Current Status of Combinatorial and High-Throughput Methods for Discovering …koasas.kaist.ac.kr/bitstream/10203/18378/1/Current stat… · · 2018-01-30Current Status of Combinatorial

Current Status of Combinatorial and High-Throughput Methodsfor Discovering New Materials and Catalysts

Seong Ihl Woo*, Ki Woong Kim, Hyun Yong Cho, Kwang Seok Oh, Min Ku Jeon, Naresh Hiralal Tarte, Tai Suk Kimand Asif Mahmood

Department of Chemical and Biomolecular Engineering & Center for Ultramicrochemical Process Systems (CUPS), KoreaAdvanced Institute of Science and Technology(KAIST), 373-1 Guseong-Dong, Yuseong-Gu, Daejeon, 305-701, Republic of Korea,Tel: þ82-42-869-3918, Fax: þ82-42-869-8890, E-mail: [email protected]

Keywords: Catalyst, Combinatorial material research, Olefin polymerization, DeNO, Direct methanolfuel cell, Dielectric, Ferrolectric, Ferromagnetic, Lithium-ion battery.

Received: 24. 09. 2004; Accepted: 02. 12. 2004

AbstractCombinatorial technology has been evaluated as the revolutionary approach to overcomethe limitation of conventional research and to advance research in the development ofnovel materials and catalysts. For the past decade, because of the advance in librarysynthetic method and characterization tools, combinatorial and high-throughputmethodology surprisingly matured and nowadays has been extended to discovering novelolefin polymerization catalysts. However, despite such an advance, the characterizationmethodology did not keep pace with the increase in library density and limited theapplication of combinatorial technology. Therefore, in combinatorial technologies, thedevelopment of novel characterization methods is urgent and very important. In thisreview, we introduce several characterization tools and synthetic apparatus that arecurrently applied to discovering inorganic materials and catalysts using combinatorialtechnology, and consider how to overcome these limitations.

1 Introduction

For the last decade, combinatorial technology has been in-creasingly applied for the discovery and optimization ofnovel materials and catalysts. In its early stages, it was fo-cused on the development of electronic materials such ashigh-Tc superconductor and phosphors, but nowadays ithas been extended to the development of new catalyst sys-tems for olefin polymerization as a result of the surprisingadvances in library synthesis and characterization tools.

In the case of thin-film library synthesis, instead of usingbinary or quaternary masks, a composition-spread schemehas been efficiently employed for high-density thin-film li-brary synthesis. In the development of novel catalysts, it isnecessary to fabricate powder libraries instead of thin-filmlibraries, owing to the fact that the physical and chemicalproperties that influence the activity of the catalyst are verydifferent from the thin film. For this reason, an automatedliquid handler was developed which can control the amountof liquid down to microliter volumes. However, despite ofthese advances in library synthesis, combinatorial technolo-gy has only been used for a restricted range of catalysts andmaterials because the rapid characterization methodologyhas not kept pace with the increase of library density.

In this contribution, we review the current status ofcombinatorial technology for the discovery and optimiza-

tion of catalysts and inorganic materials, including olefinpolymerization catalysts, deNOx catalysts, anode materialfor direct methanol fuel cells (DMFCs), dielectric/ferro-electric materials, ferromagnetic materials, and electrodematerial for lithium rechargeable batteries. In addition, weconsider how to overcome the limitations in the combina-torial approach, which are mentioned above.

2 Catalysts

2.1 Olefin Polymerization Catalysts

The development of novel catalyst systems for olefin poly-merization has been recognized as time-consuming and ex-pensive work that includes ligand substitution, catalystcharacterization, catalytic activity testing, and polymercharacterization [1]; amounting to a technical bottleneckin the development of new catalysts with high activity forolefin polymerization. Hence, some research groups inlate 1990s suggested the combinatorial approach as an al-ternative solution. It consists of two components as sum-marized in Figure 1: (1) high-throughput synthesis and (2)high-throughput screening.

High-throughput synthesis is focused on the build-up ofligands and catalysts. The various libraries of ligand are

138 QSAR Comb. Sci. 2005, 24 DOI: 10.1002/qsar.200420061 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Review

supported at functionalized polymer beads [2, 3] or homo-geneous organic compounds [4, 5]. Generally, the synthesisof liquid-phase ligands requires separation steps. However,solid-phase ligand synthesis can simplify the process by us-ing functionalized polymer beads. As various synthesizerswere developed recently [6], solid-phase ligands, homoge-neous organic compounds, and liquid-phase ligands couldbe synthesized easily [7, 8].

Brookhart et al. reported that high-molecular-weightpolyolefins were synthesized with Ni(II) and Pd(II) dii-mine derivatives activated by MAO or other ionizing re-agents [9 – 11]. These are the first late-transition-metal sys-tems that control the microstructure of polyolefin. Powerset al. applied the combinatorial concept into the catalyticsystems mentioned the above [2, 3]. Recently, group-IVand group-VI metal catalysts, as well as late-transition-metal catalysts, have been studied using a combinatorialapproach [3, 4].

High-throughput screening is used for the rapid discov-ery from a library of one or a group of catalysts whichhave high activity of polymerization. Powers et al. [2, 3],Schunk et al. [7], Murphy et al. [12 – 13], and Woo et al. [8]used parallel polymerization reactor systems for thescreening of catalyst libraries. These instruments were de-signed to control temperature and pressure independentlyand can be operated under anaerobic conditions. There-fore, polymerization conditions such as temperature, pres-sure, and molar ratio of catalyst to co-catalyst, can be opti-mized easily. Recently, various methods for characterizingpolymers synthesized by homopolymerization or copoly-merization have been suggested [14 – 26]. Potyrailo et al.[14 – 16], M�lhaupt et al. [21], and Gabriel et al. [23] re-ported that fluorescence, infrared (IR) and Raman spec-troscopies are efficient analytical spectroscopic tools forhigh-throughput screening. These are correlated withchemical properties of interest, such as molecular weight,degree of branching, and catalyst selectivity. Schubert

et al. [17, 20, 26] and Pasch et al. [22] reported using chro-matographic tools as gel-permeation chromatography(GPC) and liquid chromatography (LC). Fast GPC sys-tems can characterize molecular weights and the corre-sponding molecular weight distributions of polymer in sol-ution by using online monitoring [17], shorter columns,parallelization, flow-injection analysis, and high-speedGPC columns [20]. In addition, LC techniques, which re-quire 2 – 4 minutes per sample for analysis are suitable forthe analysis of different polymer distributions [19].

Schubert et al. [24] and Meredith et al. [25] utilized scan-ning probe microscopy (SPM) and atomic force micros-copy (AFM) as methodologies of high-throughput screen-ing, because the microstructures of polymers characterizeduisng microscopic tools are related to the mechanicalproperties of the polymers [25].

2.2 DeNOx Catalysts

In the late 1990s, as part of the effort to discovering novelmaterials with useful properties, combinatorial chemistrywas applied to solid-state heterogeneous catalysis [27 – 31].For inorganic electronic materials, high-throughputscreening is relatively simple because the physical proper-ties can be measured using a probe (i.e., for superconduc-tivity) or the naked eye (i.e., for luminescence). However,in the case of solid heterogeneous catalysts, the situation istotally different. Most of the heterogeneous catalytic reac-tions occur in the gas phase, which makes high-throughputscreening of heterogeneous catalysts even more difficult[32]. Because of this, in the 1990s, the combinatorial searchfor novel catalysts has been focused on simple reactionssuch as the dehydrogenation of ethane and the partial oxi-dation of propane [33 – 36]. Here, the solid-state catalystslibrary was synthesized by thin-film deposition [37] and aliquid dosing technique [38 – 42]. However, recently, thecombinatorial research of solid-state catalysts was expand-ed to environmental catalysis, such as those used for NOx

decomposition. In the field of DeNOx catalysis, several re-search groups have performed the HC-SCR reaction byusing multi-channel reactors [43 – 45]. Different patternsof multi-channel reactor are shown in Figure 2. Senkanand coworkers [43, 45] prepared a library of 1500 differentcatalysts in combination with 41 active elements and 13supports by using a liquid dosing technique. They foundthat Cu�Os/13X catalyst showed the best activity in thepresence of 10% water at 250~400 8C. Ritchter and cow-orkers [44] also prepared a 56-member catalyst librarycontaining Ag, Co, Cu, and In. They performed the deNOx

catalytic reaction at high temperature, between 400 and500 8C, in the presence of excess oxygen and found thatAg�Co�Cu catalysts showed good activity except for In-containing catalysts. In each case, the catalytic perform-ance of each library in a multi-channel parallel reactor wasevaluated using resolved mass spectroscopy (MS). In re-cent developments in new high-throughput screening

QSAR Comb. Sci. 2005, 24 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 139

Figure 1. The combinatorial approach for the development ofpolyolefin catalysts.

Current Status of Combinatorial and High-Throughput Methods

methods for solid catalysts, MS has drawn much attentionbecause it is a time- and cost-saving technique. Eventhough MS is a useful tool for screening catalytic perform-ance, it is still a one-by-one analyzing system. Hence, it isnot suitable for rapid screening. However, Sch�th andcoworkers [49] demonstrated a new, fast parallel detectionbased on the color change of an organic dye in the pres-ence of either educts or reaction product in a reaction gasflow at NO decomposition and NO reduction. The pres-ence of NO in a gas stream was detected by the colorchange, from colorless to blue – green, of filter paper im-pregnated with an organic dye, 2,2-azinobis(3-ethylbenz-thiazoline-6-sulfonic acid) (ABTS).

This technique is promising method for parallel screen-ing but, still has the problem that it requires a specific dyefor each reaction to be investigated.

Although high-throughput screening is a powerful toolfor preparing catalyst libraries and finding an optimumcatalyst by analyzing the product gas within each library,to date the selection of active element for a target reactionsuch as deNOx catalytic reaction has been achieved by ex-periment and reported experimental results rather thanthe rational selection of the active elements. To choose theactive components efficiently for the discovery of opti-mized deNOx catalysts, the data-mining technique be-comes more important. Such studies at atomic and elec-tronic levels are expected to play an important role in pre-dicting new catalysts with high activity, selectivity, and re-sistance to poisoning. The effect of a large number of met-als, supports, and additives on catalytic activity wasinvestigated using a computer simulation technique [50,51]. Such a technique can be helpful for improving the pos-sibility of proving the optimum composition of catalyst.

The development of novel catalysts by using combinato-rial technology is continuing. Many research groups haveinvented several methods for the fast screening of hetero-geneous catalysts. However, because these methods eachhave their own advantages and disadvantages, applying

them, with combinatorial technology, to various kinds ofcatalytic reaction is restructed. Therefore, it is a prerequi-site for combinatorial technology to develop high-through-put screening techniques for DeNOx catalysts as well asfast and effective approaches for compositional analysis ofsolid catalysts on the libraries.

2.3 Electrode Material for Direct Methanol Fuel Cells(DMFCs)

A DMFC is a power source that uses methanol and waterto produce carbon dioxide, electrons, and protons at theanode (Eq. 1). The electrons and protons, which are trans-ferred along an external circuit and the electrolyte mem-brane, respectively, react with oxygen to produce water atthe cathode (Eq. 2).

CH3OHþH2O!CO2þ6Hþþ 6e� (1)

3/2O2þ 6Hþþ 6e�! 3H2O (2)

A schematic diagram of a DMFC is shown in Figure 3 [52].DMFCs have the advantages of high energy density ofmethanol (3800 kcal/L) in comparison to hydrogen at360 atm (658 kcal/L), and easy control of liquid fuel, whichis an important point for miniaturization and applicationin portable power sources. However, poor performance ofmembrane-electrode assembly (MEA), which results fromkinetic or activation over-potential at both the anode andcathode, is a bottleneck for the commercialization ofDMFCs.

In the combinatorial search for DMFC catalysts, Mal-louk and coworkers [53] first reported a combinatorialelectrochemistry by using fluorescence acid indicator suchas acridine and quinine, showing that Pt(44)Ru(41)Os(10)Ir(5) was more electrochemically active than com-mercial Pt(50)Ru(50) black, which is widely used as ananode catalyst in DMFC. The array mapping of quaterna-

140 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim QSAR Comb. Sci. 2005, 24

Figure 2. Different types of multi-channel reactors. a) A 64-channel multi-channel reactor [46], b) a 207 micro-channel reactor [47],and c) a 49-channel reactor [48].

Review S. Ihl Woo et al.

ry phase into two dimensions is shown in Figure 4a andcurrent – voltage measurement results are shown in Fig-ure 4b. Recently, Woo et al. [54] reported a novel anodecatalyst for DMFC [Pt(77)Ru(17)Mo(4)W(2)], found byusing high-throughput screening after repeated cyclic vol-tammetry. The introduction of this method made it possi-ble to characterize long-term activity. As we are using cali-brated ink-jet printers, this method still has the problemthat the resultant dot of the combinatorial array has a het-erogeneous phase.

Sullivan et al. [55] suggested an automated electrochem-ical analysis system with combinatorial electrode arrays ina common electrolyte to measure proton concentrationand electrochemical current at each electrode of a 64-elec-trode array. The 64-electrode array and the apparatus usedare shown in Figure 5. Jiang and Chu [56] proposed a dif-


Figure 4. a) Unfolding of a quaternary phase diagram and mapping into two dimensions and and inkjet-printed array of electrocha-talysts on Toray carbon paper. b) Current – voltage data of Pt(44)Ru(41)Os(10)Ir(5) (squares) and Johnson-Matthey Pt(50)Ru(50)(circles) catalysts [53].

Figure 3. Schematic diagram of a DMFC [52].


ferent method, one that can measure the electrochemicalproperties of various electrodes in neutral, concentratedacidic, and basic solutions using a movable electrolyteprobe system.

The reaction region in DMFCs is limited to the contactarea between the electrode and the membrane. The area isvery small because a solid-state polymer membrane can-not be mixed with an electrode consisting of small catalystparticles. Therefore, ionomers such as Nafion are impreg-nated into the catalytic layer [57, 58]. Shul et al. [58] re-ported combinatorial screening results on methanol elec-tro-oxidation as a function of Nafion-ionomer concentra-tion and the chemical ratio of Pt to Ru in a PtRu alloy us-ing an acid indicator. The composition of the array isshown in Figure 6 and the most active composition is[Pt(50)Ru(5)/Nafion¼ 63.6: 6.4 wt..-%].

The number of papers reporting combinatorial studiesof DMFC catalysts is growing; however, the developmentof fully automated systems that are suitable for combina-torial arrays is required for reliable and fast electrochemi-cal analysis.

3 Materials

3.1 Dielectric/Ferroelectric Materials

Combinatorial methodology has been studied for optimiz-ing the chemical composition ratio of Ba/Sr in (Ba,Sr)TiO3

(BST) which is one of the dielectric materials for dynamicrandom-access memory (DRAM) capacitors and gate ox-ides since the late 1990s [59 – 61] as it has high dielectricconstant (er¼ 300 – 700) as well as good frequency proper-ties. In addition, electric properties, such as dielectric con-stant and loss tangent, can be controlled by varying thechemical composition.

In the early stage of this research, as shown in Figure 7a,the combinatorial thin-film library was fabricated using bi-nary or quaternary mask strategies [62 –64]. However, thepossibility to increase the library density is limited becauseof the exponential increase of the number of masks re-quired. For instance, we need ten binary masks for making210 different samples. Hence, Takeuchi et al. [65 –67] andKoinuma et al. [68] synthesized the combinatorial library byusing the compositional spread scheme. The compositionalspread scheme means refers to a way of preparing a combi-natorial library by using an automated mask (Figure 7b) ora co-deposition method with a shadow mask (Figure 8).

Takeuchi et al. [65] fabricated (Ba,Sr)TiO3 libraries bypulsed-laser deposition apparatus equipped with an auto-mated shutter to give a composition gradient over the sub-strate. In the approach, there are two ways to synthesizethese libraries. Firstly, (Ba,Sr)TiO3 libraries were deposit-ed at room temperature by using the metal precursor, suchas BaF2, SrF2, or TiO2, to mix them homogeneously on themolecular level; they must undergo multi-step thermaltreatments, but it was very difficult to mix solid-multi-lay-ers homogeneously using only thermal treatment. To over-come the problem, (Ba,Sr)TiO3 libraries were deposited atabove 600 8C by pulsed-laser deposition apparatus equip-ped with an automated shutter [66 – 68]. In this case, asshown in Figure 7b, BaTiO3 and SrTiO3 targets were de-posited alternatively to synthesize the BST thin-film libra-ries with various Ba/Sr ratios on the x-axis; the Ba0.65Sr0.35

TiO3 film exhibited the highest dielectric constant (Fig-ure 9b). Scanning evanescent microwave microscopy(SEMM) is a micro-near-field spectroscopy that is veryuseful for measuring the impedance characteristics of vari-ous materials rapidly [61, 69 – 71]. As shown in Figure 9a,it consists of a sharpened metal tip mounted on the centerconductor of high-quality-factor (Q) l/4 coaxial resonatorand protruding beyond an aperture formed on the endwall of the resonator. This design improved the signal-to-noise ratio by a factor of Q and could minimize the far-field background signal. Additionally, it allows submicronspatial resolution, even in quantitative analysis of complexdielectric constants. The resonant frequency and Q, as afunction of the properties (dielectric constant and tangentloss) of the materials placed in the vicinity of the tip, can


Figure 5. The 64-electrode array and the apparatus used to re-cord cyclic voltammograms at each electrode in sequence [55].


be detected by the electronics. On measuring the dielectricconstant, this device applied microwave energy with high aresonant frequency (above 1 GHz) into the sample tocause a resonant frequency shift. Such a change in the fre-quency can be converted into the dielectric constant. On atheoretical basis, an evanescent microwave microscope is apowerful device for measuring independent characteristicsincluding the dielectric constant; it is nondestructive, andhas excellent spatial resolution. As shown in Figure 9c, thecomposition where Ba/Sr¼ 0.65 :0.35 was the boundarybetween the ferroelectric phase and the paraelectric phase[66, 67], and this implies that an optimal composition canbe placed on the phase-transition point. Therefore, in com-binatorial materials science, phase mapping is as importantas the synthesis of libraries with homogeneous phases.

Until now, combinatorial thin-film synthesis has beenmainly carried out by varying the chemical composition of

the samples. However, to our knowledge, the annealingtemperature is also a very important factor in the dielec-tric constant of the samples. To vary the deposition tem-perature, Koinuma et al. [72] used a temperature-gradientheater (Figure 10a) that heated one side of the substrateto give a temperature gradient over the substrate. Theheating source was a Nd:YAG laser (l¼ 1064 nm) and thetemperature is changed linearly from 585 to 765 8C. Asshown in Figure 10b, it was revealed that the region withhigh er (about 400) and relatively low tan dr/tan ds (below30) was found in the 0.4< x< 0.7 and 710 8C<Tgr (deposi-tion temperature)< 750 8C range.

Besides BST thin films, the combinatorial approach todiscovering novel dielectric materials was intensively per-formed because BST has a high crystallization tempera-ture (>650 8C) and it is not suitable for IC fabrication[73 – 76].


Figure 6. Composition of a combinatorial array for methanol electro-oxidation. a) Pt-Ru alloy composition. b) Pt�Ru�Nafion terna-ry composition [58].


Bell Laboratory fabricated amorphous (Zn,Ti,Sn)O2 li-braries using a co-sputtering system [74]. In this system,the chemical composition can be varied easily by the de-gree of overlap of three targets that is as a function of thedistance between target gun and the substrate. Accordingto the results, Zn0.2Sn0.2Ti0.6O2 shows the highest dielectricconstant and lower leakage current density than other die-lectric materials such as SiOx, TaOx, and BST. In addition,the above material can be synthesized at below 400 8C andit is suitable for IC fabrication.

Based on the experimental results, Gladfelter et al. fab-ricated (Hf,Sn,Ti)O2 library by using the chemical vapordeposition (CVD) [74, 76]. In this experiment, anhydrousmetal nitrate was used as the metal precursor. The synthet-ic scheme of these combinatorial thin-film libraries is thesame as that of co-sputtering system. In the CVD process,the oxygen supplier such as H2O and O2 is not necessaryunlike a co-sputtering system and pulsed-laser depositionbecause of the metal nitrate (metal-(NO3) n) used as themetal precursor. However, this system has some disadvan-tages as follows: (1) if one metal precursor influences the


Figure 7. Masking schemes for combinatorial thin-film library synthesis. a) Binary or quaternary mask b) Movable mask (automatedshutter)


deposition rate of the others, it is impossible to make acombinatorial chip, (2) thickness non-uniformity is themain problem, as for the co-sputtering system mentionedabove. Despite these disadvantages, this is an importantapproach in the combinatorial synthesis because it is thefirst to apply the CVD process to fabricating a combinato-rial library. To date, a combinatorial thin-film array hasbeen successfully fabricated by the inter-diffusion of solid-

state multi-layers deposited by RF-sputtering, pulsed-laserdeposition, molecular beam epitaxy (MBE), and CVD.However, there is a fatal problem in the homogeneity ofthe samples. To overcome the problem, Maier et al. [77]deposit the combinatorial chip using a wet deposition tech-nique including sol – gel coating because liquid – liquid in-termixing is very efficient during the deposition process.However, to date, only the possibility of making the com-


Figure 8. Combinatorial thin-film library synthesis by co-sputtering. a) Schematic drawing of an RF-co-sputtering machine. b) Thin-film library.


binatorial library was reported; excellent performancedata has not been presented because of the poor extrinsicproperties of the thin-film library, such as film non-uni-formity and roughness. If a wet deposition process with im-proved extrinsic properties were to be developed, it wouldbe a more efficient process than any other dry depositionmethod and would also have the advantages of efficientliquid – liquid intermixing and control of chemical compo-sition.

3.2 Ferromagnetic Materials

To date, widespread DRAM as a computer-memory de-vice has advantages over high-speed read/erase with highdensity, but its fatal problem lies in memory volatility. Toovercome the problem, various nonvolatile memorieshave been suggested. Among these nonvolatile memories,magneto-resistive random-access memory (MRAM) devi-ces have been noted as promising memory devices becauseof their high densities, high speed, and nonvolatility. To re-alize this kind of memory, finding novel ferromagnetic ma-terials is very important, as information is stored using the


Figure 9. a) Schematic drawing of a SEMM. b) Dielectric constant vs. composition on a (Ba,Sr)TiO3 spread at 0.95, 2.85, and 4.95GHz at room temperature. c) Normalized dielectric dispersion ( (e0.95 GHz – e4.95 GHz)/e0.95 GHz ) vs. composition [66].


magneto-resistance effect. Therefore, as for other materi-als, many research groups have worked to find novel ferro-magnetic materials with high performance by doping animpurity into the host structure [78 – 80]. The amount ofdoping, in particular, has a dominant effect on the ferro-magnetic properties and this stimulates material scientiststo discover novel materials by using the combinatorial ap-proach.

A combinatorial thin-film library for ferromagnetic ma-terials was fabricated by the composition-spread scheme,using e.g., the co-deposition method and concentration-gradient method using the automated shutter in the depo-sition apparatus, as mentioned in the previous section, 3.1[81, 82].

In the combinatorial approaches, various metal alloysand metal oxides have been suggested as candidates for


Figure 10. a) Temperature-gradient heater (from Prof. Takeuchi�s lab). b) Combinatorial results for a (Ba,Sr)TiO3 library fabricatedby the temperature gradient heater [72].


ferromagnetic materials. Among metal alloys, theNi�Mn�Ga system [83] and CoCr alloy with small amountof additives were fabricated using a co-sputtering system[84]. In addition, Ni1–xFex library was deposited by pulsed-laser deposition (apparatus equipped with an automatedshutter) [85].

In the Co1–x–y–zPtxTayTizCr alloy, the concentration of Pthas a dominant influence on the coercivity (Hc) in hystere-sis loop. In addition, Erik et al. suggested the theoreticalmodel to give the polynomial of Cr in the following equa-tion (Eq. 3) [86].

Hc¼ 0.725�570.3Crþ 6915Cr2þ 20142Cr3þ 78.59CrTa.(Eq. 3)

In the Ni1–xFex alloy system, relative saturation magnetiza-tion is the lowest at around x¼ 0.3, at which point occursthe structural transition from face-centered cubic (FCC)to body-centered cubic (BCC) [85, 86].

Takeuchi et al. explored the entire array of Ni�Mn�Gaalloy compositions to find novel materials for ferromag-netic shape-memory device [83]. The authors measuredthe magnetization and the martensitic phase transitionfrom the martensitic to the austenitic. To detect the phasetransition, Ni�Mn�Ga array was deposited on the siliconcantilevers and then progressively heated while under ob-servation. As the phase transition occurs, the cantileverbends. Because the cantilever with the deposited metallicfilm acts as a concave mirror, the phase transition can bedetected simply by a sudden change in reflection resultingfrom the changing radius of the curvature of mirrors.Here, they found a linear relationship between martensitictransition temperature and room-temperature magnetiza-tion – the higher the magnetization, the lower the transi-tion temperature. This behavior shows that there is astrong coupling between shape-memory alloy behaviorand ferromagnetism over a large range of compositions inthe Ni�Mn�Ga system [82].

According to the preceding review by Jandeleit et al.[87], perovskite manganites have been intensively studiedby combinatorial technology because they have colossalmagneto-resistance (CMR) and charge ordering as a func-tion of temperature and composition. Perovskite mangan-ites (RE1–xAxMnO3 (RE¼Y, La, Ce, Pr, Nd, Sm, Eu, Gd,Tb, Er, Tm, Yb, and A¼Ca, Sr, Ba) library was depositedon (100)LaAlO3, SrTiO3, and NdGaO3 by pulsed laserdeposition with an automated shutter [88 – 90].

Besides the perovskite manganate, Koinuma et al. re-ported that anatase Ti1–xCoxO2 (1� x� 0.05) has ferromag-netic properties and they produced a library with tempera-ture and composition gradient by using automated shutterand temperature gradient heater, respectively. The chemi-cal composition (Cobalt-doping amount, x) at the transi-tion from magnetic region to non-magnetic region was de-creased from 0.9 to 0.5 as the deposition temperature is de-creased from 750 to 550 8C. This shows that the magnetic

property is also dependent on the substrate temperatureas well as the chemical composition in TiO2�CoO system[91 – 93]. In addition, they explored the magnetic proper-ties of various transition-metal-doped TiO2 thin films. Theresults revealed that Co-doped anatase film (x¼ 0.06) hadstrong magnetism (~0.3 mB/Co atom of saturated magnet-ism), but that Cr and Mn doped films had no magneticstructures [94].

To map the magnetic phases at low temperature, severalcharacterization tools were employed; among these tools,scanning superconducting quantum interference device(SQUID) microscopy (Figure 11) is frequently used as anevaluation technique [95, 96]. Scanning SQUID micros-copy is a quantitative probe with extremely high-field sen-sitivity of the order of a few fT/Hz to nT/Hz, so that it iscapable of sensing even a weak field of thin-film samples,although its spatial resolution is limited by the SQUIDring, several micrometers (below 10 mm). However, toovercome this spatial limitation, it has been suggested thatSQUID has junction areas of under 0.1 mm2 by using nano-bridges as weak links.

Scanning SQUID microscopy has made it possible tocharacterize the presence of a magnetic phase by imaging.However, to measure the hysteresis loop that is the impor-tant feature of ferromagnetism, a magneto-optical Kerr-ef-fect (MOKE) instrument has been used. The parameters,including coercivity, squareness, and maximum angularperpendicular Kerr rotation were derived from these loopsand used for fitting data for calculation of coercivity andsqureness by input of the composition and thickness [84,97].

Ludwig et al. used a vibration sample magnetometer(VSM) to measure magnetization and anisotropy of Fe50

Co50/Co80B20 multilayer films [98]. VSM operates on thebasis of Faraday�s law of induction, which states that achanging magnetic field will produce an electric field. Thiselectric field can be measured and can tell us informationabout the changing magnetic field. By rotating the sample908 it is possible to determine the magnetic anisotropy.

Gervais et al. fabricated the La1-xAxMnO3 (A¼Ce, K,Na, Sr) samples in powder form and carried out the phasemapping by using the electron paramagnetic resonance(EPR). This is very powerful tool to observe the transitionfrom the high-temperature paramagnetic insulating phaseto the low-temperature ferromagnetic conducting phase ofperovskite manganates [99]. However, the detection timeis a little bit longer than other tools such as SSM andtherefore it is not suitable for combinatorial technologyfor discovering the novel material.

3.3 Lithium-Ion Batteries

The introduction by Sony in 1990 of the world�s first com-mercially successful rechargeable battery represented arevolution in the power source industry [100 – 102]. It issimilar to the semiconductor revolution in which there was



a replacement of the valve by the transistor in the 1940 –50s. The new cell can store more than twice the energycompared to conventional rechargeable batteries of thesame size and mass. These advantageous results are partlyfrom the high standard potential and low electrochemicalequivalent weight of lithium. Further, lithium-ion re-chargeable batteries is safer than any other secondary bat-tery, such as Ni/MH and Ni/Cd batteries, because interca-lation compounds are used as the electrode. The battery

consists of a cathode, an anode, and an electrolyte. Amongthese components, the cathode material plays an impor-tant role in enhancing the electrochemical properties oflithium-ion battery and, therefore, the research has beenfocused on the development of a novel cathode material[103 – 107], which is multi-component metal oxide. There-fore, the combinatorial methodology can be applied to thediscovery of novel cathode material as it can for other ma-terials systems. However, such an approach was only re-


Figure 11. a) Scanning SQUID microscopy. b) Results of Ni�Mn�Ga alloy analysis by the SQUID [83].


ported in recent years [108 – 112], because an inert atmos-phere is required to measure the electrochemical proper-ties of cathode materials, and this presents a barrier tomeasuring electrochemical performance relative to thelithium ion.

Let us consider several combinatorial approaches fordiscovering cathode materials. Spong et al. [108] fabricatedeight compositions of carbon-containing LiMn2O4 elec-trode material (1 ~20 wt.-% carbon) from stock ink solu-tions using an automated liquid handler and each composi-

tion was repeated eight times. The sample of each elec-trode material was transferred to an electrode on 8� 8electrochemical testing arrays (Figures 12a and b). Cyclicvoltammetry could be performed on all 64 working elec-trodes simultaneously. A 64-channel potentiostat (Fig-ure 12c) gives real-time data on the current level at eachelectrode in the array, together with cyclic voltammogramsfor each cell in the voltage range between 3 and 4.5 V. Itwas revealed that a carbon loading of 3% is required toavoid resistance limitation at 0.1 mV/s, and a 20% carbon


Figure 12. a) Schematic drawing of electrochemical testing array [110]. b) Photo of an 8� 8 electrochemical electrode array. c) A64-multi-channel potentiostat [108, from Prof. Owen�s homepage].


loading is required to obtain effective cycling of the mate-rial, in most cases because of the low conductivity ofLiMn2O4 for the lithium-ion battery. Watanabe et al. hasstudied LiCoO2 and LiFePO4 with various amounts of car-

bon [110] and, found that the co-synthesis of LiFePO4 withcarbon increases the macroscopic conductivity but de-creases the energy density, because te addition of carbondecreases the volume and mass fraction of LiFePO4 in the


Figure 13. a) Photo and schematic drawing of micro-thin-film test cell. b) Normalized – dQ/dV analysis for the cells for x inLiMnxNi2–xO4¼ 0.4 ~1.7 [111].


electrode and interferes with the ion transfer across thecarbon film coated on the surface of cathode material. Ingeneral, carbon is main reason for capacity fading in bat-teries because it catalyzes the oxidation reaction of the or-ganic molecules in the liquid electrolyte placed on the sur-face of carbon. Therefore, it is necessary to find the way toincrease the electrochemical properties without addingcarbon. To realize this approach, Whitacre et al. [111] de-posited LiyMnxNi2–xO4 thin-film libraries on the patternedPt wafer by the co-sputtering of LiNiO2 and LiMn2O4 tar-gets. To measure the cycle life, they fabricated the micro-thin-film test cell (Figure 13a) without carbon by semicon-ductor processing involving deposition and photolithogra-phy. In the LiyMnxNi2–xO4 thin-film library, the chemicalcomposition (x¼ 1.4) with high initial discharge capacity(52 mAh/cm2mm) placed at the transition point from thelayered structure (Ni-rich region) to the spinel structure(Mn-rich region) (Figure 13b). However, this approachhas some disadvantages, as follows: (1) about 40% of themicro-cells showed electrochemical failure because of thecomplexity of making the micro-cell, and (2) an impurityphase, relative to Li concentration, exists in the combina-torial chip. However, this cell exhibits good electrochemi-cal cycling behavior because of the solid electrolyte eventhough the cut-off voltage is 5.5 V.

Besides cathode materials, combinatorial research hasbeen applied to the discovery of novel anode material[113, 114]. For the present study, graphite is commonlyused as the anode material because of relatively high ca-pacity (372 mAh/g) and long cycle life. However, recentstudies have been carried out into replacing graphite byother materials such as metal alloys, including lithium al-loy. Lithium alloys offer specific and volumetric capacitiesthat are potentially many times that of graphite. However,the capacity retention on cycling of these materials is notadequate because of heterogeneous reactions that occurwhen the lithium ion is inserted into and extracted out ofthese materials. To solve these problems, Dahn et al. [114]fabricated amorphous SiAlSn alloys on Cu foil using a co-sputtering system and made 25 different coin cells to meas-ure the electrochemical properties. According to the phasediagram and X-ray diffraction (XRD) data, the Si-rich re-gion was amorphous and the Sn-rich region exhibited acrystalline region existing in two or three phases. In addi-tion, the crystalline samples exhibited a two-phase reac-tion under cyclic voltammetry and poor cycle life, whileamorphous samples showed a homogeneous reaction un-der cyclic voltammetry and excellent cycle life. Amor-phous SiAlSn alloys exhibited capacities of over1800 mAh/g, as well as reversible capacities of over1500 mAh/g when cycled between 1.2 and 0.1 V versuslithium for 10 cycles. These results are very high, with val-ues 5 or 6 times that of graphite. So far, the above electro-chemical test was mainly performed using a 64-channel po-tentiostat or conventional device. This limits the possibilityof increasing the library density in the application of the

combinatorial method to novel cathode and anode materi-als. Therefore, it is necessary to develop novel characteri-zation tools or methods, and as a next step, it is expectedthat combinatorial research for discovering cathode oranode materials will focus on the evaluation methodology.

4 Conclusion

We have reviewed combinatorial technology as applied tovarious fields such as olefin polymerization catalysts, De-NOx catalysts, anode materials for DMFCs, dielectric/fer-roelectric materials, ferromagnetic materials, and elec-trode materials for lithium-ion rechargeable batteries. Theremarkable advances in library synthetic instruments havebeen demonstrated; however, characterization tools havenot kept pace. The particular difficulties associated withcharacterizing catalyst performance or activity, comparedwith inorganic materials, because of complicated reactionpaths or mechanisms has been highlighted. And, in thecase of electrode materials for lithium-ion rechargeablebatteries, the electrochemical reaction requires the inertatmosphere. This restriction makes the fabrication of elec-trode arrays for electrochemical evaluation difficult; it alsotakes a long time to measuring the electrochemical proper-ties such as discharge capacity. Therefore, to apply thecombinatorial approach to various inorganic materials andcatalysts, novel characterization methodologies have to bedeveloped on a physical/chemical basis, like the opticalcharacterization method applied into the discovery ofanode material for DMFCs.

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