Tuning Intermediate Bands of Protective Coatings to Reach the Bulk‐Recombination Limit of Stable Water‐Oxidation GaP Photoanodes

Abstract

Stable photoelectrochemical solar fuel production requires protective coatings to achieve effective charge separation, transport, and injection at the semiconductor–liquid interfaces, implying that the coating should energetically align its intermediate band (IB) with both the photoabsorber's band edge and co‐catalyst's potentials. Yet approaches to adjust coating IB positions to accommodate various semiconductor light absorbers for constructing efficient and stable photoelectrodes have not been developed. Herein, three types of transition metal (M = Mn2+, Mn3+, and Cr3+ ions) alloyed TiO2 coatings are discovered using atomic layer deposition (ALD). The IB energetics of these coatings are characterized by X‐ray photoelectron spectroscopy and are found to be tunable inside the TiO2 bandgap, through varying ALD growth conditions. By applying these coatings to n‐type GaP and integrating with IrOx co‐catalysts, the water‐oxidation J–E performance is comparable to an uncoated corroding GaP photoanode. It reaches the bulk recombination limit of the GaP and achieves ≈28% absorbed photon to current efficiency under 475‐nm light excitation (6.48 mW cm−2) and 100‐h stable water oxidation. The outstanding performance and stability are attributed to the efficient charge separation and hole transport, as allowed by the energy alignment of the coating IB and the GaP valence band edge.

Multicolor Bipolar Modulation of Titanium–Chromium Oxide Electrochromic Coatings

“…The electronic structure of (Ti,Cr)O x was probed by deconvoluting the respective valence XPS spectra (Figure 6a) and fitting the spectral data following protocols reported previously. 24,35,36 Consistent with previous reports, we observed shoulder peaks between the valence band maximum (VBM) and Fermi level (E F ) in both the as-grown (Ti,Cr)O x and (Ti,Cr)O x after the anodic chromic process. The shoulder peak is centered around binding energy (B.E.)…”

“…35 The potentials of all the energy levels, including band edges, Fermi levels, and IBs, were also referenced to the NHE scale (the right vertical axis). 36 The deconvolution approach for (Ti,Cr)O x was consistent with the TiO 2 analysis from previous literature and the CrO x analysis (peak fitting shown in Table S4). 36 The two primary O 2p peaks located at 7.00 ± 0.10 eV (Peak IV) and 5.00 ± 0.10 eV (Peak V) were derived from the Ti−O bonds.…”

“…This nanocrystalline microstructure enables the intermediate-band electronic structure and electrical conductivity. 36 Lastly, according to the XPS Cr 2p spectra (Figure S9a and Table S1), the Cr composition existed predominately as Cr 3+ (88.65%) with a portion of unreduced Cr 6+ (11.35%) from the ALD precursor.…”

“…difference between the IB center and E F , represented by E IB – E F . The potentials of all the energy levels, including band edges, Fermi levels, and IBs, were also referenced to the NHE scale (the right vertical axis) …”

Exploring the Photocorrosion Mechanism of a Photocatalyst

“…Since then, PEC water splitting has been investigated using metal oxides, oxychalcogenides, and nonoxide semiconductors. Currently, the solar-to-hydrogen (STH) conversion efficiency is close to or even greater than 10%, which demonstrates the potential of PEC water splitting for efficient hydrogen production by direct utilization of harvested solar energy. ,− Although the STH efficiency has been dramatically enhanced for PEC water splitting by bandgap engineering and by the development of efficient cocatalysts necessary for gas-generating reactions, the long-term stability of gas evolution should be improved for the commercial utilization of PEC water-splitting technology. − ,− Notably, nonoxide semiconductors with narrower bandgaps have been developed to absorb photons with a wide range of energies; however, these materials can self-oxidize and are unstable against photo-oxidation. ,− …”