Fundamentals of TiO2 Photocatalysis: Concepts, Mechanisms, and Challenges
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Photocatalysis has been widely applied in various areas, such as solar cells, water splitting, and pollutant degradation. Therefore, the photochemical mechanisms and basic principles of photocatalysis, especially TiO2 photocatalysis, have been extensively investigated by various surface science methods in the last decade, aiming to provide important information for TiO2 photocatalysis under real environmental conditions. Recent progress that provides fundamental insights into TiO2 photocatalysis at a molecular level is highlighted. Insights into the structures of TiO2 and the basic principles of TiO2 photocatalysis are discussed first, which provides the basic concepts of TiO2 photocatalysis. Following this, details of the photochemistry of three important molecules (oxygen, water, methanol) on the model TiO2 surfaces are presented, in an attempt to unravel the relationship between charge/energy transfer and bond breaking/forming in TiO2 photocatalysis. Lastly, challenges and opportunities of the mechanistic studies of TiO2 photocatalysis at the molecular level are discussed briefly, as well as possible photocatalysis models.Keywords:
Photocatalytic water splitting
Photocatalytic water splitting
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Photocatalytic water splitting has been widely studied as a means of converting solar energy into hydrogen as an ideal energy carrier in the future. Systems for photocatalytic water splitting can be divided into one-step excitation and two-step excitation processes. The former uses a single photocatalyst while the latter uses a pair of photocatalysts to separately generate hydrogen and oxygen. Significant progress has been made in each type of photocatalytic water splitting system in recent years, although improving the solar-to-hydrogen energy conversion efficiency and constructing practical technologies remain important tasks. This perspective summarizes recent advances in the field of photocatalytic overall water splitting, with a focus on the design of photocatalysts, co-catalysts and reaction systems. The associated challenges and potential approaches to practical solar hydrogen production via photocatalytic water splitting are also presented.
Photocatalytic water splitting
Energy transformation
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Photocatalytic overall water splitting into H2 and O2 is expected to be a promising method for the efficient utilization of solar energy. The design of optimal photocatalyst structures is a key to efficient overall water splitting, and the development of photocatalysts which can efficiently convert large portion of visible light spectrum has been required. Recently, a series of complex perovskite type transition metal oxynitrides, LaMgxT 1-xO1+3xN2-3x, was developed as photocatalysts for direct water splitting operable at wide wavelength of visible light. In addition two-step excitation water splitting via a novel photocatalytic device termed as photocatalyst sheet was developed. This consists of two types of semiconductors (hydrogen evolution photocatalyst and oxygen evolution photocatalyst) particles embedded in a conductive layer, and showed high efficiency for overall water splitting. These recent advances in photocatalytic water splitting were introduced.
Photocatalytic water splitting
Visible spectrum
Oxygen evolution
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With a view to the particularity of overall water splitting,the preparation of photocatalytic materials and their performance for hydrogen and oxygen production from overall water splitting are reviewed in relation to the design of structure and energy band of photocatalytic materials as well as their surface modification.The principle of two step reaction(Z system) for overall water splitting and several currently available Z systems are introduced.Furthermore,existing problems of photocatalytic overall water splitting are also briefed.
Photocatalytic water splitting
Oxygen evolution
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Photocatalytic water splitting
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Photocatalytic water splitting can realize a direct conversion from solar energy into green hydrogen energy, which is conducive to effectively mitigate energy crisis and environmental issues. Single‐atom catalysts (SACs) have shown great potential in photocatalytic hydrogen evolution, with unique geometric and electronic structures that help to boost mass and charge transfer during photocatalytic processes. Herein, recent advances of SACs in photocatalytic water splitting are focused upon. To decrease aggregation and realize sufficient utilization of metal atoms, different synthesis strategies for SACs are documented. For photocatalytic hydrogen evolution, the catalytic performances, active sites, and structure–property relationships of SACs including Al, Co, Ni, Pd, Ag, and Pt single sites are highlighted. The existing challenges and future directions of SACs in photocatalytic water splitting are provided.
Photocatalytic water splitting
Hydrogen atom
Solar energy conversion
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Exploiting earth-abundant and low-cost photocatalysts for high efficiency photocatalytic water splitting is of profound significance. Herein, we report an improved photocatalytic water splitting activity by P and As substitution at the N-site in the C2N monolayer using state-of-the-art hybrid density functional calculations. Our results show that the band gap can be reduced in C2N by increasing the concentrations of P and As substitution, and at the same time the obtained band gap value is higher than the free energy of water splitting except for As with concentrations of x = 0.333. This indicates that these new compositions of P/As substituted C2N monolayers are thermodynamically suitable to drive hydrogen evolution reaction. The calculated effective mass of charge carriers illustrates that charge transfer to the reactive sites would be easier in the substituted system than the pure C2N, and also our results suggest that the recombination rate would be lower in the substituted system, indicating the enhancement in the efficiencies of photocatalytic water splitting. The band edge position with respect to the redox potentials of water shows that P/As substituted C2N monolayers are the potential photocatalysts for water splitting than the pristine C2N monolayer. From the optical absorption spectra, we found that P/As substituted C2N monolayer shows optical absorption extended more into the visible region, indicating enhanced energy harvesting. Our results reflect that the P/As substituted C2N monolayer could be the potential visible-light photocatalyst for overall water splitting.
Photocatalytic water splitting
Absorption edge
Visible spectrum
Charge carrier
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Hydrogen is the ideal fuel for the future because it is clean, energy efficient, and abundant in nature. While various technologies can be used to generate hydrogen, only some of them can be considered environmentally friendly. Recently, solar hydrogen generated via photocatalytic water splitting has attracted tremendous attention and has been extensively studied because of its great potential for low-cost and clean hydrogen production. This paper gives a comprehensive review of the development of photocatalytic water splitting for generating hydrogen, particularly under visible-light irradiation. The topics covered include an introduction of hydrogen production technologies, a review of photocatalytic water splitting over titania and non-titania based photocatalysts, a discussion of the types of photocatalytic water-splitting approaches, and a conclusion for the current challenges and future prospects of photocatalytic water splitting. Based on the literatures reported here, the development of highly stable visible–light-active photocatalytic materials, and the design of efficient, low-cost photoreactor systems are the key for the advancement of solar-hydrogen production via photocatalytic water splitting in the future.
Photocatalytic water splitting
Environmentally Friendly
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Photocatalytic water splitting
Visible spectrum
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Solar light utilization for photocatalytic overall water splitting (POWS) is a promising alternative to electrolysis to produce hydrogen, since photocatalytic water splitting is simple, and can be operated at low-cost. Transformation and storage of solar energy in the form hydrogen can significantly reduce the rate of the greenhouse gas emissions. Given the low-cost and simplicity of photocatalytic hydrogen production, this thesis has focused on designing an efficient photocatalytic water splitting system.SrTiO3 has been shown to be capable of driving photocatalytic overall water splitting under UV light illumination. In this thesis, SrTiO3 is used as the photocatalyst and the performance of SrTiO3 is discussed in the POWS reaction. Several strategies have been applied to understand particular functions of the SrTiO3-based photocatalyst in photocatalytic overall water splitting with following aspects: i) the photocatalytic transients are collected to understand how Ni/NiO co-catalyst is changed during illumination; ii) Mg is doped into the structure of SrTiO3 to improve the photocatalytic activity; iii) Cr2O3 is introduced on Mg:SrTiO3-NiOx composite to improve the stability of photocatalytic gas evolution; iv) state-of-the art semiconductors, Al:SrTiO3 and Mg:SrTiO3, are compared in the same conditions. Generally, the effect of the various modifications on the photocatalytic gas evolution rates has been revealed by reliable on-line GC measurements. Due to the fast detection mode of the GC, the applied setup allows to determine transients in gas-evolution to reveal activity and stability of the tested photocatalysts.
Photocatalytic water splitting
Electrolysis of water
Non-blocking I/O
Visible spectrum
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