Enhanced Air Stability in REPb3 (RE = Rare Earths) by Dimensional

May 17, 2016 - We conceptually selected the compounds REPb3 (RE = Eu, Yb), which are unstable in air, and converted them to the stable materials in ...
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Enhanced Air Stability in REPb3 (RE = Rare Earths) by Dimensional Reduction Mediated Valence Transition Udumula Subbarao, Sumanta Sarkar, Rajkumar Jana, Sourav S. Bera, and Sebastian C. Peter* New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India S Supporting Information *

ABSTRACT: We conceptually selected the compounds REPb3 (RE = Eu, Yb), which are unstable in air, and converted them to the stable materials in ambient conditions by the chemical processes of “nanoparticle formation” and “dimensional reduction”. The nanoparticles and the bulk counterparts were synthesized by the solvothermal and high-frequency induction furnace heating methods, respectively. The reduction of the particle size led to the valence transition of the rare earth atom, which was monitored through magnetic susceptibility and X-ray absorption near edge spectroscopy (XANES) measurements. The stability was checked by X-ray diffraction and thermogravimetric analysis over a period of seven months in oxygen and argon atmospheres and confirmed by XANES. The nanoparticles showed outstanding stability toward aerial oxidation over a period of seven months compared to the bulk counterpart, as the latter one is more prone to the oxidation within a few days.

1. INTRODUCTION Designing nanostructured materials with different morphology is an important tool in the field of nanotechnology, as it gives an extra tool to manipulate the materials for unique magnetic, optical, catalytic, and transport properties. During the last two decades, considerable interests have grown in the fabrication of low-dimensional materials, such as nanorods,1 nanowires,2 nanotubes,3 nanospheres,4 and nanoplates,5 due to their potential applications in the disciplines of chemistry,6 physics,7 and medicine.8,9 A few notable technological applications of these recently developed materials are found in solar cell,10 water splitting,11 hydrogen generation,1 catalysis,12 photodetector,2 optical transmission,5 engineered electronic and magnetic phases,13 and drug delivery.14 However, use of several promising materials having desired chemical and physical properties in potential applications is limited due to their lack of stability in ambient conditions. For example, perovskitebased materials are proved with promising power efficiency of more than 15% in solar cell.15 Although these materials showed remarkable ability to convert solar energy to electrical energy, their stability under ambient conditions is still questionable due to their degradation into other chemical species in the presence of oxygen. High-temperature YBCO superconductors are another class of materials, in which the origin of the superconductivity was proposed due to the presence of trivalent copper atom. However, it is quite obvious that these materials would not survive in air and aqueous solution due the presence of unstable trivalent copper.16 The best currently known blood pool magnetic resonance imaging agents are based on iron oxides also found to be less stable in air.17 However, the issues in the stability of these materials come from the change of the oxidation states of different constituent © XXXX American Chemical Society

elements as the oxidation state of an element has significant effect on the Gibbs free energy (−ΔG°) of a system, which in turn is a direct measure of the stability of a material associated with it. This can be presented either by Latimer diagram or Frost diagram (valid for multivalent ionic species in extreme acidic or alkaline medium) in terms of volts equivalent (nE°) as −ΔG°/F = nE°. We calculated the nE° of three sets of metal family, namely, six transition metals, four p-block metals, and four rare earths in basic medium and plotted them against corresponding oxidation states assuming that addition of sodium borohydride (NaBH4), which was used as a reducing agent in ethylene glycol (EG), would give rise to alkaline medium. It is apparent from Figure 1 that a particular oxidation

Figure 1. Frost diagram on selected elements having the probability of multiple oxidation states; (a) transition metals, (b) p-block metals, and (c) rare-earth metals. Received: March 17, 2016

A

DOI: 10.1021/acs.inorgchem.6b00676 Inorg. Chem. XXXX, XXX, XXX−XXX

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Model 7590), first rapidly heated to 160 A (ca. 1100 K) and kept at that temperature for 30 min. Finally, the reaction was rapidly cooled to room temperature by switching off the power supply. The product could easily be removed from the tantalum tubes, and no reaction with the crucible material was observed. They are in polycrystalline form and light gray in color. The weight losses of the final material were found to be less than 1%. The samples obtained from the highfrequency induction heating method were used for the bulk property studies. Phase identity and purity of all the compounds were determined by powder X-ray diffraction (XRD) experiments that were performed with a Bruker D8 Discover diffractometer using Cu Kα radiation (λ = 1.5406 Å) over the angular range of 20° ≤ 2θ ≤ 80° at 300 K calibrated against corundum standard. The experimental powder patterns of all compounds and the XRD pattern simulated from the reported data were found to be in good agreement (Figures S2−S5). All stability tests were characterized by using powder XRD. Quantitative microanalyses on YbPb3 were performed with a FEI NOVA NANOSEM 600 instrument equipped with an EDAX instrument. Data were acquired with an accelerating voltage of 20 kV and a 100 s accumulation time. The EDAX analysis was performed using P/B-ZAF standardless method (where Z = atomic no. correction factor, A = absorption correction factor, F = fluorescence factor, P/B = peak to background model) on selected spots and points. The compositions obtained for the Yb/Pb ratio confirmed the Pb deficiency in nanomaterial compared to the bulk compound. The EDAX spectra of YbPb3 are shown in Figure S6. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) images, and selected area electron diffraction (SAED) patterns were collected using a JEOL 200 TEM instrument. Samples for TEM were prepared by sonicating the nanocrystalline powders in ethanol and dropping a small volume onto a carbon-coated copper grid. Magnetic measurements were performed using a Quantum Design MPMS-SQUID magnetometer. Measurements were performed on polycrystalline samples, which were ground and screened by powder XRD to verify phase identity and purity. Temperature-dependent magnetic susceptibility data were collected in the field cooled mode (FC) from 2 to 300 K in an applied field (H) of 10 and 1000 Oe. Field-dependent magnetization data were also collected for REPb3 (RE = Yb, La, and Eu) nanoparticles at 2 and 300 K with field sweeping from −50 to 50 kOe. XANES experiments at 300 K on YbPb3_NB, YbPb3_N, YbPb3_P, and YbPb3_C, EuPb3_B, and EuPb3_N were performed at PETRA III, P06 beamline of DESY, Germany. Measurements at the Eu and Yb LIII edges and at ambient pressure were performed in transmission mode using gas ionization chambers to monitor the incident and transmitted X-ray intensities. Monochromatic X-rays were obtained using a Si (111) double crystal monochromator, which was calibrated by defining the inflection point (first derivative maxima) of Cu foil as 8980.5 eV. The beam was focused employing a Kirkpatrick−Baez (K−B) mirror optic. A rhodium-coated X-ray mirror was used to suppress higher-order harmonics. A CCD detector was used to record the transmitted signals. The sample was prepared by mixing an appropriate amount of finely ground powder with cellulose and cold pressing them to a pellet. Thermogravimetric analyses (TGA) of YbPb3_B and YbPb3_N samples were recorded on PerkinElmer TGA analyzer. TGA was performed on the samples in Ar and O2 atmosphere (flow rate 40 mL/ min) with the heat rate of 10 K/min within the temperature range of 303−1000 K. The Latimer diagram for each of the elements in acidic media was taken from literature.26 The corresponding reduction potential for each redox pair was calculated using the equation, E = E° − 0.059 pH,27 where E = reduction potential and E° is standard reduction potential. In extreme acidic conditions, pH = 0, Ea = E°. All the potentials were calculated extreme basic conditions pH = 14 using Eb = Ea − (0.059 × 14). The microstructural strain in both the bulk and nano samples of YbPb3 from the corresponding PXRD patterns were calculated using Williamson−Hall (WH) method using the equation βr = Kλ/D cos θ + 4η tan θ.28,29 Here, βr is the resultant broadening after

state is more stable in aqueous medium (higher oxidation states in most of the cases) over others owing to the advantages in terms of hydration energy. Hence, we propose that a material can be stabilized in a given medium by modulating the oxidation states of its constituent elements by chemical route. To circumstantiate our proposal, we strategically synthesized the compounds REPb3 (RE = Eu and Yb) in nanodimension with different size and morphology using solvothermal method. REPb3 compounds crystallize in the cubic AuCu3 structure type with space group Pm3̅m (Figure S1), have been previously synthesized by high-frequency induction heating (HFIH).18 EuPb3 19 and YbPb3 20 were reported to be paramagnetic and diamagnetic, respectively. Among the rare earths, Eu and Yb have special interest due to their ability to exist in two energetically similar electronic configurations: the magnetic Eu2+ (4f7) and nonmagnetic Eu3+(4f6), the nonmagnetic Yb2+ (4f14) and magnetic Yb3+ (4f13). Because of the close proximity in terms of energy, Eu- and Yb-based compounds, in many occasions, show mixed valent/intermediate valent states.21−25 We used YbPb3 as for the complete study, then confirmed our observations briefly in EuPb3 and LaPb3. The compounds were characterized by various techniques, and probable mechanism of the size reduction and formation of various morphologies were proposed using the concepts of nanoparticle formation (NF) and dimensional reduction (DR). Magnetic measurement data showed the valence transition of divalent RE in bulk sample to trivalent RE in nanomaterial, which is later confirmed by X-ray absorption near edge spectroscopy (XANES) measurements. The stability of the material was checked over a period of seven months, and it was found that the nanomaterials were found to be stable in air without any decomposition. However, bulk compounds were decomposed into the oxides of RE and Pb within a short period of time.

2. EXPERIMENTAL SECTION The following materials were used for the synthesis: ytterbium(III) chloride (YbCl3), lanthanum(III) chloride (LaCl3), europium(III) chloride (EuCl3), and lead(II) acetate trihydrate (Pb(CH3COO)2· 3H2O) as solid salt precursors, ethylene glycol (EG) as solvent as purchased from Alfa Aesar with purity 99.99%. Polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) as surfactants and NaBH4 as the reducing agent purchased from Sigma-Aldrich with more than 98% purity. The compounds were synthesized by two methods. In solvothermal method, pure phase of REPb3 (RE = Yb, La, and Eu) nanoparticles was synthesized by the solvothermal method in the presence and absence of surfactants, where EG was used as solvent. An amount of 0.1 mol of YbCl3 (0.0279 g), LaCl3 (0.0245 g), EuCl3 (0.0258), and 0.3 mol of Pb(CH3COO)2·3H2O (0.1138 g) were taken in three different 25 mL Teflon lined stainless steel autoclave filled with EG ∼80% (20 mL). The surfactants (0.080 g) were added followed by NaBH4 in greater than twice stoichiometric amount into the solution. The autoclave was then immediately sealed and kept for heating at 493 K for four different reaction times of 12, 24, 36, and 48 h for YbPb3 and 24 h for LaPb3 and EuPb3. After the reaction was complete, the autoclave was cooled to the room temperature (300 K) naturally. The product obtained as black precipitate was washed thoroughly with ethanol several times by centrifuging at 8000 rpm for 5 min followed by drying under vacuum at 333 K for 8 h. All the characterizations were performed on dried powder samples. High-frequency induction heating was used for the synthesis of samples in polycrystalline with large particle size approximately in micron range. Rare earths and lead elements were mixed with three different reactions in the ideal 1:3 atomic ratio and sealed in tantalum ampules under argon atmosphere in an arc-melting apparatus. The tantalum ampules were subsequently placed in a water-cooled sample chamber of an induction furnace (Easy Heat induction heating system, B

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Figure 2. Surfactant-directed morphological evolution in the formation of YbPb3 nanoparticle. Unit cell of YbPb3. (a) Three-dimensional structure of YbPb3 in the absence of any surfactant. (b) TEM image of spherical nanoparticle without surfactant after 24 h (c) and 48 h (d). Surfactant-assisted dimensional reduction to one dimension crystal structure of YbPb3 (e), and TEM images of PVP (f) and CTAB (g) assisted growth of nanorods of YbPb3. subtracting from the instrumental broadening. The first term (Kλ/D cos θ) in the right-hand side is from Scherer equation, which accounts for broadening owing to crystallite size, while the second term (i.e., 4η tan θ) is the measure of broadening arising due to microstructural strain, and η is directly proportional to strain. Best-fitting parameters were obtained when Lorentzian and Gaussian functions were used for bulk and nano samples, respectively. Details of this procedure can be found elsewhere.30,31 Linear fitting of βr cos θ versus 4 sin θ curve yielded values of η, which were 5.6 × 10−5 and 5.3 × 10−4 for bulk and nano, respectively.

nanoparticle.43 The presence of additives or surfactant molecules used during the synthesis may further affect and sometimes complicate the magnetic properties of the system. Occurrence of ferromagnetic ordering in thiolate-modified gold nanoparticles is an ideal example in this context.44 In fact, YbPb3, to the best of our knowledge, is the first rare earth based intermetallic compound synthesized as nanomaterial in different morphologies. The TEM measurements and SAED patterns on the sample in the absence of surfactant (YbPb3_N) show that NF favored the formation of spherical nanoparticles. However, in the presence of the surfactants CTAB (YbPb3_C) and PVP (YbPb3_P), both NF and DR favored the formation of rod-shaped nanoparticles. In effect, the overall role of the surfactants may be conceived as an indirect scission along a particular direction(s) or plane(s) so that growth occurs in other direction(s). Figure 2a shows the representative unit cell of YbPb3. In the absence of surfactants, the growth of nanoparticle on the seed happens along all the planes with equal probability (Figure 2b). The particle size measured from the TEM for the sample obtained after 24 h of reaction is ∼16 nm (Figures 2c and S7). As the reaction time extends the nucleation and growth continued in all directions resulting in larger particle size (Figure 2d). The particle size for the sample obtained after 48 h of reaction is ∼45 nm as shown in Figure S8. These values are corroborating with the crystallite size obtained using Scherer formula on the powder XRD data with 24 and 51 nm, respectively (Table S2). In the presence of surfactants PVP and CTAB, the NF and DR resulted in one-dimensional nanorods of ∼40 and 20 nm width (Figure 2e). The nanorods obtained with PVP and CTAB are shown in Figure 2f,g. The mechanism of CTAB-triggered nanorod formation in aqueous solution (a polar solvent) has been previously reported.40,45 A similar mechanism can be proposed for the YbPb3 nanorod formation, as EG is highly polar solvent. In polar solvents at high temperature CTAB forms a double-layer structure,46 which acts as a soft template for the nucleation and growth of YbPb3 nanorods in (100) direction, which is also clear from the

3. RESULTS AND DISCUSSIONS 3.1. Reaction Chemistry and Nanocrystal Growth. In solvothermal method, pure phase of YbPb3 was obtained by varying different parameters such as time intervals of the reaction, surfactants, and solvents. Since the standard reduction potentials of Yb3+ and Pb2+ are −2.19 and −0.13 V,32 respectively, Pb2+ is more easily reduced to Pb° in the solution compared to the reduction of Yb3+ to Yb0. Figures S2−S5 show the XRD patterns of YbPb3 synthesized under different reaction conditions, which matched well with the simulated pattern obtained from the reported crystallographic data.33 The slight shift of all peaks toward the lower 2θ angle indicates 8% deficiency in Pb content, which was calculated from the Reitveld refinement of the XRD pattern and later confirmed by the energy-dispersive X-ray (EDX) measurements (Figure S6 and Table S1). Variation of surfactant is one of the most important parameters in controlling the size and morphology of the nanoparticles.34−40 There are two distinct cases in the NF processes: the first one is the occurrence of magnetism in the nanoparticles with nonmagnetic in their corresponding bulk phases as noticed in several nonmagnetic inorganic oxides ZnO, SnO2, In2O3, HfO2, CdS, GaN, and NbN.41,42 The presence of oxygen, sulfur, and nitrogen vacancies have been asserted as the cause for the “universal” occurrence of ferromagnetism in these type nanomaterials, which are diamagnetic in their bulk phases.41 In the second case, there is a switching from one type of magnetic phenomenon to the other as noticed in Mn C

DOI: 10.1021/acs.inorgchem.6b00676 Inorg. Chem. XXXX, XXX, XXX−XXX

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increased for the sample with PVP, which suggests that the growth occurs along the (100) direction. This is very wellsupported by TEM, as there is no change in the width of nanorods (∼40 nm) in the case of samples with PVP (Figure S9a). However, interestingly, this ratio with CTAB remains close for 24 and 48 h samples, which was further supported by TEM, as there is a 5 times enhancement in the width of nanorods from 20 nm for the 24 h sample to ∼250 nm for 48 h sample (Figure S9b) with a proportional enhancement in the length of the crystals. HRTEM (Figures 3) clearly shows the lattice fringes suggesting that the crystal growth occurs in a particular direction. In the case of spherical sample (YbPb3_N), the dspacing calculated is 0.273 nm which suggests that growth occurs along the (111) direction, while in the case of nanorods (YbPb3_P/C), the calculated d-spacing of 0.490 nm confirms the crystal growth along (100) direction. The SAED patterns confirm highly ordered crystalline state, and the presence of only one set of diffraction spots in both samples (Figure 3c,d) indicates that unique growth direction as a result of NF and DR (Figure 4). PVP is known to preferentially bind with (110) planes of the polyhedral seeds. It is proposed that these seeds orient along (100) plane, which is more accessible due to the presence of relatively lesser number of additive molecules at high temperature.49 This mechanism can also explain the inhibition of lateral growth of the nanorods synthesized at higher reaction time in the presence of PVP as a result of selective blocking of (110) planes by the additive. CTAB, however, is known to form double-layer micellar tubes, which act as a soft template toward the synthesis of rodlike morphology.50 This ability in fact leads to an increase in the width of nanorods as the reaction time increases. 3.2. Magnetism and Valence Fluctuation. Anticipating valence fluctuations of Yb and surface magnetism as proposed earlier for several rare earth oxides, we performed magnetic measurements on all compounds at an applied field of 1000 Oe (Figure 5). The diamagnetic nature of bulk YbPb3 (YbPb3_B) reported earlier18 was confirmed by our magnetic measurements (Figure 5a). The negative magnetic susceptibility at

HRTEM obtained from rod (Figure 3b). Similarly, PVP acts as a surface-regulating polymer to control rodlike morphol-

Figure 3. HRTEM images for YbPb3_N24 (a) and YbPb3_P24 (b) and the corresponding SAED patterns in (c) and (d).

ogy.47,48 PVP has a polyvinyl skeleton with polar group (N− CO), which may be preferentially absorbed on the faces parallel to the (100) axis direction47 of the YbPb3 seeds, leading to the growth along (100) direction forming rodlike morphology (Figure 2f). The HRTEM and SAED patterns for the selected sphere and rod-shaped nanocrystals are shown in Figure 3. The intensity ratio of the planes (111) to (200) in the powder XRD pattern was calculated for the samples synthesized after 24 and 48 h. As expected the ratio remains the same for the sample without surfactant but proportionally

Figure 4. Cartoon presentation of the crystal growth at 24 and 48 h of (a)YbPb3_N, (b) YbPb3_P, and (c) YbPb3_C. D

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Figure 5. (a) Temperature-dependent magnetic susceptibility (χm) of YbPb3 at 1000 Oe, synthesized in bulk form. (inset) The magnetization as a function of applied magnetic field at 300 K for YbPb3 bulk materials. (b) Temperature-dependent magnetic susceptibility (χm) and inverse susceptibility (1/χm) of YbPb3_N, YbPb3_P, and YbPb3_C at 1000 Oe. The Curie−Weiss fitting is shown as red line. The estimated magnetic moment is included in each plot. (c) Magnetization as a function of applied magnetic field at 300 K YbPb3_N, YbPb3_P, and YbPb3_C and (d) LIII absorption edge spectra of Yb for YbPb3_N, YbPb3_P, and YbPb3_C for 24 h sample.

the Yb atom. To prove the actual valence state of Yb in nanomaterials, XANES measurements were performed at the Yb LIII-edge at 300 K and ambient pressure (Figure 5d). The spectra show only one absorption peak centered at ∼8948 eV for all samples attributing to Yb3+ atoms54,55 and clearly confirms the absence of Yb2+ or mixed valence behavior in all nanosamples. This valence transition can be triggered by several reasons: the reduction in the size of the particles, surface magnetism, and the deficiency at the atomic sites. In case of Yb-based nanomaterials, the reduction in dimension leads to reduction in valence state due to loss of nearest neighbors.56 The enhanced magnetic behavior of YbPb3_N24 (4.98 μB/Yb) can be attributed due to the presence of uncorrelated paramagnetic spins at the surface that undergo a magnetic transition to a collective spin state at low temperatures yielding the large measured magnetization values and related to broken exchange bonds and reduced coordination numbers at the surface.57 The presence of surfactants suppresses the paramagnetic behavior compared to the surfactant-free sample probably because they compensate the unfulfilled surface spins by donating electrons fully or at least partially. The reduction can also be directly connected to the particle size and surface area. Nanomagnetism was previously reported in many systems originating from different surface states.56,58 As mentioned in the previous section, the particle size of YbPb3_N24 increased from 16 to 45 nm in YbPb3_N48. Since the particle size is larger for YbPb3_N48, the surface area to volume ratio becomes smaller and reduces the contribution of surface magnetism (Figure S11). The role of surface magnetism was further proved from the magnetization curve of all samples at 300 K in low magnetic field with the presence of small hysteresis loop (Figure S12). Since La is expected to have only one oxidation state (La3+), we also synthesized LaPb3 both bulk and nano forms (Figure S13). Controlled magnetic measurements with slight enhancement in the magnetic moment (Figure S14) in the susceptibility data

room temperature (−0.0015 emu/mol) and negative slope in field-dependent magnetization data at room temperature (inset in Figure 5a) and at 2 K (Figure S10) suggest that YbPb3_B is purely diamagnetic with divalent Yb. However, temperature-dependent inverse susceptibility (Figure 5b) curve above 100 K for all nanoparticles obey Curie−Weiss law, χ(T) = C/(T − θp)51,52 where C is the Curie−Weiss constant (NAμeff2/3KbT) and θp is the Weiss temperature. A fit to the curve in high-temperature range resulted in an effective magnetic moment of 4.98, 4.18, and 3.6 μ B /Yb atom for YbPb 3 _N, YbPb 3 _P, and YbPb 3 _C, respectively, suggesting the existence of Yb3+ in all these materials. These results confirmed the fact that scaling the size from micro to nanoregime triggered the valence fluctuation of nonmagnetic divalent Yb to magnetic trivalent Yb. The estimated experimental μeff value for YbPb3_N is ∼10% more than that of expected for a free ion Yb3+ moment (4.56 μB/Yb), which can be attributed to other parameters, mostly surface magnetism or deficiency also contributing to the total magnetic moment in addition to trivalent Yb. This is in fact corroborating with 8% of Pb deficiency observed in XRD and EDX measurements (Table S1), but the possible role of surface magnetism cannot be ruled out completely. In addition, the estimated experimental μeff value for YbPb3_P and YbPb3_C are, respectively, 9% and 21%, lower than that of expected value of Yb3+can be attributed to crystal field contributions. The transition from Yb2+ in bulk material to Yb3+ in nanomaterials was also confirmed from the magnetization measurements (Figure 5c). It is well-understood that the surface spin of the magnetic system will be enhanced with the reduction of dimensionality from bulk to nano scale,53 which in fact changes the magnetic properties of these materials. In bulk phase, the spins of the sublattices responsible for the bulk magnetic properties are well-compensated, whereas NF process not only leaves many uncompensated spins on the surface of the particles but also favors the valence transition of E

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Figure 6. Stability tests for bulk and nanoparticles of YbPb3. (a) PXRD of YbPb3_B in the presence of air over a period of 100 d, (b) PXRD of YbPb3_B in Ar and air conditions over a period of 15 d, (c) PXRD of YbPb3_N in the presence of air over a period of 100 d, (d) PXRD of YbPb3_N in Ar and air condition over a period of 30 d, and (e) TGA on YbPb3_B and YbPb3_N in the presence of argon and oxygen atmosphere.

from the system probably due to the strain or unreacted Pb present in the system as observed in the magnetic measurements. To understand the leaching of Pb from YbPb3 nanoparticles, we quantified the microstructural strain in both the bulk and nano samples from corresponding PXRD patterns using WH method (Figure S18).28,29 The calculations showed that YbPb3_B has almost 9.5 times strain compared to that of YbPb3_N. This excessive strain may trigger valence transition from Yb2+ to Yb3+, which in turn may give rise to shortening in Yb−Pb bond distances leading to dissolution of Pb from the system. It is widely expected that the surface of the nanoparticles is more prone to the oxidation compared to the bulk materials. However, induced valence transition of Yb in YbPb3 gives more stability toward aerial oxidation. The stability was further checked by TGA in the presence oxygen and argon within the temperature range of 303−1000 K (Figure 6e). In the presence of Ar, YbPb3_B shows negligible difference until 1000 K, but the material started to gain weight with oxygen at 373 K due to the oxidation of Yb. The total weight gain of YbPb3_B in the presence of oxygen during the temperature range of 373−900 K is ∼6%. On the contrary, the presence of PbO in the PXRD of YbPb3_B after TGA measurements in the presence of oxygen at 1073 K is probably due to the oxidation of unreacted or dissoluted Pb (Figure S19a). However, the TGA plot for YbPb3_N is completely different, as it does not show any weight gain both in oxygen and argon atmosphere, and hence we rule out the oxidation of Yb. Instead, a weight loss of 8% and 5% in the temperature range of 303− 800 K in the presence of Ar and O2, respectively, can be attributed to the dissolution of Pb, followed by the formation of PbO and finally decomposition to Pb2O3 (Figure S19b). However, there is no gain in weight for the sample in the presence of Ar after 800 K confirms no oxidation has occurred. We also noticed that surfactants give extra stability toward the aerial oxidation of the materials, as the compounds YbPb3_P and YbPb3_C have not decomposed neither to the oxide of Yb nor Pb over a period of five months (Figure S20). The stability and valence transition was further confirmed by X-ray photoelectron spectroscopy (XPS) measurements. XPS spectra of YbPb3_B shows both Yb3+ and Yb2+ are present, which may be because of the oxidation of YbPb3_bulk during experimental and/or measurement procedure, as YbPb3_bulk is highly sensitive to aerial oxidation. However, all the nanocompounds contain only Yb3+ states (Figure S21). Even after 150 d, there is almost no shift in binding energy of Yb3+ in YbPb3_N, which clearly indicates that there is no change in the coordination

confirm the occurrence of some surface contribution toward the enhanced magnetic moment in these nanomaterials. Note that this kind of valence transition also occurs in many oxides due to the deficiencies in O sites, such as CuFeO2+δ.59 Similarly, YbPb3 nanoparticles also show deficiency at Pb position, which further concludes that the role of deficiency cannot completely be ruled out. Although, we could not find the trace of elemental Pb in powder XRD, as powder XRD cannot detect the presence of negligibly small amount of elemental Pb. But magnetic susceptibility measurements at lower field (10 Oe), which is a much more sensitive technique (sensitivity: 2 × 10−14T Hz )60 than powder XRD, resulted in demagnetization at 7.2 K (Figure S15), which is the superconducting temperature of Pb metal, proving the presence of elemental Pb.61 However, notice that the extent of demagnetization for the nanoparticles is higher than that for bulk, suggesting more unreacted Pb in the total system. From the various controlled studies, we conclude that the origin of magnetism via valence transition of Yb is due to the combined effect of reduction in the size of the particles, surface magnetism, and the deficiency at the atomic sites. 3.3. Air Stability. The reactivity of an atom apparently depends on the oxidation state and size of the ion. Since Yb has the ability to exist in two different oxidation states, their reactivity should also be different. The valence transition of Yb is energetically favorable, but Yb2+ is a more stable valence state in comparison to Yb3+ because of substantial decrease in strain and higher exchange coupling interaction in Yb2+.22 However, the nonmagnetic Yb2+ has ionic radius of 194 pm,62 which is larger than the magnetic Yb3+ having an ionic radius of 174 pm.62 Because of large size, the reactivity of Yb2+ is expected to be more than Yb3+. In other words, the compounds with Yb2+ are expected to react more with oxygen compared to the trivalent compounds. To check this, we performed PXRD measurements on YbPb3_B (divalent) and YbPb3_N (trivalent) samples over a period of five months. The PXRD of bulk sample contains YbO0.5, which confirmed the oxidation of the sample (Figures 6a,b and S16), but nanomaterials remain stable without any peak corresponding to YbO0.5 (Figures 6c,d and S17). Though in case of nanomaterial except from aerial oxygen, incorporation of the same may happen from oxygencontaining solvent (EG) as well. But still there is no detectable presence of YbO0.5 even after several days indicating high stability of the YbPb3 nanomaterial. Surprisingly, the PXRD of nanomaterials contains small peaks at 2θ = 28.6°, 31.9°, and 48.7° corresponding to PbO (Figure S17). This can be explained as the oxidation of Pb, which was either slowly etched F

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Figure 7. (a) Temperature-dependent magnetic susceptibility (χm) of EuPb3 at 10 Oe, synthesized in bulk (EuPb3_B) and nano without surfactant (EuPb3_N). (inset) The susceptibility data of EuPb3_N sample at low temperature to confirm the negative values. (b) Magnetization as a function of applied magnetic field at 300 K for EuPb3_B and EuPb3_N. (inset) The same in the lower-field range for better clarity. PXRD comparison EuPb3_bulk and nano comparing a period of 40 d in air condition (c) EuPb3_B and (d) EuPb3_N.

Figure 8. Time-dependent Yb and Eu LIII absorption edge spectra for (a) YbPb3_B, (b) YbPb3_N, (c) EuPb3_B, and (d) EuPb3_N samples over a period of seven months.

environment of Yb3+ ions even after 150 d. Therefore, YbPb3 in nanodimension is highly stable without the formation of detectable amount of ytterbium oxide (Figure S22). 3.4. Control Studies. To ascertain our hypothesis, the induced valence transition of Yb toward the air stability, we synthesized the prototype compound EuPb3 both in micro (EuPb3_B) and nano (EuPb3_N) crystalline forms (Figure S23). As reported before, Eu is divalent in EuPb3_B and orders anti-ferromagnetically at 20 K.19 EuPb3_N obtained is of uniform particle size of ∼150 nm as observed in TEM

measurement (Figure S24). Temperature-dependent magnetic susceptibility confirmed that EuPb3_B is paramagnetic with divalent Eu, while EuPb3_N is diamagnetic with trivalent Eu (Figure 7a,b). The stability of these two forms was checked in oxygen and argon atmosphere over a period of five months. As expected from our hypothesis, EuPb3_B decomposed gradually (Figure 7c), and EuPb3_N remains stable (Figure 7d). Interestingly, EuPb3_B was decomposed to PbO instead of Eu2O3 (Figure 7c), which is probably due to relatively more stable Eu2+ compared to Pb ion. The stability of the samples G

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Inorganic Chemistry toward oxygen was further checked over a period of seven months using XANES measurements and shown in Figure 8. The near-edge spectra for YbPb3_B obtained at 300 K and at ambient pressure (Figure 8a) showed a main absorption peak (white line resonance) of the spectrum for both spectra is centered at ∼8941.5 eV, which is attributed to divalent Yb atoms.63−65 The spectrum also revealed the presence of a weaker feature at ∼8949 eV, indicating that some trivalent Yb is also present.63−65 However, over a period of time, the lowenergy peak originating from the Yb2+ ions is gradually decreased, whereas the high-energy one originating from Yb3+ is enhanced. The amplified intensity at 8949 eV and concomitant decrease in intensity at 8941 eV implies conversion of Yb2+ to Yb3+ upon time and almost completely converted after seven months. A similar trend was observed for EuPb3_B sample, where the complete conversion of divalent Eu to trivalent Eu occurred after five months. However, the nanomaterials YbPb3_N and EuPb3_N, respectively, showed only one peak corresponds to the trivalent oxidation state of RE, and no change was observed over a period of seven and five months (Figure 8b,d).

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AUTHOR INFORMATION

Corresponding Author

*Phone: 080-22082998. Fax: 080-22082627. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Department of Science and Technology (DST) (Grant SR/S2/RJN-24/2010), Sheikh Saqr Laboratory and Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) is gratefully acknowledged. U.S., R.K.J., and S.S. thank Council of Scientific and Industrial Research for research fellowship, R.K.J. thanks JNCASR and DST for research fellowship and S.S.B. thanks JNCASR for summer student research fellowship. We are thankful to DST and Saha Institute of Nuclear Physics and JNCASR for funding and DESY, Germany, for the XANES beamline, and Dr. G. Falkenberg for helping the measurements. We are grateful to Prof. C. N. R. Rao for his constant support and encouragement.

4. CONCLUSIONS In conclusion, we have been motivated in the development of air-stable materials as probable promising materials for various applications in the field of chemistry, physics, biology, and engineering. Although several methods have been employed for this objective, not a single method dealt with chemical transformation of metal in a molecule/compound from its one oxidation state to more stable one. Knowing this, we have selected a few metals that are known to exist in various oxidation states and compared the stability of each one using standard well-known Frost diagram. Among them, we have chosen a couple of rare earth elements, Eu and Yb, which are known to exist in both divalent and trivalent oxidation states, and strategically targeted a simple cubic compound of formula REPb3 crystallizing in AuCu3 structure type, for the detailed studies. We have employed a new and facile synthetic strategy for synthesis of these materials in nano form. The formation of different morphologies in nanoscale has been assisted with the concepts of NF and DR in connection with the concept of crystallography. Although various parameters such as particle size, presence of surfactants, deficiency of metal, and surface states could play the crucial roles, our controlled studies confirmed that the valence transition of RE metals is responsible for the stability of these materials toward air. Our work is expected to give good input on the synthesis of future air-stable materials for various scientific and technological applications.

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Oe and for 48 h in the field of 1000 Oe for YbPb3 and 1000 Oe for LaPb3, stability test by PXRD of YbPb3 plots and microstructural strain calculation of YbPb3, and XPS spectra. (PDF)

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REFERENCES

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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.inorgchem.6b00676. Powder XRD pattern of nanomaterials compared with the simulated pattern for the samples synthesized at 12, 24, 36, and 48 h for YbPb3 and 24 h for LaPb3 and EuPb3, TEM images for the samples YbPb3_P and YbPb3_C synthesized at 48 h and EuPb3 for 24 h, temperature-dependent magnetic susceptibility data for the sample synthesized at 24 h, in field of 1000 and 10 H

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