Hole-Accepting-Ligand-Modified CdSe QDs for Dramatic Enhancement of Photocatalytic and Photoelectrochemical Hydrogen Evolution by Solar Energy

Liquid-Liquid Phase Separation-Mediated Photocatalytic Subcellular Hybrid System for Highly Efficient Hydrogen Production

Plant chloroplasts have a highly compartmentalized interior, essential for executing photocatalytic functions. However, the construction of a photocatalytic reaction compartment similar to chloroplasts in inorganic-biological hybrid systems (IBS) has not been reported. Drawing inspiration from the compartmentalized chloroplast and the phenomenon of liquid-liquid phase separation, herein, a new strategy is first developed for constructing a photocatalytic subcellular hybrid system through liquid-liquid phase separation technology in living cells. Photosensitizers and in vivo expressed hydrogenases are designed to coassemble within the cell to create subcellular compartments for synergetic photocatalysis. This compartmentalization facilitates efficient electron transfer and light energy utilization, resulting in highly effective H2 production. The subcellular compartments hybrid system (HM/IBSCS) exhibits a nearly 87-fold increase in H2 production compared to the bare bacteria/hybrid system. Furthermore, the intracellular compartments of the photocatalytic reactor enhance the system's stability obviously, with the bacteria maintaining approximately 81% of their H2 production activity even after undergoing five cycles of photocatalytic hydrogen production. The research brings forward visionary prospects for the field of semi-artificial photosynthesis, offering new possibilities for advancements in areas such as renewable energy, biomanufacturing, and genetic engineering.

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Effect of a redox-mediating ligand shell on photocatalysis by CdS quantum dots

Semiconductor quantum dots (QDs) are efficient organic photoredox catalysts due to their high extinction coefficients and easily tunable band edge potentials. Despite the majority of the surface being covered by ligands, our understanding of the effect of the ligand shell on organic photocatalysis is limited to steric effects. We hypothesize that we can increase the activity of QD photocatalysts by designing a ligand shell with targeted electronic properties, namely, redox-mediating ligands. Herein, we functionalize our QDs with hole-mediating ferrocene (Fc) derivative ligands and perform a reaction where the slow step is hole transfer from QD to substrate. Surprisingly, we find that a hole-shuttling Fc inhibits catalysis, but confers much greater stability to the catalyst by preventing a build-up of destructive holes. We also find that dynamically bound Fc ligands can promote catalysis by surface exchange and creation of a more permeable ligand shell. Finally, we find that trapping the electron on a ligand dramatically increases the rate of reaction. These results have major implications for understanding the rate-limiting processes for charge transfer from QDs and the role of the ligand shell in modulating it.

Photochemistry Journey to Multielectron and Multiproton Chemical Transformation

The odyssey of photochemistry is accompanied by the journey to manipulate "electrons" and "protons" in time, in space, and in energy. Over the past decades, single-electron (1e^-) photochemical transformations have brought marvelous achievements. However, as each photon absorption typically generates only one exciton pair, it is exponentially challenging to accomplish multielectron and proton photochemical transformations. The multistep differences in thermodynamics and kinetics urgently require us to optimize light harvesting, expedite consecutive electron transfer, manipulate the interaction of catalysts with substrates, and coordinate proton transfer kinetics to furnish selective bond formations. Tandem catalysis enables orchestrating different photochemical events and catalytic transformations from subpicoseconds to seconds, which facilitates multielectron redox chemistries and brings consecutive, value-added reactivities. Joint efforts in molecular and material design, mechanistic understanding, and theoretical modeling will bring multielectron and proton synthetic opportunities for fuels, fertilizers, and chemicals with enhanced versatility, efficiency, selectivity, and scalability, thus taking better advantage of photons (i.e., sunlight) for our sustainable society.

A Hybrid Assembly with Nickel Poly-Pyridine Polymer on CdS Quantum Dots for Photo-Reducing CO2 into Syngas with Controlled H2 /CO Ratios

A hybrid photocatalytic assembly with Ni poly-pyridine polymers binding on CdS quantum dots was developed via thiophene immobilization. The fabricated hybrid assembly facilitated efficient charge separation, and each component endowed great synergy. As a result, a high syngas production rate was achieved over 5500 μmol g_cat ^-1  h^-1 from photocatalytic CO₂ reduction under visible-light irradiation, accompanied by an adjustable H₂ /CO ratio ranging from 4 : 1 to 1 : 3. A novel hybrid assembly was described for syngas synthesis with boosted activity and controlled selectivity, which provides a profile to ingeniously understand molecular-level design for photocatalysts.

Organic building blocks at inorganic nanomaterial interfaces

This tutorial review presents our perspective on designing organic molecules for the functionalization of inorganic nanomaterial surfaces, through the model of an "anchor-functionality" paradigm. This "anchor-functionality" paradigm is a streamlined design strategy developed from a comprehensive range of materials (e.g., lead halide perovskites, II-VI semiconductors, III-V semiconductors, metal oxides, diamonds, carbon dots, silicon, etc.) and applications (e.g., light-emitting diodes, photovoltaics, lasers, photonic cavities, photocatalysis, fluorescence imaging, photo dynamic therapy, drug delivery, etc.). The structure of this organic interface modifier comprises two key components: anchor groups binding to inorganic surfaces and functional groups that optimize their performance in specific applications. To help readers better understand and utilize this approach, the roles of different anchor groups and different functional groups are discussed and explained through their interactions with inorganic materials and external environments.

Promoting Photocatalytic Hydrogen Evolution Activity of Graphitic Carbon Nitride with Hole-Transfer Agents

Visible light-driven photocatalytic reduction of protons to H2 is considered a promising way of solar-to-chemical energy conversion. Effective transfer of the photogenerated electrons and holes to the surface of the photocatalyst by minimizing their recombination is essential for achieving a high photocatalytic activity. In general, a sacrificial electron donor is used as a hole scavenger to remove photogenerated holes from the valence band for the continuation of the photocatalytic hydrogen (H2 ) evolution process. Here, for the first time, the hole-transfer dynamics from Pt-loaded sol-gel-prepared graphitic carbon nitride (Pt-sg-CN) photocatalyst were investigated using different adsorbed hole acceptors along with a sacrificial agent (ascorbic acid). A significant increment (4.84 times) in H2 production was achieved by employing phenothiazine (PTZ) as the hole acceptor with continuous H2 production for 3 days. A detailed charge-transfer dynamic of the photocatalytic process in the presence of the hole acceptors was examined by time-resolved photoluminescence and in situ electron paramagnetic resonance studies.

Controlled Photoinduced Electron Transfer from InP/ZnS Quantum Dots through Cu Doping: A New Prototype for the Visible-Light Photocatalytic Hydrogen Evolution Reaction

Photoexcited electron extraction from semiconductors can be useful for converting solar energy into useful forms of energy. Although InP quantum dots (QDs) are considered alternative materials for solar energy conversion, the inherent instability of bare InP QDs demands the use of passivation layers such as ZnS for practical applications, which impedes carrier extraction from the QDs. Here, we demonstrate that Cu-doped InP/ZnS (InP/Cu:ZnS) QDs improve the electron transfer ability due to hole capture by Cu dopants. Steady-state and time-resolved photoluminescence studies confirmed that electrons were effectively transferred from the InP/Cu:ZnS QDs to a benzoquinone acceptor by retarding the electron-hole recombination within the QD. We evaluated the photocatalytic H₂ evolution performance of InP/Cu:ZnS QDs under visible light, which showed outstanding photocatalytic H₂ evolution activity and stability under visible light illumination. The photocatalytic activity was preserved even in the absence of a cocatalyst.

Semiconductor nanocrystals for small molecule activation via artificial photosynthesis

The protocol of artificial photosynthesis using semiconductor nanocrystals shines light on green, facile and low-cost small molecule activation to produce solar fuels and value-added chemicals.

ZnSe Nanorods as Visible-Light Absorbers for Photocatalytic and Photoelectrochemical H2 Evolution in Water

A precious-metal- and Cd-free photocatalyst system for efficient H₂ evolution from aqueous protons with a performance comparable to Cd-based quantum dots is presented. Rod-shaped ZnSe nanocrystals (nanorods, NRs) with a Ni(BF₄ )₂ co-catalyst suspended in aqueous ascorbic acid evolve H₂ with an activity up to 54±2 mmol H 2  g_ZnSe ^-1  h^-1 and a quantum yield of 50±4 % (λ=400 nm) under visible light illumination (AM 1.5G, 100 mW cm^-2 , λ>400 nm). Under simulated full-spectrum solar irradiation (AM 1.5G, 100 mW cm^-2 ), up to 149±22 mmol H 2  g_ZnSe ^-1  h^-1 is generated. Significant photocorrosion was not noticeable within 40 h and activity was even observed without an added co-catalyst. The ZnSe NRs can also be used to construct an inexpensive delafossite CuCrO₂ photocathode, which does not rely on a sacrificial electron donor. Immobilized ZnSe NRs on CuCrO₂ generate photocurrents of around -10 μA cm^-2 in an aqueous electrolyte solution (pH 5.5) with a photocurrent onset potential of approximately +0.75 V vs. RHE. This work establishes ZnSe as a state-of-the-art light absorber for photocatalytic and photoelectrochemical H₂ generation.

NiO nanowires as hole-transfer layer for drastic enhancement of CdSe-sensitized photocathodes

Grass-like NiO nanowires as a hole-transfer layer to improve light capture efficiency and charge transfer rate for a CdSe-sensitized photocathode.

Ultrafast Dynamics of Charge Transfer and Photochemical Reactions in Solar Energy Conversion

For decades, ultrafast time-resolved spectroscopy has found its way into an increasing number of applications. It has become a vital technique to investigate energy conversion processes and charge transfer dynamics in optoelectronic systems such as solar cells and solar-driven photocatalytic applications. The understanding of charge transfer and photochemical reactions can help optimize and improve the performance of relevant devices with solar energy conversion processes. Here, the fundamental principles of photochemical and photophysical processes in photoinduced reactions, in which the fundamental charge carrier dynamic processes include interfacial electron transfer, singlet excitons, triplet excitons, excitons fission, and recombination, are reviewed. Transient absorption (TA) spectroscopy techniques provide a good understanding of the energy/electron transfer processes. These processes, including excited state generation and interfacial energy/electron transfer, are dominate constituents of solar energy conversion applications, for example, dye-sensitized solar cells and photocatalysis. An outlook for intrinsic electron/energy transfer dynamics via TA spectroscopic characterization is provided, establishing a foundation for the rational design of solar energy conversion devices.

Interface Engineering of Colloidal CdSe Quantum Dot Thin Films as Acid-Stable Photocathodes for Solar-Driven Hydrogen Evolution

Colloidal semiconductor quantum dot (CQD)-based photocathodes for solar-driven hydrogen evolution have attracted significant attention because of their tunable size, nanostructured morphology, crystalline orientation, and band gap. Here, we report a thin film heterojunction photocathode composed of organic PEDOT:PSS as a hole transport layer, CdSe CQDs as a semiconductor light absorber, and conformal Pt layer deposited by atomic layer deposition (ALD) serving as both a passivation layer and cocatalyst for hydrogen evolution. In neutral aqueous solution, a PEDOT:PSS/CdSe/Pt heterogeneous photocathode with 200 cycles of ALD Pt produces a photocurrent density of -1.08 mA/cm² (AM-1.5G, 100 mW/cm²) at a potential of 0 V versus reversible hydrogen electrode (RHE) ( j₀) in neutral aqueous solution, which is nearly 12 times that of the pristine CdSe photocathode. This composite photocathode shows an onset potential for water reduction at +0.46 V versus RHE and long-term stability with negligible degradation. In the acidic electrolyte (pH = 1), where the hydrogen evolution reaction is more favorable but stability is limited because of photocorrosion, a thicker Pt film (300 cycles) is shown to greatly improve the device stability and a j₀ of -2.14 mA/cm² is obtained with only 8.3% activity degradation after 6 h, compared with 80% degradation under the same conditions when the less conformal electrodeposition method is used to deposit the Pt layer. Electrochemical impedance spectroscopy and time-resolved photoluminescence results indicate that these enhancements stem from a lower bulk charge recombination rate, higher interfacial charge-transfer rate, and faster reaction kinetics. We believe that these interface engineering strategies can be extended to other colloidal semiconductors to construct more efficient and stable heterogeneous photoelectrodes for solar fuel production.

Recent Advances in Sensitized Photocathodes: From Molecular Dyes to Semiconducting Quantum Dots

The increasing demand for sustainable and environmentally benign energy has stimulated intense research to establish highly efficient photo-electrochemical (PEC) cells for direct solar-to-fuel conversion via water splitting. Light absorption, as the initial step of the catalytic process, is regarded as the foundation of establishing highly efficient PEC systems. To make full use of visible light, sensitization on photoelectrodes using either molecular dyes or semiconducting quantum dots provides a promising method. In this field, however, there remain many fundamental issues to be solved, which need in-depth study. Here, fundamental knowledge of PEC systems is introduced to enable readers a better understanding of this field. Then, the development history and current state in both molecular dye- and quantum dot-sensitized photocathodes for PEC water splitting are discussed. A systematical comparison between the two systems has been made. Special emphasis is placed on the research of quantum dot-sensitized photocathodes, which have shown superiority in both efficiency and durability towards PEC water splitting at the present stage. Finally, the opportunities and challenges in the future for sensitized PEC water-splitting systems are proposed.

Self-assembled inorganic clusters of semiconducting quantum dots for effective solar hydrogen evolution

The catalytic activity of CdSe QDs could be enhanced more than 150-fold by forming self-assembled clusters with ZnSe QDs madeex situ.

Mesoporous Nanosheet Networked Hybrids of Cobalt Oxide and Cobalt Phosphate for Efficient Electrochemical and Photoelectrochemical Oxygen Evolution

A novel mesoporous nanosheet networked hybrid comprising Co₃ O₄ and Co₃ (PO₄ )₂ is successfully synthesized using a facile and scalable method through calcinating the carbon, cobalt hydroxy carbonate, and cobalt phosphate composite precursor. Electron transfer from Co₃ O₄ to Co₃ (PO₄ )₂ , together with the special networked structure and the porous nature of the nanosheets enable the Co₃ (PO₄ )₂ -Co₃ O₄ hybrid to have a high oxygen evolution reaction (OER) activity and outstanding stability in alkaline electrolyte, e.g., an overpotential of 270 mV at current density of 10 mA cm^-2 , and a Tafel slope of 39 mV dec^-1 , which are superior to most non-noble metal-based OER electrocatalysts reported thus far and as well the commercial RuO₂ electrocatalyst. Furthermore, Co₃ (PO₄ )₂ -Co₃ O₄ hybrid is demonstrated to be used as an efficient cocatalyst to enhance the photoelectrochemical OER performance of BiVO₄ photoanode. A significantly increased photocurrent density of 3.0 mA cm^-2 at 1.23 V (vs reversible hydrogen electrode, RHE), and a potential reduction of 530 mV with respect to that of bare BiVO₄ at the photocurrent density of 0.5 mA cm^-2 are achieved. The electron transfer-induced enhancement of OER by a hybrid structure may pave the new routes for the design and synthesis of low-cost catalysts for electrochemical and photoelectrochemical oxygen evolution.

A Redox Shuttle Accelerates O2 Evolution of Photocatalysts Formed In Situ under Visible Light

A redox shuttle strategy is demonstrated to be a promising approach to accelerate hole removal for efficient O₂ production with mesoporous graphitic carbon nitride, WO₃ , BiVO₄ , NiTi-LDH, and Ag₃ PO₄ water-oxidation catalysts under visible-light irradiation.

One-dimensional TiO2 Nanotube Photocatalysts for Solar Water Splitting

Hydrogen production from water splitting by photo/photoelectron-catalytic process is a promising route to solve both fossil fuel depletion and environmental pollution at the same time. Titanium dioxide (TiO2) nanotubes have attracted much interest due to their large specific surface area and highly ordered structure, which has led to promising potential applications in photocatalytic degradation, photoreduction of CO2, water splitting, supercapacitors, dye-sensitized solar cells, lithium-ion batteries and biomedical devices. Nanotubes can be fabricated via facile hydrothermal method, solvothermal method, template technique and electrochemical anodic oxidation. In this report, we provide a comprehensive review on recent progress of the synthesis and modification of TiO2 nanotubes to be used for photo/photoelectro-catalytic water splitting. The future development of TiO2 nanotubes is also discussed.

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.