OH Dissociation Research Articles

Co-Zn single atoms anchored carbon nanotubes derived from anti-perovskite carbides for boosted hydrogen evolution and oxygen reduction reactions

The optimization of the single-atom catalysts (SACs) performance has emerged as a rising star in recent years, however, the synergistic effect for the diatomic sites (DASs) electrocatalysts remains a grand challenge. Herein, we propose a self-decomposition approach for the synthesis of atomically dispersed Co-Zn single atoms anchored on Co-embedded carbon nanotubes (Co,Zn SAs@Co-CNTs) derived from anti-perovskite carbides. Experimental results and theoretical calculations demonstrate that the synergy effect between Co and Zn DASs accelerates the formation of OOH* and the dissociation of OH* in the oxygen reduction reaction (ORR), and reduces the adsorption–desorption energy barrier of H* in the hydrogen evolution reaction (HER). As expected, the Co,Zn SAs@Co-CNTs exhibits superior HER (acid: ɳ10 = 142 mV; alkaline: ɳ10 = 89 mV) and ORR (E1/2 = 0.92 V) activities with excellent operational stability, outperforming the 20 wt% Pt/C benchmark. This study highlights a new insight for modulating the electronic structure of active centers for electrocatalysts by dual-atom site.

Kirkendall effect induced the formation of hollow Co2P in Co-N-C for ORR, OER, HER and flexible Zn–Air battery

Enhancing the exposure of active sites and optimizing the electronic structure of the catalysts are indispensable for efficient catalyst design, yet remains challenging in the renewable energy field. Herein, a sequential pyrolyzing and phosphating approach for a well-designed GO/CoTPP gel precursor has been explored to prepare a Co-based carbon catalyst containing single-atom Co-Nx and hollow Co2P active species (Co2P/Co-N-C) for multifunctional electrocatalytic reactions. Kirkendall effect occurring during the template-free one-pot phosphating approach induced the formation of the hollow Co2P structure in the catalyst, simultaneously optimizing the electron structure and allowing the active sites to be exposed to a greater extent. DFT calculations demonstrated that the Co2P and Co-Nx structures enable Co2P/Co-N-C to have a suitable d-band center depending on the electron transfer from Co2P to Co-Nx, which enhances the adsorption of O* and the dissociation of OH* in oxygen electrode reactions, thereby facilitates catalytic reactions. As a result, the Co2P/Co-N-C exhibits excellent trifunctional activity with a half-wave potential of 0.89 V in alkaline for oxygen reduction reaction (ORR), overpotentials of 330 mV for oxygen evolution reaction (OER) and 190 mV for hydrogen evolution reaction (HER), respectively, which are among the best results reported to date for Co-containing carbon-based catalysts. Furthermore, the flexible Zinc-air battery with Co2P/Co-N-C electrodes also demonstrates a high open circuit voltage of 1.35 V, which is applied to build the self-powered device for water splitting continuously. Our work provides a facile strategy to prepare high-performance trifunctional catalysts by morphology/structure control of metal-based NPs on carbon-based materials.

Computational Screening of Pt1@Ti3C2T2 (T = O, S) MXene Catalysts for Water-Gas Shift Reaction.

Single-atom catalysts (SACs) provide an opportunity to elucidate the catalytic mechanism of complex reactions in heterogeneous catalysis. The low-temperature water-gas shift (WGS) reaction is an important industrial technology to obtain high purity hydrogen. Herein, we study the catalytic activity of Pt1@Ti3C2T2 (T = O, S) SACs, where one subsurface Ti atom with three T vacancies in the functionalized Ti3C2T2 (T = O, S) MXene is substituted by one Pt atom, for the low-temperature WGS reaction, using density functional theory (DFT). The results show that Pt1@Ti3C2T2 provides an excellent platform for the WGS reaction by its bowl-shaped vacancy derived from the Pt1 single atom and three T defects surrounding it. Especially, Pt1@Ti3C2S2 SAC has higher catalytic performance for the WGS reaction, due to the weaker electronegativity of the S atom than the O atom, which significantly reduces the energy barrier of H* migration in the WGS reaction, which is often the rate-determining step. In the most favorable redox mechanism of the WGS reaction on Pt1@Ti3C2S2, the rate-determining step is the dissociation of OH* into O* and H* with the energy barrier as low as 1.12 eV. These results demonstrate that Pt1@Ti3C2S2 is promising in the application of MXenes for low-temperature WGS reactions.

High energy barrier hydroxyl radical dissociation mechanism of a low shock sensitivity dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50) explosive.

As a representative of the new generation of high-energy explosives, TKX-50 has attracted widespread attention due to its remarkably low sensitivity toward shock. However, the reported decomposition barriers of TKX-50 (∼37 kcal mol-1) are comparable to those of commonly used explosives. The mechanism of its low shock sensitivity remains unclear. In this study, using an ab initio molecular dynamics method combined with a multiscale shock simulation technique and transition state calculations (at the B2PLYP-D3/Def2TZVP level), we discovered an unconventional reaction pathway of TKX-50 under shock, and its rate-controlling step is the dissociation of the hydroxyl radical (OH) from the anion ring after proton transfer, followed by ring rupture and the production of H2O and N2. The barrier for this OH dissociation reaction is as high as 51.9 kcal mol-1. In contrast, under thermal stimuli, TKX-50 prefers to open rings directly after proton transfer without losing the OH. The corresponding barrier is 35.4 kcal mol-1, which is in good agreement with previous studies. The reason for the unconventional reaction pathway of TKX-50 under shock may be the suppression of anion ring opening in thermal decomposition by steric hindrance upon shock compression. In addition, the dominant N2 generation pathway under shock releases less energy than pyrolysis which further explains the low shock sensitivity of TKX-50. This study comprehensively elucidates the different reaction mechanisms of TKX-50 under thermal and shock conditions and proposes a crucial reaction pathway leading to its low shock sensitivity. These findings will contribute to the understanding and application of tetrazole anionic energetic salts.

Ordered mesoporous carbon with binary CoFe atomic species for highly efficient oxygen reduction electrocatalysis.

The exploration of powerful, efficient and precious metal-free electrocatalysts for facilitating the sluggish kinetics of the oxygen reduction reaction (ORR) is a crucial endeavor in the development and application of energy conversion and storage devices. Herein, we have rationally designed and synthesized bimetallic CoFe species consisting of CoFe nanoparticles and atomically dispersed dual atoms anchored on an ordered mesoporous carbon matrix (CoFe/NC) as highly efficient ORR electrocatalysts. The pyrolyzation temperature for CoFe/NC plays a vital role in regulating the morphology and composition of both the carbon matrix and CoFe species. The optimized CoFe/NC-750 exhibits a favorable ORR performance in 0.1 M KOH with a high half-wave potential (E1/2) of 0.87 V vs. RHE, excellent tolerance to methanol and remarkable durability (no obvious decrease in E1/2 value after 3000 cycles), all of which are superior to the performance of commercial Pt/C. Experimental measurements and density functional theory (DFT) calculations reveal that the improved ORR performance of CoFe/NC-750 is mainly attributed to the electronic structure of atomically dispersed Fe active sites modulated by the surrounding CoFe alloys and Co single atoms, which accelerates the dissociation and reduction of intermediate OH* species and promotes the ORR process.

Anion Exchange Membrane Using Fused Expanded Pyridinium As Their Cationic Unit

Ion conductivity is expressed as a function of the Faraday constant, ion concentration, and mobility. Thus, an increase in both ion concentration and mobility is crucial for increasing the ion conductivity. Ion mobility reflects ionic motion in the restricted area of ion conduction channels. The general approach to expand ion conduction channels in AEM is to introduce a large number of ionic groups, i.e., to increase ion-exchange capacity (IEC). Because the increase of IEC also increase ion concentration, increase of IEC have been major strategy for increasing ion conductivity. However, this approach causes softening of the AEM because high IEC involves large amount of water uptake (WU).On the other hand, the ion concentration in AEM is also affected by the dissociation constant of the ionic groups. However, conventional ionic groups in AEMs, such as ammonium cations, suffer from a low dissociation constant that leads to low ion conduction in AEM. Previously, we found that a fused expanded pyridinium cation (FEP) having a delocalized positive charge showed high dissociation of OH– owing to the weak interaction between the soft acid (FEP) and the hard base (OH–) which can be explained based on the Pearson Hard Soft Acid Base (HSAB) principle. As the result, AEM with FEP unit exhibited excellent ion conductivity of 123.4 ±13.3 mS cm–1 at 80 °C with very low IEC (0.77 mmol g-1).Recently, we synthesized alkylated fused expanded pyridinium (CxFEP) with alkyl chain lengths (x) of 0, 6, 12, and 18 and introduced into polymers as side chains for AEM applications. FEP is characterized by a large π cation that offers weak electrostatic interactions with anions such as hydroxide (OH–) ions and therefore leads to high OH– conductivity in AEM. CxFEP shows synergetic effect between the π-π interactions of FEP and van der Waals interactions of the alkyl chains and forms a strong assembly, particularly for C6FEP. On the other hand, when CxFEP was introduced into the polymers, C12FEP formed an ordered stacking assembly in the films, whereas the other polymers with shorter (C0 and C6) and longer (C18) alkyl chains exhibited no such ordered structure. For the AEM, all the polymers showed higher ion conductivities than that of commercial ammonium-based AEM, even with their low ion exchange capacities (IEC), probably because of their characteristic weak interactions between the FEP cation and OH–. In particular, the AEM with C12FEP showed the highest ion conductivity of 143.3 ± 27.3 mS cm-1 even with a very low IEC of 0.41 mmol g-1. In addition to the high ion dissociation of FEP, the molecular assembly of FEP provides ionic conduction paths. This study demonstrates the importance of high ion dissociation and formation of a conduction pathway for AEM with high conductivity.

High‐Performance Alkaline Freshwater and Seawater Hydrogen Catalysis by Sword‐Head Structured Mo2N‐Ni3Mo3N Tunable Interstitial Compound Electrocatalysts

AbstractRealizing efficient electrocatalysts is a stepping stone toward achieving high‐performance alkaline water/seawater electrolysis, but remains a crucial challenge. Herein, heterogeneous Mo2N/Ni3Mo3N electrocatalysts on nickel foam (denoted MN‐NMN/NF) that is stable and active for the hydrogen evolution reaction (HER) in both alkaline water/seawater are demonstrated. The optimized MN‐NMN09/NF achieves an ultralow HER overpotential of 11 mV@10 mA cm−2 in 1.0 m KOH electrolyte, which is not only superior to the benchmark Pt/C catalysts but also the best reported ever among NiMo‐based electrocatalysts in an alkaline environment. Successively, the optimized MN‐NMN09/NF electrocatalyst can drive HER current densities of 10 and 500 mA cm−2 using low overpotentials of 9.37 and 123 mV in 1.0 m KOH seawater electrolyte, which remains durable after 120 h long‐term electrolysis at a constant current density of 500 mA cm−2. In situ Raman analysis reveals that the enhanced performance is attributed to the accelerated H2O adsorption and OH dissociation processes on the MN‐NMN surfaces. Theoretical analysis further confirms that rapid H2O adsorption‐dissociation kinetics and H adsorption‐conversion kinetics on the Ni3Mo3N/NiOOH and Ni3Mo3N/MoOx surfaces result in boosted HER capability. This work depicts a significant potential for designing stable and efficient hydrogen production electrocatalysts for both alkaline water and seawater electrolysis.

Plasma‐enhanced evaporation and its impact on plasma properties and gaseous chemistry in a pin‐to‐water pulsed discharge

AbstractThe plasma in contact with liquids has led to various novel applications such as plasma biomedicine, material synthesis, and so on. However, the phenomenon of evaporation under plasma treatment and its impact on plasma–liquid interactions has a limited understanding. In this study, the spatially and temporally resolved behavior of water vapor production and its induced influences on plasma properties and gaseous chemistry were studied in detail in an atmospheric pressure pin‐to‐water pulsed He discharge. Diagnostic methods such as laser‐induced fluorescence (LIF) and high‐resolution optical emission spectroscopy (OES) were applied to determine the water vapor and OH radical densities, as well as key plasma parameters such as the gas temperature and electron density. It shows that the physicochemical properties of plasma vary among different discharge regions due to evaporation behavior stimulated during the pulsed discharge‐on phase. In addition, using simulation based on the experimental data, the mechanisms of how water vapor affects the observed spatiotemporal behaviors of OH radicals in different discharge regions are understood. Compared to the pin‐anode and liquid‐cathode sheath regions, proper electron parameters such as density and temperature, as well as water vapor density in the plasma‐positive column, significantly enhance the production of reactive OH radical through the dominant path of electron‐stimulated H2O dissociation. However, higher levels of electron parameters in the intense discharge region near the positive‐pin boundary enhance OH dissociation and finally result in the hollow distribution of OH density. From the global kinetic plasma simulation, the production of reactive hydroxide species playing key roles in plasma medicine treatments, such as O, H, HO2, H2O2, and hydrated ions including H+(H2O)4 and H+(H2O)5, are promoted noticeably as a result of the enhanced water evaporation process.

Electronic Enhancement Engineering by Atomic Fe–N4 Sites for Highly‐Efficient PEMFCs: Tailored Electric‐Thermal Field on Pt Surface

AbstractLowering noble‐metal Pt usage and simultaneously enhancing electrocatalytic oxygen reduction reaction (ORR) activity and stability of Pt‐based ORR electrocatalysts is the key to realize the large‐scale application of fuel cells. Here, an effective strategy is developed to reduce Pt usage through the strong electron interaction between uniform Pt nanoparticles (≈4.0 nm) and abundant atomically dispersed Fe–N4 sites modified on an ordered mesoporous carbon (OMC) surface for efficiently enhancing ORR performance. Density functional theory (DFT) calculations show that the strong electron interactions between Pt and Fe–N4 sites decrease the d‐band center of Pt in Pt@Fe–N–OMC‐2 by 0.21 eV relative to that of as‐prepared Pt@OMC, indicating the weakened O2 adsorption and accelerated desorption of oxygenated species on Pt sites. In situ Raman spectra demonstrate that the introduction of Fe–N4 moieties promotes the O–OH dissociation process. Finite element method simulations reveal that the electric and thermal field of the embedded Pt nanoparticle surface is enhanced through modifying Fe–N4 sites on the OMC surface, accelerating the accumulation of ORR‐related species (O2, H+, and H2O), which is conductive to electrocatalyzing the ORR. This innovative approach not only illustrates the synergistic mechanism between Pt and Fe–N4 sites, but also simultaneously provides new avenues to design advanced electrocatalysts for fuel cells.

O i] 6300 Å emission as a probe of external photoevaporation of protoplanetary discs

ABSTRACT We study the utility of the [O i] 6300 Å forbidden line for identifying and interpreting externally driven photoevaporative winds in different environments and at a range of distances. Thermally excited [O i] 6300 Å is a well-known tracer of inner disc winds, so any external contribution needs to be distinguishable. In external winds, the line is not thermally excited and instead results from the dissociation of OH, and we study how the line luminosity resulting from that process scales with the disc/environmental parameters. We find that the line luminosity increases dramatically with FUV radiation field strength above around 5000 G0. The predicted luminosities from our models are consistent with measurements of the line luminosity of proplyds in the Orion Nebula Cluster. The high luminosity in strong UV environments alone may act as a diagnostic, but a rise in the [O i]-to-accretion luminosity ratio is predicted to better separate the two contributions. This could provide a means of identifying external photoevaporation in distant clusters where the proplyd morphology of evaporating discs cannot be spatially resolved.

Single-atom alloy with Pt-Co dual sites as an efficient electrocatalyst for oxygen reduction reaction

Reducing the usage of noble metals, such as platinum-based catalysts for oxygen reduction reaction (ORR) is pressingly demanded towards the practical applications of proton-exchange membrane fuel cells. One promising way is to develop Pt single atom catalysts (SACs), which, however, are plagued by their preference toward two-electron ORR pathway as well as stability issue. Herein, a single-atom alloy (SAA) catalyst with platinum-cobalt (Pt-Co) dual sites encapsulated in nitrogen-doped graphitized carbon nanotubes (Pt1Co100/N-GCNT) consisting of isolated Pt atoms decorated on the surface of Co nanoparticles was reported. Based on complementary spectroscopic characterizations and first-principle calculations, we propose that the unique Pt-Co dual sites in SAA facilitates the adsorption and dissociation of oxygen, particularly for the immobilization of OOH* intermediate and the dissociation of OH* intermediate, and thus result in high-efficiency four-electron ORR pathway. Consequently, the Pt1Co100/N-GCNT SAA catalyst achieves a mass activity of 0.81 A mg–1Pt at 0.90 V (versus the reversible hydrogen electrode) in 0.1 M HClO4 electrolyte, outperform commercial Pt/C catalyst for 5.4 times. The superior stability of the SAA catalyst was reflected by the results from the 30,000 potential-scanning cycles combined with the post characterization of the catalyst.

Exploring the Dual Characteristics of CH3OH Adsorption to Metal Atomic Structures on Si (111)-7 × 7 Surface.

Metal atoms were deposited on an Si (111)-7 × 7 surface, and they were adsorbed with alcohol gases (CH3OH/C2H5OH/C3H7OH). Initially, CnH2n+1OH adsorption was simply used as an intermediate layer to prevent the chemical reaction between metal and Si atoms. Through scanning tunneling microscopy (STM) and a mass spectrometer, the CnH2n+1OH dissociation process is further derived as the construction of a surface quasi-potential with horizontal and vertical directions. With the help of three typical metal depositions, the surface characteristics of CH3OH adsorption are more clearly presented in this paper. Adjusting the preheating temperature, the difference of thermal stability between CH3O– and H+ could be obviously derived in Au deposition. After a large amount of H+ was separated, the isolation characteristic of CH3O– was discussed in the case of Fe deposition. In the process of building a new metal-CH3O–-H+ model, the dual characteristics of CH3OH were synthetically verified in Sn deposition. CH3O– adsorption is prone to influencing the interaction between the metal deposition and substrate surface in the vertical direction, while H+ adsorption determines the horizontal behavior of metal atoms. These investigations lead one to believe that, to a certain extent, the formation of regular metal atomic structures on the Si (111)-7 × 7-CH3OH surface is promoted, especially according to the dual characteristics and adsorption models we explored.

Theoretical insight into adsorption and dissociation of water on NiCr binary alloy surfaces: Early-stage oxidation mechanism

The effects of alloy surface composition and pre-adsorbed oxygen on the behaviors of H2O over Ni–Cr binary alloy surfaces were investigated by using the first-principles method. The surface energies and work functions for a series of Ni–Cr (111) alloy surfaces with different Cr concentrations were addressed to track the surface reactivities. An enhancement effect on the surface reactivity from Cr doping in the top-surface layer (TSL) of nickel substrates was identified. The locations of Cr in the TSL dramatically promoted exothermic adsorption of H2O and its decomposed products, including OH, O, and H. The calculated potential dissociation pathways further demonstrated that the successive dissociation of H2O molecules was substantially triggered in the presence of Cr doping in the TSL. By contrast, the Cr doping in the sub-surface layer hindered the dehydrogenation of H2O with a relatively higher energy barrier for OH dissociation. Moreover, with pre-adsorbed oxygen atoms closer to Cr, the first elementary step of H2O dissociation was easily fostered, whereas the OH dissociation was hindered. The Cr doping and O pre-adsorption accelerated the dissociation of H2O, which plays a critical role in the initial oxidation of nickel-based alloys in water- or oxygen-bearing environments.

Dissociation of hydrogen peroxide in water and methanol through a biased molecular dynamics investigation.

The two dissociation channels of HOOH, namely, HOOH and HOOH, in water and methanol are investigated using umbrella-sampling ab initio molecular dynamics. Our potential of mean force calculations reveals the HOOH dissociation to be more favorable in methanol with a free energy barrier of 7.56 kcal/mol, while the HOOH dissociation possesses a free energy barrier of 11.46 kcal/mol. In water, the HOOH dissociation channel is more favorable (8.25 kcal/mol), while the HOOH dissociation process requires a higher free energy (11.28 kcal/mol). Such reaction favorability can be explained by inspecting the formation of secondary radical species during the course of multiple hydrogen donating-accepting processes in each reaction channel. The radical species, that is, H3 O• (observed in water) and CH3 OH2• (observed in methanol), are the first subordinate species upon the HOOH dissociation. For the HOOH dissociation channel in methanol, the secondary species such as water and formaldehyde can be observed, while the re-generation of HOOH in water can be spotted.

Hydrogen production by the steam reforming and partial oxidation of methane under the dielectric barrierdischarge

The steam reforming and partial oxidation of methane to produce hydrogen under dielectric barrier discharge were conducted in the CH4-O2-N2-H2O reaction system; the effects of H2O/CH4 molar ratio, O2/N2 molar ratio, total gas flow, discharge voltage and discharge frequency on the hydrogen production were investigated and the reaction mechanism was analyzed on the basis of the in-situ diagnostic emission spectroscopy. The results indicate that the conversion of methane and the yield of hydrogenincrease with the increase of H2O/CH4 molar ratio, O2/N2 molar ratio, and discharge voltage, but decrease with the increase of the total gas flow rate and show a volcano-shape trend with the increase of discharge frequency (peaked at 9.8 kHz). Under the conditions with an H2O/CH4 molar ratio of 1.82, O2/N2 molar ratio of 2.1, total flow rate of 136 mL/min, discharge voltage of 18.6 kV and discharge frequency of 9.8 kHz, in particular, the conversion of methane and the yield of hydrogen reach 47.45% and 21.33%, respectively. During the reaction, methane and water vapor may dissociate by the action of high energy electrons to generate CHx·, H·, OH·, O· and other free radicals and hydrogen is then produced through the collision between the free radicals. H· may come from the electronic dissociation of CH4 as well as the dissociation of OH· formed primarily from the water vapor dissociation. The partial oxidation of methane is mainly manifested by the oxidation of CH2· with O·, where O· is produced by the electronic dissociation of O2 as well as the further dissociation of OH·

Methanol oxidation over rutile Au1@TiO2 catalyst: Importance of facets and oxygen vacancy

Single-atom catalysts have attracted exceptional interest of researchers. However, the arguing whether tuning the interactions between the dispersed single metals and the support can enhance the activity, selectivity and stability of single atom catalysts is still open. Herein, single atom Au catalyst (rutile Au1@TiO2) is presented via density functional theory calculations, which exhibits high efficiency for methanol oxidation to formate, especially the exposed (110) facet. Rutile Au1@TiO2 (110) facet prefers to localize at 6-fold Ti site under rich-oxygen conditions; methanol goes through adsorption, OH dissociation, CH dissociation, O2 molecule adsorption, and withdraws H2O and formate to gas phase separately. In particular, a significant step on the way to methanol oxidation is the oxygen vacancy formation at the reduced rutile Au1@TiO2 (110) surface after CH scission and H2O desorption. The subsequent adsorbed O2 molecule does only provide oxygen species for methanol oxidation, but not fill surface oxygen vacancy, which promotes formate formation and desorption to develop very well.

Electronically Excited OH Super-rotors from Water Photodissociation by Using Vacuum Ultraviolet Free-Electron Laser Pulses.

The fragmentation dynamics of water in a superexcited state play an important role in the ionosphere of the planets and in the photodissociation region (PDR) of the planetary nebula. In this Letter, we experimentally study the fragmentation dynamics of H2O with the energy above its ionization potential initiated by vacuum ultraviolet free-electron laser pulses. The experimental results indicate that the binary fragmentation channels H + OH and the triple channels O + 2H both present at 96.4 nm photolysis. Electronically excited OH super-rotors (v = 0, N ≥ 36, or v = 1, N ≥ 34), with the internal energy just above the OH (A) dissociation energy, are observed for the first time, which are only supported by the large centrifugal barriers. An absolute cross section of these super-rotors is estimated to be 0.7(±0.3) × 10-18 cm2. The tunnelling rates of these extremely rotationally excited states are also analyzed. This work shows a spectacular example of energy transfer from a photon to fragment rotation through photodissociation.

Reactions in Tirapazamine Induced by the Attachment of Low-Energy Electrons: Dissociation Versus Roaming of OH.

Tirapazamine (TPZ) has been tested in clinical trials on radio‐chemotherapy due to its potential highly selective toxicity towards hypoxic tumor cells. It was suggested that either the hydroxyl radical or benzotriazinyl radical may form as bioactive radical after the initial reduction of TPZ in solution. In the present work, we studied low‐energy electron attachment to TPZ in the gas phase and investigated the decomposition of the formed TPZ− anion by mass spectrometry. We observed the formation of the (TPZ–OH)− anion accompanied by the dissociation of the hydroxyl radical as by far the most abundant reaction pathway upon attachment of a low‐energy electron. Quantum chemical calculations suggest that NH2 pyramidalization is the key reaction coordinate for the reaction dynamics upon electron attachment. We propose an OH roaming mechanism for other reaction channels observed, in competition with the OH dissociation.

Reversible intermolecular-coupled-intramolecular (RICI) proton transfer occurring on the reaction-radius a of 2-naphthol-6,8-disulfonate photoacid.

Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) from a reversibly dissociating photoacid, 2-naphthol-6,8-disulfonate (2N68DS). The reaction was carried out in water and in acetonitrile-water solutions. We find by carefully analyzing the geminate recombination dynamics of the photobase-proton pair that follows the ESPT reaction that there are two targets for the proton back-recombination reaction: the original O- dissociation site and the SO3 - side group at the 8 position which is closest to the proton OH dissociation site. This observation is corroborated in acetonitrile-water mixtures of χwater < 0.14, where a slow intramolecular ESPT occurs on a time scale of about 1 ns between the OH group and the SO3 - group via H-bonding water. The proton-transferred R*O- fluorescence band in mixtures of χwater < 0.14 where only intramolecular ESPT occurs is red shifted by about 2000 cm-1 from the free R*O- band in neat water. As the water content in the mixture increases above χwater = 0.14, the R*O- fluorescence band shifts noticeably to the blue region. For χwater > 0.23 the band resembles the free anion band observed in pure water. Concomitantly, the ESPT rate increases when χwater increases because the intermolecular ESPT to the solvent (bulk water) gradually prevails over the much slower intramolecular via the water-bridges ESPT process.

Adsorption Behavior of the Hydroxyl Radical and Its Effects on Monolayer MoS2.

Based on first-principles density functional theory calculations, we investigated a modified routine using hydroxyl adsorption that recently demonstrated the controlled growth of MoS2 monolayers. The new growth approach impedes the deposition of a second MoS2 layer; however, the hydroxyl adsorption and its effects have been mostly unexplored. Through this study, we first explored the adsorption behaviors of the hydroxyl radical (OH) on monolayer MoS2 and briefly discussed its effects on the stability and electronic structure. Monolayer MoS2 repels charged OH–, whereas the adsorption of the neutral OH radical is energetically favorable; the corresponding adsorption energies are 0.09 eV and −1.35 eV, respectively. The diffusion barrier of the OH radical on MoS2 is 0.52 eV, indicating that the molecule can quickly diffuse. Next, the study demonstrated that for multiple OH adsorptions, a concerted reaction including OH dissociation and H2O formation is more energetically favorable than the adsorption of two OH molecules by 2.50 eV, which in turn results in a mixed adsorption configuration of O and OH. In addition, we revealed that the OH adsorption creates a mid-gap state and facilitates the reconstruction of the MoS2 edge.