Proton Recombination Research Articles

Influence of low level of non-metal doping on g-C3N4 performance for H2 production from water under solar light irradiation

In this paper, a facile one step synthesis method for the preparation of C-, B-, P- and S-doped g-C3N4 by incorporation of small concentration of doping element precursor into urea during thermal polycondensation is reported leading to much lower doping levels than the ones usually reported. The as-obtained doped g-C3N4 photocatalytic materials are deeply characterized in terms of structural, morphological, surface and optical properties. Doping yields beneficial surface morphology modulation along with improved optical, electronic and photocatalytic properties. In particular, C-doped and S-doped g-C3N4 show, after deposition of gold nanoparticles (<1 wt%), enhanced photocatalytic performance (at least twice as high as the undoped photocatalyst, to achieve ca. 610 μmol/h/g) for the production of H2 by water splitting under solar light in the presence of low content (1 vol%) of triethanolamine (TEOA) as sacrificial agent. The most remarkable activity results from the incorporation of traces of S dopant, mainly inserted into interplanar hollow cavities. The enhanced activity is attributed to a combination of high surface area, location of the S dopant and small size of the co-catalyst NPs, which induces enhanced visible light harvesting, enhanced charge carrier separation and enhanced proton recombination. This work highlights the benefits of the optimized low-level doping strategy to overcome the main limitations of g–C3N4–based photocatalysts.

Marcus inversion is observed for excited state proton transfer in the adiabatic limit using naphthol based photoacids

Marcus theory is applied on ESPT reactions in the adiabatic limit, using cyano-naphthol derivatives. A systematic variation of the reaction exergonicity is achieved by changing the proton donor, solvent (acts as base) and the temperature. Marcus inverted regions are observed for the proton recombination steps when the solvent is changed and temperature variation was used to vary the reaction exergonicity.

One stone two birds: Vanadium doping as dual roles in self-reduced Pt clusters and accelerated water splitting

Integrating active Pt clusters into transition-metal oxides with water-dissociation ability is effective to prepare a bifunctional electrocatalyst for water splitting in alkaline. However, the additional utilization of a reductant and/or the operation at the elevating temperature causes the over-growth and agglomeration of Pt clusters, thus losing the high catalytic performance. Herein, we report that V dopant not only favors self-reducing Pt clusters on NiFe layered double hydroxide (LDH) (Pt/NiFeV) at room temperature, but also regulates interfacial charge redistribution to enhance the water-splitting performance. Experimental and theoretical studies reveal that V dopant into NiFe LDH triggers more electrons to transfer to adjacent Fe atoms, thus leading to a higher reducing ability compared to that without V-doping. When used as water-splitting electrocatalyst, V doping promotes electron loss of Pt clusters in Pt/NiFeV, optimizing the free energy of hydrogen adsorption and proton recombination kinetics at the cathode. Meanwhile, it also moves the d-band center of Ni away from the Fermi level to optimize the adsorption of *OH intermediates and facilitate the desorption of oxygen molecules at the anode. Thereby, Pt/NiFeV exhibits much higher bifunctional performance than V-free Pt/NiFe LDH toward both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This work can spark inspiration of designing other bifunctional electrocatalysts for energy conversion and storage.

Tantalum Nitride As a Promising Catalyst for Hydrogen Evolution Reaction

Stable and active catalyst material among transition-metal nitrides (TMNs) have been investigated via density functional theory (DFT) calculations for possibility of catalysing the hydrogen evolution reaction (HER). The result from DFT modelling revealed that a full monolayer of proton was formed on the (100) facet of the rocksalt structure of TaN surface. The H coverage as a function of applied potential was calculated on the surface too. Activation energy of H2 formation was also calculated at varying applied potentials. The rate of the reaction was calculated and compared with that of Pt(111). The results nominated TaN capable of catalysing HER at low overpotentials of around −0.1 V vs. a reversible hydrogen electrode, with a similar activation energy for proton recombination from the surface to make H2 as that reported for Pt(111). In addition, TaN was predicted to function as good as MoS2 with regards to activity. This is interesting result that can motivate experimentalists to test this material further for the possibility of its real-life applications.

Insights into Proton Recombination in Ceramic Proton Conducting Electrodes

The effect of catalyst addition on H2 evolution from composite electrodes of La0.7Sr0.3TiO3 (LST) and BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) was studied. Starting with symmetric cells (LST∣∣BZCYYb∣∣LST), Pt was added to one or both electrodes, after which i–V polarization measurements were performed in humidified H2 at 723 and 773 K. The base cells showed very high impedances but these decreased dramatically upon addition of Pt to both electrodes. When Pt was added to only one electrode, the cells performed as diodes, showing that Pt was necessary for H2 dissociation but not for H recombination. The effects of adding Ru, W, Re and Fe were also studied. DFT calculations helped confirm that H recombination on BaZrO3 is expected to be barrierless. The implications of these results for potential application to electrochemical synthesis of ammonia are discussed.

Tracking Subtle Membrane Disruptions with a Tethered Photoacid

AbstractFluorescent dyes are common tools for tracking biological membranes. In this work, we introduce a new ratiometric fluorescent probe for membranes based on a surface‐sensitive photoacid. As all other probes, it lights up within membranes, and furthermore, it has different ratios of fluorescent peaks for different membranes. The main unique feature of the probe is its ability to sense events taking place in the membrane far away from its actual location. This is due to the photoacidity of the probe, and the associated kinetics of its excited‐state proton release and capture that are highly sensitive to the lateral proton diffusion on the surface of the membrane. We show here that minor disruptions to the membrane integrity, induced by the Triton X‐100 surfactant at low concentration (surfactant:lipid ratio of 0.02), can be sensed by our probe using steady‐state fluorescence, while other probes are invisible to them. Time‐resolved fluorescence reveals that the surfactant insertion affects the lateral proton diffusion and accordingly the measured proton recombination of the photoacid.

Exploring the reactivity of β-cyclodextrin-encapsulated anthraquinone-2,6-disulfonate

The intracomplex reaction of anthraquinone-2,6-disulfonate (AQDS) in triplet excited state (3AQDS) with beta-cyclodextrin (CD) and the reaction of 3AQDS:CD complex with external quencher, N-Acetyl-l-Tyrosine (TyrO−), have been studied using laser flash photolysis, 1H NMR and 1H CIDNP. The two reactions were found to be competing with each other. The intracomplex hydrogen abstraction previously proposed for 3AQDS:CD complex may represent a two-step process, where at first stage an electron is transferred from the –OH group of CD to 3AQDS followed by a proton transfer and recombination of AQDS and CD radicals resulting in a stable AQDS-CD product. The rate constant of the intracomplex electron transfer from CD to 3AQDS was found to be 6 ± 1 × 106 s−1 and the rate of subsequent proton transfer is 1 ± 0.1 × 106 s−1. The rate constant of electron transfer between TyrO− and 3AQDS:CD complex (0.4 ± 0.1 × 109 M−1 s−1) drops approx. by half compared to that in the reaction between TyrO− and free 3AQDS (0.8 ± 0.1 × 109 M−1 s−1).

Accelerating hydrogen evolution at neutral pH by destabilization of water with a conducting oxophilic metal oxide

Self-supporting Ni4Mo–V2O3 nanosheets, which combine oxophilic V2O3 as a water dissociation center with Ni4Mo as a proton recombination site, display an extremely low overpotential (39.3 mV at 10 mA cm−2) for hydrogen evolution at neutral pH.

Nanometric Ni5P4 Clusters Nested on NiCo2O4 for Efficient Hydrogen Production via Alkaline Water Electrolysis

AbstractA number of non‐noble catalysts are developed for hydrogen production via acidic water electrolysis. Nevertheless, for the more economical alkaline hydrogen generation, the restricted kinetics of the water dissociation Volmer step along with its following proton recombination Tafel step for these non‐noble electrocatalysts generally lead to sluggish hydrogen‐production process. Here, a facile method is designed to nest nanometric Ni5P4 clusters on NiCo2O4 (achieving Ni5P4@NiCo2O4) by a phosphating process of NiO clusters on NiCo2O4. Acting as a high‐efficiency electrode for alkaline water electrolysis, the Ni5P4@NiCo2O4 can efficiently and preferentially convert H2O to H2 with a low overpotential of 27 mV at 10 mA cm−2 and the Tafel slope of 27 mV dec−1, which are comparable to the results for platinum and superior than those of the state‐of‐the‐art platinum‐free electrocatalysts. Density functional theory calculations confirm that NiCo2O4 species exhibit a higher ability to electrolyze water into H* intermediate and then Ni5P4 clusters facilitate the subsequent desorption of the H2 products. Profiting from the promoted kinetic steps, the Ni5P4@NiCo2O4 electrocatalyst is promising for scalable alkaline hydrogen production.

Insights into Excited State Intramolecular Proton Transfer: An Alternative Model for Excited State Proton Transfer of Green Fluorescence Protein

The excited state intramolecular proton transfer (ESIPT) that occurs in the o-sulfonamide analogue ( o-TsABDI) of the green fluorescent protein (GFP) chromophore provides an alternative model to get insights into the excited state proton transfer (ESPT) related photophysics of GFP. In this article, we explored the ESIPT-related photophysics of o-TsABDI by electronic absorption and fluorescence emission spectra in a wide polarity range of solvents, cis- trans photoisomerization experiment, and Coulomb-attenuating method (CAM)/time-dependent (TD) density functional theory (DFT) calculations. We found that the whole ESIPT process involves four steps. The first step is photoexcitation of o-TsABDI, which does not involve charge transfer (CT). The second step is ESIPT and accompanying electron transfer from the n orbital of the sulfonamide nitrogen to the half-filled π orbital of the 4-benzylideneimidazolone moiety. The third step is fluorescence emission of the zwitterionic o-TsABDI and accompanying CT from the π* orbital of the 4-benzylideneimidazolone moiety to the half-filled n orbital of the sulfonamide nitrogen. The last step involves irreversible and barrierless proton recombination. In contrast to the isolated GFP chromophore and its p- and m-amino analogues, the S1 excited state of o-TsABDI does not relax by way of cis- trans photoisomerization through the S1/S0 conical intersection CI(I) by rotating around the I-bond, but follows the ESIPT pathway. The low fluorescence quantum yield of the zwitterionic o-TsABDI might be due to (1) the fluorescence that involves the low-probability π* → n charge transfer and (2) nonradiative relaxation through the S1/S0 conical intersection CI(P″) by rotating around the P-bond.

In English

We investigate the conditions for increase of the photoluminescence of CdS nanocrystals in polyethyleneimine and polyethyleneimine with carbon multiwall nanotubes on macroporous silicon substrates. Macroporous silicon structures are made using the photoelectrochemical etching of n‑Si wafers. Macropores with diameter D p = 2¸5 mm, depth h p = 40¸120 mm and concentration N p = (1¸6)×10 6 cm ‑2 were formed. CdS nanocrystals of 1.8–2 nm in size are synthesized in reaction between Cd 2+ and HS - in a colloidal solution of polyethyleneimine in water. C arbon multiwall high purity nanotubes with submicron length and 20 nm diameter were produced by catalytic pyrolysis of unsaturated hydrocarbons. “Polymer-nanoparticles” coatings are deposited from a colloidal solution in water on single crystalline silicon, macroporous silicon and oxidized macroporous silicon. The maximal photoluminescence intensity of CdS nanocrystals is measured f or the oxidized macroporous silicon substrates with maximal electric field strength at the Si- SiO 2 interface . The photoluminescence quantum yield of CdS nanocrystals on the surface of oxidized macroporous silicon increases with time and reaches 28 %. Photoluminescence of polyethyleneimine with carbon multiwall nanotubes on macroporous silicon with microporous layer is 3–6 times more intense as compared with substrates c-Si, macroporous Si and oxidized macroporous Si due to a non-radiative proton recombination decrease at the silicon matrix boundary with microporous layer and nanocoating.

Ab initio molecular dynamics study of the mechanism of proton recombination with a weak base.

Despite its fundamental nature, many of the microscopic features of acid–base recombination remain poorly understood. In this work, we use ab initio molecular dynamics simulations to study the recombination of the proton with a weak base, the carbonate ion CO3(2–). Our simulations elucidate the network structure around CO3(2–) that provides a distribution of pathways over which recombination can occur. We observe that the penultimate neutralization step involves a correlated behavior of the transferred protons that is mediated by the water wires decorating the carbonate. These concerted proton transfers are coupled to collective compressions of these water wires. We show further that these processes are dynamically coupled to the reorganization of the water molecules hydrating the CO3(2–) ion. The insights from these simulations help to bridge the structural and dynamical complexity of the microscopic mechanisms with those of phenomenological models invoked by experiments in this field.

Impact of additives on zirconium oxidation by water: mechanistic insights from first principles

Zirconium alloys are widely used as cladding materials employed to contain the fission fuel in nuclear power plants. A limiting factor for fuel longevity is the corrosion property of the zirconium alloys. In the reactor, the main corrodent is water. The oxide forms thermodynamically during hydrogen evolution. Due to the corrosion mechanism, a fraction of the hydrogen is transferred to the alloy. It has long been known that the alloying elements actually control the hydrogen pick-up fraction, HPUF. A mechanism that explains these observations by means of density functional theory calculations is presented and validated. A hydroxylated grain boundary model decorated by various transition metal, TM, ions is employed to study the dependence of the hydrogen evolution reaction, HER, on the choice of TM ion and spin state along the hydride–proton recombination pathway. The efficiency of the system to utilize the overpotential for hydrogen evolution, originating from the overall corrosivity of the alloy, is found to be decisive for the HPUF. A dual origin of the detrimental effects of Co and Ni additives on the HPUF is identified.

Comprehensive Studies on an Overall Proton Transfer Cycle of the ortho-Green Fluorescent Protein Chromophore

Initiated by excited-state intramolecular proton transfer (ESIPT) reaction, an overall reaction cycle of 4-(2-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), an analogue of the core chromophore of the green fluorescent protein (GFP), has been investigated. In contrast to the native GFP core, 4-(4-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (p-HBDI), which requires hydrogen-bonding relay to accomplish proton transfer in vivo, o-HBDI possesses a seven-membered-ring intramolecular hydrogen bond and thus provides an ideal system for mimicking an intrinsic proton-transfer reaction. Upon excitation, ESIPT takes place in o-HBDI, resulting in a ∼600 nm proton-transfer tautomer emission. The o-HBDI tautomer emission, resolved by fluorescence upconversion, is comprised of an instantaneous rise to a few hundred femtosecond oscillation in the early relaxation stage. Frequency analysis derived from ultrashort pulse gives two low-frequency vibrations at 115 and 236 cm(-1), corresponding to skeletal deformation motions associated with the hydrogen bond. The results further conclude that ESIPT in o-HBDI is essentially triggered by low-frequency motions and may be barrierless along the reaction coordinate. Femtosecond UV/vis transient absorption spectra also provide supplementary evidence for the structural evolution during the reaction. In CH(3)CN, an instant rise of a 530 nm transient is resolved, which then undergoes 7.8 ps decay, accompanied by the growth of a rather long-lived 580 nm transient species. It is thus concluded that following ESIPT the cis-proton transfer isomer undergoes cis-trans-isomerization. The results of viscosity-dependent dynamics are in favor of the one-bond-flip mechanism, which is in contrast to the volume-conserving isomerization behavior for cis-stilbene and p-HBDI. Further confirmation is given by the picosecond-femtosecond transient IR absorption spectra, where several new and long-lived IR bands in the range of 1400-1500 cm(-1) are assigned to the phenyl in-plane breathing motions of the trans-proton transfer tautomer. Monitored by the nanosecond transient absorption, the 580 nm transient undergoes a ∼7.7 μs decay constant, accompanied by the growth of a new ∼500 nm band. The latter is assigned to a deprotonated tautomer species, which then undergoes the ground-state reverse proton recombination to the original o-HBDI in ∼50 μs, achieving an overall, reversible proton transfer cycle. This assignment is unambiguously supported by pump-probe laser induced fluorescence studies. On these standpoints, a comparison of photophysical properties among o-HBDI, p-HBDI, and wild-type GFP is discussed in detail.

Dynamics of proton recombination with NO3− anion in water clusters

Recombination events of a proton with NO3- at (H2O)8 clusters are studied by molecular dynamics, using "on-the-fly" reliable ab initio MP2 potentials. The main findings are: (1) the lifetime of the ions is less than 1.2 picoseconds; (2) the recombination step invariably involves H3O+, not H5O2+; and (3) an essentially unique transition-state structure of H3O+/NO3- for recombination is found in all cases. Proton migration involves both H3O+ and H5O2+ species: Grotthuss and other mechanisms contribute.

Ultrafast Excited-State Dynamics in the Green Fluorescent Protein Variant S65T/H148D. 3. Short- and Long-Time Dynamics of the Excited-State Proton Transfer

Steady-state emission, femtosecond pump-probe spectroscopy, and time-correlated single-photon counting (TCSPC) measurements were used to study the photophysics and the excited-state proton transfer (ESPT) reactions in the green fluorescent protein (GFP) variant S65T/H148D at three pH values: 6.0, 7.9, and 9.5. Selective mutation of GFP caused a dramatic change in the steady-state and excited-state behavior as compared to the wild-type GFP (wt-GFP) studied earlier. An excitation wavelength dependence of the quantum yield of the strong emission band at 510 nm (I* band) indicates the competition between adiabatic and non-adiabatic excited-state proton-transfer reactions. Pump-probe measurements show that the signal buildup probed at 510 nm is biphasic, where 0.8 of the signal amplitude is ultrashort, <150 fs, and 0.2 of the signal decreases with a approximately 10 ps time constant. This effect is a summary result of adiabatic ESPT to the carboxylate group of Asp148 and nonradiative processes. When compared with the luminescence of wt-GFP, the steady-state intensity at 450 nm is lower by a factor of about 10. This very weak emission at 450 nm has a nonexponential decay. It is relatively pH insensitive and, at about 25 ps, is almost twice as long as in wt-GFP. The former exhibits a surprisingly small kinetic deuterium isotope effect (KDIE), compared with the KDIE of about 5 for wt-GFP. Such weak proton dependence may indicate that this emission comes from the species not directly involved in the ESPT. In contrast, pH and H/D isotope dependence of the intense nonexponential luminescence decay of the S65T/H148D deprotonated form measured at 510 nm may result from an isomerization-induced deactivation that is accompanied by the proton recombination quenching. The data are complementary to the femtosecond time-resolved emission data obtained by ultrafast fluorescence up-conversion spectroscopy, found in the preceding paper (Shi et al.). The spectroscopic results are discussed on the basis of the detailed X-ray structure of the mutant published in the preceding paper (Shu et al.).

Geminate Proton Recombination at the Surface of SDS and CTAC Micelles Probed with a Micelle-Anchored Anthocyanin

The functionalized flavylium salt 6-hexyl-7-hydroxy-4-methyflavylium chloride (HHMF) was employed to probe some of the fundamental features of proton transfer reactions at the surface of anionic sodium dodecyl sulfate (SDS) and cationic hexadecyltrimethylammonium chloride (CTAC) micelles. In contrast to most ordinary flavylium salts, HHMF is insoluble in water, but readily incorporates into SDS and CTAC micelles. In the ground state, the rate constant for deprotonation of the acid form (AH+) of HHMF decreases 100-fold upon going from CTAC (kd = 3.0 x 10(6) s(-1)) to SDS (kd = 1.4 x 10(4) s(-1)), consistent with the presence of an activation barrier for proton transfer in the ground state and reflecting, respectively, stabilization or destabilization of the AH+ cation by the micelle. Reprotonation of A is diffusion-controlled in both micelles (kp(SDS) = (2.1 x 10(11))[H+]aq s(-1) and kp(CTAC) = (3.7 x 10(8))[H+]aq s(-1)), the difference reflecting the rate of proton entry into the micelles. In the excited singlet state, the rate constants for deprotonation of the AH+* form of HHMF are similar in the two micelles (2.4 x 10(10) s(-1)), consistent with activationless proton transfer. Reprotonation of the excited A is dominated by fast geminate recombination of the photogenerated (A*-H+) pair at the micelle surface (k(rec)(SDS) = 6.1 x 10(9) s(-1) and k(rec)(CTAC) = 3.4 x 10(10) s(-1)) and the net efficiencies of geminate recombination are quite similar in SDS (0.89) and CTAC (0.86).

Saturation Effect in the Temperature Dependence of a Proton Recombination with a Photobase

The temperature dependence of the reaction of a proton recombination with a photobase is studied in glycerol−water mixtures. For this purpose, we used a strong photobase, 7-hydroxy-4-methyl coumarin (4CUOH). The experimental data are analyzed using the bimolecular irreversible Smoluchowski theory. At high temperatures, the proton recombination rate is almost temperature independent, whereas at low temperatures, the rate constant has strong temperature dependence. The unusual temperature dependence is explained using proton-tunneling theory, based on the Landau−Zener curve crossing formulation. The high-temperature behavior of the rate constant denotes the nonadiabatic limit, whereas the low-temperature behavior denotes the adiabatic limit. We used an approximate expression for the proton-transfer rate, which bridges the nonadiabatic and the solvent controlled adiabatic limit to fit the temperature dependence curve of the experimental proton-transfer rate constant.

Photoinduced Coupled Proton and Electron Transfers. 2. 7-Hydroxyquinolinium Ion

The hydroxyl group of 7-hydroxyquinoline in acidic media (i.e., of 7-hydroxyquinolinium cation, 7-HQ(C)) undergoes photoinduced deprotonation even in strongly acidic media. It occurs from dilute perchloric acid solutions up to 8 mol L-1 concentration, which is evidence of an outstanding photoacidity. Excited-state proton ejection was shown to be reversible at acid concentrations greater than 1 mol L-1. Below this value, the proton ejection was not balanced by the proton recombination. Concerning the rates of the excited-state reactions, (i) the proton ejection rate constants k1 increased upon acid dilution, attaining at high dilution the limiting value of (k1)0 = 5.5 × 1010 s-1. k1 was shown to vary with the water activity according to k1 = (k1)0(aH2O)4. The number 4 is then a key number for the ability of water to accept a proton from 7-HQ(C*). (ii) The proton recombination rate constant varies from 3 × 109 to 4 × 1010 s-1 when the acid concentration ranges from 1 to 5 mol L-1. This observation can tenta...

Competitive geminate quenching and geminate recombination reactions of 1‐naphthol

AbstractThe back proton recombination following the photo‐dissociation of 1‐naphthol was studied by a time‐correlated single‐photon counting apparatus. The measured picosecond time‐resolved fluorescence decay profiles were analyzed using the numerical solution of the Debye‐Smoluchowski equation with coupled reversible and irreversible proton back‐recombination reactions at the source.A full kinetic analysis shows that both the reversible geminate recombination reaction, which assumes at long times a t−3/2 dependence, and the irreversible geminate quenching reaction, which assumes at long times a t−1/2 dependence, are significant in determining the excited‐state kinetics of 1‐naphthol.