As an alluring metal-free polymeric semiconductor material, graphite-like carbon nitride (g-C3N4; abbreviated as GCN) has triggered a new impetus in the field of photocatalysis, mainly favoured from its fascinating physicochemical and photoelectronic structural features. However, certain inherent drawbacks, involving rapid reassembly of photocarriers, low specific surface area and insufficient optical absorption, limit the wide-range applicability of GCN. Generation of 0D point defects mainly by introducing vacancies (C and/or N) into the framework of GCN has spurred extensive consideration owing to their distinctive qualities to manoeuvre substantially, the optical absorption, radiative carrier isolation, and surface photoreactions. The present review endeavours to summarise a comprehensive study on vacancy defect engineered GCN. Starting from the basic introduction of defects and C/N vacancy modulated GCN, numerous advanced strategies for the controlled designing of vacancy rich GCN have been explored and discussed. Afterwards, light was thrown on the various substantial technologies which are useful for characterising and identifying the introduction of defects in GCN. The salient significance of defect engineering in GCN has been reviewed concerning its impact on optical absorption, charge isolation and surface photoreaction ability. Typically, the achievement of defect engineered GCN has been scrutinised toward various applications like photocatalytic water splitting, CO2 conversion, N2 fixation, pollutant degradation, and H2O2 production. Finally, the review ends with conclusions and vouchsafing future challenges and opportunities on the intriguing and emerging area of vacancy defect engineered GCN photocatalysts.

C-, N-Vacancy defect engineered polymeric carbon nitride towards photocatalysis: viewpoints and challenges

Ammonia (NH3), one of the basic chemicals in most fertilizers and a promising carbon-free energy storage carrier, is typically synthesized via the Haber–Bosch process with high energy consumption a...

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Rational catalyst design for N2 reduction under ambient conditions: Strategies towards enhanced conversion efficiency

Manganese dioxide (MnO2 ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO2 single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO2 -based composites via the construction of homojunctions and MnO2 /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO2 -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO2 -based materials for comprehensive environmental applications is provided.

MnO2 -Based Materials for Environmental Applications.

Electrochemical ammonia synthesis: Mechanistic understanding and catalyst design

Developing active and robust catalysts for the electrocatalytic N2 reduction reaction (NRR) represents a promising strategy for ambient NH3 production but remains challenging. Herein, CeO2 was modulated by Fe-doping for the NRR in neutral media. Fe-doping was found to induce the morphological change of CeO2 from crystalline nanoparticles to partial-amorphous nanosheets which contained abundant oxygen vacancies (VO), resulting in more exposed active sites and accelerated electron transport. Density functional theory (DFT) calculations further revealed that the coexistence of the Fe dopant and its adjacent VO enabled the creation of Ce3+–Ce3+ pairs which served as the most active centers for effectively catalyzing the NRR and suppressing the hydrogen evolution reaction. These Fe-doping induced synergistic effects led to a significantly enhanced NRR performance of Fe-CeO2 with an NH3 yield of 26.2 μg h−1 mg−1 (−0.5 V) and a faradaic efficiency of 14.7% (−0.4 V). Therefore, this metal-doping induced multifunctionality will open up new opportunities to design powerful NRR catalysts for N2 fixation.

Fe-doping induced morphological changes, oxygen vacancies and Ce3+–Ce3+ pairs in CeO2 for promoting electrocatalytic nitrogen fixation

Exploring active and durable metal-free electrocatalysts represents a promising direction for electrocatalytic N2 reduction reaction (NRR). Herein, by filling the nitrogen vacancies (NVs) with S dopants in graphitic C3N4, we showed that the resultant S-NV-C3N4 could be an efficient and robust metal-free NRR catalyst in neutral solution. S-NV-C3N4 with a high S concentration of 5.2 at.% exhibited a fascinating NRR performance with an NH3 yield of 32.7 μg h−1 mg−1 and a Faradaic efficiency of 14.1 % at −0.4 V (vs. RHE), considerably outperforming pristine C3N4 and NV-containing C3N4. S-NV-C3N4 also showed exceptional durability for at least 20 h, in stark contrast to the inferior stability of NV-containing C3N4. Density functional theory calculations disclosed that the filled S dopants could break the scaling relation to effectively stabilize *N2H and destabilize *NH2 on S-NV-C3N4, leading to more optimized adsorption of NRR intermediates and a significantly reduced energy barrier. Meanwhile, the competitive HER can be effectively suppressed on S-NV-C3N4 due to the high energy barrier for water dissociation.

Filling the nitrogen vacancies with sulphur dopants in graphitic C3N4 for efficient and robust electrocatalytic nitrogen reduction

The development of highly efficient and durable electrocatalysts for the nitrogen reduction reaction (NRR) is of paramount significance for NH3 electrosynthesis. Herein, we report an effective B-doping strategy for the structural engineering of MnO2 toward the NRR through combined experimental and theoretical studies. Introducing B-dopants into MnO2 nanosheets was found to create abundant O-vacancies which cooperated with B-dopants to promote the conductivity and enhance the intrinsic NRR activity of MnO2. The developed B-doped MnO2 nanosheets grown on carbon cloth (B-MnO2/CC) exhibited a significantly enhanced NRR performance with an NH3 yield of 54.2 μg h−1 mg−1 (−0.4 V) and a faradaic efficiency of 16.8% (−0.2 V), and are among the best Mn-based NRR catalysts reported so far. Density functional theory calculations further revealed the synergistic role of B-dopants and O-vacancies in inducing asymmetric charge distribution, which could activate the neighboring Mn atoms to facilitate the stabilization of the key intermediate *N2H on MnO2, leading to reduced reaction energy barrier and enhanced intrinsic NRR activity.

Synergistic boron-dopants and boron-induced oxygen vacancies in MnO2 nanosheets to promote electrocatalytic nitrogen reduction

Vacancy engineering and heteroatom doping are two effective approaches to tailor the electronic structures of catalysts for improved electrocatalytic activity. Herein, these two approaches were rationally combined to modulate the structure of SnS2 toward the N2 reduction reaction (NRR) by means of Mo-doping, which simultaneously induced the generation of enriched S-vacancies (Vs). The developed Mo-doped SnS2 nanosheets with enriched Vs presented a conspicuously enhanced NRR activity with an NH3 yield of 41.3 μg h−1 mg−1 (−0.5 V) and a faradaic efficiency of 20.8% (−0.4 V) and are among the best SnS2-based NRR catalysts to date. Mechanistic studies revealed that the co-presence of the Mo dopant and Vs enabled the creation of Mo–Sn–Sn trimer catalytic sites, capable of strongly activating N2 even for the cleavage of the NN triple bond to the NN double bond at the N2 adsorption stage, consequently leading to a downhill process of the first hydrogenation step and a largely reduced energy barrier.

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Mo-doped SnS2 with enriched S-vacancies for highly efficient electrocatalytic N2 reduction: the critical role of the Mo–Sn–Sn trimer

The nitrogen reduction reaction (NRR) is a pivotal step in electrochemical N2 fixation to NH3. VS2 holds great promise as a NRR electrocatalyst, but its high activity requires the sufficient activation of inert basal planes. Herein, we demonstrate the first successful activation of VS2 basal planes toward the NRR by introducing S-vacancies (Vs) and B-dopants. The theoretical calculations unravel that the synergistic role of VS and B-dopants enables the most effective activation of VS2 basal planes by creating unique B-adjacent-unsaturated-V active sites that can significantly promote the NRR while suppressing hydrogen evolution. The synthesized B-doped VS2 nanoflowers with enriched surface Vs delivered an NH3 yield of 55.7 μg h−1 mg−1 (−0.4 V) and a faradaic efficiency (FE) of 16.4% (−0.2 V) and represent the best V-based catalysts to date. Our theoretical and experimental findings may facilitate the exploration and understanding of advanced transition-metal disulfide catalysts for the NRR.

Qingqing Li

Papers

Filling the nitrogen vacancies with sulphur dopants in graphitic C3N4 for efficient and robust electrocatalytic nitrogen reduction

Fe-doping induced morphological changes, oxygen vacancies and Ce3+–Ce3+ pairs in CeO2 for promoting electrocatalytic nitrogen fixation

Synergistic boron-dopants and boron-induced oxygen vacancies in MnO2 nanosheets to promote electrocatalytic nitrogen reduction

Mo-doped SnS2 with enriched S-vacancies for highly efficient electrocatalytic N2 reduction: the critical role of the Mo–Sn–Sn trimer

Activating VS2 basal planes for enhanced NRR electrocatalysis: the synergistic role of S-vacancies and B dopants