Gigantic Photoresponse and Reversible Photoswitching in Ionic

Article pubs.acs.org/JPCC

Gigantic Photoresponse and Reversible Photoswitching in Ionic Conductivity of Polycrystalline β-AgI Farzana Sabeth, Toshifumi Iimori, and Nobuhiro Ohta* Research Institute for Electronic Science (RIES), Hokkaido University, Sapporo 001-0020, Japan ABSTRACT: A photoinduced change in the ionic conductivity was measured for polycrystalline silver iodide (AgI) by complex impedance spectroscopy, and a three-order magnitude reduction (from gigaohms to megaohms) in the bulk resistance of β-AgI on photoexcitation was found at 77K. The bulk resistance gradually increased when the light was turned off. Reversible photoinduced switching (photoswitching) between low and high resistivity states was observed, depending on the time in the dark state. This gigantic photoinduced change in the ionic conductivity is explained in terms of distortion of the β-AgI lattice following the photoinduced generation of electron-hole pairs.

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INTRODUCTION Transitions between different phases can be induced in the phase-transition materials by applying external stimuli such as photoexcitation and external electric and magnetic fields. For example, phenomena such as photoinduced switching (photoswitching) and photomemory effects in organic and inorganic materials including various polymers and resistive switching have been intensively studied.1−5 In addition, light-induced superconductivity in a stripe-ordered cuprate and resistive switching in a high-Tc superconductor have been reported.6,7 Electric-field-induced reversible electrical switching in disordered structures that involves transitions between highly resistive states and conducting states has also been observed in various disordered semiconductors.8 In the modern transdisciplinary research field, the immanent properties of the solidstate ionic conductors have attracted remarkable interest as they have potential for use in batteries.9 However, for the improvement of efficiency of the devices, the control of the physical properties of such materials is highly demanded to satisfy commercial applications. Silver-based ionic conductors are excellent candidates for electrolytes used in solid-state batteries because of the high exchange rate of Ag+ ions at the electrode,10 and prototypes of all the solid state batteries using silver ionic conductors have already been reported.9 Silver iodide (AgI) is one of the most intensively studied solid-state ionic conductors. It exhibits a rich phase behavior as it has at least six crystalline polymorphs and undergoes phase transitions induced by temperature and pressure. Among them, two phases coexist at room temperature and ambient pressure: β-AgI (wurtzite structure) and γ-AgI (zinc-blende structure). The most intriguing feature of AgI is that at 420 K it undergoes a first-order phase transition to a superionic conducting state, α-AgI, which has an extremely high ionic conductivity.11,12 The photoresponse of the conductivity and the photoexcitation dynamics and its temperature dependence of silver halides have been extensively investigated.13−17 However, the photoconduc© 2012 American Chemical Society

tivity and photoinduced phase transitions of AgI have been scarcely investigated at low temperatures below 150 K. The ionic conductivity of AgI has been investigated with and without photoirradiation, and the bulk resistance and the color of polycrystalline AgI exhibited reversible photoswitching at room temperature.18 Several studies have sought to explain the macroscopic and microscopic behaviors of conducting ions and the phase transition mechanism,19−21 but many of the most important characteristics still remain highly enigmatic. In the present study, a gigantic photoinduced change in the ionic conductivity by nearly 3 orders of magnitude was found in polycrystalline AgI. In addition, the reversibility of the photoswitching of the bulk resistance was investigated by complex impedance spectroscopy.

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EXPERIMENTAL SECTION Commercially available AgI powder (purchased from Junsei Chemical Co.) was gently ground in an agate mortar and pellets that were of 13 mm in diameter and 0.7−0.8 mm thick were prepared by uniaxially pressing at 200 kgf/cm2. The pellets were immerged in potassium iodide (KI) solution for 2 days to increase the proportion of the β-component. They were then rinsed with mili-Q-water and dried. X-ray diffraction patterns of the samples were obtained using a X-ray diffractometer (Rigaku, RINT-UltimaIII/SW, light source Cu). Carbon paste was used to fabricate electrodes on the pellet surfaces. AgI pellet prepared by grinding in the agate mortar and KI-treated samples were characterized by X-ray diffraction and impedance spectroscopy measurements. On the basis of the results, the AgI sample before the KI treatment is assigned to the γ-phase of AgI, while the AgI sample after the KI treatment is assigned to the β-phase of AgI. Received: January 12, 2012 Revised: March 13, 2012 Published: April 4, 2012 9209

dx.doi.org/10.1021/jp300382x | J. Phys. Chem. C 2012, 116, 9209−9213

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Complex impedance spectra were measured with an alternating current voltage of 0.1 V in the frequency range from 5 MHz to 42 Hz or 5 MHz to 50 mHz using impedance analyzers (Hioki 3532−50, or Solartron, 1260 + 1296) at 298 and 77 K with and without photoirradiation. For the measurements at 77 K, the sample was directly immersed in a liquid nitrogen dewar having quartz windows. A xenon lamp installed in the fluorescence spectrometer (JASCO, FP777) was employed as a light source to illuminate the central portion of a pellet between the electrodes having a separation 1−2 mm. The intensity was controlled using neutral density filter, which was calibrated for 430 and 450 nm by measuring its transmittance in 300−700 nm range. Steady-state photocurrent excitation spectra of the KI-treated AgI pellets were measured using an electrometer (Keithley, model 617) at 298 and 77 K.

Figure 2. Impedance spectra of the polycrystalline AgI pellet before (blue dots with the right and top axes) and after the KI treatment (red dots with the left and bottom axes).

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of 4 × 10−4 eV deg−1 below room temperature.27 The absorption tail at wavelengths longer than the absorption edge extended to 750 nm was ascribed to the presence of Frenkel defects. Steady-state photocurrent excitation (PCE) spectra of β-AgI were measured in the wavelength range of 700−300 nm at 298 K and at 77 K using a direct current (dc) method with light intensities of 1 × 10−2 and 1 × 10−4 W/cm2 at 430 nm (which hereafter are denoted as I0 and 0.01I0, respectively). In both cases, a low dc voltage of 0.1 V was applied, which is much lower than the AgI decomposition potential (0.4 V).28 Figure 3

RESULTS AND DISCUSSION At ambient conditions, the β- and γ-phases coexist in AgI, but the proportion of the γ-phase is very high immediately after fabricating AgI pellets. In fact, the X-ray diffraction pattern of AgI pellets thus prepared is assigned to that of the γ-component (Figure 1a). On the other hand, the X-ray diffraction patterns

Figure 1. XRD pattern of polycrystalline AgI (A) before and (B) after the KI treatment.

of potassium iodide (KI) treated samples of AgI were assigned to that of the β-component (Figure 1b). It was reported that the proportion of β-phase gradually increased with the aging of the sample,18 but we used a different technique, KI treatment, to produce β-phase in this experiment to get a relatively quick response, which is also commonly accepted that excess of iodine concentration produces β-AgI.22−25 The bulk resistances estimated from complex impedance spectra (Cole−Cole plots) measured at room temperature (298 K) in the frequency range from 42 Hz to 5 MHz confirm the assignments of AgI pellets before and after the KI treatment (Figure 2). As the conductivity of γ-AgI is higher than that of βAgI,26 before the KI treatment, the bulk resistance is as small as ∼70 kΩ, indicating the γ-phase of AgI. After the KI treatment, the bulk resistance increases by more than 1 order of magnitude (Figure 2), indicating that the KI treatment converts the polycrystalline γ-phase into the stable β-phase (we refer to this sample below as β-AgI). The absorption spectrum of polycrystalline β-AgI shows a sharp edge at around 450 nm at room temperature and gives strong intensity in the shorter wavelengths, as shown in our previous paper.18 The temperature dependence of the absorption spectrum of β-AgI single crystal was examined, and the absorption edge was shown to shift to shorter wavelength with decreasing temperature with a constant value

Figure 3. Intensity dependence of PCE spectra of KI-treated polycrystalline β-AgI pellets measured in the wavelength range 700− 300 nm at (a) 298 K and (b) 77 K. Solid and dotted lines represent PCE spectra obtained with strong and weak irradiated light, respectively.

shows the results. With strong photoirradiation, the PCE spectra exhibit a sharp peak at 450 nm at 298 K and at 430 nm at 77 K, and the photocurrent relative to the absorption intensity is considerably higher at longer wavelengths than at shorter wavelengths. With weak photoirradiation, on the other hand, the PCE spectra at both temperatures are rather similar to the absorption spectrum, implying that the photocurrent is nearly proportional to the absorption intensity for weak photoirradiation. It should be noted, however, that the photocurrent intensity relative to the absorption intensity at shorter wavelengths with 0.01I0 still becomes weaker with decreasing wavelength, suggesting that the photocurrent 9210

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following excitation at shorter wavelengths are very sensitive to the photoirradiation light intensity. Note that the absorption intensity at shorter wavelengths slightly increases with decreasing wavelength.18 The PCE spectrum obtained at 77 K is considerably sharper than that obtained at 298 K. The photocurrent is over 1 order of magnitude higher at 298 K than that at 77 K and the ratio of the photocurrent for I0 to that for 0.01I0 is approximately 10 (∼20 nA and ∼1.5 nA at room temperature and ∼2 nA and ∼0.15 nA at 77 K, respectively), indicating the nonlinear light intensity dependence of the photocurrent for the wide range of photoirradiation intensity. Note that the photocurrents are compared at the maximum for each of the irradiation light intensities. The low photocurrent obtained by short-wavelength excitation (below 400 nm) with intense irradiation may indicate that the photoinduced increase in the mobility of interstitial Ag+ ions may be suppressed by efficient recombination of electron−hole pairs because strong photoirradiation generates a large amount of electron−hole pairs. Photoexcitation can generate excitons consisting of electron−hole pairs, and the dissociated electrons and/or holes (Ag+ ions) may interact with lattice ions, inducing local lattice distortion on a microscopic scale. The whole system may then have a new equilibrium structure, which gives rise to lattice relaxation. Thus, generation of electron−hole pairs following photoexcitation alters the crystal structure, which may increase the mobility and/or the concentration of the interstitial Ag+ ions. With intense irradiation at shorter wavelengths, the exciton density is so high on the surface because of the high absorption intensity that efficient recombination of holes and electrons may occur before dissociation. The positions of the peak in the PCE spectrum (450 nm at 298 K and 430 nm at 77 K), which were clearly observed for intense irradiation, are ascribed to the exciton band obtained in the vicinity of the absorption edge.29 The magnitude of the shift of the peak toward the shorter wavelength region at 77 K well agrees with the one expected from the temperature dependence of the absorption edge.27 In addition, the photocurrent with longer wavelength excitation and with intense irradiation increased by a small amount, which may arise due to the presence of the defects acting as trapping centers. Since a sharp peak of the PCE spectra was observed at 450 and 430 nm at 298 and 77 K, respectively, photoirradiation effects on the bulk resistance were investigated using complex impedance analyzer with excitation at these wavelengths. Figure 4 shows the results obtained at 77 K. Cole−Cole plots show that polycrystalline β-AgI without photoirradiation has a very high bulk resistance of the order of gigaohms at 77 K. When the sample was irradiated with 430-nm light, the bulk resistance decreased to the order of megaohms, a change of approximately 3 orders of magnitudes. The filled black squares in Figure 4 indicate the complex impedance spectrum measured prior to photoirradiation under dark conditions. The filled red circles show the impedance spectrum measured under 430-nm photoirradiation. The filled blue circles, filled blue squares, filled black circles, and blue open circles, respectively, show the spectra obtained under dark conditions 5 min, 30 min, 1 h, and 7 h after turning off the irradiation; these spectra were obtained to investigate the memory effect. Figure 5 shows that photoswitching between high and low resistive states in β-AgI at 77 K is reversible. The bulk resistance decreases to 11.7 MΩ with 430-nm excitation, but it does not recover to its original value (i.e., 4.3 GΩ) under dark conditions

Figure 4. Impedance spectra of KI-treated polycrystalline β-AgI measured at 77 K: Under dark conditions (black squares); under photoirradiation at 430 nm (red circles); dark conditions 5 min after 430-nm excitation (blue circles); dark conditions 30 min after 430-nm excitation (blue squares); dark conditions 1 h after 430-nm excitation (●); dark conditions 7 h after 430-nm excitation (blue open circles). (The data indicated by the red circles, blue circles, and blue squares is replotted in Figure 2b on expanded scales).

Figure 5. Photoswitching of bulk resistance between high and low resistive states of KI-treated polycrystalline β-AgI at 77 K. First 6 cycles indicate switching between the first dark state and the 430-nm excitation state in 5 min intervals, and the next 6 cycles indicate switching between the second dark state and the 430-nm excitation state where the second dark state is measured 30 min after the 430-nm excitation. It is noteworthy that the irradiation time was same for all the cycles.

just after the irradiation is stopped. After 5 min, we measured a bulk resistance of about 74 MΩ under dark conditions, and we observed switching between 74 and 11.7 MΩ for dark conditions and 430-nm excitation, respectively. A resistance of about 167 MΩ can be achieved by increasing the time under dark conditions (30 min). Thus, photoswitching can be controlled between any high resistance obtained under dark conditions and a photoinduced low resistance by controlling the time under dark conditions. Bode plots at 298 and 77 K are shown in Figure 6. Without photoirradiation at 77 K, the resistance in the low-frequency limit is as large as 4.3 GΩ, (which is also indicated in the Cole− Cole plots in Figure 4), and the reactance has a peak at 7.9 Hz. By application of 430-nm photoirradiation, the resistance decreased to 11.7 MΩ, and the reactance peak shifted to 6.3 9211

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appropriate in the present case since the photoinduced change occurred in a fraction of the volume near the surface and since XRD detects the change in the whole volume. It is further noted that the resistance of the γ-phase of AgI, which is more than 1 order of magnitude lower than that of βAgI at 298 K and increases monotonically with decreasing temperature, could not be measured at 77 K because it was extremely high both with and without photoirradiation (the order of teraohms or more). Thus, the gigantic photoresposne of the resistance of AgI at low temperatures seems to be limited to the β-phase.

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SUMMARY In this work the bulk resistance of β-AgI was measured using impedance spectroscopy both with and without photoirradiation at 77 K. A 3 orders of magnitude change in the bulk resistance was observed upon photoirradiation, which is ascribed to a lattice distortion probably resulting from the photoinduced electron−hole pair generation. A reversible photoswitching between low and high resistive states was observed, depending on the time in the dark state.

Figure 6. Plots of (a) resistance and (b) reactance of KI-treated β-AgI at 77 K (solid line) and 298 K (dotted line) with and without photoirradiation (red and black lines, respectively). Black solid lines, under dark conditions at 77 K, correspond to the scale in the left-hand side, and the others correspond to the scale in the right-hand side.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

kHz. By assuming an equivalent circuit consisting of a resistor and a capacitor in parallel, the peak frequency of the reactance corresponds to the inverse of the time constant of the circuit. The present results reveal that the resistance and the relaxation time drastically decrease with photoirradiation at 77 K. At 298 K, the magnitude of the resistance and the peak frequency of the reactance of this sample were ∼2.51 MΩ and 45 kHz without photoirradiation and ∼1.22 MΩ and 120 kHz with photoirradiation at 450 nm, respectively. Thus, the conductivity of AgI is enhanced by photoirradiation even at room temperature, but the enhancement is not as large as that at 77 K for which the conductivity is enhanced by approximately 3 orders of magnitudes. The gigantic enhancement of the conductivity at low temperature (77 K) may be ascribed to the photoinduced enhancement of the carrier concentration (Ag+ ion) and enhancement of the mobility which can induce structural phase transition. The observed frequency shift at 77 K suggests the enhancement of the cooperative localized motion of Ag+ ion30,31 (hopping from an occupied site to an adjacent unoccupied site) with photoirradiation, which is limited to β-phase probably due to its structure. The hopping process depends on the availability of the unoccupied sites which may be limited at 298 K due to the thermally activated hopping. The conductivity under dark conditions prior to photoirradiation was not recovered even after 7 h at 77 K, implying that the photoirradiation effect is stored by the sample. The ionic mobility is suppressed at 77 K, and thus the ionic conductivity decreases. On application of 430-nm photoirradiation, generated electrons transmit the photon energy to interstitial Ag+ ion by trapping of electrons by holes. The mobility of Ag+ ions increases on acquiring the photon energy, which may generate lattice distortions and stacking faults. Consequently, the mobility of Ag+ ions is enhanced and the ionic conductivity increases, as mentioned above. The present result, that is, the slow recovery of the lattice distortion at a low temperature of 77 K, indicates the strong possibility of structural phase transition. To elucidate the mechanism of the lattice distortion, XRD measurements may be not

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (Grant No. 20245001) from the Ministry of Education, Culture, Sports, Science and Technology in Japan.

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