Semiconductor-solution Interface Research Articles

(Europe Section Heinz Gerischer Award) The Long Reach of Electrochemistry: Semiconductors, Metallization and Energy Storage

This talk will attempt to illustrate the long reach of electrochemistry by briefly reviewing our work in a number of different areas including oxygen electrochemistry of platinum metals, various aspects of compound semiconductors and their electrochemistry, electrodeposition of copper metallization and electrochemical kinetics of vanadium redox reactions. The discussion will briefly touch on early work on oxygen evolution on ruthenium and iridium and electrochromism in anodic iridium oxides.1-3 Compound semiconductors will be discussed in a number of aspects from epitaxial crystal growth to electrochemistry. Specific topics will include chemical vapor deposition technology4,5 and electrochemistry at the semiconductor-solution interface including film growth6, oscillatory reactions7 and pore formation on indium phosphide8,9 and photoelectrochemical etching of gallium nitride.10 Aspects of electrodeposited copper metallization will be discussed including in-situ stress measurements and in-situ atomic force microscopy during deposition11 and spontaneous morphology changes during room-temperature aging12. Electrochemical kinetics of vanadium redox reactions on carbon electrodes13 will be discussed as well as other aspects of vanadium flow batteries.14,15 References D. N. Buckley, L. D. Burke, “Lowering of Overvoltage for Oxygen Evolution at Noble Metal Electrodes in the Presence of Ruthenium Salts”, J. Electroanal. Chem. 52, 433 (1974).D. N. Buckley, L. D. Burke, “Oxygen Evolution and Corrosion at Iridium Anodes”, J. Chem. Soc. Faraday Trans. 1, 72, 2431 (1976).D. N. Buckley, L. D. Burke, “Enhancement of Charge Capacity of an Iridium Surface in the Anodic Region”, J. Chem. Soc. Faraday Trans. 1, 71, 1447 (1975).D. N. Buckley, J. R. C. Filipe, F. R. Lineman, K. W. Wang, K. M. Lee, A. A. Westphal, S. M. McEwan and M. A. DiGiuseppe, “Spatial Variations in the Epitaxial Growth of InP and InGaAs by Trichloride VPE and their Physical and Chemical Origins,” J. Electrochem. Soc., 139, 1185 (1992).D. N. Buckley C. W. Seabury, J. L. Valdes, G. Cadet, J. W. Mitchell, M. A. DiGiuseppe, R. C. Smith, J. R. C. Filipe, R. B. Bylsma, U. K. Chakrabarti and K. W. Wang, “Growth of InGaAs Structures using In Situ Electrochemically Generated Arsine”, Appl. Phys. Lett. 57, 1684 (1990).D. N. Buckley, E. Harveyand S. N. G. Chu, “Growth of Anodic Films on Compound Semiconductor Electrodes: InP in Aqueous (NH4)2S”, Monatshefte fur Chemie , 133, 785 (2002)E. Harvey, D. N. Buckley, and S. N. G. Chu, “Oscillatory Behavior during the Anodization of InP”, Electrochem. Solid State Lett, 5 , G22 (2002)C. O'Dwyer, D. Sutton, M. Serantoni, S. B. Newcomb and D. N. Buckley, "An Investigation by AFM and TEM of the Mechanism of Anodic Formation of Nanoporosity in n-InP in KOH", J. Electrochem. Soc. 154, H78-H85 (2007).Robert P. Lynch, Nathan Quill, Colm O’Dwyer, Shohei Nakahara and D. Noel Buckley, "Propagation of Nanopores during Anodic Etching of n-InP in KOH," Phys. Chem. Chem. Phys., 15, 15135-15145 (2013).C. Heffernan, R. P. Lynch and D. N. Buckley, “A Study of the Photoelectrochemical Etching of n-GaN in H3PO4 and KOH Electrolytes”, ECS Journal of Solid State Science and Technology, 9, 015003 (2020).S. Ahmed, T. T. Ahmed, M. O’Grady, S. Nakahara and D. N. Buckley, "Investigation of Stress and Morphology in Electrodeposited Copper Nanofilms by Cantilever Beam Method and In-Situ Electrochemical Atomic Force Microscopy," J. Appl. Phys. 103, 073506 (2008).S. Ahmed, D. N. Buckley, S. Nakahara, T. T. Ahmed, and Y. Kuo, "An Isothermal Annealing Study of Spontaneous Morphology Change in Electrodeposited Copper Metallization," J. Electrochem. Soc., 154, D103 (2007).A. Bourke, M. A. Miller, R. P. Lynch, X. Gao, J. Landon, J. S. Wainright, R. F. Savinell, and D. N. Buckley, “Electrode Kinetics of Vanadium Flow Batteries: Contrasting Responses of VII-VIII and VIV-VV to Electrochemical Pretreatment”, J. Electrochem. Soc., 163(1), A5097-A5105 (2016); doi:10.1149/2.0131601jesC. Petchsingh, N. Quill, J. T. Joyce, D. Ní Eidhin, D. Oboroceanu, C. Lenihan, X. Gao, R. P. Lynch, and D. N. Buckley, “Spectroscopic Measurement of State of Charge in Vanadium Flow Batteries with an Analytical Model of VIV-VV Absorbance” J. Electrochem. Soc. 163, A5068-A5083 (2016) doi:10.1149/2.0091601jes.D. N. Buckley, D. Oboroceanu, N Quill, C. Lenihan1 and R. P. Lynch, “Modelling and Accelerated Testing of Catholyte Stability in Vanadium Flow Batteries,” J. Electrochem. Soc., 168, 030530 (2021). Figure 1

Influence of surface restructuring on the activity of SrTiO3 photoelectrodes for photocatalytic hydrogen reduction

Perovskite photoelectrodes are being extensively studied in search for photocatalytic materials that can produce hydrogen through water splitting. The solar-to-hydrogen efficiency of these materials is critically dependent on the electrochemical state of their surface. Here, we develop an embedded quantum-mechanical approach using the self-consistent continuum solvation model to predict the relation between band alignment, electrochemical stability, and photocatalytic activity taking into account the long-range polarization of the semiconductor electrode under electrical bias. Using this comprehensive model, we calculate the charge-voltage response of various reconstructions of a solvated ${\mathrm{SrTiO}}_{3}$ surface, revealing that interfacial charge trapping exerts primary control on the electrical response and surface stability of the photoelectrode. Our results provide a detailed molecular-level interpretation of the enhanced photocatalytic activity of ${\mathrm{SrTiO}}_{3}$ upon voltage-induced restructuring of the semiconductor-solution interface.

(Invited) Wet-Chemical Etching of III-V Semiconductors: Towards Atomic-Layer-Scale Processing

Recent developments in device technology resulted in an upsurge of interest in III-V compound semiconductors. These include improved fabrication techniques for flexible electronic devices,[1,2] and epitaxial integration of various III-V channel materials on Si-based platform wafers enabling the development of highly scaled CMOS transistors [3–5] and nanophotonic devices [6]. While fabrication strategies for traditional III-V optoelectronic devices such as light-emitting diodes, lasers and solar cells are well established, the very small dimensions of these new nanodevices pose new problems. In such applications wet-chemical etching remains an essential step. The ever decreasing size of III-V devices requires ultimately atomic-layer-scale control of surfaces in terms of etching selectivity, stoichiometry and morphology.[7,8] Considerable expertise and literature is currently available on III-V semiconductor etching[9,10]. However, much of the reported work is empirical, etch rates are high, and mechanistic insight into (electro)chemical processes occurring at the semiconductor-solution interface is often lacking. In this work an overview will be given of wet-chemical approaches for nanoscale and atomic-layer-scale etching of Ga(In)As and InP. Two types of etching systems will be discussed. The first is based on the use of acidic H2O2 solution. Inductively coupled plasma mass spectrometry (ICP-MS) measurements, used to determine etching kinetics, showed that under similar conditions the etch rate of Ga(In)As in H2SO4/H2O2 solution is more than an order of magnitude higher than that of InP. Another striking observation is the influence of the acid, H2SO4 and HCl, on etching kinetics. An increase in HCl concentration leads to an increase in the etch rate of InP while the dissolution rate of Ga(In)As is markedly lowered for the active etching range. Previous work suggested that the surface oxide or hydroxide may be important.[11,12] We have used X-ray photoemission spectroscopy (XPS) and time-of-flight elastic recoil detection analysis (ToF-ERDA) to obtain information about (hydr)oxide formation on the etched surfaces. ToF-ERDA measurements also allowed us the detect surface chlorine in the case of HCl-based etchants. The results indicate that, while the initial step (the breaking of the III-V surface bond) is the same for both semiconductors, the ease with which the resulting group V hydroxide entity at the surface can be deprotonated determines whether the etch rate will be high (Ga(In)As) or low (InP). The mechanism can also help to explain the contrasting role of HCl in the dissolution of the two semiconductors. An alternative digital type of etching approach for InGaAs and InAs involves self-limiting surface oxidation in O3/H2O followed by an oxide removal step in HCl solution[12]. ICP-MS quantification of the dissolved surface oxide species indicates that a high stoichiometry of etching is obtained and that the number of equivalent oxidized atomic layers can be controlled by adjusting the dissolved O3 concentration. For >4 cycles some surface roughening is observed, most likely due to due to the high solubility of As oxides in water. An important advantage of the 2-step approach is that defect selective etching can be effectively suppressed. Examples of applications of the two etching systems will be highlighted. [1] C.-W. Cheng, K.-T. Shiu, N. Li, S.-J. Han, L. Shi, D.K. Sadana, Nat. Commun. 4 (2013) 1577. doi:10.1038/ncomms2583[2] N.J. Smeenk, J. Engel, P. Mulder, G.J. Bauhuis, G. Bissels, J.J. Schermer, E. Vlieg, J.J. Kelly, ECS J. Solid State Sci. Technol. 2 (2013) P58–P65.[3] J.A. del Alamo, Nature. 479 (2011) 317–323. doi:10.1038/nature10677[4] M. Paladugu, C. Merckling, R. Loo, O. Richard, H. Bender, J. Dekoster, W. Vandervorst, M. Caymax, M. Heyns, Cryst. Growth Des. 12 (2012) 4696–4702. doi:10.1021/cg300779v[5] N. Waldron, C. Merckling, L. Teugels, P. Ong, S.A.U. Ibrahim, F. Sebaai, A. Pourghaderi, K. Barla, N. Collaert, A.V.-Y. Thean, IEEE Electron Device Lett. 35 (2014) 1097–1099. doi:10.1109/LED.2014.2359579[6] Y. Shi, Z. Wang, J. Van Campenhout, M. Pantouvaki, W. Guo, B. Kunert, and D. Van Thourhout, Optica 4, 1468-1473 (2017). doi:10.1364/OPTICA.4.001468[7] K.J. Kanarik, T. Lill, E.A. Hudson, S. Sriraman, S. Tan, J. Marks, V. Vahedi, R.A. Gottscho, J. Vac. Sci. Technol. Vac. Surf. Films. 33 (2015) 020802. doi:10.1116/1.4913379[8] G.S. Oehrlein, D. Metzler, C. Li, ECS J. Solid State Sci. Technol. 4 (2015) N5041–N5053. doi:10.1149/2.0061506jss[9] P.H.L. Notten, J.E.A.M. Meerakker, J.J. Kelly, Elsevier Advanced Technology, 199.[10] A.R. Clawson, Mater. Sci. Eng. R Rep. 31 (2001) 1–438[11] D.H. van Dorp, S. Arnauts, D. Cuypers, J. Rip, F. Holsteyns, S.D. Gendt, J.J. Kelly, ECS J. Solid State Sci. Technol. 3 (2014) P179–P184. doi:10.1149/2.021405jss[12] D.H. van Dorp, S. Arnauts, F. Holsteyns, S.D. Gendt, ECS J. Solid State Sci. Technol. 4 (2015) N5061–N5066. doi:10.1149/2.0081506jss

(Invited) Quantum Confinement Controls Effective Band Gap, Photocatalytic H2 Evolution and Photovoltage in CdSe Nanocrystals

Here, we provide the first quantitative analysis of quantum-size-controlled photocatalytic H2 evolution at the semiconductor–solution interface. We find that the hydrogen evolution rate from suspended mercaptoethanol-ligated CdSe quantum dots in aqueous sodium sulfite solution depends on nanocrystal size. Photoelectrochemical measurements on CdSe nanocrystal films reveal that the observed reactivity is controlled by the free energy change of the system, as determined by the proton reduction potential and the quasi-Fermi energy of the dots. The corresponding free energy change can be fitted to the photocatalytic activity using a modified Butler–Volmer equation for reaction kinetics. In parallel, Surface photovoltage spectroscopy (SPS) was used to photochemical charge transfer at the CdSe quantum dot -indium doped tin oxide (ITO) interface. Negative photovoltages are observed under super band gap illumination from majority carrier (electron) injection into the ITO substrate. The photovoltage onset energies track the optical band gaps of the samples and are assigned as effective band gaps of the films. The photovoltage values (−125 to −750 mV) vary with quantum dot sizes and are modulated by the built-in potential of the CdSe–ITO Schottky type contacts. Deviations from the ideal Schottky model are attributed to Fermi level pinning in states approximately 1.1 V negative of the ITO conduction band edge. Positive photovoltage signals of +80 to +125 mV in films of >4.0 nm nanocrystals and in thin (70 nm) nanocrystal films are attributed to electron–hole (polaron) pairs that are polarized by a space charge layer at the CdSe–ITO boundary. The space charge layer is 70–150 nm wide, based on thickness-dependent photovoltage measurements. These findings establish a quantitative experimental basis for quantum-confinement-controlled proton reduction and electron transfer with semiconductor nanocrystals. Figure 1

Charge Transport and Stability of a Semiconductor-Solution Interface Under Electrical Bias from First Principles

Solar energy is the most abundant energy source available to humankind; this energy cannot be harnessed on demand, however, due to the variability of sunlight. Artificial photosynthesis offers a sustainable way to overcome that variability through the photocatalytic conversion of solar power into chemical fuels at a semiconductor–electrolyte interface. Although considerable progress has been made in simulating the bulk properties of semiconductors from first principles, much less has been done to address the electrochemical response of semiconductor–electrolyte interfaces under realistic environmental conditions. We present broadly applicable and highly transferable computational techniques to simulate semiconductor–electrolyte interfaces. By introducing a continuum model of the semiconductor and electrolyte regions surrounding the quantum-mechanical interfacial layer, our model enables the self-consistent determination of the electrical response of a semiconductor-solution interface, including the atomistic effects of charge trapping at interfacial surface states. Using this, we develop a new description of the semiconductor-solution interface allowing us to predict the electrical response, stability, and equilibrium Schottky barrier of a photoelectrode as a function of different surface chemistries. This allows researchers to optimize materials selection for both charge transport and stability, leading to more efficient and practical artificial photosynthesis systems. Reference: Campbell and Dabo, Physical Review B 95, 205308 (2017).

Preface—Focus Issue on Processes at the Semiconductor-Solution Interface

This focus issue addresses some of the cutting edge research themes in many processes that occur at the interface between a semiconductor and a solution. This interface, a pH and redox potential-controlled liquid analog to the metal-semiconductor interface, is where semiconductor electrochemistry occurs. With the advent of alternative approaches to lower cost and more efficient hydrogen production, to the energetics of the electrolyte-material interface for batteries, the physics and electrochemistry of solar photovoltaics, and transistor-based technologies that monitor liquid-based biological interactions and so much more, this interface remains central to the underlying mechanisms for so many multidisciplinary topics. Energy storage and conversion, materials design and characterization, environmental science and technology, and much more, rely on this critical interface as part of the scientific search for a sustainable energy future.

(Invited) Corrosion of Semiconductors at the Solution Interface in Integrated Solar Fuel Devices: Challenges and Perspectives

Corrosion of technologically important small band-gap ( E G<2 eV) semiconductors is a common behavioral response to the photoelectrochemical environment such as illumination condition, potential, pH, buffer ions, and etc. Importantly, this corrosion process of semiconductors is generally recognized as the leading failure mechanism in integrated solar fuel devices. Therefore, understanding the corrosion reaction is critical to the development of stabilization strategies toward the realization of efficient and stable solar fuel conversion devices. Depending on the materials behavior in response to the corrosive environment and electrochemical stress, the corrosion process can be categorized into dissolution and passivation corrosion depending on the dissolution rate of the thermodynamically decomposed products at the semiconductor solution interface. In this work, photo-passivation and -corrosion processes of two representative semiconductors under working conditions have been identified. Functions of the decomposed product that remains at the semiconductor interface have been investigated including effects to the interfacial energetics and co-stabilization. For the semiconductor that involves dissolution corrosion under operation, two effective new designs that are promising in maintaining the functionalities of the integrated photoelectrodes have been experimentally studied. Challenges and future research directions will be also discussed.

The influence of carrier density and doping type on lithium insertion and extraction processes at silicon surfaces

The Li+ insertion and extraction characteristics at n-type and p-type Si(100) electrodes with different carrier density and doping type are investigated by cyclic voltammetry and constant current measurements. The insertion and extraction potentials are demonstrated to vary with cycling and the occurrence of an activation effect is shown in n-type electrodes where the charge capacity and voltammetric currents are found to be much higher than p-type electrodes. A rate-dependent redox process influenced by the surface region electronic density, which influences the magnitude of cyclic voltammetry current is found at Si(100) surface regions during Li insertion and extraction. At p-type Si(100) surface regions, a thin, uniform film forms at lower currents, while also showing a consistently high (>70%) Coulombic efficiency for Li extraction. The p-type Si(100) surface region does not undergo crack formation after deintercalation and the amorphization was demonstrated using transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) and Raman scattering demonstrate that highly doped n-type Si(100) retains Li as a silicide and converts to an amorphous phase as a two-step phase conversion process. The findings show the succinct dependence of Li insertion and extraction processes for uniformly doped Si(100) single crystals and how the doping type and its effect on the semiconductor-solution interface dominate Li insertion and extraction, composition, crystallinity changes and charge capacity.

Imaging Fluorescent Nanoparticles To Probe Photoinduced Charging of a Semiconductor–Solution Interface

Optically transparent semiconductors allow simultaneous control of interfacial electrical potential and spectroscopic observation of chemistry near the electrode surface. Care must be taken, however, to avoid unwanted photoexcitation-induced charging of the semiconductor electrode that could influence the results. In this work, we investigate the in situ surface charging by photoexcitation well below the band gap of an optically transparent semiconductor, indium-tin oxide (ITO) electrode. Using total-internal-reflection fluorescence microscopy, the population of ~100-nm negatively charged carboxylate-polystyrene fluorescent nanoparticles at an ITO-aqueous solution interface could be monitored in situ. At positive applied potentials (~0.7 V versus Ag/AgCl), nanoparticles accumulate reversibly in the electrical double-layer of the ITO surface, and the interfacial nanoparticle populations increase with 488-nm excitation intensity. The potential sensitivity of nanoparticle population exhibited no dependence on excitation intensity, varied from 0.1 to 10 W cm(-2), while the onset potential for particle accumulation shifted by as much as 0.3 V. This shift in surface potential appears to be due to photoexcitation-induced charging of the ITO, even though the excitation radiation photon energy, ~2.4 eV, is well below the primary band gap of ITO, >3.5 eV. A kinetic model was developed to determine the photon order of electron-hole generation relative to the electron-hole recombination. The photoexcitation process was found to be first-order in photon flux, suggesting one-photon excitation of an indirect band gap or defect sites, rather than two-photon excitation into the direct band gap. A control experiment was conducted with red-fluorescent carboxylate-polystyrene particles that were counted using 647-nm excitation, where the photon energy is below the indirect band gap or defect site energy and where the optical absorption of the film vanishes. Red illumination between 1 and 15 W cm(-2) produced no detectable shifts in the onset accumulation potential, which is consistent with the negligible optical absorption of the ITO film at this longer wavelength.

Quantum Confinement Controls Photocatalysis: A Free Energy Analysis for Photocatalytic Proton Reduction at CdSe Nanocrystals

The ability to adjust the mechanical, optical, magnetic, electric, and chemical properties of materials via the quantum confinement effect is well-understood. Here, we provide the first quantitative analysis of quantum-size-controlled photocatalytic H2 evolution at the semiconductor-solution interface. Specifically, it is found that the hydrogen evolution rate from illuminated suspended CdSe quantum dots in aqueous sodium sulfite solution depends on nanocrystal size. Photoelectrochemical measurements on CdSe nanocrystal films reveal that the observed reactivity is controlled by the free energy change of the system, as determined by the proton reduction potential and the quasi-Fermi energy of the dots. The corresponding free energy change can be fitted to the photocatalytic activity using a modified Butler-Volmer equation for reaction kinetics. These findings establish a quantitative experimental basis for quantum-confinement-controlled proton reduction with semiconductor nanocrystals. Electrochemical data further indicate that proton reduction occurs at cadmium sites on the dots, and that charge separation in these nanocrystals is controlled by surface effects, not by space charge layers.

Current-Line Oriented Pore Formation in n-InP Anodized in KOH

Electrochemically formed pores in InP in KOH switch from being crystallographically oriented (CO) to being current-line oriented (CLO) above a specific potential in both 17 mol dm-3 and 2.5 mol dm-3 KOH at a temperature of 10oC. The CLO pores formed in KOH have roughly elliptical cross sections, and are wider along the than along the perpendicular <011> direction. The CLO pores can form only when a critical porosity is reached and their formation marks the transition from porous to planar etching. Many of the features of pore formation are explained by evoking the effect that both temperature and electrolyte concentration can have on the effective diffusion length of holes at the semiconductor-solution interface.

Synthesis and characterization of electro-crystallized Cd–Sn–Se semiconductor films for application in non-aqueous photoelectrochemical solar cells

In the present investigation, thin films of CdSnSe have been developed on transparent conducting oxide (TCO) coated glasses by electrolytic deposition. The controlled incorporation of Sn in the semiconducting layer have been achieved by varying the concentration of Sn 2+ from 5 to 22 g/l of SnCl 2 in the deposition bath. The semiconductor film grown on the glass substrate consisted of n-type CdSnSe semiconductor compounds (alloyed and/or mixed type) in the form of highly dispersed, spherically shaped polycrystallites as detected from X-ray diffraction (XRD) studies, atomic force microscopy (AFM) and scanning electron microscopy (SEM). Their optoelectronic properties were determined by spectroscopic analysis and electrochemical measurements. The performance characteristics of a photoelectrochemical (PEC) cell fabricated with the prepared photo-electrode and ferrocene–ferricenium redox couple in dimethyl formamide were observed under dark and illuminated conditions. The prepared semiconductor films were electrochemically characterized through capacitance–voltage measurements. The film that was obtained from 10 g/l of Sn 2+ in the bath, showed an optimum spectral sensitivity and corresponded to a film thickness of 0.65 μm and stoichiometry of Cd:Sn:Se as 1:1:1. The pronounced PEC activity of this film compared to the others was attributed to the combined effect of space charge properties, electron–hole recombination processes and transfer of charges through the Helmholtz layer at the semiconductor–solution interface.

Supramolecular semiconductor receptor assemblies: Improved electron transfer at TiO2-β-cyclodextrin colloid interfaces

Semiconductor TiO2-colloids are stabilized by s-cyclodextrin (s-CD). Photoinduced electron transfer at the semiconductor (S.C.) solution interface is controlled in the S.C.-receptor assembly TiO2-β-CD. Trapping of TiO2-s-CD photoexcited conduction-band electrons by N,N’-dioctyl-4,4’-bipyridinium, C8V2+, is 4.4 fold more effective than by N,N’-dimethyl-4,4’-bipyridinium. The kinetics of C8V2+ reduction, inhibition experiments and laser flash photolysis studies reveal that the effective trapping of conduction-band electrons by C8V2+ originates from its association to the s-CD receptor. Electron transfer from dye, proflavine, Pfl, to the S.C., is similarly controlled by the S.C.-receptor assembly. In the presence of the Pt modified semiconductor-receptor colloid, Pt-TiO2-s-CD, effective photoinduced H2-evolution occurs using Pfl as photosensitizer. Inhibition experiments and flow-dialysis measurements indicate that the effective H2-evolution originates from the association of Pfl to the s-CD receptor sites.

In-situ Fourier transform infrared spectroscopy of electrochemical processes at the silicon—acetonitrile interface

A new approach is used for the study of the semiconductor—solution interface with Fourier transform IR spectroscopy (FTIRS) in the multiple total internal reflection mode. In this method, a thin semiconductor wafer is mounted on a germanium internal reflection crystal by wafer-bonding. Measurements are possible in a wide frequency range (800–4000 cm −1). The potential of the method is demonstrated by measurements on the anodic oxidation in acetonitrile containing a low concentration water of n-type silicon (111) surfaces pretreated in hydrofluoric acid solution. The experiments show that detection of surface bound species (SiH, SiOSi) is possible with submonolayer sensitivity.

Chemistry with Red and Near Infrared Light

Opportunities are explored to initiate controlled chemistry by photolysis with long-wavelength visible and near infrared light. In one direction, bimolecular reactions are induced by exciting collisional pairs in a solid matrix to energy surfaces well below reactant dissociation limits. Examples discussed include product specific (including stereospecific) photo-oxidation of small alkenes and alkynes by NO2, and cycloaddition reactions of singlet SO and singlet O2. Conducting the chemistry in rare gas matrices allows us to elucidate elementary reaction steps by FT-IR spectroscopy of trapped intermediates, and to gain insight into the dynamics of transients by wavelength-dependent laser photochemistry. In the case of olefin epoxidations, stereochemical details of reaction paths are uncovered by chemical trapping of transient oxirane biradicals in their nascent conformation. State-specific reactions of singlet excited SO and singlet O2 with hydrocarbons illustrate how photons deep in the near infrared can be used for controlled chemical synthesis. In parallel work, excited state redox reactions in homogeneous and colloidal semiconductor solutions are explored that pertain to chemical storage of near infrared quanta and conversion into electrical energy. Examples are studies on chemical storage of singlet delta molecular oxygen (O2(1Δ)), a metastable near infrared energy carrier, its excited state redox chemistry in aqueous solution, and direct reduction of O2(1Δ), at a semiconductor electrode. Using very sensitive time resolved optical techniques, new mechanistic insight is gained on singlet O2 reactions, and on photo-oxidation of halide at the semiconductor-solution interface.

Photoelectrochemical dissociation of water at silicon doped n-GaAs electrodes

The photoelectrochemical properties of silicon-doped n-GaAs were studied in aqueous solution of different pH values. The photosensitivity is higher in alkaline than in acidic solutions. However, visual photocorrosion, and deterioration of photocurrent could be observed after few hours of illumination in 0.1 M NaOH solution. The corrosion was decreased by coating the GaAs electrode with a 50 Å thick gold layer. If GaAs was used as an anode in an electrochemical cell, and illuminated with visible light ( 4200 A ̊ < λ < 6000 A ̊ ), photocurrent was observed due to the generation of H 2 over a Pt counter electrode. The photocurrent generated in 0.1 M NaOH solution has an onset potential of −1.1 V (NHE). Furthermore, it saturated at a potential of −0.3 V (NHE). Both the rate of H 2-production and photocurrent varied linearly with light intensity. The quantum efficiency for the hydrogen production is 0.4 hydrogen atom per second per photogenerated electron-hole pair. Our results are explained in the light of the band model of the semiconductor-solution interface.

Evidence for adduct formation at the semiconductor-solution interface: photoluminescent properties of cadmium selenide in the presence of lanthanide .beta.-diketonate complexes

Photoluminescence (PL) measurements of etched, single-crystal n-CdSe demonstrate that the semiconductor surface engages in adduct formation with a family of lanthanide {beta}-diketonate complexes, Ln(fod){sub 3} (Ln = lanthanide; fod = 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato anion), in isooctane ambient.

Intensity modulated photocurrent spectroscopy of n‐GaAs

AbstractSurface recombination at n‐GaAs photoelectrodes in alkaline selenide solutions has been investigated by intensity modulated photocurrent spectroscopy (IMPS). The technique involves sinusoidal modulation of the incident photon flux and frequency response analysis of the photocurrent. Recombination processes give rise to characteristic changes in the phase and amplitude of the periodic photocurrent, and the time constants for the relaxation of carriers in surface states can be obtained conveniently from analysis in the complex plane. The method can detect shifts in the band edge energies that result from changes in the surface dipole at the semiconductor solution interface, and the results indicate that the band edge energies of GaAs vary with electrode potential and illumination intensity. The effect of ruthenium treatment on surface recombination has also been explored by the technique.

A model for electron transfer at the illuminated p-type semiconductor-solution interface

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTA model for electron transfer at the illuminated p-type semiconductor-solution interfaceShahed U. M. Khan and John O'M. BockrisCite this: J. Phys. Chem. 1984, 88, 12, 2504–2515Publication Date (Print):June 1, 1984Publication History Published online1 May 2002Published inissue 1 June 1984https://doi.org/10.1021/j150656a016RIGHTS & PERMISSIONSArticle Views307Altmetric-Citations31LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts

Hydrogen-Evolving Solar Cells

Sunlight is directly converted to chemical energy in hydrogen-evolving photoelectrochemical cells with semiconductor electrodes. Their Gibbs free energy efficiency of solar-to-hydrogen conversion, 13.3 percent, exceeds the solar-to-fuel conversion efficiency of green plants and approaches the solar-to-electrical conversion efficiency of the best p-n junction cells. In hydrogen-evolving photoelectrodes, electron-hole pairs photogenerated in the semiconductor are separated at electrical microcontacts between the semiconductor and group VIII metal catalyst islands. Conversion is efficient when the island diameters are small relative to the wave-lengths of sunlight exciting the semiconductor; when the island spacings are smaller than the diffusion length of electrons at the semiconductor surface; when the height of the potential energy barriers that separate the photogenerated electrons from holes at the semiconductor surface is raised by hydrogen alloying of the islands; when radiationless recombination of electron-hole pairs at the semiconductor-solution interface between the islands is suppressed by controlling the semiconductor surface chemistry; and when the semiconductor has an appropriate band gap (1.0 to 1.8 electron volts) for efficient solar conversion.