Large Scale Fabrication of Quasi-Aligned ZnO Stacking Nanoplates

Large Scale Fabrication of Quasi-Aligned ZnO Stacking NanoplatesXueli Cao,Haibo Zeng,*Ming Wang,Xijin Xu,Ming Fang,Shulin Ji,and Lide Zhang*Key Laboratory of Materials Physics,Institute of Solid State Physics,Chinese Academy of Sciences,Hefei 230031,Anhui,People’s Republic of ChinaRecei V ed:January 14,2008;In Final Form:February 28,2008The large scale fabrication of quasi-aligned ZnO stacking nanoplates was achieved for the first time by low-temperature solution growth with assistance of citrate.Each nanoplate is composed of two or several parallel stacking nanosheets in back-to-back way with the same central axis.These nanosheets are of single crystalline and with two large surfaces as ((0001)planes.Remarkably,these stacking nanoplates approximately vertically stand on the substrate by their side small planes,and their surface morphology and verticality can be further adjusted by the molar ratio of citrate to Zn 2+ions.The formation mechanism was considered according to the effect of introduced ions on the nucleation and growth process.These ZnO nanoplates with multilayer stacking and vertical alignment features have large specific surface area and c plane exposure and hence would have great potential advantages for correlative properties.ZnO nanostructures have attracted more and more interest because of their expected excellent exhibition in a great deal of properties and relevant applications.1Remarkably,some are strongly dependent on the particular exposed surface of the nanostructures,such as luminescence,photocatalytic,and sens-ing properties.2,3Some properties of ZnO nanostructures are strongly preferential to the c planes.For example,Ye et al.4have reported that the ZnO nanoplates have good photocatalytic activity,and the thinner nanoplates have higher performance.These results demonstrated that the c planes have higher photocatalytic activity than the side surfaces in ZnO.On the basis of this consideration,the method to fabricate the c plane-dominated ZnO nanostructures in which the relative surface area of the c plane is larger than that of others has emerged but still is a challenge.Generally,for ZnO,the highest surface energy of the c plane advances the growth along the c -axis,and results in the familiar nanowires in which the relative surface area of the c plane is very low.5,6Although c plane-dominated ZnO nanoplates have been reported recently,7there still remains a significant challenge to directly fabricate mul-tilayer ZnO nanoplate arrays in large scale with c plane-dominated surface,especially to those standing on the substrates.Here,we report the quasi-aligned ZnO stacking nanoplates for the first time,which were fabricated in large scale by citrate-assisted low-temperature solution method.These multilayer stacking and vertical alignment features enable them to possess great c -dominated planes,and the surface morphology and verticality can be further adjusted by the molar ratio of citrate ion/Zn 2+.The novel ZnO nanostructures are expected to have promising applications in c -dominated preferential properties.For example,the stacking ZnO nanoplates on substrates may be served as effective and convenient recycled photocatalysts.An aqueous solution composed of 0.01M hydrated zinc nitrate [Zn(NO 3)2‚6H 2O],0.01M hexamethylenetetramine (HMT,C 6H 12N 4),and trisodium citrate (molar ratio of citrate ion/Zn 2+0.02or 0.05)of reagent grade was first used as the nutrient solution.Silicon substrates were washed with acetone in an ultrasonic bath and etched by piranha solution (3:1concentrated H 2SO 4/30%H 2O 2).Quasi-aligned ZnO stacking nanoplate arrays were grown on the silicon substrate using a simple two-step process.In the first step,the colloidal ZnO nanocrystals were spin-coated on the substrate several times.Between coatings,the silicon substrate was annealed at 150°C to ensure particle adhesion to the substrate surface.ZnO nanocrystals were prepared according to the previously reported method of Pacholski.8After the silicon substrate was coated with ZnO nanocrystals,the seeded substrates were placed upside-down in a sealed bottle containing the nutrient solution.Then,the bottle was put into an oven and maintained at 80°C for 6h.Finally,the products on the silicon substrates were washed with deionized water several times and then dried for further characterization.The morphology and structure of the products were examined by field emission scanning electron microscopy (FE-SEM,FEI,Sirion 200),high-resolution transmission electron microscopy (HRTEM,JEOL 2010),and X-ray diffraction (XRD,Philips X’Pert Pro diffractometer with Cu K R Radiation).For the photocatalytic activity evaluation,the concentration of photo-degrated methylene blue was recorded by a spectrophotometer (SP-2000UV).The organic dyes (primal solution: 1.0×10-4M,30mL)and the catalysts (10mg)were thoroughly stirred in the dark to reach the adsorption equilibrium of the organic dyes with the catalyst and then exposed to light from a UV light (18W).Figure 1shows the XRD patterns of the samples fabricated with different citrate concentration.Without citrate in the nutrient solution,the sample represents the normal c -axis *To whom all correspondence should be addressed.E-mail:(H.Z)hbzeng@;(L.Z.)ldzhang@.Fax:+865515591434.Tel:+865515591465.52672008,112,5267-5270Published on Web 03/13/200810.1021/jp800499r CCC:$40.75©2008American Chemical Societypreferential orientation (Figure 1a).When citrate ion/Zn 2+molar ratio is 0.02(Figure 1b),the intensities of (100)and (002)peaks are similar,which suggests no obvious orientation in this sample.When increasing the citrate ion/Zn 2+molar ratio to 0.05,the XRD pattern exhibits very distinct (100)preferential orientation,as shown in Figure 1c,which implies that this sample may have special nanostructure:the ZnO (100)plane would be parallel to the substrate.These results indicate that the molar ratio of citrate ion/Zn 2+could greatly change the ZnO preferential growth orientation,inducing the evolvement from the (001)orientation without citrate,to the unconspicuous orientation with the citrate ion/Zn 2+molar ratio of 0.02and the (100)orientation with that of 0.05.Figure 2a shows a typical low-magnification FE-SEM image of the sample with a molar ratio of citrate ion/Zn 2+of 0.02.Obviously,ZnO nanoplates with uniform size have been fabricated in large scale.Cross-sectional image (Figure 2b)shows that the nanoplates form a monolayer array on the substrate,which is derived from the ZnO seed layer and stands on the substrate in a certain gradient.From the high magnifica-tion FE-SEM image (Figure 2c),almost all the nanoplates are composed of a large thin and a small thin nanosheet in back-to-back stacking way,which are of smooth and hexagonal morphologies.The diameter and the thickness of nanosheet are of about 1µm and 200nm,respectively.An enlarged FE-SEMimage (Figure 2c)also shows that there are some small hexagonal steps on one side of the plate.The TEM image and the selected area electron diffraction (SAED)(Figure 2d)patterns show the same result and also reveal that the top and bottom large surfaces of the nanoplate are ((0001)planes and {1-100}small side facets.The result suggests that citrate molecules slow down the crystal growth along the (001)orientation,which provides a simple approach to control the dominated special surface.By increasing the molar ratio of citrate ion/Zn 2+from 0.02to 0.05,we can also prepare large scale uniform nanoplate arrays,as shown in Figure 3a.However,several obvious differences can be observed in the sample with 0.05citrate ion/Zn 2+molar ratio compared to the sample with 0.02molar ratio.First,as shown in Figure 3,panels b and c,the stacking nanoplate has become multilayer.Second,the multilayer stacking nanoplate arrayshaveFigure 1.XRD patterns of the samples by low-temperature solution growth without citrate (a),growth with citrate ion/Zn 2+molar ratio of 0.02(b),and 0.05(c).Figure 2.FE-SEM (a -c)and TEM (d)images of ZnO double-layer stacking nanoplates corresponding to citrate ion/Zn 2+molar ratio of 0.02.The inset is the SAEDpattern.Figure 3.FE-SEM images of ZnO multilayer stacking nanoplates corresponding to citrate ion/Zn 2+molar ratio of 0.05,the low magnification FE-SEM (a),the high magnification FE-SEM (b),and the cross-sectional FE-SEM image (c).5268J.Phys.Chem.C,Vol.112,No.14,2008Lettersbetter verticality than double-layer stacking nanoplate arrays.Third,the ZnO nanoplate surface becomes rough and porous at citrate ion/Zn 2+of 0.05molar ratio compared to that of 0.02molar ratio.These multilayer stacking and rough surface will induce much larger specific surface area.These features will stimulate great application potential in c -plane related properties when combined with those well-standing nanoplates.The typical nanowire arrays were obtained by the normal solution growth without citrate assistance,as shown in Figure 4.According to the great differences in orientation and morphology between stacking nanoplate arrays and nanowire arrays,we learn that the citrate ion in nutrient solution plays a vital role in the nucleation and growth process.In principle,a crystal growth process consists of nucleation and growth,which is affected by the intrinsic crystal structure and the external conditions including the kinetic energy barrier,temperature,time and capping molecules,and so on.9The crystal structures of the nanoplates involved in the work are all hexagonal in nature.In general,for the materials of hexagonal nanoplates the anisotropic growth along the c -axis is available to form the one-dimensional nanostructures.5However,the hexagonal nanoplates reported here have been fabricated by the citrate-assisted low-temperature solution method.As we know,citrate is an important biological ligand for metal ions.It can form a strong complex with Ca 2+,Mg 2+,Zn 2+,Fe 2+,Au +,and Ag +ions.10While the citrate was introduced to the growth solution,the period for the formation of ZnO nuclei was remarkably changed.Zn 2+will form [Zn(C 6H 5O 7)412-]10-with citrate ion,which can bind to the positive charged Zn 2+terminated (0001)polar plane more strongly than to another nonpolar surface due to the Coulombic force.Such adsorption would slow down crystal growth along the [001]direction.11On the other hand,the citrate adsorption would slow down the surface energy,which could result in the (0001)surface not becoming the highest-energy surface.According,the intrinsically anisotropic growth of ZnO along the <001>direction is substantially suppressed and the platelike ZnO is formed,which is similar to the formation of the ZnO nanodisks.12Citrate again plays an important role in the formation of quasi-aligned ZnO stacking nanoplates by possible bonding to the Zn 2+(0001)surface through the -COO -and -OH groups and suppressing growth along the <001>direction;crystal growth then proceeds sideways,leading to the formation of stacking nanoplates.Different from the orientated column-to-plate,11,13the stacking nanoplates we synthesized are concave in center.From the detailed examination of the top and bottom of the back-to-back double nanoplate,the surface is quite different:one side is smooth and another is rough (Figure 2c).Around the roughsurface center,there are some fine step edges gradually sticking out and these lead to the concave in center.A single hexagonal plate can be further developed into multilayer plates along the steps.According to the above analyses,it was clearly demonstrated that the citrate adsorption can significantly change the ZnO growth orientation.Along with the increase of the molar ratio of citrate ion/Zn 2+,the orientation of products has changed from (001)to (100),and the morphology has changed from nanowires into stacking nanoplates.Besides the orientation and the morphology,the molar ratio of citrate/Zn 2+also strongly affected the verticality of the arrays.With the increase of the molar ratio of citrate ion/Zn 2+,the verticality of the ZnO nanoplate arrays was greatly improved.The seed layer on the substrate has a nonperfect c -orientation,in the early stages of growth;ZnO crystals grew along the fastest growth orientation along the <10-10>direction,and these crystals are not aligned.But as the platelike randomly oriented crystals grew further,they began to overlap and their growth became physically limited as the misaligned nanoplate began to impinge on other neighboring crystals,giving rise to the preferred orientation of the nanoplate arrays.When the citrate ion/Zn 2+has been increased to 0.05,large existence of complex compounds with citrate and Zn 2+finally results in the obviously (100)orientation nanoplate arrays,which are represented in the XRD pattern.If the citrate ion concentration is further increased,the impingement of the neighboring crystals also increases,so the verticality of the multilayer ZnO nanoplate arrays improves.The surface of the ZnO becomes rough and porous along with the increase of the molar ratio of citrate ions to zinc concentra-tion.This “etching”effect is similar to the results reported by Wang et al.14The comparison of photocatalytic activity of different samples is illustrated in Figure 5.Under the same conditions,the double and multilayer stacking nanoplates have much greater photo-catalytic activity in the decomposition of methylene blue compared to the common nanowire arrays.With the conversion of ZnO nanostructure from nanowire to the nanoplate,especially the multistacking nanoplate,the photocatalytic efficiency was greatly improved.This finding is exciting for exploration of photocatalysts with high efficiency but is reasonable according to its large specific surface area and large exposure of active c plane.These results further demonstrate that such standing ZnO stacking nanoplates have great potential applications in specific surface area and c plane-related properties.As to the origins of high photocatalytic efficiency of typical multistacking nanoplate arrays,two factors should be noted.The first one is thelargeFigure 4.FE-SEM image of ZnO nanowires without citrate in the growthprocess.Figure 5.Photodegradation efficiency of methylene blue by the double-layer stacking ZnO nanoplates (a),multilayer stacking ZnO nanoplates (b),and normal ZnO nanowires (c).Letters J.Phys.Chem.C,Vol.112,No.14,20085269specific surface area,which greatly enhances the light-harvesting due to the longer surface light path and reduced reflectance, and the pollutant-adsorption on the enlarged surface.Further-more,the dominated c surface could also greatly improve the performance.Yamaguchi and co-workers15have reported that photocatalytic activity of ZnO film strongly depends on the specified crystal faces that are exposed.Our results give an indication that the polar c surfaces have higher photocatalytic activity,compared with nonpolar planes,at least in the decomposition of methylene blue.This is coincident with the high surface energy of c planes.In summary,through addition of citrate ions in the growth nutrition large scale hexagonal ZnO stacking nanoplate arrays have been obtained for the first time,which are approximately vertical on substrate.It also reveals that the top and bottom wide surface of the nanoplate are((0001)planes and{1-100} side facets.These c-dominated ZnO nanoplates standing on silicon substrate have a large active surface area exposed to the environment.Also,we have clearly demonstrated that ZnO stacking nanoplate arrays with the dominated c planes show a much higher photocatalytic activity in decomposition of meth-ylene blue,compared with common nanowire arrays.The novel ZnO nanostructures are expected to have promising applications in other fields.Acknowledgment.This work supported by the Major Research Plan of the National Natural Science Foundation of China(10604055)and the National Basic Research Program of China(Grant2007CB936601)References and 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