High quantum efficiency of hydrogen production from methanol aqueous solution with PtCu-TiO2 photocatalysts.

Methanol with 12.5 wt% H2 content is widely considered a liquid hydrogen medium. Taking into account water with 11.1 wt% H2 content, H2 synthesis from the mixture of water and methanol is a promising method for on-demand hydrogen production. We demonstrate an atomic-level catalyst design strategy using the synergy between single atoms and nanodots for H2 production. The PtCu-TiO2 sandwich photocatalyst achieves a remarkable H2 formation rate (2,383.9 µmol h-1) with a high apparent quantum efficiency (99.2%). Furthermore, the oxidation product is a high-value chemical formaldehyde with 98.6% selectivity instead of CO2, leading to a nearly zero-carbon-emission process. Detailed investigations indicate a dual role of the copper atoms: an electron acceptor to facilitate photoelectron transfer to Pt, and a hole acceptor for the selective oxidation of methanol to formaldehyde, thus avoiding over-oxidation to CO2. The synergy between Pt nanodots and Cu single atoms together reduces the activation energy of this process to 13.2 kJ mol-1.

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.

Converting Glycerol into Valuable Trioses by Cuδ+ -Single-Atom-Decorated WO3 under Visible Light.

Photocatalytic selective oxidation under visible light presents a promising approach for the sustainable transformation of biomass-derived wastes. However, achieving both high conversion and excellent selectivity poses a significant challenge. In this study, two valuable trioses, glyceraldehyde and dihydroxyacetone, are produced from glycerol over Cuδ+ -decorated WO3 photocatalyst in the presence of H2 O2 . The photocatalyst exhibits a remarkable five-fold increase in the conversion rate (3.81 mmol ⋅ g-1 ⋅ h-1 ) while maintaining a high selectivity towards two trioses (46.4 % to glyceraldehyde and 32.9 % to dihydroxyacetone). Through a comprehensive analysis involving X-ray photoelectron spectroscopy measurements with and without light irradiation, electron spin resonance spectroscopy, and isotopic analysis, the critical role of Cu+ species has been explored as efficient hole acceptors. These species facilitate charge transfer, promoting glycerol oxidation by photoholes, followed by coupling with OH- , which are subsequently dehydrated to yield the desired glyceraldehyde and dihydroxyacetone.

Single-Atom Iridium on Hematite Photoanodes for Solar Water Splitting: Catalyst or Spectator?

Single-atom catalysts (SACs) on hematite photoanodes are efficient cocatalysts to boost photoelectrochemical performance. They feature high atom utilization, remarkable activity, and distinct active sites. However, the specific role of SACs on hematite photoanodes is not fully understood yet: Do SACs behave as a catalytic site or as a spectator? By combining spectroscopic experiments and computer simulations, we demonstrate that single-atom iridium (sIr) catalysts on hematite (α-Fe2O3/sIr) photoanodes act as a true catalyst by trapping holes from hematite and providing active sites for the water oxidation reaction. In situ transient absorption spectroscopy showed a reduced number of holes and shortened hole lifetime in the presence of sIr. This was particularly evident on the second timescale, indicative of fast hole transfer and depletion toward water oxidation. Intensity-modulated photocurrent spectroscopy evidenced a faster hole transfer at the α-Fe2O3/sIr/electrolyte interface compared to that at bare α-Fe2O3. Density functional theory calculations revealed the mechanism for water oxidation using sIr as a catalytic center to be the preferred pathway as it displayed a lower onset potential than the Fe sites. X-ray photoelectron spectroscopy demonstrated that sIr introduced a mid-gap of 4d state, key to the fast hole transfer and hole depletion. These combined results provide new insights into the processes controlling solar water oxidation and the role of SACs in enhancing the catalytic performance of semiconductors in photo-assisted reactions.

Sodium-Directed Photon-Induced Assembly Strategy for Preparing Multisite Catalysts with High Atomic Utilization Efficiency.

Integrating different reaction sites offers new prospects to address the difficulties in single-atom catalysis, but the precise regulation of active sites at the atomic level remains challenging. Here, we demonstrate a sodium-directed photon-induced assembly (SPA) strategy for boosting the atomic utilization efficiency of single-atom catalysts (SACs) by constructing multifarious Au sites on TiO2 substrate. Na+ was employed as the crucial cement to direct Au single atoms onto TiO2, while the light-induced electron transfer from excited TiO2 to Au(Na+) ensembles contributed to the self-assembly formation of Au nanoclusters. The synergism between plasmonic near-field and Schottky junction enabled the cascade electron transfer for charge separation, which was further enhanced by oxygen vacancies in TiO2. Our dual-site photocatalysts exhibited a nearly 2 orders of magnitude improvement in the hydrogen evolution activity under simulated solar light, with a striking turnover frequency (TOF) value of 1533 h-1 that exceeded other Au/TiO2-based photocatalysts reported. Our SPA strategy can be easily extended to prepare a wide range of metal-coupled nanostructures with enhanced performance for diverse catalytic reactions. Thus, this study provides a well-defined platform to extend the boundaries of SACs for multisite catalysis through harnessing metal-support interactions.

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.

Effective Activation of Strong C-Cl Bonds for Highly Selective Photosynthesis of Bibenzyl via Homo-Coupling.

Carbon-carbon (C-C) coupling of organic halides has been successfully achieved in homogeneous catalysis, while the limitation, e.g., the dependence on rare noble metals, complexity of the metal-ligand catalylst and the poor catalyst stability and recyclability, needs to be tackled for a green process. The past few years have witnessed heterogeneous photocatalysis as a green and novel method for organic synthesis processes. However, the study on C-C coupling of chloride substrates is rare due to the extremely high bond energy of C-Cl bond (327 kJ mol-1 ). Here, we report a robust heterogeneous photocatalyst (Cu/ZnO) to drive the homo-coupling of benzyl chloride with high efficiency, which achieves an unprecedented high selectivity of bibenzyl (93 %) and yield rate of 92 % at room temperature. Moreover, this photocatalytic process has been validated for C-C coupling of 10 benzylic chlorides all with high yields. In addition, the excellent stability has been observed for 8 cycles of reactions. With detailed characterization and DFT calculation, the high selectivity is attributed to the enhanced adsorption of reactants, stabilization of intermediates (benzyl radicals) for the selective coupling by the Cu loading and the moderate oxidation ability of the ZnO support, besides the promoted charge separation and transfer by Cu species.

Binary Au-Cu Reaction Sites Decorated ZnO for Selective Methane Oxidation to C1 Oxygenates with Nearly 100% Selectivity at Room Temperature.

Direct and efficient oxidation of methane to methanol and the related liquid oxygenates provides a promising pathway for sustainable chemical industry, while still remaining an ongoing challenge owing to the dilemma between methane activation and overoxidation. Here, ZnO with highly dispersed dual Au and Cu species as cocatalysts enables efficient and selective photocatalytic conversion of methane to methanol and one-carbon (C1) oxygenates using O2 as the oxidant operated at ambient temperature. The optimized AuCu-ZnO photocatalyst achieves up to 11225 µmol·g-1·h-1 of primary products (CH3OH and CH3OOH) and HCHO with a nearly 100% selectivity, resulting in a 14.1% apparent quantum yield at 365 nm, much higher than the previous best photocatalysts reported for methane conversion to oxygenates. In situ EPR and XPS disclose that Cu species serve as photoinduced electron mediators to promote O2 activation to •OOH, and simultaneously that Au is an efficient hole acceptor to enhance H2O oxidation to •OH, thus synergistically promoting charge separation and methane transformation. This work highlights the significances of co-modification with suitable dual cocatalysts on simultaneous regulation of activity and selectivity.

Hollow Carbon Sphere-Modified Graphitic Carbon Nitride for Efficient Photocatalytic H2 Production.

A novel hybrid photocatalyst composed of hollow carbon nanospheres (NCS) and graphitic carbon nitride (CN) curly nanosheets has been prepared by the calcination of a NCS precursor and freeze-dried urea. The optimized photocatalyst exhibits an efficient photocatalytic performance under visible light irradiation with a highest H2 generation rate of 3612.3 µmol g-1 h-1 , leading to an apparent quantum yield of 10.04 % at 420 nm, five times higher than the widely reported benchmark photocatalyst CN (2.01 % AQY). The materials characterization shows that NCS-modified CN curly nanosheets can promote photoelectron transfer and suppress charge recombination through their special coupling interface and NCS as an electron acceptor, which significantly improves the photocatalytic efficiency. Thus, this study provides an efficient strategy for the design of highly efficient photocatalyst, particularly suitable for a totally metal-free photocatalytic system.

Covalent organic framework photocatalysts: structures and applications.

In the light of increasing energy demand and environmental pollution, it is urgently required to find a clean and renewable energy source. In these years, photocatalysis that uses solar energy for either fuel production, such as hydrogen evolution and hydrocarbon production, or environmental pollutant degradation, has shown great potential to achieve this goal. Among the various photocatalysts, covalent organic frameworks (COFs) are very attractive due to their excellent structural regularity, robust framework, inherent porosity and good activity. Thus, many studies have been carried out to investigate the photocatalytic performance of COFs and COF-based photocatalysts. In this critical review, the recent progress and advances of COF photocatalysts are thoroughly presented. Furthermore, diverse linkers between COF building blocks such as boron-containing connections and nitrogen-containing connections are summarised and compared. The morphologies of COFs and several commonly used strategies pertaining to photocatalytic activity are also discussed. Following this, the applications of COF-based photocatalysts are detailed including photocatalytic hydrogen evolution, CO2 conversion and degradation of environmental contaminants. Finally, a summary and perspective on the opportunities and challenges for the future development of COF and COF-based photocatalysts are given.

Efficient Hole Trapping in Carbon Dot/Oxygen-Modified Carbon Nitride Heterojunction Photocatalysts for Enhanced Methanol Production from CO2 under Neutral Conditions.

Artificial photosynthesis of alcohols from CO2 is still unsatisfactory owing to the rapid charge relaxation compared to the sluggish photoreactions and the oxidation of alcohol products. Here, we demonstrate that CO2 is reduced to methanol with 100 % selectivity using water as the only electron donor on a carbon nitride-like polymer (FAT) decorated with carbon dots. The quantum efficiency of 5.9 % (λ=420 nm) is 300 % higher than the previously reported carbon nitride junction. Using transient absorption spectroscopy, we observed that holes in FAT could be extracted by the carbon dots with nearly 75 % efficiency before they become unreactive by trapping. Extraction of holes resulted in a greater density of photoelectrons, indicative of reduced recombination of shorter-lived reactive electrons. This work offers a strategy to promote photocatalysis by increasing the amount of reactive photogenerated charges via structure engineering and extraction before energy losses by deep trapping.

Metal-Organic Framework Decorated Cuprous Oxide Nanowires for Long-lived Charges Applied in Selective Photocatalytic CO2 Reduction to CH4.

Improving the stability of cuprous oxide (Cu2 O) is imperative to its practical applications in artificial photosynthesis. In this work, Cu2 O nanowires are encapsulated by metal-organic frameworks (MOFs) of Cu3 (BTC)2 (BTC=1,3,5-benzene tricarboxylate) using a surfactant-free method. Such MOFs not only suppress the water vapor-induced corrosion of Cu2 O but also facilitate charge separation and CO2 uptake, thus resulting in a nanocomposite representing 1.9 times improved activity and stability for selective photocatalytic CO2 reduction into CH4 under mild reaction conditions. Furthermore, direct transfer of photogenerated electrons from the conduction band of Cu2 O to the LUMO level of non-excited Cu3 (BTC)2 has been evidenced by time-resolved photoluminescence. This work proposes an effective strategy for CO2 conversion by a synergy of charge separation and CO2 adsorption, leading to the enhanced photocatalytic reaction when MOFs are integrated with metal oxide photocatalyst.

Revealing Ammonia Quantification Minefield in Photo/Electrocatalysis.

Photo/electrocatalytic ammonia synthesis has recently developed fast while the ammonia yields over state-of-the-art photo/electrocatalysts are still very moderate. Such low concentration of synthesized NH3 brings about a challenge to the reliable quantification of the product in photo/electrocatalysis. Notably, we found that the quantitative detection of ammonia concentration below 0.2 ppm is error-prone, which is likely the case happening in the majority of photo/electrocatalytic NH3 synthesis, thus arising concerns about the rationality and accuracy for low-concentration ammonia quantification in these processes. Herein, we discuss the methodology used and analyze the reliability of various detection methods for the detection of trace ammonia in aqueous media. The challenges facing the detection of low concentration of ammonia in photo/electrocatalysis can be overcome by integration with multiple detection methods. According to the data presented, we also propose an effective criterion for precise quantification of ammonia, avoiding the unreasonable comparisons in photo/electrocatalytic ammonia synthesis.

Energy Platform for Directed Charge Transfer in the Cascade Z-Scheme Heterojunction: CO2 Photoreduction without a Cocatalyst.

A universal strategy is developed to construct a cascade Z-Scheme system, in which an effective energy platform is the core to direct charge transfer and separation, blocking the unexpected type-II charge transfer pathway. The dimension-matched (001)TiO2 -g-C3 N4 /BiVO4 nanosheet heterojunction (T-CN/BVNS) is the first such model. The optimized cascade Z-Scheme exhibits ≈19-fold photoactivity improvement for CO2 reduction to CO in the absence of cocatalysts and costly sacrificial agents under visible-light irradiation, compared with BVNS, which is also superior to other reported Z-Scheme systems even with noble metals as mediators. The experimental results and DFT calculations based on van der Waals structural models on the ultrafast timescale reveal that the introduced T as the platform prolongs the lifetimes of spatially separated electrons and holes and does not compromise their reduction and oxidation potentials.

Site Sensitivity of Interfacial Charge Transfer and Photocatalytic Efficiency in Photocatalysis: Methanol Oxidation on Anatase TiO2 Nanocrystals.

Photocatalytic oxidation of methanol on various anatase TiO2 nanocrystals was studied by in situ and time-resolved characterizations and DFT calculations. Surface site and resulting surface adsorbates affect the surface band bending/bulk-to-surface charge migration processes and interfacial electronic structure/interfacial charge transfer processes. TiO2 nanocrystals predominantly enclosed by the {001} facets expose a high density of reactive fourfold-coordinated Ti sites (Ti4c ) at which CH3 OH molecules dissociate to form the CH3 O adsorbate (CH3 O(a)Ti4c ). CH3 O(a)Ti4c localized density of states are almost at the valence band maximum of TiO2 surface, facilitating the interfacial hole transfer process; CH3 O(a)Ti4c with a high coverage promotes upward surface band bending, facilitating bulk-to-surface hole migration. CH3 O(a)Ti4c exhibits the highest photocatalytic oxidation rate constant. TiO2 nanocrystals enclosed by the {001} facets are most active in photocatalytic methanol oxidation.

Insight on Shallow Trap States-Introduced Photocathodic Performance in n-Type Polymer Photocatalysts.

Graphitic carbon nitride (g-C3N4) is a robust organic semiconductor photocatalyst with proven H2 evolution ability. However, its application in a photoelectrochemical system as a photocathode for H2 production is extremely challenging with the majority of reports representing it as a photoanode. Despite research into constructing g-C3N4 photocathodes in recent years, factors affecting an n-type semiconductor's properties as a photocathode are still not well-understood. The current work demonstrates an effective strategy to transform an n-type g-C3N4 photoanode material into an efficient photocathode through introducing electron trap states associated with both N-defects and C-OH terminal groups. As compared to the g-C3N4 photoelectrode, this strategy develops 2 orders of magnitude higher conductivity and 3 orders of magnitude longer-lived shallow-trapped charges. Furthermore, the average OCVD lifetime observed for def-g-C3N4 is 5 times longer than that observed for g-C3N4. Thus, clear photocathode behavior has been observed with negative photocurrent densities of around -10 µA/cm2 at 0 V vs RHE. Open circuit photovoltage decay (OCVD), Mott-Schottky (MS) plot, and transient absorption spectroscopy (TAS) provide consistent evidence that long-lived shallow-trapped electrons that exist at about the microsecond time scale after photoexcitation are key to the photocathode behavior observed for defect-rich g-C3N4, thus further demonstrating g-C3N4 can be both a photoanode and a photocathode candidate.

Mesoporous Polymeric Cyanamide-Triazole-Heptazine Photocatalysts for Highly-Efficient Water Splitting.

Conjugated polymers are promising light harvesters for water reduction/oxidation due to their simple synthesis and adjustable bandgap. Herein, both cyanamide and triazole functional groups are first incorporated into a heptazine-based carbon nitride (CN) polymer, resulting in a mesoporous conjugated cyanamide-triazole-heptazine polymer (CTHP) with different compositions by increasing the quantity of cyanamide/triazole units in the CN backbone. Varying the compositions of CTHP modulates its electronic structures, mesoporous morphologies, and redox energies, resulting in a significantly improved photocatalytic performance for both H2 and O2 evolution under visible light irradiation. A remarkable H2 evolution rate of 12723 µmol h-1 g-1 is observed, resulting in a high apparent quantum yield of 11.97% at 400 nm. In parallel, the optimized photocatalyst also exhibits an O2 evolution rate of 221 µmol h-1  g-1 , 9.6 times higher than the CN counterpart, with the value being the highest among the reported CN-based bifunctional photocatalysts. This work provides an efficient molecular engineering approach for the rational design of functional polymeric photocatalysts.

Mimicking Natural Photosynthesis: Solar to Renewable H2 Fuel Synthesis by Z-Scheme Water Splitting Systems.

Visible light-driven water splitting using cheap and robust photocatalysts is one of the most exciting ways to produce clean and renewable energy for future generations. Cutting edge research within the field focuses on so-called "Z-scheme" systems, which are inspired by the photosystem II-photosystem I (PSII/PSI) coupling from natural photosynthesis. A Z-scheme system comprises two photocatalysts and generates two sets of charge carriers, splitting water into its constituent parts, hydrogen and oxygen, at separate locations. This is not only more efficient than using a single photocatalyst, but practically it could also be safer. Researchers within the field are constantly aiming to bring systems toward industrial level efficiencies by maximizing light absorption of the materials, engineering more stable redox couples, and also searching for new hydrogen and oxygen evolution cocatalysts. This review provides an in-depth survey of relevant Z-schemes from past to present, with particular focus on mechanistic breakthroughs, and highlights current state of the art systems which are at the forefront of the field.

Characterization of charge carrier behavior in photocatalysis using transient absorption spectroscopy.

Photocatalysis is a promising sustainable method to generate solar fuels for the future, as well as having other applications such as water/air purification. However, the performance of photocatalysts is often limited by poor charge carrier dynamics. To improve charge carrier dynamics, it is necessary to characterize and understand charge carrier behavior in photocatalytic systems. This critical review will present Transient Absorption Spectroscopy (TAS) as a useful technique for understanding the behavior of photoexcited charges in semiconductor photocatalysts. The role of TAS amongst other techniques for characterizing charge carrier behavior will be outlined. Basic principles behind TAS will be introduced, and interpretation of TAS spectra and kinetics will be discussed in the context of exemplar literature. It will be demonstrated that TAS is a powerful technique to obtain fundamental understanding of the behavior of photoexcited charges.

Platinum- and CuOx -Decorated TiO2 Photocatalyst for Oxidative Coupling of Methane to C2 Hydrocarbons in a Flow Reactor.

Oxidative coupling of methane (OCM) is considered one of the most promising catalytic technologies to upgrade methane. However, C2 products (C2 H6 /C2 H4 ) from conventional methane conversion have not been produced commercially owing to competition from overoxidation and carbon accumulation at high temperatures. Herein, we report the codeposition of Pt nanoparticles and CuOx clusters on TiO2 (PC-50) and use of the resulting photocatalyst for OCM in a flow reactor operated at room temperature under atmospheric pressure for the first time. The optimized Cu0.1 Pt0.5 /PC-50 sample showed a highest yield of C2 product of 6.8 µmol h-1 at a space velocity of 2400 h-1 , more than twice the sum of the activity of Pt/PC-50 (1.07 µmol h-1 ) and Cu/PC-50 (1.9 µmol h-1 ), it might also be the highest among photocatalytic methane conversions reported so far under atmospheric pressure. A high C2 selectivity of 60 % is also comparable to that attainable by conventional high-temperature (>943 K) thermal catalysis. It is proposed that Pt functions as an electron acceptor to facilitate charge separation, while holes could transfer to CuOx to avoid deep dehydrogenation and the overoxidation of C2 products.