Crystal-Plane Dependence of Critical Concentration for

Crystal-Plane Dependence of Critical Concentration for Nucleationon Hydrothermal ZnO NanowiresYong He,† Takeshi Yanagida,*,† Kazuki Nagashima,† Fuwei Zhuge,† Gang Meng,† Bo Xu,†

Annop Klamchuen,† Sakon Rahong,† Masaki Kanai,† Xiaomin Li,‡ Masaru Suzuki,§ Shoichi Kai,§

and Tomoji Kawai*,†

†The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka Ibaraki, Osaka 567-0047, Japan‡State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, ChineseAcademy of Sciences, Shanghai 200050, P. R. China§Department of Applied Quantum Physics and Nuclear Engineering, Faculty of Engineering, Kyushu University, 744 Motooka,Nishi-ku, Fukuoka 819-0395, Japan

*S Supporting Information

ABSTRACT: Hydrothermal ZnO nanowires have shown great potential forvarious nanoscale device applications due to their fascinating properties and low-temperature processing. A preferential crystal growth of ZnO (0001) polar planeis essential and fundamental to realize the anisotropic nanowire growth. Here wedemonstrate that a critical concentration for a nucleation strongly depends on acrystal plane, which plays an important role on an anisotropic growth ofhydrothermal ZnO nanowires. We measure a growth rate of each crystal planewhen varying a concentration of Zn ionic species by using a regular arraystructure. Selective anisotropic growth on (0001) plane emerges within a certainconcentration range. Above the concentration range, a crystal growth on (101 ̅0)plane tends to simultaneously occur. This strong concentration dependence onthe crystal plane is understood in terms of a critical concentration differencebetween (0001) plane and (101 ̅0) plane, which is related to the surface energydifference between the crystal planes.

■ INTRODUCTION

ZnO nanowires formed via a hydrothermal method haveattracted much attention due to their fascinating physicalproperties and the potentials for nanodevice applications,including light-emitting devices, solar cells, power generators,and others.1−7 One of the reasons why the hydrothermalmethod has been so attractive even compared with othermethods is that whole hydrothermal processes can beperformed under a relatively low-temperature range less than100 °C,8,9 which is hardly attainable to other conventionalvapor-phase methods including VLS mechanism.10−27 Thislow-temperature synthesis is favorable, especially whenintegrating nanowires with other components (e.g., polymers)within the devices.9,28 In the hydrothermal growth of ZnOnanowires, a preferential nucleation at ZnO (0001) polar planeis essential and fundamental to realize the anisotropic nanowiregrowth.4,29,30 The effects of various experimental parameters,including pH, temperature, and ionic species, have beenexamined to understand their roles on the reaction scheme ofhydrothermal growth.30−45 For example, Yamabi and Imaireported the role of pH on the variation of ionic species duringhydrothermal ZnO nanowire growth.36 Xu et al. demonstratedthe importance of temperature on the morphology ofhydrothermal ZnO nanowires.37 Joo et al. recently reported a

face-selective electrostatic control of hydrothermal ZnOnanowires by intentionally adding positively charged ionsduring growth.32 In most previous studies, the origin ofanisotropic crystal growth in hydrothermal ZnO nanowires hasbeen interpreted in terms of the variations of ionic species inaqueous solutions and their electrostatic interactions with ZnOcrystal planes.31,32,38,39

Here we demonstrate that a crystal-plane dependence oncritical concentration for a nucleation plays an important roleon an anisotropic growth of hydrothermal ZnO nanowires. Wefound the strong concentration dependence on the crystalgrowth of each ZnO crystal plane when varying only aconcentration of Zn species under the same temperature andpH value. This strong concentration dependence on the crystalplane can be understood in terms of a critical concentrationdifference between (0001) plane and (101 ̅0) plane for anucleation. Our findings highlight that controlling precisely aconcentration during growth is essential to tailor themorphology of hydrothermal ZnO nanowires.

Received: November 16, 2012Revised: December 17, 2012Published: December 21, 2012

Article

pubs.acs.org/JPCC

© 2012 American Chemical Society 1197 dx.doi.org/10.1021/jp3113232 | J. Phys. Chem. C 2013, 117, 1197−1203

pubs.acs.org/JPCC

■ EXPERIMENTAL SECTION

We utilize a series of ZnO nanowire arrays to measure a growthrate of each crystal plane. We have fabricated the ZnOnanowire arrays on spatially patterned photoresist/ZnO seedlayer/Al2O3 substrates by utilizing conventional hydrothermalmethod. The schematic of fabrication process is shown inFigure 1a. First, a ZnO seed layer of 20 nm thickness isdeposited onto Al2O3 (0001) substrate by pulsed laserdeposition method. Prior to the deposition, the substrate isheated up to 600 °C. The oxygen partial pressure during thedeposition is set to be 1 Pa. The fabricated ZnO seed layer isgrown along the ⟨0001⟩ orientation, as shown in Figure 1b ofXRD data. After the preparation of the seed layer, we coatphotoresist (AZ5206/Z5200 2:1) onto ZnO/Al2O3 substrateby spin-coating at 5500 rpm for 60 s and subsequently bake it at90 °C for 3 min. Then, we create the circular hole arraypatterns with 610 nm diameter and 390 nm interval distancesby nanoimprint lithography, as shown in Figure 1c. Aftermaking the hole array patterns, the patterned photoresist isthen solidified at 120 °C for 5 min. The photoresist residuals atthe bottom of patterned hole are removed by reactive ion-etching process. The remained photoresist thickness onnonpatterned area is ∼50 nm. After these patterning processes,we perform the hydrothermal growth of ZnO nanowires. Asolution for the hydrothermal reaction is prepared by usingequimolar zinc nitrate hexahydrate (Zn(NO3)2•6H2O) andhexamethylenetetramine (HMTA, (CH2)6N4). All materials areanalytical grade and used without further purifications. Aqueoussolution is prepared at room temperature, and the concen-tration is varied from 0.01 to 40 mM to examine theconcentration dependence. We measure the pH values byusing a pH meter. The measured pH values are ranged from6.58 to 6.98 when varying the concentration. The patterned-photoresist/ZnO/Al2O3 substrate is then immersed upsidedown into the growth solution in a sealed beaker and kept at 95°C for a given time. Structural characterizations of fabricated

ZnO nanowires are performed by using X-ray diffraction andfield emission scanning electron microscopy.

■ RESULTS AND DISCUSSION

Figure 2a shows the typical SEM images of ZnO nanowirearrays when varying the concentration range from 0.01 to 40mM. The nanowires grown for 20 h are shown in the Figures.ZnO nanowires are vertically grown from the substrate. Thenanowire morphology drastically changes with varyingconcentration. Figure 2b shows the XRD data of fabricatedZnO nanowires. No additional peaks are observed in XRD dataeven after ZnO nanowires growth, indicating the nanowiregrowth orientation of ⟨0001⟩ direction. For further con-firmation, we perform TEM measurements, as shown in FigureS1 in the Supporting Information, which also highlights the⟨0001⟩ growth orientation. There are several features in regardsto the concentration dependence on the nanowire morphologyin Figure 2a. For the 0.01 mM case, there is no visible crystalgrowth due to the diluted environment. Above 0.1 mM, thenanowires start to grow and the nanowire length increases withincreasing the concentration. The multinanowires exist withinone hole array, which tend to be unified when increasing theconcentration above 5 mM. In such high concentration range,the nanowires tend to grow even along lateral direction,resulting in an increase in nanowire diameter and a decrease inthe interval between nanowires. Figure 2c shows the aspectratio data of nanowires, defined as a ratio of length to diameter.The aspect ratio data shows the maximum around between 1and 5 mM, highlighting the significant role of concentration onthe morphology of nanowires. To examine this concentrationdependence as a time series data, Figure 3a,b shows the SEMimages of nanowires grown under 1 and 20 mM for differentgrowth time (1, 5, and 20 h). There is clearly a significantdifference between the two concentrations on the time seriesdata. In the case of 1 mM, the nanowires grow mainly onlyalong the ⟨0001⟩ direction without any significant lateral

Figure 1. (a) Schematic of fabrication process of hydrothermal method for ZnO nanowires. (b) XRD data of ZnO seed layer on Al2O3 substrate. (c)SEM images of fabricated hole array patterns on substrate.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp3113232 | J. Phys. Chem. C 2013, 117, 1197−12031198

sidewall growth for all growth time. For 20 mM, the nanowirestend to unify within an array even at short growth time, 1 h, by

growing not only a ⟨0001⟩ direction but also a lateral sidewalldirection. Similar trends on the time series data are also

Figure 2. (a) Concentration dependence on ZnO nanowire morphology. The varied concentration range is 0.01−40 mM. (b) XRD data of ZnOnanowires. (c) Aspect ratio data when varying the concentration.

Figure 3. (a) Time series data on ZnO nanowire morphology with the concentration of 1 mM. (b) Time series data on ZnO nanowire morphologywith the concentration of 20 mM.



confirmed for different concentrations, as seen in theSupporting Information, Figure S2. Thus, the variation ofconcentration seems to affect not only the growth rate but alsothe growth direction.To specify more quantitatively the above trends in regards to

the concentration dependence, we extract the length and radiusdata of fabricated nanowires as a function of a concentration,and the results are shown in Figure 4a. It is noted that there areseveral reports on the concentration dependence of hydro-thermal ZnO nanowires.40−43 However, the intrinsic mecha-nisms in regards to the concentration dependence on thegrowth direction have not been discussed. Data of nanowiresgrown for 20 h are shown in the Figure. The length data aremeasured over 100 nanowires in cross-sectional SEM images.The radius data are measured over 100 nanowires in top-viewSEM images. The nanowire growth along the ⟨0001⟩ direction

starts at around between 0.01 and 0.1 mM, and the nanowirelength increases up to over 2 μm when increasing theconcentration. On the contrary, the nanowire radius remainsalmost constant below 5 mM and starts to increase above thethreshold concentration around 5 mM. The magnified figuresfor radius data can be found in the Supporting Information,Figure S3, which more clearly shows the increase in radiusvalues above 5 mM. Although these data are taken fromnanowires grown for 20 h, the similar trend is also observedeven for nanowires grown for shorter time, 5 h, as shown inFigure 4b. Obviously, there are two threshold concentrationsfor nanowire growth, one (around 0.01 to0.1 mM) for a growthof (0001) plane and the other (around 1−5 mM) for a lateralsidewall direction-mainly (101 ̅0) plane. As for the growth timedependence of concentration in a closed system, we performexperiments by exchanging a solution every 5 h to maintain a

Figure 4. (a) Concentration dependence on the nanowire morphology data (length and radius). The growth time is 20 h. (b) Concentrationdependence on the nanowire morphology data (length and radius). The growth time is 5 h.

Figure 5. (a) Morphology time series data of nanowires when exchanging a solution every 5 h for 1 mM. (b) Morphology time series data ofnanowires when exchanging a solution every 5 h for 20 mM.

Figure 6. (a) pH data variation when varying the concentration. (b) Concentration dependence on the nanowire morphology data (length andradius) under a constant pH experiments. The growth time is 20 h.



concentration. The time series data of nanowire morphologiesare shown in Figure 5. The growth time is 20 h, and theconcentrations are 1 and 20 mM. For comparison, previousdata without exchanging a solution are shown. Although thereis a quantitative effect of maintaining a concentration on thegrowth rate, especially for the length data at longer growth timeabove 15 h, the difference tends to be smaller, <20% below 10h. Thus the concentration decrease in our closed system can beestimated to be <20% in the early stage below 10 h of reactiontime by assuming first-order reaction. In addition, theconcentration dependence on the morphology data is almostindependent of the growth time (5 and 20 h), as seen in thesimilarity between Figure 4a,b. These results consistentlydemonstrate that the major semiquantitative trends ofconcentration dependence shown in Figure 4 are valid. Thesemorphology data also highlight that the concentrationsignificantly influences not only the growth rate but also thegrowth direction. The concentration dependence on thegrowth rate of nanowires is solely a matter of supplied flux.The concentration dependence on the growth direction seemsto be not readily interpreted in terms of existing models basedon the variations of ionic species in a solution and theirelectrostatic interactions with ZnO crystal planes.32,38

Here we discuss what essentially causes the concentrationdependence on the growth direction in Figures 3 and 4. First,we examine the effect of pH when varying the concentration of(Zn(NO3)2•6H2O) and HMTA. This is because Zn(NO3)2might act as an weak acid and HMTA is a weak base,29 and thepH value of solution strongly affects a reaction scheme ofhydrothermal ZnO growth via changing the concentration ofexisting ionic species in a solution.32,36,38 Figure 6a shows thevariation of pH values when varying the concentration. As canbe seen, even increasing the concentration from 0.01 to 40 mM,the pH value decreased from 6.98 to 6.58. Additional data inregards to the pH value variation during ZnO nanowire growthcan be seen in the Supporting Information, Figure S4.Obviously the pH variation range is quite small, and such pHvariation does not seem to impact the reaction scheme of ZnOhydrothermal synthesis as reported in previous works.32,38 Toconfirm the effect of pH variation on our observations inregards to the nanowire morphology change, we haveperformed hydrothermal experiments under a constant pHcondition via adjusting the pH value 7 using NaOH. Thenanowire data of such pH-controlled experiments are shown inFigure 6b. The nanowires were grown for 20 h. Clearly theconcentration dependence on the nanowire morphology dataand the growth direction is consistent with those of pH-uncontrolled experiments in Figure 4. Other possibleexperimental parameters, which affect a reaction scheme ofZnO hydrothermal synthesis, are typically a temperature and anionic species.33,38,45 Because we control these parameters to beconstant in our experiments, these contributions do not seemto be major factors to cause the drastic change of the nanowiremorphology in our experiments. Therefore, we consider how achange of concentrations affects a crystal nucleation event atdifferent crystal planes, which essentially determines theanisotropy of crystal growth.Here we consider a nucleation of ZnO crystal at a solid−

liquid interface. In principle, a nucleation in a solution occurswhen a surrounding concentration exceeds a critical concen-tration for a nucleation, which is above a saturatedconcentration.46−50 When a nucleation occurs within a solutionin the presence of solid surfaces, an interaction between a

nucleus and a solid surface has to be considered to understandthe free energy gain of growth system.48,50 It is well known thatthe presence of solid surfaces significantly reduces a free energybarrier for a nucleation because a free energy gain due to aninteraction between a nucleus and a solid surface is larger thanthat between a nucleus and a liquid in most cases.48−50 If thereis a surface energy difference between crystal planes, then eachcrystal plane may have the different free energy barrier for anucleation. In the case of ZnO crystal, a (0001) polar plane iswell-known to have a highest surface energy (2.0 J/m2)compared with other crystal planes, for example, (101 ̅0) plane,1.16 J/m2.51−53 The crystal plane with larger surface energyshould have a lower free energy barrier for a nucleation (i.e., alower critical concentration for a nucleation), and a crystalgrowth appears when a concentration exceeds a criticalconcentration for each crystal plane. On the basis of thisscenario, when increasing a concentration, first a nucleation at a(0001) plane only emerges without any nucleation at othercrystal planes, and such <0001> oriented crystal growth inprinciple can continue until the concentration exceeds nextlower critical concentration of other crystal planes, for example,(101 ̅0) plane. Above such concentration, the anisotropic crystalgrowth along ⟨0001⟩ no longer exists and the crystal growthalong lateral directions can coexist. Figure 7 shows the

schematic of the above implications to interpret theconcentration dependence on the morphology of hydrothermalZnO nanowires. As clearly seen in the schematic image, tomaintain the anisotropic crystal growth along a ⟨0001⟩direction, controlling a concentration to be within betweenthe critical concentration for (0001) plane and the next lowestcritical concentration for (101̅0) plane is essential. Thus thepresent concept based on a crystal-plane dependence of acritical concentration for a nucleation can rigorously explainwhy we have observed the concentration dependence on thegrowth direction in hydrothermal ZnO nanowires.

■ SUMMARYIn summary, we show that a critical concentration for anucleation strongly depends on a crystal plane, which plays acritical role in the anisotropic growth of hydrothermal ZnOnanowires. We measure the crystal growth of each crystal planewhen varying the concentration of Zn ionic species by using aregular array structure. Selective anisotropic growth on (0001)plane emerges within a certain concentration range. Above the

Figure 7. Schematic of a concentration dependence on a crystalgrowth of hydrothermal ZnO nanowires as a crystal-plane dependenceof a critical concentration for nucleation.



concentration range, a crystal growth on (101̅0) plane tends tosimultaneously occur. This strong concentration dependenceon the crystal plane is understood in terms of a differencebetween (0001) plane and (101̅0) plane at a criticalconcentration for a nucleation. Considering the universality ofthe present design concept, controlling a concentration duringthe nanowire growth process would impact not only hydro-thermal ZnO nanowires but also any anisotropic 1D nanowiregrowths.

■ ASSOCIATED CONTENT*S Supporting InformationLow-magnification TEM image of ZnO nanowire, high-magnification TEM image of ZnO nanowire, selected areaelectron diffraction pattern, indicating the [0001] growthorientation, and EDS pattern of ZnO nanowire. Time seriesSEM images of morphology change of nanowires when varyinga concentration ranged from 1 to 40 mM. Magnified data of theconcentration dependence on the radius data, and pH valuechange as a function of nanowire growth time. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] (T.Y.); E-mail:[email protected] (T.K).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis study was partially supported by a Grant-in-Aid forScientific Research on Innovative Areas [20111004] from theMinistry of Education, Culture Sports, Science and Technology(MEXT) of Japan. F.Z., B.X., G.M., and Y.H. were supportedby NEXT. K.N. was supported by TEPCO MemorialFoundation. T.K. was supported by FIRST program.

■ REFERENCES(1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind,H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897−1899.(2) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D.Nat. Mater. 2005, 4, 455−459.(3) Qin, Y.; Wang, X.; Wang, Z. L. Nature 2008, 451, 809−813.(4) Xu, S.; Wang, Z. L. Nano Res. 2011, 4, 1013−1098.(5) Park, W. I.; Yi, G. C. Adv. Mater. 2004, 16, 87−90.(6) Wei, Y.; Xu, C.; Xu, S.; Li, C.; Wu, W.; Wang, Z. L. Nano Lett.2010, 10, 2092−2096.(7) Yang, R.; Qin, Y.; Dai, L.; Wang, Z. L. Nat. Nanotechnol. 2009, 4,34−39.(8) Vayssieres, L. Adv. Mater. 2003, 15, 464−466.(9) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.;Zhang, Y. F.; Saykally, R. J.; Yang, P. D. Angew. Chem., Int. Ed. 2003,42, 3031−3034.(10) Heo, Y. W.; Varadarajan, V.; Kaufman, M.; Kim, K.; Norton, D.P.; Ren, F.; Fleming, P. H. Appl. Phys. Lett. 2002, 81, 3046−3048.(11) Yao, B. D.; Chan, Y. F.; Wang, N. Appl. Phys. Lett. 2002, 81,757−759.(12) Huang, M. H.; Wu, Y. Y.; Feick, H.; Tran, N.; Weber, E.; Yang,P. D. Adv. Mater. 2001, 13, 113−116.(13) Nagashima, K.; Yanagida, T.; Tanaka, H.; Seki, S.; Saeki, A.;Tagawa, S.; Kawai, T. J. Am. Chem. Soc. 2008, 130, 5378−5382.(14) Marcu, A.; Yanagida, T.; Nagashima, K.; Oka, K.; Tanaka, H.;Kawai, T. Appl. Phys. Lett. 2008, 92, 173119.(15) Oka, K.; Yanagida, T.; Nagashima, K.; Tanaka, H.; Kawai, T. J.Am. Chem. Soc. 2009, 131, 3434−3435.

(16) Klamchuen, A.; Yanagida, T.; Nagashima, K.; Seki, S.; Oka, K.;Taniguchi, M.; Kawai, T. Appl. Phys. Lett. 2009, 95, 053105.(17) Oka, K.; Yanagida, T.; Nagashima, K.; Tanaka, H.; Seki, S.;Honsho, Y.; Ishimaru, M.; Hirata, A.; Kawai, T. Appl. Phys. Lett. 2009,95, 133110.(18) Nagashima, K.; Yanagida, T.; Oka, K.; Taniguchi, M.; Kawai, T.;Kim, J. S.; Park, B. H. Nano Lett. 2010, 10, 1359−1363.(19) Oka, K.; Yanagida, T.; Nagashima, K.; Kawai, T.; Kim, J. S.;Park, B.H. J. Am. Chem. Soc. 2010, 132, 6634−6635.(20) Nagashima, K.; Yanagida, T.; Oka, K.; Kanai, M.; Klamchuen,A.; Kim, J. S.; Park, B. H.; Kawai, T. Nano Lett. 2011, 11, 2114−2118.(21) Nagashima, K.; Yanagida, T.; Tanaka, H.; Kawai, T. Appl. Phys.Lett. 2007, 90, 233103.(22) Yanagida, T.; Nagashima, K.; Tanaka, H.; Kawai, T. Appl. Phys.Lett. 2007, 91, 061502.(23) Nagashima, K.; Yanagida, T.; Oka, K.; Tanaka, H.; Kawai, T.Appl. Phys. Lett. 2008, 93, 153103.(24) Yanagida, T.; Marcu, A.; Matsui, H.; Nagashima, K.; Oka, K.;Yokota, K.; Taniguchi, M.; Kawai, T. J. Phys. Chem. C 2008, 112,18923−18926.(25) Klamchuen, A.; Yanagida, T.; Kanai, M.; Nagashima, K.; Oka,K.; Kawai, T.; Suzuki, M.; Hidaka, Y.; Kai, S. Appl. Phys. Lett. 2010, 97,073114.(26) Nagashima, K.; Yanagida, T.; Tanaka, H.; Kawai, T. J. Appl. Phys.2007, 101, 124304.(27) Zhuge, F.; Yanagida, T.; Nagashima, K.; Yoshida, H.; Kanai, M.;Xu, B.; Klamchuen, A.; Meng, G.; He, Y.; Rahong, S.; Li, X.; Suzuki,M.; Kai, S.; Takeda, S.; Kawai, T. J. Phys. Chem. C 2012, 116, 24367−24372.(28) Zhang, S.; Shen, Y.; Fang, H.; Xu, S.; Song, J.; Wang, Z. L. J.Mater. Chem. 2010, 20, 10606−10610.(29) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 4430−4431.(30) Greene, L. E.; Yuhas, B. D.; Law, M.; Zitoun, D.; Yang, P. D.Inorg. Chem. 2006, 45, 7535−7543.(31) Wu, W.; Hu, G.; Cui, S.; Zhou, Y.; Wu, H. Cryst. Growth Des.2008, 8, 4014−4020.(32) Joo, J.; Chow, B. Y.; Prakash, M.; Boyden, E. S.; Jacobson, J. M.Nat. Mater. 2011, 10, 596−601.(33) Xu, S.; Adiga, N.; Ba, S.; Dasgupta, T.; Wu, C. F. J.; Wang, Z. L.ACS Nano 2009, 3, 1803−1812.(34) Lockett, A. M.; Thomas, P. J.; O’Brien, P. J. Phys. Chem. C 2012,116, 8089−8094.(35) Lincot, D. MRS Bull. 2010, 35, 778−789.(36) Yamabi, S.; Imai, H. J. Mater. Chem. 2002, 12, 3773−3778.(37) Xu, S.; Wei, Y.; Kirkham, M.; Liu, J.; Mai, W.; Davidovic, D.;Snyder, R. L.; Wang, Z. L. J. Am. Chem. Soc. 2008, 130, 14958−14959.(38) Richardson, J. J.; Lange, F. F. Cryst. Growth Des. 2009, 9, 2570−2575.(39) Demianets, L. N.; Kostomarov, D. V.; Kuz’mina, I. P.; Pushko,S. V. Crystallogr. Rep. 2002, 47, S86−S98.(40) Coltrin, M. E.; Hsu, J. W. P.; Scymgeour, D. A.; Creighton, J. R.;Simmons, N. C.; Matzke, C. M. J. Cryst. Growth 2008, 310, 584−593.(41) Wei, Y.; Wu, W.; Guo, R.; Yuan, D.; Das, S.; Wang, Z. L. NanoLett. 2010, 10, 3414−3419.(42) Xu, S.; Lao, C.; Weintraub, B.; Wang, Z. L. J. Mater. Res. 2008,23, 2072−2077.(43) Zhang, D.-B.; Wang, S.-J.; Cheng, K.; Dai, S.-X.; Hu, B.-B.; Han,X.; Shi, Q.; Du, Z.-L. ACS Appl. Mater. Interfaces 2012, 4, 2969−2977.(44) Govender, K.; Boyle, D. S.; Kenway, P. B.; O’Brien, P. J. Mater.Chem. 2004, 14, 2575−2591.(45) Richardson, J. J.; Lange, F. F. Cryst. Growth Des. 2009, 9, 2576−2581.(46) Anisimov, M. P. Usp. Khim. 2003, 72, 664−705.(47) Glynn, P. D.; Reardon, E. J. Am. J. Sci. 1990, 290, 164−201.(48) Mullin, J. W. Crystallization, 4th ed.; Butterworth−Heinemann:Oxford, U.K., 2001; pp 181−284.(49) Vayssieres, L.; Keis, K.; Lindquist, S. E.; Hagfeldt, A. J. Phys.Chem. B 2001, 105, 3350−3352.



http://pubs.acs.org

mailto:[email protected]

mailto:[email protected]

(50) De Yoreo, J. J.; Vekilov, P. G. Rev. Mineral. Geochem. 2003, 54,57−93.(51) Na, S. H.; Park, C. H. J. Korean Phys. Soc. 2009, 54, 867−872.(52) Wander, A.; Schedin, F.; Steadman, P.; Norris, A.; McGrath, R.;Turner, T. S.; Thornton, G.; Harrison, N. M. Phys. Rev. Lett. 2001, 86,3811−3814.(53) Wander, A.; Harrison, N. M. Surf. Sci. 2000, 457, L342−L346.



Documents

Crystal-Plane Dependence of Critical Concentration for