Double-atom dealloying-derived Frank partial dislocations in cobalt nanocatalysts boost metal-air batteries and fuel cells.

Oxygen reduction reaction (ORR), an essential reaction in metal-air batteries and fuel cells, still faces many challenges, such as exploiting cost-effective nonprecious metal electrocatalysts and identifying their surface catalytic sites. Here we introduce bulk defects, Frank partial dislocations (FPDs), into metallic cobalt to construct a highly active and stable catalyst and demonstrate an atomic-level insight into its surface terminal catalysis. Through thermally dealloying bimetallic carbide (Co3ZnC), FPDs were in situ generated in the final dealloyed metallic cobalt. Both theoretical calculations and atomic characterizations uncovered that FPD-driven surface terminations create a distinctive type of surface catalytic site that combines concave geometry and compressive strain, and this two-in-one site intensively weakens oxygen binding. When being evaluated for the ORR, the catalyst exhibits onset and half-wave potentials of 1.02 and 0.90 V (versus the reversible hydrogen electrode), respectively, and negligible activity decay after 30,000 cycles. Furthermore, zinc-air batteries and H2-O2/air fuel cells built with this catalyst also achieve remarkable performance, making it a promising alternative to state-of-the-art Pt-based catalysts. Our findings pave the way for the use of bulk defects to upgrade the catalytic properties of nonprecious electrocatalysts.

Guiding the Driving Factors on Plasma Super-Photothermal S-Scheme Core-Shell Nanoreactor to Enhance Photothermal Catalytic H2 Evolution and Selective CO2 Reduction.

Light-induced heat has a non-negligible role in photocatalytic reactions. However, it is still challenging to design highly efficient catalysts that can make use of light and thermal energy synergistically. Herein, the study proposes a plasma super-photothermal S-scheme heterojunction core-shell nanoreactor based on manipulation of the driving factors, which consists of α-Fe2 O3 encapsulated by g-C3 N4 modified with gold quantum dots. α-Fe2 O3 can promote carrier spatial separation while also acting as a thermal core to radiate heat to the shell, while Au quantum dots transfer energetic electrons and heat to g-C3 N4 via surface plasmon resonance. Consequently, the catalytic activity of Au/α-Fe2 O3 @g-C3 N4 is significantly improved by internal and external double hot spots, and it shows an H2 evolution rate of 5762.35 µmol h-1 g-1 , and the selectivity of CO2 conversion to CH4 is 91.2%. This work provides an effective strategy to design new plasma photothermal catalysts for the solar-to-fuel transition.

Super-Photothermal Effect-Mediated Fast Reaction Kinetic in S-Scheme Organic/Inorganic Heterojunction Hollow Spheres Toward Optimized Photocatalytic Performance.

Using full solar spectrum for energy conversion and environmental remediation is a major challenge, and solar-driven photothermal chemistry is a promising route to achieve this goal. Herein, this work reports a photothermal nano-constrained reactor based on hollow structured g-C3 N4 @ZnIn2 S4 core-shell S-scheme heterojunction, where the synergistic effect of super-photothermal effect and S-scheme heterostructure significantly improve the photocatalytic performance of g-C3 N4 . The formation mechanism of g-C3 N4 @ZnIn2 S4 is predicted in advance by theoretical calculations and advanced techniques, and the super-photothermal effect of g-C3 N4 @ZnIn2 S4 and its contribution to the near-field chemical reaction is confirmed by numerical simulations and infrared thermography. Consequently, the photocatalytic degradation rate of g-C3 N4 @ZnIn2 S4 for tetracycline hydrochloride is 99.3%, and the photocatalytic hydrogen production is up to 4075.65 µmol h-1 g-1 , which are 6.94 and 30.87 times those of pure g-C3 N4 , respectively. The combination of S-scheme heterojunction and thermal synergism provides a promising insight for the design of an efficient photocatalytic reaction platform.

Under-Coordinated CoFe Layered Double Hydroxide Nanocages Derived from Nanoconfined Hydrolysis of Bimetal Organic Compounds for Efficient Electrocatalytic Water Oxidation.

Hierarchically structured bimetal hydroxides are promising for electrocatalytic oxygen evolution reaction (OER), yet synthetically challenging. Here, the nanoconfined hydrolysis of a hitherto unknown CoFe-bimetal-organic compound (b-MOC) is reported for the controllable synthesis of highly OER active nanostructures of CoFe layered double hydroxide (LDH). The nanoporous structures trigger the nanoconfined hydrolysis in the sacrificial b-MOC template, producing CoFe LDH core-shell octahedrons, nanoporous octahedrons, and hollow nanocages with abundant under-coordinated metal sites. The hollow nanocages of CoFe LDH demonstrate a remarkable turnover frequency (TOF) of 0.0505 s-1 for OER catalysis at an overpotential of 300 mV. It is durable in up to 50 h of electrolysis at step current densities of 10-100 mA cm-2 . Ex situ and in situ X-ray absorption spectroscopic analysis combined with theoretical calculations suggests that under-coordinated Co cations can bind with deprotonated Fe-OH motifs to form OER active Fe-O-Co dimmers in the electrochemical oxidation process, thereby contributing to the good catalytic activity. This work presents an efficient strategy for the synthesis of highly under-coordinated bimetal hydroxide nanostructures. The mechanistic understanding underscores the power of maximizing the amount of bimetal-dimer sites for efficient OER catalysis.

Sc-HOPO: A Potential Construct for Use in Radioscandium-Based Radiopharmaceuticals.

Three isotopes of scandium─43Sc, 44Sc, and 47Sc─have attracted increasing attention as potential candidates for use in imaging and therapy, respectively, as well as for possible theranostic use as an elementally matched pair. Here, we present the octadentate chelator 3,4,3-(LI-1,2-HOPO) (or HOPO), an effective chelator for hard cations, as a potential ligand for use in radioscandium constructs with simple radiolabeling under mild conditions. HOPO forms a 1:1 Sc-HOPO complex that was fully characterized, both experimentally and theoretically. [47Sc]Sc-HOPO exhibited good stability in chemical and biological challenges over 7 days. In healthy mice, [43,47Sc]Sc-HOPO cleared the body rapidly with no signs of demetalation. HOPO is a strong candidate for use in radioscandium-based radiopharmaceuticals.

The Underlying Molecular Mechanism of Fence Engineering to Break the Activity-Stability Trade-Off in Catalysts for the Hydrogen Evolution Reaction.

Non-precious-metal (NPM) catalysts often face the formidable challenge of a trade-off between long-term stability and high activity, which has not yet been widely addressed. Herein we propose a distinct molecule-selective fence as a promising concept to solve this activity-stability trade-off. The fence encloses the catalyst and prevents species poisonous to the catalyst from reaching it, but allows catalytic reaction-related species to diffuse freely. We constructed a CoS2 fence layer on the external surface of highly active cobalt-doped MoS2 , achieving a remarkable catalytic stability towards the alkaline hydrogen evolution reaction and improved activity. In situ spectroscopy uncovered the underlying molecular mechanism of the CoS2 fence for breaking the activity-stability trade-off of the MoS2 catalyst. This work offers valuable guidance for rationally designing efficient and stable NPM catalysts.

Uptake and Removal of Uranium by and from Human Teeth.

Uranium-238 (238U), a long-lived radiometal, is widespread in the environment because of both naturally occurring processes and anthropogenic processes. The ingestion or inhalation of large amounts of U is a major threat to humans, and its toxicity is considered mostly chemical rather than radiological. Therefore, a way to remove uranium ingested by humans from uranium-contaminated water or from the air is critically needed. This study investigated the uranium uptake by hydroxyapatite (HAP), a compound found in human bone and teeth. The uptake of U by teeth is a result of U transport as dissolved uranyl (UO22+) in contaminated water, and U adsorption has been linked to delays in both tooth eruption and development. In this present work, the influence of pH, contact time, initial U concentration, and buffer solution on the uptake and removal of U in synthetic HAP was investigated and modeled. The influence of pH (pH of human saliva, 6.7-7.4) on the uptake of uranyl was negligible. Furthermore, the kinetics were extremely fast; in one second of exposure, 98% of uranyl was uptaken by HAP. The uptake followed pseudo-second-order kinetics and a Freundlich isotherm model. A 0.2 M sodium carbonate solution removed all the uranyl from HAP after 1 h. Another series of in vitro tests were performed with real teeth as targets. We found that, for a 50 mg/L U in PBS solution adjusted to physiological pH, ∼35% of the uranyl was uptaken by the tooth after 1 h, following pseudo-first-order kinetics. Among several washing solutions tested, a commercially available carbonate, as well as a commercially available fluoride solution, enabled removal of all the uranyl taken up by the teeth.

In Situ Coupling of MnO and Co@N-Doped Graphite Carbon Derived from Prussian Blue Analogous Achieves High-Performance Reversible Oxygen Electrocatalysis for Zn-Air Batteries.

Coupling dual active components into one integrated catalyst as well as understanding their electronic interaction behavior on reversible oxygen electrocatalysis is central to achieving high energy-conversion efficiency for Zn-air batteries (ZABs). Herein, we demonstrate an effective couple of MnO and Co nanocrystals embedded in N-doped graphite carbon to integrate a highly efficient bifunctional catalyst (denoted as MnO/Co@NGC) toward oxygen reduction and evolution reaction (ORR/OER). MnO/Co@NGC was first successfully prepared by the one-step pyrolysis of Mn3[Co(CN)6]2·9H2O@PVP (poly(vinyl pyrrolidone)), and X-ray absorption near-edge structure analysis revealed that the charges were transferred from MnO to Co@NGC, which makes MnO more electrophilic to facilitate the initial electrochemical adsorption of OH- for improving the OER activity. As expected, the as-designed MnO/Co@NGC displays excellent bifunctional ORR/OER activity with a small overpotential gap of only 0.736 V, providing the ZABs with a high trip efficiency of 57.2% as well as excellent cycling stability. This work not only offers a bifunctional ORR/OER electrocatalyst but also further highlights the interfacial charge distribution in oxygen electrocatalysis, affording a promising approach for developing advanced energy-related materials.

Unraveling the Beneficial Microstructure Evolution in Pyrite for Boosted Lithium Storage Performance.

Pyrite FeS2 as a high-capacity electrode material for lithium-ion batteries (LIBs) is hindered by its unstable cycling performance owing to the large volume change and irreversible phase segregation from coarsening of Fe. Here, the beneficial microstructure evolution in MoS2 -modified FeS2 is unraveled during the cycling process; the microstructure evolution is responsible for its significantly boosted lithium storage performance, making it suitable for use as an anode for LIBs. Specifically, the FeS2 /MoS2 displays a long cycle life with a capacity retention of 116 % after 600 cycles at 0.5 A g-1 , which is the best among the reported FeS2 -based materials so far. A series of electrochemical tests and structural characterizations substantially revealed that the introduced MoS2 in FeS2 experiences an irreversible electrochemical reaction and thus the in situ formed metallic Mo could act as the conductive buffer layer to accelerate the dynamics of Li+ diffusion and electron transport. More importantly, it can guarantee the highly reversible conversion in lithiated FeS2 by preventing Fe coarsening. This work provides a fundamental understanding and an effective strategy towards the microstructure evolution for boosting lithium storage performances for other metal sulfide-based materials.

Toward Understanding the Enhanced Pseudocapacitive Storage in 3D SnS/MXene Architectures Enabled by Engineered Surface Reactions.

The optimization of three-dimensional (3D) MXene-based electrodes with desired electrochemical performances is highly demanded. Here, a precursor-guided strategy is reported for fabricating the 3D SnS/MXene architecture with tiny SnS nanocrystals (≈5 nm in size) covalently decorated on the wrinkled Ti3 C2 Tx nanosheets through Ti-S bonds (denoted as SnS/Ti3 C2 Tx -O). The formation of Ti-S bonds between SnS and Ti3 C2 Tx was confirmed by extended X-ray absorption fine structure (EXAFS). Rather than bulky SnS plates decorated on Ti3 C2 Tx (SnS/Ti3 C2 Tx -H) by one-step hydrothermal sulfidation followed by post annealing, this SnS/Ti3 C2 Tx -O presents size-dependent structural and dynamic properties. The as-formed 3D hierarchical structure can provide short ion-diffusion pathways and electron transport distances because of the more accessible surface sites. In addition, benefiting from the tiny SnS nanocrystals that can effectively improve Na+ diffusion and suppress structural variation upon charge/discharge processes, the as-obtained SnS/Ti3 C2 Tx -O can generate pseudocapacitance-dominated storage behavior enabled by engineered surface reactions. As predicted, this electrode exhibits an enhanced Na storage capacity of 565 mAh g-1 at 0.1 A g-1 after 75 cycles, outperforming SnS/Ti3 C2 Tx -H (336 mAh g-1 ), SnS (212 mAh g-1 ), and Ti3 C2 Tx (104 mAh g-1 ) electrodes.

Identifying the Transfer Kinetics of Adsorbed Hydroxyl as a Descriptor of Alkaline Hydrogen Evolution Reaction.

The key descriptor that dominates the kinetics of the alkaline hydrogen evolution reaction (HER) has not yet been unequivocally identified. Herein, we focus on the adsorbed hydroxyl (OHad ) transfer process (OHad + e- ⇄ OH- ) and reveal its crucial role in promoting the overall kinetics of alkaline HER based on Ni/Co-modified MoSe2 model catalysts (Ni-MoSe2 and Co-MoSe2 ) that feature almost identical water dissociation and hydrogen adsorption energies, but evidently different activity trends in alkaline (Ni-MoSe2 ≫ Co-MoSe2 ) and acidic (Co-MoSe2 ≥ Ni-MoSe2 ) media. Experimental and theoretical calculation results demonstrate that tailoring MoSe2 with Ni not only optimizes the hydroxyl adsorption, but also promotes the desorption of OH- and the electron-involved conversion of OHad to OH- , all of which synergistically accelerate the kinetics of OHad + e- ⇄ OH- and thereby the overall kinetics of the alkaline HER.

Optimizing Dispersion, Exfoliation, Synthesis, and Device Fabrication of Inorganic Nanomaterials Using Hansen Solubility Parameters.

Hansen solubility parameters (HSPs) were established by Hansen in 1967 and predict miscibility between different material systems. So far, HSP theory works across polymers, crystalline bulk solids and nanomaterials and can be used to identify single solvents or, more likely, blends of solvents that deliver not only the initial solubility but also control it during reaction processes. This minireview summarizes the recent progress on HSP theory to optimize dispersion, exfoliation, synthesis, and device fabrication of inorganic nanomaterials. First, we briefly introduce HSP theory and determination of HSPs. Then, we discuss in detail the utilization of HSPs for inorganic nanomaterials, focusing on carbon nanomaterials, two-dimensional non-graphene nanomaterials, and metal oxide nanoparticles. Finally, challenges and perspectives of HSP theory in inorganic nanomaterials are reviewed.

Arousing the Reactive Fe Sites in Pyrite (FeS2) via Integration of Electronic Structure Reconfiguration and in Situ Electrochemical Topotactic Transformation for Highly Efficient Oxygen Evolution Reaction.

Despite significant advances in the development of highly efficient and robust oxygen evolution reaction (OER) electrocatalysts to replace noble-metal catalysts, commercializing OER catalysts with high catalytic activity for sustainable development still remains a great challenge. Especially, transition-metal Fe-based OER catalysts, despite their earth-abundant, cost-efficient, and environmentally benign superiorities over Co- and Ni-based materials, have received relatively insufficient attention because of their poor apparent OER activities. Herein, by rational design, we report Ni-modified pyrite (FeS2) spheres with yolk-shell structure that could serve as pre-electrocatalyst precursors to induce a highly active nickel-iron oxyhydroxide via in situ electrochemical topological transformation under the OER process. Notably, as confirmed by the results of X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations, Ni doping could effectively regulate the intrinsic electronic structure of FeS2 to realize a semiconductor-to-semimetal transition, which endows FeS2 with dramatically improved conductivity and water adsorption ability, providing prequisites for subsequent topological transformation. Moreover, systematic post-characterizations further reveal that the optimal Ni-FeS2-0.5 sample completely converts to amorphous Ni-doped FeOOH via an in situ electrochemical transformation with yolk-shell structure well-preserved under the OER conditions. The electronic structure modulation combined with electrochemical topotactic transformation strategies well stimulate the reactive Fe sites in Ni-FeS2-0.5, which show impressively low overpotentials of 250 and 326 mV to drive the current densities ( j) of 10 and 100 mA cm-2, respectively, and a Tafel slope as small as 34 mV dec-1 for the OER process. When assembled as a water electrolyzer for the overall water splitting, Ni-FeS2-0.5 can display a low voltage of 1.55 V to drive a current density of 10 mA cm-2, outperforming most of the transition-metal-based bifunctional electrocatalysts to date. This work may provide new insight into the rational design of other high-performance Fe-based OER electrocatalysts and inspire the exploration of cost-effective, ecofriendly electrocatalysts to meet the demand for future sustainable development.

Constructing Conductive Bridge Arrays between Ti3C2Tx MXene Nanosheets for High-Performance Lithium-Ion Batteries and Highly Efficient Hydrogen Evolution.

Ti3C2Tx is a member of the MXene family with high potential for electrochemical applications, including lithium-ion batteries (LIBs) and hydrogen evolution reaction (HER). However, severe interlayer restacking not only causes a great loss of the active sites but also decreases the ionic diffusion channels, both of which significantly degrades the electrochemical performances of LIBs and HER. The common interlayer spacers could increase interlayer space but reduce the conductivity. Herein, we introduce in situ carbon nanotube (CNT) arrays between Ti3C2Tx MXene nanosheets (3D CNTs@Ti3C2Tx) as the conductive bridges for achieving a unique architecture with high conductivity, fast ion/mass transfer channels, and high exposure of the activity sites. In this architecture, 1D CNTs can not only be viewed as the interlayer spacer that prevents Ti3C2Tx MXene nanosheets from recombining but also connect with the neighbor Ti3C2Tx MXene nanosheets providing more ion/electron transport channels. Benefiting from this unique structure that could improve ion/electron transfer kinetics and capacitive contribution, 3D CNTs@Ti3C2Tx displays high specific capacity as an anode for LIBs (491 mA h g-1 at 320 mA g-1). Furthermore, 3D CNTs@Ti3C2Tx also exhibits excellent HER performance in alkaline medium (the overpotential is 93 mV at 10 mA cm-2) and excellent water splitting performance. This strategy that in situ construction of CNT arrays between the MXene nanosheets proves an effective method for the rational design of multifunctional energy storage/conversion materials.

Improved synthesis of the bifunctional chelator p-SCN-Bn-HOPO.

The bifunctional ligand p-SCN-Bn-HOPO, which has four 1,2-hydroxypyridinone groups on a spermine backbone with an isothiocyanate linker, has been shown to be an efficient and stable chelator for Zr(iv) and, more importantly, the radioisotope 89Zr for use in radiolabeling antibodies for positron emission tomography (PET) imaging. Previous studies of 89Zr-HOPO-trastuzumab in mice showed low background, good tumor to organ contrast, and very low bone uptake which show p-SCN-Bn-HOPO to be an important next-generation bifunctional chelator for radioimmunoPET imaging with 89Zr. However, the reported synthesis of p-SCN-Bn-HOPO involves nine steps and multiple HPLC purifications with an overall yield of about 1.4%. Herein we report an improved and efficient synthesis of p-SCN-Bn-HOPO in four steps with 14.3% overall yield which will improve its availability for further biological studies and wider application in PET imaging. The new synthetic route also allows variation in linker length and chemistries which may be helpful in modifying in vivo clearance behaviors of future agents.

Transition Metal Carbide Complex Architectures for Energy-Related Applications.

Transition metal carbides (TMCs), as a family of special interstitial alloys, exhibit novel intrinsic characteristics such as high melting point, high electronic conductivity, excellent mechanical and chemical stability, and good corrosion resistance, and hence have attracted ever-growing attention as promising electrode materials for energy-related applications. In this regard, we give a comprehensive overview of the structural design of transition metal carbide complex architectures and their structure advantages for energy-related applications. After a brief classification, we summarize in detail recent progress in controllable design and synthesis of TMCs with complex nanostructures (e.g., zero-, one-, two-, and three-dimensional, and self-supported electrodes) for electrochemical energy storage and conversion applications including metal-ion batteries, supercapacitors, rechargeable metal-air batteries, fuel cells, and water splitting. Finally, we end this review with some potential challenges and research prospects of TMCs as electrode materials for energy-related applications.

Solubility-Parameter-Guided Solvent Selection to Initiate Ostwald Ripening for Interior Space-Tunable Structures with Architecture-Dependent Electrochemical Performance.

Despite significant advancement in preparing various hollow structures by Ostwald ripening, one common problem is the intractable uncontrollability of initiating Ostwald ripening due to the complexity of the reaction processes. Here, a new strategy on Hansen solubility parameter (HSP)-guided solvent selection to initiate Ostwald ripening is proposed. Based on this comprehensive principle for solvent optimization, N,N-dimethylformamide (DMF) was screened out, achieving accurate synthesis of interior space-tunable MoSe2 spherical structures (solid, core-shell, yolk-shell and hollow spheres). The resultant MoSe2 structures exhibit architecture-dependent electrochemical performances towards hydrogen evolution reaction and sodium-ion batteries. This pre-solvent selection strategy can effectively provide researchers great possibility in efficiently synthesizing various hollow structures. This work paves a new pathway for deeply understanding Ostwald ripening.

A template-free method for preparation of cobalt nanoparticles embedded in N-doped carbon nanofibers with a hierarchical pore structure for oxygen reduction.

Designing and preparing porous materials without using any templates is a challenge. Herein, single-nozzle electrospinning technology coupled with post pyrolysis is applied to prepare cobalt nanoparticles embedded in N-doped carbon nanofibers with a hierarchical pore structure (HP-Co-NCNFs). The resultant HP-Co-NCNFs have lengths up to several millimeters with an average diameter of 200 nm and possess abundant micro/meso/macropores on both the surface and within the fibers. Such a microstructure endows the surface area as high as 115 m(2) g(-1) . When used as an electrocatalyst for the oxygen reduction reaction (ORR), the HP-Co-NCNFs exhibit outstanding electrochemical performance in terms of activity, methanol tolerance, and durability. The hierarchically porous structure and high surface area can effectively decrease the mass transport resistance and increase the exposed ORR active sites. The sufficient amount of exposed ORR active sites along with accessible transport channel and enhanced electrical conductivity may be responsible for the good electrocatalytic performance.

Ultrafine Mo2C nanoparticles encapsulated in N-doped carbon nanofibers with enhanced lithium storage performance.

Rechargeable lithium ion batteries (LIBs) have attracted extensive attention globally due to their good cycling stability, high energy density, and rapid-rate capability, while the rational design of electrode materials can significantly improve their electrochemical performance. In this work, ultrafine Mo2C nanoparticles (NPs) were successfully encapsulated in one dimensional (1D) N-doped porous carbon nanofibers to form a hybrid (Mo2C-NCNFs) through a single-nozzle electrospinning approach coupled with post-pyrolysis. The sizes of the Mo2C NPs were in the range of 2-4 nm and the ultrafine Mo2C NPs were uniformly encapsulated in the N-doped carbon nanofibers forming a highly conductive and interconnecting network, which can facilitate fast electronic transport. When evaluated as an anode material for LIBs, the resultant hybrid exhibits stable cycling performance and excellent rate behavior. More remarkably, the Mo2C-NCNFs hybrid is capable of delivering a specific capacity of 658.0 mA h g(-1) under 100 mA g(-1) after 50 cycles. Even under 2000 mA g(-1), a relatively high specific capacity of 411.9 mA h g(-1) can be achieved, which surpasses the theoretical capacity of graphite (372 mA h g(-1)). The excellent lithium storage performance can be attributed to its unique nanostructure with a strong interaction between the ultrafine Mo2C NPs and N-doped carbon that effectively tolerates the volume change, suppresses the agglomeration of Mo2C NPs, and provides conductive pathways for highly efficient charge transfer during lithium insertion and extraction.

One-pot synthesis and Nb4N5 surface modification of Nb(4+) self-doped KNbO3 nanorods for enhanced visible-light-driven hydrogen production.

Herein, rhombohedral self-doped KNbO3 (S-KN) nanorods were fabricated via a one pot, solvothermal method without using any surfactant. The presence of Nb(4+) in S-KN greatly narrows its band gap and thus extends its photoresponse from UV to the visible light region. Moreover, S-KN/Nb4N5 nanorod heterostructures were obtained by nitriding S-KN nanorods for different times, which exhibited significantly enhanced photocatalytic activity for hydrogen production under visible light irradiation. The junction formed between S-KN and Nb4N5 and the Nb(4+) self-doping of KN are supposed to be responsible for the enhanced photocatalytic activity of S-KN/Nb4N5. This study also paves the way for the synthesis of other similar photocatalysts.