Key Strategies for Enhancing H2 Production in Transition Metal Oxide Based Photocatalysts

Unveiling the reaction mechanism of photocatalytic H2 evolution coupled with selective bio-mass monosaccharide upgrading over single palladium atom-engineered carbon-rich graphitic carbon nitride

The simultaneous utilization of photogenerated electrons and holes in the coupled redox reactions of H2 production and selective biomass upgrading is a promising strategy to offer both economic and environmental benefits. Nevertheless, the method faces challenges of low H2 evolution efficiency and poor selectivity to biomass-derived chemicals. Herein, a supramolecular preorganisation-thermal polymerization-photo-assisted reduction strategy was designed to fabricate single palladium atom-engineered carbon-rich g-C3N4 (Pd1/BCNx) catalysts, and the interlayer Pd-N4 coordination configuration was confirmed for stabilizing the isolated Pd atoms. Under simulated sunlight irradiation, the optimized 0.39 %Pd1/BCN2 catalyst demonstrated superior performance in the co-generation of H2 and lactic acid by the substitution of monosaccharide (fructose or xylose) for traditional hole scavenger. In a dilute NaOH system (1 or 1.5 mol/L) and after 4 h light irradiation, the conversion of monosaccharide reached 100 %, the H2 evolution rate and selectivity to lactic acid approached 5.7 mmol gcat-1 h-1 and 71.8 % (fructose system) and 7.0 mmol gcat-1 h-1 and 64.1 % (xylose system). The reaction mechanism studies unveiled that the Pd1/BCNx catalysts with the accelerated photogenerated charge transfer dynamics and the maximum Pd atoms utilisation efficiency greatly facilitated the coupled redox reaction; moreover, the synergy of the as-generated reactive oxygen species (ROSs) and ROSs-induced xylose (or fructose)-based radical intermediates played a pivotal role on the production of LA with high activity and selectivity via a C-C bond cleavage pathway.

Covalent organic framework without cocatalyst loading for efficient photocatalytic sacrificial hydrogen production from water

Metals are typically essential as either integral components within photocatalysts or as cocatalyst modifiers to enable efficient artificial photosynthesis, such as water splitting and carbon dioxide reduction. However, developing photocatalysts that function effectively without metal cocatalysts remains challenging due to their cost and scarcity. Here we show a nonstoichiometric β-ketoenamine-linked covalent organic framework that operates without cocatalysts, achieving hydrogen production rates of 15.48 mmol·g⁻¹·h⁻¹ from seawater and 22.45 mmol·g⁻¹·h⁻¹ from water with an ascorbic acid scavenger under visible light. It outperforms many reported platinum-modified covalent organic frameworks and metal-containing inorganic photocatalysts. The enhanced performance is attributed to its broad light absorption edge extending to approximately 660 nm, efficient charge separation, and the presence of abundant active oxygen sites derived from carbonyl groups, which exhibit a low hydrogen-binding Gibbs free energy change. This work lays the groundwork for designing cost-effective photocatalytic systems suitable for large-scale hydrogen production.

Co Nanoparticles on MnO: Electron Transfer through Ohmic and S-Scheme Heterojunction for Photocatalytic Hydrogen Evolution

The photocatalytic hydrolysis method represents a significant potential solution to the dual challenges of energy security and environmental sustainability. The selection of suitable photocatalytic materials and systems is of paramount importance for the successful implementation of photocatalytic hydrogen production technology. In this study, in situ reduction of Co nanoparticles on MnO was successfully performed by calcining MnCo-PBA. Furthermore, graphdiyne (GDY) was successfully introduced by physical agitation. The introduction of GDY reduced Co/MnO agglomeration and made the Co/MnO/GDY catalyst exhibit high activity in hydrogen production, with an optimum production rate of 2117.33 μmol·g-1·h-1, which was 4.88 and 2.67 times higher than that of GDY and Co/MnO, respectively. The results of the photoelectrochemical test indicate that the composite catalyst has a better photogenerated carrier separation efficiency. In situ X-ray photoelectron spectroscopy, density functional theory calculations, and electron paramagnetic resonance were used to investigate the electron transfer mechanism during the photocatalytic process, confirming the presence of an S-scheme heterojunction and an ohmic junction, which enhance the separation of photogenerated carriers. The GDY-based heterojunction catalyst constructed in this study has the potential to significantly enhance the hydrogen production activity of bimetallic catalysts.

Influence of π-π interactions on organic photocatalytic materials and their performance

Currently, organic photocatalyst-based photocatalysis has garnered significant attention as an environmentally friendly and sustainable reaction system due to the preferable structural flexibility and adjustable optoelectronic features of organic photocatalysts. In addition, π-π interactions, as one of the common non-bonded interactions, play an important role in the structure and property adjustments of organic photocatalysts due to their unique advantages in modulating the electronic structure, facilitating charge migration, and influencing interfacial reactions. However, studies summarizing the relationship between the π-π interactions of organic photocatalysts and their photocatalytic performance are still rare. Therefore, in this review, we introduced the types of π-π interactions, characterization techniques, and different types of organic photocatalytic materials. Then, the influence of π-π interactions on photocatalysis and the modification strategies of π-π interactions were summarized. Finally, we discussed their influence on photocatalytic performance in different photocatalytic systems and analyzed the challenges and prospects associated with harnessing π-π interactions in photocatalysis. The review provides a clear map for understanding π-π interaction formation mechanism and its application in organic photocatalysts, offering useful guidance for researchers in this field.

Template-free synthesis of single-crystal SrTiO3 nanocages for photocatalytic overall water splitting

In this study, we present a novel approach to achieve the template-free fabrication of nanocage-shaped SrTiO3 (N-STO) single crystals via molten salt flux treatment. Systematic characterizations demonstrate the high crystallinity and low defect density of N-STO. The N-STO single crystals enable overall water splitting (OWS) with hydrogen and oxygen evolution rates of 100.86 μmol h-1 g-1 and 44.2 μmol h-1 g-1, respectively, which is 5.7-fold higher than the porous SrTiO3 (P-STO) control.

Recent Advances in Vanadate-Based Materials for Photocatalytic Hydrogen Production

Metal vanadates are a developing group of semiconducting metal oxide materials that are gaining increasing attention due to their great redox potential, effective separation of photogenerated electron-hole pairs, and tunability of structural and physicochemical properties. Their rational design as effective photocatalysts can find use in various applications, including energy conversion/storage and environmental remediation. In particular, one of the viable ways to address energy-related issues can be through the sustainable production of hydrogen (H2), a clean fuel produced by photocatalysis using metal vanadates. However, the rapid recombination of photogenerated electron-hole pairs limits their practical use as effective photocatalysts, and thus, many efforts have been devoted to optimizing metal vanadates to enhance their efficiency. Herein, we provide a comprehensive review that deals with the recent development strategies of metal (Ni, Fe, Zn, Ag, In, Bi, rare earth, etc.) vanadates with the working mechanisms. Their synthesis, doping, cocatalyst loading, heterojunction creation, and carbon loading are also reviewed for photocatalytic H2 production. The challenges that metal vanadate-based photocatalysts have been facing are also discussed along with their significant potential for environmentally friendly and sustainable clean fuel production.

Photocatalysis with Covalent Organic Frameworks

ConspectusUtilizing light to enable chemical conversions presents a green and sustainable approach to produce fuels and chemicals, and photocatalysis is one of the key chemical technologies that needs to be well developed in this century. Despite continuous progress in the advancement of various photocatalysts based on small inorganic and organic compounds, polymers, and networks, designing and constructing photocatalysts that combine activity, selectivity, and reusability remains a challenging goal. For catalytic activity, the difficulty originates from the complexity of photochemical reactions, where the light-harvesting system, multielectron and multihole-involving processes, and pinpoint mass delivery simultaneously need to be established in the system. For selectivity, the difficulty stems from the elaborate design of catalytic sites and space, especially their orbital energy levels, spatial arrangement, and environment; developing a molecular strategy that enables an overall design and control of these factors of different aspects is necessary yet arduous. For reusability, the difficulty arises from the stability and recyclability of the photocatalysts upon continuous operation under photoredox reaction conditions. How to recover photocatalysts in an energy-saving way to enable their cyclic use while retaining activity and selectivity is at the core of this problem. These bottleneck issues reflect that molecular design of a photocatalyst is not a simple summation of the above requirements, but a systematic scheme that can organically interlock various aspects is needed.To enable such an elaborate design and precise control, a basic requirement of the scaffold for constructing a promising photocatalyst is that its primary and high-order structures should be molecularly predesignable and synthetically controllable. Such a molecular regime has successfully evolved in natural photosynthesis, where light-harvesting chlorophyll antennae and photocatalytic centers are spatially well-organized and energetically well-defined to build ways for exciton migration, photoinduced electron transfer and charge separation, electron and hole flows, and oxidation of water and reduction of carbon dioxide, thereby converting water into oxygen to release ATP and NADPH via the light reaction and carbon dioxide into glucose with ATP and NADPH through the dark reaction. Similarly, a predesignable polymeric scaffold would be promising for integrating these complex photochemical processes to construct photocatalysts.Covalent organic frameworks (COFs) are a class of extended yet polymeric materials that enable the organization of organic units or metallo-organic moieties into well-defined architectures. In principle, COFs are molecularly designable with topology diagrams and synthetically controllable through polymerization reactions, offering an irreplaceable platform for designing and synthesizing photocatalysts. This feature enticed researchers to develop various photocatalysts based on COFs and drove the rapid progress in this field over the past decade. In this Account, we summarize the recent advances in the molecular design and synthetic control of COF photocatalysts, by highlighting the key achievements in developing ways to enable light harvesting, trigger photoinduced electron transfer and charge separation, allow charge carrier transport and mass delivery, control energy level, catalytic space, and environmental engineering, and develop stability and recyclability with an aim to reveal a full picture of this field. By scrutinizing typical photocatalytic reactions, we show the key problems to be addressed for COFs and predict future directions.

NiCo-MOFs in situ anchored on graphdiyne with metal-like properties form a strongly coupled electron transport interface and construct an ohmic contact to achieve efficient charge-hole spatial separation

Metals exhibit unique characteristics in photocatalysis, and their incorporation into semiconductors can result in remarkable features. This study focuses on the preparation of graphdiyne with Cu (CG) by using Cu powder as a catalyst. The addition of Cu reduces the narrow band gap of graphdiyne and imparts metal-like properties to the material. By leveraging the electronegativity of CG, a spherical NiCo-MOF (NC) is grown and in situ anchored on CG, forming a strongly coupled electron transport interface. In addition, the CG with metal-like properties also displays distinct characteristics. The integration of CG and NC through an ohmic contact significantly enhances the spatial separation of photogenerated carrier holes. Efficient hydrogen evolution is achieved through a synergistic effect of the strongly coupled electron transport interface and the spatial separation of photogenerated carrier holes. This research provides a new perspective on the design and development of metal-like narrow band gap semiconductors.

Photocatalytic Methanol Dehydrogenation Promoted Synergistically by Atomically Dispersed Pd and Clustered Pd

Supported metal in the form of single atoms, clusters, and particles can individually or jointly affect the activity of supported heterogeneous catalysts. While the individual contribution of the supported metal to the overall activity of supported photocatalysts has been identified, the joint activity of mixed metal species is overlooked because of their different photoelectric properties. Here, atomically dispersed Pd (Pd1) and Pd clusters are loaded onto CdS, serving as oxidation and reduction sites for methanol dehydrogenation. The Pd1 substitutes Cd2+, forming hole-trapping states for methanol oxidation and assisting the dispersion of photodeposited Pd clusters. Therefore, methanol dehydrogenation on CdS with supported Pd1 and Pd clusters exhibits the highest turnover frequency of 1.14 s-1 based on the Pd content and affords H2 and HCHO with a similar apparent quantum yield of 87 ± 1% at 452 nm under optimized reaction conditions. This work highlights the synergistic catalysis of supported metal for improved photocatalytic activity.

Highly Active CoNi-CoN3 Composite Sites Synergistically Accelerate Oxygen Electrode Reactions in Rechargeable Zinc-Air Batteries

Reaching rapid reaction kinetics of oxygen reduction (ORR) and oxygen evolution reactions (OER) is critical for realizing efficient rechargeable zinc-air batteries (ZABs). Herein, a novel CoNi-CoN3 composite site containing CoNi alloyed nanoparticles and CoN3 moieties is first constructed in N-doped carbon nanosheet matrix (CoNi-CoN3/C). Benefiting from the high electroactivity of CoNi-CoN3 composite sites and large surface area, CoNi-CoN3/C shows a superior half-wave potential (0.88 V versus RHE) for ORR and a small overpotential (360 mV) for OER at 10 mA cm-2. Theoretical calculations have demonstrated that the introduction of CoNi alloys has modulated the electronic distributions near the CoN3 moiety, inducing the d-band center of CoNi-CoN3 composite site to shift down, thus stabilizing the valence state of Co active sites and balancing the adsorption of OER/ORR intermediates. Accordingly, the reaction energy trends exhibit optimized overpotentials for OER/ORR, leading to superior battery performances. For aqueous and flexible quasi-solid-state rechargeable ZABs with CoNi-CoN3/C as catalyst, a large power density (250 mW cm-2) and high specific capacity (804 mAh g-1) are achieved. The in-depth understanding of the electroactivity enhancement mechanism of interactive metal nanoparticles and metal coordinated with nitrogen (MNx) moieties is crucial for designing novel high-performance metal/nitrogen-doped carbon (M─N─C) catalysts.

Piezoelectric Polarization and Sulfur Vacancy Enhanced Photocatalytic Hydrogen Evolution Performance of Bi2S3/ZnSn(OH)6 Piezo-photocatalyst

The combination of piezoelectric catalysis and photocatalysis could effectively enhance the carrier separation efficiency and further improve the hydrogen production activity. However, piezoelectric polarization always suffers from a low polarization strength, which severely restricts its actual applications. In this study, we successfully synthesized a novel sulfur vacancy-rich Bi2S3/ZnSn (OH)6 (BS-12/ZSH) piezo-photocatalyst for hydrogen evolution through water splitting. Notably, the piezo-photocatalytic hydrogen generation rate of the 8% BS-12/ZSH catalyst (336.21 μmol/g/h) was superior to that of pristine ZSH (29.71 μmol/g/h) and BS-12 (21.66 μmol/g/h). In addition, the hydrogen generation for 8% BS-12/ZSH (336.21 μmol/g/h) under ultrasonic coupling illumination was significantly higher than that under single illumination (52.09 μmol/g/h) and ultrasound (121.90 μmol/g/h), owing to the cooperative interaction of the sulfur vacancy and piezoelectric field. Various characterization analyses confirmed that (1) the introduction of sulfur vacancies in BS-12 provided more active sites, (2) BS-12 with sulfur vacancies acted as a co-catalyst to accelerate the hydrogen production rate, and (3) the piezoelectric field eliminated the electrostatic shielding and offered an additional driving force, which effectively promoted the separation of electron-hole pairs. This research clearly reveals the synergistic effect between piezocatalysis and photocatalysis as well as offers a promising sight for the rational design of high-efficiency piezo-photocatalysts.

Ba3SnGa10-xInxO20 (0 ≤ x ≤ 2): site-selective doping, band structure engineering and photocatalytic overall water splitting

Developing new photocatalysts and deciphering the structure-property relationship are always the central topics in photocatalysis. In this study, a new photocatalyst Ba3SnGa10O20 containing two d10 metal cations was prepared by a high temperature solid state reaction, and its crystal structure was investigated by Rietveld refinements of monochromatic X-ray powder diffraction data for the first time. There are 2 Ba, 4 metal cations and 6 O independent atoms in a unit cell. Sn4+ and Ga3+ co-occupy the octahedral cavities named M1 and M2 sites, and the other two metal sites are fully occupied by Ga3+. Rational In3+-to-Ga3+ substitution was performed to reduce the potential of the conduction band minimum and enhance the light absorption ability, which was indeed confirmed using UV-vis diffuse reflectance spectra and Mott-Schottky plots for Ba3SnGa10-xInxO20 (0 ≤ x ≤ 2). Interestingly, In3+ exhibits site selective doping at M1 and M2 sites exclusively. With the light absorption ability enhanced, the photocatalytic overall water splitting activity was also improved, i.e. the photocatalytic H2 generation rate was 1.7(1) μmol h-1 for Ba3SnGa10O20, and the optimal catalyst Ba3SnGa8.5In1.5O20 loaded with 1.0 wt% Pd exhibited the H2 generation rate of 27.5(4) μmol h-1 and the apparent quantum yield at 254 nm was estimated to be 2.28% in pure water.