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Nonideal Mixing of Se-Te in Aqueous Micellar Phase for Nanoalloys Over the Whole Mole Mixing Range with Morphology Control from Nanoparticles to Nanoribbons Gurinder Kaur† and Mandeep Singh Bakshi*,‡ Nanotechnology Research Laboratory, College of North Atlantic, Labrador City, NL A2V 2K7, and Department of Chemistry and Physics, Mount Saint Vincent UniVersity, Halifax, NoVa Scotia, B3M 2J6, Canada ReceiVed: September 7, 2009; ReVised Manuscript ReceiVed: October 17, 2009

Se-Te alloy nanocrystals (NCs) were synthesized in aqueous micellar phase at 85 °C by using Na2SeO3/ Na2TeO3 as Se/Te source in the presence of different amounts of hydrazine (i.e., 0.1-3.6 M) as reducing agent over the entire mole fraction range of Se (xSe) from xSe ) 0 to 1. The shape, structure, and composition of alloy NCs were characterized by scanning electron microscopic (SEM), transmission electron microscopic (TEM), and energy dispersive X-ray spectroscopic (EDS) measurements. Despite many similar characteristic features of elemental Se and Te, a drastic change in the morphology of NCs was observed from xSe ) 0 to 1. Amorphous water-soluble Se nanoparticles were obtained at xSe ) 1 in the presence of excess of hydrazine, whereas long nanoribbons of several micrometers of Te were produced at xSe ) 0. In the intermediate mole fractions, a smooth transition in the morphology from predominantly rhombohedral in the Se rich-region to long nanoribbons in the Te rich-region of the mixtures was observed. At relatively much lower hydrazine concentration, large plate-like morphologies were obtained at xSe ) 1, while the morphologies at other mole fractions were slightly distorted with greater effect in the Se rich-region of the mixtures. A careful EDS analysis on each kind of morphology revealed a homogeneous mixing between elemental Se and Te but with a significantly nonideal behavior over the whole mole fraction range. All results were summarized in a phase diagram depicting the relationship between stochiometric amounts of Se and Te with their atomic percent in crystalline phase. NCs were always highly rich in the Te contents even in the Se rich-region of the mixtures and the overall growth was predominantly driven by metalloid (crystalline) nature of Te rather than nonmetallic (amorphous) nature of Se. N2H4(aq) + SeO32- /TeO32-(aq) f Se/Te(s) + N2(g) +

Introduction Both Selenium (Se) and tellurium (Te) belong to sixth group p-block elements and exist in helical chains in crystalline form. Se exhibits photovoltaic action by converting light into electricity through photoconductivity due to its reasonably small band gap of 2.6 eV in the crystalline form. This allows Se to be used in versatile photocells and solar cells.1,2 Te also exhibits photoconductivity to a lesser extent. Both Se and Te are p-type semiconductors and hence used in electronics and solid-state applications. Independently, both predominantly form 1D nanostructures3-8 due to a preferential growth along the c-axis of trigonal hexagonal geometry and exist in the spiral chains. One-dimensional geometries have potential ramifications in electronic device formation.9 Facile production of such morphologies under economical favorable conditions such as in aqueous phase and at relatively low temperature (<100 °C) is always required. Recently, Se-Te alloys have been found to demonstrate interesting temperature dependence of electrical resistivity.10 In addition, fine homogeneous nearly monodisperse Se-Te nanorods have also been synthesized simply by following hydrazine reduction.11 Hydrazine is a relatively weaker reducing agent than ammonia but it is best suited for a slow reduction of Se or Te precursors into their atomic forms. The following reaction is expected to take place * To whom correspondence should be addressed. E-mail: ms_bakshi@ yahoo.com. † College of North Atlantic. ‡ Mount Saint Vincent University.

2OH-(aq) + H2O (1) In our recent work12 related to the synthesis and characterization of PbSe and CdSe nanocrystals (NCs), we have also observed the formation of Se nano- and microrods in both cases. It shows that Se is more readily reduced in comparison to Pb and Cd, which is understood from its nonmetallic nature as well as its location in the periodic table. The same can be said about Te. Se-Te alloy formation11,13 is emerging as a potential field with applications in nanoelectronics, therefore a comprehensive investigation of their simultaneous reduction requires attention of both reduction chemistry as well as crystallization kinetic perspectives. In the present work, we have focused our attention on both aspects. To investigate these aspects, an aqueous micellar phase simultaneous reduction of sodium selenite and tellurite by hydrazine has been carried out to produce Se-Te alloy nanomorphologies at 85 °C in the presence of cetyltrimethylammoium bromide (CTAB) as stabilizing as well as capping agent. This is done over the whole mole fraction range of Se + Te mixtures under varying amounts of hydrazine in order to understand the influence of its reducing ability on the crystallization kinetics of Se-Te nanoalloys at different mole fractions. A drastic change in the shape, structure, and composition of nanomorphologies is observed. At low amounts of Se, mostly rhombohedral geometries are produced that slowly take the form of spindle-shaped rods. As the mole fraction reaches in the Te-

10.1021/jp9086249  2010 American Chemical Society Published on Web 11/03/2009

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Figure 1. Photos of as prepared reactions over the whole mole fraction range of Se + Te mixtures in the presence of 3.6 M hydrazine.

rich region, spindle-shaped particles then take the form of long needles that ultimately acquire ribbon-shaped structures. EDS analyses demonstrate the presence of a greater amount of Te than Se in each case, even in the Se-rich region of the mixture, when a large amount of hydrazine is used. As the amount of hydrazine reduces, it allows progressively greater amount of Se to incorporate in Se-Te alloy NCs. The results are discussed on the basis of reducing efficiency of hydrazine as well as different growth kinetics of Se and Te to form Se-Te nanoalloy. This study provides a detailed analysis of Se-Te nanoalloy formation over the whole mole fraction range with entirely different nanomorphologies and is a first in-depth analysis to our information. Experimental Section Synthesis of Se-Te alloy NCs. Sodium selenite (Na2SeO3), sodium tellurite (Na2TeO3), hydrazine, and cetyltrimethylammonium bromide (CTAB), all 99% pure, were purchased from Aldrich. Double distilled water was used for all preparations. Se-Te NCs were synthesized by a simultaneous hydrazine reduction of both aqueous Na2SeO3 and Na2TeO3 in a micellar solution. First of all, two aqueous stock solutions of Na2SeO3 ) 25 mM and Na2TeO3 ) 25 mM were prepared separately in the presence of CTAB ) 50 mM. Required quantities of both stock solutions were mixed to produce different Se mole fractions, xSe ) 0, 0.2, 0.4, 0.6, 0.8, and 1, with total [Na2SeO3 + Na2TeO3] ) 25 mM, along with a constant addition of 3.6 M hydrazine in each case. It is to be mentioned that all reactions were carried out by using fresh hydrazine. The reaction bottles with final volume of 10 mL of each mole fraction were then kept in an oil bath maintained at 85 °C for 24 h under static conditions. The choice of 85 °C provides better prospects of getting crystalline nanoparticles. Similar set of reactions were also carried out in the presence of 0.1 and 2.2 M hydrazine while keeping the concentrations of all other ingredients constant. The color of the solution changed from colorless to reddish-brown for the mole fractions lying in the Se rich-region, while colorless to gray-black for those lying in the Te richregion of the mixtures within 1-2 h and remained same until 48 h. Figure 1 shows a series of bottles with different mole fractions from xSe ) 0-1 prepared with hydrazine ) 3.6 M. It is to be mentioned that we were unable to collect pure Se nanoparticles (xSe ) 1) even spinning the solution at more than 15 000 rpm over an extended period of time. The colloidal suspensions thus obtained at other mole fractions were cooled

to room temperature. NCs were collected after repeated washing with pure water (at least 3 times) to remove surfactant followed by spinning at 10 000 rpm for 5 min in each case. Methods. FESEM, TEM, EDS, and UV-Visible Measurements. Scanning electron microscopic (SEM) analysis was carried out on a Zeiss NVision 40 Dual Beam FIB/SEM instrument. Photomicrographs were obtained in bright field scanning/imaging mode, using a spot size of ∼1 nm and 12 cm of a camera length. Energy dispersive X-ray spectroscopic (EDS) microanalysis was carried out using an Oxford-INCA Atmospheric Ultrathin Window (UTW) and the data were processed using Oxford INCA Microanalysis Suite Version 4.04. Transmission electron microscopic (TEM) analysis was done on a JEOL 2010F at an operating voltage of 200 kV. The samples were prepared by mounting a drop of a solution on a carbon-coated Cu grid and allowed to dry in the air. UV-visible spectra of as prepared Se colloidal suspensions were recorded by UV spectrophotometer (Multiskan Spectrum, model no. 1500) in the wavelength range of 200-900 nm. Time dependent scans of some reactions at regular intervals were also carried out to understand the growth kinetics of Se nanoparticles. The measurements were performed simultaneously withdrawing a fixed amount of colloidal nanoparticles suspension (∼1 mL) from the reaction vessel and then determined its UV-visible spectrum at room temperature. After the measurement, the sample was reintroduced into the reaction vessel each time. In this way, a number of time dependent scans were carried out over a time span of more than two hours. Results and Discussion Reactions with [Hydrazine] ) 3.6 M. Figure 2a shows the SEM images of Se-Te alloy NCs with xSe ) 0.8. Most of the NCs are either in cubic or rhombohedral form14,15 with size 208 ( 52 nm (see size distribution histogram in Figure 2aH) but some of the particles are having triangular ring shaped structures (indicated by block arrow). Close inspection of these shapes (Figure 2b,c) shows the presence of small surface protrusions arranged in a spiral fashion. A TEM micrograph (Figure 2d) of the same sample gives similar information though surface protrusions are less clear. Lattice resolved image (Figure 2e) of {100} planes indicates the crystalline nature of NCs. Atomic composition of different morphologies was determined from EDS analysis. Figure 2f-h demonstrates the EDS line spectrum over triangular ring, rhombohedral, and cubic particles, respectively. A homogeneous mixing between elemental Se and Te

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Figure 2. (a) Se-Te alloy NCs of rhombohedral or cubic morphologies prepared with a mole fraction of Se, xSe ) 0.8, in the presence of 3.6 M hydrazine. Almost all rhombohedral NCs have rhombohedral angle, RT ) 60°. Some NCs possess triangular ring structure (shown as filled block arrow) with surface protrusions. (aH) Size distribution histogram of NCs shown in (a). (b,c) Close up images of a rhombohedral and open triangular ring NCs showing surface protrusions. (d) TEM image of few rhombohedral NCs, and (e) a lattice resolved image. (f-h) The EDS line analysis across triangular ring shaped, rhombohedral, and a cubic shaped NCs, respectively, demonstrating almost homogeneous mixing between elemental Se and Te with significantly lower amount of Se in rhombohedral and cubic NCs than in triangular ring shaped NC. See details in text.

with significantly lesser amount of Se in comparison to Te is evident in each case. Compositional analysis based on various EDS spectra (see Supporting Information, Figure S1) computes 11 and 89 atomic % of Se and Te, respectively. From these values, one can calculate the mole fraction of Se in crystalline alloy state, which comes out to be 0.17. Thus, there is a huge difference between 0.8 (stochiometric mole fraction) and 0.17 values that indicates that the actual amount of Se incorporated in alloy NCs is less than one-fourth of its stochiometric amount used in the first place. Decrease in the mole fraction from xSe ) 0.8 to 0.6 mainly produces spindle shaped16 NCs (Figure 3a,b) with aspect ratio 3.4 ( 0.7 (Figure 3aH). TEM micrograph fully supports such morphologies (Figure 3c). High-resolution TEM image further confirms their crystalline nature and lattice-resolved image (Figure 3d) shows that the particles are bound with {100} crystal planes. Atomic compositions have been deduced from EDS line

spectrum along the long as well as short axes of a single spindle (Figure 3e,f). In both cases, a significantly reduced amount of Se is present in comparison to Te. A detailed analysis (Supporting Information, Figure S2) suggests the presence of 2.5 and 95 atomic % Se and Te, respectively. Thus, a decrease in the xSe from 0.8 to 0.6 further reduces the amount of Se from 11 to 2.5 atomic %. As the mole fraction of Se is reduced to 0.4, the spindle shape of NCs remains intact but there is a substantial increase in the number of micrometer-sized spindles (Figure 3g,h). Interestingly, there is no significant change in the aspect ratio of small spindles (∼3.4 ( 0.7) while that of large spindles increases to 4.1 ( 0.8 (Supporting Information, Figure S3). Hence, the overall size of the spindle shaped NCs has increased three-dimensionally as xSe decreases from 0.6 to 0.4. Simple area calculations (based on two dotted fused triangles in Figure 3h) suggest that large spindles are at least 12 times larger than short spindles. Such a huge growth in the

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Figure 3. (a,b) Spindle-shaped Se-Te alloy NCs prepared with a mole fraction of Se, xSe ) 0.6, in the presence of 3.6 M hydrazine. (aH) shows the aspect ratio of spindles. (c) TEM image of few spindles, and (d) a lattice-resolved image showing the growth along 〈001〉 zone axis. (e,f) The EDS line analysis along the short and long axes, respectively, of a spindle. Note the presence significantly low amount of Se in comparison to Te, while both elemental Se and Te homogenously mixed throughout the whole spindle. (g) SEM image of Se-Te alloy spindles prepared with mole fraction of Se, xSe ) 0.4 in the presence of 3.6 M hydrazine. Note the presence of large number of extraordinarily big spindles along with smaller ones. (h) A single big spindle along with two dotted triangles used to calculate the area. (i) A schematic comparison between a rhombohedral and spindle depicting almost equal length of their long axes. See details in text.

spindle size is mainly attributed to a facilitated nucleation along {111} crystal planes of trigonal hexagonal geometry, and it is promoted in the Te rich-region of the mixture, that is, xSe ) 0.4. Though, both Se and Te belong to 6-group elements of periodic table, Te is predominantly included in the category of metalloid rather than nonmetal as of Se. Se normally possesses greater affinity to exist in the amorphous state17-19 and Te doping is required to increase its glass transition temperature.20-23 Although both possess trigonal hexagonal geometry, Te is thus expected to grow in a much orderly manner along the c-axis

due to its predominantly crystalline nature than Se, and that is why cubic and rhombohedral geometries in Se-rich region of the mixture are converted into spindles of comparable sizes. Note that the long axis of most of the rhombohedral NCs can easily be equated with ∼250 nm at xSe ) 0.8 (see Figure 2aH for size distribution), which is quite close to the long axis of majority of spindles (Figure 3i). In fact, rhombohedral primitive unit cell is closely related to a cubic unit cell in a sense that the compression or expansion of the latter along cube body diagonals converts it into rhombohedral shape.24 Thus as the

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Figure 4. (a,b) Se-Te alloy long needles prepared with a mole fraction of Se, xSe ) 0.2, in the presence of 3.6 M hydrazine. The average aspect ratio is calculated from the size distribution histogram shown in (aH). (c) TEM image of several long needles, and (d) a lattice resolved image showing a twin boundary separating {111} and {110} crystal planes from each other. (e) Selected area diffraction image taken along the 〈110〉 zone axis showing the single crystal nature of a needle. (f) EDS line analysis across the short axis of a single needle to compute the atomic % of Se and Te. Note insignificant emission due to elemental Se while each needle is entirely constituted by Te. (g,h) A single spot EDS analysis on two different needles with corresponding EDS spectra. See details in text.

amount of Te increases from xSe ) 0.8 to 0.6, cubic geometries also tend to convert into rhombohedral due to preferential crystal growth along the cube body diagonals. In addition, when rhombohedral angle, RT, becomes equal to 60°, the lattice points can be indexed in terms of face centered cubic lattice and is very much true in our case. A close inspection of most of the rhombohedral NCs in Figure 2a indeed show that RT ) 60° (see Figure 2a) which means that rhombohedral geometry in fact evolved from cubic morphology. This mechanism becomes even more clear as the amount of Te further increases from xSe ) 0.4 to 0.2. This sample produces mostly long needles of hexagonal geometry (Figure 4a,b) with

aspect ratio of 11.3 ( 3.4 (Figure 4aH). Note a substantial increase in the aspect ratio of long needles from long spindles (4.1 ( 0.8) of the previous case. Low-resolution TEM is shown in Figure 4c whereas HRTEM with lattice-resolved image of {111} crystal planes is shown in Figure 4d. Twin boundary, which separates {111} from {110} planes, is also clearly visible in Figure 4d, and thus confirms the presence of hexagonal morphology of long needles. Selected area diffraction image along 〈110〉 zone axis (Figure 4e) proves a single crystal nature of each needle. EDS line spectrum (Figure 4f) as well as single spot analysis from different needles (Figure 4g,h) give identical results of atomic composition. Atomic % of Se is almost

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Figure 5. (a,b) Pure Te long interconnected nanoribbons of several micrometer prepared with a mole fraction of Se, xSe ) 0, in the presence of 3.6 M hydrazine. (c,d) EDS line analysis across two different nanoribbons showing emission only due to elemental Te.

negligible in comparison to Te (Supporting Information, Figure S4), thus all needles are mainly constituted by Te. These long needles ultimately convert into pure Te nanoribbons (NRBs) of width ≈ 80 nm and length of several micrometers at xSe ) 0 (Figure 5). Such long ribbons are evident from SEM (Figure 5a) as well as from TEM (Figure 5b) images. Line EDS spectrum across two different NRBs demonstrates that they are of pure Te. It could be further related to their preference toward the crystal structures. Usually a growth in 〈111〉 direction leads

to the formation of elongated geometries than along the 〈001〉 direction. Te is preferentially opting the former than latter. Reactions with [Hydrazine] ) 2.2 M. A significant change in the atomic composition of Se and Te is observed in Se-Te alloy NCs with a decrease in the amount of hydrazine while keeping all other reaction conditions exactly identical. Here, xSe ) 0.8 is mostly a mixture of NCs shown in Figure 6 with almost equal amounts of Se ) 47.62 and Te ) 50.67 atomic % in each kind of NC (Supporting Information, Figure S5). A

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Figure 6. Se-Te alloy polyhedral NCs prepared with a mole fraction of Se, xSe ) 0.8, in the presence of 2.2 M hydrazine. (a,b,e,f) Sequence of spiral crystal growth along the c-axis of trigonal hexagonal geometry of Se-Te alloy NCs. It starts along the rim of open triangular NC (a) and fills the hole with layers of spiral growth (b). (c) Schematic representations of hexagonal lattice with a unit cell, and (d) dislocations or kinks which act as active sites for further nucleation. Disappearance of several layered growth on the surface of the NC in (e) is due to the participation of dislocations or kinks in further nucleation which ultimately produces a NC (f) with appreciable smooth surface. See details in text.

careful scrutiny of these NCs demonstrates a fine spiral crystal growth from Figure 6a through f. A triangular ring NC shown in Figure 6a in fact bears surface protrusions arranged in a typical spiral pattern similar to that of Figure 2c. But there are some drastic differences between them. First, the size of NC of Figure 2c was ∼800 nm while it is 2 µm in the present case. Second, thickness of the ring in the previous case was just ∼250 nm with less prominent spiral arrangement in comparison to ∼800 nm in the present case. It means that a large increase in the amount of Se and its homogeneous mixing with equal amounts of Te produces fine spiral topography due to a less probability of crystal defects. Continuing crystal growth from Figure 6a to b is predominantly taking place along the spiral paths with a continuous decrease in the radius of an empty hole that ultimately fills it. This mechanism can very well be explained in a schematic arrangement (Figure 6c) of Se and Te atoms in trigonal hexagonal lattice. It seems that [1j1j0] crystal planes of hexagonal unit cells undergo preferential spiral growth along the c-axis. A layer-by-layer deposition25,26 of freshly cleaved atoms on continuous spiral paths first reduces the radius of the hole and then eventually fills it. It leaves unequal lattice layers along the spiral paths as indicated by white arrow in Figure 6b. Once the spiral growth is done, now the dislocations of the planes along the spiral paths become the active sites for further nucleation. This surface topography is somewhat similar to the steps shown in Figure 6d. Usually, a typical crystallization process proceeds through a layer-by-layer growth with an approximate thickness of 1 nm.26 It is carried out by freshly

produced atomic species or nucleating centers in the solution, which approach the surface and look for the active sites with greater surface energy over rest of the crystal planes. The active sites then attract them favorably and allow them to integrate into the lattice plane. These active sites are generally kinks and dislocations and are present even under equilibrium conditions when thermally activated nucleating centers shift positions in the lattice plane or revert back to the solution phase.27,28 But in the present case, dislocations or steps created by a spiral growth on the surface of a NC shown in Figure 6b become the active sites for further nucleation. Thus a facilitated growth over them ultimately leads to a much smoother NC of roughly trigonal pyramidal shape as shown in Figure 6f through 6e. White dotted semicircles show the smoothened dislocations left by spiral growth. Note that the spiral growth always runs perpendicular to the c-axis (dotted arrow in Figure 6f). As the amount of Se decreases in the mixture, that is, xSe ) 0.6, there is a large change in the morphology of NCs and practically no spiral growth is observed. Now the NCs have acquired rombohedral shape (Figure 7a). Most of them are much smaller in size (0.35 ( 0.07 µm, Supporting Information, Figure S6) while some are quite big enough (0.94 ( 0.37 µm, Supporting Information, Figure S6). Both small and big morphologies appear to preserve {100} facets that indicate that the growth is primarily oriented along 〈001〉 facets (Figure 7b). Big morphologies are expected to be the outcome of the interparticle fusion among the smaller ones because even the two large NCs in Figure 7b appear to fuse with each other. EDS analysis of

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Figure 7. (a) Rhombohedral Se-Te alloy NCs prepared with a mole fraction of Se, xSe ) 0.6, in the presence of 2.2 M hydrazine. Two kinds of NCs, that is, small and large are clearly visible. (b) A close-up image of few big and small-sized NCs showing incomplete crystal growth on {100} facets. (c,d) Spindle shaped Se-Te alloy NCs prepared with a mole fraction of Se, xSe ) 0.4, in the presence of 2.2 M hydrazine. Again two sizes of spindles are present. Large spindles and large rhombohedral NCs have almost equal length of their long axes. See details in text.

both kinds of NCs gives Se ) 4.02 ( 1.98 atomic % and Te ) 94.8 ( 2.81 atomic % (Supporting Information, Figure S7). This shows that the amount of Se has significantly decreased from 47.62 atomic % (Supporting Information, Figure S5) in the previous case (i.e., xSe ) 0.8) to 4.02 atomic %. Comparing the present rhombohedral morphology with the spindle-shaped NCs produced by the same mole fraction (xSe ) 0.6) with hydrazine ) 3.6 M (Figure 3a) suggests that the amount of Se has in fact increased from 2.5 to 4.02 atomic %. It means that the composition has direct relationship with overall morphology of NCs. Spindle shape is considered to be an extended rhombohedral shape to attain hexagonal geometry and greater amount of Se prefers rhombohedral rather than spindle shaped geometry. This aspect becomes more clear as we further decrease the mole fraction to xSe ) 0.4, and that eventually converts the rhombohedral NCs into spindle shape particles (Figure 7c). Again two sizes of spindle shape NCs are obtained because two sizes of rhombohedral NCs have transformed into corresponding spindles as the amount of Se reduces from xSe ) 0.6 to 0.4. Bigger and smaller spindles are with aspect ratio ) 3.13 ( 0.71 and 2.65 ( 0.7, respectively (Supporting Information, Figure S8). In addition, the long axes of both rhombohedral NCs (Figure 7b) and spindles (Figure 7d) are almost of equal length (i.e., ∼1.4 µm). Now the particles have much smoother surfaces (Figure 7d) and no significant crystal defects are visible. The atomic % of Se has further decreased to 1.72 ( 1.42 and that of Te increased to 98.3 ( 1.42 (Supporting Information, Figure S9) in comparison to the values obtained for xSe ) 0.6. Reactions with [Hydrazine] ) 0.1 M. To further clarify the effect of hydrazine on the mixing ratio of Se-Te as well as on the morphology, the concentration of hydrazine is reduced more than 20 times. The NCs thus obtained at different mole ratios have been shown in Figure 8. Here, one important difference is observed in the synthesis of pure Se nanoparticles. At xSe ) 1 (Figure 8a), large petals of several micrometers are obtained which was not the case in the presence of either 3.6 or 2.2 M of hydrazine. As mentioned in the experimental section, it was not possible to spin down the Se nanoparticles even at more than 15 000 rpm in the presence of 3.6 or 2.2 M hydrazine because they were too small and appreciably water-soluble.

Therefore, they have only been characterized by UV-visible studies and discussed in the following section. This contrasting difference between the natures of Se NCs is simply explained on the basis of insufficient amount of hydrazine available to produce insufficient number of Se nucleating centers in the present case. Low amount of hydrazine ) 0.1 M will generate less number of nucleating centers that will accommodate a large amount of Se through an autocatalytic reduction of SeO3-2 ions that may eventually lead to the formation of large petal-like morphologies. Further decrease in the amount of Se, that is, xSe ) 0.6 (Figure 8b,c) and xSe ) 0.2 (Figure 8d,e), follows almost the same trend in the morphology that has been seen previously for higher amounts of hydrazine (i.e., 3.6 and 2.2 M). Again predominantly spindle shaped morphology is prevalent at xSe ) 0.6 but with more distorted shapes (Figure 8b,c) which were not seen in Figure 3a,b as well as in Figure 7a for the same mole fraction with higher amounts of hydrazine. But such spindle shaped NCs now possess greater amount of Se ) 8.85 ( 3.52 atomic % (Te ) 91.14 ( 3.53 atomic %, Supporting Information, Figure S10) than the same samples at higher amounts of hydrazine (Supporting Information, Figure S2, S7). For xSe ) 0.2 (Figure 8d,e), there is also a large difference in the aspect ratio of long NRs (aspect ratio ) 3.0 ( 0.56, Figure 8f) with those synthesized in the presence of 3.6 M hydrazine (aspect ratio ) 11.3 ( 3.4, Figure 4b). The latter ones are more well-defined and close to monodisperse while this is not so in the present case (Figure 8e), which is again related to the amount of Se present. Present NRs consist of 8.72 ( 1.70 atomic % of Se (Te ) 91.28 ( 1.70 atomic %, Supporting Information, Figure S11) which is much higher than Se ) 0.88 ( 0.9 atomic % present in long NRs of Figure 4b. It further confirms the previous conclusion that low amounts of Se or high amounts of Te in fact support the elongated hexagonal geometry. Characterization of Se Water-Soluble Colloids. UV-visible studies have been frequently performed to explore the nature of amorphous Se (a-Se) nanoparticles.29,30 The extinction spectrum of a-Se nanoparticles is highly dependent on their shape and size,31-33 and covers a broad range of visible spectrum because the Se colloids are generally of bright reddish-brown

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Figure 8. (a) Large petals of pure Se (xSe ) 1) prepared in the presence of 0.1 M hydrazine. (b) Distorted Se-Te alloy spindles NCs prepared with a mole fraction of Se, xSe ) 0.6, in the presence of 0.1 M hydrazine. (c) A large spindle covered with surface defects. (d,e) Long Se-Te alloy needles of large size distribution (f) prepared with a mole fraction of Se, xSe ) 0.2, in the presence of 0.1 M hydrazine. See details in text.

color. In addition, one can see a clear blue or red shift in the absorbance as the size of Se colloids decreases or increases, respectively. Thus, the progress of the synthesis of Se colloids can very well be monitored with the help of time dependent UV-visible studies. In the present case, excess of hydrazine produces water-soluble a-Se particles while a little amount of hydrazine leads to the formation of large flakes as discussed previously. Both reactions can be easily studied with the help of UV-visible measurements. Figure 9a,b shows two contrasting time dependent absorbance spectra of Se colloids synthesized in the presence of 3.6 and 0.1 M hydrazine, respectively. In both cases, absorbance increases with time. Figure 9a shows two prominent absorbances located around 400 and 560 nm with a weak blue shift in the second absorbance, while Figure 9b shows an initial one peak around 550 nm which red shifts within 25 min, then divides into two peaks (one sharp and other broad) until 75 min, and ultimately converts into multiple peaks. All peaks of Figure 9b show a significant red shift. One thing is common in both figures that the peak around ∼550 nm actually initiates either a blue or red shift in the first place. This peak is mainly attributed to interchain interactions perpendicular to the c-axis. Therefore, as a smaller morphology evolves, it follows a blue shift (Figure 9a) and when a much bigger morphology sets in, it shows a red shift31-33 (Figure 9b). Thus, the sharp peaks around 400 nm (Figure 9a) and 600 nm (Figure 9b) represent trigonal Se small and big nanoparticles, respectively. Practically no shift in the first one indicates no change in the particles size while a significant red shift in the second peaks suggests an evolution of a much larger morphology. Figure 9c illustrates a sigmoidal growth of the a-Se nanoparticles with respect to time in the presence of 3.6 M hydrazine. A regular increase in the intensity at 400 nm is observed until 20 min of the reaction, but an instantaneous increase is observed thereafter, and growth becomes constant within 40 min of the reaction. Similar variation in the intensity at 560 nm (second

peak of Figure 9a) is observed (Supporting Information, Figure S12). On the other hand, almost identical growth kinetics is observed for a reaction in the presence of 0.1 M hydrazine (Figure 9d). Here the variation in the intensity of the peak originating close to 550 nm (Figure 9b) is followed. Because this peak splits into two within 25 min of the reaction, we have chosen the intensity of the left sharp peak (indicated by blue arrows) to monitor the growth kinetics instead of the right broad peak (indicated by the red arrows). It is to be noted that this bifurcated peak is practically the same which appears for a-Se colloids in Figure 9a. The only difference is that the peaks appear at higher wavelengths in Figure 9b than in Figure 9a due to a much larger size of the nanoparticles of the former than the latter. Both sigmoidal curves of Figure 9c,d show an instantaneous increase around 20 min of the reaction suggesting almost similar nature of nucleating centers produced during both reactions. Then, an instantaneous growth begins which lasts around 23 min in the presence of 3.6 M hydrazine (Figure 9c) while around 43 min in the presence of 0.1 M hydrazine (Figure 9d) before attaining equilibrium. The longer growth period is obviously understood from the much smaller amount of hydrazine. But unlike Figure 9c, the equilibrium is still not reached in Figure 9d, and two data points above 100 min appear with even higher intensity values which are extracted from the sharp peaks indicated by the empty block arrow in Figure 9b. Note, both peaks are further red-shifted from the previous one and can be attributed to the formation of even larger morphologies which eventually lead to the formation of large petals as shown in Figure 8a. Comparative Phase Diagram. A sequence of all above results indicates that at high hydrazine concentrations, Se-Te alloy NCs are always made up of a much greater amount of Te even in the Se rich-region of the mixture. However, as the amount of hydrazine reduces, the relative amount of Se in its

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Figure 9. (a) Time dependent UV-visible spectra of a reaction with xSe ) 1 in the presence of 3.6 M hydrazine. The lower most spectrum is taken at 5 min of the reaction time and the last one is taken at 135 min from the beginning of the reaction. Both peaks represent to the Se nanoparticles. (b) Time dependent scan of a similar reaction with xSe ) 1 in the presence of 0.1 M hydrazine. Again the lower most spectrum begins at 1 min of the reaction time until the last one is taken at 135 min of the reaction. Single peak is observed until 25 min which bifurcates thereafter and continues to red shift. (c) A plot of intensity of peak at 400 nm versus reaction time. The data points are extracted from time dependent spectra of panel a. Three regions of the plot refer to nucleation, growth, and monodisperse nanoparticles formation. (d) Similar plot of data points for a peak originating at 550 nm of panel b versus reaction time. After bifurcation of this peak at 25 min, the intensity of the left-hand side peak (indicated by blue arrows) has been taken for this plot. This plot is again divided into nucleation, growth, and nanoparticle formation. See details in text.

Figure 10. A plot of atomic % of Se and Te in Se-Te alloy NCs versus stochiometric mole fraction of Se (xSe) over the whole composition range of Se + Te mixtures in the presence of 3.6 (red symbols), 2.2 (green symbols), and 0.1 M (blue symbols) hydrazine. Empty and filled symbols belong to Te and Se atomic %, respectively. Straight dotted lines represent the ideal mixing between elemental Se and Te over the whole mixing range of Se + Te mixtures. SEM or TEM images (1 through 6) of different Se-Te alloy morphologies belong to different mole fractions. A color of the arrow indicates to which hydrazine concentration that mole fraction belongs. For instance, images 1 and 6 are indicated by the “blue” and “red” arrows, respectively. The “blue” colored symbols belong to 0.1 M hydrazine and “red” colored symbols to 3.6 M hydrazine (see legends of the graph). Thus, image 1 belongs to xSe ) 1 (hydrazine ) 0.1 M) and image 6 belongs to xSe ) 0 (hydrazine ) 3.6 M). See details in text.

rich-region starts increasing but still remains far below than Te. Figure 10 illustrates a comparison between the stochiometric mole fraction and atomic % of both components. Figure 10 can be regarded as a phase diagram of solid nanoalloy solution in which straight dotted lines represent the ideal mixing between

elemental Se and Te in Se-Te alloy. Experimental values of atomic % of Se (filled symbols) and Te (empty symbols) determined from EDS analysis at each mole fraction show a significant negative and positive deviations, respectively, from this ideal behavior at three hydrazine concentrations, that is,

Nonideal Mixing of Se-Te in Aqueous Micellar Phase 3.6, 2.2, and 0.1 M. A strong negative deviation in Se contents at all hydrazine concentrations indicates that the amount of Se is always much less than that of Te over the whole mole fraction range and Se-Te alloy NCs prefer Te over Se contents. This is an interesting observation that appears contradictory to usual identical properties of these two elements. Although both Se and Te undergo spiral trigonal hexagonal atomic arrangement, Se is largely considered as nonmetal with smaller atomic radius (117 pm) than Te (143 pm). Thus, Te possesses greater tendency to show metallic properties than Se. Moreover, there are plenty of examples11,17,19,29-33 where Se nanoparticles exist in amorphous state whereas Te possesses predominantly greater tendency to crystallize as 1D morphologies.34-37 On the basis of this fact, the phase diagram shows a drastic variation in the morphology of Se-Te alloy NCs over the whole composition range. But there is always a smooth transition in the morphology from one mole fraction to another which is again closely related to the atomic % of Se and Te present. A large dependence of composition on the amount of hydrazine is mainly observed in the Se rich-region (i.e., xSe ) 0.8 - 1) of the mixtures. Pure Se (xSe ) 1) always produced small predominantly amorphous nanoparticles in the presence of 3.6 and 2.2 M of hydrazine, while micrometer sized flakes (image 1) are formed with 0.1 M hydrazine. Thus, amount of hydrazine has a significant influence on pure Se nanomorphologies. This is all related to the nucleation kinetics because excess of hydrazine will reduce maximum number of SeO3-2 ions into atoms with little possibility of further nucleation, but low hydrazine will leave many SeO3-2 ions for further nucleation. Hence, the former nucleating centers will remain in aqueous phase stabilized with CTAB but latter ones will continue the autocatalytic process to produce petal like morphologies (image 1). As the amount of Se reduces at xSe ) 0.8, there are also 20% Te nucleating centers that possess predominantly metallic properties with little solubility in the aqueous phase. Now a contrasting solubility preference between Te and Se nucleating centers does not allow their stochiometric mixing to pass on to heterogeneous crystalline phase. A greater aqueous solubility preference of Se nucleating centers will leave much greater amount of Te rather than Se in heterogeneous crystalline phase even in the Se rich-region of the mixture. But as the amount of hydrazine reduces to 2.2 M, greater number of unreduced SeO3-2 ions will undergo autocatalytic reduction on already available Te nucleating centers and hence will increase the atomic % of Se in crystalline phase along with Te to produce Se-Te nanoalloy. Thus, different atomic % of Se induces a change in the overall shape and structure of resulting NCs. Note image 2 with greater amount of Se shows polyhedral morphologies (see also Figure 6) but image 2′ with much lesser amount of Se shows predominantly rhombohedral morphologies. Same is true with xSe ) 0.6. Low atomic % Se produces fine spindles (image 3) but relatively greater atomic % distorts the same morphology (image 3′). Likewise, xSe ) 0.2 produces fine hexagonal needles (image 5) with low atomic % of Se but distorted ones (image 5′) with relatively high atomic %. In addition, there is a smooth transformation in the rhombohedral morphology to spindle shape from xSe ) 0.8 to 0.6, short spindles to long spindles from xSe ) 0.6 to 0.4, long spindles to long needles from xSe ) 0.4 to 0.2, and finally long needles to long ribbons from xSe ) 0.2 to 0. This sequence can very well be related to both predominantly amorphous nature of Se and its amount in Se-Te alloy NCs. Thus, right-hand side (Se rich-region) of the phase diagram

J. Phys. Chem. C, Vol. 114, No. 1, 2010 153 prefers polyhedral or rhombohedral shape while left-hand side (Te rich-region) favors 1D nanomorphologies. Equimolar region of the mixtures clearly identifies the transitions between these two extremes. Conclusions The present results conclude that fine crystalline Se-Te alloy morphologies can be obtained over the whole composition range of Se and Te mixtures. But the shape, structure, and elemental composition of different morphologies depend very much on the amount of reducing agent (i.e., hydrazine). Three concentrations of hydrazine viz. 3.6, 2.2, and 0.1 M have been used to determine the influence of reducing agent on the overall morphology at a particular mole fraction. The concentrations 3.6 and 2.2 M are considered to be in excess in comparison to 0.1 M of hydrazine, as well as in comparison to 25 mM of Se and Te sources. Excess of hydrazine produces amorphous watersoluble Se nanoparticles and fine crystalline Te nanoribbons of several micrometers in length. Their different mixing ratios on the other hand produce different morphologies with regular transitions from rhombohedral NCs in the Se rich-region to spindle-shaped NCs close to equimolar proportions, and then to fine crystalline needles in the Te rich-region of the mixtures. All morphologies show a complete homogeneous mixing between elemental Se and Te in Se-Te alloy but with a marked nonideal behavior with respect to their stochiometric mole fractions over the total mixing range. All morphologies show a significantly low Se and high Te contents than their ideal amounts. The degree of nonideality decreases in the order of 3.6 M > 2.2 M > 0.1 M of hydrazine, which means excess of hydrazine produces crystalline Se-Te alloys with significantly higher amounts in elemental Te. These findings have been attributed primarily to the nonmetallic nature of Se and metalloid nature of Te. A reduction of SeO3-2 ions by excess of hydrazine produces many nucleating centers with amorphous nature that have predominantly greater affinity for aqueous phase. In contrast, TeO3-2 ions produce crystalline nucleating centers with greater probability of existing heterogeneously in aqueous phase. Thus, in the mixed state Se-Te alloy NCs possess greater amount of Te even in the Se rich-region of the mixtures. However, as the amount of hydrazine is reduced to 0.1 M, relatively fewer Se nucleating centers are produced. They undergo autocatalytic process with other SeO3-2 ions to produce large flake-like morphologies. A low hydrazine concentration also influences the shape, structure, and composition of different morphologies predominantly lying in the Se rich-region but with little effect on the morphologies of Te rich-region. Thus, this study provides an easy access to facile synthesis of different morphologies of Se-Te alloy NCs simply by varying the amount of hydrazine. One -dimensional Se-Te alloy nanoribbons and long needles can find their way to low cost versatility and large scale production for their industrial applications in optoelectronic and device formation. Supporting Information Available: EDS spectra of various samples and size distribution histograms. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Selenium; Zingaro, R. A., Cooper, W. C., Eds.; Litton Educational Publishing: New York, 1974; pp 25 and 174-217. (2) Berger, L. I. Semiconductor Materials; CRC Press: Boca Raton, FL, 1997; p 86. (3) Lu, Q.; Gao, F.; Komarneni, S. Chem. Mater. 2005, 18, 159.

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