Surface Extraction Process During Initial Oxidation of Pt(111): Effect of Hydrophilic/Hydrophobic Cations in Alkaline Media.

The surface oxidation states of the metal electrodes affect the activity, selectivity, and stability of the electrocatalysts. Oxide formation and reduction on such electrodes must be comprehensively understood to achieve next-generation electrocatalysts with outstanding performance and stability. Herein, the initial electrochemical oxidation of Pt(111) in alkaline media containing hydrophilic and hydrophobic cations is investigated by X-ray crystal truncation rod (CTR) scattering, infrared (IR) spectroscopy, and nanoparticle-based surface-enhanced Raman spectroscopy (SERS). Structural determination using X-ray CTR revealed surface buckling and Pt extraction at the initial stage of surface oxidation, depending on the cationic species. Vibrational spectroscopy is performed to identify the potential- and cation-dependent formation of three oxide species (IR-active OHad, Raman-active OHad/Oad(H2O), and Raman-active Oad). Hydrophilic alkali metal cations (Li+) inhibit surface roughening via irreversible oxide formation. Hydrophilic Li+ can strongly stabilize IR-active OHad, hindering the extraction of Pt surface atoms. Interestingly, bulky hydrophobic cations such as tetramethylammonium (TMA+) cation also reduce the extent of irreversible oxidation despite the absence of IR-active OHad. Hydrophobic TMA+ inhibits the formation of Raman-active OHad/Oad(H2O) associated with Pt extraction. In contrast, the moderate hydrophilicity of K+ has no protective effect against irreversible oxidation. Moderate hydrophilicity enables the coadsorption of Raman-active OHad/Oad(H2O) and Raman-active Oad. The electrostatic repulsion between Raman-active OHad/Oad(H2O) and neighboring Raman-active Oad promotes Pt extraction. These results provide insights into controlling the surface structures of electrocatalysts using cationic species during the oxide formation and reduction processes.

Enhanced oxygen reduction reaction on caffeine-modified platinum single-crystal electrodes.

Enhancing the activity of the oxygen reduction reaction (ORR) is crucial for fuel cell development, and hydrophobic species are known to increase the ORR activity. This paper reports that caffeine enhanced the specific ORR activity of Pt(111) 11-fold compared to that without caffeine in a 0.1 M HClO4 aqueous solution. Moreover, caffeine increased the ORR activity of Pt(110) 2.5-fold; however, the activity of Pt(100) was unaffected. The infrared (IR) band of PtOH (blocking species of the ORR) decreased for all the surfaces. Caffeine was adsorbed with its molecular plane perpendicular to the Pt(111) and Pt(110) surfaces and tilted relative to the Pt(100) surface. Thus, the effects of caffeine on the ORR activity can be rationalized by a decrease in PtOH coverage and the difference in adsorption geometry of caffeine.

Interfacial Structure of Pt(110) Electrode during Hydrogen Evolution Reaction in Alkaline Solutions.

In alkaline solutions, interfacial cations affect the hydrogen evolution reaction (HER) activity of platinum electrodes. However, the effects of cations on the HER activity have not been previously investigated based on interfacial structures. In situ surface X-ray diffraction was performed on Pt(110), of which the HER activity is the highest in the low-index planes of Pt, at hydrogen evolution potentials in alkaline solutions, and revealed the interfacial structure of alkali metal cations (Li+ and Cs+). The interfacial structure of the Pt(110) electrode changed reversibly depending on the electrode potential. In the LiOH solution, where the HER activity was higher, the densely packed water layer in the electrical double layer acted as a hydrogen supplier. In the CsOH solution, where the HER activity was lower, the Cs+ cations were aligned in the missing rows of the 1 × 2 reconstructed Pt(110) surface, suggesting that the Cs+ hindered water from accessing the surface, resulting in a lower HER activity.

Tailoring the active site for the oxygen evolution reaction on a Pt electrode.

Highly active electrocatalysts for the oxygen evolution reaction (OER) are essential to improve the efficiency of water electrolysis. The properties of OER active sites on single-crystal Pt electrodes were examined herein. The OER is markedly enhanced by repeated oxidative and reductive potential cycles on the Pt(111) surface. The OER activity on Pt(111) is nine times higher in the third cycle than that before the potential cycles. OER activation by potential cycling depends on the (111) terrace width, with wider (111) terraces significantly enhancing the OER. The oxidation/reduction of the Pt(111) surface produces atomic-sized vacancies on the terraces that activate the OER. Structural analysis using X-ray diffraction reveals that the active sites formed by potential cycling are defects in the second subsurface Pt layer. Potential cycling induces the bowl-shaped roughening of the electrode surface, wherein high-coordination number Pt atoms at the bottom of the cavities activate the OER.

Effect of Hydrophobic Cations on the Inhibitors for the Oxygen Reduction Reaction on Anions and Ionomers Adsorbed on Single-Crystal Pt Electrodes.

Weakening of the poisoning by the specifically adsorbed anions assists in developing next-generation electrocatalysts for use in low-temperature fuel cells. In this study, we evaluated how hydrophobic cations with different alkyl chain lengths affect the oxygen reduction reaction (ORR) activities on the single-crystal Pt surfaces in contact with sulfuric acid solution and Nafion ionomers. Interfacial tetraalkylammonium cations with longer alkyl chains activated the ORR on the Pt(111) surface. In a solution containing tetrahexylammonium cations (THA+), the ORR activities on Pt(111) in sulfuric acid solution and on Nafion-modified Pt(111) in perchloric acid solution were four and eight times higher than those in the solutions without THA+, respectively. Infrared spectroscopy revealed the reduction of the amount of (bi)sulfate anions and the sulfonate group of Nafion adsorbed on Pt(111) due to the presence of THA+. The hydrophobic cations weaken the noncovalent interactions between specifically adsorbed species and promote the ORR.

In Situ Spectroscopic Study on the Surface Hydroxylation of Diamond Electrodes.

Carbon-based materials are regarded as an environmentally benign alternative to the conventional metal electrode used in electrochemistry from the viewpoint of sustainable chemistry. Among various carbon electrode materials, boron-doped diamond (BDD) exhibits superior electrochemical properties. However, it is still uncertain how surface chemical species of BDD influence the electrochemical performance, because of the difficulty in characterizing the surface species. Here, we have developed in situ spectroscopic measurement systems on BDD electrodes, i.e., in situ attenuated total reflection infrared spectroscopy (ATR-IR) and electrochemical X-ray photoelectron spectroscopy (EC-XPS). ATR-IR studies at a controlled electrode potential confirmed selective surface hydroxylation. EC-XPS studies confirmed deprotonation of C-OH groups at the BDD/electrolyte interface. These findings should be important not only for better understanding of BDD's fundamentals but also for a variety of applications.

Effect of hydrophobic cations on the oxygen reduction reaction on single‒crystal platinum electrodes.

Highly active catalysts for the oxygen reduction reaction are essential for the widespread and economically viable use of polymer electrolyte fuel cells. Here we report the oxygen reduction reaction activities of single‒crystal platinum electrodes in acidic solutions containing tetraalkylammonium cations with different alkyl chain lengths. The high hydrophobicity of a tetraalkylammonium cation with a longer alkyl chain enhances the oxygen reduction reaction activity. The activity on Pt(111) in the presence of tetra‒n‒hexylammonium cation is eight times as high as that without this cation, which is comparable to the activities on Pt3Co(111) and Pt3Ni(111) electrodes. Hydrophobic cations and their hydration shells destabilize the adsorbed hydroxide and adsorbed water. The hydrophobic characteristics of non‒specifically adsorbed cations can prevent the adsorption of poisoning species on the platinum electrode and form a highly efficient interface for the oxygen reduction reaction.

Impact of helical organization on the photovoltaic properties of oligothiophene supramolecular polymers.

Helical self-assembly of functional π-conjugated molecules offers unique photochemical and electronic properties in the spectroscopic level, but there are only a few examples that demonstrate their positive impact on the optoelectronic device level. Here, we demonstrate that hydrogen-bonded tapelike supramolecular polymers of a barbiturated oligo(alkylthiophene) show notable improvement in their photovoltaic properties upon organizing into helical nanofibers. A tapelike hydrogen-bonded supramolecular array of barbiturated oligo(butylthiophene) molecules was directly visualized by STM at a liquid-solid interface. TEM, AFM and XRD revealed that the tapelike supramolecular polymers further organize into helical nanofibers in solution and bulk states. Bulk heterojunction solar cells of the helical nanofibers and soluble fullerene showed a power conversion efficiency of 4.5%, which is markedly high compared to that of the regioisomer of butyl chains organizing into 3D lamellar agglomerates.

In situ observation of Pt oxides on the low index planes of Pt using surface enhanced Raman spectroscopy.

In situ vibrational spectra of Pt oxides that cannot be measured with IR spectroscopy have been studied on the low index planes of Pt using surface enhanced Raman spectroscopy with bare Au nanoparticles (NPSERS). Two bands appear around 570 and 340 cm-1 at higher potentials in 0.1 M HClO4 saturated with Ar, which are assigned to the stretching vibration of Pt-O(H) and the libration vibration of Pt-O, respectively. NPSERS spectra are measured in O2 saturated solution for the first time. The band intensities of Pt-O(H) and Pt-O in O2 saturated solution are enhanced significantly compared with those in Ar saturated solution. The onset potentials of Pt-O and Pt-O(H) formation are 1.15 V(RHE) on Pt(100) and 1.2 V(RHE) on Pt(111) and Pt(110). The onset potential of Pt-O and Pt-O(H) and band shape differ from the results obtained using shell isolated surface enhanced Raman spectroscopy (SHINERS). The Pt-O and Pt-O(H) band intensities are normalized using COad as an internal standard. The Pt-O(H) band intensity depends on surface structures as Pt(110) < Pt(111) ≪ Pt(100), whereas the Pt-O band gives a different intensity order for Pt(111) and Pt(110) as Pt(111) ≤ Pt(110) ≪ Pt(100) in O2 saturated solution.

Real-time observation of interfacial ions during electrocrystallization.

Understanding the electrocrystallization mechanisms of metal cations is of importance for many industrial and scientific fields. We have determined the transitional structures during underpotential deposition (upd) of various metal cations on Au(111) electrode using time-resolved surface X-ray diffraction and step-scan IR spectroscopy. At the initial stage of upd, a characteristic intensity transient appears in the time-resolved crystal truncation rod depending on metal cations. Metal cations with relatively high coordination energies of hydration water are deposited in two steps: first, the hydrated metal cations approached the surface and are metastably located at the outer Helmholtz plane, then they are deposited via the destruction of the hydration shell. However, Tl+ and Ag+, which have low hydration energy, are rapidly adsorbed on Au(111) electrode without any metastable states of dehydration. Therefore, the deposition rate is strongly related to the coordination energy of the hydration water. Metal cations strongly interacting with the counter coadsorbed anions such as Cu2+ in sulfuric acid causes the deposition rate to be slower because of the formation of complexes.

Structural Effects on the Incident Photon-to-Current Conversion Efficiency of Zn Porphyrin Dyes on the Low-Index Planes of TiO2.

The structural effects of substrates on the incident photon-to-current conversion efficiency (IPCE) of Zn porphyrin (ZnP) dyes (ZnP-ref, YD2, and ZnPBAT) have been studied on well-defined single-crystal surfaces of rutile TiO2 (TiO2(111), TiO2(100), and TiO2(110)). IPCE of ZnP-ref depends on the structure of the substrates remarkably: TiO2(100) < TiO2(110) < TiO2(111). IPCE of ZnP-ref/TiO2(111) is 13 times as high as that of ZnP-ref/TiO2(100) at 570 nm. YD2 and ZnPBAT also give the highest IPCE on TiO2(111). The relative coverages of the porphyrin dyes give the following order: TiO2(111) < TiO2(110) < TiO2(100). This order is opposite to that of IPCEs. The orientation of the dyes is predicted using density functional theory calculations on simplified models of TiO2 surfaces. The highest IPCE on TiO2(111) is attributed to the high rate of electron transfer through the space due to the fluctuation of the tilt angle of the adsorbed dyes.

Interfacial Structure of PtNi Surface Alloy on Pt(111) Electrode for Oxygen Reduction Reaction.

The interfacial structure and activity for the oxygen reduction reaction (ORR) were investigated on a PtNi surface alloy on a Pt(111) electrode (PtNi/Pt(111)). The PtNi surface alloy was prepared by thermal annealing of Ni2+ modified on Pt(111) at 573-803 K. After optimizing the alloying temperature and the amount of added Ni, the ORR current density of PtNi/Pt(111) at 0.9 V (reversible hydrogen electrode) is enhanced 9.5 times compared with that of Pt(111), and the activity is decreased by 24% after 1000 potential cycles. The atomic composition and subsurface structure of PtNi/Pt(111) were determined by in situ infrared reflection-absorption spectroscopy and X-ray diffraction. The surface contains a (111)-oriented Pt-skin and the subsurface of the 2nd-5th layers of the PtNi alloy contains less than 11% Ni atoms. The layer spacings of the surface alloy layers are slightly expanded compared with those of bare Pt(111). Homogeneous alloying with a small amount of Ni in the subsurface layers achieves the high ORR activity and durability.

Vibrational Spectroscopic Observation of Atomic-Scale Local Surface Sites Using Site-Selective Signal Enhancement.

Molecule-substrate interactions are sensitively affected by atomic-scale surface structures. Unique activity in heterogeneous catalysts or electrocatalysts is often related with local surface sites with specific structures. We demonstrate that adsorption geometry of a model molecule with an isocyanide anchor is drastically varied among one-fold atop, two-fold bridge, and three-fold hollow configurations with increasing the size of atomic-scale local surface sites of Pd islands on an Au(111) model surface. The vibrational spectroscopic observation of such local information is realized by site-selective and self-assembled formation of hotspots, where Raman scattering intensity is significantly enhanced via excitation of localized surface plasmons.

Structural effects on the oxygen reduction reaction on the high index planes of Pt3Co.

The oxygen reduction reaction (ORR) has been studied on the n(111)-(111) and n(111)-(100) series of Pt3Co using a hanging meniscus rotating disk electrode (HMRDE) in 0.1 M HClO4 (n is the number of terrace atomic rows). The activity for the ORR on the n(111)-(111) series, which is estimated by the specific activity at 0.90 V (RHE) j(k), increases linearly with an increase in the step atom density d(S). On the other hand, the activity for the ORR on the n(111)-(100) series increases linearly with an increase of d(S) on the surfaces with n≥ 9. On the surfaces with n < 9, however, the activity for the ORR decreases with an increase of d(S). We find a correlation between the ORR activity and θ(OH) at the step on the assumption that all the oxide species are Pt-OH.

Effect of non-specifically adsorbed ions on the surface oxidation of Pt(111).

The oxidation processes of a Pt(111) electrode in alkaline electrolytes depend on non-specifically adsorbed ions according to in situ X-ray diffraction and infrared spectroscopic measurements. In an aqueous solution of LiOH, an OHad adlayer is formed in the first oxidation step of the Pt(111) electrode as a result of the strong interaction between Li(+) and OHad , whereas Pt oxidation proceeds without OHad formation in CsOH solution. Structural analysis by X-ray diffraction indicates that Li(+) is strongly protective against surface roughening caused by subsurface oxidation. Although Cs(+) is situated near the Pt surface, the weak protective effect of Cs(+) results in irreversible surface roughening due to subsurface oxidation.

Quantitating the lattice strain dependence of monolayer Pt shell activity toward oxygen reduction.

Lattice strain of Pt-based catalysts reflecting d-band status is the decisive factor of their catalytic activity toward oxygen reduction reaction (ORR). For the newly arisen monolayer Pt system, however, no general strategy to isolate the lattice strain has been achieved due to the short-range ordering structure of monolayer Pt shells on different facets of core nanoparticles. Herein, based on the extended X-ray absorption fine structure of monolayer Pt atoms on various single crystal facets, we propose an effective methodology for evaluating the lattice strain of monolayer Pt shells on core nanoparticles. The quantitative lattice strain establishes a direct correlation to monolayer Pt shell ORR activity.

Surface X-ray scattering of stepped surfaces of platinum in an electrochemical environment: Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111).

Real surface structures of the high-index planes of Pt with three atomic rows of terraces (Pt(331) = 3(111)-(111) and Pt(511) = 3(100)-(111)) have been determined in 0.1 M HClO(4) at 0.1 and 0.5 V(RHE) with the use of surface X-ray scattering (SXS). The surfaces with two atomic rows of terraces, Pt(110) = 2(111)-(111) and Pt(311) = 2(100)-(111) = 2(111)-(100), are reconstructed to a (1 × 2) structure according to previous studies. However, the surfaces with three atomic rows of terraces have pseudo-(1 × 1) structures. The interlayer spacing between the first and the second layers, d(12), is expanded 13% on Pt(331) compared to that of the bulk, whereas it is contracted 37% on Pt(511). The surface structures do not depend on the applied potential on either surface.

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.

Estimation of surface structure and carbon monoxide oxidation site of shape-controlled Pt nanoparticles.

Surface structures of shape-controlled Pt nanoparticles have been estimated using cyclic voltammetry (CV) and infrared reflection absorption spectroscopy (IRAS). Cubic and cuboctahedral Pt nanoparticles are prepared using a capping polymer. These nanoparticles give CVs similar to those of single crystal electrodes of Pt in sulfuric acid solution. The CV of cubic nanoparticles is similar to that of the Pt(510) [=5(100)-(110)] electrode, while the CV of cuboctahedral nanoparticles is reproduced well with the convolution of Pt(766) [=13(111)-(100)] and Pt(17 1 1) [=9(100)-(111)] electrodes. These results suggest that the planes of the cubic and cuboctahedral nanoparticles are composed of step-terrace and atomically flat terraces, respectively. Adsorbed carbon monoxide (CO) on the shape-controlled nanoparticles gives the IR bands that are assigned to on-top and bridged CO. The band of on-top CO is deconvoluted to two bands: the higher and the lower frequency bands are assigned to the CO on the plane and the edges of the nanoparticles, respectively. On-top CO adsorbed on the edges is oxidized at more negative potential than that on the planes. Edge sites of the nanoparticles promote CO oxidation.