Surfctnt Assisted Friction of Different Nnostructures of Boehmite y Hydrotherml Process Kveti Rjrm 1 nd Jihoon Kim* Division of Advnced Mterils Engineering, Kongju Ntionl University, Cheonn, Chungchungnm-do 32588, Kore. *Corresponding uthor: Jihoon.kim@kongju.c.kr (J. Kim) 1 ORCID: 0000-0003-3649-3356, *ORCID: 0000-0003-1477-1423 Astrct The morphologicl controlled growth of Boehmite (γ-alooh) nnocrystls hs ttrcted extensive reserch interest ecuse of their unique shpe-dependent nd widespred pplictions. Surfctnts hve een frequently used in the morphologicl controlled synthesis of oehmite nnocrystls in solution. In this work, y using hexdecyl trimethyl mmonium romide (CTAB) s structure-directing gent nd precipittor, we present synthesis of well-crystllized oehmite nnostructures with vrious morphologies vrying from three-dimensionl (3D) flower like rchitectures, two-dimensionl (2D) flkes to one-dimensionl (1D) rods, wires with enhnced spect rtios nd even lmellr rchitectures y hydrotherml process y simply controlling the CTAB concentrtion, rection time nd temperture, nd the ph of the rection mixture. The otined oehmite nnostructures were chrcterized y techniques, such s X-ry diffrction (XRD), Fourier trnsform infrred technique (FTIR), Scnning Electron Microscopy (SEM). The convincing evidence for the crystl phse of the s-prepred different nnostructures ws provided y FTIR spectr. A possile formtion mechnism of the different oehmite nnostructures is proposed sed on the rection dynmics process nd the surfctnt-ssisted growth. Keywords: γ-alooh, Hydrotherml Process, Different Nnostructures, Boehmite, Nnowires, Nnorods, Nnoflkes nd Lmellr Architectures INTRODUCTION Over the pst few decdes, tremendous efforts hve een devoted to the friction of inorgnic nnomterils with different shpes such s 1D nnowires, nnorods, nnotues, nnostrips, 2D nnoflkes, nnosheets, nd their selfssemled 3D lmellr rchitectures, nd nnoflowers, s the mteril properties, such s physicl, chemicl, mgnetic, opticl nd ctlytic properties nd their potentil pplictions re lrgely dependent on sizes nd/or morphologies [1-5]. Severl Techniques hve een developed to fricte the inorgnic mterils with controlled morphologies, chemicl vpor deposition (CVD) [3], templte-confined method [4], vpor-liquid-solid (VLS) methods, soft lithogrphy, lser ltion nd solution-phse pproches [6-8], etc. Hydrotherml nd solvotherml crystlliztion is well-known method used to grow inorgnic crystls. As moderte nd djustle method, the hydrotherml route hs occurred s n lterntive synthesis route to control the properties of the inorgnic mterils, including size, structure, morphology, components, etc. In those processes, unstle prticles re dissolved nd recrystllized to form more stle prticles, leding to crystl growth. The shpe of the prticle is usully connected with the intrinsic structure of the crystls nd the environmentl conditions nd the shpe of the crystls re determined so tht their totl surfce energy cn e minimized. The processing time depends on the finl size of the crystls required usully, it tkes few hours or dys. Some of the experimentl results found tht the growth prmeters such s surfce energy, growth rte, cpping molecules, rection temperture, nd the ph of the rection mixture to e criticl in determining the ehvior of the ultimte nnostructures [8]. Understnding the growth mechnism nd the shpe-guiding process is essentil for the synthesis of the prticles with desired shpes nd sizes [9]. Boehmite, s one of the two polymorphs of the luminum oxyhydroxide (the other one is dispore, α-alooh), nd it is prtly dehydrted luminum hydroxides which is primry precursor for the preprtion of different lumins, such s γ- Al 2O 3 nd corundum (α-al 2O 3), is mteril widely useful in severl commercil pplictions, for exmple, cermics, rsive mterils, fire-retrdnts, dsorents, cotings, ctlytic process (ctlysts nd ctlyst supports), nd fillers for polymeric composites [10-12]. According to the recent reports, oehmite nnoprticles hve lso found some of the technologicl utilities s dielectric microelectronic, iomterils, nd opticl devices [13]. Since there is close reltionship etween the morphology nd/or size nd properties nd further pplictions, enormous ttention hs een pid to synthesize oehmite smples with different shpes nd dimensions, such s nnorods, nnotues, nnofiers, nnowires, dispersed nnosheets, nnoflkes, nd even their ssemled superstructures, such s flowers, hllow microspheres, nd cntloupe-like rchitectures [14-17]. For exmple, T. He et l. reported hydrotherml synthesis of oehmite nnocrystls with vrying morphologies y freshly precipitted Al(OH 3) gel t 240 0 C for 16 h in different solutions nd their studies shows, the nisotropic dsorption of the nions on the fcets of the oehmite ws considered to e responsile for the formtion of oehmite nnocrystls with vrious rchitectures [18]. X. Y. Cheon et l. reported the hydrotherml synthesis of 1D nnorods nd 2D nnoflkes nd they concluded tht the cidity of the rection mixture plys significnt role in determining the size nd morphology of the 2781
resulting nnocrystls [19]. More recently, W. Jio et l. reported the different sizes nd morphologies of oehmite with enhnced spect rtios y djusting the molr rtios in n orgnic dditive-free ctionic-nionic doule hydrolysis method [20]. As stted ove, mny groups hve een developed nd documented numerous procedures of controlling the morphologies nd sizes of oehmite nnoprticles, however herein we present simple surfctnt-ssisted hydrotherml synthesis of well-crystllized oehmite nnostructures with different morphologies vrying from flower like rchitectures, lmellr rchitectures, nnoflkes to nnorods, nnostrips, nd even nnowires with enhnced spect rtios (i.e., [length long the -xis]/[length long the c-xis]) y simply controlling the Al 3+ /CTAB concentrtion, rection time, temperture, nd the ph of the rection mixture. EXPERIMENTAL SECTION All of the chemicl regent used in our experiments were nlyticl grde nd used s purchsed without further purifiction. Aluminum nitrte nonhydrte (Al(NO 3) 3 9H 2O), hexdecyl trimethyl mmonium romide (CTAB), nd mmoni (NH 3) were purchsed from Sigm-Aldrich. Deionized wter ws used directly without further purifiction. In typicl synthesis, clculted mount of Al(NO 3) 3 9H 2O ws dissolved into 20 ml of distilled wter under mgnetic stirring. Similrly, certin mole rtio of CTAB ws dissolved into 20 ml of distilled wter under mgnetic stirring. Then, the CTAB solution ws susequently dded drop y drop to the Al(NO 3) 3 9H 2O solution, nd the resulting mixture ws stirred for severl minutes until cler solution ws formed. Susequently, some mount of concentrted NH 3 9H 2O ws dded dropwise to the ove solution to get certin ph of the rection mixture. Finlly, the resultnt solution ws trnsferred into 60 ml Teflon-lined, stinless steel utoclve, which ws then seled nd kept in the electric oven t 200 0 C for 24 h. After the hydrotherml tretment, the utoclve ws slowly ir cooled to room-temperture. The resultnt colloidl product ws centrifuged nd wshed severl times with deionized wter, followed y ethnol, nd then dried it under vcuum t 60 0 C for 8h. The phse identifiction of smples ws crried out on X-ry diffrction (XRD) ptterns, using Rigku miniflex 600 with Cu-Kα rdition (λ=1.54056 Å). The morphology nd sizes of the smples were studied y field emission scnning electron microscopy (FESEM; Nov 200). Fourier trnsform infrred (FTIR) spectr were performed with PerkinElmer-Frontier FT- IR Spectrometer. Figure 1- XRD ptterns of the hydrotherml products formed with 3.0 mmol Al(NO 3) 3 nd 1.25 mmol CTAB t 200 0 C for 24 h, t ph 3 (); t ph 5 (). RESULT AND DISCUSSION Fig.1 shows the typicl XRD ptterns of the otined smples prepred t ph= ~3 () nd ~5 () t 200 0 C for 24 h. Under these cidic conditions, ll the shrp nd strong reflection peks cn e esily indexed to the orthorhomic oehmite phse, which is in good greement with the previous reports [19-21]. Figure 2- FE-SEM imges of the hydrotherml products formed t 200 0 C with Al 3+ /CTAB molr rtio of 3.0/1.25 for 12h (); 24 h (). 2782
No peks from the other phses were oserved, indicting the high purity nd crystllinity of the s-prepred γ-alooh. The XRD ptterns were lmost identicl for the oth smples which re nonowires (), nnorods (), the little rodening nd less intensity of XRD pttern () compred to () should e relted to the smll size of the s-prepred product. Figs. 2 nd 2 shows the FE-SEM imges of the γ-alooh smples synthesized t ph= ~5 with Al 3+ /CTAB molr rtio of 3.0/1.25 t 200 0 C for 12h nd 24 h respectively. The FE-SEM imge Fig. 2, revels tht the γ-alooh smple is composed of so mny uniform nnorods hving the lengths nd the dimeters re pproximtely 470 nm nd 50 nm respectively leding to the spect rtio of ~9.5. The FE-SEM imge Fig. 2, revels tht the less gglomertion nd enhncement in the spect rtio of γ-alooh nnorods to ~15, which is proly due to the incresed in the rection time from 12h to 24h leding to morphology growth of the Boehmite. To understnd the effect of the rection time on the morphology of the resulting product, we hve incresed the rection time from 24 h to 36 h nd y keeping ll the other rection conditions s constnt nd its FE-SEM imges were otined. FE-SEM imge Fig.3 clerly shows the result of ove effect y hving the mixed spect rtio rnging from 12 to 17, lso the imge clerly demonstrtes the undles of nnorods due to the self-ssemly of severl individul nnorods, possily s the result of the presence of hydrogen onds etween the djcent single nnostrips through the fllowing suggested mechnism [22]. In the oehmite lttice, oxygen ions re locted in distorted octhedrl rrngement round luminum nd orgnized in prllel lyers connected y hydrogen onds [23]. Therefore, the individul oehmite nnorods self-ssemle lterlly due to the surfce hydrogen onds nd form undles of oehmite nnorods s indicted in Fig. 3. c Figure 3- FE-SEM imge of the hydrotherml product formed t 200 0 C with Al 3+ /CTAB molr rtio of 3.0/1.25 for 36h. In order to investigte the effect of the cidity on the morphologies of the resulting products, we hve mintined the ph of the hydrotherml rection mixture t 3.0 y keeping ll other rection conditions s sme, which were, rection temperture 200 0 C, Al 3+ /CTAB molr rtio of 3.0/1.25 nd the Figure 4- FE-SEM imges of the hydrotherml product formed t 200 0 C with Al 3+ /CTAB molr rtio of 3.0/1.25 for 24h t ph 3.0 (); () nd (c) re the mgnified FE-SEM imges t different orienttions. rection time ws 24 h, lter FE-SEM imges of the resulting products were otined. From the FE-SEM imges in Fig. 4 2783
clerly, demonstrtes the smple consist of mny high spect rtio nnowires with length scles rnging from 800 nm to ~2 µm nd dimeter of nnowires re out 18 nm to 35 nm. The spect rtio of the nnowires is out 100. The Figures 4-4. clerly shows the self-ssemle undles of nnowires in n enormous numer due to the presence of hydrogen onds etween the djcent single nnowires, which cn e understood y the mechnism explined erlier. The Fig. 4c revels the mgnified imge of the self-ssemled undle of nnowire in the morphology growth direction. The good crystllinity of the nnowires cn e seen from the XRD pttern in the Fig. 1(). From the figures 2 nd 4, we cn conclude tht, the high cidic conditions re more fvorle towrd the formtion of high spect rtio 1D nnostructures. A similr conclusion ws given y X.Y. Chen et l. even without using the surfctnt [19]. To investigte the influence of the CTAB concentrtion on the otined product morphology nd sizes, series of experiments were crried out. Vrying mounts of 2.50, 3.75, nd 5.0 mmol CTAB were dded to the system to replce the 1.25 mmol CTAB, nd the rection mixture ws mintined t 200 0 C. The crystllinity of the products decresed with the incresing CTAB concentrtion, which cn e clerly seen from the XRD ptterns in the Fig. 5 y compring the XRD ptterns in the Fig. 1. Initilly, to investigte the time-dependent evolution of the hydrotherml products, we hve done few experiments for 12 h nd 24 h nd y keeping hydrotherml rection t 200 0 C with Al 3+ /CTAB molr rtio of 3.0/1.25. The result of this products cn e seen in the Fig. 6. The FE-SEM imge Fig. 6 demonstrted thin lyered structure which ws the product otined for 12 h. c Figure 5- XRD ptterns of the hydrotherml products formed t 200 0 C for 24 h nd for Al 3+ /CTAB molr rtio 3.0/2.50 (c), 3.0/3.75 (d), nd 3.0/5.0 (e). When the growth period ws prolonged to 24 h, the structures of the smple comprised severl flkes y ssemle of few ly ers. The sizes of the flkes (Fig. 6) were incresed for 24 h rection hving smller gps in etween flkes when compred to flkes otined for (Fig. 6) 12 h rection. XRD ptterns of Figure 6- FE-SEM imges of the hydrotherml product formed t 200 0 C with Al 3+ /CTAB molr rtio of 3.0/2.50 for 12 h (); 24 h (); Al 3+ /CTAB molr rtio 3.0/5.0 for 24 h (c). the flke cn e seen from the Fig. 5 (c). By incresing the mount of CTAB further, the sheets ecome compct. When the usge of CTAB is more thn or equl to 5.0 mmol complete product ws lmellr structures, the gps etween lmellr structures ecme lrger, decresing the ppernce of flkes. The Fig. 7 shows the mgnified imge of the lmellr γ-alooh 2784
rchitecture nd it revels tht the pproximtely 1.5 µm length lmellr superstructures consist of flkes with thickness of pproximtely 20 nm. The XRD pttern of the lmellr rchitectures cn e seen from the Fig. 5 (e), the crystllinity of the products ws decresed due to the increse in CTAB concentrtion s discussed previously. Figure 7- Mgnified FE-SEM imge of fig. 6 (c) product. The FE-SEM imges in Fig. 8 ws otined for the hydrotherml products synthesized under the conditions where Al 3+ /CTAB molr rtio of 3.0/3.75 t 200 0 C with for 24 h. The Fig. 8 consists of very fine lmellr rchitectures long with few flower-like rchitectures. The flower-like superstructures re pproximtely 2.0 µm in size consisting of flkes spred in ll the, nd c xis. Figure 5d shows the XRD pttern of the Flower like γ-alooh rchitectures. These results indicte tht CTAB concentrtion hs very significnt effect on controlling the morphology of synthesized γ-alooh rchitectures. A lower cidic nd lower CTAB concentrtion results in 1D structures such s nnorods nd nnowires, little higher CTAB concentrtion results in 2D flkes nd 3D rchitectures such s nnoflowers long with lmellr rchitectures, nd higher surfctnt concentrtion gives complete multilyered 3D superstructures such s lmellr rchitectures. Figure 9 shows the FT-IR spectr of the hydrotherml products otined with 3.0 mmol Al(NO 3) 3 nd 1.25 mmol CTAB t 200 0 C for 24 h. The FT-IR spectrum Fig. 9() is otined for the resultnt product synthesized t ph 3, nd nother spectrum Fig. 9() is otined for the resultnt product which ws synthesized t ph 5. There is no distinct difference etween these FTIR spectr. It is cler tht the 1D oehmite nnostructures (Fig. 2 nd Fig. 4) otined y simple hydro therml conditions showed sorption nds of Fig. 9 () t 3296, 3092, 2095, 1644, 1154, 1067, 747, 628, nd 467 cm -1, nd sorption nds of Fig. 9 () t 3304, 3090, 2101, 1973, 1646, 1157, 1067, 742, 619, 473 cm -1, which gree precisely with those reported in the literture [24]. In detil, the intensive nds t 3304, 3296 nd 3090, 3092 cm -1 cn e ssigned to the Figure 8- FE-SEM imges of the hydrotherml product formed t 200 0 C with Al 3+ /CTAB molr rtio of 3.0/3.75 for 24 h (); Mgnified FE-SEM imge (). Figure 9- FT-IR spectr of the hydrotherml products otined with 3.0 mmol Al(NO 3) 3 nd 1.25 mmol CTAB t 200 0 C for 24 h, t ph 3 (); t ph 5 (). 2785
υ s (Al)O-H nd υ s (Al)O-H stretching virtions respectively. The two wek nd t 2101 nd 1973 cm -1 cn e ssigned to comintion nds. The intense nd t 1067 cm -1 nd the shoulder t 1154 cm -1 re scried to the δ s Al-O-H nd δ s Al- O-H ending virtions in the oehmite lttice. The three strong nds t 747, 628, 476 cm -1 cn e ssigned to the virtion mode of AlO 6. In ddition, the shoulder t 1644 cm -1 cn e ssigned to the ending mode of dsored wter. A similr interprettion cn e pplicle to the FTIR spectrum () in Fig. 9. Consequently, FTIR nlysis lso confirms tht the s prepred hydrotherml products re pure-phse γ- AlOOH. Figure 10- FT-IR spectr of the hydrotherml products otined t 200 0 C for 24 nd for Al 3+ /CTAB molr rtio 3.0/2.50 (c), 3.0/3.75 (d), nd 3.0/5.0 (e). The Fourier trnsform infrred spectrometry spectr of the hydrotherml products synthesized t 200 0 C for 24 nd with Al 3+ /CTAB molr rtio 3.0/2.50, 3.0/3.75, nd 3.0/5.0 were shown nd denoted s (c), (d), (e) respectively in the Figure 10. The FTIR spectrum (c) corresponds to the otined product of flkes like morphology, FTIR spectrum (d) corresponds to the hydrotherml product of flower like rchitectures, nd The FTIR spectrum (e) corresponds to the lmellr rchitectures. The spectrums in the Fig. 10 hs slight difference proly due to the different morphology of the otined products which is consistent with the XRD results (Fig.5) which hve poor crystllinity. As discussed ove the sorption nds of Fig. 9; the fig.10 sorption nds t 3314, 3088, 2924, 2854, 1652, 1068, 749, 627, nd 480 cm -1, lso gree precisely with those reported in the literture. The typicl C-H nds re not visile in the spectr of prepred smples, which indicte tht CTAB is not present in the products. The rod 3310 cm -1 the wek nds t 1640 cm -1 ccount for the stretching nd ending modes of the sored wter. The product peks 3314 cm -1 nd 3088 cm -1 cn e ssigned to the υ s (Al)O-H nd υ s (Al)O-H stretching virtions respectively. The nd 2105 cm -1 lso oserved in the spectrum of γ-alooh product. The three torsionl modes 747, 627, 480 cm -1 cn e ssigned to the virtion mode of AlO 6. The nd t 1069 cm -1 is ssigned to the δ s Al-O-H of γ-alooh. Therefore, FTIR nlysis of Fig.10 resultnt products lso confirms tht the s prepred hydrotherml products re pure-phse γ-alooh. These results confirm CTAB serves vitl role in the otining the different oehmite nnostructures. Usully, CTAB is used s templte micelle molecules to synthesize nnomterils. When CTAB dissolves in wter it cn completely ionize nd the resultnt ction is positively chrged tetrhedron tht possesses long, hydrophoic til. When it is in the solution, OH - cn dsor to ctionic hed of CTAB y electrosttic forces. The orthorhomic AlOOH exhiits lmellr structure in which the Al3+ ions exist distorted, edge shring octhedrl rrys of oxide ions tht form doule lyer with zigzg chins of H-onds tht connect the lyers [21]. At the initil growth nd higher concentrtion of CTAB, lmellr structures re redily formed in solution, followed y growth process, which results in multi-lyered metstle structure (Fig. 6(c)). When the hydrotherml rection temperture is high nd in the presence of CTAB, the metstle structures could e controlled to form the lmellr structures y conventionl Ostwld ripening (Fig.7). The solution with the lower concentrtion of CTAB, the metstle lyered structure could e distured nd split into rod like forms (Fig.2) nd even wire like (Fig.4) structures, lter when self-ssemly procedure hppened flower like rchitectures cn e produced. In the crystlliztion process CTB serves s growth controller nd n gglomertion controller, with the growth of the γ-alooh crystl, the morphous Al(OH) 3 dehydrtes to form AlOOH. Due to the preferentil dsorption of CTA + hed groups, only uniform rchitectures with preferentil growth direction my e formed. In ddition, this hydrotherml tretment is used to improve the crystllinity [25]. CONCLUSION In summry, the synthesis of different morphology of oehmite (γ-alooh) such s nnorods, nnowires, nnoflkes, lmellr nd flower like rchitectures y hydrotherml route ws studied. The effect of the cidity of the rection mixture nd the CTAB concentrtion on the morphologies of the resulting products were investigted, which reveled tht the under cidic conditions with lower CTAB concentrtion 1D, with higher CTAB concentrtion 2D nd 3D morphology of oehmite nnostructures formed. CTAB served s structure directing gent nd precipittor in this controlled formtion of the rchitectures nd morphologies. ACKNOWLEDGMENTS This work ws supported y Internl Reserch Progrm funded y Kongju Ntionl University. REFERENCES [1] Jolivet, J.-P., Froidefond, C., Pottier, A., Chnec, C., Cssignon, S, Tronc, E., Euzen, P, J. Mter. Chem. 2004, 14, 3281-3288. 2786
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