Hollow mesoporous zirconia nanocontainers ofr storing and controlled releasing of corrosion inhibito

CERAMICSINTERNATIONALAvailable online at Ceramics International 40(2014)10457–10463Hollow mesoporous zirconia nanocontainers for storing and controlledreleasing of corrosion inhibitorsArunchandran Chenan,S.Ramya,R.P.George,U.Kamachi Mudali nCorrosion Science and Technology Group,Indira Gandhi Centre for Atomic Research,Kalpakkam 603102,IndiaReceived 8February 2014;accepted 4March 2014Available online 17March 2014AbstractRelease of inhibitor molecules on demand from nanocontainers dispersed in a passive barrier coatings offer great promise for active corrosion protection coatings of metals and alloys with self healing ability.In the present study,hollow mesoporous zirconia nanospheres (hm -ZrO 2)with a hollow core /porous shell structure are proposed as effective containers for corrosion inhibitor loading and releasing.Hollow mesoporous zirconia nanocontainers were synthesized using solid silica nanoparticles as templates.The morphology and phase of zirconia nanocontainers were studied using high resolution transmission electron microscopy and laser Raman spectroscopy.2-Mercaptobenzothiazole was selected as the model corrosion inhibitor for the encapsulation.The storage and release properties of hm -ZrO 2were investigated using UV –visible spectroscopy.The encapsulation ef ficiency of hm -ZrO 2was 63%and we observed a faster and higher release of 2-MBT from hm -ZrO 2when the pH was shifted from neutral value.&2014Elsevier Ltd and Techna Group S.r.l.All rights reserved.Keywords:A.Calcination;C.Corrosion;C.Diffusion;D.ZrO 21.IntroductionCorrosion of metals and alloys is a global problem resulting in huge economic loses and causing environmental concerns.Application of coatings on metals and alloys are one of the most effective methods employed to prevent corrosion [1].The main attribute of anticorrosion coatings is to avoid the contact of metals from its environment and they function either passively or actively.In passive corrosion protection,coatings act as an impermeable physical barrier to the corrosive environment.However,these class of coatings work only when the coatings remain intact.Any damage in the coatings will lead to corrosion.On the contrary,in active corrosion protection corrosion inhibitors are incorporated in the barrier coatings and corrosion inhibition would be initiated when the barrier coating is damaged [2].The corrosion inhibitors cannot be added directly into the barrier coatings,as it can disrupt coating integrity due toundesired leaching of inhibitors and inhibitor deactivation [2–7].But this disadvantage can be smartly overcome by the encapsu-lation of corrosion inhibitor in nanocontainers and uniformly dispersing them in the coating.These nanocontainers dispersed in the coating can release inhibitor on demand during corrosion process and protect the underlying metal.Many encapsulation techniques have been reported so far for effective storage and on demand release of corrosion inhibitors.Emulsion polymeriza-tion encapsulation,layer by layer assembly of oppositely charged polyelectrolyte and inhibitor layers on the surface of nanoparticles and adsorption of inhibitor molecules on the porous metal oxides are some of the encapsulation techniques reported [4–7].This smart strategy helps in storing the inhibitor and thereby avoiding any unfavorable interaction of inhibitor with the coating.Nanocontainers are used to load high amount of inhibitor,avoid detrimental leaching of inhibitor and ensure sustained and intelligent release of inhibitor on demand.The release of inhibitor is induced by changes in the local environ-ment in the damaged area of the coating,such as changes in local pH,ionic strength,or presence of aggressive ions [8].Heterocyclic organic compounds consisting of oxygen,nitrogen,/locate/ceramint/10.1016/j.ceramint.2014.03.0160272-8842/&2014Elsevier Ltd and Techna Group S.r.l.All rights reserved.nCorresponding author.Tel.:þ914427480121;fax:þ914427480301.E-mail addresses:kamachi@.in ,ukmudali@ (U.Kamachi Mudali).sulfur,and phosphorus are generally used as corrosion inhibi-tors.2-mercaptobenzothiazole,a thiazole derivative,was chosen as model inhibitor molecule for encapsulation in this study. Zheludkevich et al.employed silica nanoparticles covered layer-by-layer with polyelectrolyte and layers of inhibitor as nanocontainer for active corrosion protection[2].Even though a layer-by-layer assembled shell could ensure sustained release of inhibitor molecules for active corrosion protection,their complexities limit the scaling up and industrial application. These difficulties can be overcome by using hollow and porous nanomaterials with high pore volume and large surface area to load corrosion inhibitors.Porous metal oxides have been successfully employed as nanocontainers to encapsulate corro-sion inhibitors.Recently,inorganic porous materials have been widely investigated as reservoirs for loading corrosion inhibi-tors[9–16].Compared to porous metal oxides and inorganic porous materials,hollow mesoporous nanomaterials can func-tion as a better container for encapsulation.The hollow inner cavity of these porous nanomaterials can effectively store the inhibitor molecules and release in a controlled way through its mesopore channels.This work describes a new contribution to the design of nanocontainer for the encapsulation of corrosion inhibitor. Hollow mesoporous zirconia nanocapsules with a hollow core/porous shell structure were synthesized through a hard template method[17–19].In this method,the size and the size distribution of the hollow spheres are controlled by the size of the template.The template is then removed through the pores of thefinal spheres obtained.Monodispersed solid silica nanoparticles were used as template for the synthesis of hm-ZrO2.The silica nanoparticles were coated with zirconium oxide layer and later these silica templates were removed using sodium hydroxide.The present study reports the synthesis, characterization and inhibitor storage and release properties of hollow mesoporous zirconia spheres.2.Experimental section2.1.MaterialsAll chemicals were used as received without further purifica-tion.Zirconium butoxide solution(80wt.%in1-butanol), tetraethoxysilane(TEOS),ethanol,concentrated ammonia solu-tion(25%NH3in water),sodium hydroxide(NaOH),and Brij30were used to synthesize hollow mesoporous zirconia spheres.2-mercaptobenzothiazole(MBT)was used as corrosion inhibitor,acetone was used for inhibitor loading experiments and sodium chloride(NaCl)was used for releasing experiments. All the chemicals used were of reagent grade and the water used in the study was purified in a three stage Millipore Milli-Q plus 185purification system,which had a resistivity higher than 18MΏ.cm.2.2.Preparation of SiO2template particlesThe stober method was used to synthesize monodispersed silica nanoparticles[20].75mL of ethanol,10mL of water and3.15mL of concentrated ammonia solution were mixed and stirred.After30minutes of stirring,6mL of TEOS was added dropwise and the reaction mixture was stirred for8h. The resultant white silica powders were collected by centrifu-gation and washed several times with water and ethanol.Then the solid silica powders were again dispersed in water and dried at room temperature.2.3.Preparation of hm-ZrO22g of SiO2template particles were dispersed in200mL of ethanol and1mL Brij30and1mL water was added into it. After30min of stirring of the reaction mixture,6mL of Zr (BuO)4was added and the mixture was further stirred for8h. The white colored powder was collected after centrifugation and redispersed in water.These particles were aged in water for3days at298K and this aging step has a very important role on the structure of the shell.After aging,the removal of the organic material and the crystallization of the amorphous zirconium oxide layer were carried out by calcination.The white powder was calcined at8501C for2h.Finally,the silica templates were removed by a treatment with NaOH(5M)for 48h.The powders were kept in NaOH solution with stirring and after24h NaOH solution was exchanged with fresh alkaline solution and the stirring continued for another24h. After removing the silica template,the core-shell zirconia spheres were washedfive times with water and dried at1001C for further use.2.4.Characterization of hm-ZrO2The high resolution images of silica template and hm-ZrO2 were taken using a JEOL JEM2100(Japan)electron micro-scope operated at200kV.The powder samples were dispersed in ethyl alcohol under ultrasonication and few drops of the dispersed samples were placed on the sample holder and analyzed without further treatment.The phase analysis of the hm-ZrO2was carried out using laser Raman spectroscopy(Lab RAM HR800,HORIBA JOBIN YVON Raman spectrometer equipped with1800grooves/mm holographic grating).The sample was placed under Olympus BXFM-ILHS microscope mounted at the entrance of the Raman spectrograph.Arþlaser of488nm was used as excitation source and the power of the laser at the sample was$8.6mW.The Raman spectra were recorded using a super cooled(oÀ1101C)1024Â256 pixels charge coupled detector(CCD)in static mode over the range of80–1000cmÀ1with10s exposure time and10 CCD accumulations.Nitrogen adsorption/desorption isotherms were recorded at77K on a Sorptomatic1990(Thermo Quest, CE Instruments,Italy).The specific surface area was calculated by Brunauer–Emmett–Teller(BET)method[21].The HMS sample wasfirst degassed for24h at1501C in vacuum.The average pore size and pore size distributions were obtained from the desorption branch using Barrett–Joyner–Halenda (BJH)method[22].A.Chenan et al./Ceramics International40(2014)10457–10463 104582.5.Loading and releasing of2-MBT50mg of hm-ZrO2was mixed with3mL of1mg mLÀ1 MBT solution.The mixture was then sonicated for30m and stirred further for24h in a glass vial.After24h of stirring of the mixture,the nanocontainers loaded with inhibitor molecules were taken out by centrifugation and cleaned with water.The MBT loaded hm-ZrO2was obtained by drying at room temperature.Thefiltrate was extracted from the vial and used for UV-visible spectroscopy studies to confirm the encapsulation of MBT.UV–visible spectroscopy(UV-2450,SHIMADZU make,Japan)was used for UV–vis analysis.25mg of MBT loaded hm-ZrO2was dispersed in150mL of releasing medium(0.05M NaCl)at different pH values of3, 7and10at room temperature and the solution was stirred at a rate of500rpm using a magnetic stirrer.1mL of the solution was pipetted out at given time intervals for UV-visible analysis.After the measurement,the solution was poured back immediately to the solution.3.Results and discussion3.1.Synthesis and characterization of hm-Zirconia spheresHollow mesoporous zirconia nanocontainers with approxi-mate size of400nm were synthesized using a modified method published elsewhere[17–19].Thefive steps involved in the preparation of hollow mesoporous zirconia are described below.1.Monodispersed silica template nanoparticles were synthe-sized using the well known stober method.2.The prepared monodispersed silica nanoparticles were thencoated with amorphous layer of zirconium hydroxide during the hydrolysis of zirconium butoxide in the presence of Brij30.3.The silica core with the zirconia shell containing Brij30was aged in water for3days at room temperature.ter the silica core with zirconia shell was calcined at8501C to remove the organics and to achieve the crystal-lization of the shell.5.Finally,the template,silica core was removed using NaOH(5M)treatment for48h.The silica particles were leached out through the pores present in the zirconia shell.The removal of solid silica core particles resulted in the formation of hollow mesoporous zirconia nanocontainers. Fig.1depicts the HRTEM images of silica template particles having average diameter of375nm.Fig.2presents the HRTEM images of hollow mesoporous zirconia obtained after the removal of silica templates.As presented in the HRTEM images,the average thickness of zirconia layer was 30nm.The perfectly shaped spherical morphology of the zirconia nanocontainer was maintained after calcination and the removal of silica template with NaOH treatment(Fig.2). The porous structure of the shell helped the NaOH solution to access the core of the nanocontainer and dissolve the silica. The removal of silica particle from the interior confirmed that the zirconia nanocontainer had a porous structure.As shown in the HRETM image(Fig.(2d))the hollowness was the most important structural feature of the as prepared zirconia nano-spheres.Both the porous structure and hollowness are very essential for high loading capacity of the inhibitor molecules. During encapsulation,the pores on the shell facilitate the diffusion of the inhibitor molecules and the empty space inside the zirconia sphere facilitates the storage of inhibitor mole-cules.Moreover,the porous structure of the shell makes ease on demand release of the inhibitor molecules.The phase of the as prepared hollow mesoporous zirconia was investigated using laser Raman spectroscopy.Fig.3shows Fig.1.HRTEM images of solid silica nanoparticles at different magnifications.A.Chenan et al./Ceramics International40(2014)10457–1046310459the Raman spectrum of hollow mesoporous zirconia.As revealed by Fig.3,the characteristic phonon modes of tetragonal phase of zirconia were observed in the Raman spectra.Five Raman active modes of tetragonal phase were observed.The bands at 149cm À1,274cm À1,and 463cm À1are assigned to the E g mode of vibration of tetragonal phase [23].The bands at 318cm À1and 648cm À1were of B 1g mode.The mesoporosity of the sample was con firmed by N 2adsorption-desorption isotherm analysis presented in Fig.4.The hollow zirconia particles showed a sorption isotherm of type IV with a hysteresis loop characteristic for materials with mesoporous structure.The hysteresis loop was due to the capillary condensation in mesopore structures.The type IV adsorption isotherm is a characteristic feature of mesoporous materials.The speci fic surface area of hollow zirconia products was obtained using the BET method and it was found tobeFig.2.HRTEM images of hollow mesoporous zirconia nanocontainers at different magni fications.Fig.3.Raman spectra of as prepared hollow mesoporouszirconia.Fig. 4.N 2adsorption-desorption isotherms of hm -ZrO 2and pore size distribution (inset).A.Chenan et al./Ceramics International 40(2014)10457–1046310460145.97m 2g À1.Barrett –Joyner –Halenda (BJH)analyses revealed that the hollow zirconia products exhibited pore size centered at 3.9nm and cumulative pore volume of 0.0993cm 3g À1.The high surface area,mesopore distributions and hollow structure of the nanocontainer are very important for inhibitor loading and controlled inhibitor release properties.3.2.Inhibitor loading and releasing properties of hm-ZrO 2nanocontainers2-Mercaptobenzothiazole was successfully loaded into hm -ZrO 2nanocontainers by mixing hm -ZrO 2nanocontainers with 2-MBT solution in acetone and followed by stirring for 24h.Fig.5presents the UV –vis spectra of MBT solution before and after the interactions with hm -ZrO 2nanocontainers.λmax for2-mercaptobenzothiazole is at 325nm.The UV –visible spectra (Fig.5)showed that,the absorption maxima for 2-MBT was decreased after the interaction of MBT molecules with hm -ZrO 2nanocontainers.Moreover,there were no new absorption bands and the position of the peaks was not changed.This decrease in the absorption intensity of MBT after the interaction with hm -ZrO 2nanocontainers con firmed the decrease of the MBT concentration in the solution as well as the successful loading of the MBT molecules in the pores and interiors of hm -ZrO 2nanocontainers.The loading of the organic inhibitor molecules in inorganic nanocontainers are made possible through weak non covalent interactions such as physical adsorption,electrostatic interaction,hydrogen bond-ing and π–πstacking [19].The loading ef ficiency was calculated as follows:Loading efficiency ð%Þ¼ðTotal amount of MBT ÀFree MBT in solution =Total amount of MBT ÞÂ100The loading ef ficiency of MBT in the HMS nanocontainer was found to be 63%.The release behavior of 2-MBT loaded hm -ZrO 2was investigated by UV –visible analysis of NaCl (0.05M)suspen-sions of 2-MBT loaded hm -ZrO 2at different pH values.The absorption intensity at ca.325nm was measured at given time interval for 28h and the results are shown in Fig.6.Fig.6shows the release of MBT and the increase of concentration of MBT with time and attained equilibrium after 5h.When the releasing medium had in filtrated into the pores and channels of hm -ZrO 2nanocontainers,the inhibitor mole-cules (2-MBT)gets dissolved in the medium and released by diffusion through the pores along the aqueouspathways.Fig.5.UV –visible spectra of MBT before and after interaction with hm-ZrO 2.Fig.6.UV –vis spectra at different times of the 0.05M NaCl media in which 2-MBT was released from 2-MBT loaded hm -ZrO 2nanocontainers (a)at pH 3,(b)at pH 7,(c)at pH 10and (d)corresponding pH-dependent releasing behavior of 2-MBT from hm -ZrO 2-MBT system.A.Chenan et al./Ceramics International 40(2014)10457–1046310461The releasing of MBT from hm-ZrO2followed similar release kinetics at different pH values.A faster release was observed under alkaline and acidic conditions compared to neutral condition.The release of MBT was leveled off and reached equilibrium in4h at pH10.It was observed that higher amount of2-MBT was released from the hm-ZrO2at pH3and10compared to pH7.The amount of MBT released after28h under acidic and alkaline conditions were0.66mg mLÀ1and0.53mg mLÀ1respec-tively,while in neutral conditions it was0.45mg mLÀ1only. The difference in the release rate and quantity of MBT released from hm-ZrO2can be explained by the variation of solubility of MBT and the differences in the surface charge of both MBT and zirconia particles with pH.The solubility of2-MBT is low in neutral pH,but relatively higher in alkaline and acidic conditions[24].Both the zirconia particles and the inhibitor molecules have the same surface charge when the pH values vary from neutral pH values.Under acidic pH conditions zirconia particles have positive surface charge whereas;at alkaline pH zirconia particles acquire negative surface charge [25].At acidic pH,MBT could be protonated due to the presence of a lone pair of electrons on the N atom and carries a positive charge[26,27].Thus under acidic conditions,both the zirconia particles and MBT had positive charge and this led to larger electrostatic repulsion between them,thereby facilitating faster release of MBT from hm-ZrO2nanocontainers in high amount.Similarly,under alkaline conditions,MBT exists in ionized form with a negative charge on the S atom and thus the MBT molecules carry negative surface charge[28].It is clear from Fig.6(c)that the absorption band at325nm is shifted to 309nm since in basic medium(pH49)2-MBT exists in ionized form with negative charge on the sulfur atom.Again this led to electrostatic repulsion between the hm-ZrO2 nanocontainers and2-MBT,thereby facilitating the diffusion of MBT through the pores of hm-ZrO2nanocontainers and faster release under alkaline condition.Hence,it could be stated that the release of loaded2-MBT from hm-ZrO2 nanocontainers was pH specific since when the pH was shifted from neutral value,the amount of2-MBT release was more. This dependency of inhibitor release on pH confirmed the stimuli responsive intelligent releasing property of hm-ZrO2 nanocontainers.This observation can be extrapolated to a corrosion process,as corrosion of metals is always accom-panied by changes in local pH.Thus the above mentioned results are encouraging for the use of inhibitor loaded hm-ZrO2 nanocontainers in anticorrosive active coating systems with self-healing ability.4.ConclusionsThe present paper reports the successful synthesis of hollow mesoporous zirconia nanocapsules and its utility as containers for storing and on demand releasing of inhibitor molecules. Hollow mesoporous zirconia was synthesized using silica nanoparticles as templates.The synthesized nanocontainers were monodispersed spherical particles with average size of 400nm.The phase of the hollow mesoporous zirconia was tetragonal.2-MBT,corrosion inhibitor molecules,was suc-cessfully loaded into hollow mesoporous zirconia and the loading efficiency was found to be63%.The release study revealed that hollow mesoporous nanocontainers release higher amounts of2-MBT in acidic as well as alkaline pH conditions compared to neutral pH.Work is in progress to apply this new nanocontainer-inhibitor system as smart anticorrosion coatings forfield testing.AcknowledgmentsThe D.A.E research fellowship awarded to Arunchandran Chenan is greatly acknowledged.The authors thank Dr.P. 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