Experimental determination of charge carrier dynamics in carbon nitride heterojunctions

Microscopic time-resolved spectroscopy of organic crystals at the nanometer and micrometer scale

How can the photoexcitation dynamics processes of solid substances be measured at the microscopic level? This is a popular topic in many fields of basic and applied sciences. Organic crystals are one of many materials, and understanding their intrinsic properties at the microscopic level is the focus of this discussion. Microscopic time-resolved spectroscopy can study the morphology-dependent photoexcitation dynamics and energy spatial transport processes in organic crystals. In this tutorial review, we introduce the principles and methods of microscopic time-resolved photoluminescence and microscopic transient absorption techniques, which are currently or potentially used to study organic crystals at the nanometer and micrometer scale. Meanwhile, the research status of organic crystals investigated via microscopic time-resolved spectroscopy is discussed, with several examples provided. We then compare the advantages and disadvantages of different methods in detail, offering insights into potential improvements for instrument performance.

Effectively Enhanced Photocatalytic Performance of BP/BiOBr 2D/2D Z-Scheme Heterojunction

Black phosphorus (BP) is a novel two-dimensional (2D) material with remarkable potential for use in environmental remediation and energy conversion. However, the practical application of BP is significantly limited by its low catalytic efficiency and poor structural stability. In this study, a Z-scheme BP/BiOBr 2D/2D heterojunction was fabricated using a simple solution reaction method at room temperature. The BP/BiOBr heterojunction exhibited significantly enhanced photocatalytic performance in the degradation of various organic pollutants and the production of hydrogen under visible light irradiation. This improved activity can be attributed to the efficient separation of photogenerated charges and the extended lifetime of charge carriers within the heterojunction. The durability and structural stability of the BiP-10 heterojunction were demonstrated through cycling tests, which maintained high photocatalytic efficiency over multiple uses. This study presents a promising approach to the development of BP-based photocatalytic materials for sustainable environmental and energy applications.

Surface properties of carbon nitride materials used in photocatalytic systems for energy and environmental applications

The use of photocatalytic systems involving semiconductor materials for environmental and energy applications, such as water remediation and clean energy production, is highly significant. In line with this, a family of carbon-based polymeric materials known as carbon nitride (CNx) has emerged as a promising candidate for this purpose. Despite CNx's remarkable characteristics of performance, stability, and visible light responsiveness, its chemical inertness and poor surface properties hinder interfacial interactions, which are key to effective catalysis. This highlight reviews the literature focusing on the surface chemistry of CNx, especially its structural formation pathway, reactivity, and solvent interactions. It also explores recent advancements in the use of modified CNx for hydrogen production and arsenic remediation, offering recommendations for future material design improvements.

Charge Trapping in Semiconductor Photocatalysts: A Time- and Space-Domain Perspective

Harnessing solar energy to produce value-added fuels and chemicals through photocatalysis techniques holds promise for establishing a sustainable and environmentally friendly energy economy. The intricate dynamics of photogenerated charge carriers lies at the core of the photocatalysis. The balance between charge trapping and band-edge recombination has a crucial influence on the activity of semiconductor photocatalysts. Consequently, the regulation of traps in photocatalysts becomes the key to optimizing their activities. Nevertheless, our comprehension of charge trapping, compared to that of well-studied charge recombination, remains somewhat limited. This limitation stems from the inherently heterogeneous nature of traps at both temporal and spatial scales, which renders the characterization of charge trapping a formidable challenge. Fortunately, recent advancements in both time-resolved spectroscopy and space-resolved microscopy have paved the way for considerable progress in the investigation and manipulation of charge trapping. In this Perspective, we focus on charge trapping in photocatalysts with the aim of establishing a direct link to their photocatalytic activities. To achieve this, we begin by elucidating the principles of advanced time-resolved spectroscopic techniques such as femtosecond time-resolved transient absorption spectroscopy and space-resolved microscopic methods, such as single-molecule fluorescence microscopy and surface photovoltage microscopy. Additionally, we provide an overview of noteworthy research endeavors dedicated to probing charge trapping using time- and space-resolved techniques. Our attention is then directed toward recent achievements in the manipulation of charge trapping in photocatalysts through defect engineering. Finally, we summarize this Perspective and discuss the future challenges and opportunities that lie ahead in the field.

Ultratrace Aromatic Anhydride Dopant as Intermediate Island to Promote Charge Transfer of Graphitic Carbon Nitride for Enhancing the Photocatalytic Degradation of Rhodamine B

In this work, 0.75 wt ‰ 2,3-pyridinedicarboxylic anhydride (PDA) as a novel dopant was utilized to obtain modified graphitic carbon nitride with ultratrace doping (3MCN-PDA3) by facile thermal polymerization. Characterization of the microstructure, surface state, and porosity properties of the samples indicated that 3MCN-PDA3 has a thinner sheet-like, larger-scale, and tighter lamellar stacking structure than that of pristine graphitic carbon nitride (3MCN). Based on photo/electrochemical analysis, the PDA dopant formed an extended coplanar conjugated system by anhydride-amine thermal condensation with heptazine rings, and the channels of amide covalent bonds and superconjugation of the solitary pair of electrons of the nitrogen atoms of PDA synergistically promoted the charge transport performance of 3MCN-PDA3. Under visible light, the photodegradation efficiency of Rhodamine B (RhB) over 3MCN-PDA3 reached 92.4% in 60 min and realized almost entire removal in 200 min (99.2%), 1.43 times that of 3MCN. Furthermore, the experimental results and generalized density theory calculations confirmed that PDA acts as an intermediate molecular island and constructs an efficient carrier transfer pathway between different heptazine units. The results indicate that PDA is a promising candidate to enhance the charge transfer performance through ultratrace doping in the large-scale preparation and application of the graphitic carbon nitride photocatalyst.

Promoting Photocatalytic H2 Evolution through Retarded Charge Trapping and Recombination by Continuously Distributed Defects in Methylammonium Lead Iodide Perovskite

Inspired by its great success in the photovoltaic field, methylammonium lead iodide perovskite (MAPbI3 ) has recently been actively explored as photocatalysts in H2 evolution reactions. However, the practical application of MAPbI3 photocatalysts remains hampered by the intrinsically fast trapping and recombination of photogenerated charges. Herein, we propose a novel strategy of regulating the distribution of defective areas to promote charge-transfer dynamics of MAPbI3 photocatalysts. By deliberately designing and synthesizing the MAPbI3 photocatalysts featuring a unique continuation of defective areas, we demonstrate that such a feature enables retardation of charge trapping and recombination via lengthening the charge-transfer distance. As an outcome, such MAPbI3 photocatalysts turn out to achieve an impressive photocatalytic H2 evolution rate as high as 0.64 mmol ⋅ g-1  ⋅ h-1 , one order of magnitude higher than that of the conventional MAPbI3 photocatalysts. This work establishes a new paradigm for controlling charge-transfer dynamics in photocatalysis.

Heterostructure charge transfer dynamics on self-assembled ZnO on electronically different single-walled carbon nanotubes

The charge transfer kinetics of the catalyst particles play a key role in advanced oxidation processes (AOP) for the complete destruction of recalcitrant and persistent contaminants in water. Here, a significant improvement in the photocatalytic performance is observed in the Single-Walled Carbon Nanotube (SWCNT)-ZnO heterostructure photocatalyst. The charge transfer dynamics and factors affecting AOP are studied using ZnO nanoparticles self-assembled onto three electronically different SWCNTs (metallic, semiconducting, and pristine) via the precipitation method, introducing a heterojunction interface. The creation of the SWCNT/ZnO heterostructure interface improves charge transfer and separation, resulting in a charge carrier lifetime of 7.37 ns. Also, surface area, pore size, and pore volumes are increased by 4.2 times compared to those of ZnO. The nanoparticles-coated face-mask fabric used as the floating photocatalyst exhibited high stability and recyclability with 99% RhB degradation efficiency under natural sunlight and 94% under UV light after the 5th cycle. The surface and crystal defects-oxygen or zinc defects/interstitials open new reaction active sites that assist in charge carrier transfer and act as pollutant absorption and interaction sites for enhanced performance. The ideal band edge positions of the valence band and conduction band favor the generation of H2O/OH•, OH·/OH, and O2/HO2• reactive oxygen species. OH• radicals are found to play a vital role in this AOP by using ethanol as an OH• scavenger.

Donor-Acceptor Covalent-Organic Frameworks Based on Phthalimide as an Electron-Deficient Unit for Efficient Visible-Light Catalytic Hydrogen Evolution

Donor-acceptor two-dimensional covalent-organic frameworks (COFs) have great potential as photocatalysts for hydrogen evolution because of their tunable structures, ordered and strong stacking, high crystallinity, and porosity. Herein, an acceptor unit, namely phthalimide, has been employed for the first time to construct COFs. Two donor-acceptor COFs (TAPFy-PhI and TAPB-PhI) have been successfully synthesized via a Schiff base reaction using phthalimide as the acceptor and 1,3,6,8-tetrakis(4-aminophenyl)pyrene (TAPFy) and 1,3,5-tris(4-aminophenyl)benzene (TAPB) as donors. The synthesized COFs exhibited high crystallinity, permanent porosity, excellent chemical stability, suitable band gaps, and broad visible-light absorption. In the presence of ascorbic acid (sacrificial reagent), the TAPFy-PhI COF exhibited an efficient photocatalytic performance with a hydrogen evolution rate of 1763 μmol g^-1 h^-1. Moreover, the photocatalytic performance was further improved by the addition of Pt (1 wt %) as a cocatalyst, and the hydrogen evolution rate reached 2718 μmol g^-1 h^-1.

Charge carrier dynamics and reaction intermediates in heterogeneous photocatalysis by time-resolved spectroscopies

Sunlight as the most abundant renewable energy holds the promise to make our society sustainable. However, due to its low power density and intermittence, efficient conversion and storage of solar energy as a clean fuel are crucial. Apart from solar fuel synthesis, sunlight can also be used to drive other reactions including organic conversion and air/water purification. Given such potential of photocatalysis, the past few decades have seen a surge in the discovery of photocatalysts. However, the current photocatalytic efficiency is still very moderate. To address this challenge, it is important to understand fundamental factors that dominate the efficiency of a photocatalytic process to enable the rational design and development of photocatalytic systems. Many recent studies highlighted transient absorption spectroscopy (TAS) and time-resolved infrared (TRIR) spectroscopy as powerful approaches to characterise charge carrier dynamics and reaction pathways to elucidate the reasons behind low photocatalytic efficiencies, and to rationalise photocatalytic activities exhibited by closely related materials. Accordingly, as a fast-moving area, the past decade has witnessed an explosion in reports on charge carrier dynamics and reaction mechanisms on a wide range of photocatalytic materials. This critical review will discuss the application of TAS and TRIR in a wide range of heterogeneous photocatalytic systems, demonstrating the variety of ways in which these techniques can be used to understand the correlation between materials design, charge carrier behaviour, and photocatalytic activity. Finally, it provides a comprehensive outlook for potential developments in the area of time-resolved spectroscopies with an aim to provide design strategies for photocatalysts.

Polymer Photoelectrodes for Solar Fuel Production: Progress and Challenges

Converting solar energy to fuels has attracted substantial interest over the past decades because it has the potential to sustainably meet the increasing global energy demand. However, achieving this potential requires significant technological advances. Polymer photoelectrodes are composed of earth-abundant elements, e.g. carbon, nitrogen, oxygen, hydrogen, which promise to be more economically sustainable than their inorganic counterparts. Furthermore, the electronic structure of polymer photoelectrodes can be more easily tuned to fit the solar spectrum than inorganic counterparts, promising a feasible practical application. As a fast-moving area, in particular, over the past ten years, we have witnessed an explosion of reports on polymer materials, including photoelectrodes, cocatalysts, device architectures, and fundamental understanding experimentally and theoretically, all of which have been detailed in this review. Furthermore, the prospects of this field are discussed to highlight the future development of polymer photoelectrodes.

Incorporation of Sequence Aza-Substitution and Thiophene Bridge in Linear Conjugated Polymers Toward Highly Efficient Photo-Catalytic Hydrogen Evolution

The hydrogen evolution performance of organic photo-catalysts is lagged by numerous factors, such as the narrow photon absorption window, low charge transport, and so on. In this paper, four linear conjugated polymers were designed and synthesized based on dibenzothiophene-S,S-dioxide as acceptor, and aza-substituted thiophene-phenyl-thiophene with different substituted numbers as co-units. The polymers with thiophene bridge and aza-substitution exhibited broad visible-absorption because of the extended conjugated length and improved planar structures resulting from the intramolecular non-covalent interactions (S···N or CH···N). The mono-substitution polymer without addition of any co-catalysts showed the highest photo-catalytic performances with the hydrogen evolution rates of 8950 and 7388 μmol g^-1  h^-1 under the UV-vis (>295 nm) and visible (>420 nm) irradiation, respectively. The corresponding apparent quantum yields were as high as 8.34, 5.37, and 1.96% for the 420, 500, and 550 nm monochromatic light irradiation, respectively, which were much higher than those of the classic polymer (P7) without thiophene bridge and aza-substitution. This work indicated that the incorporation of thiophene bridge enhanced visible absorption and aza-substitution optimized co-planarity and activate reactive sites, which should be an effective strategy to improve the photo-catalytic performance of linear conjugated polymer. This article is protected by copyright. All rights reserved.

Covalent triazine frameworks constructed via benzyl halide monomers showing high photocatalytic activity in biomass reforming

Here we report the synthesis of covalent triazine frameworks (CTFs) using benzyl halide monomers which are more cost-effective and with higher availability than previous ones. The resulting CTFs were successfully applied for efficient photocatalytic reforming of glucose for the first time, with a high hydrogen evolution rate up to 330 μmol g^-1 h^-1 under pH = 12. This work presented a new way to synthesize CTFs and further exhibited their potential applications in photocatalytic biomass reforming.