Photocatalytic water splitting is considered one of the most important and appealing approaches for the production of green H2 to address the global energy demand. The utmost possible form of artificial photosynthesis is a two-step photoexcitation known as "Z-scheme", which mimics the natural photosystem. This process solely relies on the effective coupling and suitable band positions of semiconductors (SCs) and redox mediators for the purpose to catalyze the surface chemical reactions and significantly deter the backward reaction. In recent years, the Z-scheme strategies and their key role have been studied progressively through experimental approaches. In addition, theoretical studies based on density functional theory have provided detailed insight into the mechanistic aspects of some breathtakingly complex problems associated with hydrogen evolution reaction and oxygen evolution reaction. In this context, this critical review gives an overview of the fundamentals of Z-scheme photocatalysis, including both theoretical and experimental advancements in the field of photocatalytic water splitting, and suggests future perspectives.

Photocatalytic Z-Scheme Overall Water Splitting: Recent Advances in Theory and Experiments.

Ammonia, one of the most important chemicals and carbon-free energy carriers, is mainly produced by the traditional Haber-Bosch process operated at high pressure and temperature, which results in massive energy consumption and CO2 emissions. Alternatively, the electrocatalytic nitrogen reduction reaction to synthesize NH3 under ambient conditions using renewable energy has recently attracted significant attention. However, the competing hydrogen evolution reaction (HER) significantly reduces the faradaic efficiency and NH3 production rate. The design of high-performance electrocatalysts with the suppression of the HER for N2 reduction to NH3 under ambient conditions is a crucial consideration for the development of electrocatalytic NH3 synthesis with high FE and NH3 production rate. Five kinds of recently developed electrocatalysts classified by their chemical compositions are summarized, with particular emphasis on the relationship between their optimal electrocatalytic conditions and NH3 production performance. Conclusions and perspectives are provided for the future design of high-performance electrocatalysts for electrocatalytic NH3 production. The Review can give practical guidance for the design of effective electrocatalysts with high FE and NH3 production rates.

Current Progress of Electrocatalysts for Ammonia Synthesis Through Electrochemical Nitrogen Reduction Under Ambient Conditions.

Electro-reduction of N2 on nanostructured materials and the design strategies of advanced catalysts based on descriptors

Ammonia (NH3) is a crucial chemical as a fertilizer and energy carrier, which is mainly produced by the traditional Haber–Bosch process that consumes hydrogen and leads to the emission of carbon dioxide The electrochemical nitrogen reduction reaction (ENRR) is considered as an alternative method with low energy consumption and mild condition However, the NH3 yield and faradaic efficiency (FE) of the electrocatalyst are dominated by the competition between the hydrogen evolution reaction (HER) and NRR In order to achieve both high catalytic activity and high selectivity, the poor-performance catalyst needs to be improved via modification engineering For the catalysts of oxygenated compounds, introducing oxygen vacancies (OVs) into the structure can enhance the catalytic property of NRR via regulating the local electronegativity and coordination environment In this review, recent advances in the detection, preparation and application of OVs for NRR are summarized We focus on the generating strategies of OVs and their evaluation method, including vacuum annealing, high-temperature hydrogen reduction, wet chemical, plasma treatment, and ion-doping strategy Finally, the opportunities and challenges towards the suitable design of OVs in oxides for NRR are highlighted

Oxygen vacancies for promoting the electrochemical nitrogen reduction reaction

Research progress on g–C3N4–based photocatalysts for organic pollutants degradation in wastewater: From exciton and carrier perspectives

The origin of enhanced photocatalytic activity in g-C3N4/TiO2 heterostructure revealed by DFT calculations.

Computational insights into nitrogen reduction reaction catalyzed by transition metal doped graphene: Comparative investigations

Effects of oxygen defects on electronic band structures and dopant migration in Sn-doped TiO2 by density functional studies

The group ⅥB elements have good compatibility with carbon to form two-dimensional carbides. In this work, two kinds of two-dimensional metal carbides (2D-MC6, M = Ti and Hf) were theoretically proposed. Using first-principles calculations, we optimized their structures and checked their dynamical/thermal stabilities. Furthermore, their electronic, mechanical and optical properties were also computationally investigated. Calculations on electronic properties show that the 2D-TiC6 and 2D-HfC6 exhibit direct band gap of 0.82 eV and indirect band gap of 1.20 eV, respectively. Calculations on mechanical properties indicate that 2D-TiC6 and 2D-HfC6 have good elasticity. By applying biaxial strain or external electric field, the electronic band gap of 2D-TiC6 and 2D-HfC6 can be effectively tuned. Due to the moderate direct band gap, the 2D-TiC6 exhibits smaller exciton binding energy (0.58 eV) and larger absorbed photon flux (0.66 mA cm−2) than those of 2D-HfC6. Particularly, the 2D-TiC6 shows potential for short wave infrared detection or emission applications when certain tensile strain is applied. This study provides new insight into the rational design of functional nanomaterials.

Juan Xie

Papers

The origin of enhanced photocatalytic activity in g-C3N4/TiO2 heterostructure revealed by DFT calculations.

Computational insights into nitrogen reduction reaction catalyzed by transition metal doped graphene: Comparative investigations

Effects of oxygen defects on electronic band structures and dopant migration in Sn-doped TiO2 by density functional studies

Structures and optoelectronic properties of two-dimensional MC6 (M = Ti and Hf) predicted by computational approaches