Enhancing Charge Separation in Metallic Photocatalysts: A Case Study of the Conducting Molybdenum Dioxide

A new visible‐light responsive metallic photocatalyst, nanostructured MoO₂, has been discovered. The metallic nature of MoO₂ is confirmed by valance X‐ray photoelectron spectroscopy spectrum and theoretical calculations. However, MoO₂ itself shows only moderate activity due to the serious charge recombination, a general disadvantage of metallic photocatalysts. The findings suggest that its effective charge diffusion length L_p is smaller than 1.0 nm while the separation efficiency η_sep is less than 10%. Therefore, only the periphery of the metallic MoO₂ can effectively contribute to photocatalysis. This limitation is overcome by integrating MoO₂ in a hydrothermal carbonation carbon (HTCC) matrix (mainly contains semiconductive polyfuran). This simple chemical modification brings two advantages: (i) an internal electric field is formed at the interface between MoO₂ and HTCC due to their appropriate band alignment; (ii) the nanostructured MoO₂ and the HTCC matrix are intertwined with each other intimately. Their small size and large contact area promote charge transfer, especially under the internal electric field. Therefore, the separation rate of photoexcited charge carrier in MoO₂ is greatly enhanced. The activity increases by 2.4, 16.8, and 4.0 times in photocatalytic oxygen evolution, dyes degradation, and photoelectrochemicl cell, respectively. The new approach is helpful for further development of metallic photocatalysts.     

Cation Substitution Induced d-Band Center Modulation on Cobalt-Based Spinel Oxides for Catalytic Ozonation

Co₃O₄ spinel is a promising transition metal oxide (TMO) catalyst for the catalytic ozonation of volatile organic compounds (VOCs). Herein, metal–organic frameworks (MOFs)-derived Ni- and Mg- substituted Co₃O₄ catalysts retain similar spinel structures, but display improved and reduced ozonation performance of methyl mercaptan (CH₃SH), respectively. Remarkably, the NiCo₂O₄ catalyst can still ≈90% removal of CH₃SH after running for 20 h at room temperature under an initial concentration of 50 ppm CH₃SH and 40 ppm O₃, relative humidity of 60%, and space velocity of 300 000 mL h⁻¹ g⁻¹, exceeding the reported values. Experimental characterizations have unveiled that the substitution of Ni and Mg into the Co₃O₄ spinel altered surface acidity, oxygen species mobility, and Co²⁺/Co³⁺ ratio. The in situ Raman spectra reveal the dynamic formation Co(III)-O_ad* via the transformation of O₃ into surface atomic oxygen (O_ad*) and peroxide species (O₂*). Theoretical calculations verify that Ni-substitution increases nonuniform charges and Fermi density, leading to a moderate increase in d-band center energy levels, thereby promoting O₃ specific adsorption/activation to convert O_ad*/O₂* and •OH/¹O₂/•O₂⁻, which contributes to eliminate CH₃SH and prevent poisoning. The concept of tuning the d-band center can provide valuable insights for the design of other catalysts for catalytic ozonation.     

Reactive Oxygenated Species Generated on Iodide-Doped BiVO4/BaTiO3 Heterostructures with Ag/Cu Nanoparticles by Coupled Piezophototronic Effect and Plasmonic Excitation

An effective generation of reactive oxygen species (ROS) is of interest from the perspective of environmental technology and industrial chemistry, and here piezocatalysis and photocatalysis using heterostructures based on iodide-doped BiVO₄/BaTiO₃ with photodeposited Ag or Cu nanoparticles (BiVO₄:I/BTO-Ag or BiVO₄:I/BTO-Cu) is studied. The generation rates of •OH and •O₂⁻ radicals over BiVO₄:I/BTO-Ag during piezophotocatalysis are 371 and 292 µmol g⁻¹ h⁻¹, respectively, and significantly higher than those of sole piezocatalysis and photocatalysis. These rates are among the highest reported for the production of free radicals with the piezophototronic effect. Among the catalysts, BiVO₄:I/BTO shows the highest reactivity for the production of H₂O₂ in piezocatalysis (with a concentration of 468 µm after 100 min of irradiation, and still constantly increasing). On BiVO₄:I/BTO-Ag and BiVO₄:I/BTO-Cu, it seems that redundant electrons and holes had reacted effectively with the generated H₂O₂ and in turn had reduced their activities; however, the amounts of H₂O₂ that are formed on BiVO₄:I/BTO-Ag or BiVO₄:I/BTO-Cu under piezophotocatalysis are superior to those of individual piezocatalysis and photocatalysis. A piezophototronic coupling via an ultrasound-mediated and piezoelectric-based polarization field and photoexcitation accounting for the enhanced photocatalytic activity of the iodine-doped heterostructures with plasmonically sized Ag or Cu nanoparticles is suggested.     

Manganese Functionalized Silicate Nanoparticles as a Fenton‐Type Catalyst for Water Purification by Advanced Oxidation Processes (AOP)

Wet hydrogen peroxide catalytic oxidation (WHPCO) is one of the most important industrially applicable advanced oxidation processes (AOPs) for the decomposition of organic pollutants in water. It is demonstrated that manganese functionalized silicate nanoparticles with interparticle porosity act as a superior Fenton‐type nanocatalyst in WHPCO as they can decompose 80% of a test organic compound in 30 minutes at neutral pH and room temperature. By using X‐ray absorption spectroscopic techniques it is also shown that the superior activity of the nanocatalyst can be attributed uniquely to framework manganese, which decomposes H₂O₂ to reactive hydroxyls and, unlike manganese in Mn₃O₄ or Mn₂O₃ nanoparticles, does not promote the simultaneous decomposition of hydrogen peroxide. The presented material thus introduces a new family of Fenton nanocatalysts, which are environmentally friendly, cost‐effective, and possess superior efficiency for the decomposition of H₂O₂ to reactive hydroxyls (AOP), which in turn readily decompose organic pollutants dissolved in water.     

Influence of temperature on the etching rate of SiO2 in CF4 + O2 plasma

The plasmochemical etching of SiO₂ in CF₄ + O₂ plasma is considered. During the experiment SiO₂ films are etched in CF₄ + O₂ plasma at temperatures of 300 and 350 K. The dependences of plasmochemical etching rates of SiO₂ on O₂ content in the feed are measured. The experimental measurements are compared with theoretical calculations. The obtained theoretical results are used to predict the real dimensions of etched trenches. It is found that decrease in temperature reduces lateral undercutting due to decreased desorption of formed SiF₄ molecules from the sidewalls.     

Single-Atom Catalysts for H2O2 Electrosynthesis via Two-Electron Oxygen Reduction Reaction

Oxygen reduction reaction via the two-electron route (2e⁻ ORR) provides a green method for the direct production of hydrogen peroxide (H₂O₂) along with in situ utilization. The effective catalysts with high ORR activity, 2e⁻ selectivity, and stability are essential for the application of this technology. Single-atom catalysts (SACs) have attracted intensively attention for H₂O₂ electrosynthesis owing to the unique geometric and electronic configurations. In this review, the mechanism and theoretical predictions for 2e⁻ ORR over SACs are first introduced. Then, the recent advances of various SACs for the electrosynthesis of H₂O₂ are documented. And the correlation between the central atom, coordination atoms, and coordination environment of SACs and the corresponding electrocatalytic ORR performance including activity, selectivity, and stability are emphatically analyzed and summarized. Finally, the major challenges and opportunities regarding the future design of SACs for the H₂O₂ production are pointed out.     

Synthesis and gas sensing behavior of nanostructured V2O5 thin films prepared by spray pyrolysis

Nanocrystalline vanadium pentoxide (V₂O₅) thin films have been deposited by a spray pyrolysis technique on preheated glass substrates. The substrate temperature was changed between 300 and 500 °C. The structural and morphological properties of the films were investigated by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). The influence of different substrate temperatures on ethanol response of V₂O₅ has also been investigated. XRD revealed that the films deposited at T_pyr=300 °C have low peak intensities, but the degree of crystallinity improved when the temperature was increased to 500 °C and films had orthorhombic structures with preferential orientations along the (0 0 1) direction. The fractal analysis showed a decreasing trend versus the pyrolysis temperature. Sensing properties of the samples were studied in the presence of ethanol vapor. The operating temperature of the sensor was optimized for the best gas response. The response increased linearly with different ethanol concentrations. It was found that films deposited at the lowest substrate temperature (300 °C) had the highest sensing response to ethanol.     

Nanostructured Spinel Manganates and Their Composites for Electrochemical Energy Conversion and Storage

Spinel manganates (AMn₂O₄; A = Co, Ni, Cu, Zn, and Fe; collectively referred to as AMO) are promising electrode materials for water electrolyzers, pseudocapacitors, and batteries owing to their inherent advantages such as valence variability, high catalytic activity, conductivity, stability, low-cost, and environmental friendliness. Nanostructured materials, with a large surface area and short ion diffusion length, offer great potential for achieving enhanced electrochemical performance. This review summarizes spinel manganates with various nanostructured morphologies and discusses the impact of the structure and composition on the electrochemical performance. The review demonstrates that nanostructured spinel manganates with preferred A-site cation significantly improve the thermodynamics and electrochemical reaction kinetics at solid–liquid and solid–solid interfaces. Notably, faceted, hollow, 1D nanostructured CoMn₂O₄ and its nanocomposites (CoMn/CoMn₂O₄ and NiMn₂O₄/C) exhibit outstanding electrochemical performance. The review also provides an overview of the importance of energy conversion and storage, and the advantages of spinel manganates as electrode materials. Additionally, the review describes feasible methods of synthesizing AMO nanostructures and nanocomposites. The insights provided in this review are expected to contribute to the synthesis of spinel manganates with desired morphologies and compositions, enabling the future development of efficient electrode materials for energy conversion and storage devices.     

Residual impurities in GaN/Al2O3 grown by metalorganic vapor phase epitaxy

Residual impurities in GaN films on sapphire (A1₂O₃) substrates grown by two-step metalorganic vapor phase epitaxy (MOVPE) have been investigated. We have mainly investigated the incorporation of carbon into the GaN films with GaN buffer layers on A1₂O₃ during MOVPE growth, comparing trimethygallium (TMGa) and triethygallium (TEGa) as the typical gallium precursors. The films were characterized by secondary ion mass spectroscopy analysis, photolu-minescence, and Hall measurements. The carbon, hydrogen, and oxygen concentrations increase with decreasing growth temperature in using TMGa. Especially the carbon concentration increases with decreasing a V/III ratio, for both TMGa and TEGa. There is about two times more carbon in the GaN films grown using TEGa than those using TMGa. The carbon from TMGa mainly enhances the D-A pair emission (∼378 nm), which shows the carbon makes an acceptor level at nitrogen sites in GaN. On the other hand, the carbon from TEGa enhances a deep emission (∼550 nm), which shows the carbon makes not only an acceptor level but deep levels at interstitial sites in GaN. The carbon impurities originate from methyl radicals for TMGa, or ethyl radicals for TEGa. It is supposed that, in the case of TEGa, the carbon impurities are not always located at nitrogen sites, but are also located at interstitial sites because of the C-C bonding in ethyl radicals.     

Temperature Stability of V2O5-Doped KNN-LS-BF Lead-Free Piezoelectric Ceramics

V₂O₅-doped Na_0.5K_0.5NbO₃-LiSbO₃-BiFeO₃ (KNN-LS-BF) lead-free piezoelectric ceramics were prepared by the traditional sintering method, and their temperature stability was studied. Characterization of the temperature dependences of dielectric and piezoelectric properties of the V₂O₅-doped KNN-LS-BF ceramics showed that V₂O₅ doping could significantly improve the temperature stability in the temperature range of 30°C to 420°C and cause a downward shift in the orthorhombic–tetragonal phase transition to below room temperature. It was also found that the V₂O₅-doped KNN-LS-BF ceramics possess good dielectric and piezoelectric properties (ε _r > 1066, tan δ < 4%, d ₃₃ > 185 pC/N, k _p > 0.25) in the temperature range of 30°C to 300°C.     

Infrared spectroscopy and X-ray diffraction studies on the crystallographic evolution of La2O3 films upon annealing

Rare earth oxides (REOs) have lately received extensive attention in relation to the continuous scaling down of non-volatile memories (NVMs). In particular, La₂O₃ films are promising for integration into future NVMs because they are expected to crystallize above 400 °C in the hexagonal phase (h-La₂O₃) which has a higher κ value than the cubic phase (c-La₂O₃) in which most of REOs crystallize. In this work, La₂O₃ films are grown on Si by atomic layer deposition using La(C₅H₅)₃ and H₂O. Within the framework of the h-La₂O₃ formation, we systematically study the crystallographic evolution of La₂O₃ films versus annealing temperature (200-600 °C) by Fourier transform infrared spectroscopy (FTIR) and grazing incidence X-ray diffraction (GIXRD). As-grown films are chemically unstable in air since a rapid transformation into monoclinic LaO(OH) and hexagonal La(OH)₃ occurs. Vacuum annealing of sufficiently thick (>100 nm) La(OH)₃ layers induces clear changes in FTIR and GIXRD spectra: c-La₂O₃ gradually forms in the 300-500 °C range while annealing at 600 °C generates h-La₂O₃ which exhibits, as inferred from our electrical data, a desirable κ ∼ 27. A quick transformation from h-La₂O₃ into La(OH)₃ occurs due to H₂O absorption, indicating that the annealed films are chemically unstable. This study extends our recent work on the h-La₂O₃ formation.     

Impedance Spectroscopy of Dielectric BaTi<Subscript>5</Subscript>O<Subscript>11</Subscript> Film Prepared by Laser Chemical Vapor Deposition Method

BaTi₅O₁₁ film was prepared on Pt/Ti/SiO₂/Si substrate by the laser chemical vapor deposition method. A single-phase BaTi₅O₁₁ film with ([`3] 22)/([`2] 23) (\overline{3} 22)/(\overline{2} 23) preferred orientation and columnar cross-section was obtained at high deposition rate (154.8 μm h⁻¹). The dielectric constant (ε _r) of the BaTi₅O₁₁ film was 21, measured at 300 K and 1 MHz. The electrical properties of the BaTi₅O₁₁ film were investigated by ac impedance spectroscopy from 300 K to 1073 K at 10² Hz to 10⁷ Hz. Plots of the real and imaginary parts of the impedance (Z′ and Z″) and electrical modulus (M′ and M″) in the above frequency and temperature range suggested the presence of two relaxation regimes, which were attributed to grain and grain boundary responses. The ac conductivity plots as a function of frequency showed three types of conduction process at elevated temperature. The frequency-independent plateau at low frequency was due to dc conductivity. The mid-frequency conductivity was due to grain boundaries, while the high-frequency conductivity was due to grains.     

Effect of post-growth annealing on the structural,optical and electrical properties of V2O5 nanorods and its fabrication,characterization of V2O5/p-Si junction diode

We report the synthesis of V₂O₅ nanorods by utilizing simple wet chemical strategy with ammonia meta vanadate (NH₄VO₃) and polyethylene glycol (PEG) exploited as precursor and surfactant agent, respectively. The effect of post-annealing on structural, optical and electrical properties of V₂O₅ nanorods was characterized by XRD, HRSEM-EDX, TEM, FT-IR, UV (DRS), PL, TG–DTA and DC conductivity studies. The X-ray diffraction analysis revealed that the prepared sample annealed at 150 °C for 5 h which exhibited anorthic phase of V₅O₉ and annealed at 300–600 °C showed the anorthic phase change to orthorhombic phase of V₂O₅ due to the post-annealing effect. The surface morphology results indicated that increasing temperature caused a change from microrods to a nanorods shape in the morphology of V₂O₅. FT-IR spectrum confirmed that the presence of V₂O₅ functional groups and the formation of V–O bond. The optical band gap was found in the range 2.5–2.48 eV and observed to decreases with various annealed temperature. The DC electrical conductivity was studied as a function of temperature which indicated the semiconducting nature. Further, the potential of V₂O₅ nanostructures were grown on the p-Si substrate using the nebulizer spray technique. The junction properties of the V₂O₅/p-Si diode were evaluated by measuring current (I)–voltage (V) and AC characteristics.     

Growth of epitaxial Pr2O3 layers on Si(1 1 1)

The growth of Pr₂O₃ layers on Si(1 1 1) has been studied by X-ray diffraction, Low-energy electron diffraction (LEED) and atomic force microscopy (AFM). Pr₂O₃ starts to grow as a 0.6-nm thick layer corresponding to one unit cell of the hexagonal phase (1 ML). The X-ray results indicate that layers thicker than 0.6 nm do not grow with the hexagonal phase. Growth takes place at a sample temperature of 500–550 °C. Annealing of the monolayer in UHV at a temperature above 700 °C leads to the formation of Pr₂O₃ and PrSi₂ islands. Silicide islands are found only at annealing in UHV and do not occur at annealing in oxygen atmosphere of 10⁻⁸ mbar. The LEED pattern after heating to 730 °C shows a (2×2) and (√3×√3) superstructure and after heating to 1000 °C a (1×5) superstructure occurs. The superstructures seen in the LEED pattern arise from silicide structures in the area between the islands. The silicide remains on the surface and cannot be removed with flashing to 1100 °C. Further deposition of Pr₂O₃ on the surface covered with silicide phases does not lead to growth of ordered layers.     

Theoretical Design of Catalysts for the Heterolytic Splitting of H2

Here, we briefly review recent advances in H₂ storage technologies relying on mixed proton–hydride and destabilized hydride materials. We establish a general relationship across different materials: the higher the effective H content, the higher the temperatures needed to completely desorb H₂. Nevertheless, several systems show promising thermodynamics for H₂ desorption; however, the desorption kinetics still needs to be improved by the use of appropriate catalysts. Prompted by the importance of heterolytically splitting stable dihydrogen molecules for proton–hydride technologies, we attempt to theoretically design novel H₂ transfer catalysts. We focus mainly on M₄Nm₄H₈ catalysts (M = V, Ti, Zr, Hf, and Nm = Si, C, B, N), which should be able to preserve their functionality in the strongly reducing environment of a H₂ storage system. We are able to determine the energy of H₂ detachment from these molecules, as well as the associated energy barriers. In order to optimize the properties of the catalysts, we use isoelectronic atom‐by‐atom substitutions, vary the valence electron count, and borrow the concept of near‐surface alloys from extended solids and apply it to molecular systems. We are able to obtain control over the enthalpy and electronic barriers for H₂ detachment. Molecules with the coordinatively unsaturated > Ti?Si < unit exhibit particularly favorable thermodynamics and show unusually small electronic barriers for H₂ detachment (> 0.27 eV) and attachment (> 0.07 eV). These and homologous ZrSi frameworks may serve as novel H₂ transfer catalysts for use with emerging lightweight hydrogen storage materials holding 5.0–10.4 wt % hydrogen, such as Li₂NH, Li₂Mg(NH)₂, Mg₂Si, and LiH/MgB₂ (discharged forms). Catalytic properties are also anticipated for appropriate defects on the surfaces and crystal edges of solid Ti and Zr silicides, and for Ti?Si ad‐units chemisorbed on other support materials.     

IGZO thin film transistors with Al2O3 gate insulators fabricated at different temperatures

Thin film transistors (TFTs) with bottom gate and staggered electrodes using atomic layer deposited Al₂O₃ as gate insulator and radio frequency sputtered In–Ga–Zn Oxide (IGZO) as channel layer are fabricated in this work. The performances of IGZO TFTs with different deposition temperature of Al₂O₃ are investigated and compared. The experiment results show that the Al₂O₃ deposition temperature play an important role in the field effect mobility, I_on/I_off ratio, sub-threshold swing and bias stability of the devices. The TFT with a 250 °C Al₂O₃ gate insulator shows the best performance; specifically, field effect mobility of 6.3 cm²/Vs, threshold voltage of 5.1 V, I_on/I_off ratio of 4×10⁷, and sub-threshold swing of 0.56 V/dec. The 250 °C Al₂O₃ insulator based device also shows a substantially smaller threshold voltage shift of 1.5 V after a 10 V gate voltage is stressed for 1 h, while the value for the 200, 300 and 350 °C Al₂O₃ insulator based devices are 2.3, 2.6, and 1.64 V, respectively.     

Single Semi-Metallic Selenium Atoms on Ti3C2 MXene Nanosheets as Excellent Cathode for Lithium–Oxygen Batteries

Rechargeable Li–O₂ batteries are promising due to their superior high energy density but subject to sluggish oxygen reduction/evolution kinetics. Developing highly efficient catalysts to improve catalytic activity and alleviate oxidation–reduction overpotential of Li–O₂ batteries is of great challenge and importance. Herein, a CO₂-assisted thermal-reaction strategy is developed to fabricate isolated semi-metallic selenium single-atom-doped Ti₃C₂ MXene catalyst (SASe-Ti₃C₂) as cathodes for high-performance Li–O₂ batteries. The isolated moieties of single Se atom catalysis centers can function as active catalytic centers to drastically enhance the intrinsic LiO₂-absorption ability and thus fundamentally modulate the formation/decomposition mechanism of lithium peroxide (Li₂O₂) discharge product, thus demonstrating greatly enhanced redox kinetics and efficiently ameliorated overpotentials. Theoretical simulations reveal that the interaction between Se-involved moieties and Ti₃C₂ substrate greatly enhances the intrinsic LiO₂-absorption ability and fundamentally promotes the charge transfer between electrode and Li₂O₂ product, deeply ameliorating the round-trip overpotential. The well-designed SASe–Ti₃C₂ electrode exhibits decreased charge/discharge polarization (1.10 V vs Li/Li⁺), ultrahigh discharge capacity (17 260 mAh g⁻¹ at 100 mA g⁻¹), and superior durability (170 cycles at 200 mA g⁻¹) as cathode for Li–O₂ batteries. The promising results will shed light on the design of highly efficient catalysts for oxygen-involved systems of future investigation.     

Selective etching of (Ba,Sr)TiO3 thin films over silicon in an inductively coupled plasma

In this work, we investigated etching characteristics of BST thin films and higher selectivity of BST over Si using inductive coupled O₂/Cl₂/Ar plasma (ICP) system. The maximum etch rate of BST thin films and selectivity of BST over Si were 61.5 nm/min at a O₂ addition of 1 sccm, 9.52 at a O₂ addition of 4 sccm into the Cl₂(30%)/Ar(70%) plasma, respectively. Plasma diagnostics was performed by Langmuir probe (LP), optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS). These results confirm that the increased etch rates at O₂ addition of 1 sccm is the result of the enhanced chemical reaction between BST and Cl radicals and an ion bombardment effect.     

Deposition of thin Bi2Te3 and Sb2Te3 films by pulsed laser ablation

Bi₂Te₃ and Sb₂Te₃ films were obtained by pulsed laser ablation. The films were deposited in vacuum (1 × 10⁻⁵ Torr) on single crystal substrates of Al₂O₃ (0001), BaF₂ (111), and fresh cleavages of KCl or NaCl (001) heated to 453–523 K. The films were 10–1500 nm thick. The structures of the bulk material of targets and films were studied by X-ray diffractometry and transmission high-energy electron diffraction, respectively. Electrical properties of the films were measured in the temperature range of 77–300 K. It is shown that the films possess semiconductor properties. Several activation portions are observed in the temperature dependences of resistivity; the energies of activation portions depend on the film thickness and crystallite size.     

Metal Ion‐Terpyridine‐Functionalized L‐Tyrosinamide Aptamers: Nucleoapzymes for Oxygen Insertion into C?H Bonds and the Transformation of L‐Tyrosinamide into Amidodopachrome

A series of metal ion‐terpyridine‐modified L‐tyrosinamide aptamers (M^{n +} = Cu²⁺ or Fe³⁺) act as enzyme‐mimicking catalysts (nucleoapzymes) for oxygen‐insertion into C? H bonds and the transformation of L‐tyrosinamide into amidodopachrome. The reaction proceeds in the presence of H₂O₂ and coadded L‐ascorbic acid. In one series of experiments, the catalyzed oxidation of L‐tyrosinamide to amidodopachrome by a set of nucleoapzymes consisting of Fe³⁺‐ or Cu²⁺‐terpyridine complexes tethered directly or through a 4 × thymidine (4 × T) bridge, to the 5′‐ or 3′‐end of the 49‐mer L‐tyrosinamide aptamer or to a shorter 23‐mer L‐tyrosinamide aptamer is examined. All nucleoapzymes reveal catalytic Michaelis–Menten enzyme‐like activities and the separated Fe³⁺‐ or Cu²⁺‐terpyridine and L‐tyrosinamide aptamer units show only minute catalytic properties. The catalytic activities of the nucleoapzymes are attributed to the concentration of the L‐tyrosinamide substrate by the aptamer units in proximity to the catalytic sites (K_d = (14 ± 0.1) × 10^?6 m for all 49‐mer catalysts and K_d = (2.5 ± 0.1) × 10^?6 m and K_d = (0.8 ± 0.04) × 10^?6 m for the 23‐mer catalysts). Electron spin resonance experiments reveal that ?OH radicals and ascorbate radicals participate in the transformation of tyrosine derivatives to catechol products. An autocatalytic feedback mechanism for the amplified generation of the two radicals is suggested.