AgI Nanoparticles: Size-Controlled Stabilization of ... · α-AgI rapidly begins to transform into...
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100 AgI Nanoparticles: Size-Controlled Stabilization of Superionic Phase to Room Temperature Chemical Science Solid-state ionic conductors are actively studied for their large application potential in batteries and sensors. From the viewpoint of future nanodevices, nanoscaled ionic conductors are attracting much interest. Silver iodide (AgI) is a well-known ionic conductor whose high-temperature α -phase shows a superionic conductivity greater than 1 Ω -1 • cm -1 . Below 147℃, α -AgI undergoes a phase transition into the poorly conducting β - and γ -polymorphs, thereby limiting its applications. We have recently reported the facile synthesis of variable-size AgI nanoparticles coated with poly-N- vinyl-2-pyrrolidone (PVP) and the controllable tuning of the α - to β -/ γ -phase transition temperature (T c↓ ) [1]. When the size approaches 10-11 nm, the α - phase survives down to 30℃ - the lowest temperature for any AgI family material. Moreover, the conductivity of 11 nm β -/ γ -AgI nanoparticles at 24℃ is ~1.5×10 -2 Ω -1 •cm -1 - the highest ionic conductivity for a binary solid at room temperature. Ionic conductors are crucial for operating electrochemical devices, such as batteries and sensors. Although liquid phases principally give high ionic conductivity, solid-state materials are much more suitable from the perspectives of device fabrication (facile shaping, patterning and integration), stability (non-volatility) and safety (non-explosiveness). Silver-based ionic solids are excellent candidates of electrolytes for solid state batteries both because of their high conductivity and the high polarizability of Ag + ions that brings high exchange rate at the electrode reaction [2]. Actually, prototypes of all-solid batteries employing silver ionic conductors have been reported [3]. However, the performance of such devices is still in need of improvement for satisfying commercial applications. Efforts to obtain a high conductivity at lower temperatures have focused on the synthesis of ternary phases derived from AgI, of metal halide solid solutions, and of inorganic composite materials. An alternative way to attempt to stabilize the superionic phase at lower temperatures can be through the fabrication of nanoscaled AgI materials with controlled size and shape. For instance, when the size of materials is down to nanoscale, the temperatures of fusion and order-disorder phase transitions can be lowered as the proportion of surface atoms increases, resulting in the increase of surface energy. Our aim in this work was to investigate in a systematic way the size effect on T c in AgI nanoparticles without any pre-shaped inorganic mold, but with the application of a soft organic polymer as a stabilizer. We successfully synthesized PVP-coated AgI nanoparticles in a variety of particle sizes by a solution-based technique. Figure 1 shows the extracted size dependence of the phase transition temperatures from the differential scanning calorimetry (DSC) measurements of as- prepared AgI nanoparticles measured on cooling. The size dependence of the α - → β -/ γ -phase transition temperature T c↓ is very pronounced and T c↓ shifts considerably to lower temperatures with decreasing nanoparticle size, leading to a progressively enlarged thermal hysteresis. Specifically for the 10 and 11 nm nanoparticles, T c↓ is drastically suppressed down to 40 ± 1℃. Detailed insight into the phase transition behavior of AgI nanoparticles was obtained from the temperature-dependent synchrotron X-ray powder diffraction at beamline BL02B2. Figure 2(a) shows the temperature evolution of the α -AgI fraction for 11 nm nanoparticles during the first thermal cycle, as derived from Rietveld refinements of the diffraction data. The results clearly show the same huge hysteresis observed by DSC, as shown in Fig. 2(b). The dominant structure of the as-prepared AgI nanoparticles at 21℃ is the γ -polymorph (64.1(1)%). The β -polymorph (35.9(1)%) is also present. This phase assemblage remains stable on heating up to 138℃. Further heating leads to a gradual transformation to the α -phase that is complete at 165℃. Between 165 and 190℃, only α -AgI was observed. However, the β -/γ - → α -phase transition is strongly thermally hysteretic. On cooling, small fractions of β - and γ -phases appear initially at 128℃ and grow only slightly down to 42℃, where the 150 Bulk 147°C 100 β/γ-phase 50 0 0 10 20 30 40 50 Particle Size (nm) bulk nanoparticle T c↓ (°C) α α-phase Superionic state α-phase Superionic state ) Lowest T c (Agl family materials) Fig. 1. Size dependence of phase transition characteristics in AgI nanoparticles on cooling. The mean diameters of the nanoparticles were extracted from the TEM images to be 9.9±2.4, 11.3±3.8, 19.4±3.7, 23.6±7.3 and 41.2±7.8 nm.
Transcript of AgI Nanoparticles: Size-Controlled Stabilization of ... · α-AgI rapidly begins to transform into...
100
AgI Nanoparticles: Size-Controlled Stabilization of Superionic Phase to Room Temperature
Chemical Science
Solid-state ionic conductors are actively studiedfor their large application potential in batteries andsensors. From the viewpoint of future nanodevices,nanoscaled ionic conductors are attracting muchinterest. Silver iodide (AgI) is a well-known ionicconductor whose high-temperature α -phase showsa superionic conductivity greater than 1 Ω -1•cm-1.Below 147, α -AgI undergoes a phase transitioninto the poorly conducting β - and γ -polymorphs,thereby limiting its applications.
We have recently reported the facile synthesis ofvariable-size AgI nanoparticles coated with poly-N-vinyl-2-pyrrolidone (PVP) and the controllable tuningof the α - to β -/γ -phase transition temperature (Tc↓)[1]. When the size approaches 10-11 nm, the α -phase survives down to 30 - the lowest temperaturefor any AgI family material. Moreover, the conductivityof 11 nm β-/γ-AgI nanoparticles at 24 is ~1.5×10-2
Ω -1•cm-1 - the highest ionic conductivity for a binarysolid at room temperature.
Ionic conductors are crucial for operatingelectrochemical devices, such as batteries andsensors. Although liquid phases principally give highionic conductivity, solid-state materials are much moresuitable from the perspectives of device fabrication(facile shaping, patterning and integration), stability(non-volatility) and safety (non-explosiveness).Silver-based ionic solids are excellent candidatesof electrolytes for solid state batteries both because oftheir high conductivity and the high polarizability of Ag+
ions that brings high exchange rate at the electrodereaction [2]. Actually, prototypes of all-solid batteriesemploying silver ionic conductors have been reported[3]. However, the performance of such devices is stillin need of improvement for satisfying commercialapplications. Efforts to obtain a high conductivity atlower temperatures have focused on the synthesis ofternary phases derived from AgI, of metal halide solidsolutions, and of inorganic composite materials. Analternative way to attempt to stabilize the superionicphase at lower temperatures can be through thefabrication of nanoscaled AgI materials with controlledsize and shape. For instance, when the size ofmaterials is down to nanoscale, the temperatures offusion and order-disorder phase transitions can belowered as the proportion of surface atoms increases,resulting in the increase of surface energy.
Our aim in this work was to investigate in asystematic way the size effect on Tc in AgI nanoparticleswithout any pre-shaped inorganic mold, but with theapplication of a soft organic polymer as a stabilizer.
We successfully synthesized PVP-coated AgInanoparticles in a variety of particle sizes by asolution-based technique.
Figure 1 shows the extracted size dependence ofthe phase transition temperatures from the differentialscanning calorimetry (DSC) measurements of as-prepared AgI nanoparticles measured on cooling. Thesize dependence of the α - → β -/ γ -phase transitiontemperature Tc↓ is very pronounced and Tc↓ shiftsconsiderably to lower temperatures with decreasingnanoparticle size, leading to a progressively enlargedthermal hysteresis. Specifically for the 10 and 11 nmnanoparticles, Tc↓ is drastically suppressed downto 40 ± 1.
Detailed insight into the phase transition behaviorof AgI nanoparticles was obtained from thetemperature-dependent synchrotron X-ray powderdiffraction at beamline BL02B2. Figure 2(a) showsthe temperature evolution of the α -AgI fraction for11 nm nanoparticles during the first thermal cycle,as derived from Rietveld refinements of the diffractiondata. The results clearly show the same hugehysteresis observed by DSC, as shown in Fig. 2(b).The dominant structure of the as-prepared AgInanoparticles at 21 is the γ -polymorph (64.1(1)%).The β -polymorph (35.9(1)%) is also present. Thisphase assemblage remains stable on heatingup to 138. Further heating leads to a gradualtransformation to the α -phase that is complete at165. Between 165 and 190, only α -AgI wasobserved. However, the β -/γ - → α -phase transitionis strongly thermally hysteretic. On cooling, smallfractions of β- and γ-phases appear initially at 128and grow only slightly down to 42, where the
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Fig. 1. Size dependence of phase transition characteristicsin AgI nanoparticles on cooling. The mean diameters ofthe nanoparticles were extracted from the TEM images tobe 9.9±2.4, 11.3±3.8, 19.4±3.7, 23.6±7.3 and 41.2±7.8 nm.
α -phase is still the dominant structure (α -phase70.7(1)%, β -phase 18.4(1)%, and γ -phase 10.9(1)%)(Fig. 2(c)). Below this temperature, which coincideswith the sharp endothermic peak observed in DSC,α-AgI rapidly begins to transform into β-AgI but survivesdown to 30 (Fig. 2(a)). The structural transition iscomplete at 24.
Figure 3 shows the results of conductivitymeasurements for 11 nm AgI nanoparticles andAgI polycrystals. The huge thermal hystereticbehavior observed in DSC and diffractionmeasurements is also evident in the temperaturedependence of the conductivity σ . We also notethat the room-temperature conductivity foras-prepared 11 nm nanoparticles is significantlyincreased to ~1.5×10-2 Ω -1•cm-1. To the best of ourknowledge, this is the highest ionic conductivityreported for a binary solid at room temperature andis five orders of magnitude higher than that of AgI
polycrystals (~1.6×10-7 Ω -1•cm-1). On heating of theas-prepared nanoparticles, σ gradually increases anda notable change is observed at around 160, whereboth DSC and diffraction show a thermal event. Oncooling, σ remains very high, which is consistent withthe presence of superionic supercooled α-AgI whoseactivation energy is 0.06 eV, comparable to thatof bulk α -AgI (~0.05 eV). Finally, σ approaches thesame value as that in the heating process at 36.
The solution-based chemistry using PVP as astabilizer that has been employed here is sufficientlysuitable for isolating new families of nanoscaled(super)ionic conductors with controlled size andmorphology. This will facilitate a wide range offundamental studies about ionic mobility at thenanoscale at a previously inaccessible temperaturewindow and promises applications in nanobatteriesor other electrochemical devices.
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Rie Makiuraa,b,*,†, Hiroshi Kitagawaa,b,c,d and Kenichi Katoe
a Department of Chemistry, Kyushu Universityb JST-CRESTc Department of Chemistry, Kyoto Universityd INAMORI Frontier Research Center, Kyushu Universitye SPring-8 / RIKEN•JASRI
*E-mail: [email protected]
† Present address: Nanoscience and Nanotechnology Research Center, Osaka Prefecture Univ.
References[1] R. Makiura, T. Yonemura, T. Yamada, M. Yamauchi, R.Ikeda, H. Kitagawa, K. Kato and M. Takata: NatureMater. 8 (2009) 476.[2] S. Hull: Rep. Prog. Phys. 67 (2004) 1233. [3] B.B. Owens: J. Power Sources 90 (2000) 2.
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Fig. 2. Structural characterization of 11 nm AgI nanoparticles. (a) Temperature dependence of α-AgI fractions for the first thermalcycle, as extracted from Rietveld refinements of synchrotron X-ray diffraction data. (b) DSC thermograms for first thermal cycle.(c) Final observed (red circles) and calculated (blue solid line) synchrotron X-ray (λ = 0.5122 Å) powder diffraction profiles at 42following heating to 190 and subsequent cooling (agreement factors: Rwp = 2.30% and Rexp = 4.44%). The lower solid lines showthe different profiles and the tick marks show the reflection positions of α-AgI (top), β-AgI (middle) and γ-AgI (bottom) phases.
Fig. 3. Transport behavior of 11 nm AgI nanoparticles. Temperaturedependence of conductivity σ for first thermal cycle. The activationenergies derived using an Arrhenius expression are indicated. Theconductivity data for AgI polycrystals are also included for comparison.
