ChemInform Abstract: Soft Solution Processing: Low-Energy Direct Fabrication of Advanced Inorganic Materials

Abstract

Department of Fine Chemicals Engineering and Chemistry, Chungnam National University,220 kung-dong, Yusong-gu, Taejon 305-764, KoreaReceived March 27, 1999A new concept ‘‘soft solution processing’’ has been introduced to fabricate advanced solid state materials inan economical, environmentally friendly, and energy and material efficient way. The prepared films show thedesired and prospective properties despite of low temperature synthesis and no post-synthesis annealing. Suc-cessful examples demonstrate that soft solution processing is capable of preparing advanced materials withplanned properties through the easy control of reaction conditions in a suitable aqueous solution in a single syn-thetic step without huge energy consumption and without any sophisticated equipment.IntroductionAs advances in the instrumentation and material haveameliorated modern human life, the demand to developmore sophisticated and complicated functions increases.Especially, one of the most required techniques is a minia-turization of advanced inorganic materials such as micro-electronics. It means that technical improvements in thefabrication of functional thin films are important. However,although an intensive attention is being paid mainly to thefabrication of more delicate and advanced material, the eco-nomic, environmental, and energy factors of the fabricationprocesses have not been adequately considered. Since theearth is an almost closed system, the used materials wouldbe recycled, moreover wasted energy and heat should beminimized. Thus, if the production of desired substance, thesimultaneous shape-forming and in situ compositional andfunctional grading can be performed in only one syntheticstep at low temperature with high energy and material effi-ciency, it should be the most effective synthetic route.Unfortunately, the usual synthetic approaches described inprevious works to fabricate thin films have been mainlyfocused on multistep processes using gas phase and/or vac-uum system. These processes include the following: prepa-ration of a bulk material or its precursors; deposition orintegration onto/into substrates using chemical vapor deposi-tion, sputtering, or laser ablation deposition; the formation ofshape; size; and orientation. In addition, post-synthesis heattreatments are often required to allow proper functions.Since huge energy consumption and sophisticated instru-mentation are required in these methods, these techniquesare not economical nor environmentally friendly. In thecourse of trying to developing such a novel syntheticapproach, ‘‘soft solution processing’’ seems to fit these crite-ria. In this contribution, we explain what is soft solution pro-cessing and how soft solution processing can be realized toobtain advanced inorganic materials. In addition, severalsuccessful examples and some possible synthetic improve-ment using designed soft solution processing methods willbe given. Definition and Nature of “Soft Solution Processing’’‘‘Soft solution processing’’ is a term encompassing a widegroup of synthetic processes used to prepare advanced solidstate materials using a solution treatment at a low tempera-ture. Here, ‘‘soft’’ means less environmental charge, lowenergy and material consumption, no complicated instru-mentation, and no sophisticated multistep processes. Thesolution treatment of the starting substances can be easilycombined with different activation methods such as electro-chemical reactions, photochemical excitation, hydrothermalreactions at a low temperature, application of microwaves orultrasonics, chimie douce reactions, bioactivation or hot-pressing to prepare desired materials. Since all living organ-isms on earth consist of aqueous solution systems at ambienttemperature and pressure, water should be the most suitablesolvent for the soft solution processing. Therefore, spontane-ous chemical reactions in aqueous solution such as dissolu-tion, precipitation and/or interfacial reaction should beinvolved in the soft solution processing. In addition, themajor advanced ceramic materials contain at least one metalelement with its particular oxidation state, coordination, andchemical composition. It is attributed to the specialized elec-tronic configuration of the given metal species. Here, itshould be noted that the dissolved metal in aqueous solutionare generally combined with oxygen derivatives such ashydroxyl group to form a certain anion such as metalite ormetalate.