Template-free Synthesis of Mesoporous and Crystalline Transition Metal Oxide Nanoplates with Abundant Surface Defects

Abstract

•Template-free syntheses of mesoporous transition metal oxides are demonstrated•Crystal interconversion results in highly crystalline frameworks•Elimination of small molecules creates continuous porosity within oxides•Unique surface-step defects boost the activity of porous oxides There has been a tremendous upsurge of interest in crystalline mesoporous transition metal oxides that have enormous potential for applications such as catalysis, energy conversion and storage, separation, and biomedicines. Traditional syntheses of mesoporous oxides rely on the use of templates, which ultimately bring a great challenge in the balance of crystallinity and porosity at the mesoscale. Improving crystallinity of oxides usually occurs along with the growth of crystalline grains, leading to the collapse of mesostructures. Here, we demonstrate the template-free method to meet these challenges. By low-temperature crystal interconversion, the prepared crystalline mesoporous oxides are shown to have abundant surface-step defects that strongly correlate with the activity of oxides. This synthetic approach is applicable to a number of transition metal oxides, providing a powerful tool to engineer their crystallinity and porosity as well as their catalytic activity. Mesoporous transition metal oxides with crystalline walls show promising applications in catalysis and energy storage. Syntheses of those crystalline mesoporous oxides, however, currently remain as an unmet challenge because of the large volume shrinkage during crystallization in a sol-gel process. Here, we demonstrate a facile and general template-free method to prepare six mesoporous transition metal oxides, e.g., CuO, CoO, and spinel MCo2O4 (M = Co, Cu, Mn, Zn), with highly crystalline walls and continuous mesopores in the forms of two-dimensional nanoplates or nanosheets. The method is based on the thermal crystalline transformation from basic carbonates to mesoporous metal oxides. The crystal interconversion not only endows the crystallinity and mesoporosity of oxides but also generates abundant surface-step defects that show remarkable enhancement of the catalytic activity of porous oxides. Our method likely provides an alternative paradigm for cost-effective and universal synthesis of crystalline mesoporous oxides beyond the traditional templating methods. Mesoporous transition metal oxides with crystalline walls show promising applications in catalysis and energy storage. Syntheses of those crystalline mesoporous oxides, however, currently remain as an unmet challenge because of the large volume shrinkage during crystallization in a sol-gel process. Here, we demonstrate a facile and general template-free method to prepare six mesoporous transition metal oxides, e.g., CuO, CoO, and spinel MCo2O4 (M = Co, Cu, Mn, Zn), with highly crystalline walls and continuous mesopores in the forms of two-dimensional nanoplates or nanosheets. The method is based on the thermal crystalline transformation from basic carbonates to mesoporous metal oxides. The crystal interconversion not only endows the crystallinity and mesoporosity of oxides but also generates abundant surface-step defects that show remarkable enhancement of the catalytic activity of porous oxides. Our method likely provides an alternative paradigm for cost-effective and universal synthesis of crystalline mesoporous oxides beyond the traditional templating methods. There has been a substantial amount of interest in nanostructured mesoporous oxides with large accessible surface areas/pore volumes.1Kresge C.T. Leonowicz M.E. Roth W.J. Vartuli J.C. Beck J.S. 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To avoid the thermal crystallization of oxides from its amorphous sol, our synthetic concept is to use conversion of highly crystalline basic carbonate salts (i.e., M(OH)2CO3) to metal oxides (Figure 1A).38Xiong S. Chen J.S. Lou X.W. Zeng H.C. Mesoporous Co3O4 and [email protected] topotactically transformed from chrysanthemum-like Co(CO3)0.5(OH)·0.11H2O and their lithium-storage properties.Adv. Funct. Mater. 2012; 22: 861-871Crossref Scopus (486) Google Scholar,39Hu L. Peng Q. Li Y. Selective synthesis of Co3O4 nanocrystal with different shape and crystal plane effect on catalytic property for methane combustion.J. Am. Chem. Soc. 2008; 130: 16136-16137Crossref PubMed Scopus (765) Google Scholar Simultaneously, the removal of small molecules such as CO2 and H2O creates mesoscopic porosity in the size range of 3–8 nm within metal oxides in the absence of any templates. Since most basic carbonate salts have a low thermal decomposition temperature, the elimination of non-metal components effectively avoids the overgrowth of crystal grains and the collapse of pores while maintaining the high crystallinity of oxides. Six mesoporous oxides including Co3O4, CoO, Cu0.92Co2.08O4, MnCo2O4.5, ZnCo2O4, and CuO were synthesized in two-dimensional (2D) nanostructures as nanoplates and nanosheets. Compared with other templating syntheses, our method is unique in that there are abundant surface-step defects as identified on the surface of mesoporous Co3O4 nanoplates. Those defect-rich Co3O4 nanoplates are proven to have superior catalase- and peroxidase-like activity, about 130-fold more active than the commercial one and 10-fold more active than the mesoporous Co3O4 prepared from hard templating. Density functional theory (DFT) calculations further reveal the key role of the surface cobalt step in promoting the activity of Co3O4. Our method is therefore expected to provide a new way of preparing nanostructured mesoporous oxides with crystalline walls and defect-rich surfaces that are potentially useful in heterogeneous catalysis. The synthetic method of oxide nanoplates is schematically presented in Figure 1A. Using mesoporous Co3O4 as an example, nanoplates of cobalt hydroxide carbonate (Co2(OH)2CO3) were synthesized through a hydrothermal method. Cobalt(II) nitrate (Co(NO3)2) was first mixed with urea in water/ethanol in the presence of oleic acid as stabilizers. The mixture was heated to 160°C for 9 h in an autoclave. Under this temperature, urea gradually decomposed to form isocyanic acid and ammonia that controlled the pH of the solution (Equation 1). Meanwhile, isocyanic acid hydrolyzed in water further produced CO2 (Equation 2).40Yim S.D. Kim S.J. Baik J.H. Nam I.S. Mok Y.S. Lee J.-H. Cho B.K. Oh S.H. Decomposition of urea into NH3 for the SCR process.Ind. Eng. Chem. Res. 2004; 43: 4856-4863Crossref Scopus (260) Google Scholar As such, Co(NO3)2 reacted with CO2 under slightly basic condition to form Co2(OH)2CO3 nanoplates capped by oleic acid (Equation 3). As-obtained Co2(OH)2CO3 nanoplates were calcined at 300°C for 3 h to produce mesoporous Co3O4 nanoplates (Equation 4). The corresponding reactions are as follows:NH2-CO-NH2 → HNCO + NH3(Equation 1) HNCO + H2O → CO2 + NH3(Equation 2) CO2 + 4OH− +2Co2+ → Co2(OH)2CO3 + H2O(Equation 3) 3Co2(OH)2CO3 + O2 → 3CO2 + 3H2O + 2Co3O4(Equation 4) Figure 1 shows electron microscopy characterizations of Co2(OH)2CO3 and Co3O4 nanoplates. Low-magnification scanning electron microscopy (SEM) images confirm monodispersity of the two types of nanoplates with dog-bone-like nanostructures. Co2(OH)2CO3 nanoplates have a length of 3.96 ± 0.46 μm with a thickness of 130 ± 30 nm (Figures S1 and S2). The center width of Co2(OH)2CO3 is 0.85 ± 0.15 μm while that of the two ends is 1.05 ± 0.17 μm. Co2(OH)2CO3 nanoplates are densely packed without any obvious porosity under transmission electron microscopy (TEM, Figure 1C). The high-resolution TEM (HRTEM) image in Figure 1D shows clear lattice fringes of Co2(OH)2CO3 of about 0.26 nm, assigned to its (121) plane. Figure 1E displays the selected area electron diffraction (SAED) pattern in which (001) and (2‾00) planes are well resolved along the [010] direction. This is in good agreement with the P21/a space group of monoclinic Co2(OH)2CO3. Since the SAED is taken perpendicular to the nanoplate, the growth direction of the Co2(OH)2CO3 nanoplate is along [001] while the lateral direction is oriented along [100]. The ordered SAED pattern also reveals single-crystal-like crystallinity of Co2(OH)2CO3 nanoplates. After calcination, the nanostructures of 2D plates were largely retained while forming the mesoscale porosity in Co3O4. The removal of large anions, i.e., OH− and CO32−, brought about a minor volume shrinkage. The average length and thickness of these mesoporous Co3O4 nanoplates are measured as 3.88 ± 0.24 μm and 90 ± 20 nm (Figure S3), respectively, slightly smaller than those of Co2(OH)2CO3 nanoplates. Under dark-field scanning TEM (STEM) (Figure 1H), Co3O4 nanoplates showed interconnected small frameworks separated with finite pores throughout in the range of 3–8 nm. The ordered SAED patterns along [110] zone axes confirm the high crystallinity of spinel Co3O4 with the space group of Fd3m. Thermogravimetric analysis (TGA) confirms the thermal decomposition of Co2(OH)2CO3 (Figure 2A). The decomposition occurs sharply at 300°C with a weight loss of about 29%, consistent with the theoretical weight loss (24 wt %) for the crystalline transition from Co2(OH)2CO3 to Co3O4. The additional weight loss of 5 wt % is possibly due to the removal of oleic acid that are thermally decomposed around 250°C.41Wilson D. Langell M.A. XPS analysis of oleylamine/oleic acid capped Fe3O4 nanoparticles as a function of temperature.Appl. Surf. Sci. 2014; 303: 6-13Crossref Scopus (310) Google Scholar This was also confirmed by infrared spectroscopy (Figure S4). The crystalline transition from Co2(OH)2CO3 to Co3O4 was examined by X-ray diffraction (XRD). In line with the TGA analysis, Co2(OH)2CO3 (PDF no. 01-079-7085) is readily transformed to spinel Co3O4 (PDF no. 090418) as displayed in Figure 2B. The appearance of the small-angle X-ray scattering (SAXS) peak at q value of about 0.55 nm−1 in Figure 2C is indicative of the mesoscale periodicity in Co3O4.42Liu B. Kuo C.-H. Chen J. Luo Z. Thanneeru S. Li W. Song W. Biswas S. Suib S.L. He J. Ligand-assisted co-assembly approach toward mesoporous hybrid catalysts of transition-metal oxides and noble metals: photochemical water splitting.Angew. Chem. Int. Ed. 2015; 127: 9189-9193Crossref Google Scholar,43Mohanty P. Fei Y. Landskron K. Synthesis of periodic mesoporous coesite.J. Am. Chem. Soc. 2009; 131: 9638-9639Crossref PubMed Scopus (25) Google Scholar Figure 2D shows the nitrogen sorption isotherms of as-made Co3O4 nanoplates. A representative type-IV isotherm of Co3O4 nanoplates is characteristic of mesoporosity. The Brunauer-Emmett-Teller (BET) specific surface area is 127 m2 g−1 with an average pore diameter of 4.4 nm calculated by the Barrett-Joyner-Halenda method (Figure S5A). The BET specific surface area of mesoporous Co3O4 nanoplates is also comparable with other mesoporous Co3O4 materials prepared using soft or hard templating methods, as summarized in Table S1. In comparison, the BET specific surface area of Co2(OH)2CO3 nanoplates is only 16 m2 g−1 without an obviously fine pore distribution (Figures 2D and S5B). The surface oxidation state of Co was examined using X-ray photoelectron spectroscopy (XPS) (Figures 2E and S6). The Co 2p peaks centered at 779.3 and 794.6 eV are assigned to Co 2p3/2 and Co 2p1/2, respectively.44Wang L. Wan J. Zhao Y. Yang N. Wang D. Hollow multi-shelled structures of Co3O4 dodecahedron with unique crystal orientation for enhanced photocatalytic CO2 reduction.J. Am. Chem. Soc. 2019; 141: 2238-2241Crossref PubMed Scopus (146) Google Scholar The splitting of Co 2p3/2 and Co 2p1/2 is 15.3 eV, as typically for Co3O4.45Liu L. Jiang Z. Fang L. Xu H. Zhang H. Gu X. Wang Y. Probing the crystal plane effect of Co3O4 for enhanced electrocatalytic performance toward efficient overall water splitting.ACS Appl. Mater. Interfaces. 2017; 9: 27736-27744Crossref PubMed Scopus (84) Google Scholar,46Jin Y. Wang L. Shang Y. Gao J. Li J. He X. Facile synthesis of monodisperse Co3O4 mesoporous microdisks as an anode material for lithium ion batteries.Electrochim. Acta. 2015; 151: 109-117Crossref Scopus (50) Google Scholar The ratio of Co3+/Co2+ is estimated to be 3.2:1 (Figure 2E), which is higher than the theoretical value of Co3+/Co2+.47Xu L. Jiang Q. Xiao Z. Li X. Huo J. Wang S. Dai L. Plasma-engraved Co3O4 nanosheets with oxygen vacancies and high surface area for the oxygen evolution reaction.Angew. Chem. Int. Ed. 2016; 55: 5277-5281Crossref PubMed Scopus (1074) Google Scholar By comparison, the Co3+/Co2+ ratio of commercial Co3O4 powder is measured as 1.6:1. In addition, the Co2+ satellite features became less pronounced in Co3O4 nanoplates. The surface composition has been confirmed using secondary ion mass spectrometry (SIMS). As shown in Table S2, the surface Co-to-O ratio of Co3O4 nanoplates is approximately 1:5.6 at a sputtering depth of 0.5 nm, while the surface Co-to-O ratio of commercial Co3O4 is 1:2.6. These results reveal that mesoporous Co3O4 nanoplates have a Co-defect-rich surface (see discussion below). Synthesis of hydroxide carbonates is a delicate balance of the solution pH and CO2 concentration.48Wang X. Zhuang J. Peng Q. Li Y. A general strategy for nanocrystal synthesis.Nature. 2005; 437: 121-124Crossref PubMed Scopus (2318) Google Scholar First of all, the solution pH controls the delivery of hydroxide anions. When adding sodium hydroxide directly, the formation of cobalt hydroxides was seen (Figure S7). At the same time, the balanced delivery rate of carbonates anions is extremely important. Again, the direct use of sodium carbonate directly resulted in the formation of cobalt carbonate (Figure S8). In our synthesis, the thermal decomposition of urea was chosen to generate carbonate-containing alkaline solution medium under hydrothermal conditions. Additionally, oleic acid plays a key role in determining the nanoplate structures of Co2(OH)2CO3 and balancing the hydrolysis rate of Co2+ ions as demonstrated in our control experiments (Figures S9–S11). In the absence of oleic acid, pure Co3O4 nanoparticles were formed, while Co2(OH)2CO3 was obtained in the presence of oleic acid regardless of solvents, e.g., water, ethanol, or water-ethanol mixtures. Oleate as a strong chelation ligand can bind to Co2+ ions and moderate their reactivity. In the absence of oleic acid, Co2+ favored the formation of Co(OH)2 thermodynamically with a low-solubility product constant (Ksp, Co(OH)2 = 5.92 × 10−15).49Jiang L. Sui Y. Qi J. Chang Y. He Y. Meng Q. Wei F. Sun Z. Jin Y. Structure dependence of Fe-Co hydroxides on Fe/Co ratio and their application for supercapacitors.Part. Part. Syst. Charact. 2017; 34: 1600239Crossref Scopus (30) Google Scholar The Co(OH)2 intermediates further decomposed to Co3O4 nanoparticles under high temperatures.50Guan J. Zhang Z. Ji J. Dou M. Wang F. Hydrothermal synthesis of highly dispersed Co3O4 nanoparticles on biomass-derived nitrogen-doped hierarchically porous carbon networks as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions.ACS Appl. Mater. Interfaces. 2017; 9: 30662-30669Crossref PubMed Scopus (72) Google Scholar Furthermore, oleic acid is critical to control the size of Co2(OH)2CO3 nanoplates (Figures S12 and S13). When the concentration of oleic acid varied from 0.17 M to 0.46 M, the average length of Co2(OH)2CO3 nanoplates decreased from 4.12 ± 0.41 μm to 3.22 ± 0.28 μm. The center width of Co2(OH)2CO3 nanoplates also decreased slightly from 0.88 ± 0.08 μm to 0.83 ± 0.12 μm. In addition, the thickness of Co2(OH)2CO3 nanoplates also decreased from approximately 150 nm to 90 nm with an increase of oleic acid concentration from 0.17 M to 0.46 M, as displayed in Figure S14. The formation of mesoporosity relies only on the thermal decomposition of hydroxide carbonates where the removal of small molecules (e.g., CO2 and H2O) can template the formation of mesoporosity. As confirmed by TEM in Figures S15 and S16, when annealed at 150°C (below thermal decomposition temperature) for 2 h, Co2(OH)2CO3 nanoplates remained stable without phase transformation to Co3O4. No mesopores were observed. When the thermal annealing temperature increased to 550°C, Swiss-cheese-like Co3O4 nanoplates with much larger pores (43 ± 14 nm) were formed. These results suggest that mesoporous Co3O4 nanoplates remain thermally unstable and that the pores will collapse at a higher calcination temperature (e.g., 550°C). The collapse of mesoporous structures is a result of the growth of Co3O4 crystalline domains, as reported previously.51Song W. Poyraz A.S. Meng Y. Ren Z. Chen S.-Y. Suib S.L. Mesoporous Co3O4 with controlled porosity: inverse micelle synthesis and high-performance catalytic CO oxidation at −60°C.Chem. Mater. 2014; 26: 4629-4639Crossref Scopus (216) Google Scholar Our synthetic strategy is general and applicable to the preparation of other mesoporous transition metal oxides. First of all, the oxidation state of transition metals is controllable through the calcination atmosphere. Taking Co2(OH)2CO3 as an example, calcining Co2(OH)2CO3 nanoplates under nitrogen results in the formation of CoO (Figures 3A and S17). In the absence of oxygen, the oxidation state of Co remains as +2 as presented in Co2(OH)2CO3. Second, other transition metals, e.g., Cu, Zn, and Mn with atomic size similar to that of Co, can be doped in Co2(OH)2CO3. This further leads to the formation of doped spinel structures after calcination. As examples, three mixed spinel oxides in the form of MCo2O4 were prepared (Figures 3 and S18–S21). Among them, ZnCo2O4 formed 2D nanosheets while the other two showed similar 2D plate-like structures. Lastly, our method has been validated for other hydroxide carbonates, e.g., Cu2(OH)2CO3. Similarly, Cu2(OH)2CO3 as precursors can be converted to crystalline mesoporous CuO nanosheets, as displayed in Figure S22. All porous oxides show irregular and disordered mesopores. To investigate the detailed pore structures and distribution, we further used STEM to characterize Co3O4 nanoplates. The images were acquired by rotating the samples from 2° to −78° and reconstructed into a 3D tomogram (Video S1). Representative STEM images at tilt angles from 2° to −78° are presented in Figure 4A. The interconnected mesopores were observed throughout Co3O4 nanoplates. Figures 4B–4D show the 3D reconstructed surface and cross-section images. It is interesting to point out that Co3O4 frameworks and mesopores are bicontinuous, although the porous structures are disordered. The wall thickness is measured as 5.9 ± 1.2 nm (Figure 4E) while the pore diameter is approximately 5.4 ± 1.4 nm (Figure 4F), slightly larger than that measured from BET. The volume ratio for void is measured as 47.6% ± 10%, consistent with the void ratio from BET measurement (approximately 48.3% using the density of Co3O4, see Supplemental Information for details). From the reconstructed cross-section images, mesopores are interconnected through vertical channels along the z direction (Video S2). This is likely attributed to the formation mechanism of mesopores. In the process of thermal decomposition of Co2(OH)2CO3, the outflow of small molecules such as CO2 and H2O templated the formation of mesoscale porosity while simultaneously converting Co2(OH)2CO3 to Co3O4. The diffusion of those small molecules through vertical channels likely minimizes the diffusion length to the surface of nanoplates. eyJraWQiOiI4ZjUxYWNhY2IzYjhiNjNlNzFlYmIzYWFmYTU5NmZmYyIsImFsZyI6IlJTMjU2In0