J. Phys. Chem. B 2005, 109, 2881-2884
2881
Electrochemical Self-Assembly of Highly Oriented ZnO-Surfactant Hybrid Multilayers Huai-Yu Jing, Xiao-Long Li, Ying Lu, Zhen-Hong Mai, and Ming Li* Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China ReceiVed: September 15, 2004; In Final Form: NoVember 23, 2004
Highly oriented zinc oxide-surfactant hybrid multilayers were electrochemically self-assembled on silicon substrates from Zn(NO3)2 solutions containing extremely low concentration of sodium dodecyl sulfate (SDS). The X-ray diffraction results showed that the structure of the hybrid film is sensitive to the concentration of SDS. When the concentration of SDS is below a critical value, 0.002 wt %, a surfactant bilayer is adsorbed on the silicon surface, together with electrodeposited crystalline ZnO particles. Above this concentration, lamellar ZnO-surfactant hybrid films are formed, the period of which decreases from 31.7 ( 0.2 Å at 0.003 wt % to 27.5 ( 0.2 Å at 0.005 wt %, another critical concentration. It then increases monotonically and reaches its maximum of 33.0 ( 0.2 Å above 0.05 wt %. The results implied that the kinetics of the electrochemical self-assembly depends on the relative speed of the reduction of the zinc ions and the aggregation of the surfactant. The two processes occur cooperatively at the electrolyte-electrode interface to form the hybrid films.
I. Introduction There has been growing interest in inorganic/organic hybrid multilayers due to their fascinating physical properties and potential applications.1-3 Numerous inorganic-organic hybrid films with nanometer scale periodicity have been produced by chemical self-assembly approaches. For example, various silica-surfactant hybrid films were fabricated by evaporationinduced self-assembly.4,5 Langmuir-Blodgett (LB) films containing nickel nanoparticles were synthesized by reducing the nickel stearate LB films in NaBH4 solutions.6 Nanoscale composites of polyaniline and vanadium oxide were assembled via the electrostatic layer-by-layer (LBL) technique.7 Recently a new method, i.e., electrochemical self-assembly,8 has been developed to produce nanostructured hybrid films. The basic idea is to use the interfacial potential to control the molecular assembly. The surface of the working electrode serves as the solid-liquid interface for the aggregation of the reagents. Incorporation of the surfactant layers into the deposited film is realized when the potential applied to the electrode can simultaneously induce a desired phase of surfactant-inorganic aggregates and reduce the metal ions.9 Hybrid films can be fabricated by controlling the assembly of surfactant and metal ions in a very dilute solution, even below the critical micelle concentration of the surfactant. In the past two years, the electrochemical self-assembly method has been used to produce several hybrid films that contain, for instance, ZnO, WO3, and Pt sublayers.8-10 But the quality of the structures is still to be improved. The films were reported to be a mixture of two different lamellar phases due to two slightly different pathways to form stable surfactant bilayers.8 Moreover, the films were not oriented with the interfaces parallel to the substrate. Undoubtedly, ordering and orientation are prerequisites to large scale applications of the organic-inorganic hybrid films in microelectronic industries. * To whom correspondence should be addressed. E-mail: mingli@ aphy.iphy.ac.cn.
We found that well ordered and highly oriented hybrid multilayers can be synthesized on ultra smooth surfaces in the electrochemical self-assembly process. We used silicon wafers as the working electrodes in this work. This is of importance for their potential applications in the microelectronic industry in which the silicon technique has become very mature. The results showed that the smooth surface of the silicon substrate helps the surfactant and metal ions to assemble at the electrode surface equably and uniformly. It also helps the stacking directions of the layers to orient parallel to the surface. To understand the mechanism of the electrochemical selfassembly process, we performed detailed X-ray diffraction analysis on the samples to monitor the structure of the films fabricated under different conditions. The concentrations of SDS in this work were chosen to be much lower than the ones in the previous reports.8-10 This enabled us to have a deeper insight into the mechanism of the electrochemical self-assembly process. The analysis showed that the period of the lamellar structure varied significantly with the concentration of SDS, and there were two critical transition concentrations in the selfassembly process. In contrast, the concentrations of SDS in the previous reports were higher than the surface micelle concentration (smc)9 of SDS such that the surface-induced self-assembly was not influenced by the variation of the SDS concentration. II. Experimental Details Zn(NO3)2‚6H2O and an anionic surfactant, sodium dodecyl sulfate (SDS), were analytical reagent grade and used as supplied. A Milli-Q water system (Millipore Corp.) was used to produce purified water with a resistance of 18 MΩ cm for all the experiments. P-type, (100)-oriented silicon wafers with resistance of 10 Ω cm were used as the cathodes, and a polished platinum sheet was used as the anode. The electrical contact was realized by painting conducting silver paste on the rear side of the wafer. The wafers were ultrasonically cleaned with acetone and ethanol. They were then boiled in a mixture of H2O: H2O2:H2SO4 ) 5:1:1 (by volume) for 15 min and boiled again
10.1021/jp0458351 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/21/2005
2882 J. Phys. Chem. B, Vol. 109, No. 7, 2005 in a mixture of H2O:H2O2:NH3‚H2O ) 5:1:1 (by volume) for 15 min. The wafers were finally etched for 1 min with a dilute HF solution (HF:H2O ) 1:1 by volume) to remove the native oxide, rinsed with deionized water, and dried in a stream of nitrogen before use. The electrodeposition was performed on a CHI-660 electrochemical station, using a conventional three-electrode cell with a saturated calomel electrode as the reference electrode. The ZnO-surfactant hybrid films were cathodically deposited from 0.02 M Zn(NO3)2 solutions containing various weight percent (from 0.001 to 0.1 wt %) of SDS. The depositions were carried out potentiostatically at -0.9 V against the reference electrode without stirring for 600 s. The electrolytic solution was kept at 60 °C during the experiments. ZnO can be deposited from a nitrate bath by reduction of nitrate ions.11,12 Electroreduction of nitrate to nitrite ions produces hydroxide ions and increases of concentration of OH- at the cathode, resulting in the formation of ZnO. The expression of the reaction is Zn2++ NO3- +2e - f ZnO + NO2-. The lamellar structure of the multilayers was examined by low-angle X-ray diffraction on a Bruker D8-Advance diffractometer equipped with a Goebel mirror to get parallel X-ray beams and to suppress the Cu Kβ radiation. The rocking curve around the Bragg points was used to characterize the degree of orientation of the multilayers. The crystallographic structure of the films was studied by glancing angle X-ray diffraction (GAXRD), in which the incident angle between the X-ray beam and the sample surface was fixed at 1° while the intensity of the scattered X-rays was monitored as a function of 2θ. The surface morphology of the films was imaged by a Hitachi S-4200 scanning electron microscope (SEM). The X-ray photoelectron spectroscopy (XPS) was performed using the PHI5300/ESCA surface analysis system to investigate the composition of the inorganic layer in the films. An Al KR X-ray excitation source (photo energy, 1486.6 eV) was used at a takeoff angle of 90° with respect to the film surface normal. The position of the C1s peak was taken as a standard (with a banding energy of 285.0 eV). III. Results and Discussion XPS was used to estimate the chemical composition in the multilayer. The results indicated that the hybrid films were mainly composed of Zn, O, C, and S. Subtracting the part of O associated with SDS (C12H25SO4Na), the estimated atom ratio of Zn to O is approximately 1:1. The banding energy of Zn 2P3/2 is at 1021.8 eV and the Auger parameter is 2009.5 eV, coinciding with that of ZnO.13 All these results demonstrate that the composition of the inorganic layer is ZnO. The X-ray reflectivity (XRR) patterns of the multilayers are shown in Figure 1. The films exhibit well-defined lamellar structure when the concentration of SDS is higher than 0.002 wt %. The films are all highly oriented with their interfaces parallel to the surface of the silicon. This is evidenced by the rocking curves measured at the second Bragg peaks of the samples (Figure 2). The half-width at half-maximum of the second Bragg peak is about 0.05°, only slightly wider than the instrumental resolution of 0.03°. The interlayer spacing, i.e., the period of the lamellar structure, can be calculated from the position of the Bragg peak. It decreases from 31.7 ( 0.2 to 27.5 ( 0.2 Å as the concentration of SDS increases from 0.002 to 0.005 wt %. When the concentration of SDS is below 0.002 wt %, one does not see any Bragg peaks. But the reflectivity curves show features of a bilayer on the substrate. It is observed that 0.002 wt % is a critical concentration for the formation of the periodic lamellar structures. The X-ray
Jing et al.
Figure 1. XRR pattern of the hybrid multilayers fabricated with various concentrations of SDS: (a) 0.001 wt %, (b) 0.002 wt %, (c) 0.003 wt %, (d) 0.004 wt %, (e) 0.005 wt %, (f) 0.01 wt %, (g) 0.05 wt %, and (h) 0.1 wt %. Inset: The XRR patterns of the multilayers deposited at the nominal SDS concentration of 0.002 wt %.
Figure 2. Rocking scans around the second Bragg peak of the films deposited at various concentration of SDS: (a) 0.05 wt %, (b) 0.005 wt %, and (c) 0.003 wt %.
reflectivity of the sample is very sensitive to the concentration of SDS near this critical concentration (see the inset in Figure 1). Because it is hard to control precisely the concentration of the solution, the XRR curves show evidence of either a bilayer or a periodic structure near the nominal SDS concentration of 0.002 wt %. Away from this value, the structures obtained are quite repeatable. The transition from a bilayer to a multilayered stack is also conformed by the surface morphology of the sample grown at very low concentration of SDS. For example, the SEM images of the surface morphology of the films deposited with 0.001 and 0.05 wt % SDS are compared in Figure 3. When the concentration of SDS is below 0.002 wt %, the surface of the film is covered with many islands. But the surfaces of the multilayers deposited with higher concentrations of SDS become smooth, confirming the formation of an oriented lamellar structure. The islands are electrodeposited, small crystals of ZnO. Figure 4 is the high-angle X-ray diffraction pattern of the samples formed in the solutions with different concentrations of SDS. From Figure 4a one sees three crystalline diffraction peaks of the sample fabricated with 0.001 wt % SDS. They correspond to the hexagonal ZnO. As the concentration of SDS is increased, the intensities of diffraction peaks of the hexagonal ZnO decrease gradually and disappear eventually. When the concentration of SDS amounts to 0.003 wt %, a diffraction peak at about 33.2° appears. Its intensity increases with the increasing concentration of SDS. We could not index this peak, since only one single peak was observed. But it must correspond to the lamellar structure, because the appearance of this peak is companied by the smoothening of the surface of the film, as is
Self-Assembly of ZnO-Surfactant Hybrid Multilayers
J. Phys. Chem. B, Vol. 109, No. 7, 2005 2883
Figure 3. SEM images of the surface morphology of the films deposited with (a) 0.001 wt % SDS and (b) 0.05 wt % SDS.
TABLE 1: Parameters Used in the Simulations of the Reflectivity Profiles Shown in Figure 1 concn (wt %) 0.003 0.004 0.005 0.01 0.05
sublayer
electron density, e/Å3
thickness ((0.2), Å
roughness ((0.6), Å
organic chain inorganic part organic chain inorganic part organic chain inorganic part organic chain inorganic part organic chain inorganic part
0.35 ( 0.02 0.65 ( 0.03 0.35 ( 0.02 0.58 ( 0.03 0.35 ( 0.02 0.53 ( 0.03 0.33 ( 0.02 0.53 ( 0.03 0.31 ( 0.02 0.53 ( 0.03
20.5 11.2 20.5 9.2 20.5 7.0 22.0 7.0 26.0 7.0
4.0 4.0 3.5 3.5 3.3 3.3 3.3 3.3 3.4 3.4
Figure 4. X-ray diffraction pattern of the hybrid multilayers deposited from solutions of various concentration of SDS.
observed by SEM. Figure 4 confirms that the morphological transition of the films is due to the crystallographic structural transformation of the inorganic layers in the self-assembly process. An implication of the above observations is that the solidliquid interface can serve as a unique synthesis environment for metal ions and surfactants to cooperatively assemble into a hybrid structure. During such a process, the growth of ZnO is modified by surfactant, which adsorbs onto the working electrode easily. As the electrodeposition process begins, ZnO is formed on the surface of the chemically modified electrode. But at the same time, the surfactant adsorbs rapidly onto the freshly formed surface of ZnO. If the adsorption of the surfactant is fast enough, the deposited ZnO is covered and smoothened by it in the form of a bilayer. A new ZnO layer is formed on top of the bilayer as the electrodeposition proceeds. The process is repeated to form a multilayer. The adsorption speed of the surfactant molecules is mainly determined by their concentration near the electrolyte/electrode interface at constant potential, which in turn is determined by the bulk concentration of SDS. When the concentration of SDS is lower than 0.002 wt %, the number of the adsorbed surfactant molecules is not large enough to fully cover the surface of the growing ZnO. Islands are formed at the positions not covered by surfactant molecules (Figure 3a). Above 0.002 wt %, the adsorbed surfactant molecules are dense enough to cover the whole surface so that the periodic lamellar structures can be observed. It is interesting to note that there is another transition concentration, 0.005 wt %. The period of the lamellar structure decreases from 31.7 ( 0.2 to 27.5 ( 0.2 Å as the concentration of SDS increases from 0.003 to 0.005 wt %. It then increases with the increasing concentration of SDS and saturates eventually at 33.0 ( 0.2 Å when the concentration of SDS is higher than 0.05 wt %. We have fitted the XRR profile in Figure 1 according to the method reported in refs 14 and 15. The parameters used in the simulation are listed in Table 1. The
Figure 5. Electrondensity profile derived from the reflectivity data. Two periods are shown. The maxima are inorganic sublayers and the minima are organic sublayers.
electron density profiles derived from the reflectivity data are shown in Figure 5. They provide a more complete view of the internal structure of the multilayers. When the concentration of SDS is below 0.005 wt %, the thickness of the inorganic layer decreases with the increasing of the SDS concentration because the time needed for the surfactant to form a full bilayer becomes shorter as the concentration of SDS increases. The electron density of the inorganic layer, which is the average of ZnO and the hydrophilic headgroups of the surfactant, decreased accordingly. When the concentration of SDS was above 0.005 wt %, the thickness of the inorganic layer was basically unchanged. But the thickness of the organic layer increased with the increasing concentration of SDS. We believe that the structure of the electrochemically selfassembled multilayer depends on the relative speed of the growth of the inorganic layer and the adsorption of the SDS molecules. When the concentration of SDS is between 0.003 and 0.005 wt %, a bilayer of surfactant is quickly formed. But the time needed to form a bilayer is inversely proportional to the concentration of SDS, so the higher the SDS concentration, the less ZnO deposited in a growth circle and therefore a thinner
2884 J. Phys. Chem. B, Vol. 109, No. 7, 2005 inorganic sublayer. This explains why the period of the lamellar structure decreases as the concentration of SDS increases from 0.003 to 0.005 wt %. But when the concentration of SDS is above 0.005 wt %, the adsorption of SDS is fast enough to ensure the contemporary formation of the organic and the inorganic sublayers. However, the surfactant molecules become more compact as the amount of the adsorbed surfactant molecules increase. This results in reduced tilt angle of the chains against the normal of the surface, explaining why the thickness of the organic sublayer increases from 20.5 ( 0.2 to 26.0 ( 0.2Å as the concentration of SDS is increased from 0.005 wt % to higher than 0.05 wt % (see Figure 5). IV. Conclusion Well-ordered and highly oriented zinc oxide-surfactant hybrid multilayers have been fabricated via electrochemical selfassembly on silicon substrate from Zn(NO3)2 solutions containing the anionic surfactant sodium dodecyl sulfate. The extremely low concentration of SDS used in this work has enabled us to have a deeper insight into the mechanism of the electrochemical self-assembly process. The aggregation of the surfactant molecules at the liquid-solid interfaces can be affected by many factors, such as the hydrophilicity of the substrate, the type of surfactant, and the counterions. Previous reports8-10 have led to the conclusion that by controlling the interfacial charge density via application of bias voltage to the substrate, one can change the assembly patterns of the surface aggregates. But in this work we have been concerned with the kinetics of the selfassembly process and its effect on the lamellar structure formed at an adequate electrode potential. The main results are that there exist two critical SDS concentrations. One corresponds to the transition from a single bilayer to a stack of bilayers. The other corresponds to the minimum of the period of the film. The results showed that the structure of the electrochemically self-assembled film is controlled by the relative speed of the reduction of metal ions and the aggregation of surfactant molecules. It also told us that the growth speed of the film should be very low to ensure the orientation of the sublayers.
Jing et al. The control experiments using metal electrodes all resulted in fast and unoriented growth. Another reason for the oriented growth is the smoothness of the silicon surface. Films grown on roughened silicon surfaces lose their orientation very soon. Studies on the kinetic roughening of such films are in progress, the results of which will be published elsewhere. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 10325419 and 10274096). References and Notes (1) Mitzi, D. B. Chem. Mater. 2001, 13, 3283. (2) Decher, G.; Schlenoff, J. B. Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim, 2003. (3) Zhang, J.; Wang, Z. L.; Liu, J.; Chen, S. W.; Liu, G. Y. Self-Assembled Nanostructures; Kluwer Academic: New York, 2003. (4) Lu, Y. F.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W. L.; Guo, Y. X.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364. (5) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. AdV. Mater. 1999, 11, 579. (6) Khomutov, G. B.; Bykov, I. V.; Gainutdinov, R. V.; Polyakov, S. N.; Sergeyev-Cherenkov, A. N.; Tolstikhina, A. L. Colloids Surf. A 2002, 198, 559. (7) Ferreira, M.; Huguenin, F.; Zucolotto, V.; da Silva, J. E. P.; de Torresi, S. I. C.; Temperini, M. L. A.; Torresi, R. M.; Oliveira, O. N., Jr. J. Phys. Chem. B 2003, 107, 8351. (8) Choi, K. S.; Lichtenegger, H. C.; Stucky, G. D. McFarland, E. W. J. Am. Chem. Soc. 2002, 124, 12402. (9) Choi, K. S.; McFarland, E. W.; Stucky, G. D. AdV. Mater. 2003, 15, 2018. (10) Baeck, S. H.; Choi, K. S.; Jaramillo, T. F.; Stucky, G. D.; McFarland, E. W. AdV. Mater. 2003, 15, 1269. (11) Izaki, M.; Omi, T. J. Electrochem. Soc. 1996, 143, L53. (12) Yoshida, T.; Tochimoto, M.; Schlettwein, D.; Wohrle, D.; Sugiura, T.; Minoura, H. Chem. Mater. 1999, 11, 2657. (13) Moudler, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer, Eden Prairie, MN, 1992. (14) Nitz, V.; Tolan, M.; Schlomka, J. P.; Seeck, O. H.; Stettner, J.; Press, W. Phys. ReV. B 1996, 54, 5038. (15) Tolan, M. X-ray Scattering from Soft-Matter Thin Films; Springer Tracts in Modern Physics; Hohler G.; Ed.; Springer-Verlag: Karlsruhe, 1999.
