Dissolution, nanostructures

The concepts and basic approach used in studies of electrical fluctuations in corrosion processes proved to be very successful as well in mechanistic studies of electrode reactions taking place at materials covered by passivating films. A typical example is the electrochemical dissolution of silicon. From an analysis of the noise characteristics of this process, it has been possible to identify many features as well as the conductivity of the nanostructures of porous silicon being formed on the original silicon surface. 

This is not easy to understand, since on thermodynamic grounds clusters should be less stable than the bulk material. It is possible that clusters generated by electrochemical nanostructuring may undergo a certain degree of alloying with the material of the surface that could increase their stability. Indeed, Monte Carlo simulations show that if achievable, such alloying will improve the stability of the clusters toward dissolution. 

Experiments and simulations show that the characteristics of the nanostructures generated by this procedure are basically given by live parameters the distance between the STM and the substrate, the quantity of material loaded on the tip, the maximum ion current density for the dissolution of the material on the tip, the potential of the substrate, and the diameter of the STM apex. The controlled variation of these five parameters allows tailoring of the diameter and height of the clusters. 

The second procedure is different from the previous one in several aspects. First, the metallic substrate employed is Au, which does not show a remarkable dissolution under the experimental conditions chosen, so that no faradaic processes are involved at either the substrate or the tip. Second, the tip is polarized negatively with respect to the surface. Third, the potential bias between the tip and the substrate must be extremely small (e.g., -2 mV) otherwise, no nanocavity formation is observed. Fourth, the potential of the substrate must be in a region where reconstruction of the Au(lll) surface occurs. Thus, when the bias potential is stepped from a significant positive value (typically, 200 mV) to a small negative value and kept there for a period of several seconds, individual pits of about 40 nm result, with a depth of two to four atomic layers. According to the authors, this nanostructuring procedure is initiated by an important electronic (but not mechanical) contact between tip and substrate. As a consequence of this interaction, and stimulated by an enhanced local reconstruction of the surface, some Au atoms are mobilized from the Au surface to the tip, where they are adhered. When the tip is pulled out of the surface, a pit with a mound beside it is left on the surface. The formation of the connecting neck between the tip and surface is similar to the TILMD technique described above but with a different hnal result a hole instead of a cluster on the surface (Chi et al., 2000). 

In an LLS study, Zhang et al. [94] dissolved this well-characterized PNIPAM-seg-St copolymer in deionized water for 10 days to ensure complete dissolution. The final concentration used was 7.2 E - 7g/mL and it was clarified with a 0.5 pm Millipore Millex-LCR filter to remove dust. Note that in their experiment, the scattering volume ( 10 pL) still contained 105-106 copolymer chains so that the number of density fluctuations was not a problem even in such a dilute solution. Their original objective was to determine whether such a copolymer chain could self-fold into the predicted singleflower-like core-shell nanostructure. 

Using this approach, hydrophilic (neutral or ionic) comonomers, such as end-captured short polyethylene oxide (PEO) chains (macromonomer), l-vinyl-2-pyrrolidone (VP), acrylic acid (AA) and N,N-dimethylacrylamide (DMA), can be grafted and inserted on the thermally sensitive chain backbone by free radical copolymerization in aqueous solutions at different reaction temperatures higher or lower than its lower critical solution temperature (LCST). When the reaction temperature is higher than the LOST, the chain backbone becomes hydrophobic and collapses into a globular form during the polymerization, which acts as a template so that most of the hydrophilic comonomers are attached on its surface to form a core-shell structure. The dissolution of such a core-shell nanostructure leads to a protein-like heterogeneous distribution of hydrophilic comonomers on the chain backbone. 

Zhao et al. obtained NaYE4 nanostructured arrays by solvothermal reaction of NaE, R(NOs)3 and oleic acid. A reverse micelle dissolution-... 

This process produces stabilized highly porous amorphous nanostructured particles with improved wetting and rate of dissolution. The crystallinity can be tuned by varying the processing solvents, stabilizers, and conditions. Drug loading has been reported to be as high as 91%.[ l... 

Fig. 2 shows another kind of si2S-dependent effects in the potentiodynamic multi-frequency AC response of Si-Me/Me" nanostructured system. PDEIS spectra of Cu cathodic deposition and anodic dissolution on Si with nanoporous... 

In the Pt-doped hexagonal mesophase formed from CPCI (cetyl pyridinium chloride), platinum ions are adsorbed at the surface of the surfactant cylinders. They are reduced radiolytically into a metal layer as a nanotube of around 10 nm diameter and a few hundred nm long (Fig. 3f). Extraction of all these nanostructures is achieved by dissolution of the soft template using alcohol. This possible easy extraction constitutes a marked advantage over the synthesis in hard templates, such as mesoporous silica or carbon nanotubes, the dissolution of which is more hazardous for the metal nanostructures. 

Electrochemical STM nanofabrication techniques usually involve either localized etching (dissolution), or plating (deposition) on the surface via a sharp STM tip. The first demonstration of nanostructuring was reported by Penner s group. They fabricated silver and copper nanoclusters of 20-50 nm wide and 1-7 nm high at predetennined position in the deposition process. In addition to metal clusters the deposition method was used to fabricate nano-sized polyaniline. The authors used a pulse voltage technique and fabricated polyaniline spots 10-60 run in diameter and 1-20 run in height. 

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