Epoxide assisted metal oxide replication (EAMOR): a new technique for metal oxide patterning

Abstract

We report a facile and new technique termed Epoxide Assisted Metal Oxide Replication (EAMOR) to create metal oxide replicate structures with a “volume” or “surface” deposition selectivity controlled by the type of precursor salt used. The respective metal oxide is formed in situ within the voids defined by the polystyrene template (PS) through an interplay of interfacial and colloidal chemistry.

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The fabrication of colloidal materials with desired structural, optical and surface properties has attracted broad interest for their wide range of applications in filtration, catalyst supports, battery materials, sensors, optics, advanced coatings and photovoltaics.¹ A variety of wet chemical and electrochemical approaches have been employed for preparing such surfaces. One of the commonly used approaches for surface structuring is to replicate the structure of a colloidal crystal or superlattice of particles with a compound or material of choice.²

In this work, we demonstrate a facile and flexible strategy for fabricating patterned metal oxide structures termed Epoxide Assisted Metal Oxide Replication (EAMOR). In contrast to standard alkoxide sol–gel replication, EAMOR offers excellent versatility and access to the whole range of oxide deposition, from surface to volume templating. As a proof of concept, we demonstrate the templating of polystyrene array (PS) by two different metal oxides, ZnO (volume templating) and SnO₂ (surface templating). These oxides are being formed in situ via a colloidal process either inside the voids defined by the particle lattice (ZnO) or on the surface of carboxylate terminated PS by adsorption and fixation (SnO₂). To ensure fine control and tuneability of overgrown oxides, generally small amounts of material are deposited with each cycle, and thus the process is repeated multiple times: adsorption of metal ions from a dilute solution and a subsequent interfacial chemical reaction (epoxide acting as a “fixating” agent), leading to the formation of metal–oxygen–metal (M–O–M) bonds. A detailed reaction mechanism of the classical epoxide assisted sol–gel process for SnO₂ can be found elsewhere.³ Rather showing the patterning quality, which can be still further improved, the primary objective of the work is to show the high versatility and structural control with a new, chemically different and distinct, yet very simple method of creating metal oxide templates.

By example of ZnO, a widely used semiconducting material, let us discuss the principle of the EAMOR method and its structure directing approach (Fig. 1). The base template is prepared by Langmuir–Schaefer deposition of commercial carboxyl terminated PS particles with a mean diameter of 4.3 μm (Fig. 1, top).

Fig. 1 Schematic illustration for the preparation of particle templated metal oxide patterns by means of the EAMOR method.

In a second step, the void space defined by the PS template lattice was then charged (e.g. by spin coating) with a metal salt precursor solution followed by a dilute propylene oxide (PO) solution. This process was repeated several times, leading to in situ layer-by-layer deposition of the respective metal oxide (Fig. 1, middle). The nature and morphology of the final replicates obviously depends on the metal precursor concentration as well as on the number of cycles. One coating cycle is a single iteration of applying metal oxide precursor followed by a dilute aqueous solution of propylene oxide (1 mL PO in 10 mL of distilled water), respectively. Finally, the oxide filled PS templates are annealed at 500 °C in air, as a result of which the PS template is removed and ZnO replicate is left behind (Fig. 1, bottom).

SEM images of ZnO structures obtained under different conditions are presented in Fig. 2. The naked PS template (Fig. 2a) is replicated with ZnO after 12 cycles of spin coating 0.3 M Zn(NO₃)₂ (pH = 6.13) followed by PO and a final annealing step (Fig. 2b).

Fig. 2 SEM images (top view) of polystyrene latex templated ZnO microstructures: (a) Langmuir–Schäefer monolayer of PS beads on a glass substrate, (b) ZnO replicate, (c)–(e) inhomogeneous ZnO pattern replicates with 0.03, 0.3 and 3 M zinc precursor concentration, respectively, (f) partially fused PS beads.

It was found that the mode of void filling greatly influences the final replicate structure and homogeneity. A series of test experiments was done by drop casting or dipping of the templated substrates. The disadvantage of this type of immersion is that the removal of excess solution is rather cumbersome. If excess metal precursor solution is left behind, addition of propylene oxide solution leads to nonspecific overgrowth of metal oxide. This can be avoided by blow drying the substrates with nitrogen after each dip. However, with this, a significant amount of PS template particles were blown away or washed off in the process. Fig. 2c and d show a substantial loss of PS template particles after 6 dipping cycles with 0.03 and 0.3 M Zn(NO₃)₂ precursor concentration, respectively. With 6 cycles of 3 M zinc precursor concentration (Fig. 2e), surprisingly the blowing off of excess liquid does no longer lead to partial loss of PS particles but to a completely overgrown template structure. Higher zinc nitrate precursor concentrations result in improved adhesion of PS particles on the glass substrates. At higher ionic strengths, ionic interactions between carboxylate modified PS surfaces and metal ions are stronger. This leads to a more viscous sol which in turn boosts PS particle cohesion. Given the higher solution viscosity, a significantly thicker and faster growing oxide filler layer is formed with more concentrated salt solutions. To reduce template particle loss and increase replicate fidelity spin coating was chosen as the method of choice for the deposition of the active components. Driven by the high centrifugal forces during spin coating, excess solution is almost instantaneously ejected, leaving a uniform film of solution, which is necessary for satisfactory process reproducibility. If the substrate is exposed to aqueous propylene oxide solution for longer periods of time (>1 min), fusing of PS beads can be observed. This is due to a non-negligible solubility and swelling of the PS beads in PO. In this way, even non-perfect particle lattices attain perfect hexagonal symmetry. The effect is shown in Fig. 2f.

Let us now discuss the effect of the metal oxide precursor concentration as well as the effect of the number of coating cycles on the final replicate structures. The former is presented in the upper half of Fig. 3(a)–(c), whereas the latter is shown in the lower half Fig. 3(d)–(g). At 12 cycles with low precursor concentrations (0.03 M, Fig. 3a) an array of saucer-like rings is formed rather than the continuous volume backfilling. The rings surround the positions where the latex spheres were originally located. At 0.03 M, clearly the concentration is too low to allow for complete replication of the template pattern. At 12 cycles with 0.3 M precursor concentration (Fig. 3b), inverse opal structures are always formed. At higher concentration (3 M, 12 cycles), a dense overgrown thick film of ZnO was deposited on the template (Fig. 3c). Clearly in this case, the propensity for forming needle like structures is rather predominant and leads to a lower replicate quality.

Fig. 3 SEM images (top view) of ZnO coatings obtained at different precursor concentration (a) 0.03 M, (b) 0.3 M, and (c) 3 M, and # coating cycles (d) 3 cycles, (e) 6 cycles, (f) 9 cycles, and (g) 15 cycles.

In the second study, the zinc nitrate concentration was kept constant at 0.3 M and the PS templates were overgrown with 3–15 cycles with a cycle interval of 3. Up to 9 cycles, the template replication was incomplete and the amount of material deposited was not sufficient to fill up the void space. Clearly, the backfilling level increases with increasing cycle numbers: at 3 cycles, ZnO mimics the bottom third of the PS sphere (Fig. 3d), whereas in 6 coating cycles, the material clearly is building up the wall between adjacent PS particles (Fig. 3e). With 9 coating cycles (Fig. 3f) the wall height is further increased and at 12 cycles, the replicate quality is best. More than 12 coating cycles generally result in a marked increase in nonspecific overgrowth (Fig. 3g). In conclusion, 0.3 M Zn(NO₃)₂ and 12 coating cycles were determined ideal for patterning ZnO on 4.3 μm diameter polystyrene latex by EAMOR.

A control experiment was performed with the goal to substantiate the active role of the epoxide “fixating agent”. The control experiment involves the same procedure as for the standard replicates with the difference that in the second step pure water without propylene oxide was used. In Fig. 4b, a coherent, well defined replicate structure is lacking. The small randomly deposited ZnO clearly fails to produce the desired long range ordered replicate pattern. In addition, another control experiment demonstrating the necessity of the template matrix was executed on a plain borosilicate glass substrate without the PS particle template. In this case, we observe the formation of random patches of needle-like porous ZnO structures on the substrate alternating with blank areas (Fig. 4c). In the magnified image, the porous, dendrite/needle-like nature of the ZnO deposits can be seen.

Fig. 4 SEM images (top view) of ZnO replicate obtained from 0.3 M zinc solution after 12 cycles: (a) with propylene oxide, (b) without propylene oxide, (c) without PS template.

Now let us corroborate the claimed versatility for metal oxide deposition of the EAMOR methodology by adopting a different material for the PS template replication namely tin oxide SnO₂. 12 cycles of tin oxide were deposited using 0.03 M tin tetrachloride (SnCl₄, pH = 1.32) as the metal precursor solution. The experimental procedure was identical to the one used for ZnO replication. Fig. 5(a)–(b) shows SEM images of the obtained SnO₂ replicate before and after annealing. Contrary to the ZnO case, the tin oxide deposits preferably on the PS template rather than in the interstitial voids (compare Fig. 5a and insert of Fig. 4a). After annealing, the surface templated tin oxide skin on the PS particle results in a hollow capsule structure. In comparison with 0.3 M ZnO (Fig. 3), the surface selective tin oxide deposition which results in hollow spheres can be attributed to three main factors, namely i) the higher intrinsic affinity of the tetravalent Sn⁴⁺ cation to carboxylate surfaces, ii) the propensity for smaller charge stabilized colloids in the tin oxide system (partly corroborated by the smaller XRD crystallite sizes) and iii) the faster hydroxide formation and condensation kinetics linked to the considerably more acidic character of the hexaqua Sn⁴⁺ ion. Long range covering of these capsules/shells is depicted in Fig. 5c. The FIB cross section of the hollow spheres (Fig. 5d) clearly shows the surface pattering of SnO₂ and also some residual porous oxide deposition on the substrate.

Fig. 5 SEM images (top view) of a tin oxide replicates obtained (a) and (b) before and after annealing, respectively, (c) and (d) FIB-SEM images of SnO₂ shells obtained after annealing, (e) and (f) Grazing angle XRD diffractogram of templated ZnO and SnO₂ replicates, respectively.

Chemical identity and crystalline character of the templated metal oxides (ZnO, SnO₂) were confirmed by grazing incidence X-ray diffraction (XRD) with a scan range between 20° and 90°. Fig. 5e shows the XRD pattern of typical ZnO replicate (12 cycles, 0.3 M zinc nitrate). As expected, the templated film shows the exact same Bragg reflection signatures which are typical for a wurtzite ZnO structure. The diffraction peaks at 2θ values of 31.77°, 34.42°, 36.25°, 47.54°, 56.60°, 62.85° and 67.95° respectively arise from diffractions off the (100), (002), (101), (102), (110), (103), and (112) crystallographic planes (File number: COD9008877).⁴ These peaks are readily index matched to the diffraction off hexagonal phase ZnO. It can be seen that the ZnO films obtained from by EAMOR possess a significant degree of crystallinity, indicated by the relatively sharp peaks and the ability to see higher order diffraction peaks. Peak width analysis suggested a typical crystallite size on the order of 27.4 nm. Furthermore, no characteristic peaks corresponding to impurities, (e.g. Zn(OH)₂, etc.), were observed in the XRD patterns. Fig. 5f shows XRD pattern of SnO₂ signatures which are typical for the cassiterite type SnO₂. The diffraction signatures arise from the various crystal plane reflections (110), (101), (200), (111), (211), (301) and (311), respectively (File number: COD1000062).⁴ The individual diffraction peaks in the SnO₂ case are rather broad, indicating very small nanoscopic crystallite sizes. Typical crystallite sizes predicted by the Scherrer equation are on the order of 5.4 nm.

Conclusions

In conclusion, we have developed a simple and easy to use deposition method for replicating microscopic patterns in the form of overgrown metal oxide coatings termed EAMOR. The process utilizes simple metal salts (precursor) and an organic epoxide (fixating agent). Proof of concept of the technique was demonstrated by example of ZnO and SnO₂ PS particle template replication. The morphology of the deposited oxide layer is readily controlled by changing the metal salt, the concentration of the precursor solution and/or the number of coating cycles. It allows covering the whole range from surface to volume templating. Alternating coating cycles with different metal oxide precursors should allow the deposition of mixed oxide and multilayer oxide materials. Additional studies towards the controlled deposition of multilayer oxide films by this method will be the focus of a follow-up publication.

Notes and references

1. (a) M. Wang and X. Wang, Sol. Energy Mater. Sol. Cells, 2008, 92, 357; (b) P. T. Tanev, M. Chibwe and T. J. Pinnavaia, Nature, 1994, 368, 321; (c) S. Schumann, S. A. F. Bon, R. A. Hatton and T. S. Jones, Chem. Commun., 2009, 6478; (d) D. R. Rolison, J. W. Long, J. C. Lytle, A. E. Fischer, C. P. Rhodes, T. M. McEvoy, M. E. Bourg and A. M. Lubers, Chem. Soc. Rev., 2009, 38, 226; (e) J. Park, S. Lim and K. Park, J. Nanopart. Res., 2013, 15, 1; (f) N. Linares, C. P. Canlas, J. Garcia-Martinez and T. J. Pinnavaia, Catal. Commun., 2014, 44, 50; (g) B. T. Holland, C. F. Blanford and A. Stein, Science, 1998, 281, 538; (h) J. Elias, M. Bechelany, I. Utke, R. Erni, D. Hosseini, J. Michler and L. Philippe, Nanomater. Energy, 2012, 1, 696; (i) M. Bechelany, P. Brodard, J. Elias, A. Brioude, J. Michler and L. Philippe, Langmuir, 2010, 26, 14364; (j) N. Leventis, P. Vassilaras, E. F. Fabrizio and A. Dass, J. Mater. Chem., 2007, 17, 1502; (k) N. Leventis, N. Chandrasekaran, A. Sadekar, S. Mulik and C. Leventis, J. Mater. Chem., 2010, 20, 7456.
2. (a) Z. Zhong, Y. Yin, B. Gates and Y. Xia, Adv. Mater., 2000, 12, 206; (b) M. Yang and J.-J. Zhu, J. Cryst. Growth, 2003, 256, 134; (c) H. G. Yang and H. C. Zeng, J. Phys. Chem. B, 2004, 108, 3492; (d) R. T. Tom, A. S. Nair, N. Singh, M. Aslam, C. L. Nagendra, R. Philip, K. Vijayamohanan and T. Pradeep, Langmuir, 2003, 19, 3439; (e) Q. Peng, Y. Dong and Y. Li, Angew. Chem., Int. Ed., 2003, 42, 3027; (f) T. Nakashima and N. Kimizuka, J. Am. Chem. Soc., 2003, 125, 6386; (g) P. Jiang, J. F. Bertone and V. L. Colvin, Science, 2001, 291, 453; (h) M. Iida, T. Sasaki and M. Watanabe, Chem. Mater., 1998, 10, 3780; (i) M. Davis, D. Ramirez and L. Hope-Weeks, ACS Appl. Mater. Interfaces, 2013, 5, 7786.
3. (a) M. M. Koebel, D. Y. Nadargi, G. Jimenez-Cadena and Y. E. Romanyuk, ACS Appl. Mater. Interfaces, 2012, 4, 2464; (b) A. E. Gash, T. M. Tillotson, J. H. Satcher Jr, L. W. Hrubesh and R. L. Simpson, J. Non-Cryst. Solids, 2001, 285, 22; (c) A. E. Gash, J. H. Satcher and R. L. Simpson, Chem. Mater., 2003, 15, 3268.
4. S. Gražulis, A. Daškevič, A. Merkys, D. Chateigner, L. Lutterotti, M. Quirós, N. R. Serebryanaya, P. Moeck, R. T. Downs and A. LeBail, Nucleic Acids Res., 2012, 40, 420.

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Footnote

† Electronic supplementary information (ESI) available: SEM images of (i) self-assembled monolayer of PS microspheres (ii) overgrown structures of ZnO, (iii) control experiments, (iv) FIB SEM of ZnO deposition within the voids of PS and at the interface of three PS spheres, (v) SnO₂ hollow sphere, EAMOR chemistry and postulates. See DOI: 10.1039/c4ra07630c

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