Films of Tunable ZnO Nanostructures Prepared by a Surfactant

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Films of Tunable ZnO Nanostructures Prepared by a Surfactant-Mediated Soft Synthesis Route Benoit P. Pichon,*,†,‡ Cedric Leuvrey,‡ Dris Ihiawakrim,‡ Didier Tichit,† and Corine Gerardin*,† †

Institut Charles Gerhardt de Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France ‡ Institut de Physique et de Chimie des Materiaux de Strasbourg, UMR 7504, CNRS-UdS-ECPM, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France ABSTRACT: Films of ZnO nanostructures were prepared by a soft chemical synthesis route from ZnO crystal seeds in aqueous medium, in the presence of alkylsulfates of different chain length acting as structure-directing agents. Films of arrayed single crystal ZnO nanorods were formed with short alkyl sulfates, from C6 to C8 alkylene chains, while hybrid lamellar ZnO with a platelike morphology were obtained with C10 to C18 alkyl sulfates. In the case of the short alkyl sulfates, due to the interaction between the sulfate groups and the Zn2+ planes of the ZnO structure, the growth along the c axis is partially inhibited and smaller aspect ratios of the nanorods are obtained than in alkylsulfate-free conditions. In the case of the hybrid lamellar ZnO structures which consist in ZnO layers intercalated with alkylsulfate bilayers, the structural characteristics depend on the alkylene chain length. Basal spacings increase linearly with the chain length, while the plate size decreases dramatically when the chain length exceeds C14. The different characteristics of these ZnO nanostructured films allow modifying their optical properties.

1. INTRODUCTION Films of ZnO nanostructures1 have become very attractive because of their high potential in a wide range of applications like photocatalysis, photovoltaics,2 4 optoelectronic devices,5,6 biosensors and gas sensors,7 9 and so on.10 13 Indeed ZnO is a wide band gap (3.37 eV) semiconductor having a high electron hole binding energy (60 meV), which has excellent electronic, optical, and piezoelectric properties and is also biocompatible, biodegradable, and nontoxic. The physical properties of materials are also highly dependent on the morphology and on the size of the nanostructures. Moreover, the combination of inorganic and organic substructures with different physical properties may present the advantage of superior performances. The fabrication of highly structured ZnO films can be achieved by the aqueous solution chemistry based route, which started to be developed as a simple low cost industrial processing and fits with large scale production and environmental respect. The colloidal wetting chemical synthesis was demonstrated to be efficient to control ZnO crystal growth under mild conditions by epitaxy on oriented substrates.14 It was extended to produce large arrays of well-crystallized and oriented ZnO nanostructures such as nanorods,15 18 nanotubes,19,20 nanocolumns,21 nanowires,22,23 and nanoplates.24 27 While arrays of 1D ZnO nanostructures have been widely reported, nanoplate arrays are very attractive because of their high surface to volume ratio which has been demonstrated to enhance properties such as photocatalytic activity.28 The controlled growth of the inorganic structures driven by organic molecules through physical and chemical interactions r 2011 American Chemical Society

represents an effective route to construct nanoscale materials with high surface to volume ratio. Complexing agents such as sodium citrate have been used to control the growth of ZnO nanoplates stacked as columns or grown onto a first array of nanorods.26 Vertical multilayer stacking nanoplates arrays with increasing roughness and porosity have been also produced.25 These morphologies result from the carboxylate groups, which interact with the ZnO surface and inhibit the longitudinal growth of rods but promote the growth of nanoplates. Arrays of nanoplates grown from a film of ZnO crystal seeds were also obtained in the presence of polymers.29 Up to now, most of these arrayed nanostructures consist in pure ZnO and therefore yield a limited surface to volume ratio. Although a few studies have recently reported on the directed growth of hybrid ZnO by using organic templates,30 32 the preparation of hybrid ZnO nanostructured films remains a significant challenge. Films of hybrid nanostructures which associate ZnO and surfactants have been mostly produced by an electrodepositing method.33 43 However, the simplicity of the soft chemical route for preparing films of lamellar ZnO nanostructures appears as a much more attractive approach. Recently, films of lamellar ZnO nanostructures were prepared at low processing temperatures by growth from ZnO nanocrystal seeds using an anionic surfactant, sodium dodecyl sulfate.44 The obtained nanostructures consist of plates Received: July 28, 2011 Revised: October 18, 2011 Published: October 19, 2011 23695

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The Journal of Physical Chemistry C of ZnO layers intercalated with bilayers of dodecyl sulfate anions. These results were an incitement to investigate more largely the synthesis of ZnO nanostructures by varying the properties of the anionic additive. Indeed, it could be expected that varying the surfactant properties would give rise to different characteristics of the hybrid surfactant/ZnO lamellar structures: size and morphology of the lamellar plates, thickness of the two substructures (ZnO layer and surfactant bilayer), and arrangement of the nanoplates in the film. For this purpose, alkyl sulfates of increasing alkylene chain lengths (from six carbon atoms, C6, to 18 carbon atoms, C18) were chosen to direct the structure of ZnO. Due to the well-known different behaviors of short and longer alkyl sulfates in solution, different structuring processes of ZnO films should be observed, since C6 and C8 alkyl sulfates behave as well soluble anions in water, while alkyl sulfates with more than 10 carbon atoms are surface active agents. Then, in this paper, the formation of films of ZnO nanostructures prepared with different alkyl sulfates has been examined. Depending on the hydrocarbon chain length of the alkyl sulfate, well-aligned nanorods or ordered lamellar hybrid structures were obtained. A main goal is then to give some new insights on the phenomena involved in the structuring processes of the different nanostructures, taking into account the properties of the alkyl sulfates. Moreover, the optical properties of the different ZnO nanostructures have been investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. Starting materials used in this investigation were analytical grade and used as received without further purification. Zinc nitrate hexahydrate Zn(NO3)2 3 6H2O and hexamethylenetetramine (HMTA) were purchased from Merck. Sodium akyl sulfate salts CnH2n+1SO4Na with different alkylene chain lengths (n = 6, 8, 10, 12, 14, 16, 18) were named as Cn with n referring to the number of carbon atoms. These surfactants were purchased from Aldrich. They were used to control the synthesis of the hybrid ZnO plates HZCn from the film of nanoparticles and the corresponding HZCns powders. Water was deionized water and had a specific resistivity exceeding 5 MΩ. 2.2. Synthesis. ZnO objects were grown onto a film of ZnO nanocrystal seeds ZnOF which was deposited on a glass substrate by dip coating from a colloidal solution and followed by annealing at 550
C as previously reported.44 The thickness of the film of ZnO nanoparticles was 500 700 nm. In a typical procedure, 24 mL of a 2.5  10 2 M equimolar solution of Zn(NO3)2 3 6H2O and HMTA in water were added to a vial of 30 mL. Then an aqueous solution (1 mL) of surfactant with a molar ratio Cn/Zn2+ of 1/1 was added in and stirred for 10 min. Subsequently, ZnOF substrates were placed upside down in the vial. The system was placed in an oven for 15 h at 90
C. The substrate HZCn was then rinsed with water and dried at 90
C for 15 h. The colloidal suspension and the precipitate formed in solution were also rinsed with water and centrifugated before drying. White powders of HZCns samples were obtained in addition to substrates. 2.3. Characterization. The morphologies of the HZCn nanostructures grown on substrates were observed by scanning electron microscopy (SEM) using a JEOL electron microscope 6700 equipped with a field emission gun operating at 3 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded with a TOPCON model

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002B transmission electron microscope coupled with energy dispersive X-ray (EDX) spectroscopy, operating at 200 kV, with a point-to-point resolution of 0.18 nm. Electron diffraction was performed at a camera length of 66 cm. X-ray diffraction (XRD) data were collected at room temperature using a D5000 Siemens diffractometer in the Bragg Brentano geometry (Cu Kα1, λ = 1.540598 Å). The coherence length of the lamellar structure was estimated using the Debye Scherer equation, Δ(2θ) = 0.9λ/(L cos(2θ)). L is the full half-maximum width (fwhm) of the first Bragg peak at low 2θ angle which corresponds to the first reflection on the basal planes of the lamellar structure. FT-IR spectra were collected on a Digilab FTS 3000 computer-driven instrument (0.1 mm thick powder samples in KBr). All spectra were normalized according to the νZn O absorption band at 470 and 436 cm 1. UV vis studies were performed on a Shimadzu UV3600 spectrometer in the transmission mode with a resolution of 4 nm and a sampling rate of 300 nm 3 min 1.

3. RESULTS AND DISCUSSION ZnO nanostructures were grown following two steps according to the synthesis method that was reported previously.44 First, several films of ZnO crystals seeds were prepared by dipping glass plates in an alcoholic solution of zinc acetate and diethanolamine followed by heating at 500
C with a heating rate of 5
C/min. In a second step, ZnO nanostructures were grown onto such films of ZnO nanocrystal seeds. The growth of the nanostructures was directed by the addition of different alkyl sulfates, which act as structuring agents in the aqueous solution. Alkyl sulfates with different alkylene chain lengths were used to control the formation of specific assemblies. They differ by their solubility and the longer ones present different critical micelle concentrations (cmc's). The smallest ones with C6 and C8 short alkylene chain lengths were found to yield ZnO structures, which are different from those obtained with alkyl sulfates of longer alkylene chains (C10 18). Controlled Growth of Nanorods in the Presence of C6 8 Alkyl Sulfates. The morphology and the orientation of ZnO

nanostructures grown onto the film of ZnO nanocrystal seeds were studied by SEM (Figure 1). First, a sample HZC0 was prepared without using any alkyl sulfate and was considered as a reference (Figure 1a,b). Well-aligned hexagonal nanorods grown preferentially in a direction normal to the film of ZnO nanoparticles are observed. These nanorods are featured by a narrow size distribution, which is centered at a diameter of 80 ( 5 nm. The reaction performed in the presence of alkyl sulfate C6 and C8 also led to nanorods (Figure 1c f). However, an increase of the diameter up to 400 nm and a reduction in height (1.5 1 μm) were observed when increasing the length of the alkylene chain. A closer observation shows the broadening of the diameter distribution and an increasing proportion of interconnected nanorods. Therefore, the presence of short alkyl sulfates has an influence on the growth of nanorods which, in addition, depends on the length of the alkylene chain. The structures of the HZC0, HZC6, and HZC8 films were investigated by XRD (Figure 2). The patterns exhibit strong and narrow peaks consistent with the high crystallinity of these samples. No impurities were observed, and all diffraction peaks can be indexed to the hexagonal phase of ZnO (wurtzite-type) with lattice constants a = 3.25032 Å and c = 5.20661 Å, which agree well with the reported values (a = 3.24982 Å and c = 5.20661 Å) from JCPDS card no. 36-1451. In addition, the higher 23696

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Figure 2. XRD patterns of ZnO nanorods HZC0, HCZ6, and HCZ8 grown onto the film of ZnO nanocrystal seeds.

Figure 1. SEM pictures of nanorods grown onto the film of ZnO nanocrystal seeds: top views (a, c, e) and side views (b, d, f); blank, without surfactant HZC0 (a, b), HZC6 (c, d), HZC8 (e, f). Scale bars are 500 nm.

relative intensities of the (002) diffraction peaks illustrate the anisotropic shape and selective orientation of the nanorods along the c axis. These observations are in agreement with a preferential growing of the ZnO nanorods in the direction normal to the film of ZnO nanocrystal seeds. Nevertheless, the presence of weakly intense (100) and (101) reflections shows that nanorods are not perfectly perpendicular to the film because of the lack of selective crystallographic orientation of ZnO crystal seeds.44 More details on morphological and structural features are given in Figure 3 using TEM. Individual nanorods grown from the film of nanoparticles were collected by scratching sample HZC8. A close observation of a typical low-magnification TEM image (Figure 3a) shows that both extremities of the nanorods are different. Arrows indicate the roughened extremities that are attached to ZnO nanocrystal seeds. The other rounded extremities correspond to those that were exposed to the reaction medium. Nanorods are featured by diameters from 80 to 180 nm and lengths up to 1 μm. High-resolution TEM images provide information on the atomic arrangements of the ZnO nanorods (Figure 3b). Lattice fringes oriented perpendicular to the rod axis are observed, and the average spacing was found to be about 5.2 Å corresponding to Æ00.1æ direction of ZnO crystal. In addition, selected area electron diffraction (SAED) patterns show spots (Figure 3c), which can be indexed to the crystallographic planes of the ZnO structure. Moreover, the zone axis deduced from this picture corresponds to the (01.0) plane, which is associated to the lateral side of the nanorods. These results confirm that nanorods are single crystals with specific orientations along the c axis. Arrays of Hybrid ZnO Lamellar Nanostructures Prepared by Using C10 18 Alkyl Sulfates. As it was already noticed, the

nature of the surfactant plays an important role in controlling the morphology and the structure of ZnO.44 Moreover, the alkylene chain length of the alkyl sulfates seems to be a determining parameter. Nanorods grew from the film of ZnO nanocrystal seeds when alkylene chains of less than eight carbon atoms were used. When the number of carbon atoms increases from 10 to 18, ZnO nanostructures still grew from the film but their morphologies dramatically changed and nanoplates emerged (Figure 4). Figure 4a reveals small plates of about 90 nm in size which grew from the film of ZnO nanocrystal seeds in the presence of surfactant C10. In contrast, sodium dodecyl sulfate (SDS) surfactant (C12) induced the growth of well-defined and much bigger plates over large areas (Figure 4b). The plates are quite uniform in size, with a length of up to 20 μm and a width up to 5 μm giving an aspect ratio (l/w) of about 4. C14 (Figure 4c) and C16 (Figure 4d) surfactants both induced rectangular plates similar to those with the C12 surfactant but with different aspect ratios of 5 and 2, respectively. A closer observation of sample HCZ16 shows that large plates are replaced by smaller ones of approximately 100 nm (inset, Figure 4d). A cross-sectional SEM view of the array of HZC16 shows that nanoplates are clearly grown from ZnO nanoparticles (Figure 4f). In addition, all plates have a thickness that amounts to 100 ( 5 nm. However, in the presence of C18, only small plates were grown from the ZnO nanoparticle seeds (Figure 4e). The structure of the large plates grown in the presence of surfactants C12 and C16 was studied by TEM. The nanoplates consist in well-ordered lamellar structures exhibiting interlayer spacings of 31 ( 2 and 40 ( 2 Å for C12 and C16, respectively (Figure 5a,b). These features suggest the presence of alternative layers of ZnO and surfactants. HRTEM images indicate that the thickness of the inorganic layers is approximately 13 ( 2 Å for all plates and is not affected by the nature of the surfactant. No electron diffraction pattern could be recorded, certainly due to the low relative amount of inorganic substructure in these materials. XRD patterns of the nanoplates grown from the ZnO nanocrystal seeds revealed interesting features (Figure 6). Sample HZC10 presents Bragg peaks above 30
(2θ), which can all be 23697

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Figure 3. (a) TEM, (b) high-resolution TEM images, and (c) electron diffraction pattern of nanorods HZC8 grown onto the film of ZnO nanoparticles (arrows). The circle in the electron diffraction pattern is due to the carbon-coated membrane of the TEM grid.

Figure 4. SEM pictures of plates grown from the film of ZnO nanocrystal seeds: (a) HZC10; (b) HZC12; (c) HZC14; (d, f) HZC16; (e) HZC18.

Figure 5. TEM images of lamellar structured hybrid ZnO nanoplates of (a) HZC12 and (b) HZC16 grown onto the film of ZnO nanocrystal seeds.

indexed to the ZnO structure, but does not present any low angle peak. In contrast, the XRD patterns of HZC12 18 exhibit intense Bragg peaks, between 2
(2θ) and 25
(2θ) that were clearly associated to the basal planes of a lamellar structure. This is in agreement with the observations by TEM. The first (00l) peaks at 2.97, 2.81, 2.47, and 2.40
2θ for HC12, HC14, HC16, and HC18, respectively, correspond to basal spacings of 29.7, 31.4, 35.7, and 36.8 Å. The presence of several well-defined (00l)

reflections giving up to nine harmonics is indicative of the longrange order of the lamellar structure in these films as previously observed in TEM images, where they give similar basal spacings. A straight correlation is observed between the basal spacings and the number of carbon atoms in the alkylene chain of the surfactants as shown in Figure 6c. This suggests the alternative arrangement of surfactants and ZnO layers. The extrapolation to zero carbon atoms of the curve obtained in Figure 6c gives a value of about 14 Å for the thickness of the inorganic layer and the remaining sulfate headgroup of the surfactants. This leads to a thickness of the inorganic layer which is consistent with the value measured on TEM images (13 ( 2 Å). Subtracting this value from the basal spacings allows calculating the thickness of the organic layers which increases from 15 to 22 Å when changing from C12 to C18 alkylene chains. These values are smaller than the length of two surfactants C12 18 arranged tail to tail in their extended conformation. It shows that the surfactant chains are interdigitated or tilted with respect to the direction normal to the inorganic layers (Scheme 1). In addition, the lamellar structures prepared from HZC12 16 surfactants are featured by coherence lengths of 88.2, 80.2, and 81.0 nm, respectively, which are in the same range of the plate thickness (≈ 90 nm) observed in TEM images. In contrast, the plate thickness value (63.6 nm) is lower for HZC18 and is correlated with the smaller size of the 23698

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Figure 6. (a) XRD patterns of the hybrid plates HZC10 18 grown from the film of ZnO nanocrystal seeds. (b) High 2Θ angle region corresponding to the ZnO structure. (c) Basal spacing corresponding to the (001) reflection of the lamellar structure plotted against the number of carbon atoms of the alkylene chains.

nanoplates. Moreover, at 2θ values higher than 30
, several peaks corresponding to the ZnO hexagonal structure of the nanoplates were observed (Figure 6b). Their broadening with the increase of the hydrocarbon chain length can be correlated to crystallites with smaller sizes. With the aim to study separately the composition of nanorods and plates, EDX spectrometry associated to TEM was performed on the nanostructures scratched from the film grown onto ZnO nanocrystal seeds (Figure 7). HZC6 nanorods are only composed of zinc and oxygen (O 55%, Zn 44%) with little excess of oxygen in the range of the experimental error. No sulfur is detected, which shows that very low amounts of surfactant are adsorbed. In contrast, HZC10 and HZC16 samples contain

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significant amounts of sulfur in addition to zinc and oxygen. It shows unambiguously the presence of surfactants possessing sulfate headgroups. Moreover, the HZC10 nanostructure presents a lower S/Zn atomic ratio (0.21) than that of the HZC16 structure (0.39). This agrees with a higher amount of surfactants intercalated in the hybrid HZC16 structure, than that adsorbed on the surface of HZC10. Infrared spectroscopy provides more precise information on the composition of the nanoplates and also on the interactions between the organic and inorganic phases (Figure 8). Analyses were performed on nanoplates HZCn grown onto ZnO nanocrystal seeds. FT-IR spectra clearly indicate the presence of both organic and inorganic substructures. All spectra corresponding to HZC12 18 show νZn O absorption bands at 470 and 436 cm 1, which are indicative of ZnO layers. In addition, the presence of surfactants is depicted by strong and characteristic vibration bands at higher wavenumbers. The asymmetric (νas(OSO3)) and symmetric (νs(OSO3)) stretching vibrations at 1355 1135 and ∼1065 cm 1, respectively, are indicative of the sulfate headgroups. FT-IR also provides information on the interactions between the surfactant and the ZnO layer. For all samples HZC12 18, the comparison with pure SDS molecules in the solid state clearly shows a shift toward higher frequencies of about 15 26 and 15 21 cm 1 for the νas(OSO3) and νs(OSO3) modes, respectively (Table 1). These observations confirm the bonding of the sulfates to the ZnO layer surfaces via electrostatic interactions. The hydrocarbon chains are featured by the asymmetric and symmetric C H stretching vibrations, which correspond to bands located at 2960 2850 cm 1. The increasing intensity of the C H bands with respect to the OSO3 bands reveals the relative increase of the surfactant alkylene chain length. Moreover, the asymmetric CH2 vibration (νas(CH2)) band at 2921 2917 cm 1 is associated to the all-trans conformation of the alkylene chain and is present in all compounds.45 In contrast, the contribution at ≈2932 cm 1, which corresponds to the less ordered gauche trans conformation is not observed. This is consistent with the presence of δ(CH2) scissoring mode at 1467 cm 1 that is typical of an all-trans methylene chain suggesting that the alkyl chain is long enough to be organized in an ordered assembly.46 In addition, FT-IR peak absorption frequencies and widths of the νas(CH2) band can also be used for the evaluation of the degree of order of the different coatings.47,48 The comparison of the νas(CH2) frequency in spectra of ZnO nanostructures with the one of free SDS molecules in the solid state showed a shift of about 2 4 cm 1 to higher frequencies (Table 1). The broadening of the bandwidth is about 23 cm 1 for all ZnO nanostructures, whereas it is of 31 cm 1 for solid SDS salt. Therefore, both shift and broadening can be interpreted as the signature of the fact that the association of both organic and inorganic substructures results in a more ordered organization of the surfactant molecules than in pure SDS salt. All surfactant molecules behave similarly in the hybrid structures and are close packed and well ordered. Hydroxyl groups and water molecules are evidenced by the broad bands observed at high frequency (3690 3035 cm 1). A relatively narrow band is observed at 3466 cm 1 in the SDS spectrum, as well as in all hybrid ZnO samples (3480 cm 1), although it is shifted and broader. The broadening is due to hydrogen bond formation between the surfactant and the inorganic layer and to intercalated water.49 In addition, a second contribution is observed at 3345 cm 1, which may be attributed to adsorbed hydroxo and aquo groups at the surface of the inorganic layer. 23699

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Scheme 1. Computational Model of ZnO Crystal Structure: (a) View along the c Axis, (b) Top View of the (00.1) Plane, and (c) Growth of ZnO Hexagonal Crystal in the Presence of Surfactant

Figure 7. EDX spectra of (a) HZC6, (b) HZC10, and (c) HZC16 grown in the solution. The carbon signal partly originated from the sample holder.

Figure 8. IR-TF spectra of hybrid ZnO nanoplates grown from the film of nanocrystal seeds: (a) expansion from 3900 to 2500 cm 1; (b) expansion from 1800 to 400 cm 1.

The IR-TF spectrum of HZC10 differs from the others, although vibration modes corresponding to the C10 surfactant are clearly observed. The different bands observed in the range 500 400 and 3600 3300 cm 1 corresponding to the Zn O and O H vibration modes, respectively, suggest a different structure of the inorganic substructure. In addition, the bands corresponding to the alkylene chain (2950 2850 and 1460 cm 1) and to the sulfate headgroups (1250 1150 cm 1) are very weak. It can be related to the low amount of surfactant as observed by EDX and agrees with the absence of the lamellar structure. Growth Mechanisms of ZnO Nanostructures. The microstructural features previously reported yield important information on the mechanisms of the ZnO growth process. The growth

of specific ZnO 1D or 2D nanostructures can be controlled very simply by tuning the length of the alkylene chain. Such behavior is directed by the structural characteristics of ZnO and the interactions between surfactants and the crystal upon the growth process. The spontaneous growth of ZnO without any organic additives leads to the thermodynamically stable wurtzite (a = 3.25 Å and c = 5.21 Å) corresponding to the hexagonal crystal system with the P63mc space group.50,51 It results in a polar and ionic structure of alternating planes composed of 4-fold tetrahedrally coordinated O2 and Zn2+ ions stacked alternatively along the c axis (Scheme 1a). Indeed, the usual crystal habit consists in a basal negatively charged polar oxygen (00. 1) face, a top 23700

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Table 1. Absorption Frequencies for Free SDS Molecules and HZCn Samples Measured by FT-IR Spectroscopy absorption intensities (cm 1) vibration mode OH

SDS 3466

HZC12

HZC14

HZC16

HZC18

3480

3445

3488

3483

3349

3340

3355

3345

asCH3

2956

2956

2956

2956

2955

asCH2

2917

2921

2919

2918

2921

sCH3

2873

2873

2872

2873

2872

sCH3

2850

2852

2851

2850

2851

1250 1219

1231 1193

1235 1197

1127 1193

1229 1193

1082

1061

1067

1067

1061

asOSO3 sOSO3

Scheme 2. Growth of ZnO Nanostructures: (a) Nanorods without Additive; (b) Nanorods in the Presence of Alkyl Sulfates C6 and C8, and (c) Lamellar Nanostructures in the Presence of Alkyl Sulfates C12-18

tetrahedron corner-exposed positively charged polar zinc (001) face and low-index nonpolar (10.0) and (10.1) faces (and other C6v symmetric ones) which are parallel to the c axis. In addition, the reactivity of ZnO crystal upon growth is dictated by the polar faces which are metastable, whereas the nonpolar faces are the most stable ones. The wurtzite crystal structure has no center of inversion associated to an inherent asymmetry along the c axis and results in an anisotropic growth of the crystal along this direction. According to the different velocities of crystal growth in different directions [10.0] > [10.1] > [00.1] and [00. 1],50 the theoretical and most stable crystal habit is a hexagon elongated along the c axis such as nanorods. Influence of the Alkyl Chain Length of Alkyl Sulfates on the ZnO Crystal Growth. In the absence of alkyl sulfates, wide arrays of nanorods were grown in a direction perpendicular to the film of ZnO crystal seeds (Scheme 2a). As demonstrated by XRD, all nanorods expose their (00.1) face corresponding to their top extremity. When no additive is used, growth occurs spontaneously. In the presence of alkyl sulfates C6 and C8, the growth of nanorods is modified because of their adsorption through electrostatic interactions onto the (00.1) face, which has a higher energy than the lateral and nonpolar faces. This behavior

can be attributed to the different charge of the zinc (00.1) plane and the anionic sulfate headgroup. Therefore, the presence of short alkyl sulfates partially inhibits the growth of ZnO along the c axis (Scheme 1b). Both alkyl sulfates C6 and C8 exhibit no cmc and do not form spontaneous molecular assemblies in pure aqueous solution. Nevertheless, their concentration tends to increase locally when they adsorb onto crystal surfaces.52 Their high solubility in aqueous medium induces equilibrium between surfactants adsorbed onto the surface and those free in solution. It explains the partial growth inhibition of the nanorods. In addition, a slight increase of the length of the alkylene chain (C6 to C8) yields nanorods with a lower aspect ratio. It results from a lower solubility of the surfactants in the reaction medium, which strongly adsorb onto the crystals and act as a more efficient growth inhibitor. The growth along the c axis occurs more slowly, and the expansion along the perpendicular directions as [100] and [101] results in nanorods with broader diameter and shorter lengths (Scheme 2b). As shown by SEM images, the addition of alkyl sulfates C6 and C8 does not significantly influence the density in nanorods grown from the crystal seeds. Thus, in the presence of short alkyl sulfates, the enlarged nanorods get very 23701

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Figure 9. UV vis absorption spectra of the film of ZnO nanocrystal seeds HCZ0 and of the ZnO nanostructures HCZn.

close from each other, whereas, without surfactant, they were all individual and grew separately. Surfactants with C10 to C18 alkylene chains have a dramatic influence on ZnO growth. Their use results in platelike morphologies with lamellar structures described as assemblies of ZnO and surfactants. As reported recently, more efficient complexing agents with lower solubility29,53 or at higher concentration24 in water influence the growth in a continuous film of ZnO onto the film of nanocrystal seeds. This suggests that C10 to C18 surfactants inhibit much more the crystal growth along the c axis than alkyl sulfates C6 and C8. These surfactants are expected to form molecular assemblies as micelles that direct the growth of ZnO into hybrid structures (Scheme 1c). The growth process of such hybrid materials is obviously different from the one of nanorods. In contrast to the case of alkyl sulfates C6 to C8, C12 to C18 surfactants which are featured by a cmc have the ability to form molecular assemblies such as bilayers and to adsorb onto the (00.1) plane of the ZnO crystal seeds. The alkylene chains provide hydrophobic interactions which favor a long-range organization, as shown by XRD and FT-IR. Therefore, the anionic headgroups of the second layer are exposed to the solution and interact with polycationic zinc oxide species and surfactant Zn based hybrid assemblies may already form in solution (scheme 2c). As suggested by FT-IR spectra, electrostatic interactions between surfactants and Zn polycationic species may direct the growth of the hybrid structure. Double side functionalized anionic bilayers clearly inhibit the growth of ZnO as well as promote the nucleation of a new layer of ZnO to form a lamellar structure. In addition, tuning the length of the alkylene chains influences the basal spacing of the structure. The length of the alkylene chain also influences the morphology of the plates grown onto the film of ZnO nanoparticles and the surface covered by the plates. Indeed the cmc and the solubility of surfactants are closely related to the length of the alkylene chains.54 C12 and C14 surfactants are the most efficient surfactants to form lamellar structures and to induce a full coverage of the substrate with large nanoplates. They assemble easily in bilayers at concentrations higher than their cmc, which are 5.0  10 3 and 6.5  10 4 M at 23
C, respectively. In contrast, the C16 surfactant leads to a significant decrease of the amount of large nanoplates. Indeed, smaller nanoplates of about 100 nm mostly cover the surface of the ZnO crystal seeds film. It may result from the lower cmc (8.0  10 5 M) and solubility of these surfactants which modifies the growth mechanism of ZnO. In the case of the C18 surfactant, the surface is only covered by very small

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plates. The nonobservation of nanorods or lamellar nanostructures in the presence of surfactant C10 is an interesting point. Indeed the concentration in C10 surfactant (2.5  10 2 M) is very close to its cmc at 23
C (3.1  10 2).55 However, because the reaction occurs at 90
C, we expect the cmc to increase and the concentration may not be sufficient to allow the assembling of C10 surfactants. Therefore, they behave as complexing agents that strongly inhibit the growth of ZnO from the seeds. Study of the Optical Properties of the Films as a Function of the ZnO Structure. The ZnO nanostructures HCZn grown from the film of ZnO nanocrystal seeds ZnOF display interesting structure dependent optical properties which were characterized by using UV vis spectroscopy (Figure 9). The optical absorption spectrum of ZnOF was recorded as a reference. A sharp absorption edge at λ = 368 nm typical of ZnO nanocrystal seeds was observed.56 It is ascribed to the semiconductor properties of ZnO and consists in the electron transfer from the valence band to the conduction band featured by a band gap energy of 3.37 eV. All optical absorption spectra exhibit an intrinsic band with similar intensity below λ = 390 nm. Nevertheless, films that consist in nanostructures grown from the ZnO crystal seeds present some differences in absorption at higher wavelengths, which can be related to the grown nanostructures. While a film of ZnO crystal seeds is transparent in the visible region, nanorod arrays HZCO grown without any surfactant show a strong band edge absorption in the region above 390 nm. The increase in absorption originates from the array of nanorods and is a function of their length. This has been observed for films of ZnO aggregates.57,58 It corresponds to partial light scattering induced by the free space between nanorods, which results in a lower transmittance of the film and partial absorption. Hybrid nanostructures also modify the optical properties in this region. Films made of large hybrid lamellar plates (HCZ12 16) display intermediate absorption, while small plates (HCZ10 and HCZ18) have similar optical properties to the film of ZnO nanocrystal seeds. These observations imply that the variation in optical absorption originates from the film thickness, which is directly correlated to the size and the density of the hybrid plates. Large hybrid plates may also attenuate the transport of light through the film by light scattering.57,58 This phenomenon can also be explained by the hybrid inorganic organic structure, which implies the chemical binding of surfactants at the surface of the inorganic layers that results in some modifications of the electron transfer at the ZnO/surfactant interface.

4. CONCLUSION In summary, by changing the alkylene chain length of alkyl sulfates from C6 to C18 carbon atoms, two different types of ZnO based nanostructures were formed by growth onto films of ZnO nanocrystal seeds. Uniform and dense arrayed ZnO nanostructures with single crystalline nanorods were obtained in the case of the shortest alkyl sulfates (C6 and C8), while highly ordered hybrid ZnO/alkyl sulfate lamellar structures were formed when alkyl sulfates with more than 10 carbon atoms were used. Comparatively to the synthesis in alkyl sulfate-free conditions, the addition of the short alkyl sulfates leads to a decrease of the aspect ratio of the nanorods, a broadening of their diameter distribution, and the appearance of interconnections between nanorods. Two concurrent features account for this behavior: the high solubility of the short alkyl sulfates in water and the strong electrostatic interaction between the anionic 23702

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The Journal of Physical Chemistry C sulfate headgroup and the Zn2+ plane, which concur to inhibit the growth along the c direction. In the case of the long alkyl sulfates, the obtained lamellar hybrid structures exhibit a linear relationship between the basal spacing, varying between about 30 and 37 Å, and the increase of alkylene chain length. It was also shown that the size of the plates depends on the carbon chain length: while the plate size is very large in the case of C12 and C14 alkyl sulfates (plate length up to 20 μm), it dramatically decreases in the case of C16 and C18 (∼100 nm). These latter results are consistent with the self-assembly behavior in solution of the longer alkyl sulfates, and account for the different cmc and solubility in water of the surfactants. Finally, the control of the type of the ZnO nanostructure (nanorods or hybrid lamellar structures) and of their characteristic sizes allows tuning the optical properties of the films. In conclusion, this very simple soft chemical synthesis route using different anionic additives appears to be very promising for the preparation of ZnO thin films with tunable physical properties depending on the nanostructure.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

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