Hydrogen is readily obtained from renewable and non‐renewable resources via water splitting by using thermal, electrical, photonic and biochemical energy. The major hydrogen production is generated ...from thermal energy through steam reforming/gasification of fossil fuel. As the commonly used non‐renewable resources will be depleted in the long run, there is great demand to utilize renewable energy resources for hydrogen production. Most of the renewable resources may be used to produce electricity for driving water splitting while challenges remain to improve cost‐effectiveness. As the most abundant energy resource, the direct conversion of solar energy to hydrogen is considered the most sustainable energy production method without causing pollutions to the environment. In overall, this review briefly summarizes thermolytic, electrolytic, photolytic and biolytic water splitting. It highlights photonic and electrical driven water splitting together with photovoltaic‐integrated solar‐driven water electrolysis.
Energy‐driven hydrogen production via water splitting with thermal, electrical, photonic and biochemical energy and their combined forms such as thermoelectrolysis, biophotolysis, and photoelectrolysis are summarized in this review. There are focuses on recent advances in water splitting with the use of renewable energy for photocatalytic and electrocatalytic hydrogen production such as photovoltaic‐integrated solar driven water electrolysis.
Graphene, an allotrope of carbon, is an intriguing material because it has potentially influential properties including high electrical conductivity and zero band gap on water splitting. ...Nanostructured hematite (Fe2O3) has been reported as a benchmark catalyst for investigating solar water splitting. In this study, we aim to understand the role of graphene as an underlayer material of hematite using density functional theory+U methodology. To understand the effect of graphene substrate on hematite's catalytic efficiency, we consider pristine graphene as well as graphene with defects (carbon vacancies in different positions and concentrations). Overpotential is found to be reduced for graphene supported hematite as compared to stand-alone hematite. Charge density difference analysis confirms more charge delocalization when carbon vacancies is present in the system. This observation is supported by smaller magnetization values and net Bader charge of the active site. Furthermore, graphene plays an important role in reducing the band gap significantly which is beneficial for catalytic efficiency. In particular, hematite supported by graphene having vacancies had nearly zero band gap which is expected to help charge carrier to be transported to the surface. We have calculated the cumulative probability of a charge to reach hematite's surface using a wave propagation simulator. Graphene supported hematite has higher cumulative probability of charge transfer than bare hematite. Graphene supported hematite having carbon vacancies in graphene shows higher cumulative probability than its pristine counterpart. These indicatives for improved carrier transport and catalysis are beneficial for water splitting. These observations also support the previously reported experimental electron impedance spectroscopy (EIS) results where graphene overlayered hematite has reported to have lower band gap, higher photocurrent density and promotable solar-driven water oxidation reaction.
Display omitted
•Graphene underlayer helps to reduce the overpotential of nanostructured hematite.•Graphene plays a key role to reduce band gap significantly benefitting catalysis.•Vacancies in graphene lead to more charge delocalization.•Carbon vacancies increase the probability of a charge to reach hematite's surface.
Conventional photodynamic therapy (PDT) has limited applications in clinical cancer therapy due to the insufficient O2 supply, inefficient reactive oxygen species (ROS) generation, and low ...penetration depth of light. In this work, a multifunctional nanoplatform, upconversion nanoparticles (UCNPs)@TiO2@MnO2 core/shell/sheet nanocomposites (UTMs), is designed and constructed to overcome these drawbacks by generating O2 in situ, amplifying the content of singlet oxygen (1O2) and hydroxyl radical (•OH) via water‐splitting, and utilizing 980 nm near‐infrared (NIR) light to increase penetration depth. Once UTMs are accumulated at tumor site, intracellular H2O2 is catalyzed by MnO2 nanosheets to generate O2 for improving oxygen‐dependent PDT. Simultaneously, with the decomposition of MnO2 nanosheets and 980 nm NIR irradiation, UCNPs can efficiently convert NIR to ultraviolet light to activate TiO2 and generate toxic ROS for deep tumor therapy. In addition, UCNPs and decomposed Mn2+ can be used for further upconversion luminescence and magnetic resonance imaging in tumor site. Both in vitro and in vivo experiments demonstrate that this nanoplatform can significantly improve PDT efficiency with tumor imaging capability, which will find great potential in the fight against tumor.
Enhanced and amplified photodynamic therapy: A multifunctional nanoplatform, UCNPs@TiO2@MnO2 core/shell/sheet nanocomposites, is designed to overcome the drawbacks of photodynamic therapy by generating O2 in situ, amplifying the content of singlet oxygen (1O2) and hydroxyl radical (•OH) via water‐splitting, and utilizing 980 nm near‐infrared light to increase penetration depth, which significantly improves PDT efficiency as well as reduces the side effects.
Nanosheets of silicon have attracted a great deal of attention due to its tunable optical and electronic properties. However, the development of facile and easily scalable synthesis process has ...remained a great contest. Endeavor has been made in this research to find a waste inferred effective photocatalyst to deliver hydrogen (H2) through visible light responsive water splitting.
One-pot solid phase reaction was applied to synthesis catalyst and adopted ultrathin structure. The photocatalytic efficiency of catalyst was examined by XRD, XPS, and UV–VIS absorption spectra, PL, FESEM, HRTEM and EDX. The HRTEM and FESEM images revealed the interconnected nanosheets with Si having the average thickness of 5 nm and their band gaps were 2.3–2.5 eV corresponding to the absorption of visible light range. The H2 production rate on photocatalyst was originated to 3200 μmol h−1 without utilizing any conciliatory electron givers, voltage or pH alteration, which beats the Pt, Ru, Rh, Pd and Au stacked photocatalyst ever detailed up until this point. The significant increase in photocatalytic activity could be the fast charge migration and separation from the silicon-hydrogen and silicon-hydroxyl bonds on Si surface and facilitation of charge separation could results from the multiple reflections of visible light on ultrathin nanosheets. It has been confirmed that the electron/hole recombination rate in ultrathin nanosheets of Si declined due to the oxidation of Si surface. It would be presumed that the approach of surface chemistry of silicon could not be limited towards the photocatalytic water splitting and could be applicable to remedy water pollution.
Display omitted
•Ultrathin nanostructured noble metal free Si/MgO photocatalyst was reported.•Visible light responsive Si/MgO was prepared via one step solid phase reaction method.•XPX showed that the formation of Si-H and Si-OH bonds of Si facilitated the H2 production.•The highest H2 production was found to 3200 μmol h−1g−1 without being affected by any reagent.
Exploring earth‐abundant and highly efficient electrocatalysts is critical for further development of water electrolyzer systems. Integrating bifunctional catalytically active sites into one ...multi‐component might greatly improve the overall water‐splitting performance. In this work, amorphous NiO nanosheets coupled with ultrafine Ni and MoO3 nanoparticles (MoO3/Ni–NiO), which contains two heterostructures (i.e., Ni–NiO and MoO3–NiO), is fabricated via a novel sequential electrodeposition strategy. The as‐synthesized MoO3/Ni–NiO composite exhibits superior electrocatalytic properties, affording low overpotentials of 62 mV at 10 mA cm−2 and 347 mV at 100 mA cm−2 for catalyzing the hydrogen and the oxygen evolution reaction (HER/OER), respectively. Moreover, the MoO3/Ni–NiO hybrid enables the overall alkaline water‐splitting at a low cell voltage of 1.55 V to achieve 10 mA cm−2 with outstanding catalytic durability, significantly outperforming the noble‐metal catalysts and many materials previously reported. Experimental and theoretical investigations collectively demonstrate the generated Ni–NiO and MoO3–NiO heterostructures significantly reduce the energetic barrier and act as catalytically active centers for selective HER and OER, synergistically accelerating the overall water‐splitting process. This work helps to fundamentally understand the heterostructure‐dependent mechanism, providing guidance for the rational design and oriented construction of hybrid nanomaterials for diverse catalytic processes.
A new MoO3/Ni–NiO hybrid electrocatalyst is designed and synthesized via a novel sequential electrodeposition strategy, which exhibits excellent activity and durability for the overall water splitting process. Experimental and theoretical analysis demonstrate the improved hydrogen evolution performance should be mainly attributed to the Ni–NiO heterostructure, and the generated MoO3–NiO heterointerface is responsible for enhancing the oxygen evolution activity, synergistically facilitating bifunctional electrocatalysis.
Electrocatalytic water‐splitting has gained a firm hold in the area of renewable hydrogen production owing to its integrative compatibility with intermittent energy sources. However, wide‐scale ...implementation of this technology demands discovery of new electrode materials that strike a good balance between efficiency, stability, and cost. In the pool of inexpensive electrodes capable of catalyzing hydrogen and oxygen evolution reactions, metal borides/borates have made a big splash in the last decade. However, the research in this family of electrocatalysts remains unorganized owing to the diversity of reports. This review summarizes the past and present research progress in metal borides/borates for electrocatalytic water‐splitting. The fundamental reasons for electrochemical behavior in different metal borides/borates are highlighted here, also including some comments regarding erroneous practices in the performance evaluation of metal borides/borates. Various strategies used to enhance the electrocatalytic performance of metal borides/borates are discussed in detail. Different methods evolved over the years for the synthesis of metal borides/borates are also discussed. Finally, an assessment of the commercial viability of metal borides/borates is made and future research directions are suggested.
The unprecedented emergence of metal borides/borates as a highly efficient family of materials for electrocatalytic water‐splitting is reviewed. A discussion on the theories to understand their reaction mechanism, various material engineering strategies to improve their performance, and different synthesis routes for their fabrication is included in the review. A concrete future perspective is provided for further research and development.
Efficient charge separation and utilization are critical factors in photocatalysis. Herein, it is demonstrated that the complete spatial separation of oxidation and reduction cocatalysts enhances the ...efficacy of charge separation and surface reaction. Specifically, a Pt@NH2‐UiO‐66@MnOx (PUM) heterostructured photocatalyst with Pt and MnOx as cocatalysts is designed for the optimization of the NH2‐UiO‐66 photocatalyst. Compared with the pristine NH2‐UiO‐66, Pt@NH2‐UiO‐66 (PU), and NH2‐UiO‐66@MnOx (UM) samples, the PUM sample exhibits the highest hydrogen production activity. As cocatalysts, Pt favors trapping of electrons, while MnOx tends to collect holes. Upon generation from NH2‐UiO‐66, electrons and holes flow inward and outward of the metal–organic framework photocatalyst, accumulating on the corresponding cocatalysts, and then take part in the redox reactions. The PUM photocatalyst greatly prolongs the lifetime of the photogenerated electrons and holes, which favors the electron–hole separation. Furthermore, the PUM sample facilitates overall water splitting in the absence of sacrificial agents, thereby demonstrating its potential as a modification method of MOF‐type semiconductors for the overall water‐splitting reaction.
A heterostructured photocatalyst, Pt@NH2‐UiO‐66@MnOx (PUM), is constructed based on the rational optimization of NH2‐UiO‐66 with spatially separated Pt and MnOx nanoparticles as cocatalysts. The optimization results in a long lifetime of the photogenerated electrons and holes in the composite, which can drive overall water splitting in the absence of sacrificial agents.
Heterostructures are widely fabricated for promotion of photogenerated charge separation and solar cell/fuel production. (Oxy)nitrides are extremely promising for solar energy conversion, but the ...fabrication of heterostructures based on nitrogen-containing semiconductors is still challenging. Here, a simple ammonia thermal synthesis of a heterostructure (denoted as Ta
N
/BTON) composed of 1D Ta
N
nanorods and BaTaO
N (BTON) nanoparticles (0D), which is demonstrated to result in a remarkable increase in photogenerated charge separation and solar hydrogen production from water, is introduced. As analyzed and discussed, the Ta
N
/BTON heterostructure is type II and tends to create intimate interfaces between the 1D nanorods and 0D nanoparticles. The 1D Ta
N
nanorods are demonstrated to transfer electrons along the rod orientation direction. Furthermore, the intimate interfaces of the heterostructure are believed to originate from the similar Ta-based octahedron units of Ta
N
and BTON. All of the above features are expected to integrally endow increased photoinduced charge separation and one order of magnitude higher solar overall water splitting activity with respect to counterpart systems. These results may open a new avenue to fabricate heterostructures on the basis of nitrogen-containing semiconductors that is extremely promising for solar energy conversion.
PDF
