Ultrathin In-Plane Heterostructures for Efficient CO2 Chemical Fixation.

Chemical fixation of carbon dioxide (CO2 ) into value-added organics is regarded as a competitive and viable method in large scale industrial production, during which the catalysts with promoting CO2 activation ability are needed. Herein, we proposed an in-plane heterostructure strategy to construct Lewis acid-base sites for efficient CO2 activation. By taking ultrathin in-plane Cu2 O/Cu heterostructures as a prototype, we show that Lewis acid-base sites on heterointerface can facilitate a mixed C and O dual coordination on surface, which not only strengthen CO2 adsorption, but also effectively activate the inert molecules. As revealed by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and quasi in situ X-ray photoelectron spectroscopy (XPS), Lewis acid-base sites could readily activate CO2 to . CO2 - species, which is the key intermediate radical for CO2 fixation. As a result, abundant Lewis acid-base sites endow Cu2 O/Cu nanosheets with excellent performances for dimethyl carbonate generation, a high conversion yield of 28 % with nearly 100 % selectivity under mild conditions. This study provides a model structure for CO2 fixation reactions.

Optically Switchable Photocatalysis in Ultrathin Black Phosphorus Nanosheets.

Recently low-dimensional materials hold great potential in the field of photocatalysis, whereas the concomitantly promoted many-body effects have long been ignored. Such Coulomb interaction-mediated effects would lead to some intriguing, nontrivial band structures, thus promising versatile photocatalytic performances and optimized strategies. Here, we demonstrate that ultrathin black phosphorus (BP) nanosheets exhibit an exotic, excitation-energy-dependent, optical switching effect in photocatalytic reactive oxygen species (ROS) generation. It is, for the first time, observed that singlet oxygen (1O2) and hydroxyl radical (•OH) are the dominant ROS products under visible- and ultraviolet-light excitations, respectively. Such an effect can be understood as a result of subband structure, where energy-transfer and charge-transfer processes are feasible under excitations in the first and second subband systems, respectively. This work not only establishes an in-depth understanding on the influence of many-body effects on photocatalysis but also paves the way for optimizing catalytic performances via controllable photoexcitation.

Oxygen-Vacancy-Mediated Exciton Dissociation in BiOBr for Boosting Charge-Carrier-Involved Molecular Oxygen Activation.

Excitonic effects mediated by Coulomb interactions between photogenerated electrons and holes play crucial roles in photoinduced processes of semiconductors. In terms of photocatalysis, however, efforts have seldom been devoted to the relevant aspects. For the catalysts with giant excitonic effects, the coexisting, competitive exciton generation serves as a key obstacle to the yield of free charge carriers, and hence, transformation of excitons into free carriers would be beneficial for optimizing the charge-carrier-involved photocatalytic processes. Herein, by taking bismuth oxybromide (BiOBr) as a prototypical model system, we demonstrate that excitons can be effectively dissociated into charge carriers with the incorporation of oxygen vacancy, leading to excellent performances in charge-carrier-involved photocatalytic reactions such as superoxide generation and selective organic syntheses under visible-light illumination. This work not only establishes an in-depth understanding of defective structures in photocatalysts but also paves the way for excitonic regulation via defect engineering.

Boosting Hot-Electron Generation: Exciton Dissociation at the Order-Disorder Interfaces in Polymeric Photocatalysts.

Excitonic effects, arising from the Coulomb interactions between photogenerated electrons and holes, dominate the optical excitation properties of semiconductors, whereas their influences on photocatalytic processes have seldom been discussed. In view of the competitive generation of excitons and hot carriers, exciton dissociation is proposed as an alternative strategy for hot-carrier harvesting in photocatalysts. Herein, by taking heptazine-based melon as an example, we verified that enhanced hot-carrier generation could be obtained in semicrystalline polymeric photocatalysts, which is ascribed to the accelerated exciton dissociation at the abundant order-disorder interfaces. Moreover, driven by the accompanying electron injection toward ordered chains and hole blocking in disordered chains, semicrystalline heptazine-based melon showed an ∼7-fold promotion in electron concentration with respect to its pristine counterpart. Benefiting from these, the semicrystalline sample exhibited dramatic enhancements in electron-involved photocatalytic processes, such as superoxide radical production and selective alcohol oxidation. This work brightens excitonic aspects for the design of advanced photocatalysts.

Giant Electron-Hole Interactions in Confined Layered Structures for Molecular Oxygen Activation.

Numerous efforts have been devoted to understanding the excitation processes of photocatalysts, whereas the potential Coulomb interactions between photogenerated electrons and holes have been long ignored. Once these interactions are considered, excitonic effects will arise that undoubtedly influence the sunlight-driven catalytic processes. Herein, by taking bismuth oxyhalide as examples, we proposed that giant electron-hole interactions would be expected in confined layered structures, and excitons would be the dominating photoexcited species. Photocatalytic molecular oxygen activation tests were performed as a proof of concept, where singlet oxygen generation via energy transfer process was brightened. Further experiments verify that structural confinement is curial to the giant excitonic effects, where the involved catalytic process could be readily regulated via facet-engineering, thus enabling diverse reactive oxygen species generation. This study not only provides an excitonic prospective on photocatalytic processes, but also paves a new approach for pursuing systems with giant electron-hole interactions.

Ultrathin Black Phosphorus Nanosheets for Efficient Singlet Oxygen Generation.

Benefiting from its strong oxidizing properties, the singlet oxygen has garnered serious attentions in physical, chemical, as well as biological studies. However, the photosensitizers for the generation of singlet oxygen bear in low quantum yields, lack of long wavelength absorption band, poor biocompatibility, undegradable in living tissues, and so on. Here we first demonstrate the exfoliated black phosphorus nanosheets to be effective photosensitizers for the generation of singlet oxygen with a high quantum yield of about 0.91, rendering their attractive applications in catalysis and photodynamic therapy. Through in vitro and in vivo studies, the water dispersible black phosphorus nanosheets show notable cancer therapy ability. In addition, the photodegradable character of black phosphorus from element to biocompatible phosphorus oxides further highlights its therapeutic potential against cancer. This study will not only expand the breadth of study in black phosphorus but also offer an efficient catalyst and photodynamic therapy agent.

Efficient Exciton Dissociation in Heterojunction Interfaces Realizing Enhanced Photoresponsive Performance.

Excitonic effects, originating from the interactions between charge carriers, influence and even dominate the photoresponsive properties of low-dimensional materials. For efficient carrier-related photoresponse, it is imperative to develop appropriate strategies to promote exciton dissociation in these systems. Herein, by taking black phosphorus nanosheets/poly(3-hexylthiophene) (BP/P3HT) as a prototype, we propose that the construction of a heterojunction with a certain band alignment and transport property can facilitate exciton dissociation into free carriers. Analyses on band structures and carrier kinetics confirmed the directional injection of holes from BP to P3HT and the excellent transport property associated with the injected holes in P3HT. Benefiting from these features, the BP/P3HT heterojunction yielded a high photocurrent on-off ratio of ∼18.3, contrasting with the much lower values in pristine BP nanosheets and P3HT. This work provides a feasible scenario for exciton regulation via constructing a heterojunction and establishes an in-depth understanding of exciton dissociation in photoresponsive properties.

Oxygen vacancy associated single-electron transfer for photofixation of CO2 to long-chain chemicals.

The photofixation and utilization of CO2 via single-electron mechanism is considered to be a clean and green way to produce high-value-added commodity chemicals with long carbon chains. However, this topic has not been fully explored for the highly negative reduction potential in the formation of reactive carbonate radical. Herein, by taking Bi2O3 nanosheets as a model system, we illustrate that oxygen vacancies confined in atomic layers can lower the adsorption energy of CO2 on the reactive sites, and thus activate CO2 by single-electron transfer in mild conditions. As demonstrated, Bi2O3 nanosheets with rich oxygen vacancies show enhanced generation of •CO2- species during the reaction process and achieve a high conversion yield of dimethyl carbonate (DMC) with nearly 100% selectivity in the presence of methanol. This study establishes a practical way for the photofixation of CO2 to long-chain chemicals via defect engineering.

Highly Active Fe Sites in Ultrathin Pyrrhotite Fe7S8 Nanosheets Realizing Efficient Electrocatalytic Oxygen Evolution.

Identification of active sites in an electrocatalyst is essential for understanding of the mechanism of electrocatalytic water splitting. To be one of the most active oxygen evolution reaction catalysts in alkaline media, Ni-Fe based compounds have attracted tremendous attention, while the role of Ni and Fe sites played has still come under debate. Herein, by taking the pyrrhotite Fe7S8 nanosheets with mixed-valence states and metallic conductivity for examples, we illustrate that Fe could be a highly active site for electrocatalytic oxygen evolution. It is shown that the delocalized electrons in the ultrathin Fe7S8 nanosheets could facilitate electron transfer processes of the system, where d orbitals of FeII and FeIII would be overlapped with each other during the catalytic reactions, rendering the ultrathin Fe7S8 nanosheets to be the most efficient Fe-based electrocatalyst for water oxidation. As expected, the ultrathin Fe7S8 nanosheets exhibit promising electrocatalytic oxygen evolution activities, with a low overpotential of 0.27 V and a large current density of 300 mA cm-2 at 0.5 V. This work provides solid evidence that Fe could be an efficient active site for electrocatalytic water splitting.

Insights into the excitonic processes in polymeric photocatalysts.

Understanding the photoexcitation processes in semiconductors is critical for the design of advanced photocatalytic materials. Nevertheless, traditional viewpoints focus on photogenerated free charge carriers, which are somehow invalid once the many-body effects are taken into account, especially for polymeric photocatalysts. Here we systematically investigate the photoexcitation processes involved in the polymer matrix of graphitic carbon nitride (g-C3N4) by combining photoluminescence spectroscopy and ultrafast transient absorption spectroscopy, validating the strong excitonic effects in the well-known photocatalyst for the first time. The identification of the robust triplet-triplet annihilation process, in which two triplet excitons collide to produce a singlet exciton, highlights an important nonradiative depopulation pathway of excited species and thereby offers potential strategies to regulate the photocatalytic activities of polymeric g-C3N4. The work establishes a new understanding of the photocatalytic mechanism in the polymeric g-C3N4 matrix, and thus paves the way for designing effective polymeric photocatalysts through excitonic engineering.

Delocalized Spin States in 2D Atomic Layers Realizing Enhanced Electrocatalytic Oxygen Evolution.

The electrocatalytic activity of transition-metal-based compounds is strongly related to the spin states of metal atoms. However, the ways for regulation of spin states of catalysts are still limited, and the underlying relationship between the spin states and catalytic activities remains unclear. Herein, for the first time, by taking NiII -based compounds without high or low spin states for example, it is shown that their spin states can be delocalized after introducing structural distortion to the atomic layers. The delocalized spin states for Ni atoms can provide not only high electrical conductivity but also low adsorption energy between the active sites and reaction intermediates for the system. As expected, the ultrathin nanosheets of nickel-chalcogenides with structural distortions show dramatically enhanced activity in electrocatalytic oxygen evolution compared to their corresponding bulk samples. This work establishes new way for the design of advanced electrocatalysts in transition-metal-based compounds via regulation of spin states.

Enhanced Singlet Oxygen Generation in Oxidized Graphitic Carbon Nitride for Organic Synthesis.

Experimental data reveal that the incorporation of carbonyl groups into polymer matrix can significantly enhance singlet oxygen ((1) O2 ) generation and suppress production of other reactive oxygen species. Excitonic processes investigated by phosphorescence spectroscopy reveal enhanced triplet-exciton generation in the modified g-C3 N4 , which facilitate (1) O2 generation through an energy transfer process. Benefiting from this, the modified g-C3 N4 shows excellent conversion and selectivity in organic synthesis.