Dissolution illumination

Although it presents an obstacle in practical applications, the photoanodic corrosion of colloids has often been used to obtain information about the interaction of dissolved compounds with the photo-produced charge carriers, as it was found that solutes can influence the rate of the dissolution. Both promoting and retarding effects were observed The rate of dissolution is readily followed by recording the decrease in the intensity of the absorption spectrum of the colloid upon illumination, or more precisely, by determining the yields of metal and sulfate ions in solution. 

The photochemical yield in the experiments of Fig. 6 is 0.04 CdS molecules dissolved per photon absorbed. The yield itself is not constant during the illumination but decreases as the degree of dissolution increases, i.e. with decreasing particle size. 

The recombination of trapped electrons and holes produces the fluorescence. Adsorbed oxygen scavenges electrons producing O2" which also is adsorbed. OJ is a much better quencher than Oj. Its accumulation under illumination therefore leads to the decrease in fluorescence intensity. During the dark period disappears. During the illumination in the presence of oxygen, the colloid undergoes photoanodic dissolution (see Sect. 3.2). The ZnS particles become smaller in this way, and this finally leads to an increase in fluorescence yield as already described for CdS. 

The yield of the photocathodic dissolution of CdS in a solution containing 1 x 10" M SOi" is only 0.005 molecules dissolved per photon absorbed. In the presence of 5 X 10 M excess Cd " ions it amounts to 0.05. Sulfate and dithionate are formed in the ratio 2.2 to 1. The oxidation of SO3" is effected by the positive holes produced upon illumination, two holes being necessary to convert one SO ion into SO " or 1/2 SjOg . If the SOj anion captured the two holes from the same colloidal particle ( two-hole mechanism ), only sulfate would appear as the oxidation product. However, if SO3" captured only one hole to form the radical SOj", the final products would be formed bj reactions of two such radicals, and these two radicals could even originate from different colloidal particles ( one-hole mechanism )... 

The photocathodic dissolution of CdS also occurs in the presence of sodium thiosulfate, but not in the presence of excess SH ions. ZnS cannot be dissolved by illumination in the presence of sulfite. However, in the presence of excess Zn " ions in solution, Zn metal is deposited on the colloidal ZnS particles This process also occurs when propanol-2 is used as a scavenger for positive holra. 

Anodic dissolution of n-Si can also proceed at a polarization under illumination. The maximum current is limited by illumination intensity when the saturation photo current density is lower than the critical current, Ji. The characteristics of i-V curves of n-Si under a high illumination intensity, when the reaction is no longer limited by the availability of photo generated carriers, is identical to that for p-Si. Similar also to p-Si, formation of PS on n-Si occurs only below the critical current, Jx 24... 

There is also an etched layer of Si on the surface under illumination as illustrated in Figure 18. This etched layer is mainly due to photo-induced corrosion. As a result of the photo induced dissolution the top surface of PS layer recedes with time. The rate of dissolution depends on doping, F1F concentration, current density and illumination intensity. 

The dissolution of PS during PS formation may occur in the dark or under illumination. Both are essentially corrosion processes, by which the silicon in the PS is oxidized and dissolved with simultaneous reduction of the oxidizing species in the solution. The material in the PS, which is distant from the growing front is little affected by the external bias due to the high resistivity of PS and is essentially at the open circuit potential (OCP). Such corrosion process is responsible for the formation of micro PS of certain thickness (stain film) in HF solutions containing oxidants under an unbiased condition. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

For the p-type substrate a significant number of electrons are collected at the backside, as shown in the top part of Fig. 3.2. This is true not only for the illuminated p-type electrode but also if the electrode is kept in the dark, which indicates that electrons are injected during the tetravalent dissolution reaction. In the regime of oscillations the electron injection current is found to oscillate, too [CalO]. 

Fig. 4.5 Dissolution valence nv as a function of anodic current density for low doped p-type and strongly illuminated, low doped n-type samples (<1017cnT3, 2.5% HF, at RT). For current densities belowJPS the samples were measured with and without the microporous layer. This produces a minor difference in indicated by two data points.

If the same experiment is performed with an n-type Si electrode under identical illumination intensity the anodic photocurrent is found to be larger than for the p-type electrode under cathodic conditions. This increase is small (about 10%) for current densities in excess of JPS. Figure 3.2 shows that in this anodic regime injected electrons are also detected at p-type electrodes. This allows us to interpret the 10% increase in photocurrent observed at n-type electrodes as electron injection during anodic oxide formation and dissolution. 

Electron injection has been observed during the chemical dissolution of an oxide film in HF [Mai, Ozl, Bi5]. The injected electrons are easily detected if the anodized electrode is n-type and kept in the dark. Independently of oxide thickness and whether the oxide is thermally grown or formed by anodization, injected electrons are only observed during the dissolution of the last few monolayers adjacent to the silicon interface. The electron injection current transient depends on dissolution rate respectively HF concentration, however, the exchanged charge per area is always in the order of 0.6 mC cm-2. This is shown in Fig. 4.14 for an n-type silicon electrode illuminated with chopped light. The transient injection current is clearly visible in the dark phases. 

For doping-dependent anodic etch stops in HF, a general hierarchy of dissolution is observed [La5] illuminated n-doped and n+-doped areas are most easily dissolved, followed by p+-doped areas. Next likely to be dissolved are p-type areas. Moderately n-type doped areas kept in the dark are least likely to be etched. This hierarchy corresponds to the potential shift of the I-V curve in the regime of PS formation [Gal, Zh5]. 

Holes, which initiate the dissolution process, are minority charge carriers in n-type electrodes. The concentration of holes nh is very low in n-type Si under equilibrium conditions. The hole concentration can be increased by illumination or by... 

A specific feature of macropore formation in n-type silicon is the possibility of controlling the pore tip current by illumination and not by applied bias. This adds another degree of freedom that is not available for mesopore or macropore formation on p-type substrates. The dark current density of moderately doped n-type Si electrodes anodized at low bias is negligible, as shown in Fig. 4.11, therefore all macropore structures discussed below are formed using illumination of the electrode to generate the flux of holes needed for the dissolution process. Illumination, however, is not the only possible source of holes for example, hole injection from a p-doped region is expected to produce similar results. 

The exciplex intensity showed quite different behavior as the setting proceeded. A comparison of the (monomer peak/monomer peak) ratio to the (exciplex peak/monomer peak) ratio was quite illuminating. We considered the initial maximum wavelength of the exciplex emission at 540 nm, and compared its intensity to the monomer intensity at 405 nm as the dissolution/ polymerization proceeded. A substantial decrease in exciplex intensity, compared to monomer intensity, was observed over the first 40 min of the cure. The ratio then leveled off, indicating that the local viscosity had reached a maximum after 40 min and that the dissolution/polymerization was considered to have reached completion at the ambient temperature of the laboratory. Since the working time for the cement was considerably less than the 40-min time period over which the exciplex/monomer intensity ratio was steadily decreasing, the intensity ratios served as in situ monitors of the cure. 

Figure 16. Photocurrent/cell potential difference for n-type GaP anode and p-type GaP cathode in O.IM HfSOi, illuminated as in Figure 15. Hydrogen evolution occurred at the GaP cathode without visible degradation, but in this cell the anodic reaction is oxidation of P to HsPOs and dissolution of the anode material (16).

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