Chalcogenide microspheres

A General Chemical Conversion Method toVarious Semiconductor Hollow Structures**

Qing Peng, Sheng Xu, Zhongbin Zhuang, Xun Wang,and Yadong Li*

In this paper, a convenient chemical conversion methodthat allows the direct preparation of organized, nanocrystal-line semiconductor hollow microspheres, as well as theircore/shell structures, is reported. By using hollow micro-spheres of an active semiconductor (e.g., ZnSe) as a startingreactant and in situ template, a series of chalcogenide andoxide hollow microspheres with different bandgap valueshas been prepared successfully through either a solution-phase precipitation conversion or a conventional gas-phaseconversion process. In contrast to previous studies, thischemical conversion method is a relatively general way toobtain semiconductor hollow structures, and also a powerfultool for property modification of the microspheres. Thesehollow structures have great potential for many applications,such as in the fields of optoelectronic technology, photovol-taic devices, photonic bandgap crystals, and photochemicalsolar cells.

Semiconductor hollow microspheres, together with theircore/shell structures, have attracted much research interestin recent years because of their unique optical, electrical,and magnetic properties,[1] which can be tailored over abroad range by altering the structure, composition, and sizeof the interior nanocrystals.[2] They are known to have appli-cations in the fields of biomedicine, delivery vehicle systems,catalysts, and photonic bandgap (PBG) crystals.[3] Further-more, these hollow structures have been considered as po-tential candidates for electrode materials in photochemicalsolar cells. Such materials are expected to exhibit high light-collection efficiency and a fast motion of charge carriersdue to their hollow structures and the closely packed inter-penetrating networks with large internal surface area attheir interior.[4] An improved photon-to-photocurrentcharge-carrier-generation efficiency has also been found intheir core/shell structures.[5] Many efforts have been devoted

[*] Dr. Q. Peng, S. Xu, Z. Zhuang, Dr. X. Wang, Prof. Y. LiDepartment of Chemistry and the Key Laboratory of Atomic &Molecular Nanosciences (Ministry of Education, China)Tsinghua University, Beijing, 100084 (P. R. China)andNational Center for Nanoscience and Nanotechnology, Beijing,100084 (P. R. China)Fax: (+86) 10-62788765E-mail: ydli@tsinghua.edu.cn

[**] This work was supported by NSFC (50372030, 20025102,20151001), the Foundation for the Author of National ExcellentDoctoral Dissertation of P. R. China and the State Key Project ofFundamental Research for Nanomaterials and Nanostructures(2003CB716901).

Supporting information for this article is available on the WWWunder http://www.small-journal.com or from the author.

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to the synthesis of semiconductor hollow microspheres.Template synthesis through inorganic precipitation process-es, such as with silica spheres, polymer latex spheres, core/shell gel particles, liquid droplets, and emulsion droplets,have been commonly used.[6] However, these methods areusually suitable for certain specific semiconductor micro-spheres, and after formation, the templates still remain inthe system and need to be removed later. A direct and rela-tively general synthetic method is urgently needed and con-tinues to be a challenging endeavor.

Recently, we successfully synthesized monodisperse,nanocrystalline ZnSe semiconductor hollow microspheres inaqueous solution at the gas–liquid interfaces of bubbles.[7]

Unlike inert SiO2 and polystyrene microspheres, the interiorof the ZnSe nanocrystals are more reactive, and further-more, they have a relatively large Ksp value (10�29.4) com-pared to other metal selenides (or tellurides), such as, forexample, CdSe (10�35.2), Ag2Se (10�63.7), CuSe (10�48.1), PbSe(10�42.1), HgSe (10�64.5), CoSe (10�31.2), and NiSe (10�32.7).This implies that the ZnSe microspheres can act as both re-actants to synthesize more stable chalcogenides and oxides,and templates to obtain structures with a hollow-spheremorphology. On the basis of this concept, a convenientchemical conversion mechanism (see Figure 1) has been es-tablished. In such a process, unique electrical or opticalproperties can be endowed to the products.

To control the size of the target semiconductor hollowmicrospheres, the diameter of the starting template micro-spheres (usually, an active semiconductor with a relativelylarge Ksp value, such as ZnSe, is selected) must be control-led. Based on the gas–liquid interface aggregation mecha-nism proposed previously,[7] the size of N2 bubbles, as wellas the concentration and aggregation radius of the as-formed ZnSe monomers, play important roles in the final

size of the hollow ZnSe microspheres. The size can be ad-justed by controlling the viscosity of the solution, the supplyrate of Zn2+ ions, and the mobility of the ZnSe monomers.In our experiments, rather than simply changing the concen-tration of the raw materials and/or the reaction tempera-ture, complexing agents were used to control the supply rateof Zn2 + ions, which yielded good results. Through competi-tion between complexation and precipitation in solution,different complexing agents can achieve different supplyrates and thus the diameter of the microspheres can bemodulated. Figure 2a shows ZnSe microspheres of �2 mm

diameter that were prepared using ethylenediaminetetraace-tic acid (EDTA) as a controlling reagent. With no complex-ing agent added, 3 mm microspheres were obtained in-stead.[7] By using glycerol or agar to increase the viscosity ofsolution, smaller N2 bubbles and a reduced mobility ofZnSe monomers is achieved, which results in ZnSe micro-spheres with smaller diameters. Figure 2b shows the 300 nmZnSe microspheres prepared when agar was used in the re-action system. Under relatively high magnification, ZnSenanocrystals of about 20 nm diameter can be seen on thesphere surface, which makes the surface appear rough.

We selected 2 mm ZnSe hollow microspheres as startingreactants to illustrate the chemical conversion route toother semiconductor hollow microspheres and their core/shell structures. Through a solution-phase precipitation con-version involving the reaction of ZnSe with metal ions, anumber of selenide hollow microspheres with lower Ksp val-ues can be conveniently obtained at low temperatures. Forheavy metal ions such as Ag+ , Cu2+ , Pb2 +, and Hg2+ , whose

Figure 1. Schematic illustration of the chemical conversion method tosemiconductor hollow microspheres and their core/shell structures.Through a precipitation conversion technique, selenide microspheres(or core/shell-structured microspheres) can be obtained. Theseas-obtained microspheres (or ZnSe microspheres) can further beconverted to corresponding chalcogenide or oxide microspheresthrough a gas-phase reaction at a relatively high temperature.

Figure 2. ZnSe microspheres with diameters of about 2 mm (a) and300 nm (b) obtained by controlling the supply rate of Zn2 + sourcesand the mobility of ZnSe monomers.

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selenides have Ksp values that are significantly lower thanthat of ZnSe, the K value for the conversion reaction[Eq. (1)] is nonetheless large enough. These ions will reactwith ZnSe at room temperature rapidly, and correspondingselenide microspheres can easily be obtained by immersingthe ZnSe microspheres into solutions containing an excessof metal ions.

ZnSeþ 2 Agþ $RTAg2Seþ Zn2þ

Kq ¼ KqspðZnSeÞ=Kq

spðAg2SeÞ ¼ 1034ð1Þ

However, to obtain microspheres of transition-metal se-lenides whose Ksp values are very close to or only a littlelower than that of ZnSe (e.g., CdSe, CoSe, and NiSe), highionic concentrations and additional energy (by heating to140 8C) is required to successfully complete the conversionreaction [Eq. (2)]. In these experiments, hydrazine was alsoused to prevent the formation of elemental Se in the so-lution, which originates from the redox reaction betweenZnSe and hot water (including a small amount of dissolvedO2) at 140 8C.

ZnSeþ Cd2þ $140 oCCdSeþ Zn2þ

Kq ¼ KqspðZnSeÞ=Kq

spðCdSeÞ ¼ 105:8ð2Þ

Figure 3a shows the XRD patterns of Ag2Se andCu2�xSe microspheres prepared at room temperature, andCdSe microspheres prepared at 140 8C. The patterns indi-cate that all three are pure-phase selenides, and for theCu2�xSe and CdSe products, cubic structures are inheritedfrom the original ZnSe templates due to the mild reactionconditions. Figure 3b, c, and d show SEM images of as-pre-

pared Ag2Se, Cu2�xSe, and CdSe products, respectively (theinsets of Figure 3 c and d are high-magnification images ofCu2�xSe and CdSe microspheres, respectively). They all stillhave a spherical morphology and good monodispersity.Compared to the original ZnSe microspheres, they have arelatively rough surface and a similar diameter (�2 mm).The inset of Figure 3b shows an individual cracked Ag2Sesphere (achieved by squeezing the particle in a mortar),which shows that these spheres also inherit the hollow struc-tures from the template ZnSe microspheres.

Since ZnSe microspheres are composed of numerousnanocrystals, they exhibit good permeability in solution.Ions and H2O molecules can therefore move inside thespheres with relative ease, which helps to ensure that theAg2Se, Cu2�xSe, and CdSe microspheres are pure-phasecompounds. By introducing other soluble chalcogen sourcesinto the reaction system, rather than form a chalcogenideshell outside the surface of the microsphere, this precipita-tion conversion method can also be used to synthesize mi-crospheres with core/shell structures. When an appropriateamount of thioacetamide was introduced to the system,ZnSe/ZnS and ZnSe/CdS core/shell microspheres can be ob-tained at 140 8C (see the SEM images in the Supporting In-formation). Energy dispersion spectroscopy (EDS) was per-formed on the samples; the results (see Supporting Informa-tion; the ZnS and CdS phases cannot be detected by XRDdue to their low content) indicate the existence of S (about3.7% in ZnSe/ZnS and 4.9% in ZnSe/CdS) and Cd (about6.4% in ZnSe/CdS) within the microspheres.

In addition to solution-phase conversion, gas-phase reac-tions are also effective for carrying out this chemical conver-sion method. By reacting the as-prepared selenide micro-

Figure 3. XRD patterns (a) and SEM images of Ag2Se (b), Cu2�xSe (c), and CdSe (d) microspheres.

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spheres with oxygen or gaseous sulfur at a relatively highertemperature, more stable oxide or sulfide (or their compo-site) microspheres can be prepared. In our experiments,ZnSe and CdSe microspheres were chosen as the startingmaterials. After heating ZnSe microspheres in air for 3 h, aZnO sample was obtained ðZnSeþO2

600 oC���!ZnOÞ. Under

the protection of an argon atmosphere, ZnS and CdS sam-ples were obtained after heating the starting selenide micro-spheres with sulfur powders at 400 8C for 3 hðZnSeðCdSeÞ þ S 400 oC

���!ZnSðCdSÞ). Figure 4a shows XRDpatterns of the three samples. All the reflections can be in-dexed to pure-phase ZnO, ZnS, and CdS, respectively. It isworth noting that after heating, ZnO and CdS adopt thethermodynamically stable wurtzite structures instead oftheir original zinc blende structures. From the SEM images(Figure 4b, c and d), it is clear that the samples retain theirspherical morphology. The ZnO microspheres (Figure 4b)were found to have slightly reduced in size, while the ZnS(Figure 4c) and CdS (Figure 4 d) microspheres have similardiameters, as compared to the starting spheres. The inset ofFigure 4b shows a typical individual sphere of ZnO, whichhas a diameter of about 1.8 mm. The ZnS and CdS micro-spheres have a relatively rough surface. From Figure 4 c andthe inset of Figure 4d, it can be seen that the interior nano-crystals of the ZnS and CdS microspheres have an averagesize of about 200 nm.

According to the needs of practical applications, uniqueproperties can be conveniently introduced to the systemduring the chemical conversion process to enhance the per-formance of the semiconductor microspheres. For example,during the conversion of ZnSe to ZnO microspheres, con-trolling the amount of O2 leads to the formation of a thinZnSe1�xOx layer on the outside of the ZnSe microspheres,

thus changing the luminescence properties of the sample.[8]

Figure 5a shows the room-temperature PL spectrum of theZnSe/ZnSe1�xOx sample. The emission peak at 595 nm is

Figure 4. XRD patterns (a) and SEM images of ZnO (b), ZnS (c), and CdS (d) microspheres.

Figure 5. PL spectra of ZnSe/ZnSe1�xOx (a) and ZnSe/ZnS samples (b)at room temperature (I: reaction time=2 h, II: reaction time=0.5 h).

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due to the self-activated luminescence of ZnSe nanocrystals,probably as a result of some donor–acceptor pairs related toZn vacancy and interstitial sites.[9] The new strong orangeemission at 618 nm (not present in pure ZnSe) comes fromthe point-defect complexes {V00ZnZnCC

i} and {V00ZnZnCi} of

ZnSe1�xOx.[10] Figure 5b shows the PL spectra of ZnSe/ZnS

samples with different degrees of sulfuration (I: reactiontime=2 h, II: reaction time=0.5 h) in the conversion proc-ess from ZnSe to ZnS microspheres. The strong lumines-cence band at 402 nm comes from the ZnSe/ZnS core/shellnanocrystals inside the spheres,[11a] which imbues the sam-ples with a potential application as laser diodes operating inthe deep-blue spectral region (l<420 nm).[11b] The peak at453 nm is attributed to the trap-state emission of ZnS[12a] ,and the peak at 469 nm is considered to result from emis-sion from ZnSe nanocrystals.[12b] With increased reactiontime, the intensity of the peaks at 402 nm (shoulder inline I) and 453 nm becomes stronger due to the increasedamount of ZnS. By controlling the value of x carefully, thisconversion process can also be employed to adjust thebandgaps of these ternary semiconductors, which will be re-ported later in more detail.

In summary, solution-phase precipitation and gas-phaseconversion processes have been employed to prepare aseries of semiconductor hollow microspheres and their core/shell structures. Unique properties can be conveniently ob-tained from the target compounds using this conversionprocess. The synthetic strategy can also be applied to thepreparation of other compounds, especially to those withwell-understood structures. The semiconductor hollow struc-tures have potential applications in photovoltaic devices,photonic bandgap crystals, and photochemical solar cells.

Experimental Section

Ag2Se and Cu2�xSe microspheres: ZnSe microspheres(0.144 g, 0.001 mol) were immersed in a AgNO3 (1 m, 50 mL) orCu(NO3)2 (1 m, 50 mL) solution for 3 days. CdSe microspheres:ZnSe microspheres (0.144 g, 0.001 mol) and Cd(NO3)2·4 H2O(0.61 g, 0.002 mol) were added to 35 ml of an EDTA solution(made by dissolving EDTA (1.24 g, 0.003 mol) in a NaOH (40 mL,0.5 m) solution). The mixture was transferred into a 50 mL Teflon-lined autoclave, and N2H4·H2O (5 mL) was added. The autoclavewas sealed and heated at 140 8C for 48 h, and then was allowedto cool to room temperature. The final product was collected byfiltration, washed with deionized water, and then dried at 60 8C.

ZnSe/ZnS, ZnSe/CdS core/shell microspheres: ZnSe micro-spheres (0.144 g, 0.001 mol), ZnSO4·7 H2O (0.14 g, 0.0005 mol)or Cd(NO3)2·4 H2O (0.15 g, 0.0005 mol) were added in a 50 mLTeflon-lined autoclave, and H2O (40 mL) and thioacetamide(0.038 g, 0.0005 mol) were then added. The autoclave wassealed and heated at 140 8C for 30 min, and then allowed tocool to room temperature. The final product was collected by fil-tration, washed with deionized water, and then dried at 60 8C.

ZnO microspheres: ZnSe microspheres (0.144 g, 0.001 mol)were placed in a ceramic boat in a furnace and heated in air at600 8C for 3 h. ZnS and CdS microspheres: ZnSe microspheres(0.144 g, 0.001 mol) or CdSe microspheres (0.191 g, 0.001 mol)

were placed in a ceramic boat downstream from another ceramicboat, which was filled with an excess of sulfur powder. In anargon flow (100–150 mL min�1), they were heated at 400 8C for3 h. The products were collected after the furnace was cooleddown to room temperature. ZnSe/ZnSe1�xOx microspheres: ZnSemicrospheres (0.144 g, 0.001 mol) were placed in a ceramicboat in a furnace and then heated in a N2 (common grade) flowat 600 8C for 3 h. ZnSe/ZnS microspheres: These were preparedby the same procedure as for the ZnS microspheres, except thata shorter reaction time was employed (sample I: 2 h, sam-ple II: 0.5 h).

XRD patterns were obtained on a Bruker D8-advance X-raypowder diffractometer with CuKa radiation (l =1.5418 �). Thesize and morphology of the ZnSe microspheres was determinedat 20 kV by a JEOL JSM-6301F scanning electron microscope.Photoluminescence experiments were conducted in air on a Hita-chi F-4500 fluorescence spectrophotometer. The samples wereplaced in quartz cuvettes. The wavelength of excitation wasshorter than the onset of absorption for each sample.

Keywords:chalcogenides · microspheres · nanostructured materials ·selenides · semiconductors

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Received: June 11, 2004

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