Formation and Growth of Bi2Te3 in Biomolecule ... - ACS Publications

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Formation and Growth of Bi2Te3 in Biomolecule-Assisted Near-Critical Water: In Situ Synchrotron Radiation Study Jian-Li Mi, Mogens Christensen, Christoffer Tyrsted, Kirsten Ø. Jensen, Jacob Becker, Peter Hald, and Bo B. Iversen* Centre for Materials Crystallography, Department of Chemistry, and iNANO, Aarhus UniVersity, Langelandsgade 140, DK-8000, Aarhus, Denmark ReceiVed: April 28, 2010; ReVised Manuscript ReceiVed: June 3, 2010

A novel “green” chemical route was designed to prepare nanostructured Bi2Te3 in near-critical water using the biomolecule alginic acid as reductant. The supercritical or near-critical synthesis gives a rapid method for the preparation of nanomaterials, and the formation of Bi2Te3 can be finished within several minutes. The formation and growth of Bi2Te3 was studied by in situ synchrotron radiation powder X-ray diffraction (SRPXRD). Comprehensive in situ experiments give direct evidence of the phase formation and grain growth of Bi2Te3. Near-critical preparation of Bi2Te3 results in nanoscale particles with a plateletlike structure. The concentration of NaOH appears to play a crucial role on the particle size while it is little affected by the synthesis temperature. 1. Introduction Alloys based on Bi2Te3 are state-of-the-art and industrially applied materials for thermoelectric cooling applications, and persistent efforts therefore have been dedicated to improving the thermoelectric performance of Bi2Te3 and its alloys.1,2 Recent progress in thermoelectric materials has often involved nanostructuring of materials to dramatically lower the thermal conductivity and, thereby, to improve the thermoelectric figure of merit (ZT).3–8 The thermoelectric performance can be enhanced by controlling the transport of phonons and electrons in superlattice materials, and a maximum ZT value of about 2.4 was reported for p-type Bi2Te3 superlattice devices.3 The primary mechanism for ZT enhancement in superlattices can also exist in random nanostructures, and indeed, nanostructured bulk materials with structures of various length scales have significantly decreased thermal conductivity.6,8 Recently, a high ZT value of 1.4 was reported in p-type nanocrystalline BixSb2-xTe3 prepared by ball-milling of crystalline BixSb2-xTe3 ingots followed by hot pressing of the nanopowders.1 The enhanced ZT value is mainly a result of a significant reduction in the thermal conductivity due to strong phonon scattering by grain boundaries and defects in the nanostructures. The nanocrystallite size must be tuned to primarily affect the phonons and only cause minor scattering of the charge carriers. Currently, there is an increased emphasis on the topic of “green” chemical processes. However, common chemical routes for the preparation of nanostructured Bi2Te3-based compounds rely on organic solvents and toxic reductants such as hydrazine, NaBH4, or DMF.9–12 Environmentally benign reducing agents and nontoxic solvent media are key issues that merit important consideration in a green synthesis strategy.13,14 Because many biomolecules are nontoxic, environmentally benign, cheap, and abundant, biomolecule-assisted synthesis methods attract much attention in the preparation of various nanomaterials. In such reactions biomolecules are often exploited as templates or reagents.15,16 Biomolecules were reported as a reducing agent * To whom [email protected].

correspondence

should

be

addressed.

E-mail:

for the synthesis of some elemental metals, for example, silver nanoparticles17 and tellurium nanowires.18 It would therefore be advantageous if biomolecules could replace toxic chemicals as the reducing agent in the preparation of novel alloys and compounds. Recently, we have successfully synthesized Bi2Te3 nanoparticles by a biomolecule-assisted hydrothermal method.19 Some of the problems with conventional solvothermal or hydrothermal methods are the limited temperature and pressure control as well as the slow process time. This leads to limited particle size control. In contrast, synthesis in supercritical or near-critical solutions is a very promising method for manipulation of the size and size distribution of nanoparticles.20 Supercritical fluids exhibit particularly attractive properties such as gaslike transport properties in diffusivity, viscosity, and surface tension, while maintaining liquidlike properties such as high-solvation capability and density. These unique properties make supercritical fluids attractive solvents in chemical processes. In general, very little is known about the chemical reactions taking place during nanoparticle formation at high temperature and high pressure. However, using high intensity synchrotron radiation together with specially built in situ reactors, it is possible to open the “black boxes” of hydro/solvothermal in super/subcritical conditions, allowing a precise and direct study of different nanocrystal growth processes.21–25 In this study, we present a new green synthesis route for the production of Bi2Te3 nanoparticles in high temperature and high pressure water (near the critical point of water) and apply synchrotron radiation powder X-ray diffraction (SR-PXRD) to study the formation and the grain growth of the Bi2Te3 nanoparticles in situ. To complement the in situ study, various hydrothermal autoclave experiments were also carried out. The morphologies and structures, as well as the thermoelectric properties, of the products are reported. 2. Experimental Section All chemical reagents used in the experiments were analytical grade. For the preparation of Bi2Te3, the precursors

10.1021/jp103858z  2010 American Chemical Society Published on Web 06/24/2010

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BiCl3 and Te(OH)6 were used as reactants and ethylenediamine-tetra-acetic acid sodium (EDTA) and alginic acid were used as surfactant and reductant, respectively. EDTA (12 mmol) was first dissolved in 40 mL of distilled water, 6 mmol BiCl3 and 9 mmol Te(OH)6 were then mixed with 4 g alginic acid in the above-mentioned solution, and proper amounts of NaOH were added. Finally, the solution was diluted with water to 50 mL. The NaOH concentrations were adjusted to 0.5 and 3 M for each temperature. For the in situ experiments, a custom-designed high-pressure flow cell with a sapphire capillary was used to study the process by time-resolved in situ X-ray diffraction.22–25 After injecting the precursor solution, the sapphire capillary was sealed, pressurized, and subsequently heated to the targeted temperature using a HPLC pump and a heated air flow, respectively. Experiments with different reaction temperatures of 250 and 300 °C were performed under a pressure of 25 MPa. The in situ synchrotron data were measured at beamline I711, MAXlab, Sweden, using a home-built sample stage and a Mar165 CCD detector. The monochromator produced a beam with a wavelength of 0.9496 Å, and the setup ran with a time resolution of 11.35 s between each frame, of which 7.35 s was detector dead time due to readout. The synchrotron diffraction data were in the refinement corrected for instrumental broadening using a LaB6 standard. Separate hydrothermal synthesis experiments were furthermore performed for comparison. A precursor solution with the same concentration as the in situ experiments was introduced into two Teflon-lined, stainless-steel autoclaves filling them to 85% of their capacities. The NaOH concentrations were again adjusted to 0.5 and 3 M, respectively. The autoclaves were heated and maintained at 200 °C for 24 h and then allowed to cool in air to room temperature. The black products were collected and washed with distilled water and ethanol by centrifugation. The products were analyzed by X-ray diffraction (XRD) on a STOE powder diffractometer using Cu KR radiation (λ ) 1.5406 Å). The morphology of the products was observed on a NOVA600 field-emission scanning electron microscope (FESEM). Transmission electron microscopy (TEM) and highresolution TEM (HRTEM) were performed on a Philips CM20 microscope. The powders were sintered into bulk pellets by spark plasma sintering (SPS) at 350 °C under 40 MPa for 5 min. The thermoelectric properties were measured using a Quantum Design physical property measuring system (PPMS) and a laser flash apparatus (Netzsch LFA 457). 3. Results and Discussion 3.1. In Situ Investigation of Formation and Growth of Bi2Te3. For convenience, the samples in the in situ study are named as “ISx_y”, where “IS” denotes the in situ study, x is the synthesis temperature in degrees centigrade, and y is the NaOH molar concentration. Figure 1 shows the time evolution of the SR-PXRD patterns for the sample IS250_05. During the first 1.5 min, the SR-PXRD patterns show a BiOCl precipitate in the solution due to the hydrolysis of BiCl3. Hexagonal structured Te forms rapidly after 1.5 min of reaction. The Bi2Te3 phase starts appearing after about 4.0 min, with subsequent diminishing of Te and concurrent growth of Bi2Te3. Pure single phase Bi2Te3 is achieved after about 13.5 min. All the peaks after 13.5 min can be well indexed to the rhombohedral Bi2Te3 crystal structure. The other reaction conditions show similar processes, but the phase transformations occur much earlier, when either raising the reaction temperature or increasing the NaOH concentration. The phase transformation from hexagonal

Mi et al.

Figure 1. Time evolution of the SR-PXRD patterns for the synthesis of sample IS250_05 in the condition of 250 °C and 0.5 M NaOH. The crystallite formation of Te, subsequent phase transformation from Te to Bi2Te3, and completion of the formation of Bi2Te3 are observed at 1.5, 4.0, and 13.5 min, respectively.

TABLE 1: Start Time (t1) of the Phase Transformation from Hexagonal Te to Rhombohedral Bi2Te3 and Completion Time (t2) for the Formation of Bi2Te3 for Different Samplesa sample

t1 (min) t2 (min)

IS250_05

4.0

13.5

IS250_3

1.9

6.4

IS300_05

2.1

4.3

IS300_3

1.5

3.6

direction (a,b)-planes c-axis (a,b)-planes c-axis (a,b)-planes c-axis (a,b)-planes c-axis

Dmax (nm) k (min-1) 25(2) 11(1) 38(1) 26(1) 23(1) 10(1) 42(1) 20(1)

0.09(2) 0.07(2) 0.38(2) 0.11(2) 0.35(2) 0.31(1) 0.33(3) 0.24(3)

a Dmax is the equilibrium particle size and k is a kinetic fitting parameter (see text).

Te to rhombohedral Bi2Te3 started at about 1.9, 2.1, and 1.5 min for IS250_3, IS300_05, and IS300_3, respectively, and a single phase of Bi2Te3 was finally formed at about 6.4, 4.3, and 3.6 min. The start time (t1) of the phase transformation from Te to Bi2Te3 and the completion time (t2) for the formation of Bi2Te3 for different samples are listed in Table 1. The results show that the biomolecule alginic acid is an effective reducing agent in high temperature and high pressure water. For the formation of Bi2Te3 (or Sb2Te3), various formation mechanisms have been proposed, including the combination of elemental Bi and Te, the interaction of Bi3+ and Te2-, and the reaction between Bi3+ and elemental Te.26,27 In this study, the in situ SR-PXRD data give direct evidence that Te acts as an intermediate in the formation process of Bi2Te3, whereas no metallic Bi is observed in the process. Thus, based on the diffraction data it may be suggested that the formation mechanism of Bi2Te3 involves the reaction between Te(OH)6 and Bi3+ ions. First, Te(OH)6 is rapidly reduced into elemental Te by alginic acid under the near-critical condition of the NaOH aqueous solution. Subsequently, the formation of Bi2Te3 results from the direct reaction between metallic Te and ionic Bi3+ (or [Bi(EDTA)]+). The observations verify the proposed growth mechanism where intermediate Te is used as templates to form Bi2Te3 and other materials.26,28,29 The time-resolved SR-PXRD data were refined using the Rietveld method implemented in Fullprof.30 The anisotropic size

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Figure 2. Representative Rietveld refinement SR-PXRD patterns with resulting observed, calculated, and difference patterns of samples IS250_3 (a) and IS300_05 (b).

broadening of the SR-PXRD was modeled using a linear combination of spherical harmonics.23,31

βh )

λ λ ) Dh · cos θ cos θ

∑ almp · Ylmp(Θh, Φh)

(1)

lmp

where βh is the size contribution to the integral breadth of reflection h, and Ylmp(Θh,Φh) are normalized real spherical harmonics. The refined coefficients, almp, were used to calculate the volume-weighted particle morphology. Figure 2a and b show representative SR-PXRD data with observed, calculated, and difference patterns of IS250_3 and IS300_05, respectively. The figures show good agreement between the refined calculated patterns and the observed data. For the experiments performed at 250 °C, the phase transformation between Te and Bi2Te3 was slow. Therefore, a secondary Te phase was added in the refinement of the SR-PXRD data. For the experiments performed at 300 °C, the phase transformation was much faster, and only a single Bi2Te3 phase was treated in the refinement. Table 2 displays the Rietveld refinement parameters of the SRPXRD data of sample IS250_3 and IS300_05 corresponding to the two plots in Figure 2. Figure 3 shows the particle sizes along the (a,b)-plane and along the c-axis for the four different samples. The dimensions along the (a,b)-plane are much larger than those along the c-axis for all the samples, revealing the products to be plateletlike nanostructures. The structure can be well explained by the anisotropic crystallographic structure of Bi2Te3. Bi2Te3 crystals consist of a layered structure along the c-axis with weak van der Waals bonds between the neighboring Te layers. As a result, the crystal growth rate parallel to the (a,b)-plane is anticipated to be higher than that parallel to the c-axis direction. The refined morphology agrees well with a large number of reports finding plateletlike nanostructures for Bi2Te3-based compounds.19,27,28 As shown in Figure 3, the particle sizes of samples IS250_05 and IS300_05 are much smaller than those of samples IS250_3 and IS300_3, indicating that the NaOH concentration appears to play a crucial role on the particle size. The nanocrystallites of Bi2Te3 for sample IS250_05 grow slowly up to equilibrium compared with the other samples, which initially grow rapidly, followed by a slow approach toward maximum. For the grain-growth of nanocrystalline materials, a “size-dependent impediment” model can be used to describe the kinetics of the grain growth process.32,33 The model can be expressed as 2 2 D(t)2 ) Dmax - (Dmax - D20) exp[-k(t - t0)]

(2)

where D(t) is the average grain size and D0 and Dmax are the initial and equilibrium particle sizes, respectively. The k parameter describes the specific interface energy and grain

boundary mobility and t is the reaction duration. The solid lines in Figure 3 represent the fits using this model. With known values of D0 and t0, the models contain two adjustable parameters, Dmax and k. In the initial frames of the SR-PXRD data, the phase fraction of Bi2Te3 is relatively low compared with Te, and this makes it difficult to get accurate particle sizes of the Bi2Te3 products during the early stage of the grain growth process. Nevertheless, the data are roughly described by the model. The resulting k values are 0.09(2), 0.35(2), 0.38(2), and 0.33(3) min-1 for the growth curves along the (a,b)-planes for IS250_05, IS300_05, IS250_3, and IS300_3, respectively, and along the c-axis, 0.07(2), 0.31(1), 0.11(2), and 0.24(3) min-1, respectively. Except for the k value of 0.38(2) for sample IS250_3 along the (a,b)-plane, the k values appear to be affected by the temperature rather than the NaOH concentration. This indicates that the specific interface energy and grain boundary mobility are enhanced with increasing the temperature. The fitted equilibrium particle sizes along the (a,b)-planes are 25(2), 23(1), 38(1), and 42(1) nm, and along the c-axis they are 11(1), 10(1), 26(1), and 20(1) nm for IS250_05, IS300_05, IS250_3, and IS300_3, respectively. The results indicate that the final particle size is little affected by the reaction temperature, whereas the NaOH concentration plays an important role. The equilibrium particle size Dmax and the parameter k are also displayed in Table 1. 3.2. Hydrothermal Synthesis of Bi2Te3. The samples prepared by the hydrothermal method are named as “Hx_y”, where “H” denotes the hydrothermal method and x and y are the synthesis temperature in degrees centigrade and the NaOH concentration, respectively. Figure 4 shows the XRD patterns of samples H200_05 and H200_3 prepared by the hydrothermal method at 200 °C under NaOH concentrations of 0.5 and 3 M, respectively. Single phase Bi2Te3 can be obtained at both conditions. All the detected peaks can be well indexed to the rhombohedral Bi2Te3 crystal (JCPDS No. 82-0358). The calculated lattice parameters from the XRD patterns are a ) 4.396(1) Å, c ) 30.52(1) Å for sample H200_05, and a ) 4.392(1) Å, c ) 30.53(1) Å for sample H200_3, which are in good agreement with the standard data of a ) 4.395 Å and c ) 30.44 Å.34 The XRD peaks observed for sample H200_05 are much broader than those of H200_3, indicating the particle size of H200_05 is much smaller than that of H200_3, which agrees with the in situ study that the concentration of NaOH plays a crucial role on the resulting particle size. The particle sizes of the samples prepared under conventional hydrothermal condition were determined by the Scherrer equation through fitting the peaks by a Lorentz function. A silicon standard was used to account for the instrumental broadening. The particle sizes calculated along the [0 1 5] and [1 0 10] directions are around

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Figure 4. XRD patterns of Bi2Te3 powders of samples H200_05 and H200_3 prepared by the hydrothermal method at 200 °C under NaOH concentrations of 0.5 and 3 M.

Figure 3. Particle dimensions along the (a,b)-plane (a) and c-axis (b) for the different samples. The solid lines are the results of fits with the “size-dependent impediment” grain-growth model.

TABLE 2: Refined Parameters of the Rietveld Analysis of the SR-PXRD Data of Samples IS250_3 and IS300_05, Corresponding to the Two Plots in Figure 2 sample

IS250_3

IS300_05

reaction time (min) No. of data points No. of refined parameters No. of reflections RWP (%) formula space group RI (%) RF (%) cell parameter a (Å) cell parameter c (Å) z (Bi) z (Te2) second phase RI′ (%) RF′ (%)

3.8 1460 38 110 10.5 Bi2Te3 R-3m 4.73 5.14 4.3686(1) 30.3783(1) 0.4012(7) 0.7896(5) Te 2.41 2.07

18.9 1460 31 71 13.0 Bi2Te3 R-3m 5.82 6.46 4.3858(1) 30.5363(1) 0.4005(3) 0.7882(2)

45.1 and 28.7 nm, respectively, for sample H200_05. From the geometrical relationship between the [0 1 5] direction and the (a,b)-plane, the particle size along the (a,b)-plane is 36.6 nm. And from the geometrical relationship between the [1 0 10] direction and the c-axis, the particle size of the c-direction is evaluated to be about 23.6 nm. The particle sizes of sample H200_3, estimated by the Scherrer equation, are 291.6 and 214.0 nm along the (a,b)-plane and the c-axis, respectively, which are too large for reliable size estimation with the Scherrer equation. Nevertheless, the results indicate that the particles have anisotropic growth. Compared with the in situ study, the particles prepared in autoclaves have increased in size. This is probably due to the long time reaction process. The differences in morphology and particle size between samples H200_05 and H200_3 can be clearly seen from the SEM and TEM images shown in Figure 5. Figure 5a and c show

Figure 5. (a,b) SEM images of Bi2Te3 powders of samples H200_05 (a) and H200_3 (b). In (c), TEM and HRTEM (inset) images of H200_05 are shown, while (d) shows a TEM image of H200_3.

that the product of H200_05 is composed of a lot of nanorods. The nanorods are polycrystalline and are made up of very small nanoparticles with particles sizes ranging from 20 to 50 nm, as well as some large platelets with diameters up to 100 nm and a thickness of about 20 nm. The well-resolved diffraction fringes shown in the HRTEM image in the inset of Figure 5c indicate that the nanoparticles almost have the same orientation relationships. However, as shown by the SEM and TEM images in Figure 5b and d, the particles of sample H200_3 prepared under higher NaOH concentration are much larger than those of H200_05. The particle sizes of sample H200_3 are approximately from 200 to 300 nm. For sample H200_05 it can also be seen that the surface of the nanorods are coated with a thin layer of amorphous material. The amorphous layer, which is absent in sample H200_3, might be due to some organic species formed during the reaction under the low NaOH concentration. 3.3. Thermoelectric Properties. Nanostructured Bi2Te3 is expected to have reduced thermal conductivity. The intrinsic thermoelectric properties of the nanocrystallites are difficult to evaluate, therefore, samples H200_05 and H200_3 were sintered into bulk pellets by SPS. The relative densities are around 65 and 92% for H200_05 and H200_3, respectively. The low density of sample H200_05 is probably due to some organic products remaining at the sample, as detected in the TEM image of Figure 5c, and it makes it very difficult to measure reliable properties. The values of the Seebeck coefficient R, electrical conductivity σ, and low temperature thermal conductivity κ were

Bi2Te3 in Biomolecule-Assisted Near-Critical Water

J. Phys. Chem. C, Vol. 114, No. 28, 2010 12137 are still low in this study compared with Bi2Te3 single crystals, which have a ZT of about 0.4 at room temperature35 and polycrystalline Bi2Te3 prepared by mechanical alloying with a ZT of about 0.56 at 460 K.36 The low thermoelectric properties in this study are due to the limited power factor, which originates mainly from relatively poorly sintered samples and remaining organic residues. The main purpose of the present study has been to develop a new reaction route to nanostructured Bi2Te3. It is necessary to optimize the composition to get well doped and dense Bi2Te3 to improve the thermoelectric properties. 4. Conclusions A novel, rapid, and “green” chemical route to synthesize nanostructured Bi2Te3 in near-critical water was presented. The method employs the biomolecule alginic acid, which serves as an effective reducing agent in water at high temperature and high pressure. The in situ SR-PXRD data provide fundamental and direct information on the formation and growth process of Bi2Te3. Tellurium is an intermediate product during the reaction process, and the synthesis of Bi2Te3 results from the reaction between elemental Te and ionic Bi3+ (or [Bi(EDTA)]+). The extracted particle size shows that Bi2Te3 prepared in near-critical water is nanoscale and has a plateletlike structure. The concentration of NaOH, unlike temperature, plays a crucial role on the resulting particle size, whereas the temperature affects the growth kinetics. Acknowledgment. This work was supported by the Danish National Research Foundation (Center for Materials Crystallography), the Danish Strategic Research Council (Center for Energy Materials), and the Danish Research Council for Nature and Universe (Danscatt). MAX-lab is thanked for the beam time, and Yngve Cerenius and Dorthe Haase are thanked for assistance during measurements. References and Notes

Figure 6. Temperature dependence of the Seebeck coefficient (a), the electrical resistivity (b), and the thermal conductivity (c) of samples H200_05 (solid square) and H200_3 (open circle).

measured using a PPMS, whereas κ at temperatures higher than 300 K was measured with an LFA. The temperature dependencies of R, σ, and κ are displayed in Figure 6. Sample H200_05 has n-type conduction over the measured temperature range, as indicated by its negative Seebeck coefficients, while H200_3 exhibits p-type conduction. The Seebeck coefficient reaches a maximum value of about 50 µV K-1 at 320 K for H200_3, while the absolute value of Seebeck coefficient increases with temperature over the measured temperature range for H200_05. A value of -70 µV K-1 is achieved at 380 K for H200_05. Sample H200_3 has much lower electrical resistivity than H200_05, mainly due to the low density of sample H200_05 and some remaining organic chemicals in the sample. For the same reasons, H200_05 also has much lower thermal conductivity than H200_3, and κ values of about 0.30 and 1.08 W m-1 K-1 are obtained for H200_05 and H200_3, respectively. The calculated figures of merit ZT are 0.001 and 0.008 for H200_05 and H200_3 at room temperature. The thermoelectric properties

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