The basic concepts concerning formation and operation of a junction between a semiconductor and a solution are presented in this entry. The behavior of semiconductor electrodes in the dark and under illumination, different from that of metallic electrodes, is explained with reference to the peculiar properties of semiconductors such as very low concentration of mobile electrons; presence of a gap of forbidden energies for electronic states; control by electrode potential, under appropriate conditions, of the surface concentration of carriers; and the ability of illumination to change drastically the population of minority carriers. The dependence of photoeffects on material properties and light intensity is discussed, considering also the role and effects of surface states. The relevance of interface energetics for the performances and stability of a photoelectrode in contact with different redox couples is pointed out. Some semiconducting materials like oxides and calchogenides of transition metals, possess the ability of intercalating guest charged species (either in the dark or under illumination). Such a possibility is analyzed for consideration of their employment as electrodes of secondary battery cells. The analysis takes into account the various components of the free energy which determine the cell voltage in case of a lithium-ion battery. Moreover, considerations on the electronic and crystal structures are exposed to define the relevant requirements intercalation semiconductors should satisfy in the context of the technology of lithium-ion battery. Some oxides of transition metals manifest also the capability of transporting the electrical current simultaneously via electronic and ionic charge carriers. Such a characteristic of mixed conduction renders possible the practical utilization of some semiconducting oxides either as electrodes or solid-state electrolytes of solid oxide fuel cells (SOFCs). Some significative examples of semiconducting materials for SOFCs are reported underlining with particular emphasis their ability of modulating the conduction properties or switching the type of electrical conduction under the operative conditions of a fuel cell in dependence of the nature of interfaces the semiconductors create in a SOFC. Finally, the impact the advancements in nanotechnologies and nanoparticles synthesis has had in the development of semiconducting materials for photoelectrochemical cells is outlined in the particular case of the quantum dots that have been employed as sensitizers in dye-sensitized solar cells. In this entry organic semiconductors have not been included.

The material produced through the electrochemical polymerization of 3′4′-DDTT has been characterized with the EQCM during the process of n-doping. The supporting electrolyte (SE) was chosen considering mainly the two characteristics of hydrophobicity (to avoid the presence of water as potential contaminant) and chemical affinity with the alkyl and aromatic moieties present in poly-3′4′-DDTT. On these bases the salt (n-C4H9)4NClO4 was selected as SE since it contains the organic molecular cation (n-C4H9)4N+ that is expected to represent the charge compensating species in poly-3′4′-DDTT during n-doping. The feature of the reversibility of the electrical current profiles originated by the process of injection/extraction of electronic charge carriers in poly-3′4′-DDTT, is not encountered in the associated EQCM data. The interpretation of the EQCM data requires the consideration of phenomena of different nature. In the present work a thorough discussion of the factors influencing the EQCM response during polymer n-doping is provided taking into account the spontaneous adsorption of cations, the eventual reorientation of poly-3′4′-DDTT on the substrate and the consequences of the chains rearrangement on the electrical polarizability of poly-3′4′-DDTT during the cycles of electrochemical n-doping and undoping.

The electrochemical oxidation of 3 ',4 '-didodecyl-2,2 ':5 ',2 ''-terthiophene (3 ' 4 '-DDTT) at applied potential values lower than 0.8 V vs Ag/Ag+ leads to the formation of an anodic deposit that is prevalently constituted by the products of monomer coupling at positions 5 and 5 ''. Upon electrochemical cycling poly-3 ' 4 '-DDTT manifests three phenomena: the cathodic shift of the main peak of poly-3 ' 4 '-DDTT oxidation, the bathochromic shift of 60 nm for the main UV-vis absorption peak, and the IR spectral evolution consisting of the progressive increase of the signals of 2,5-substituted thiophenes at expense of the signals from 2-substituted thiophenes. The observed behavior was interpreted in terms of the presence of reactive oligomers trapped in pristine as-deposited poly-3 ' 4 '-DDTT, with the oligomers undergoing further coupling in the polymeric phase upon electrochemical cycling. This combination of findings led us to conclude that poly-3 ' 4 '-DDTT manifests solid-state polymerization. After voltammogram stabilization poly-3 ' 4 '-DDTT was electrochemically stressed. The system showed a reversible electrochemical behavior up to 0.9 V vs Ag/Ag+. A depiction of the evolution of the electronic bands in poly-3 ' 4 '-DDTT when passing from the pristine state to the electrochemically cycled state has been proposed, taking into account the combination of electrochemical and optical data.

Contact glow discharge electrolysis (CGDE) can be exploited in environmental chemistry for the degradation of pollutants in wastewater. This study focuses on the employment of cheap materials (e.g., steel and tungsten) as electrodes for experiments of CGDE conducted in electrochemical cells with variable electrolytic composition. A clear correlation between breakdown voltage (V-B)/discharge (or midpoint) voltage (V-D) and the conductivity of the electrolyte is shown. Regardless of the chemical nature of the ionogenic species (acid, base or salt), the higher the conductivity of the solution, the lower the applied potential required for the onset of the glow discharge. Concerning practical application, these salts could be added to poorly conductive wastewaters to increase their conductivity and thus reduce the ignition potential necessary for the development of the CGDE. Such an effect could render the process of chemical waste disposal from wastewaters more economical. Moreover, it is evidenced that both V-B and V-D are practically independent on the ratio anode area to cathode area if highly conductive solutions are employed.

The cost-effective production of chemicals in electrolytic cells and the conversion of the radiation energy into electrical energy in photoelectrochemical cells (PECs) require the use of electrodes with large surface area, which possess either electrocatalytic or photoelectrocatalytic properties. In this context nanostructured semiconductors are electrodic materials of great relevance because of the possibility of varying their photoelectrocatalytic properties in a controlled fashion via doping, dye-sensitization or modification of the conditions of deposition. Among semiconductors for electrolysers and PECs the class of the transition metal oxides (TMOs) with a particular focus on NiO interests for the chemical-physical inertness in ambient conditions and the intrinsic electroactivity in the solid state. The latter aspect implies the existence of capacitive properties in TMO and NiO electrodes which thus act as charge storage systems. After a comparative analysis of the (photo)electrochemical properties of nanostructured TMO electrodes in the configuration of thin film the use of NiO and analogs for the specific applications of water photoelectrolysis and, secondly, photoelectrochemical conversion of carbon dioxide will be discussed.

A reliable and cheap equipment is hereby proposed for the deposition of thin films on several substrates. This system is capable of deposition using three different techniques sequentially on the same substrate. The techniques available are ionic layer adsorption and reaction (SILAR), electrodeposition, and dip coating on both rigid and flexible conductive substrates (FTO, ITO-PEN). Using low-cost electronic components, we built a working prototype, driven by a simple software that is open source and user-friendly. In order to test our system, we used it to fabricate dye-sensitized solar cells (DSSCs) and counter electrodes of platinum and cobalt sulfide using both rigid glass-FTO and flexible ITO-PEN substrates. The electrodes as well as the complete devices have been characterized through cyclic voltammetry (CV). The solar cell devices have been characterized through current versus voltage curves ( I - V ) under simulated solar illumination.

Nonstoichiometric nickel oxide (NiOx) has been deposited as thin film utilizing indium-doped tin oxide as transparent and electrically conductive substrate. Spray deposition of a suspension ofNiOxnanoparticles in alcoholic medium allowed the preparation of uniformNiOxcoatings. Sintering of the coatings was conducted at temperatures below 500°C for few minutes. This scalable procedure allowed the attainment ofNiOxfilms with mesoporous morphology and reticulated structure. The electrochemical characterization showed thatNiOxelectrodes possess large surface area (about 1000 times larger than their geometrical area). Due to the openness of theNiOxmorphology, the underlying conductive substrate can be contacted by the electrolyte and undergo redox processes within the potential range in whichNiOxis electroactive. This requires careful control of the conditions of polarization in order to prevent the simultaneous occurrence of reduction/oxidation processes in both components of the multilayered electrode. The combination of the open structure with optical transparency and elevated electroactivity in organic electrolytes motivated us to analyze the potential of the spray-depositedNiOxfilms as semiconducting cathodes of dye-sensitized solar cells of p-type when erythrosine B was the sensitizer.

Nanoporous nickel oxide (NiO x ) has been deposited with two different procedures of sintering (CS and RDS). Both samples display solid state oxidation at about 3.1 V vs Li+/Li. Upon sensitization of CS/RDS NiO x with erythrosine b (ERY), nickel oxide oxidation occurs at the same potential. Impedance spectroscopy revealed a higher charge transfer resistance for ERY-sensitized RDS NiO x with respect to sensitized CS NiO x . This was due to the chemisorption of a larger amount of ERY on RDS with respect to CS NiO x . Upon illumination the photoinduced charge transfer between ERY layer and NiO x could be observed only with oxidized CS. Photoelectrochemical effects of sensitized RDS NiO x were evidenced upon oxide reduction. With the addition of iodine RDS NiOx electrodes could give the reduction iodine → iodide in addition to the reduction of RDS NiO x . p-type dye sensitized solar cells were assembled with RDS NiO x photocathodes sensitized either by ERY or Fast Green. Resulting overall efficiencies ranged between 0.02 and 0.04 % upon irradiation with solar spectrum simulator (I in: 0.1 W cm−2).

In this work a novel preparation method is proposed for the one step synthesis and thin-film deposition of cost effective counter electrodes for dye sensitized solar cells (DSSC). This method is fast and allows depositing CoS nanoparticles onto F-doped SnO2 (FTO) substrates within 2hours. The cost of reagents needed is significantly less than the cost of the products based on hexachloroplatinic acid used in the production of platinum transparent counter electrodes, and the method is compatible with the ink-jet and screen-printing technologies. The whole process does not require expensive equipment and is of simple implementation. Electrochemical Impedance Spectroscopy, Cyclic Voltammetry and I-V curves under simulated sunlight were used to characterize the electrode efficiency and stability. The counter electrodes prepared according to our procedure were transparent and show good catalytic activity with the I-/I3- redox couple in a high stability electrolyte for DSSC (HSE). Under the best deposition conditions the charge transfer resistance of the electrodes was 1.3Ωcm2, less than that of the screen printed platinum on FTO glass (2.3Ωcm2). Power conversion efficiencies up to 6.6% were reached using the CoS counter electrodes. The optimized CoS counter electrodes were demonstrated to work also with a ferrocene besed redox liquid electolyte.

TiO2-based dye-sensitized solar cells (DSSCs) sensitized with the Ru-dyes N719 and Z907, are subjected to prolonged reverse bias (RB) by imposing a constant current density of 25 mA cm(-2) for up to 1000 h. The temporal changes of the DSSCs during the aging process are followed by the periodical recording of current-voltage curves, electrochemical impedance spectra, charge extraction measurements, and photovoltage decays at the various stages of cell aging. Beside the expected decrease in the concentration of I-3(-) during RB aging, the measurements show a surprising shift of the conduction band edge toward lower energies and a concomitant slowing down of the recombination process with the electrolyte for both DSSCs. This leads to a noticeable improvement in the energy conversion efficiency under low illumination conditions. These unexpected results are discussed on the basis of recent literature reports and attributed tentatively to the adsorption of H+ ions, which are produced during the RB aging, onto the TiO2 surface.

The adsorption behavior of two symmetrical indolenine-based squaraines, indicated with VG1-C2 and VG1-C10, sensitizing electrodeposited mesoporous zinc oxide (ZnO), was studied and compared with that of di-tetrabutyl-ammonium cis-bis(isothiocyanato) bis(2,2'-bipyridyl-4,4'-dicarboxylato) ruthenium (II) (N719). The choice of squaraines as metal-free dye-sensitizers was motivated by their far-red NIR sensitivity with respect to the traditional Ru complex-based dyes, the ease of their preparation and their higher molar extinction coefficient. The electrochemically grown ZnO here described were porous due to the high volume yield of electrodeposition (about 10(4) cm(3) of ZnO per unit charge of electrolysis). In the present analysis the process of sensitization of TiO2 with the same set of dyes and the performance of the corresponding DSCs were also considered for sake of comparison. VG1-C2 sensitizer proved to be particularly effective electron injector in ZnO electrodes giving larger photocurrents in VG1-C2-sensitized ZnO with respect to TiO2. The latter system presented higher kinetic stability of the photoinjected charges as evidenced by the larger photovoltages of TiO2-based DSCs with respect to ZnO-based devices with the same sensitizer. The long alkyl substituents in squaraine VG1-C10 inhibit electron injection in ZnO and a specific effect of electrical passivation of the ZnO surface introduced by the bulky apolar groups was claimed. Overall efficiencies in the order of 1% were measured with the ZnO-based DSCs under AM 1.5 solar simulator when the photoactive area and the thickness of ZnO films were 0.5 cm(2) and 2 mu m, respectively. These results on ZnO- based DSCs with the oxide prepared at temperatures below 260 degrees C are particularly interesting considering the relatively large area and small thickness of the related electrodes, which makes them in principle useful for application in weakly absorbing devices utilizing flexible substrates.

In the present work we report a new, simple method to heal the defects of SAMs constituted by small aromatic thiols, namely benzenethiol (BT) and 2-naphthalenethiol (2-NT), on polycrystalline Au surfaces. The method consists in the alternate immersion of the Au substrate in the thiolic solution and in ultrapure water. The layers obtained with the new protocol were characterized by means of Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS), and were compared with SAMs prepared with the traditional adsorption procedure. The results show that our treatment leads to a remarkable increase of the blocking behavior of the layers, indicating a decrease in density of defective sites, and to an improvement of the SAM stability. On the basis of the obtained results, a possible explanation to this phenomenon is proposed. (C) 2013 Elsevier B.V. All rights reserved.

The effect of several p-substituent groups (-F, -CH3, -OH, -COOH, -NHCOCH3) on the performances and the stability of aromatic monolayers on copper has been studied. All the results agreed indicating that such aromatic films are very stable and effective as corrosion inhibitors even in harsh conditions (aerated H2SO4 0.5 M). Nevertheless, while XP spectroscopy showed an excellent chemical stability for all the layers regardless the terminal groups, EIS measurements highlighted notably different protective properties and durability for the various SAMs. Such passivation depends not only on the hydrophobicity of the terminal group but, surprisingly, also on the electronic effect of the substituent on the ring as well. In particular, molecules having an electron-withdrawing substituent showed good passivity at first, a steep increase of their protective properties following the exposure to the electrolyte but a relatively poor stability. The presence of an electron-donating substituent, on the other hand, led to relatively poor initial protection, a slow reorganization but an exceptionally long durability. (C) 2013 Elsevier B.V. All rights reserved.

Dye-sensitized solar cells have been tested before, during, and after stress tests performed either under intense Xe-lamp illumination (equivalent to 1 sun up to 2.5 sun) or under thermal cycles between room T and 80 °C. In-situ emission spectra and transient photovoltage decay curves have been taken to monitor the cell aging conditions. Incipient degradation phenomena in aged cells can be detected by changes in emission intensity, maximum photovoltage and in the time constant of photovoltage decay. UV-filtering of the Xe beam can prevent such cell degradation, provided the cell overheating is avoided.

Electrodeposition from ZnCl2 aqueous solution was performed to grow ZnO thin films on the surface of polycrystalline copper plates. Electrochemical parameters for deposition were optimized by means of cyclic voltammetry (CV). The morphology of the deposits was studied via scanning electron microscopy (SEM), and their chemical composition was ascertained by means of X-ray photoelectron spectroscopy (XPS). The effects of changing the deposition bath temperature (T bath) and the role played by post-deposition treatments, such as temperature and time of annealing in air, were studied. SEM images of freshly deposited vs. annealed samples have shown that in the former case the films display a rough morphology with mixed grain/hexagonal platelets structures and in the latter smaller but more uniformly dispersed cubic grains. T bath is found to be the key parameter to induce the different morphology in the deposited films, which reflects in a different chemical reactivity of surface species, as found on the basis of the binding energies and relative quantitative ratios between Zn 2p and O 1s peaks. In fact, a higher T bath favours a more efficient desorption of OH groups upon annealing, the O 1s peak resulting to much more drastically modified oxide/hydroxide intensity ratio with respect to the case of the sample deposited at lower T bath.

In the present work we used different techniques to study self-assembled monolayers (SAMs) of two aromatic thiols, namely, benzenethiol (BT) and 2-naphthalenethiol (2-NT), and one alkylic thiol, 1-undecanethiol (1-UT), on polycrystalline copper, comparing their corrosion inhibition efficiency and their stability up to a week in H2SO4 0.5 M. Both electrochemical impedance spectroscopy (EIS) and linear polarization highlighted different aging trends for 1-UT on one side and aromatic thiols on the other. 1-UT was initially the best corrosion inhibitor among the three thiols, as expected from its larger thickness, but it degraded very rapidly. On the contrary, BT and 2-NT showed a noticeable increase of their protective properties during the first hours of exposure to the electrolyte leading to a superior performance over any 1-UT sample. Raman spectroscopy suggested this behavior to be related to an enhancement of the structural order of the aromatic layer. In addition, both XP spectroscopy and electrochemical measurements revealed BT and 2-NT layers to be stabler than 1-UT. In particular, BT layers exposed to H2SO4 0.5 M exhibited better protective properties, with respect to the freshly prepared samples, lasting for over 1 week of aging.

The effect of Co-60 gamma rays irradiation on the polymetallayne [-Pt(PBu3)-C C-C6H4-C6H4-C C-](n) (Pt-DEBP) of defined chain length corresponding to 10 repeat units, has been studied in detail. The UV-vis absorption spectra of Pt-DEBP have been recorded in solution upon exposure of the polymetallayne at increasing radiation doses in the range up to 90 Gy, with special care to the features related to low doses. Complex modifications of the chemical structure of Pt-DEBP could be accessed through NMR, FTIR, GPC, and XPS characterizations, which support the attack of Cl and H radicals coming from the radiolysis of the solvent, CHCl3, to the triple C C bonds of the backbone, leading to the formation of chlorinated double and single C-C bonds, with a concomitant increase of the molecular weight due to a recombinant effect of oligomer fragments upon irradiation. The presence of vinyl and single chlorinated moieties has been sustained from the simulation of the UV-vis spectra based on theoretical calculations.

A long-term life test (3200 h) on large-area dye-sensitized cells is performed both under outdoor conditions, in the sunny Mediterranean climate in Rome (Italy), and under continuous light soaking (1 Sun, 85 °C). Different degradation rates are investigated for the outdoor samples with horizontally and vertically oriented cells (azimuth South, tilt angle 25°). Thirty identical photocells (active area=3.6 cm(2), conversion efficiencies=(4.8±0.2)%) are aged using a robust master-plate configuration. After the first 1000 h of testing in open-circuit conditions, some of the test samples are set near the maximum power point (MPP) and the life test continued further until 3200 h. A detailed analysis of the physical parameters obtained by electrochemical impedance is given together with electrolyte transmittance variation with time as a function of the ageing conditions. Faster degradation in devices working at the MPP is observed, due mainly to a progressive decrease of the triiodide concentration in the electrolyte and a likely alteration at the titania/electrolyte interface. Outdoor devices working with vertically oriented cells show clearly that the orientation of long-striped cells can affect the lifetime. The aged cells suffer an increase of recombination rate, change in the chemical capacitance, and positive shift of the titania conduction band level. A strong correlation between the increase of the electrolyte diffusion resistance and degradation phenomena is found.

Spray deposition followed by sintering of nickel oxide (NiO x ) nanoparticles (average diameter: 40 nm) has been chosen as method of deposition of mesoporous NiO x coatings onto indium tin oxide (ITO) substrates. This procedure allows the scalable preparation of NiO x samples with large surface area (~103 times the geometrical area) and its potential for applications such as electrocatalysis or electrochemical solar energy conversion, which require high electroactivity in confined systems. The potential of these NiO x films as semiconducting cathodes for dye-sensitized solar cell (DSC) purposes has been evaluated for 0.3–3-μm-thick films of NiO x sensitized with erythrosine B (ERY). The electrochemical processes involving the NiO x coatings in the pristine and sensitized states were examined and indicated surface confinement as demonstrated by the linear dependence of the current densities with the scan rate of the cyclic voltammetry. Cathodic polarization of NiO x on ITO can also lead to the irreversible reduction of the underlying ITO substrate because of the mesoporous nature of the sintered NiO x film that allows the shunting of ITO to the electrolyte. ITO-based reduction processes alter irreversibly the properties of charge transfer through the ITO/NiOx interface and limit the range of potential to NiO x coatings sintered for DSC purposes.

The adsorption of Benzenethiol and 1-Undecanethiol on polycrystalline copper and their ability to inhibit the Cu corrosion process for several days in strongly acidic solution were studied by XPS and EIS. Surprisingly, the SAM of aromatic thiols assured higher protection and higher stability than the SAM of long-chain alkylic thiols in spite of their lower thickness, thanks to a noticeable increase in the charge-transfer resistance observed by EIS after few hours of exposure to the electrolyte. XPS data revealed that the bond with the substrate is more stable in the case of Benzenethiol than with 1-Undecanethiol. This is probably due to the presence of strong ring interactions, which allows the SAM to assume a highly ordered structure. The higher stability of the aromatic SAMs appears to be more important than the layer thickness for substrate passivation.