[Show abstract][Hide abstract] ABSTRACT: The antioxidants in biological organisms can scavenge excess free radicals and effectively reduce oxidative stress, which protects DNA, protein and lipids in the human body from damage, thus preventing diseases from being induced. Therefore, it is particularly significant to assay the antioxidant capacities of our habitual foods during dietary evaluation. Herein, ultrathin graphitic carbon nitride (utg-C3N4)/TiO2 composites have been introduced as sensing elements into a photoelectrochemical platform with a thin layer structured flow-cell, for the real-time assay of the global antioxidant capacity in practical samples. In this system, the two-dimensional utg-C3N4 nanosheet/TiO2 nanoparticle composite material provided a much better optoelectronic function than the individual materials. In comparison with previous reports, this photoelectrochemical strategy shows considerable advantages, including excellent anti-interference properties, a high level of stability and reproducibility, and it is also proved to be the most prompt, convenient and cost-effective method for antioxidant capacity detection up to now. Moreover, utilizing theoretical and experimental examinations, we revealed its photoelectrochemical sensing mechanism in depth. It is proposed that the developed method will pave the way for the development of excellent antioxidant assays with the advantages of photoelectrochemistry and fluidic cells . It is expected to be further applied in food quality inspections and health guides, as well as in other fields.
[Show abstract][Hide abstract] ABSTRACT: TiO2 is an abundant and environmentally benign material, but has a wide band gap, which greatly confines its applications in photocatalysis. Doping and modifying the material composition are both generally used to change and control the photocatalytic activity of semiconductors. Herein, we describe a method and resulting activity of depositing Ce-/S-codoped TiO2 nanoparticles (NPs) on water-soluble sulfonated graphene (SGE) sheets, which guarantees a direct contact and satisfactory electron transfer between the semiconductor and graphene. The Ce/S–TiO2 NPs are homogeneously fixed on the surface of SGE sheets with an average particle size of 7 nm. The resulting composite showed noticeable activity in photodegrading methyl orange (κ = 0.425 h−1). This improved performance can be attributed to the synergistic effects of Ce- and S-codoping toward TiO2 and the composite action between TiO2 NPs and SGE. This type of novel composite is expected to stimulate the development of doped and graphene-involved photocatalysts for addressing environmental problems.
[Show abstract][Hide abstract] ABSTRACT: The usage of coplanar π-conjugated segments represents a feasible strategy on reducing the energy gap of organic push–pull dyes for mesoscopic titania solar cells. In this paper, we report two new dyes coded as C254 and C255 with the respective 1,4-di(thiophen-2-yl)benzene and indacenodithiophene π-linkers, in combination with the electron-releasing triphenylamine and electron-withdrawing cyanoacrylic acid units. The energy-gap reduction stemming from the rigidity of the π-linker is accompanied by a negative shift of the ground-state redox potential, which however does not affect the yield of hole injection from the oxidized state of dye molecules to a cobalt redox electrolyte. On the other side, we have identified from femtosecond transient absorption measurements a diminished rate of electron injection from the relaxed, low-energy excited state of C255 to titania, albeit a comparable rate of electron injection from the high-energy excited states of these two dyes. The bulkier C255 dye with four hexyl side chains tethered on the two sp3 carbons of the fused indacenodithiophene unit can form a more compact self-assembling monolayer on titania, considerably attenuating the charge recombination of photoinjected electrons in titania with the cobalt electrolyte and thus enhancing the cell photovoltage and efficiency.
The Journal of Physical Chemistry C. 02/2014; 118(6):2977–2986.
[Show abstract][Hide abstract] ABSTRACT: Correlation between current–voltage curves and recombination kinetics of dye-sensitized solar cells was a key subject for understanding the operation mechanisms and improving the device performance. A galvanostatic constant intensity light perturbation (GCILP) technique carried out on the current–voltage curve was developed to discover the correlation. The technique focused on synchronously deriving recombination kinetics and energetic distribution of trap state from the photovoltage responses and reconstructing the current–voltage curve by these derived kinetic parameters. In this technique, the photovoltage response amplitude was analyzed to obtain recombination kinetic parameters such as equilibrium dark recombination current density (or exchange current density) and recombination reaction order; the photovoltage response time trace was used to determine energetic distribution of trap states. Based on these analysis results, not only the effects of conduction band shifts and changes in the recombination rate on the open-circuit voltage could be analyzed but also the current–voltage curves could be successfully reconstructed. So this technique provided a new more convenient approach for efficiently evaluating and deeply understanding the important characteristics of solar cells.
The Journal of Physical Chemistry C. 07/2013; 117(31):15924–15932.
[Show abstract][Hide abstract] ABSTRACT: A novel photoelectrochemical sensor has been designed with polyaniline-reduced graphene oxide-titanium dioxide, which was further applied to sense gallic acid and exhibited extraordinary rapid response, high sensitivity and excellent anti-inference. Meanwhile, the mechanism has been elaborately explored.
Chemical Communications 07/2013; · 6.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A numerical model for interpretation of the light-intensity-dependent nonlinear characteristics of the short-circuit current in dye-sensitized solar cells is suggested. The model is based on the continuity equation and includes the influences of the nongeminate recombination between electrons and electron acceptors in the electrolyte and the geminate recombination between electrons and oxidized dye molecules. The influences of the order and rate constant of the nongeminate recombination reaction, the light-absorption coefficient of the dye, the film thickness, the rate constant of geminate recombination, and the regeneration rate constant on the nonlinear characteristics of the short-circuit current are simulated and analyzed. It is proposed that superlinear and sublinear characteristics of the short-circuit current should be attributed to low electron-collection efficiency and low dye-regeneration efficiency, respectively. These results allow a deep understanding of the origin of the nonlinear characteristics of the short-circuit current in solar cells.
[Show abstract][Hide abstract] ABSTRACT: We report two triarylamine-cyanoacrylic acid based push–pull dyes C252 and C253 featuring the π-conjugated linkers of 2,6-di(thiophen-2-yl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole and 4H,4′H-2,2′-bidithieno[3,2-b:2′,3′-d]pyrrole, respectively. Benefitting from an improved coplanarity of the conjugated units, the C253 dye displays a red-shifted absorption peak and an enhanced maximum molar absorption coefficient in comparison with C252. However, this pattern of conjugated linker alternation is associated with an 80 mV negative shift of the ground-state oxidation potential, which dominates an almost 5 times reduced rate of hole injection from the oxidized state of C253 to the divalent tris(2,2′-bipyridine)cobalt (Co-bpy) cation in the redox electrolyte, resulting in a considerably poor net charge separation yield. On the other side, a dye-sensitized solar cell employing the C252 photosensitizer and the Co-bpy electrolyte exhibits a good power conversion efficiency of 9.5% measured under the 100 mW cm−2 simulated AM1.5 sunlight. The dissimilarity of cell photovoltage is scrutinized by evaluating the shift of the titania conduction band edge and the variation of interfacial charge recombination kinetics, the latter of which presents a clear correlation with dye coating thickness on titania derived from X-ray photoelectron spectroscopy measurements. Our work has underlined the important energetic and kinetic interplays which should be seriously considered in the further optimization of active components in dye-sensitized solar cells.
Energy & Environmental Science 04/2013; 6(5):1604-1614. · 11.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: OH radicals as reactive oxygen species in an organism were applied to assay antioxidant capacity since the obtained results present high biological relevance. As a proper photocatalyst, titanium dioxide was employed to generate OH radicals under ultraviolet light irradiation. However, ultraviolet light can damage molecular probe (DNA or protein) during the detection of antioxidant capacity, which interferes with the results. In this article, a novel composite graphene oxide-titanium dioxide (GO-TiO2) nanostructure was synthesized, which can generate numerous OH radicals just under visible light irradiation. In addition, a novel electrochemical antioxidant capacity sensor was designed with GO-TiO2 composites as source of OH radicals and DNA as a molecular probe. Antioxidants were measured by using the suppression of the decline of reduction current of methylene blue used as an intercalating agent for DNA after irradiation and ˙OH-mediated DNA damage. Using gallic acid (GA) as a mode antioxidant species, the detection of GA at levels as low as 0.85 mg L-1 was possible. The antioxidant capacity of other antioxidants was also assayed. Finally, the novel sensor was applied to the determination of antioxidant capacity in tea.
[Show abstract][Hide abstract] ABSTRACT: A new research strategy for determining the conduction band movement of TiO(2) films and charge recombination between electrons in the TiO(2) film and electron acceptors in the electrolyte was proposed. Steady-state short-circuit current density versus open-circuit voltage was employed to attain the exchange current density and recombination reaction order. Transient photovoltage decay and open-circuit voltage decay measurements were carried out to obtain the energetic distribution of trapped electrons. Reduced voltage-dependent trapped electron concentration and trapped electron concentration-dependent recombination current density were used to analyze influence factors of open-circuit voltage, including contributions from conduction band movement and charge recombination. The simulated and measured electron concentration were in agreement and confirmed the validity of this method for extracting conduction band movement and recombination parameters. This new approach provides a physical insight which could help us to more conveniently and efficiently understand the operation of DSCs.
Physical Chemistry Chemical Physics 11/2012; · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A new strategy based on a thick organic film modified electrode allowed us, for the first time, to explore the voltammetric processes for a series of hydrophilic ions by electrochemically induced redox transformations of common molecular probes. During the limited time available for voltammetry, this thick organic film ensured that the generated product of the molecular probe, which is within a limited diffusion layer, was kept far away from the aqueous-organic solvent interface; therefore, regardless of the degree of hydrophobicity, the generated product never participates in ion exchange across the interface and the charge neutrality of the organic film (containing an extremely hydrophobic electrolyte) can only be maintained by the injection of ions from the aqueous phase. Taking advantage of this fact, common redox probes, such as ferrocene (Fc) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), which are almost useless for both three-phase electrode (TPE) and thin-layer cyclic voltammetry (TLCV) methods, can induce the transfer of numerous highly hydrophilic anions and cations. Consequently, the majority of their Gibbs transfer energies have been accurately determined for the first time to the best of our knowledge. With this in mind, using TCNQ as a redox probe to induce facilitated cation transfer, a stategy that is more advantageous than traditional methods has been developed. The main advantages are that: (i) voltammetric experiments performed on this system were free from the polarized potential window (ppw) in the aqueous phase and, as a result, this allowed the assessment of weakly assisted ion transfers, which appear at the terminal of the ppw at single polarized interfaces; (ii) without introducing the tetraphenylarsonium-tetraphenylborate (TPAs-TPB) thermodynamic assumption, one can conveniently evaluate both the association constant and the stoichiometric parameter between the ion and its ionophore by comparison of their direct and facilitated ion transfer voltammograms. These encouraging results illustrated the exciting innovation for assessing direct and facilitated ion transfers based on this new thick organic film modified electrode.
Physical Chemistry Chemical Physics 03/2012; 14(10):3659-68. · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the dependence of the photovoltaic performance of dye-sensitized solar cells on the cations with different charge densities, such as lithium (Li(+)), sodium (Na(+)), potassium (K(+)), and dimethylimidazolium (DMI(+)). The efficiencies of light harvesting, electron injection and charge collection were evaluated to clarify the influence of cation selection on photocurrent generation. It is found that the short-circuit photocurrents of DSCs gradually diminish with decreasing cation charge densities, partially owing to reduced electron injection rates which are intimately related to the reaction Gibbs free energies. Further experiments indicate that the upward movement of conduction band edge results in decreased reaction Gibbs free energy of electron injection from Li(+) to DMI(+). At an irradiation of 100 mW cm(-2) AM1.5 sunlight, the open-circuit photovoltage and the fill factor of a typical dye-sensitized solar cell increase in the order of Li(+) < Na(+) < K(+) < DMI(+). Analyses of impedance data reveal that the increase of cell photovoltage mainly correlates with the upward shift of the conduction band edge induced by the adsorption of low-charge-density cations on the surface of titania nanocrystals. A J-V model was proposed to understand the improvement of the fill factor. It is found that the increase of the fill factor stems from the decrease of recombination current density under the equilibrium state in the dark by fitting the J-V data with the model.
Physical Chemistry Chemical Physics 08/2011; 13(32):14590-7. · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we report a theoretical study exploring the stochastic signal from electrochemical detection toward the nanometer scale systems. By using a discrete simulation as well as a statistical theory, this work investigates the distribution of the capturing times for redox molecules that released at a given distance from an electrode. We find that the capturing times tend to distribute over a wider range with a larger distance between the molecule’s source and the electrode, and thus the corresponding electrochemical signal would be somewhat less repeatable.
[Show abstract][Hide abstract] ABSTRACT: We present a more general expression for the relationship of potential dependence, which implies that a change in the interfacial drop across the interface has little effect on the free energy of the reaction, but mainly affects the surface concentration of reactant in each phase. Abundant experimental results from several well-known groups are analyzed in great detail to confirm our conclusion. At the same time, we define a new parameter named Frumkin correction factor to describe this relationship of potential dependence, which expresses the thermodynamic effect of double diffuse layers within both phases in contrast with the so often suggested kinetic electron-transfer (ET) coefficient; we also find that it depends on two intimately related aspects: the charges of reactive species and the ratio of the diffuse layer potential to the total potential within each phase, so it is quite arbitrary to ignore the diffuse layer effect in the aqueous phase just because of its relatively small values. In addition, a fascinating question on the inverted region at liquid/liquid interfaces has been successfully interpreted by an opposite surface concentration effect, which was often considered as a kinetic Marcus inverse by most groups.
Physical Chemistry Chemical Physics 02/2011; 13(7):2774-9. · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Here we propose a new and facile route based on a water-droplet modified approach to explore the voltammetric processes for a series of typical heavy metal ions at the water|1,2-dicholorethane (W|DCE) interface. This convenient method having a selective transfer characteristic indicates that a higher concentration of target ion setting can avoid the masking transfers from other interferential species. The determined Gibbs transfer energies follow an order, Pb2+
[Show abstract][Hide abstract] ABSTRACT: For a sphere electrode enclosed in finite-volume electrolyte, the measured current will deviate from the result predicted by the semi-infinite diffusion theory after some time. By random-walk simulation, we compared this time to the one needed for diffusion layer to reach electrolyte boundary, and revealed a clear signal delay of electrochemical current. Further we presented a quantitative description of this delay time. The simulation results suggested that the semi-infinite diffusion theory can even be applied when the theoretical diffusion layer grows to 1.28 electrolyte thicknesses, with an accuracy better than 0.5%. We attributed this time delay to the molecules’ finite propagation velocity. Finally, we discussed how this delay can influence and facilitate the following electrochemical detection towards the nanometer and single-cell scale.
Journal of Electroanalytical Chemistry. 01/2009; 633(1):235-239.
[Show abstract][Hide abstract] ABSTRACT: In this paper, based on Einstein relationship be-tween diffusion and random walk, the electrochemical behavior of a system with a limited number of molecules was simulated and explored theoretically. The transition of the current vs time responses from discrete to continuous was clearly obtained as the number of redox molecules increased from 10 to 10 6 . By correlation analysis between the simu-lation results and the results of analytical expressions, a quantized extent parameter was proposed to investigate the underlying rules of these discrete signals, which looked stochastic. The results revealed that this parameter would be useful to describe such systems.
Journal of Solid State Electrochemistry 09/2008; 12(6):701-706. · 2.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work, a new method for the simultaneous determination of Pb(II) and Cd(II) on the multiwalled carbon nanotubes (MWNT)-Nafion-bismuth modified glassy carbon electrode (GCE) using square-wave anodic stripping voltammetry has been studied. Scanning electron microscopy was used to investigate the characteristics of the MWNT-Nafion-bismuth modified GCE. Well-defined sharp stripping peaks were observed in the determination of Pb(II) and Cd(II) simultaneously on this electrode. Under optimized conditions, the lowest detectable concentrations were 50 ng/l for Pb(II) and 80 ng/l for Cd(II) under a 10 min preconcentration. The attractive performances of MWNT-Nafion-bismuth modified GCE demonstrated its application for a simple, rapid, and harmless determination of trace heavy metals.