含有金属盐的溶胶凝胶制备过程呢

1、SolGel Processing of Thin Films with Metal SaltsINTRODUCTIONSolgel process is an attractive alternative to other methods for synthesis of ceramics and glasses for many reasons: for example, low temperature synthesis, simple equipments to be used, thin film formability and so on. Particularly, solgel

2、 process is very useful for thin film deposition because of the capability to coat materials of various shapes and/or large area, to control the composition easily for obtaining solutions of homogeneity and controlled concentration without using expensive equipment. Historically, metal alkoxides hav

3、e been employed in solgel process, which readily undergo catalyzed hydrolysis and condensation to form nanoscale oxide or hydroxide particles. Still in general, metal alkoxides are often used as raw materials in solgel process, but many of the alkoxides are very difficult to be obtained and dealt wi

4、th because of the high sensitivity to the atmospheric moisture. In addition, when multi-component ceramics are to be prepared, the rate of alkoxide hydrolysis should be controlled, which is not very easy (Nishio, 1999a, 2000a, 2000b, 2003; Kodaira, 2003).Metal salts are very useful, cheaper, very ea

5、sy to handle than metal alkoxides, and hence are good alternatives if they are readily converted to oxides by thermal decomposition and can be solved in many kinds of organic solvents in which metal complexes are formed. In other words, the solgel process with metal salts can be realized by chelatin

6、g the metal ions by organic ligands.The metal salts include chlorides, acetates, nitrates, sulfides and so on. Chlorides, nitrates and sulfides have high solubility in water or organic solvents. In some cases acetates have lower solubility in water or organic solvents than other metal salts. However

7、, acetate ions can stabilize the metal ions in solutions through coordination by C=O groups. If metal salts are just dissolved in water or organic solvents without chemical reaction like a chelating, they are re-crystallized on solvent evaporation. It is important how to stabilize the metal ions in

8、solutions without coordination by anions like Cl or .Many researchers have carried out the studies on formation of metal complexes with organic ligands. Furthermore, the metal complexes with organic ligands have been used for preparation of ceramics and metal oxide thin films by solgel process, usin

9、g metal salts like nitrates (Rajendran, 2001; Norman, 1999; Kim, 1999; Gash, 2001; Xu, 2002; Lio, 2001), chlorides (Nishio, 1996a, 1999b, 1999c, 2000b; Hu, 2000; Ramanan, 2001; Alam, 2002; Kikkawa, 2002), and acetates (Bao, 1998; Nishio, 1999a, 2000a; Rho, 2002; Wang, 2002; Kodaira, 2003) as the sta

10、rting materials.In this chapter, some examples are described on preparation of thin films by solgel process with metal salts using chelating agents.PECHINIS METHODPechinis method (Pechini, 1967) is very famous as a simple method for preparing metal oxide powders where polymeric precursors are made f

11、rom metal salts, ethylene glycol and citric acid by low temperature heat-treatment. This method allows the metal cations to be mixed at a molecular level and the stoichiometric compositions to be achieved by chelating the metal ions in solution by citric acid, on which many studies have been reporte

12、d (Baythoun, 1982; Tai, 1992; Roy, 1999; Robert, 2001). Furthermore, this process offers several advantages in fabrication of ceramic thin films, including low cost, homogeneous compositions, high purity, and low heat-treatment temperatures. Pechinis method has been applied to preparation of thin fi

13、lms, for example, of electrical materials (Liu, 1995; Chai, 2002; Bernardi, 2002; Rosario, 2002), ferroelectric materials (Spagnol, 2002), optical materials (Lima, 2003), electrolytes (Agarwal, 1997) and so on.Pechinis method is based on polymerization of metal citrates using ethylene glycol. A hydr

14、ocarboxylic acid such as citric, tartaric and glycolic acids form polybasic acid chelates with metal cations in aqueous solutions. When compared with the majority of the acids, citric acid (C3H4(OH)COOH)3) is more widely used in Pechinis processing because of its high stability. The typical metal co

15、mplexes with citric ligands tend to be fairly stable due to the strong coordination of the citric ion to the metal cation involving two carboxyl groups and one hydroxyl group, as shown in Figure 3-1 (Liu, 1995; Spagnol, 2002). The addition of a glycol such as ethylene glycol (HOC2H4OH) leads to the

16、formation of an organic ester. Condensation reaction occurs with the formation of a water molecule. The hydroxide ions arise from the carboxylic acid and the protons from the alcohol, generating water molecules (Anderson, 1987). The condensation and polymerization reaction is promoted by heat-treatm

17、ent.An example is shown below. Chai and co-workers (2002) have synthesized La0.8Sr0.2Co0.5Ni0.5O3 (LSCN) thin films using Pechinis process. Precursors used for the preparation of LSCN solutions were metal nitrates: La(NO3)3·6H2O, Sr(NO3)2, Co(NO3)2·6H2O, and Ni(NO3)2·6H2O. Citric acid

18、 (C3H4(OH)COOH)3) and ethylene glycol (HOC2H4OH) were used as complexation/polymerization agents for the process. The preparation procedure for the LSCN solution is shown in Figure 3-2. The authors thought that the process is refined by a particular parameter, namely, the molar ratio (Rc) between ci

19、tric acid and metal ions, focusing on the effect of Rc. Figure 3-3 shows the TGADTA curves of the LSCN precursor powders prepared from solutions of different Rc values. In the TGA curves, the temperature at which the weight loss terminated increased with decreasing Rc values; 600, 650, 675 and 750&#

20、176;C for Rc = 4.0, 3.3, 2.5 and 1.7, respectively. It may be qualitatively stated that heat flow increased with decreasing Rc from 4.0 to 1.7. XRD measurements indicated that higher Rc values are desirable for crystal growth. Figure 3-4 shows the SEM images of the thin films heat-treated at 750

21、6;C. It is noticed that the grain size increases with increasing Rc value. These results indicate that a high Rc value, having higher citrate and lower metal ion concentrations in the precursor, is more favorable for crystallization and grain growth. It is shown by these results that the precursor r

22、eagents with the largest Rc value ( = 4.0) provide the precursor gels the lowest temperature of weight loss termination. The weight loss is the highest at Rc = 4.0 being 78%, decreasing to 75, 71 and 68% for Rc = 3.3, 2.5 and 1.7, respectively. And the grain size of the thin film is increased with i

23、ncreasing Rc value. Obviously, higher concentrations of metal ions led to larger numbers of nuclei during crystallization, resulting in smaller average grain sizes.Figure 3-1. Schematic illustration showing the solution chemistry and reactions involved in the Pechini process (Liu, 1995).Figure 3-2.

24、Preparation procedure of La0.8 Sr0.2 Co0.5 Ni0.5O3 (Chai, 2002).MODIFIED PECHINIS METHODSome studies have been reported on modified Pechinis method for preparation of metal oxide thin films (Petrenko, 1990a, 1990b; Takahashi, 1994; Jimenez, 1995; Agarwal, 1997; Sato, 1997; Lima, 2000; Shao, 2000). I

25、n these studies, chelating agents in Pechinis process (citric acid) is replaced by ethylenediamine tetraacetic acid (EDTA) in order to increase the extent of chelation to metal ions in the solution. Because EDTA has stronger chelating power than citric acid to metal ions, the uniformity of the metal

26、 ions in the solution is expected to be improved.Figure 3-3. TGADTA curves of the LSCN precursor powders prepared with different Rc (Chai, 2002).Fransaer et al. (1989) have been proposed that three basic reactions are involved during formation of the gel precursor in the EDTA solution process; metal

27、 chelate formation, solvent evaporation and gel formation.Figure 3-4. SEM images of the surface of the LSCN thin films (Chai, 2002): (a) Rc = 1.7, (b) Rc = 2.5, (c) Rc = 3.3 and (d) Rc = 4.0.(3-1)Parameters of the solutions, such as concentration, precipitation tendency, viscosity and pH, all affect

28、 the charatceters of the EDTA solution process. Sheen and coworkers (1997) have described the importance of the stability of metal ions in the solution given by the chelation. The concentration of metal ions and chelating agents in the solution after solvent evaporation is very important. The major

29、problem associated with the process is the occurrence of precipitation during solvent evaporation. It should be emphasized that the optimal pH value of the initial solution is important to the chemical homogeneity. The concept of the study is to lower the concentration of the free metal ions in the

30、solution by the formation of soluble complexes. In the paper (Sheen, 1997), an EDTA solution process was proposed in which three basic reactions were involved during formation of the gel precursor.Precipitation:(3-2)Complex formation:(3-3)(3-4)In a metal oxide system, there are two competitive react

31、ions between precipitation and complexation.To find the relationship between free metal ion concentration, let “A” be the interfering complexant which reduced the free metal ion concentration (Ringbon, 1963). Therefore(3-5)By use of the overall stability constants, m of the complexes formed in the s

32、ide-reaction, the above equation can be re-written as(3-6)In this equation, “A” can be dealt with proton.At lower pH values, the metal ions are not precipitated as hydroxides, and the chelating ability of EDTA is lowered because protolytic stability constant for metal ion increases. Therefore, the s

33、olubility of metal ions should be solution system (i.e. solvents, metal salts and pH) dependent. (Petrenko, 1990a, 1990b). Figure 3-5 shows calculated conditional stability constants of LSGM for the EDTA of La, Sr, Ga, and Mn by following equation.(3-7)(MEDTA)' is total number of complexes consi

34、sted with M:EDTA = 1:1 (molar ratio).(3-8)(3-9)where M is apparent free metal ion concentration that has not reacted with complexant EDTA, and EDTA is the apparent ligand concentration not bound to the metal ion M.Thus, conditional coefficient is shown by(3-10)M is coefficient of side reaction betwe

35、en metal ion and hydroxide ion, EDTA is coefficient of the reaction between EDTA and proton, and KMEDTA is stability coefficient of EDTA for metal ions. The stability coefficient is dependent on the pH of the solution. In general, ammonia solution is used to control the pH of the solution. In Figure

36、 3-5, it is obvious that metal ions in the solution is stabilized at about pH = 10.Figure 3-5. Calculated conditional stabilized constants for the EDTA complex with metal ions as a function of pH.Agarwal et al. (1997) have prepeared barium cerate based thin films using a modified Pechinis method. Th

37、ey used barium nitrate, cerium nitrate, and gadolinium nitrate as precursors for the material. Ethylene glycol and EDTA were used as polymerization/complexation agents for the process. Ammonia water was used to facilitate the dissolution of EDTA in deionized water. Crack-free, uniform, and non-porou

38、s BaCe0.8Gd0.2O3 thin films could be prepared using the modified Pechinis method. In this method, the most important factor was the ratio of complexation agent (EDTA) to total metal ions because the ratio affects the resin structure and the amount of organics to be removed during firing, which, in t

39、urn, affects the final film microstructure. Crack-free and uniform films were obtained only at intermediate ratios form 1.5 to 2.5.COMPLEXES WITH AMINEA few studies have been reported about preparation of metal oxide thin films by solgel process with alkanolamines (Kamalasanan, 1996; Schuler, 1999;

40、Huang, 2001; Silva, 2002; Viart, 2002).The thin film prepared had the composition of (100 x)ZnOx·Al2O3, where x = 07. Zinc acetate, which was used as the zinc source, was very soluble in water and alcohol. Aluminium nitrate was used as the aluminium source (Nishio, 1996b). Zinc acetate and alum

41、inium nitrate were mixed to obtain the desired composition and dissolved in ethanol. Then diethylentriamine, diethanolamine, monoethanolamine and acetylacetone (Fig. 3-6) were added to prepare coating solutions called solution A, B, C and D, respectively. Transparent silica glass plates were used as

42、 substrates. The substrate coated with the solution was heat-treated at a given temperature in air or in hydrogen gas to form the thin film.Figure 3-6. Molecular structure of chelating agents.The crystallization temperature and the crystallinity of the film was investigated by X-ray diffraction patt

43、ern analyses. The crystallization of the film started from 300°C when prepared from solution A, and from 200°C when prepared solution B, C, and D. Remarkably, it was observed that only the (0 0 2) plane grew rapidly from 400°C for thin films prepared from solution C and D. This indica

44、tes that the crystal growth in the direction of the c-axis was predominant. The growth in the direction of the c-axis was preferential around 600°C for the film prepared from solution B. However, all the peaks showed homogeneous growth in the film prepared from solution A and no preferential gr

45、owth was observed. In summarizing, the films prepared from solution C and D exhibited enhanced orientation in the c-axis direction with increasing heat-treatment temperature.The results of fourier transform-infrared spectroscopy (FTIR) agreed with those of XDR analyses. The solution B-derived film s

46、howed an absorption peak due to ZnO bond vibration around 400 cm1 when heat-treated at 300°C, while the solution C­ and D-derived films showed the absorption at 200°C. In the TGADTA curves, a large exothermic peak with large weight loss was found at about 460°C for solution A, ab

47、out 510°C for solution B, about 390°C for solution C, about 340°C and 400°C for solution D. These results indicate the difference in temperatures for the burning of the organics.The Hall voltage was measured in the thin film heat-treated at 700°C. The increase and decrease o

48、f mobility and carrier density corresponded to that of resistivity. The resistivity of the thin films prepared under the same condition has lowered in the order, solution D, solution C, solution B and solution A. The thin film prepared from solution D showed large carrier density (22.6 × 1017 c

49、m3) and mobility (1.49 cm V1 s1) when heat-treated in air.The differences in the orientation and resistivity are considered to be due to the differences in the coordination power and the boiling point of the chelating agents. The ligand atoms of the chelating agents are nitrogens and oxygens. Diethy

50、lenetriamine has one nitrogen atom, nonoethanolamine has one nitrogen atom and one oxygen atom and acetylacetone has two oxygen atoms. In general, the coordination power of the oxygen atom is stronger than that of the nitrogen atom. The boiling point of the chelating agents is as follows; diethanola

51、mine (271°C) > diethyleneamine (207.0°C) > nomoethanolamine (171.1°C) > acetylacetone (140°C). Ring type chelates with acetylacetone may form two ZnO bonds at lower temperatures. The orientation to the substrate increased at the relatively low temperature of 200°C,

52、and then increased with increasing heat-treatment temperature. As a result, the resistivity decreased with increasing high orientation in the direction of the c-axis.FORMATION OF ALKOXIDES FROM TRANSITION METAL IONS WITH HIGH REACTIVITYIn general, many metal alkoxides are synthesized from metal chlo

53、rides because metal chlorides have high reactivity with many organic solvents.A coating solution for preparing WO3 thin films was prepared from anhydrous hexachlorotungsten and C2H5OH. Hexachlorotungsten (WCl6) was dissolved in ethanol (Nishio, 1999c) which was performed in dry N2 gas in order to av

54、oid violent reaction of WCl6 with oxygen and moisture. The color of the solution was light yellow immediately after preparation, and changed from light yellow to dark blue soon. The color of the solution became light blue after several days. After about 30 days, the color of the solution became colo

55、rless. At this time, the pH was l and never changed. In the transmission spectra of the solutions, there were several absorption peaks below 440 nm. The transmittance of the solution after 30 days was almost equal to that of ethanol. Furthermore, the several absorption peaks at short wavelengths bec

56、ame one. An exthothermic peak at 42°C was observed in the DTA spectrum of the solution kept standing at room temperature for 2 days. When the solution kept standing for 2 days was refluxed at 50°C for 24 h, the solution changed in color from blue to colorless. When H2O was added to the ref

57、luxed solution, a white gel formed. In FTIR spectra of the solutions (a) kept standing for 2 days, (b) refluxed at 50°C, and (c) kept standing for 30 days), absorption due to stretching vibration of WO bonds was observed only in (b) and (c) (Reagan, 1970). Figure 3-7 shows the 13C NMR spectrum

58、of ethanol and the coating solution. The shift due to OCH2 was larger than that of CH3. A peak due to ethanol OCH2 was observed at 56.993 ppm. The calculated value for OCH2 is 55.8 ppm (Nakanishi, 1982). When WCl6 was dissolved in ethanol (WCl6:C2H5OH = 1:2), the OCH2 peak was observed at 59.444 ppm

59、, where the peak was broadened in comparison with that of ethanol. Figure 3-8 shows the relationship between the chemical shift of 13C (OCH2) and the composition of the solution (WCl6:ethanol). The chemical shift increased linearly with increasing WCl6/C2H5OH ratio.Figure 3-7. 13 C NMR spectra of (a) ethanol and (b) the WCl6 ethanol solution (mole ratio; C2H5OH: WCl6 = 6:1).From XRD analyses, it was proved that WO3 crystallized at temperatures above 350°C. The amorphous and crystalline thin films prepared were transparent. Chlorine or carbon wa