Hierarchical ZnO@Hybrid Carbon Core-Shell Nanowire Array on a Graphene Fiber Microelectrode for Ultrasensitive Detection of 2,4,6-Trinitrotoluene.

A hierarchical architecture composed of nitrogen (N)-rich carbon@graphitic carbon-coated ZnO nanowire arrays on a graphene fiber (ZnO@C/GF) was fabricated by direct growth of a ZnO@zeolitic imidazolate framework-8 (ZIF-8) core-shell nanowire array on a GF followed by annealing and used as a microelectrode for detection of 2,4,6-trinitrotoluene (TNT). In such a design, ZnO accumulated TNT through a strong nitroxide-zinc interaction and ZIF-8 served as the precursor of the N-rich carbon@graphitic carbon layer that seamlessly connected ZnO with the GF to improve the poor conductivity of ZnO, thus enhancing the sensitivity of the ZnO@C/GF microelectrode. The constructed hierarchical hybrid fiber microsensor exhibited a wide linear response to TNT in a concentration range of 0.1-32.2 µM with a low detection limit of 3.3 nM. This ZnO@C/GF microelectrode was further successfully applied to the detection of TNT in lake and tap water, indicating its promise as a portable sensor for the electrochemical detection of explosive compounds.

Covalent grafting of molecular catalysts on C3N x H y as robust, efficient and well-defined photocatalysts for solar fuel synthesis.

The covalent attachment of molecules to 2D materials is an emerging area as strong covalent chemistry offers new hybrid properties and greater mechanical stability compared with nanoparticles. A nickel bis-aminothiophenol catalyst was grafted onto a range of 2D carbon nitrides (C3N x H y ) to form noble metal-free photocatalysts for H2 production. The hybrids produce H2 beyond 8 days with turnover numbers reaching 1360 based on nickel, a more than 3 fold higher durability than reported molecular catalyst-carbon nitride mixtures, and under longer wavelengths (>475 nm). Time-resolved spectroscopy reveals sub-microsecond electron transfer to the grafted catalyst, six orders of magnitude faster compared with similar reports of non-grafted catalysts. The photoelectrons on the catalyst have a ca. 1000 times longer half-time (7 ms) compared with bare carbon nitride (10 µs). The grafting strategy operates across a range of molecular catalyst-carbon nitride combinations, thus paving the way for robust efficient photocatalysts based on low-cost tunable components.

A 3D-graphene fiber electrode embedded with nitrogen-rich-carbon-coated ZIF-67 for the ultrasensitive detection of adrenaline.

A novel nitrogen-rich-carbon-coated ZIF-67 embedded three-dimensional-graphene (ZIF-67/NC/3DG) fiber was fabricated via a facile one-pot electrodeposition self-assembly method, and used as a prominent electrode for the non-enzymatic detection of adrenaline (Ad). In this design, the prepared ZIF-67 adsorbs Ad through hydrogen bonding and electrostatic interaction, while polypyrrole functions as the precursor of the conductive NC that seamlessly connects ZIF-67 with the 3DG fiber electrode to ameliorate the poor conductivity of the ZIF-67 moiety and thus improve the sensitivity of the ZIF-67/NC/3DG fiber electrode for detecting Ad. The constructed fiber sensor shows a double linear response over the Ad concentration range of 0.06-95 µM with a high sensitivity of 44.6 mA mM-1 cm-2 and 95.0-5900 µM with a good sensitivity of 11.0 mA mM-1 cm-2, giving a low detection limit of 0.02 µM and excellent repeatability. The ZIF-67/NC/3DG fiber electrode was further successfully applied for the determination of Ad in a real sample of human serum, indicating that this fiber electrode is a promising miniaturized sensor for electrochemical analysis.

Earth-Abundant Molecular Z-Scheme Photoelectrochemical Cell for Overall Water-Splitting.

A push-pull organic dye and a cobaloxime catalyst were successfully cografted on NiO and CuGaO2 to form efficient molecular photocathodes for H2 production with >80% Faradaic efficiency. CuGaO2 is emerging as a more effective p-type semiconductor in photoelectrochemical cells and yields a photocathode with 4-fold higher photocurrent densities and 400 mV more positive onset photocurrent potential compared to the one based on NiO. Such an optimized CuGaO2 photocathode was combined with a TaON|CoO x photoanode in a photoelectrochemical cell. Operated in this Z-scheme configuration, the two photoelectrodes produced H2 and O2 from water with 87% and 88% Faradaic efficiency, respectively, at pH 7 under visible light and in the absence of an applied bias, equating to a solar to hydrogen conversion efficiency of 5.4 × 10-3%. This is, to the best of our knowledge, the highest efficiency reported so far for a molecular-based noble metal-free water splitting Z-scheme.

Ultra-small dispersed Cu x O nanoparticles on graphene fibers for miniaturized electrochemical sensor applications.

A graphene microfiber (GF) modified with ultrafine Cu x O nanoparticles (Cu x ONPs/GF) has been fabricated by direct annealing of electrodeposited nano-sized copper-based metal organic frameworks (HKUST-1) and used as an electrode for nonenzymatic H2O2 sensing. Benefiting from the unique microfiber architecture and synergetic effects, as well as strong coupling between components with many active sites and boosted electron transport, the Cu x ONPs/GF electrode shows prominent sensitivity, selectivity and long-term operational stability for the detection of H2O2. Further work successfully applied this Cu x ONPs/GF electrode to detection of H2O2 in real samples such as milk and human serum. These results indicate that the Cu x ONPs/GF is a promising mini-sized sensor in electrochemical analysis.

Tunable Covalent Triazine-Based Frameworks (CTF-0) for Visible-Light-Driven Hydrogen and Oxygen Generation from Water Splitting.

Covalent triazine-based frameworks (CTFs), a group of semiconductive polymers, have been identified for photocatalytic water splitting recently. Their adjustable band gap and facile processing offer great potential for discovery and development. Here, we present a series of CTF-0 materials fabricated by two different approaches, a microwave-assisted synthesis and an ionothermal method, for water splitting driven by visible-light irradiation. The material (CTF-0-M2) synthesized by microwave technology shows a high photocatalytic activity for hydrogen evolution (up to 7010 µmol h-1 g-1), which is 7 times higher than another (CTF-0-I) prepared by conventional ionothermal trimerization under identical photocatalytic conditions. This leads to a high turnover number (TON) of 726 with respect to the platinum cocatalyst after seven cycles under visible light. We attribute this to the narrowed band gap, the most negative conduction band, and the rapid photogenerated charge separation and transfer. On the other hand, the material prepared by the ionothermal method is the most efficient one for oxygen evolution. CTF-0-I initially produces ca. 6 times greater volumes of oxygen gas than CTF-0-M2 under identical experimental conditions. CTF-0-I presents an apparent quantum efficiency (AQY) of 5.2% at 420 nm for oxygen production without any cocatalyst. The activity for water oxidation exceeds that of most reported CTFs due to a large driving force for oxidation and a large number of active sites. Our findings indicate that the band positions and the interlayer stacking structures of CTF-0 were modulated by varying synthesis conditions. These modulations impact the optical and redox properties, resulting in an enhanced performance for photocatalytic hydrogen and oxygen evolution, confirmed by first-principles calculations.

Insights into the mechanism and aging of a noble-metal free H2-evolving dye-sensitized photocathode.

Dye-sensitized photo-electrochemical cells (DS-PECs) form an emerging technology for the large-scale storage of solar energy in the form of (solar) fuels because of the low cost and ease of processing of their constitutive photoelectrode materials. Preparing such molecular photocathodes requires a well-controlled co-immobilization of molecular dyes and catalysts onto transparent semiconducting materials. Here we used a series of surface analysis techniques to describe the molecular assembly of a push-pull organic dye and a cobalt diimine-dioxime catalyst co-grafted on a p-type NiO electrode substrate. (Photo)electrochemical measurements allowed characterization of electron transfer processes within such an assembly and to demonstrate for the first time that a CoI species is formed as the entry into the light-driven H2 evolution mechanism of a dye-sensitized photocathode. This co-grafted noble-metal free H2-evolving photocathode architecture displays similar performances to its covalent dye-catalyst counterpart based on the same catalytic moiety. Post-operando time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of these photoelectrodes after extensive photoelectrochemical operation suggested decomposition pathways of the dye and triazole linkage used to graft the catalyst onto NiO, providing grounds for the design of optimized molecular DS-PEC components with increased robustness upon turnover.

A protocol for quantifying hydrogen evolution by dye-sensitized molecular photocathodes and its implementation for evaluating a new covalent architecture based on an optimized dye-catalyst dyad.

A protocol that combines gas chromatography and a high-sensitivity micro Clark-type electrode is described to quantify hydrogen production across gas and solution phases for systems operating at very low currents such as dye-sensitized H2-evolving photocathodes. Data indicate that a significant fraction of H2 remains in aqueous solution even after several hours of experiments. Using this protocol, re-evaluation of a dye-sensitized H2-evolving photocathode based on a dye-catalyst dyad showed a reproducible 66% increase of the faradaic efficiency compared with previously reported headspace GC measurements [Kaeffer et al., J. Am. Chem. Soc., 2016, 138, 12308-12311]. This dyad was based on an organic push-pull dye where donor and acceptor are separated by one thiophene group. Insertion of a second thiophene group between the donor and acceptor led to a more efficient system with 30% improved faradaic efficiency for H2 evolution.

Time-Resolved IR Spectroscopy Reveals a Mechanism with TiO2 as a Reversible Electron Acceptor in a TiO2-Re Catalyst System for CO2 Photoreduction.

Attaching the phosphonated molecular catalyst [ReIBr(bpy)(CO)3]0 to the wide-bandgap semiconductor TiO2 strongly enhances the rate of visible-light-driven reduction of CO2 to CO in dimethylformamide with triethanolamine (TEOA) as sacrificial electron donor. Herein, we show by transient mid-IR spectroscopy that the mechanism of catalyst photoreduction is initiated by ultrafast electron injection into TiO2, followed by rapid (ps-ns) and sequential two-electron oxidation of TEOA that is coordinated to the Re center. The injected electrons can be stored in the conduction band of TiO2 on an ms-s time scale, and we propose that they lead to further reduction of the Re catalyst and completion of the catalytic cycle. Thus, the excited Re catalyst gives away one electron and would eventually get three electrons back. The function of an electron reservoir would represent a role for TiO2 in photocatalytic CO2 reduction that has previously not been considered. We propose that the increase in photocatalytic activity upon heterogenization of the catalyst to TiO2 is due to the slow charge recombination and the high oxidative power of the ReII species after electron injection as compared to the excited MLCT state of the unbound Re catalyst or when immobilized on ZrO2, which results in a more efficient reaction with TEOA.

Electrocatalytic and Solar-Driven CO2 Reduction to CO with a Molecular Manganese Catalyst Immobilized on Mesoporous TiO2.

Electrocatalytic CO2 reduction to CO was achieved with a novel Mn complex, fac-[MnBr(4,4'-bis(phosphonic acid)-2,2'-bipyridine)(CO)3 ] (MnP), immobilized on a mesoporous TiO2 electrode. A benchmark turnover number of 112±17 was attained with these TiO2 |MnP electrodes after 2 h electrolysis. Post-catalysis IR spectroscopy demonstrated that the molecular structure of the MnP catalyst was retained. UV/vis spectroscopy confirmed that an active Mn-Mn dimer was formed during catalysis on the TiO2 electrode, showing the dynamic formation of a catalytically active dimer on an electrode surface. Finally, we combined the light-protected TiO2 |MnP cathode with a CdS-sensitized photoanode to enable solar-light-driven CO2 reduction with the light-sensitive MnP catalyst.

Heterogenised Molecular Catalysts for the Reduction of CO2 to Fuels.

CO(2) conversion provides a possible solution to curtail the growing CO(2) levels in our atmosphere and reduce dependence on fossil fuels. To this end, it is essential to develop efficient catalysts for the reduction of CO(2). The structure and activity of molecular CO(2) reduction catalysts can be tuned and they offer good selectivity with reasonable stability. Heterogenisation of these molecules reduces solvent restrictions, facilitates recyclability and can dramatically improve activity by preventing catalyst inactivation and perturbing the kinetics of intermediates. The nature and morphology of the solid-state material upon which the catalyst is immobilised can significantly influence the activity of the hybrid assembly. Although work in this area began forty years ago, it has only drawn substantial attention in recent years. This review article gives an overview of the historical development of the field.

Improving the photocatalytic reduction of CO2 to CO through immobilisation of a molecular Re catalyst on TiO2.

The photocatalytic activity of phosphonated Re complexes, [Re(2,2'-bipyridine-4,4'-bisphosphonic acid) (CO)3(L)] (ReP; L = 3-picoline or bromide) immobilised on TiO2 nanoparticles is reported. The heterogenised Re catalyst on the semiconductor, ReP-TiO2 hybrid, displays an improvement in CO2 reduction photocatalysis. A high turnover number (TON) of 48 molCO molRe(-1) is observed in DMF with the electron donor triethanolamine at λ>420 nm. ReP-TiO2 compares favourably to previously reported homogeneous systems and is the highest TON reported to date for a CO2-reducing Re photocatalyst under visible light irradiation. Photocatalytic CO2 reduction is even observed with ReP-TiO2 at wavelengths of λ>495 nm. Infrared and X-ray photoelectron spectroscopies confirm that an intact ReP catalyst is present on the TiO2 surface before and during catalysis. Transient absorption spectroscopy suggests that the high activity upon heterogenisation is due to an increase in the lifetime of the immobilised anionic Re intermediate (t50% >1 s for ReP-TiO2 compared with t50% = 60 ms for ReP in solution) and immobilisation might also reduce the formation of inactive Re dimers. This study demonstrates that the activity of a homogeneous photocatalyst can be improved through immobilisation on a metal oxide surface by favourably modifying its photochemical kinetics.

Comparison of rhenium-porphyrin dyads for CO2 photoreduction: photocatalytic studies and charge separation dynamics studied by time-resolved IR spectroscopy.

We report a study of the photocatalytic reduction of CO2 to CO by zinc porphyrins covalently linked to [ReI(2,2'-bipyridine)(CO)3L]+/0 moieties with visible light of wavelength >520 nm. Dyad 1 contains an amide C6H4NHC(O) link from porphyrin to bipyridine (Bpy), Dyad 2 contains an additional methoxybenzamide within the bridge C6H4NHC(O)C6H3(OMe)NHC(O), while Dyad 3 has a saturated bridge C6H4NHC(O)CH2; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of Dyad 3 Br and [Dyad 3 pic]OTf are described. The photocatalytic performance of [Dyad 3 pic]OTf in DMF/triethanolamine (5 : 1) is approximately an order of magnitude better than [Dyad 1 pic]PF6 or [Dyad 2 pic]OTf in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re-Br units is very similar to that of the dyads with [Re-pic]+ units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO2 into the latter to form a carbonate. Time-resolved IR spectra of [Dyad 2 pic]OTf and [Dyad 3 pic]OTf (560 nm excitation in CH2Cl2) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of ν(CO) bands to low wavenumbers. The rise time of the charge-separated species for [Dyad 3 pic]OTf is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of Dyad 1 Br and Dyad 2 Br are quite different showing a mixture of 3MLCT, IL and charge-separated excited states. In the case of Dyad 3 Br, the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium.

Heterogenised Molecular Catalysts for the Reduction of CO2 to Fuels.

CO2 conversion provides a possible solution to curtail the growing CO2 levels in our atmosphere and reduce dependence on fossil fuels. To this end, it is essential to develop efficient catalysts for the reduction of CO2. The structure and activity of molecular CO2 reduction catalysts can be tuned and they offer good selectivity with reasonable stability. Heterogenisation of these molecules reduces solvent restrictions, facilitates recyclability and can dramatically improve activity by preventing catalyst inactivation and perturbing the kinetics of intermediates. The nature and morphology of the solid-state material upon which the catalyst is immobilised can significantly influence the activity of the hybrid assembly. Although work in this area began forty years ago, it has only drawn substantial attention in recent years. This review article gives an overview of the historical development of the field.

CO2 photoreduction with long-wavelength light: dyads and monomers of zinc porphyrin and rhenium bipyridine.

Photocatalytic CO(2) reduction has been studied for two dyads with porphyrin covalently attached to rhenium tricarbonyl bipyridine moieties, and on separate components consisting of [Re(CO)(3)(Picoline)Bpy](+) and either zinc porphyrin or zinc chlorin. TONs decrease in the order: zinc porphyrin + Re > long spacer dyad > zinc chlorin + Re > short spacer dyad.