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The photoelectrochemical behavior of n-ZnO, -CdS, and -GaP single crystal semiconductor electrodes was investigated in acetonitrile which contained various electroactive compounds whose standard redox potential varied by over 3.2 V.The cyclic voltammograms of the n-type semiconductors in the dark and illuminated were compared to the Nernstian behavior at a Pt disk electrode. The photodissolution of the semiconductor electrodes did not occur until the electrode potential was well positive of the flat band potential. An underpotential was developed for the photooxidation of solution species at each semiconductor. A model for electron transfer at the semiconductor-solution interface is proposed which incorporates a surface state or intermediate level which can mediate electron transfer and surface recombination. The model is used to discuss previous work with semiconductors, electron transfer, and stabilization of the semiconductor surface.Key factors in the utilization of semiconductor electrodes in electrochemical cells and devices, including those for solar energy conversion, are knowledge of the relative locations of the energy levels in the semiconductor and solution and an understanding of the role that surface states or intermediate

Tailored molecular structure is the key to tune the electronic and spectroscopic properties of a molecule. This is always a goal in electrogenerated chemiluminescence (ECL), that is, to have a material with high photoluminescence (PL) quantum yield and stable, long-lived radical ions in solution upon oxidation and reduction. Among various ECL active materials, polyaromatic hydrocarbons (PAHs) were the earliest subjects of ECL studies, [1] and substituted PAHs have been widely investigated.[2] Indeed, the PAHs 9,10-diphenylanthracene (DPA) and rubrene (R) have usually been used as ECL standards.[1a] These molecules are intense ECL emitters because of their high fluorescence quantum yields and stable radical cations and anions in aprotic media. However, the ECL emission of DPA and R eventually decays. This might be attributed to the irreversibility of the second oxidation because of the instability of the dication, for example, DPA 2+. Although for generation of ECL spectra, the radical ions are typically obtained by pulsing the electrode potentials, slightly beyond ( % 100 mV) the first reduction and oxidation peak potentials, there is always some disproportionation of the radical cation leading to dication production. Thus, instability of doubly charged ions leads to slow decomposition. If the second oxidation wave is made more electrochemically reversible, the stability of ECL should be significantly improved. A similar argument has been made in considering the stability of electrochromic devices involving the generation of radical ions. [3] In aprotic media, ECL involves light emission due to the electron transfer process in the vicinity of the electrode with the intensity largely governed by the stability of the electrogenerated radical ions over the time scale of the potential sweeps (or steps) and the quantum efficiency of the generated excited state.[1a] The ECL spectrum of a compound is usually the same as the fluorescence spectrum because both methods produce the same excited state. However, the ECL spectra of some PAHs, like pyrene, show another extra broad and structureless peak at longer wavelengths, which is attributed to the formation of excimer.[4] Long wavelength emission can also be attributed to the formation of a byproduct during the electrolysis, for example, by the decomposition of the radical cation, as seen in ECL of anthracene.[1e]Fluorene-based molecules, like oligofluorenes [2a] and terfluorenes [5] are of interest in ECL and organic light emitting devices (OLED) because of their good electrochemical and thermal stabilities and high quantum yields. The present work reports that capping a DPA derivative, pyrene and anthracene, with two fluorene derivatives produces new aromatic hydrocarbons (FDF, FPF, FAF) (Figure 1) with very interesting ECL behavior, with enhanced ECL efficiency and stability as compared to their parent PAHs.Figure 2 depicts a comparison of the cyclic voltammograms (CV) of FAF, FDF, and FPF and their parent counterparts. The electrochemical data are summarized in Tabl...

Nafion and CdS-Nafion membranes were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. Depending on the preparation method, samples dominated by either cubic or hexagonal phases of CdS can be prepared. Samples dominated by the cubic form of CdS have very rough surfaces compared to those dominated by the hexagonal form. The S(2p) binding energies indicate that the surface of the hexagonal form is rich in sulfate ions, whereas the surface of the cubic form is dominated by the sulfide ions of CdS. A gray-blue deposit formed on the cubic CdS-Nafion surface under UV irradiation is identified as atomic sulfur Algorithms for finding local minima, maxima, and saddle points on surfaces, starting from an arbitrary point, are presented. These algorithms are based on making a local approximation to the surface in the form of a rational function constructed from the local first and second derivatives of the surface. All parameters of these algorithms required for stepping across the surface are determined in nonarbitrary ways. The convergence of these procedures to the desired stationary point is shown to be quadratic. Applications for stationary-point searches on two model surfaces are also given for illustrative purposes.

Das photoelektrochemische Verhalten einer Anzahl von nach verschiedenen Methoden (CVD, direkte Oxidation des Metalls, Erhitzen von Metallsalzlösungen) in polykristalliner Form hergestellten halbleitenden Oxiden wird untersucht.