Long-Lived Photo-Response of Multi-Layer N-Doped Graphene-Based Films.

New insights into the mechanism of the improved photo(electro)catalytic activity of graphene by heteroatom doping were explored by transient transmittance and reflectance spectroscopy of multi-layer N-doped graphene-based samples on a quartz substrate prepared by chitosan pyrolysis in the temperature range 900-1200 °C compared to an undoped graphene control. All samples had an expected photo-response: fast relaxation (within 1 ps) due to decreased plasmon damping and increased conductivity. However, the N-doped graphenes had an additional transient absorption signal of roughly 10 times lower intensity, with 10-50 ps formation time and the lifetime extending into the nanosecond domain. These photo-induced responses were recalculated as (complex) dielectric function changes and decomposed into Drude-Lorentz parameters to derive the origin of the opto(electronic) responses. Consequently, the long-lived responses were revealed to have different dielectric function spectra from those of the short-lived responses, which was ultimately attributed to electron trapping at doping centers. These trapped electrons are presumed to be responsible for the improved catalytic activity of multi-layer N-doped graphene-based films compared to that of multi-layer undoped graphene-based films.

Unraveling the Optimal Cerium Content for Boosting the Photoresponse Activity of Mixed-Metal Zr/Ce-Based Metal-Organic Frameworks through a Photodynamic and Photocurrent Correlation: Implications on Water Splitting Efficiency.

Mixed-metal-organic frameworks (MMOFs) have emerged as promising photocatalyst candidates in multiple reactions. For instance, the doping of Zr-UiO-type MOFs with Ce atoms increases their photoactivity owing to a better overlap between the organic linker and Ce orbitals. However, it is not clear which is the ideal content of Ce to reach the optimal photocatalytic performance. Herein, a series of MMOFs isostructural to UiO-66 and with napthalene-2,6-dicarboxylate (NDC) as a linker were synthesized and characterized. The Ce content was varied from 0 to 100% and their corresponding structural, chemical, photodynamic, and photoresponse properties were investigated. Powder X-ray diffraction shows that when the content of Ce is 12% onward, in addition to the UiO-type structure, a second crystalline structure is cosynthesized (NDC-Ce). Steady-state and femtosecond (fs) to millisecond (ms) spectroscopy studies reveal the existence of two competing processes: a linker excimer formation and an ultrafast ligand-to-cluster charge transfer (LCCT) phenomenon from the organic linker to Zr/Ce metal clusters. The ultrafast (fs-regime) LCCT process leads to the formation of long-lived charge-separated states, which are more efficiently photoproduced when the content of Ce reaches 9%, suggesting that the related material would show the highest photoactivity. Photoaction spectroscopic measurements corroborate that the sample with 9% of Ce exhibits the maximum photocatalytic efficiency, which is reflected in a 20% increment in overall water splitting efficiency compared with the monometallic Zr-based MOF. The current study demonstrates the relationship between the photodynamical properties of the MMOFs and their photocatalytic performance, providing new findings and opening new ways for improving the design of new MOFs with enhanced photocatalytic activities.

Role of Defects on the Particle Size-Capacitance Relationship of Zn-Co Mixed Metal Oxide Supported on Heteroatom-Doped Graphenes as Supercapacitors.

Supercapacitors are considered among the most promising electrical energy storage devices, there being a need to achieve the highest possible energy storage density. Herein small mixed Zn-Co metal oxide nanoparticles are grown on doped graphene (O-, N- and, B-doped graphenes). The electrochemical properties of the resulting mixed Zn-Co metal oxide nanoparticles (4 nm) grown on B-doped graphene exhibit an outstanding specific capacitance of 2568 F g-1 at 2 A g-1 , ranking this B-doped graphene composite among the best performing electrodes. The energy storage capacity is also remarkable even at large current densities (i.e., 640 F g-1 at 40 A g-1 ). In contrast, larger nanoparticles are obtained using N- and O-doped graphenes as support, the resulting materials exhibiting lower performance. Besides energy storage, the Zn-Co oxide on B-doped graphene shows notable electrochemical performance and stability obtaining a maximum energy density of 77.6 W h Kg-1 at 850 W Kg-1 , a power density of 8500 W Kg-1 at 28.3 W h Kg-1 , and a capacitance retention higher than 85% after 5000 cycles. The smaller nanoparticle size and improved electrochemical performance on B-doped graphene-based devices are attributed to the higher defect density and nature of the dopant element on graphene.

Photoelectrochemical alcohols oxidation over polymeric carbon nitride photoanodes with simultaneous H2 production.

The photoelectrochemical oxidation of organic molecules into valuable chemicals is a promising technology, but its development is hampered by the poor stability of photoanodic materials in aqueous solutions, low faradaic efficiency, low product selectivity, and a narrow working pH range. Here, we demonstrate the synthesis of value-added aldehydes and carboxylic acids with clean hydrogen (H2) production in water using a photoelectrochemical cell based solely on polymeric carbon nitride (CN) as the photoanode. Isotope labeling measurements and DFT calculations reveal a preferential adsorption of benzyl alcohol and molecular oxygen to the CN layer, enabling fast proton abstraction and oxygen reduction, which leads to the synthesis of an aldehyde at the first step. Further oxidation affords the corresponding acid. The CN photoanode exhibits excellent stability (>40 h) and activity for the oxidation of a wide range of substituted benzyl alcohols with high yield, selectivity (up to 99%), and faradaic efficiency (>90%).

Synergistic effect of Cu and Fe small nanoparticles supported on porous N-doped graphitic framework for selective electrochemical CO2 reduction at low overpotential.

Electrochemical CO2 reduction is an appealing approach to diminish CO2 emissions, while obtaining valuable chemicals and fuels from renewable electricity. However, efficient electrocatalysts exhibiting high selectivity and low operating potentials are still needed. Herein it is reported that Cu and Fe nanoparticles supported on porous N-doped graphitic carbon matrix are efficient and selective electrocatalysts for CO2 reduction to CO at low overpotentials. XRD and Raman spectroscopy confirmed independent Cu and Fe metals as the main phases. HRSEM and HRTEM images show the coral-like morphology of the porous N-doped graphitic carbon matrix supporting Cu and Fe metal nanoparticles (about 10 wt%) homogeneously distributed with an average size of 1.5 nm and narrow size distribution. At the optimum Fe/Cu ratio of 2, this material present high activity for CO2 reduction to CO at -0.3 V vs. RHE with a faradaic efficiency of 96%. Moreover, at -0.5 V vs. RHE this electrocatalyst produces 27.8 mmol of CO gcat-1 h-1, the production rate being stable for 17 h. A synergy between Cu and Fe nanoparticles due to their close proximity in comparison with independent Cu or Fe electrocatalysts was observed.

All-carbon microporous graphitic photocatalyst-promoted reduction of CO2 to CO in the absence of metals or dopant elements.

Microporous graphitic carbon (mp-C) derived from the pyrolysis of α-, ß-, and γ-cyclodextrins exhibited photocatalytic activity in CO2-saturated acetonitrile-water upon irradiation with UV-Vis light and in the presence of triethanolamine, forming H2 (19 µmol h-1) and CO (23 µmol h-1) accompanied by a lesser proportion of CH4 (4 µmol h-1). The most efficient was the mp-C material derived from α-cyclodextrin (mp-Cα) and having a pore dimension of 0.68 nm. The process also occured, although to a much lesser extent, under simulated sunlight or with UV-Vis irradiation in the absence of a sacrificial agent, with H2O being the electron donor. The origin of the CO was proved by isotopic 13C labelling experiments. Photocurrent measurements proved the occurrence of charge separation and the increase in photocurrent intensity in the presence of CO2. Transient absorption spectroscopy was used to detect the charge separate state decay in the microsecond time scale and proved that a fraction of the photogenerated electrons were able to react with CO2.

CsCu2I3 Nanoparticles Incorporated within a Mesoporous Metal-Organic Porphyrin Framework as a Catalyst for One-Pot Click Cycloaddition and Oxidation/Knoevenagel Tandem Reaction.

Metal-organic frameworks (MOFs) and metal halide perovskites are currently under much investigation due to their unique properties and applications. Herein, an innovative strategy has been developed combining an iron-porphyrin MOF, PCN-222(Fe), and an in situ-grown CsCu2I3 nontoxic lead-free halide perovskite based on an earth-abundant metal that becomes incorporated within the MOF channels [CsCu2I3@PCN-222(Fe)]. Encapsulation was designed to decrease and control the particle size and increase the stability of CsCu2I3. The hybrid materials were characterized by various techniques including FE-SEM, elemental mapping and line scanning EDX, TEM, PXRD, UV-Vis DRS, BET surface area, XPS, and photoemission measurements. Hybrid CsCu2I3@PCN-222(Fe) materials were examined as heterogeneous multifunctional (photo)catalysts for copper(I)-catalyzed alkyne-azide cycloaddition (CuAAC) and one-pot selective photo-oxidation/Knoevenagel condensation cascade reaction. Interestingly, CsCu2I3@PCN-222(Fe) outperforms not only its individual components CsCu2I3 and PCN-222(Fe) but also other reported (photo)catalysts for these transformations. This is attributed to cooperation and synergistic effects of the PCN-222(Fe) host and CsCu2I3 nanocrystals. To understand the catalytic and photocatalytic mechanisms, control and inhibition experiments, electron paramagnetic resonance (EPR) measurements, and time-resolved phosphorescence were performed, revealing the main role of active species of Cu(I) in the click reaction and the superoxide ion (O2•-) and singlet oxygen (1O2) in the photocatalytic reaction.

Visible and NIR Light Assistance of the N2 Reduction to NH3 Catalyzed by Cs-promoted Ru Nanoparticles Supported on Strontium Titanate.

NH3 production accounts for more than 1% of the total CO2 emissions and is considered one of the most energy-intensive industrial processes currently (T > 400 °C and P > 80 bars). The development of atmospheric-pressure N2 fixation to NH3 under mild conditions is attracting much attention, especially using additional renewable energy sources. Herein, efficient photothermal NH3 evolution in continuous flow upon visible and NIR light irradiation at near 1 Sun power using Cs-decorated strontium titanate-supported Ru nanoparticles is reported. Notably, for the optimal photocatalytic composition, a constant NH3 rate near 3500 µmolNH3 gcatalyst -1 h-1 was achieved for 120 h reactions, being among the highest values reported at atmospheric pressure under 1 Sun irradiation.

Tuning the Photocatalytic Activity of Ti-Based Metal-Organic Frameworks through Modulator Defect-Engineered Functionalization.

Defect engineering is a valuable tool to tune the photocatalytic activity of metal-organic frameworks (MOFs). Inducing defects through the attachment of functionalized modulators can introduce cooperative units that can tune the bandgap of the material and enhance their chemical, thermal, and photostabilities among other properties. However, the majority of defect engineering studies for photocatalytic applications are limited to Zr-based MOFs, and there is still a lack of interrelation between synthetic variables, the resultant MOF properties, and their effect on their photocatalytic performance. We report a comprehensive study on the defect engineering of the titanium heterometallic MOF MUV-10 by fluoro- and hydroxy-isophthalic acid (Iso) modulators, rationalizing the effect of the materials' properties on their photocatalytic activity for hydrogen production. The Iso-OH modified MOFs present a volcano-type profile with a 2.3-fold increase in comparison to the pristine materials, whereas the Iso-F modified samples have a gradual increase with up to a 4.2-fold enhancement. It has been demonstrated that ∼9% of Iso-OH modulator incorporation produces ∼40% defects, inducing band gap reduction and longer excited states lifetime. Similar defect percentages have been generated upon near 40% Iso-F modulator incorporation; however, negligible band gap changes and shorter excited states lifetimes were determined. The higher photocatalytic activity in Iso-F modulator derived MOF has been attributed to the effect of the divergent defect-compensation modes on the materials' photostability and to the increase in the external surface area upon introduction of Iso-F modulator.

Mn(II) sub-nanometric site stabilization in noble, N-doped carbonaceous materials for electrochemical CO2 reduction.

The preparation of stable and efficient electrocatalysts comprising abundant and non-critical row-materials is of paramount importance for their industrial implementation. Herein, we present a simple synthetic route to prepare Mn(II) sub-nanometric active sites over a highly N-doped noble carbonaceous support. This support not only promotes a strong stabilization of the Mn(II) sites, improving its stability against oxidation, but also provides a convenient coordination environment in the Mn(II) sites able to produce CO, HCOOH and CH3COOH from electrochemical CO2 reduction.

Synthetic Fuels from Biomass: Photocatalytic Hydrodecarboxylation of Octanoic Acid by Ni Nanoparticles Deposited on TiO2.

Decarboxylation of low-value fatty acids from biomass is a simple process to produce synthetic fuels suitable to be blended with gasoline or diesel. The present study reports the photocatalytic decarboxylation of octanoic acid in the presence of H2 by a series of modified TiO2 to form mixtures of n-heptane and tetradecane as major products in variable proportions, depending on the photocatalyst and the reaction conditions. It was found that the photocatalytic activity increases upon an optimal reductive NaBH4 treatment, presumably by generation of surface oxygen vacancies and by the deposition of Ni nanoparticles in the appropriate loading. Under the optimized conditions, an almost complete octanoic acid conversion and a combined selectivity to n-heptane and tetradecane over 80 % were reached at 10 h of UV/Vis light irradiation with a 300 W Xe lamp. No changes in the photocatalytic performance were observed for six consecutive runs. The present results illustrate the possibility that photocatalytic decarboxylation offers for the transformation of biomass into synthetic fuels under mild conditions.

Fe clusters embedded on N-doped graphene as a photothermal catalyst for selective CO2 hydrogenation.

In comparison with the Co analog, small Fe clusters incorporated in a graphene matrix exhibit a photo-assisted increase of 110% in reverse water gas shift CO2 hydrogenation under UV-Vis light irradiation. Available data indicate that the photo-assistance derives from light absorption by the N-doped graphene followed by charge recombination at the Fe clusters, increasing their local temperature.

Effect of Linker Distribution in the Photocatalytic Activity of Multivariate Mesoporous Crystals.

The use of Metal-Organic Frameworks as crystalline matrices for the synthesis of multiple component or multivariate solids by the combination of different linkers into a single material has emerged as a versatile route to tailor the properties of single-component phases or even access new functions. This approach is particularly relevant for Zr6-MOFs due to the synthetic flexibility of this inorganic node. However, the majority of materials are isolated as polycrystalline solids, which are not ideal to decipher the spatial arrangement of parent and exchanged linkers for the formation of homogeneous structures or heterogeneous domains across the solid. Here we use high-throughput methodologies to optimize the synthesis of single crystals of UiO-68 and UiO-68-TZDC, a photoactive analogue based on a tetrazine dicarboxylic derivative. The analysis of the single linker phases reveals the necessity of combining both linkers to produce multivariate frameworks that combine efficient light sensitization, chemical stability, and porosity, all relevant to photocatalysis. We use solvent-assisted linker exchange reactions to produce a family of UiO-68-TZDC% binary frameworks, which respect the integrity and morphology of the original crystals. Our results suggest that the concentration of TZDC in solution and the reaction time control the distribution of this linker in the sibling crystals for a uniform mixture or the formation of core-shell domains. We also demonstrate how the possibility of generating an asymmetric distribution of both linkers has a negligible effect on the electronic structure and optical band gap of the solids but controls their performance for drastic changes in the photocatalytic activity toward proton or methyl viologen reduction.

Metal-Organic Framework Derived Bimetallic Materials for Electrochemical Energy Storage.

Supercapacitors (SCs), showing excellent power density, long service life, and high reversibility, have received great attention because of the increasing demand for energy storage devices. To further improve their performance, it is essential to develop advanced electrode materials. One group of materials, porous crystalline solids referred to as metal-organic frameworks (MOFs), have proved to be excellent templates for synthesizing functional materials to be employed in the preparation of electrodes for SCs. In comparison to monometallic MOFs, bimetallic MOFs and their derivatives offer a number of advantages, including tunable electrochemical activity, high charge capacity, and improved electrical conductivity. This review focuses on the use of MOF-derived bimetallic materials in SCs, the origin of the improved performance, and the latest developments in the field. Furthermore, the challenges and perspectives in this research area are discussed.

Plasma-Induced Defects Enhance the Visible-Light Photocatalytic Activity of MIL-125(Ti)-NH2 for Overall Water Splitting.

Defect engineering in metal-organic frameworks is commonly performed by using thermal or chemical treatments. Herein we report that oxygen plasma treatment generates structural defects on MIL-125(Ti)-NH2 , leading to an increase in its photocatalytic activity. Characterization data indicate that plasma-treated materials retain most of their initial crystallinity, while exhibiting somewhat lower surface area and pore volume. XPS and FT-IR spectroscopy reveal that oxygen plasma induces MIL-125(Ti)-NH2 partial terephthalate decarboxylation and an increase in the Ti-OH population. Thermogravimetric analyses confirm the generation of structural defects by oxygen plasma and allowed an estimation of the resulting experimental formula of the treated MIL-125(Ti)-NH2 solids. SEM analyses show that oxygen plasma treatment of MIL-125(Ti)-NH2 gradually decreases its particle size. Importantly, diffuse reflectance UV/Vis spectroscopy and valence band measurements demonstrate that oxygen plasma treatment alters the MIL-125(Ti)-NH2 band gap and, more significantly, the alignment of highest occupied and lowest unoccupied crystal orbitals. An optimal oxygen plasma treatment to achieve the highest efficiency in water splitting with or without methanol as sacrificial electron donor under UV/Vis or simulated sunlight was determined. The optimized plasma-treated MIL-125(Ti)-NH2 photocatalyst acts as a truly heterogeneous photocatalyst and retains most of its initial photoactivity and crystallinity upon reuse.

Vapor-Phase Photocatalytic Overall Water Splitting Using Hybrid Methylammonium Copper and Lead Perovskites.

Films or powders of hybrid methylammonium copper halide perovskite exhibit photocatalytic activity for overall water splitting in the vapor phase in the absence of any sacrificial agent, resulting in the generation of H2 and O2, reaching a maximum production rate of 6 µmol H2 × g cat-1h-1 efficiency. The photocatalytic activity depends on the composition, degreasing all inorganic Cs2CuCl2Br2 perovskite and other Cl/Br proportions in the methylammonium hybrids. XRD indicates that MA2CuCl2Br2 is stable under irradiation conditions in agreement with the linear H2 production with the irradiation time. Similar to copper analogue, hybrid methylammonium lead halide perovskites also promote the overall photocatalytic water splitting, but with four times less efficiency than the Cu analogues. The present results show that, although moisture is strongly detrimental to the photovoltaic applications of hybrid perovskites, it is still possible to use these materials as photocatalysts for processes requiring moisture due to the lack of relevance in the photocatalytic processes of interparticle charge migration.

Synthesis, post-synthetic modification and stability of a 2D styryl ammonium lead iodide hybrid material.

A new hybrid lead iodide material (HP1) having 4-vinylphenylene ammonium as the organic cation has been prepared. The structural formula based on chemical analysis of HP1 corresponds to PbI2.5(4-styrylammonium)0.5. The crystallinity of HP1 was confirmed by powder X-ray diffraction and high resolution transmission electron microscopy. The presence of the styryl ammonium moiety in HP1 allows post-synthetic modification by radical copolymerization with styrene to obtain the HP2 material with higher hydrophobicity. Stability tests reveal that both HP1 and HP2 show hydrogen evolution in the dark, indicating about 0.6% partial decomposition of the hybrid material. This hydrogen evolution increases by a factor of 3 when HP1 and HP2 are exposed to visible light. X-ray photoelectron spectroscopy analysis shows an increase of NH2 groups and a decrease of NH3+ units suggesting that the origin of hydrogen evolution is the deprotonation of ammonium ions.

Long-Term Photostability in Terephthalate Metal-Organic Frameworks.

Prolonged (weeks) UV/Vis irradiation under Ar of UiO-66(Zr), UiO66 Zr-NO2 , MIL101 Fe, MIL125 Ti-NH2 , MIL101 Cr and MIL101 Cr(Pt) shows that these MOFs undergo photodecarboxylation of benzenedicarboxylate (BDC) linker in a significant percentage depending on the structure and composition of the material. Routine characterization techniques such as XRD, UV/Vis spectroscopy and TGA fail to detect changes in the material, although porosity and surface area change upon irradiation of powders. In contrast to BCD-containing MOFs, zeolitic imidazolate ZIF-8 does not evolve CO2 or any other gas upon irradiation.

De novo synthesis of mesoporous photoactive titanium(iv)-organic frameworks with MIL-100 topology.

Most developments in the chemistry and applications of metal-organic frameworks (MOFs) have been made possible thanks to the value of reticular chemistry in guiding the unlimited combination of organic connectors and secondary building units (SBUs) into targeted architectures. However, the development of new titanium-frameworks still remains limited by the difficulties in controlling the formation of persistent Ti-SBUs with predetermined directionality amenable to the isoreticular approach. Here we report the synthesis of a mesoporous Ti-MOF displaying a MIL-100 topology. MIL-100(Ti) combines excellent chemical stability and mesoporosity, intrinsic to this archetypical family of porous materials, with photoactive Ti3(µ3-O) metal-oxo clusters. By using high-throughput synthetic methodologies, we have confirmed that the formation of this SBU is thermodynamically favored as it is not strictly dependent on the metal precursor of choice and can be regarded as an adequate building block to control the design of new Ti-MOF architectures. We are confident that the addition of a mesoporous solid to the small number of crystalline, porous titanium-frameworks available will be a valuable asset to accelerate the development of new porous photocatalysts without the pore size limitations currently imposed by the microporous materials available.

Graphene-Based Materials as Efficient Photocatalysts for Water Splitting.

Photocatalysis has been proposed as one of the most promising approaches for solar fuel production. Among the photocatalysts studied for water splitting, graphene and related materials have recently emerged as attractive candidates due to their striking properties and sustainable production when obtained from biomass wastes. In most of the cases reported so far, graphene has been typically used as additive to enhance its photocatalytic activity of semiconductor materials as consequence of the improved charge separation and visible light harvesting. However, graphene-based materials have demonstrated also intrinsic photocatalytic activity towards solar fuels production, and more specifically for water splitting. The photocatalytic activity of graphene derives from defects generated during synthesis or their introduction through post-synthetic treatments. In this short review, we aim to summarize the most representative examples of graphene based photocatalysts and the different approaches carried out in order to improve the photocatalytic activity towards water splitting. It will be presented that the introduction of defects in the graphenic lattice as well as the incorporation of small amounts of metal or metal oxide nanoparticles on the graphene surface improve the photocatalytic activity of graphene. What is more, a simple one-step preparation method has demonstrated to provide crystal orientation to the nanoparticles strongly grafted on graphene resulting in remarkable photocatalytic properties. These two features, crystal orientation and strong grafting, have been identified as a general methodology to further enhance the photocatalytic activity in graphenebased materials for water splitting. Finally, future prospects in this filed will be also commented.