Photocatalytic water reduction using a polymer coated carbon quantum dot sensitizer and a nickel nanoparticle catalyst

Hydrogen gas is produced photocatalytically using 470 nm light, PVP-coated carbon quantum dots (CQDs) as the photosensitizer, and nickel nanoparticles (NiNPs) as the catalyst. The effect of the amount of polyvinylpyrrolidone (PVP) on the ability of the CQD/NiNP composites to catalyze proton reduction was studied. A maximum of 330 mmols H₂/g CQD is produced using 68 μg ml^-1 of CQDs and 6 μg ml^-1 of NiNPs, with activity persisting for 4 h when 20 wt%-PVP-coated CQDs were used. The H₂ production quantum yield under these conditions is 6%. It was found that composites having higher weight percent PVP had decreased rates of H₂ production, but increased duration. Increasing the weight percent of PVP coating also increases the fluorescence quantum yield of CQDs. Fluorescence quenching titrations reveal that H₂ production could occur by either a reductive or oxidative quenching mechanism. The nanomaterials, prepared using simple methods, are used as the photosensitizer and catalyst in the proton reduction system that operates using visible light.

Enhanced device lifetime of double-heterojunction nanorod light-emitting diodes

Colloidal quantum dots (QDs) are emerging as solution-processable, high-performance materials for light-emitting diodes (LEDs). Understanding the failure mechanism(s) is of both fundamental and practical importance, yet little is known of how QD-LEDs fail. Here, we have carried out accelerated device lifetime measurements on double heterojunction nanorod- (DHNR) and QD-LEDs. A common dependence of device lifetime on the initial driving voltage is observed over more than two orders of magnitude range in the initial luminance. This behavior is independent of whether the emitting materials are DHNRs or QDs prepared under different conditions. Reducing the hole injection barrier by doping HTL allows lower voltage operation, leading to longer device lifetimes. DHNRs with a band structure that further lowers the hole injection barrier require even lower driving voltages and therefore lead to longer device lifetimes than core/shell QDs. At 1000 cd m^-2, the DHNR-LED exhibits no significant degradation even after more than 200 h of continuous operation. QD-LEDs, on the other hand, are completely degraded in less than ∼100 h under the same initial luminance conditions. Hole accumulation/trapping leading to HTL degradation, which in turn deteriorates electroluminescence but not the photoluminescence, is suggested to be the main cause of degradation of both DHNR- and QD-LEDs.

Recent Progress in Energy-Driven Water Splitting

Hydrogen is readily obtained from renewable and non-renewable resources via water splitting by using thermal, electrical, photonic and biochemical energy. The major hydrogen production is generated from thermal energy through steam reforming/gasification of fossil fuel. As the commonly used non-renewable resources will be depleted in the long run, there is great demand to utilize renewable energy resources for hydrogen production. Most of the renewable resources may be used to produce electricity for driving water splitting while challenges remain to improve cost-effectiveness. As the most abundant energy resource, the direct conversion of solar energy to hydrogen is considered the most sustainable energy production method without causing pollutions to the environment. In overall, this review briefly summarizes thermolytic, electrolytic, photolytic and biolytic water splitting. It highlights photonic and electrical driven water splitting together with photovoltaic-integrated solar-driven water electrolysis.

Delayed Molecular Triplet Generation from Energized Lead Sulfide Quantum Dots

The generation and transfer of triplet excitons across the molecular-semiconductor interface represents an important technological breakthrough featuring numerous fundamental scientific questions. This contribution demonstrates curious delayed formation of TIPS-pentacene molecular triplet excitons bound on the surface of PbS nanocrystals mediated through the initial production of a proposed charge transfer intermediate following selective excitation of the PbS quantum dots. Ultrafast UV-vis and near-IR transient absorption spectroscopy was used to track the dynamics of the initial PbS exciton quenching as well as time scale of the formation of molecular triplet excited states that persisted for 10 μs on the PbS surface, enabling subsequent energy and electron transfer reactivity. These results provide the pivotal proof-of-concept that PbS nanocrystals absorbing near-IR radiation can ultimately generate molecular triplets on their surfaces through processes distinct from direct Dexter triplet energy transfer. More broadly, this work establishes that small metal chalcogenide semiconductor nanocrystals interfaced with molecular chromophores exhibit behavior reminiscent of supramolecular chemical systems, a potentially impactful concept for nanoscience.

Elemental Mass Size Distribution for Characterization, Quantification and Identification of Trace Nanoparticles in Serum and Environmental Waters

Accurate characterization, quantification, and identification of nanoparticles (NPs) are essential to fully understand the environmental processes and effects of NPs. Herein, the elemental mass size distribution (EMSD), which measures particle size, mass, and composition, is proposed for the direct size characterization, mass quantification, and composition identification of trace NPs in complex matrixes. A one-step method for the rapid measurement of EMSDs in 8 min was developed through the online coupling of size-exclusion chromatography (SEC) with inductively coupled plasma mass spectrometry (ICP-MS). The use of a mobile phase with a relatively high ionic strength (a mixture of 2% FL-70 and 2 mM Na₂S₂O₃) ensured the complete elution of different-sized NPs from the column and, therefore, a size-independent response. After application of a correction for instrumental broadening by a method developed in this study, the size distribution of NPs by EMSD determination agreed closely with that obtained from transmission electron microscopy (TEM) analysis. Compared with TEM, EMSD allows a more rapid determination with a higher mass sensitivity (1 pg for gold and silver NPs) and comparable size discrimination (0.27 nm). The proposed EMSD-based method was capable of identifying trace Ag₂S NPs and core-shell nanocomposite Au@Ag, as well as quantitatively tracking the dissolution and size transformation of silver nanoparticles in serum and environmental waters.

Well-Defined Nanographene-Rhenium Complex as an Efficient Electrocatalyst and Photocatalyst for Selective CO2 Reduction

Improving energy efficiency of electrocatalytic and photocatalytic CO₂ conversion to useful chemicals poses a significant scientific challenge. We report on using a colloidal nanographene to form a molecular complex with a metal ion to tackle this challenge. In this work, a well-defined nanographene-Re complex was synthesized, in which electron delocalization over the nanographene and the metal ion significantly decreases the electrical potential needed to drive the chemical reduction. We show the complex can selectively electrocatalyze CO₂ reduction to CO in tetrahydrofuran at -0.48 V vs NHE, the least negative potential reported for a molecular catalyst. In addition, the complex can absorb a significant spectrum of visible light to photocatalyze the chemical transformation without the need for a photosensitizer.

Electron-transfer dependent photocatalytic hydrogen generation over cross-linked CdSe/TiO2 type-II heterostructure

Developing type-II heterostructures with a spatial separation of photoexcited electrons and holes is a useful route to promote photocatalytic hydrogen generation. However, few investigations on the charge transfer process across the heterojunction have been carried out, which can allow us to uncover the reaction mechanism. Herein, CdSe quantum dots (QDs) and TiO₂ nanocrystals were synthesized and combined in water yielding CdSe/TiO₂ type II heterostructures. It was found that mercaptopropionic acid as bifunctional molecules could bind with CdSe and TiO₂ to form a cross-linked morphology. The charge carrier dynamics of bare CdSe and CdSe/TiO₂ were detected using femtosecond transient absorption spectroscopy. In the presence of TiO₂, the average exciton lifetime of CdSe QDs was apparently decreased, owing to the electron transfer from photoexcited CdSe to TiO₂. Particularly, the electron-transfer rate from small CdSe QDs (3.0 nm) was much faster than that from big CdSe QDs (4.2 nm). The improved photocatalytic hydrogen generation was observed for CdSe/TiO₂ compared to bare CdSe QDs. The enhancement factor for small CdSe QDs was higher than that for big CdSe QDs, which was in good agreement with the electron-transfer rates. This result indicated that the electron transfer between CdSe and TiO₂ played an important role in photocatalytic hydrogen generation on CdSe/TiO₂ type-II heterostructure. Our study provides a fundamental guidance to construct efficient heterostructured photocatalysts by delicate control of the band alignment.

Design of a Hole Trapping Ligand

A new ligand that covalently attaches to the surface of colloidal CdSe/CdS nanorods and can simultaneously chelate a molecular metal center is described. The dithiocarbamate-bipyridine ligand system facilitates hole transfer through energetic overlap at the inorganic-organic interface and conjugation through the organic ligand to a chelated metal center. Density functional theory calculations show that the coordination of the free ligand to a CdS surface causes the formation of two hybridized molecular states that lie in the band gap of CdS. The further chelation of Fe(II) to the bipyridine moiety causes the presence of seven midgap states. Hole transfer from the CdS valence band to the midgap states is dipole allowed and occurs at a faster rate than what is experimentally known for the CdSe/CdS band-edge radiative recombination. In the case of the ligand bound with iron, a two-step process emerges that places the hole on the iron, again at rates much faster than band gap recombination. The system was experimentally assembled and characterized via UV-vis absorbance spectroscopy, fluorescence spectroscopy, time-resolved photoluminescence spectroscopy, and energy dispersive X-ray spectroscopy. Theoretically predicted red shifts in absorbance were observed experimentally, as well as the expected quench in photoluminescence and lifetimes in time-resolved photoluminescence.

Light-driven hydrogen production from aqueous solutions based on a new Dubois-type nickel catalyst

We developed a new water soluble CdSe/Ni hybrid, which yields remarkable photon-to-H₂ efficiency among all noble-metal free systems based on synthetic Ni molecular catalysts.

Cobalt Schiff-base complexes for electrocatalytic hydrogen generation

Two cobalt(iii) complexes containing inexpensive Schiff-base ligands have been found to be active for proton reduction at low overpotentials.

Cu(iii)triarylcorroles with asymmetric push–pull meso-substitutions: tunable molecular electrochemically catalyzed hydrogen evolution

The A₂B type Cu(iii)triarylcorroles with meso-aryl push–pull substituents demonstrate that these compounds are highly efficient catalysts for modulating electrocatalyzed hydrogen evolution reactions (HERs).

An amorphous dual action electrocatalyst based on oxygen doped cobalt sulfide for the hydrogen and oxygen evolution reactions

Here we electrodeposit an amorphous bifunctional electrocatalyst that is active for both the HER and OER under alkaline conditions which is based on oxygen doped cobalt sulfide.

The mechanism change by switching the reactants from water to hydroxyl ions for electrocatalytic water oxidation: a case study of copper oxide microspheres

The as-obtained CuO microspheres can serve as an active and stable water oxidation catalyst under electrochemical reaction conditions and operate at modest overpotential providing an alternative to the Co-WOC catalyst for applications in solar energy storage.

Cobalt nitride as an efficient cocatalyst on CdS nanorods for enhanced photocatalytic hydrogen production in water

Noble-metal-free cobalt nitride (Co₃N) can be used as a novel cocatalyst on CdS nanorods for photocatalytic H₂production in water under visible light irradiation.

Developing a scalable artificial photosynthesis technology through nanomaterials by design

An artificial photosynthetic system that directly produces fuels from sunlight could provide an approach to scalable energy storage and a technology for the carbon-neutral production of high-energy-density transportation fuels. A variety of designs are currently being explored to create a viable artificial photosynthetic system, and the most technologically advanced systems are based on semiconducting photoelectrodes. Here, I discuss the development of an approach that is based on an architecture, first conceived around a decade ago, that combines arrays of semiconducting microwires with flexible polymeric membranes. I highlight the key steps that have been taken towards delivering a fully functional solar fuels generator, which have exploited advances in nanotechnology at all hierarchical levels of device construction, and include the discovery of earth-abundant electrocatalysts for fuel formation and materials for the stabilization of light absorbers. Finally, I consider the remaining scientific and engineering challenges facing the fulfilment of an artificial photosynthetic system that is simultaneously safe, robust, efficient and scalable.

Synergistic Effect of a Molecular Cocatalyst and a Heterojunction in a 1 D Semiconductor Photocatalyst for Robust and Highly Efficient Solar Hydrogen Production

Photocatalytic production of hydrogen by water splitting is a promising pathway for the conversion of solar energy into chemical energy. However, the photocatalytic conversion efficiency is often limited by the sluggish transfer of the photogenerated charge carriers, charge recombination, and subsequent slow catalytic reactions. Herein, we report a highly active noble-metal-free photocatalytic system for hydrogen production in water. The system contains a water-soluble nickel complex as a molecular cocatalyst and zinc sulfide on 1D cadmium sulfide as the heterojunction photocatalyst. The complex can efficiently transport photogenerated electrons and holes over a heterojunction photocatalyst to hamper charge recombination, leading to highly improved catalytic efficiency and durability of a heterojunction photocatalyst- molecular cocatalyst system. The results show that under optimal conditions, the average apparent quantum yield was approximately 58.3 % after 7 h of irradiation with monochromatic 420 nm light. In contrast, the value is only 16.8 % if the molecular cocatalyst is absent. Such a remarkable performance in a molecular cocatalyst-based photocatalytic system without any noble metal loading has, to the best of our knowledge, not been reported to date.

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems

Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Because of the significance of energy-related small molecule activation, this review covers recent progress in hydrogen evolution, oxygen evolution, and oxygen reduction reactions catalyzed by porphyrins and corroles.

Simultaneous Optimization of Colloidal Stability and Interfacial Charge Transfer Efficiency in Photocatalytic Pt/CdS Nanocrystals

Colloidal stability and efficient interfacial charge transfer in semiconductor nanocrystals are of great importance for photocatalytic applications in aqueous solution since they provide long-term functionality and high photocatalytic activity, respectively. However, colloidal stability and interfacial charge transfer efficiency are difficult to optimize simultaneously since the ligand layer often acts as both a shell stabilizing the nanocrystals in colloidal suspension and a barrier reducing the efficiency of interfacial charge transfer. Here, we show that, for cysteine-coated, Pt-decorated CdS nanocrystals and Na₂SO₃ as hole scavenger, triethanolamine (TEOA) replaces the original cysteine ligands in situ and prolongs the highly efficient and steady H₂ evolution period by more than a factor of 10. It is shown that Na₂SO₃ is consumed during H₂ generation while TEOA makes no significant contribution to the H₂ generation. An apparent quantum yield of 31.5%, a turnover frequency of 0.11 H₂/Pt/s, and an interfacial charge transfer rate faster than 0.3 ps were achieved in the TEOA stabilized system. The short length, branched structure and weak binding of TEOA to CdS as well as sufficient free TEOA in the solution are the keys to enhancing colloidal stability and maintaining efficient interfacial charge transfer at the same time. Additionally, TEOA is commercially available and cheap, and we anticipate that this approach can be widely applied in many photocatalytic applications involving colloidal nanocrystals.

Carbon Nitride Supramolecular Hybrid Material Enabled High-Efficiency Photocatalytic Water Treatments

Surface defects in relation to surface compositions, morphology, and active sites play crucial roles in photocatalytic activity of graphitic carbon nitride (g-C₃N₄) material for highly reactive oxygen radicals production. Here, we report a high-efficiency carbon nitride supramolecular hybrid material prepared by patching the surface defects with inorganic clusters. Fe (III) {PO₄[WO(O₂)₂]₄} clusters have been noncovalently integrated on surface of g-C₃N₄, where the surface defects provide accommodation sites for these clusters and driving forces for self-assembly. During photocatalytic process, the activity of supramolecular hybrid is 1.53 times than pure g-C₃N₄ for the degradation of Rhodamine B (RhB) and 2.26 times for Methyl Orange (MO) under the simulated solar light. Under the mediation of H₂O₂ (50 mmol L^-1), the activity increases to 6.52 times for RhB and 28.3 times for MO. The solid cluster active sites with high specific surface area (SSA) defect surface promoting the kinetics of hydroxide radicals production give rise to the extremely high photocatalytic activity. It exhibits recyclable capability and works in large-scale demonstration under the natural sunlight as well and interestingly the environmental temperature has little effects on the photocatalytic activity.

Photocatalytic Hydrogen Generation by CdSe/CdS Nanoparticles

The photocatalytic hydrogen (H2) production activity of various CdSe semiconductor nanoparticles was compared including CdSe and CdSe/CdS quantum dots (QDs), CdSe quantum rods (QRs), and CdSe/CdS dot-in-rods (DIRs). With equivalent photons absorbed, the H2 generation activity orders as CdSe QDs ≫ CdSe QRs > CdSe/CdS QDs > CdSe/CdS DIRs, which is surprisingly the opposite of the electron-hole separation efficiency. Calculations of photoexcited surface charge densities are positively correlated with the H2 production rate and suggest the size of the nanoparticle plays a critical role in determining the relative efficiency of H2 production.

Catalytic Light-Driven Generation of Hydrogen from Water by Iron Dithiolene Complexes

The development of active, robust systems for light-driven hydrogen production from aqueous protons based on catalysts and light absorbers composed solely of earth abundant elements remains a challenge in the development of an artificial photosynthetic system for water splitting. Herein, we report the synthesis and characterization of four closely related Fe bis(benzenedithiolate) complexes that exhibit catalytic activity for hydrogen evolution when employed in systems with water-soluble CdSe QDs as photosensitizer and ascorbic acid as a sacrificial electron source under visible light irradiation (520 nm). The complex with the most electron-donating dithiolene ligand exhibits the highest activity, the overall order of activity correlating with the reduction potential of the formally Fe(III) dimeric dianions. Detailed studies of the effect of different capping agents and the extent of surface coverage of these capping agents on the CdSe QD surfaces reveal that they affect system activity and provide insight into the continued development of such systems containing QD light absorbers and molecular catalysts for H2 formation.

A highly efficient noble metal free photocatalytic hydrogen evolution system containing MoP and CdS quantum dots

We report the construction of a highly efficient noble metal free photocatalytic hydrogen (H2) evolution system using CdS quantum dots as the light absorber and metallic MoP as the cocatalyst. MoP can be prepared by a facile temperature programmed reduction method and small clusters of MoP nanoparticles sized 10-30 nm were obtained by probe ultrasonication. The effect of synthesis conditions on the electrocatalytic and photocatalytic H2 evolution activity of MoP was investigated. The highest H2 evolution rate of 1100 μmol h(-1) can be achieved by the optimized system under visible light (λ≥ 420 nm), which is comparable to that when Pt was used as the cocatalyst. A high quantum efficiency of 45% is obtained at 460 nm irradiation.

Electronic Processes within Quantum Dot-Molecule Complexes

The subject of this review is the colloidal quantum dot (QD) and specifically the interaction of the QD with proximate molecules. It covers various functions of these molecules, including (i) ligands for the QDs, coupled electronically or vibrationally to localized surface states or to the delocalized states of the QD core, (ii) energy or electron donors or acceptors for the QDs, and (iii) structural components of QD assemblies that dictate QD-QD or QD-molecule interactions. Research on interactions of ligands with colloidal QDs has revealed that ligands determine not only the excited state dynamics of the QD but also, in some cases, its ground state electronic structure. Specifically, the article discusses (i) measurement of the electronic structure of colloidal QDs and the influence of their surface chemistry, in particular, dipolar ligands and exciton-delocalizing ligands, on their electronic energies; (ii) the role of molecules in interfacial electron and energy transfer processes involving QDs, including electron-to-vibrational energy transfer and the use of the ligand shell of a QD as a semipermeable membrane that gates its redox activity; and (iii) a particular application of colloidal QDs, photoredox catalysis, which exploits the combination of the electronic structure of the QD core and the chemistry at its surface to use the energy of the QD excited state to drive chemical reactions.

Solar-Driven Reduction of Aqueous Protons Coupled to Selective Alcohol Oxidation with a Carbon Nitride-Molecular Ni Catalyst System

Solar water-splitting represents an important strategy toward production of the storable and renewable fuel hydrogen. The water oxidation half-reaction typically proceeds with poor efficiency and produces the unprofitable and often damaging product, O2. Herein, we demonstrate an alternative approach and couple solar H2 generation with value-added organic substrate oxidation. Solar irradiation of a cyanamide surface-functionalized melon-type carbon nitride ((NCN)CNx) and a molecular nickel(II) bis(diphosphine) H2-evolution catalyst (NiP) enabled the production of H2 with concomitant selective oxidation of benzylic alcohols to aldehydes in high yield under purely aqueous conditions, at room temperature and ambient pressure. This one-pot system maintained its activity over 24 h, generating products in 1:1 stoichiometry, separated in the gas and solution phases. The (NCN)CNx-NiP system showed an activity of 763 μmol (g CNx)(-1) h(-1) toward H2 and aldehyde production, a Ni-based turnover frequency of 76 h(-1), and an external quantum efficiency of 15% (λ = 360 ± 10 nm). This precious metal-free and nontoxic photocatalytic system displays better performance than an analogous system containing platinum instead of NiP. Transient absorption spectroscopy revealed that the photoactivity of (NCN)CNx is due to efficient substrate oxidation of the material, which outweighs possible charge recombination compared to the nonfunctionalized melon-type carbon nitride. Photoexcited (NCN)CNx in the presence of an organic substrate can accumulate ultralong-lived "trapped electrons", which allow for fuel generation in the dark. The artificial photosynthetic system thereby catalyzes a closed redox cycle showing 100% atom economy and generates two value-added products, a solar chemical, and solar fuel.

Secondary coordination sphere accelerates hole transfer for enhanced hydrogen photogeneration from [FeFe]-hydrogenase mimic and CdSe QDs in water

Achieving highly efficient hydrogen (H₂) evolution via artificial photosynthesis is a great ambition pursued by scientists in recent decades because H₂ has high specific enthalpy of combustion and benign combustion product. [FeFe]-Hydrogenase ([FeFe]-H₂ase) mimics have been demonstrated to be promising catalysts for H₂ photoproduction. However, the efficient photocatalytic H₂ generation system, consisting of PAA-g-Fe₂S₂, CdSe QDs and H₂A, suffered from low stability, probably due to the hole accumulation induced photooxidation of CdSe QDs and the subsequent crash of [FeFe]-H₂ase mimics. In this work, we take advantage of supramolecular interaction for the first time to construct the secondary coordination sphere of electron donors (HA⁻) to CdSe QDs. The generated secondary coordination sphere helps realize much faster hole removal with a ~30-fold increase, thus leading to higher stability and activity for H₂ evolution. The unique photocatalytic H₂ evolution system features a great increase of turnover number to 83600, which is the highest one obtained so far for photocatalytic H₂ production by using [FeFe]-H₂ase mimics as catalysts.