Article pubs.acs.org/cm
Colloidal Crystal Templates Direct the Morphologies of Fabricated Porous Cuprous Oxide Particles Ming Fu,* Ailun Zhao, Dawei He, and Yongsheng Wang Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, P. R. China S Supporting Information *
ABSTRACT: Well-controlled porous morphologies and tailored shapes are important for the mesostructures that are produced in material chemistry. Hard template processes provide an efficient way to fabricate porous structures, but the only a few particle shapes can be produced using 3D porous templates. Little is discussed about how such templates affect the shapes of particles. Here, porous Cu2O crystals with different shapes and degrees of branching can be electrodeposited using colloidal crystal templates. These templates can produce particle shapes that are exactly the same as those created without templates, but they can also produce different shapes than those fabricated on bare substrates (without a colloidal crystal template). The presence of the colloidal crystal template blocks ion diffusion and changes the deposition environment. Both increased and decreased degrees of branching can be produced when different electrolyte systems are used in conjunction with a colloidal crystal template.
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INTRODUCTION Creating solid particles with specific morphologies is a key consideration for applications in sensors, catalysts, and energy harvesting and storage. These applications are directed by structure-dependent properties such as film thicknesses, crystal geometries and sizes, surface-to-volume ratios, carrier transportation performance, grain boundaries, and defects. Porous morphologies, especially those with highly ordered structures,1,2 have additional advantages owing to their large surface areas, three-dimensional (3D) connected networks, and their possible ability to regulate photons and phonons. Selforganization and its following templating methods provide efficient ways to fabricate porous solids with characteristic dimensions on the scale of nanometers to micrometers.1,3−6 Mesostructures with various two-dimensional (2D) or 3D pore symmetries or even hierarchical porous structures can be obtained.4,7,8 However, controlling the shapes of porous materials via self-organization is more complex and therefore more difficult than controlling the shapes of nonporous materials. Only a few types of outer mesoscale shapes can be constructed, including films, spherical powders,9−13 polyhedral particles,12,14,15 rods,10,16 cubic boxes,17,18 sheets,19−22 and hierarchically porous monoliths.23,24 These porous mesostructures are still far rarer than mesostructures fabricated from the many common liquid and vapor techniques that do not introduce porous morphologies. Fabricating porous structures using templating methods has significantly broadened the available set of materials that can be formed into templates. However, templated materials generally lack diversity in their overall shape because the interior © 2014 American Chemical Society
structures are usually exact replicas of the template. In fact, the original shapes of nonporous particles can be retained during the templating process,19,20 though they are seldom reported. When colloidal crystals serve as the template, the general structure that forms is a 3D inverse opal film; only a few reports have suggested that shapes other than inverse opal films can be formed. In previous studies, single crystals of calcite with dendritic or particle shapes, which were not present in a film, were fabricated using colloidal crystals via an anamorphous-tocrystalline or precipitation strategy.12,25 In other studies, porous cuprous oxide (Cu2O) with only cubic shapes was fabricated by electrodepostion.22,26 Furthermore, spherical or hierarchical macroporous structures of metal oxides have been developed using the pechini gel method.13 Zinc oxide nanomeshes can be formed by colloidal crystal-assisted electrodeposition when nanosheet habits are retained,21,22 and template effects have been found to guide the orientation of nanomeshes.19,20 However, to date, 3D porous structures with tailored shapes are still rarely fabricated. Furthermore, 3D templates for creating porous structures, such as colloidal crystals, represent environments that are different from those in which nonporous crystals can be produced. Therefore, different 3D templates may be able to produce interior pores as well as particles with different general shapes in order to tailor their outer appearances. Cu2O is a traditional and promising semiconducting material for energy harvesting,27,28 because it has a direct band gap Received: December 4, 2013 Revised: March 31, 2014 Published: April 1, 2014 3084
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conditions. Aside from the pores in the body of the particle, the outer appearance of the porous Cu2O particle is similar to that of the particle fabricated without the 3D colloidal crystal template. While these images show that a 3D colloidal crystal template can be used to create porous particles that have similar morphologies to those created on bare substrates, a 3D colloidal crystal template can also be used to fabricate porous particles that have vastly different morphologies. Figure 1 (g-i) shows three porous Cu2O particles that have different branching morphologies. The three particles in Figure 1 (g-i) show quatrefoil-like morphologies of decreasing size and decreasing degrees of branching. The outer shapes of particles deposited using the colloidal crystal-covered substrates are affected by the porous templates when compared with the outer shapes of particles created by deposition onto bare ITO substrates. Because crystal habits are not easily modified by the use of templates, we studied the template effects using only cubic Cu2O. Figure 2 presents typical crystal shapes reflecting the cubic growth habit in the absence and in the presence of a colloidal crystal template when applying six different potentials (a-f; −0.17 to 0.08 V) in a cupric nitrate solution. Different aspectsa surface, an edge, and a vertexof Cu2O particles fabricated without a colloidal crystal template are labeled 1−3 in Figure 2 panels. The corresponding particles fabricated in the presence of a colloidal crystal template are labeled 4−6. As the overpotentials decreased (from a to f), branching morphologies appeared in the Cu2O crystals, as shown in panels 1−3. This phenomenon can be attributed to overpotential-limited29 or shape-dependent charge distribution31 branching. However, the shapes of each of the porous Cu2O particles fabricated using the colloidal crystal template (Figure 2, panels 4−6) have a similar cubic shape. No branching geometries were detected, even for structures deposited at 0.02 and 0.08 V, which show remarkable branching in the absence of the colloidal crystal template. Therefore, colloidal crystal templates seem to prevent the formation of branching morphologies under these conditions. Besides the retained facet growth, tiny distortions were shown in the cubic particles in the presence of the template, especially at lower overpotential (more positive potential). Obtuse apex angles tended to exist on the top surface or top edge of the particles. This may have occurred because more unsettled Cu+/Cu2O was generated close to the ITO substrate or the Gibbs energy varied differently when different pore surfaces (along the different colloidal crystal planes) explosed.20 The deposition of Cu2O comprises two steps, listed below:
around 2 eV, a high absorption coefficient, low-cost fabrication, low toxicity, and is widely available. More important for this work, electrodeposition can produce Cu2O particles that have various shapes, including different crystal habits, orientations, and branching patterns.29,30 Branching morphologies are typically related to ion diffusion, which can also be regulated by a 3D porous template. Therefore, in this paper, we demonstrate the electrodeposition of Cu2O particles having various morphologies in the presence of 3D colloidal crystals. Different structures with ordered pores were formed, and the template effects on the appearance of these structures are illuminated here.
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RESULTS AND DISCUSSIONS Cu2O can achieve octahedral and cubic crystal habits with a controlled degree of branching.29,30 Here, Cu2O is electrodeposited into the interior voids of the colloidal template after colloidal crystals are coated onto the indium tin oxide (ITO) substrate. The exact morphologies of Cu2O crystals could be retained after electrodeposition in the presence of a 3D macroporous template. Figure 1 (a-c) shows three different aspectsa vertex, an edge, and a surface, respectivelyof a typical octahedral Cu2O crystal particle fabricated via electrodeposition (without a 3D template). Figure 1 (d-f) shows the same three aspects of a Cu2O crystal particle formed in the presence of a 3D colloidal crystal template under the same
Cu 2 + + e− → Cu+
(1)
2Cu+ + H 2O → Cu 2O + 2H+
(2)
In a cupric nitrate solution, the concentration of H+ ions is not constant. The presence of a porous template (a colloidal crystal template) may hinder the diffusion of H+ ions and consequently decrease the local pH near the particles. This lower pH during deposition can potentially limit a particle’s degree of branching.29 The pH in the electrodeposition process remained constant when a cupric acetate electrolyte was used together with a buffer solution. Figure 3 shows particles fabricated in a buffer solution at pH 4.5 when different potentials, from −0.2 to −0.03 V, were applied. In buffers with higher pH, the deposition tended to show branching geometries in the absence of the colloidal crystal template. The scanning electron
Figure 1. Porous Cu2O can be controlled to have identical or different shapes compared with nonporous Cu2O particles. (a-c) Scanning electron microscopy (SEM) images showing the outer shape of a typical octahedral Cu2O particle formed without a colloidal crystal template; (d-f) SEM images of a porous Cu2O particle, fabricated using a colloidal crystal template, with a similar morphology to the one shown in (a-c). All the particles in (a-f) were deposited in the same electrochemical conditions in a cupric nitrate solution with sodium dodecyl sulfate. (g-i) SEM images of three different porous Cu2O particles with branching morphologies whose shapes cannot be obtained when deposition occurs on bare indium tin oxide (ITO) substrates; particles were deposited in cupric acetate buffer solution at pH 4.5. All scale bars are 2 μm. 3085
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Figure 2. Comparing the shapes of Cu2O particles fabricated in a cupric nitrate solution in the absence or in the presence of a colloidal crystal template. (a-f) SEM images of structures fabricated at (a) −0.17 V, (b) −0.13 V, (c) −0.08 V, (d) −0.03 V, (e) 0.02 V, and (f) 0.08 V. The numbers in each panel show (1−3) structures fabricated in the absence of colloidal crystals and (4−6) structures fabricated in the presence of colloidal crystals. All scale bars are 2 μm.
electrodeposition, the formed particles showed both ordered porous morphologies and higher degrees of branching. When the buffer solution was used, the porous template did not affect the concentration of H+ ions. Though the branching at apexes that occurred in this situation was different from how branching normally occurs,29,31 ion diffusion still played an important role in the formation process. The effect of the template on diffusion and crystallization resulted in more changes to the final particle shape. The movements of Cu2+ ions toward the seeded particles are blocked by the porous template. Therefore, the concentration gradients that arise in the presence of templates are greater than those where templates are absent, which may enhance the apex growth of Cu2O particles. As apex branching was observed to be extremely sensitivity to pH, the attachment rate for produced Cu2O from Cu+ ions seems to be important for the final particle shape. Therefore, and even more importantly, templates block the Cu2O formation process from Cu+ in which Cu2O would be able to find a thermodynamically favorable place to attach. In addition, at a high pH of 4.5, the lifetime of Cu+ in solution is short. So Cu2O tends to grow at the position Cu+ is produced than another thermodynamically favorable place it hardly moves onto. This did finally produce particle morphologies with higher degrees of apex branching in the presence of a template. Furthermore, we found that the degree of branching near the bottom of the substrate was much higher than it was further away from the substrate. This phenomenon makes sense because there are more Cu+ ions produced near the ITO substrate and the colloidal crystal template produces a stronger hindrance to ion diffusion. It is even more difficult for Cu+/ Cu2O nearby the ITO substrate to find a favorite place to attach on the particle position away from ITO than in the absence of
Figure 3. Comparing the shapes of Cu2O particles fabricated in a cupric acetate buffer solution at pH 4.5 in the absence or in the presence of a colloidal crystal template. SEM images of structures fabricated at (a) −0.2 V, (b) −0.1 V, (c) −0.05 V, and (d) −0.03 V. The numbers in each panel show (1−2) structures fabricated in the absence of colloidal crystals and (3−4) structures fabricated in the presence of colloidal crystals. All scale bars are 2 μm.
microscopy (SEM) images presented in panels labeled 1−2 in Figure 3 show the typical branching structures on a surface or a top edge. When colloidal crystal templates were used in 3086
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the template does not affect the resulting morphologies as much as it does when deposition is fast at higher pH values. Many morphological details other than porous structures can be modified when a colloidal crystal template is involved in the formation process. Heterostructures containing fractional porous structures can be obtained when the thickness of the template is less than the height of the Cu2O particles, as shown in Figure s1 (a-b). When the colloidal crystal template is thick enough, the size of the porous structures is completely determined by the size of the deposited Cu2O particles. The size of the porous structures is related to deposition time, as shown in Figure s1 (c-d). Although particle morphologies can be modified by a porous template, we have not observed significant differences when changing only the diameter of the colloidal microspheres. The cubic particles of Cu2O fabricated using a colloidal template with microspheres of different diameters are shown in Figure s2 (a-f). Aside from increased pore sizes, the particle morphologies did not show any significant differences. In addition, all the porous particles formed on the substrate usually have similar habits and branching degrees as shown in the low-magnification images in Figure s3. In conclusion, we have successfully fabricated variously shaped Cu2O particles with ordered pores using colloidal crystal template-assisted electrodeposition. Of the morphologies produced, we formulated cubic and octahedral shapes with different degrees of branching. Porous Cu2O particles can be fabricated to have exactly the same general shape as nonporous Cu2O particles. In addition, shapes that cannot be obtained when deposition occurs on a bare ITO substrate can be produced when a colloidal crystal template is used. The presence of the colloidal crystal template blocks ion diffusion and changes the deposition environment. Both increased and decreased degrees of branching can be found when different electrolyte systems are used in conjunction with a colloidal crystal template. These 3D mesoscale architectures hold significant promise for applications in energy harvesting, sensors, and photonics.
template. Therefore, the highly branched morphologies were observed at the base of the particles. Figure 4 shows the morphologies of Cu2O structures fabricated in a cupric acetate buffer solution at pH 3.9. As
Figure 4. Comparing the shapes of Cu2O particles fabricated in a cupric acetate buffer solution at pH 3.9 in the absence and in the presence of the colloidal crystal template. SEM images of structures fabricated at (a) −0.6 V, (b) −0.3 V, and (c) −0.2 V. The numbers on each panel of (a-b) show (1−3) structures fabricated in the absence of colloidal crystals and (4−6) structures fabricated in the presence of colloidal crystals. SEM images with labels c1-c2 show Cu2O particles fabricated in the presence of a colloidal crystal template. All scale bars are 2 μm.
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EXPERIMENTAL SECTION
Monodisperse polystyrene microspheres were prepared by emulsifierfree polymerization technology with styrene, methacrylic acid, styrenesulfonate, ammonium persulfate, and sodium bicarbonate. Colloidal crystal templates with 3D ordered periodicity were prepared using a self-assembly method. Cleaned indium tin oxide (ITO) substrates were vertically immersed in colloidal suspensions of monodispersed polystyrene (PS) microspheres with diameters of 170−700 nm and placed in an incubator at approximately 55 °C with 40% to 50% humidity. Cu2O structures were fabricated by electrodeposition using a threeelectrode system (CHI660A Electrochemical Workstation). Bare ITO substrates or colloidal crystal-covered ITO substrates were used as the electrodeposition working electrode, and a saturated calomel electrode was used as the reference electrode. The electrodeposition solution for Cu2O with a cubic habit consisted of either 0.02 M cupric nitrate or 0.02 M cupric acetate, together with an acetic acid buffer solution. Cu2O particles with octahedral habits (Figure 1 a-f) were deposited in a cupric nitrate solution with sodium dodecyl sulfate at −0.017 V. The acetic acid buffer solution contained acetic acid and sodium acetate. The pH of the electrodeposition solution was adjusted to 4.5 (Figure 3) or 3.9 (Figure 4) by adding appropriate amounts of acetic acid and sodium acetate. The porous particles in Figure 1 (g-i) were also deposited in cupric acetate buffer solution at pH 4.5 at −0.05, −0.1, and −0.2 V, respectively. After electrodeposition, the colloidal crystal
the pH decreased and the concentration of H+ ions increased, the reaction leading to Cu 2 O particles slowed down considerably, as suggested by eq 2. In this case, the rates of ion diffusion in the absence and in the presence of the colloidal crystal template did not differ much, so their formation processes were similar. Therefore, the morphologies fabricated using the templates [Figure 4, panels a-b(4−6)] were similar to those deposited on bare substrates [Figure 4, panels a-b (1− 3)]. The degree of branching in the porous Cu2O particles was even lower than for the nonporous Cu2O particles, as shown in Figure 4 (b), which is the opposite of what occurred when higher pH values were used. The branching degree of porous particles fabricated at −0.2 V, shown in Figure 4 (c), just reached the branching degree associated with nonporous Cu2O particles fabricated at −0.3 V. In this way, we found that the 3D porous template affected the deposited morphologies in several ways, depending on a variety of factors contributing to the whole reaction process. When the deposition rate is very slow, 3087
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templates were removed by immersing the substrates in a toluene solution for 24 h. The morphology of the structures was characterized using a scanning electron microscopy (SEM) system (HITACHI S-4800; Hitachi; Tokyo, Japan). The samples were characterized without metal-coating before imaging. The surfaces of the ITO substrates and the copper sample holder were connected by a carbon tape.
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ASSOCIATED CONTENT
S Supporting Information *
Fabricated Cu2O structures when changing the dimension of template or Cu2O particles and using a colloidal template with microspheres of different diameters, low-magnification images showing uniformity of particles on one substrate, the XRD analysis of the particles. This material is available free of charge via the Internet at http://pubs.acs.org.
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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 This work was supported by the National Science Foundation of China under grant Nos. 91123025, 50902008, 61335006, and 61378013, Beijing Higher Education Young Elite Teacher Project No. YETP0574, and the National Basic Research Program of China Nos. 2011CB932700 and 2011CB932703
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