Tailoring Anisotropic Morphology at the Nanoregime: Surface Bonding

Feb 21, 2013 - School of Chemical Engineering, Dalian University of Technology, Dalian 116024 ..... Congting Sun , Yan Wang , Chaoyang Tu , Dongfeng X...
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Tailoring Anisotropic Morphology at the Nanoregime: Surface Bonding Motif Determines the Morphology Transformation of ZnO Nanostructures Congting Sun†,‡ and Dongfeng Xue*,†,‡ †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ABSTRACT: A quantitative relationship between bond length, crystal morphology, and particle size has been established to investigate the morphology transformation of ZnO nanostructures at the nanoregime. Surface bonding conditions dominate the anisotropic growth of ZnO nanoparticles. Critical surface bond lengths at which ZnO can respectively exhibit pyramid-, truncated pencil-, pencil-, and rod-like morphologies were quantitatively calculated under different extension/shrinkage degrees of Zn−O bond length in the lattice. The size of particular ZnO nanostructures was obtained by tracking the variation of bond length in the lattice. At the nanoregime, ZnO thermodynamically prefers to exhibit nanorods with a diameter smaller than 3.60 nm, nanopyramids with size smaller than 2.94 nm, nanopencils with a diameter larger than 3.15 nm, and truncated nanopencils with a diameter larger than 3.92 nm. Our results are in agreement with experimental observations and indicate the fundamental role of surface bonding control in tailoring anisotropic growth of ZnO nanostructures.

1. INTRODUCTION Crystal growth is an exothermic process during which chemical bonds are formed to realize the incorporation of free ions,1 molecular clusters, or nanoparticles2 into a lattice. Attention has been given to the chemical bonding conditions at particular (hkl) surfaces to investigate the phase transformation from free ions or molecules to crystalline state.3 Controlling anisotropy is a key concept in the generation of complex functionality in advanced materials.4 Chemical bonding conditions at growth interfaces have been demonstrated to dominate the anisotropic growth of single crystals.5 On the basis of crystal structure and chemical bonding data such as the bonding length, direction, and strength between constituent atoms, the ideal morphologies of a series of inorganic crystal were successfully predicted from the chemical bond viewpoint.5,6 Distinct chemical bonding conditions at different crystal surfaces derive from the anisotropic crystal structure. Tailoring the chemical bonding conditions can therefore provide an approach to modify the anisotropic growth of single crystals. On the basis of chemical bonding conditions at (100), (001) and (001)̅ surfaces, morphology evolutions of ZnO particles in both acid and alkaline conditions were studied from both experiments and calculations.3 Both crystal size and shape have been proposed to influence materials properties; therefore, the size range in which particular nanostructures can grow is highly desired. The particle size of isotropic spherical colloids can be derived from the mass− volume−density correlation, where crystal mass can be obtained from the rate equations.7 For some anisotropic morphologies, the variation of percentage of surface atoms with total number of © 2013 American Chemical Society

atoms was used to predict the size regime in which the nanostructures were obtained, yet this approach required an assumption about the shape of nanostructures.8 Recent progress showed surface-induced structural modification in nanoparticles, resulting in the variations of bond length near the surface of nanoparticles.9 Chemical bonds play fundamental roles in determining materials properties10,11 and crystal growth behaviors.12,13 At the nanoregime, surface/volume ratio is sizedependent. The size of nanoparticles can therefore be estimated by tracking the variation of bond length in the particles. Zinc oxide (ZnO) is a II−VI semiconductor with a wide, direct band gap (3.3 eV) and a large exciton binding energy (60 meV).14−16 Nanocrystalline ZnO materials have been widely studied for high-technology applications such as in photovoltaic devices,17 light-emitting diodes,18 sensors,19 and biological labels.20 A variety of ZnO morphologies were fabricated, including nanopyramids,21 nanopencils,22 nanorods,23 and nanowires.24 Previous studies demonstrated that the Young’s modulus,25 band gap,26 optical extinction,27 and critical temperature of melting of ZnO nanostructures28 are size-dependent. Moreover, ZnO nanostructures with distinct geometries have been proved to possess unique performances. For example, ZnO nanopencils were used as efficient electron mediators for the fabrication of efficient liquid ammonia chemical sensors.22 ZnO nanowires can be used to fabricate an-inorganic light-emitting diode in a Received: February 1, 2013 Revised: February 15, 2013 Published: February 21, 2013 5505

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polymeric microfluidic manifold.29 In order to integrate the advantages of both crystal size and anisotropic morphology in tailoring materials performances, it is important to explore an effective approach to tailoring the anisotropic morphologies at the nanoregime. In this study, we establish a relationship between the bond length, crystal morphology, and particle size from the chemical bonding viewpoint. Quantitative anisotropic morphology−chemical bonding length−particle size correlations are provided to map the morphology transformation of ZnO nanostructures at the nanoregime.

2. METHOD On the basis of chemical bonding theory of single crystal growth, we can quantitatively calculate ZnO thermodynamic morphology:3,5 R hkl = K ·

bond Ehkl Ahkl ·dhkl

(1)

where Ebond hkl is the chemical bonding energy of the kJ mol−1, Ahkl is the area of the (hkl) plane in m2,

(hkl) plane in and dhkl is the (hkl) interplanar distance in m. From eq 1, it can be deduced that the relative growth rate of (hkl) plane is proportional to the chemical bonding energy along the normal direction of the (hkl) plane per unit cell volume. The chemical bonding conditions at (100), (011), (001), and (001)̅ surfaces are shown in Figure 1. 30 Ebond hkl can be calculated by bond valence model

Figure 1. Structural characteristics of ZnO. (a) Bond length and bond angle in bulk ZnO crystal. (b−e) Chemical bonding conditions at the (100) (b), (011) (c), (001) (d) and (001)̅ (e) planes. bond Ehkl = 364.98· exp[(1.704 − d Zn − O)/0.37] −1

(2)

Figure 2. Relationship between critical Zn−O bond length on different surfaces and ZnO morphologies. (a) Pyramid-like ZnO particles, (b) pencil-like ZnO particles, (c) truncated pencil-like ZnO particles, (d) rod-like ZnO particles. The error bars indicate the longer bond length and the shorter bond length on the (011) plane.

−1

where 364.98 (kJ mol vu ), 1.704 and 0.37 are the bond valence parameters of the Zn−O bond, and dZn−O is the Zn−O bond length in Å.31 Figure 2 shows the bond length ranges in 5506

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Zn−O bond in the bulk becomes sZn−O(1 ± 1/n) (“+” for case 1, and “−” for case 2). According to the bond valence model,30 the Zn−O bond length (in units of Å) in ZnO nanostructures d′Zn−O can be calculated by

which a ZnO single crystal can be grown with pyramid-, pencil-, truncated pencil-, and rod-like morphologies, respectively. Recent progress showed surface-induced structural modification in nanoparticles, resulting in the variations of bond length in nanoparticles.9 As shown in Figure 3, Zn−O bond lengths in the

d′Zn − O = 1.704 − 0.37 ln[sZn − O(1 ± 1/n)]

(3)

The shrinkage or extension degree of the Zn−O bond, ΔdZn−O, is defined as Δd Zn − O = (d′Zn − O − d Zn − O)/d Zn − O × 100%

(4)

Positive ΔdZn−O represents the stretch of the Zn−O bond, and negative ΔdZn−O represents the shrinkage of the Zn−O bond. From Figure 3, the relationship between crystal size L (in units of nm) and n can be expressed as L = 0.2n·d′Zn − O ·sin 107.98°

(5)

where 107.98° is the angle between the bond along the c axis and the bond in the ab plane as shown in Figure 1a. By combining eqs 3 and 5, we can obtain the relationship between bond length and crystal size. In the case of shortened Zn−O bonds in the lattice (case 1), L=

0.2·d′Zn − O ·sin 107.98° exp[(1.704 − d′Zn − O )/0.37]/sZn − O − 1

(6)

and in the case of elongated Zn−O bonds in the lattice (case 2), L=−

0.2·d′Zn − O ·sin 107.98° exp[(1.704 − d′Zn − O )/0.37]/sZn − O − 1

(7)

where sZn−O is the bond valence of the Zn−O bond in the bulk ZnO single crystals. Figure 4 shows the calculated relationship between bond length and crystal size. For wurtzite ZnO (Figure 1a), the relationship between bond length and lattice constants a(b) and c (in units of Å) can be expressed as a = b = 2 [dZn −O(s)]2 − (0.125c)2 cos120°, and c = dZn−O(l)/ 0.375, where dZn−O (s) is the shorter Zn−O bond length in the lattice, and dZn−O (l) is the longer Zn−O bond length in the lattice. It can be concluded that the lattice constants of wurtzite ZnO increase with increasing Zn−O bond lengths in the lattice. Therefore, the lattice constants of ZnO nanostructures are slightly different from that of bulk ZnO single crystal due to different bond lengths.

Figure 3. Schematics of variations in Zn−O bond length at the nanoregime. (a) Compensating the inadequate bond valence generated by the Zn (O) atoms at the surface via shortening the Zn−O bond length in the lattice. (b) Compensating the inadequate bond valence generated by the Zn (O) atoms at the surface via ligands in the growth environment.

3. RESULTS AND DISCUSSION As shown in Figure 2, ZnO nanostructures prefer to exhibit pyramid- and pencil-like morphologies in the case of shrinkage of Zn−O bonds in the lattice, whereas ZnO nanostructures prefer to exhibit truncated pencil- and rod-like morphologies in the case of stretching of Zn−O bonds in the lattice. For pyramid-like ZnO nanostructures, only {011} and (001̅) planes are exposed, and pencil-like ZnO nanostructures are bounded by {100}, {011} and (001)̅ planes. For pyramid-like ZnO nanostructure, the R(011) should be smaller, which requires a longer Zn−O bond on {011} than that on the other surfaces. As a consequent, the shrinkage of Zn−O bonds on the {100} and (001̅) facets and the stretch of Zn−O bonds on the {011} facets can provide optimum bonding conditions to grow pyramid-like ZnO nanostructures. On the contrary, for truncated pencil- and rod-like ZnO nanostructures, R(011) should be larger to diminish and even eliminate the exposure of {011} surfaces, which requires shorter Zn−O bonds on (011) than that on the other surfaces. The stretch of Zn−O bonds on the {100}, (001) and (001̅) planes and the shrinkage of Zn−O bonds on the {011} planes can generate

lattice change in order to compensate the inadequate bond valence generated by unsaturated atoms on the surface to maintain the total bond valence of ZnO particles. There are two cases. In case 1, the inadequate bond valence generated by the Zn (O) atoms at the surface is compensated by shortening the Zn−O bond length in the lattice (Figure 3a). In case 2, the inadequate bond valence generated by the Zn (O) atoms at the surface is compensated by ligands in the growth environment. Since the surface unsaturated Zn (O) atoms coordinate with the atoms in ligands, redundant bond valence on the atoms in ligands can be formed. Consequently, Zn−O bond in the lattice will be elongated to offset the redundant bond valence. As shown in Figure 3, along the chemical bonding direction of the unsaturated Zn (O) atoms in {100} surfaces (guided by gray, yellow, and blue backgrounds), it can be found that the number of chemical bonds, n, in the bulk are the same for any Zn−O chemical bonding direction on the surface. Moreover, all Zn−O bonds in the bulk participate in compensating the inadequate (case 1) or redundant (case 2) bond valence generated by the unsaturated surface atoms. Therefore, the bond valence of any 5507

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bond valence generated by the unsaturated atoms in surface, n is relatively low. The variation of bond valence on the Zn−O bond (i.e., ± 1/n·sZn−O) is larger, which results in the higher ΔdZn−O in the lattice. With the increase of L, more chemical bonds participate in compensating inadequate or redundant bond valence generated by the unsaturated atoms in surface. Larger n value leads to the smaller ±1/n·sZn−O on Zn−O bond and consequently the lower ΔdZn−O in the lattice. With L beyond the nanoregime, the Zn−O bond length equals that in the bulk materials. Because the Zn−O bond length in the lattice depends on the particle size, the Zn−O bond length in the lattice of ZnO nanostructures can be controlled by experimental conditions that can control particle size, such as reaction time,21 additives,3 supersaturation,7 and solvent.33 In order to obtain the size range in which ZnO nanostructures with particular geometry are thermodynamically preferred to (100) exist, we plot the critical Δd(011) Zn−O versus ΔdZn−O that can be used to divide ZnO pyramid, pencil, truncated pencil, and rod in Figure 5. For ZnO pyramids, the critical stretch degree of Zn−O bonds on the (011) plane decreases with increasing shrinkage degree of the Zn−O bond on the (100) plane. When the Zn−O bond length is shortened by 5%, the Zn−O bond length on (011) should be stretched by 18.17%. In the previous report, Zn−O bond length can achieve 2.361 Å (ΔdZn−O = 19.42%), and the shortest Zn−O bond length can achieve 1.896 Å (ΔdZn−O = −4.10%).32 We therefore assume that the largest stretch degree of Zn−O bonds on {011} surfaces is 20%, and the largest shrinkage degree of Zn−O bonds on {011} surfaces is 5%. From Zn−O Figure 5a, when ΔdZn−O (011) = 20%, Δd(100) = −3.2%. According to the bond length and crystal size correlation, we confirm that the size of pyramid-like ZnO nanostructures is smaller than 2.94 nm (as shown in Figure 6a). The size of previously synthesized ZnO nanopyramids is smaller than 10 nm.21 In practical experiment,21 thermal injection was used to generate ZnO nanocrystals within short reaction time, which can control the particle size. As shown in Figure 4a, smaller particle size leads to larger shrinkage degree of Zn−O bonds in the lattice, satisfying the chemical bonding conditions to grow ZnO nanopyramids. Since ΔdZn−O (011) ≥ −5%, the corresponding ΔdZn−O (100) = 3.3% can be confirmed for ZnO nanorods from Figure 5d. Therefore, ZnO thermodynamically preferred to exhibit nanorods with a diameter smaller than 3.60 nm (Figure 6d). Single crystalline ZnO nanorods with a diameter of about 5 nm were synthesized in ethanol solvent at room temperature.33 Using ethanol as solvent, the inadequate bond valence generated by the Zn (O) atoms at the surface via ethanol, leading to the larger stretch degree of Zn−O bonds on the {100}, (001), and (001)̅ planes at smaller particle size (Figures 3 and 4b). This can provide an optimum bonding condition to grow ZnO nanorods. Compared with nanopyramids and nanorods, ZnO can grow with truncated pencil-like and pencil-like morphologies within −3% ≤ ΔdZn−O (011) ≤ 3% (Figure 5b,c). On the basis of eqs 6 and 7, ZnO nanostructures thermodynamically prefer to exhibit nanopencils with diameters larger than 3.15 nm, and truncated nanopencils with diameters larger than 3.92 nm (Figure 6b,c). Experimentally, pencil-like and truncated pencil-like ZnO nanostrutures22,33 and microstructures34 have been prepared in previous works. Surface Zn−O bonding length can be controlled by selecting proper surfactants, whose ligands can hinder the chemical bonding between Zn and O atoms at the surface. In alkaline solutions,22,33 OH− can adsorb on {011} surfaces. The chemical bonding between Zn and O at {011} surfaces is slightly longer than that in the bulk, which leads to the formation of ZnO pencils. In acid solutions,34 the chemical

Figure 4. Relationship between Zn−O bond length and particle size. (a) Decreased crystal size with shortening the Zn−O bond length in the lattice. (b) Decreased crystal size with stretching the Zn−O bond length in the lattice.

these chemical bonding conditions to grow truncated pencil- and rod-like ZnO nanostructures. Figure 2a shows the critical bond lengths on the (011) plane under different shrinkage degrees of Zn−O bond on the (100) plane (Δd(100) Zn−O), where the error bars indicate the longer bond length and the shorter bond length on the (011) plane as shown in Figure 1c. On the basis of eq 1, these critical bond lengths result in R(011) = 0.88R(100) − 0.47R(001), which can satisfy the critical condition for forming a ZnO pyramid. When dZn−O (011) is larger than the critical bond length in our calculations, R(011) < 0.88R(100) − 0.47R(001), ZnO exhibits a pyramid. On the contrary, when dZn−O (011) is smaller than the critical bond lengths in Figure 2a, {100} surfaces will be exposed, and pencil-like ZnO particles appear. In order to inhibit the exposure of (001) surface, we further calculate the lower limit of dZn−O (011) as shown in Figure 2b. Once dZn−O (011) is smaller than this critical value, R(011) > 0.458R(001), the (001) surface is exposed, and the ZnO morphology transforms from pencil to truncated pencil. Furthermore, we calculate the critical bond length on the (011) plane that can divide the truncated pencil-like and rod-like ZnO (Figure 2d). When dZn−O (011) is larger than the critical value, R(011) < 0.88R(100) + 0.476R(001), ZnO exhibits truncated pencil-like morphologies, whereas when dZn−O (011) is smaller than the critical value, R(011) > 0.88R(100) + 0.476R(001), ZnO exhibits rod-like morphologies. Figure 4 shows that ΔdZn−O becomes larger with decreasing the crystal size L. At smaller size, the number of chemical bonds that were used to compensate the inadequate or redundant 5508

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Figure 5. Relationship between shrinkage or extension degree of Zn−O bond and ZnO morphologies. (a) Pyramid-like ZnO particles, (b) pencil-like ZnO particles, (c) truncated pencil-like ZnO particles, (d) rod-like ZnO particles.

Figure 6. Relationship between bond length, ZnO morphologies, and particle size. (a) Pyramid-like ZnO nanostructures, (b) pencil-like ZnO nanostructures, (c) truncated pencil-like ZnO nanostructures, (d) rodlike ZnO nanostructures. The error bars indicate the longer bond length and the shorter bond length on the (011) plane.

bonding between Zn and O at {011} surfaces is nearly equal to that in the bulk due to the small size of H+, leading to the 5509

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ZnO morphologies listed at the same horizontal line). When Zn−O bonds on both {100} surfaces and {011} elongate, the exposure of {011} surfaces increases (referring to the calculated ZnO morphologies listed at the diagonal).

formation of ZnO truncated pencils (thermodynamic morphology of ZnO). Our calculated results agree with these experimental observations. Figure 7 shows two kinds of morphology transformations by tailoring surface bonding. One kind is the morphology

4. CONCLUSION This work demonstrates that the anisotropic morphology of ZnO nanostructures can be tailored by modifying surface bonding lengths at the nanoregime. A relationship between bond length, crystal morphology, and particle size has been established to investigate the morphology transformation of ZnO nanostructures from nanorod to nanopyramid. Critical surface bond lengths at which ZnO can respectively exhibit pyramid-, truncated pencil-, pencil-, and rod-like morphologies were quantitatively calculated. We correlate ZnO anisotropic morphology and the particle size on the basis of Zn−O bond length. At the nanoregime, ZnO thermodynamically prefers to exhibit nanorods with diameters smaller than 3.60 nm, nanopyramids with size smaller than 2.94 nm, nanopencils with diameters larger than 3.15 nm, and truncated nanopencils with diameters larger than 3.92 nm, in agreement with experimental observations. The present study provides an insight into the morphology transformation of nanostructures from the viewpoint of surface bonding motif.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant Nos. 51125009, 50872016 and 20973033), the National Natural Science Foundation for Creative Research Group (Grant Nos. 20921002 and 21221061), and the Hundred Talents Program of the Chinese Academy of Sciences is acknowledged.

Figure 7. Morphology transformation of ZnO via tailoring surface bonding length. (a) Transformation between pencil-like and pyramidlike ZnO nanostructures. (b) Transformation between pencil-like and rod-like ZnO nanostructures.

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transformation from ZnO pencil-like nanostructures to pyramidlike nanostructures, and the other is from ZnO truncated pencillike nanostructures to rod-like nanostructures. As shown in Figure 7a, with increasing Zn−O bond length on {011} surfaces, ZnO nanostructures undergo the morphology evolution from nanopencils to nanopyramids (referring to the calculated ZnO morphologies listed at the same vertical line). With increasing Zn−O bond length on {100} surfaces, ZnO nanostructures undergo the transformation from nanopencils to nanopyramids (referring to the calculated ZnO morphologies listed at the same horizontal line). With increasing both Zn−O bond length on {100} surfaces and Zn−O bond length on {011}, the exposure of {100} surfaces diminishes (referring to the calculated ZnO morphologies listed at the diagonal). In Figure 7b, in the case of stretch of Zn−O bond length on (100) surface, ZnO undergoes the evolution from nanorods to truncated nanopencils and to nanopencils by increasing bond length on the (011) surface. Increased Zn−O bond length on the (011) surface can result in the larger exposure of {011} surfaces (referring to the calculated ZnO morphologies listed at the same vertical line), whereas increased Zn−O bond length on the (100) surface leads to the larger exposure of {100} surfaces (referring to the calculated

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