Hierarchical Metal Oxide Topographies Replicated from Highly Textured Graphene Oxide by Intercalation Templating Po-Yen Chen,*,†,‡ Muchun Liu,†,§ Thomas M. Valentin,†,‡ Zhongying Wang,†,‡ Ruben Spitz Steinberg,†,‡ Jaskiranjeet Sodhi,†,‡ Ian Y. Wong,*,†,‡ and Robert H. Hurt*,†,‡ †
School of Engineering, ‡Institute for Molecular and Nanoscale Innovation, and §Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *
ABSTRACT: Confined assembly in the intersheet gallery spaces of two-dimensional (2D) materials is an emerging templating route for creation of ultrathin material architectures. Here, we demonstrate a general synthetic route for transcribing complex wrinkled and crumpled topographies in graphene oxide (GO) films into textured metal oxides. Intercalation of hydrated metal ions into textured GO multilayer films followed by dehydration, thermal decomposition, and air oxidation produces Zn, Al, Mn, and Cu oxide films with high-fidelity replication of the original GO textures, including “multi-generational”, multiscale textures that have been recently achieved through extreme graphene compression. The textured metal oxides are shown to consist of nanosheet-like aggregates of interconnected particles, whose mobility, attachment, and sintering are guided by the 2D template. This intercalation templating approach has broad applicability for the creation of complex, textured films and provides a bridging technology that can transcribe the wide variety of textures already realized in graphene into insulating and semiconducting materials. These textured metal oxide films exhibit enhanced electrochemical and photocatalytic performance over planar films and show potential as high-activity electrodes for energy storage, catalysis, and biosensing. KEYWORDS: hierarchical surface architectures, ion intercalation, sacrificial templating, graphene, nanoscale assembly, electrochemical and photocatalytic activities
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only been demonstrated for the fabrication of simple planar films. The present paper explores a templating approach for creating complex textured surface topographies in metal oxide films. One possible approach is to build on well-established methods for texturing graphene films12 and then transcribing those textures into metal oxide chemistry through sacrificial intercalation templating. Previously, textured graphene films have been fabricated via depositing or transferring GO coatings onto softer substrates (e.g., prestretched polymer), followed by substrate contraction actuated either mechanically or thermally, which compresses the stiffer surface film.13−16 The results of this compression are wrinkled textures (for unidirectional (1D) contraction) or crumpled textures (for isotropic (2D)
wo-dimensional (2D) nanomaterials can serve as versatile templates that nucleate, orient, and confine the assembly of ultrathin material architectures.1−3 In particular, graphene oxide (GO) has often been used to direct growth due to its rich surface chemistry, ease of aqueous processing, and large interlayer gallery spaces.4−8 By restacking GO sheets into a multilayer film and introducing precursors through intercalation, the 2D interlayer gallery spaces can confine and guide the assembly and conversion of precursors into lamellar material structures.9−11 This intercalation approach is particularly suited to metal oxides, which display excellent mechanical strength as well as thermal stability and are stable under conditions where graphene templates can be removed by air oxidation. Recently, this concept has been demonstrated using thick GO papers as intercalation templates to synthesize YBa2Cu3O7−δ (Y123)9 and ZnO11 lamellar structures in a planar configuration. This technique offers excellent control over oxide thickness and chemistry but has © 2016 American Chemical Society
Received: August 2, 2016 Accepted: November 7, 2016 Published: November 7, 2016 10869
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Figure 1. Creation of hierarchical metal oxide topographies using highly textured GO as versatile ion intercalation templates. Multigenerational GO films are fabricated by one-time uniaxial contraction (G1-1D-GO), one-time isotropic contraction (G1-2D-GO), and repeated isotropic−isotropic contractions (G2-2D2D-GO). The as-fabricated textured GO films are then utilized as intercalation templates for hydrated metal cations (e.g., Ag+, Zn2+, Al3+, Mn2+, Cu2+). After immersion in aqueous precursor salt solutions, the metal ion-intercalated GO films are dehydrated, annealed, and calcined at high temperature to oxidatively remove the carbon-based template and create textured metal oxide films (ZnO, Al2O3, Mn2O3, CuO) with high-fidelity replication of the original graphene wrinkled, crumpled, and hierarchical topographies.
contraction). We have recently demonstrated a highly versatile approach for more complex, hierarchical wrinkle/crumpletextured films using multiple cycles of contraction, film removal, transfer, and further contraction.12 This method achieves a degree of complexity, deformation, and surface area enhancement that far exceeds that of simple one-step deformation processes.15,17,18 Currently, however, it is not clear if these highly textured graphene films can be used as templates to fabricate textured metal oxides by ion intercalation. In particular, high concentrations of ions would have to intercalate uniformly into the complex deformed gallery spaces in textured GO films, and also the complex multiscale structural features would have to be accurately replicated and survive high-temperature oxidative removal of the graphene template. In this article, a facile fabrication method is demonstrated for replicating complex, hierarchical graphene wrinkled/crumpled textures in metal oxide films (see Figure 1). The fabrication involves (1) texturing of graphene oxide films by programmed compression of underlying polymer substrates, (2) spontaneous intercalation of hydrated metal ions into these GO films in a water-swollen state, (3) dehydration of cation−GO complexes and thermal decomposition of GO to dissociate the complexes, and (4) calcination to remove the graphene template by O2
oxidation. To determine the generality and versatility of this method, a variety of mono- and multivalent metal ions were explored as intercalants (Ag+, Zn2+, Al3+, Mn2+, and Cu2+), and a range of GO templates was applied with prefabricated textures that include uniaxial wrinkles, isotropic crumples, and complex hierarchies.12 We find that many but not all of the metal ions and synthesis conditions produce textured metal oxide films, and the optimal precursors and concentration for successful replication are identified and reported here. Remarkably, even the finest structural features in multiscale hierarchical GO templates can be replicated with high fidelity in Zn, Al, Mn, and Cu oxide films when using the optimal ion loading. The final films are shown to consist of nanosheet-like aggregates of distinct metal oxide nanoparticles, whose mobility, attachment, and sintering are guided by the 2D confinement imposed by the template during their assembly. Finally, we show that these nanostructured metal oxide surfaces display enhanced performance over planar films as electrochemical capacitors and photocatalytic substrates. Overall, this intercalation templating approach has broad applicability for the creation of complex, textured metal oxide films and provides a generalized synthetic route that can replicate textures already realized in highly flexible graphene sheets into other materials systems. 10870
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Figure 2. Metal cation intercalation behavior studied in GO multilayer films. (a) Time-resolved XRD patterns of Zn2+-intercalated GO films during drying. The black solid line represents the dry neat GO film (before metal ion intercalation). The peak at 2θ ∼ 15° (denoted with an asterisk) is from the silicon substrate and can be used as a reference for peak shifting. (b) XRD patterns of neat GO, Ag+-GO, Zn2+-GO, and A13+-GO thick, planar films after overnight drying, and the corresponding interlayer spaces are 7.6 (neat GO), 7.9 (Ag+), 9.2 (Zn2+), and 10.1 Å (Al3+). Again the peak at 15° with an asterisk originates from the silicon substrate. (c) Ag 3d, (d) Zn 2p, and (e) Al 2p XPS spectra measured at different nominal depths of metal ion-intercalated crumpled templates. (f) Ag/C, Zn/C, and Al/C atomic ratios as a function of etching depth, showing the relatively uniform doping that results from intercalation. (g) Thermogravimetric analysis profiles of neat GO, Ag+GO, Zn2+-GO, and A13+-GO films in the air as a second method to confirm and quantify metal loading. Photo of final, as-synthesized Al2O3 crumple-textured film is shown in the inset; scale bar is 2 cm.
RESULTS AND DISCUSSION Figure S1 provides more details on the synthesis protocol. First, an aqueous GO suspension is drop-cast on a thermally responsive polymer shrink film to form a planar multilayer GO coating, denoted as a “zeroth-generation (G0)” structure. Heating the sample above the glass transition temperature, Tg, of polystyrene (∼100 °C) triggers substrate contraction to release the prestretched strain, leading to first-generation (G1) wrinkled textures in the case of uniaxial deformation (G1-1DGO) or to first-generation crumpled textures in the case of
isotropic deformation (G1-2D-GO). Further contraction can be used to create hierarchical second-generation (G2) textures. This is accomplished by detaching the G1 film through the dissolution of the polystyrene substrate in dichloromethane followed by film transfer to a new shrink film for further deformation. As an example, two cycles of biaxial contraction result in a multiscale crumpled−crumpled structure we refer to as G2-2D2D-GO. After these deformation steps, we explore whether the first-generation, G1, or the hierarchical second10871
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accumulate until most of the original charge is neutralized. Full charge neutralization of carboxylate groups (estimated as 1 group/25 carbon atoms)20 would produce Zn contents in GO films of ∼8 wt %, which is on the same order as the experimental uptake (see below) and indicates this is a significant mechanism. Second, cations are known to form coordination complexes with oxygen-containing functional groups on GO surfaces and, in particular, with charged carboxyl groups, which have a binding energy (∼10 kcal/ mol) larger than that of hydroxyl or epoxy groups (∼1 kcal/ mol).21 The equilibrium binding constant of Zn2+ to carboxyl groups has been reported to be about 3−10 M−1 [Zn2+ + RCOO− ↔ Zn2+(−OOCR)],22 and those for Ag+ and Al3+ are much larger (500 and 10−1000 M−1, respectively).23 At the chosen [Al3+] bulk concentration of 0.1 M (see below) and using a typical binding constant of 100 M−1, the equilibrium relation predicts ∼90% of RCOO− groups covered by Al3+. This suggests that chemically specific ion−carboxylate complexation is also an important mechanism and helps explain the high Al3+ and Ag+ uptake relative to Zn2+ in GO templates. Afterward, the Mn+-GO complexes undergo a two-step annealing in air, first at 150 °C to complete drying and cause partial GO decomposition that liberates the bound metal and second at 500 °C to remove the reduced GO (rGO) templates by O2 oxidation and produce CO, CO2, and metal oxide films. Figure S4a−d shows XRD patterns of neat GO, Ag+-GO, Zn2+GO, and Al3+-GO films before and after the first stage of annealing at 150 °C. At 150 °C, the GO templates are thermally reduced and become rGO templates gradually (Figure S4e). Surprisingly, annealing of the Ag+-GO sample produces some crystalline metallic Ag0 (see Figure S4b), which indicates the presence of a reducing environment inside these films created by GO decomposition intermediates or evolved CO. Figure S5 shows that the surface structure of these films is preserved during 150 °C annealing. The second stage annealing process at 500 °C burns off the graphene template and creates final metal oxide films. Thermogravimetric analysis (TGA) profiles of neat GO, Ag+GO, Zn2+-GO, and Al3+-GO films are shown in Figure 2g. A mass loss of about 5 wt % before 100 °C is seen in all samples due to water desorption, and thermal decomposition begins between 150 and 200 °C. The carbon (rGO) oxidation step begins at about 400 °C and is influenced by metal loading. Aluminum inhibits oxidation, likely acting as a partial barrier to O2 diffusion, while Ag loading slightly enhances the oxidation rate, consistent with its previously reported behavior as a catalyst for low-temperature carbon oxidation.24 The rGO template and polystyrene substrate are completely removed as the temperature increases above 500 °C. From the TGA curves, the loadings of the metal oxides synthesized from Ag+-GO, Zn2+-GO, and Al3+-GO films are found to be 14.1, 11.2, and 17.1 wt %, respectively, which are close to the mass ratios predicted from the XPS spectra. The ability of aluminum to inhibit oxidation by a physical mechanism may be also related to its higher loading. Heat treatment of Ag+-GO films yields metallic Ag characterized by XRD (Figure S6a), which shows face-centered cubic Ag (JCPDS 04-0783). The ZnO product from Zn2+-GO (Figure S6b) is consistent with the wurtzite hexagonal structure (JCPDS 36-1451). The Al2O3 product from Al3+-GO requires an annealing temperature over 900 °C to improve the crystallinity of Al2O3, and its XRD pattern corresponds to αAl2O3 in JCPDS 42-1468 (Figure S6c). We also intercalated
generation, G2, structures can be successfully replicated in the form of a textured metal oxide film. The templated synthesis of textured metal oxides starts with the intercalation of metal ions into the prefabricated textured GO film. To understand and control this process, a series of intercalation experiments were first carried out on thick nontextured (planar) GO films to generate samples for detailed X-ray diffraction (XRD) characterization. The XRD pattern (Figure 2a) of neat GO (before metal ion intercalation) shows a strong peak at 11.7°, which corresponds to a 7.6 Å interlayer spacing from Bragg’s law. The spacing is typical of other GO multilayer films/papers and much larger than that of graphite (∼3.4 Å) due to the pillaring and hydration effects of oxygen functional groups on the GO basal plane.19 The films were immersed overnight in ZnNO3 solutions, washed with water, and in a fully wet state placed on a silicon substrate for XRD analysis. Time-resolved XRD spectra during the drying process initially show no peak due to the highly expanded waterswollen structure of the fully hydrated GO films. After 18 min of drying, a strong peak that represents multilayer stacking reappears at 9.2°, and the peak shifts to 9.6° after overnight drying. The shift of the peak (compared with the peak at 11.7° in neat GO) reflects the expansion effect of ion intercalation. Similar time-resolved XRD spectra of Ag+-, Al3+-, and Mn2+intercalated GO films are shown in Figure S2a−c, respectively. In Figure 2b, after partial drying during overnight storage at room temperature, the peak shifts for a series of salt solutions from 11.7° (neat GO) to 11.2, 9.6, and 8.8° for films containing Ag+, Zn2+, and Al3+, respectively. The corresponding interlayer spaces increase from 7.6 Å (neat GO) to 7.9 Å (Ag+), 9.2 Å (Zn2+), and 10.1 Å (Al3+). The increase in d-spacing correlates with the hydrated diameter of Ag+, Zn2+, and Al3+ (2.5, 6.0, and 9.0 Å), and this correlation likely reflects the partially hydrated state of these ions and complexes in the GO gallery spaces. X-ray photoelectron spectroscopy (XPS) was used to make quantitative estimates of metal loading in the GO film. To quantify the uptake of metal ions, G1-2D-GO crumpled films were first fabricated and soaked in various precursor salt solutions, and the metal ion-intercalated GO films (Mn+-GO) were then rinsed with water and dried overnight prior to XPS characterizations. Figure S3a shows the presence of metallic elements in XPS survey scans of Ag+-GO, Zn2+-GO, and Al3+GO films. The uniformity of ion intercalation across the GO film was then evaluated by ion beam etching and XPS depth profiling. Figure 2c−e shows Ag 3d, Zn 2p, and Al 2p spectra from 40 to 120 nm etch depth. The intensity and shape of the XPS spectra remain nearly constant, which indicates the uniformity in the amount and local chemical environment of the metal atoms throughout the films. For neat GO films, no metallic elements were detected by either the XPS survey scan or depth profiling. Fitting along with the C 1s spectrum of each metal ion−GO film (Figure S3b−d) allows the calculation of Ag/C, Zn/C, and Al/C atomic ratios, which remain nearly constant from 40 to 120 nm at around 3, 2, and 7%, respectively. In general, intercalation of metal ions into hydrated multilayer GO is driven by the combination of electrostatic attraction and chemical complexation. Net negative charge on GO nanosheets (ζ-potential ∼ −50 mV) provides a significant driving force to recruit metal ions from bulk solution to the nanosheet surfaces (ion concentration enhancement factor by the Nernst equation: [M]surface/[M]bulk = exp(EzF/RT) is ∼52 for divalent cations at −50 mV). This large factor drives ions to 10872
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Figure 3. Surface morphology of ZnO wrinkled/crumpled textures. The ZnO textures can be designed by selecting GO sacrificial templates with different film thicknesses and compression modes during prefabrication. Top-to-bottom comparison shows how the wrinkling wavelength of G1-1D-ZnO and the crumpling size of G1-2D-ZnO can be systematically tuned. The scale bars in all of the panels are 5 μm.
two other multivalent transition metal ions (Cu2+, Mn2+) into GO film templates, followed by calcination in air at 600 and 800 °C, respectively. The XRD patterns of these products are shown in Figure S6d,e and are well-indexed to the cubic Mn2O3 phase (JCPDS 41-1442) and the monoclinic CuO structure (JCPDS 89-2529), respectively. In addition, by fitting the XRD spectra with multiple standard patterns of metal carbides (listed in Supporting Information), we do not find any metal carbide species present in the products. The final products of this templating route are metal oxide films that show a rich set of wrinkled and crumpled textures (Figures 3, 4, and S7−S11). We noticed that the surface topography is affected by the metal ion concentration in the immersion solution. As shown in the scanning electron microscope (SEM) images in Figure S7, a low concentration of Zn(NO3)2 solution (0.05 M) gives rise to a crack-prone, noncontinuous ZnO film. An optimal concentration (0.10 M) produces a crack-free and homogeneous 2D ZnO crumpled structure, which replicates the surface morphology of the G12D-GO template. Further increase in Zn(NO3)2 concentration to 0.91 M impedes the replication process and produces visible ZnO large particles that disrupt the crumpled texture. The concentration effects were similar in the Al2O3, Mn2O3, and
CuO systems, but the optimal concentrations vary from 0.1 M for Al(NO3)3 (Figure S8), 0.03 M for MnCl2 (Figure S9), to 0.1 M for CuCl2 (Figure S10). Interestingly, templated Ag films do not replicate the crumpled structure of 2D-compressed GO templates at any AgNO3 concentration from 0.03 to 0.90 M (Figure S11). We believe the failure to replicate in the Ag system is related to the formation and extensive sintering of zerovalent Ag nanoparticles, as shown later. In addition, Figure S12 displays the SEM images of metal oxides templated by hydrazine-treated G1-2D templates, which exhibit a lower O/C atomic ratio.25 The templated metal oxides do not replicate GO crumpled textures, confirming the importance of oxygencontaining functional groups on GO nanosheets in the processes of multilayer expansion, ion uptake, and binding. Having demonstrated the basic ability to replicate complex graphene wrinkled/crumpled topographies in metal oxide films, we set out to see if the deformation mode and length scales of those textures could be systematically tuned. Previous work has shown that the texture in GO films is determined by the nature and extent of mechanical compression and the initial film thickness, which in turn is set by the concentration of the GO dispersion used in solution casting.12Figure 3 shows the ability to control the deformation mode (1D wrinkling vs 2D 10873
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Figure 4. Highly textured metal oxides templated by multi-generational GO structures. Sequential deformation can generate multiscale GO films, which act as versatile templates to synthesize different metal oxide nanostructures. First row shows the SEM images of (a) G1-1D-GO wrinkled template and resulting (b) G1-1D-ZnO, (c) G1-1D-Al2O3, and (d) G1-1D-Mn3O4 wrinkled structures. Second row shows the SEM images of (e) G1-2D-GO crumpled template and resulting (f) G1-2D-ZnO, (g) G1-2D-Al2O3, and (h) G1-2D-Mn3O4 crumple-textured structures. Third row shows the SEM images of (i) G2-2D2D-GO template, (j) G2-2D2D-ZnO, (k) G2-2D2D-Al2O3, and (l) G2-2D2D-Mn3O4 structures; (m−p) fourth row shows the corresponding cross-sectional SEM images. The scale bars in the first three rows are all 5 μm; the scale bars in the insets are 1 μm. The scale bars in the fourth row are 2 μm.
crumpling) and the feature length scale in metal oxide films. Uniaxial contraction creates periodic first-generation (G1) wrinkled (1D) graphene templates that lead to 1D ZnO wrinkled textures (G1-1D-ZnO); isotropic compression results in G1-2D crumpled templates and then 2D ZnO crumpled textures (G1-2D-ZnO). The wrinkling wavelength of G1-1DGO and crumpling length scale of G2-2D-GO are most easily adjusted by varying the G0 film thickness. By drop-casting the GO dispersion with concentration at 0.1−2.6 mg mL−1, the thickness of G0 films can be precisely tuned from 50 to 600 nm (Figure S13a). The corresponding G1-1D-GO templates (Figure 3a−d) display ordered wrinkled structures with characteristic wavelengths ranging from 2.5 to 25.7 μm, characterized by fast Fourier transform (FFT) analysis (Figure S13b). The G1-1D-ZnO wrinkled textures (Figure 3i−l) made from these templates also vary in a regular and systematic fashion with characteristic wavelength changing from 1.1 to 10.0 μm (Figure S13c). Remarkably, this process accurately replicates fine textural features, while reducing the wavelengths to 60% of those in the parent graphene film (Figure S13d). We attribute this behavior
to the formation of a connected metal oxide structure that undergoes a uniform volumetric shrinkage in the late stages of template removal. The volumetric shrinkage is large since the overall mass loss on calcination is ∼90% and is observed to be nearly constant as graphene film thickness is varied. By incorporating an empirical factor that represents the linear shrinkage during the calcination process, the characteristic wavelength of templated metal oxide wrinkled textures can be predicted by a modification of the theory used to describe graphene wrinkling (eq 1): λmetal oxide = 2πsh(Ec̅ /Esf̅ )1/3
(1)
where h is the G0 thickness and E̅ i = Ei/(1 − ν2i ) is given in terms of the Young’s modulus E and Poisson’s ratio ν of the coating (c) or shrink film substrate (sf), respectively.26 Linear shrinkage factor (s) can be defined in eq 2 as the ratio of wrinkling wavelength of metal oxide to GO template: s = λmetal oxide /λGO template 10874
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Figure 5. TEM images of GO crumpled template and crumple-textured metal oxides. (a,b) G1-2D-GO crumpled template. (c,d) G1-2D-ZnO. (e,f) G1-2D-Al2O3. (g,h) G1-2D-Mn2O3. (i,j) Ag product. The scale bars in the first rows are all 250 nm; 50 nm in the second row. The top row images clearly show the nanosheet-like nature of the metal oxide product, while the bottom row high-resolution images show the substructure of the nanosheets to be fused 2D nanoparticle arrays.
textures (Figure 4c), G1-1D-Mn2O3 wrinkled textures (Figure 4d), G1-2D-Al2O3 crumpled textures (Figure 4g), and G1-2DMn2O3 crumpled textures (Figure 4h) were all synthesized using either G1-1D-GO (Figure 4a) or G1-2D-GO templates (Figure 4e). The SEM images show a strong morphological similarity between the original GO template and the final metal oxide thin film, demonstrating the broad applicability of the method. To achieve higher-order deformation modes, multiscale G2 templates were fabricated by sequential deformation. For instance, repeated biaxial contraction (2D2D) results in multiscale crumpled−crumpled textures with two characteristic length scales: smaller features (∼3 × 3 μm) conserved from the first generation, G1, and larger features created in the second generation, G2 (∼15 × 15 μm) (Figure 4i). Using these multiscale G2 structures as sacrificial templates produces a variety of complex textures in metal oxides, including G22D2D-ZnO (Figure 4j), G2-2D2D-Al2O3 (Figure 4k), and G22D2D-Mn2O3 structures (Figure 4l). In each case, the metal oxide films exhibit a crumpled−crumpled hierarchical structure that is similar to the G2-2D2D-GO template. From the crosssectional SEM images (Figure 4n−p), the metal oxide films have a substructure of stacked nanosheets, indicating that this synthesis approach preserves not only the overall film structure and surface morphology but also the internal lamellar nanostructure that is derived from stacked GO films. To further characterize these underlying nanostructures, we performed transmission electron microscopy (TEM) analyses on the GO crumpled templates and crumple-textured metal oxides (Figure 5). The intercalation synthesis has the ability to transcribe nanoscale curvature (such as creases and folds) of the G1-2D-GO template (Figure 5a) into versatile metal oxide systems. It can be seen that the metal oxide products, such as ZnO (Figure 5c), Al2O3 (Figure 5e), and Mn2O3 (Figure 5g), consist of sheet-like substructures composed of interconnected nanoparticle arrays. The nanoparticle sizes vary with the chemical composition: 5−10 nm for ZnO (Figure 5d), 10−20 nm for Al2O3 (Figure 5f), and 15−25 nm for Mn2O3 (Figure 5h). These TEM images provide important insights into the assembly and growth mechanisms. The decomposition of
Similar FFT analysis was also conducted on G1-1D-Al2O3 and G1-1D-Mn2O3 wrinkled textures (Figure S13e), and the dominant characteristic wavelengths for the G1-1D-GO template, G 1 -1D-ZnO, G 1 -1D-Al 2 O 3 , and G 1 -1D-Mn2 O 3 wrinkled textures are 7.0, 3.3, 2.2, and 3.9 μm. The shrinkage factor of each metal oxide can be obtained and is summarized in Figure S13f: 0.43 for ZnO, 0.31 for Al2O3, and 0.56 for Mn2O3. It also indicates that the intercalation templating synthesis can transcribe the complex surface textures of GO templates into versatile metal oxides, while reducing the feature size (i.e., wrinkling wavelength and crumpling size). The length scale of 2D crumpled textures can be tailored in a similar way. The G1-2D-GO template results in a ZnO structure with chaotic crumples and no orientational order (G1-2DZnO). By adjusting the thickness of G0 coating from 50 to 600 nm, the crumpling feature size of the G1-2D-GO template can be tuned in the range of 3 × 3 to 20 × 20 μm, and the corresponding G1-2D-ZnO crumpling features range in size from 1 × 1 to 8 × 8 μm. Also, the processing order between ion intercalation and mechanical deformation affects the feature size of G1-2D-ZnO structures. We found that the ion intercalation can be carried out prior to mechanical deformation, and one still achieved a textured film. The preintercalation processing leads to the increase of Young’s modulus of GO templates (to the reinforcing or cross-linking effect of ion) and the creation of larger crumpling features (Figure S14b). The preintercalated G1-2D-GO template (intercalation then deformation) creates a G1-2D-ZnO structure with larger crumpling features (20 × 20 μm, Figure S14d) than the ZnO structure templated by post-intercalated GO samples (5 × 5 μm, deformation then intercalation, Figure S14c) after high-temperature calcination. Therefore, the processing order can be also utilized to tailor the crumpled metal oxide structures. The broad applicability of this approach can be further illustrated by extending the data to other metal oxides and by exploring more complex “second-generation (G2)” texture modes created in graphene films by multiple compression steps.12 To test other materials systems, G1-1D-Al2O3 wrinkled 10875
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Figure 6. Electrochemical and photocatalytic characterizations of G1-2D structures. (a) Cyclic voltammetry curves of planar Mn2O3 and G12D-Mn2O3 electrodes at the scan rate of 25 mV s−1. (b) Cycling stability of G1-2D-Mn2O3 electrodes at the scan rate of 75 mV s−1. (c) Photocatalytic degradation of rhodamine B using G1-2D-ZnO film. The duration of UV irradiation is 3 h. (d) Rate kinetics of photodegradation of rhodamine B by G1-2D-ZnO and planar ZnO films. No significant change occurred in the UV−vis spectra of the dye under light irradiation (in the absence of catalyst), suggesting the observed photodegradation of dyes is not due to the self-oxidation of dye. k is the first-order reaction constant of photodegradation of rhodamine B.
complex multicomponent metal oxides with tunable surface textures. The interconvertibility and high mobility of Ag can be utilized as an independent component in the hybrid structure, which can be advantageous in multiple technological applications, including antibacterial surfaces27 and photocatalysts. The high surface area of these complex textured metal oxide structures may be advantageous for enhanced electrochemical or catalytic activity. As a proof-of-concept, the capacitive current densities of the G1-2D-Mn2O3 film was evaluated via cyclic voltammetry in KOH electrolyte and compared with planar Mn2O3 film (templated from G0 GO coatings). G1-2DMn2O3 film achieves 300−400% improvement in current density compared to planar Mn2O3 (Figure 6a). Both Mn2O3 structures remain electromechanically robust under a wide range of scan rates (Figure S16a,b) and show excellent cycling stability after 300 cycles (Figure 6b). Similarly, semiconducting G1-2D-ZnO film (band gap ∼3.3 eV,28Figure S16c) demonstrates photocatalytic performance better than that of planar ZnO film, which is evaluated by the photodegradation of rhodamine B under UV-light irradiation. Figure 6c shows the time-dependent absorbance spectra of rhodamine B with a continuous degradation catalyzed by G1-2D-ZnO film. The degradation rate of rhodamine B catalyzed by crumpled ZnO is about 2 times faster than that catalyzed by planar ZnO film (Figure 6d). The improvement on electrochemical and photocatalytic activities mainly originates from the highly convoluted metal oxide structures, which exhibit increased area density of active material over the planar films.29,30 Altogether, a generalized templating method has been developed to replicate complex graphene topographies in
cation−GO complexes initially produces atomically dispersed metal species or ultrafine clusters confined in 2D gallery spaces. The existence of the graphene template through most of the annealing process causes preferential in-plane (X,Y) mobility of these precursors, so that the growth and sintering of metal oxide nanoparticles produce 2D interconnected particle arrays that are the basic nanoscale building blocks of the final metal oxide films. The final metal oxide films have nanoscale features that consist of 2D particle superlattices as porous nanosheets. While all of the metal oxide films have this particle-based nanosheet structure, the zerovalent Ag (Figure 5i,j) undergoes more extensive sintering and mobility, yielding large fused nanoparticles that cannot accurately replicate the fine textural features in the GO templates. We believe that the ease of reduction of Ag+ to metallic silver and the low melting point of ultrafine metallic Ag clusters are the main reasons for the failure of structural transcription. Such versatile templated syntheses also provide an opportunity to synthesize and direct the design of multicomponent mixed metal oxides. As a proof-of-concept, a hydrated GO film was first immersed in a mixture of 0.1 M AgNO3, Zn(NO3)2, and Al(NO3)3 solution and then subjected to a two-step annealing treatment in air at 150 and 800 °C to form a functional metal oxide and decompose GO sheets, respectively. The XRD pattern of the sintered product is consistent with the spinel ZnAl2O4 structure (JCPDS 36-1451) and metallic Ag (JCPDS 04-0783), as shown in Figure S15a. The composite product is also a macroscopically sized, continuous, free-standing film which resembles the crumpled morphology of original GO templates (Figure S15b,c), demonstrating the capability of the templated synthesis for 10876
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ACS Nano
Figure 7. Generalized synthetic routes for fabricating crumple/wrinkle-textured metal oxides. Wrinkled, crumpled, and hierarchical GO structures are fabricated via uniaxial (1D), biaxial (2D), and sequential deformation, respectively. The textured GO films can be then utilized as versatile intercalation templates for different metal ions (e.g., Zn2+, Al3+, Mn2+). After graphene-based templates were removed, multiple textured metal oxides (ZnO, Al2O3, Mn2O3) are achieved with replicated wrinkled, crumpled, and hierarchical structures. The radius of each SEM is 20 μm.
an indirect synthetic route for the creation of crumpled and wrinkled textures in materials systems that cannot sustain the large mechanical deformation needed for direct texturing by compression. Also, our technique has the advantage (relative to chemical vapor deposition methods) of simple fabrication steps in the open atmosphere and low temperature using the potential future commodity material of GO. Therefore, we expect that such wet coating processes can be scalable up to meter length scale or even in a roll-to-roll fashion in the future. We envision this versatile, facile, and scalable fabrication of highly textured metal oxides or metal oxide−graphene composites could enable electrochemical or catalytic electrodes with enhanced performance for energy storage,35−37 catalysis,38 and biosensing.39
various metal oxides (Figure 7). Intercalation of hydrated metal ions into textured GO multilayer films followed by dehydration, thermal decomposition, and air oxidation produces metal oxide thin films with high-fidelity replication of the original GO wrinkled/crumpled textures. It is remarkable that this technique achieves replication of fine textural features at only ∼10 wt % metal loading since this is far lower than the metal loading needed to replace the graphene template with fully dense metal oxide at the same physical sample size. Our results show that metal−oxygen complexes decompose upon annealing to ultrafine metal oxide precursors within intersheet graphene gallery spaces, which then grow into sintered nanoparticle arrays during high-temperature graphene oxidation. The particles assemble into porous nanosheet-like superstructures that reflect the 2D confinement present during early growth and sintering. With the exception of Ag, which shows evidence of particle fusion, these porous particle assemblies show sufficient structural integrity to reproduce fine wrinkled/ crumpled features even during consolidation and bulk shrinkage in the late stages of template removal.
METHODS Materials. Ethanol, dichloromethane (DCM), acetone, hydrazine monohydrate, silver nitrate (AgNO3), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), aluminum nitrate (Al(NO3)3), manganese(II) chloride tetrahydrate (MnCl2·4H2O), and copper(II) chloride dihydrate (CuCl2·4H2O) were purchased from Sigma-Aldrich. Thermally responsive polyethylene heat shrink films were bought from Grafix. All water was deionized (18.2 MΩ, milli-Q pore). All reagents were used as received without further purification. Fabrication of Multi-generational Graphene Oxide Templates. GO suspensions were prepared by a modified Hummer’s method, purified, and characterized as described previously.40 The concentration of stock GO suspension is 3.1 mg mL−1, with a C/O atomic ratio of approximately 1.8. Different concentration of GO suspensions was prepared by diluting with ethanol. The polymer shrink film was cut into 4 cm2 squares and washed with ethanol. Once dry, samples were treated with pure oxygen plasma in a Deiner Atto standard plasma system with a borosilicate glass chamber and a 13.56 MHz, 0−50 W generator. The chamber pressure was pumped down to and maintained at 0.13 mbar while being flushed with pure oxygen for 5 min. Plasma was then generated at 100% power (50 W) for 90 s followed by slow venting of the chamber. Next, 150 μL of a GO suspension was drop-cast onto the substrates. Once dry, the planar
CONCLUSIONS This templated synthesis has great versatility and enables the intercalation of various metal ions ranging from Ag+, Zn2+, and Al3+ to Mn2+ and Cu2+ and even combinations of multiple ions. The intercalation templates can be selected from a recently reported “genealogy” of multi-generational complex graphene textures,12 thus enabling the growth of Zn, Al, Mn, and Cu oxide thin films with highly complex wrinkled and crumpled textures and systematic control of the metal oxide structural parameters (e.g., deformation mode, length scale). There has been extensive recent work on the folding, crumpling, and building of graphene structures,31−34 and we anticipate that the present technique will allow many of these structures to be replicated into metal oxide platforms. This approach introduces 10877
DOI: 10.1021/acsnano.6b05179 ACS Nano 2016, 10, 10869−10879
Article
ACS Nano GO films (G0) were obtained, and the samples were placed and allowed to shrink in an oven at 140 °C for 30 min. For 1D uniaxial deformation, two sides of the sample were constrained by firmly clamping the substrate, and the compression strain was actuated only in one direction; for 2D biaxial deformation, the sample was shrunk without any clamps or constrains. At this stage, the G1 wrinkled/ crumpled patterns were obtained. Afterward, the samples were removed from the oven and allowed to cool for approximately 30 min on the benchtop. To achieve higher-generation GO hierarchy, DCM was then used to dissolve the polystyrene substrates away, and free-standing GO films were left in the solvent. The free-standing GO films were washed in DCM for 2 h, in acetone for 15 min, and then transferred to ethanol. To achieve subsequent deformation, the freestanding films in ethanol were transferred to new plasma-treated shrink films, and the G2 shrinkage (at 140 °C for 30 min) was conducted after drying. After being shrunk, the G2 hierarchy was obtained. Preparation of Chemically Reduced GO. Hydrazine dilute solution (2 wt %) was prepared and stirred overnight. The GO templates were immersed fully in the hydrazine solution; the reduction was allowed to proceed at 80 °C for at least 24 h, and a dark browncolored rGO film was produced. The rGO structures were sequentially rinsed with water and dried in an oven at 70 °C. Synthesis of Textured Metal Oxides. Solutions of AgNO3, Zn(NO3)2, Al(NO3)3, CuCl2, and MnCl2 were prepared in different concentration from 0.003 to 0.3 M. The GO templates (including G11D, G1-2D, G2-2D2D) were immersed in each precursor solution overnight (at least 5 mL solution for a 1 cm2 sized GO template). After the ion uptake overnight, the GO templates were removed from the precursor solutions and rinsed in milli-Q water for 15−30 s. The samples were dried at room temperature overnight and annealed at 150 °C in an oven for at least 2 h. The samples (including Ag+-GO, Zn2+-GO, Al3+-GO, Cu2+-GO) were then calcined at 600 °C in a furnace for at least 2 h. The Mn2+-GO sample was calcined at 800 °C to form Mn3O4. The heating/cooling ramps were set at 40 °C min−1. Characterization. Surface morphology of the multi-generational GO templates, rGO templates, and templated metal oxides (Ag, ZnO, Al2O3, Mn3O4, and CuO) was investigated using a field emission scanning electron microscope (LEO 1530 VP) operating at 10.0 kV for low-, medium-, and high-resolution imaging. Before the SEM imaging, all samples were coated with a layer of AuPd (
