An Insight into Carbon Nanomaterial-Based Photocatalytic Water Splitting for Green Hydrogen Production

At present, the energy shortage and environmental pollution are the burning global issues. For centuries, fossil fuels have been used to meet worldwide energy demand. However, thousands of tons of greenhouse gases are released into the atmosphere when fossil fuels are burned, contributing to global warming. Therefore, green energy must replace fossil fuels, and hydrogen is a prime choice. Photocatalytic water splitting (PWS) under solar irradiation could address energy and environmental problems. In the past decade, solar photocatalysts have been used to manufacture sustainable fuels. Scientists are working to synthesize a reliable, affordable, and light-efficient photocatalyst. Developing efficient photocatalysts for water redox reactions in suspension is a key to solar energy conversion. Semiconductor nanoparticles can be used as photocatalysts to accelerate redox reactions to generate chemical fuel or electricity. Carbon materials are substantial photocatalysts for total WS under solar irradiation due to their high activity, high stability, low cost, easy production, and structural diversity. Carbon-based materials such as graphene, graphene oxide, graphitic carbon nitride, fullerenes, carbon nanotubes, and carbon quantum dots can be used as semiconductors, photosensitizers, cocatalysts, and support materials. This review comprehensively explains how carbon-based composite materials function as photocatalytic semiconductors for hydrogen production, the water-splitting mechanism, and the chemistry of redox reactions. Also, how heteroatom doping, defects and surface functionalities, etc., can influence the efficiency of carbon photocatalysts in H2 production. The challenges faced in the PWS process and future prospects are briefly discussed.

g-C3N4/TiO2-B{100} heterostructures used as promising photocatalysts for water splitting from a hybrid density functional study

Fabrication of heterostructures has been shown to be a good strategy to improve photocatalytic performance. By using first-principles calculation based on hybrid density functionals, the photocatalytic mechanism of g-C₃N₄/TiO₂-B{100} heterostructures is investigated to understand the process of water decomposition. We find that the reduction of the band gap of g-C₃N₄/TiO₂-B{100} heterostructures enhances the visible light response range. g-C₃N₄/TiO₂-B{100} heterostructures have direct band gaps, staggered band alignment, electron flow from g-C₃N₄ to TiO₂-B{100} surfaces and straddling water decomposition potential, and are potential Z-scheme photocatalysts. Photoinduced carriers can be effectively separated using the Z-scheme photocatalytic mechanism. Our results demonstrate that g-C₃N₄/TiO₂-B{100} heterostructures can enhance light absorption, prolong the life of photoinduced carriers, and further improve the photocatalytic activity. We believe that our findings can provide a reference for explaining the enhancement mechanism of the g-C₃N₄/TiO₂ photocatalyst as observed in the experiment.

Rapid screening alloying elements for improved corrosion resistance on the Mg(0001) surface using first principles calculations

The poor corrosion resistance of Mg alloys is a major challenge for their applications. The corrosion of Mg alloys is mainly controlled by the anodic dissolution of Mg and the cathodic hydrogen evolution reaction (HER), which is closely related to the stability and the hydrogen adsorption of the Mg surface. In this work, the effects of alloying elements (As, Ge, Cd, Zn, Ga, Al, and Y) on the stability and the hydrogen adsorption of a Mg(0001) surface have been studied based on first principles calculations. We have developed a horizontally integrated approach to evaluate their effects on corrosion resistance using parameters such as the surface energy, vacancy formation energy, Bader charge, electron density distribution, and the adsorption free energy of H atom at different adsorbed sites. We found that the doped atoms could significantly change the surface atomic structure and electron transfer on the Mg surface. These behaviors modified the energy required to detach the nearest neighbors of doped atoms from the Mg surface, the adsorption free energy of H atoms, and the stable adsorption sites of H atoms on the Mg surface, which regulate the corrosion resistance of Mg alloys. Interestingly, we found that Y doping on the Mg surface increased the corrosion resistance and our new method had tremendous potential in the rapid screening of alloying elements that could improve the stability of Mg alloys and inhibit the hydrogen evolution reaction.

Mechanism for hydrogen evolution from water splitting based on a MoS2/WSe2 heterojunction photocatalyst: a first-principle study

In this study, density functional theory and hybrid functional theory are used to calculate the work function and energy band structure of MoS₂ and WSe₂, as well as the binding energy, work function, energy band structure, density of states, charge density difference, energy band alignment, Bader charge, and H adsorption free energy of MoS₂/WSe₂. The difference in work function led to the formation of a built-in electric field from WSe₂ to MoS₂, and the energy band alignment indicated that the redox reactions were located on the MoS₂ and WSe₂ semiconductors, respectively. The binding energy of MoS₂ and WSe₂ indicated that the thermodynamic properties of the heterogeneous structure were stable. MoS₂ and WSe₂ gathered electrons and holes, respectively, and redistributed them under the action of the built-in electric field. The photogenerated electrons and holes were enriched on the surface of WSe₂ and MoS₂, which greatly improved the efficiency of hydrogen production by photocatalytic water splitting.

The mechanism of heterojunction photocatalytic splitting of water for hydrogen evolution.

Investigation on photocatalytic mechanism of graphitic SiC (g-SiC)/MoS2 van der Waals heterostructured photocatalysts for overall water splitting

Two-dimensional MoS₂-based heterostructures have been given great attention due to their excellent properties.

The mechanism of hydrogen adsorption on transition metal dichalcogenides as hydrogen evolution reaction catalyst

Two-dimensional transition metal dichalcogenides (TMDs) have been widely considered as potential hydrogen evolution reaction (HER) catalysts because of their low cost and good electrochemical stability in acid conditions.

Few-layered metallic 1T-MoS2/TiO2 with exposed (001) facets: two-dimensional nanocomposites for enhanced photocatalytic activities

Titanium dioxide (TiO₂) with exposed (001) facets (TiO₂(001)) has attractive photocatalytic properties.

Non-covalent functionalization of WS2 monolayer with small fullerenes: tuning electronic properties and photoactivity

Atomically thin 2-D transition metal dichalcogenide (TMDCs) heterostructures have attracted growing interest due to their massive potential in solar energy applications due to their visible band gap and very strong light–matter interactions.

Black phosphorene/monolayer transition-metal dichalcogenides as two dimensional van der Waals heterostructures: a first-principles study

The electronic structure of black phosphorene/XT₂(X = Mo, W; T = S, Se, Te) two dimensional heterostructures is presented using the first-principles method.

In-plane interfacing effects of two-dimensional transition-metal dichalcogenide heterostructures

Two-dimensional TMD in-plane heterostructures demonstrate true type-II band alignment and the built-in electric field makes the defect states consecutive.

Electronic properties and photoactivity of monolayer MoS2/fullerene van der Waals heterostructures

van der Waals (vdW) heterostructures have attracted immense interest recently due to their unusual properties and new phenomena.

The interactions between TiO2 and graphene with surface inhomogeneity determined using density functional theory

TiO2/graphene composites have shown promise as photocatalysts, leading to improved electronic properties. We have modeled using density functional theory TiO2/graphene interfaces formed between graphene with various defects/functional groups (C vacancy, epoxide, and hydroxyl) and TiO2 clusters of various sizes. We considered clusters from 3 to 45 atoms, the latter a nanoparticle of ∼1 nm in size. Our results show that binding to pristine graphene is dominated by van der Waals forces, and that C vacancies or epoxide groups lead to much stronger binding between the graphene and TiO2. Such sites may serve to anchor TiO2 to graphene. Graphene surfaces with hydroxyls however lead to OH transfer to TiO2 and weak interactions between the graphene and the hydroxylated TiO2 cluster. Charge transfer may occur between TiO2 and graphene in various directions (graphene to TiO2 or TiO2 to graphene), depending on the state of the graphene surface, based on overlap of the density of states. Our work indicates that graphene surface defects or functional groups may have a significant effect on the stability, structure, and photoactivity of these materials.

Prediction of spin–orbital coupling effects on the electronic structure of two dimensional van der Waals heterostructures

We report a first-principles study on the electronic structure of van der Waals (vdW) heterostructures consisting of two dimensional (2D) materials.