Semiconductor Electrocatalyst Interfaces: Theory, Experiment,...
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Semiconductor−Electrocatalyst Interfaces: Theory, Experiment, and Applications in Photoelectrochemical Water Splitting Michael R. Nellist,† Forrest A. L. Laskowski,† Fuding Lin, Thomas J. Mills,‡ and Shannon W. Boettcher*
Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403, United States
CONSPECTUS: Light-absorbing semiconductor electrodes coated with electrocatalysts are key components of photo- electrochemical energy conversion and storage systems. Efforts to optimize these systems have been slowed by an inadequate understanding of the semiconductor−electrocatalyst (sem|cat) interface. The sem|cat interface is important because it separates and collects photoexcited charge carriers from the semi- conductor. The photovoltage generated by the interface drives “uphill” photochemical reactions, such as water splitting to form hydrogen fuel. Here we describe efforts to understand the microscopic processes and materials parameters governing interfacial electron transfer between light-absorbing semi- conductors, electrocatalysts, and solution. We highlight the properties of transition-metal oxyhydroxide electrocatalysts, such as Ni(Fe)OOH, because they are the fastest oxygen-evolution catalysts known in alkaline media and are (typically) permeable to electrolyte. We describe the physics that govern the charge-transfer kinetics for different interface types, and show how numerical simulations can explain the response of composite systems. Emphasis is placed on “limiting” behavior. Electrocatalysts that are permeable to electrolyte form “adaptive” junctions where the interface energetics change during operation as charge accumulates in the catalyst, but is screened locally by electrolyte ions. Electrocatalysts that are dense, and thus impermeable to electrolyte, form buried junctions where the interface physics are unchanged during operation. Experiments to directly measure the interface behavior and test the theory/simulations are challenging because conventional photoelectrochemical techniques do not measure the electrocatalyst potential during operation. We developed dual-working- electrode (DWE) photoelectrochemistry to address this limitation. A second electrode is attached to the catalyst layer to sense or control current/voltage independent from that of the semiconductor back ohmic contact. Consistent with simulations, electrolyte-permeable, redox-active catalysts such as Ni(Fe)OOH form “adaptive” junctions where the effective barrier height for electron exchange depends on the potential of the catalyst. This is in contrast to sem|cat interfaces with dense electrolyte- impermeable catalysts, such as nanocrystalline IrOx, that behave like solid-state buried (Schottky-like) junctions. These results elucidate a design principle for catalyzed photoelectrodes. The buried heterojunctions formed by dense catalysts are often limited by Fermi-level pinning and low photovoltages. Catalysts deposited by “soft” methods, such as electrodeposition, form adaptive junctions that tend to provide larger photovoltages and efficiencies. We also preview efforts to improve theory/ simulations to account for the presence of surface states and discuss the prospect of carrier-selective catalyst contacts.
■ INTRODUCTION High-efficiency photoelectrochemical water-splitting systems require integrating electrocatalysts (cat) onto light-absorbing semiconductors (sem). Despite the central role that the sem|cat interface plays in collecting one carrier over the other and generating photovoltage, the energetics and charge-transfer processes at catalyzed semiconductor interfaces are poorly understood. A simple picture is that the semiconductor absorbs light and separates charge while the catalyst increases the rate of the hydrogen- or oxygen-evolution reaction (HER or OER, respectively). Experiments by different groups, however, show that, after deposition of OER catalysts onto n-type semi- conductors, the photoelectrode characteristics (e.g., the photo- voltage, photocurrent, and fill-factor) change in a way often inconsistent with this view.1,2 Parallel hypotheses have attributed this behavior to changes in surface recombination,3,4
band bending,5 interface-charge trapping,6 optical effects,7 or kinetics.8−11 Several factors make unravelling these different effects difficult. First, electrocatalysts are not well-defined electronic materials (e.g., a metal or semiconductor), but are often porous, hydrated, and redox-active solids. How does one describe such nontraditional electronic interfaces? Second, most of the semiconductor systems that have been studied are polycrystalline and/or nanostructured, which makes interpret- ing elementary processes difficult. Third, there is a lack of experimental tools to directly measure the interfacial processes. In this Account we discuss our use of simulation and new
photoelectrochemical experiments to clarify the microscopic details of electron transfer in catalyzed water-oxidizing
Received: January 2, 2016 Published: April 1, 2016
© 2016 American Chemical Society 733 DOI: 10.1021/acs.accounts.6b00001 Acc. Chem. Res. 2016, 49, 733−740
photoelectrodes. We connect the microscopic processes to the observable current−voltage responses, and discuss possible design principles for high-performance systems. Figure 1 shows basic processes in a catalyzed photoelectrode.
The semiconductor, catalyst, and solution are all characterized
by electrochemical potentials (Fermi levels) which equilibrate in the dark (Ef,n, Ecat, and Esol, respectively). Under illumination the concentration of minority holes increases and thus the hole quasi-Fermi level (Ef,p) drops down from the electron level (Ef,n) to create a photovoltage Vph at the sem|cat interface. During steady-state photodriven oxygen evolution, Ecat is driven lower on the electron energy scale (more positive on the electrochemical scale) than Esol (the thermodynamic oxygen potential), such that there is a net positive current from catalyst to solution. The degree to which Ef,p separates from Ef,n at the semiconductor surface is governed by the relative forward and reverse rates of electron and hole transport at the sem|cat interface in addition to the rates of bulk recombination (Rb) and generation (G). The hole current density is given by Jp = μpp∇Ef,p, where μp is the hole mobility and ∇Ef,p is the hole quasi-Fermi-level gradient. Traditional photoelectrochemical measurements use an
ohmic contact to the back of the semiconductor (i.e., the left side of the diagram in Figure 1) to sweep the semiconductor potential Esem (which is also the majority-carrier Fermi level Ef,n in the bulk), and measure the resulting current in both the light and dark. It is difficult from such measurements to determine how the individual charge-transfer, catalysis, and recombination steps affect the J−V response. First, it is not possible to determine which portion of the total applied potential (i.e., qVapp = Esem − Esol) drops at the sem|cat interface versus at the cat|sol interface because one cannot determine Ecat. Further, the current measured is the sum of the net electron and hole currents and it is not possible to distinguish whether the holes or electrons flow into the catalyst or directly into the solution. A number of techniques have been used to augment
traditional photoelectrochemical measurement. Transient absorption spectroscopies12 provide insight into the various recombination processes,5,6 though data interpretation is complicated by the pulsed-laser excitation. Photoelectrodes operate at steady state under low light intensity. Methods based on impedance are powerful13 but rely on fitting equivalent circuits, which are complicated for multicomponent systems. Here we describe alternative methods that provide direct information about the interface, as well as theory and simulation to corroborate the measurements.
■ MATERIALS: SEMICONDUCTORS AND ELECTROCATALYSTS
Among device geometries proposed for a solar-water-splitting system, one compelling option employs two semiconductors in series, with different bandgaps, to absorb different portions of the solar spectrum.14 One semiconductor, operating as a photoanode, drives water oxidation to form O2(g), while the other, operating as a photocathode, drives water reduction to form H2(g). Electrocatalysts decorate both semiconductors to increase the kinetics of the fuel-forming reactions. While the sem|cat interface is important in both, we focus here on the photoanode.
Oxides, such as Fe2O3, BiVO4, and WO3, have been studied extensively as water-oxidizing photoanodes, in part because they can be simply made and, being already oxidized, are reasonably stable under the appropriate-pH OER conditions.15
The oxides are typically polycrystalline and the sem|cat interface thus likely nonuniform. Recently, there has been a revived interest in using thin oxide films to stabilize n-Si and n- GaAs photoanodes which have superior electronic properties (mobility, carrier lifetime) but corrode under anodic con- ditions.16 Fabrication of high-quality pn junctions, that provide for large photovoltages, is straightforward on Si/GaAs. For oxide photoelectrodes there are limited methods to fabricate solid-state pn junctions; tuning the properties of the sem|cat interface is therefore particularly important.
To understand the interface, it is critical to understand the electrocatalyst’s electronic and electrochemical properties. In the simplest case the catalyst is a dense solid with high electrical conductivity (e.g., a metal or deg