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In Situ Quantification of Surface Intermediates and Correlation to Discharge Products on Hematite Photoanodes using a Combined Scanning Electrochemical Microscopy Approach Mihail R Krumov, Burton H. Simpson, Michael J Counihan, and Joaquin Rodriguez-Lopez Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04896 • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 3, 2018
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Analytical Chemistry
In Situ Quantification of Surface Intermediates and Correlation to Discharge Products on Hematite Photoanodes using a Combined Scanning Electrochemical Microscopy Approach Mihail R. Krumov, &,‡ Burton H Simpson, &,‡,† Michael J. Counihan&, Joaquín RodríguezLópez&,#,* &
Department of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, IL 61801*Corresponding author e-mail:
[email protected]; Phone: +1 (217) 300-7354. #
Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign
ABSTRACT: Hematite is a promising photoanode for solar driven water splitting. Elucidating its surface chemical pathways is key to improving its performance. Here, we use redox titrations in the Surface Interrogation mode of Scanning Electrochemical Microscopy (SI-SECM) to quantitatively probe in situ the reactivity and time evolution of surface species formed on hematite during photo assisted water oxidation. Using SI-SECM, two distinct populations of oxidizing surface species were resolved with measured ksi of 316 m3/(mol·s) and 2 m3/(mol·s) for the more and less reactive species respectively. While the surface coverage of both species was found to increase as a function of applied bias, the rate constants did not change appreciably, suggesting that the mechanism of water oxidation is independent of bias potential. In the absence of applied potential, both populations exhibit decay that is well described by second order kinetics, with kd values of 1.2×105 ± 0.2×105 and 6.3×103 ± 0.9×103 m2/(mol·s) for the fast and slow reacting adsorbates, respectively. Using transient substrate generation/ tip collection mode, we detected the evolution of as much as 1.0 µmol/m2 of H2O2 during this decay process, which correlates with the coverage observed by one of the titrated species. By deconvoluting the reactivity of multiple adsorbed reactants, these experiments demonstrate how SI-SECM enables direct observation of multiple adsorbates and reaction pathways on operating photoelectrodes.
Hematite is a promising photoanode for solar driven water splitting because of its narrow band gap, chemical stability and elemental abundance.1,2 However, even the best performing hematite photocatalysts exhibit efficiencies far below the theoretical maximum.3 Slow water oxidation kinetics that take seconds to complete a full catalytic cycle are a major factor contributing to its poor performance.4 Photocatalytic water oxidation is a complex electron and proton transfer process that proceeds through reactive surface states (RSS) of hematite.4 These surface states are often reactive oxygen species (ROS) that are direct intermediates of the water oxidation reaction, but these states can also be parasitic surface species that trap holes and serve as recombination centers.4 Because RSSs are crucial to the water oxidation reaction, elucidating their nature is central to strategies aimed at enhancing hematite’s photocatalytic performance. The need to fill this knowledge gap has led to several recent works elucidating the surface species involved in water oxidation on hematite. Studies utilizing photoelectrochemical impedance spectroscopy5-7 or derivatives such as intensity modulated photocurrent spectroscopy8-10 have observed an accumulation of surface states during
water oxidation on hematite that were proposed to be Fe (IV) and Fe(V) states in the form of M-OHx species. Klahr, et. al. observed that the decay kinetics of the accumulated states were consistent with the formation of Fe(IV) species that react bimolecularly to generate O2 and that a critical coverage of these species was necessary for the onset of O2 evolution.7 Transient absorption spectroscopy also shows evidence of long-lived trapped holes on the surface of hematite that appear to be intermediates of water oxidation.11-13 It was only recently that Zandi, et. al. reported their use of operando infrared spectroscopy to positively identify FeIV=O one form of the long-lived holes.14 This study also observed the presence of an additional photogenerated species hypothesized to be FeIV-OFeIV or FeIV-O-O-FeIV, but were unable to unambiguously identify this species or rule out M-OHx species. Despite identifying FeIV=O as a key intermediate of water oxidation on hematite, Zandi concluded that further investigation is needed to resolve both the identities and kinetics of other surface species involved in water oxidation on hematite. Although the aforementioned studies established FeIV=O as a key intermediate of water oxidation on hema-
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tite, a more thorough understanding of the interfacial reactivity of surface species can better elucidate the role each species plays in the overall reactive mechanism. Knowing whether these species cooperate to oxidize water or represent intermediates of mechanisms competing for active sites will inform attempts to produce better hematite photoelectrodes. The techniques employed previously to probe decay kinetics at hematite surfaces lacked selectivity for surface species, provided limited quantification of coverage, and did not offer a direct measurement of reactivity. Scanning electrochemical microscopy (SECM) is a versatile electrochemical platform used in the characterization of electrocatalysts and photoelectrodes to collect products15,16 and measure electron transfer kinetics.17,18 Recently, the surface interrogation mode of SECM (SI-SECM) was introduced to probe reactive intermediates at catalytic surfaces,19 and our group and others have since demonstrated its use for detection of ROS on operating photoelectrode surfaces.20-24 SISECM uses a redox mediator to redox titrate species on substrates of interest, and enables the quantification of the surface coverage,25 reaction kinetics, and spatial distribution24 of adsorbed chemical intermediates of (electro)catalytic reactions.
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species, which can be from disproportionation reactions between hydroxyl radicals on the surface. As oxo species are considered to be the primary intermediate in water oxidation on hematite, these results suggest how side reactions might contribute to the low efficiency of hematite photoanodes.
SI-SECM is performed in two steps, as shown in Figure 1A. In the first step, light and/or external bias are applied to the substrate to initiate catalysis and populate the surface with intermediates. After this, the driving force is removed and a small tip electrode positioned close to the surface is activated to electrochemically generate a titrant in situ from a mediator in solution. The titrant diffuses to the substrate and reacts with redox-active surface species. This regenerates the mediator, producing a transient positive feedback current, shown in Figure 1B, that decays as the surface species are consumed. A background signal, where no catalysis occurs, is recorded by repeating the experiment without illuminating the substrate. Background subtraction removes contributions from charging current, other solution processes, and any non PEC surface process from the tip current. Because it is selective to surface species, quantitative for both surface coverage and reactivity, and operates over the large range of spatial and temporal scales relevant to the kinetics of water oxidation,23 SI-SECM is ideal for studying adsorbed reaction intermediates formed on hematite. Here, we use the multimodal functionality of SECM to investigate the role of RSS in the chemistry of water oxidation on thin film samples of hematite. Two distinct populations of photogenerated RSS were identified by their drastically differing reaction kinetics with our titrant species. Both species show potentialdependent coverages that increase proportionally with photocurrent, suggesting that both are related to water oxidation activity on the hematite surface. Delayed titrations showed that both populations decayed over time following second order kinetics with rate constants differing by an order of magnitude. These results prompted transient collection experiments that detected small quantities of H2O2 generated during the decay of these
Figure 1: (A) Schematic of SI-SECM. In the first step, the substrate is activated through illumination and/or external bias to initiate catalysis, generating adsorbed intermediates. The probe remains inactive. In the second step, bias and illumination are removed so PEC processes cease at the substrate. The SECM probe is biased to activate an electrochemical mediator which diffuses to the surface and titrates the adsorbate in situ. (B) Titration of adsorbate generates a transient feedback loop of mediator which is recorded by the tip as a current signal. A dark run is performed for background subtraction to produce the interrogation current. (C) Block diagram of the SECM and illumination setup.
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Analytical Chemistry
EXPERIMENTAL SECTION Chemicals Fluorine doped tin oxide (FTO) coated glass slides (Rs
