Principles and Applications of Semiconductor Photoelectrochemistry

Light Conversion by Electrochemiluminescence at Semiconductor Surfaces

ConspectusElectrochemiluminescence (ECL) is the electrochemical generation of light. It involves an interfacial charge transfer that produces the excited state of a luminophore at the electrode surface. ECL is a powerful readout method that is widely employed for immunoassays and clinical diagnostics and is progressively evolving into a microscopy technique. On the other hand, photoelectrochemistry at illuminated semiconductors is a field of research that deals with the transfer of photogenerated charge carriers at the solid-liquid interface. This concept offers several advantages such as a considerable lowering of the onset potential required for triggering an electrochemical reaction as well as light addressable chemistry, via the spatial confinement of redox reactions at locally illuminated semiconductor electrodes. The combination of ECL with photoelectrochemistry at illuminated semiconductors is termed photoinduced ECL (PECL). It deals with the triggering of an ECL reaction through the transfer of photogenerated minority charge carriers at the illuminated solid/liquid interface. PECL results in the conversion of incident photons (λexc), that are absorbed by the semiconductor photoelectrode to emitted photons (λPECL), produced by the ECL reaction. Although demonstrated in the 1970s by Bard et al. in ultradry organic solvents, PECL remained unexplored until the last five years. Nowadays, as a result of the considerable progress achieved in semiconductor photoelectrodes and ECL systems, a large variety of PECL systems can be designed by combining photoelectrode materials with ECL luminophores, making it a versatile tool for light conversion in aqueous media.In this Account, we introduce the fundamentals of ECL and photoelectrochemistry at illuminated semiconductors and review the recent developments in PECL. We discuss the two main PECL light conversion schemes: downconversion (where λexc < λPECL) and upconversion (where λexc > λPECL). Besides, PECL can be used to simplify considerably the common electrochemical setups employed for ECL. Indeed, by engineering the photoelectrode material and carefully considering the reactivity involved for ECL and its counter-reaction, PECL enables the ultimate concept of all-optical ECL (AO-ECL), i.e., ECL generation at an illuminated monolithic device immersed into the electrolyte solution. As discussed in this Account, AO-ECL is an important breakthrough that allows the simplest ECL experimental configuration ever reported, eliminating constraints such as an electrical power supply, wires, electrodes, connections, and specific electrochemical knowledge. As shown at the end of this Account, due to the robustness of recently manufactured PECL systems, several applications can already be envisioned for microscopy, elucidation of solar conversion mechanisms, near-infrared imaging, and bioanalysis.

Electrochemical Impedance of Well-Passivated Semiconductors Reveals Bandgaps, Fermi Levels, and Interfacial Density of States

For well-passivated semiconductor materials, the density of states (DOS) at the band edge determines the concentration of electrons (or holes) available to participate in photo/electrochemical redox and chemical reactions. Electrochemical impedance enables the characterization of photo-electrode DOS in a functional, in situ, electrochemical environment. However, the in situ electrochemical approach remains underutilized for band structure characterization of inorganic semiconductors. In this work, we demonstrate that the DOS of the well-passivated, highly ordered semiconductors silicon and germanium is directly probed by electrochemical impedance spectroscopy (EIS). More specifically, EIS measurements of the chemical capacitance in contact with electrolyte enable direct analysis of the DOS properties. From the capacitance-potential plot, the following parameters can be extracted: Fermi level, valence band maximum, conduction band minimum, and a quantitative value of the number of states at each potential. This study aims to establish the groundwork for future EIS investigations of electronically modified semiconductor interfaces with covalently bound organic molecules, organometallic catalysts, or more complex biorelated functionalizations.

In Situ Analytical Techniques for the Investigation of Material Stability and Interface Dynamics in Electrocatalytic and Photoelectrochemical Applications

Electrocatalysis and photoelectrochemistry are critical to technologies like fuel cells, electrolysis, and solar fuels. Material stability and interfacial phenomena are central to the performance and long-term viability of these technologies. Researchers need tools to uncover the fundamental processes occurring at the electrode/electrolyte interface. Numerous analytical instruments are well-developed for material characterization, but many are ex situ techniques often performed under vacuum and without applied bias. Such measurements miss dynamic phenomena in the electrolyte under operational conditions. However, innovative advancements have allowed modification of these techniques for in situ characterization in liquid environments at electrochemically relevant conditions. This review explains some of the main in situ electrochemical characterization techniques, briefly explaining the principle of operation and highlighting key work in applying the method to investigate material stability and interfacial properties for electrocatalysts and photoelectrodes. Covered methods include spectroscopy (in situ UV-vis, ambient pressure X-ray photoelectron spectroscopy (APXPS), and in situ Raman), mass spectrometry (on-line inductively coupled plasma mass spectrometry (ICP-MS) and differential electrochemical mass spectrometry (DEMS)), and microscopy (in situ transmission electron microscopy (TEM), electrochemical atomic force microscopy (EC-AFM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical microscopy (SECM)). Each technique's capabilities and advantages/disadvantages are discussed and summarized for comparison.

Design Principles and Developments of Integrated Solar Flow Batteries

ConspectusDue to the intermittent nature of sunlight, practical round-trip solar energy utilization systems require both efficient solar energy conversion and inexpensive large-scale energy storage. Conventional round-trip solar energy utilization systems typically rely on the combination of two or more separated devices to fulfill such requirements. Integrated solar flow batteries (SFBs) are a new type of device that integrates solar energy conversion and electrochemical storage. In SFBs, the solar energy absorbed by photoelectrodes is converted into chemical energy by charging up redox couples dissolved in electrolyte solutions in contact with the photoelectrodes. To deliver electricity on demand, the reverse redox reactions are carried out to release chemical energy stored in redox couples as one would do in the discharge of a normal redox flow battery (RFB). The integrated design of SFBs enables all the functions demanded by round trip solar energy utilization systems to be realized within a single device. Leveraging rapidly developing parallel technologies of photovoltaic solar cells and RFBs, significant progress in the field of SFBs has been made in the past few years. This Account aims to provide a general reference and tutorial for researchers who are interested in SFBs, and to describe the design principles and thus facilitate the development of this nascent field.The operation principle of SFBs is built on the working mechanism of RFBs and photoelectrochemical (PEC) cells, so we first describe the basic concept and important features of RFBs and redox couples with the emphasis on the quantitative understanding of RFB cell potentials. We also introduce different types of PEC cells and highlight two different photoelectrode designs that are commonly seen in SFB literature: simple semiconductor photoelectrodes and PV cell photoelectrodes. A set of experimental protocols for characterizing the redox couples, RFBs, photoelectrodes, and SFBs are presented to promote comparable assessment and discussion of important figures of merits of SFBs.Solar-to-output electricity efficiency (SOEE) defines the round trip energy efficiency of SFBs and has received substantial research attention. We introduce a quantitative simulation method to find the relationship between the SOEE and cell potential of SFBs and reveal the design principles for highly efficient SFBs. Several other important performance metrics of SFBs are also introduced. Then we review the historical development of SFBs and identify the state-of-the-art demonstrations at each development stage with more emphasis on our own research efforts in developing SFBs built with PV photoelectrodes. Finally, we preview some promising future directions and the challenges for advancing both the scientific understanding and practical applications of SFBs.

Single nanoparticle photoelectrochemistry: What is next?

Semiconductor photoelectrochemistry is a fascinating field that deals with the chemistry and physics of photodriven reactions at solid/liquid interfaces. The interdisciplinary field attracts (electro)chemists, materials scientists, spectroscopists, and theorists to study fundamental and applied problems such as carrier dynamics at illuminated electrode/electrolyte interfaces and solar energy conversion to electricity or chemical fuels. In the pursuit of practical photoelectrochemical energy conversion systems, researchers are exploring inexpensive, solution-processed semiconductor nanomaterials as light absorbers. Harnessing the enormous potential of nanomaterials for energy conversion applications requires a fundamental understanding of charge carrier generation, separation, transport, and interfacial charge transfer at heterogeneous nanoscale interfaces. Our current understanding of these processes is derived mainly from ensemble-average measurements of nanoparticle electrodes that report on the average behavior of trillions of nanoparticles. Ensemble-average measurements conceal how nanoparticle heterogeneity (e.g., differences in particle size, shape, and surface structure) contributes to the overall photoelectrochemical response. This perspective article focuses on the emerging area of single particle photoelectrochemistry, which has opened up an exciting new frontier: direct investigations of photodriven reactions on individual nanomaterials, with the ability to elucidate the role of particle-dependent properties on the photoelectrochemical behavior. Here, we (1) review the basic principles of photoelectrochemical cells, (2) point out the potential advantages and differences between bulk and nanoelectrodes, (3) introduce approaches to single nanoparticle photoelectrochemistry and highlight key findings, and (4) provide our perspective on future research directions.

Geometrical and Chemical-Dependent Hydrolysis Mechanisms of Silicon Nanomembranes for Biodegradable Electronics

Biodegradable electronic devices that physically disappear in physiological or environmental solutions are of critical importance for widespread applications in healthcare management and environmental sustainability. The precise modulation of materials and devices dissolution with on-demand operational lifetime, however, remain a key challenge. Silicon nanomembranes (Si NMs) are one of the essential semiconductor components for high-performance biodegradable electronics at the system level. In this work, we discover unusual hydrolysis behaviors of Si NMs that are significantly dependent on the dimensions of devices as well as their surface chemistry statuses. The experiments show a pronounced increase in hydrolysis rates of p-type Si NMs with larger sizes, and mechanical stirring introduces a significant decrease in dissolution rates. The presence of phosphates and potassium ions in solutions, or lower dopant levels of Si NMs will facilitate the degradation of Si NMs and will also lead to a stronger size-dependent effect. Molecular dynamics simulations are performed to reveal ion adsorption mechanisms of Si NMs under different surface charge statuses and confirm our experimental observations. Through geometrical designs, Si NM-based electrode arrays with tunable dissolution lifetime are formed, and their electrochemical properties are analyzed in vitro. These results offer new controlling strategies to modulate the operational time frames of Si NMs through geometrical design and surface chemistry modification and provide crucial fundamental understandings for engineering high-performance biodegradable electronics.

Computational insights into charge transfer across functionalized semiconductor surfaces

Photoelectrochemical water-splitting is a promising carbon-free fuel production method for producing H₂ and O₂ gas from liquid water. These cells are typically composed of at least one semiconductor photoelectrode which is prone to degradation and/or oxidation. Various surface modifications are known for stabilizing semiconductor photoelectrodes, yet stabilization techniques are often accompanied by a decrease in photoelectrode performance. However, the impact of surface modification on charge transport and its consequence on performance is still lacking, creating a roadblock for further improvements. In this review, we discuss how density functional theory and finite-element device simulations are reliable tools for providing insight into charge transport across modified photoelectrodes.

Surviving High-Temperature Calcination: ZrO2 -Induced Hematite Nanotubes for Photoelectrochemical Water Oxidation

Nanotubular Fe₂ O₃ is a promising photoanode material, and producing morphologies that withstand high-temperature calcination (HTC) is urgently needed to enhance the photoelectrochemical (PEC) performance. This work describes the design and fabrication of Fe₂ O₃ nanotube arrays that survive HTC for the first time. By introducing a ZrO₂ shell on hydrothermal FeOOH nanorods by atomic layer deposition, subsequent high-temperature solid-state reaction converts FeOOH-ZrO₂ nanorods to ZrO₂ -induced Fe₂ O₃ nanotubes (Zr-Fe₂ O₃ NTs). The structural evolution of the hematite nanotubes is systematically explored. As a result of the nanostructuring and shortened charge collection distance, the nanotube photoanode shows a greatly improved PEC water oxidation activity, exhibiting a photocurrent density of 1.5 mA cm^-2 at 1.23 V (vs. reversible hydrogen electrode, RHE), which is the highest among hematite nanotube photoanodes without co-catalysts. Furthermore, a Co-Pi decorated Zr-Fe₂ O₃ NT photoanode reveals an enhanced onset potential of 0.65 V (vs. RHE) and a photocurrent of 1.87 mA cm^-2 (at 1.23 V vs. RHE).

Charge Recombination Control for High Efficiency Quantum Dot Sensitized Solar Cells

Benefiting from the unique excellent optoelectronic properties of quantum dot light absorbers, quantum dot sensitized solar cell (QDSCs) are a promising candidate for the low-cost third-generation solar cells. Over the past few years, the power conversion efficiency (PCE) of QDSCs presents a rapid evolution from less than 1% to beyond 8%. Charge recombination is regarded as one of the most significant factors in limiting the photovoltaic performance of QDSCs. A significant improvement in the PCE of QDSCs has been obtained by charge recombination control. Some effective routes to suppress charge recombination processes, such as adopting preprepared high-quality QD sensitizers, tailoring the electronic properties of QDs, and interface engineering with the use of organic or inorganic thin layer overcoating the sensitized photoanode have been overviewed in this perspective. Also, the possible accesses to better performance (higher efficiency and stability) of the QDSCs have been proposed on the basis of achievements obtained previously.

Ordered arrays of tilted silicon nanobelts with enhanced solar hydrogen evolution performance

Well-ordered tilted silicon nanobelt arrays have been fabricated over a large area (≥2.5 cm(2)) by metal assisted chemical etching of pre-patterned silicon, which demonstrated markedly enhanced solar hydrogen evolution performance, compared with planar silicon of the same type and previously reported silicon nanowires prepared in a similar way.

Atomic layer deposition of a submonolayer catalyst for the enhanced photoelectrochemical performance of water oxidation with hematite

Hematite photoanodes were coated with an ultrathin cobalt oxide layer by atomic layer deposition (ALD). The optimal coating-1 ALD cycle, which amounts to <1 monolayer of Co(OH)2/Co3O4-resulted in significantly enhanced photoelectrochemical water oxidation performance. A stable, 100-200 mV cathodic shift in the photocurrent onset potential was observed that is correlated to an order of magnitude reduction in the resistance to charge transfer at the Fe2O3/H2O interface. Furthermore, the optical transparency of the ultrathin Co(OH)2/Co3O4 coating establishes it as a particularly advantageous treatment for nanostructured water oxidation photoanodes. The photocurrent of catalyst-coated nanostructured inverse opal scaffold hematite photoanodes reached 0.81 and 2.1 mA/cm(2) at 1.23 and 1.53 V, respectively.

Semiconductor photocatalysis--mechanistic and synthetic aspects

Preceding work on photoelectrochemistry at semiconductor single-crystal electrodes has formed the basis for the tremendous growth in the three last decades in the field of photocatalysis at semiconductor powders. The reason for this is the unique ability of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a large variety of electron-donor and -acceptor substrates. Whereas great attention was paid to water splitting and the exhaustive aerobic degradation of pollutants, only a small amount of research also explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts as well as the investigation of reaction mechanisms are discussed. Novel atom-economic C-C and C-N coupling reactions illustrate the relevance of semiconductor photocatalysis for organic synthesis, and demonstrate that the multidisciplinary field combines classical photochemistry with electrochemistry, solid-state chemistry, and heterogeneous catalysis.

CdS nanoparticles sensitization of Al-doped ZnO nanorod array thin film with hydrogen treatment as an ITO/FTO-free photoanode for solar water splitting

Aluminum-doped zinc oxide (AZO) nanorod array thin film with hydrogen treatment possesses the functions of transparent conducting oxide thin film and 1-D nanostructured semiconductor simultaneously. To enhance the absorption in the visible light region, it is sensitized by cadmium sulfide (CdS) nanoparticles which efficiently increase the absorption around 460 nm. The CdS nanoparticles-sensitized AZO nanorod array thin film with hydrogen treatment exhibits significantly improved photoelectrochemical property. After further heat treatment, a maximum short current density of 5.03 mA cm-2 is obtained under illumination. They not only are much higher than those without CdS nanoparticles sensitization and those without Al-doping and/or hydrogen treatment, but also comparable and even slightly superior to some earlier works for the CdS-sensitized zinc oxide nanorod array thin films with indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) as substrates. This demonstrated successfully that the AZO nanorod array thin film with hydrogen treatment is quite suitable as an ITO/FTO-free photoanode and has great potentials in solar water splitting after sensitization by quantum dots capable of visible light absorption.

The electrochemistry of nanostructured titanium dioxide electrodes

Several of the multiple applications of titanium dioxide nanomaterials are directly related to the introduction or generation of charge carriers in the oxide. Thus, electrochemistry plays a central role in the understanding of the factors that must be controlled for the optimization of the material for each application. Herein, the main conceptual tools needed to address the study of the electrochemical properties of TiO(2) nanostructured electrodes are reviewed, as well as the electrochemical methods to prepare and modify them. Particular attention is paid to the dark electrochemical response of these nanomaterials and its direct connection with the TiO(2) electronic structure, interfacial area and grain boundary density. The physical bases for the generation of currents under illumination are also presented. Emphasis is placed on the fact that the kinetics of charge-carrier transfer to solution determines the sign and value of the photocurrent. Furthermore, methods for extracting kinetic information from open-circuit potential and photocurrent measurements are briefly presented. Some aspects of the combination of electrochemical and spectroscopic measurements are also dealt with. Finally, some of the applications of TiO(2) nanostructured samples derived from their electrochemical properties are concisely reviewed. Particular attention is paid to photocatalytic processes and, to a lesser extent, to photosynthetic reactions as well as to applications related to energy from the aspects of both saving (electrochromic layers) and accumulation (batteries). The use of TiO(2) nanomaterials in solar cells is not covered, as a number of reviews have been published addressing this issue.