Crystalline films of Co3O4 are
deposited
by electrochemically oxidizing a tartrate complex of Co2+ in an aqueous, alkaline solution at elevated temperatures. The crystallinity
and stability of the films are a strong function of the deposition
temperature. Films deposited at temperatures from 50 to 90 °C
are amorphous, but films deposited from refluxing solution at 103
°C are crystalline. The crystalline films adhere strongly to
the substrate, whereas the amorphous films peel off of the substrate
when dried due to drying stresses. The crystalline films deposit with
the normal spinel structure, with a lattice parameter of 0.8097 nm
and crystallite size of 26 nm. The catalytic activity of Co3O4 for the oxygen evolution reaction (OER) of the crystalline
and amorphous films is compared by Tafel analysis in alkaline solution
at pH 14. The crystalline Co3O4 film has a Tafel
slope of 49 mV/decade and an exchange current density of 2.0 ×
10–10 A cm–2, whereas an amorphous
film deposited at 50 °C has a Tafel slope of 36 mV/decade and
an exchange current density of 5.4 × 10–12 A
cm–2. Because the films deposited from refluxing
electrolyte deposit directly as crystalline films, it is possible
to deposit them epitaxially on single-crystal Au(100). This opens
up the possibility to study the catalytic activity of different Co3O4 planes exposed to the electrolyte.
The development of facile methods for screening organic functional molecules through C-H bond activation is a revolutionary trend in materials research. The prediction of mechanochromism as well as mechanochromic trends of luminogens is an appealing yet challenging puzzle. Here, we present a strategy for the design of mechanochromic luminogens based on the dipole moment of donor-acceptor molecules. For this purpose, a highly efficient route to 2,7-diaryl-[1,2,4]triazolo[1,5-a]pyrimidines (2,7-diaryl-TAPs) has been established through programmed C-H arylation, which unlocks a great opportunity to rapidly assemble a library of fluorophores for the discovery of mechanochromic regularity. Molecular dipole moment can be employed to explain and further predict the mechanochromic trends. The 2,7-diaryl-TAPs with electron-donating groups on the 2-aryl and electron-withdrawing groups on the 7-aryl possess a relatively small dipole moment and exhibit a red-shifted mechanochromism. When the two aryls are interchanged, the resulting luminogens have a relatively large dipole moment and display a blue-shifted mechanochromism. Seven pairs of isomers with opposite mechanochromic trends are presented as illustrative examples. The aryl-interchanged congeners with a bidirectional emission shift are structurally similar, which provides an avenue for understanding in-depth the mechanochromic mechanism.
We report a facile and low-cost bottom-up synthesis of ultrathin Zn(bim)(OAc) MOF nanosheets (with thicknesses of ∼5 nm and a high yield of ∼65%) and their derived N-doped porous ultrathin (2.5 ± 0.8 nm) carbon nanosheets (UT-CNSs) for energy storage.
a b s t r a c tOur current paper is devoted to studying the numerical and analytical solutions for a class of Generalized Fractional Diffusion Equations (GFDEs) with new Generalized TimeFractional Derivative (GTFD). The GTFD we propose here is defined in the Caputo sense. We consider the GFDEs on a bounded domain. The numerical solutions are obtained by using the Finite Difference Method (FDM) of full discretization. The stability of FDM is discussed and the order of convergence is evaluated numerically. Numerical experiments are given, which illustrate that the FDM is simple and effective for solving the GFDEs with different coefficients and source functions. An interesting phenomenon is that we can observe the period-like solution in GFDEs with a particular positive periodic weight function. Using the method of separation of variables, we convert the homogeneous GFDE into two ordinary differential equations, and solve them via the help of solutions of the initial value problem with Caputo derivative. In the analytical solution, we observe that the weight function in the denominator, and scale function mapping the response domain differently. Since the derivative considered in this article is new, many existing results of FDEs are generalized.
Although the planar square Fe-N 4 is considered to be the basic unit of the active Fe-N 4 -based moieties, the exact local structure of such moieties has not yet been determined due to that the axial ligands and the second coordination sphere (i.e., the surrounding carbon matrix) of Fe-N 4 are unclear. [8] Based on the computational hydrogen electrode (CHE) model, the theoretical ORR onset potentials of Fe-N 4 in different models were as low as 0.25-0.43 V versus RHE (V RHE ), [9,10] much lower than the experimentally measured ones on Fe-N-C (0.82-0.95 V RHE ). Besides, some new active moieties in Fe-N-C (e.g., FeON 4 , Fe(OH)N 4 , and FeN 4 Cl) were proposed by using in situ characterizations or CHE modeling, [11][12][13][14] revealing that the axialligand coordination played a critical role in high-activity Fe-N 4 -based moieties. However, these studies were carried out on Fe-N-C that contained not only the Fe-N 4 sites but also a large number of other ORR active sites (such as the N-doped carbon and intrinsic defects of carbon), in which the implemented modifications would affect the ORR activities of different active sites and sometimes even alter the structure of the carbon matrix. [15] That is, the observed ORR activity improvement of the Fe-N-C after such modifications could not be solely attributed to the enhancement of the intrinsic ORR activity of Fe-N 4 and thus the mechanism associated with the improvement is still ambiguous. To this end, developing a model catalyst as a studying platform is highly demanded to reveal the regulation mechanism of axial-ligand coordination on the catalytic activity of Fe-N 4 sites.Iron porphyrin (FePr) and iron phthalocyanine (FePc) are widely used molecular model catalysts with Fe-N 4 as the only kind of ORR active sites. [16] Although some modified FePc and FePr with axially coordinated Fe-N 4 moieties have been reported in recent papers, [17][18][19] only a few strong ligands (e.g., CN − and pyridine) could perform such axial coordination due to that the FePc and FePr are more inclined to exist as the D4h symmetry structure with highly concentrated local electrons. [17] In order to systematically study the correlation between the axial ligands and the catalytic activity of Fe-N 4 sites, it is necessary to explore a Fe-N 4 -based molecular model catalyst with a stronger axial coordination ability. Poly(phthalocyanine iron) (PFePc) is an alkali-soluble Identifying the actual structure and tuning the catalytic activity of Fe-N 4based moieties, well-recognized high-activity sites in the oxygen reduction reaction (ORR) are challenging problems. Herein, by using poly(iron phthalocyanine) (PFePc) as an Fe-N 4 -based model electrocatalyst, a mechanistic insight into the effect of axial ligands on the ORR catalytic activity of Fe-N 4 is provided and it is revealed that the ORR activity of Fe-N 4 sites with OH desorption as a rate-determining step is related to the energy level gap between the OH p x p y and Fe 3d z 2 , which can be tuned by regulating the field strength of the...
The recent progress on the fabrication of two-dimensional metal–organic frameworks and their derivatives as well as their applications in electrochemical energy storage and electrocatalysis are reviewed.
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