Microstructures and mechanical performance of polyelectrolyte/nanocrystalline TiO2 nanolayered composites.

2.4 MB·English

by  Zhang, Bin

Zhangetal.NanoscaleResearchLetters2013,8:44
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NANO EXPRESS Open Access
Microstructures and mechanical performance
of polyelectrolyte/nanocrystalline TiO
2
nanolayered composites
Bin Zhang1*, Hai-Feng Tan1,2, Jia-Wei Yan2, Ming-Dong Zhang1, Xu-Dong Sun1 and Guang-Ping Zhang2*
Abstract
Biological materials with hierarchicallylaminated structures usually exhibit a good synergy between strength and
fracture toughness. Here, we showthata bio-inspired (polyelectrolyte (PE)/TiO ) nanolayered composite with a
24
thickness ratio of TiO and amorphous PE layers ofabout 1.1 has been prepared successfully on Sisubstrates by
2
layer-by-layer self-assembly and chemical bath deposition methods. Microstructures of thenanolayered composite
were investigated by scanning electron microscopy, secondaryion mass spectroscopy,and high-resolution
transmission microscopy. Mechanical performance of thecompositewas characterized byinstrumented
indentation.Thecompositeconsistingof17.9-nm-thicknanocrystallineTiO and16.4-nm-thickamorphousPElayers
2
hasastrengthofabout245MPa,whichisclosetothatofshells,whilethefracturetoughnessofthecomposite,K =
IC
1.62±0.30MPa·m1/2,isevidentlyhigherthanthatofthebulkTiO .Apossiblestrategytobuildthecompositeat
2
nanoscaleforhighmechanicalperformancewasaddressed.
Keywords:Bio-inspirednanolayeredcomposite,Layer-by-layerself-assembly,Chemicalbathdeposition,Mechanical
property
Background insensitive to flaws, i.e., the strength of a perfect mineral
Biological materials (such as bones or shells, etc.) with platelet was maintained despite defects. This intrigued us
multiscale and hierarchical structures consisting of thick, to design and synthesize the artificial counterparts of this
hard inorganic mineral layers and thin, soft organic layers compositewithnanometer-thickconstituentlayerslessthan
exhibit an excellent combination of strength and tough- 30nm.
ness [1,2]. Although a number of metallic multilayered/ In this work, a variation method of combination of tra-
laminated composites produced by various methods [3,4] ditionalchemicalbathdeposition(CBD)[10,13]andlayer-
hadrevealedahighstrengthandapotentialimprovement by-layer (LBL) self-assembly [14] methods was conducted
ofplasticitywithoutlosingstrengthowingtothecontri- topreparealayeredstructurestackedalternatelybynano-
bution of the laminated structures and interfaces [5], a crystallineTiO andpolyelectrolyte(PE)layerswiththick-
2
feweffortshadbeenmadetodesignandsynthesizebio- nesses less than 30 nm. Microstructures and mechanical
mimetic laminated materials with submicron-thick in- properties of the nanolayered composites (NLCs) were
organic layers [6-11]. Theoretically, Gao et al. [12] investigated.
demonstrated that when the critical length scale of the
mineral inorganic platelets in natural materials drops
Methods
below approximately 30 nm, the biomaterials became
Silicon (001) substrates (3×10 mm2) were immersed in
Piranha solution [15] for 20 min at 60°C after ultrasonic
*Correspondence:[e-mail];[e-mail]
cleaning in acetone. A negatively charged hydrophilic Si-
1KeyLaboratoryforAnisotropyandTextureofMaterials(Ministryof
Education),NortheasternUniversity,3-11WenhuaRoad,Shenyang110819, OHlayerwasformedontheSisurface.Owingtotheelec-
People'sRepublicofChina trostatic attraction of oppositely charged polyions, three
2ShenyangNationalLaboratoryforMaterialsScience,InstituteofMetal
different PEs, poly(ethyleneimine) (PEI), poly(sodium 4-
Research,ChineseAcademyofSciences,72WenhuaRoad,Shenyang110016,
People'sRepublicofChina styrenesulfonate)(PSS),andpoly(allylaminehydrochloride)
©2013Zhangetal.;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommons
AttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproduction
inanymedium,providedtheoriginalworkisproperlycited.
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(PAH),wereselectedaspolycation,polyanion,andpolyca- TI 900, Eden Prairie, MN, USA) with a Berkovich tip (tip
tion, respectively, and the organic polymer layers were radius approximately 50 nm) was used to determine the
assembled by LBL deposition [14] of the three different hardness (H) and modulus (E) of the NLCs at a constant
PEs. The negatively charged Si substrates (after Piranha loading rate of 20 μN/s at room temperature. The mean
treatment) were alternately immersed into the three dif- values of H and E were then obtained at an indentation
ferent PE solutions in the sequence (PEI/PSS)(PAH/PSS) depthof10%to20%wholethicknessoftheNLCinorder
3
[10,14],andtheimmersionintherespectivepolymersolu- to eliminate substrate effects [16]. Microindentation tests
tions was at room temperature for 20 min. A positively (LECO AMH43, St. Joseph, MI, USA) were conducted to
charged surface was formed by adsorption of PEI on sili- evaluate fracture toughness of the NLCs following the
con since PEI can givegood coveringof oxidizedsurfaces methodproposedbyXiaetal.[17].
[14]. The thickness of the PE layers wascontrolled by the
number of dipping cycles into PAH/PSS solutions, while Results and discussion
three dipping cycles were carried out in the present work Microstructures
to ensure the thickness of the PE layers to be less than A scanning electron microscopy (SEM) observation
30 nm. Deposition of inorganic TiO layers onto the PE (Figure1a) shows that the surface ofthe (PE/TiO ) NLC
2 2 4
surfacewasaccomplishedina10mMsolutionoftitanium isquitesmooth.Acrackingregioncausedbya scratching
peroxo complex (TiO2+) and 30 mM HCl by the CBD of a needle reveals that the NLC is a typical multilayered
2
procedure [10]. In order to ensure the thickness of the structure with four layers, as indicated by arrows in
deposited TiO layer to be less than 30 nm, the adopted Figure 1b. The surface morphology of the NLC examined
2
deposition time and temperature were 2 h and 60°C, re- byatomicforcemicroscopy(Figure1c)showsthatthetop
spectively.ThePE/TiO NLCswithfourbilayeredperiods TiO layer is a densely packed spherical particle with a
2 2
((PE/TiO ) ) were prepared finally by sequentially apply- diameter of approximately 40 nm. The surface roughness
2 4
ingtheLBLself-assemblyandtheCBDtechniques. ofthetopTiO layerisabout4.5nm(Figure1d).
2
Secondary ion mass spectroscopy (SIMS; ION-TOF SIMS characterizations of the intensity variations of the
TOF.SIMS 5, Münster, Germany) was utilized to deter- ejected secondary ions of the present elements as a func-
mine the existence of Ti, O, C, and Si ions, as a function tion of sputtering time of the primary ion beam exhibit
ofdepthbelowthefilmsurface.Ananoindenter(Hysitron thatthereisaperiodicalvariationoftheintensityofOion
Figure1SEMobservations,AFMcharacterization,andsurfaceroughnessofthenanocomposite.SEMobservationsonsurfaceofthe(PE/TiO)
24
nanolayeredcomposite:(a)surfacemorphologyand(b)layerstructure.(c)AFMcharacterizationofsurfaceofthenanocomposite.(d)Surface
roughnessofthenanocompositemeasuredbyAFM.
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and Ti ion with the sputtering time (Figure 2), while the
intensity of C ion exhibits an inverse periodical variation
with the sputtering time. After the appearance of four
peaks of the periodical variation of the elements, the in-
tensityoftheTiandCionsbecomesdecreased,whilethat
of the Si ion becomes strong and finally reaches a cer-
tain intensity level, indicating the appearance of the Si
substrate. The profile clearly demonstrates the presence
of a multilayered structure of alternating TiO -enriched
2
and C-enriched layers, i.e., the existence of an ordered
composite structure of well-defined inorganic and or-
ganiclayers.
A transmission electron microscopy (TEM) cross-
sectional observation at a low magnification (Figure 3a)
also clearly reveals the multilayered structure in the (PE/
TiO ) NLC,thoughthereisinterpenetrationbetweenthe
2 4
PEand TiO layers(seeFigure3b).TheorganicPElayers
2
appear as bright regions with an average thickness of
16.4 nm, while the inorganic TiO layers are visible as
2
dark regions with an average thickness of 17.9 nm esti-
matedfromTEMcross-sectionalimages.Thus,thethick-
ness ratio (R) of the TiO layer to the PE layer is about
t 2
1.1.AhighmagnificationofthePE/TiO NLC(Figure3b)
2
shows that the interface between the PE and TiO layers
2
is not sharp completely, but somewhat diffuse, indicating
a sizeable interpenetration between theTiO and organic
2
PE components [10]. A selected-area electron diffraction
pattern taken from the dotted-circle region in Figure 3a
was presented in the inset of Figure 3b, revealing the dif-
fuse diffraction ring corresponding to the amorphous PE
layers, while some diffraction spots exhibit the existence
ofcrystallites.Ahigh-resolutiontransmissionelectronmi-
croscopy (HRTEM) image (Figure 3c) shows that some
nanocrystallines (NCs) with different orientations have
Figure3TEMcross-sectionalimagesofthecompositeand
HRTEMimageoftheinterface.TEMcross-sectionalimagesofthe
(PE/TiO ) nanolayeredcompositeat(a)lowmagnificationand
24
(b)highmagnification.(c)HRTEMimageofinorganicTiO layer
2
andorganic/inorganicinterface.
formed in theTiO layer and their sizes are in a range of
2
about 5 to 15 nm. The NC TiO might form during the
2
CBD process rather than the TEM electron-beam irra-
Figure2SIMScharacterizations.Variationoftheintensityof diation since the TEM accelerating voltage we used was
ejectedsecondaryionsofthepresentelementsasafunctionof 200 keV rather than 400 keV [10]. The formation of the
sputteringtimeofprimaryionbeamcharacterizedbysecondaryion
NC TiO might be related to the very thin TiO layers
massspectroscopy. 2 2
(approximately17.9nm)depositedinashorttime(2h)of
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theCBDprocess.Inaddition,theroughandthinPElayers indentation depth of about 22 nm. Young's modulus of
assembled by few numbers of cycles (3 cycles) for the theNLCdeterminedfromthecontactareaandtheelastic
PAH/PSSmightalsoplayanimportantroleinthehetero- contact stiffness [16] is 17.56±1.35 GPa, which is much
geneousnucleationoftheTiO nanocrystallines. lowerthanthatofthenacre(E=50GPa)[18].Suchalow
2
Young's modulus may be attributed to the large volume
fraction of organic PElayersdue to R ≈1.1.Based on the
t
ruleofmixture,Young'smodulusisestimatedtobeabout
MFigeuchrean4iacaslhpoewrfsoarmtyapnicceal load-indentation depth curve of 16.74 GPa by using ETiO2 = 27.5 GPa and EPE=5 GPa
[11], and this is close to the experimental result of the
the (PE/TiO ) NLC. In the loading stage, no pop-in be-
2 4 (PE/TiO ) NLC (17.56 GPa). The mean hardness of the
24
havior was detected, indicating that the NLC can be
(PE/TiO ) NLC determined by nanoindentation is
24
deformed continuously to the indentation depth of about
0.73 GPa with a standard deviation of 0.09 GPa. Using a
30 nm. In the unloading stage, the initially linear unload- general relation between hardness (H) and strength (σ)
ing reveals an elastic recovery. With a further unloading, found in a lot of materials, σ≅H= , the mean strength of
3
the nonlinear variation of the load with the displacement
theNLCwascalculatedasabout245MPa,whichisquite
reveals the non-elastic recovery, leading to a residual
close to the strength of shells reported in the literature
(100to300MPa)[10,18].AlthoughR ≈1.1,itisexpected
t
thattheNC TiO layerswouldalsocontributetothehigh
2
strengthofthecomposite.
Following the method to determine the fracture tough-
ness(K )ofathinfilmbondedtoabrittlesubstrate[17],
IC
when the indentation load was large enough applied to
the Si substrate uncoated by the (PE/TiO ) NLC, micro-
2 4
cracks initiated from four corners of the indent in the Si
substrate and advanced into the (PE/TiO ) nanolayer-
2 4
coated region, as indicated by an arrow in Figure 4b.
Based on the measurements of the crack length, K of
IC
the (PE/TiO ) NLC was obtained as K =1.62±
2 4 IC
0.30 MPa·m1/2, which is almost a threefold increase in
comparison to that of the single TiO layer of approxi-
2
mately 400 nm thick [11]. One reason for the enhance-
ment of K of the present NLC was attributed to energy
IC
dissipationviacrackdeflectionalongtheinorganic/organic
interface, as a general mechanism operated in artificial
and natural multilayered architectures [11]. Furthermore,
since the present (PE/TiO ) NLC has an inorganic/
24
organic layer thickness ratio of about 1.1 and the TiO
2
thicknessisonly17.9nm,itisbelievedthatevenifacrack
initiatesintheTiO layerwithathicknessof17.9nm,the
2
NLC would become more insensitive to flaws, as pre-
dicted by Gao et al. [12]. The hierarchical structures in
biological materials have shown a good synergy of high
strength and good fracture toughness (damage tolerance).
Li et al. [19] have revealed that the mineral layer in the
nacre consists of nanocrystalline CaCO platelets, which
4
facilitates grain boundary sliding. This also implies the
possible activation of the grain boundary sliding mecha-
nism in our NC TiO layers during deformation. The
Figure4Load-indentationdepthcurveofthecompositeand 2
present results indicate that building the composite con-
SEMimageoftheindentation-inducedmicrocrack.(a)Load-
indentationdepthcurveofthe(PE/TiO2)4nanolayeredcomposite sisted of the amorphous PE and the NC TiO2 layers at
measuredbynanoindentation.(b)SEMimageshowingthat nanometer scales may provide a possible strategy toward
indentation-inducedmicrocrackadvancedintothe(PE/TiO2)4 enhancing damage tolerance of the material even if the
nanolayer-coatedregion,bywhichfracturetoughnessofthe
best optimum ratio of the organic layer to the NC inor-
nanocompositecanbeobtained.
ganiclayerstillneedstobefound.
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Conclusions 13. BillJ,HoffmannRC,FuchsTM,AldingerF:Depositionofceramicmaterials
The bio-inspired (PE/TiO ) nanolayered composite with fromaqueoussolutioninducedbyorganictemplates.ZMetallkd2002,93:12.
24
14. DecherG:Fuzzynanoassemblies:towardlayeredpolymeric
an inorganic/organic layer thickness ratio of about 1.1, multicomposites.Science1997,277:1232–1237.
which consisted of nanocrystalline TiO2 and amorphous 15. SeuKJ,PandeyAP,HaqueF,ProctorEA,RibbeAE,HovisJS:Effectof
PElayerswiththicknessesof17.9and16.4nm,respectively, surfacetreatmentondiffusionanddomainformationinsupportedlipid
bilayers.BiophysJ2007,92:2445–2450.
was prepared on a Si (001) substrate by LBL self-assembly
16. OliverWC,PharrGM:Measurementofhardnessandelasticmodulusby
and CBD methods. The (PE/TiO2)4 nanocomposite has a instrumentedindentation:advancesinunderstandingandrefinements
strength of about 245 MPa, being close to that of the na- tomethodology.JMaterRes2004,19:3–20.
17. XiaZ,CurtinWA,SheldonBW:Anewmethodtoevaluatethefracture
turalshell,whilethefracturetoughnessofthenanocompo- toughnessofthinfilms.ActaMater2004,52:3507–3517.
site, KIC=1.62±0.30 MPa·m1/2, is evidently higher than 18. JiBH,GaoHJ:Mechanicalpropertiesofnanostructureofbiological
thatofthesingleTiO ofabout400nmthick. materials.JMechPhysSolid2004,52:1963–1990.
2
19. LiXD,XuZH,WangRZ:Insituobservationofnanograinrotationand
deformationinnacre.NanoLett2006,6:2301–2304.
Abbreviations
CBD:Chemicalbathdeposition;LBL:Layer-by-layer;NC:Nanocrystalline;
NLC:Nanolayeredcomposite;PE:Polyelectrolyte. doi:10.1186/1556-276X-8-44
Citethisarticleas:Zhangetal.:Microstructuresandmechanical
performanceofpolyelectrolyte/nanocrystallineTiO nanolayered
Competinginterests 2
composites.NanoscaleResearchLetters20138:44.
Theauthorsdeclarethattheyhavenocompetinginterests.
Authors’contributions
Allauthorscontributedequallytothiswork.BZ,XDS,andGPZconceivedthe
project.BZ,HFT,andMDZperformedtheexperiments.JWYperformedthe
TEMobservations.Allauthorsanalyzedthedata,discussedtheresults,and
wrotethepaper.Allauthorsreadandapprovedthefinalmanuscript.
Acknowledgements
ThisworkwassupportedbytheNationalBasicResearchProgramofChina
(grantno.2010CB631003)andpartiallysupportedbytheNationalNatural
ScienceFoundationofChina(grantnos.51171045and51071158).
Received:20November2012Accepted:6January2013
Published:21January2013
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