Minmin Gao

Structural design of TiO2-based photocatalyst for H2 production and degradation applications

TiO2-based photocatalysts, being inexpensive and abundant, in conjunction with having high photostability and environmentally friendly characteristics, are the most extensively studied photocatalytic material for hydrogen production and pollutant degradation. However, its existing issues, such as wide bandgap, high overpotential and rapid recombination of photogenerated carriers limit its photocatalytic properties. The opportunities for structural development of a TiO2 nanomaterial towards highly efficient and pragmatic photocatalysis applications are evidently plentiful. Hence, in this review, we will look into critical structural engineering strategies that give favorable physicochemical properties such as improved light absorption, photostability, charge-carrier dynamics, increase surface area etc. that benefit photocatalysis functionalities. Amongst the various structural engineering options, we will be covering the most prevalent and elegant core–shell and hierarchical structural designs, which rationally combine the advantages of structural manipulation and multi-material composition engineering. This review aims to provide a comprehensive and contemporary overview, as well as a guide of the development of new generation TiO2 based photocatalysts via structural design for improved solar energy conversion technologies.

Bidentate-complex-derived TiO2/carbon dot photocatalysts: in situ synthesis, versatile heterostructures, and enhanced H2 evolution

In this paper, we demonstrate a series of metal-free and inexpensive TiO2/carbon dot (CD) nanocomposites via facile hydrothermal synthesis from bidentate complexes of a green carbon source, vitamin C (VC). Other than the importance of deriving CDs from alternate carbon materials instead of graphitic precursors, the in situ transformed CDs from VC ensure the formation of chemically coupled heterostructures, TiO2/CDs, which serve as efficient photocatalysts exhibiting a higher H2 evolution rate from photocatalytic reactions up to 9.7 times than that of bare TiO2. Interestingly, the H2 evolution rate can effectively be tuned by VC amounts, hydrothermal temperatures and reaction durations. The mechanism of the enhanced H2 evolution rate was also discussed, in which the synergetic effects of the hydrothermal treatment along with the favourable electron transfer ability and upconverted photoluminescence of CDs contribute to the improved photocatalytic behaviour.

Plasmonic photothermic directed broadband sunlight harnessing for seawater catalysis and desalination

Using readily available renewable resources, i.e. solar energy and seawater, to secure sustainable fuel and freshwater for humanity is an impactful quest. Here, we have designed solar thermal collector nanocomposites (SiO2/Ag@TiO2 core–shell) that possess efficient photothermic properties for highly targeted interfacial phase transition reactions that are synergistically favorable for both seawater catalysis and desalination reactions. The photothermic effect arising from plasmonic metal nanoparticles causes localized interfacial heating which directly triggers surface-dominated catalysis and steam generation processes, with minimal heat losses, reduced thermal masses and optics implementation. The solar thermal collector nanocomposites are seawater/photo stable for practical solar conversion of seawater to simultaneously produce clean energy and water. Finally, a proof-of-concept all-in-one compact solar hydrogen and distillate production prototype demonstrates the viability of sustainable photothermic driven catalysis and desalination of seawater under natural sunlight. Importantly, this approach holds great promise for enhancing energy and water productivity without considerable capital, infrastructure and environmental ramifications.

Green chemistry synthesis of a nanocomposite graphene hydrogel with three-dimensional nano-mesopores for photocatalytic H2 production

In this work, we have developed a nanocomposite graphene hydrogel (NGH) based on green chemistry, employing vitamin C (VC) to attain a supramolecular 3D network of hybrid nanostructured materials. Here, it is shown that the hydrogel is an appropriate and robust host for stable a TiO2 semiconductor catalyst sensitized with visible light responsive nanostructured particles. The NGH is tailored with well-defined nano-mesopores, a large surface area, a highly dispersive nanosheet–nanorods–nanoparticle composite, and enhance visible light absorption. Finally, we demonstrate practical applications of utilizing the NGH with water containing pores for photocatalytic H2 production. An important pragmatic consideration of using a NGH is the ease of separation and recovery of the nanosized catalyst after the photoreaction which would otherwise require extensive and expensive nanofiltration.

Photocatalytic H2 production of composite one-dimensional TiO2 nanostructures of different morphological structures and crystal phases with graphene

One-dimensional TiO2 nanostructures of different structural phases with graphene composites were employed as photocatalysts for photocatalytic H2 production. Two strategies have been explored for enhancement of photocatalytic reactivity. The first is demonstrated through the use of one-dimensional nanostructures, which feature good vectorial electron transport due to decreased grain boundaries. The second strategy is to form a composite/network with two-dimensional reduced graphene oxide (RGO), which features visible light photosensitization and is an efficient charge transporter and separator. It is noted that other than structural design, it is crucial to attain anatase phase structures and superior interfacial contact between GO and TiO2 nanostructures to enable optimal synergy between one-dimensional TiO2 nanostructures and graphene for enhanced photocatalytic H2 production performance. In this work, hydrothermal synthesis and an in situ solution-based photoreduction method are used owing to their scalability, low temperature, high yield and ease of temperature and reactant concentration control characteristics.

Design of a Metal Oxide–Organic Framework (MoOF) Foam Microreactor: Solar‐Induced Direct Pollutant Degradation and Hydrogen Generation

A macroporous carbon network combined with mesoporous catalyst immobilization by a template method gives a metal-oxide-organic framework (MoOF) foam microreactor that readily soaks up pollutants and localizes solar energy in itself, leading to effective degradation of water pollutants (e.g., methyl orange (MO) and also hydrogen generation. The cleaned-up water can be removed from the microreactor simply by compression, and the microreactor used repeatedly.

Harvesting broadband absorption of the solar spectrum for enhanced photocatalytic H2 generation

Absorption of the solar spectrum in the visible and near infrared region is highly desirable to improve photocatalytic H2 generation. Traditionally, this can be fulfilled by designing photocatalyst materials with narrower band gaps, or with upconversion capabilities. However, such materials often pose challenges such as in synthesis, structural defects, and stability which may lead to adverse photocatalytic performance. This paper focuses on broadband utilization of the solar spectrum for enhanced photocatalysis solar H2 production where the spectrum not utilized by the photocatalysts is absorbed and converted to heat energy. This approach delves into harvesting the broadband spectrum for synergistic photocatalysis and thermal heat generation, with minimal photocatalyst material manipulation. The profound impact of temperature on photocatalysis was manifested in a drastic increase of H2 production by a maximum of 40-fold. The apparent quantum yield was also calculated to reach 66.9% using an ultraviolet LED light source. Outdoor testing verifies the potential of broad spectrum operation under natural sunlight as well as the convenience and simplicity of various reactor designs for practical photocatalysis applications.

Solar-driven photothermal nanostructured materials designs and prerequisites for evaporation and catalysis applications

Solar energy is a major source of renewable energy with the potential to meet the energy demand and to support the sustainable development of the world. The efficient harvesting and conversion of solar energy is one of the key factors to maximize the utilization of solar energy. In general, solar energy can be harnessed and converted into various kinds of energy, including electricity, fuels and thermal energy, through photovoltaic, photochemical and photothermal processes, respectively. Among these technologies, photothermal conversion is a direct conversion process that has attained the highest achievable conversion efficiency. The photothermal effect has been used as a novel strategy to augment vaporization and catalysis performance. In this review, we look into the basis of the photothermal conversion process, the design of efficient photothermal conversion materials in terms of both light harvesting and thermal management, a fundamental understanding of various system schemes, and the recent progress in photothermal evaporation and catalysis applications. This review aims to afford researchers with a better understanding of the photothermal effect and provide a guide for the rational design and development of highly efficient photothermal materials in energy and environmental fields.

Self-regulating reversible photocatalytic-driven chromism of a cavity enhanced optical field TiO2/CuO nanocomposite

Over the years, the exploration of inorganic chromogenic materials commonly interfaced with expensive noble metals has been limited, while the study of organic photochromic materials has proliferated. The key challenge lies in the optimization of inorganic materials’ constituents and structural design to achieve enhanced light–matter interaction chromism with performance commensurate with that of their organic counterparts. Here, we demonstrate a tailored inorganic transition metal cavity that boosts optically controlled reversible and repeatable chromism driven by a photocatalytic reaction. The solution-processable TiO2/CuO nanocomposite is endowed with a mesoporous cavity that is highly adept at performing self-regulating reversible photochromism under solar irradiation. The improved photoreactive chromism stems from the tailored critical structural parameters of the highly accessible mesoporous shell, reduced charge carrier diffusion length thin shell, and cavity enhanced optical field for photon–matter interactions. Consequently, the nanocomposite exhibits dual-functional photoregulated effects, i.e. photochromism mediated light transmittance modulation and rewritable printing/patterning. The nanocomposite offers high sensitivity, resulting in a short response time due to the efficient charge transfer, and such chromism effects are stable in the ambient environment. Furthermore, controlled switching of the chromism effects may be obtained simply by low-temperature heating. The concerted combination of inexpensive materials/production, low toxicity and a highly transparent noble metal-free inorganic nanocomposite renders chromogenic properties that will trigger a renewed interest in smart light-stimulus integrated technology.

Visible‐to‐NIR Photon Harvesting: Progressive Engineering of Catalysts for Solar‐Powered Environmental Purification and Fuel Production

Utilization of diffusive solar energy through photocatalytic processes for environmental purification and fuel production has long been pursued. However, efficient capture of visible-near-infrared (NIR) photons, especially for those with wavelengths longer than 600 nm, is a demanding quest in photocatalysis owing to their relatively low energy. In recent years, benefiting from the advances in photoactive material design, photocatalytic reaction system optimization, and new emerging mechanisms for long-wavelength photon activation, increasing numbers of studies on the harnessing of visible-NIR light for solar-to-chemical energy conversion have been reported. Here, the aim is to comprehensively summarize the progress in this area. The main strategies of the long-wavelength visible-NIR photon capture and the explicitly engineered material systems, i.e., narrow optical gap, photosensitizers, upconversion, and photothermal materials, are elaborated. In addition, the advances in long-wavelength light-driven photo- and photothermal-catalytic environmental remediation and fuel production are discussed. It is anticipated that this review presents the forefront achievements in visible-NIR photon capture and at the same time promotes the development of novel visible-NIR photon harnessing catalysts toward efficient solar energy utilization.