Controlled Synthesis and Photocatalytic Activity of ZnSe

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J. Phys. Chem. C 2009, 113, 5434–5443

Controlled Synthesis and Photocatalytic Activity of ZnSe Nanostructured Assemblies with Different Morphologies and Crystalline Phases Lihui Zhang, Heqing Yang,* Jie Yu, Fanghong Shao, Li Li, Fenghua Zhang, and Hua Zhao Key Laboratory of Macromolecular Science of Shaanxi ProVince, School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an, 710062, China ReceiVed: NoVember 26, 2008; ReVised Manuscript ReceiVed: January 16, 2009

Microspheres constructed from ZnSe nanoparticles with a cubic structure and spherical flowerlike nanoarchitectures assembled from ZnSe(en)0.5 nanoflakes were synthesized via a solvothermal reaction of ZnCl2 with Na2SeO3 and ethylenediamine in ethylene glycol in the presence of poly(vinylpyrrolidone) at 180 °C for 12 h. Their controlled synthesis is achieved by varying the volume ratio of ethylenediamine to ethylene glycol. Spherical flowerlike nanoarchitectures assembled from ZnSe nanoflakes with a hexagonal structure are obtained by calcining the ZnSe(en)0.5 in Ar at 350 °C. Their reaction mechanisms and the self-assembly evolution processes are discussed. The photocatalytic activity of the as-prepared products in various geometrical morphologies for the degradation of organic pollutants was studied, and the results indicate that the photocatalytic ability of the ZnSe-spherical flowerlike nanoarchitectures is stronger than that of the ZnSe microspheres. 1. Introduction Recently, nanostructured materials with a defined size and shape have attracted much interest due to their great potential for fundamental research into the effect of dimensionality and size on physical and chemical properties, as well as for electronic and optoelectronic nanodevices.1 As one of the important Znbased II-VI semiconductors, zinc selenide (ZnSe) has been considered to be a perspective material for optoelectronic devices, including blue laser diodes, light-emitting diodes, and photodetectors, due to its wide direct band gap (2.67 eV) and large exciton binding energy (21 meV).2-4 Moreover, ZnSe is also a promising material for windows, lenses, output couplers, beam expanders, and optically controlled switching, due to its low absorptivity at infrared wavelength, its visible transmission, and giant photosensitivity.5 ZnSe nanostructures with various morphologies, such as nanoparticles,6 nanorods,7 nanowires,8 nanobelts,9 nanoneedles,10 nanotube,11 nanoplates,12 and complicated hierarchical nanostructures constructed with nanoscale building blocks,13,14 have been fabricated by a variety of methods. Previous studies were mainly focused on the photoluminesecence of ZnSe nanostructures while investigations on other physical and chemical properties, such as current-voltage and photocatalytic behaviors, are quite rare.15,16 Recently, Xiong et al.15 found that the photocatalytic activity of ZnSe nanobelts in the photodegradation of the fuchsine acid is higher than that of TiO2 nanoparticles. Hierarchical assembly of nanoscale building blocks (nanoparticles, nanowires, nanotubes, and nanosheets) is a crucial step toward functional nanosystems and still remains a challenge in the field of nanoscale science.17 Peng et al.13 synthesized the hollow ZnSe microspheres assembled with nanoparticles by using N2 bubbles as templates for the first time. Subsequently, ZnSe nanowire bundles,2 and flowerlike clusters of radically aligned ZnSe nanoflakes,14 were also produced. However, up to now, there are no reports on the synthesis of monodispersive * Corresponding author. Fax: +86-29-85307774, e-mail: hqyang@ snnu.edu.cn.

spherical flowerlike superstructures constructed with ZnSe nanoflakes, and the controlled synthesis of ZnSe hierarchical nanostructures with different morphologies and crystalline phases. In this article, we report on a very simple solvothermal route for the controlled synthesis of microspheres constructed with ZnSe nanoparticles and spherical flowerlike nanoarchitectures assembled with ZnSe nanoflakes. The controlled synthesis of both the nanoarchitectures was achieved by adjusting the volume ratio of ethylenediamine (en) to ethylene glycol (EG) in the ZnCl2-Na2SeO3-EG-en-polyvinylpyrrolidone (PVP) solvothermal system and subsequent annealing in Ar. The formation mechanism of the nanoarchitectures and their photocatalytic activities in the degradation of methyl orange were investigated in detail. 2. Experimental Section All reagents used were of analytical purity and were directly used without further purification. Microspheres constructed with ZnSe nanoparticles: In a typical procedure, 1 mL of en and 24 mL of EG were put into a beaker of 50 mL capacity. Then 0.068 g (0.5 mmol) of ZnCl2, 0.086 g (0.5 mmol) of Na2SeO3, and 0.4 g of PVP (molecular weight )30 000) were added into the beaker under stirring. After vigorously stirring for 10 min, a clear solution was formed. The mixed solution was transferred into a Teflon-lined stainless steel autoclave of 50 mL capacity. The autoclave was sealed and heated at 180 °C for 12 h. After the heating treatment, the autoclave was cooled to room temperature naturally. The products were collected by centrifugation, washed three times with deionized water and absolute ethanol, respectively, and then dried at 50 °C; as a result, yellow powders were obtained. ZnSe spherical flowerlike nanoarchitectures assembled with nanoflakes: 4 mL of en and 21 mL of EG were used instead of 1 mL of en and 24 mL of EG. The products obtained via the same experimental procedure were heated to 350 °C at 30 °C h-1 for 1 h in Ar in a horizontal furnace, leading to green powders.

10.1021/jp810385v CCC: $40.75  2009 American Chemical Society Published on Web 03/16/2009

ZnSe Nanostructured Assemblies

J. Phys. Chem. C, Vol. 113, No. 14, 2009 5435

Figure 1. SEM images of the products prepared at 180 °C for 12 h at Ven/VEG of (a) 0/25, (b) 1/24, (c) 2.5/22.5, and (d) 4/21.

The as-prepared products were characterized and analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and thermal analysis and infrared (IR) spectroscopy. The XRD analysis was performed using a Rigaku D/MAX-IIIC X-ray diffractometer with Cu KR radiation (λ ) 1.540598 Å) at 40 kV and 40 mA. The scanning speed was 5°/min. Nanocrystalline size, L, was calculated using Scherrer’s formula: L ) Kλ/(β cos θ), where λ is the wavelength of X-rays used, β is the full width at half-maximum (fwhm) intensity of the diffraction line, θ is the Bragg angle for the measured hkl peak, and K is a constant equal to 0.9. SEM images were obtained using a FEI Quanta 200 scanning electron microscope at an accelerating voltage of 20 kV. An energy-dispersive X-ray spectroscopy (EDX) system attached to the SEM was employed to analyze chemical composition. TEM and electron diffraction images were obtained using a JEOL JEM-3010 transmission electron microscope at an accelerating voltage of 300 kV. Samples for TEM were prepared by dispersing powders on a carbon-coated copper grid. The IR spectrum was recorded using a Bruker Equinx55 Fourier transform IR spectrophotometer at room temperature. Thermogravimetric and differential thermal analysis (TG-DTA) was carried out at a heating rate of 20 °C min-1 in N2 gas at a flowing rate of 100 mL min-1 using a TA Q600 system. The Brunauer-Emmett-Teller (BET) specific surface area was performed by N2 gas adsorption using a ST03A surface analytical instrument (Beijing analysis instrument factory, China). Photocatalytic properties of the products with various geometrical morphologies were examined by measuring the decol-

oration of methyl orange aqueous solution. Thirty milligrams of the as-prepared products was added to 30 mL of 5 × 10-5 M methyl orange solution. Prior to irradiation, the suspensions were magnetically stirred for 10 min to establish a adsorption/ desorption equilibrium between the dye and the ZnSe powders, after which they were transferred into a 50 mL quartz test tube and irradiated with four 15 W ultraviolet (UV) lamps (365 nm) at a distance of about 25 cm. At a given irradiation time interval, 5 mL samples were withdrawn from the test tube for analysis. Absorption spectra of these solutions were measured by using a TU-1901 ultraviolet-visible spectrophotometer. 3. Results and Discussion The products prepared by varying the volume ratio of en to EG in the ZnCl2-Na2SeO3-en-EG-PVP solvothermal system were characterized by SEM and XRD, and the results are shown in Figure 1 and 2. Figure 1a shows the SEM image of the products obtained in the absence of en. It can be seen that the product is composed of rodlike structures with the diameters of 3.5-4.5 µm and small particles with irregular morphology. When en was introduced, the morphologies of the products were changed. Figure 1b shows the SEM images of the product obtained in the solvothermal system with the volume ratio of en to EG ) 1:24. The SEM results reveal that the products consist of microspheres with a diameter of 1.4 ( 0.2 µm. These microspheres have a smooth surface and a very narrow size distribution. When the volume ratio of en to EG is increased to 2.5:22.5, besides the microspheres with diameters of 0.5-1.2 µm, spherical flowerlike nanoarchitectures with diameters of

5436 J. Phys. Chem. C, Vol. 113, No. 14, 2009

Zhang et al.

Figure 2. XRD patterns of the products obtained at 180 °C for 12 h at Ven/VEG of (a) 0/25, (b) 1/24, (c) 2.5/22.5, and (d) 4/21. The stick pattern is the simulated XRD pattern from the Rietveld refinement and positional parameters of ZnSe(en)0.5 reported by Li et al.18

about 3 µm are observed (Figure 1c). The spherical flowerlike nanoarchitectures are assembled from nanoflakes with thicknesses of 100-200 nm. As Ven/VEG is increased to 4:21, the products are monodispersive spherical flowerlike nanoarchitectures with diameters of 2.5 ( 0.5 µm (Figure 1d), which are assembled from nanoflakes with a smooth surface and thicknesses of 75 to 150 nm. All diffraction peaks shown in Figure 2a can be indexed as Se element with a hexagonal structure, which are in a good agreement with JCPDS card No. 65-1876. The result indicates that the rodlike structured powders obtained in the absence of en are hexagonal Se. The XRD pattern of the microspheres obtained at Ven/VEG of 1:24 shown in Figure 2b has only three peaks at 2θ ) 27.2°, 45.3°, 53.7°. According to JCPDS Card No. 37-1463 (a ) 5.669 Å), the product is cubic ZnSe, and the peaks are (111), (220), and (311) diffraction lines of the cubic structure. Additionally, the XRD pattern exhibits broad diffraction peaks, which reveal that the ZnSe microspheres might be assembled from nanoparticles with even smaller size. The size of the ZnSe nanocrystallites was calculated using Scherrer’s equation from the three peaks to be 6.5 ( 0.5 nm. The XRD pattern of the product obtained at Ven/VEG ) 2.5:22.5 shown in Figure 2c has strong and sharp diffraction peaks. The positions of the diffraction peaks are consistent with that of the simulated powder XRD pattern (stick pattern in Figure 2) from the Rietveld refinement and positional parameters of ZnSe(en)0.5 reported by Li et al.18 Moreover, three weak broad diffraction peaks at 2θ ) 27.2°, 45.3°, 53.7° from cubic ZnSe are also observable in Figure 2c. Figure 2d shows the XRD pattern of the spherical flowerlike nanoarchitectures obtained at Ven/VEG ) 4:21. The weak broad diffraction peaks from cubic ZnSe are not observed, and the pattern fits the simulation very well. The XRD pattern was indexed by VBTreor90 method using program PowderX,19 and the 2θ values of selected well-defined diffrac-

Figure 3. IR spectrum (a) and TG-DTA curves (b) of the precursors with spherical flowerlike nanoarchitectures obtained at Ven/VEG of 4:21.

tion peaks were used as input data. The results are listed in Table 1. It can be seen that the indexed constants are consistent with that of ZnSe(en)0.5 reported by Li et al.18 and Lu et al.20 Therefore, it is reasonable to conclude that the product obtained at Ven/VEG ) 2.5:22.5 consists of microspheres constructed from ZnSe nanoparticles with a cubic structure and spherical flowerlike nanoarchitectures assembled from ZnSe(en)0.5 nanoflakes with an orthorhombic structure. The product obtained at Ven/ VEG ) 4:21 possesses only spherical flowerlike nanoarchitectures assembled from ZnSe(en)0.5 nanoflakes. These results reveal that en plays a significant role in composition and microstructural evolution of ZnSe, and controlled synthesis of microstructured ZnSe can be achieved by adjusting Ven/VEG. In order to further ascertain the chemical compositions of the spherical flowerlike nanoarchitectures, the as-prepared products were examined by using IR spectroscopy and TG-DTA, and the results are shown in Figure 3a,b. In the IR spectrum shown in Figure 3a, the two sharp peaks at 3234 and 3115 cm-1 are assigned to the NH2 asymmetric and symmetric stretching vibrations, the bands at 1601 and 1025 cm-1 are attributed to the NH2 scissors and wagging vibrations,21 the absorption bands at 1359, 2931, and 2863 cm-1 correspond to wagging,22 asymmetric, and symmetric stretching vibrations of the CH2, respectively, and the absorption band at 1086 cm-1 can be assigned to the C-N stretching vibrations. The NH2 asymmetric

TABLE 1: Indexing Results of the XRD Pattern of the Spherical Flowerlike ZnSe Precursor Nanoarchitectures Obtained at Ven/VEG of 4:21 vol (Å3)

cryst syst

a (Å)

b (Å)

c (Å)

ZnSe precursors

ortho

6.6198(2)

6.4624(1)

17.3514(6)

742.29

values of ZnSe(en)0.5 reported by Li et al.18

ortho

6.6298(9)

6.4608(9)

17.350(2)

744.0(2)

FOM M20 ) 12 F20 ) 18


Figure 4. XRD pattern (a) and SEM image (b) of the products after annealing. The stick pattern is the standard XRD pattern for ZnSe powders (JPCDS card file no.15-0105).

J. Phys. Chem. C, Vol. 113, No. 14, 2009 5437 and symmetric stretching vibrations shift toward lower frequency, as compared with that of en,12 which could suggest that en has been intercalated into the precursors. In addition, the absorption band at about 3438 cm-1, which corresponds to OH stretching vibration,2 could be due to the absorption of EG and H2O in the sample. TG-DTA results are shown in Figure 3b. The TG curve shows that the products undergo a singlestep weight-loss process. The measured weight loss of the en is 16.5% from 250 to 400 °C, which is close to the expected value of 17.2% calculated for the change of ZnSe(en)0.5 to ZnSe. The DTA curve shows a continuous change from room temperature to 250 °C, indicating the release of freely bound water. The endothermic peak at 322 °C corresponds to the decomposition of ZnSe(en)0.5. These results reveal that the spherical flowerlike nanoarchitectures obtained at Ven/VEG ) 4:21 are a ZnSe-en hybrid compound, ZnSe(en)0.5. Figure 4a shows the XRD pattern of the annealed spherical flowerlike assemblies. According to JCPDS card No. 15-0105 (a ) 3.996 Å, c ) 6.55 Å), the products belong to hexagonal wurtzite ZnSe. Corresponding SEM images are displayed in Figure 4b. As can be seen, after ZnSe(en)0.5 decomposed into ZnSe, the geometrical morphology is well preserved. Figure 5a shows a typical TEM image of an isolated ZnSe microsphere with a diameter of 1.35 µm. The surface of the microsphere is relatively rough. A higher magnification TEM image of the white rectangle in part a shown in Figure 5b indicates that the microsphere is assembled from tiny nanocrystallites with diameters of 6-8 nm. Figure 5c is the corresponding selected area electron diffraction (SAED) pattern, displaying five diffused rings. These rings are assigned to (111), (220) (311), (331), and (422) diffraction lines of cubic ZnSe, respectively. Figure 5d shows a corresponding HRTEM image.

Figure 5. TEM, SAED, and HRTEM images of the ZnSe microspheres prepared at Ven/VEG ) 1:24.


Zhang et al.

Figure 6. TEM, SAED and HRTEM images of the ZnSe spherical flowerlike assembly obtained at Ven/VEG ) 4:21 and after annealing.

A spacing of the crystallographic planes measured from the HRTEM image is about 0.33 nm, which corresponds to the interplanar distance of (111) lattice plane of cubic ZnSe. Figure 6a shows a typical TEM image of an isolated spherical flowerlike assembly. The spherical flowerlike assembly has a diameter of about 2.5 µm and is constructed with small nanoflakes, which are consistent with the SEM results. The TEM image of the constituent nanoflake in the white rectangle in part a is shown in Figure 6b; it is visible that the surface of the nanoflake is relatively rough. The SAED shown in Figure 6c can be indexed as the [21j1j0] zone axis of single crystalline ZnSe with a hexagonal structure. The HRTEM image is displayed in Figure 6d. The spacings between two conjoint planes are about 0.33 and 0.34 nm, which correspond to the (002) and (010) lattice planes of the hexagonal ZnSe, respectively. The SAED and HRTEM results suggest that the constituent nanoflakes are single crystalline ZnSe with the hexagonal structure of the wurtzite type and have two preferential growth directions, [001] and [010]. Figure 7a-c shows the representative SEM images of the products synthesized at Ven/VEG of 1:24 for 0.5, 1, and 2 h, respectively. As shown in Figure 7a, the product obtained after 0.5 h has tubelike structures with a diameter of about 1.75 µm. When the reaction time is prolonged to 1 h, the amount of tubelike products is decreased, and a large quantity of microspheres with diameters of 0.6 to 1.0 µm are found (Figure 7b). When the reaction time is increased to 2 h or longer, the products completely transformed into microspheres (Figures 7c and 1b). The average diameter of microspheres increases slightly, and its uniformity is improved with increasing reaction time. After reacting for 2 and 12 h, the average diameters of microspheres are about 1.0 and 1.4 µm, respectively. EDX spectra of the samples are shown in Figure 7d. Curves I and II in Figure 7d

indicate that the tubelike structures are elemental Se. Curves III and IV reveal that the microspheres are composed of Zn and Se elements. The molar ratio of Zn:Se was calculated to be 0.85:1. The corresponding XRD patterns are shown in Figure 8, which indicates that the tubelike product obtained after 0.5 h is hexagonal Se (Figure 8a). The tubelike Se gradually transformed into cubic ZnSe microspheres with reaction time (Figure 8b-d). In addition, we observed that the ZnSe diffraction peaks become narrower with increasing reaction time, indicating the growth of the constituent ZnSe nanocrystallites. The sizes of the ZnSe nanocrystallites were calculated using Scherrer’s equation, to be 3.5, 4.8, 5.7, and 6.5 nm for 1.0, 2.0, 6.0, and 12 h, respectively. According to Wang et al.23 and Sun et al.,24 the EG acted as both solvent and reducing agent in this case, and SeO32- ions were reduced by EG to form elemental Se. The Se was activated by a nucleophilic attack of the amine to form Se2- ions because en is a strong base, which might be similar to the way sulfur is activated by amine or hydroxide.25-27 The Se2- ions reacted with Zn2+ ions to form ZnSe. The chemical reactions to form ZnSe are formulated as follows:

2HOCH2CH2OH ) 2CH3CHO + 2H2O

(1)

2CH3CHO + SeO23 ) 2CH3COO + Se + H2O (2)

Se + NH2CH2CH2NH2 f Se2-

(3)

Zn2+ + Se2- ) ZnSe(cubic structure)

(4)

In the reaction system, the reaction (3) was a slow step, which determined the formation of ZnSe microspheres. In order to



Figure 7. SEM images of the products prepared at Ven/VEG of 1:24 at 180 °C for (a) 0.5 h, (b) 1 h, and (c) 2 h; (d) EDX spectra of the rectangles in a, b, c (I and II are for black rectangles in a, b, respectively; III and IV for white rectangles in b, c, respectively).

Figure 8. XRD patterns of the products prepared at Ven/VEG )1:24 at 180 °C for (a) 0.5 h, (b)1 h, (c) 2 h, and (d) 6 h.

identify the role of PVP in the formation of ZnSe microspheres, the products obtained at Ven/VEG ) 1:24 without PVP were characterized by SEM and XRD, as shown in Figure 9. The results indicate that the as-synthesized product consists of microspheres with wide diameter distribution assembled from cubic ZnSe nanocrystallites with a mean crystallite size of 7.5 nm. The ZnSe microspheres are not well dispersed and aggregated to form assemblies with irregular morphologies. Therefore, the PVP played an important role in the formation and the assembly of ZnSe nanoparticles into monodispersed microspheres. It prevented the growth of the ZnSe nanocrystallite and agglomeration of the microspheres. The formation of the ZnSe nanoparticle-based microspheres can be described as follows: initially, Zn2+ ions in the reaction medium formed complex coordinate bonds with PVP, which reacted with the Se2- ions, generated via reaction 3, forming ZnSe monomers. After the nucleation, the ZnSe monomers grew into ZnSe nanocrystallites stabilized by PVP. Driven by the

Figure 9. SEM image (a) and XRD pattern (b) of the products obtained at Ven/VEG ) 1:24 without PVP.

minimization of the total energy of the system, these ZnSe nanocrystals aggregated together to form nanospheres. The formation and growth of ZnSe nanospheres were a slow process because reaction 3 was a slow step. With increasing reaction time, the initially formed primary nanoparticles might aggregate together, which then acted as the cores for the subsequent


Zhang et al.

Figure 10. SEM images of the products prepared at Ven/VEG ) 4:21 at 180 °C for (a) 20 min, (b) 0.5 h, (c) 1 h, (d) 2 h, and (e) 6 h; (f) EDX spectra of the rectangles in a, b, d. (I and II are for black rectangles in a, b, respectively; III and IV for white rectangles in b, d, respectively).

aggregation of the new ones to form dispersed ZnSe microspheres. During the formation of ZnSe microspheres, PVP molecules acted as a stabilizing agent, prohibiting the ZnSe microsphere aggregation and the constituent nanoparticle growth as a result of its steric effect.28,29 Figure 10a-e shows the representative SEM images of the products synthesized at Ven/VEG ) 4:21 for 20 min, 0.5, 1, 2, and 6 h, respectively. As shown in Figure 10a, the products obtained after 20 min are of the tubelike product with diameters of about 2 µm. When the reaction time was prolonged to 0.5 h, spherical flowerlike nanoarchitectures with diameters of about 1.6 µm are observed besides the tubelike structures (Figure 10b). With increasing reaction time, the number of the tubelike particles decreased while that of the spherical flowerlike nanoarchitectures increased. After reacting for 2-12 h, the products are completely spherical flowerlike nanoarchitectures (Figures 10d,e and 1d). EDX spectra of the tubelike structures and the spherical flowerlike nanoarchitectures shown in Figure 10f indicate that the tubelike structures are elemental Se while the spherical flowerlike nanoarchitectures contain C, N, Se, and Zn elements. XRD patterns of the tubelike Se and the spherical flowerlike nanoarchitectures shown in Figure 11 can be indexed to hexagonal Se and orthorhombic ZnSe(en)0.5, respectively.

Figure 11. XRD patterns of the products prepared at Ven/VEG ) 4:21 at 180 °C for (a) 20 min, (b) 0.5 h, (c) 1 h, (d) 2 h, and (e) 6 h. The stick pattern is the simulated XRD pattern from the Rietveld refinement and the parameters of ZnSe(en)0.5 reported by Li et al.18



Figure 12. SEM image (a) and XRD pattern (b) of the products obtained at Ven/VEG ) 4:21 without PVP. The stick pattern in part b is the simulated XRD pattern from the Rietveld refinement and the parameters of ZnSe(en)0.5 reported by Li et al.18

These results indicated that at Ven/VEG of 4:21, en acted not only as a strong alkali to activate Se to form Se2- ions but also as a ligand to react with Zn2+ and Se2- ions, forming ZnSe(en)0.5. The ZnSe(en)0.5 decomposed into ZnSe with a hexagonal structure after calcination. Therefore, the chemical reactions to form hexagonal ZnSe also include the following reactions:

Zn2+ + en + Se2- f ZnSe:(en)0.5

(5)

ZnSe:(en)0.5 f ZnSe(hexagonal structure) + en

(6)

Figure 12 shows the SEM and XRD results of the sample obtained at Ven/VEG of 4:21 without PVP. The XRD and SEM results indicate that the as-synthesized products are composed of nanoflakes, nanoflake aggregations with irregular morphol-

ogy, and spherical flowerlike nanoarchitectures constructed from ZnSe(en)0.5 nanoflakes. It implies that PVP promoted the selfassembly of ZnSe(en)0.5 nanoflakes into spherical flowerlike nanoarchitectures. The formation of the ZnSe(en)0.5 spherical flowerlike nanoarchitectures can be explained as follows. Initially, Zn2+ ions reacted with en and Se2- ions to form ZnSe(en)0.5 monomers via reaction 5 in the presence of PVP. After nucleation, the ZnSe(en)0.5 monomers grew into ZnSe(en)0.5 nanocrystallites stabilized by PVP. The initially formed nuclei aggregated into nuclei in a cluster, and subsequent growth from the assembled nuclei resulted in ZnSe(en)0.5 spherical flowerlike nanoarchitectures. It is well-known that the wurtzite ZnSe crystal is described as alternating stacking of positively charged Zn and negatively charged Se ions along its c axis.30 The crystal structure of ZnSe(en)0.5 is a three-dimensional network containing monolayers of ZnSe slabs that are interconnected by bridging en molecules.18 In addition, for ZnSe(en)0.5, ZnS(en)0.5, etc., there are two preferential growth directions, [0001] and [01j10].31 Therefore, it is reasonable to conclude that the initially formed primary ZnSe(en)0.5 nuclei also have positively charged Zn and negatively charged Se surfaces. These ZnSe(en)0.5 nuclei aggregated into nuclei in a cluster driven by the electrostatic attraction32 during the nucleation without PVP. The assembled nuclei grew along [0001] and [01j10] directions into nanoflake assembled aggregations with irregular morphology and spherical flowerlike nanoarchitectures constructed with nanoflakes with increasing reaction time. The single nanoflake grew from a single nucleus. PVP served as a ligand to Zn, so that concentrated Zn2+ ions and ZnSe(en)0.5 molecules as a result of its molecular chain effect thus promoted the formation of a single aggregate of nuclei during the nucleation. The nanoflake-based spherical flowerlike nanoarchitectures with good monodispersity grew from a single aggregate of nuclei. In conclusion, the final products obtained in the ZnCl2-Na2SeO3-EG-en-PVP solvothermal system depend on Ven/VEG and reaction time, which are summarized in Figure 13. Figure 14a shows the UV-vis absorption spectra of the methyl orange aqueous solution (initial concentration: 5 × 10-5 M, 30 mL) in the presence of the ZnSe spherical-flower nanoarchitecture samples (30 mg) under exposure to UV light for various durations. The absorption peaks corresponding to methyl orange diminished gradually as the exposure time was

Figure 13. Evolution of phase compositions and particle morphologies of the products at different reaction conditions.


Zhang et al. spherical flowerlike nanoarchitectures assembled from nanoflakes with a hexagonal structure has been developed by changing the volume ratio of en to EG in the ZnCl2-Na2SeO3-EG-en-PVP solvothermal system and subsequent annealing in Ar. This simple, mild solution approach to fabricate three-dimensional ZnSe superstructures with controllable morphologies and crystalline phases can be easily scaled up and potentially extended to the hierarchical assembly of nanoscale building blocks of other zinc compounds. The as-prepared ZnSe spherical flowerlike nanoarchitectures showed an excellent photocatalytic ability to degrade methyl orange, which are expected to be useful in sewage water treatment. Acknowledgment. We thank the National Natural Science Foundation of China (Grant No. 20573072) and Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20060718010) for their support to this work. References and Notes

Figure 14. (a) Absorption spectrum of the methyl orange solution in the presence of ZnSe spherical flowerlike nanoarchitectures under exposure to UV light. (b) Photodegradation of the methyl orange solution under different conditions: (1) without catalyst and under UV light, (2) with ZnSe microspheres and under UV light, (3) with ZnSe spherical flowerlike nanoarchitectures and under UV light.

extended, and the maximum absorbance disappeared almost completely after irradiation for about 10 h. The loss of absorbance implies the destruction of the dye chromogen. Since no new peak was observed, the methyl orange has been decomposed. Figure 14b displays the results of the methyl orange solution degradation in a series of experimental conditions. The concentration of methyl orange changed slightly under exposure to UV light without catalyst (curve 1). However, as indicated by curves 2 and 3, the concentration of methyl orange solution obviously decreased with ZnSe microspheres or ZnSe spherical flowerlike nanoarchitectures under exposure to UV light, revealing the obvious photocatalytic ability of the samples. Moreover, the ZnSe spherical-flowerlike nanoarchitecture sample exhibits superior photocatalysis over the ZnSe microsphere sample. It is generally accepted that the catalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalyst. The high specific surface area can provide more reactive adsorption/desorption sites for photocatalytic reactions. The BET surfaces of ZnSe microspheres and spherical flowerlike nanoarchitectures were measured to be 83.13 and 58.67 m2 g-1, respectively. In addition, we observed that the color of ZnSe microspheres constructed from ZnSe nanoparticles with a cubic structure changed slowly from yellow to orange and red with time in air. This observation revealed that the constituent ZnSe naocrystallites with a size of about 6.5 nm were not stable in air because of its high surface energy effect. The Se2- ions on the surface of the microspheres was oxidized in air to form amorphous Se, and thus a ZnSe/Se core-shell structure was formed. This result is consistent with the finding by Li et al.13 The amorphous Se layer on the surface of the constituent ZnSe nanoparticles would decrease its photocatalysis capability. 4. Conclusion In summary, controlled synthesis of ZnSe microspheres constructed from nanoparticles with a cubic structure and

(1) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66. (2) Xiong, S. L.; Shen, J. M.; Xie, Q.; Gao, Y. Q.; Tang, Q.; Qian, Y. T. AdV. Funct. Mater. 2005, 15, 1787. (3) Xiang, B.; Zhang, H. Z.; Li, G. H.; Yang, F. H.; Su, F. H.; Wang, R. M.; Xu, J.; Lu, G. W.; Sun, X. C.; Zhao, Q.; Yu, D. P. Appl. Phys. Lett. 2003, 82, 3330. (4) Wang, C. R.; Wang, J.; Li, Q.; Yi, G. C. AdV. Funct. Mater. 2005, 15, 1471. (5) Tafreshi, M. J.; Balakrishna, K.; Dhansekaran, R. J. Mater. Sci. 1997, 32, 3517. (6) (a) Wang, W. Z.; Geng, Y.; Yan, P.; Liu, F. Y.; Xie, Y.; Qian, Y. T. J. Am. Chem. Soc. 1999, 121, 4062. (b) Li, Y. D.; Ding, Y.; Qian, Y. T.; Zhang, Y.; Yang, L. Inorg. Chem. 1998, 37, 2844. (7) (a) Yang, J.; Xue, C.; Yu, S. H.; Zeng, J. H.; Qian, Y. T. Angew. Chem. 2002, 114, 4891. (b) Panda, A. B.; Glaspell, G.; El-Shall, M. S. J. Am. Chem. Soc. 2006, 128, 2790. (8) (a) Zhang, X. T.; Liu, Z.; Leung, Y. P.; Li, Q.; Hark, S. K. Appl. Phys. Lett. 2003, 83, 5533. (b) Zhu, Y. C.; Bando, Y. S. Chem. Phys. Lett. 2003, 377, 367. (9) (a) Zhai, T. Y.; Zhong, H. Z.; Gu, Z. J.; Peng, A. D.; Fu, H. B.; Ma, Y.; Li, Y. F.; Yao, J. N. J. Phys. Chem. C 2007, 111, 2980. (b) Jiang, Y.; Meng, X. M.; Yiu, W. C.; Liu, J.; Ding, J. X.; Lee, C. S.; Lee, S. T. J. Phys. Chem. B 2004, 108, 2784. (10) Fu, H. Z.; Li, H. Y.; Jie, W. Q.; Yang, L. J. Cryst. Growth 2006, 289, 440. (11) (a) Colli, A.; Hofmann, S.; Ferrari, A. C.; Ducati, C.; Martelli, F.; Rubini, S.; Franciosi, A.; Robertson, J. Appl. Phys. Lett. 2005, 86, 153103. (b) Hu, J. Q.; Bando, Y. S.; Zhan, J. H.; Liu, Z. W.; Golberg, D.; Ringer, S. P. AdV. Mater. 2005, 17, 975. (12) (a) Deng, Z. X.; Wang, C.; Sun, X. M.; Li, Y. D. Inorg. Chem. 2002, 41, 869. (b) Chen, M. H.; Gao, L. Mater. Chem. Phys. 2005, 91, 437. (13) Peng, Q.; Dong, Y. J.; Li, Y. D. Angew. Chem., Int. Ed. 2003, 42, 3027. (14) Du, J.; Xu, L. Q.; Zou, G. F.; Chai, L. L.; Qian, Y. T. Mater. Chem. Phys. 2007, 103, 441. (15) Xiong, S. L.; Xi, B. J.; Wang, C. M.; Xi, G. C.; Liu, X. Y.; Qian, Y. T. Chem. Eur. J. 2007, 13, 7926. (16) Philipose, U.; Ruda, H. E.; Shik, A.; Souza, C F. D.; Sun, P. J. Appl. Phys. 2006, 99, 066106. (17) Xu, L.; Su, Y.; Li, S.; Chen, Y. Q.; Zhou, Q. T.; Yin, S.; Feng, Y. J. Phys. Chem. B 2007, 111, 760. (18) Huang, X. Y.; Heulings IV, H. R.; Le, V. N.; Li, J. Chem. Mater. 2001, 13, 3754. (19) (a) Dong, C. J. Appl. Crystallogr. 1999, 32, 838. (b) Dong, C.; Wu, F.; Chen, H. J. Appl. Crystallogr. 1999, 32, 850. (c) Dong, C.; Chen, H.; Wu, F. J. Appl. Crystallogr. 1999, 32, 168. (20) Lu, J.; Wei, S.; Peng, Y. Y.; Yu, W. C.; Qian, Y. T. J. Phys. Chem. B 2003, 107, 3427. (21) Mizushima, S.; Nakangwa, I.; Sweeny, D. M. J. Chem. Phys. 1956, 25, 1006. (22) Allen, A. D.; Senoff, C. V. Can. J. Chem. 1965, 43, 888. (23) Wang, J. W.; Wang, X.; Peng, Q.; Li, Y. D. Inorg. Chem. 2004, 43, 7552. (24) Sun, Y. G.; Yin, Y. D.; Mayers, B. T.; Herricks, T.; Xia, Y. N. Chem. Mater. 2002, 14, 4736.

ZnSe Nanostructured Assemblies (25) Li, Y. D.; Ding, Y.; Liao, H. W.; Qian, Y. T. J. Phys. Chem. Solids 1999, 60, 965. (26) Li, B.; Xie, Y.; Huang, J. X.; Qian, Y. T. AdV. Mater. 1999, 11, 17. (27) Jiang, T.; Ozin, G. A.; Bedard, R. L. AdV. Mater. 1994, 6, 860. (28) Zhang, Z. T.; Zhao, B.; Hu, L. M. J. Solid State Chem. 1996, 121, 105. (29) Wang, X. B.; Zhang, S. H.; Zhang, Z. C. Mater. Chem. Phys. 2008, 107, 9.

J. Phys. Chem. C, Vol. 113, No. 14, 2009 5443 (30) Leung, Y. P.; Choy, W C. H.; Markov, I.; Pang, G K. H.; Ong, H. C.; Yuk, T. I. Appl. Phys. Lett. 2006, 88, 183110. (31) Lu, F.; Cai, W. P.; Zhang, Y. G.; Li, Y.; Sun, F. Q.; Heo, S. H.; Cho, S. O. J. Phys. Chem. C 2007, 111, 13385. (32) (a) Barick, K. C.; Aslam, M.; Dravid, V. P.; Bahadur, D. J. Phys. Chem. C 2008, 112, 15163. (b) Tay, Y. Y.; Li, S.; Boey, F.; Cheng, Y. H.; Liang, M. H. Phys. B 2007, 394, 372.

JP810385V

Controlled Synthesis and Photocatalytic Activity of ZnSe

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