Synthesis and Raman Spectroscopy of Multiphasic Nanostructured Bi

Article pubs.acs.org/JPCC

Synthesis and Raman Spectroscopy of Multiphasic Nanostructured Bi−Te Networks with Tailored Composition Gayatri D. Keskar,† Ramakrishna Podila,‡ Lihua Zhang,∥ Apparao M. Rao,‡,§ and Lisa D. Pfefferle*,† †

Department of Chemical Engineering, Yale University, P.O. Box 208286, New Haven, Connecticut 06520-8286, United States Department of Physics and Astronomy, and §COMSET, Clemson University, Clemson, South Carolina 29634, United States ∥ Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973-5000, United States ‡

S Supporting Information *

ABSTRACT: Development of synthetic routes to control the morphology and composition of nanostructured thermoelectric materials and to leverage their unique performance enhancements presents challenges in the realization of practical thermoelectric systems. We report here the fabrication of intricate networks of nanostructured tellurium, bismuth telluride, and bismuth-rich compounds with diverse morphologies. The nanostructured networks synthesized via solution-phase techniques consist of nanocrystalline Bi2Te3 with a grain size of about 15−20 nm, 3−5 nm thick rolled-up nanosheets of Te forming tubular structures, nanotubes of Bi2Te3 about 300−400 nm in diameter, Te and Bi4Te3 nanowires ranging from 50 to 200 nm diameter, and microspheres of 3−7 μm diameter composed of self-assembled BiOCl nanorods. The formation and crystallinity of Bi-rich and Te-rich compounds were investigated using powder X-ray and electron back-scattered diffraction. We present the first detailed analysis of microRaman scattering of BixTey nanostructures of above morphologies using six different laser wavelengths. The BixTey nanostructures exhibit the most intense infrared (IR) active A1u mode at 120 cm−1 in the Raman spectra, which disperses with a change in the chemical composition and laser power. In addition, we observe new internal strain-induced peaks in the Raman spectra of BixTey nanostructures. The rich morphologies and compositions present within the nanostructured Bi−Te compounds are expected to result in novel thermoelectric materials.

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INTRODUCTION Thermoelectric (TE) materials are evolving as a renewable source of energy with a potential to generate significant power from various waste heat sources such as the human body, computer chips, automobile exhaust, and industrial utilities. The past decade witnessed a huge impetus for achieving enhanced figure of merit (ZT)1 in nanostructured TE materials for practical applications. One of the strategies for realizing the next generation TE nanomaterials is to improve Seebeck coefficient with a simultaneous reduction of thermal conductivity using mixtures of multiphasic nanostructured materials.2−4 Bismuth telluride (Bi2Te3) is an established TE material and has been widely studied for cooling and power generation applications at room temperature. Enhanced TE performance has been demonstrated in p-type and n-type Bi2Te3-based nanostructures with the highest ZT of ∼2.4 in p-type Bi2Te3 superlattice thin films.5,6 It is well-known that Bi2Te3 is a stable phase within a narrow composition range centered at ∼40% at. Bi at room temperature.7 A substantial effect of doping is typically observed on the transport properties in near-stoichiometric compositions of the Bi−Te system. The structure−composition−property−processing relationships for nonstoichiometric Bi−Te compounds are not yet established, and the diverse morphologies and tunable © 2013 American Chemical Society

compositions of the Bi−Te nanostructures described in this Article may prove beneficial for further improvement in TE properties. Different synthesis techniques have been used to prepare single phase and heterophase8 Bi−Te nanostructures with various morphologies9−18 such as nanoparticles, nanotubes, nanorods, nanowires, sheets, hexagonal nanoplates, etc. The hydrothermal/solvothermal methods have received the most attention because of their simplicity, morphological control,9,11,17 and scalable production at low cost and low temperature. Recently, the self-sacrificed template technique has been employed where a 1D Te precursor is synthesized and used as a sacrificial template to obtain 1D metal telluride nanostructures.19−21 The phonon dispersion relations in nanostructured Bi−Te compounds can provide important insights into their physical properties. Besides probing the vibrational properties, Raman spectroscopy can also elucidate the phase composition and stoichiometry of TE nanomaterials. Several theoretical and experimental reports on the Raman spectroscopy of bulk Bi2Te3 and thin films have appeared in the literature,22,23 and recent Received: March 9, 2012 Revised: April 4, 2013 Published: April 5, 2013 9446

dx.doi.org/10.1021/jp402879h | J. Phys. Chem. C 2013, 117, 9446−9455

The Journal of Physical Chemistry C

Article

Table 1. Experimental Conditions Used for Different Nanostructured Bi−Te Samples sample ID

syn. route

1 2 3 4 5 6

sol sol sol sol sol sol

7 8

CR-RT annealing in Ar (1 h)

bismuth precursor

Te precursor

BM Bi2Te3 (2 mmol) BiCl3 (2 mmol) Te (3 mmol) BiCl3 (1 mmol) Te (3 mmol) BiCl3 (1.5 mmol) Te (3 mmol) Te (3 mmol) BiCl3 (3 mmol) commercially available polycrystalline Bi2Te3 sample 4 sample 2

reducing agent

temp (°C)

solvent

pH adjustment

N2H4·H2O EG EG EG EG EG

120 180 180 180 180 180

DMF HNO3 + EG HNO3 + EG HNO3 + EG HNO3 + EG HNO3 + EG

NH4OH + NaOH (13.3)

NaBH4

RT 350

H2O

slowly cooled to room temperature, and the precipitates were thoroughly washed with DI water. The growth procedure for the next five samples involves second sol synthesis based on using 1D Te metal precursor as a sacrificial template to fabricate BixTey nanostructures as described in ref 19. For sample 2, 2.5 mL of concentrated nitric acid (HNO3) was added to 5 mL of ethylene glycol (EG), and 3 mmol of Te was dissolved into this mixture to form Te solution. Next, 50 mL of EG was added together with 2 mmol of bismuth trichloride (BiCl3) to Te solution, and the resultant solution after stirring for 30 min was reacted in the autoclave at 180 °C for 12 h. The autoclave was naturally cooled to room temperature, and the precipitates were thoroughly washed with ethanol. The same procedure was repeated for samples 3, 4, and 5 with a slight modification in the starting ratios of BiCl3 to Te, as 1:3 in case of sample 3, 1.5:3 for sample 4, and 3:3 for sample 5. Commercially available polycrystalline Bi2Te3 was used as a precursor material for sample 6 and synthesized following the same sol technique as for sample 2. The final products of each sample were dried in an oven at 80 °C for 4 h. We have previously demonstrated that a postsynthesis treatment with NaBH4 greatly improves the quality and yield of Bi nanotubes.28 Therefore, NaBH4 was slowly added to the aqueous solution of the as-prepared product of sample 4. It chemically reduced the sample in air at room temperature (CR-RT). After 24 h, the supernatant was removed, and the residue was dried in an oven at 80 °C for 4 h. The final product was labeled as sample 7 for further characterization. Sample 8 was prepared by postsynthesis annealing of sample 2 in argon at 350 °C for 1 h. Characterization. The final products were characterized using environmental scanning electron microscopy (ESEM FEG − FEI, XL-30) with energy dispersive spectroscopy (EDX, Princeton Gamma Tech., Spirit) for average compositional analysis. Structural investigation was performed with transmission electron microscopy (FEI Tecnai TF20 FEG, 200 kV TEM/STEM). JEOL 2100F, a high-resolution analytical TEM, was used for elemental mapping of individual Bi x Te y nanostructures and for selected area electron diffraction (SAED) at the Center for Functional Nanomaterials, Brookhaven National Laboratory. The different crystal structures and corresponding phases in each sample were analyzed by powder X-ray diffraction (XRD) with a Bruker D8-Focus powder diffractometer with a Cu radiation source and scintillation (NaI) detector (Cu Kα, λ = 0.154056 nm). The phase identification and quantification (wt %) were carried out using MDI’s Jade 8 software for all of the samples. ESEM (FEG − FEI, XL-30) was used to acquire electron back-scattered diffraction patterns (EBSP) for individual Bi−Te-based nanostructures to confirm their phases. Micro-Raman spectra for nanostructured Bi−Te samples were recorded with a Horiba Jobin Yvon triple grating T64000 spectrometer at various wavelengths and laser

articles on 0D, 1D, and 2D Bi2Te3 nanostructures report the observation of unconventional features in corresponding Raman spectra.16,24−27,36 Here, we describe a facile and readily scalable approach to grow networks of multiphasic BixTey nanostructures. The experimental conditions leading to the growth of crystalline BixTey nanostructures are discussed, showing how the structure and chemical composition can be controlled by changing the metal precursors, the stoichiometry, the reaction temperature, the reaction medium, and the reducing agent. Multiphasic networks of different low-dimensional morphologies are formed with tailored chemical composition in the as-synthesized product. The IR active mode (A1u), which is Raman forbidden for bulk Bi2Te3 crystal, is observed at ∼120 cm−1 as the strongest peak in the Raman spectrum of BixTey nanostructures. The novel Raman modes below Eg2 mode are also observed in nanostructured BixTey through laser excitation-dependent Raman spectroscopy, and their origin is attributed to the presence of internal strain in the crystalline nanostructures. The synthesis strategies presented here can be further extended to design nontelluride-based TE materials with layered structure. Tunable solvothermal synthesis of the multiphasic nanostructured Bi−Te based compounds has provided finer control over the composition, paving the way toward designing nanostructured TE materials with useful properties.

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EXPERIMENTAL SECTION Synthesis of Bi−Te Nanostructures. Bi−Te-based nanostructures in this Article were fabricated using solvothermal synthesis (sol). The effect of different presynthesis and postsynthesis treatments such as ball-milling (BM), annealing, and the addition of sodium borohydride (NaBH4) on the structure and composition of Bi−Te nanostructures was investigated. Table 1 summarizes the synthesis parameters for eight Bi−Te nanostructures described in this study. The reaction time for all of the as-synthesized samples was 12 h. The first sol synthesis carried out was as follows: Bi and Te powders were mixed together in the ratio of 2:3 and ball-milled at room temperature for 1 h in air. This BM mixture was used as a starting material for sample 1. Two millimoles of BM mixture was dissolved into 20 mL of dimethylformamide (DMF) and stirred for 30 min. 6.46 g of hydrazine hydrate (N2H4·H2O) was mixed with 10 mL of DMF, and this reductant solution was added dropwise to the precursor solution and stirred for 30 more minutes. Next, the pH of the mixture was adjusted to 13.3 by first adding 10 mL of aqueous ammonia (NH4OH, 28−30%) followed by sodium hydroxide (NaOH, 5 g in 5 mL of H2O) solution. The final solution was transferred to the Teflon liner of 70 mL capacity, and the liner was filled with DMF to 80% of the total volume. After 12 h of reaction at 120 °C, the autoclave was 9447



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Figure 1. TEM image of nanostructured Bi2Te3 and Te nanowires formed in sample 1 (a), and EDX spectra with corresponding chemical analysis (b). Inset shows the nanocrystalline grains of Bi2Te3 (∼15−20 nm).

Figure 2. TEM/SEM/STEM images of the Bi−Te nanostructured networks formed in sample 2: (a) TEM image illustrating different morphologies such as nanotubes, rolled-up nanosheets, and nanowires; (b) SEM image of the as-synthesized product revealing high yield growth of nanostructures; (c) Microspheres composed of self-assembled nanorods; (d) halfway rolled-up Te nanosheet forming tubular structures (inset shows the corresponding SAED pattern); (e) STEM image displaying the Bi2Te3 nanoparticles formed on the tubular nanostructures (scale bar is 1 μm). The top inset shows the SAED pattern corresponding to polycrystalline BixTey nanotubes, and the bottom inset indicates Bi2Te3 nanoparticles with high crystallinity; and (f) BixTey nanostructure with Te-rich core along with the chemical analysis (scale bar is 200 nm for (d) and (f)).

morphology of sample 1 with as-grown nanocrystalline BixTey and Te nanowires (about 30−50 nm in diameter). The average grain size of BixTey nanoparticles is ∼15−20 nm as illustrated in the inset. Representative EDX spectra for sample 1 with corresponding chemical analyses are shown in Figure 1. The mechanical ball-milling prior to the synthesis resulted in direct alloying of elemental Bi and Te to form Bi2Te3 nanocrystals as confirmed by the XRD analysis. However, the phase investigation report (Figure S1 in the Supporting Information) for the BM mixture indicated the abundance of elemental Te (∼60 wt %). When hydrazine hydrate is added during the synthesis, it gets

power intensities were measured on the sample. Six different excitation wavelengths in the range of 633−785 nm (excitation at 633 nm (1.96 eV), 660 nm (1.88 eV) from diode lasers and 725 nm (1.71 eV), 745 nm (1.67 eV), 765 nm (1.62 eV), and 785 nm (1.58 eV) from Spectra physics tunable Ti:sapphire laser) were used to probe the core features of BixTey nanostructures.

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RESULTS AND DISCUSSION The physical properties of Bi−Te nanostructures are extremely sensitive to the synthesis parameters such as the time, growth temperature, and the bismuth precursor. Figure 1a shows the 9448



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Figure 3. TEM images of an individual BixTey nanostructure from sample 7 with corresponding quantitative analysis: (a) Te tubular nanostructures with Bi2Te3 nanoparticles, (b) nanowire composed of Bi-rich phase (Bi3Te4 or Bi4Te5), and (c) elemental mapping of Y junction formed by two Bi2Te3 nanotubes with O(K), Bi(L), and Te(L) series.

nanostructures from Te powder. The higher concentration of HNO3 causes the generated Te seeds to grow larger in the EG reaction medium, resulting in large diameter Te nanotubes as compared to Te (50−100 nm diameter) nanowires.29 The asgrown Te nanostructures have high chemical activities, initiating the alloying process with reduced Bi, where Bi and Te mutually diffuse to form bismuth telluride nanoparticles on large rolled-up Te nanosheets (300−400 nm diameter) as seen in Figure 2e. The in situ reaction of Bi3+ with Te nanowires during solvothermal synthesis leads to the formation of BixTey nanowires.35 Effect of the Starting Stoichiometry. A detailed structural characterization indicated that samples 3−8 have a similar network of low-dimensional morphologies as seen for sample 2. The microspheres composed of nanorods are shown below to contain Cl due to BiCl3 precursor. The formation of microspheres can be prevented by using different Bi precursors such as commercially available polycrystalline Bi2Te3 (sample 6) or bismuth nitrate with Te; however, it adversely affects the yield and quality of BixTey nanostructures. The size and morphology of different nanostructures formed in each sample (as well as how they are assembled) are important in determining the thermoelectric properties, particularly electrical transport and thermal conductivity for the network of nanomaterials.4,8,18 The average chemical composition (at. %) of BixTey nanostructures from sample 2 is Bi ∼30−35%, Te ∼58−60%, O ∼5−12%, whereas the microspheres are Bi-rich and have about Bi ∼45%, O ∼35%, Cl ∼20%. A change in the starting stoichiometry of Bi:Te has directly affected the chemical composition of samples 3, 4, and 5 by forming Bi-deficient (samples 3 and 4) or -rich (sample 5) BixTey nanostructures with ∼8−15% Cl. Sample 6 synthesized with polycrystalline Bi2Te3 does not show any Cl in the chemical analysis along with Bi ∼25%, Te ∼60%, and O ∼15% consistent with the microscopy results. The post treatment with NaBH4 led to the disintegration of microspheres composed of nanorods, thereby reducing the Cl content of sample 7 to ∼1% as compared to the parent compound (sample 4). In contrast to the previous result for Bi nanotubes, no improvement in the quality or yield of BixTey nanostructures was observed after the NaBH4 treatment. After annealing at 350 °C (above the melting point of bismuth

adsorbed on the surface of Te seeds forming a homogeneous dispersion in DMF solution. As a result, thin tellurium nanowires of ∼40 nm diameter are formed via the dissolution− recrystallization process.29 An intricate network of Bi−Te-based nanostructures is formed in sample 2 as seen in Figure 2a, which exhibits different morphologies, nanowires, nanotubes, and microspheres composed of self- assembled nanorods. A low magnification SEM image (Figure 2b) shows the high yield fabrication of nanostructures, which are typically 10−20 μm long. Nanorods are about 50−100 nm wide and 500 nm long and act as building blocks in the construction of microspheres with 3−7 μm diameter as seen in Figure 2c. Nanowires have two different diameter distributions: 50−100 and 150−200 nm. Highly crystalline Te nanowires with 50−100 nm diameter are observed in Figure 2a, and Figure 2f shows BixTey nanowire of ∼200 nm diameter. TEM studies revealed that Te tubular structures (∼300−400 nm diameter) formed have rolled-up sheet nature and the sheet thickness is typically ∼3−5 nm thick. Hence, the rolled-up sheet structures started melting under the influence of the electron beam (see Figure 2d). The single crystallinity of Te nanotubes is verified by the SAED pattern presented in Figure 2d. The as-synthesized BixTey nanotubes exhibit a diameter distribution similar to that of the rolled-up Te tubular nanostructures. Figure 2e demonstrates that most of the BixTey tubular nanostructures formed during the synthesis have rough surfaces as they are decorated with single crystalline nanoparticles. The randomly orientated Bi2Te3 nanoparticles on the lining of rolledup nanosheets contribute toward the polycrystalline nature of the tubular nanostructures as confirmed by the electron diffraction ring patterns (top inset of Figure 2e). We point out that some BixTey nanostructures formed in sample 2 have a Te-rich core as seen in Figure 2f, which may be due to the incomplete conversion of Te to Bi2Te3 during the alloying process. Our results are in agreement with the previously proposed growth mechanism studies for polycrystalline Bi2Te3 nanotubes and nanowires using nanoscale Te as a sacrificial template.12,19,21,35 It is found that nitric acid plays a crucial role in the formation of Te 9449



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Figure 4. (a) XRD patterns of bulk Bi2Te3 from PRUFF database and sample 2. The indexed diffraction peaks correspond to trigonal-hexagonal crystal structure. Inset to the left panel shows the phase investigation report for sample 2. (b) XRD analysis of the rest of the as-synthesized Bi−Te nanostructured samples (1, 3−8) in comparison with commercially available bulk constituent material.

oxychloride (BiOCl)) for 1 h in Ar, we observed almost no microspheres of BiOCl in sample 8 without compromising BixTey nanostructures. The chemical composition of each individual nanostructure was investigated with due attention to the surface oxidation of nanostructures to identify the likely phase composition and stoichiometry of a single nanostructure.30 Chemical mapping for different nanomorphologies formed in sample 7 is presented in Figure 3 along with the respective chemical analysis. Figure 3a shows Te nanotubes decorated with Bi2Te3 nanoparticles at the beginning of the alloying process. The atomic ratio for BixTey nanowire is close to the molar ratio of Bi-rich phases such as Bi3Te4 or Bi4Te5 as revealed in Figure 3b. Elemental maps of O(K), Bi(L), and Te(L) series are reported in Figure 3c for Y junction of Bi2Te3 nanotubes with irregular surfaces. Effect of the Reaction Time and Temperature. The structure and composition of BixTey nanostructures can be tailored by adjusting the reaction time and temperature. A decrease in the average diameter of BixTey nanostructures is observed when the reaction temperature is reduced to 140 °C, at the expense of Bi2Te3 phase amount (by almost 50%). To maximize the Bi2Te3 content in our samples via complete consumption of Te nanostructures into Bi2Te3 nanotubes, we varied the reaction temperature from 180 to 200 °C, and time from 12 to 18 and 24 h while keeping other synthesis parameters the same as sample 2. The structural and chemical compositional studies revealed the significant drop in the concentration of microspheres at 200 °C as compared to 180 °C for the same time. The concentration of microspheres was reduced with increase in time, and the Cl content dropped from ∼20% at 12 h to 6% at 24 h. Effect of the Nitric Acid and Hydrazine Hydrate. When the amount of nitric acid in EG is increased to 4 mL for sample 2, a 50% increase in the BixTey nanotubes is observed with almost no BiOCl microsphere formation. It is clear that no BixTey nanostructures are formed in the absence of nitric acid. The addition of (0.5 g) hydrazine hydrate to the mixture of EG and HNO3 leads to the preferential growth of Te nanowires resulting in a 10-fold increase in the BixTey nanowires for sample 5. Therefore, by varying the amount of nitric acid (hydrazine hydrate) added to the EG solution, the concentration of BixTey

nanotubes (nanowires) in the as-synthesized powder can be effectively controlled. Sample 1 shows noticeable peak broadening in the XRD spectrum due to the formation of nanocrystalline tellurobismuthite (Bi2Te3) as compared to constituent bulk polycrystalline material along with Te (∼48%) and 4% pilsenite (Bi4Te3). Figure 4a presents the comparison of XRD patterns of as-synthesized sample 2 with bulk Bi2Te3 from PRUFF (ID = R060948) database. It is evident from the XRD results that sample 2 has Bi2Te3 as the main constituent. The identified phases for sample 2 are Bi2Te 3, bismoclite (BiOCl), and Te. The phase quantification report confirms that Bi2Te 3 is the most dominating phase followed by the Bi-rich BiOCl phase. The XRD patterns and phase investigation reports for the rest of the samples are presented in Figure 4b and Table 2, respectively. All of the nanostructured samples were synthesized twice, and the XRD measurements were performed on samples from both of the runs to validate reproducibility of the data. The uncertainty in the phase investigation data is less than 5% for each sample. Samples 3, 4, and 5 exhibit almost the same XRD spectra as sample 2, whereas the peaks corresponding to BiOCl are absent in samples 6, 7, and 8 in accordance with the EDX results. It is interesting to note that, although samples 3, 4, and 5 show similar morphology and XRD spectra, their chemical composition and respective phase amounts vary significantly. The phase analysis results for samples (2−5) synthesized with the second sol procedure indicate that the phase amount of Bi2Te3 in sample 5 is the highest followed by sample 2, which is almost 25% more than in sample 4, and Te content in sample 3 is about twice that of sample 2. It provides a direct evidence of change in the stoichiometric ratio of Bi:Te in the starting material. We employed different strategies to drastically decrease the BiOCl content and to increase the Bi2Te3 content: (i) sample 6 synthesized with commercially available polycrystalline Bi2Te3 forms the lowest phase amounts of BiOCl and Bi2Te3, and the highest phase amount of Bi4Te3 (∼15%) because of the higher concentration of Bi4Te3 (∼10%) impurities in the commercial bulk Bi2Te3 powder; (ii) sample 7 shows reduced BiOCl phase amount after post treatment with NaBH4; however, (Bi2Te3 + Bi4Te3) content is also lowered as compared to the starting material (sample 4); (iii) the highest Te content is found in 9450



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content to about 2 wt % and increased the Bi2Te3 amount by almost 100% as compared to the parent material (sample 2). The phase investigation report revealed that higher reaction/ annealing temperature with longer time increases the Bi4Te3 phase content.31 The synthesis of sample 2 when carried out at 200 °C for 12 h resulted in 10 times higher Bi4Te3 phase amount with a 65% reduction in BiOCl content as compared to that at 180 °C. Further, the post annealing at 350 °C for 2 h resulted in about 20 wt % of Bi4Te3 as compared to 1 h annealing. We observed a 15% increase in the Bi2Te3 content with longer reaction time (18 h) at 180 °C, but then the phase content remained almost constant from 18 to 24 h. The appearance of Te diffraction peaks even after increasing the reaction time to 24 h indicated that the Te did not completely transform into Bi2Te3, contrary to previous results.19 The highest Bi2Te3 phase content obtained in our multiphasic nanostructured samples is ∼85%. XRD analysis provided bulk phase information, but electron back-scattered diffraction (EBSD) was used to elucidate the crystallinity, chemical composition, and phase identification of an individual nanostructure. The samples were prepared by dispersing the as-synthesized product on gold-coated silicon (Si) substrate. The Si substrate was deposited with thick layer (>100 nm) of gold to ensure that no diffraction pattern was observed from the substrate itself, and hence no interference of bands from substrate while measuring the individual nanostructure. HKL Technology Channel 5 program was used for data analysis and indexing the diffraction patterns. Electron backscattered diffraction patterns (EBSP) were acquired from individual nanostructure using ESEM FEG − FEI, XL-30 at 20 kV and spot size 1 with Flamenco software. The surface roughness and curvature of the micro- and nanostructures offered challenges in the acquisition of diffraction pattern and detection of bands due to blurring and shadow formation (see indexed pattern of Figure 5). Therefore, several micro-/ nanostructures with same morphology were measured for accurate phase identification. Figure 5 shows EBSP acquired from polycrystalline nanotubes with rough surfaces in sample 3. The phase investigation report provided information about fit parameter MAD (mean angular deviation), chemical composition, and crystal structure parameters of the tubular nanostructures confirming the Bi2Te3 phase. EBSPs shown in Figures S2 and S3 (see the Supporting Information) confirmed the other two phases corresponding to different nanomorphologies in sample 3. The polycrystalline microspheres composed of self-assembled nanorods exhibit the BiOCl phase with tetragonal structure as depicted in Figure S2. EBSP for nanowires with 200 nm diameter (see Figure S3) indicates Bi4Te3 phase with trigonal-hexagonal structure. The three distinct phases identified by EBSD corresponding to low-dimensional morphologies in sample 3 are consistent with the fingerprint obtained from the XRD phase investigation report for sample 3. All of our samples contain varying amounts of nanostructured BixTey, and their room-temperature Raman spectra exhibit several distinct features in the low frequency region (50−150 cm−1) under 633 nm (1.96 eV) excitation (Figure 6a). The laser power at the sample was limited to 0.2 mW to prevent any overheating of the samples, which was shown to modify the sample.25 The Raman spectrum of bulk polycrystalline Bi2Te3 (bottom spectra in Figure 6a) exhibits three signature optical phonon peaks (A1g1 at 61.5 cm−1, Eg2 at 102 cm−1, and A1g2 at 134 cm−1) in good agreement with the previously observed Raman peaks for bulk crystalline Bi2Te3.22,23 The reported micro-Raman

Table 2. Phase Investigation Report with Quantitative Analysis for All Nanostructured Bi−Te Samples Prepared Using Different Synthesis Routes sample 1

2

3

4

5

6

7

8

phase ID

chemical Formula

tellurium

Te

tellurobismuthite

Bi2Te3

pilsenite

Bi4Te3

tellurium

Te

tellurobismuthite

Bi2Te3

pilsenite

Bi4Te3

bismoclite

BiOCl

tellurium

Te

tellurobismuthite

Bi2Te3

pilsenite

Bi4Te3

bismoclite

BiOCl

tellurium

Te

tellurobismuthite

Bi2Te3

pilsenite

Bi4Te3

bismoclite

BiOCl

tellurium

Te

tellurobismuthite

Bi2Te3

pilsenite

Bi4Te3

bismoclite

BiOCl

tellurium

Te

tellurobismuthite

Bi2Te3

pilsenite

Bi4Te3

bismoclite

BiOCl

tellurium

Te

tellurobismuthite

Bi2Te3

pilsenite

Bi4Te3

bismoclite

BiOCl

tellurium

Te

tellurobismuthite

Bi2Te3

pilsenite

Bi4Te3

bismoclite

BiOCl

file ID 99-0003656 99-0003657 99-0002951 99-0003656 99-0003657 99-0002951 99-0000389 99-0003656 99-0003657 99-0002951 99-0000389 99-0003656 99-0003657 99-0002951 99-0000389 99-0003656 99-0003657 99-0002951 99-0000389 99-0003656 99-0003657 99-0002951 99-0000389 99-0003656 99-0003657 99-0002951 99-0000389 99-0003656 99-0003657 99-0002951 99-0000389

I%

wt %

tag

12

47.8

major

21.5

48.1

major

0.5

4.1

minor

5.8

25.2

major

24.6

43.6

major

0

0.1

trace

11

31.1

major

27.1

54.4

major

14.6

29.4

major

0.1

0.3

trace

6.0

15.8

major

16.8

44.2

major

14.3

37.3

major

0.0

0.3

trace

5.3

18.7

major

16.8

23.6

major

14.3

61.4

major

0.0

0

absent

5.3

15

minor

16.1

58.7

major

18.7

25.5

major

1.4

15.8

major

0.1

0.1

absent

24.0

67.9

major

12.1

28.1

major

0.2

3.9

minor

0.1

0.2

trace

16.8

19.4

major

14.3

80.6

major

0.0

0.6

trace

5.3

1.6

trace

sample 7 followed by sample 6; and (iv) sample 8 has the lowest Te and the highest Bi2Te3 phase content in all eight samples. The post annealing treatment significantly reduced the BiOCl 9451



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Figure 5. EBSP of the Bi2Te3 nanotubes decorated with nanoparticles from sample 3. The inset in the top right corner presents the indexed pattern; unit cell of the trigonal-hexagonal structure is shown in blue (middle right inset). SEM image of Bi2Te3 nanotubes is displayed in bottom right corner with a red cross, which marks the spot used to acquire the EBSP. The phase investigation report is revealed at the bottom left corner.

Figure 6. Room-temperature Raman spectra of Bi−Te nanostructures measured using the 633 nm (1.96 eV) excitation: (a) Raman spectra normalized to the intensity of A1u mode for eight nanostructured Bi−Te samples along with bulk Te and polycrystalline Bi2Te3 for reference measured with 0.2 mW laser power on the sample, and (b) Raman spectra for sample 4 at different laser power intensities measured on the sample. The red dashed circles in the optical images show the areas of the focused laser beam, which facilitated collection of Raman data with increasing incident power on the sample (scale bar is 1 μm). The dotted lines in the left and right panels serve as a guide to the eye.

the prevalent presence of defects in nanostructures (such as Bi vacancies, antisites, excess Te in our case, etc.) is known to lift symmetry-based selection rules. Bulk Te exhibits excellent Raman scattering properties with several peaks present in the range 50−150 cm−1 (see Figure 6a). Clearly, the Te spectrum exhibits a strong Raman peak at about 118 cm−1. Hence, the X peak observed in Raman spectra for BixTey nanostructures cannot be exclusively assigned to the A1u mode based upon the data in Figure 6a. Furthermore, the absence of strong dispersion for this mode makes it difficult to rule out the possibility of X peak originating from Te. Therefore, we measured the Raman spectra of our samples at several laser power intensities to confirm the assignment of A1u mode to X peak. Figure 6b shows representative evolution in Raman spectra of sample 4

data are the average of three measurements from various spots per sample at different wavelengths and laser powers. Interestingly, the nanostructured Bi2Te3 exhibits an extra peak at ∼120 cm−1 not found in the bulk phase. Previously, this mode (X peak) was observed in nanocrystalline and few-quintuple thick Bi2Te3, and identified as a A1u mode.24−27,36 It is worth noting that the A1u mode is infrared (IR) active phonon mode and is Raman forbidden for centro-symmetric Bi2Te3. The presence of Te1 atoms in the center of unit cell demands the IRactive modes (A1u) to be odd parity. Because the polarizability does not change for odd parity modes, in centro-symmetric molecules, they are often Raman inactive. It is possible that the low-dimensional structure of our samples breaks the centrosymmetric nature of Bi2Te3 (due to defects) allowing the observation of A1u mode. This result is not surprising becauase 9452



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Figure 7. Raman spectra of sample 2 collected at different excitation wavelengths: (a) normalized Raman spectra with respect to A1u mode measured at six different excitation wavelengths varied from 633 to 785 nm, (b) new Raman peak is observed at 135 cm−1 using the 725 nm excitation, and (c) Raman spectrum at 765 nm shows two additional peaks below the Eg2 mode (∼102 cm−1), reconfirming the presence of internal strain in nanostructured Bi−Te samples.

Figure 7 for clear deconvolution). This peak may possibly arise from Te atoms in our Te-rich samples; however, in the remaining set of samples, we ascribe the presence of this mode (∼90 cm−1) to the internal strain present in the nanostructures. Recently, Pradhan et al. observed the appearance of new modes (labeled M1, M2, and M3) in bulk Bi2Te3 under high pressure.32 The presence of internal strain in the sample may have the same effect as the external pressure leading to the appearance of new modes below the Eg2 mode. In fact, rapid deterioration of our nanostructured samples upon increasing the laser power is also in agreement with the presence of internal strain. Such effects of internal strain were previously observed in subnanometer singlewall carbon nanotubes and other semiconducting nanostructures like Ga2O3.33,34 The upshifted A1g2 modes provide further evidence of the internal strain in all of the nanostructured samples. The core features of BixTey nanostructures are probed by the excitation wavelength-dependent Raman scattering. As shown in Figure 7a, the Raman features of sample 2 exhibit weak dispersions with respect to the excitation wavelength/energy (see Figure S7, Supporting Information) similar to the composition. The Raman spectra at higher wavelengths (>725 nm, 1.71 eV) show new peak above the A1u mode at ∼135 cm−1 as seen in Figure 7b. The penetration depth of laser probe increases with increasing wavelength. Thus, one may expect longer wavelengths to probe not only the surface but also considerable interior part of the sample. The new peak observed at 135 cm−1 (above 725 nm) may be arising due to the Te-rich core of some nanostructures. It is important to note that the Raman spectrum at 765 nm (1.62 eV) shows two new peaks below the Eg2 mode, reconfirming the presence of internal strain in nanostructured Bi−Te network similar to M1, M2, and M3 peaks in ref 32 (see Figure 7c).

(corresponding to BixTey nanostructures inside the red dotted area) with increasing laser power intensity measured on sample at 633 nm. At low powers (below 0.5 mW), no laser-induced structural disorder is observed optically as shown in Figure 6b. We notice that the X peak (∼121 cm−1 at 0.1 mW) frequency softens gradually with further increase in the power (see Figure S4, Supporting Information). The Raman spectrum of sample 4 appears to change for laser power above 0.5 mW due to local melting of the nanostructures, which is evident from the optical images. However, at this power level no laser-induced structural damage is observed for bulk polycrystalline Bi2Te3, confirming that the sample heating occurs due to its nanostructured nature. The X peak is red-shifted to 117 cm−1 at higher power, and the nanostructures completely disappear when the power is further raised to 1.5 mW. We attribute this red-shift to deterioration of Bi2Te3 nanostructure into elemental Te upon laser heating. The red-shift of X peak at higher laser power confirms that the X peak (shown in Figure 6b) is different from the Te peak and is indeed the A1u mode. It is worth pointing out that the A1g1 mode that occurs due to the in-phase motion of Bi−Te2 atoms is hardened (softened) with increasing amount of Bi2Te3 (Te) as seen in Figure S5 (see the Supporting Information). This could be attributed to possible changes in the Bi−Te2 bonding forces during the formation of nanostructures. Notably, the A1g1 signal is relatively weak for samples 6 and 7 due to excess Te as compared to Bi2Te3. The data analysis suggests that the typical Raman features of Bi2Te3 are intense only for samples containing at least 30 wt % Bi2Te3 (see Figure 6a and Figure S6, Supporting Information). For preceding discussion, we refer to samples containing higher weight percentage of Te as compared to Bi2Te3 as Te-rich samples (3, 4, 6, and 7). In Te-rich samples, Te overwhelms the Raman features of Bi2Te3 due to its higher Raman crosssection,23 and these samples exhibit softened and broadened A1u mode. Furthermore, samples 6 and 7 clearly exhibit a lower shoulder (∼115 cm−1) to the A1u mode (due to excess Te as compared to Bi2Te3) leading to A1u peak broadening. The peak at ∼90 cm−1 is observed in the Raman spectra for all of the nanostructured Bi−Te samples below the Eg2 mode (see

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CONCLUSION AND FUTURE WORK We have synthesized multiphasic Bi−Te nanostructured networks with diverse morphologies. The compositional dependence of BixTey nanostructures is studied by varying the temperature, the Bi precursor, the starting stoichiometry of Bi:Te, and the reducing agent. A systematic study of the 9453



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crystallinity and composition by XRD, EBSD, and Raman allowed us to perform a detailed phase analysis, specific to individual nanostructure. We provide a conclusive way to confirm the strongest peak observed at 120 cm−1 in the Raman spectra of nanostructured BixTey samples to be the IR active A1u mode. Novel spectral features below Eg2 mode and upshifting of A1g2 mode observed for the first time in the Raman spectra of nanostructured samples are assigned to the internal strain present in BixTey nanostructures. The investigation of the “doping effect” due to deviation from the stoichiometric Bi2Te3 composition and coexistence of multiple phases on the thermoelectric properties is under progress. These new findings offer a fresh perspective in the search of high-performance nanostructured thermoelectric materials, exploiting the tunable properties of the multiphasic low-dimensional Bi−Te system.

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ASSOCIATED CONTENT

S Supporting Information *

XRD spectra of Bi2Te3 ball-milled mixture and bulk Bi2Te3 from PRUFF database, EBSP of microspheres composed of BiOCl nanorods and EBSP of Bi4Te3 nanowires, dispersion plots of A1u mode frequency as a function of laser power for sample 4, A1g1 mode frequency and intensity as a function of chemical composition, and A1g1 and A1u mode frequencies as a function of laser energy for sample 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: lisa.pfeff[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from AFOSR (Weinstock, U. Texas, Dallas) and AFOSR DURIP for the T64000 triple Raman spectrometer. We thank Dr. Zhenting Jiang for assistance with the electron back-scattered diffraction measurements. We also acknowledge Prof. Jian He of Clemson University for his contribution toward this research project. Research was carried out (in whole or in part) at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC0298CH10886.

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REFERENCES

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Synthesis and Raman Spectroscopy of Multiphasic Nanostructured Bi

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