Photoelectrochemical Kinetics of Eosin Y-Sensitized Zinc Oxide Films Investigated by Scanning Electrochemical Microscopy

Transparent Ultramicroelectrodes for Studying Interfacial Charge-Transfer Kinetics of Photoelectrochemical Water Oxidation at TiO2 Nanorods with Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) has been extensively applied to the electrochemical analysis of the surfaces and interfaces of a photoelectrochemical (PEC) system. A semiconductor photoelectrode with a well-defined geometry and active surface area comparable to SECM's tip is highly desired for accurately quantifying interfacial charge-transfer activities and photoelectrochemically generated redox species, where the broadening effects due to the mass transfer gradient and nonlocal electron transfer at a planar semiconductor surface can be minimized. Here, we present a newly developed platform as a SECM substrate for investigating semiconductor PEC activities, which is based on a transparent ultramicroelectrode (UME) fabricated by using two-step photolithographic patterning and ion milling methods. This transparent UME with a 25 μm recessed disk shape is fully characterized with SECM for quantifying the interfacial charge-transfer rates of IrCl₆^2-/IrCl₆^3- by comparing with theoretical results from finite element simulations in COMSOL Multiphysics. When coated with TiO₂ nanorods as a model semiconductor material, the transparent UME can be used to quantify the catalytic PEC water oxidation in a feedback mode of SECM by sampling tip and substrate current signals simultaneously. This transparent UME-SECM study provides insights into the potential-dependent PEC water oxidation reaction mechanism and the quantitative analysis of photocurrent contributions from water oxidation and the SECM tip-generated redox mediator. The transparent UME-SECM method can be potentially expanded to other SECM operation modes such as surface interrogation for understanding the dynamics of the electrode surfaces and interfaces of a PEC system.

Effect of a Cocatalyst on a Photoanode in Water Splitting: A Study of Scanning Electrochemical Microscopy

With a proper band gap of ∼2.4 eV for solar light absorption and suitable valence band edge position for oxygen evolution, scheelite-monoclinic bismuth vanadate (BiVO₄) has become one of the most attractive photocatalysts for efficient visible-light-driven photoelectrochemical (PEC) water splitting. Several studies have indicated that surface modification of BiVO₄ with a cocatalyst such as NiFe layered double hydroxide (LDH) can significantly increase the PEC water splitting performance of the catalyst. Herein, we experimentally investigated the charge transfer dynamics and charge carrier recombination processes by scanning electrochemical microscopy (SECM) with the feedback mode on the surface of BiVO₄ and BiVO₄/NiFe-LDH as model samples. The ratio of rate constants for photogenerated hole (k_h+⁰) to electron (k_e-⁰) via the photocatalyst of BiVO₄/NiFe-LDH reacting with the redox couple is found to be five times larger than that of BiVO₄ under illumination. In this case, the ratio of the rate constants k_h+⁰/k_e-⁰ stands for the interfacial charge recombination process. This implies the cocatalyst NiFe-LDH suppresses the electron back transfer greatly and finally reduces the surface recombination. Control experiments with cocatalysts CoPi and RuO_x onto BiVO₄ further verify this conclusion. Therefore, the SECM characterization allows us to make an overall analysis on the function of cocatalysts in the PEC water splitting system.

Effect of TiO2 Photoanodes Morphology and Dye Structure on Dye-Regeneration Kinetics Investigated by Scanning Electrochemical Microscopy

The dye regeneration in dye-sensitized solar cells (DSSCs) is improved by optimizing the charge separation at the level of the sensitized semiconductor treatment of the mesoporous electrode by TiCl4 that passivates the surface for back electron transfer reactions. The dye-regeneration kinetics is analyzed for DN216- and D358-sensitized porous TiO2 electrodes with and without a TiCl4 treatment by means of scanning electrochemical microscopy (SECM). Different mass transport limitation of the [Co(bpy)3]3+ mediator through the porous electrode is found for the comparison of the structurally similar dyes but cannot be detected for the thin layer introduced by the TiCl4 treatment. Phototransient measurements are conducted directly in the SECM cell without any intermediated sample manipulation. The results from those measurements corroborate the findings from steady state SECM measurements.

Increase of power conversion efficiency in dye-sensitized solar cells through ferroelectric substrate induced charge transport enhancement

Ferroelectric functionalized dye-sensitized solar cells were fabricated by using a positively-poled LiNbO₃ substrate coated with ITO (ITO-LiNbO₃) as a collector electrode and demonstrated enhanced power conversion efficiency. Surface potential properties of TiO₂ nanoparticle film coated on the ITO-LiNbO₃ (TiO₂/ITO-LiNbO₃) examined by Kelvin probe force microscopy (KPFM) confirmed that a large electric field (a few 10 V/µm) generated from LiNbO₃ can penetrate through the ITO layer and is applied to TiO₂ film. This polarization-induced electric field leads to an increased photocurrent density by attracting and promoting electrons to direct transport through the mesoporous TiO₂ network toward the collector electrode and a decreased charge recombination by facilitating electrons to pass through fewer boundaries of nanoparticles, resulting in high power conversion efficiency. The power conversion efficiency was enhanced by more than 40% in comparison with that without polarization-induced electric field. Incorporating functional ferroelectrics into photovoltaic cells would be a good strategy in improving photovoltaic performance and is applicable to other types of photovoltaic devices, such as perovskite solar cells.

New Insight into Procedure of Interface Electron Transfer through Cascade System with Enhanced Photocatalytic Activity

Recombination of photogenerated electron-hole pairs is extremely limited in the practical application of photocatalysis toward solving the energy crisis and environmental pollution. A rational design of the cascade system (i.e., rGO/Bi₂ WO₆ /Au, and ternary composites) with highly efficient charge carrier separation is successfully constructed. As expected, the integrated system (rGO/Bi₂ WO₆ /Au) shows enhanced photocatalytic activity compared to bare Bi₂ WO₆ and other binary composites, and it is proved in multiple electron transfer (MET) behavior, namely a cooperative electron transfer (ET) cascade effect. Simultaneously, UV-vis/scanning electrochemical microscopy is used to directly identify MET kinetic information through an in situ probe scanning technique, where the "fast" and "slow" heterogeneous ET rate constants (K_eff ) of corresponding photocatalysts on the different interfaces are found, which further reveals that the MET behavior is the prime source for enhanced photocatalytic activity. This work not only offers a new insight to study catalytic performance during photocatalysis and electrocatalysis systems, but also opens up a new avenue to design highly efficient catalysts in photocatalytic CO₂ conversion to useful chemicals and photovoltaic devices.

Surface Interrogation Scanning Electrochemical Microscopy for a Photoelectrochemical Reaction: Water Oxidation on a Hematite Surface

To understand the pathway of a photoelectrochemical (PEC) reaction, quantitative knowledge of reaction intermediates is important. We describe here surface interrogation scanning electrochemical microscopy for this purpose (PEC SI-SECM), where a light pulse to a photoactive semiconductor film at a given potential generates intermediates that are then analyzed by a tip generated titrant at known times after the light pulse. The improvements were demonstrated for photoelectrochemical water oxidation (oxygen evolution) reaction on a hematite surface. The density of photoactive sites, proposed to be Fe⁴⁺ species, on a hematite surface was successfully quantified, and the photoelectrochemical water oxidation reaction dynamics were elucidated by time-dependent redox titration experiments. The new configuration of PEC SI-SECM should find expanded usage to understand and investigate more complicated PEC reactions with other materials.

Determining Li+-Coupled Redox Targeting Reaction Kinetics of Battery Materials with Scanning Electrochemical Microscopy

The redox targeting reaction of Li⁺-storage materials with redox mediators is the key process in redox flow lithium batteries, a promising technology for next-generation large-scale energy storage. The kinetics of the Li⁺-coupled heterogeneous charge transfer between the energy storage material and redox mediator dictates the performance of the device, while as a new type of charge transfer process it has been rarely studied. Here, scanning electrochemical microscopy (SECM) was employed for the first time to determine the interfacial charge transfer kinetics of LiFePO₄/FePO₄ upon delithiation and lithiation by a pair of redox shuttle molecules FcBr₂⁺ and Fc. The effective rate constant k_eff was determined to be around 3.70-6.57 × 10^-3 cm/s for the two-way pseudo-first-order reactions, which feature a linear dependence on the composition of LiFePO₄, validating the kinetic process of interfacial charge transfer rather than bulk solid diffusion. In addition, in conjunction with chronoamperometry measurement, the SECM study disproves the conventional "shrinking-core" model for the delithiation of LiFePO₄ and presents an intriguing way of probing the phase boundary propagations induced by interfacial redox reactions. This study demonstrates a reliable method for the kinetics of redox targeting reactions, and the results provide useful guidance for the optimization of redox targeting systems for large-scale energy storage.

High-Throughput Screening and Optimization of Binary Quantum Dots Cosensitized Solar Cell

Quantum dots (QDs) are considered as the alternative of dye sensitizers for solar cells. However, interfacial construction and evaluation of photocatalytic nanomaterials still remains challenge through the conventional methodology involving demo devices. We propose here a high-throughput screening and optimizing method based on combinatorial chemistry and scanning electrochemical microscopy (SECM). A homogeneous TiO2 catalyst layer is coated on a FTO substrate, which is then covered by a dark mask to expose the photocatalyst array. On each photocatalyst spot, different successive ionic layer adsorption and reaction (SILAR) processes are performed by a programmed solution dispenser to load the binary PbxCd1-xS QDs sensitizers. An optical fiber is employed as the scanning tip of SECM, and the photocatalytic current is recorded during the imaging experiment, through which the optimized technical parameters are figured out. To verify the validity of the combinatorial SECM imaging results, the controlled trials are performed with the corresponding photovoltaic demo devices. The harmonious accordance proved that the methodology based on combinatorial chemistry and SECM is valuable for the interfacial construction, high-throughput screening, and optimization of QDSSCs. Furthermore, the PbxCd1-xS/CdS QDs cosensitized solar cell optimized by SECM achieves a short circuit current density of 24.47 mA/cm(2), an open circuit potential of 421 mV, a fill factor of 0.52, and a photovoltaic conversion efficiency of 5.33%.

Photoelectrochemical Water Splitting System--A Study of Interfacial Charge Transfer with Scanning Electrochemical Microscopy

Fast charge transfer kinetics at the photoelectrode/electrolyte interface is critical for efficient photoelectrochemical (PEC) water splitting system. Thus, far, a measurement of kinetics constants for such processes is limited. In this study, scanning electrochemical microscopy (SECM) is employed to investigate the charge transfer kinetics at the photoelectrode/electrolyte interface in the feedback mode in order to simulate the oxygen evolution process in PEC system. The popular photocatalysts BiVO4 and Mo doped BiVO4 (labeled as Mo:BiVO4) are selected as photoanodes and the common redox couple [Fe(CN)6](3-)/[Fe(CN)6](4-) as molecular probe. SECM characterization can directly reveal the surface catalytic reaction kinetics constant of 9.30 × 10(7) mol(-1) cm(3) s(-1) for the BiVO4. Furthermore, we find that after excitation, the ratio of rate constant for photogenerated hole to electron via Mo:BiVO4 reacting with mediator at the electrode/electrolyte interface is about 30 times larger than that of BiVO4. This suggests that introduction of Mo(6+) ion into BiVO4 can possibly facilitate solar to oxygen evolution (hole involved process) and suppress the interfacial back reaction (electron involved process) at photoanode/electrolyte interface. Therefore, the SECM measurement allows us to make a comprehensive analysis of interfacial charge transfer kinetics in PEC system.

Investigation of photoinduced electron transfer on TiO2 nanowire arrays/porphyrin composite via scanning electrochemical microscopy

A photo-induced electron transfer (PET) system was constructed by the combination of vertically aligned single-crystal TiO₂ nanowire arrays and porphyrin to investigate the mechanism of the charge transfer process in artificial photosynthesis.

Investigation of regeneration kinetics in quantum-dots-sensitized solar cells with scanning electrochemical microscopy

A fast quantum dots (QDs) regeneration process is necessary for highly efficient QDs-sensitized solar cells. Herein, CdSe and CdS QDs regeneration rates (kQD') in three redox electrolytes, which are triiodide and iodide ions (I3(-)/I(-)), Co(bpy)3(PF6)2 and Co(bpy)3(PF6)3 (Co(3+)/Co(2+)), and 1-methy-1-H-tetrazole-5-thiolate and its dimer (T2/T(-)), have been first investigated with scanning electrochemical microscopy (SECM). The results reveal that the kinetics of QDs regeneration depends on the nature of the QDs and the redox shuttles presented in QDSSCs. For QDs of CdSe and CdS, the regeneration rate (kQD') in the case of a T2/T(-)-based electrolyte is about two times larger than that of Co(3+)/Co(2+) and I3(-)/I(-). Additionally, the kQD' for CdSe is about two times larger than that of CdS in the same redox shuttle electrolyte, which could be due to a large driving force for the reaction between the exited state quantum dots (QD(+)) and redox electrolytes.

A cyclopenta[1,2-b:5,4-b′]dithiophene–porphyrin conjugate for mesoscopic solar cells: a D–π–D–A approach

In this article, cyclopenta[1,2-b:5,4-b′]dithiophene (CPDT) was introduced as a spacer between the porphyrin chromophore and cyanoacetic acid to obtain a porphyrin dye (coded as LW9).

Investigation of the regeneration kinetics of organic dyes with pyridine ring anchoring groups by scanning electrochemical microscopy

The regeneration dynamics can be understood with the help of scanning electrochemical microscopy, showing that anchoring group significantly affects the effective rate constant.

Efficient p-type dye-sensitized solar cells based on disulfide/thiolate electrolytes

Herein, an organic redox couple 1-methy-1H-tetrazole-5-thiolate (T(-)) and its disulfide dimer (T2) redox shuttle, as an electrolyte, is introduced in a p-type dye-sensitized solar cell (DSC) on the basis of an organic dye (P1) sensitizer and nanocrystal CuCrO2 electrode. Using this iodide-free transparent redox electrolyte in conjunction with the sensitized heterojunction, we achieve a high open-circuit voltage of over 300 mV. An optimal efficiency of 0.23% is obtained using a CoS counter electrode and an optimized electrolyte composition under AM 1.5 G 100 mW cm(-2) light illumination which, to the best of our knowledge, represents the highest efficiency that has so far been reported for p-type DSCs using organic redox couples.

Dye doped Eosin yellowish silica nanoparticles as novel fluorophore for a peroxyoxalate chemiluminescence system

In this work, we report the preparation and characterization of novel dye doped fluorophore Eosin yellowish silica nanoparticles (ESNPs). We synthesized ESNPs by the Stöber method via encapsulation of Eosin Yellowish in silica particles by the condensation of tetraethyl orthosilicate under alkaline condition at room temperature. The resulted ESNPs were characterized by transmission electron microscopy, atomic force microscopy; UV-Visible, fluorescence and Fourier transform infrared spectroscopy. The sizes of the nanoparticles have been found to be 300.0 (±1.0), 400.0 (±1.1) and 500.0 (±5.2) nm depending the reaction conditions under which they were synthesized. Furthermore, because of intense light emission, the ESNPs were used as fluorophore in a peroxyoxalate chemiluminescence system. The effect of solvent and concentrations of necessary reagents, bis(2,4,6-trichlorophenyl)oxalate, sodium salicylate, hydrogen peroxide and the effects of size of the ESNP and temperature on the luminescence efficiency of the system were examined. The activation kinetic parameters of the system were also evaluated from the temperature investigation.

Analytical expressions for quantitative scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM), is a recent analytical technique in electrochemistry, which was developed in the 1990s and uses microelectrodes to probe various surfaces. Even with the well-known disc microelectrodes, the system geometry is not as simple as in regular electrochemistry. As a consequence even the simplest experiments, the so-called positive and negative feedback approach curves, cannot be described with exact analytical expressions. This review gathers all the analytical expressions available in the SECM literature in steady-state feedback experiments. Some of them are claimed as general expressions, other are presented as approximate. Their validity is discussed in the light of the current understanding and computer facilities.

Investigation of Localized Catalytic and Electrocatalytic Processes and Corrosion Reactions with Scanning Electrochemical Microscopy (SECM)

Scanning electrochemical microscopy (SECM) has developed into a very versatile tool for the investigation of solid-liquid, liquid-liquid and liquid-gas interfaces. The arrangement of an ultramicroelectrode (UME) in close proximity to the interface under study allows the application of a large variety of different experimental schemes. The most important have been named feedback mode, generation-collection mode, redox competition mode and direct mode. Quantitative descriptions are available for the UME signal, depending on different sample properties and experimental variables. Therefore, SECM has been established as an indispensible tool in many areas of fundamental electrochemical research. Currently, it also spreads as an important new method to solve more applied problems, in which inhomogeneous current distributions are typically observed on different length scales. Prominent examples include devices for electrochemical energy conversion such as fuel cells and batteries as well as localized corrosion phenomena. However, the direct local investigation of such systems is often impossible. Instead, suitable reaction schemes, sample environments, model samples and even new operation modes have to be introduced in order to obtain results that are relevant to the practical application. This review outlines and compares the theoretical basis of the different SECM working modes and reviews the application in the area of electrochemical energy conversion and localized corrosion with a special emphasis on the problems encountered when working with practical samples.

The influence of charge transport and recombination on the performance of dye-sensitized solar cells

Electrochemical impedance spectroscopy (EIS) and transient voltage decay measurements are applied to compare the performance of dye sensitized solar cells (DSCs) using organic electrolytes, ionic liquids and organic-hole conductors as hole transport materials (HTM). Nano-crystalline titania films sensitized by the same heteroleptic ruthenium complex NaRu(4-carboxylic acid-4'-carboxylate) (4,4'-dinonyl-2,2'-bipyridyl)(NCS)(2) , coded Z-907Na are employed as working electrodes. The influence of the nature of the HTM on the photovoltaic figures of merit, that is, the open circuit voltage, short circuit photocurrent and fill factor is evaluated. In order to derive the electron lifetime, as well as the electron diffusion coefficient and charge collection efficiency, EIS measurements are performed in the dark and under illumination corresponding to realistic photovoltaic operating conditions of these mesoscopic solar cells. A theoretical model is established to interpret the frequency response off the impedance under open circuit conditions, which is conceptually similar to photovoltage transient decay measurements. Important information on factors that govern the dynamics of electron transport within the nanocrystalline TiO(2) film and charge recombination across the dye sensitized heterojunction is obtained.

Scanning electrochemical microscopy

This review describes work done in scanning electrochemical microscopy (SECM) since 2000 with an emphasis on new applications and important trends, such as nanometer-sized tips. SECM has been adapted to investigate charge transport across liquid/liquid interfaces and to probe charge transport in thin films and membranes. It has been used in biological systems like single cells to study ion transport in channels, as well as cellular and enzyme activity. It is also a powerful and useful tool for the evaluation of the electrocatalytic activities of different materials for useful reactions, such as oxygen reduction and hydrogen oxidation. SECM has also been used as an electrochemical tool for studies of the local properties and reactivity of a wide variety of materials, including metals, insulators, and semiconductors. Finally, SECM has been combined with several other nonelectrochemical techniques, such as atomic force microscopy, to enhance and complement the information available from SECM alone.

Scanning electrochemical microscopy for direct imaging of reaction rates

Not only in electrochemistry but also in biology and in membrane transport, localized processes at solid-liquid or liquid-liquid interfaces play an important role at defect sites, pores, or individual cells, but are difficult to characterize by integral investigation. Scanning electrochemical microscopy is suitable for such investigations. After two decades of development, this method is based on a solid theoretical foundation and a large number of demonstrated applications. It offers the possibility of directly imaging heterogeneous reaction rates and locally modifying substrates by electrochemically generated reagents. The applications range from classical electrochemical problems, such as the investigation of localized corrosion and electrocatalytic reactions in fuel cells, sensor surfaces, biochips, and microstructured analysis systems, to mass transport through synthetic membranes, skin and tissue, as well as intercellular communication processes. Moreover, processes can be studied that occur at liquid surfaces and liquid-liquid interfaces.