Highly Shape-Selective Synthesis, Silica Coating, Self-Assembly

Highly Shape-Selective Synthesis, Silica Coating, Self-Assembly

(Parte 1 de 2)

DOI: 10.1021/la903544a ALangmuir X, X(X), X–X pubs.acs.org/Langmuir ©XXXX American Chemical Society

Highly Shape-Selective Synthesis, Silica Coating, Self-Assembly, and Magnetic Hydrogen Sensing of Hematite Nanoparticles

Jianhui Zhang,*,†,‡ Aaron Thurber,‡ Charles Hanna,‡ and Alex Punnoose‡

†National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China and ‡Department of Physics, Boise State University, Boise, Idaho 83725

Received September 18, 2009. Revised Manuscript Received November 17, 2009

The open forced hydrolysis method and controllable silica growth based on bound water to polyvinylpyrrolidone moleculeshavebeendevelopedforthehighlyshape(includingrhombohedra,semispheres,androds)selectivesynthesis,selfassembly,anduniformsilicacoating(intheunprecedentedrangeof5-200nm) ofhematitenanoparticles.Theopensystem realizes the direct short-range self-assembly of hematite semispheres in their growth process. The bound water method has been extended to coat gold nanoparticles with tunable silica shell and directly assemble the cores into one-dimensional, dimer, and trimer nanostructures during the coating process. The silica coating increases the particle stability and monodispersity even as hematite is modified into ferromagnetic Fe3O4. The hematite@silica core-shell spheres are assembled into long-range ordered structures with considerable photonic bandgap for the first time due to their high monodispersity. By exploiting the hematite antiferromagnetism caused by the superexchange interaction via intervening oxygen ions that are sensitive to hydrogen, a novel hydrogen sensing based on magnetization variations is achieved in the hematiteassemblies. Weakening theantiferromagnetismbyreducing thehematite size and/or coveringthehematitesurface by silica coating suppresses the sensitivityto hydrogen, showing that the antiferromagnetic spin variations on the hematite surface are responsible for the gas sensing.

Introduction

Iron oxide has been widely applied in magnetic recording media, catalysts, pigments, gas sensors, optical devices, rechargeable lithium batteries, and electromagnetic devices.1-9 To enhance the performance in existing applications and to explore novel applications, morphology-selective synthesis of iron oxide has been intensively studied because its physicochemical properties are strongly size- and shape-dependent. For example, a hydrothermal method was modified by using polyisobutylene bissuccinimide (L113B) or span80 as a soft template to prepare hematite (R-Fe2O3) nanotubes and nanorods, where shape-dependent magnetic properties were observed.9 Size-dependent magnetic properties and electrochemical properties were also revealed in hematite nanorods (with gradients in size and porosity) madebyahydrothermalmethodimprovedbyadopting a high concentration of inorganic salts and high temperature.10 With the assistance of polyvinylpyrrolidone (PVP), Zheng et al. madesingle-crystallinequasicubichematitenanoparticlesthatare superior catalysts to the other forms of nano- and microsized hematite catalysts in terms of activation temperature, conversion efficiency, andthermalstabilityinthecatalyticreaction.11Hollow hematite spheres with higher photocatalytic properties than other hematite crystals were synthesized by a CTAB (hexadecyltrimethylammonium bromide)-assisted hydrothermal method.12 γ-Fe2O3 particles were coated with silica to increase their stability ordispersioninawidepHrangeinaqueoussolutionsandorganic media.13 Fe3O4 particles were coated with mesoporous silica for targeted drug delivery and multiphase separation.14

Theforcedhydrolysisofferricsaltinasealedsystemisthemost popular method for the shape-selective synthesis of iron oxide with shapes such as spheres, cubes, ellipsoids, and rods.7 To obtain more controllable shapes and sizes, a variety of additives such as phosphate,2 L13B or span80,9 PVP,1 sodium dodecylsulfonate, sodium dodecylebenzenesulfonate, CTAB,12,15 hexa- decyipyridinium chloride,16 NaClO3,17 and PEG18 have been used to improve this method, and many distinctive iron oxides such as ellipsoidal,7 rodlike and tubelike,9,15 quasicubic,1 hollow spheric,12 rhombohedral,16 cantaloupe-like,17 and ringlike18 hematite, and β-FeOOH rods15 have been synthesized. However, the residual additives will contaminate the final product.15 Here we developed an open forced hydrolysis method because opening the reaction system facilitates the volatilization of the byproduct HCl, which not only accelerates the hydrolysis and reduces the reaction time but also helps one tune the product shape which is sensitive to the ratio of [Fe3þ]/[Cl-].7

*Corresponding author. E-mail: zhangjh@nju.edu.cn. (1) Bronstein, L. M.; Huang, X.; Retrum, J.; Schmucker, A.; Pink, M.; Stein,

B. D.; Dragnea, B. Mater. Chem. 2007, 19, 3624. (2) Morales, M. P.; Gonz alez-Carre~no, T.; Serna, C. J. J. Mater. Res. 1992, 7, 2538. (3) Sadakane, M.; Horiuchi, T.; Kato, N.; Takahashi, C.; Ueda, W. Chem.

Mater. 2007, 19, 5779. (4) Kiwi, J.; C€artzel, M. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1101. (5) Syirok y, K.; Jire sov a, J.; Hudec, L. O. Thin Solid Films 1994, 245,2 1. (6) Neri, G.; Bonavita, A.; Galvagno, S.; Siciliano, P.; Capone, S. Sens.

Actuators, B 2002, 82, 40. (7) Matijevic, E.; Scheiner, P. J. Colloid Interface Sci. 1978, 63, 509. (8) Chen, J.; Xu, L.; Li, W.; Gou, X. Adv. Mater. 2005, 17, 582. (9) Liu, L.; Kou, H. Z.; Mo, W. L.; Liu, H. J.; Wang, Y. Q. J. Phys. Chem. B 2006, 110, 15218. (10) Wu, C. Z.; Yin, P.; Zhu, X.; Ouyang, C. Z.; Xie, Y. J. Phys. Chem. B 2006, 110, 17806. (1) Zheng, Y.; Cheng, Y.; Wang, Y.; Bao, F.; Zhou, L.; Wei, X.; Zhang, Y.; Zheng, Q. J. Phys. Chem B 2006, 110, 3093.

(12) Li, L.; Chu, Y.; Liu, Y.; Dong, L. J. Phys. Chem. C 2007, 1, 2123. (13) Klotz, M.; Ayral, A.; Guizard, C.; M enager, C.; Cabuil, V. J. Colloid

Interface Sci. 1999, 220, 357. (14) Zhao, W.; Gu, J.; Zhang, L.; Chen, H.; Shi, J. J. Am. Chem. Soc. 2005, 127, 8916. (15) Wang, X.; Chen, X.; Gao, L.; Zheng, H.; Ji, M.; Tang, C.; Shen, T.; Zhang,

Z. J. Mater. Chem. 2004, 14, 905. (16) Jing, Z.; Wu, S. Mater. Lett. 2004, 58, 3637. (17) Zhu, L.-P.; Xiao, H.-M.; Fu, S.-Y. Cryst. Growth Des. 2007, 7, 177. (18) Li, L.; Chu, Y.; Liu, Y. Nanotechnology 2007, 18, 105603.

B DOI: 10.1021/la903544a Langmuir X, X(X), X–X

Article Zhang et al.

A variety of routes such as microemulsion,19 molecular templating,20 and modified st€ober methods13,14,21-26 have been developed to coat iron oxide with silica for high stability, drug delivery, multiphase separation, and possible photonic crystal with magnetically controlled bandgap. However, these methods are limited by aggregations, deformed spherical shape, polydispersity, and additional surface modification processes used to suppress the formation of new nuclei. Here, to effectively break through these limitations, thus obtaining monodisperse hematite@silica core-shell particles, a two-phase system (water/ PVP/n-pentanol or WPN) has been successfully developed to confine the silica growth on the hematite surface. The development of the WPN system is spurred by the following two motivations: (i) water can be bound by PVP in aqueous solutions with ahighconcentration ofPVP, whereinwaterdoesnot act asa solvent in the usual sense. Indeed, it is customary to refer to such water as “bound” water.27 Therefore, we can confine the hydrolysis of the silica precursor along with PVP by binding water with PVP in the WPN system, where the aqueous solution of highly concentrated PVP is formed since PVP tends to entirely exists in the water phase of the water/n-pentanol system.28 (i) PVP has been extensively used as the stabilizer and/or affinity agent in the nanotechnology field due to its excellent adsorption ability. For example, we managed to coat silica/polystyrene colloids with uniform metal, alloy, and TiO2 while preventing agglomeration29-32 by using PVP. In our WPN reaction system, as shown in Scheme 1, the direct adsorption of PVP molecules with bound waterontothehematite particles not onlycan successfullyprotect the hematite particles from agglomeration but also can realize the direct growth of silica on the hematite particles by consuming the water bound to PVP with the hydrolysis of the silica precursor.

The self-assembly of colloids or nanoparticles has been intensely investigated for its importance in a wide variety of applications such as biological assays, biochemical sensors, paints, ceramics, photolithography, and photonic crystals.3-35 Here we accomplisheddirectshort-range ordered assembly ofhematite semispheres in their synthesis process by simply modifying the ferric chloride concentration. We also tried to directly assemble the hematite nanoparticles coated with silica into ordered structures such as one-dimensional structures, dimer, and trimer clusters in their coating process by adjusting the coating conditions. Finally, we managed to assemble the as-prepared monodisperse hematite-silica core-shell spheres into long-range ordered colloidal crystals with considerable photonic bandgap by using the negative pressure assembly method developed by ourselves to assemble colloids.34

It is well-known that hematite has an antiferromagnetic nature arisingfromthesuperexchangeinteractionsbetweenFe3þspinsin adjacent layers, mediated by the intervening layer of oxygen ions.36 The intervening oxygen ions on the hematite surface will be easily reduced or removed by hydrogen, thus weakening the antiferromagnetic coupling and increasing the magnetization. This magnetization enhancement on the surface is insignificant and undetectable in bulk hematite. However, when hematite is prepared in nanparticle form, the increased specific surface area makes the surface magnetization enhancement significant and detectable. Namely, the magnetization enhancement on the hematite surface is detectable when introducing the reducing gas, hydrogen. By modifying the nanoparticle size, here the magnetic hydrogen sensing is successfully observed in assemblies of 220 nm hematite semispheres. Compared to the conventional hydrogen sensing based on the variations of electrical properties, the novel magnetic hydrogen sensing developed here overcomes limits such as low sensitivity and complex electrode contacts8 and provides rapid and reliable response without physical contact.

Experimental Section

Materials and Characterization. FeCl3 (g98.5%), hydrochloric acid (HCl, 38-40%), PVP (average molecular weight

NH3 solution (25-28%), n-pentanol (g99%), and anhydrous denatured ethanol (g99.7%) were used as received. TEM images were obtained on JEM-200CX (JEOL, Japan) transmission electron microscopes. The sample was placed on a carbon-coated coppergridforTEMobservation.SEM imageswereobtainedon a LEO1530 VP (Phillips, The Netherlands) instruments. X-ray powder diffraction (XRD) patterns were recorded using an X-ray diffractometer (Rigaku D/max-RA, Japan) with Cu KR radiation(λ=1.5418A ).Transmissionspectrawererecordedona U-3410 spectrophotometer (Hitachi, Japan). Sample films (with thickness of ∼10 μm measured by a optical microscope) were deposited on the 12 4 m quartz sample holder for the magnetization measurements with and without 5% H2 flow in Ar (at 100 mL/min) by using a vibrating sample magnetometer

(VSM,Lakeshoremodel7404)equippedwithahigh-temperature oven.TheVSMovenwascontinuouslypurgedthroughitsnormal purge system with ultrapure nitrogen at 100 mL/min.

Synthesis and Self-Assembly of Hematite Nanoparticles.

The reaction solutions of HCl (0.0005 M) and FeCl3 with concentrations ranging from 0.005 to 0.045 M were aged at 100 Ci n

Pyrex tubes for 24h.Toavoid large lossesofthe reactionsolution via evaporation, the tubewas closedby a glass stopper. A piece of filter paper was inserted between the tube and stopper to keep the reaction solution open to atmosphere during the entire reaction process. The direct short-range self-assembly of the as-formed hematite nanoparticles was achieved when the concentration of

FeCl3 wasc ontrolledt ob ea round0 .03M .

Scheme 1. Proposed Silica Coating Procedure in WPN System

(19) Santra, S.; Tapec, R.; Theodoropoulou, N.; Dobson, J.; Hebard, A.; Tan, W.

Langmuir 2001, 17, 2900. (20) Wu, P.; Zhu, J.; Xu, Z. Adv. Funct. Mater. 2004, 14, 345. (21) Lu, Y.; Yin, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 183. (2) Hao, L.; Zhu, C.; Jiang, W.-Q.; Chen, C.-N.; Hu, Y.; Chen, Z.-Y. J. Mater.

Chem. 2004, 14, 2929. (23) Yang, C.; Wang, G.; Lu, Z.; Sun, J.; Zhuang, J.; Yang, W. J. Mater. Chem. 2005, 15, 4252. (24) Han, Y.-S.; Jeong, G.-Y.; Lee, S.-Y.; Kim, H.-K. J. Solid State Chem. 2007, 180, 2978. (25) Phylipse, A. P.; van Bruggen, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92. (26) Dai, Z.; Meiser, F.; M€ohwald, H. J. Colloid Interface Sci. 2005, 288, 298. (27) de Dood, M. J. A.; Kalkman, J.; Strohh€ofer, C.; Michielsen, J.; van der

Elsken, J. J. Phys. Chem. B 2003, 107, 5906. (28) Cui, Y. D.; Yi, G. B.; Liao, L. W. The Synthesis and Applications of

Polyvinylpyrrolidone; Science: Beijing, 2001. (29) Zhang, J.; Liu, J.; Wang, S.; Zhan, P.; Wang, Z.; Ming, N. Adv. Funct.

Mater. 2004, 14, 1089. (30) Zhang, J.; Wang, S.; Liu, J.; Wang, Z.; Ming, N. J. Mater. Res. 2005, 20, 965. (31) Zhang, J.; Zhan, P.; Liu, H.; Wang, Z.; Ming, N. Mater. Lett. 2006, 60, 280. (32) Zhang, J.; Liu, H.; Wang, Z.; Ming, N.J. Solid StateChem. 2007,180, 1291. (3) Zhang, J. H.; Zhan, P.; Wang, Z. L.; Zhang, W. Y.; Ming, N. B. J. Mater.

Res. 2003, 18, 649. (34) Zhang, J.; Liu, H.; Wang, Z.; Ming, N. B. J. Appl. Phys. 2008, 103, 013517. (35) Yi, G.-R.; Manoharan, V. N.; Michel, E.; Elsesser, M. T.; Yang, S.-M.;

Pine, D. J. Adv. Mater. 2004, 16, 1204. (36) Robinson, P.; Harrison, R. J.; McEnroe, S. A.; Hargraves, R. B. Am. Mineral. 2004, 89, 725.

DOI: 10.1021/la903544a CLangmuir X, X(X), X–X

Zhang et al. Article

Synthesis and Self-Assembly of Hematite@Silica Core-

Shell Nanostructures. The as-prepared hematite particles were dispersed in 4 mL of ethanol solution of ammonia-water (20 vol %). The obtained suspension was added into 20 mL of n-pentanol solution of PVP (0.2 g) under stirring. After 30 min, the ethanol solution ofTEOS (20 vol %) was added. The dilution ofTEOSwithethanolcanhelponesuppresstheformationofnew nucleiandtheaggregationoradhesionofparticles.33Thereaction solution was then stirred for 12 h and centrifuged. The obtained product was washed three times with ethanol by centrifugation and ultrasonication.

In general, the surface charge of the hematite nanoparticles made from the forced hydrolysis method is positive. If they are dispersed into ammonia-water, the OH- ions in ammoniawater will neutralize the positive surface charge and lead to the aggregation of nanoparticles. Therefore, ammonia-water was greatlydiluted byethanolbeforethe dispersionoftheas-prepared hematite particles to prevent aggregation. However, by reasonablycontrollingtheammonia-watervolumeratioinethanol,the aggregation causedbyammonia-water can bepurposely usedto facilitate self-assembly of colloidal clusters. Here, we tried to directly assemble the as-formed hematite-silica core-shell particles into one-dimensional, dimer, and trimer cluster structures during the coating processes by simply increasing the ammoniawater volume ratio to 40%. The long-range ordered assembly of the as-prepared core-shell colloids was realized by using the negative pressure assembly method.34 In brief, a glass substrate was held vertically in the ethanol suspension of the as-prepared colloids (1 wt %) in a Bunsen flask by using a small paper clip. Then, the flask was placed in a vacuum drier connected with a water-jetair pump. Afterthepumpwasopened, theself-assembly of colloids under negative pressure started with a typical ethanol evaporation rate of 0.002 cm3/min.

Results and Discussion

As shown in Figure 1a, the product made using 0.005 M FeCl3 is dominated by monodisperse rhombohedral particles with average edge length of 45 nm. The selected area electron diffraction(SAED,seeinsetofFigure1a)patternobtainedfroma single particle lying flat on the support film, with the electron beam perpendicular to the rhombic facets, clearly shows the single-crystal nature of the rhombohedral particles. With increas- ing the concentration of FeCl3 from 0.005 to 0.01 M, the product remains rhombohedral shape and increases in average edge- length from 45 to 70 nm. The SEM image (Figure1b) clearly shows the rhombohedral nature, with the particles comprising of sixrhombicfacets.Thesequentialincreaseoftheconcentrationof

FeCl3 suppresses the formation of the rhombohedral shape and induces the formation of the spherical particles. The pure mono- disperse semispherical particles with average diameter of 120 nm are obtained by using 0.02 M FeCl3 (see Figure 1c). By further increasing the concentration of FeCl3 from 0.02 to 0.03 M, the particles improve in spherical shape and monodispersity and increase in average diameter to 150 nm. Furthermore, some short-range ordered structures of the semispheres arranging in the hexagonal closed-packed way (see Figure 1d) are formed. Interestingly, there is barely interstice among the semispheres in the self-assemblies. There is some precipitation at the bottom of the tube after this synthesis reaction is completed. So we deduce that these assemblies may arise from the ordered deposit of the semispheres on the tube bottom (due to their large diameter and highmonodispersity)whiletheyarestillgrowing.Thepostgrowth of the semispheres will fill their interstice in the assemblies. The averagediameterofthesemispherescanbefurtherincreasedfrom 150 to 220 nm by prolonging the reaction time to 48 h. When the concentration of FeCl3 is increased above 0.03 M, the growth of the spherical shape is suppressed and the rodlike shape begins to appear. With a concentration of 0.045 M FeCl3, the product is dominated by rodlike particles (see Figure 1e).

The XRD patterns have been recorded for the typical samples with rhombohedral (Figure 1a), semispherical (Figure 1c), and rodlike (Figure 1e) morphologies to identify their crystal structure. As shown in Figure 1f, all the samples can be indexed to a pure hexagonal structure of hematite with cell constants of a= 0.5035 nm and c= 1.3740 nm (JCPDS No. 3-0664). No diffraction peaks from impurities are found in the samples. Compared with conventional forced hydrolysis methods in a sealed system2,7 for the synthesis of hematite, the open system demonstrated here accelerates the hydrolysis reaction, reduces the reaction time from 2 to 10 days to 24 h, expands the strict formation condition for hematite from0.02-0.04to 0.005-0.045

MFeCl3,improvestheproductmonodispersity,andproducesthe novel ordered self-assemblies of hematite semispheres and the distinctive rhombohedral structure which usually only forms in the presence of additives.16

ThesampleshowninFigure1chasbeenfirstchosenasthecore for the silica coating due to its good spherical shape. As shown in Figure 2a-d, every hematite particle (with dark contrast) is uniformly coated with silica (with light contrast). By simply modifying the amount of hematite and/or TEOS solution, the silica shell can be continuously tuned from 10 to 200 nm. With increasing silica shell thickness, the core-shell complex particles obviously improve in monodispersity and spherical shape. We also coated the hematite samples with irregular shapes including the short-range ordered (shown in Figure 1d) and rodlike (shown in Figure 1e) structures with uniform silica shell. As shown in Figure 2e,f, all the irregular hematite nanoparticles can be well coated with uniform silica shell regardless of the shape such as dimer, Y-like, and X-like morphologies.

Allthe hematite samplestendto tightly sticktothe vessel wall if they are dispersed in water or ethanol (they cannot flake away even under intense sonication) and quickly aggregate into large

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