Cation Driven Charge Trans fer in (Co, Fe) Prus sian Blues

Transcription

1 Jour nal of the Chi nese Chem i cal So ci ety, 2002, 49, Cation Driven Charge Trans fer in (Co, Fe) Prus sian Blues Hwo-Shuenn Sheu* ( ), Shih-Hui Jen ( ), Ting-Wei Kang ( ), Ling-Yun Jang ( ) and Jyh-Fu Lee ( ) Synchrotron Ra di a tion Re search Cen ter, Hsinchu Sci ence-based In dus trial Park, Hsinchu 30077, Taiwan, R..C. The charge trans fer be tween Co balt and Iron ions in a se ries of (Co, Fe) Prus sian blues has been in ves ti - gated by syn chro tron X-ray ab sorp tion spec tros copy and pow der dif frac tion. (Co, Fe) Prus s ian blues, A xco y] nh 2 where A = Li, Na, K, Rb and Cs, were pre pared by dropwise mix ing method. X-ray ab - sorp tion near edge spec tra showed that co balt was in a higher ox i da tion state when the in ter sti tial al kali cat ion changed from Li, Na, to K, Rb, Cs. At the same time, the va lence of Fe shifted from 3+ to 2+. The cat ion driven charge trans fer abil ity fol lows the or der of Li Na < K < Rb Cs. A sim i lar trend was also found when the KCl con cen tra tion was in creased dur ing the syn the sis of K-containing Prus sian blue. The higher the con cen - tra tion of KCl, the more Fe 2+ -CN-Co 3+ frag ment in the (Co, Fe) Prus sian blue, and hence the higher ef fi ciency for photo-induced mag ne ti za tion. The Co-N dis tance in Prus sian blue be came shorter, about 0.15 Å in Co 2+ salt com pared to Co 3+ salt, and the unit cell di men sion was re duced about 0.3 Å as well. IN TR DUC TIN The photo-induced mag ne ti za tion in (Co, Fe) Prus sian blues was first re ported by Fujishima and Hashimoto in Since then, this phe nom e non has been ex ten sively in - ves ti gated by means of var i ous ex per i men tal tech niques 2-8 as well as the o ret i cal cal cu la tions The com po si tion of (Co, Fe) Prus sian blue is nonstoichiometric, and the struc ture is very com pli cated due to the pres ence of many de fects in crys - tal. Both ex per i men tal re sults and the o ret i cal cal cu la tions sug gested that photo-induced mag ne ti za tion was at trib ut able to the (CN) 5Fe 2+ -CN-Co 3+ (NC) 4(H 2) 2 unit. Dur ing the ir ra - di a tion by red light, the Fe 2+ -CN-Co 3+ (dia mag netic) switched to Fe 3+ -CN-Co 2+ (fer ro mag netic) along with elec tron trans fer and spin in ver sion at very low tem per a ture. n the other hand, blue light drives in the op po site di rec tion. The more Fe 2+ -CN-Co 3+ frag ment in the (Co, Fe) Prus - sian blue the more ac tive the photo-induced frag ment is; there fore, the higher ef fi ciency of photo-induced mag ne ti za - tion is. Sato et al. 15 re ported that the elec tronic struc ture of (Co, Fe) Prus sian blue thin film was switched from Fe 3+ (low spin)-cn-co 2+ (high spin) to Fe 2+ (low spin)-cn-co 3+ (low spin) by re plac ing the Na + to K + at in ter sti tial sites. The re - verse pro cess oc curred when re plac ing K + by Na +. The mag - netic prop erty, in terms of Tc, was also in creased from 280 (Na + ) to 340 K (K + ). Verdaguer et al. 7 have ex am ined a se ries of (Co, Fe) Prus sian blues with var i ous amounts of Cs cat ions in serted in the tet ra he dral sites of the struc ture. Their re sults showed that the change of the mag ne ti za tion in duced by light de pends on a com pro mise be tween the num ber of ex cit able dia mag netic pairs and the amount of ] va can cies giv ing the net work flex i bil ity. It is well known that (Fe, Fe) Prus sian blues could be syn the sized from FeCl 3 mixed with K 4], or from FeCl 2 with K 3]. The fi nal prod ucts are iden ti cal for both syn the sis pro cesses. Such a phe nom e non of charge trans fer be tween two iron ions is called self-redox. The struc - ture of Prus sian blue can be de scribed as two iron ions chem i - cally co or di nated on the C and N ends of the cy a nide group. The Fe 3+ is sur rounded by six C at oms, and the Fe 2+ is sur - rounded by six N at oms. The ligand field is strong at Fe 3+ co - or di nated on the C end and weak at Fe 2+ on the N end. In this work we syn the sized (Co, Fe) Prus sian blues start ing from CoCl 2 and K 3] salts in which the self-redox pro cess is also ob served. The ox i da tion states of co balt and iron are highly de pend ent on their start ing forms and ra tio as well as the type of coun ter ions. These fac tors sig nif i cantly af fect their ox i da tion states and spin states. In this pa per, we will re port how to con trol the Fe 2+ - CN-Co 3+ frag ment by chang ing coun ter ions dur ing syn the - sis. These Prus sian blues were char ac ter ized by syn chro tron X-ray ab sorp tion spec tros copy and pow der dif frac tion. Ded i cated to the cel e bra tion of the sev en ti eth an ni ver sary of Chem i cal So ci ety the lo cated in Tai pei. * Cor re sponding au thor. hsheu@srrc.gov.tw

2 814 J. Chin. Chem. Soc., Vol. 49, No. 5, 2002 Sheu et al. EX PER I MEN TAL SEC TIN A se ries of A x Co y ] nh 2 (A = Li, Na, K, Rb, and Cs) were pre pared ac cord ing to the lit er a ture 16 via pre cip - i ta tion from a so lu tion by dropwise mix ing equiv a lent amounts of 0.01 mol dm -3 so lu tions of K 3 ] and CoCl 2 (each con tain ing the cor re spond ing al kali chlo ride of 1 mol dm -3 ). The so lu tion was then kept at 40 C and con tin u ously stirred over night. All the pre cip i tates were dried in 80 ºC for 10 hours and then placed in at mo sphere (tem per a ture of ca. 25 C and rel a tive hu mid ity of ap prox i mately 80%) so that these ma te ri als were in hy drated form. The other se ries of K xco y] nh 2 were pre pared by us ing the same pro - cess ex cept the 1 M ACl was re placed by KCl so lu tion of var - i ous con cen tra tions (1.5 M, 1 M, 0.5 M, 0.1 M and 0 M). The metal con tents in the re sul tant pow der were char ac ter ized by ICP-AES (see Ta ble 1). X-ray ab sorp tion spec tra of the pow der sam ples were re corded at wig gler beamline BL17C at the Syn chro tron Ra - di a tion Re search Cen ter (SRRC), Tai wan. Trans mis sion mode was used for de tec tion. The XAS mea sure ments were car ried out at both the Fe K-edge and the Co K-edge. A Si(111) dou ble-crystal mono chro ma tor was em ployed for scan ning en ergy. Data anal y sis was per formed fol low ing stan dard pro ce dures 17,18 us ing the UWXAFS pack age (in - clud ing AUTBK, ATMS, 22 FEFF, 23,24 and FEFFIT 19,25 pro grams). X-ray pow der dif frac tion was car ried out at BL17A of SRRC. The wave length of the in ci dent X-ray was Å. XRD pat terns were col lected in re flec tion mode of a flat sam - ple in the range (2 ) from 10 to 120 at an in ter val of 0.01 for about 10 sec per step. Rietveld re fine ment was ap plied to the dif frac tion pat terns us ing the GSAS pro gram. 26 RE SULTS AND DIS CUS SIN The vari a tion of the Co/Fe ions ra tio in (Co, Fe) Prus - sian blues are quite large among var i ous sam ples, as can be seen in Ta ble 1, al though these com pounds were syn the sized start ing from the same stoichiometric ra tio and fol low ing the same pro ce dures. X-ray ab sorp tion spec tros copy is a pow er - ful tool for char ac ter iza tion of the va lence, lo cal sym me try, bond dis tance, and the type and num ber of neigh bor ing at oms around the se lected ab sorb ing atom. Fig. 1 shows the X-ray ab sorp tion near edge spec tra (XANES) at Fe and the Co K- edges for A xco y] nh 2 pow ders. The XANES of start ing ma te ri als, CoCl 2 and K 3], are also in cluded for com par i son. The XANES at Fe K-edge (top of Fig. 1) are sub stan tially iden ti cal for the Li and Na salts. How ever, the ab sorp tion edge (E 0) shifted to lower en ergy (by ca. 1 ev) for K, Rb, and Cs salts. These re sults in di cated that the ox i da tion state of Fe al tered from 3+ to 2+ on the re place ment of Li and Na by K, Rb, and Cs. At the same time, the po si tion of Co K-edge (bot tom of Fig. 1) shifted to higher en ergy for K, Rb, and Cs salts as com pared to the Li and Na salts. b vi ously the charge trans fer be tween Fe and Co ions in A xco y] nh 2 pow ders oc curred. It is also in ter est ing to note that the white lines of K, Rb, and Cs salts at Co K-edge be came broader and shifted to higher en ergy, in di cat ing some de gree of mix ing Co 2+ and Co 3+ spe cies. n the whole, the per cent - age of Co 3+ spe cies in creased as the in ter sti tial al kali cat ions changed ac cord ing to the se quence of Li Na < K < Rb Cs. This trend also re flected the rel a tive driv ing abil ity for charge trans fer by cor re spond ing al kali cat ions. Fig. 2 shows the Fou rier trans formed EXAFS func tions at both Fe and Co K-edges for var i ous (Co, Fe) Prus sian blues, and the bond dis tances de rived from EXAFS data anal - Table 1. Contents of the Alkali Metal, Co, and Fe in Various (Co, Fe) Prussian Blues A A% K% Co% Fe% Proposed Formula Li Li 0.59 K 10.0 Co 1.55 Na Na 7.3 K 0.37 Co 1.52 K K 11.6 Co 1.97 Rb Rb 1.72 Co 1.30 Cs +* Cs Co M KCl K 3.46 Co M KCl K 2.05 Co M KCl K 1.04 Co 2.10 * Measured by ICP-MS. The sample may be co-precipitated with ACl.

3 Cat ion Driven Charge Trans fer in (Co, Fe) Prus sian Blues J. Chin. Chem. Soc., Vol. 49, No. 5, y sis are listed in Ta ble 2. Both C N bond length and Fe-C dis - tance did not show sig nif i cant dif fer ences among all these com pounds. The C N bond ing in (Co, Fe) Prus sian blue is so strong that its bond length is hardly af fected by the ox i da tion state of Co. The Fe-C bond length is only slightly af fected (ca Å) by the ox i da tion state of ei ther Co or Fe. In con - trast, Co-N dis tance is short ened for ca Å when the ox i - da tion state of co balt be comes higher as in the case of K and Rb Prus sian blues salts. Co 3+ was known to have a stron ger ligand field. As a re sult, the stron ger Co-N bond ing led to a shorter bond dis tance and a low spin state of Co was ob tained. The in flu ence of Na + and K + cat ions on the ligand field have also been ob served in Prus sian blues thin films by Hashimoto et al. 15 n the other hand, Sinha et al. 27 re ported a re dox po - ten tial shift in duced by the re place ment of al kali cat ions in the elec tro lyte for (Ni, Fe) Prus sian blues. A small por tion of Na xco y] nh 2 pow ders were dipped into a 3 M KCl aqua so lu tion at room tem per a - ture over night. Af ter dry ing, the sam ple was ex am ined by XRD again. The (2 0 0) re flec tion was found to shift to a higher Bragg an gle, sug gest ing a shrink age of the lat tice which prob a bly arose from a trans for ma tion of Co 2+ to Co 3+. This ex per i ment also pro vided ev i dence that the al kali cat - Table 2. Bond Distances (in Å) of the A x Co y ] nh 2 (A=Li, Na, K, and Rb) Derived from EXAFS Data Analysis Bond Li + Na + K + Rb + Co-N 2.095(6) 2.103(6) 1.867(4) 1.829(7) Co- 2.41(3) 2.42(3) 2.15(2) 2.10(2) N-C* 1.190(4) 1.190(4) 1.179(4) 1.176(4) Fe-C* 1.929(2) 1.930(2) 1.913(2) 1.907(2) Derived from Co K-edge EXAFS. * Derived from Fe K-edge EXAFS. ions at the in ter sti tial sites played a cru cial role in de ter min - ing the elec tronic and spin states of the host com pounds. The ion ex change be hav ior does not only hap pen in thin film and elec tro chem is try but also in its pow der form. The al kali ionic ra dius are 0.68, 0.98, 1.33, 1.48, and 1.67 Å for Li +, Na +, K +, Rb +, and Cs +, re spec tively. (Co, Fe) Prus sian blue pos sesses a three di men sional net work struc - ture with a unit cell di men sion of ca. 10 Å. There fore the cage size from Co-NC-Fe edge to edge is ca. 5 Å. It is enough to in - cor po rate more than one al kali ion. How ever, the unit cell Fig. 1. XANES of Fe and Co K-edges of (Co, Fe) Prus - sian blues of var i ous al kali salts. a: Li, b: Na, c: K, d: Rb, e: Cs. Fe K-edge of K 3] (top) and Co K-edge of CoCl 2 (bot tom) are also in - cluded for com par i son. Fig. 2. Ra dial dis tri bu tion func tions around Fe and Co at oms in (Co, Fe) Prus sian blues of var i ous al - kali salts: (a) Li, (b) Na, (c) K, and (d) Rb.

4 816 J. Chin. Chem. Soc., Vol. 49, No. 5, 2002 Sheu et al. was found to shrink with an in crease in the atomic num ber of the al kali ion. The rea son is thought to be that (Co, Fe) Prus - sian blue is hy drated and al kali ions have a strong hydration af fin ity. The af fin ity de creases as the atomic num ber in - creases. We spec u late that the driv ing force for unit cell shrink age may come from the hydration af fin ity of the al kali ion, be cause desorption of wa ter mol e cules was re ported to be the main cause for the change of Co va lence in the (Co, Fe) Prus sian blues. 28 The driv ing force from al kali cat ions could be also ex - am ined by us ing dif fer ent con cen tra tions of the same kind of cat ion. In deed, we ob served a change in the ox i da tion states of both Co and Fe in (Co, Fe) Prus sian blues by us ing KCl so - lu tions with var i ous con cen tra tions in syn the sis. The XANES spec tra of Prus sian blue salts with var i ous KCl con cen tra - tions are pre sented in Fig. 3. A higher con cen tra tion of KCl so lu tion used and hence higher K + con tent in the fi nal prod uct led to more Co 3+ spe cies. The shrink age of lat tice vol ume by al ter ing the al kali ions or in creas ing their con cen tra tion is ac com pa nied by a change in the ox i da tion state of Co in (Co, Fe) Prus sian blue and an in crease in its ligand field. There fore the spin states of Co ion switches from a high spin to a low spin state. The change of lat tice di men sion could be ex am ined by dif frac tion method. X-ray pow der dif frac tion was also ap plied to in ves ti - gate the (Co, Fe) Prus sian blues pre pared by us ing var i ous KCl con cen tra tions. A pro gres sive shift of the (2 0 0) re flec - tion to high an gle (see Fig. 4) in di cated that the cell shrunk with the in creased KCl con cen tra tion. Charge trans fer be - tween Co and Fe not only al tered the lo cal struc ture, but the pe riod of long ranged or der ing in crys tal was also changed. Rietveld re fine ment was per formed on the sam ple syn the - Fig. 4. (2 0 0) Re flec tion in X-ray pow der dif frac tion pat terns of K xco y] with var i ous KCl con cen tra tions in syn the sis: (a) 0 M, (b) 0.1 M, (c) 0.5 M, (d) 1.0 M, and (e) 1.5 M. Fig. 3. Co K-edge XANES of K x Co y ], KCl: 0 M (a), 0.1 M (b), 0.5 M (c), and 1 M (d). Fig. 5. Rietveld re fine ment of the XRD pat tern of (Co, Fe) Prus sian blue with a KCl so lu tion of 0.1 M in syn the sis. The (+) sym bol de notes ex per i - men tal data; solid line de notes the fit ting re sult; ticks mark the re flec tion po si tions; and the lower curve rep re sents the dif fer ence be tween ex per i men tal and cal cu lated data.

5 Cat ion Driven Charge Trans fer in (Co, Fe) Prus sian Blues J. Chin. Chem. Soc., Vol. 49, No. 5, sized with 0.1 M KCl, as shown in Fig. 5. The XRD data can be fit ted by two phases with a cell di men sion of (1) and (6) Å, re spec tively, cor re spond ing to Co 2+ and Co 3+ spe cies in the (Co, Fe) Prus sian blue (wrp = 0.05, Rp = 0.033). The bond dis tances of Fe-C de rived from Rietveld re - fine ment are close to each other for these two phases but the Co-N dis tances change to about 0.13 Å (1.987(5) and 1.86(3) Å for phase one and phase two, re spec tively). The amount of Co-N bond dis tance shrinkage by an in crease in the Co ox i da - tion state is less in var i ous K salt than that be tween two dif fer - ent kinds of cat ions. In this cal cu la tion the C N bond dis - tance was fixed. These re sults are con sis tent with those de - rived from EXAFS data anal y sis in Ta ble 2. CN CLU SINS In this re port, we dem on strated that the charge trans fer be tween Fe and Co ions in (Co, Fe) Prus sian blues were clearly ob served by chang ing the in ter sti tial al kali cat ions. The driv ing force for charge trans fer be tween Fe and Co ions is in the or der of Cs Rb > K > Na Li. The unit cell also shrunk for ca. 0.3 Å on switch ing from Co 2+ (high spin) to Co 3+ (low spin), as ev i denced by both EXAFS and XRD. The net re sult from charge trans fer was the for ma tion of more (Co 3+, Fe 2+ ) spe cies which were sug gested to be re spon si ble for the photo-induced mag ne ti za tion. This work provids a new route to gen er ate (Co, Fe) Prus sian blues with a lightinduced mag ne ti za tion at higher ef fi ciency. AC KNWL EDGE MENT This work was fi nan cially sup ported by the Na tional Sci ence Coun cil of the Re pub lic of China and the Syn chro - tron Ra di a tion Re search Cen ter. Re ceived May 3, Key Words Prus sian blue; Charge trans fer; Cat ion driven; EXAFS; XRD. REF ER ENCES 1. Sato,.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Sci ence 1996, 272, Einaga, Y.; Sato,.; Iyoda, T.; Kobayashi, Y.; Ambe. F.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1997, Einaga,Y.; hkoshi, S.; Sato,.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1998, Sato,.; Einaga, Y.; Fujishima, A.; Hashimoto, K. Inorg. Chem. 1999, 38, Bleuzen, A.; Lomenech, C.; Escax, V.; Vil lain, F.; Varret, F.; Mou lin, C.; Verdaguer, M. J. Am. Chem. Soc. 2000, 122, Mou lin, C.; Vil lain, F.; Bleuzen, A.; Arrio, M.; Sainctavit, P.; Lomenech, C.; Escax, V.; Baudelet, F.; Dartyge, E.; Gallet, J.; Verdaguer, M. J. Am. Chem. Soc. 2000, 122, Escax, V.; Bleuzen, A.; Mou lin, C.; Vil lain, F.; Goujon, A.; Varret, F.; Verdaguer, M. J. Am. Chem. Soc. 2001, 123, Cham pion, G.; Escax, V.; Mou lin, C.; Bleuzen, A.; Vil lain, F.; Baudelet, F.; Dartyge, E.; Verdaguer, M. J. Am. Chem. Soc. 2001, 123, Yoshizawa, K.; Mohri, F.; Nuspl, G.; Yamabe, T. J. Phys. Chem. B 1998, 102, Kawamoto, T.; Asai, Y.; Abe, S. Phys. Rev. B 1999, 60, Nishino, M.; Yamaguchi, K. Phys. Rev. B 1998, 58, Gutsev, G. L.; Reddy, B. V.; Khanna, S. N.; Rao, B. K.; Jena, P. Phys. Rev. B 1998, 58, Nishino, M.; Yoshioka, Y.; Yamaguchi, K. Chem. Phys. Lett. 1998, 297, Nishino, M.; Kitagawa, Y.; nishi, T. Mol. Crys tal Liq. Cryst. 2000, 343, Sato,.; Einaga, Y.; Iyoda, T.; Fujishima, A.; Hashimoto, K. J. Phys. Chem. B 1997, 101, Kulesza, P. J.; Malik, M. A.; Berrettoni, M.; Giorgetti, M.; Zamponi, S.; Schmidt, R.; Marassi, R. J. Phys. Chem. B 1998, 102, Sayers D. E.; Bunker, B. A. In X-ray Ab sorp tion: Prin ci ples, Ap pli ca tions, Tech niques of EXAFS, SEXAFS and XANES; Koningsberger, D. C.; Prins, R., Eds.; John Wiley & Sons: New York, 1988; p Asakura, A. In X-ray Ab sorp tion Fine Struc ture for Cat a - lysts and Sur face; Iwasawa, Y., Ed.; World Sci en tific: Sin ga - pore, 1996; p Newville, M. Ph. D. The sis, Newville, M.; Livins, P.; Yacoby, Y.; Rehr, J. J.; Stern, E. A. Phys. Rev. B 1993, 47, Newville, M. AUTBK doc u ment, Ravel,B. ATMS doc u ment, Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. Phys. Rev. B 1995, 52, Ankudinov, A. FEFF601 doc u ment, Newville, M. FEFFIT doc u ment, Larson A. C.; Von Dreele, R. B. Gen eral Struc ture Anal y sis

6 818 J. Chin. Chem. Soc., Vol. 49, No. 5, 2002 Sheu et al. Sys tem, Los Alamos Na tional Lab o ra tory, Los Alamos, NM, USA Sinha, S.; B. Humphrey, D.; Bocarsly, A. B. Inorg. Chem. 1984, 23, Sheu, H.-S.; Lee, J.-F.; Jean, Y.-C.; Kang, T.-W. IUCr XVIII meet ing, Glas gow, UK