Effects of Acetic Acid on the Crystallization Temperature ofSol–Gel-Derived MgO Nano-Powders and Thin Films

Jin Young Kim, Hyun Suk Jung, and Kug Sun Hongw

School of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul, Korea

The influences of acetic acid addition to Mg-methoxide on thestability of the precursor and the crystallization behavior of sol–gel-derived MgO nano-powders and thin films were investigatedusing X-ray powder diffraction, transmission electron micros-copy, Fourier-transformed infrared spectroscopy, and therm-ogravimetry. The addition of acetic acid enhanced the stabilityof the alkoxide against precipitation. Moreover, during postheattreatment of the gel powders treated with acetic acid, a signif-icantly lowered crystallization temperature (2501C) was ob-served as compared to the untreated counterpart (3501C). Thelow-temperature crystallization of MgO, induced by the modi-fication of Mg-methoxide with acetic acid, was related to thedecomposition of organics at a lower temperature. These resultscould be explained in terms of the decrease of the O–R bondstrength depending on the increase in the alkyl group size. MgOthin films having a high degree of crystallinity were successfullyobtained from the Mg-methoxide treated with acetic acid at3001C. The low-temperature crystallization of sol–gel-derivedMgO thin films showed the feasibility for their application as aprotective layer in alternative current plasma display panel cells.

I. Introduction

RECENTLY, MgO thin films have received much attention foruse as protective layers in alternative current (AC) plasma

display panels (PDPs) because of their low erosion rate and highsecondary electron emission coefficients.1,2 MgO thin films havebeen prepared by several vacuum deposition methods, such asPLD (pulsed laser deposition),3 e-beam evaporation,4 and MO-CVD (metal organic chemical vapor deposition).5 However,these processes are incompatible with the manufacture of AC-PDP cells, which is based on thick film processes. The sol–gelprocess is one of the manufacturing methods for cost-efficientproduction of thick films.

Since the substrates used in these panels are made of a soda-lime silicate with a low melting temperature, one of the majorrequirements of the MgO thin film fabrication is a low crystal-lization temperature. In our previous work, the O–R (R5 alkylgroup) bond was shown to be broken easily when the size ofalkyl group increased.6 In such a case, decomposition of Mg-alkoxide is achieved at lower temperatures during post heattreatment; thereby, crystallization of MgO is facilitated. In thissense, although the acetate group in acetic acid does not belongto alkyl group, its bulky nature is expected to exhibit a similarrole to facilitate the crystallization ofMgO at a low temperature.Actually, acetic acid has been used as a stabilizing agent againsthydrolysis and/or precipitation for other metal organic sys-

tems,7,8 while its influence on the crystallization of MgO dur-ing post heat treatment has not been systemically studied.

In the present study, the influence of acetic acid treatment onthe crystallization behavior of sol-gel derived MgO gel powderduring postheat treatment has been systematically investigated.After confirming the role of acetic acid addition during the re-action in facilitating the crystallization of MgO gel powder, ithas been demonstrated that the same role is achieved in sol–gel-derived MgO thin films as well.

II. Experimental Procedure

MgO powders and thin films were prepared using sol–gelprocessing. Mg-methoxide (Aldrich, St. Louis, MO, 6 wt% so-lution in methanol) was selected as a precursor material. Drymethanol (Samchun, Seoul, South Korea, 99.9%) and aceticacid (Aldrich, glacial) were used as a solvent and an additive(stabilizer), respectively. Two kinds of sols with different com-positions, denoted as AA and Alk, were prepared: sol AA con-tained acetic acid while sol Alk contained no additive. Forpreparing sol AA, Mg-methoxide (5.94 mL) was diluted in drymethanol (7 mL) and acetic acid (0.081 mL) was added to drymethanol (7 mL) in another flask. Then, the acetic acid solutionwas added to the Mg-solution, followed by reaction for 2 h. Theratio of acetic acid to alkoxide was fixed to be 0.32 since, in apreliminary study, the sol was most stable against precipitationat this ratio. Sol Alk was prepared by the same procedure ex-cept that methanol (7 mL) without acetic acid was added to theMg-solution. All reactions were carried out at room tempera-ture in an Ar-filled glove box. Powders were prepared by dryingeach sol at 601C for 12 h in a vacuum oven that was connectedto a glove box in order to avoid any exposure to atmosphericwater vapor. Thin films were prepared by spin coating the pre-cursor solutions on (100)-oriented Si substrates at 2000 rpm for30 s. In the previous study, we confirmed that the Mg-methox-ide (Alk) did not react with water in air during the coatingprocess using XPS and Fourier-transformed infrared spectros-copy (FT-IR) study.9 Samples were annealed under variousconditions in a N2 atmosphere. The thickness of thin films wasapproximately 150 nm after annealing.

Thermal decomposition behaviors were analyzed usingthermogravimetry [(TG), Perkin-Elmer, Boston, MA, TGA 7],and FT-IR (Bomem, Quebec, Canada, DA 8). The crystalliza-tion behaviors were investigated by X-ray diffraction [(XRD),M18XHF-SRA, Macscience Co., Tokyo, Japan], and transmis-sion electron microscopy [(TEM), JEM-200CX, JEOL, Tokyo,Japan]. The morphologies of the MgO powders were alsoinvestigated using TEM and the thickness of the films wasmeasured with field emission scanning electron microscopy[(FE-SEM), JSM-6330F, JEOL].

III. Results and Discussion

In our ancillary experiments to test the stability of Mg-methox-ide, white precipitates formed in the Alk precursor when

Journal

J. Am. Ceram. Soc., 88 [3] 784–787 (2005)

DOI: 10.1111/j.1551-2916.2005.00119.x

784

V. A. Hackley—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: kshongss@plaza.snu.ac.kr

Manuscript No. 11027. Received May 14, 2004; approved October 1, 2004.

exposed to air for 0.5 h. The addition of acetic acid enhanced thestability: no precipitate was observed in the AA precursor untilthe sol transformed to a transparent gel in 40 h. Such stabilitywould result from chemical modification of acetic acid toalkoxide, which is confirmed by FT-IR spectra of AA powders.FT-IR spectrum of as-dried AA powder exhibited symmetricand asymmetric-COO-vibration modes of carboxyl groupsat around 1400 and 1600 cm�1,10 which were absent in Alk. Inaddition, the observation of an exothermic reaction during theaddition of acetic acid to Mg-methoxide in this work also sug-gests a chemical modification to the Mg-methoxide indirectly.Exothermic reactions due to the addition of acetic acid in othersol–gel systems such as Ti-and Zr-alkoxides were also interpret-ed as an indirect indication of a chemical modification of thealkoxide.7,8

Crystallization behaviors of MgO powder product duringpostheat treatment were investigated by XRD and TEM, andthe results are shown in Figs. 1 and 2. XRD peak intensity ofAA product is much higher than Alk (Fig. 1). Also, selected areaelectron diffraction (SAED) pattern from AA powder shows aspot-like ring pattern while Alk powder exhibits a diffusive ringpattern (Fig. 2). These indicate that acetic acid in AA solutionindeed take on a role to facilitate the crystallization of MgOduring post heat treatment. Thus, it is necessary to investigatehow the postcrystallization is fostered by the acetic acid as dis-cussed below.

In Fig. 1, as-dried samples from both solutions exhibit nocrystalline peaks but they show typical XRD patterns of Mg-methoxide,11 which disappear by postheat treatment at hightemperatures. Crystallization of AA powder starts from a tem-perature below 2501C, while Alk counterpart above 3001C. Thelower-temperature crystallization of AA powder accompaniesthe lower-temperature removal of the residual organics such asmethoxy and/or acetate groups. Thus, Alk solution is interpret-ed to yield more stable residual organics, which, in turn, retardsthe crystallization of MgO. However, acetic acid in AA solutionresults in less stable residual organics, which are decomposedrelatively easily at a lower temperature; thereby, crystallizationis more suitable.

The role of residual organic removal induced by acetic acid infacilitating the postcrystallization, as discussed above, is furtherconfirmed by FT-IR and TG studies shown in Figs. 3 and 4. InFig. 3, absorption peaks observed in as-dried Alk powders at2925.5, 2584.1, and 2794.3 cm�1 are assigned to C–H bending

and two C–H stretching modes of the methoxy group, respec-tively.11 Similar absorption peaks are observed in as-dried AA

powder. However, as temperature rises, these peaks disappearmore apparently in AA powder as compared with Alk so thatonly a broad band results at 4001C in AA powder (dotted circlearea in Fig. 3). This observation confirms that it is more difficultto eliminate residual organics in Alk powders, while it is viceversa for the AA powders. Furthermore, TG results shown inFig. 4 indicate that residual organics in Alk are eliminated at ahigher temperature (3201C) than the AA counterpart (2201C).The quenching temperatures of each sample, indicated by a ‘k’symbol (Fig. 4), shows the relationship between the crystalliza-tion to MgO and the thermal decomposition of organic groupssince the onset temperatures of thermal decomposition are closeto the crystallization temperatures. It is clear that the modifica-tion of Mg-methoxide with acetic acid facilitated the thermaldecomposition of organic groups and enhanced the crystalliza-tion process of MgO.

In our previous study on the relationship between the size ofthe functional groups of the alkoxides and the thermal decom-position behavior,6 bulky ethoxy groups decomposed at lower

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Fig. 1. X-ray diffraction patterns of MgO powders from (a) pure alk-oxide (Alk) precursor and (b) acetic acid added (AA) precursor fired atvarious temperatures in N2 atmosphere. Dotted lines indicate diffractionpeaks of MgO powders.

Fig. 2. TEM bright field images and SAED patterns of MgO powdersfrom (a) pure alkoxide (Alk) precursor and (b) acetic acid added (AA)precursor quenched at 3001C in N2 atmosphere.

March 2005 Communications of the American Ceramic Society 785

temperatures than methoxy groups. This phenomenon was ex-plained in terms of the O–R bond strength depending on the sizeof the functional group. Desu12 also reported similar resultsduring the CVD process using a Si-alkoxide (Si(OR)4) system.He explained that the O–R (R5 alkyl group or CH3CO group)bond energy decreased as the stabilization of the functionalgroup, which is related to the heat of formation, increased.Comparing the heat of formation of CH3 and CH3CO radicals(i.e., 147 kJ/mol for CH3 and �24.3 kJ/mol for CH3CO),

12 thestrength of bonding between the oxygen and CH3CO radicals inMg-methoxide treated with acetic acid (AA) is expected to beweaker than that between the oxygen and CH3 radicals in un-treated Mg-methoxide (Alk). Thus, the acetate groups were de-composed more readily than the methoxy group, which inducedthe low-temperature crystallization.

Having shown the role of acetic acid in facilitating postcrys-tallization of MgO powders, it is necessary to investigate wheth-er the same role is achieved in sol–gel-coated MgO thin films.

Thus, the crystallinity of thin films fired at 3001C was investi-gated using TEM analysis. As shown in the TEM bright fieldimages in Fig. 5, nano-sized crystallites are found in AA thinfilms while no crystallites are observed in Alk thin films. Theprogression of low-temperature crystallization in the MgO thinfilms due to acetic acid additions is also proved by SAED pat-terns in Fig. 5. The AA film shows an apparent ring pattern,while the Alk film shows a diffuse pattern. As confirmed by theTEM analysis, AA thin films are crystallized below 3001C, whileAlk thin films did not demonstrate crystallization at the sametemperature. Enhanced crystallinity of MgO thin films at a lowtemperature through addition of acetic acid was in agreementwith the behavior demonstrated in gel powders.

IV. Conclusions

The influences of acetic acid modifications on the stability ofMg-methoxide and the low-temperature crystallization behaviorof MgO during postheat treatment were investigated. The chem-ical modification through acetic acid addition improved the sta-bility of the Mg-alkoxide since no precipitation by hydrolysis

3400 3200 3000 2800 2600 3400 3200 3000 2800 2600

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Fig. 3. Infrared (IR) spectra of MgO powders from (a) pure alkoxide(Alk) precursor and (b) acetic acid added (AA) precursor fired at varioustemperatures in N2 atmosphere. Dotted lines indicate C–H bending andstretching modes of functional groups.

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Fig. 4. Thermogravimetry of MgO powders from (a) pure alkoxide(Alk) precursor and (b) acetic acid added (AA) precursor in N2 atmos-phere.

Fig. 5. TEM bright field images and SAED patterns of MgO thin filmsdeposited from (a) pure alkoxide (Alk) precursor and (b) acetic acidadded (AA) precursor fired at 3001C in N2 atmosphere.

786 Communications of the American Ceramic Society Vol. 88, No. 3

was observed in air when acetic acid was added to Mg-methox-ide. From XRD and TEM analysis, it was also found that theacetic acid addition lowered the crystallization temperature andenhanced the crystallinity of the resultant MgO powder. UsingFT-IR and TG analysis, the low-temperature crystallization ofmodified Mg-alkoxide powder was revealed to originate fromthe low-temperature thermal decomposition of the acetategroups. This phenomenon was interpreted to occur becauseO–R bond strength was decreased in AA solution as the sizeof the functional group increased. Using the acetic acid modifiedMg-methoxide, a well-crystallized MgO thin film was success-fully obtained at 3001C.

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