The comparative catalytic activities of iron phosphides, Fe x P (x = 1-3), have been established with phase-pure material grown by Chemical Vapor Deposition (CVD) from single-source organometallic precursors. This is the first report of the preparation of phase-pure thin films of FeP and Fe 2 P and their identity was established with scanning-electron microscopy, X-ray photoelectron spectroscopy, and powder X-ray diffraction. All materials were deposited on fluorine-doped tin oxide (FTO) for evaluation of their activities towards the hydrogen evolution reaction (HER) of water splitting in 0.5 M H 2 SO 4. HER activity follows the trend Fe 3 P > Fe 2 P > FeP, with Fe 3 P having the lowest overpotential of 49 mV at a current density of 10 mA cm-2. Density functional theory (DFT) calculations are congruent with the observed activity trend with hydrogen binding favoring the iron-rich terminating surfaces of Fe 3 P and Fe 2 P over the iron-poor terminating surfaces of FeP. The results present a clear trend of activity with iron-rich phosphide phases outperforming phosphorus rich phases for hydrogen evolution. The films of Fe 2 P were grown using Fe(CO) 4 PH 3 (1), while the films of FeP were prepared using either Fe(CO) 4 P t BuH 2 (2) or the new molecule {Fe(CO) 4 P(H) t Bu} 2 (3) on quartz and FTO. Compound 3 was prepared from the reaction of PCl 2 t Bu with a mixture of Na[HFe(CO) 4 ] and Na 2 [Fe(CO) 4 ] and characterized by single-crystal X-Ray diffraction, ESI-MS, elemental analysis, and 31 P/ 1 H NMR spectroscopies. Films of Fe 3 P were prepared as previously described from H 2 Fe 3 (CO) 9 P t Bu (4).
A variety of catalysts have recently been developed for electrocatalytic oxygen evolution, but very few of them can be readily integrated with semiconducting light absorbers for photoelectrochemical or photocatalytic water splitting. Here, we demonstrate an efficient core/shell photoanode with a highly active oxygen evolution electrocatalyst shell (FeMnP) and semiconductor core (rutile TiO) for photoelectrochemical oxygen evolution reaction. Metal-organic chemical vapor deposition from a single-source precursor was used to ensure good contact between the FeMnP and the TiO. The TiO/FeMnP core/shell photoanode reaches the theoretical photocurrent density for rutile TiO of 1.8 mA cm at 1.23 V vs reversible hydrogen electrode under simulated 100 mW cm (1 sun) irradiation. The dramatic enhancement is a result of the synergistic effects of the high oxygen evolution reaction activity of FeMnP (delivering an overpotential of 300 mV with a Tafel slope of 65 mV dec in 1 M KOH) and the conductive interlayer between the surface active sites and semiconductor core which boosts the interfacial charge transfer and photocarrier collection. The facile fabrication of the TiO/FeMnP core/shell nanorod array photoanode offers a compelling strategy for preparing highly efficient photoelectrochemical solar energy conversion devices.
Developing stable and efficient bifunctional catalysts for overall water splitting into hydrogen and oxygen is a critical step in the realization of several clean-energy technologies. Here we report a robust and highly active electrocatalyst that is constructed by deposition of the ternary metal phosphide FeMnP onto graphene-protected nickel foam by metal-organic chemical vapor deposition from a single source precursor. FeMnP exhibits high electrocatalytic activity toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Utilizing FeMnP/GNF as both the anode and the cathode for overall water splitting, a current density of 10 mA cm-2 is achieved at a cell voltage of as low as 1.55 V with excellent stability. Complementary density functional theory (DFT) calculations suggest that facets exposing both Fe and Mn sites are necessary to achieve high HER activity. The present work provides a facile strategy for fabricating highly efficient electrocatalysts from earth-abundant materials for overall water splitting.
The reactions of (S)-2-(1,8-naphthalimido)propanoic acid (HL(ala)) and (S)-2-(1,8-naphthalimido)-3-hydroxypropanoic acid (HL(ser)), protonated forms of ligands that contain a carboxylate donor group, an enantiopure chiral center, and a 1,8-naphthalimide π···π stacking supramolecular tecton and in the case of HL(ser) an alcohol functional group, with the appropriate alkali metal hydroxide followed by a variety of crystallization methods leads to the formation of crystalline K(L(ala))(MeOH) (1), K(L(ala))(H2O) (2), Na(L(ala))(H2O) (3), KL(ser) (4), CsL(ser) (5), and CsL(ala) (6). Each of these new complexes has a solid state structure based on six-coordinate metals linked into homochiral helical rod secondary building unit (SBU) central cores. In addition to the bonding of the carboxylate and solvent (in the case of L(ser) the ligand alcohol) to the metals, both oxygens on the 1,8-naphthalimide act as donor groups. One naphthalimide oxygen bonds to the same helical rod SBU as the carboxylate group of that ligand forming a chelate ring. The other naphthalimide oxygen bonds to adjacent SBUs. In complexes 1-3, this inter-rod link has a square arrangement bonding four other rods forming a three-dimensional enantiopure metal-organic framework (MOF) structure, whereas in 4-6 this link has a linear arrangement bonding two other rods forming a two-dimensional, sheet structure. In the latter case, the third dimension is supported exclusively by interdigitated π···π stacking interactions of the naphthalimide supramolecular tecton, forming enantiopure supramolecular MOF solids. Compounds 1-3 lose the coordinated solvent when heating above 100 °C. For 1, the polycrystalline powder reverts to 1 only by recrystallization from methanol, whereas compounds 2 and 3 undergo gas/solid, single-crystal to single-crystal transformations to form dehydrated compounds 2* and 3*, and rehydration occurs when crystals of these new complexes are left out in air. The reversible single-crystal to single-crystal transformation of 2 involves the dissociation/coordination of a terminal water ligand, but the case of 3 is remarkable considering that the water that is lost is the only bridging ligand between the metals in the helical rod SBU and a carboxylate oxygen that is a terminal ligand in 3 moves into a bridging position in 3* to maintain the homochiral helical rods. Both 2* and 3* contain five-coordinate metals. There are no coordinated solvents in compounds 4-6, in two cases by designed ligand modification, which allows them to have high thermal stability. Compounds 1-3 did not exhibit observable Second Harmonic Generation (SHG) efficiency at an incident wavelength of 1064 nm, but compounds 4-6 did exhibit modest SHG efficiency for MOF-like compounds in the range of 30 × α-SiO2.
The reactions of the potassium salts of the ligands (S)-2-(1,8-naphthalimido)propanoate (KL(ala)), (S)-2-(1,8-naphthalimido)-3-hydroxypropanoate (KL(ser)), and (R)-2-(1,8-naphthalimido)propanoate (KL(ala)*), enantiopure carboxylate ligands containing a 1,8-naphthalimide π···π stacking supramolecular tecton, and, in the case of L(ser)(-), an alcohol functional group with calcium or strontium nitrate under solvothermal conditions produce crystalline [Ca(L(ala))2(H2O)]·(H2O) (1); [Ca(L(ser))2]·(H2O)2 (2); [Sr(L(ala))2(H2O)]·(H2O)3 (3); [Sr(L(ala)*)2(H2O)]·(H2O)3 (3*); and [Sr(L(ser))2(H2O)] (5). Placing 3 under vacuum removes the interstitial waters to produce [Sr(L(ala))2(H2O)] (4) in a single-crystal to single-crystal transformation; introduction of water vapor to 4 leads to the reformation of crystalline 3. Each of these new complexes has a solid-state structure based on homochiral rod secondary building unit (SBUs) central cores. Supramolecular π···π stacking interactions between 1,8-naphthalimide rings link adjacent rod SBUs into three-dimensional structures for 1, 3, 4, and 5 and two-dimensional structure for 2. Compounds 1 and 3 have open one-dimensional channels along the crystallographic c axis that are occupied by disordered solvent. For 3, these channels close and open in the reversible single-crystal conversion to 4; the π···π stacking interactions of the naphthalimide rings facilitate this process by rotating and slipping. Infrared spectroscopy demonstrated that the rehydration of 4 with D2O leads to 3d8, and the process of dehydration and rehydration of 3d8 with H2O leads to 3, thus showing exchange of the coordinated water in this process. These forms of 3 and 4 were characterized by (1)H, (2)H, and (13)C solid-state NMR spectroscopy, and thermal and luminescence data are reported on all of the complexes.
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