Multiform Oxide Optical Materials via the Versatile Pechini-Type Sol-Gel Process PDF

J. Phys. Chem. C 2007, 111, 5835-5845 5835...

J. Phys. Chem. C 2007, 111, 5835-5845 5835 FEATURE ARTICLE Multiform Oxide Optical Materials via the Versatile Pechini-Type Sol-Gel Process: Synthesis and Characteristics Jun Lin,* Min Yu, Cuikun Lin, and Xiaoming Liu Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ReceiVed: January 4, 2007; In Final Form: January 26, 2007 This feature article highlights work from the authors’ laboratories on the various kinds of oxide optical materials, See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. mainly luminescence and pigment materials with different forms (powder, core-shell structures, thin film and patterning) prepared by the Pechini-type sol-gel (PSG) process. The PSG process, which uses the common metal salts (nitrates, acetates, chlorides, etc.) as precursors and citric acid (CA) as chelating ligands of metal Downloaded via UNIV MONTPELLIER on May 31, 2024 at 16:15:04 (UTC). ions and polyhydroxy alcohol (such as ethylene glycol or poly ethylene glycol) as a cross-linking agent to form a polymeric resin on molecular level, reduces segregation of particular metal ions and ensures compositional homogeneity. This process can overcome most of the difficulties and disadvantages that frequently occur in the alkoxides based sol-gel process. Using the PSG process, we are able to prepare luminescent powder materials that cannot be well synthesized by the solid-state reaction method, environmentally friendly and highly efficient phosphors that lack metal activator ions, core-shell structured monodisperse and spherical optical materials with tunable physical chemical properties, and thin film phosphors and their patterning combined with soft lithography techniques. The extensive applicability of this process and potential material applications are demonstrated. 1. Background frequently used as the alternatives to the metal-alkoxides based sol-gel process.4 As one of the most well-known soft solution processes, so far the sol-gel technique has found a very extensive application The PC technique, also known as the Pechini method5 (later for the design and synthesis of various kinds of advanced we call it Pechini-type sol-gel process, and abbreviate as PSG), functional and engineering materials, including powders, films, is well-known and used for the synthesis of homogeneous fibers, and monoliths of almost any shape, size, and chemical multicomponent metal oxide materials. This method includes a composition, and of course, nanostructures and organic- combined process of metal complex formation and in situ inorganic hybrids.1 There are some 30 000 sol-gel related polymerization of organics. Normally an R-hydroxycarboxylic research papers published in ISI-indexed journals and many acid such as citric acid (CA) is used to form stable metal special books in the last two decades (1986-2006). The sol- complexes, and their polyesterification with a polyhydroxy gel process basically involves the synthesis of an inorganic and/ alcohol such as ethylene glycol (EG) or poly(ethylene glycol) or organic network by a chemical reaction in solution at low (PEG) forms a polymeric resin. Immobilization of metal (generally ambient) temperatures followed by the transition from complexes in such rigid organic polymer networks reduces solution to colloidal sol and to a multiphasic gel form. According segregation of particular metal ions, ensuring compositional to the different precursors utilized, the sol-gel techniques can homogeneity. The calcination of the polymeric resin at a be basically divided into three types: (1) the sol-gel route based moderate temperature (500-1000 °C) then generates a pure upon hydrolysis-condensation of metal-alkoxides; (2) the phase multicomponent metal oxides. The above process is gelation route based upon concentration of aqueous solutions clearly shown in Scheme 1.4a,6 Many metal ions, except involving metal-chelates, often called as “chelate gel” route; and (3) the polymerizable-complex (PC) route.2 Apart from the monovalent cations such as sodium and potassium, form very tetraethyl orthosilicate (silicon alkoxide), most of the metal stable chelate complexes with CA, since CA is a polybasic alkoxides suffer from high cost, unavailability, toxicity, and fast compound having three carboxylic acid groups and one alcoholic hydrolysis rate (thus difficult in controlling the homogeneity group in one molecule. The potential ability of CA to solubilize of different components during experimental processes).3 As a a wide range of metal ions in a mixed solvent of EG and H2O result, in the preparation of multicomponent oxide materials is of prime importance, especially for systems involving cations comprising of more than one type of metal ion, the latter two that can be readily hydrolyzed to form insoluble precipitates in routes (especially the polymerizable-complex route) are very the presence of water.4a An advantage of the PSG process is that the viscosity and the molecular weight of the polymer can * Corresponding author. E-mail: [email protected]. be tailored by varying the CA/EG molar ratio and the synthesis 10.1021/jp070062c CCC: $37.00 © 2007 American Chemical Society Published on Web 03/31/2007 5836 J. Phys. Chem. C, Vol. 111, No. 16, 2007 Lin et al. SCHEME 1: Basic Principle of Pechini-Type Sol-Gel Process and Multiform Optical Materials Derived from It Jun Lin was born in Changchun, China, in 1966. He received B.S. and M.S. degrees in inorganic chemistry from Jilin University, China in 1989 and 1992, respectively, and a Ph.D. degree (inorganic chemistry) from the Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences, in 1995. He was then appointed as a research assistant in City University of Hong Kong (1996) and a DAAD research fellow in Institute of New Materials (INM, Germany, 1997), followed by a postdoctoral researcher at Virginia Commonwealth University (U.S.A., 1998) and University of New Orleans (U.S.A., 1999). He came back to China in 2000, and since then has been working as a professor in CIAC. His research interests include bulk and nanostructrued luminescent materials (powder and thin film), surface modification, crystal growth with different morphologies, luminescence and spectral properties of lanthanide ions in inorganic solids via soft chemistry routes, and organic/inorganic semiconducting layered hybrid materials (especially those with perovskite structure). He is the author or coauthor of more than 200 peer-reviewed journal articles in the areas of luminescent and nanostructured materials Min Yu received her B.S. and M.S. in chemistry from Northeast Normal University (China) in 1988 and 1995, respectively. Since 2000 she has been a graduate student in Jun Lin’s group at Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences, and received a Ph.D. degree in 2003. She won the President Prize of Chinese Academy of Sciences and served as an associate professor in Northeast Normal University after graduation. Her scientific interests include preparation and patterning of phosphor films as well as surface In the past 5 years, we have extended the application of the modification of silica particles with luminescent layers via sol-gel and PSG process to the systematic synthesis of various kinds of soft lithography techniques. oxide optical materials, mainly luminescence and pigment materials with different forms (powder, core-shell structures, Cuikun Lin is a senior graduate student in Jun Lin’s group at thin film, and patterning).8 The purpose of our research is to Changchun Institute of Applied Chemistry (CIAC), Chinese Academy reveal the feasibility, versatility, advantages, and disadvantages of Sciences. He graduated from Shandong Normal University (China) of this method for the synthesis of such optical materials, in an with a B.S. degree and came to CIAC to study for a Ph.D. degree in 2002, with a research focus on the synthesis of environmentally friendly effort to gain fine control of the material morphology and find luminescent materials without metal activator ions and core shell novel optical materials. Here in this feature article, we will structured pigment materials via Pechini-type sol-gel process. demonstrate the multiform of the optical materials derived from the PSG precursor solutions, including powder luminescent Xiaoming Liu is a Ph.D. candidate in Jun Lin’s group at Changchun materials (combined with the spray drying process), monodis- Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences. perse and spherical core-shell structured phosphor and pigment He graduated from Jiujiang College and joined Lin’s group in 2003. particles (via the surface modification process), thin film His research interests include synthesis of powder luminescent materials for field emission displays via soft chemistry process. phosphors (via dip-coating), and their patterning (combined with the soft-lithography process). The basic principle of the temperature, and the materials can be made into powder and multiform of optical materials prepared by the PSG process is thin film forms and others.2 shown in Scheme 1. Following this scheme, we will discuss Now the PSG process has been used for the synthesis of each kind of optical (mainly luminescent) material in the electric and magnetic materials rather extensively, including following sections. ferroelectric and capacitor materials, superconducting materials, 2. Powder Luminescent Materials photocatalytic materials, magneto-optical materials, electrolytic materials for solid oxide fuel cells, and so on.2,4-7 The improved A luminescent material, also called a phosphor, is a solid material properties for the PSG process with respect to the other which converts certain types of energy into electromagnetic methods (such as solid-state reaction method and amorphous radiation over and above thermal radiation. The electromagnetic citrate method) have been demonstrated by Kakihana in a review radiation emitted by a luminescent material is usually in the article for the synthesis of superconductors and photocatalysts.4a visible range but can also be in other spectral regions, such as Feature Article J. Phys. Chem. C, Vol. 111, No. 16, 2007 5837 the ultraviolet or infrared.9 Nowadays, luminescent materials have found a wide variety of applications, including displays (such as television tubes, computer monitor tubes, and radar screens) and lighting (such as fluorescent lamps), X-ray- intensifying, and scintillation.9,10 For all of these purposes, some tens of thousands of phosphors have been synthesized and characterized via various kinds of methods, but only about 50 materials exhibit properties that are sufficient for practical applications. Although activities aimed at producing novel phosphors for classical applications are still going on, now much attention has been shifted and has begun to be focused on the investigation and optimization of topology of phosphor layers, morphology of phosphor particles, light generation and propaga- tion, and so on.11 The most common and useful form of phosphors is powder, which is mixed with organic binders to spread on a substrate for display and lighting application purposes. Conventionally, powder phosphors are prepared by the solid reaction process, Figure 1. XRD patterns of β-Ga2O3:Dy3+ phosphors prepared by PSG i.e., direct mixing the precursor components (oxides and (a, 1000 °C) and SSR (b, 1200 °C) process and their corresponding inorganic salts etc.) and firing them at high temperature for long SEM micrographs (c for a and d for b). The peaks with (*) marks in time with repeat grinding process.12 This process suffers from (b) belong to Dy3Ga5O12 phase. The JCPDS (41-1103) card for a waste of energy (>1000 °C, >10 h in general), contamination β-Ga2O3 is given as a reference. of impurities (from the repeat grinding process and crucibles at the sample remained amorphous after 500 °C heat treatment, it high temperature), and inhomogeneous composition and mor- began to crystallize at 600 °C, the crystallinity increased with phology (>3 µm micron particles with irregular shapes, wide raising the annealing temperature, and the crystallization was size distribution, and serious aggregation) for the final phosphor complete at 1000 °C (4 h). No second phase was detected, products.12 As a result, many soft chemistry processes, such as indicating that the Dy3+ ions have been successfully dissolved sol-gel, hydrothermal, spray pyrolysis, coprecipitation, high in the β-Ga2O3 host lattices by substitution for the Ga3+ (Figure boiling solvent, combustion, and microwave assisted heating 1a).14 But for β-Ga2O3:Dy3+ via the SSR process (mixing Ga2O3 have been developed to prepare luminescent powders.12 All of and Dy2O3 together followed by grinding and high temperature these processes have their own advantages and disadvantages. annealing), even after being fired at 1200 °C for 8 h, the Dy3+ In general, they can produce phosphors with better morphology ions were unable to be well dissolved in the β-Ga2O3 host (more regular, spherical in some cases) from the nano- (around lattices, and minor amounts of Dy3Ga5O12 were always present 10 nm) to microscale (