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Published asKlahrHowever, the physical-chemical mechanisms responsible for the photoelectrochemical performance of this material ( J(V ) response) are still poorly understood. In the present study we prepared thin film hematite electrodes by Atomic Layer Deposition to study the photoelectrochemical properties of this material under water splitting conditions. We employed Impedance Spectroscopy to determine the main steps involved in photocurrent production at different conditions of voltage, light intensity and electrolyte pH. A general physical model is proposed, which includes the existence of a surface state at the semiconductor/liquid interface where holes accumulate. The strong correlation between the charging of this state with the charge transfer resistance and the photocurrent onset provides new evidence of the accumulation of holes in surface states at the semiconductor/electrolyte interface, which are responsible for water oxidation. The charging of this surface state under illumination is also related to the shift of the measured flat band potential. These findings demonstrate the utility of Impedance Spectroscopy in investigations of hematite electrodes to provide key parameters of photoelectrodes with a relatively simple measurement. 3 I ntroductionAs part of the quest to develop better and cleaner energy conversion and storage systems, the direct conversion of sunlight into chemical fuels has become a subject of renewed interest.One attractive example is the use of semiconductors to harness solar photons to split water, thereby producing hydrogen as a chemical fuel. In order to achieve this, a given material must satisfy a number of stringent requirements including visible light absorption, efficient charge carrier separation and transport, facile interfacial charge-transfer kinetics, appropriate positions of the conduction and valence band energy levels with respect to required reaction potentials and good stability in contact with aqueous solutions. 1 While such systems were heavily investigated several decades ago, no material so far has fulfilled all the required conditions. 2,3 Recent advances in nanotechnology and catalysis, however, greatly increase the prospects of developing a combination of materials capable of efficient conversion of sunlight to chemical fuels. 4 Hematite ( -Fe 2 O 3 ) is a very promising material for photoelectrochemical (PEC) water splitting due to its combination of sufficiently broad visible light absorption, up to 590 nm, and excellent stability under caustic operating conditions. 5,6 However, hematite electrodes are adversely affected by a number of factors including a long penetration depth of visible light due to its indirect band gap transition and a very short minority carrier lifetime and mobility; this combination hinders efficient collection of the minority carriers via the required interfacial charge-transfer reactions. Considerable effort has been devoted to improving the actual efficiency by employing nanostructuring strategies, which disconnects the ligh...

Impedance spectroscopy was applied to investigate the characteristics of dye-sensitized nanostructured TiO2 solar cells (DSC) with high efficiencies of light to electricity conversion of 11.1% and 10.2%. The different parameters, that is, chemical capacitance, steady-state transport resistance, transient diffusion coefficient, and charge-transfer (recombination) resistance, have been interpreted in a unified and consistent framework, in which an exponential distribution of the localized states in the TiO2 band gap plays a central role. The temperature variation of the chemical diffusion coefficient dependence on the Fermi-level position has been observed consistently with the standard multiple trapping model of electron transport in disordered semiconductors. A Tafel dependence of the recombination resistance dependence on bias potential has been rationalized in terms of the charge transfer from a distribution of surface states using the Marcus model of electron transfer. The current-potential curve of the solar cells has been independently constructed from the impedance parameters, allowing a separate analysis of the contribution of different resistive processes to the overall conversion efficiency.

This paper analyzes the small signal ac impedance of electron diffusion and recombination in a spatially restricted situation with application in systems such as porous TiO 2 nanostructured photoelectrodes and intrinsically conducting polymers. It is shown that the diffusion-recombination model with the main types of boundary conditions assumes a finite set of possible behaviors in the frequency domain, which are classified according to relevant physical parameters. There are four possible cases: (i) the impedance of finite diffusion with reflecting boundary, (ii) the impedance of finite diffusion with absorbing boundary, (iii) the impedance of diffusion-reaction in semiinfinite space or Gerischer impedance, and (iv) the impedance that combines Warburg response at high frequency and a reaction arc at low frequency. The generality of the approach is discussed in terms of the spatial distribution of the electrochemical potential or quasi-Fermi level and also in terms of the transmission line representation. An extension is considered to the diffusion in lithium intercalation electrodes coupled to a homogeneous solid-state reaction. The connection is established with other frameworks for the description of transport and reaction in electrochemical systems.

Recently, a new class of photoelectrochemical cells based on nanoscaled porous metal oxide semiconductors (dye-sensitized solar cell) has promoted intense research due to the prospects of cheap and efficient conversion of visible light into electricity and of new applications such as transparent solar cells.[1] It is widely agreed that the electron-transfer kinetics play a major role in determining the energy conversion efficiency of dye-sensitized solar cells. [2, 3] Herein, we develop a new powerful tool to study the electron lifetime in dye solar cells as a function of the photovoltage (V oc ); the open-circuit voltage-decay (OCVD) technique. This technique has certain advantages over frequenIn summary, the temperature effect on the arrangement of stilbenoid dendrimers on HOPG is presented in this work. It is seen that SD12 molecules form well-ordered hexagonal nanostructures at 16 8C. However, if the adlayer is annealed at 65 8C, the adlayer structure is changed into a well-ordered parallelogram nanostructure in a close-packed arrangement with a higher surface coverage. The phenomenon described here supports the earlier reports on two liquid-crystalline phases for SD12.[15] The results in this research are useful in understanding the phase transition of SD12 as well as metastable complex systems with temperature. Experimental SectionSynthetic methods: SD12, SD14, and SD16 were prepared as described in the literature. [15] Sample preparation for STM observation: The molecules were dissolved in toluene (HPLC grade, Aldrich) with a concentration of less than 0.01 mg mL À1. The self-assembled adlayers were prepared by depositing a droplet of this solution onto a freshly cleaved surface of HOPG (quality ZYB, Digital Instruments). The STM images in Figure 1 a, 2 a and 3 were acquired after the solvent evaporated at 16 8C; the STM images of Figure 1 b and Figure 2 c after the substrate was kept at 65 8C for about 2 h and slowly cooled down to room temperature. The experiment was performed with a Nanoscope IIIa SPM (Digital Instruments, Santa Barbara, CA) under ambient conditions. STM tips were mechanically formed Pt/Ir wires (90/10). All STM images were recorded using the constant current mode. The specific tunneling conditions are given in the figure captions. The preliminary simulations were performed using the Hyperchem software to model the structures of the molecules.

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A combination of electron lifetime measurement in nanoparticles as a function of the Fermi level position at high resolution in the potential scale with a new model to describe this dependence provides a powerful tool to study the microscopic processes and parameters governing recombination in dye-sensitized solar cells. This model predicts a behavior divided in three domains for the electron lifetime dependence on open-circuit voltage that is in excellent agreement with the experimental results: a constant lifetime at high photovoltage, related to free electrons; an exponential increase due to internal trapping and detrapping and an inverted parabolla at low photovoltage that corresponds to the density of levels of acceptor electrolyte species, including the Marcus inverted region.