Photocatalytic and photoelectrochemical processes are two key systems in harvesting sunlight for energy and environmental applications. As both systems are employing photoactive semiconductors as the ...major active component, strategies have been formulated to improve the properties of the semiconductors for better performances. However, requirements to yield excellent performances are different in these two distinctive systems. Although there are universal strategies applicable to improve the performance of photoactive semiconductors, similarities and differences exist when the semiconductors are to be used differently. Here, considerations on selected typical factors governing the performances in photocatalytic and photoelectrochemical systems, even though the same type of semiconductor is used, are provided. Understanding of the underlying mechanisms in relation to their photoactivities is of fundamental importance for rational design of high‐performing photoactive materials, which may serve as a general guideline for the fabrication of good photocatalysts or photoelectrodes toward sustainable solar fuel generation.
Photoactive semiconductors have been extensively used in solar energy conversion applications. They are typically employed in either photocatalytic or photoelectrochemical systems. Although some generic strategies to improve the materials have been formulated, the requirements to achieve excellent activities are different in both processes. Insightful discussions on the aspects for consideration in developing excellent photocatalytic and photoelectrochemical systems are provided.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Photoreforming is a process that harnesses the redox ability of photocatalysts upon illumination, to simultaneously drive the reduction of H
+
into hydrogen gas and oxidation of organic compounds. ...Over the past few decades, significant effort has been devoted to improving the photocatalytic hydrogen evolution efficiency, while substantially less focus has been directed towards the oxidation reactions. More recently, the realization of the potential for simultaneous hydrogen production with value-added organics has inspired researchers to use photooxidation pathways to tune the selectivity of oxidized products. As a distinct benefit, the less energetically demanding organic reforming is highly favorable when compared to the slow kinetics of oxygen evolution which negates the need for expensive and/or harmful hole scavengers. Photocatalyst modifications, such as secondary component deposition, doping, defect, phase and morphology engineering, have been the main strategies adopted to tune the photooxidation pathways and oxidation products. Direct control of the process conditions, including pH, temperature and reactant concentration, and favorable reactor designs can further improve the selectivity towards desired products. While other published reviews focus on the types of photocatalysts or feedstocks used to enhance the hydrogen evolution efficiency, this review highlights the importance of controlling the selectivity of the photoreforming reaction, particularly as an alternative path for waste abatement or valorization for industry. This review links the strategies used to improve the selectivity of photoreforming of organic waste into high-value and desirable chemicals, as well as offers an outlook on the future research direction required to deliver highly selective photocatalyst. A holistic strategy that comprises photocatalyst and system designs, appropriate characterizations and implementation of artificial intelligence has also been proposed and discussed to further aid establishment of the structure-mechanism-function relationship, thereby accelerating the discovery of optimum selective photoreforming systems.
This review appraises recent literature and provides guidelines for the rational design of photocatalytic system for selective photoreforming reaction.
Cuprous Oxide (Cu2O) is a photocatalyst with severe photocorrosion issues. Theoretically, it can undergo both self‐oxidation (to form copper oxide (CuO)) and self‐reduction (to form metallic copper ...(Cu)) upon illumination with the aid of photoexcited charges. There is, however, limited experimental understanding of the “dominant” photocorrosion pathway. Both photocorrosion modes can be regulated by tailoring the conditions of the photocatalytic reactions. Photooxidation of Cu2O (in the form of a suspension system), accompanied by corroded morphology, is kinetically favourable and is the prevailing deactivation pathway. With knowledge of the dominant deactivation mode of Cu2O, suppression of self‐photooxidation together with enhancement in its overall photocatalytic performance can be achieved after a careful selection of sacrificial hole (h+) scavenger. In this way, stable hydrogen (H2) production can be attained without the need for deposition of secondary components.
This catalyst will self‐destruct in… Photoexcited Cu2O can photocorrode by two possible pathways; namely, self‐photoreduction into Cu metal or self‐photooxidation into CuO. A systematic study of Cu2O photocorrosion demonstrates that this process is dominated by self‐photooxidation through consumption of photogenerated holes (h+). Scale bar: 100 nm.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Bismuth-based semiconductors, as an emerging group of photocatalysts possessing a layered-structure, have been widely demonstrated to show excellent photo-oxidation capability in water oxidation and ...pollutant decomposition. Deriving from the suitable band gap and well-dispersed valence band, Bi-based materials are generally visible-light-responsive and consequently have received considerable interest in photocatalysis. Different from conventional binary metal oxide semiconductors, the synthesis of Bi-based materials needs to be carefully controlled as most of them are prepared as ternary oxide materials that comprise multiple elements and are sensitive to the synthesis conditions. Specifically, different methods need to be employed to obtain suspended particles and electrodes for photocatalytic and photoelectrochemical reactions, respectively. Given that the synthesis process for photoelectrodes usually involves more than one step, fabricating the desired ternary Bi-based photoelectrodes can be more challenging when compared to powdered photocatalysts. With the significant emergence of Bi-based materials, understanding the effect of different synthesis methods on the catalyst properties is crucial to ensure great performance. This review summarizes recent developments in Bi-based photocatalysts by comparing and discussing various synthesis approaches for powdered photocatalysts and thin film photoelectrodes.
A comprehensive survey on preparation methods of powdered or thin-film Bi-based photocatalysts is provided, comparing he diverse approaches and their advantages and limitations in the context of photocatalytic and photoelectrochemical applications.
Surface coating of a protective layer can prevent the corrosion of Cu2O at electrode liquid junctions (ELJs) in photoelectrochemical water splitting. However, a facile methodology for the deposition ...of a conformal protective layer is still a challenge. Here, an ultrathin layer of amorphous ZnO is introduced on Cu2O by pulsed electrodeposition, to construct a “sandwich” structure of a composite photoelectrode of TiO2/ZnO/Cu2O on an FTO substrate. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) visualises the spatial distribution of Ti, Zn, Cu, and Sn elements of the composite. Benefiting from the homogeneous coating of a ZnO layer, visible cracks in TiO2 coating are significantly reduced, thus preventing the direct contact between the electrolyte and Cu2O. Moreover, due to the ultrathin property of the amorphous ZnO layer, the energetic electrons from the excited Cu2O can be injected via the ZnO layer into TiO2, as elucidated by time-resolved photoluminescence (TRPL) results. The resulting composite photoelectrode shows enhanced photoelectrochemical activity and stability, compared to the bare Cu2O, as well as the TiO2/Cu2O photoelectrode. This study offers a versatile and effective method for improving the stability and charge separation efficiency of Cu2O, which is useful in guiding the surface coating of other nanostructured materials for solar energy conversion.
Exploring advanced technologies to efficiently produce green hydrogen energy is imperative to alleviate the energy crisis and environmental pollution. Conventional overall water electrolysis (OWE) ...has been regarded as a promising approach for effective H2 production, however, it is largely restricted by the sluggish kinetics of the anodic oxygen evolution reaction (OER). Coupling kinetically favorable anodic reactions, such as biomass‐derived compound oxidation and pollutant degradation, with the hydrogen evolution reaction (HER) in hybrid water electrolysis (HWE), can not only solve the biomass recycling and pollutant emission problems but also save the energy cost for clean H2 generation. Hence, various advanced earth‐abundant electrocatalysts have been developed to catalyze those promising anodic reactions, yet some problems such as tedious preparation and unsatisfactory performance still exist. Given the gap between research and practical applications, this review summarizes the recent progress in electrocatalysts for diverse alternative anodic oxidation reactions over the last five years together with their application in HWE systems. An in‐depth understanding of different reaction mechanisms and assessments toward electrocatalysts is discussed to further enhance anodic efficiency. The advantages, differences, and critical issues of different HWE systems are thoroughly discussed as well, providing a new avenue for low‐voltage H2 production from renewable resources and waste products.
Hybrid water electrolysis (HWE) integrating biomass refining/pollutant degradation with the hydrogen evolution reaction is a feasible way to boost hydrogen production. This review summarizes recent development in promising anodic substrates and current progress in feasible electrocatalysts, providing an in‐depth understanding of different reaction mechanisms and assessments toward electrocatalysts to enable possible implementation of various anodic substrate‐based HWE systems for industrial H2 production.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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•A series of MoS2-loaded CdS NRs were prepared to study the photogenerated charge dynamics.•MoS2-tipped CdS NRs exhibited an excellent photocatalytic H2 evolution activity.•MoS2 ...deposited on the tips of CdS NRs gave a slow charge recombination.•Spatially longitudinal charge separation and transfer intensified by the MoS2 tip was verified as a crucial factor.
A series of MoS2-loaded CdS nanorods (NRs) have been successfully fabricated with the MoS2 spatially distributed only on the tips or on the tips and walls of the CdS NRs, which impacted on photocatalytic H2 evolution activity. MoS2-tipped CdS NRs were found to exhibit a better H2 evolution performance (31.46 mmol h−1 g−1) than MoS2-coated CdS NRs (7.32 mmol h−1 g−1) and bare CdS NRs (2.96 mmol h−1 g−1). Kelvin probe force microscopy (KPFM) was used to identify the presence of a spatial electric field between the CdS NR and MoS2 tip, with the electric field strongly inducing photogenerated electron-hole separation along the long axis of the CdS NRs and electron transfer to MoS2 tips. The effect of longitudinal transfer of photogenerated electrons was confirmed by Pt photodeposition where it was found that Pt particles were photodeposited on the MoS2 tips, whereas, in the case of pure CdS NRs and MoS2-coated CdS NRs, Pt particles were photodeposited on the walls and tips of the NRs, indicating a lack of spatially directional charge transfer. Time-resolved photoluminescence (TRPL) spectroscopy using band pass and long pass filters was employed to demonstrate that MoS2 located on the tips of CdS NR can better separate photogenerated electron-hole pairs and suppress charge recombination. Consequently, slow charge recombination and spatially separated redox sites, deriving from MoS2 tip engendered long distance of electron separation and transfer within the CdS NRs, give rise to the superior photoactivity by the MoS2-tipped CdS NRs. This study reveals the relationship between the MoS2 distribution and photogenerated charge dynamics, and also provides greater insight into the performance of CdS-MoS2 composites for photocatalytic H2 evolution.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Robust screening of materials on the basis of structure–property–activity relationships to discover active photocatalysts is a highly sought out aspect of photocatalysis research. Recent advancements ...in machine learning offer considerable opportunities to evolve photocatalysts discovery practices. Machine learning has largely facilitated various areas of science and engineering, including heterogeneous catalysis, but adaptation of it in photocatalysis research is still at an elementary stage. The scarcity of consistent training data is a major bottleneck, and we foresee the integration of photocatalysis domain knowledge in mainstream machine learning protocols as a viable solution. Here, we present a holistic framework incorporating machine learning and domain knowledge to set directions toward accelerated discovery of solar photocatalysts. This Perspective begins with a discussion on domain knowledge available in photocatalysis which could potentially be leveraged to liaise with machine learning methods. Subsequently, we present prevalent machine learning practices in heterogeneous catalysis tailored to assist discovery of photocatalysts in a purely data-driven fashion. Lastly, we conceptualize various strategies for complementing data-driven machine learning with photocatalysis domain knowledge. The strategies involve the following: (i) integration of theoretical and prior empirical knowledge during the training of machine learning models; (ii) embedding the knowledge in feature space; and (iii) utilizing existing material databases to constrain machine learning predictions. The aforementioned human-in-loop framework (leveraging both human and machine intelligence) could possibly mitigate the lack of interpretability and reliability associated with data-driven machine learning and reinforce complex model architectures irrespective of data scarcity. The concept could also offer substantial benefits to photocatalysis informatics by promoting a paradigm shift away from the Edisonian approach.
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IJS, KILJ, NUK, PNG, UL, UM
Dramatically increased CO
2
concentration from several point sources is perceived to cause severe greenhouse effect towards the serious ongoing global warming with associated climate destabilization, ...inducing undesirable natural calamities, melting of glaciers, and extreme weather patterns. CO
2
capture and utilization (CCU) has received tremendous attention due to its significant role in intensifying global warming. Considering the lack of a timely review on the state-of-the-art progress of promising CCU techniques, developing an appropriate and prompt summary of such advanced techniques with a comprehensive understanding is necessary. Thus, it is imperative to provide a timely review, given the fast growth of sophisticated CO
2
capture and utilization materials and their implementation. In this work, we critically summarized and comprehensively reviewed the characteristics and performance of both liquid and solid CO
2
adsorbents with possible schemes for the improvement of their CO
2
capture ability and advances in CO
2
utilization. Their industrial applications in pre- and post-combustion CO
2
capture as well as utilization were systematically discussed and compared. With our great effort, this review would be of significant importance for academic researchers for obtaining an overall understanding of the current developments and future trends of CCU. This work is bound to benefit researchers in fields relating to CCU and facilitate the progress of significant breakthroughs in both fundamental research and commercial applications to deliver perspective views for future scientific and industrial advances in CCU.
This review covers the sustainable development of advanced improvements in CO
2
capture and utilization.
An extremely close relationship exists between energy usage and water supply with a tremendous amount of energy being consumed to process water for drinking and other purposes. The current energy ...crisis and inefficient water management place enormous stress on the sustainability of our society and environment. As such, the development of high-efficiency, cost-effective, and environmentally friendly materials which possess co-existing functionalities for applications ranging from energy capture to water treatment in one material, provides an opportunity to achieve sustainable development. As multifunctional materials, the layer-structured Magnèli titanium oxides with stoichiometry of Ti
n
O
2
n
−1
(
n
≥ 2) have been extensively studied in view of their potential for photocatalytic, thermoelectric and photothermal applications over the past few years. This group of materials occurs naturally as layered structures with planar oxygen defects, however, understanding of the correlation between the planar arrangements of the oxygen defects and various energy-related properties remains limited. Here, we demonstrate how the formation of layer structured Ti
n
O
2
n
−1
with various planar oxygen defect arrangements correlates with the changes of their physical and chemical properties. The experimental results from inelastic neutron scattering analysis and electrical characterizations provide evidence that the planar oxygen defects are responsible for phonon scattering and exert a strong influence on their electrical conductivities. Manipulating these planar defects allows interconversion between different phases, which changes the interplay between electronic and phononic sub-systems. These manipulations potentially enable optimization of the corresponding physical properties of these materials such that they are rendered suitable for applications that require co-operative multifunctionality. More specifically, the experimental results demonstrate that the valence band positions and the onset potentials in the materials are raised, further enhancing their ability for catalysis of electrochemical reactions. This work also demonstrates the combinational effects of the thermoelectric and photothermal properties of these materials on their photocatalytic and electrochemical performance thereby providing a novel means of controlling the multi-response functionality of these materials for a variety of applications in different environments.
This work demonstrates that the layer structured Magnèli titanium oxides possess co-existing functionalities, which can be applied to both energy harvesting and water treatment using one material.
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