Tunable Synthesis of Various Wurtzite ZnS Architectural Structures and Their Photocatalytic...

DOI: 10.1002/adfm.200600891

Tunable Synthesis of Various Wurtzite ZnS Architectural Structuresand Their Photocatalytic Properties**

By Shenglin Xiong, Baojuan Xi, Chengming Wang, Dechen Xu, Xiaoming Feng, Zhichao Zhu, andYitai Qian*

1. Introduction

In recent years, controlling the morphology and size of nano-materials is a crucial issue in nanoscience research attributedto expectation of novel properties. Although 1D nanomateri-als, such as naowires,[1] nanobelts and nanotubes,[2,3] exhibit lotsof special properties in the optical, electrochemical, mechanicaland thermal facets, in comparison with 1D nanostructures, thecomplex 3D architectures constructed by 1D structures possi-bly provide a great deal of opportunity to explore their novelproperties due to their complex structures and thus have beenattracting the broad attention in the materials science. Toachieve the goal of the self-assembly of 1D nanostructures, de-veloping rational routes to construct complex 2 or 3D architec-tural structures is of great importance. At present, considerableeffort has been devoted to synthesizing semiconductor 3Dstructures self-assembled by 1D structures such as hierarchicalZnO nanostructures, selenium nanowire network, and 3D CdSarchitecture.[4–6] In particular, self-assembly of 1D structure

into 3D structure via the solution-based route is more attrac-tive owing to its gentle synthesis conditions, easiness to oper-ate, and the large production.

As an important II–VI semiconductor with a wide bandgap(3.72 eV for cubic phase[7] and 3.77 eV for the hexagonal wurt-zite phase[8] at room temperature), ZnS has been called a per-spective material for optoelectronic devices. As we all know,the cubic phase (zinc blende) of ZnS is sable at room tempera-ture while the hexagonal phase (wurtzite) is a high-tempera-ture form. The equilibrium phase transformation between cu-bic and hexagonal phases occurs at about 1020 °C.[9]

Considerable effort has contributed to design effective meth-ods to synthesize ZnS nanocrystallines with different shapeand size.[9a,10] however, most of the reports produced cubic zincblended phase of ZnS, with very in the hexagonal wurtzitestructure due to its metastable character.[10°] Several pioneer-ing work developed various synthesis routes for the prepara-tion of wurtize ZnS in solution at low temperature.[10e,m,°,t,11–16]

Although urchin-like structures and spherical architecturesself-assembled with nanorods have been observed for theabove-mentioned II–VI semiconductors, the synthesis of com-plex 3D structures made of hexagonal wurtzite phase of ZnScrystals through the solution-based route at low temperaturehave been few reported until recently.[17]

In this study, novel and complex wurtzite ZnS 3D architec-tures and 1D nanostructures have been prepared via a mono-dentate-amine-based solvothermal approach in a binary solu-tion made of distilled water and ethanol amine. L-cysteine(C3H7NO2S), an amino acid is used as a sulfur source in ourwork. More importantly, ZnS 3D nanostructures are confirmedto possess extraordinary photocatalytic ability to photo-degra-dation of acid fuchsine compared with that of commercial ZnSpowders under infrared-light irradiation and commercial TiO2

2728 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17, 2728–2738

–[*] Prof. Y. T. Qian, Dr. S. L. Xiong, B. Xi, C. Wang, D. Xu, X. Feng,

Z. ZhuHefei National Laboratory for Physical Sciences at Microscale andDepartment of ChemistryUniversity of Science and Technology of ChinaHefei, Anhui, 230026 (P.R.China)E-mail: [email protected].

[**] The financial support of this work, by National Natural Science Foun-dation of China and the 973 Project of China (No. 2005CB623601), isgratefully acknowledged. Supporting Information for this article isavailable online from Wiley InterScience or from the author.

Semiconductor ZnS with novel and complex 3D architectures such as nanorods (or nanowires) networks, urchinlike nanosturc-tures, nearly monodisperse nanospheres self-assembled from nanorods and 1D nanostructures (rods and wires) had beensynthesized in a binary solution by controlling the reaction conditions, such as the volume ratio of the mixed solvents and thereaction temperature. The morphology of ZnS changed from 3D architectural structures to 1D rodlike (or wirelike) shapewhen the temperature was increased from 160 to 200–240 °C. The possible growth mechanisms for the formation of nano-spheres self-assembled from nanorods are tentatively discussed according to the experimental results. The photocatalytic activ-ity of various ZnS nanostructures has been tested by degradation of acid fuchsine under infrared light compared to that ofcommercial ZnS powders under infrared-light irradiation and commercial TiO2 powders under UV-light irradiation, indicatingthat the as-obtained ZnS nanostructures exhibit excellent photocatalytic activity for degradation of acid fuchsine.

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powders under UV-light irradiation. The present work showsthat the solvothermal route is facile, cheap, and versatile. Thus,it is very easily scaled up to industrial application and brings usnew light to the synthesis and self-assembly of functionalmaterials.

2. Results and Discussion

2.1. Structural Study

Figure 1 shows the X-ray diffraction (XRD) patterns of theas-prepared products at different volume ratio water/EA withthe same initial reactant concentrations (the concentrations of

Zn2+ and L-cysteine were 0.02 mol L–1, 0.04 mol L–1, respective-ly) after reaction for 24 h at 160 °C. The reflection peaks of thedifferent products can be indexed as wurtzite ZnS with latticecontants a = 3.82 Å and c = 6.26 Å, which is in good agreementwith the literature values (JCPDS Card No. 36-1450). No peaksof impurities were detected, revealing the high purity of the as-synthesized products. The XRD patterns demonstrate thatwell-crystallized hexagonal ZnS were obtained under presentsynthetic conditions.

2.2. Morphology

The morphology and size of the as-obtained products wereobserved by FESEM and TEM. Figure 2a shows that the prod-uct is composed of spherical structures assembled by loosely in-tertwisting nanorods in a binary solution with a volume ratio ofVwater:VEA of 2:3. But these spherical structures have no ob-vious region between each other, and most of them are inter-connected together. The magnified image (Fig. 2b) clearly indi-cates the exact configuration of the surface of a sphere, namely,the nanorod-networks. Interestingly, urchin-like structures

were obtained in the final products when only the volume ratioof Vwater:VEA was increased to 3:2. From Figure 3a and b, it canbe observed that these urchin-like structures have the tendencyto interconnect with each other. Close observations on a typicalbroken urchin-like structure exhibited in Figure 3c indicatethat these structures are constructed from tightly self-as-sembled nanorod arrays.

TEM images (Fig. 4a and b and Supporting Information,Fig. S1) further confirm that the urchin-like structures are con-structed by radial nanorods arrays from center to the surface ofspheres. Close examination of the image of Figure 4c showsthat the length of the constituent nanorods is 300–350 nm. TheHigh-resolution TEM (HRTEM) image of Figure 4d reveals atypical single nanorod with a diameter of 5.25 ± 0.25 nm andthe lattice spacings of 0.31 nm, corresponding to the (002). Theanalyses of HRTEM and the inserted FFT ED pattern bothconfirm that the as-obtained products are single-crystalline innature and grow preferentially along the [001] direction. Theelemental analysis with EDS shown in Figure 4e demonstratesthat the molar ratio of Zn an S is 1.157:1 (the signals of Cu arefrom the copper grid), which is close to that of the standard

Adv. Funct. Mater. 2007, 17, 2728–2738 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.afm-journal.de 2729

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Figure 1. XRD patterns of the products obtained under different condi-tions: a) pure EA; b) 1:4; c) 2:3; d) 3:2; e) 9:1. The molar ratio Zn(Ac)2/L-cysteine = 1:2, with reaction for 24 h at 160 °C.

Figure 2. FESEM images of the ZnS nanorod networks. a) panoramic im-age and b) magnified image. The products were fabricated at 160 °C for24 h in mixed solutions with the volume ratios of Vwater:VEA= 2:3.

Figure 3. a), b), and c) were the FESEM images of the ZnS urchin-likestructures with different magnification. The products were fabricatedat 160 °C for 24 h in mixed solutions with the volume ratios ofVwater:VEA= 3:2.

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S. Xiong et al./Wurtzite ZnS Architectural Structures and Their Photocatalytic Properties

stoichiometric composition, in consistent with the results ofXPS (see Supporting Information, Fig. S2a).

Figure 5a and b is typical FESEM images of the sample pre-pared with the volume ratio of Vwater:VEA reaching 9:1, indicat-ing that these products are composed of nearly monodispersenanospheres with narrow diameter distributions of about 500–600 nm. High-magnification FESEM images (Figs. 4b) (seealso Supporting Information, Fig. S3a and b) clearly revealsthat the nanospheres are constructed from the assembly ofnanorods. TEM images (Fig. 4c–e and Supporting Information,

Fig. S3c–h) further confirm that these spheresare assembled with nanorods which are highlydirected to form radial arrays from center tothe surface of spheres. Close examination ofthese images shows that the length and the di-ameter of the constituent nanorods are250–300 nm and 7–9.5 nm, respectively. TheHigh-resolution TEM image of individualnanorod (Fig. S3h) confirms the single-crystalnature and the preferential [001] directiongrowth.

In our experiments, the morphology of theproducts varies greatly with the change of thevolume ratio of Vwater:VEA, keeping the otherconditions constant, suggesting the volume ra-tio of Vwater:VEA plays a crucial role in theformation of kinds of ZnS nanostructures.When the volume ratio of Vwater:VEA de-creased to 1:4, what had been obtained isshown in Figure 6a and b, revealing that theproducts consist of nanowire networks whichare constructed by ultrathin and flexiblenanowires with diameter in the range of2.5–7.5 nm (see Fig. 6c and d), obviously dif-ferent from the morphology of the above-mentioned products. Energy-dispersive spec-tra (EDS) analysis of network structuresshown in Figure 4f demonstrates that theproducts contain only Zn and S and their mo-lar ratio is 1.132:1 (the signals of Cu and C arefrom the copper grid), in agreement with theresults of XPS (see Supporting Information,Fig. S2b). It is worthy to point out that theproducts are composed of nanoribbon net-works only using ethonal amine as solvent(see Supporting Information, Fig. S4), whileaggregate of particles have been fabricated inthe final products only using water as solvent(see Supporting Information, Fig. S5). Thisbinary solution reaction system is quite com-plicated and the formation of certain nano-structures needs further revolution. Ob-viously, the synergetic effects of ethonalamine and water could exert a key influenceon the morphology of ZnS nanostructures.

2.3. Time-Dependent Experiments

In order to study the growth mechanism of the ZnS nano-spheres self-essembled from nanorods, a series of experimentswere carried out with different lengths of time:

a) 15–35 min at 160 °C: a milk-white suspension was ob-served to form after 15 min, showing the formation of ZnS at afast growth rate. Figure 7a shows the TEM image of the prod-uct obtained after 15 min, which reveals the product consistedof urchinlike particles with size ranging from 90 to 240 nm.

2730 www.afm-journal.de © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2007, 17, 2728–2738

Figure 4. a), b) TEM and c), d) HRTEM images of the ZnS urchin-like structures. The productswere fabricated at 160 °C for 24 h in mixed solutions with the volume ratios of Vwater:VEA= 3:2.e) The corresponding EDS spectrum taken from the urchin-like structures indicated in (c). Theinserted in (d) is the corresponding SAED pattern recorded from a single ZnS nanorod.

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The inserted ED pattern in Figure 7a demonstrated the crystal-linity of these urchinlike particles. When the reaction time wasprolonged to 35 min, a great deal of white precipitate gener-ated in the bottom of autoclave. Nearly monodisperse urchin-like particles formed and up-grew in the size compared to thesample in Figure 7a (see Fig. 7b and c).

b) 45 min at 160 °C: When the reaction time was increasedto 45 min, the morphology of the products changed remark-ably. As shown in Figure 7d, a minority of nanospheres self-as-sembled with nanorods and urchinlike nanoparticles coexistedin the products after aging for 45 min.

c) 1–24 h at 160 °C: From Figure 7e–g, it is clearly observedthat subsequently these nanospheres gradually increased in thequantity and meanwhile urchinlike nanoparticles gradually re-duced and finally monodiperse nanospheres self-assembledwith nanorods predominated in the same region on the surfaceof TEM grid.

XRD patterns of the products prepared at 35, 45, 60 and120 min are shown in Figure S9. XRD data show that the hex-agonal ZnS phase begins to form within 35 min and the prod-ucts gradually crystallized with the extension of time. Both EDand XRD analyses demonstrate that urchinlike nanoparticlesdirectly grew from the solution and no amorphous ZnS parti-cles generated during the whole growth process different from

other similar inorganic nanomaterials, suggesting a new growthmechanism of ZnS nanospheres is present in our synthetic sys-tem.

In the current study, it is found that L-cysteine and ethanolamine are indispensable for the formation of nanospheres self-assembled with nanorods. When the other sulfur source (for in-stance, thioacetamide and thiourea) replaced L-cysteine, or an-other amine (for instance, ethylenediamine) was substitutedfor ethanol amine, no nanospheres self-assembled with nano-rods obtained in the final products (see Supporting Informa-tion, Figs. S7 and S8), which indicates that the formation ofthese novel 3D structures may be relevant to the special struc-ture of L-cysteine and ethanol amine.

The exact mechanism for the formation of these ZnS nano-spheres is still completely unclear. However, it is obvious thatthe formation of the urchinlike particles is of great importancein the growth of the nanospheres self-assembled from nanorods.Through investigating the time-dependent the formation pro-cess of ZnS nanospheres, we speculate that the present synthe-sis involves the L-cysteine-governed nucleation growth processand EA-dominated oriented-assembly process. Based on a pre-vious report, metal ions could react with L-cysteine to form stea-dy complexes.[18] Therefore, it is reasonable to conclude that, inour approach, at the initial stage of the solvothermal reaction,the free Zn2+ can coordinate with L-cysteine to form L-cysteine-Zn2+ complex and then decompose to form initial ZnS nuclei insuch a fast rate that the urchinlike nanoparticles immediatelyformed due to the orientation-directed effect of ethanol amine(see Fig. 7a). The color change of the solution also confirmedthis process, when L-cysteine was added into the Zn(Ac)2 solu-tion. The reagent ethanol amine maybe adsorb onto the (010)and (100) of the incipient ZnS nuclei possibly resulting from thematch between the special ZnS atomic surface structures andthe linear molecular structure of ethanol amine, and thus mak-ing (001) a high energy face as compared to the EA-covered(010) and (100). Direct evidence for the hypothesis requiresfurther study and the work is under way. Meanwhile the analy-ses of FTIR also demonstrate the existence of ethonal amine inthe prepared ZnS products in comparison with the IR spectrumof pure ethonal amine (see Supporting Information Fig. S10).The selective absorption of ethonal amine onto some facets notonly prevents the particles from the agglomeration but also in-fluence the growth of these planes, which was strongly in favorof the anisotropic growth of the new born ZnS nuclei along[001] axis further confirmed by the HRTEM result so that theurchinlike particles instead of irregular particles formed in theinitial reaction stage (only 15 min). Then, oriented attachmentof the ripening process of urchinlike nanopartiles followed bycrystallographic fusion of the (001) faces conforms to that de-scribed first by Penn and Banfield for TiO2 nanocrystals[19] andguides the formation of 1D nanorods and these nanorods self-assemble to form ordered 3D structures due to the direction-oriented role of ethanol amine. This may accord with the factthat the crystal growth is modulated extrinsically by solvent ab-sorption on certain crystallographic facets which inhibits thegrowth of some crystal planes and leads to different growthrates during the growth process of particle thus generating cer-


Figure 5. SEM and TEM images of the nanospheres. a) General view ofthe nanospheres. b)–e) Magnified SEM and TEM images of the nano-spheres, clearly showing the nanospheres are constructed from shortnanorods. The products were fabricated at 160 °C for 24 h in mixed solu-tions with the volume ratios of Vwater:VEA= 9:1.

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tain novel crystal shapes.[20] Additionally, it is noteworthy point-ing out that the products consist of spherical structures aggre-gated with nanoparticles when didentate ethylenediamine (twoN-chelating atoms) was used instead of ethanol amine (oneN-chelating atom) in reaction system with other conditions un-changed (see Fig. S8). The result suggests that ethonal aminewith one N-chelating atom prevents growth of nanoparticlesand favors formation of 1D nanostructures and further self-as-sembly into 3D architectures, which may originate from the dif-ferent structure of the two kinds of amine.

2.4. Influence of Reaction Temperature

To investigate the influence of temperatureto the morphology, we took products at thevolume ratio of Vwater:VEA= 3:2 for an in-stance. It was found that temperature signifi-cantly affect the shape of the particles. Whenthe temperature was increased to 200, 240 °C,respectively, the shapes were obviously differ-ent from that observed at 160 °C. Figure 8aand b is a typical TEM image of the sample at200 °C for 20 h, showing that these productsconsist of nanorods with diameter of about8–9 nm and length up to 300–400 nm. Thesenanorods take on uniform morphology andhigh quality. Thus it can be seen that elevatingthe temperature could favor 1D growth ofZnS nanocrystals and lead to the formation oflong nanorods. At a too-high temperature,however, the adsorption of the capping li-gands (ethanol amine molecules) on the lat-eral surfaces of the growing 1D nanocrystalsmight be much weakened, and thus the 1Dnanocrystals have a relatively fast lateralgrowth rate, resulting in the formation ofthicker nanorods. As shown in Figure 8c andd, the products obtained at 240 °C were muchthicker nanorods with thickness in the rangeof 11.5–13 nm and length in 24–150 nm (alsosee Supporting Information, Fig. S6). Fromthese TEM images, it can be seen that thenanorods has self-assembly into nanocraftsalong their long axes and then into large-scaleordered 2D patterns on the surface of TEMgrid. Typical HRTEM of the nanorods re-vealed their single-crystalline nature and thepreferential growth along the c axis (Fig. 8d).The experimental revealed that the mixed sol-vothermal temperature ranging from 200 to240 °C were optimum for the formation ofnanorods (or nanowires). The temperature ofthe solvothermal treatment would influencethe pressure of the reaction system, the solu-bility of the products, and the kinetics of crys-tallization, through which the morphologiesof the products would be changed with differ-ent solvothermal temperatures. The condi-

tions for preparing various typical nanostructures contain thecomposition of a mixed solution and reaction temperature andtheir influence on shape of the products are listed in Table 1.

2.5. Optical Properties of ZnS Nanostructures

The UV-vis absorption spectra for the samples prepared inthe current mixed solution with different volume ratios of Vwa-

ter:VEA are indicated in Figure 9. Figure 9a shows a typical UVspectrum of the ZnS nanospheres constructed nanorods. A


Figure 6. a)–d) TEM images of the ZnS nanowire networks. e) The corresponding EDS spec-trum taken from the network structures indicated in (a). The products were fabricated at160 °C for 20 h in mixed solutions with the volume ratios of Vwater:VEA= 1:4.

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stable peak was observed at 338 nm (3.68 eV) with a slight redshift compared with that of the bandgap for bulk wurtzite ZnS(3.8 eV).[21] In addition, an additional very broad absorptionpeak (or tail) between 370 and 800 nm consisted of severalshoulder peaks centered at 390, 419, 490, and 610 nm. The longabsorption tail in the case of the nanospheres constructed fromnanorods was due to scattering by the crystals.[22] On the otherhand, the products (b–e) show a broad absorption peak in therange of 300–320 nm with a central peak position at about 304,311, 314, and 312 nm, respectively (Fig. 9b–e), which are ob-viously different to the bulk ZnS (at ∼ 340 nm). Compared withthe bandgap of bulk wurtzite ZnS, the bandgap of various ZnSnanostructures were greatly blue-shifted possibly due to quan-tum-size effects, which could be further confirmed by theabove HRTEM results.

As we all know, the study of luminescence properties canshed some light on defects in the ZnS crystals and their poten-tial as photonic materials. Figure 10 shows the room-tempera-ture photoluminescence spectra of the different nanostructures,which were measured using a Perkin-Elmer LS-55 lumines-cence spectrometer with an excitation slit width of 5 nm andan emission slit width of 5 nm. Curves 1–4 correspond to thenanorod networks, the nanowire networks the urchin-likestructures, and nanorods, respectively. It can be seen that all


Figure 7. a)–h) Typical TEM images of the samples in the formation of ZnS nanospheres self-assembled nanorods taken at different time: a) 15 min;b) 25 min; c) 35 min; d) 45 min; e) 1 h; f) 2 h; g) 24 h.

Figure 8. TEM images of the ZnS nanorods. The products were fabricatedin mixed solutions with the volume ratios of Vwater:VEA= 3:2. a), b) 200 °Cfor 20 h; c)–f) at 240 °C for 20 h.

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the curves display have almost same PL band position; onecentered at about 435 nm is the stable and strong blue emissionand the other centered at about 570 nm is a weaker yellow-blue emission. The weak yellow-green emission is similar tothat previously reported for photoluminescence of ZnSnanoribbons[11b] and Mn2+-doped ZnS thin films.[23] However,in this study, no other peaks for impurities were detected byXPS and EDS analysis; the prepared various ZnS nanostruc-tures were of high purity in the present reaction system. Thenoticeable yellow-green emissions at 570 nm could be origi-nated from some self-activated centers, vacancy states, or inter-stitial states related with the special nanostructures.[24] On theother hand, the stable and strong blue emission at 435 nm mayderive from the large surface-to-volume ratio in the obtainedvarious nanstructures. The emission at 435 nm is similar to thereported near-UV emission band at 438 nm that is associatedwith the transitions involving vacancy states in ZnS nanocrys-

tals.[25] Such emission bands have also reported for ZnS nano-tubes,[10i] nanowires,[26a] and nanobelts.[26b,c]

2.6. Photocatalytic Activity of ZnS Nanostrures

To demonstrate the potential applicability in photocatalysisof as-obtained ZnS nanomaterials and the effect of theirmorphologies on their photocatalytic activities, we investigatedtheir photocatalytic activity by choosing photocatalytic degra-dation of acid fuchsine as reference and selected the character-istic absorption of acid fuchsine at about 545 nm for monitor-ing the adsorption and photocatalytic degradation process.Figure 11 indicates the absorption spectra of an aqueous solu-tion of acid fuchsine (intial concentration: 1.0 × 10–4 M, 20 mL)in the presence of 10 mg ZnS nanoparticles prepared underdifferent conditions under exposure to the infrared light lamp(250 W) for 30 min. The absorption peak corresponding to acid


Table 1. Summary of experimental results indicating the influence of the composition of a mixed solution and reactiontemperature on shape of the products.

Sample Vwater:VEA Temperature

[°C]

morphology

a 2:3 160 spherical structures assembled by

loosely intertwisting nanorods

b 3:2 160 urchin-like structures

c 9:1 160 monodisperse nanospheres assembled with nanorods

d 1:4 160 nanowire networks

e Pure ethanol amine 160 nanoribbon networks

f Pure water 160 aggregate of particles

g 3:2 200 nanorods with diameter of about 8–9 nm and length

up to 300–400 nm

h 3:2 240 nanorods with diameter of about 11.5–13 nm

and length up to 24–150 nm

i 1:4 240 nanorods

j 2:3 240 nanorods

300 400 500 600 700 800

2

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6

8

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12

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338nm312nm

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b

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Figure 9. The UV-vis absorption spectra of the products obtained in amixed solution with different volume ratios of Vwater:VEA: a) 9:1; b) 3:2;c) 2:3; d) 1:4; e) pure EA. The molar ratio Zn(Ac)2/L-cysteine = 1:2, with re-action for 24 h at 160 °C.

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Figure 10. Room-temperature PL spectra of the products obtained in amixed solution with different volume ratios of Vwater:VEA: a) 2:3; b) 1:4;c) 3:2; d) 3:2. The molar ratio Zn(Ac)2/L-cysteine = 1:2, with reaction for24 h at 160 °C a)–c) and at 240 °C (d).

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fuchsine at 545 nm of the solution with ZnS of differentmorphologies diminish sharply even disappear completely indensity compared to the initial acid fuchsine solution after irra-diation for about 30 min. it is worthy mentioning that thoseZnS nanostructures possess the high photocatalytic activityand the minor difference in the absorption ability and the pho-tocatalytic activity maybe result from the subtle distinction ofsurface-to-volume between various morphologies. The BETsurface areas of nanowire networks and nanospheres con-structed from nanorods are about 185 and 49 m2 g–1, respective-ly. ZnS nanowire networks present a large surface area due totheir special netlike shape.

To understand the potential advantage of various ZnS nano-structures as photodegradation nanomaterials, we took nano-wire networks as an instance, in comparison with commercialZnS powders (size < 1 lm) measured under the same condi-tions. Time-dependent UV-vis absorption spectra of photocata-lytic degraded acid fuchsine were recorded (bottom part inFig. 12A). As shown in Figure 12A, the absorbance intensityof the peak corresponding to the acid fuchsine molecule at545 nm decreased very quickly once ZnS nanowire networkswere added, corresponding to the immediate color fading andonly in 4 min the color faded almost completely (see upperpart in Fig. 12A). These results implied that ZnS nanowire net-works have very strong photocatalytic ability possibly due tothe high BET surface and thus the efficient absorption of dyemolecules. With exposure time increasing, the typical sharppeak at 545 nm completely vanished after 8–12 min. However,as for commercial ZnS powders, they require 120 min to de-grade acid fuchsine completely (Fig. 12B). A series of color se-quential changes (upper part in Fig. 12A) are detected by theUV-vis absorption measurements.

For further comparison, Figure 12C shows the curves of theconcentration of residual acid fuchsine with infrared light irra-diation time over the as-prepared ZnS nanowire networks andcommercial ZnS powders. Within the detective time, thephotodegradation rate of commercial ZnS powders is ratherlow. On the contrary, at the initial stage the photocatalytic deg-radation velocity is very quick and when the time reached only4 min the acid fuchsine had almost completely degraded. Asregards commercial ZnS powders, a great amount of acid fuch-sine still remained even though the time was elongated toaround 40 min. The superior photodegradation properties ofZnS nanowire networks to that of commercial ZnS powderscan be illustrated by their higher surface area and the nanofea-tures.

In addition, further experiments were performed to comparethe catalytic activity of the ZnS nanowire networks (10 mg)and commercial TiO2 powders (10 mg) under UV-light illumi-nation. The results demonstrate the high photodegradation ve-locity of ZnS nanowire networks and the acid fuchsine almostcompletely degraded when the time is prolonged to only15 min (Fig. 13A). In contrast, the degradation velocity ofcommercial TiO2 is very slow and a small quantity of acid fuch-sine remains even though the time extends to 12 h (Fig. 13B).In all, the ZnS nanowire networks possess superior catalyticefficiency to commercial TiO2, which could be attributed to thehigh surface area of ZnS nanowire networks.

3. Conclusions

In summary, semiconductor ZnS with complex 3D architec-tures such as nanorods (nanowires) networks, urchinlike nanos-turctures, monodisperse nanospheres self-assembled fromnanorods and 1D nanostructures (rods and wires) have beensynthesized in a binary solution system by controlling the reac-tion conditions, such as the volume ratio of the mixed solventsand reaction temperature. A possible growth mechanism wasproposed to explain the formation of the ZnS nanospheresself-assembled with nanorods. The facile monodentate-amine-based mixed solvothermal synthesis route could supply a newmethod to prepare other semiconductor nanomaterials withnovel morphologies. These ZnS nanostructures display veryhigh photocatalytic activity and are much more efficient thanthat of commercial ZnS powders under infrared-light irradia-tion and commercial TiO2 powders under UV-light irradiationas demonstrated by the photodegradation of acid fuchsine atambient temperature. The excellent photocatalytic perfor-mance of ZnS nanostructures is related to their special struc-tures and implies the potential photocatalytic application.

4. Experimental

All chemicals were of analytical and were directly used withoutany treatment. In a typical procedure, 1 mmoL cadmium acetate(Zn(Ac)2

.2H2O) and 2 mmoL L-cysteine (C3H7NO2S) were added to agiven amount of distilled water and the mixture was dispersed to form


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Figure 11. The absorption spectra of a solution a acid fuchsine(1.0 × 10–4

M, 20 mL) in the presence of ZnS nanoparticels (10 mg) underexposure to the infrared light lamp for 30 min: a) initial acid fuchsine;b)–e) corresponding to the spectra measured after adding the samplesprepared in mixed solution with different volume ratios of Vwater:VEA ofb) 2:3, c) 1:4, d) 3:2, and e) 9:1.

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a homogeneous solution by constant strong stirring. Then, a givenamount of ethanol amine (EA) was added to the above solution atroom temperature and was continually stirred 10 min. and then, the re-

sulting mixture was transferred into a Teflon-lined stainless autoclave(60 mL capacity). The autoclave was sealed and maintained at 160–240 °C for 24 h. The system was then cooled to ambient temperature


(A)

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a: 4min

k: 120min

i: 45min

g: 28minh: 35min

f: 24min

j: 60min

e: 24mind: 18minc: 12minb: 8min

d

c

b

a

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sorb

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(C)

0 20 40 60 80 100 120

0.0

0.2

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1.0

C/C

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Time (min)

ZnS nanowire networks

Commercial ZnS

Figure 12. A) Absorption spectrum of a solution of acid fuchsine (1.0 × 10–4M, 20 mL) in the presence of ZnS nanowire networks (10 mg) after its expo-

sure to the infrared light: a) intial solution; b)–j) after adding ZnS naowire networks after exposure to infrared light for b) 1, c) 2, d) 3, e) 4, f) 8, g) 12,h) 18, i) 24, and j) 28 min. the inset shows the enlarged temporal evolution of absorption (e–j); and B) commercial ZnS powders, which shows the acidfuchsine concentration decreases with the irradiation time. C) Curves of the concentration of residual acid fuchsine with different infrared light irradiationtime over the as-prepared ZnS nanowire networks and commercial ZnS powders.

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naturally. The final product was collected and washed with distilledwater and absolute alcohol several times, vacuum-dried, and kept forfurther characterization.

The products were characterized by X-ray diffraction (XRD) re-corded on a Japanese Rigaku D/max-cA rotating anode X-ray diffrac-tometer equipped with the monochromatic high-intensity Cu Ka radia-tion (k = 1.54178). SEM images were taken with a field emissionscanning electron microscope (FESEM, JEOL-6300F, 15 kV). Micros-copy was performed with a Hitachi (Tokyo, Japan) H-800 transmission

electron microscope (TEM) at an accelerating volt-age of 200 kV, and a JEOL-2010 high-resolutionTEM, also at 200 kV. Photoluminescence (PL)measurements were carried out on a Perkin-ElmerLS-55 luminescence spectrometer using a pulsedXe lamp. UV-vis spectra were recorded on a Solid-Spec-3700 spectrophotometer at room temperature.

The Photocatalytic Measurements: the photocata-lytic activity experiments on the obtained ZnSnanostructures for the decomposition of acid fuch-sine in air were performed at ambient temperature.A cylindrical Pyrex flask (capacity ca. 25 mL) wasused as the photoreactor vessels. ZnS nanoparticlesas catalyst (10 mg) was added in the aqueous acidfuchsine solution (C20H17N3O9S3Na2) (Sigma-Al-drich Chemical Co.; 1.0 × 10–4

M, 20 mL) and wasmagnetically stirred in the dark for 10 min to reachthe adsorption equilibrium of acid fuchsine with thecatalyst, and then exposed to infrared light(250 W). As a comparison, the photocatalytic activ-ity of commercial ZnS powers (Aldrich, size< 1 lm, 99.9 %) was also tested at the same experi-mental conditions. Additional experiments wereperformed to further compare the catalytic activityof the ZnS nanowire networks and commercialTiO2 powders (Degussa P25, Degussa Co. the sur-face area is ca. 45 m2g–1) under UV-light illumina-tion (high-pressure Hg lamp (60 W)). UV-vis ab-sorption spectra were recorded at differentintervals to monitor the reaction using SolidSpec-3700 UV/vis spectrophotometer.

Received: September 28, 2006Revised: January 5, 2007

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(A)

400 500 600 700

0.0

0.5

400 500 600 700

0.00

0.02

0.04

0.06

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0.12

0.14

h

e

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rban

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b

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sorb

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(B)

400 500 600 700

0.0

0.5

e: 50min

i :24hh :12h

f: 3hg :4.5h

d: 28minc: 15minb :5min

a

a: initial solution

Ab

sorb

an

ce

Wavelength (nm)

Figure 13. A) Absorption spectra of a solution of acid fuchsine (1.0 × 10–4M, 20 mL) in the

presence of ZnS nanowire networks (10 mg) after its exposure to UV light: a) intial solution,b)–h) after adding ZnS naowire networks after exposure to UV light for b) 1, c) 2, d) 3, e) 5,f) 15 min, g) 28, and h) 35 min. The inset shows the enlarged temporal evolution of absorp-tion (b–h); and B) commercial TiO2 powders (10 mg), which shows the acid fuchsine concen-tration decreases with the irradiation time.

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Tunable Synthesis of Various Wurtzite ZnS Architectural Structures and Their Photocatalytic...

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