Nanoscale Stabilization of Nonequilibrium Rock Salt BiAgSeS

Stabilization of nonequilibrium phases of inorganic solids is an art in synthetic chemistry. Herein, we demonstrate the entrapping of nonequilibrium r...
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Nanoscale Stabilization of Nonequilibrium Rock Salt BiAgSeS: Colloidal Synthesis and Temperature Driven Unusual Phase Transition Satya N. Guin,† Swastika Banerjee,† Dirtha Sanyal,§ Swapan K. Pati,†,‡ and Kanishka Biswas*,† †

New Chemistry Unit and ‡Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore 560064, India § Variable Energy Cyclotron Centre, 1/AF Bidhannagar, Kolkata 700064, India S Supporting Information *

ABSTRACT: Stabilization of nonequilibrium phases of inorganic solids is an art in synthetic chemistry. Herein, we demonstrate the entrapping of nonequilibrium rock salt phase of BiAgSeS in the nanocrystalline form at room temperature via kinetic colloidal synthesis. High symmetry rock salt nanocrystals undergo an unusual irreversible phase transition to a lower symmetry thermodynamic trigonal structure upon thermal treatment. To get fundamental insights into such unusual finding, we have performed temperature dependent synchrotron powder X-ray diffraction, positron annihilation spectroscopy, and density functional theory based structural energy and phonon modes investigations. Kinetically trapped strained rock salt BiAgSeS nanocrystals irreversibly transform to thermodynamically stable trigonal structure upon heating via Ag−Bi exchange, which is alleviated by a significant increase in the Ag vacancy.

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high diffusion rate as the precursors are generally exist as soluble form inside a suitable solvent. Thus, like molten salt and flux methods, low temperature solution-based synthesis can also stabilize nonequilibrium phases in nanoscale. High surface energy, surface stress in nanodimension, reaction temperature, solvent, ligands, and precursor source play crucial role for the determination of crystal structure of the product and the dynamics of reaction.8,16−18 At low-temperature solution phase synthesis, the reaction is often controlled kinetically rather than thermodynamically.19 Hence, thermodynamic phase stability of the material can reverse in nanodimension, which sometimes results in stabilization of unusual high energy kinetic phases at ambient condition. Although stabilization of kinetic phases is quite challenging and uncommon, it was observed in PbmSb2nTem+3n,14 bixbyite type V2O3,17 cobalt nanoparticle,19 PbmBi2nTe3n+m,20 Pb2‑xSnxS2,21 wurtzite-type MnSe (γ-MnSe),22 CuInSe2,23 Cu2SnSe3,24 Cu2ZnSnS4,25,26 tetragonal Ag2Se (βAg2Se),27 and orthorhombic MnAs.28 Recently, the copper and silver based I−V−VI2 (where I = Cu, Ag; V = Sb, Bi; and VI = S, Se, Te) semiconductors have emerged as a new contestant for wide spectrum of technologically relevant applications like thermoelectrics,29−36 solar cell,37,38 and phase change memory devices.39 The crystal structure and structural variability of these compounds depend on the cationic and anionic elements, which consequence

he paradigm shifts in the research area of energy, environment, and technologically relevant disciplines in recent decade have emerged a library of new functional inorganic materials. Major fraction among them has been synthesized by high-temperature solid state method.1−6 Although synthesis of inorganic solids using high-temperature method is a popular process, which generally leads to the formation of the thermodynamically stable product, it often leaves little room for kinetic modifications.1−5 The plenteous structural chemistry and exotic chemical and physicochemical properties of kinetic phases are precious for a better understanding of fundamental solid state chemistry and technological prospects.1,2,4,7,8 The crystal symmetry is an important parameter to tune electronic structure of the material; hence, synthesis of the metastable or kinetic phases is important. For kinetic modifications, reactant diffusion rate should be high so that activation energy barrier of the reaction can be lowered to avoid the thermodynamic path. In this context, metal flux and molten salt techniques have been explored successfully for the synthesis of many novel inorganic structures belonging to oxides,3 chalcogenides,2,4,5 intermetallics,1,9 and pnictides10 family with new structure types. Apart from the bulk inorganic solids, the chemistry in nanoscale shaped a great scope for tuning of the properties of material. The control of crystal structure, size, and morphology resulted in many technologically relevant properties in the nanoscale material.11−15 Seeded-growth, polyol, solvothermal, and hot injection methods are well explored for the synthesis of nanocrystals with tailored composition, size, shape, and crystal structure.11−15 The advantage of solution based synthesis is the © 2017 American Chemical Society

Received: March 1, 2017 Revised: March 30, 2017 Published: April 2, 2017 3769

DOI: 10.1021/acs.chemmater.7b00862 Chem. Mater. 2017, 29, 3769−3777

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Chemistry of Materials

Figure 1. Powder XRD pattern (λ= 1.5406 Å) for bulk BiAgSeS: (a, b) cubic and (c) trigonal bulk BiAgSeS with simulated patterns. (d) Temperature dependent (RT−300 °C) heating−cooling cycle synchrotron (λ = 0.9543 Å) powder X-ray diffraction pattern of bulk trigonal BiAgSeS.

transition to a thermodynamically stable low symmetry trigonal structure at 200 °C. To understand the mechanistic path of the unusual phase transition, we have performed temperature dependent synchrotron powder X-ray diffraction, positron annihilation spectroscopy, and first-principles theoretical calculations. Temperature dependent positron annihilation spectroscopy indicates the formation of a large numbers of Ag vacancies during rock salt to trigonal transition, which actually assists Ag−Bi exchange that leads to the phase transition. Density functional perturbation theory (DFPT) based calculations confirm that nanocrystalline cubic BiAgSeS has three imaginary Γ-phonon modes, which indicate the metastable nature of the cubic nanocrystalline phase. A significant relaxation of strain is evidenced by the change of 1.28 GPa pressure during cubic to trigonal phase transformation.

substantial effect on the electronic structure and phonon dispersion, which all together result in an interesting structure− property relationship. Dimensionality of these materials has also significant influence on the structure and properties. For example, the nanocrystalline forms of AgBiSe2 and AgBiS2 display a different structure−property relationship compared to their bulk counterparts.30−33,36 BiAgSeS is an interesting material from this family, which gained significant attention recently for thermoelectric applications.34,35 In the bulk phase, BiAgSeS crystallizes in a disordered rock salt cubic structure (space group Fm3m ̅ ) at high temperature.34,35,40 We found that bulk BiAgSeS is a polymorphic material and shows a reversible phase transition from room temperature trigonal structure (space group P3̅m1) to a rock-salt cubic structure above 250 °C (Figure 1). Firstprinciples total energy calculations also reveal that trigonal phase is more stable than the rock salt phase at room temperature (discussed later). To date, all the studies on BiAgSeS are mainly focused on the bulk sample. However, we emphasize that size reduction to nanoscale has impact on the crystal structure and property of the material. Therefore, it would be worthy to explore the phase stability of the BiAgSeS in nanodimension. Herein, we demonstrate the stabilization of the nonequilibrium rock salt structure of BiAgSeS in the nanocrystalline form at room temperature, which is otherwise the high temperature phase for bulk BiAgSeS. Kinetically trapped stained rock salt nanocrystal undergoes an irreversible phase

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EXPERIMENTAL SECTION

Chemicals. Bismuth neodecenate (90%, Alfa Aesar), silver acetate anhydrous (Alfa Aesar, 99%), sulfur powder (S; 99.999%, Alfa Aesar), selenium powder (Se, Aldrich, 99.5+%), and oleic acid (Sigma-Aldrich, 90%) were used for synthesis of nanocrystalline BiAgSeS. For bulk BiAgSeS synthesis, bismuth (Bi; 99.9999%, Alfa Aesar), silver shot (Ag; 99.999%, Sigma-Aldrich), selenium shot (Se; 99.999%, Alfa Aesar), and sulfur (S; 99.999%, Alfa Aesar) were used. All the chemicals were used as purchased for the synthesis. Synthesis of BiAgSeS nanocrystals. We have synthesized cubic BiAgSeS nanocrystals using a bottom-up soft chemical synthesis. At first, silver acetate (50 mg, 0.2995 mmol) and oleic acid (8 mL) were 3770

DOI: 10.1021/acs.chemmater.7b00862 Chem. Mater. 2017, 29, 3769−3777

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Chemistry of Materials taken in a round-bottom flask, and the mixture was heated to 100 °C for 1/2 h with stirring in N2 atmosphere. The resulting clear solution cooled down to room temperature, and in the solution bismuth neodecenate (216.83 mg, 0.2995 mmol), selenium powder (23.69 mg, 0.2995 mmol) and sulfur powder (9.62 mg, 0.2995 mmol) were added. The reaction mixture was then heated rapidly to 220 °C and held for 2 h. Afterward, the solution of reaction mixture was cooled to room temperature. The black color nanocrystals of BiAgSeS were precipitated out from the reaction mixture by adding excess ethanol and n-hexane (1:1) and followed by centrifugation. BiAgSeS nanocrystals were washed several times with n-hexane and ethanol and finally dried under vacuum at 60 °C for 2 h. Synthesis of Bulk Cubic BiAgSeS. Ingots (7 g) of bulk cubic BiAgSeS were synthesized by mixing 0.0162 mol (3.384 g) of bismuth, 0.0162 mol (1.7509 g) of silver, 0.0162 mol (1.279 g) of selenium, 0.0162 mol (0.52 g) of sulfur in a quartz tube. The tube was flamesealed under high vacuum (10−5 Torr) and slowly heated up to 400 °C over 12 h, then heated up to 850 °C in 4 h, soaked for 10 h, and subsequently ice water quenched to trap the high-temperature cubic phase at room temperature. Synthesis of Bulk Trigonal BiAgSeS. Ingots (7 g) of bulk trigonal BiAgSeS were synthesized by mixing appropriate ratios of the highly pure starting materials, that is, (1:1:1:1) such as bulk cubic BiAgSeS in a quartz tube. The tube was slowly heated up to 400 °C over 12 h, then heated up to 850 °C in 4 h, soaked for 10 h, and slowly cooled to room temperature in a period of 15 h to get the trigonal phase of BiAgSeS. Powder X-ray Diffraction. A Bruker D8 diffractometer was used for room temperature powder X-ray diffraction data collection for all the samples using a Cu Kα (λ = 1.5406 Å) radiation source. Rietveld refinement of room temperature PXRD patterns was done using FULLPROF suit program. Temperature-dependent X-ray powder diffraction studies were carried out using a synchrotron radiation source at BL-18B (Indian beamline), Photon Factory, KEK, Tsukuba, Japan. The measurements were carried out under N2 atmosphere using a monochromatic X-ray λ of 0.9543 Å. All the high-temperature measurements were carried out with Anton Paar DHS 1100 heat cell. A Si(111) double crystal monochromator was used to check the energy of the beam and was cross-checked with Si (640b NIST) standard. The measurements were carried out in Bragg−Brentano geometry with a divergence slit (300 μm), an antiscattering slit (350 μm), and a receiving slit (300 μm). Field Emission Scanning Electron Microscopy. FESEM imaging was performed using a NOVA NANO SEM 600 (FEI, Germany) operated at 15 kV. Energy dispersive spectroscopy (EDX) analysis has been performed with an EDX Genesis instrument attached to the FESEM column. Transmission Electron Microscopy. TEM imaging was done using a JEOL (JEM3010) TEM fitted with a Gatan CCD camera operating at 300 kV accelerating voltage and also using a FEI TECNAI G2 20 STWIN TEM operating at 200 kV. EDX elemental mapping was performed during STEM imaging. The background was subtracted (using multipolynomial model) during the data processing for EDX elemental mapping (with 500 eV minimum region of interest width). Inductively Coupled Plasma Atomic Emission Spectroscopy. A PerkinElmer Optima 7000DV instrument was used for ICP-AES measurements. For compositional analysis, the nanocrystal was dissolved in aqua regia (HNO3/HCl = 1:3) followed by dilution with Millipore water. Ag standard (1000 mg/L, Merck), Bi standard (1000 mg/L, Fluka), Se standard (1000 mg/L, Sigma-Aldrich), and S standard (1000 mg/L, Sigma-Aldrich) were used to determine the compositions in ICP. X-ray Photoelectron Spectroscopy. An Omicron Nanotechnology spectrometer with monochromatic Mg−Kα (1253.6 eV) X-ray source with a relative composition detection better than 0.1% was used for XPS measurement. Band Gap Measurement. Diffuse reflectance measurements were performed on nanocrystalline and powdered bulk sample to measure the optical energy gap. A PerkinElmer Lambda 900, UV−vis/NIR spectrometer has been used for the measurement. By using the

Kubelka−Munk equation, α/S = (1 − R)2/(2R), where R is the reflectance, and α and S are the absorption and scattering coefficients, respectively, absorption (α/S) data were calculated from reflectance data. The band gap of the sample was derived from α/S (a.u.) versus E (eV) plot. Positron Annihilation Study. In the present case, the positron annihilation experiments were done with a 22NaCl source of strength about 10 μCi sealed in a 1.5 μm (micrometer) thick nickel foil. The sealed source was placed in between two identical plane faced samples (8 mm diameter × 1 mm thick pellet). The positron annihilation lifetime was measured with a conventional fast−fast coincidence assembly consisting of two gamma ray detectors (25 mm long and 25 mm tapered to 13 mm diameter BaF2 scintillator optically coupled with XP2020 Q photomultiplier tube) and two constant fraction differential discriminators (Fast ComTech; model 7029A) having time resolution (full width at half-maximum) of ∼220 ps measured by the prompt gamma ray of 60Co source.31 About five-million coincidence counts were recorded in a multichannel analyzer. The recorded lifetime spectrum was analyzed by the computer code PATFIT-8845 with proper source corrections. To identify the chemical nature of the defect, the coincidence Doppler broadening (CDB) measurement with ± ΔE selection was used. The CDB setup consisting of two identical HPGe detectors41 (model number PGC 1216sp of DSG, Germany of 12% efficiency and having energy resolution of 1.15 at 514 keV of 85Sr) has a very high peak to background ratio (better than 105:1). About 2 × 107 counts were recorded in a dual ADC based multiparameter data acquisition system (Model number MPA-3 of FAST ComTec, Germany). The CDB spectrum of the nanocrystalline BiAgSeS sample was analyzed by constructing the ratio curve41,42 with the area normalized CDB spectrum of the bulk BiAgSeS sample. The temperature dependent (30−350 °C) Doppler broadening of positron annihilation radiation (DBPAR) measurement was carried out with a single HPGe detector (efficiency, 12%; type, PGC 1216sp of DSG, Germany) having energy resolution of 1.1 at 514 keV of 85Sr. In this particular measurement, the source sample sandwich has been kept in a vacuum sealed heating oven with a temperature accuracy of ±2 °C. The DBAR spectrum was recorded in a dual ADC based multiparameter data acquisition system (MPA-3 of FAST ComTec, Germany). The Doppler broadening of the annihilation 511 keV γ-ray spectrum was analyzed by evaluating the conventional line-shape parameters (S-parameter).42 The S-parameter is calculated as the ratio of the counts in the central area of the 511 keV photo peak (| 511 keV − Eγ | ≤ 0.85 keV) and the total area of the photo peak (| 511 keV − Eγ | ≤ 4.25 keV). The S-parameter represents the fraction of positrons annihilating with the lower momentum electrons with respect to the total electrons annihilated. Computational Details. Density functional theory based firstprinciples calculations of the trigonal and cubic phases of BiAgSeS have been carried out with a general gradient approximation (GGA/ PBE)43 method as implemented in Quantum Espresso package.44 Onsite electron correlations of d-electrons of Ag atom were modeled with Hubbard U parameter (5 eV). Plane-wave basis was truncated with energy cut-offs of 60 and 480 Ry in the representation of wave functions and density, respectively. 7 × 7 × 3 (trigonal) and 7 × 7 × 7 (cubic) Monkhorst−Pack k-meshes are used to sample Brillouin zone integration. To understand the structural stability, we have determined vibrational density of states (VDOS) at high symmetry point (Γ) using density-functional perturbation theory (DFPT).45 To study the effect of nanoscale, we have used a 48 atom (trigonal) and 32 atom (cubic) supercells by considering different crystallographic directions, such as [100, 010, 001, 111] axes for termination of the periodicity of the 3D crystal lattice that eventually mimic the effect of low dimensionality. Cell parameter of the simulation box is taken to be the average of the experimental cell parameters of AgBiS2 and AgBiSe2 crystallizing in the trigonal structure and that of the cubic structure. The structural optimizations are performed using the Broyden−Fetcher−Goldfarb− Shanno (BFGS) algorithm. Convergence threshold is set at 10−3 Ry bohr−1 for force. We have checked that these conditions give good convergence of the total energy within 10−4 Ry per atom. 3771

DOI: 10.1021/acs.chemmater.7b00862 Chem. Mater. 2017, 29, 3769−3777

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Figure 2. (a) Powder XRD pattern (λ = 1.5406 Å) of as synthesized BiAgSeS nanocrystals. (b) Optical absorption spectra of cubic BiAgSeS nanocrystals with bulk cubic and trigonal BiAgSeS. (c) TEM image of BiAgSeS nanocrystals; the lower inset shows the indexed SAED pattern. (d) HRTEM image of nanocrystalline BiAgSeS; the upper inset shows particle size distribution histogram.

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RESULTS AND DISCUSSION Nanocrystalline rock salt phase of BiAgSeS was synthesized using a simple solution-phase method in oleic acid medium. Bismuth neodecenate and silver acetate were used for Bi3+ and Ag+ sources, respectively. The elemental powder of selenium and sulfur were used as anion sources. In the synthesis, silver acetate was converted into Ag-oleate. The resulting Ag-oleate, bismuth neodecenate, Se, and S powder were then heated to 220 °C for 2 h under N2 in Schelenk line, then quickly cooled down to room temperature. The solution based low-temperature synthesis and size reduction to nanoscale stabilize kinetic rocksalt phase of BiAgSeS at room temperature. The experimental evidence for this was acquired from powder Xray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray analysis (EDX), scanning and transmission electron microscopy (FESEM/TEM), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and selected area electron diffraction analysis (SAED). Controlled bulk form of BiAgSeS with both the cubic and trigonal symmetry were synthesized using an elemental melting reaction at 850 °C by mixing the appropriate ratios of high-purity Ag, Bi, Se, and S in evacuated quartz tube. To trap the hightemperature bulk cubic phase at room temperature, reaction tube was quenched in ice water bath. The room temperature trigonal phase of the material was obtained by slow cooling of the tube to room temperature. In Figure 1, we present power XRD patterns of bulk trigonal and cubic BiAgSeS samples with simulated patterns. All the

XRD peaks of the ice water quenched sample could be indexed based on the cubic BiAgSeS pattern (space group Fm3̅m, Figure 1a). The 2θ positions for the peaks in cubic sample were located exactly in the middle of cubic AgBiSe2 and AgBiS2 phases (Figure 1b and Figure S1a, Supporting Information). We would like to note that no simulated PXRD pattern is reported for room temperature trigonal phase. However, it is expected that the 2θ peak positions for trigonal phase will be located in middle of that trigonal phases of AgBiSe2 and AgBiS2 as similar to cubic phase. The comparison pattern shows the peaks for the slow cooled bulk sample are located exactly in the middle of trigonal AgBiSe2 and AgBiS2 phases, which indeed indicates the trigonal structure (P3̅m1) to be the room temperature phase (Figure 1c and Figure S1b, Supporting Information). Rietveld profile refinement further confirms the trigonal structure of the slow cooled sample bulk BiAgSeS (Figure S2, Supporting Information). Temperature dependent structural evolution study using synchrotron PXRD indicates bulk trigonal BiAgSeS undergoes a reversible trigonal to cubic phase transition at ∼250 °C (Figure 1d). The above discussions conclude that the cubic structure is the high-temperature phase of bulk BiAgSeS, which transforms reversibly to a trigonal structure upon cooling to room temperature. In Figure 2a, we represent the room temperature PXRD pattern of as-synthesized nanocrystals with controlled bulk trigonal and cubic BiAgSeS. All the peaks in PXRD pattern could be indexed based on the cubic phase of BiAgSeS, which indicates the cubic rock salt structure to be the room 3772

DOI: 10.1021/acs.chemmater.7b00862 Chem. Mater. 2017, 29, 3769−3777

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Figure 3. (a) Temperature dependent (RT−300 °C) heating−cooling cycle synchrotron (λ = 0.9543 Å) powder X-ray diffraction pattern of nanocrystalline BiAgSeS; shaded region and black arrows indicate the splitting of PXRD peaks. (b) Positron annihilation lifetime spectra for cubic nanocrystalline BiAgSeS. (c) Area normalized ratio between the CDB spectrum of nanocrystalline and bulk cubic BiAgSeS. (d) Temperature dependent Doppler broadening S-parameter of BiAgSeS nanocrystals.

temperature phase of BiAgSeS nanocrystals. Rietveld profile refinement further indicates the cubic structure of the nanocrystals at room temperature (Figure S3, Supporting Information). The low-temperature soft chemical synthesis and size reduction to nanoscale lead to the formation of nonequilibrium cubic phase of BiAgSeS at room temperature, which is otherwise the stable phase at high temperature for bulk BiAgSeS. The as-synthesized nanocrystals exhibit well-defined optical band gap ∼0.8 eV, which is blue-shifted than that of bulk cubic (∼0.65 eV) and trigonal (∼0.6 eV) BiAgSeS (Figure 2b). BiAgSeS nanocrystals are nearly monodisperse in nature, which can be seen from FESEM image (Figure S4, Supporting Information). TEM and electron diffraction experiments were carried out for structural investigation at room temperature (Figure 2c). The size of the nanocrystals ranges from 8−14 nm (inset of Figure 2d). The particle size histogram generated form the TEM micrographs shows that the average size of nanocrystals is ∼10.8 nm (inset of Figure 2d). The average crystallite size calculated from PXRD using Scherrer peak broadening is to be 12.4 nm. SAED pattern collected from the nanocrystals clearly shows that the Bragg diffractions correspond to the rock salt structure at room temperature (inset of Figure 2c). High-resolution TEM (HRTEM) image shows a clear lattice spacing of 3.3 Å, which corresponds to (111) interplanar distance of cubic BiAgSeS (Figure 2d). X-ray photoelectron spectroscopy on the nanocrystals indicates the presence of all the elements in the nanocrystals

(Figure S5, Supporting Information). Two strong peaks at centered at 368.9 and 375 eV (Figure S5b, Supporting Information) with a peak splitting of 6.1 eV were due to Ag(I) 3d5/2 and Ag(I) 3d3/2 states. The peaks at 159 and 164.3 eV corresponded to Bi 4f7/2 and Bi 4f5/2 (Figure S5c, Supporting Information). The deconvoluted XPS spectra of Se 3d region show two peaks at 54.8 and 57 eV that correspond to Se 3d5/2 and Se 3d3/2 (Figure S 5d, Supporting Information), respectively. A peak located at 229.6 eV is due to S 2s (Figure S5e, Supporting Information). The composition of the nanocrystals obtained from energy dispersive X-ray (EDX) spectra (Figure S6, Supporting Information) and ICP-AES measurement indicate that stoichiometry is close to nominal composition. The EDX compositional mapping over the group of nanocrystals taken during scanning transmission electron microscopy (STEM) also indicates the single-phase homogeneity of the BiAgSeS nanocrystals (Figure S7, Supporting Information). To understand the structural evolution in BiAgSeS nanocrystals, we have performed variable temperature synchrotron PXRD in heating−cooling cycle (Figure 3a). We observed that nanocrystalline rocksalt BiAgSeS remains stable below 200 °C. The onset of the phase transformation occurs at ∼200 °C, and further increase of temperature results in the transformation to trigonal structure (marked with black arrow in the Figure 3a). Notably, the phase transition is irreversible in nature for nanocrystals, as the trigonal structure does not revert back to cubic during cooling cycle. A similar result was found on 3773

DOI: 10.1021/acs.chemmater.7b00862 Chem. Mater. 2017, 29, 3769−3777

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Figure 4. (a) Γ-point phonon modes for nanocrystalline cubic and trigonal phases of BiAgSeS; V1, V2, and V3 indicate imaginary phonon mods for cubic BiAgSeS nanocrystals. (b) Crystals structural evolution among nanocrsyaltalline cubic and trigonal BiAgSeS. The numerical value in the picture indicates the bond distances (in Å unit). Energy diagram for (c) nanocrystalline and (d) bulk BiAgSeS.

I1 (25 ± 1), I2 (72 ± 1), and I3 (3 ± 0.2), respectively. The shortest lifetime component, τ1, is the free annihilation of positrons, while the intermediate lifetime, τ2, is due to annihilation of positrons at the defect site.46,47 The τ2 value indicates the presence of Ag vacancy-type defect in BiAgSeS sample.31,48,49 The long lifetime component τ3 (1606 ± 70 ps) is due to the formation of positronium in the voids or surface of the sample. To understand the specific nature of defect in the system, we have performed coincidence Doppler broadening (CDB) measurement at room temperature. In Figure 3c, we represent the area normalized ratio between the CDB spectrum of nanocrystalline cubic BiAgSeS and the same for bulk cubic BiAgSeS. The ratio curve shows a dip in the momentum with minimum centroid at 20 × 10−3 moc. The dip in the ratio curve in the higher momentum region (centroid at 20 × 10−3 moc) indicates less annihilation of positrons with core electrons of atom. The chemical nature of the defect site can be identified by estimating the kinetic energy (Ekin) of the electrons corresponding to that momentum value. Assuming positrons are thermalized before annihilation and using Virial theorem approximation (in the atom the expectation value of the kinetic energy of an electron, Ekin, is equal to the binding energy of the electron), we have calculated Ekin using the formula, pL= (2 m0Ekin)1/2, where, m0 is the rest mass of an electron and pL is the longitudinal component of electron momentum p along the

thermal annealing of the nanocrystalline sample at high temperature (Figure S8a, Supporting Information). We would like to note that the cubic nanocrystals were exclusively transformed to trigonal BiAgSeS without any phase separation to trigonal AgBiSe2 and AgBiS2 (Figure S8b, Supporting Information). All these observations imply that the low temperature solution phase synthesis kinetically arrests the nonequilibrium rock salt cubic phase of BiAgSeS at nanodimension, which transforms to thermodynamically stable trigonal phase on heating. To understand the role of defects or vacancies during phase transition, we have performed positron annihilation spectroscopy, which is a unique tool for characterization and identification of vacancies in material. Here, we have performed room temperature lifetime spectroscopy, coincidence Doppler broadening (CDB) spectroscopy, and temperature-dependent (RT−300 °C) one-detector Doppler broadening of positron annihilation radiation (DBPAR) line-shape spectroscopy to identify the type of defects and its role during the structural phase transformation. In Figure 3b, we present the positron annihilation lifetime spectrum for the nanocrystalline BiAgSeS sample at room temperature. The best fit (variance of fit is less than 1 per channel) of the spectrum obtained with three-lifetime components. The lifetime components are τ1 (166 ± 1 ps), τ2 (344 ± 4 ps), and τ3 (1606 ± 70 ps) with relative Intensities 3774

DOI: 10.1021/acs.chemmater.7b00862 Chem. Mater. 2017, 29, 3769−3777

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Chemistry of Materials detector axis. We found the momentum value of 20 × 103 moc corresponds to Ekin of 102 eV, which is close to the binding energy of Ag 4s (97 eV) core electrons. Thus, the ratio curve indicates the presence of higher concentration of Ag vacancy (VAg) in the nanocrystalline cubic BiAgSeS sample with respect to the bulk cubic BiAgSeS sample. Figure 3d demonstrates the temperature dependent (room temperature to 350 °C) variation of Doppler broadening of positron annihilation line shape parameter (S-parameter) for the nanocrystalline cubic BiAgSeS. It clearly shows a step-like increase of S-parameter beyond 210 °C up on heating of the sample. The increase of S-parameter at higher temperature suggests positrons are annihilating more with the open volume defects, which is VAg in the present case. This observation suggests that increase in the Ag vacancy concentration plays a vital role during kinetic cubic to thermodynamic trigonal phase transformation. To get insights into the structural feasibility and associated phase transition, we have carried out density-functional theory (DFT) and density-functional perturbation theory (DFPT) based first-principles calculations. From the thermodynamic analysis, we found that cubic phase is less stable with respect to the trigonal phase at both the nano and bulk scale, while relative stability of trigonal phase with respect to cubic phase is same for both bulk and nanoscale (Figure 4). The analysis of Γ -point phonon modes for nanocrystalline phases of BiAgSeS indicates that particularly the cubic nanocrystals possess three imaginary phonon modes, V1, V2, V3 (Figure 4a). Such instability arises due to the stretching and shear modes mainly from the coordinatively unsaturated surface atoms of the nanocrystal (Figure S9, Supporting Information) and thus impart the metastable nature of the nanocrystalline cubic phase. However, instability due to surface strain can be relieved by specific synthesis condition.8,16,18 In the kinetic low temperature solution phase synthesis, the passivation of the uncoordinated surface atoms by chemically bound ligands can result in stabilization of nonequilirium phase at ambient condition. Consequently, cubic phase at nanodimension appears to be a kinetically trapped state due to specific synthesis condition and effect of the nanoscale regime. Γ-point phonon analysis of trigonal nanocrystals indicates that all the vibrational modes have positive frequencies (Figure 4a), which indicates trigonal phase to be thermodynamically stable phase at nanodimension. In Figure 4b, we show the crystal structural evolution among nanocrystalline cubic and trigonal BiAgSeS. Ag−S and Ag−Se bond lengths at the surface are in similar range compared to that of the interior in cubic phase (Figure 4b). On the other hand, for trigonal nanocrystal, surface bonds are shorter with respect to the interior to a large extent. Thus, room for structural deformation is less for cubic phase compared to that of the trigonal phase because of the symmetry constraint. Consequently, the cubic phase in nanodimension cannot release the strain through bond deformation, which gives rise to unstable modes of vibration due to the stretching and shear modes involving mainly the surface atoms. In contrast, structural relaxation thermodynamically stabilizes the trigonal phase and results in high surface tension. A change of 1.28 GPa pressure during cubic to trigonal phase transformation in nanocrystals also indicates a significant relaxation of strain after phase transformation. Therefore, while metastable cubic nanocrystalline phase transforms to the thermodynamically stable trigonal phase upon thermal treatment, the trigonal

phase does not revert back to the strained cubic phase on cooling (Figure 3a). Notably, the Γ-point phonon analysis of the bulk cubic and trigonal BiAgSeS does not show any imaginary modes, which indicate both the structures exist in stable minima and these phases are interconvertible depending on the temperature (Figure S10, Supporting Information). In fact, the variation of pressure for trigonal to cubic at bulk phase transition is zero. Accordingly, trigonal−cubic phase transition in bulk BiAgSeS is reversible in nature (Figure S11, Supporting Information). This analysis suggests that surface strain prevails in nanocrystal, which has a strong influence on the phase transition. For a better visualization of the energetics of the phase transition process, we have shown schematic of energy diagram for nanocrystalline and bulk BiAgSeS in Figure 4c and d, respectively. Ab initio molecular dynamics simulation reveals that metastable cubic to stable trigonal phase transition at nanoscale occurs via a possible transient rhombohedral intermediate structure, which is not observed in experiment. This is due to the high energy rhombohedral phase, which transforms to trigonal phase in a picosecond time scale. Thus, the process can be expressed as cubic → rhombohedral → trigonal. The first step, that is, cubic → rhombohedral transition, takes place via Ag−Bi exchange, which is alleviated by a large increase in the Ag vacancy, as confirmed through positron annihilation study (discussed above). Similar type of Ag−Bi exchange during phase transition is evidenced in AgBiSe2 nanocrystals.35 In the second step, the transient rhombohedral phase transforms into thermodynamically stable trigonal phase by two successive steps: (a) bond stretching/shortening without any bond dissociation and reformation and (b) reorientation of the bond angles and dihedrals. Since Ag vacancy is an intrinsic defect present in BiAgSeS system, this ensures the reversible transformation between cubic and rhombohedral. However, trigonal → rhombohedral transformation appears to be ratelimiting factor at nanodimension due to the contribution of huge strain. Consequently, the reverse process involving trigonal → rhombohedral → cubic in nanodimension is not feasible, and cubic phase cannot be recovered from the trigonal phase.

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CONCLUSION We have synthesized nonequilibrium strained rock salt phase of BiAgSeS nanocrystals by a low-temperature solution-based synthesis. The metastable rock salt structure is stabilized due to low temperature kinetic synthesis condition and effect of the nanoscale regime. The irreversible cubic−trigonal phase transition in nanocrystal goes via a transient rhombohedral phase that is facilitated by the formation of a large number of Ag vacancies at high temperature. Nanocrystalline BiAgSeS remained in thermodynamic trigonal phase after thermal treatment and does not revert back to metastable rocksalt phase upon cooling. A significant relaxation of strain is evidenced by the change of 1.28 GPa pressure during cubic to trigonal phase transformation. The trigonal−cubic transition is locked in nanoscale, as the formation of transient rhombohedral phase from trigonal nanocrystals is not feasible in nanodimension. The demonstration of the synthesis of the nonequilibrium structure in form of nanocrystal, unusual phase transition, and understanding of the mechanistic path of the structural evolution definitely enriches solid-state chemistry and materials science. 3775

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00862. Zoomed powder-XRD; Rietveld-refinement data; FESEM image; XPS spectra; EDX spectra; EDX compositional mapping; PXRD with different annealing temperature; imaginary Γ-point phonon modes for nanocrystalline cubic BiAgSeS; Γ-point phonon modes for bulk BiAgSeS; structural evolution in bulk BiAgSeS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Satya N. Guin: 0000-0003-0122-7728 Dirtha Sanyal: 0000-0003-2490-3610 Swapan K. Pati: 0000-0002-5124-7455 Kanishka Biswas: 0000-0001-9119-2455 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.B. thanks DST Ramanujan Fellowship and Sheik Saqr Laboratory. S.K.P. thanks DST for funding. S.N.G. and S.B. thank the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) and CSIR, respectively, for research fellowships. The authors thank the DST, India, for financial support and the Saha Institute of Nuclear Physics and JNCASR, India, for facilitating the experiments at the Indian Beamline, Photon Factory, KEK, Japan.

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