9R Structures - Journal of The

Journal of The Electrochemical Society, 154 共9兲 K51-K60 共2007兲

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0013-4651/2007/154共9兲/K51/10/$20.00 © The Electrochemical Society

AgI Nanoplates in Unusual 7H/9R Structures Highly Ionically Conducting Polytype Heterostructures Yu-Guo Guo,a,z Jong-Sook Lee,b Yong-Sheng Hu,a and Joachim Maiera,* a

Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany Chonnam National University, Gwangju 500-757, Korea

b

Homogeneous AgI nanoplates 300 nm in diameter and 50 nm in thickness can be prepared by a solution-based route using poly共diallyldimethylammonium chloride兲 as capping agent. The formation of unusual 7H/9R polytype modifications is the origin of the huge room temperature superionic conductivity and the large hysteresis in the phase transition with respect to the superionic high-temperature phase ␣-AgI. All these properties indicate a substantial Ag disorder in the 7H/9R polytype AgI unlike the normal ␤-AgI in wurtzite structure. The Ag disorder in 7H/9R polytype structure can be formally understood in terms of an equilibrium redistribution of charge carriers at heterojunctions in ionically conducting zinc blende 共␥-AgI兲/wurtzite共␤-AgI兲 heterostructures in the subnano regime. Upon martensitic transformation of ␣-AgI with its body-centered-cubic iodine lattice, the 7H/9R stacking sequence rather than the 2H sequence of ␤-AgI occurs in the presence of Ag+ attracting surface adsorbents of Cl−. The usability of these materials as electrolytes in rechargeable all-solid-state Ag batteries working at 100°C was demonstrated by using Ag as anode and TiS2 as cathode. © 2007 The Electrochemical Society. 关DOI: 10.1149/1.2750514兴 All rights reserved. Manuscript submitted December 18, 2006; revised manuscript received January 31, 2007. Available electronically July 9, 2007.

The high-temperature modification of silver iodide, ␣-AgI, is a prototype “superionic” conductor. Owing to the large degree of disorder in the silver sublattice being responsible for the high ionic conductivity 共⬃10−1 ⍀−1 cm−1兲, with the body-centered-cubic 共bcc兲 iodine lattice staying rigid, this state is often referred to as “quasimolten” or “liquid-like.” Below 147°C at ambient pressure silver iodide transforms to the wurtzite structure 共␤-AgI兲, in which the silver ions as well as the close-packed iodine ions are ordered according to a hexagonal crystal symmetry; a considerably smaller amount of silver disorder 共of the order of 10−4兲共Ref. 1兲 when compared to the high-temperature situation, as well as high-barrier migration pathways,2 explain the moderate ionic conductivity 共10−6–10−7 ⍀−1 cm−1 at T = 25°C兲. Zinc blende structured ␥-AgI is often found to coexist with ␤-AgI in commercial AgI powders and also easily formed, e.g., by grinding or compressing ␤-AgI powders or quenching ␣-AgI material. The metastability of this phase is reflected by the nonexistence of macroscopic-scale single crystals and in particular by the irreversible transformation to ␤-AgI around 90–100°C on heating. Recently, Guo et al.3 prepared well-defined ␥-AgI nanotetrahedra, which, surprisingly, are stable upon thermal cycles up to 180°C, which involves transformation to and from the high-temperature ␣-AgI phase. The high magnitude of the reproducible conductivity of the ␥-AgI nanotetrahedra3 共⬃10−5 ⍀−1 cm−1 at T = 25°C兲 is not explainable through the advantage of three-dimensional transport pathways in comparison to the situation in ␤-AgI;2,4 rather, it should be attributed to a high degree of Ag disorder. The occurrence of both wurtzite and zinc blende structure is also met in some AnB8−n compounds with pronounced polytypism such as SiC and ZnS, and in fact, several polytypes of AgI had been identified in as-grown single crystals of AgI 共see the review by Prager5兲 such as 4H, 8H, 12H, 16H, 80H, etc. It is notable that 共i兲 they are all of even-numbered periods and in hexagonal or pseudohexagonal symmetry, and 共ii兲 they transform into ␤-AgI 共2H兲 structure irreversibly at moderate temperatures of 60 to 100°C, well below the ␣-␤ transition temperature of 147°C. An unusual, oddnumbered period polytype in trigonal symmetry, 7H-AgI, was identified by Davis and Johnson6 in fine powders dispersed in fiberglass filter in conjunction with aerosol research for ice nucleation seed. This unusual 7H modification of AgI was found to be responsible for the extremely high conductivity enhancement and the phase transition with a large hysteresis in AgI:alumina composite electrolytes.7

* Electrochemical Society Active Member. z

E-mail: [email protected]

Unlike the ideal 共semi-infinite兲 space-charge effects at the halide/ alumina interface that quantitatively explained the conductivity enhancement for the composites of other silver halides 共AgCl, AgBr兲,8 the huge conductivity enhancement in AgI:alumina composites9 deserved special consideration. Lee et al.10 found the coexistence of a new polytype of AgI, 9R-AgI, in the AgI:alumina composites as well as in the specimen prepared according to Davis and Johnson. The 9R structure has been observed in various layered compounds as well as in some metals and alloys, but it is not a common polytype in AnB8−n compounds: Only a few singular cases have been reported to the best of the authors’ knowledge, viz. SiC,11 ZnS,12 and GaN,13 and none of them seems to have been prepared as macroscopically stable phase. Except for one report of 7H–Ba7Nb4MoO20,14 hardly any other example of 7H stacking structure can be found. Recently, Guo et al.17 prepared nanocrystals of AgI in large scale by a solution-based route followed by a annealing procedure under an argon environment. The nanoparticles consist of those exotic polytypes of AgI, i.e., 7H and 9R, and exhibit extreme conductivity 共⬃2.2 ⫻ 10−3 ⍀−1 cm−1 at T = 25°C兲 and large hysteresis in phase transition similarly as observed in AgI:alumina composites 共⬃7 ⫻ 10−4 ⍀−1 cm−1 for 40 mol % alumina content at T = 25°C兲.7 Unlike the case of the composites, AgI nanoparticles with a welldefined morphology can be separated and individually characterized. In this way, an astonishing conductivity isotropy of the material has been revealed.17 The purpose of the present contribution is first to tackle thoroughly the stability and the formation mechanism of these unusual polytypes of AgI. Both as-prepared samples and annealed ones are investigated in detail. Second, in order to explain the extraordinary conductivity effects, the concept of ionically conducting polytype heterostructures is used, being the first counterpart of the semiconducting polytype heterostructures such as SiC,18,19 ZnS,20 CdSe,21 CdTe, etc. 共see Fig. 1兲. Finally, the usability of these AgI nanoplates as electrolytes in rechargeable all-solid-state Ag batteries is demonstrated by using Ag as anode and TiS2 as cathode. Experimental Preparation of materials.— Poly共diallyldimethylammonium chloride兲共PDADMAC, 20 wt % in water, Mw ca. 100,000– 200,000兲, AgI 共99.5%兲, and NH4I 共99%兲 were purchased from Sigma-Aldrich Chemical Co. KI 共99.8%兲 and AgNO3 共99.8%兲 were obtained from Merck KGaA, ethanol and acetone used as solvents from Carl Roth GmbH. All these chemicals were used directly without further purification. AgI nanoplates were synthesized by the procedure reported previously.17 Briefly, an aqueous solution of PDAD-

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Figure 2. 共Color online兲共a兲 Topographic and 共b兲 phase AFM image of AgI nanoparticles dispersed on a silicon wafer. 共c兲 The section analysis of a single particle clearly indicates the plate shape of the nanoparticle of 共d兲 50 ± 2 nm in thickness.

Figure 1. Classification scheme of heterostructures. Upper row represents conventional heterostructures composed of two different materials in the same crystal modification such as GaAs/AlAs in zinc blende structure, GaN/AlN in wurtzite structure, or Si/Ge in diamond structure. The ionic counterpart is MBE-grown CaF2 /BaF2 heterostructure15,16. The lower row represents polytype heterostructure composed of the same material in different crystal modification, i.e., zinc blende/wurzite of SiC, ZnS, CdSe, CdTe, etc. Rather unique example of ionic counterparts is AgI material of the current work.

MAC 共10 mL, 20 wt %兲 was added to an ethanol solution of KI 共150 mL, 0.033 M兲 under vigorous stirring at room temperature. Then, AgNO3 共50 mL, 0.1 M兲 in an ethanol-water mixed solvent 共1:1 in volume兲 was added, and after 60 min stirring, the mixture was left to rest for up to 6 days at room temperature before the precipitates were collected for characterization. The precipitates were carefully washed repeatedly with double-distilled water and then dried at 50°C for 24 h. The annealing procedure was carried out at 180°C under Ar environment for 5 h. AgI in the fiberglass filter was prepared according to Ref. 6: a fiberglass filter paper 共Whatman GF/C兲 was soaked in AgI–NH4I acetone solution 共mole ratio AgI:NH4I = 2:1兲, heated up to 550°C, and quenched. Characterization.— The morphologies of AgI nanoplates before and after annealing were investigated by using a TopoMetrix scanning probe microscope 共TMX 2000兲. X-ray diffraction 共XRD兲 measurements were carried out with a Philips PW3710 using filtered Cu K␣ radiation. In situ XRD measurements were employed on a Philips PW302 in the temperature range 25°C ⬍ T ⬍ 180°C. Temperatures were kept constant during the scanning of each pattern, which took around 1 h. Differential scanning calorimetry 共DSC兲 was carried out on Setaram DSC 121 under Ar atmosphere in different temperature ranges of 25°C ⬍ T ⬍ 180°C, 25°C ⬍ T ⬍ 300°C, 25°C ⬍ T ⬍ 400°C, and 25°C ⬍ T ⬍ 500°C. For electrical measurements the pellets of AgI nanoplates were prepared by pressing the powder uniaxially at 5400 kg cm−2 into disks of 6 mm diameter, and films by a drop-dry method on quartz slides with the size of 10 ⫻ 10 mm, respectively. The typical thickness for pellets was 1–2 mm, while that for films was 0.5–1 ␮m. Impedance spectra were recorded by a Solartron 1260 and a Novocontrol Alpha-A analyzer using pasted silver or evaporated Pt film. The heating and cooling rates for the conductivity and the DSC measurements 共for 25°C ⬍ T ⬍ 180°C兲 were consistently chosen to be 0.5°C min−1.

The DSC measurements for thermal cycles to higher temperatures were performed at 5°C min−1 and the conductivity below room temperature was measured using a closed-cycle refrigerator 共CCR兲, RDK 10-320, Leybold兲 every 2 degrees. For the conductivity measurements on single nanoplates, the scanning probe microscope and the Novocontrol Alpha-A analyzer were combined. AgI nanoplates were ultrasonically treated and dispersed on a silver-coated Si wafer serving as bottom electrode. A gold-coated atomic force microscopy 共AFM兲 tip 共NSC11/Cr-Au, Mikro Masch兲 with the curvature radius less than 50 nm and a cantilever of high spring constant 共ca. 50 N m−1兲 was used as top electrode. X-ray photoelectron spectra 共XPS兲 were collected on an Axis Ultra instrument 共Kratos Analytical, Ltd., U.K.兲 using Al K␣ as the exciting source. Etching was performed by using a high-energy Ar gun 共5 kV兲. Battery preparation and test.— For the assembly of Ag batteries, AgI nanoplates were used as electrolyte, a mixture of silver powder 共Alfa, 5–8 ␮m兲 and annealed AgI nanoplates in the weight ratio of 1:1 was used as anode, and a mixture of gold powder 共ChemPUR, 99.9%, ⬍1 ␮m兲, TiS2 powder 共Aldrich, 99.9%, ⬃1 ␮m兲, and annealed AgI nanoplates in the weight ratio of 8:1:8 was used as cathode. Each component was uniaxially pressed at 5400 kg cm−2 into pellets of 8 mm diameter and 0.5–1 mm in thickness and the stack of cathode-electrolyte-anode pellets was pressed together into a three-layered cell at 6000 kg cm−2. The cells were assembled in an argon-filled glove box. The battery cell was put into a tube furnace, heated to 180°C, and then cooled and kept at 100°C. Discharge-charge cycling was carried out on an Arbin MSTAT system. Results and Discussion Morphology and crystal structure.— Figure 2a and b shows the typical topographic and phase AFM image of AgI nanoparticles dispersed on a silicon wafer. A large quantity of platelike nanoparticles with a diameter of ⬃300 nm was observed. As the values of the height of section analysis of AFM images are very precise, the vertical distance is an appropriate measure of the thickness of nanoplates 共Fig. 2c and d兲. The mean thickness of the nanoplate was measured to be 50 ± 2 nm, which clearly indicates the platelike shape of AgI nanoparticles. The crystal structure of the polytype nanoplates was investigated by XRD. Figure 3a shows the XRD pattern of the as-prepared nanoplates, which exhibits a significant peak broadening from those of


Figure 3. X-ray diffraction patterns of 共a兲 as-prepared and 共b兲 annealed AgI nanoplates in comparison with those of 共c兲 AgI:alumina composites and 共d兲 AgI:fiberglass prepared according to Ref. 6.

␤-AgI in wurtzite structure. In addition to the broadening due to the small crystallite size and possibly strain, stacking faults are supposed to explain the more pronounced broadening of the reflection type h − k = 3N ± 1共l ⫽ 0兲 22 or 共h0l兲 class as indicated. After annealing at 180°C for 5 h under an argon atmosphere 共Fig. 3b兲, the 共h0l兲 reflections disappeared and split into two new reflections instead. The results indicate that a new ordering of the stacking layer sequence has developed from the strongly disordered or faulted structure of the as-prepared nanoplates 共Fig. 3a兲. The new structure however does not correspond to any 共pseudo-兲hexagonal variants observed in single crystals such as 4H, 8H, 12H, etc.5 The structural feature is rather similar to those of AgI:alumina composites 共Fig. 3c兲 7 and AgI dispersion in fiberglass 共Fig. 3d兲 prepared ac-

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cording to Ref. 6. In both cases, very unusual polytype phases of AgI, viz. 7H and 9R, have been identified. The structural characteristics of the nanoplates were reproducible upon further heating and cooling cycles between room temperature and 180°C, which involves the phase transition to and from high-temperature ␣-AgI. 共The details of the phase transition characteristics are in the following section.兲 The stronger 共002兲 intensity of preferentially oriented film of the nanoplates in comparison with that of randomly oriented powder compact indicates that the large surfaces of the nanoplates are 共002兲 basal planes. Quantitative phase analysis using a Rietveld refinement program GSAS 23 on the XRD data of annealed nanoplates 共Fig. 3b兲 yielded the nominal composition of 37.6% 9R-AgI, 30.5% 7H-AgI, 24.8% ␤-AgI, and 7.1% ␥-AgI with residuals, RP = 3.8% and weighted residuals, RwP = 4.9%.17 “␤-AgI” and “␥-AgI” constituting ⬃30% of the material according to the Rietveld analysis should be considered to represent an overall ␤-like stacking 共2H兲 and ␥-like stacking 共3C兲 portions epitaxially intergrown with 7H and 9R structures, which is a commonly observed phenomenon in other polytypic materials such as SiC. These nominal ␤-AgI and ␥-AgI phases do not behave as regular independent phases, because phase transitions characteristic of regular ␤-AgI and ␥-AgI phases3 do not appear at all. In Fig. 4, AgI polytypes are schematically illustrated in terms of close-packed layers of tetrahedral sites occupied by silver ions. The left column illustrates the usual low-temperature polymorphs of AgI of the hexagonal close-packing 共h兲 in 2H sequence of ABAB… 共␤-AgI兲 and cubic close-packing 共c兲 in 3C sequence, and the most common higher polytypes observed in single crystals, 4H polytype with the ABCB sequence or hc共hc兲 in Jagodzinski’s notation,24 which takes into account the stacking in relation with the neighboring layers. The unusual 7H and 9R polytypes, observed in nanoplates as well as AgI:alumina composites and AgI dispersed in fiberglass, are illustrated in the middle and right columns of Fig. 4. The stacking sequence of ABCBCAC and ABCBCACAB can as well be represented as cchhchh and chh共chhchh兲, respectively. The notation indicates that all higher AgI polytypes are considered as a ␤-AgI共h兲/␥-AgI共c兲 superlattice or heterostructure on the crystal lattice scale. Furthermore, longer period polytypes can be considered as heterostructure of shorter-period ones, i.e., 4H/3C heterostructure for 7H-AgI, 4H/3C/2H or 7H/2H superlattice for 9R-AgI polytype.17 Heterostructures or superlattices have been mainly referred to in the field of semiconductors while the current material AgI is an ionic conductor. Such an ionically conducting heterostructure in the me-

Figure 4. 共Color online兲 AgI polytypes represented in terms of close-packed iodine tetrahedra occupied by silver ions: 共left兲 from the top 2H-AgI 共␤-AgI兲, 3CAgI 共␥-AgI兲, and 4H-AgI; 共middle兲 7HAgI 共right兲 9R-AgI. The stacking sequences are represented by ABC notation and hc notation. All higher AgI polytypes can be considered as ␤-AgI共h兲/␥-AgI共c兲 superlattice or heterostructure.

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Figure 5. DSC curves of as-prepared AgI nanoplates. After the first heating where two thermal events around the nominal ␤-␣ transition temperature, 147 and 157°C are distinguished, single thermal effect around 147°C on heating and around 95°C on cooling, thus with a large hysteresis of ⬃50°C, were reproducibly observed.

soscopic regime is suggested to be responsible for the extraordinary conductivity effects of AgI nanoplates which will be detailed later. Phase transition characteristics.— Figure 5 shows the DSC curves from the first three heating-cooling cycles of as-prepared AgI nanoplates. On first heating, two overlapped thermal events were observed with peaks near the ␤-␣ transition temperature 共⬃147°C兲 and the other at higher temperature by ca. 10°C. The two distinct thermal events in the DSC curve can be well correlated with the two-step transitions in the conductivity curve of the pellet of as-prepared AgI nanoplates 共see the inset of Fig. 6兲. On cooling, the thermal event occurred around 95°C, which is lower than the nominal ␣-␤ phase transition temperature by as much as ⬃50°C. Figure 6 shows a corresponding drop in conductivity as well. From the second thermal cycle on, in both DSC and conductivity, distinct events on heating around 147°C and on cooling around 95°C, respectively, were reproducibly observed, corresponding to a large hysteresis of ⬃50°C. The conductivity hysteresis loops close only around 35°C on cooling after a broad transition around 60°C. The conductivity drop around 60°C without appreciable heat effects in DSC measurements is attributed to the transition between different polytype structures or the change in stacking fault arrangements. Negligible heat effects can be ascribed to low stacking fault energies, which is typical for materials exhibiting many polytypes and stacking faults. The hysteresis was found to hardly depend on heating and cooling rates. Figure 7 shows that the thermal effect and conductivity step around 147°C on heating and around 95°C on cooling are unequivocally related to the phase transition to and from ␣-AgI. The specimen was annealed at 180°C for 5 h before measurement. While the transition point on heating is similar to the nominal ␤-␣ transition point, the supercooling of ␣-AgI structure down to 95°C is remarkable. The transformation crystallography of AgI materials is quite similar to that of martensitic phase transformations of many alloys based on the noble metals such as Cu, Ag, and Au, as well as in pure metals of Sm, Co, Cu, and Li, viz. between high-temperature body centered structure 共␣兲 and low-temperature close-packed-layer structures such as face-centered 共␥兲, hexagonal 共␤兲, or 9R.25 The transformation can be described by the shear-and-shuffle of 兵110其 planes of bcc to the close-packed planes of the low-temperature structure. The occurrence and prevalence of a long-period 9R struc-

Figure 6. Conductivity curves of as-prepared AgI nanoplates. Inset shows the two-step transition on first heating 共corresponding to two DSC thermal events in Fig. 5兲. From the second thermal cycle and following-on large hysteresis loops closing beyond 160°C on heating and below 40°C on cooling were reproducible. Room-temperature conductivity of a nanoplate film 共␴⬜c兲 and the average conductivity of single nanoplates measured along the c axis 共␴储c兲 by AFM tips are shown. The conductivity curve of ␤-AgI polycrystal and room-temperature conductivity values of ␤-AgI single crystals parallel and perpendicular to c axis are indicated.

ture is ascribed to very small average distortion accompanied by stacking sequence ABCBCACAB or chh共chhchh兲.25 Although identified only recently,10 the occurrence of 9R modification in AgI may be phenomenologically understood in terms of phase relationships. The 9R polytypes have been reported in a variety of com-

Figure 7. In situ X-ray diffraction patterns of AgI nanoplates on a thermal cycle between room temperature and 180°C.

Journal of The Electrochemical Society, 154 共9兲 K51-K60 共2007兲 pounds as well: GaSe,26 M1−xX 共M = Ti, V; X = S, Se兲,27 ABX3perovskite-related compounds such as BaRuO3 under high pressure,28 or A-type 共hexagonal兲 sesquioxides such as ThO2– La2O3,29 or CeO2–La2O3.30 As mentioned in the beginning, however, 9R polytypes are not common in typical AnB8−n polytypic compounds such as SiC and ZnS. The 7H modification of AgI, first reported in 1974,6 remains a rare and unusual polytype form. The 7H form has been found to coexist with the 9R form of AgI with a similar prevalence in three cases we have examined, i.e., ammonia solution derived fine powder according to Ref. 6, AgI:alumina composites,31 and AgI nanoplates of the present work. The crystallographic relations of AgI similar to metals system, i.e., bcc to closepacked structures, strictly refer to iodine lattice only. The silver ions in the ␣-AgI phase with the bcc iodine lattice are distributed over many interstitial sites, which is responsible for its high ionic conductivity. The huge hysteresis in the transformation to and from hightemperature ␣-AgI, first observed in AgI:alumina composites by Shahi and Wagner,9 had been intriguing for a long time. AgI bulk phase in the composites, as in other silver halide composites such as AgCl and AgBr, was not considered to be altered in the composites. An unidentifiable interfacial phase at the interface between AgI and alumina was supposed to be responsible for this phenomenon, but then the disappearance of the bulk phase transitions should have been explained. Lee et al.7 identified 7H polytype formation in the composites and explained the hysteresis as the characteristic of the 7H modification of AgI. The statement remains valid except for the coexistence of the 9R form with the 7H polytype. The observation in the present work further supports the transformation hysteresis to be ascribed to the formation of 7H and 9R polytype formation of AgI. In a previous publication,7 the mechanism of the large hysteresis of AgI with unusual polytype phases was discussed in terms of martensitic transformations. The hysteresis in the transformation between ␤-AgI共2H,hcp兲 and ␣-AgI, depending on heating and cooling rate, is less than ⬃5°C, and the hysteresis observed in the transformation between ␥-AgI共3C,fcc兲 and ␣-AgI amounts to ⬃12°C.3 Macroscopically, the hysteresis represents the energy dissipation by the “friction” in the phase boundary motion during transformation, hence the friction in the transformation to and from ␣-AgI apparently being the least for 2H ␤-AgI, larger for 3C ␥-AgI, and largest for 7H/9R polytypes. The tendency is, however, exactly opposite to the one expected from the friction in a hard-sphere atomistic model. As mentioned above, bcc-9R transformation exhibits a minimum average distortion at the phase boundary by shuffling of 兵110其 bcc planes in the stacking sequence. The bcc-fcc shuffling transformation has a less homogeneous distortion, while bcc-2H requires additional displacement shear for the undistorted interface. Therefore, there should be another dominating source of friction in 7H/9R and to a lesser degree in 3C共␥兲 AgI compared to 2H 共␤兲 AgI to explain the hysteresis behavior. We suggest that the huge phase transition hysteresis in AgI nanoplates of 7H/9R polytypes compared to the phase transition behavior of 2H ␤-AgI, despite the opposite effects by the crystallography, should be another strong indication of a high degree of silver disorder. Recently, Ahlers25 showed that the longrange order in metal alloy systems decreases the hysteresis in the martensitic transformation by forcing the atoms to move back on the same path to restore original structure without defects. In disordered systems, the multiplicity of the transformation paths leads to a large hysteresis in transformation and re-transformation. Conduction mechanism.— Together with the large hysteresis, the huge conductivity enhancement by more than four orders of magnitude in AgI:alumina composites had been puzzling for some time. The conductivity enhancement in other silver halide composites such as AgBr:alumina and AgCl:alumina has been quantitatively explained by enhanced defect concentrations along space-charge layers. Specifically, space-charge layers with accumulated 共negatively charged兲 silver vacancies are formed which chargecompensate silver ion adsorption by chemical interaction with nu-

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cleophilic alumina. In analogy with the vacancy generation by doping of a higher valent cation homogeneously in the bulk, the process has been named “heterogeneous doping”.32,33 AgI nanoplates prepared in this work as pure AgI materials without any dispersoids clearly indicate that the conductivity effects as well as the phase transition characteristics are intrinsic property of 7H/9R polytype AgI. The large disorder in the Ag lattice responsible for the extremely high conductivity is also reflected in the reduced transformation enthalpy3 and the huge hysteresis in phase transformation. AgI nanoplates with well-defined composition and geometry allow in-depth characterization of the conduction behavior of 7H/9R polytypes. Comparison of the conductivity values of the pellets, films, and single nanoplates 共by using conductive AFM tips17兲 reveals that the conductivity of the nanoplates is crystallographically isotropic and a “bulk” property of the entire nanoplates. The geometry of the nanoplates 共Fig. 2兲 clearly represents the surface energy anisotropy. In Fig. 6, ␴pellet represents room-temperature conductivity of a pellet of randomly oriented nanoplates. ␴⬜c is measured from a film with preferentially aligned nanoplates, thus representing the conductivity of the nanoplates perpendicular to the c axis, and ␴储c is the average conductivity of single individual nanoplates measured along the c axis of the nanoplates. All of these conductivities are close in magnitude. The behavior can be compared to that of ␤-AgI, in which the anisotropy in the transport pathways leads to significant conductivity anisotropy at room temperature as indicated in Fig. 6. With increasing temperature and thus increasing defect concentrations, the conductivity of ␤-AgI becomes isotropic, however. The conductivity isotropy of AgI nanoplates hence indicates a substantial disorder of Ag lattice even at room temperature. Compared to the pronounced non-Arrhenius behavior in the conductivity of ␤-AgI as shown in Fig. 6, which can be ascribed to thermally activated formation of defects and the transitions in the conduction mechanism,2 AgI nanoplates exhibit a well-defined small activation energy of ⬃0.25 eV 共estimated from ln ␴T vs 1/T representation兲 over a wide temperature range. The range extends from −150°C up to 130°C on heating 共see Fig. 6兲. This is significantly less than that for ␤-AgI, for which apparent activation energies vary from 0.5 to 1 eV over the same range. Rather, the low activation energy of the nanoplates may be compared with that of superionic ␣-AgI 共0.1 eV兲, in which the small activation energy is attributed to the small migration barrier in the bcc iodine frame, whereas the temperature dependence of defect concentration in highly disordered Ag lattice is negligible. The activation energy of ⬃0.25 eV represents the migration barrier of substantially disordered silver ions in the close-packed iodine lattice. Although the grayish-yellow nanoplate powders turn almost black if pressed to pellets, the electronic conductivity of polytype AgI nanoplates was found to be less than 10−5 of the total conductivity.17 Thus far, sufficient evidence has been presented of a significant Ag disorder in the 7H/9R polytype phases constituting the AgI nanoplates. The fundamental question of why these unusual polytypes exhibit a high degree of disorder and are hence highly conducting may be answered by atomistic modeling or first-principles calculations. Meanwhile, we propose a qualitative “top-down” approach to understand the present feature, viz., by regarding the polytype as a heterostructure of ␤-AgI/␥-AgI or zinc blende/wurtzite as mentioned earlier. Similarly to electronic effects at semiconductor-semiconductor contacts, at the contact of two ionic conductors a redistribution of ions occurs due to the difference in partial free-energy levels of the ionic defects.8,34-37 As shown in Fig. 8a for two ionic compounds MX and MX⬘ with Frenkel disorder, the redistribution of ions leads to a severe modification of the ionic defect concentrations in the respective space-charge regions with the characteristic screening lengths of ␭, ␭⬘. In the space-charge region of MX 共MX⬘兲, the vacancy concentration increases 共decreases兲 and the interstitial concentration decreases 共increases兲, which, depending on the respective

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Figure 8. 共Color online兲共left兲 Ion redistribution at the contact MX/MX⬘ with semi-infinite space-charge effects and consequently an anisotropic conductivity 共cf., ␴储c, ␴⬜c兲. 共right兲 Mesoscopic heterolayer of MX/MX⬘ with the layer thickness in the sub-Debye length regime. Bulk level is nowhere present and the gross disorder leads to a more isotropic conductivity 共cf., ␴储c, ␴⬜c兲. Bottom diagram indicates the silver transfer from ␤-AgI layer to ␥-AgI layer.

situations of the materials, may lead to greatly increased spacecharge conductivities.15,16,38 Even in the case where the spacecharge region is negligibly small compared to the bulk region and thus the overall conductivity perpendicular to the interface is hardly affected, the short-circuiting paths of the space-charge region can increase the effective conductivity along the interface significantly as indicated at the bottom of Fig. 8. Conductivity enhancements in AgBr:AgI,39 AgCl:AgI 40兲 composite electrolytes can be attributed to such interfacial effects. The occurrence of semi-infinite spacecharge effects has been quantitatively demonstrated in the molecular beam epitaxy 共MBE兲-grown CaF2 /BaF2 heterostructure with layer thickness larger than 50 nm.15,16 When the interfacial spacing is comparable to or smaller than the screening length, as schematically shown in Fig. 8b, the system will be charged throughout. In such mesoscopic regimes, the material is nowhere in the original bulk situation with regard to charge carrier distribution. The two phases lose their individuality and an ensemble with new transport properties can form. Such effects are discussed for MBE-grown fluorine conducting CaF2–BaF2 heterostructures at tiny layer thickness.15,16 Such effects have also been seen in AgI:alumina composites7 in the stacking fault region directly at the contacts, and are suggested to occur in polytype AgI nanoplates by considering 7H and 9R phase as heterostructures of ␤-AgI共2H兲 and ␥-AgI共3C兲, or 4H layers as indicated in Fig. 4, which are at most several close-packed layers thick and in the sub-Debye length range. In Fig. 8b, bottom, the silver ion transfer from ␤-AgI layer to ␥-AgI layer is assumed to thus charge the former negative with silver vacancies 共VAg ⬘ 兲 and the latter positive with silver interstitials 共Agi兲. One distinct property expected for such a grossly disordered state is the conductivity isotropy, which is in fact observed for AgI nanoplates. The model assumes the isolated “ionic” Fermi level, i.e., the electrochemical potential of Ag ions36 in ␤-AgI to be higher than that of ␥-AgI. The assumption is based on the condition in which the environment with a high chemical potential of Ag+ favors the formation of ␥-AgI.41 The data for constructing ionic band structure of heterojunction between ␥-AgI and ␤-AgI are not available. Granzer35 constructed ionic level schemes for AgCl/AgBr heterojunctions from known and derivable parameters of the respective halides. However, the level structure thus constructed is extremely idealized, because mutual solubilities and interface mismatch between two phases are not considered. The heterojunctions of two polytypes, i.e., zinc blende/wurzite, can be completely defect-free,

latticed matched, coherent, and chemically homogeneous. The potential and feasibility of such heterostructures are in fact hot issues in semiconductor physics.18,19,42 Mainly semiconductor heterostructures have been prepared that are composed of different materials but exhibit the same crystal structures. Formation of wurtzite GaN in the metastabilized zinc blende heterostructure was considered a nuisance to be prevented.43 Currently, zinc blende/wurzite polytype heterostructures are under active research and development, the key of which is to control the polytypism in the materials with pronounced polytypism, e.g., SiC, ZnS, CdSe, CdTe, etc. Because most known zinc blende/wurtzite polytype materials are semiconductors, the ionic conducting heterostructure of AgI is in rather a unique position 共see Fig. 1兲. Formation mechanism.— In order to identify the formation mechanism of the unusual 7H/9R polytypes of AgI, three different cases were closely examined: 共i兲 AgI composites with “wet” fine ␥-alumina,7,9,44 共ii兲 AgI powder dispersed in fiberglass filter obtained from AgI–NH4I acetone solution according to Davis and Johnson,6 and 共iii兲 AgI nanoplates produced from PDADMACcontaining solution 共this work兲. The OH− group of the wet alumina acting as Lewis base, which is also responsible for semi-infinite space charge effects in AgCl and AgBr composites with alumina, has been suggested to trigger the formation of the polytypes in AgI:alumina composites 共Fig. 9b兲. Dispersoids with acidic surface activity such as SiO2 did not result in the high conductivity or in the formation of the 7H /9R polytypes.3 The 7H/9R polytype AgI powder prepared according to Ref. 6 can thus not be attributed to the fiberglass filter of silica material alone. When the filter soaked with

Figure 9. 共Color online兲 Schematic diagrams representing the mechanism of polytype formation for three different cases: 共a兲 Cl− adsorbates stemming from PDADMAC attracts Ag+ ions of AgI nanoplates. 共b兲 OH− group of the wet alumina acts as Lewis base and attracts Ag+ ions of AgI in the composite. 共c兲 NH3 stemming from AgI–NH4I acetone solution is Lewis base attracting Ag+ ions of AgI dispersed in fiberglass filter.


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Figure 10. 共Color online兲 XPS spectra of AgI nanoplates before and after Ar etching. The three bottom graphs represent the detailed feature of I 3d, Ag 3d, and Cl 2p spectra. After etching, Cl is not detectable. Quantitative analysis shows the Ag excess before etching and nearly 1:1 AgI stoichiometry after etching.

AgI–NH4I acetone solution is heated to 550°C and quenched, NH4I is decomposed to NH3 共and HI兲, which behaves as a Lewis base similar to OH− groups on the alumina surface 共Fig. 9c兲. In fact, the increase in the surface conductivity of AgCl by adsorption of NH3 gas is similar to the space-charge effects in composites with alumina.45 As expected from the weak interaction of the NH3 adsorbate and the evaporation under open air conditions, the polytype AgI phase in the fiberglass filter did not survive the moderate heating up to ⬃180°C. The material was found to predominantly turn to ␤-AgI on cooling. We have emphasized that AgI nanoplates, in contrast to AgI:alu-

mina composites and AgI in fiberglass filter, are composed of pure AgI phase in 7H/9R polytype structure. No macroscopic second phase could be identified that could be responsible for the chemical effects. As different polyelectrolytes lead to the AgI nanopowders in different crystal structure and morphology,3 PDADMAC used in the preparation of AgI nanoplates is expected to exhibit functional groups which trigger the formation of 7H/9R polytype. In fact, XPS revealed absorbed Cl− ions on the surface of AgI nanoplates, which stem from PDADMAC. As compared in Fig. 10, after etching with high-energy Ar+ 共5 kV兲 for several minutes, no Cl− ions were detected, which indicates that Cl− ions appear only on the surface but

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not in the bulk of the nanoplates. Quantitative analysis revealed a surface composition of 51.7% Ag, 46.8% I, and 1.5% Cl 共in atom %兲 before etching. After etching, the Ag:I ratio was found very close to 1:1 stoichiometric composition, which represents the chemical composition of the nanoplate bulk. As the Ag excess in the surface composition indicates, Cl− ions from PDADMAC are prone to attract Ag+ ions similarly for OH− and NH3, Fig. 9. The morphology of nanoplates can be ascribed to the reduced surface energy of the basal c planes of polytypes 共or 兵110其 planes of bcc ␣-AgI兲 by chlorine adsorption. It was found that other polyelectrolytes e.g., poly共sodium 4-styrenesulfonate兲 and poly共acrylic acid sodium兲, lead to the formation of AgI nanoparticles of different morphologies and crystal structures.3 Recall that the well-defined 7H/9R polytype diffraction pattern developed after first heating of the as-prepared nanoplates 共up to 180°C兲 and subsequently cooling 共Fig. 3兲. The XRD pattern of the as-prepared nanoplates indicates extremely high densities of stacking faults with respect to ␤-AgI structure, which can be regarded as zinc blende/wurzite heterostructure with random layer thicknesses. The conductivities of as-prepared nanoplates are only slightly less than those of the specimens with a distinct 7H/9R periodicity 共Fig. 6兲. The other two routes to the formation of 7H/9R polytypes discussed above involve high-temperature treatments during the preparation, which means in both cases the as-prepared material has already experienced the ␣/␤ transformation. Therefore, it can be concluded that 7H/9R stacking arrangements of AgI are established upon martensitic transformation from bcc ␣-AgI phase in the presence of Ag+ ion attracting surface activity. The occurrence of “nano” 7H/9R polytype AgI is a direct consequence of the surface-induced formation-stabilization mechanism. The effect of chemical interactions at the surface should be limited to a certain penetration depth. The uniform thickness of our nanoplates 共⬃50 nm兲 appears to represent a critical thickness above which 7H/9R polytype phases become unstable. In the case of AgI:alumina composites, 100% formation of the unusual polytype modification occurred in the composites with alumina content larger than 30% in which the AgI particle size is less than 100 nm. Therefore, this unusual material with unusual properties relies on a double size effect: 共i兲 macroscopically unstable 7H and 9R stacking fault arrangements are stable only in the crystallites of the thickness less than the penetration depth of the surface effects which is less than 100 nm. 共ii兲 the stacking fault arrangements can be conceived as heterolayers composed of ␤-AgI 共wurtzite兲 and ␥-AgI 共zinc blende兲共i.e., ␤/␥/␤/␥. . .兲, with individual layer thicknesses of a few atomic distances, much below the Debye length, which can be estimated ⬃10 nm for ␤-AgI. Phase stability.— It is emphasized that all the structural features and properties of the nanoplates are reproducible upon repeated thermal cycling up to 180°C involving the transition to and from ␣-AgI phase. The supercooled ␣-AgI phase at 100°C 共⬃50°C below the ␤-AgI/␣-AgI transition temperature兲 has been found to be stable over 1 month. In order to investigate the thermal stability of the material, DSC and conductivity measurements were performed in thermal cycles similarly as shown in Fig. 5 and 6, but consecutively up to higher temperatures as 300, 400, and 500°C. The results of DSC in Fig. 11 and conductivity in Fig. 12 show consistently the decrease in the hysteresis mainly by the reduced supercooling in transition from ␣-AgI to the low-temperature modification. The cooling transitions occurred at ⬃90°C for the cycle up to 180°C, and ⬃114°C for 300°C cycle, ⬃125°C for 400°C cycle and ⬃140°C for 500°C cycle. The phase transition behavior of the cycle after heating to 500°C is in fact identical to the normal ␤-AgI. 共Relatively large hysteresis by 7°C is due to the high heating and cooling rate.兲 Figure 12 shows the decrease in the conductivity of the low-temperature phase accompanying the decrease in phase transition hysteresis. Complete transformation to ␤-AgI on 500°C annealing also has been directly confirmed by conductivity and XRD measurements.

Figure 11. DSC curves upon thermal cycles consecutively up to 180, 300, 400, and 500°C. The decrease in the thermal hysteresis mainly by the decreased supercooling on cooling transition has been observed. The cooling transitions occurred at 90°C for the cycle up to 180°C, at 114°C for the 300°C cycle, at 125°C for the 400°C cycle, and at 140°C for the 500°C cycle.

We also observed a change in the particle morphology by AFM after the respective annealing treatments. While annealing the nanoplates 共dispersed on a silicon wafer兲 up to 180°C did not change the plate morphology of the as-prepared nanoplates 共Fig. 13a兲 except edges being smoothened 共b兲, significant rounding of the exposed surface of the particles occurred with increasing temperatures 共c,d,e兲. After annealing at 500°C, Fig. 13e, the particles exhibit spherical morphology. The annealing temperature of 500°C is still below the melting temperature of ␣-AgI, 558°C. The morphology change suggests that the free-surface energy of AgI particle becomes isotropic with increasing temperature due to the “surface roughening” effect46 as well as due to desorption-diffusion of chlorine ions.

Figure 12. 共Color online兲 Conductivity curves upon thermal cycles consecutively up to 180, 300, 400, and 500°C. The decrease in the hysteresis is consistent with DSC behavior in Fig. 11. The conductivity of the lowtemperature phase decreased substantially with increasing annealing temperatures. After 500°C annealing the conductivity behavior is the same as that of normal ␤-AgI.


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Figure 13. 共Color online兲 The morphology of AgI nanoplates of 共a兲 asprepared and after annealing at 共b兲 180°C, 共c兲 300°C, 共d兲 400°C, and 共e兲 500°C. The plates become more rounded with increasing temperature and almost spherical after annealing at 500°C. The scale bar corresponds to 200 nm.

Complete loss of the surface adsorbed Cl− on 500°C annealing results in the formation of normal ␤-AgI as well as the spherical morphology. Application: All-solid-state rechargeable Ag battery with TiS2 cathode.— Silver iodide, in fact a mixture of ␤-AgI with mechanically generated metastable ␥-AgI, was until 1960 considered the most ionically conductive material at room temperature and was employed in one of the first all-solid-state batteries.47 Then, in order to stabilize the highly disordered state, occurring in ␣-AgI at elevated temperature, double salts such as Ag3SI and RbAg4I5 had been prepared. The highest Ag conductivity is achieved in RbAg4I5 but the material is unstable against humidity or I2. Unavailability of stable and sufficiently conductive solid electrolytes had hampered the development of Ag solid-state batteries. The polytype heterostructured AgI of the present work exhibits conductivities almost comparable to RbAg4I5. It is the best binary solid electrolyte material at room temperature known so far and, unlike double-salt Ag electrolytes, the polytype phase and thus the conductivity have been found stable over a year under ambient condition. High polarizability of Ag ions ensures a high exchange rate at the electrode reaction, which is a critical factor in all-solid-state devices. Therefore, all-solid-state Ag batteries are still systems of choice for miniaturized devices in which switching rate and reversibility rather than energy density and specific materials cost are important. Moreover, shape and morphology of polytype AgI nanoplates are very advantageous for possible nanointegration as demonstrated by the conductivity measurements on single nanoplates.17 The chemical stability of nanoplates in ambient condition is not only to be compared to other silver conductors but is another important strong point in competition with lithium batteries. As a first step, rechargeable all-solid-state Ag batteries have been successfully constructed by using the pellets of highly conducting AgI nanoplates as electrolyte, Ag as anode, and TiS2 as cathode. The battery operates successfully with high-temperature ␣-AgI phase both from commercial AgI powder and from AgI nanoplates. At room temperature the operation with nanoplates is still feasible, while impossible in the case of commercial “␤-AgI” powder. The difference is greatest for operation at 100°C. While a reasonable operation with commercial AgI powder is still impossible, the supercooled ␣-AgI in the case of nanoplates due to the unusual large hysteresis behavior allowed similar discharge-charge behavior as at temperatures above 150°C. The cells of nanoplates were first heated to 180°C above the transition temperature and then cooled to 100°C. Figure 14 shows that TiS2 is able to reversibly accommodate Ag up to Ag0.4TiS2 at 70–270 mV vs Ag+ /Ag with a good capacity retention on cycling at 100°C. Improved performance by searching for different cathode materials 共e.g., TiTe2兲 and the detailed battery characterization are reported separately.48 Conclusions A substantial Ag ion disorder even in the low-temperature modification of AgI nanoplates has been attributed to the formation of stable, unusual 7H/9R polytype structures 共whereas ␤-AgI in wurtz-

Figure 14. Discharge 共Ag insertion兲/charge 共Ag extraction兲 curves of a Ag battery with the cell structure of TiS2 兩AgI nanoplates兩Ag at 100°C at a current density of 100 ␮A cm−2.

ite structure is the stable modification for macroscopic samples兲. The large disorder is represented in the conduction behavior exhibiting a huge conductivity at room temperature that is higher by a factor of 104 than that of ␤-AgI, with an activation energy of 0.25 eV 共which is less than half that of ␤-AgI兲 over a wide temperature range, and a striking conduction isotropy in spite of the crystallographic anisotropy. The large hysteresis, characterized by a ⬃50°C supercooling in the transition from high-temperature ␣-AgI, and a heat of transformation that is substantially smaller than that of ␤-AgI to ␣-AgI, can be as well ascribed to the pronounced Ag disorder. The Ag disorder in AgI nanoplates can be viewed as a mesoscopic effect as a consequence of the formation of zincblende共␥-AgI兲/wurtzite共␤-AgI兲 heterostructure on the crystal lattice scale, i.e., in the subnano regime. As the thickness of individual layers is well below the screening length of the space-charge layers at the heterojunction, the material is expected to be charged throughout, resulting in substantially disordered lattice of mobile Ag ions. The common formation mechanism of 7H/9R polytypes in different cases has been identified to be the Ag ion attracting surface chemistry, especially in the case of AgI nanoplates Cl− adsorption from the capping agent, PDADMAC. As the surface effect has a limited penetration depth, the stability of the material relies on the small thickness of crystallites of ⬃50 nm. This, together with the mesoscopic ionic conductivity effect, constitutes a double size effect. The 7H/9R polytype stacking arrangement is facilitated by the martensitic transformation from ␣-AgI in bcc iodine lattice, which is reproducibly formed upon thermal cycles up to 180°C. Annealing at higher temperatures results in the decrease both in phase transition hysteresis and conductivity, and heat treatment at 500°C results in the complete disappearance of 7H/9R polytypes. The application of AgI nanoplates as solid electrolytes has been successfully demonstrated in a rechargeable all-solid-state Ag battery using TiS2 cathode material. This battery works well at room temperature 共AgI: in the stacking fault phase兲 and in a very superior way at 100°C 共AgI: in the quenched ␣-phase兲. 共Note that bulk ␣-AgI is stable only above 150°C兲. More interesting is the prospect of this unique material as components of nanoscaled batteries 共nanobatteries兲 or nanointegrated batteries to supply appropriate power for future nanodevices. In view of shape, ionic conductivity, and stability of the AgI nanoplates, they are ideal candidates for such applications.

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Journal of The Electrochemical Society, 154 共9兲 K51-K60 共2007兲 Acknowledgments

This paper is dedicated to the late Professor J. Bruce Wagner, Jr. The authors thank the following persons for technical support and discussions: G. Götz 共XRD兲, U. Klock 共DSC兲, U. Traub 共programming兲, Dr. M. Konuma 共XPS兲 in MPI-FKF, and Professor S. Adams, National University of Singapore 共GSAS analysis兲. Max Planck Institute for Solid State Research assisted in meeting the publication costs of this article.

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