DSpace at VNU: Application of impedance technique for study of Ionic conducting properties of lixla1-x TiO3 Perovskite Thin films

VNU JOURNAL OF SCIENCE, Mathematics - Physics, T.XXII, N01, 2006 A P P L IC A T IO N O F IM P E D A N C E T E C H N IQ U E FO R ST U D Y O F IO N IC C O N D U C T I N G P R O P E R T I E S O F L i.L a ^ T I O g P E R O V S K I T E T H IN F IL M S N g u y e n N a n g Dinh*, N g u y e n T h i B a o N g o c D efartm ent o f E ngineering Physics & Ncinotechnogy, College o f Technology, V N U H Le D i n h T r o n g D efa rtm en t o f Physics, Pedagogical College, Hanoi II, X u a n Hoa, Virth Phuc A b s t r a c t Impedance spectroscopy (IS) technique has been analyzed and applied to measure the ionic conductivity of the films For a superionic conductor, like a thin-film electrolyte, it is necessary to characterize IS by using a two-electrodes ccll, where the thin film is deposited between two metallic electrodes In a very thin film, the Helmholtz layer strongly affects to the conductivity during the measurement So that, an equivalent scheme should be chosen with a separation of three frequency zones to fit the theoretical curves to experimental data The best curve fitted with two to three circles is taken and elaborated to determine all the parameters of the scheme, and R4 —a parameter of the ionic conductivity - in particular Electron-beam-deposited LixL a lxTiO:, crystalline thin films were used for IS measurements Using the fitting method, the experimental data were fitted with the circles obtained by the chosen equivalent scheme From these circles, the ionic conductivity at room temperature of a 350-nm thick Li012La088TiO3 film was found to be of Ơ = 6,52 xl ° s.cm \ I n t r o d u c tio n It is known t h a t for analyzing an ac cu rrent, Y-conductivity has been used, w here Y = l/z If z is z = I z I e'*, Y = / 1z I e"* T h u s Y is a vector with a value of 1/1z I a n d a n opposit p h a se to the phase of z z is de p en d e n t on the frequence of the ac c u rr e n t T h e common a nd suitable ran ge used for study is of 10 mHz to 10 MHz In th e electrolytes, in solide sta te in p a rticu la r the ionic mobility is much smaller t h a n the electron mobility, so t h a t to respond to the change of the electrical field, it is neccessary to m e a s u r e in the ran ge of low frequencies From the experimatal data obtained in th e impe dance spectra (IS) one can d ete rm ine the ionic conductivity and o t h e r p a r a m e t e r s such as the charge tran s p o rt, diffusion coefficient in solids However, it is so m etim es very difficult to analyze the res ults for thin films, because for very th in films all above mentioned p a r a m e t e r s are strongly affected by the *Corresponding author, email: dinhnn@Yint.edu \'H 54 A p p lic a tio n o f Im p e d a n c e T ech n iq u e fo r S tu d y o f 55 thickness and contacts in the m e a su r e m e n t cells Thus, the choice of th e e qu iv a le n t schemes consisting with subschemes of resistance, capacity, etc plays a n i m p o r t a n t role to find out real values of the pa m eters One of the most i m p o r t a n t principles to design th e eq uiv alent schemes is t h a t the total c u r r e n t value as well as th e phase shift m u st have the sam e values as the measured sample Recently, multicompound superionic conductors with a perovskite s t r u c tu r e such as LixLaỊ.xT i have increasingly been studied This is b ecause these materials have a high Li-ion conductivity, even a t not very high t e m p e r a t u r e Especially, the thin-film formed LixL a l xT i can be used in m an y scopes like allsold-state ionic batteries, electrochromic display, elctrochemical sensors, etc [ 1] However, in the experimental researches, to d e te rm in e the ionic conductivity of the thin and/or very thin films of LixL a lxT i by the IS techniq ue is alw ay s to face many difficulties, t h a t obtained res ults often have a large error So t h a t, the aim of this work is to utilize this technique with an alysing a series of equ iv a le n t schemes applied to fit with the obtained experimental d a ta from IS m e a s u r e m e n t s on the thin films b Fig Equivalent schemes of samples measured in an electrochemical cell (a) and the corresponding elements of Zj part A n a ly sis o f th e im p e d a n c e th eo ry 2.1 E q u iv a le n t sc h e m s fo r a three-electrodes cell a n d I S c h a r a c te r is tic s It is well-known t h a t an equivalent scheme for the sa mple m e a s u r e d in th e three-electrodes cell h a s a Randle scheme as presented in Fig In th is schem e Cd is the capacity of the double layer, Zf characterizes the electrochemical proccess For simplicity, z r is splitte d into two p a rt s connected in serial Rs a nd Cs or Rct a n d Zw, where Rs a n d Cs, respectively is a real and an image p a rt of th e impedan ce, Rct is Nguyen Nang Dinh, Nguyen Thi Ba o Ngoe, Le Dinh Trong 56 the resistance of the charge tran sp o rt , Zw is the W a r b u r g im p e d a n c e c h arac terizin g a mass t r a n s p o r t , R is th e resistance of the liquid electrolyte Among th ese p a r a m e t e r s , only Cd a nd R are not d e p e n t onto th e frequency By calculation as shown in [2], the expression for the frequency r ela tio n ship of Rs and C8, respectively is: Cs = l/(ơ( 1/2) Rs = Rct + G /o 1/2 a nd (1) where a is d e te r m i n e d as follows: RT Ơ= (2 ) 1/ ^ Co * [>2 F A n /2 V D ưo D 1R * J Where R is th e ideal gas constant, T is th e absolute t e m p e r a t u r e , n is th e ion valence, F - F a r a d a y constant, A - electrode area, DO, DR - diffusion coefficents, r e s p e c t i v e ly o f o x id a t io n and r e d u c tio n r e a c t io n s , C q , C r - c o r r e s p o n d in g ion co n cen tra tio n s One can express the frequency dependences of th e real a n d image p a r t s of the equivalent schem as follows: K0 + ( c dơ( 112 + )2 +C0 C Ỉ ( r , :i + ° “ ‘ c o C j 1( z im + w "1/2 r cI + ơo>- , / ) 1/2 (3) Ơ + 1j (4) 2 C d c o l / + l ) ■K C Ỉ ( R t , + c c o - 1' )1 At th e low frequencies, » 0, the equations (1) a n d (2) can be expressed as: CD— ZRe = Ro + Ret + © ‘ 1/2 (5) z im = CTC0-"2 4- 2Cd (6) By combining two above equations, one can elim in a te z im = ZRe - R - Ret CO an d o b t a i n ’ + 2Cd T h a t is meaning, the image-vs-real part dependence is a linnear one, when the image p a rt also approaches to zero, then the real p a rt h a s a value equals to: (R0 + (7) CD-> Rct - 2ơ 2Cd) In th e r a n g e of high AC frequency, the expressions (3) a n d (4), respectively t ransfo rm into: A p p l i c a t i o n o f I m p e d a n c e T e c h n iq u e f o r S t u d y of z : = 57 c o C d R c im | + n r R ( ) c d R ct From these e q u a t io n s one can combine in one expression of t h e other kind r elationship be tw ee n th e real a n d image parts, as folows: Í n Z rc- R - ¥ \ / n R cl (10) ) This clearly d e m o n s t r a t e s t h a t in the graphic presentation, Zim- ZRe plot is a half of a round T his ro un d crosses the horizontal axis a t two points, th e first one is a t R„ correspo nd ing to CO > 00 a n d the second one is at (R0 + Rct) - w h e n to -> The ionic conductivity (ơ) can be found from e x p erim en tal d a ta obtained for Rct, it is d e te rm ine d as follows: ( 11) SRa where d a n d s , res pectiv ely is th e thickness and a re a of the working electrode 2.2 E q u iv a le n t sch em fo r a two-electrodes cell a n d its im p ed a n ce sp ectru m The theory of th e IS m ea su re d on a two-flat-electrodes cell has been developed by MacDonall Using this, the a uth ors of [3,4] have studied ionic conductance of solid state electrolyte (or superionic matGrials) Basing on the MacDonall’s a r g u m e n t in general, the im p ed an ce IS a n AC frequency function with threG characteristic ranges of high, middle a n d low frequencies Depending on both the composition and structu re of the ionic m ateria ls, th e IS may have one, two or three circles in the Z| ZKp plots (Fig ) One can analyze the IS characteristics as follows: - High frequency range: In this range t h e r e is no charge t r a n s p o r t a t the interface b o u n d a r i e s b e tw ee n t h e electrodes and electrolyte The e q u iv a le n t scheme IS p r e s e n t e d in Fig 2a, w h e r e Cg is the geometrical capacity of t h e two parallel electrodes in t h e solid electrolyte, Rb is the resistance of the electrolyte T he IS h a s one circle (Arc3) - Middle frequency range: In this range, th e affect of th e geometrical capacity can be neglected T h e im p e d a n c e is determined m ainly by th e capicity of th e double layer formed a t t h e c on ta cts be tw ee n the solid electrolyte a n d electrodes T h u s the charge t r a n s p o r t a t th is in te rface is depen dent on th e reactions a t th e contacting b oundaries T h e e q u iv a le n t scheme is pre sented in Fig 2b, w h e r e R t is the r esistance of th e c h a r g e t r a n s p o r t , Cdl is the capicity of the double layer (Helmholtz layer) The c o rr e s p o n d i n g IS is Acr circle - Low f req ue ncy range: in th e first half circle th e action of the ions e n a b l e s to form a co n ce n tra tio n gradient of the ions with the The im p e d a n c e plot h a s l i n e a r form with a slop e q u als to unit When c u r r e n t a cts like a quazy-DC one T hus the ZIm -> and ZRp AC c u r r e n t on im p ed an ce Zd co-» the AC t h e n h a s the 58 Nguyen Nang Dinh, Nguyen Thi B a o Ngoe , Le D inh Trong corresponding value of Rct in the horizontal axis The im pc d an c e sp e c tr u m h a s the form of A r c l in Fig Fig Equivalent schemes at different friquencies ranges and corresponding impedance spectra of the samples measured by a twoelectrodes cell T he im p e d a n c e s p e c tr a o f LixLaj.xT i th in film s ,1 E x p e r i m e n t a l e v i d e n c e The th in films of LixL a lxT i with X “ 0.12 has been prepared by electron beam deposition The e x p er im e n tal im pedance spectra obtained for two-electrodes cells vs the film thickness a re show n in Fig For the films thic ker 350 nm, the IS becomes na rrow er with th e decrease of the film thickness This is consistent with the MacDonall’s th eory for the two-electrodes cells From the th ic k nes s of 350 nm down, instead, the conductivity is increased, the IS is not only condensed bu t also expanded into tim es of th e circle r a d iu s (see d3 plot) This shows that the MacDonall’s theory can be applied onlv for the films with some critical thickness In p r e s e n t sa m ple s it is aro und 350 nm This may probably ^ be a tt r ib u t e d to the films thickness o effect So that, the choice of N eq uiv ale nt schemes sim ilar to the schemes from Fig is not suitable more for such a t h in film It is necessary to tak e th e influence of both the capacity of Helmholtz layer a n d t h e c h a r g e of t h e c o n c e n t r a t i o n Fig Im pedance spectra o f L ixL a J^ r iU tn in g radien t into th e eq uiv alent films with different thickness: d = 900 nm (dl), ^ 650 nm (cL2) and 350 nm (d3) A p p l i c a t i o n o f I m p e d a n c e T ec h n iq u e for S t u d y of 59 3.2 A n a l y s i s a n d e s t im a ti o n o f ionic c o n d u c t i v i t y Figure p r e s e n ts the IS data (dark points) of a 350 nm - thick LixL a l xT i film In the frequency range from 10 mHz to 10 MHz, the IS splits on two clear circles Moreover, th e second circle s t a r t s at th e frequency when the first circle is not finished yet This shows t h a t all th e processes occuring in the middle a nd low frequencies have strongly affected to the samples In deed, the a p p ea n c e of large circle concernning to the low frequency n g e proves a much considerably large influence of the electrochem ical processes in P ig Im pedance spectra d a ta (dark p o in ts) o f a 350the interface b e tw ee n th e nm thick LiJLdj.xTiOfr measured in frequency range from solid electrolyte * a n d 10 mHz to 10 MHz The solid circles are the fitted IS electrodes T h u s, for fitting obtained from the equivalent scheme of Fig the theoretic al IS with the ex p erim e ntal d a t a it is neccessary to design a series of e quiv ale nt schem es where above mentioned processes m u s t be take n in considering Using software available in the A uto.L ab-Pote ntiostat-PGS-3 0, one can choose different parallel a n d serial pa rts of th e sch em es to design an equ ivalent schem e t h a t is most fitted with e xperim ental circles By th is method we have obtained a scheme as p r e s e n t e d in fig The r e s u l t of th is fitting is shown by the solid circles in Fig he e q u iv a le n t schem e in Fig consists of t h r e e p a r t s corresponding to three ranges of AC freque ncies, as follows: - The p a r t I h a s only R l , the resistance of contact a nd depend on th e frequency So th e impedance has only real part lead wires does not The p a r t II c h a r a c t e r i z e s the contribution of th e electrochemical processes in the interface b e tw ee n t h e t h i n film and electrodes Here R2 is th e res istan c e of metal/solid electrolyte contact; C l and R3, respectively are the capacity and the resistan ce cau sed by th e concentration of ions in th e electrolyte; C2 is the capacity of the Helmholtz layer ( t h a t strongly affect to th e sa mple when th e l a s t is thin enough) Besides, Q c h a r a c t e r i z e s the grain a n d b ou n da rie s size effect Its corre sponding im p e d a n c e is Z(Q) = A*(jco)'n, where n = , Q is intrinsic resistance, n = 0.55, Q is th e W a r b u r g impedan ce, a nd n = , Q is the capacity - The p a r t III c h a r a c t e r i z e s ionic conductivity of the t h in films in th e low frequency range H e re C3 is th e capacity formed betw een th e two parallel electrodes and R4 is the r e s i s t a n c e of th e ionic conductance Nguyen N a n g Dinh , Nguyen Thi Bao Ngoe , Le Dinh Trong 60 T a b l e P re se n ted all the values of the p a r a m e t e r s obtained from the calculation by this eq uiv alent scheme F ig Equivalent schemes for a 350 nm - thick film o f LixL a l xT i0 Table Physical parameters determined from the equivalent scheme in Fig for the Lio 12^"^0 thin film (I) (III) (II) R1(Q) R2(Q) R3(Q) Cl(nF) C2(|iF) 794 -2,5 37 103 0,61 Q A(Q) n , x l 0'6 0,52 R4(Q) C3( h F) 5,37 0,12 T aking the value of R4 = 5.37 Q from the table, for the Li 12L a 88T i sample with a thickness of 350 nm a nd an electrode a re a of cm2, using formula (11), one can d e te rm in e th e Li-ion conductivity, it e q uals to Ơ = 6,52 x i o s.cm '1 t h a t is much h igher t h a n t h a t of th e t h ic k er sample This va lu e is quite close to the re su lt t h a t was r ep o rte d in [5] C on clu sion The ionic conducting properties of thin fimls of perovskite st r u c tu r e s has been studied by analysi ng a nd application of th e impedance techniq ue for the twoelectrodes cells The resu lt has shown how to fit the theoretical IS with the experimental d a ta for very th in film samples The ionic conductivity of the LixLaj xT i thin films deposited by electron beam was de te rm in e d with a high accuracy due to the application of th e fitting method For a 35C nm-thick film of Li 012La 88TiO3, the Li-ion conductivity was found to be Ơ % 6,52 xlO '6 s.cm'1 A c k n o w l e d g e m e n t s This work is fulfilled due to th e financial support of the Vietnam National Uni ve rsity Hanoi by a Special project for researches in 20062007 A p p l i c a t i o n o f I m p e d a n c e T echn iq u e for S t u d y of 61 R e feren ces C.G Granq vist, H andbook o f inorganic electrochromic m aterials, Elsevier A m sterd am -1995, 618 p M F Daniel, B D esb at a nd J c L as segues, I n f r a r e d a nd R a m a n spectroscopies of R f-sp u ttered tu n g s te n oxide films, J o u r n a l o f solid state chemistry, v (1 8 ) , pp 127 - 139 Gary s K a n n e r a nd Darryl p Butt, R a m a n a n d electrochemical probes of the dissolutin kinetics of tu n g sten in hydrogen peroxide, J Phys Chem B V.102(1998), pp 9501 - 9507 S Papaethim ious, G leftheriotis, p Yianoulis, Study of electrochromic cell incorporating WO„ M o 3> WO 3-M0 O a nd V20 coating, T h in solid film s, v.343-344(1999), pp 183-186 J M Amarilla, F Tedjar a nd c Poisignon, Influence of KOH concentration on the y - M n redox mechanism, Electrochimica Acta, Vol 15(1994), p 2321 2331  ... conductivity of the LixLaj xT i thin films deposited by electron beam was de te rm in e d with a high accuracy due to the application of th e fitting method For a 35C nm-thick film of Li 012La 8 8TiO3, ... a n t h a t of th e t h ic k er sample This va lu e is quite close to the re su lt t h a t was r ep o rte d in [5] C on clu sion The ionic conducting properties of thin fimls of perovskite st... e te rm in e the ionic conductivity of the thin and/or very thin films of LixL a lxT i by the IS techniq ue is alw ay s to face many difficulties, t h a t obtained res ults often have a large