visible light absorption ability. Thus, theconduction band edge position of photocatalyst mainlyinfluences on electron transfer rate from the catalyst particleto water molecule. The energy difference between theconduction band edge and redox potential of oxygen in thewater, i.e., ΔG in Figure 8, strongly effects the photoreductionreaction which produces superoxide (O2

−) or peroxides (O22−)

anions, and Zn-rich materials are better photocatalysts in the

perspective of this point. Of course, hole transfer from thevalence band of the photocatalyst to water to produce hydroxylradical (•OH) also effects the photocatalytic efficiency;however, the valence band positions of the mesoporousCdxZn1−xS are quite similar to each other in spite of theirchemical composition difference of the frameworks. So, wecould expect that the electron transfer rate between thephotocatalyst particle and oxygen is the main factor fordetermining the decomposition efficiency. In practice, twofactors competitively effect the photocatalytic activities of themesoporous CdxZn1−xS, and an optimized compositional value(x = 0.7 in this case) for increasing activity exists between thetwo factors.In addition, the photodecomposition efficiency of rhodamine

B is higher than that of methylene blue despite the chemicalcomposition difference of the mesoporous CdxZn1−xS photo-catalysts. The electron transfer phenomenon from theconduction band of the photocatalyst to the LUMO level ofthe organic one can reduce the photocatalytic efficiencybecause this electron quenched out in organic molecules bythe relaxation processes. As shown in Figure 8, this unfavorableelectron transfer phenomenon more quickly occurred inmethylene blue than rhodamine B, because of the LUMOlevel of methylene blue than the conduction band of themesoporous CdxZn1−xS materials. However, this unfavorableelectron transfer could be significantly suppressed in rhodamineB due to its high LUMO level energy.The photocatalytic activities of nanostructured TiO2, CdS,

ZnS, and CdxZn1−xS are summarized in Table 3. The 10 nmTiO2 nanoparticles completely decomposed within 2 h underUV-A (wavelength from 310 to 400 nm) irradiation, whereasmesoporous TiO2 degrades only 96% of methylene blue underUV lamp.31,32 96% of rhodamine B dye was degraded bycommercial TiO2 (Degussa P25) photocatalyst after 2 h of UVirradiation.33 However, due to the wide band gap energy,mesoporous TiO2 shows no photocatalytic activity under visiblelight (>400 nm) irradiation despite its high surface area (>100m2/g).34 In order to overcome the limitation of TiO2-basedphotocatalyst, semiconducting materials that can absorb in thevisible light range such as CdS, ZnS, and so on were alsoinvestigated as photocatalysts. A mixture of CdS and ZnSnanoparticles (20:80) decomposed about 85% of the initialconcentration of methylene blue after 6 h under strong visiblelight irradiation.35 Mesoporous ZnxCd1−xS nanoparticlesprepared by Wang et al. show the best photocatalytic activityfor degradation of rhodamine B under visible light irradiation(100% degraded before 15 min irradiation time).6 Themesoporous CdxZn1−xS microparticles which were preparedin this work also perfectly removed methylene blue and

Table 2. Photocatalytic Degradation of Organic Dyes

materialkapp (MB)(min−1)a

t1/2 (MB)(min)b

kapp (RhB)(min−1)a

t1/2 (RhB)(min)b

P25(TiO2) 2.62 × 10−4 2641 5.07 × 10−4 1367CdS 5.12 × 10−2 13.5 6.01 × 10−2 11.5Cd0.9Zn0.1S 5.01 × 10−2 13.8 7.12 × 10−2 9.74Cd0.7Zn0.3S 7.88 × 10−2 8.80 9.53 × 10−2 7.27Cd0.5Zn0.5S 9.30 × 10−3 74.5 9.57 × 10−3 72.5Cd0.3Zn0.7S 2.01 × 10−3 345 2.63 × 10−3 264Cd0.1Zn0.9S 1.19 × 10−3 583 1.44 × 10−3 480ZnS 9.04 × 10−4 767 9.32 × 10−4 743

aApparent rate constant is calculated by first order equation, ln(C/C0)= −kappt. bHalf-life times of organic dyes were estimated by theequation t1/2 = (ln 2)/kapp.

Figure 7. Schematic relative band alignment of the mesoporousCdxZn1−xS and redox potentials of hydrogen and oxygen (vs NHE).

Figure 8. Schematic illustration of relative band alignment of themesoporous CdxZn1−xS and HOMO−LUMO level of methylene blue(MB) and rhodamine B (RhB). HOMO−LUMO level of organic dyesare calculated by the DFT B3LYP/16301G* method.

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rhodamine B within 70 min. However, the amount ofphotocatalyst used in this work was half of that used in thestudy by Wang et al., and the concentration of the dye solutionwas double that of them. We believe that this study will revealthe prospect of large-scale photocatalytic degradation of organicpollutants in aqueous solution.

4. CONCLUSIONHighly ordered mesoporous CdxZn1−xS materials exhibitinghigh surface areas, large pore volumes, uniform pore sizes,developed crystallinity, and micrometer-sized particles withcompositional homogeneity were successfully obtained via ananoreplication method using a 3-D cubic Ia3d mesoporoussilica KIT-6 as a hard-template. The optical property and bandgap energy profile of the replicated template-free mesoporousCdxZn1−xS materials were easily controlled by manipulating thechemical composition of the frameworks. Diffuse reflectanceUV−visible spectroscopy studies indicate that the band gapenergies of the mesoporous CdxZn1−xS materials widened withdecreasing the amounts of Cd. We investigated the mesoporousCdxZn1−xS material as a visible-light-driven photocatalyst forthe degradation of methylene blue and rhodamine B. Undervisible light illumination, the mesoporous CdxZn1−xS materialsshowed composition-dependent photocatalytic activities for thedecomposition of organic dyes. The photocatalytic efficiency ofthe mesoporous CdxZn1−xS increases evidently with a decreasein the Cd contents from 1.0 to 0.7 and reduced with the furtherdecrease of Cd composition from 0.7 to 0.0. This findingpresents new possibilities for novel chalcogenide semi-conductors as the visible-light sensitive catalysts in photo-catalytic applications, especially in the field of environmentalremediation.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.7b00038.

X-ray diffraction patterns, N2 adsorption−desorptionisotherms and TEM images of the KIT-6 silica template.UV−visible absorption spectra of methylene blue andrhodamine B. (PDF)

■ AUTHOR INFORMATIONCorresponding Authors* E-mail: jinylee@skku.edu.* E-mail: sson@skku.edu.* E-mail: jimankim@skku.edu.ORCIDJin Yong Lee: 0000-0003-0360-5059

Ji Man Kim: 0000-0003-0860-4880Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Research Council ofScience and Technology (NST) through Degree and ResearchCenter (DRC) Program (2014; No. DRC-14-03-KRICT).

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Table 3. Photocatalytic Degradation of Methylene Blue and Rhodamine B by Various Nanostructured Materials

degradation

material light source dyecatalyst

concentration time (min)ratio(%)

TiO2 NPs32 9 V UV-A MB, 10 mg/L 0.5 g 120 100

mesoporous TiO231 250 W Fe-doped halide UV bulb MB, 5 × 10−5 M 0.1 g/200 mL 180 96

P-25 TiO233 UV-LED RhB, 2.08 × 10−5 M 1.6 g/L 180 96

mesoporous TiO234 300 W Xe lamp RhB, 1.5 × 10−5 M 0.05 g/50 mL 120 <1

mixture of CdS and ZnS NPs35 500 W halogen lamp MB, 10 mg/L 100 mL/g, 0.6 L 300 ∼85Mesoporous ZnxCd1‑xS NPs6 500 W Xe lamp RhB, 10−5 M 0.05 g/50 mL <15 100Mesoporous CdxZn1‑xS [this work] 200 W Xe lamp MB, 2 × 10−5 M RhB, 2 × 10−5 M 0.05 g/100 mL 70 MB 60 RhB 100

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