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Controlled nanostructuring of multipha core–shell nanowires by a template-assisted electrodeposition approach
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2012 Nanotechnology 23 305601
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IOP P UBLISHING N ANOTECHNOLOGY Nanotechnology23(2012)305601(6pp)doi:10.1088/0957-4484/23/30/305601
Controlled nanostructuring of multipha core–shell nanowires by a template-assisted electrodeposition approach
Dawei Shi1,Junyang Chen1,Saira Riaz1,Wenping Zhou2and
Xiufeng Han1
1Beijing National Laboratory for Condend Matter Physics,Institute of Physics,Chine Academy of
Sciences,Beijing100190,People’s Republic of China
therebe句型2Key laboratory of Rare-earth Physics,School of Physical Science and Technology,Inner Mongolia
University,Hohhot010021,People’s Republic of China
粉扑怎么用E-mail:xfhan@aphy.iphy.ac
Received6April2012,infinal form1June2012
Published2July2012
Online at stacks.iop/Nano/23/305601
Abstract
Multipha core–shell nanowires have been fabricated by controlling the ion transport
process of the microfluids in the nanochannels of the template.Both forced convection and
puld potential induced migration can be applied to tune the morphologies of the
nanostructures obtained by manipulating the ion transport during electrodeposition.The
morphology and content of the core–shell structure were studied byfield emission scanning
electron microscope(FESEM)analysis,transmission electron microscope(TEM)analysis and
energy dispersive spectrometry(EDS),respectively.The magnetic properties were analyzed
by vibrating sample magnetometer(VSM)analysis.A magnetically hard core and soft shell
2015年6月六级真题
constitutes the multipha composite nanostructure.The unique magnetic hysteresis curve
indicates the decoupled magnetic reversal process of the two components.Our work
provides deeper insights into the formation mechanisms of a new core–shell nanostructure,
which may have potential applications in novel spintronics devices.
金山在线翻译(Somefigures may appear in colour only in the online journal)
1.Introductiontaibao
Low-dimensional materials with morphological features on the nanoscale have attracted extensive and increasing interest due to their unique properties as well as potential applications in electronic,magnetic and optical devices[1–3].More re-cently,core–shell nanostructures,hybrid nanomaterials which may exhibit additional effects and rve as multifunction ma-terials,have been synthesized and investigated.The core–shell structures of two different chemical materials are usually incorporated in a multistep route[4–7].Though numerous methods have been investigated,template bad methods are low cost and simple for the fabrication of the nanostructures. By integration with other techniques,a large variety of new one-dimensional nanostr
uctures such as core–shell nanowires[8–12]and multilayer nanowires[13–17]have been fabricated.
浙江教育出版社网站Since the dimensions of the nanostructure can be predetermined by the template ud,control over the morphologies of the nanostructures fabricated by the template bad method is very difficult.Though a few recent works have shown that the morphologies of the nanostructures may also be strongly affected by the experimental parameters, our knowledge of the growth mechanism of this kind of nanomaterials is still very limited.
In the prent work,we report multipha core–shell nanowires fabricated by controlling ion transport process of the microfluids in nanochannels of the template.An anodic aluminum oxide(AAO)template with a diameter of300nm was ud in the synthesis process.An aqueous solution compod of lanthanum and nickel ions
Figure1.The fabrication procedure of the Ni–La multipha core–shell nanowires through forced convection manipulated microfluid ion transport.A Cu layer isfirst sputtered on one side of the AAO template.The electrodeposition process is carried out while stirring the electrolyte in a conventional three electrode configuration.The electrolyteflow created affects the deposition process and leads to the formation of the core–shell structure.
面试问题及回答技巧
rved as the electrolyte to perform the electrodeposition. Forced convection and puld potential induced migration were applied to manipulate the ion transport during the deposition.Rare-earth elemental metals and alloys have attracted much attention due to their fundamental properties with unique4f electronic structures and a wide range of applications in areas such as hydrogen storage,catalysis, magnetism,optics and other devices.Here we choo Ni–La to study the manipulation of the morphologies of the nanostructures fabricated by the template bad method. Lanthanum with a reduction potential at−2.52V(versus hydrogen electrode)[18]is very difficult to electrodeposit from an aqueous solution.By using complexing agents,the reduction potentials of lanthanum and nickel can satisfy the codeposition of two metals.Moreover,it has been found that the composition of electrodeposited Ni–La alloys was extremely nsitive to applied potentials.Thus,the weak effects of microfluidic mass transport during the electrodeposition exhibit a significant influence on the morphology of the nanostructure formed.
2.Experimental details
The procedure for the Ni–La multipha core–shell nanowires,synthesized by forced convection manipulated ion transport,is shown schematically infigure1.Electrolytes of lanthanum chloride(LaCl3,0.15M),nickel chloride (NiCl2,0.15M),ammonium chloride(NH4Cl,0.15M)and boric aci
d(H3BO3,0.2M)were prepared to carry out the electrodeposition.First,a Cu layer with a thickness of about 300nm was sputtered on one side of the anodic aluminum oxide(AAO)template rving as the conductive contact. Then the electrodeposition of Ni–La alloy was performed in a conventional three electrode configuration at room temperature.Applied potentials were measured with respect to a saturated calomel electrode(SCE)as the reference electrode and we ud platinum(Pt)foil as the counter electrode.The commercial anodic alumina membranes (Anodisc membranes from Whatman Corp)coated with a Cu layer rved as the working electrode.VersaSTAT3 (Princeton Applied Rearch)was ud for electrodeposition by using VersaStudio Electrochemistry Software to control the applied potentials.The electrodeposition was carried out potentiostatically at a constant voltage of−1.10V versus SCE.Unlike the usually performed conventional conditions of electrodeposition,a small magnet was added to stir the solution during the process.Analysis of the growth of the nanowires as a function of the pore diameter and the stirring speed was also performed.The electrodeposition of the nanowire arrays was carried out by using a fast potential puls method with the pul durations applied at U1∼−1.10V for5s and at U2∼−0.6V for7s.
A high resolution scanning electron microscope(SEM; Hitachi S-4800)was ud to obtain the SEM images shown in
Figure2.Structural characterizations of the Ni–La core–shell nanowires.(a)–(c)SEM images showing the Ni–La core–shell nanowires as the deposition proceeds.(d)TEM image of the isolated core–shell nanowires.(e)Line scanning profile details across the nanowires.The int shows the lected line scans.(f)EDS analysis of the core part of the core–shell structure.(g)EDS spectrum of the shell nanotubes.
this work.Transmission electron microscope(TEM)images were obtained by using a JEOL2010F with an accelerating voltage of200kV.The composition analysis was done with spatially resolved energy dispersive spectrometry(EDS).Both the SEM and TEM analys were performed after dissolving the AAO template in1M NaOH solution.
3.Results and discussion
The structure and morphology of typical Ni–La multipha core–shell nanowires are illustrated infigure2.The SEM images of the Ni–La core–shell nanowires as the deposition proceeds under stirring of the solution are shown in figures2(a)–(c).The SEM analysis was performed after dissolving the AAO template in NaOH solution.The diameter of the shell is around300nm,which is equal to that of the nanopores,whereas the diameter of the core nanowires is about200nm.The formed core–shell
nanostructure was further explored by line scanning across the nanowires with spatially resolved EDS.Figure2(d)shows TEM images of the core–shell nanowires.The details of the composition line profiles are displayed infigure2(e),and the int shows the lected line scans across the nanowires.It is clear from thisfigure that the concentration of Ni at the core part is much higher than the shell.To confirm the composition of the formed core–shell nanostructure,EDS spectra are shown in figures2(f)and(g).There are some other elements prent besides La and Ni in the spectrum.The Cu and C peaks originate from the holey carbon grid ud for the TEM analysis;the Al peak is obrved due to the small amount of alumina left after the removal of the template;the O peak appears becau of the oxidization of the samples; the Si peak comes from unexpected impurities.Figure2(f) shows the spectrum of the core of the nanostructure,while figure2(g)shows the composition of the shell.The La and Ni are obrved in both of them;however,the concentration of La is found to be quite different.The ratio of La to Ni is approximately1:24for the core composition while that of the shell is about1:2,which means a pha paration phenomenon occurs.
The magnetic properties of the Ni–La lf-asmbled core–shell nanowires were also characterized.The magnetic hysteresis curve of the as-deposited core–shell structure arrays was measured and a unique shape of M–H loop was obrved. As shown infigure3,the M–H loops are qu
ite different from the hysteresis behavior of a single pha material that has been reported before[19,20].This oddity is ascribed to the coaxial nanostructure made up of the composite nanowires consisting
Figure3.Magnetic properties of Ni–La core–shell nanowires. Hysteresis loops with an external magneticfield applied parallel and perpendicular to the nanowire axis.The curve with the parallelfield indicates the prence of magnetically hard and soft phas.
of magnetically hard and soft phas.It is interesting that the soft pha may be partly or even completely decoupled from the neighboring hard pha.The unique magnetic properties of the core–shell nanowires rve as further evidence of the formation of the multipha core–shell structure.
Concerning the ion transport in the nanochannels of the template during electrodeposition,migration,diffusion and forced convection mainly contribute to theflux of a species in the solution.Thus,the general equation for theflux of species j with a concentration C j is given as:渗透压力
J j=−D j∇C j−Z j F
RT
D j C j∇φ+C jυ(1)
where J j is the molarflux per unit area of species j,D j is the diffusion coefficient of species j,C j is its concentration, Z j is its charge,F,R and T have their usual meanings, is the potential andυis thefluid velocity.On the right-hand side of the equation,thefirst term reprents diffusion,the cond,migration,and the last term,convection[21].By adding a small magnet to stir the solution,the mass transport in the nanochannels during deposition will mainly be driven by forcedfluid convection.
The intensity of the convection caud by the stirring at the center was stronger than near the channel wall due to the friction between microfluidflow and the channel wall.Thus,an inhomogeneous ion concentration in the nanochannels was established during electrodeposition. The ion concentration at the center of the nanochannels remains almost constant as electrolysis proceeds,becau the high velocity of the ion transport in this region can always offt the consumption caud by electrodeposition.On the other hand,there will be a progressive depletion of ions near the wall.Even though the deposition potentials have been brought clo by complexing the more noble metal ions, Ni was still deposited preferentially near the center of the nanochannels due to the low polarization.On the contrary,the relatively quiescent solution near the wall of the nanochannels increas the concentration polarization,which would favor the deposition of less noble metal La.Furthermore,the growth rate of the shell was lower than that of the core,mainly due
to Figure4.(a)SEM image of the nanotubes formed without stirring;
(b)SEM image of the nanotubes formed with a low stirring speed;
(c)SEM image of core–shell nanowires formed with a high stirring speed.
the low ion concentration near the wall of the nanochannels. This is confirmed by the structural characterizations as shown infigures2(b)and(c);the length of the core nanowires is larger than the shell nanotubes.
Studies on the growth of nanowires as a function of the pore diameter and the stirring speed confirm the formation mechanism propod here,since the nanopores can befilled at smaller diameter even without stirring of the solution.For electrodeposition of Ni–La nanowires with a small diameter, it ems that the stirring of the electrolyte has little effect on the growth of the nanowires.Then,an investigation of the growth of nanowires as a function of the stirring speed was carried out by using a template with a diameter of200nm. Simply,nanotubes were formed without stirring as shown in figure4(a).The thickness of the nanotubes increas with the increa in stirring speed,and eventually the core–shell Ni–La nanowires are formed.The electrodeposition of ions occurs preferentially at the sites near the cathode surface and the bottom of the template nanopores[22].Th
e ion transport driven by the migration and diffusion is not enough
Figure5.Multipha Ni–La core–shell nanowires prepared though the puld potential method.(a)SEM image showing the core–shell nanostructure.(b)TEM image of novel core–shell nanowires with a gmented core component.The corresponding current–time curve recorded during the deposition process is also shown.
tofill the nanopores completely without stirring.As a result, Ni–La nanotubes were fabricated.The stirring of the solution during the electrodeposition enhances the ion transportation, and the thickness of the nanotubes increas.At high stirring speed,figure4(c)shows that core–shell nanowires are fabricated concerning the concentration polarization effects as mentioned before.
Similar results have been found in Ni–La lf-asmbled nanowires prepared by the puld potential method.As shown infigures5(a)and(b),the SEM and TEM results confirm the formation of core–shell nanostructures.Under appropriate experimental parameters,the core component appears to be a gmented structure,which is quite interesting and not what was anticipated.The puld potential method has long been widely ud to fabricate gmented nanowires[23].So far,only a limited work involved with electricfield induced ion transport has been reported[24].Here its effect on the ion trans
port is studied carefully.Though the overall mass transport in the nanochannels was driven by diffusion in this ca,the migration caud by the gradient in the electrochemical potential played an important role in the formation of the core–shell structure.The pul durations applied were at U1∼−1.10V for5s and at U2∼−0.6V for 7s.The corresponding current–time curve recorded during the deposition process is depicted infigure5(b).It should be noted that during the U2pul,ion transport driven by migration still exists.However,as shown infigure5(b),no electrodeposition occurred since the equilibrium potential of La and Ni is much lower than U2.Theflow profile was assumed to be parabolic in shape in the nanochannels[25], which means that the ion velocity is higher at the center of the nanochannels than near the wall.It should be taken into account that the less noble metal La will be deposited preferentially due to the enhanced concentration polarization effects near the wall.As for the gmented core obrved,we assume that it is associated with the ion transport at the center of the channel caud by the applied periodic puld potential. This indicates that the influence of the microfluidic mass transport may become significant,leading to the formation of the novel nanostructure.
4.Conclusions
In conclusion,multipha core–shell nanowires have been fabricated by a simple template-assisted e
lectrodeposition method.A systematic study of the microfluidic mass transport in the nanochannels of the template during electrodeposition is carried out.The phenomena shown here also provide deeper insights into the formation mechanism in template electrodeposition,which has been widely ud to fabricate various nanomaterials.Moreover,the unique magnetic properties of the core–shell nanowires demonstrate that control over the properties of the nanomaterials can be realized through manipulating their morphologies. Acknowledgments
The project was supported by the State Key Project of Fundamental Rearch of Ministry of Science and Technology (MOST,No.2010CB934400)and the National Natural Science Foundation of China(NSFC,Grant Nos10934099, 11104338and51021061),and the partial support of the Graduate Education Project of Beijing Municipal Commission of Education,the international joint projects of the NSFC–The Royal Society(UK)and the partial support of the K C Wong Education Foundation,Hong Kong. References
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