Semiconductor Electrodes Research Articles

Balancing Competing Reactions in Hydride Transfer Catalysis via Catalyst Surface Doping: The Ionization Energy Descriptor.

Hydride transfer (HT) is ubiquitous in catalytic reduction reactions. In heterogeneous electrocatalysis, the hydride donor could be a molecular catalytic intermediate adsorbed on an electrode surface. The stability and hydride-donating capability of such an intermediate may determine overall catalytic efficiency. Here, we report how to fine-tune a hydride donor's performance via doping an electrode surface. For semiconductor electrodes, we find that the ionization energy of the surface dopant can serve as a good descriptor for both the stability and hydride-donating capability of the catalytic intermediate adsorbed on the doped site. For the specific case of CO2 reduction on p-GaP, where adsorbed 2-pyridinide (2-PyH-*) was predicted to be the most likely hydride-donating species, we predict that its catalytic performance should be particularly enhanced by substituting Ga with Ti on the electrode surface; Sc, Al, and V surface dopants also could be worthy of further investigation.

Highly efficient charge separation in model Z-scheme TiO2/TiSi2/Si photoanode by micropatterned titanium silicide interlayer

Atomic layer deposited (ALD) TiO2 is an attractive material for improving the photoactivity and chemical stability of semiconductor electrodes in artificial photosynthesis. Using photoelectrochemical (PEC) measurements, we show that an interfacial, topographically microstructured TiSi2 layer inside the TiO2/Si heterojunction improves the charge carrier separation and shifts the water dissociation onset potential to more negative values. These observations are correlated with the X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) measurements, which reveal an increased band bending due to the TiSi2 interlayer. Combined with the UV–Vis absorption results, the photoelectron spectroscopy measurements allow the reconstruction of the complete energy band diagram for the TiO2/TiSi2/Si heterojunction and the calculation of the valence and conduction band offsets. The energy band alignment and improvements in PEC results reveal that the charge transfer across the heterojunction follows a Z-scheme model, where the metal-like TiSi2 islands act as recombination centers at the interface.

Determination of energy thresholds of electron excitations at semiconductor/insulator interfaces using trap-related displacement currents

Spectral measurements of illumination-induced displacement currents related to trapping of charge carriers optically excited in semiconductor electrodes are shown to deliver information regarding energy onsets of electron transitions at the interface. Presented examples include determination of the conduction band offset at the GaN/Al2O3 interface and determination of charge carrier excitation spectra of two-dimensional (2D) semiconductors MoS2 and WS2 at the interface with insulating SiO2.

Hydrogen Evolution Reaction Catalyzed By Plasmoinic Photo-Electrodes Under Visible Light Illumination

From the environmental point of view, the efficient control of the visible light driven hydrogen evolution reaction is one of the promising issue. For that, the semiconductor electrode can be regarded as the sufficient material but various semiconductor electrodes cannot be used for the visible light driven reactions due to their wide band gap energy. Recently, plasmoic photoelectric conversion system, which is the introduction of the plasmon active metal nano-structures onto the surface of the semiconductor substrate, has been received much attention due to its wide range photo response abilities. In this system, the electrons are excited and injected into the conduction band of the n-type semiconductor under visible light illuminations. Up to now, various plasmon-induced chemical reactions, even the water splitting, have been established. However, although well-established systems have been reported, the direct control of the reduction reaction at the metal-semiconductor interface using the excited electrons has not been well examined because the almost plasmonic photoconversion systems consist of the n-type semiconductor electrode and used the generated holes for the reactions. In this study, we have tried to establish the plasmon-induced hydrogen evolution system by using p-type semiconductor substrate and plasmon active nano-structures. By the fabrication of the plasmonic Ag dimer structures, we have observed the generation of the photo-current derived from the hydrogen evolution reactions under visible light illumination. Through the photocurrent measurements and the incident photon to current conversion efficiency obtained under different pH conditions, we have confirmed the pH dependence of the photo current value and the photo response characters. In addition to these, as the very interesting point for the current study, we have found the very unique catalytic property for the plasmonic hydrogen evolution especially under specific pH condition. Through the various photoelectrochemical measurements including isotropic selectively, we are sure that our plasmonic hydrogen evolution system could be one of the good candidate for future light energy conversion system.

Modulation of Excited State Dynamics in Semiconductor Electrodes with Electrical Bias

Semiconducting materials are essential building blocks of the majority of devices in the 21st century. The ability of semiconductors to generate charge carriers under band gap irradiation makes them suitable for a myriad of applications (e.g., solar cells, photocatalysts, photodetectors). In recent years the emergence of organic-inorganic lead halide perovskites revitalized these research fields. As a result, the efficiency of derived solar cells reached 22.3% in less than 10 years. To unravel the reasons behind their outstanding performance, it is vital to understand their optoelectronic properties. Spectroelectrochemical methods are suitable to determine fundamental optoelectronic properties (band edge and trap state energies) and electrochemical bias induced chemical changes in these materials. However, to probe charge carrier dynamics in these materials, it is essential to utilize ultrashort laser techniques, as these fast processes fall into the femto- or picosecond timescale. In my presentation the effect of applied bias on the charge carrier dynamics of semiconductor electrodes will be discussed. Ultrafast spectroelectrochemical experiments were carried out on FTO/TiO2/CsPbBr3 and ITO/CuI systems by coupling ultrafast transient absorption spectroscopy with electrochemical techniques.1 These two systems have practical relevance in perovskite based solar cells, and their excited state behavior under operating conditions was scrutinized. It was found that the excitonic features of both CsPbBr3 and CuI are responsive to the applied external bias, within their electrochemical stability window. The accumulation of electrons on the TiO2/CsPbBr3 or ITO/CuI interface have a pronounced effect on charge carrier lifetimes in these studied materials. The change in charge carrier lifetimes in both CsPbBr3 and CuI was completely reversible showing the dependence of excited state dynamics on the externally controlled charge carrier density. This validates the in situ electrochemical transient absorption measurement as a useful tool to probe the charge carrier injection process in different semiconductor systems. Scheidt, R. A., Samu, G. F., Janáky, C. & Kamat, P. V. Modulation of Charge Recombination in CsPbBr3 Perovskite Films with Electrochemical Bias. J. Am. Chem. Soc. 140, 86–89 (2018).

(Invited) Confined Molecular Electrocatalyst for Electrochemical and Photoelectrochemical Hydrogen Evolution Reaction

Hydrogen is the most important clean fuels in the future and production of hydrogen from water by solar energy is required. Photoelectrochemical (PEC) and photocatalytic decomposition of water has been studied for long time. Most serious problem of PEC production of hydrogen from water is that while semiconductor electrodes with small energy gap can absorb large fraction of solar energy but unstable, those with large energy gap are stable but can absorb only small fraction of solar energy. This can be solved by using semiconductor of small gap and separating the reaction site from the surface. We have constructed viologen molecular layer on hydrogen terminated Si(111) surface and then Pt complex was introduced within the molecular layer by ion-exchange reaction [1-3]. Significant decrease of overpotential for dark hydrogen evolution reaction (HER) at n-Si electrode was observed by this modification. Further improvement was achieved by constructing multi-viologen-Pt -layers. Viologen layers and Pt species act as electro relay and HER catalyst, respectively. Phoelectrochemical HER at p-Si(111) electrode is also accelerated by the modification with viologen-Pt layers. In situ XAFS study during HER showed that Pt complexes not Pt particles acted as HER catalysts, i.e., confined molecular catalysts, as schematically shown in the figure [3]. The decrease of white line intensity suggests the formation of a hydride complex as an intermediate of HER. The formation of Pt particles may have been inhibited since the complexes are separated by molecular layers. Confined molecular catalyst by viologen-Pt layers for HER was also constructed on indium tin oxide (ITO). While two redox peaks corresponding to viologen dication/ radical cation (V2+/V+•) and viologen radical cation/neutral form (V+•/V0) were observed in cyclic voltammogram (CV) of viologen monolayer modified ITO electrode without Pt complex, no redox peaks but large current due to HER was observed at viologen monolayer modified ITO electrode with Pt complex. Spectroelectrochemical measurements at constant potentials and during potential scan showed stable spectra corresponding to radical cation (V+•) and neutral form (V0) of viologen at potentials between V2+/V+• and V+•/V0 redox peaks and more negative than V+•/V0 redox peak, respectively, at an ITO electrode modified by viologen monolayer without a Pt complex but no absorption peaks corresponding to V+• or V0 at an ITO electrode modified by viologen monolayer with a Pt complex. Time-resolved spectroelectrochemical measurements, however, showed that V+• is formed upon the potentials step to the potentials more negative than V2+/ V+• redox potential and disappeared within ca. 1 ms at the ITO electrode modified by viologen monolayer with Pt complex, confirming that electron is transferred from ITO electrode to proton via viologen moiety and Pt complex [4]. By incorporating various metal complexes, not only HER but also CO2 reduction can be accelerated [5, 6] References [1] T. Masuda, K. Uosaki, Chem. Lett., 33 (2004) 788-789. [2] T. Masuda, K. Shimazu, K. Uosaki, J. Phys. Chem. C, 112 (2008) 10923-10930. [3] T. Masuda, H. Fukumitsu, S. Takakusagi, W.J. Chun, T. Kondo, K. Asakura, K. Uosaki, Adv. Mater., 24 (2012) 268-272. [4] C. Kurniawan, H. Noguchi, T. Masuda, K. Uosaki, Electrochem. Commun., 62 (2016) 56–59. [5] Y. Sun, T. Masuda, K. Uosaki, Chem. Lett., 41 (2012) 328-330. [6] T. Masuda, Y. Sun, H. Fukumitsu, H. Uehara, S. Takakusagi, W.-J. Chun, T. Kondo, K. Asakura, K. Uosaki, J. Phys. Chem. C, 120 (2016) 16200-16210. Figure 1

(Invited) Photoelectrochemistry of Semiconducting Oxide Materials for Solar Water Splitting

There is a large family of binary, ternary, quaternary, etc. metal oxides that exhibit semiconducting properties, which makes them possible candidates to be applied in solar water splitting. With the recent advances in the controlled and economical synthesis of a large variety of metal oxides and their physical properties, new efforts in the photoelectrochemical characterization of such materials has become an important focus of research in the solar water splitting community.1,2 Although photoelectrochemical water splitting is an attractive method to convert solar energy to storable chemical energy in the form of hydrogen, the materials requirements to achieve this efficiently are very challenging. The semiconducting material needs to absorb sunlight efficiently, must to be capable of reducing and/or oxidizing water, and has to be stable under illumination under current flow in an aqueous electrolyte solution. In particular, for small bandgap semiconductors, stability is often an issue. In addition, the water reduction and oxidation reactions need to occur fast, in order to effectively compete with recombination or surface degradation processes. The kinetic rate constants for charge transfer and surface recombination therefore are important parameters, but these are difficult to obtain from steady-state measurements. Intensity-modulated photocurrent spectroscopy (IMPS) is a powerful option to study the carrier dynamics in a photoelectrochemical cell. The frequency-dependent photocurrent admittance corresponds to the frequency-dependent external quantum efficiency, and time constants for charge transfer and surface recombination can be determined using a simple model, depending on the complexity of the system under study.3 We have used IMPS on a variety of systems including WO3, p-CuBi2O4 and WO3-BiVO4 heterojunctions, in order to elucidate the rate determining steps in the charge carrier dynamics. For CuBi2O4 photocathodes, an unfavorable balance between the rate constants for charge transfer and surface recombination is found to limit the conversion efficiency.4 On the other hand, IMPS analysis of screen-printed WO3 photoanodes shows that the recombination rate constant is significantly smaller than the charge transfer rate constant; however, the film thickness plays an important role in collection efficiency.5 Recent results on WO3-BiVO4 heterojunctions, CuWO4, and electrocatalyzed Sn-doped Fe2O3 photoanodes are also discussed. References “Solar Hydrogen Generation: Toward a Renewable Energy Future”. K. Rajeshwar, R. McConnell and S. Licht, Editors, Kluwer Academic, New York (2008).“Solution combustion synthesis of oxide semiconductors for solar energy conversion and environmental remediation”. K. Rajeshwar and N. R. de Tacconi, Chem. Soc. Rev. 38(7) 1984-1998 (2009).“Photoelectrochemical Water Splitting at Semiconductor Electrodes: Fundamental Problems and New Perspectives”. L. M. Peter and K. G. Upul Wijayantha, ChemPhysChem, 15, 1983–1995 (2014).“Charge Transfer and Recombination Dynamics at Inkjet-Printed CuBi2O4 Electrodes for Photoelectrochemical Water Splitting”. Ingrid Rodríguez-Gutiérrez, Rodrigo García-Rodríguez, Manuel Rodríguez-Pérez, AlbertoVega-Poot, Geonel Rodríguez Gattorno, Bruce A. Parkinson, and Gerko Oskam. J. Phys. Chem. C, 122, 27169−27179 (2018).“Charge Transfer and Recombination Kinetics at WO3 for Photoelectrochemical Water Oxidation”. Manuel Rodríguez-Pérez, Ingrid Rodríguez-Gutiérrez, Alberto Vega-Poot, Rodrigo García-Rodríguez, Geonel Rodríguez-Gattorno, Gerko Oskam. Electrochim. Acta, 258, 900-908 (2017).

Top-Down Fabrication of III-V Semiconductor Nanopillars for Photoelectrochemical Water Splitting Applications

Research into hydrogen generation via photoelectrochemical (PEC) water splitting is ever growing owing to its potential to generate clean and portable form of energy. In PEC water splitting, a semiconductor absorbs sunlight and splits water into hydrogen and oxygen to produce hydrogen fuel.1, 2 However, the lack of a single robust semiconductor material with a narrow bandgap that straddles both water redox potentials is the main barrier to develop practical PEC water splitting systems. III-V semiconductors can make ideal materials for PEC water splitting due to their outstanding optoelectronic properties including tunable band gaps to cover the entire solar spectrum, band edges straddling water redox potentials, high absorption coefficients and high crystalline quality. In addition, III-V photoelectrodes based on one-dimensional nanostructures are shown to greatly enhance the PEC performance from improved light absorption, increased semiconductor/electrolyte interface area and reduced carrier diffusion length over their planar counterparts. In this talk, I will introduce cost-effective and scalable fabrication of random semiconductor nanopillars (NPs) with controlled dimensions developed in our group using ICP etching and self-assembled random mask techniques for PEC water splitting (as illustrated in Figure 1). I will also present our results on the investigation of GaN, InGaN quantum wells and InP nanopillars (NPs), fabricated using top-down method, as photoelectrodes for PEC water splitting application. The band gap tunability with varying In content and high chemical stability makes InxGa1-xN a superior candidate for photoelectrodes. The PEC performance of random GaN NPs photoanodes was tested in 1 M NaOH under simulated one sun illumination. NPs generated the highest ever reported photocurrent density for GaN photoanodes due to exceptional absorption by the NPs, and increased depletion layer area and semiconductor/electrolyte interface area. Moreover, the PEC performance of NPs was found to be strongly influenced by carrier concentration and NP dimensions.3 We further engineered the band gap of GaN NPs by incorporating InGaN/GaN multiple quantum wells (MQWs) to further improve the PEC performance of GaN NPs. Photoluminescence and diffuse reflectance measurements confirmed that the introduction of InGaN/GaN MQWs into GaN extended the optical absorption by the NPs into the visible part of the solar spectrum, thereby contributing to the substantial improvement in photocurrent density for InGaN/GaN MQW NPs.4 Furthermore, GaN NP photoanodes were found to exhibit improved photostability after being decorated with co-catalysts such as Co3O4. The accumulation of photogenerated charge carriers at semiconductor surface trigger the self-oxidation of GaN photoanode during water oxidation reaction and results in photocorossion of GaN photoanodes. Deposition of co-catalyst significantly reduced the self-oxidation of GaN photoanode by rapidly extracting the photogenerated holes out of semiconductor that participate in water oxidation reaction.5 We also developed InP NPs using the random mask technique via top-down approach, removed plasma surface damage using wet treatment in sulphur dissolved oleylamine (S-OA) solution and investigated the PEC performance of InP NP photocathodes. We achieved stable and excellent PEC performance for NP photocathodes without any oxide protection layer after wet treatment in S-OA. The NPs exhibited saturation photocurrent density of ~34 mA/cm2, which is close to theoretical limit and power-saved cathodic efficiency of over 5%. The saturation photocurrent density and power-saved cathodic efficiency of NPs improved by 60% and 33%, respectively, compared to their counterpart planar photocathodes. We also carried out the time-resolved photoluminescence, optical and impedance spectroscopy characterisations to elucidate the PEC performance of InP photocathodes. References Fujishima, K. Honda, “Electrochemical Photolysis of Water at a Semiconductor Electrode”, Nature 1972, 238, 37–38.Walter, E. Warren, J. McKone, S. Boettcher, Q. Mi, E. Santori and N. S. Lewis, “Solar Water Splitting Cells”, Chemical Society Reviews 2010, 110, 6446–6473.Parvathala Reddy Narangari, Siva Krishna Karuturi, Mykhaylo Lysevych, Hark Hoe Tan, and Chennupati Jagadish, “Improved Photoelectrochemical Performance of GaN Nanopillar Photoanodes” Nanotechnology 2017, 28, 154001.Joshua Butson, Parvathala Reddy Narangari, Siva Krishna Karuturi, Rowena Yew, Mykhaylo Lysevych, Hark Hoe Tan, and Chennupati Jagadish, “Photoelectrochemical Studies of InGaN/GaN MQW Photoanodes”, Nanotechnology 2018, 29 045403.Guanyu Liu and Siva Krishna Karuturi, Alexandr N. Simonov, Monika Fekete, Hongjun Chen, Noushin Nasiri, Nhien H. Le, N Parvathala Reddy, Mykhaylo Lysevych, Thomas R. Gengenbach, Adrian Lowe, Hark Hoe Tan, Chennupati Jagadish, Leone Spicciac and Antonio Tricoli, “Robust Sub-Monolayers of Co3O4 Nano-Islands: a Highly Transparent Morphology for Efficient Water Oxidation Catalysis”, Advanced Energy Materials 2016, 6, 1600697. Figure 1

Protein Film, and Diffusion–Controlled Voltammetry of Hemeproteins on a Single Surface

Protein film voltammetry has made possible electrochemical studies of complex protein reactions, such as redox-induced conformational dynamics1 and enzyme kinetics.2 However, without diffusion-controlled experiments as a reference, one cannot be sure of the extent to which the protein–electrode interaction affects conformational dynamics or enzyme kinetics. Diffusion-controlled voltammetric experiments can provide an alternate view into these processes. Previously, semiconductor electrodes have been successfully employed for diffusion-controlled experiments.3 Herein, we report novel application of electrochemically deposited cadmium oxide (CdO) as a working electrode for both diffusion-controlled voltammetry as well as protein-film voltammetry (PFV). The hydrophilic CdO surface allows transient protein interactions for diffusion-controlled measurements, as well as a suitable surface for drop casting protein films. Polycrystalline CdO electrodes were electrodeposited onto a fluorine-doped tin oxide (FTO) working electrode held at –0.7 V vs. SCE for 15 min in aqueous solutions of CdCl2, then annealed at 450 °C for 2 h. CdO surfaces were characterized by means of x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and scanning electron microscopy (SEM). XPS and XRD confirm formation of polycrystalline CdO. Electron micrographs confirm complete coverage of the FTO by CdO. To generate protein films, 10 mg/mL solutions of either cytochrome c or hemoglobin were drop cast onto CdO electrodes and left for 2 h. Cyclic voltammograms recorded for both hemoglobin and cytochrome c display nearly reversible electron transfer, with redox potentials similar to those previously reported.4 1 A. El Kasmi, M. C. Leopold, R. Galligan, R. T. Robertson, S. S. Saavedra, K. El Kacemi, E. F. Bowden, Electrochem. Commun. 2002, 4, 177–181. 2 H. Liu, G. Zou, J. Biochem. Biophys. Methods. 2006, 68, 87–99. 3 G. Valincius, V. Reipa, M. P. Mayhew, V. L. Walker, Electrochem. Commun. 2002, 4, 314–317. 4 G. Battistuzzi, M. Bosari, M. Sola, F. Francia, Biochemistry, 1997, 36, 16247–16258.

Electrochemical Investigation of Graphene Composited Plasmonic Photo Conversion Electrode

The light energy conversion using the wide-gap semiconductor, such as TiO2, has been recognized as the promising way for the realization of the sustainable society. Recently, the combination of the plasmon active metal nanostructures with the semiconductor electrode has been received much attention due to its wide range photo response ability. On the other hand, graphene is the one of the interesting two-dimensional material because of its physicochemical properties. In this study, we have fabricated the graphene composited plasmonic photo electric conversion electrodes as the new light energy conversion electrode. The system shows characteristic behavior at photo energy conversion due to modification of surface electronic structure at graphene/Au nanostructure inerfaces.

Modification of the Electrochemical Behavior of InP Electrodes Induced By Ionic Bombardment: The Case of the Response of the Ce4+/Ce3+ Couple

The concept of modified electrode is very important for a lot of topics related to interfacial electrochemistry. It is particularly true when semiconductor (SC) interfacial electrochemistry is considered. Due to the specificities of the SC/liquid interfaces, modified SC electrodes have to be examined from different angles. The external part of the surface can be strongly modified due to the variations of surface /solution equilibrium inducing drastic transformations of the energy diagram at the interface. A direct consequence is a possible alteration of electronic transfers. Moreover the modification of the SC electrode can also concern deeper region through variation of the depletion layer distribution even of buried diffusion layer. Evolution of these parameters could again modify the energy diagram and the related electronic transfer abilities. Finally surface modifications can induced also evolution of the photoelectrochemical properties through evolution of the optical properties of the SC surface. All this different parameters are more or less related to the global response of the interface. It is in this context that our paper is being taken. For this study, our tool of surface modification is based on ion bombardment through mono-atomic or cluster Argon beams. Modulation of the perturbation induced by ion / SC surface interaction can be largely modulated by using mono or cluster Argon beam conditions [1]. In UHV the level and the nature of the perturbation can be easily in-situ evaluated from XPS responses evolutions from the core levels and also from the valence band. At this in-situ approach can be also associated optical responses such as ex-situ Ellipsometric Spectroscopy giving a global evaluation of the SC modification. As supporting SC material we choose InP whose surface, optical and interfacial electrochemical responses are well known in their initial stage before ionic bombardment. Based on well characterized InP (-n or -p) electrodes effect of ionic bombardment can be explored through a lot of electrochemical responses. Among the more attractive modification towards electronic transfer on InP, one can consider the addition in the electrolyte of a redox probe which acts as a perfect tool giving quantitative information for post bombardment. Ce4+/Ce3+ couple is particularly efficient due to its transfer specificities when its oxidant state (Ce4+) is only present [2-6]. Indeed pure holes injection inside the valance band (BV) is observed during the Ce4+ reduction mechanism which is only governed by the Ce4+ diffusion at the surface. As a direct consequence, the evolution of the reduction rate of Ce4+ can be easily measured vs the level of ionic bombardment. Moreover when InP is properly polarized all the holes injected are used in the anodic dissolution process with a very attractive and unusual conduction band (BC) injection phenomenon due to corrosion intermediates [6]. This is again a very sensitive probe to the level of modification of the InP surface after ionic bombardment. Note that this induced anodic process as a very similar potential distribution as the one obtained under illumination for which the holes creation is nevertheless only associated to the photon absorption. Common and difference points are also a subject of discussion for a characterization of the induced modifications. Finally we will discuss how an anodic and/or a chemical etching are able to regenerate the initial chemical state of the surface. The underlying question is how the anodic process linked to Ce4+/Ce3+ is sufficient to perform the surface regeneration even if it is initially modified by the ionic bombardment or if it requires the assistance of high positive polarization of the modified InP electrode.

(Invited) Materials By Design Principles in Artificial Photosynthesis: Discovery and Synergistic Integration of Light Absorbers, Electrocatalysts and Membranes for Complete, Stable, Efficient, and Safe Solar Fuels Generator

We are developing an artificial photosynthetic system that will utilize sunlight and water as the inputs and produce hydrogen and oxygen as the outputs. We are taking a modular, parallel development approach in which three distinct primary components-the photoanode, the photocathode, and the product-separating but ion-conducting membrane-are fabricated and optimized separately before assembly into a complete water-splitting system. The design principles incorporate two separate, photosensitive semiconductor/liquid junctions that will collectively generate the 1.7-1.9 V at open circuit necessary to support both the oxidation of H2O (or OH-) and the reduction of H+ (or H2O). The photoanode and photocathode will consist of rod-like semiconductor components, with attached heterogeneous multi-electron transfer catalysts, which are needed to drive the oxidation or reduction reactions at low overpotentials. The high aspect-ratio semiconductor rod electrode architecture allows for the use of low cost, earth abundant materials without sacrificing energy conversion efficiency due to the orthogonalization of light absorption and charge-carrier collection. Additionally, the high surface-area design of the rod-based semiconductor array electrode inherently lowers the flux of charge carriers over the rod array surface relative to the projected geometric surface of the photoelectrode, thus lowering the photocurrent density at the solid/liquid junction and thereby relaxing the demands on the activity (and cost) of any electrocatalysts. A flexible composite polymer film will allow for electron and ion conduction between the photoanode and photocathode while simultaneously preventing mixing of the gaseous products. Separate polymeric materials will be used to make electrical contact between the anode and cathode, and also to provide structural support. Interspersed patches of an ion conducting polymer will maintain charge balance between the two half-cells.

(Invited) Progress in Photoelectrochemical Energy Conversion

The interest in the direct storage of solar energy as a chemical fuel in a semiconductor based photoelectrochemical system started with the demonstration of solar photoelectrolysis of water with large band gap oxide semiconductor electrodes in the late 1970s. In the last decade or so there has been both and increased interest and increased funding towards achieving a goal of efficiently producing cost effective fuels from solar energy with either water or carbon dioxide as a feedstock. This talk will review the progress towards this goal considering recent developments. One of these developments is that the cost of photovoltaic systems has been decreasing rapidly to where currently the cost of the solar panels is now exceeded by balance of systems cost. The cost of electrolyzers will be also decrease as they are improved and scaled. Connecting these two existing technologies has the advantage of producing hydrogen where and when it is needed and at pressure. These facts mean that the window for direct solar photoelectrolysis is rapidly closing. One possible breakthrough is that a new stable, efficient, inexpensive, defect-tolerant and scalable new materials are identified that can quickly improve the efficiency of photoelectrolysis much like the hybrid perovskites are have done for photovoltaic devices. This talk will review the progress in combinatorial approaches to discover new materials for photoelectrolysis with some examples including one from the Solar Hydrogen activity Research Kit (SHArK) Project, a distributed science project that provides undergraduates and high school students with the resources to produce and screen metal oxide semiconductors for photoelectrolysis activity. In addition the reasons for producing hydrogen from water rather than direct carbon dioxide reduction to produce fuels will be reviewed. A new system for storing solar energy directly as redox equivalents, a solar chargeable redox flow battery, will also be introduced and its advantages and disadvantages compared to solar hydrogen generation will be discussed.

Hot electron-driven electrocatalytic hydrogen evolution reaction on metal\u2013semiconductor nanodiode electrodes

Hot electrons generated on metal catalysts influence atomic and molecular processes, leading to hot electron-driven catalytic reactions. Here, we show the acceleration of electrocatalytic hydrogen evolution caused by internal injection of hot electrons on Pt/Si metal–semiconductor electrodes. When a forward bias voltage is applied to the Pt/Si contact, hot electrons are injected. The excess energy of these electrons allows them to reach the Pt/electrolyte interface and reduce the adsorbed hydrogen ions to form H2 (2H+ + 2e−→H2). We show that the onset potential of the hydrogen evolution reaction shifts positively by 160 mV while the cathodic current exhibits an 8-fold increase in the presence of hot electrons. The effect disappears when the thickness of the Pt film exceeds the mean free path of the hot electrons. The concept of a hot electron-driven reaction can lead to the development of a novel mechanism for controlling reactivity at liquid–solid interfaces.

Quantitative Analysis of Cyclic Voltammetry of Redox Monolayers Adsorbed on Semiconductors: Isolating Electrode Kinetics, Lateral Interactions, and Diode Currents.

The design of devices whose functions span from sensing their environments to converting light into electricity or guiding chemical reactivity at surfaces often hinges around a correct and complete understanding of the factors at play when charges are transferred across an electrified solid-liquid interface. For semiconductor electrodes in particular, published values for charge-transfer kinetic constants are scattered. Furthermore, received wisdom suggests slower charge-transfer kinetics for semiconductors than for metal electrodes. We have used cyclic voltammetry of ferrocene-modified silicon photoanodes and photocathodes as the experimental model system and described a systematic analysis to separate charge-transfer kinetics from diode effects and interactions between adsorbed species. Our results suggest that literature values of charge-transfer kinetic constants at semiconductor electrodes are likely to be underestimates of their actual values. This is revealed by experiments and analytical models showing that the description of the potential distribution across the semiconductor-monolayer-electrolyte interface has been largely oversimplified.

Features of the Characteristics of Field-Resistant Silicon–Ultrathin Oxide–Polysilicon Structures

Results of studying the features of the current–voltage (I–V) and capacitance–voltage characteristics of field-resistant silicon–ultrathin oxide–polysilicon structures are presented. It turns out that the total recharging of localized electron states and minority carriers concentrated near the substrate–insulator interface, which occurs with a variation in the field voltage, is an order of magnitude higher than that of samples susceptible to damage by the field stress effect. The tunneling I–V characteristic is significantly asymmetric; notably, the current flowing from the field electrode into the silicon substrate is several orders of magnitude lower when compared with the current flowing from silicon to polysilicon at identical external voltages dropping across the insulating layer. To explain this asymmetry, it is assumed that a potential barrier in the transition layer from polysilicon to oxide, which separates the semiconductor electrode and substrate, has a height of ~1 eV and therefore always hinders electrical transport; for reverse currents, this barrier stops limiting the conductivity as soon as the tunneling level becomes higher than it.

Effects of photo-generated gas bubbles on the performance of tandem photoelectrochemical reactors for hydrogen production

Presence of gas bubbles in the vicinity of semiconductor electrodes interferes with their active surface areas and introduces inert voids in the electrolyte hindering its ionic conductivity. Furthermore, gas bubbles obstruct the radiation path through scattering. The aim of this work is to study the characteristic hydrogen- and oxygen-gas bubble behavior and their effects on photoelectrochemical reactor performance. Findings of gas bubble formation, electrode coverage and curtain profiles based on macroscopic bubble graphical images are reported. Effects of increased convective forces are also observed. Further, the scattering of incident light implemented through simulations based on Mie scattering theory is reported. Results show that hydrogen gas bubbles are more extensive in coverage due to formation of a froth while oxygen bubbles coalesce and rise easily. The growth of the bubble cover increases ohmic resistance reducing the current magnitude. Even at a modest current density of 10 mA/cm2, the curtain thickness may rise to 2 mm or 3 mm for oxygen and hydrogen, respectively. Light scattering increases with increasing bubble size and is more pronounced for shorter wavelengths. It is also found that presence of multiple bubbles reduces light intensity by up to 2% and highest when the bubble radius is 150 μm. Increase in both photoelectrode and electrolyte resistances as well as radiation losses due to presence of bubbles hence undermine the performance of photoelectrochemical reactors.

Photo-deposition of cobalt-phosphate group modified hematite for efficient water splitting

In this work, Cobalt Phosphate Group (Co−Pi) Modified nanostructured hematite (α-Fe2O3) photoanodes films were deposited on fluorine-doped tin oxide (FTO) coated glass by means of the hydrothermal process. The α-Fe2O3 film morphology can be tuned from nanoparticle – based patterns to high-density cabbage-like structures as Co−Pi deposition time is increased.Systematic studies were performed to clarify the mechanisms that enhance water-splitting performance of α-Fe2O3 electrodes as Co−Pi deposition time increases. All samples followed morphological and structural studies using field-emission scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. Chronoamperometry measurements showed that Co-Pi/α-Fe2O3 photoanode exhibited higher photoelectrochemical activity than the α-Fe2O3 films. The maximum photocurrent density and incident photon conversion efficiencies (IPCE) were obtained for 30 s Co-Pi/α-Fe2O3 photoanode. All samples reached their best IPCE at 400 nm. IPCE values for 30 s Co-Pi hematite films were 5 times higher than that of α-Fe2O3 sample. For the α-Fe2O3 photoanode, the H2 evolution rate was ∼5.14 μmol cm−2 h−1. After decoration by Co-Pi, the rate increases to ∼7.31 μmol cm−2 h−1, showing an enhancement of about ∼ 42.2%. It was found that under illumination, the Co−Pi catalyst can efficiently collect and store photogenerated holes from the α-Fe2O3 electrode. This charge separation reduces surface state recombination, leading to the enhancement of the efficiency of water oxidation. Thicker catalytic Co−Pi films were found to increase charge separation and surface area, which in turn increase water oxidation efficiency. These results provide important new understanding of the catalytic enhancement and limitations of the Co−Pi catalyst once coupled to semiconductor electrodes for water-splitting applications.

Stabilization of Ruthenium(II) Polypyridyl Chromophores on Mesoporous TiO2 Electrodes: Surface Reductive Electropolymerization and Silane Chemistry.

Stabilization is a critical issue in the long term operation of dye-sensitized photoelectrosynthesis cells (DSPECs) for water splitting or CO2 reduction. The cells require a stable binding of the robust molecular chromophores, catalysts, and chromophore/catalyst assemblies on metal oxide semiconductor electrodes under the corresponding (photoelectro)chemical conditions. Here, an efficient stabilization strategy is presented based on functionalization of FTO|nanoTiO2 (mesoporous, nanostructured TiO2 deposited on fluorine-doped tin oxide (FTO) glass) electrodes with a vinylsilane followed by surface reductive electropolymerization of a vinyl-derivatized Ru(II) polypyridyl chromophore. The surface electropolymerization was dominated by a grafting-through mechanism, and rapidly completed within minutes. Chromophore surface coverages were controlled up to three equivalent monolayers by the number of electropolymerization cycles. The silane immobilization and cross-linked polymer network produced highly (photo)stabilized chromophore-grafted FTO|nanoTiO2 electrodes. The electrodes showed significant improvements over structures based on atomic layer deposition and polymer dip-coating stabilization methods in a wide pH range from pH ≈ 1 to pH ≈ 12.5 under both dark and light conditions. Under illumination, with hydroquinone added as a sacrificial electron transfer donor, a photoresponse for sustained electron transfer mediation occurred for at least ∼20 h in a pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4). The overall procedure provides an efficient way to fabricate highly stabilized molecular assemblies on electrode surfaces with potential applications for DSPECs in solar fuels.

Effects of thermal annealing conditions on the electrical characteristics of TES-ADT thin-film transistors

We investigated the annealing temperature and exposure period dependence of electrical characteristics of bottom-gate/top-contact solution-processed organic thin-film transistors (TFTs) fabricated on ITO-patterned substrates using 5,11-bis(triethylsilylethynyl)anthradithiophene(TES-ADT), PVP-co-PMMA, and silver as the semiconductor, gate dielectric, and drain and source electrodes, respectively. The semiconductor films were thermally annealed at 60, 80, and 90 °C for 60 min; then, the optimal annealing temperature, which proved to be 80 °C, was considered to investigate the impact of exposure time by exposing the film for 30, 60, and 90 min. Experimental results indicate that the medium temperature resulted in the better electrical performance, whereas the high temperature proved to be detrimental by causing ruptures through the film. According to our study, TES-ADT film annealed at 80 °C for 60 min results in the finest semiconductor microstructure, which improves the electrical characteristics, whereas lower or higher temperatures and shorter or longer annealing times lead to an unfavorable structure that hampers charge transport in the film.