Zhao-Wu Tian

Electrocatalytic properties of Pt(111), Pt(332), Pt(331) and Pt(110) single crystal electrodes towards ethylene glycol oxidation in sulphuric acid solutions

Abstract In the present paper four platinum single crystal electrodes, two basal planes of Pt(111) and Pt(110) and two stepped surfaces of Pt(332) and Pt(331), were prepared and used in the study of electro-oxidation of ethylene glycol (EG). All of these Pt single crystal electrodes belong to the [1 1 0] zone of crystallography, and exhibit on their surface (111) symmetry sites or certain combinations of terraces of (111) symmetry with steps of (111) symmetry type. It has been found that as a result of a favourable steric matching of surface sites the Pt(110) electrode manifested a higher activity both for EG dissociative adsorption and oxidation than that of the Pt(111) electrode. The stepped surfaces of Pt(332) and Pt(331) operated with certain combinations of characteristics of Pt(111) and Pt(110). The best electrocatalytic properties have been obtained with a Pt(331) electrode, and this is attributed both to the configuration of the atomic arrangement and to the stability of this surface. In summary, the above results show that the performance of a given Pt single crystal electrode in EG oxidation at a potential below 1.0 V may be evaluated by three factors. 1. (1) The ability to resist self-poisoning (AB) which describes the difficulty of EG dissociative adsorption on the electrode surface. 2. (2) The activity for EG oxidation (AC). In considering that the threshold potential for EG oxidation on all electrodes is at 0.3 V and that the self-poisoning is encountered in PGPS, the activity for EG oxidation may be reasonably characterized by the intensity of the peak current acquired in NGPS near 0.6 V, which corresponds to the maximum current of EG oxidation on an activated (non-poisoned) surface of the electrode. 3. (3) The stability of activity during potential cycling (SA) between 0.05 and 1.0 V, which describes the resistance to the decrease of intensity of the EG oxidation current during voltammetric cycling. For the two basal planes studied, the AB and SA of Pt(111) are higher than those of Pt(110), but its AC is much lower than that of Pt(110). These differences are clearly related to the surface atomic arrangement of the two electrodes. As has been discussed above, the surface of Pt(111) is atomically smooth and stable during voltammetric cycling. The surface of Pt(110) presents, however, atomic steps and is reconstructed under experimental conditions, i.e. certain steric configurations are encountered on the Pt(110) surface. The high AC and the low AB may be assigned to a favourite stereographic matching during EG adsorption and oxidation on Pt(110). The two electrodes with stepped surfaces, Pt(332) and Pt(331), contain different densities of (110) sites, which are formed on the border between terrace and step, as shown in Fig. 8. The AB of these two electrodes has been observed at a moderate range between that of Pt(111) and the AB of Pt(110). With a majority of (111) sites on its surface, the electrode of Pt(332) operates at a relatively higher AC than Pt(111) does, and its SA is not as good as that of Pt(111) but is much better than the SA of a Pt(110) electrode. In all cases the highest AC and SA are obtained with a Pt(331) electrode. It may be seen from the profile of a (331) plane (shown by the cross-section of A-A in Fig. 8) that all atoms on the top of the surface participated in forming (110) sites, and the atom on the step has two functions — one is to form a (110) site with an atom located in the terrace of second layer and the other is to form a (111) site in the terrace of the same layer. It has been mentioned in the above discussions that the Pt(110) electrode keeps a higher AC due to favourite stereographic matching in EG adsorption and oxidation, but its SA is the worst, due to the instability of the surface. The highest AC and SA obtained with Pt(331) may be ascribed not only to the high density of (110) sites existing on the surface, but also to the stabilization of these (110) sites, and moreover, the synergy generated by the atomic arrangement of the Pt(331) surface may also contribute to the performance of the Pt(331) electrode.

In situ FTIR studies on the adsorption and oxidation of n-propanol and isopropanol at a platinum electrode in sulphuric acid solutions

Abstract The adsorption and oxidation of n-propanol and isopropanol at a platinum electrode have been investigated mainly by in situ FTIR spectroscopy. Molecular structure effects due to the different bonding position of the hydroxyl group to the carbon chain of the two molecules studied were demonstrated clearly. n-Propanol can dissociate into linearly bonded CO and CH3CH3 species which block the platinum surface at lower potentials; however, for steric reasons no dissociative adsorption takes place with isopropanol at a Pt electrode. CO2 was detected as the final oxidation product for both reagents. The reactive intermediate in the oxidation of n-propanol was identified as H-bonded propionic acid, and the majority intermediate in the isopropanol oxidation was acetone. It is concluded that the oxidation of n-propanol takes place via a dual path mechanism; in the case of isopropanol, a successive dehydrogenation mechanism is suggested.

Studies on silicon etching using the confined etchant layer technique

Silicon surface etching in HBr solutions using the confined etchant layer technique (CELT) as well as scanning electrochemical microscopy (SECM) has been carried out and comparison between the two methods has been made in terms of the etching resolution. It has been shown that the lateral diffusion of the etchant in SECM configuration can be suppressed in CELT by a homogeneous scavenging reaction and thus the etching resolution of surface, especially for those with slow etching rate such as Si can be improved. H3AsO3 was added to the solution containing HBr which reacts rapidly and homogeneously with the electrogenerated bromine, resulting in a very thin bromine diffusion layer surrounding the tip. The size of the etching spot at the Si wafer surface obtained using the CELT matches that of the tip very well.

Confined etchant layer technique for two-dimensional lithography at high resolution using electrochemical scanning tunnelling microscopy

In order to realize two-dimensional lithography at high resolution (several tens of nanometres), a new approach to the lithography of a fine pattern using a confined etchant layer technique (CELT) in an electrochemical system is presented. A mould plate of conductive material with a high-resolution line pattern (which can be prepared with the aid of e.g. electron beams) is used instead of the tip in scanning tunnelling microscopy (STM). The etchant species is generated at the surface of the mould plate by electrochemical photochemical or photoelectrochemical methods, then diffuses away from the surface of the mould plate. The key feature of CELT is the design of a chemical reaction which rapidly destroys the etchant (e.g. within microseconds on average) following its generation. Therefore, the gradient of the concentration of etchant can be greatly enhanced and the thickness of the diffusion layer can be greatly decreased to several tens of nanometres. Thus, the etchant layer is confined and its outer boundary can essentially retain the fine structure of the pattern of the mould plate. Then the substrate to be corroded is adjusted by ECSTM to approach the mould plate within several tens of nanometres and the corroded pattern can retain the fine structure giving a resolution of several tens of nanometres.

ELECTROCHEMICAL PREPARATION OF A POLYPARAPHENYLENE FILM AND A POLYPARAPHENYLENE POLYACRYLAMINE FUNCTIONAL FILM

Abstract We have first found the way to obtain a uniform and flexible polyparaphenylene (PPP) film by electrochemical polymerization of benzene in concentrated sulphuric acid with an anhydrous Lewis acid as catalyst. It is evident that concentrated sulphuric acid plays an important role in the electrochemical polymerization of benzene. It protonates benzene into benzenium. The rotation of the working electrode is an important condition for obtaining uniform and flexible PPP films. The thickness of the PPP film can be controlled by the electricity used for polymerization and the amounts and kinds of catalysts employed. The colour of the PPP film changes from transparent yellow to black, depending on the thickness. The PPP film is electroactive in concentrated sulphuric acid. The stable polymer film (PPP) is a suitable underlayer for polyacrylamine (PAA) formation by the electrochemical polymerization method. This PPP/PAA film has high potential for applications in the extraction of trace metal ions, ion exchange and in catalysis.

Three-dimensional micromachining for microsystems by confined etchant layer technique

Abstract The micromachining of GaAs with three different truly three-dimensional (3D) molds were performed by the confined etchant layer technique (CELT). The etched patterns were found, approximately, to be the negative copy of the 3D molds. The general comparison of CELT with the existing micromachining techniques, such as two-dimensional (2D) projection lithography and electro-discharge machining, was made. The replication of the complex microstructures down to micrometer scale has been done by CELT in a single step. The photoresist layer, together with the procedures of exposure, developing and removal of resist, could be eliminated. The advantages of CELT over the existing lithography techniques and its potential applications are discussed briefly. It has been shown that CELT could be developed as a complementary technique to the existing micromachining techniques in fabricating microdevices for microsystems.

Surface Raman spectroscopic investigation of pyridine adsorption at platinum electrodes—effects of potential and electrolyte

Surface enhanced Raman spectra of pyridine (Py) at Pt electrodes have been investigated as a function of potential and supporting electrolyte. The results show a large difference from those reported for coinage metal electrodes of Ag, Au and Cu, emphasising the effective involvement of chemical enhancement on Pt surfaces. At very negative (or positive) potentials, Raman spectra show the competitive coadsorption of hydrogen (oroxygen-containing species) with Py, and in acidic solutions, PyH+ ions prefer to dissociate into Py adsorbed on Pt surfaces even in the presence of chloride ions. The differences in the surface bonding strength for Py on Pt and coinage metal electrodes are explained in terms of the different electronic configurations of the metals.

Scanning microelectrode studies of early pitting corrosion of 18/8 stainless steel

Abstract The scanning microreference electrode (SMRE) technique was used to study the early stage of pitting corrosion of 18/8 stainless steel (SS) in solutions containing chloride (Cl−) ions. Local breakdown of passivity and micropitting corrosion occurred in the potential region that was much more negative than the ordinary pitting potential (Ep). A critical potential (Er was measured by imaging the potential near the metal surface in solution to characterize the local rupture of passivity or micropitting initiation. The concept of metastable micropitting was adopted to discuss the mechanism of pitting corrosion in its early stage.

A Leveling Method Based on Current Feedback Mode of Scanning Electrochemical Microscopy

Substrate leveling is an essential but neglected instrumental technique of scanning electrochemical microscopy (SECM). In this technical note, we provide an effective substrate leveling method based on the current feedback mode of SECM. By using an air-bearing rotary stage as the supporter of an electrolytic cell, the current feedback presents a periodic waveform signal, which can be used to characterize the levelness of the substrate. Tuning the adjusting screws of the tilt stage, substrate leveling can be completed in minutes by observing the decreased current amplitude. The obtained high-quality SECM feedback curves and images prove that this leveling technique is valuable in not only SECM studies but also electrochemical machining.

EFFECT OF SUPPRESSION EFFICIENCY ON SENSITIVITY IN ION CHROMATOGRAPHY

The ionization equilibrium is calculated for a suppressor effluent containing remnant ions, e.g., Na+ in the case of anion analysis, due to imperfect suppression. It is shown that: (1) The remnant ion not only affects the background specific conductivity but also seriously depresses the peak height of samples of low concentration. The practical peak height may be much less than the ideal peak height. For PKHA > 11, the practical peak height may be negative. (2) The sensitivity, linearity and reproducibility of conductivity detection for low concentration samples are seriously affected by the remnant ion, i.e., by the conversion efficiency of the suppressor. (3) The ionization of the weak acid derived from the eluent is depressed by the common ion effect of the strong acid derived from the sample. The peak height depression depends on the ionization constant, KHA. However, in the case of CNa > 0, the buffer effect of compensates the effect of remnant Na+. Therefore it is appropriate to use an eluent with PKHA in the range 6–8.