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The flat band potential is one of the key characteristics of photoelectrode performance. However, its determination on nanostructured materials is associated with considerable uncertainty. The complexity, applicability and pitfalls associated with the four most common experimental techniques used for evaluating flat band potentials, are illustrated using nanostructured synthetic hematite (α-Fe2O3) in strongly alkaline solutions as a case study. The motivation for this study was the large variance in flat band potential values reported for synthetic hematite electrodes that could not be justified by differences in experimental conditions, or by differences in their charge carrier densities. We demonstrate through theory and experiments that different flat band potential determination methods can yield widely different results, so could mislead the analysis of the photoelectrode performance. We have examined: (a) application of the Mott–Schottky (MS) equation to the interfacial capacitance, determined by electrochemical impedance spectroscopy as a function of electrode potential and potential perturbation frequency; (b) Gärtner–Butler (GB) analysis of the square of the photocurrent as a function of electrode potential; (c) determination of the potential of transition between cathodic and anodic photocurrents during slow potentiodynamic scans under chopped illumination (CI); (d) open circuit electrode potential (OCP) under high irradiance. Methods GB, CI and OCP were explored in absence and presence of H2O2 as hole scavenger. The CI method was found to give reproducible and the most accurate results on hematite but our overall conclusion and recommendation is that multiple methods should be employed for verifying a reported flat band potential.

It has been more than 40 years since Fujishima and Honda demonstrated water splitting using TiO2, yet there is still no clear mechanism by which surface holes on TiO2 oxidize water. In this paper, we use a range of complementary techniques to study this reaction that provide a unique insight into the reaction mechanism. Using transient photocurrent and transient absorption spectroscopy, we measure both the kinetics of electron extraction (t(50%) approximate to 200 mu s, 1.5V(RHE)) and the kinetics of hole oxidation of water (t(50%) approximate to 100 ms, 1.5V(RHE)) as a function of applied potential, demonstrating the water oxidation by TiO2 holes is the kinetic bottleneck in this water-splitting system. Photoinduced absorption spectroscopy measurements under 5 s LED irradiation are used to monitor the accumulation of surface TiO2 holes under conditions of photoelectrochemical water oxidation. Under these conditions, we find that the surface density of these holes increases nonlinearly with photocurrent density. In alkali (pH 13.6), this corresponded to a rate law for water oxidation that is third order with respect to surface hole density, with a rate constant k(WO) = 22 +/- 2 nm(4).s(-1). Under neutral (pH = 6.7) and acidic (pH = 0.6) conditions, the rate law was second order with respect to surface hole density, indicative of a change in reaction mechanism. Although a change in reaction order was observed, the rate of reaction did not change significantly over the wide pH range examined (with TOFs per surface hole in the region of 20-25 s(-1) at similar to 1 sun irradiance). This showed that the rate-limiting step does not involve OH- nucleophilic attack and demonstrated the versatility of TiO2 as an active water oxidation photocatalyst over a wide range of pH.

Transparent nano-structured hematite (α-Fe2O3) films of approximately 550 nm thickness on tin-doped indium oxide (ITO) have been obtained conveniently by ink-jet printing of a Fe(NO3)3/Brij® O10 precursor ink and subsequent annealing at 500 °C in air. When illuminated with a blue LED (λ = 455 nm, ca. 100 mW cm−2), the hematite films exhibited photocurrents of up to 70 μA cm−2 at 0.4 V vs. SCE in 0.1 M NaOH electrolyte. Thermal annealing in vacuum at 500 °C for 2 h increased photocurrents more than three times to 230 μA cm−2 in agreement with previous literature reports for pure hematite materials. These results suggest that a simple ink-jetting process with surfactants is viable. The effects of vacuum-annealing on the photoelectrical properties of α-Fe2O3 films are discussed in terms of a sub-surface state templating hypothesis based on data gathered from photo-transients, field emission scanning electron microscopy, X-ray photoelectron spectroscopy analysis, X-ray diffraction, photocurrent spectra, and cyclic voltammetry.

The p-GaAs photocathode/acetonitrile interface was characterised in the dark and with tens of μW/cm2 laser powers at 830 nm wavelength, using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Two redox mediators were used that present completely different behaviours under low optical excitation: one (Tris(2,2’-bipyridine)ruthenium(II) hexafluorophosphate) is sensitive to the surface states of the photo-cathode and the other (benzoquinone) is not. The combined electrochemical methods allow us to characterize the energetics of the electrode-electrolyte interface as well as the dominant electron transfer process.

Climate change is one of the most complex challenges that pose serious threats to livelihoods of poor people who rely heavily on agriculture and livestock particularly in climate-sensitive developing countries of the world. The negative effects of water scarcity, due to climate change, are not limited to productivity food crops but have far-reaching consequences on livestock feed production systems. Selenium (Se) is considered essential for animal health and has also been reported to counteract various abiotic stresses in plants however, understanding of Se regulated mechanisms for improving nutritional status of fodder crops remains elusive. We report the effects of exogenous selenium (Se) supply on physiological and biochemical processes that may influence green fodder yield and quality of maize (Zea mays L.) under drought stress conditions. The plants were grown in lysimeter tanks under natural conditions and were subjected to normal (100% field capacity) and water stress (60% field capacity) conditions. Foliar spray of Se was carried out before the start of tasseling stage (65 days after sowing) and was repeated after one week, whereas water spray was used as a control. Drought stress markedly reduced the water status, pigments and green fodder yield and resulted in low forage quality in water stressed maize plants. Nevertheless, exogenous Se application at 40 mg L-1 resulted in less negative leaf water potential (41%) and enhanced relative water contents (30%), total chlorophyll (53%), carotenoid contents (60%), accumulation of total free amino acids (40%) and activities of superoxide dismutase (53%), catalase (30%), peroxidase (27%) and ascorbate peroxidase (27%) with respect to control under water deficit conditions. Consequently, Se regulated processes improved fodder yield (15%) and increased crude protein (47%), fibre (10%), nitrogen free extract (10%) and Se content (36%) but did not affect crude ash content in water stressed maize plants. We propose that Se foliar spray (40 mg L-1) is a handy, feasible and cost-effective approach to improve maize fodder yield and quality in arid and semi-arid regions of the world facing acute shortage of water.

Heterostructures formed of films of organic-capped ZnO and TiO2 nanocrystals (both with the size of ca. 6 nm) and photosynthetic pigments were prepared and characterized. The surface of optically transparent electrodes (Indium Tin Oxide) was modified with nanocrystals and prepared by colloidal synthetic routes. The nanostructured electrodes were sensitized by a mixture of chlorophyll a and carotenoids. The characterization of the hybrid structures, carried out by means of steady-state optical measurements, demonstrated such class of dyes able to extend the photoresponse of the large band-gap semiconductors. The charge-transfer processes between the components of the heterojunction were investigated, and photoelectrochemical measurements taken on the sensitized ZnO and TiO2 nanocrystals electrodes elucidated the photoactivity of the heterojunctions as a function of the dyes and of the red–ox mediator used in solution. The effect of methyl viologen as different red–ox mediator was also evaluated in order to show its effect on the heterojunction photoactivity. The overall results contributed to describe the photoelectrochemical potential of the investigated heterojunctions, highlighting a higher response of the dye-sensitized ZnO nanocrystals, and then provided the TiO2-modified counterparts.

The spatial self-organization of molecular species on an Si oriented surface can be less ideal than that of an SAM on a metal, likely affecting the electronic structure of the resulting hybrids and their electrochemical response as electrodes in solution. In order to investigate such effects, a series of molecular precursors was investigated, consisting of three substituted ferrocenes with a lateral C–C group fully saturated (ethylferrocene) or with a single (vinylferrocene) or double unsaturation (ethynylferrocene). The corresponding functionalized Si(100) wafers were produced following new or literature recipes, starting from hydrogenated Si surfaces. The relationship between the degree of unsaturation in the anchored arm of each adduct and its electronic structure and electrochemical behaviour was investigated by comparing experimental (XPS, electrochemical) and ab initio results of the redox potentials in the series. Density functional theory (DFT) was applied, with inclusion of solute–solvent interactions. Different bond arrangements of the C–C arm with Si surface dimer atoms have been produced theoretically within the series of ferrocenes. Distinct values of redox potentials were displayed by the hybrids, which can be consistently related to the structural differences presented. In fact, measured and computed potentials showed a very satisfactory match only for specific adduct isomers, providing strong indications that the carbon–carbon unsaturation initially present in the anchoring arm is preserved upon addition reaction, an unprecedented result. The demonstrated tunability of a well-defined switching potential in these molecules on silicon can be beneficial to the development of Si-based memory devices.

Different approaches have been explored to increase the water oxidation activity of nanostructured hematite (alpha-Fe2O3) photoanodes, including doping with various elements, surface functionalization with both oxygen evolving catalysts (OEC) and functional overlayers and, more recently, the introduction of ultrathin oxide underlayers as tunneling back contacts. Inspired by this latter strategy, we present here a photoanode design with a nanometric spin-coated iron oxide underlayer coupled with a mesoporous hematite film deposited by electrophoresis. The electrodes equipped with the thin underlayer exhibit a four-fold improvement in photoactivity over the simple hematite porous film, reaching a stable photocurrent density of ca. 1 mA cm(-2) at 0.65 V versus the saturated calomel electrode (SCE) at pH 13.3 (NaOH 0.1 M) under air mass (AM) 1.5G illumination. A further improvement to 1.5 mA cm(-2) is observed after decoration of the hematite surface with a Fe(III)-OEC. These results demonstrate that by combining different iron oxide morphologies, it is possible to improve the selectivity of the interfaces towards both electron collection at the back contact and hole transfer to the electrolyte, obtaining an efficient all-iron based photoelectrode entirely realized with simple wet solution scalable procedures.

Electrodes based on Q-sized ZnO and TiO2, synthesised by non-hydrolytic routes, were prepared and characterised. The nanostructured electrodes were modified with photosynthetic pigments, namely, chlorophyll a, carotenoids and their mixture, to probe the photochemical sensitisation process. The photocurrent response of the hetero-junctions and the current–potential (I–V) characteristics of the sensitised electrodes are presented for different dyes. The Q-sized semiconductor sensitised electrodes showed different behaviour when compared to correspondent commercial oxides. Co-sensitisation acted by chlorophyll a and carotenoids on nanostructured electrodes was found to greatly enhance the photocurrent generation.

06.03.2018 tarihli ve 30352 sayılı Resmi Gazetede yayımlanan “Yükseköğretim Kanunu İle Bazı Kanun Ve Kanun Hükmünde Kararnamelerde Değişiklik Yapılması Hakkında Kanun” ile 18.06.2018 tarihli “Lisansüstü Tezlerin Elektronik Ortamda Toplanması, Düzenlenmesi ve Erişime Açılmasına İlişkin Yönerge” gereğince tam metin erişime açılmıştır. Üstün performans ve enerji depolama karakteristiğine sahip olan lityum iyon piller uzun şarj-deşarj çevrim ömrü, hafıza etkisi göstermemesi, çevre dostu olması gibi sebeplerden ötürü cep telefonları, bilgisayar, dijital kamera gibi taşınabilir elektronik cihazlar için vazgeçilmez bir güç kaynağı haline gelmiştir. Lityum iyon piller hibrit ve elektrikli araçlar için de tercih edilebilecek güç kaynakları olarak görülmektedir. Yüksek kapasiteli pil elde etmek için Li iyon pillerde geleneksel olarak kullanılan grafit anot yerine alternatif anot malzemelerin üretimi ilgi çekici bir araştırma konusu olmuştur. Lityum iyon pillerde anot olarak kullanılan SnO2 teorik lityum depolama kapasitesi ile en güven verici adaylardan biri olarak düşünülmektedir. SnO2 anot üretiminde birçok yöntem kullanılabilmektedir. Bu yöntemlerden sol-jel prosesi nanokompozit üretimi için kolay işleyiş basamakları, çözelti kimyasının esnekliği, düşük sıcaklık uygulamaları ve donanım masrafının az olması gibi avantajlar sunmaktadır. Tez çalışması, sol-jel yöntemiyle SnO2 toz ve SnO2-ÇDKNT nanokompozit toz üretimi ile ÇDKNT kağıt altlıklar üzerine orijinal olarak döndürerek kaplama yoluyla hazırlanan ve serbest anot olarak kullanılan SnO2-ÇDKNT nanokompozit film üretimi aşamalarından oluşmaktadır. Üretilen anot aktif nanomateryallerin x-ışını difraksiyonu (XRD) ile yapısal karakterizasyonu; taramalı elektron mikroskobu (SEM), atomik kuvvet mikroskobu (AFM) ve geçirimli elektron mikroskobu (TEM) ile morfolojik karakterizasyonu yapılmıştır. Üretilen anotların elektrokimyasal analizleri ise dönüşümlü voltametri (CV), elektrokimyasal empedans spektroskopisi (EIS) ve pil test cihazı kullanılarak yapılmıştır. Dizayn edilen CR2016 tipi hücrelerde anot elektrotların şarj-deşarj kabiliyeti, spesifik kapasitesi ve kapasite korunumu gibi parametreleri incelenmiştir. Bu çalışmada, SnO2 anot yapısında Li interkalasyonu sırasında yaklaşık % 300 oranında oluşan hacim artışı sebebiyle meydana gelen parçalanma ve dağılmanın önüne geçmek için mekanik destek olarak ÇDKNT kağıt kullanılmıştır. ÇDKNT kağıt altlıklar üzerine sol-jel döndürerek kaplama yöntemiyle SnO2-ÇDKNT nanokompozit film anotların hazırlanması ile ilgili olarak literatürde herhangi bir bilgiye rastlanmamıştır. İlk defa bu çalışmada kullanılan söz konusu yöntemle hazırlanan ÇDKNT ile takviye edilmiş SnO2 anot materyallerinin sol-jel yöntemiyle üretimi için faydalı çıktılar elde edilmiştir. SnO2-ÇDKNT nanokompozit film anotlar bu yöntemle üretilen diğer SnO2 toz ve nanokompozit anotlara göre uzun çevrim ömrü ve yüksek kapasite sağlamıştır. Because of their outstanding performance and energy storage characteristics, lithium ion batteries have become very important power sources for portable electronics such as cell phones, notebook computers and digital cameras due to their long charge-discharge cycle life, no memory effect and environmentally friendly structure. Lithium ion batteries are considered as future power sources for hybrid and electric vehicles. For obtaining high capacity batteries, the manufacturing of alternative anode materials instead of traditionally used graphite anode for Li ion batteries has been an attractive research topic. SnO2 anode which can be used in Li ion batteries with a high theoretical lithium storage capacity is considered to be one of the most reassuring candidates. Several methods can be used to produce SnO2 anodes. Among these, sol-gel process offers a lot of advantages such as easy operation steps, the flexibility of solution chemistry, low temperature applications and low cost equipment for the production of nanocomposites. This study consists of SnO2 and SnO2-MWCNT nanocomposite powder production steps and preparing free-standing SnO2-MWCNT nanocomposite films. Structural characterization of produced anode active nanomaterials were performed by XRD and for morphological characterization scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used. Electrochemical analyses were applied using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and battery tester device. Charge-discharge capabilities, specific capacity and capacity retention parameters were investigated of the anode electrodes assembled in the type of CR2016 cells. In this work, to prevent disintegration and pulverization of SnO2 anodes, MWCNT buckypapers were used as mechanical support because of volume expansion (≈ 300 %) of SnO2 anode structure during Li intercalation process. To the best of our knowledge there is no any previous work in the literature associated with preparing of SnO2-MWCNT nanocomposite film anodes on MWCNT buckypapers via sol-gel spin coating method. The results gave useful outcomes for the sol-gel production of SnO2 anode materials reinforced with MWCNT. SnO2-MWCNT nanocomposite film anodes provided long cycle life and high capacity compared with SnO2 powder and nanocomposite anodes which were produced by this method.

We propose a three-step model of electrochemical nanopore formation in n-InP in KOH that explains how crystallographically oriented etching can occur even though the rate-determining process (hole generation) occurs only at pore tips. The model shows that competition in kinetics between hole diffusion and electrochemical reaction determines the average diffusion distance of holes along the semiconductor surface and this, in turn, determines whether etching is crystallographic. If the kinetics of reaction are slow relative to diffusion, etching can occur at preferred crystallographic sites within a zone in the vicinity of the pore tip, leading to pore propagation in preferential directions. Symmetrical etching of three {111}A faces forming the pore tip causes it to propagate in the (remaining) 〈111〉A direction. As a pore etches, propagating atomic ledges can meet to form sites that can become new pore tips and this enables branching of pores along any of the 〈111〉A directions. The model explains the observed uniform width of pores and its variation with temperature, carrier concentration and electrolyte concentration. It also explains pore wall thickness, and deviations of pore propagation from the 〈111〉A directions. We believe that the model is generally applicable to electrochemical pore formation in III–V semiconductors.

Colloidal crystal templates were prepared by gravitational sedimentation of 0.5 micron polystyrene particles onto fluorine-doped tin oxide (FTO) electrodes. Scanning electron microscopy (SEM) shows that the particles were close packed and examination of successive layers indicated a predominantly face-centered-cubic (fcc) crystal structure where the direction normal to the substrate surface corresponds to the (111) direction. Oxidation of aqueous ferrous solutions resulted in the electrodeposition of ferric oxide into the templates. Removal of the colloidal templates yielded ordered macroporous electrodes (OMEs) that were the inverse structure of the colloidal templates. Current integration during electrodeposition and cross-sectional SEM images revealed that the OMEs were about 2 μm thick. Comparative X-ray diffraction and infrared studies of the OMEs did not match a known phase of ferric oxide but suggested a mixture of goethite and hematite. The spectroscopic properties of the OMEs were insensitive to heat treatments at300∘C. The OMEs were utilized for photoassisted electrochemical oxidation. A sustained photocurrent was observed from visible light in aqueous photoelectrochemical cells. Analysis of photocurrent action spectra revealed an indirect band gap of 1.85 eV. Addition of formate to the aqueous electrolytes resulted in an approximate doubling of the photocurrent.

A closely related series of three ferrocene derivatives, bearing a short carbon–carbon tethering arm for covalent attachment to H-terminated Si(1 0 0), has been used to produce electroactive hybrid materials. Three different grafting procedures have been applied for the first time on this type of molecules in order to obtain the full series of possible unsaturations in the C–C lateral chain bound to Si: ethyl, vinyl and ethynyl group, respectively, associated with the surface species (η5-C5H5)Fe2+(η5-C5H4)–CH2–CH2–Si (EtFC/Si), (η5-C5H5)Fe2+(η5-C5H4)–CH CH–Si (VFC/Si), and (η5-C5H5)Fe2+(η5-C5H4)–C C–Si (EFC/Si). The surface reactions here applied involve Grignard derivatization, Lewis acid catalysis and nucleophilic substitution by an acetylide anion. The redox potentials of the three molecule/surface hybrids, measured by cyclic voltammetry, are 0.059, 0.136 and 0.251 V vs. Ag/Ag+, respectively, from EtFC/Si to EFC/Si and are supposedly following the increasing trend of the extent of unsaturation in the tethering arm. The above trend in redox potentials parallels the one experimentally found and theoretically reproduced for the corresponding isolated molecules, which shows that the adopted functionalization procedures may be used to control the number of residual unsaturations in the C–C arm. The presence and chemical nature of the redox species covalently attached to Si are strongly supported after evaluation of XPS spectra and electrochemical data. The formation of electroactive hybrids on Si with tuneable well-defined switching potentials can be highly beneficial to the development of Si-based memory devices.

New or rarely reported synthetic routes have been applied to produce electroactive hybrid materials from ferrocene (Fc) derivatives with a known and pre-established variation in the tethering arm bound to Si. The series is characterized for having the shortest molecular link for a direct covalent attachment to H-terminated Si(100). Grignard derivatization, Lewis acid catalysis and nucleophilic substitution by an acetilyde anion have been applied for the first time in order to obtain the full series of possible unsaturations in the C–C lateral chain bound to Si: ethyl (EtFC/Si), vinyl (VFC/Si) and ethynyl group (EFC/Si). The redox potentials of these three molecule/surface hybrids, measured by cyclic voltammetry, are respectively 0.059, 0.136 and 0.251 V vs. Ag/Ag+, and increase with the extent of unsaturation in the tethering arm. XPS and electrochemical measurements have been used to assess the chemical nature and electronic properties of the hybrids.

Ce travail concerne l'étude expérimentale des réactions de transfert de charges à l'interface électrode semiconductrice électrolyte aqueux. La voltamétrie cyclique dans le noir et sous éclairement a permis d'étudier les réactions d'oxydoréduction des anions complexes Fe(CN)3-/4-6 et Co(CN)3-/4-6 sur les électrodes de tellurure de zinc et de cadmium. Fe(CN)3-6 se comporte comme un injecteur de trous et Co(CN)3-6 comme capteur d'électrons. Au-delà d'une concentration de 10 -2 M, l'étape déterminant la vitesse de la réaction de transfert de charges est la transition électronique entre la bande de valence ou la bande de conduction et les états en surface. Ce processus permet d'expliquer les faibles rendements obtenus sur les électrodes de CdTe en conversion de l'énergie solaire.

Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.