Selenium Microstructures

DOI: 10.1002/anie.200504131

Photothermally Assisted Solution-PhaseSynthesis of Microscale Tubes, Rods, Shuttles, andan Urchin-Like Assembly of Single-CrystallineTrigonal Selenium**

Bin Zhang,Wei Dai, Xingchen Ye, Fan Zuo, and Yi Xie*

Trigonal selenium (t-Se) is one of the most widely studiedmaterials owing to its fundamental significance in theoreticalstudies and its potential applications in many fields.[1] Con-

siderable effort has been devoted to generating seleniumnanorods, nanowires, nanobelts, and nanowire networks byemploying techniques such as laser ablation,[2] sonochemicalapproaches,[3] solution-mediated transformations,[4] reflux-ing,[5] hydrothermal methods,[6] CVD,[7] and biomolecule-assisted methods.[8] Recently, a great deal of attention hasbeen focused on the synthesis of tubular selenium.[9] Ultra-long Se microtubes were recently fabricated by a one-stepsolution-phase method in our group,[10] and we also found thatthe morphology of the selenium products has a remarkableinfluence on their capacity for electrochemical hydrogenstorage,[10] which inspired us to search for a simple, inex-pensive, and efficient approach to the morphology-controlledsynthesis of Se nanostructures/microstructures, especiallythose with novel and interesting morphologies.

Recently, photochemical and irradiation synthesis hasproven to be a promising route for the preparation of variousnanomaterials. These simple processes yield uniform productswhile the growth mechanism of the resulting materials can beresolved conveniently.[11–14] Current photochemical methodsfocus mainly on the use of UV light. For instance, Yang et al.have successfully prepared uniform gold nanorods upon UVirradiation,[11] and the morphology-controlled synthesis of Aunanorods has also been realized in a combined chemicalreduction/UV photochemical approach.[12] g-Irradiation froma 60Co source has been used in the preparation of semi-conductor/polymer nanocables and Ag/Pt alloy nanoparti-cles.[14] It is therefore important to design a safer, cheaper, andmore convenient light-assisted approach to 1D nanomateri-als/micromaterials.

As for the solution-phase synthesis of trigonal seleniummaterials, the transformation of amorphous Se (a-Se) into t-Se is still considered to be an important step.[1,3–6, 9–11] Ishidaet al. have reported, on the basis of Raman spectrocopicobeservations, that laser light with a wavelength of 633 nmcan induce the anisotropic crystallization of a-Se, which thenforms t-Se.[15] Thus, it is reasonable to assume that visible light(380–800 nm) should also be able to accelerate the trans-formation of a-Se into t-Se. In this communication, we reportthe first facile one-step method for the controlled synthesis ofsingle-crystalline Se 1D microstructures upon irradiation withvisible light from a commercial lawn lamp in the presence ofpoly(vinylpyrrolidone) (PVP, K-30). Selenium microtubes,nanorods, novel shuttle-like needles, and urchin-like assem-blies of selenium nanorods can be fabricated on a large scaleby varying the experimental parameters. The photo andthermal effects of visible light are thought to be significant inthis morphology-controlled synthesis of Se materials. Inaddition, we demonstrate that the selenium microtubes andmicroshuttles have distinctly different electrochemical hydro-gen-storage capacities.

Various selenium materials with unusual morphologiescan be synthesized in this visible-light-assisted approach bycontrolling the relative amounts of SeO2 and PVP. The as-prepared products were first examined by field emissionscanning electron microscopy (FESEM; Figure 1). When amixture of 0.1 g of SeO2 and 0.01 g of PVP was illuminatedwith a lawn lamp (see the Supporting Information), thesamples obtained were found to contain Se microtubes with a

[*] Dr. B. Zhang, W. Dai, X. Ye, F. Zuo, Prof. Y. XieDepartment of Nanomaterials and NanochemistryHefei National Laboratory for Physical Sciences at MicroscaleUniversity of Science and Technology of ChinaHefei, Anhui 230026 (P.R. China)Fax: (+86)551-360-3987E-mail: yxie@ustc.edu.cn

[**] This work was supported by the National Nature Science Founda-tion (NSF) of China.

Supporting Information for this article is available on the WWWunder http://www.angewandte.org or from the author.

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uniform size distribution, as shown by the low-magnificationFESEM image (Figure 1a). The FESEM image at highermagnification (Figure 1b) clearly shows the products? tubularstructure, with a diameter of 900 nm and a length of about2.5 mm. When the amount of PVP was increased to 0.05 g,with the other conditions remaining unchanged, seleniummicrorods were obtained; some cross-like rod structurescould also be observed in the final products (Figure 1c and d).As shown in Figure 1e and f, shuttle-like t-Se, which has notbeen observed in previous reports, was obtained when only0.005 g of PVP was used. These shuttles are about 6.5 mm longand end in two very sharp tips. Interestingly, seleniumnanorods with a diameter of 160 nm assembled spontaneouslyto form a novel urchin-like shape (Figure 1g and h), which hasnot been reported in previous approaches, when the amountof SeO2 was decreased to 0.05 g with other conditionsremaining unchanged. The successful preparation of theseabundant and new Se morphologies suggests that this visible-light-assisted approach could be applied to synthesize Se, Te,and other anisotropic materials with novel morphologies.

The XRD pattern and the Raman and EDAX spectra ofthe as-prepared products identifies them as trigonal seleniumof high crystallinity and purity (see the Supporting Informa-tion). The morphology and structure of the Se microshuttles

were further characterized by TEM and selected-area elec-tron diffraction (SAED). Figure 2a display the TEM imagesof the obtained selenium shuttles, which reveal that the endsof the shuttle-like samples are very sharp. The indexing of thespots in the SAED pattern (Figure 2b) and HRTEM obser-vations (Figure 2 c and d) indicate that the microshuttle is asingle crystal that grows predominantly along the [001]direction (c axis).

To determine the role of light and heat in the formation ofthese selenium materials we performed a series of controlexperiments in which the amount of the reagents wascontrolled to be identical to that of the microtubes, unlessotherwise stated. In this approach, we found that the solution-phase temperature was about 100 8C when a 500-W lawn lampwas used (the intensity of the light shining on the reactionmixture was around 340 mW cm�2). Therefore, to clarify therole of the light a conventional reflux technique was appliedto the four different solution systems, but only brokenselenium tubes (Supporting Information) were obtainedwhen refluxing the solution with different amounts of SeO2

and PVP, which are obviously different from the microtubes,microrods, microshuttles, and urchin-like assemblies of nano-rods obtained with the visible-light-assisted technique(Figure 1).

We also found that refluxing the reaction mixtures gaveonly small differences in the products? morphologies fordifferent amounts of SeO2 and PVP (Supporting Informa-tion). In contrast, the morphology obtained is very sensitive tothe amount of SeO2 and PVP in the photothermally assistedmethod (Figure 1). This indicates that the visible light fromthe lawn lamp induces and greatly enhances the directing roleof PVP in the preparation of selenium materials. Moreover,we found that only Se particles were obtained when the

Figure 1. FESEM images of four selenium samples with differentmorphologies prepared from different amounts of SeO2 and PVP:microtubes (0.1 g SeO2 + 0.01 g PVP) (a and b); microrods (0.1 gSeO2 + 0.05 g PVP) (c and d); microshuttles (0.1 g SeO2 + 0.005 gPVP) (e and f); urchin-like assembly of selenium nanorods (0.05 gSeO2 + 0.01 g PVP) (g and h).

Figure 2. a) TEM image of one t-Se microshuttle. The inset shows thelow-magnification TEM image. b) The associated SAED pattern. The[11̄0] zone axis of the trigonal selenium microshuttle is identified, thusindicating the preferential growth along the [001] direction. (c andd) HRTEM images of different areas (indicated in a) of an individualshuttle. The fringe spacing of 0.5 nm observed in this image corre-sponds to the separation of the (001) lattice planes.

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reaction mixture was irradiated with an ultrahigh-pressuremercury lamp of an ultraviolet spot curing system (HG-250,Mejiro Precision Inc.), which confirms the critical role ofthermal effects in controlling the morphology. This conclusionis also supported by the formation of broken multi-armedtubes and Se particles (Figure 3a) when the light intensity ofthe lawn lamp was decreased from 340 to 210 mW cm�2 (thepower of the lamp was decreased from 500 to 300 W).

When the same system was irradiated with g-radiationfrom a 2.59 D 1015 Bq 60Co g-ray source with a dose of 30 kGy,the brick-red products obtained were found to be uniformamorphous selenium spheres (Figure 3b). Thus, our visible-light-assisted approach is obviously different from both theconventional refluxing technique and the reported methodsthat require UV or g-ray irradiation. We suppose that photo-irradiation and thermal effects contribute simultaneously tothe controlled synthesis of Se tubes, rods, shuttles, and urchin-like assemblies. As for the role of light, we assume that itaccelerates the transformation of a-Se into t-Se and that itorients the growth along the c axis of t-Se. In fact, besides thelaser-induced anisotropic crystallization of a-Se,[15a] a similaranisotropic crystallization of selenium has been observedwhen investigating the Raman spectrum of an amorphousselenium film.[16] We therefore decided to carry out a Ramananalysis of our amorphous Se intermediates to confirm thatvisible light can also accelerate the transformation of a-Seinto t-Se (Supporting Information). It is reasonable to assumethat visible light breaks covalent bonds[15,16] in our approachand makes the products grow along the c-axis, but furtherinvestigations are necessary.

Although the exact mechanism of the photothermallyassisted morphology-controlled preparation of t-Se 1D struc-tures is still unclear, the directing role of PVP is undoubtedlysignificant. In the absence of PVP, only sphere-like Separticles were synthesized rather than 1D Se materials. Asdiscussed above, tube-like, rod-like, shuttle-like, and urchin-like selenium materials can be fabricated by adjusting theratio of SeO2 to PVP, which suggests the key role of PVP incontrolling the Se products? shape. Furthermore, intersecting,needle-like selenium was obtained (Supporting Information)when PVP was replaced by the nonionic surfactant polyoxy-ethylene (20) sorbitan monolaurate (Tween-20). However,under hydrothermal conditions no needle-like selenium wasobtained when Tween-20 was adopted as the capping agent.[12]

This further testifies that PVP or other surfactants may

exhibit more significant directing roles upon illumination withvisible light, and that this photothermally assisted technique isa better way of synthesizing selenium materials with newmorphologies. Our preliminary TEM observations of theintermediates have also shown that Se nanostructures/micro-structures experience an Ostwald-type ripening during theconversion of a-Se into t-Se and the following nucleation andgrowth process, similar to what Xia et al. have proposed forother Se and Te systems (Supporting Information).[1a,b, 17]

Interestingly, our electrochemical study demonstratedthat the discharge capacity of Se microtubes is higher thanthat of shuttle-like selenium and that the morphology of theselenium products exerts a noticeable influence on theelectrochemical hydrogen storage ability (Supporting Infor-mation).

In summary, a facile photothermally assisted solution-phase approach has been developed for the synthesis ofselenium micromaterials. Tubular, rod-like, and shuttle-likeSe as well as an urchin-like assembly of selenium nanorodscan be synthesized on a large scale by varying the relativeamounts of SeO2 and PVP. Both light and thermal effects playsignificant roles in this visible-light-assisted technique. Wehave observed that the light from a lawn lamp can induce andenhance the directing role of PVP in the morphology-controlled synthesis of selenium materials. In addition, theelectrochemical hydrogen-storage capacity has been demon-strated to be susceptible to the morphology of the Sematerials. Although detailed investigations concerning theexact photothermal role and the growth mechanism andhydrogen-storage process are still in progress, this novelapproach can be potentially extended to fabricate 1D nano-structures/microstructures of Te and other anisotropic mate-rials with interesting morphologies in the presence of PVP orother surfactants. Furthermore, the preparation and electro-chemical studies of Se materials with different shapes willoffer great opportunities to explore the dependence of theproperties of the material on its morphology and be essentialfor the subsequent manufacture of potential optoelectronicdevices.

Experimental SectionAll chemicals were of analytical grade and were used as received. In atypical procedure, a given amount of poly(vinylpyrrolidone) (PVP, K-30) and hydrazine hydrate (N2H4·H2O, 85 %, 1 mL) were added todistilled water (50 mL) and the mixture was dispersed to form ahomogeneous solution by constant strong stirring. Then, distilledwater (50 mL) containing a given amount of SeO2 was quickly addeddropwise to the above solution at room temperature, and the solutionquickly became orange-yellow. Once the solution had turned salmonpink (about 30 min), the mixture was irradiated for 6 h with visiblelight from a commercial lawn lamp (halogen bulb) at a distance of5 cm (340 mWcm�2). The black precipitate was collected and washedthree times each with distilled water and absolute ethanol. Thesample was then dried in vacuo at 50 8C for six hours. The XRD,Raman, SEM, TEM, and EDAX apparatus and the electrochemicalmeasurements are described in the Supporting Information.

Received: November 21, 2005Revised: January 5, 2006Published online: March 17, 2006

Figure 3. SEM images of selenium samples prepared by other meth-ods: a) mixture of broken multi-armed tubes and Se particles synthe-sized by illumination with a lawn lamp at 210 mWcm�2; b) uniformselenium spheres fabricated by gamma irradiation.

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.Keywords: materials science · microstructures · selenium ·semiconductors

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