Polarized Cu-Bi Site Pairs for Non-Covalent to Covalent Interaction Tuning toward N2 Photoreduction

Defect Engineering in Bi-Based Photo/Electrocatalysts for Nitrogen Reduction to Ammonia

Main group Bi-based materials have gained popularity as N2 reduction reaction (NRR) photo/electrocatalysts due to their ability to inhibit competitive H2 evolution reactions (HER) and the unique N2 adsorption activities. The introduction of defects in Bi-based catalysts represents a highly effective strategy for enhancing light absorption, promoting efficient separation of photogenerated carriers, optimizing the activity of free radicals, regulating electronic structure, and improving catalytic performance. In this review, we outline the various applications of state of the defect engineering in Bi-based catalysts and elucidate the impact of vacancies on NRR performance. In particular, the types of defects, methods of defects tailoring, advanced characterization techniques, as well as the Bi-based catalysts with abundant defects and their corresponding catalytic behavior in NRR were elucidated in detail. Finally, the main challenges and opportunities for future development of defective Bi-based NRR catalysts are discussed, which provides a comprehensive theoretical guidance for this field.

Construction of Fe(III) Active Sites on Phenanthroline-Grafted g-C3N4: Reduced Work Function and Enhanced Intramolecular Charge Transfer for Efficient N2 Photofixation

Photocatalytic nitrogen fixation is one of the important pathways for green and sustainable ammonia synthesis, but the extremely high bonding energy of the N≡N triple bond makes it difficult for conventional nitrogen fixation photocatalysts to directly activate and hydrogenate. Given this, we covalently grafted the phenanthroline unit onto graphitic carbon nitride nanosheets (CN) by the simple thermal oxidation method and complexed it with transition metal Fe3+ ions to obtain stable dispersed Fe active sites, which can significantly improve the photocatalytic activity. The Fe(III)-4-P-CN photocatalyst morphology consists of porous lamellar structures internally connected by nanowires. The special morphology of the catalysts gives them excellent nitrogen fixation performance, with an average NH3 yield of 492.9 μmol g-1 h-1, which is 6.5 times higher than that of the pristine CN, as well as better photocatalytic cycling stability. Comprehensive experiments and density-functional theory results show that Fe(III)-4-P-CN is more favorable than pristine CN for *N2 activation, effectively lowering the reaction energy barrier. Moreover, other byproducts (such as nitrate and H2O2) are also produced during the photocatalytic nitrogen fixation process, which also provides a new way for nitrogen-fixing photocatalysts to achieve multifunctional applications.

Electron deficient Bi3+δ serves as N2 absorption sites and inhibits carriers recombination to enhance N2 photo-fixation in BiOBr/TiO2 S-scheme heterojunction

Efficient carriers separation and multiple nitrogen (N2) activation sites are essential for N2 photo-fixation. Here, we found that the BiOBr/TiO2 (BBTO) displayed an attractive reversible photochromism (white → grey) due to the generation of electron deficient Bi3+δ, which was produced by the hole trapping of Bi3+ under light irradiation. Interestingly, more Bi3+δ were detected in the BBTO heterojunction than in pure BiOBr, attributing that the hole trapping was promoted by the built-in electric field in the Step scheme (S-scheme) heterojunction. In the BBTO, the electron deficient Bi3+δ enhanced carriers separation and served as the reactive active site to adsorb more N2. Consequently, the BBTO possessed an excellent N2 photo-fixation activity (191 μmol gcat-1 h-1), which was 7.7 and 18 times higher than that of pure BiOBr (24.8 μmol gcat-1 h-1) and TiO2 (10.6 μmol gcat-1 h-1), respectively. Therefore, this work provides a new perspective for enhancing N2 photo-fixation by the electron deficient photocatalysts with S-scheme heterojunction.

Asymmetric Electron Redistribution in Niobic-Oxygen Vacancy Associates to Tune Noncovalent Interaction in CO2 Photoreduction

The role of vacancy associates in photocatalytic CO2 reduction is an open question. Herein, the Nb─O vacancy associates (VNb─O) are engineered into niobic acid (NA) atomic layers to tailor the CO2 photoreduction performance. The intrinsic charge compensation from O to Nb around Nb─O vacancy associates can manipulate the active electronic states, leading to the asymmetric electron redistribution. These local symmetry breaking sites show a charge density gradient, forming a localized polarization field to polarize nonpolar CO2 molecules and tune the noncovalent interaction of reaction intermediates. This unique configuration contributes to the 9.3 times increased activity for photocatalytic CO2 reduction. Meantime, this VNb─O NA also shows excellent photocatalytic activity for NO3 --NH4 + synthesis, with NH4 + formation rate up to 3442 µmol g-1 h-1. This work supplies fresh insights into the vacancy associate design for electron redistribution and noncovalent interaction tuning in photocatalysis.

Regulated Dual Defects of Bridging Organic and Terminal Inorganic Ligands in Iron-based Metal-Organic Framework Nodes for Efficient Photocatalytic Ammonia Synthesis

Engineering advantageous defects to construct well-defined active sites in catalysts is promising but challenging to achieve efficient photocatalytic NH3 synthesis from N2 and H2O due to the chemical inertness of N2 molecule. Here, we report defective Fe-based metal-organic framework (MOF) photocatalysts via a non-thermal plasma-assisted synthesis strategy, where their NH3 production capability is synergistically regulated by two types of defects, namely, bridging organic ligands and terminal inorganic ligands (OH- and H2O). Specially, the optimized MIL-100(Fe) catalysts, where there are only terminal inorganic ligand defects and coexistence of dual defects, exhibit the respective 1.7- and 7.7-fold activity enhancement comparable to the pristine catalyst under visible light irradiation. As revealed by experimental and theoretical calculation results, the dual defects in the catalyst induce the formation of abundant and highly accessible coordinatively unsaturated Fe active sites and synergistically optimize their geometric and electronic structures, which favors the injection of more d-orbital electrons in Fe sites into the N2 π* antibonding orbital to achieve N2 activation and the formation of a key intermediate *NNH in the reaction. This work provides a guidance on the rational design and accurate construction of porous catalysts with precise defective structures for high-performance activation of catalytic molecules.

Modulating band structure through introducing Cu0/CuxO composites for the improved visible light driven ammonia synthesis

The photocatalytic performance of ceria-based materials can be tuned by adjusting the surface structures with decorating the transition-metal, which are considered as the important active sites. Herein, cuprous oxide-metallic copper composite-doped ceria nanorods were assembled through a simple hydrothermal reduction method. The photocatalytic ammonia synthesis rates exhibit an inverted "V-shaped" trend with increasing Cu0/CuxO mole ratio. The best ammonia production rate, approximately 900 or 521 µmol·gcal-1·h-1 under full-spectra or visible light, can be achieved when the Cu0/CuxO ratio is approximately 0.16, and this value is 8 times greater than that of the original sample. The absorption edge of the as-prepared samples shifted towards visible wavelengths, and they also had appropriate ammonia synthesis levels. This research provides a strategy for designing noble metal-free photocatalysts through introducing the metal/metallic oxide compositesto the catalysts.

Fe-Bi dual sites regulation of Bi2O2.33 nanosheets to promote photocatalytic nitrogen fixation activity

In the process of photocatalytic ammonia synthesis, efficient activation of nitrogen molecules constitutes a fundamental challenge. During the N2 activation, the close interdependence between the acceptance and donation of electron results in their mutual limitation, leading to high energy barrier for N2 activation and unsatisfactory photocatalytic performance. This work decoupled the electron acceptance and donation processes by constructing Fe-Bi dual active sites, resulting in enhancing N2 activation through the high electron trapping ability of Fe3+ and strong electron donating ability of Bi2+. The photocatalytic nitrogen reduction efficiency of 3%Fe/Bi2O2.33 (118.71 μmol gcat-1h-1) is 5.3 times that of Bi2O2.33 (22.41 μmol gcat-1h-1). In-situ Fourier transform infrared (In situ FTIR) spectroscopy and density functional theory (DFT) calculations manifest that Fe3+-Bi2+ dual active sites work together to promote nitrogen adsorption and activation, and the reaction path is more inclined toward alternate hydrogenation path. N2 adsorption and activation properties are optimized by heteronuclear bimetallic active sites, which offers a new way for the rational design of nitrogen-fixing photocatalysts.

Noble-Metal-Free Single- and Dual-Atom Catalysts for Artificial Photosynthesis

Artificial photosynthesis enables direct solar-to-chemical energy conversion aimed at mitigating environmental pollution and producing solar fuels and chemicals in a green and sustainable approach, and efficient, robust, and low-cost photocatalysts are the heart of artificial photosynthesis systems. As an emerging new class of cocatalytic materials, single-atom catalysts (SACs) and dual-atom catalysts (DACs) have received a great deal of current attention due to their maximal atom utilization and unique photocatalytic properties, whereas noble-metal-free ones impart abundance, availability, and cost-effectiveness allowing for scalable implementation. This review outlines the fundamental principles and synthetic methods of SACs and DACs and summarizes the most recent advances in SACs (Co, Fe, Cu, Ni, Bi, Al, Sn, Er, La, Ba, etc.) and DACs (CuNi, FeCo, InCu, KNa, CoCo, CuCu, etc.) based on non-noble metals, confined on an arsenal of organic or inorganic substrates (polymeric carbon nitride, metal oxides, metal sulfides, metal-organic frameworks, carbon, etc.) acting as versatile scaffolds in solar-light-driven photocatalytic reactions, including hydrogen evolution, carbon dioxide reduction, methane conversion, organic synthesis, nitrogen fixation, hydrogen peroxide production, and environmental remediation. The review concludes with the challenges, opportunities, and future prospects of noble-metal-free SACs and DACs for artificial photosynthesis.

Double metals sites synergistically enhanced photocatalytic N2 fixation performance over Bi24O31Br10@Bi/Ti3C2Tx Ohm junctions

At present, it is a research hotspot to realize green synthetic ammonia by using solar energy. Exploring cheap and efficient co-catalysts for enhancing the performance of photocatalysts is a challenge in the field of energy conversion. In order to boost the charge separation/transfer of the photocatalyst and widen the visible light absorption, Bi24O31Br10@Bi/Ti3C2Tx with double Ohm junction is successfully fabricated by in situ growth of metal Bi and loading Ti3C2Tx MXene on the surface of Bi24O31Br10. The dual active sites of Bi and Ti3C2Tx MXene not only broaden the light adsorption of Bi24O31Br10 but also serve as excellent 'electronic receptor' for synergically enhancing the separation/transfer efficiency of photogenerated electrons/holes. Meanwhile, temperature programmed desorption (TPD) result revealed that MXene and Bi can promote N2 adsorption/activation and NH3 desorption over Bi24O31Br10@Bi/Ti3C2Tx. As a result, under mild conditions and without the presence of hole scavenger, the ammonia synthesis efficiency of Bi24O31Br10@Bi/Ti3C2Tx-20 % reached 53.86 μmol g-1cat for three hours which is 3.2 and 53.8 times of Bi24O31Br10 and Ti3C2Tx, respectively. This study offers a novel scheme for the construction of photocatalytic systems and demonstrates Ti3C2Tx MXene and metal Bi as a promising and cheap co-catalyst.

Adjacent Mn site boosts photocatalytic hydrogen evolution of MnXCd1-XS solid solution through a dual-metal-site design

CdS has emerged as a possible candidate for photocatalytic hydrogen generation. However, further improvement in the performance of the Cd metal site is challenging due to limited optimization space. To solve this limitation, in this work, the Mn-Cd dual-metal photocatalyst was synthesized by a one-step solvothermal method, and the effects of different proportions of bimetals on hydrogen production activity were systematically studied. The ingenious design of the bimetallic sites enhances the carrier separation efficiency and the built-in electric field intensity, which leads to significant improvement in the photocatalytic hydrogen production performance of MCS0.19. Density functional theory (DFT) calculations confirm that the introduction of the Mn element can drive electrons through the Fermi level, resulting in enhanced conductivity of the catalyst. Meanwhile, electron channels are built between Mn and S, which speeds up the rate of electron transfer and is conducive to improving hydrogen production activity. This work provides a technical-methodological entrance to improve the photocatalytic hydrogen production performance of dual-metal S solid solutions and also promises to open a novel approach to creating high-efficiency solid solution photocatalysts.

Free-Electron Inversive Modulation to Charge Antibonding Orbital of ReS2 Cocatalyst for Efficient Photocatalytic Hydrogen Generation

The free electron transfer between cocatalyst and photocatalyst has a great effect on the bonding strength between the active site and adsorbed hydrogen atom (Hads ), but there is still a lack of effective means to purposely manipulate the electron transfer in a beneficial direction of H adsorption/desorption activity. Herein, when ReSx cocatalyst is loaded on TiO2 surface, a spontaneous free-electron transfer from ReSx to TiO2 happens due to the smaller work function of ReSx , causing an over-strong S-Hads bond. To prevent the over-strong S-Hads bonds of ReSx in the ReSx /TiO2 , a free-electron reversal transfer strategy is developed to weaken the strong S-Hads bonds via increasing the work function of ReSx by incorporating O to produce ReOSx cocatalyst. Research results attest that a larger work function of ReOSx than that of TiO2 can induce reversal transfer of electrons from TiO2 to ReOSx to produce electron-rich S(2+δ)- , causing the increased antibonding-orbital occupancy of S-Hads in ReOSx /TiO2 . Accordingly, the stability of adsorbed H on S sites is availably decreased, thus weakening the S-Hads of ReOSx . In this case, an electron-rich S(2+δ)- -mediated "capture-hybridization-conversion" mechanism is raised . Benefiting from such property, the resultant ReOSx /TiO2 photocatalyst exhibits a superior H2 -evolution rate of 7168 µmol h-1 g-1 .

Electronic and energy level structural engineering of graphitic carbon nitride nanotubes with B and S co-doping for photocatalytic hydrogen evolution

The ideal photocatalyst used for photocatalytic water splitting requires strong light absorption, fast charge separation/transfer ability and abundant active sites. Heteroatom doping offers a promising and rational approach to optimize the photocatalytic activity. However, achieving high photocatalytic performance remains challenging if just relying on single-element doping. Herein, Boron (B) and sulfur (S) dopants are simultaneously introduced into graphitic carbon nitride (g-C₃N₄) nanotubes by supramolecular self-assembly strategy. The developed B and S co-doped g-C₃N₄ nanotubes (B,S-TCN) exhibited an outstanding photocatalytic performance in the conversion of H₂O into H₂ (9.321 mmol g^-1h^-1), and the corresponding external quantum efficiency (EQE) reached 5.3% under the irradiation of λ = 420 nm. It is well evidenced by the closely combined experimental and (density functional theory) DFT calculations: (1) the introduction of B dopants can facilitate H₂O adsorption and drive interatomic electron transfer, leading to efficient water splitting reaction. (2) S dopants can stretch the VB position to promote the oxidation ability of g-C₃N₄, which can accelerate the consumption of holes and thus inhibit the recombination with electrons. (3) the simultaneous introduction of B and S can engineer the electronic and energy level structural of g-C₃N₄ for optimizing interior charge transfer. Finally, the purpose of maximizing photocatalytic performance is achieved.

Effects of Cascading Optical Processes: Part I: Impacts on Quantification of Sample Scattering Extinction, Intensity, and Depolarization

Light scattering is a universal matter property that is especially prominent in nanoscale or larger materials. However, the effects of scattering-based cascading optical processes on experimental quantification of sample absorption, scattering, and emission intensities, as well as scattering and emission depolarization, have not been adequately addressed. Using a series of polystyrene nanoparticles (PSNPs) of different sizes as model analytes, we present a computational and experimental study on the effects of cascading light scattering on experimental quantification of NP scattering activities (scattering cross-section or molar coefficient), intensity, and depolarization. Part II and Part III of this series of companion articles explore the effects of cascading optical processes on sample absorption and fluorescence measurements, respectively. A general theoretical model is developed on how forward scattered light complicates the general applicability of Beer's law to the experimental UV-vis spectrum of scattering samples. The correlation between the scattering intensity and PSNP concentration is highly complicated with no robust linearity even when the scatterers' concentration is very low. Such complexity arises from the combination of concentration-dependence of light scattering depolarization and the scattering inner filter effects (IFEs). Scattering depolarization increases with the PSNP scattering extinction (thereby, its concentration) but can never reach unity (isotropic) due to the polarization dependence of the scattering IFE. The insights from this study are important for understanding the strengths and limitations of various scattering-based techniques for material characterization including nanoparticle quantification. They are also foundational for quantitative mechanistic understanding on the effects of light scattering on sample absorption and fluorescence measurements.