Morphology Evolution of One-Dimensional-Based ZnO Nanostructures

Aug 6, 2009 - Xuezhen Huang and Jeffery L. Coffer. The Journal of Physical Chemistry C 2010 114 (50), 22019-22024. Abstract | Full Text HTML | PDF | P...
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J. Phys. Chem. C 2009, 113, 15514–15523

Morphology Evolution of One-Dimensional-Based ZnO Nanostructures Synthesized via Electrochemical Corrosion Kuan Zhong,† Jian Xia,† Hao H. Li,† Chao L. Liang,‡ Peng Liu,*,† and Ye X. Tong*,† School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, P. R. China, and Instrumental Analysis and Research Center, Sun Yat-Sen UniVersity, Guangzhou 510275, P. R. China ReceiVed: February 26, 2009; ReVised Manuscript ReceiVed: July 14, 2009

Various morphologies of ZnO nanostructures can be obtained through a novel method, incorporating electrochemical corrosion with three modes: liquid membrane and above and below the water line in partial immersion. X-ray diffraction (XRD) patterns, high-resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) are employed to characterize their structure. The mechanism of the growth of NRs is proposed as electrochemical corrosion and oriented attachment, which occur in a liquid membrane or partial immersion in a vapor membrane. The evolution of ZnO nanostructures such as nanorods, nanowires, nanopins, and nanodentrites is observed, and the influence of concentration, reaction time, additives, state of substrate, membrane thickness, and solvent on the morphology of ZnO is investigated. Optical properties of ZnO nanostructures are studied by using UV-visible absorption spectra and photoluminescence (PL). Their optical gaps vary from different morphologies. Among the studied samples, short nanorods show the largest optical gap, while big nanorods present the smallest value of optical gap. PL properties demonstrate that peaks of near-band emission and defect-related luminescence are basically in the same position. However, intensities for different morphologies are of different values, and short nanorods exhibit the best near-band emissions. Introduction ZnO is a promising semiconductor that has a direct wide band gap (3.37 eV), large excitation binding energy, (60 meV), and piezoelectric characteristics. To date, various shapes of ZnO nanostructures have been reported such as nanowires (NWs), nanorods (NRs), nanobelts, nanohelixes, nanosprings, nanorings, nanobows, nanoparticles (NPs), dendritic structures, comb structures, and hierarchical structures.1-12 The powerful ZnO nanostructures could find applications in sensing, piezoelectricity, photocatalysis, electrocatalysis, field emissions, waveguides, photoluminescence, and solar cells.13-22 The properties of nanostructures can be influenced via different morphology, distribution, and/or size. Thus, it is very interesting to synthesize nanostructures with different morphologies and study the effects on their properties. The approaches for growing one-dimensional (1D) or 1Dbased ZnO nanostructures include chemical vapor deposition,23 physical vapor deposition,24 electron beam lithography,25 nanotemplates,26 phase-shifting photolithography,27 biomimetic synthesis,28 hydrothermal or solvothermal synthesis,29,30 electrochemical synthesis,31 sol-gel electrophoretic deposition,32 and the sonochemical method,2 solid-vapor process,33 and molten salt synthetic process.34 Though various methods have been employed to assemble quantities of 1D or 1D-based nanostructures, the gas-based methods always encounter high temperature and require advanced apparatus. The hydrothermal process has the defects of difficult control of reaction, requirement of surfactant in many cases, and potential danger. Herein, we develop a novel * To whom correspondence should be addressed. E-mail: ceslp@ mail.sysu.edu.cn (P.L.), [email protected] (Y.X.T.). Telephone: (+86) 20-84110071. Fax: (+86) 20-84112245. † School of Chemistry and Chemical Engineering. ‡ Instrumental Analysis and Research Center.

method to assemble 1D-based ZnO nanostructures, incorporating electrochemical corrosion (EC), in which the corrosion proceeds as an electrochemical reaction without applied potential rather than a conventional chemical reaction. The method is simple, facile, environmentally friendly, consumes few electrolytes, and has a high yield. Briefly, a Zn foil is wetted by a layer of electrolyte or by partial immersion in solution. Then, the surface is electrochemically corrupted with O2, and the resulting particles are produced. Simultaneously, assembly of the as-grown particles leads to the 1D-based ZnO nanostructures. Through such unique method proposed here, we obtained ZnO nanostructures with various morphologies via liquid membrane (LM) or partial immersion (PI). Formation mechanism of ZnO nanostructures and morphology evolution have been studied. In addition, UV-vis absorption and photoluminescence (PL) properties of the as-grown samples with various shapes also have been detected. The results indicate that optical properties vary conspicuously with obvious changes in the morphology of nanostructures. Experimental Section Different morphologies of 1D-based ZnO nanostructures are obtained via EC process at room temperature. Zn foil (99.9%) is subjected to EC with three different modes as illustrated in Scheme 1. The first mode is the liquid membrane mode (LM), where Zn foil is covered by a thin liquid membrane of electrolyte, and products are grown on the Zn substrate. The second mode is the partial immersion mode (PI), where Zn foil is partially immersed in solution, and the products grown on the Zn substrate above and below the water line are collected. The whole immersion mode (WI), where the foil is completely immersed in solution, is employed for a contrast experiment.

10.1021/jp9017794 CCC: $40.75  2009 American Chemical Society Published on Web 08/06/2009

Morphology Evolution of ZnO Nanostructures SCHEME 1: Three Modes of EC, LM, and Outer and Inner Parts of PIa

a

WI is indicated for contrast.

The electrolyte is based on a KOH aqueous solution, and related additives involve ammonia, Tween-20, and ammonium chloride. The method for electrolyte preparation involves first diluting a given concentration of KOH with distilled water to 10 mL and then adding 25% concentrated ammonia measured by milliliter or/and Tween counted by drop when necessary. A nonaqueous system of ethanol is utilized for specific experiments. Zn foil is pretreated first by activation via polish and then by deoiling with acetone. For some experiments, active Zn foil is further treated for deactivation though immersion in the concentrated H2SO4 for several seconds. Next, the foil is wetted with an electrolyte via one of the three modes mentioned above and then laid in the container and left for certain period, followed by washing the product with deioned water and then acetone. All reagents used are in the degree of AR. The morphology, component, and structure of the samples are characterized via field emission scanning electron microscopy (FE-SEM) (Shimadzu JSM-6330F), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) (JEOL JEM-2010HR) equipped with an energy dispersive x-ray spectrometer (EDS) and powder X-ray diffractometer (XRD) (Bruck D8). In addition, the optical properties of samples with different shapes are also investigated. The related instruments involve a spectrofluorophotometer (RF-5301PC) and UV-visNIR spectrophotometer (UV-3150). The wavelength of excitation light is 355 nm for PL. The samples detected in optical behavior are in the form of film, without removing the substrate under room temperature. BaSO4 is used as reference. Results and Discussion Morphology Evolution in ZnO Nanostructures. Influence of Concentration of KOH and NH3 · H2O on Growth of ZnO NRs. When Zn foil was covered by a thin layer of KOH solution, the whole surface of the foil changed to a dark color, and no gas release was clearly observed. Figure 1a shows the morphologies of the samples grown via LM. It is found that uniform and abundant NRs were formed as bundles with average diameters of ∼30 nm and lengths of ∼300 nm. To explore the morphology evolution of ZnO NRs, we carried out the experiment at different conditions and found that the growing processes of nanostructures are sensitive to the growing environment such as the concentration and component of electrolyte. We found that the morphology of the sample varied from a rod-like structure to a film-like architecture with a decrease in the concentration of ammonia, KOH, or both (Figure S1 of the Supporting Information). These results imply that the concentrations of KOH and ammonia are significant factors for the growth of a ZnO 1D structure. Generally, compared with a 1D structure, a two-dimensional (2D) structure is relatively more stable from the point of low surface energy due to less surface area. Thus, alkali and ammonia act as stable reagents from this point.

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15515 This phenomenon can be explained by the reaction between Zn2+ formed by the corrosion reaction (mechanism discussed later) and NH3 as well as OH-. Large quantities of Zn(OH)2 precipitation are inclined to be generated when the concentration of NH3 or OH- is low, giving rise to a film-like structure because of a rather high reconstructive speed. In contrast, [Zn(NH3)4]2+ and [Zn(OH)4]2- complex ions could form at high concentration of NH3 and OH-, leading to slow Zn(OH)2 precipitation, and so the ZnO formed by dehydration. Consequently, structure reconstructure proceeds under a slow rate, which is benefical for anisotropic growth. In addition, the electrostatic force induced by the negative-charged surface through hydroxyl adsorption35 and the complex effect of ammonia could prevent NRs from coacervation. However, the subfilm could degenerate to transition gradually to NRs in the relatively high concentration of KOH and ammonia at the beginning of the growth of sprouts (Figure S2 of the Supporting Information), which may be ascribed to the interaction of the diffusion of colloids, strong polar surface planes [such as (001) planes], and relatively high active colloids. It is known that one system is unallowable with respect to thermodynamics, while it could take place from the point of dynamics. Influence of Tween on Growth of ZnO NRs. It is interesting that samples can also be grown in the unwetted part of the Zn foil via the PI mode (Figure 2). The distribution of the NRs is also greatly uniform, with average diameters of ∼20 nm and lengths of ∼500 nm. While the NRs obtained via PI are much more independent compared with those of grown via LM, the top of several NRs for a partial sample aggregate together and also form bundle-like structures. However, some NRs agglomerate with neighboring ones, and film-like structures could be formed when no Tween is involved (Figure S3 of the Supporting Information). This means that Tween could stabilize NRs, mainly through adsorption onto the surface and, therefore, act as a surface ligand.36 When Tween is substituted for ammonia, NRs are also obtained (Figure S3 of the Supporting Information), however, with coacervation to some extent. This is mainly due to the insufficiency of disposed ammonia in the outer part of PI. Nevertheless, film-like structures could form when the foil was wetted (Figure S3 of the Supporting Information), which may be ascribed to the fast corrosion speed and relatively low surfactant concentration. In this case, the grown mode is much more similar to the LM mode. In addition, for anisotropic etching, different anions may change the etching rate or even cause the change of etching mechanism.37-39 However, in our case, the additives such as ammonia or Tween should not change the mechanism of the electrochemical corrosion process because of the negligible interaction between them and O2 and the fast corrosion reaction. Therefore, the additives influence the diffusion and coalescent of colloids by regulating the adsorption of surfactant molecules40 on different crystal facets, further controlling product morphology. Influence of Substrate on Morphology of ZnO NRs. Panel a and b of Figure 3 present the SEM images of ZnO NRs with high aspect ratios obtained via LM mode for a long growth time. The diameters of the NRs are about 80 nm, much larger than that of the samples grown for short time (Figure 1) and the lengths stretch to around 1 µm. The high aspect ratio is easily understood because of the adequate time for diffusion of Zn and O atoms to form ZnO NRs. It is well-known that Zn can be oxidized by concentrated H2SO4, and a porous oxide film is formed on the surface of the Zn substrate. Panel c and

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Figure 1. SEM images of a sample grown via LM for short duration. (a) Image with low magnification; inset is the enlarged image. (b) Cross section of panel a. Sample is grown in 10 mL of 2 M KOH and 2 mL of NH3 · H2O for 5 h.

Figure 2. SEM images of a sample grown via PI for short duration. (a) Sample grown on the surface over the water line without being wetted for 5 h; inset is the image magnified. (b) Cross section of panel a. Components of the solution are 10 mL of 3 M KOH and 10 drops of Tween.

d of Figure 3 show the SEM images of ZnO NRs grown on the Zn substrate after preoxidized treatment. It can be seen that the NRs in Figure 3c are more independent and loose than those in Figure 3a, and the average diameters of the NRs are relatively smaller (∼50 nm), which is mainly attributed to the different nucleus seeds formed at the beginning of the growth of NRs and the availability of resources. For the case of the untreated substrate, the whole surface of film is in an active state, leading to a high reaction rate and abundant output. So, as the sprouts acted as nucleus seeds, they resulted in close adjacent NRs with relatively larger diameters. In contrast, the EC reaction could only occur at the end of the holes on the porous oxide film. Thus, the seeds are separated, and the size is confined by the hole, resulting in more independent NRs with smaller diameters. Influence of PI Mode and Growth Time on Morphology. We found that the thickness of LM also affects the morphology of the sample. When the liquid membrane was relatively thick, the top of the NRs became extremely sharp and the diameter was larger and formed the needle-like structures as depicted in Figure 4a. The main reason for this is because of the high reaction rate and fast colloid diffusion. If the thickness of the membrane was further increased, the growth mode turned to PI. In this case, needle-like structures were also obtained as depicted in Figure 4b. However, the trunks are much larger compared with the LM case. This feature of the morphology is mainly due to the interrelationship of corrosion rate, diffusion velocity, and additive regulation. The rate of EC in the immersed part is relative to the alkaline concentration. Under relatively high KOH concentration, the corrosion reaction is rather fast, and considerable precursors for rod growth are available. Therefore, many colloids could give

rise to the high diffusion velocity preferential along the highest polar surface of wurtzite ZnO, resulting in the sharp tip. However, because of comparatively low ammonia quantity, suppressive capacity to the lateral surfaces is relatively weak, which promotes transverse diffusion, with the consequence of large trunks of rods. On the basis of these principles, the rods could assemble to small diameter ones with preservation of a sharp top, when increasing the amount of additive as shown in panal a and b of Figure 5. While in the condition of low KOH concentration but a high ammonia amount, small diameter, long, blunt top rod-like structures are obtained as shown in panels c and d of Figure 5. In addition, the diameters of the NRs in Figure 5d are larger than those in Figure 5b, which is also ascribed to the suppressive capacity of weak lateral surfaces in relatively low ammonia concentration. However, no rod-like structure forms when only ammonia is involved, suggesting that the function of ammonia is mainly for morphology regulation. Influence of SolWent on Morphology. When water solvent was substituted for ethanol, ZnO NRs were also obtained for the immersed part in PI but with short lengths and round tops as shown in Figure 6a. It is indicated that corrosion still could occur without H2O, which deviates from the case of no etching action of Si in an ethanol atmosphere.41 However, the corrosion rate became slow in terms of a small amount of corrosion output (reflected indirectly from the obtained short NRs) and so did the diffusion speed (judged from the round end points). The short lengths of the NRs reflect the trend that anisotropic growth of ZnO NRs decreases greatly. This may be explained by the small difference in the surface energy on different growth facets of ZnO NRs in the weak polar ethanol compared to water. Figure

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Figure 3. SEM images of samples assembled via LM. (a) Sample grown in polished Zn film with a duration of 3 days. (b) Magnification of panel a. (c) Sample grown in deactive Zn film caused by preoxidization with concentrated H2SO4. (d) High-resolution image of panel c. Components of solution are 10 mL of 4 M KOH and 2 mL of NH3 · H2O.

Figure 4. SEM images of needle-like structures grown via (a) LM for 5 days with a solution of 10 mL of 4 M KOH and 2 mL of NH3 · H2O and (b) PI also for 5 days, with the immersed part as the object in 10 mL of 4 M KOH and 1 mL of NH3 · H2O.

6b is the SEM image of the sample obtained on a Zn surface above the ethanol solution. Only NPs rather than NRs can be found. Ethanol is prone to evaporate. Thus the “moisture” atmosphere that covered the Zn surface above the solution was quite weak, and thus, only small amounts of ZnO colloids can be generated. However, the weak polar atmosphere decreased the anisotropic growth of ZnO NRs. Both of these facts could be responsible for the formation of NPs rather than NRs. Influence of Cl- on Morphology. In our experiment, we studied the influence of chloride on the nanostructure architecture of ZnO. It is shown in Figure 7 that there are plenty of ZnO NWs with larger aspect ratios than those of ZnO NRs obtained in the solution without chloride. Some of the NWs are separated, and some are aggregated. This result indicates that the chloride

indeed promoted the corrosion reaction in terms of more output of the production, and it also affected the morphology of the ZnO nanostructure. In the field of corrosion, the effects of Clon the morphology and corrosion mechanism have been studied intensively.42-44 When chloride was added in the solution, the chlorine ion generally acted as a corrosive promoter and accounted for the formation of localized corrosion. The point corrosion was enhanced, and the generating velocity of Zn2+ was accelerated. So the concentrations of [Zn(NH3)4]2+ and [Zn(OH)4]2- complex ions were high. The amount and growth velocity of ZnO NRs was large, and the NRs evolved to NWs with an increase in longitudinal length. However, the NWs could aggregate together due to the large amount of growth of ZnO. We also observed that ZnO NWs could be obtained when the concentration of ammonia was relatively high. However, the

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Figure 5. Big NRs grown via EC with PI mode for 5 days in (a,b) 10 mL of 4 M KOH and 3 mL of NH3 · H2O and (c,d) 10 mL of 2 M KOH and 2 mL of NH3 · H2O. Immersed parts are the research objects.

Figure 6. SEM images of a sample obtained from (a) the immersed part and (b) the outer region in a nonaqueous solution of ethanol with 4 M KOH for 5 days via PI.

morphologies changed to comb-like structures and further to film-like topographies when decreasing the ammonia concentration to a certain degree or even to zero with the same quantity of chlorine (Figure S4 of the Supporting Information). Such trend phenomena could be explained from the point of view that large quantities of colloids obtained in the high Clconcentration reconstruct preferentially in the two-dimensional mode in order to reduce surface energy as quickly as possible. However, ammonia could suppress such two-dimensional growing as mentioned previously. Formation of Nanodentrites. It is expected that rapid crystallization at high supersaturation and molecular anisotropy could give birth to dendritic crystals.45-47 In consideration of these, we increased the concentration of alkali as well as ammonia and thickened the thickness of LM. Nanodendrites were indeed obtained as shown in Figure 8. It is shown that the distribution of the branch structure is highly uniform. The average diameter of the trunks is approximate 100 nm. Branches

are relatively small, around 50 nm. The array of branches is well-regulated, i.e, 6-fold symmetry. In addition, it is discernible that the branches are a little inclined downward. Structure Characterizations of Obtained Nanostructures. For short NRs (Figure 9), the clear lattice fringes show the wellcrystallized structure. The energy dispersive spectrum (EDS) shows that the components of NRs are Zn and O (Figure S5 of the Supporting Information). In addition, the interplanar distances for the NRs obtained via LM and assembled via PI are determined to be 0.52 nm, which should be indexed to be (001) planes of wurtzite ZnO. Thus, the growth direction of the asassembled NRs is along (001) planes. For long NRs (Figure 10), the indexes of the peaks obtained in the XRD pattern conform to the hexagonal ZnO crystal structure (JCPDS 65-3411). Other peaks derive from the Zn substrate. Further examination the structures of ZnO NRs was conducted via TEM and HRTEM. The clear lattice fringes and bright diffraction points (Figure 10b-g) reveal well-crystallized

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Figure 7. SEM images of ZnO NWs obtained via LM for 2 days with a solution of 10 mL of 2 M KOH, 2 mL of NH3 · H2O, and 3 M NH4Cl.

Figure 8. Morphology characterization of dentrites grown via LM with a thick electrolyte layer for 3 days. (a) SEM image. (b) Magnification of panel a. Adopted solution is 10 mL of 4 M KOH and 4 mL of NH3 · H2O.

NRs. The d values of the adjacent two spots in the insets of panels c and d of Figure 10 are calculated to be 0.26 and 0.28 nm and 0.26 and 0.16 nm, respectively, which should correspond to (002) and (100) planes and (002) and (110) planes, respectively, taking the relationship of the angle between the two spots into consideration (both with the perpendicular relationship). The spaces of lattice fringes in panels d and e of Figure 10 almost consist with the results of SAED [0.54 nm for (001) planes and 0.27 nm for (002) planes, respectively]. The indexes of the SAED in the inset in Figure 10f show a similar condition compared to that in the inset in Figure 10b. Whereas, the lattice spacing perpendicular to the growth direction is measured to be 0.52 nm (Figure 10f). For the top view (Figure 10g), the measured d values of the two adjacent spots are both 0.28 nm, and every two adjacent spots all space 60°. Thus, these spots belong to a {100} plane family. It is observed that some measured interplanar distances (Figure 10d,e) show a little discrepancy to the normal cases, which is possibly ascribed to measurement error of the HRTEM. However, the SAED gives a good result for the structure characterization according to the small error of the d value and interfacial angle as mentioned above. In addition, the indexes of SAED and lattice fringes together conclude that the obtained NRs are grown preferentially along the [001] crystallographic direction, the same as in the case of the short NRs. For nanodendrites (Figure 11), along with the distribution of the patterns of electron diffraction (insets in Figure 11b), the spaces of 0.52 nm of lattice fringes for branch and trunk correspond to the (001) planes, suggesting that branch and trunk are elongated along the unexpected [001] axis in wurtzite ZnO. It is known that wurtzite ZnO has two polar facets and six nonpolar lateral facets. The trunk elongates along the c axis,

and the branches should grow extentially from the six lateral facets {100}. The unexpected elongation along the (001) planes in the branches in our experiment is mainly due to the very fast growing speed of the nanostructure, which results from growing from the six lateral facets with (001) elongation, which also results in a 6-fold symmetry nanostructure, with a little incline rather than the normal perpendicular relationship.48 Formation Mechanism of ZnO Nanostructures. EC Process in LM. Zinc is an amphoteric metal. As for the mechanism of the growth of the NRs, the problem is whether it obeys a chemical reaction (2KOH + Zn + 2H2O ) K2Zn(OH)4 + H2) or an EC mode (electrochemical process). In our experiment, we found that there was no obvious change on the surface of a Zn foil observed by naked eyes when the Zn foil was completely immersed (WI mode) in the same solution for a rather long duration (10 h). In addition, no obvious change was observed on the surface of a Zn foil when it was hung closely near the solution for 10 h. However, when the Zn foil was partially immersed into the solution even for 5 h, the Zn foil surface over the water line changed to a gray color, and the morphology was also altered (Figure 2). These experiments indicate that electrochemical corrosion did occur, and the content of oxygen in the electrolyte played an important role in the pathway of corrosion. In other words, the mechanism of ZnO NRs obeys an EC mode via an oxygen consumption type of corrosion. The corrosion mechanism could be described as Zn -2e- f Zn2+ and O2 + 2H2O + 4e- f 4OH- for anode and cathode, respectively. With further reaction, Zn2+ and OH- combined to form [Zn(OH)4]2-, and then hydroxide precipitates because of oversaturation. The precipitates were dehydrated, and ZnO colloids were obtained. Consequently, the surface of Zn was covered with an oxide/hydroxide layer. Of course, the kinetics

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Figure 9. Structure characterization of obtained short NRs. (a) TEM image of short NRs grown via LM (Figure 1). (b) HRTEM recorded from the rectangle in panel a. (c) TEM image of short NRs grown via PI (Figure 2). (d) HRTEM recorded from the rectangle in panel c.

of the O2 reduction on the surface of the Zn foil covered by the oxide/hydroxide layer may be rather complicated.49 Nevertheless, the oxide/hydroxide layer impedes the reduction of O2 to H2O with formation of the H2O2 intermediate.50,51 Additionally, there is no obvious deep pit (Figure S2 of the Supporting Information) in the course of corrosion. They both confirm the nonexistence of H2O2 or a negligible amount, if any. In addition, the corrosion reaction was fast via the LM mode. The electrolyte-wetted Zn film could change color to gray black in several minutes. The reasons may be (1) sufficient oxygen in LM because the thickness of LM drops to the scope of the static diffusion length of oxygen (0.1-0.5 mm), (2) the proximity of cathode and anode leads to high charge transfer, and (3) excess Zn atoms in interstitial positions and oxygen vacancies serve mainly as electron donors in the pure bulk and interfacial ZnO.52,53 Formation of 1D Nanostructures. As for the growth of ZnO NRs, the ZnO particles attached preferentially to the plane with the highest surface energy, resulting in a 1D nanostructure. This forming process could be considered as the well-known oriented attachment,54,55 which is characterized as the driving force of reduction in the overall surface energy by eliminating surface area. Though, it is generally accepted that crystal growth by the oriented attachment mechanism occurs under rather high temperature. Nevertheless, according to our experiments, we

propose that oriented attachment could be initiated at much low temperature such as room temperature as long as the driving force of the attachment is strong enough. Furthermore, it is observed that the formation of a corrosion layer is prior to the sprouts, and the population of the former is much larger compared with the latter (Figure S2a of the Supporting Information). That is to say, the kinetics of the surface reactions is much faster than the mass transport of reactants or products in a liquid layer. Therefore, the growth of NRs in that case could be considered as diffusion controlled. Beside oriented attachment, NPs could coalesce with each other for reducing the interface area56 and could move along the surface of a growing nanostructure, which contributes to the various shapes of the products. For ZnO, the (001) planes with relatively higher surface energy have a strong tendency to capture smaller particles in order to reduce their surface energy. Thus, the small ZnO precursors diffuse along the surface of the growing 1D nanostructure to the top position, with partial precursors constructed with the lateral surface in the course of diffusion, resulting in growth of the NRs and broadening of their diameters. In this process, additives could influence the diffusion of NRs. EC Course for Outer Part of PI. PI is not similar to LM. The concentration of O2 in the outer part of a Zn film is higher than that of LM. Moreover, a compact zinc oxide layer forms

Morphology Evolution of ZnO Nanostructures on the surface because of the easy oxidization of bare Zn in air. The compact barrier zinc oxide layer is depleted of electrons at the oxide-solution interface due to the formation of positively charged electron donors. O2 could only be reduced in two electron steps to hydrogen peroxide species. The as-produced peroxide species can be adsorbed, and the following reactions take place in an alkaline medium at the nonstoichiometric zinc oxide surface57

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2O2- + H2O f HO2- + OH- + O2

(1)

2HO2- f 2OH- + O2

(2)

Then the dissolution reaction takes place through the point defect model,52 leading to the degradation of the lattice at the surface and precipitation of Zn(OH)2 and ZnO. However, the produced

Figure 10. Structure characterization of long ZnO NRs. (a) XRD pattern. (b) TEM image and SAED. (c) TEM image and SAED. (d) HRTEM of panel b. (e) HRTEM of panel c. (f) HRTEM, TEM, and SAED. (g) TEM image of a section of ZnO NRs with a top view and corresponding SAED. Panels a,b,d, and f show samples obtained via LM. Solution is 10 mL of 4 M KOH and 2 mL of NH3 · H2O with a growth time of 3 days on a polished Zn substrate. Panels c,e, and g show samples obtained via LM. Solution is 10 mL of 4 M KOH and 2 mL of NH3 · H2O with a growth time of 3 days on a Zn substrate after being treated in concentrated H2SO4.

Figure 11. TEM images of obtained dentritic structure. (a) TEM image. (b) HRTEM recorded from rectangle in panel a. The two insets are the corresponding fast fourier transform of the branch (top) and trunk (bottom).

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peroxide species can cause pitting. It may be one of the contributions for the deep pit (Figure S6 of the Supporting Information). In this case, a vapor membrane rather than a liquid layer is formed on the surface of the substrate due to adsorption and capillary phenomenon. Moreover, electrolytes could diffuse along the surface of substrate, and then corrosion occurs. Though the amount of oxygen is relatively large and the rate of corrosion reaction is fast, the surface reaction is still the control step because of the relatively small amount of electrolyte until the thickness of the membrane grows to or exceeds a critical value (in that case, it changes to LM). The status of KOH under such an environment could be considered as an anisotropic etchant as shown in Figure S6 of the Supporting Information so that the corrosion rate along the direction perpendicular to the substrate is much faster than that along the other orientations. However, it is not like the case of the effect of alkali on the crystalline Si etching. After the reaction, the generated products as a solid phase diffuse and reassemble to form nanostructures with certain morphologies rather than dissolve and vanish on the surface as in the case of Si etching. The interior of the relatively larger NRs formed preliminarily contain the unreacted zinc due to the anisotropic reaction. ZnO at the exterior of the NRs could absorb O2, H2O, KOH, and additives to supply reactants to the surface of virgin zinc. Corrosion continues with accompaniment of the decomposition of the relatively larger NRs to several smaller ones. Beside the anisotropic effect, capillary coacervation is another main contribution for the deeporiented corrosion. The formation of ZnO is directly oriented assembled to NRs without requiring a rather long diffusion, which is the case for LM, and thus, the NRs grown via PI are longer than those obtained via LM in the same amount of time (Figures 1 and 2). EC Process in Immersed Part of PI. As mentioned previously, no obvious reaction was observed when a Zn film was subjected to WI as long as 10 h. For a much longer reaction time, for example, 5 days, only a small amount bubbles were found on the surface of the substrate. The SEM observation of the architecture of the product indicated a particle-like feature rather than rod-like construction. It is implied that a chemical reaction could occur. It is the same anaerobic environment for WI and PI, but the morphology of the product is quite different. Such interesting phenomena could be primarily attributed to the EC mechanism. The O2 atmosphere is quite different among PI, WI, and LM modes. For the PI mode, the oxygen concentration in the vapor membrane (above the water line) is much higher than that in the solution. Besides, the EC reaction (Zn -2e- f Zn2+ and O2 + 2H2O + 4e- f 4OH- takes place in many different tiny areas corresponding to the anode and cathode, respectively) occurred in the vapor membrane, and another EC process proceeds at the same time (which contributes to the growth of big NRs in the immersed part). The anodic reaction Zn -2e- f Zn2+ could take place in the immersed part because of oxygen shortage (potential for the outer part is relatively high in contrast with the inner part), and its counterpart cathodic reaction occurred in the outer regime above the water line, especially near the solution surface. In this way, the socalled water line corrosion is formed. ZnO colloids are formed, and oriented attachment occurs in the immersed part, leading to one-dimensional growth. Therefore, the growth mechanism is considered to be diffusion controlled. The shape of obtained NRs could be regulated via ammonia and concentration of electrolyte, similar to the case of LM.

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Figure 12. (a) UV-vis spectra and (b) PL of 1D-based ZnO nanostructures with various morphologies: short NRs from Figure 2 (black line), big NRs from Figure 5c (red line), NWs from Figure 7 (blue line), long NRs from Figure 3a (green line), and nanodentrites from Figure 8 (yellow line). Gray line in panel a is recorded from a Zn substrate.

Optical Properties of ZnO Nanostructures with Various Shapes. Case for UV-Wis Absorption Spectra. The optical properties of various shapes ZnO nanostructures were studied. It is clearly shown in Figure 12a that ZnO nanostructures with various shapes display different profiles of UV-vis absorption spectra, especially for the short NRs and big NRs. On the basis of the principle of linear fit in the plot of (Rhν)2 against energy hν (where R is the absorption coeffcient, h is Planck’s constant, and ν is light frequency),58 the optical gaps of samples with various shapes are measured: 3.29 eV for short NRs, 3.10 eV for big NRs, 3.18 eV for NWs and long NRs, and 3.13 eV for nanodentrites. This information implies that the optical gap could be modulated via morphology. The more the morphology changed, the more variety in the optical gap. As is shown in Figure 12, a great contrast appears between short NRs and big NRs, and some discrepancy occurs between long NRs and nanodentrites, while no obvious change is observed for long NRs and NWs. However, the Zn substrate shows negligible absorbance (gray line in Figure 12a); therefore, the optical absorption could be derived from ZnO. Case for PL. For PL spectra (Figure 12b), it is evident that various samples exhibit a relatively sharp and strong UV emission centered at 385 nm, except for the big NRs, which is unambiguously attributed to the free excitonic emission in the near-band edge of ZnO. Even for the big NRs, the emission around 385 nm still appears but in the form of shoulder peak. However, another obvious feature is that the short NRs demonstrate the strongest excitonic emission, indicating the highest optical quality among various morphologies. In contrast, the big NRs present the poorest emission. The area illuminated and the order of NRs may be responsible for such phenomenon. The density of short NRs is the highest, and their order is also in the first rank among the five type samples, resulting in the best quality emission. The four peaks at 450, 466, 480, and 490 nm are likely attributed to defect-state luminescence.11 The relatively intensities and positions of the four peaks remain constant, basically indicating that the luminescences caused by defects are derived from the same mechanism. The presence of a green emission at ∼500 nm can be attributed to radiative recombination of photogenerated holes in the valence band with electrons in singly occupied oxygen vacancies.59,60 The intensity of the deep-level emission is determined by the concentration of the oxygen vacancies in ZnO crystals.61 Therefore, it can be inferred that the defect-states in the short NR samples are the highest, though they present the best near band-edge emissions.

Morphology Evolution of ZnO Nanostructures Conclusion In conclusion, ZnO nanostructures with various morphologies such as NRs (short or long, small or big, independent or coacervation, and sharp or blunt), NWs, NPs, and nanodentrites were obtained through simple electrochemical corrosion occurring on a Zn substrate with three different modes. All ZnO nanostructures have the same wurtzite structure, and the growth direction for the 1D-based nanostructures is along the (001) facet. The mechanism for explaining the 1D-based nanostructures growth was proposed as being oxygen consumption corrosion generated on primary colloids, which then reconstructed via oriented attachment to form various morphologies with different environments. The morphology of the ZnO nanostructures can be modulated by the growth condition, including concentration, reaction time, additives, state of substrate, membrane thickness, and solvent. Optical gaps of the ZnO nanostructures vary from their morphologies, with short NRs showing the largest optical gap. In contrast, the big NRs exhibit the smallest value of optical gap. The PL properties demonstrate that the peaks of the nearband emission and defect-related luminescence are basically in the same position. However, intensities for different morphologies are of different values, and short NRs exhibit the best near-band emission, greatly contrasting to that of the big NRs, which display the weakest luminescence. Acknowledgment. This work was supported by the National Nature Science Foundation of China (Grants 20603048 and 20573136), the Natural Science Foundations of Guangdong Province (Grants 06300070, 06023099, and 04205405), and the National Foundations of China-Australia Special Fund for Scientific and Technological Cooperation (Grant 20711120186). Supporting Information Available: Morphology evolution caused by reducing the concentration of ammonia and KOH in LM, morphology transition images for explaining the formation of NRs for LM and PI, morphology evolution via PI for short time, competitive effect of chlorine ion and ammonia on the morphology of ZnO in LM, and EDS of the short NR. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Huang, M.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (2) Jung, S.; Oh, E.; Lee, K.; Park, W.; Jeong, S. AdV. Mater. 2007, 19, 749. (3) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (4) Gao, P. X.; Ding, Y.; Mai, W. J.; Hughes, W. L.; Lao, C. S.; Wang, Z. L. Science 2005, 309, 1700. (5) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625. (6) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Science 2004, 303, 1348. (7) Hughes, W. L.; Wang, Z. L. J. Am. Chem. Soc. 2004, 126, 6703. (8) Hu, Z. S.; Ramı´rez, D. J. E.; Cervera, B. E. H.; Oskam, G.; Searson, P. C. J. Phys. Chem. B 2005, 109, 11209. (9) Li, G.-R.; Lu, X.-H.; Qu, D.-L.; Yao, C.-Z.; Zheng, F.-L.; Bu, Q.; Dawa, C.-R.; Tong, Y.-X. J. Phys. Chem. C 2007, 111, 6678. (10) Yan, H.; He, R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. J. Am. Chem. Soc. 2003, 125, 4728. (11) Jiang, P.; Zhou, J.; Fang, H.; Wang, C.; Wang, Z. L.; Xie, S. AdV. Funct. Mater. 2007, 17, 1303. (12) Jing, Y. L.; Jian, G. W.; Zhi, F. R. Nano Lett. 2002, 2, 1287. (13) Hsueh, T.-J.; Chen, Y.-W.; Chang, S.-J.; Wang, S.-F.; Hsu, C.-L.; Lin, Y.-R.; Lin, T.-S.; Chen, I.-C. J. Electrochem. Soc. 2007, 154, 393. (14) Zang, J.; Li, C. M.; Cui, X.; Wang, J.; Sun, X.; Dong, H.; Sun, C. Q. Electroanalysis 2007, 19, 1008. (15) Wang, Z. L.; Song, J. H. Science 2006, 312, 242. (16) Yang, J. L.; An, S. J.; Park, W. I.; Yi, G. C.; Choi, W. AdV. Mater. 2004, 16, 1661. (17) Hsueh, T.-J.; Chang, S.-J.; Hsu, C.-L.; Lin, Y.-R.; Chen, I.-C. Appl. Phys. Lett. 2007, 91, 053111.

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