Peroxide electroreduction on bi-modified Au surfaces: vibrational spectroscopy and density functional calculations

Fabrication strategies and surface tuning of hierarchical gold nanostructures for electrochemical detection and removal of toxic pollutants

The intensive research on the synthesis and characterization of gold (Au) nanostructures has been extensively documented over the last decades. These investigations allow the researchers to understand the relationships between the intrinsic properties of Au nanostructures such as particle size, shape, morphology, and composition to synthesize the Au nano/hybrid nanostructures with novel physicochemical properties. By tuning the properties above, these nanostructures are extensively employed to detect and remove trace amounts of toxic pollutants from the environment. This review attempts to document the achievements and current progress in Au-based nanostructures, general synthetic and fabrication strategies and their utilization in electrochemical sensing and environmental remediation applications. Additionally, the applications of Au nanostructures (e.g., as adsorbents, sensing platforms, catalysts, and electrodes) and advancements in the field of electrochemical sensing of different target analytes (e.g., proteins, nucleic acids, heavy metals, small molecules, and antigens) are summarized. The literature survey concludes the existing methods for the detection of toxic contaminants at various concentration levels. Finally, the existing challenges and future research directions on electrochemical sensing and degradation of toxic contaminants using Au nanostructures are defined.

Hydrogen Peroxide Oxidation Reaction on a 4-Mercaptopyridine Self-Assembled Monolayer on Au(111) Metallized by Platinum Nanoislands

A systematic investigation of the hydrogen peroxide oxidation reaction (HPOR) in phosphate buffer (pH = 7.3) on an Au(111) single crystal modified with a 4-mercaptopyridine self-assembled monolayer (SAM) has been conducted before and after metallization with Pt. While bare Au(111) shows considerable electrocatalytic activity towards the HPOR, the inhibition of the oxidation reaction after modification with the SAM implies that adsorbed 4-mercaptopyridine molecules do not catalyze the HPOR. However, SAM-modified Au(111) recovers catalytic activity for the HPOR already after a single metallization step fabricating Pt islands on-top. Hydrogen peroxide (HP) may then either react at the (non-metallic) Pt nanoislands or on reactivated Au sites, made accessible by structural changes of the SAM induced by the metallization. The shape of the voltammetric profiles for the HPOR on repeatedly metallized SAMs suggests that the contribution of Au to the total current density gradually diminishes with increasing Pt coverage while the contribution of the Pt islands increases. The electrochemical behavior is dominated by the Pt islands at a coverage of 0.5 ML obtained by three subsequent metallization steps.

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Oxygen Reduction Reaction on Au Revisited at Different pH Values using in situ Surface-Enhanced Raman Spectroscopy

The mechanisms of the oxygen reduction reaction (ORR) on Au surfaces are revisited in electrolytes with different pH values by using a combination of electrochemical and in situ surface-enhanced Raman scattering spectroscopy. Surprisingly, the in situ Raman signal of the O-O stretching vibration was detected during the ORR on a Au surface by using a λ=785 nm laser. Both the intermediate products O₂ ^- and H₂ O₂ could be detected, which indicates the difficulty of the further reduction H₂ O₂ and results in a lower electron transfer number, especially in neutral and acid electrolytes. The weak absorption ability of HO₂ on the Au surface may explain the poor ORR in neutral and acid electrolytes. This work not only provides a deep insight to understand the reduction mechanisms of O₂ on Au in electrolytes with different pH values but also supplies a new idea for the selection and optimization of electrolytes and efficient electrocatalysts for oxygen reduction.

The Effect of Anions and pH on the Activity and Selectivity of an Annealed Polycrystalline Au Film Electrode in the Oxygen Reduction Reaction-Revisited

Aiming at a better understanding of correlations between the activity and selectivity of Au electrodes in the oxygen reduction reaction (ORR) under controlled transport conditions, we have investigated this reaction by combined electrochemical and in situ FTIR measurements, performed in a flow cell set‐up in an attenuated total reflection (ATR) configuration in acid and alkaline electrolytes. The formation of incomplete reduction products (hydrogen peroxyde/peroxyls) was detected by a collector electrode, the onset of OH_ad formation was probed by bulk CO oxidation. Using an electroless‐deposited, annealed Au film on a Si prism as working electrode and three different electrolytes for comparison (sulfuric acid, perchloric acid, sodium hydroxide solution), we could derive detailed information on the anion adsorption behavior, and could correlate this with the ORR characteristics. The data reveal pronounced effects of the anions and the pH on the ORR characteristics, indicated e. g., by a grossly different activity and selectivity for the 4‐electron pathway to water/hydroxyls, with the onset ranging from ca. 1.0 V in alkaline electrolyte to 0.6 V in sulfuric acid electrolyte, and the selectivity for the 4‐electron pathway ranging from 100 % (alkaline electrolyte, low overpotentials) to 40 % (acidic electrolytes, alkaline electrolyte at high overpotentials). In contrast, the effect of the ORR on the anion adsorption characteristics is small. Anion effects as well as correlations between anion adsorption and ORR are discussed.

In situ Spectroscopic Insight into the Origin of the Enhanced Performance of Bimetallic Nanocatalysts towards the Oxygen Reduction Reaction (ORR)

It is vital to understand the oxygen reduction reaction (ORR) mechanism at the molecular level for the rational design and synthesis of high activity fuel-cell catalysts. Surface enhanced Raman spectroscopy (SERS) is a powerful technique capable of detecting the bond vibrations of surface species in the low wavenumber range, however, using it to probe practical nanocatalysts remains extremely challenging. Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) was used to investigate ORR processes on the surface of bimetallic Pt₃ Co nanocatalyst structures. Direct spectroscopic evidence of *OOH suggests that ORR undergoes an associative mechanism on Pt₃ Co in both acidic and basic environments. Density functional theory (DFT) calculations show that the weak *O adsorption arise from electronic effect on the Pt₃ Co surface accounts for enhanced ORR activity. This work shows SHINERS is a promising technique for the real-time observation of catalytic processes.

In situ H2O2 generation with gold nanoflowers as the coreactant accelerator for enzyme-free electrochemiluminescent immunosensing

In traditional electrochemiluminescence (ECL) analysis, gold nanomaterials are commonly used as a tool for signal amplification and linking antibodies due to their good electrical conductivity and biocompatibility. Here, we found that multitipped gold nanoparticles-gold nanoflowers (AuNFs) as coreactant accelerator have good catalytic activity for the reduction of dissolved oxygen (O₂) to hydrogen peroxide (H₂O₂) using tris (hydroxymethyl) aminomethane (Tris) as electron donor. Based on this, a new enzyme-free and label-free ECL immunosensor have been constructed for the detection of α-fetoprotein (AFP). In this system, due to the unique geometric and spatial effects of AuNFs, the dissolved O₂ as endogenous coreactant was catalyzed by AuNFs to produce H₂O₂ using Tris as an electron donor. The in situ generated H₂O₂ can more efficiently produce various electrogenerated reactive oxygen species (ROSs) as the important intermediates on the electrode surface. Then, oxidation of luminol reacts with ROSs significantly amplifies the luminol ECL signal. Under optimal experimental conditions, the proposed ECL immunosensor was able to detect the AFP concentration from 0.01 to 100 ng mL^-1, with a low detection limit of 3.4 pg mL^-1 (S/N = 3). In addition, the prepared ITO-based sensor is similar to a micro-test chip and convenient to use, thus making it suitable for clinical use as a disposable device in point-of-care tests (POCTs).

Discrimination between hydrogen bonding and protonation in the spectra of a surface-enhanced Raman sensor

Adsorbed mercaptopyridine can sense hydrogen-bonding because the ring breathing mode has a different frequency from bare and protonated species.

Intermediate stages of electrochemical oxidation of single-crystalline platinum revealed by in situ Raman spectroscopy

Understanding the atomistic details of how platinum surfaces are oxidized under electrochemical conditions is of importance for many electrochemical devices such as fuel cells and electrolysers. Here we use in situ shell-isolated nanoparticle-enhanced Raman spectroscopy to identify the intermediate stages of the electrochemical oxidation of Pt(111) and Pt(100) single crystals in perchloric acid. Density functional theory calculations were carried out to assist in assigning the experimental Raman bands by simulating the vibrational frequencies of possible intermediates and products. The perchlorate anion is suggested to interact with hydroxyl phase formed on the surface. Peroxo-like and superoxo-like two-dimensional (2D) surface oxides and amorphous 3D α-PtO₂ are sequentially formed during the anodic polarization. Our measurements elucidate the process of the electrochemical oxidation of platinum single crystals by providing evidence for the structure-sensitive formation of a 2D platinum-(su)peroxide phase. These results may contribute towards a fundamental understanding of the mechanism of degradation of platinum electrocatalysts.

Understanding degradation of platinum catalysts during oxygen reduction is vital for improving proton-exchange membrane fuel cells. Here, the authors identify intermediate stages in the oxidation of Pt(111) and Pt(100) in perchloric acid using in situ shell-isolated nanoparticle-enhanced Raman spectroscopy.

In situ video-STM studies of the mechanisms and dynamics of electrochemical bismuth nanostructure formation on Au

The microscopic mechanisms of Bi electrodeposition on Au(111) and Au(100) electrodes in the overpotential regime were studied by in situ scanning tunneling microscopy with high spatial and temporal resolution. Atomic resolution images of the needle-like Bi(110) deposits formed on Au(111) reveal the central influence of covalent Bi–Bi bonds on the deposit morphology. In the straight steps along the needle edges the Bi atoms are interlinked by these bonds, whereas at the needle tip and at kinks along the needle edges dangling bonds exist, explaining the rapid structural fluctuations at these sites. For ultrathin Bi deposits on Au(100) a more open atomic arrangement was found within the surface plane, which was tentatively assigned to an epitaxially stabilised Bi(111) film. Furthermore, well-defined nanowires, consisting of zigzag chains of Bi surface atoms, were observed on this surface.

Aerobic oxidation of 5-hydroxymethylfurfural (HMF) effectively catalyzed by a Ce0.8Bi0.2O2−δ supported Pt catalyst at room temperature

98% yield of FDCA was obtained within 30 min at room temperature when HMF oxidation was catalyzed by Pt/Ce_0.8Bi_0.2O_2−δ.

In situ surface-enhanced Raman spectroscopic study of formic acid electrooxidation on spontaneously deposited platinum on gold

Present formic acid fuel cell efficiency is limited by low kinetics at the anode, indicating the need for effective catalysts to improve the formic acid oxidation. As a prerequisite, the nature of adsorbed species and specifically the reaction intermediates formed in this process needs to be examined. This work focuses on the electrooxidation of formic acid and the nature of the intermediates at a platinum-modified gold surface prepared through spontaneous deposition using a combination of electrochemistry and in situ surface enhanced Raman spectroscopy (SERS). This Pt-modified gold electrode surface assists in oxidizing formic acid at potentials as low as 0.0 V vs. Ag/AgCl which is 0.15 V more negative than a bare Pt surface. The oxidation current obtained on the Pt-modified gold electrode is 72 times higher than on a bare Au surface and 5 times higher than on a bare Pt surface at the same potential. In situ SERS has revealed the involvement of formate at a low frequency as the primary intermediate in this electrooxidation process. While previous studies mainly focused on the formate mode at ca. 1322 cm(-1), it is the first time that a formate peak at ca. 300 cm(-1) was observed on a Pt or Pt-associated surface. A unique relationship has been observed between the formic acid oxidation currents and the SERS intensity of this formate adsorbate. Furthermore, the characteristic Stark effect of the formate proves the strong interaction between the adsorbate and the catalyst. Both electrochemical and spectroscopic results suggest that the formic acid electrooxidation takes place by the dehydrogenation pathway involving a low frequency formate intermediate on the Pt-modified gold electrode catalyst.

Direct observation of intermediates formed during steady-state electrocatalytic O2 reduction by iron porphyrins

Heme/porphyrin-based electrocatalysts (both synthetic and natural) have been known to catalyze electrochemical O2, H(+), and CO2 reduction for more than five decades. So far, no direct spectroscopic investigations of intermediates formed on the electrodes during these processes have been reported; and this has limited detailed understanding of the mechanism of these catalysts, which is key to their development. Rotating disk electrochemistry coupled to resonance Raman spectroscopy is reported for iron porphyrin electrocatalysts that reduce O2 in buffered aqueous solutions. Unlike conventional single-turnover intermediate trapping experiments, these experiments probe the system while it is under steady state. A combination of oxidation and spin-state marker bands and metal ligand vibrations (identified using isotopically enriched substrates) allow in situ identification of O2-derived intermediates formed on the electrode surface. This approach, combining dynamic electrochemistry with resonance Raman spectroscopy, may be routinely used to investigate a plethora of metalloporphyrin complexes and heme enzymes used as electrocatalysts for small-molecule activation.

Coordination of thiosemicarbazones and bis(thiosemicarbazones) to bismuth(III) as a strategy for the design of metal-based antibacterial agents

Complexes [Bi(2Fo4Ph)Cl(2)] (1), [Bi(2Ac4Ph)Cl(2)] (2), [Bi(2Bz4Ph)Cl(2)] (3), [Bi(H(2)Gy3DH)Cl(3)] (4), [Bi(H(2)Gy4Et)(OH)(2)Cl] (5), and [Bi(H(2)Gy4Ph)Cl(3)] (6) were prepared with pyridine-2-carbaldehyde 4-phenylthiosemicarbazone (H2Fo4Ph), 1-(pyridin-2-yl)ethanone 4-phenylthiosemicarbazone (H2Ac4Ph), phenyl(pyridin-2-yl)methanone 4-phenylthiosemicarbazone (H2Bz4Ph), as well as with glyoxaldehyde bis(thiosemicarbazone) (H(2)Gy4DH) and its 4-Et (H(2)Gy4Et) and 4-Ph (H(2)Gy4Ph) derivatives. The complexes exhibited antibacterial activities against Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, and Pseudomonas aeruginosa. Coordination to Bi(III) proved to be an effective strategy to increase the antibacterial activity of the thiosemicarbazones and bis(thiosemicarbazones).

Catalytically active structure of Bi deposited on a Au(111) electrode for the hydrogen peroxide reduction reaction

The surface structure of underpotentially deposited Bi has been determined on Au(111) in perchloric acid solution using surface X-ray diffraction (SXD), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. SXD analysis and STM images reveal that the catalytically active structure for the hydrogen peroxide reduction reaction (HPRR) is the (2 x 2)-Bi honeycomb structure with theta(Bi) = 0.50. The stability is supported by DFT calculations. The hydrated perchlorate anion is located in the center of the honeycomb structure without hydrogen peroxide. DFT calculations predict that the Bi honeycomb structure promotes the dissociation of the O-O bond of hydrogen peroxide. Hydrogen peroxide expels the hydrated perchlorate anion, and then HPRR takes place at the honeycomb center.

Transport effects in the oxygen reduction reaction on nanostructured, planar glassy carbon supported Pt/GC model electrodes

The role of transport and re-adsorption processes on the oxygen reduction reaction (ORR), and in particular on its selectivity was studied using nanostructured model electrodes consisting of arrays of Pt nanostructures of well-defined size and separation on a planar glassy carbon (GC) substrate. The electrochemical measurements were performed under controlled transport conditions in a double-disk electrode thin-layer flow-cell configuration; the model electrodes were fabricated by colloidal lithography techniques, yielding Pt nanostructures of well defined and controlled size and density (diameter: 140 or 85 nm, height: 20 or 10 nm, separation: from 1-2 to more than 10 diameters). The nanostructured model electrodes were characterized by scanning electron microscopy and electrochemical probing of the active surface area (via the hydrogen adsorption charge). The electrocatalytic measurements revealed a pronounced variation of the hydrogen peroxide yield, which increases by up to two orders of magnitude with increasing separation and decreasing size of the Pt nanostructures. Similar, though less pronounced effects were observed upon varying the electrolyte flow and thus the mass transport characteristics. These effects are discussed in a reaction model which includes (i) direct reduction to H(2)O on the Pt surface and (ii) additional H(2)O(2) formation and desorption on both Pt and carbon surfaces and subsequent partial re-adsorption and further reduction of the H(2)O(2) molecules on the Pt surface.

Rational Design of Selective, Sulfur-Resistant Oxidation Emissions Catalysts

A new catalyst design strategy based on optimizing electronic structure has been proposed and then applied to a very important environmental application, the design of selective, sulfur-resistant oxidation emissions catalysts. The modified d-band center model developed by us in a previous study, together with an energy decomposition scheme, is used to correlate measures of reactivity with reaction barriers of SO2 + O --> SO3 and NO + O --> NO2 on surfaces. Our objective is to find a catalyst which is active in oxidizing NO to NO2 but relatively inactive in oxidizing SO2 to SO3. The Ir alloyed Pt(111) surface is found to have the highest selectivity for oxidation of NO over SO2 at 700 K. Unfortunately, there is a slope change in the correlation of the weighted d-band center with the adsorption of NO at the transition state, which narrows down the range of the theoretical selectivity. Our ongoing study aims at understanding the reason for this. The general importance of this study for surface catalysis is also discussed.

Mechanism of Oxygen Electroreduction on Gold Surfaces in Basic Media

The mechanism of the electroreduction of oxygen on Au surfaces in basic media is examined using surface-enhanced Raman scattering (SERS) measurements and density functional theory (DFT) calculations. The spectroscopy reveals superoxide species as a reduction intermediate throughout the oxygen electroreduction, while no peroxide is detected. The spectroscopy also shows the presence of superoxide after the addition of hydrogen peroxide. The calculations show no effect of OH addition to the Au(100) surface with regard to O-O length. These results suggest that the four-electron reduction of O(2) on Au(100) in base arises from a disproportionation mechanism which is enhanced on Au(100) relative to the other two low Miller index faces of Au.

Oxidation of Formic Acid and Carbon Monoxide on Gold Electrodes Studied by Surface-Enhanced Raman Spectroscopy and DFT

The oxidation of formic acid and carbon monoxide was studied at a gold electrode by a combination of electrochemistry, in situ surface-enhanced Raman spectroscopy (SERS), differential electrochemical mass spectrometry, and first-principles DFT calculations. Comparison of the SERS results and the (field-dependent) DFT calculations strongly suggests that the relevant surface-bonded intermediate during oxidation of formic acid on gold is formate HCOO- ad*. Formate reacts to form carbon dioxide via two pathways: at low potentials, with a nearby water to produce carbon dioxide and a hydronium ion; at higher potentials, with surface-bonded hydroxyl (or oxide) to give carbon dioxide and water. In the former pathway, the rate-determining step is probably related to the reaction of surface-bonded formate with water, as measurements of the reaction order imply a surface almost completely saturated with adsorbate. The potential dependence of the rate of the low-potential pathway is presumably governed by the potential dependence of formate coverage. There is no evidence for CO formation on gold during oxidation of formic acid. The oxidation of carbon monoxide must involve the carboxyhydroxyl intermediate, but SERS measurements do not reveal this intermediate during CO oxidation, most likely because of its low surface coverage, as it is formed after the rate-determining step. Based on inconclusive spectroscopic evidence for the formation of surface-bonded OH at potentials substantially below the surface oxidation region, the question whether surface-bonded carbon monoxide reacts with surface hydroxyl or with water to form carboxyhydroxyl and carbon dioxide remains open. The SERS measurements show the existence of both atop and bridge-bonded CO on gold from two distinguishable low-frequency modes that agree very well with DFT calculations.

NO Chemisorption on Pt(111), Rh/Pt(111), and Pd/Pt(111)

The chemisorption of NO on clean Pt(111), Rh/Pt(111) alloy, and Pd/Pt(111) alloy surfaces has been studied by first principles density functional theory (DFT) computations. It was found that the surface compositions of the surface alloys have very different effects on the adsorption of NO on Rh/Pt(111) versus that on Pd/Pt(111). This is due to the different bond strength between the two metals in each alloy system. A complex d-band center weighting model developed by authors in a previous study for SO2 adsorption is demonstrated to be necessary for quantifying NO adsorption on Pd/Pt(111). A strong linear relationship between the weighted positions of the d states of the surfaces and the molecular NO adsorption energies shows the closer the weighted d-band center is shifted to the Fermi energy level, the stronger the adsorption of NO will be. The consequences of this study for the optimized design of three-way automotive catalysts, (TWC) are also discussed.

Electronic Composition−Property Relationship Applied to SO2 Chemisorption on Pt(111) Surfaces, Alloys, and Overlayers

We have developed a new electronic composition-property relationship between the adsorption energy of molecules binding unequally to multiple atoms on metal surfaces and the electronic properties of the surface. This relationship allows the estimation of the relative stability of adsorbates on various surfaces, assuming that the adsorbate adopts the same local configuration on each surface, and therefore also allows the estimation of the adsorption energy of molecules through large regions of parameter space in alloy systems with data from only a few explicit calculations. We have applied this relationship to the adsorption of SO(2) on Pt surfaces alloyed with Pd, Cu, Ru, and Ni. Using a new formula for weighting the metal d-band, we found a strong linear relationship between the weighted positions of the d states of surfaces and the most stable molecular adsorption energies. The consequences of our electronic composition-property relationship for catalyst design are also discussed.