Solution Phase Species Research Articles

Characterization and solubility measurement of synthetic uranophane and sklodowskite under oxic groundwater conditions

Naturally occurring uranyl silicates can serve as solubility-limiting solid phases (SLSPs) of U(VI) in the oxic environment of granitic groundwater, which is likely to occur in crystalline host rock media in Korea. In this study, two synthetic uranyl silicates, uranophane (Ca[UO2SiO3OH]2·5H2O) and sklodowskite (Mg[UO2SiO3OH]2·6H2O) were prepared and used to measure the U(VI) solubility in simulated groundwater (sGW), mimicking the composition of samples collected from the underground research tunnel at the Korea Atomic Energy Research Institute (KAERI). The solid- and solution-phase species involved were examined in detail using various characterization methods. According to the powder X-ray diffraction pattern and elemental analysis results, the synthetic mineral phases, which were identified as uranophane-α and sklodowskite having layered crystal structures, were retained intact in sGW; however, in 0.1 M NaClO4, emergence of other solid phases was observed. Enhanced FTIR and Raman spectroscopic data, particularly in the regions of 790–860 and 940–1020 cm−1 for the UO22+ and SiO44− ions, respectively, enable monitoring the changes in the solid phases. Ternary Ca–U(VI)-tricarbonato complexes were identified as the dominant dissolved U(VI) species in sGW using time-resolved laser-induced luminescence spectroscopy. Furthermore, the solubility constant of uranophane was calculated (log Ks,0° = 10.3 ± 0.4) and compared with the predicted values based on our geochemical modeling analysis and previously reported ones.

Flow-Xl: a new facility for the analysis of crystallization in flow systems.

Characterization of crystallization processes in situ is of great importance to furthering knowledge of how nucleation and growth processes direct the assembly of organic and inorganic materials in solution and, critically, understanding the influence that these processes have on the final physico-chemical properties of the resulting solid form. With careful specification and design, as demonstrated here, it is now possible to bring combined X-ray diffraction and Raman spectroscopy, coupled to a range of fully integrated segmented and continuous flow platforms, to the laboratory environment for in situ data acquisition for timescales of the order of seconds. The facility used here (Flow-Xl) houses a diffractometer with a micro-focus Cu Kα rotating anode X-ray source and a 2D hybrid photon-counting detector, together with a Raman spectrometer with 532 and 785 nm lasers. An overview of the diffractometer and spectrometer setup is given, and current sample environments for flow crystallization are described. Commissioning experiments highlight the sensitivity of the two instruments for time-resolved in situ data collection of samples in flow. Finally, an example case study to monitor the batch crystallization of sodium sulfate from aqueous solution, by tracking both the solute and solution phase species as a function of time, highlights the applicability of such measurements in determining the kinetics associated with crystallization processes. This work illustrates that the Flow-Xl facility provides high-resolution time-resolved in situ structural phase information through diffraction data together with molecular-scale solution data through spectroscopy, which allows crystallization mechanisms and their associated kinetics to be analysed in a laboratory setting.

Destabilization of Oxygen Reduction Reaction Intermediates on a PGM-Free Electrocatalyst By Axial Spectator Ligands

Platinum-group free Fe-N-C catalysts have been shown to be a promising class of electrocatalysts for the oxygen reduction reaction (ORR) in acidic media. However, typical synthesis methods can result in a variety of FeN4 sites embedded in graphene, including bulk-hosted and edge-hosted FeN4. The planar structure of the FeN4 active site allows two additional ligands to adsorb to Fe on either side of the graphene, which can be present from ORR reaction intermediates and other solution phase species. Here, we studied the impact of axially adsorbed ligands on bulk-hosted and edge-hosted FeN4 sites by screening over all pairs of H, O, OH, OOH, O2, H2O, NO, SO4, HSO4, and ClO4 axial ligands using solvated grand canonical density functional theory (GCDFT) to self-consistently account for applied potential. The first axial Fe adsorbate can strongly influence the binding of a second axial adsorbate and increase its adsorption energy by up to 2 eV, with more destabilization occurring for a more positive applied potential and the O* ORR intermediate being affected most. However, GCDFT calculations on these same sites without axial spectator ligands show that O* and OH* are both thermodynamically persistent, suggesting that axial spectator ligands might play a role in explaining this catalyst’s high experimental activity. Additionally, we show how the combined effects of potential, nature of the FeN4 site, and axial ligand impact O2 adsorption favorability and availability of sites in the presence of competing molecules. This work highlights the need for the continued development of advanced electrocatalyst models that account for a wide range of active site structures to facilitate comparison with experiment.

Catalytic, Spectroscopic, and Theoretical Studies of Fe4S4-Based Coordination Polymers as Heterogenous Coupled Proton-Electron Transfer Mediators for Electrocatalysis.

Iron-sulfur clusters play essential roles in biological systems, and thus synthetic [Fe4S4] clusters have been an area of active research. Recent studies have demonstrated that soluble [Fe4S4] clusters can serve as net H atom transfer mediators, improving the activity and selectivity of a homogeneous Mn CO2 reduction catalyst. Here, we demonstrate that incorporating these [Fe4S4] clusters into a coordination polymer enables heterogeneous H atom transfer from an electrode surface to a Mn complex dissolved in solution. A previously reported solution-processable Fe4S4-based coordination polymer was successfully deposited on the surfaces of different electrodes. The coated electrodes serve as H atom transfer mediators to a soluble Mn CO2 reduction catalyst displaying good product selectivity for formic acid. Furthermore, these electrodes are recyclable with a minimal decrease in activity after multiple catalytic cycles. The heterogenization of the mediator also enables the characterization of solution-phase and electrode surface species separately. Surface enhanced infrared absorption spectroscopy (SEIRAS) reveals spectroscopic signatures for an in situ generated active Mn-H species, providing a more complete mechanistic picture for this system. The active species, reaction mechanism, and the protonation sites on the [Fe4S4] clusters were further confirmed by density functional theory calculations. The observed H atom transfer reactivity of these coordination polymer-coated electrodes motivates additional applications of this composite material in reductive H atom transfer electrocatalysis.

Real-time infrared spectroscopy coupled with blind source separation for nuclear waste process monitoring

On-line infrared absorbance spectroscopy enables rapid measurement of solution-phase molecular species. Many spectra-to-concentration models exist for spectral data, with some models able to handle overlapping spectral bands and nonlinearities. However, model accuracy is limited by the quality of training data used in model fitting. The process spectra of nuclear waste simulants at the Savannah River Site display incongruity between training and process spectra; the glycolate spectral signature in the training data does not match the glycolate signature in Savannah River National Laboratory process data. A novel blind source separation algorithm is proposed that preprocesses spectral data so that process spectra more closely resemble training spectra, thereby improving model quantification accuracy when unexpected sources of variation appear in process spectra. The novel blind source separation preprocessing algorithm is shown to improve nitrate quantification from an R2 of 0.934 to 0.988 and from 0.267 to 0.978 in two instances analyzing nuclear waste simulants from the Slurry Receipt Adjustment Tank and Slurry Mix Evaporator cycle at the Savannah River Site.

Car-Parrinello molecular dynamics study of CuF, AgF, CuPF6 and AgPF6 in acetonitrile solvent and Cluster-Continuum calculation of the solvation free energy of Cu(I), Ag(I) and Li(I)

The CuF species in acetonitrile solution is a potential intermediate in copper catalyzed fluorination. In the process, different species such as Cu+, Ag+, CuF and AgF could be involved. The objective of this work is to provide relevant data on the structure of these species in solution phase, as well as the corresponding solvation thermodynamics to better understand the equilibrium for formation of CuF. To attain this goal, ab initio Car-Parrinello molecular dynamics method and cluster-continuum quasichemical approach were used. It was found that Cu+ and Ag+ ions are tetracoordinated by acetonitrile molecules, being possible to observe the release of one coordinated acetonitrile to the solution to form a tricoordinate structure, suggesting a dissociative mechanism for ligand exchange. The CuF species was found to form a predominant tricoordinate complex, with copper coordinated to two acetonitrile molecules. The AgF species have presented more labile ligand feature, with silver ion interconverting between bi- and tricoordinate structure, involving coordination to one and two acetonitrile molecules, respectively. The solvation free energy calculated for Cu+ and Ag+ are in good agreement with available experimental data. The present results indicate that CuF can be easily formed from dissolved AgF and Cu+ species in acetonitrile solution. Consequently, CuF could play an important role in copper catalyzed fluorination.

Photodegradation of Riboflavin under Alkaline Conditions: What Can Gas-Phase Photolysis Tell Us about What Happens in Solution?

The application of electrospray ionisation mass spectrometry (ESI-MS) as a direct method for detecting reactive intermediates is a technique of developing importance in the routine monitoring of solution-phase reaction pathways. Here, we utilise a novel on-line photolysis ESI-MS approach to detect the photoproducts of riboflavin in aqueous solution under mildly alkaline conditions. Riboflavin is a constituent of many food products, so its breakdown processes are of wide interest. Our on-line photolysis setup allows for solution-phase photolysis to occur within a syringe using UVA LEDs, immediately prior to being introduced into the mass spectrometer via ESI. Gas-phase photofragmentation studies via laser-interfaced mass spectrometry of deprotonated riboflavin, [RF − H]−, the dominant solution-phase species under the conditions of our study, are presented alongside the solution-phase photolysis. The results obtained illustrate the extent to which gas-phase photolysis methods can inform our understanding of the corresponding solution-phase photochemistry. We determine that the solution-phase photofragmentation observed for [RF − H]− closely mirrors the gas-phase photochemistry, with the dominant m/z 241 condensed-phase photoproduct also being observed in gas-phase photodissociation. Further gas-phase photoproducts are observed at m/z 255, 212, and 145. The value of exploring both the gas- and solution-phase photochemistry to characterise photochemical reactions is discussed.

Thermal and Photochemical Switching of Chiral Sugar Azoalkenes: A Mechanistic Interrogation

This manuscript describes a thorough study on the thermal and photochemical evolution of carbohydrate‐based azoalkenes, which have been proposed as putative intermediates en route to other heterocyclic derivatives and nucleoside analogues. These substances represent new protagonists in the azo chemical space, under intense study due to reversible photoswitching of this functional group suitable for designing artificial nanomachines and responsive materials. Although structurally simple azadienes derived from monosaccharides have long been known, the dynamics of such species in solution and solid phase remains poorly characterized. Herein, we show that such azoalkenes undergo 3E→3Z thermal isomerization, which is accelerated by the presence of Brönsted acids. On the other hand, they undergo 1E→1Z isomerization when irradiated by sunlight, while the reverse 1Z→1E isomerization occurs thermally in the dark or under acid catalysis. As a result, sugar monoazadienes not only exhibit inherent chirality, but also dual on‐off isomerization under external stimuli. Both experiments and DFT‐based computational analyses provide a consistent and unifying mechanistic picture of these transformations.

Volumetric studies of solvation of carboxylic acids in aqueous, carbon tetrachloride, methanolic and methanolic-urea binary solutions at 298.15 K

Abstract Experimental measurements of density property at 298.15 K for aqueous, carbon tetrachloride, methanolic, and 3.0004 mol·kg−1 urea in methanol as solvent systems for binary solutions of carboxylic acids (propionic acid, n-butyric acid, n-valeric acid and n-caproic acid) are reported. The data of density are used to obtain apparent molar volume of acids at the finite concentrations of solute (Vϕ) which were appropriately extrapolated to infinitely dilute concentration of acid to obtain the limiting apparent molar volumes (Vϕ0). The calculations of volume changes due to dimerization equilibria (in carbon tetrachloride and aqueous solutions) are made and used to study the volume changes due to dimerization of acid molecules. The results of Vϕ in methanol-urea solvent system are explained on the basis of channel-like structure-formation for the species (urea and acid molecules) in solution phase. The negative volume changes observed in case of aqueous solutions are explained on the basis of hydrophobic hydration while the same in carbon tetrachloride solutions occurring with positive volume changes are interpreted in terms of hydrogen-bonded dimer formation equilibria. The Vϕ variation with acid concentrations has been explained on the basis of solute-solvent interactions foe studied acids in methanol while in aqueous solutions, the same has been attributed to ionization (limiting concentration range), hydrogen bonding and hydrophobic interactions.

Dissolution of silica component of glass network at early stage of corrosion in initially silica‐saturated solution

AbstractCorrosion of glass in silica‐saturated solution has been performed with the assumption that dissolution of silicate species from the glass network would not occur. Using surface‐sensitive analytical techniques, we report experimental evidence suggesting the dissolution of silicate network species from a model nuclear waste glass, called international simple glass (ISG), in an aqueous solution initially saturated with soluble silica species. Results from low energy ion scattering and X‐ray photoelectron spectroscopy reveal a complete depletion of mobile element species (B, Na) from the ISG surface and an enrichment of Zr on the outmost surface. In support of spectroscopic analyses, results from topographic imaging with atomic force microscopy show a stochastic dissolution of glass surface resulting in a higher surface roughness with increasing corrosion time in aqueous solution. This study shows that a true equilibrium between soluble silica species in the solution phase and silicate species in the glass network could not be warranted by performing corrosion experiments in the solution where dissolved silica species are initially equilibrated with amorphous silica in the presence of KOH. The leaching of mobile species (B, Na) could affect the saturation level of aqueous solution and induce further dissolution of the glass surface.

Elucidation of Chemical Species and Reactivity at Methylammonium Lead Iodide and Cesium Tin Bromide Perovskite Surfaces via Orthogonal Reaction Chemistry

We quantified the chemical species present at and reactivity of the (100) face of tetrahedral single-crystal methylammonium lead iodide, MAPbI3(100), and polycrystalline cesium tin bromide, CsSnBr3. For these ABX3 perovskites, experiments utilized the orthogonal reactivity of the A+-site cation, the B2+-site cation, and the X–-site halide anion. Ambient pressure exposure to BF3 solutions probed the reactivity of interfacial halides. Reactions with p-trifluoromethylanilinium chloride probed the exchange reactivity of the A+-site cation. A complex-forming ligand, 4,4′-bis(trifluoromethyl)-2,2′-bipyridine, probed for interfacial B2+-site cations. Fluorine features in X-ray photoelectron spectroscopy (XPS) quantified reaction outcomes for each solution-phase species. XPS revealed adsorption of BF3, indicating surface-available halide anions on both MAPbI3(100) and on CsSnBr3. Temperature-programmed desorption quantified a ∼200 kJ mol–1 desorption activation energy from MAPbI3(100) and a ∼215 kJ mol–1 desorpti...

Mechanistic Insights into Solution-Phase Oxidative Esterification of Primary Alcohols on Pd(111) from First-Principles Microkinetic Modeling

We present an ab initio microkinetic model for the oxidative esterification of 1-propanol to methyl propionate over Pd(111). The model fully accounts for solvation of solution-phase species and added catalytic base and provides key insights into the factors that limit the activity of unpromoted Pd aerobic oxidation catalysts. In particular, we find that the activity is limited by the large steady-state surface H coverage, which destabilizes other adsorbed intermediates via lateral interactions, and substantial barriers governing the formation of O–H bonds, which is required for the reduction of O2 and removal of H byproducts from the catalyst surface.

Synthesis, characterization and electrochemical investigations of mixed-ligand copper(II)-organic supramolecular frameworks

Two mixed-ligand copper(II)-organic coordination compounds with 5,5′-dimethyl-2,2′-bipyridine (5,5′-Me2bpy) as a primary ligand while aliphatic malonate (Hmal) and aromatic 2-hydroxynicotinate (2-OHNA) as secondary ligands, were synthesized. These complexes are formulated as: [Cu(Hmal)(5,5′-Me2bpy)(H2O)](ClO4) 1 and [Cu2(2-OHNA)2(5,5′-Me2bpy)2(NO3)](NO3) 2. These two complexes were structurally characterized by single crystal X-ray diffraction analysis. Characterization was further supported by powder X-ray diffraction analysis, elemental analyses, FT-IR, FAB-MASS and TGA, DSC studies. Cyclic voltammetric and UV-visible spectral studies of these two complexes have also been done. The electrochemical studies of complex 1 in DMSO and DMF have shown that this complex undergoes quasi-reversible diffusion-controlled one-electron transfer reaction without any chemical complication while complex 2 in DMSO undergoes quasi-reversible diffusion-controlled one electron transfer reaction, following EC mechanism. The electrochemical behaviour of complex 2 in DMF is complicated probably due to presence of more than one species in solution phase.

Size Effects in Nanoparticle Catalysis at Nanoparticle Modified Electrodes: The Interplay of Diffusion and Chemical Reactions

Electron transfer reactions mediated via nanoparticles immobilized on an electrode surface are considered in respect of catalytic processes in which solution phase species are either oxidized or reduced to form products exclusively via electron transfer with negligible reaction at the underlying supporting electrode. Specifically simulation is used to explore the effect of the nanoparticle size for individual nanoparticles as well as ensembles of nanoparticles, and a kinetic diagram is developed. For a single nanoparticle its size controls the rates of diffusion of species to and from the particle so that the relative extent of the catalysis is reduced for larger particles. For an array of nanoparticles the response of the whole is sensitive not only to the particle size but also to the particle coverage since the interparticle distance influences the extent, or otherwise, of the local overlap of the diffusion layers of neighboring particles and hence also the extent of the catalysis. Both particle size a...

Modelling ac voltammetry with MECSim: facilitating simulation–experiment comparisons

Summary Here we introduce the Monash Electrochemistry Simulator (MECSim) software package that allows most aspects of ac voltammetry to be simulated when a single sine wave or a combination of sine waves are superimposed onto a dc ramp. The MECSim software and companion tool kits can be downloaded free of charge from http://www.garethkennedy.net/MECSim.html where there is also instructional documentation. Features accommodated in the simulation model include non-linear background capacitance, uncompensated resistance, Butler–Volmer and Marcus–Hush electron transfer, chemical reactions involving surface confined and solution phase species and stationary or rotating disc electrode mass transport with linear diffusion. Companion tool kits are also provided that allow Fourier transformed ac voltammograms with harmonic analysis to be generated along with other forms of data presentation. The MECSim format in combination with parameter optimisation tool kit also facilitates the implementation of sophisticated forms of experiment-theory comparisons needed to estimate the electrode kinetic and other parameters. MECSim software is computationally efficient and based on flexible code that can be used in cluster computing as well as on a single machine. Typical simulations take seconds to run on a laptop and parameter searches needed to tackle the inverse problem can be achieved efficiently. Capabilities of MECSim are illustrated in this article by comparing simulated results with those obtained experimentally.

Locating the Proton in Nicotinamide Protomers via Low-Resolution UV Action Spectroscopy of Electrosprayed Solutions.

Even in relatively simple molecules, the sites of protonation or deprotonation formed upon electrospray ionization can be controversial. This situation means that it is important to develop new approaches for identifying "protomers" and "deprotomers". In this study, we demonstrate that routine, low-resolution UV laser photodissociation spectroscopy can be applied to identify the gaseous protomers of nicotinamide formed upon electrospray. Nicotinamide is an important biological molecule that possesses multiple protonation sites associated with its pyridine and amide groups. We obtain a gas-phase absorption spectrum for protonated nicotinamide that closely resembles the solution-phase spectrum. However, photoexcitation of protonated nicotinamide produces numerous ionic photofragments, and the spectral profiles for production of these photofragments from protonated nicotinamide reveal the existence of two distinctive chromophores, which can be traced to the existence of pyridine and amide protomers. We observe that these protomers are associated with absorption bands centered at 4.96 and 4.73 eV, respectively, with the protomers appearing in an approximate ratio of ∼2:1. The fact that the considerably less favorable amide protomer is observed in substantial quantities in the gas phase is surprising given that the pyridine protomer is the lower-energy species in both solution and gas phase. The high amounts of amide protomers observed here can be explained as arising from asymmetric pyridine protomer-amide bound dimers, present in solution or in the electrosprayed droplets, which lead to enhanced formation of the unexpected amide-protonated isomers.

Modified Activated Carbon Electrodes for Advanced Supercapacitors

The development of highly sophisticated portable electronic devices and the electro-mobility revolution, require new power sources with very high power density. Consequently, we see in recent years an accelerated development of super-capacitors of several kinds. These include electric-double layer (EDL) capacitors, based on purely electrostatic interactions, and the so-called pseudo-capacitors, which electrodes include surface red-ox species adsorbed to their surface within the porous structure and enhance the electrodes’ specific capacitance. The voltage (E) that can be applied to these devices is very important, because the energy density is proportional to E2. The potential applied depends on the electrolyte solutions used. Indeed, super-capacitors can be classified also according to the electrolyte solutions used and the potential applied. Using acidic or basic aqueous solutions enable to obtain fast electrostatic interactions at relatively high specific capacitance, but the potential applied is below 1V. Aqueous super-capacitors based on neutral solutions (e.g., electrolytes like Li2SO4) can operate at potentials >1.5V but their specific capacitance is lower compared to systems with acidic or basic solutions. The use of non-aqueous solutions enables to operate high voltage super/pseudo-capacitors, that may reach energy density of the same order of magnitude as that of aqueous rechargeable batteries (e.g. lead acid battery systems). In this presentation we present results related to super and pseudo capacitors that reflect novel approaches. We demonstrate how the use of red-ox species in solution phase which adsorb to porous carbonaceous electrodes can enhance the specific capacity of both aqueous and non-aqueous super-capacitors1. We will discuss what are the real limiting reactions of capacitive electrodes in non-aqueous solutions, including ionic liquids. In fact, possible intercalation of cations and anions at extreme potentials, into graphitic nano-crystallites that always exist in activated carbons is a major limiting factor in these systems, because such intercalation processes damage the structure of the activated carbon electrodes and lead to their degradation. In an another example, we demonstrate modification of high surface area activated carbon electrodes with 3D carbon nano-dots (C-dots). We explore these composite electrodes in supercapacitor ystems. For the first time, we propose herein the modification of activated carbon (AC) by a uniform deposition of highly crystalline carbon dots (C-dots) and used this novel material as electrode for supercapacitors. C-dots were synthesized by sonication of polyethylene glycol followed by sonochemical modification of AC matrices with the as prepared C-dots. We studied the sonochemical effect of the C-dots deposition into the AC and their incorporation into the pores. The porosity of the AC/C-dots and the reference AC materials and the impact of C-dots loading on the performance of electrodes comprising these AC/C-dots composites were explored. It was found that AC/C-dots electrodes demonstrate specific capacitance which is almost 3 times higher compared to that of unmodified AC electrodes. It was established that the new electrode material "activated carbon/carbon dots" exhibits very stable behavior. In all our durability experiments we test the electrodes along many thousands of cycles. We demonstrate herein highly stable , high capacity composite electrodes for super-capacitors with a Coulombic efficiency around 100%.

Electrochemical Imaging and Redox Interrogation of Surface Defects on Operating SrTiO3 Photoelectrodes.

We introduce electrochemical imaging and nano-resolved measurements of catalytic intermediates on operating SrTiO3 photoelectrodes. Spatially resolved redox titrations of photogenerated reactive oxygen species (ROS) were used to profile changes in ROS coverage and reactivity at pristine and ion-milled defective areas on n-doped (100) SrTiO3. Adsorbed ROS reached a potential-dependent limiting coverage of ∼0.1 monolayer and did not differ significantly between milled and pristine areas. However, the reaction kinetics between a solution-phase mediator and adsorbed ROS were found to be significantly decreased over ion-milled areas. Using a nanoelectrode, we resolved ksi values of 5 and 300 m(3)/s·mol for these bimolecular reactions at defective and pristine sites, respectively. Ion-milled areas also showed significantly decreased activity toward photo-oxidations, providing evidence that photogenerated ROS mediate fast charge-transfer reactions with solution-phase species at the semiconductor-electrolyte interface. Our results provide spatially resolved direct evidence of the impact of surface defects on the performance of photoelectrochemical systems. Scanning electrochemical microscopy offers a powerful method for evaluating the reactivity of an operating electrochemical interface by using redox titrations that detected as few as 30 attomoles of adsorbed ROS.

Graphite Screen-Printed Electrodes Applied for the Accurate and Reagentless Sensing of pH.

A reagentless pH sensor based upon disposable and economical graphite screen-printed electrodes (GSPEs) is demonstrated for the first time. The voltammetric pH sensor utilizes GSPEs which are chemically pretreated to form surface immobilized oxygenated species that, when their redox behavior is monitored, give a Nernstian response over a large pH range (1-13). An excellent experimental correlation is observed between the voltammetric potential and pH over the entire pH range of 1-13 providing a simple approach with which to monitor solution pH. Such a linear response over this dynamic pH range is not usually expected but rather deviation from linearity is encountered at alkaline pH values; absence of this has previously been attributed to a change in the pKa value of surface immobilized groups from that of solution phase species. This non-deviation, which is observed here in the case of our facile produced reagentless pH sensor and also reported in the literature for pH sensitive compounds immobilized upon carbon electrodes/surfaces, where a linear response is observed over the entire pH range, is explained alternatively for the first time. The performance of the GSPE pH sensor is also directly compared with a glass pH probe and applied to the measurement of pH in "real" unbuffered samples where an excellent correlation between the two protocols is observed validating the proposed GSPE pH sensor.

Electron transfer during metal-assisted and stain etching of silicon

The etching of silicon in fluoride solutions is limited by the kinetics of charge transfer not thermodynamics. This characteristic is what gives fluoride etching its great versatility in making different types of nanostructures as the result of self-limiting chemistry. This review approaches the kinetics of electron transfer from silicon and metal-coated silicon to a solution phase species from a fundamental point of view in order to establish a better understanding of the mechanisms of nanostructure formation during metal-assisted and stain etching of silicon. Band bending calculations demonstrate that diffusion of holes away from low work function metals such as Ag is not possible. Similarly diffusion of holes outside of the space charge layer is not possible for high work function metals such as Au, Pd and Pt. While direct hole injection may be important for etch track pore formation in the immediate vicinity of the metal, the charge imbalance on or near the metal causes the metal to act like a nanopower supply that polarizes the surrounding Si. This second mechanism is implicated in nonlocal etching of Si during metal-assisted etching.