Bimetallic metal-organic frameworks for controlled catalytic graphitization of nanoporous carbons

Single metal-organic frameworks (MOFs), constructed from the coordination between one-fold metal ions and organic linkers, show limited functionalities when used as precursors for nanoporous carbon materials. Herein, we propose to merge the advantages of zinc and cobalt metals ions into one single MOF crystal (i.e., bimetallic MOFs). The organic linkers that coordinate with cobalt ions tend to yield graphitic carbons after carbonization, unlike those bridging with zinc ions, due to the controlled catalytic graphitization by the cobalt nanoparticles. In this work, we demonstrate a feasible method to achieve nanoporous carbon materials with tailored properties, including specific surface area, pore size distribution, degree of graphitization, and content of heteroatoms. The bimetallic-MOF-derived nanoporous carbon are systematically characterized, highlighting the importance of precisely controlling the properties of the carbon materials. This can be done by finely tuning the components in the bimetallic MOF precursors, and thus designing optimal carbon materials for specific applications

Lead sulfide colloidal quantum dots passivation and optoelectronic characterization for photovoltaic device application

Photovoltaic energy conversion is one of the best alternatives to fossil fuel consumption. Petroleum resources are now close from depletion and their combustion is known to be responsible for releasing a considerable amount of greenhouse gas and carcinogenic airborne particles. Novel third-generation solar cells include a vast range of device architectures and materials aiming to overcome the factors limiting the current technologies. Among them, quantum dot based devices showed promising potential both as sensitizers and as colloidal nanoparticle film. p-doped PbS colloidal quantum dot (CQD) forming a heterojunction with a n-doped wide-band-gap semiconductor such as TiO2 or ZnO. Ultimately, this technology would lead to the assembly of a tandem-type cell with CQD films absorbing in different region of the solar. The confinement in these nanostructures is also expected to result in marginal mechanisms such as hot carrier collection and multiple exciton generation which would increase the theoretical conversion efficiency limit. In this work, certain mechanisms linked with CQD film passivation are addressed using various measurement methods. X-ray photoelectron spectroscopy is investigated in depth in order to pinpointed species specific to the role of methanol during the ligand exchange and notable differences are observed in the surface states of films treated with 3-mercaptopropionic acid, thioglycolic acid and thiolactic acid. The removal of the initial oleic acid ligand following methanol rinsing clearly leaves the CQD unprotected against adventitious oxidation altering a single atomic monolayer of the nanoparticles, as confirmed by a broadening and blue shift of the first exciton energy observed on UV-Vis absorption spectroscopy. Through fluorescence spectroscopy, two in-gap states are identified in each sample and the non-uniform quenching after ligand exchange suggests that 1% of the charges injected in TiO2 are recapture by the deeper trap state. Thiolactic ligand treatment shows a notable enhanced protection against surface contamination and displayed negligible electronic configuration change compared to the untreated sample while providing similar quenching properties. It can be related to the fact that this molecule is more bulky due to its -CH3 group and undergoes more steric interactions with neighbouring ligands. A whole assembly procedure is optimized, from the PbS CQDs synthesis to characterization of selective contacts based devices. Current-voltage analysis in dark conditions indicates that transport in such device appears to be strongly affected by space-charge limiting effects due to in-gap trap states distribution. The impact of TiO2 and MoOX selective contacts is also addressed. MoOX not only improves performance due to electron screening barrier, it also enhances stability significantly. No sign of free extracted charge is observed, indicating that the doping in the CQD film is negligible. Through time resolved charge extraction measurements, one can observe that recombination appears to be first dominated by relatively slow mechanisms and undergoes a fast acceleration. The time at which this acceleration occurs looks to be partly related to the MoOX thickness, suggesting that recombination with the external circuit might play a dominant role. Recombination regimes are addressed and appears to involve multiple mechanisms which cannot be simply fit with common first or second order reaction rates

Understanding chemically processed solar cells based on quantum dots

Photovoltaic energy conversion is one of the best alternatives to fossil fuel combustion. Petroleum resources are now close to depletion and their combustion is known to be responsible for the release of a considerable amount of greenhouse gases and carcinogenic airborne particles. Novel third-generation solar cells include a vast range of device designs and materials aiming to overcome the factors limiting the current technologies. Among them, quantum dot-based devices showed promising potential both as sensitizers and as colloidal nanoparticle films. A good example is the p-type PbS colloidal quantum dots (CQDs) forming a heterojunction with a n-type wide-band-gap semiconductor such as TiO2 or ZnO. The confinement in these nanostructures is also expected to result in marginal mechanisms, such as the collection of hot carriers and generation of multiple excitons, which would increase the theoretical conversion efficiency limit. Ultimately, this technology could also lead to the assembly of a tandem-type cell with CQD films absorbing in different regions of the solar spectru

Synthesis of ternary PtPdCu spheres with three-dimensional nanoporous architectures toward superior electrocatalysts

We report a simple method for the preparation of ternary PtPdCu spheres with a three-dimensional (3D) nanoporous structure in a solution phase. The structure, composition, and electronic states of the obtained material are characterized and studied by various physical techniques. By varying the amount of Cu precursor involved in the reaction, the composition of the spheres can be adjusted without affecting the morphology. Due to the nanoporous structure and the multimetallic synergetic effect, our ternary nanoporous PtPdCu spheres exhibited 1.7, 2.8, and 4.9 times higher activity than that of binary nanoporous PtPd spheres, dendritic Pt, and commercial Pt black catalysts, respectively

Electrochemical energy storage performance of 2D nanoarchitectured hybrid materials

The fast-growing interest for two-dimensional (2D) nanomaterials is undermined by their natural restacking tendency, which severely limits their practical application. Novel porous heterostructures that coordinate 2D nanosheets with monolayered mesoporous scaffolds offer an opportunity to greatly expand the library of advanced materials suitable for electrochemical energy storage technologies

Stable blue luminescent CsPbBr3 Perovskite nanocrystals confined in mesoporous thin films

Creating CsPbBr perovskite nanocrystals with bright blue emission is challenging because their optical properties depend sensitively on structure. Growing perovskites in mesoporous templates bypasses some of these purification issues because the size of the nanocrystal is governed by the dimensions of the pores. Mesoporous silica consisting of aligned channels with tunable diameter can be easily synthesized and used as a template. When the perovskite solution evaporates and retreats, some of the liquid remains trapped in the interconnecting pores by discontinuous dewetting. The precursor crystallizes, generating stable ca. 3.1 nm blue-emitting perovskite nanocrystals. The mesoporous template also serves as a protective barrier to preserve the optical properties of the CsPbBr from atmospheric conditions. Compared to the bulk crystals and the powder composite, the strong blue-shift of the emission peak in the film is accompanied by a decrease in the longer lifetime component and an 8-fold increase in the external quantum efficiency

Hybrid methylammonium lead halide perovskite nanocrystals confined in gyroidal silica templates

We successfully synthesized hybrid methylammonium lead halide nanocrystals confined in a gyroidal mesoporous silica template. The nanocrystals adopted the same crystal structure as their bulk perovskite counterparts, highlighting the ability to control the bandgap by tuning the pore-size of the template. Besides providing more accessible spaces for the materials to diffuse within the channels, the 3D mesh of the gyroidal template should also allow easier escape of the solvent and pinning of the particles at different points, thus promoting the formation of isolated nanoparticles over larger structures (e.g., nanowires, nanorods)

Dealloying of Mesoporous PtCu Alloy Film for the Synthesis of Mesoporous Pt Films with High Electrocatalytic Activity

Mesoporous Pt film with highly electrocatalytic activity is successfully synthesized by dealloying of mesoporous PtCu alloy film prepared through electrochemical micelle assembly. The resulting mesoporous electrode exhibits high current density and superior stability toward the methanol oxidation reaction