Adaptive semiconductor/electrocatalyst junctions in water-splitting photoanodes

Journal name:
Nature Materials
Volume:
13,
Pages:
81–86
Year published:
DOI:
doi:10.1038/nmat3811
Received
Accepted
Published online

Abstract

High-efficiency photoelectrochemical water-splitting devices require the integration of electrocatalysts (ECs) with light-absorbing semiconductors (SCs), but the energetics and charge-transfer processes at SC/EC interfaces are poorly understood. We fabricate model EC-coated single-crystal TiO2 electrodes and directly probe SC/EC interfaces in situ using two working electrodes to independently monitor and control the potential and current at both the SC and the EC. We discover that redox-active ion-permeable ECs such as Ni(OH)2 or NiOOH yield ‘adaptive’ SC/EC junctions where the effective Schottky barrier height changes in situ with the oxidation level of the EC. In contrast, dense, ion-impermeable IrOx ECs yield constant-barrier-height ‘buried’ junctions. Conversion of dense, thermally deposited NiOx on TiO2 into ion-permeable Ni(OH)2 or NiOOH correlated with increased apparent photovoltage and fill factor. These results provide new insight into the dynamic behaviour of SC/EC interfaces to guide the design of efficient SC/EC devices. They also illustrate a new class of adaptive semiconductor junctions.

At a glance

Figures

  1. Conventional single-working-electrode J–V data for TiO2/EC samples and schematics of the DWE experiment.
    Figure 1: Conventional single-working-electrode JV data for TiO2/EC samples and schematics of the DWE experiment.

    a, Evolution of illuminated JV curves on repetitive cycling of TiO2/NiOx and TiO2/IrOx electrodes at a scan rate of 100 mV s−1 and 50 mV s−1, respectively, in aqueous 0.1 M KOH under 100 mW cm−2 of AM1.5G illumination. The cycle number is indicated for each curve. The semiconductor potential Vsem is referenced to the thermodynamic redox potential for the OER, . JV curves collected for TiO2/NiOx samples at 10 mV s−1 are shown in Supplementary Fig. 1. b, Device schematic showing independent electrical connections to the SC and EC. c, Band-bending diagram and wiring schematic for DWE PEC analysis. The TiO2 potential Vsem and the EC potential Vcat are controlled relative to by the primary working electrode (WE1) and the secondary working electrode (WE2), respectively, of a bi-potentiostat (using a single Hg/HgO reference electrode). Ec and Ev are the conduction and valence bands of the SC, respectively. Ef,n and Ef,p are the quasi Fermi levels for electrons and holes, respectively.

  2. Cyclic voltammograms of DWE samples and steady-state Vcat as a function of Vsem under illumination.
    Figure 2: Cyclic voltammograms of DWE samples and steady-state Vcat as a function of Vsem under illumination.

    a, The Ni(OH)2 EC was oxidized by the TiO2 electrode under 1 sun illumination by sweeping Vsem negatively then positively (through points 1–3), and then reduced by the Au film in contact with the EC (by sweeping Vcat through points 4–5). The scan rate was 20 mV s−1. The starting point of each scan is indicated by a green arrow. The photocurrent is reduced compared with the data presented in Fig. 1 owing to optical absorption in the Au top contact. Integration of the oxidation wave on the Au, the reduction wave on the Au, and the reduction wave on the TiO2 all yield 13 mC cm−2 (after background correction), indicating that the same Ni(OH)2 or NiOOH species are oxidized/reduced using either electrode. b, Steady-state EC potential (Vcat) as a function of the TiO2 potential (Vsem) under illumination. Each Vcat data point was collected after holding Vsem for 180 s. Illuminated voltammograms collected through the TiO2 electrode (WE1) are shown for reference. In both cases, Vcat reaches a constant value for sufficiently positive Vsem, consistent with the requirement for continuity of the interfacial and OER currents, and the activity of the catalysts.

  3. The measured steady-state open-circuit SC potential Vsem and calculated Voc across the SC/ECjunction as a function of the EC potential.
    Figure 3: The measured steady-state open-circuit SC potential Vsem and calculated Voc across the SC/ECjunction as a function of the EC potential.

    a, For IrOx-coated TiO2. b, For Ni(OH)2-coated TiO2. The Vsem values were recorded after holding Vcat at each potential for 3 min under 1 sun illumination. The inset band diagrams illustrate how the different electrostatic potential drops at the EC/solution interface (depicted by the spatial dependence of the vacuum electron energy Evac) for ion-permeable and ion-impermeable ECs affects the SC/EC Voc. In a, changes to Ecat lead to different electrostatic potential drops at the EC/solution interface and a constant Voc. In b, changes to Ecat affect the energetics of the SC/ECinterface and thus the apparent Voc. Similar measurements made in the dark are shown in Supplementary Fig. 2.

  4. Current–voltage curves collected across the SC/ECinterface by sweeping Vsem and holding Vcat fixed.
    Figure 4: Current–voltage curves collected across the SC/ECinterface by sweeping Vsem and holding Vcat fixed.

    a, Data for the IrOx-coated TiO2. b, Data for the Ni(OH)2-coated TiO2. Each curve corresponds to a different value of Vcat. Vsem is referenced to . The same data are plotted in the inset, except shifted so that the x axis shows the potential across the SC/ECjunction, −Vjxn = VsemVcat. The increments in Vcat between curves in a are 0.2 V and those in b are 0.1 V.

  5. Voltammetry of the Ni(OH)2 EC (through the porous Au WE2) while Vsem was held at 0 V versus  in the dark (blue) and under 1 sun illumination (black).
    Figure 5: Voltammetry of the Ni(OH)2 EC (through the porous Au WE2) while Vsem was held at 0 V versus in the dark (blue) and under 1 sun illumination (black).

    The blue (light) and orange (dark) curves show Jsem at the TiO2 WE1 during the scan. The green arrows indicate the scan direction.

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Affiliations

  1. Department of Chemistry and Biochemistry, Materials Science Institute, University of Oregon, Eugene, Oregon 97403, USA

    • Fuding Lin &
    • Shannon W. Boettcher

Contributions

F.L. and S.W.B. designed the study, analysed the data and wrote the manuscript. F.L. performed the experiments.

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The authors declare no competing financial interests.

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