Article pubs.acs.org/IC
Downscaling Effect on the Superconductivity of Pd3Bi2X2 (X = S or Se) Nanoparticles Prepared by Microwave-Assisted Polyol Synthesis Maria Roslova,*,† Lars Opherden,‡,§ Igor Veremchuk,∥ Lena Spillecke,‡,§ Holm Kirmse,⊥ Thomas Herrmannsdörfer,§ Joachim Wosnitza,‡,§ Thomas Doert,† and Michael Ruck†,∥ †
Department of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany Institute for Solid State Physics, Technische Universität Dresden, 01062 Dresden, Germany § Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz Center Dresden-Rossendorf, 01314 Dresden, Germany ∥ Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany ⊥ Institute of Physics, Humboldt University of Berlin, 12489 Berlin, Germany ‡
S Supporting Information *
ABSTRACT: Pd3Bi2S2 and Pd3Bi2Se2 have been successfully prepared in the form of nanoparticles with diameters of ∼50 nm by microwave-assisted modified polyol synthesis at low temperatures. The composition and morphology of the samples have been studied by means of powder X-ray diffraction as well as electron microscopy methods, including X-ray intensity mapping on the nanoscale. Superconducting properties of the as-prepared samples have been characterized by electrical resistivity measurements down to low temperatures (∼0.2 K). Deviations from the bulk metallic behavior originating from the submicrometer nature of the samples were registered for both phases. A significant critical-field enhancement up to 1.4 T, i.e., 4 times higher than the value of the bulk material, has been revealed for Pd3Bi2Se2. At the same time, the critical temperature is suppressed to 0.7 K from the bulk value of ∼1 K. A superconducting transition at 0.4 K has been observed in nanocrystalline Pd3Bi2S2. Here, a zero-temperature upper critical field of ∼0.5 T has been estimated. Further, spark plasma-sintered Pd3Bi2S2 and Pd3Bi2Se2 samples have been investigated. Their superconducting properties are found to lie between those of the bulk and nanosized samples. properties is a microwave-assisted polyol route.13,16,17 As compared to the synthesis with conventional heating methods, the microwave-assisted synthesis has obvious advantages of rapidness, with much reduced preparation times, often by orders of magnitude, and, therefore, reduced energy consumption, and high product yield.13,18 However, to the best of our knowledge, metal-rich ternary compounds with superconducting properties have never been synthesized before by this method, and the development of a new synthesis strategy for them is required. Objects of our research are metal-rich parkerite-type related materials, namely, Pd3Bi2S2 and Pd3Bi2Se2, in their nanocrystalline form. Bulk Pd3Bi2Se2 was found to be an s-wave weakcoupling superconductor with a Tc (μ0H = 0) of ∼1 K.19 Electronic band structure calculations for Bi-containing parkeritetype materials revealed Bi(6p) bands crossing the Fermi level, thus implying metallic-type conductivity, and the DOS at EF mainly dominated by Bi(6p) states. A weak DOS maximum at EF points toward flat bands.20 From the standpoint of crystallography, the Pd3Bi2Se2 structure possesses monoclinic symmetry
1. INTRODUCTION Superconducting properties of nanoparticles and nanocomposites have attracted great interest in light of the exploration of novel finite-size phenomena.1−6 Nanoscaling may affect the density of states (DOS) around the Fermi energy (EF) as well as electron−electron and electron−phonon interactions that make pairing more or less favorable and, thereby, changes the energy gap (Δ) and the superconducting temperature (Tc).1,7−9 Moreover, it is shown that the superconducting energy gap as well as the critical field (Hc) can be varied by tuning the particle size.2,9,10 To date, experimental studies in the field of nanostructured superconductors have been primarily focused on elemental superconductors,1−6,10,11 although extending them to more complex systems seems to be interesting. It is worth noting that good synthetic approaches based on wet chemistry have been developed for nanosized elemental superconductors such as Pb, Sn, In, etc.,11−14 but the synthesis of submicrometer particles of binary and ternary compounds in many cases remains challenging because of the presence of impurity phases and uncontrollable stoichiometry.15−17 One of the promising approaches for the synthesis of singlephase nanostructured bimetallic alloys with interesting functional © XXXX American Chemical Society
Received: June 6, 2016
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DOI: 10.1021/acs.inorgchem.6b01326 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
the nanoparticle size distribution, ∼100 particles per sample were measured. Transmission electron microscopy (TEM) images, electron diffraction (ED) patterns, and HRTEM images were collected on a JEOL JEM2200FS microscope with a field emission gun operating at 200 kV. Each sample was dispersed in ethanol with an ultrasonic bath (Elma) at a frequency of 35 kHz for 5 min and then deposited on a holey carbon grid. 2.4. Characterization of Physical Properties at Low Temperatures. For resistivity measurements, the nanocrystalline powder was pelletized by pressing for 1 min at room temperature. The applied pressure was approximately 0.1 GPa for the Pd3Bi2S2 sample and 0.2 GPa for the Pd3Bi2Se2 sample. The resistivity was measured as well on Pd3Bi2X2 (X = S or Se) bars obtained after the spark plasma sintering procedure. The measurements were taken using a 14 T Quantum Design PPMS instrument equipped with a variable-temperature insert for cooling to 1.8 K. The sample was attached to a platform, which allows for further cooling below 200 mK by making use of adiabatic demagnetization of a paramagnetic salt solution. The resistance was measured using a standard four-probe technique. The temperature was measured with a RuO2 semiconductor resistor (5 kΩ at 300 K) that was attached next to the sample. The transition temperature (Tc) is defined in the following as the temperature at which the resistivity has dropped to 50% of its normal state value. The behavior of the upper critical field, Hc2, was estimated using the Werthamer−Helfand−Hohenberg (WHH) theory in the dirty limit for a type II superconductor24 with the reduced temperature t = T/Tc dH 4 and h = 2 × Hc2/ dTc2 × Tc
(space group C2/m) and can be described as consisting of quasi-two-dimensional nets formed by Pd and Bi as well as Pd and Se atoms.20 In contrast, the analogous sulfide compound Pd3Bi2S2 shows a cubic structure (space group I213).21 A superconducting transition in Pd3Bi2S2 had not been observed so far down to 0.35 K.19 Very recently, an exotic three-band Fermions appearance close to the Fermi level has been predicted for Pd3Bi2S2 based on the results of ab initio calculations.22 It is worth noting that its nonsymmorphic crystal symmetry is considered to be essential for stabilizing these new fermions.22 In this paper, we demonstrate for the first time that parkeritetype related superconducting materials can be prepared in nanocrystalline form by a one-step low-temperature approach using microwave-assisted polyol synthesis. A detailed characterization of Pd3Bi2X2 (X = S or Se) by resistivity measurements performed down to 0.2 K allowed us to rationalize a difference in the superconducting properties between nanoparticles and sintered samples and to compare them with bulk materials.
2. EXPERIMENTAL SECTION 2.1. Synthesis. All reagents were of analytical grade and used without further purification. Ethylene glycol (EG, Fluka, 99%) used as a solvent was additionally dried at 100 °C under vacuum before synthesis. Bi(NO3)3·5H2O (Riedel-deHaen, ≥98.5%), Pd(OAc)2 (Chempur, 99.95%), thiosemicarbazide (Sigma-Aldrich, ≥99%), and elemental selenium (Chempur, ≥99%) were used as precursors. Microwave synthesis was performed in a dynamic-mode CEM Discover System operating at 300 W and 2.45 GHz. The solution containing the precursors was placed in a 35 mL Pyrex vessel and heated to the preassigned temperature (180−220 °C) where it was kept for 20−40 min. After termination of the reaction, the vessel was rapidly cooled using high-pressurized air flow. The obtained black precipitate was washed several times with ethanol for removing residual solvent. Finally, the product was dried in a vacuum chamber at room temperature overnight. For the synthesis of Pd3Bi2S2, 67 mg (0.3 mmol) of palladium acetate, 96 mg (0.2 mmol) of Bi(NO3)3·5H2O, and 18 mg (0.2 mmol) of thiosemicarbazide (TSC) were suspended in 15 mL of EG. The resulting orange solution was heated within 3 min to 220 °C and kept at this temperature for 20 min. For the synthesis of Pd3Bi2Se2 submicrometer particles, a modified synthetic approach, including a pretreatment of selenium powder and ethylenediamine mixture under microwave heating, was applied. For a typical synthesis, 16 mg (0.2 mmol) of selenium was dissolved in 5 mL of ethylenediamine followed by microwave irradiation of this mixture for 20 min while it was being intensely stirred. The freshly obtained selenium feedstock was dropped into a metal salt solution containing 67 mg (0.3 mmol) of palladium acetate and 96 mg (0.2 mmol) of bismuth nitrate in 10 mL of EG. The product was heated within 3 min to 180 °C and kept at this temperature for 20 min. 2.2. XRPD Data Collection and Analysis. X-ray powder diffraction (XRD) patterns were collected using a PANalytical X’Pert Pro MPD diffractometer with Cu Kα1 radiation (λ = 1.54056 Å) at room temperature in the 2θ range between 10° and 100°, with a scan speed of 0.01° per second and a step size of 0.026°. JANA200623 was used for further Rietveld refinement and Williamson−Hall analysis. The background modeled by a Legendre polynomial function, the lattice parameters, the zero-point 2θ shift, and the profile parameters as well as the atomic parameters were refined. The peak shape was modeled with a pseudo-Voigt analytical profile function. The profile parameters of LaB6 were used as a standard to obtain the angular dependence of the instrumental line broadening. 2.3. Electron Microscopy. The particle size and shape were determined with a scanning electron microscope (Hitachi model SU8020). Energy dispersive X-ray spectra (EDX) were recorded using an Oxford Silicon Drift Detector X-MaxN. For the determination of
π
T = Tc
⎧ ⎡ ⎤−1⎫ αh 2 +∞ ⎪ ⎢ ⎥ ⎪ ⎛1⎞ h 1 t − ⎢|2n + 1| + + ln⎜ ⎟ = ∑ ⎨ ⎥ ⎬ ⎝t ⎠ | n + | t | n + | + h + λ t 2 1 2 1 ( )/ SO ⎢⎣ ⎥⎦ ⎪ n =−∞ ⎪ ⎪ ⎪ ⎭ ⎩
( )
(1) The spin−orbit coupling parameter (λSO) was optimized for best agreement with the measured data, while the Maki parameter,
α=
2
Hc2,orb Hp
, was used as a fixed value, where
Hc2,orb = − 0.7Tc ×
dHc2 dT
(2)
T = Tc
and
Hp = 1.84(T /K) × Tc/μ0
(3) 25,26
is the weak-coupling Chandrasekhar−Clogston limit. 2.5. Compaction of Pd3Bi2X2 (X = S or Se) Nanoparticles into a Pellet by Spark Plasma Sintering (SPS). In a typical experiment, 120 mg of sample powder was filled into a WC/Co die (8 mm × 1.5 mm) that was loaded into an SPS-515ET apparatus (Fuji, Syntex). The samples were sintered under the following conditions. The system was heated to 200 °C with a heating rate of 20 °C/min where the powders were pressed for 2 min. A uniaxial pressure of 583 MPa was applied. After compaction, the pressure was released and the material was allowed to cool to room temperature.
3. RESULTS AND DISCUSSION 3.1. Details of Synthesis. While the detailed reaction mechanism of the polyol process has not been thoroughly investigated yet, our experiments have shown that Pd(II) and Bi(III) precursors have to be reduced with comparable rates in the reaction, despite the fact that the standard reduction potentials for Pd2+/Pd and Bi3+/Bi differ significantly. As long as the reaction temperature is below 150 °C and the heating rate is lower than 20 °C/min, only palladium is reduced in the polyol process,17 resulting in binary selenides or sulfides instead of ternary ones. Several ways that allowed us to overcome the premature palladium reduction were explored. Eventually, a complexation reaction between Pd2+ and Bi3+ B
DOI: 10.1021/acs.inorgchem.6b01326 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry ions with thiosemicarbazide known from the literature27−30 helps to prevent the immediate reduction processes because of the high stability of their metal organic complexes. For the Pd3Bi2Se2 synthesis, the ethylenediamine probably plays the role of a complexation agent for Pd2+ ions. Moreover, the ability of ethylenediamine to dissolve selenium under solvothermal conditions as shown in refs 31 and 32 makes such a solution a suitable selenium feedstock. It is worth noting that incomplete selenium dissolution leads to a PdBi impurity phase probably because of a lack of selenium with respect to the target composition. Our experiments revealed that microwave irradiation helps to increase the selenium dissolution velocity significantly, which results in a higher product yield (usually approximately 95−98%) and permits us to conduct the reaction at relatively low temperatures (180−220 °C). Finally, by adapting the reaction temperature and solvent, we were able to synthesize phase-pure nanocrystalline samples of the ternary metal-rich chalcogenides Pd3Bi2S2 and Pd3Bi2Se2. 3.2. Composition and Morphology Studies. Figure 1 shows X-ray powder diffraction patterns of typical Pd3Bi2S2 and
In the case of the monoclinic Pd3Bi2Se2 phase, the reliable extraction of peak width is not possible because of several peak overlaps. The Williamson−Hall equation was employed in the form βtot cos θ =
Kλ + Cε sin θ D
(4)
where D is the crystallite size, K is the shape factor (0.93), Cε is the strain term, and λ is the wavelength of Cu Kα1 radiation. The crystallite size and the lattice strain are estimated from the intercept and slope of the linearly fitted data obtained by plotting βtot cos θ versus sin θ as shown in Figure 2. No
Figure 2. Williamson−Hall plots and SEM images showing the morphology of the Pd3Bi2S2 samples after microwave irradiation for (a) 20 and (b) 100 min at 220 °C.
reflection groups seem to lie regularly above or below the mean straight line on the Williamson−Hall plot, suggesting that the particles of Pd3Bi2S2 are quite uniform in all crystallographic directions. The isotropic shape of the particles explains a rather low value of ∼10−3 for the fitted strain term Cε. From the calculations, the average crystalline size of the Pd3Bi2S2 particles after microwave irradiation for 20 min is 50 nm, whereas after microwave treatment for 120 min, it may be estimated to be 55 nm. Indeed, it has been revealed by scanning electron microscopy (SEM) that with an increase in synthesis time, a surface reorganization takes place instead of new nucleation events. However, instead of a uniform growth of all particles, we observed that single primary particles on the surface coalesce locally into larger formations with a spherical or more irregular shape (see the bottom inset in Figure 2). Figure 3 shows SEM images of the as-prepared Pd3Bi2S2 and Pd3Bi2Se2 samples after microwave irradiation for 20 min. Both powders consist of partially agglomerated primary particles with an average grain size of ∼50 nm. The Pd3Bi2S2 particles have a size distribution more homogeneous than that of the Pd3Bi2Se2 particles that might be related to synthesis peculiarities. In particular, it might be caused by usage of the selenium/ethylenediamine feedstock with an intrinsically inhomogeneous particle size distribution for the synthesis. The grain size revealed by SEM is in line with the value determined using XRD peak shape analysis. Quantitative analysis of the EDX spectra recorded on the surface of sintered pellets as well as as-prepared samples gave Pd:Bi:S and Pd:Bi:Se ratios very close to 3:2:2. EDX mapping was performed on particle conglomerates observed in
Figure 1. Rietveld fits of the XRD patterns for nanostructured Pd3Bi2S2 (top) and Pd3Bi2Se2 (bottom) samples (experimental data denoted as black crosses), including the profile fit (red solid line) and profile difference (blue solid line). The refined peak positions are indicated by black ticks. The structures are shown as insets.
Pd3Bi2Se2 samples synthesized by the microwave-assisted polyol method. Pd3Bi2S2 crystallizes in cubic symmetry (space group I213), with lattice parameter a = 8.3095(1) Å, whereas Pd3Bi2Se2 adopts a monoclinic structure (space group C2/m) with the following lattice parameters: a = 11.748(2) Å, b = 8.432(2) Å, c = 8.425(2) Å, and β = 133.96(1)°. The computed atomic coordinates and displacement parameters (see Tables S1−S3) show no significant discrepancies with respect to the previously published data.20,21 The reflections of the as-prepared compounds are broad, suggesting the formation of small particles. Particle size calculations employing the Scherrer equation33 yield average particle sizes of 34 and 30 nm for Pd3Bi2S2 and Pd3Bi2Se2, respectively. However, it has to be assumed that rapid sample heating by microwave irradiation generates an intrinsic stress so that the strain effect contribution to peak broadening might be significant. To study the microstructure in terms of both grain size and microstrain, we used the Williamson−Hall profile analysis34,35 for cubic Pd3Bi2S2. C
DOI: 10.1021/acs.inorgchem.6b01326 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. SEM images representing the shape details of the (a) Pd3Bi2S2 and (b) Pd3Bi2Se2 nanoparticles synthesized at 220 °C for 20 min.
the as-prepared samples (Figure 4). EDX analysis reveals the presence of uniformly distributed Pd, Bi, and either Se or S, suggesting that the samples are compositionally homogeneous without any regions related to impurities. Ensuing TEM studies of both samples strengthen the conclusions drawn above. Individual nanoparticles dispersed on a carbon grid (Figure 5b,e) exhibit uniform morphology and narrow size distribution (∼50 nm on average). The sulfide particles appear somehow smaller than the peer selenide ones, although the latter tend to form coral-like networks that impede shape and size estimation. Both samples are very homogeneous and do not show traces of any amorphous or crystalline impurities, or particles of any other shape. The ring diffraction patterns collected from particle bundles correspond undoubtedly to the title compounds (Figure 5c,f)
Figure 4. X-ray intensity maps collected on the surface of the (a−d) Pd3Bi2S2 and (e−h) Pd3Bi2Se2 particle agglomerates.
and demonstrate a high degree of sample crystallinity. The latter is further confirmed by high-resolution imaging of individual particles (Figure 5a,d) that revealed only a very thin amorphous layer (∼1 nm) on a specimen surface. This finding is very encouraging as it shows that quite crystalline metal-rich nanoparticles can be synthesized in fast, strongly kinetically driven microwave-assisted processes. D
DOI: 10.1021/acs.inorgchem.6b01326 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 5. Electron microscopy and diffraction for Pd3Bi2S2 (top) and Pd3Bi2Se2 (bottom). Sample overview (b and e) and HRTEM images of individual particles marked in red (a and d) are accompanied by ring diffraction patterns (c and f) of the respective particle pile. The indices are assigned according to the structure refinement in this work (see Table 1).
hindered by the small particle size. These entities hardly reach 100 nm in diameter, which makes acquisition of SAED patterns and tilting of single particles almost impossible. 3.3. Characterization of Nanostructured Bulk Materials. Bulk nanostructured materials have been obtained using the SPS procedure. Bars of Pd3Bi2X2 (X = S or Se) nanoparticles demonstrate silver-metallic shine and have a density of 7.21 g/cm3 (75% of theoretical density) for Pd3Bi2S2 and 7.47 g/cm3 (75% of theoretical density) for Pd3Bi2Se2. Notably, the measured densities of the as-sintered samples are only approximately 3−4 g/cm3. When the samples are heated to 200 °C with a 2 min hold time, the morphology of the individual nanoparticles can be preserved. However, the grain growth is not totally suppressed in sintering contrary to cold pressing. Moreover, the SPS procedure is believed to accelerate other mechanisms of densification such as mass transport and grain boundary and lattice diffusion due to usage of direct current pulses.36 3.4. Electrical Transport Measurements. For nanocrystalline Pd3Bi2Se2, we observe a superconducting transition at Tc = 0.71 K in zero field (Figure 6). Its superconducting critical temperature is lower than in the bulk compound (Tc = 0.96 K19). The transition temperature of nanosized superconductors may be affected by nanoscaling. For certain metallic elements, enhancement,37,38 negligible impact,6,39 and indeed reduction10,40,41 of Tc were observed. The latter can occur
Table 1. Indexing of Ring Diffraction Patterns ring number
d (Å)
hkl
Pd3Bi2S2 1 2 3 4 5 6 7 8
5.73 4.17 3.44 2.92 2.50 2.35 2.18 2.04
1 2 3 4 5 6 7 8
6.04 4.27 3.51 3.02 2.70 2.46 2.24 2.10
011 002 112 022 013, 103 222 123, 213 004 Pd3Bi2Se2 001, 200, 111, 002, 201, 022, 33̅ 1, 400,
110, 020, 1̅12, 220, 20̅ 3, 422 4̅21, 040,
1̅10, 20̅ 2 021, 2̅22, 310,
20̅ 1 22̅ 1, 3̅11 40̅ 2 130, ...
33̅ 2, 4̅23, ... 40̅ 4
The sulfide particles appear to be single crystals with a high degree of structural perfection, whereas the selenide particles may have less regular shapes and be subject to twinning and intergrowth. A more in-depth study of these phenomena was E
DOI: 10.1021/acs.inorgchem.6b01326 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 6. Resistivity vs temperature at various applied fields for the as-prepared nanoparticles of Pd3Bi2Se2. The inset shows the zero-field resistivity over a wider temperature range.
Figure 7. Phase diagram of nanocrystalline Pd3Bi2Se2 as compared with bulk. The solid lines are WHH fits.
for multiple reasons. First, the Debye temperature decreases with lattice expansion, which was observed in particular for Nb nanoparticles.40 This effect is not expected in our case because the lattice parameters for Pd3Bi2Se2 are found to be very close to the bulk values at room temperature. Second, a change in Tc can be discerned because of either surface effects or pairbreaking effects arising from the presence of paramagnetic impurities.10 Paramagnetic contributions can be excluded because if the Pd3Bi2Se2 powder is sintered, Tc is shifted back to its bulk value (see Figure S1, in which a partially superconducting transition occurs at 0.93 K). A decrease in Tc with a decrease in particle size was also reported for a weakly coupled disordered network of nanostructured Nb. Because of the finite size of the particles, the occurrence of discrete conduction electron energy levels leads to a changed density of states at the Fermi level. As a consequence, a reduced energy gap and, hence, a reduced Tc are observed as described by the discrete version of the BCS equation.42 This could be a possible scenario in the case of Pd3Bi2Se2 nanoparticles. The temperature dependence of the critical field Hc2(T) of nanosized Pd3Bi2Se2 can be described by a WHH approach for a type II superconductor in the dirty limit (electronic mean free path ≪ superconducting coherence length). It leads to Hc2(0) = 1.2(1) T, using a μ0Hc(T) slope near Tc of approximately −2.4(2) T/K. As a consequence, the critical field of our nanocrystalline sample is 4 times larger than that of bulk Pd3Bi2Se2 (Figure 7). This can be understood because the particle size is much smaller than the London penetration depth (estimated to be 1.26 μm at zero temperature19). Therefore, the magnetic field at the center position of a particle is weakened by only
