Boosting photoelectrochemical efficiency by near-infrared-active lattice-matched morphological heterojunctions

Photoelectrochemical catalysis is an attractive way to provide direct hydrogen production from solar energy. However, solar conversion efficiencies are hindered by the fact that light harvesting has so far been of limited efficiency in the near-infrared region as compared to that in the visible and ultraviolet regions. Here we introduce near-infrared-active photoanodes that feature lattice-matched morphological hetero-nanostructures, a strategy that improves energy conversion efficiency by increasing light-harvesting spectral range and charge separation efficiency simultaneously. Specifically, we demonstrate a near-infrared-active morphological heterojunction comprised of BiSeTe ternary alloy nanotubes and ultrathin nanosheets. The heterojunction’s hierarchical nanostructure separates charges at the lattice-matched interface of the two morphological components, preventing further carrier recombination. As a result, the photoanodes achieve an incident photon-to-current conversion efficiency of 36% at 800 nm in an electrolyte solution containing hole scavengers without a co-catalyst.

S18 region) and the absorption depth of the material for the relevant photon energies of the solar spectrum. 1 Hence, we investigate the influence of the photoanode thickness on the PEC performances, as shown in Supplementary Figure 15. As the thickness of BST-MHs increases, the corresponding photocurrent density is slightly enhanced. As a result, the optimal thicknesses for BST-MHs, NTs, NSs, and NTs+NSs are determined to be 3, 2, 3 and 2 layers, respectively. When electrode thickness is further increased beyond the optimum value, the photocurrent density decreases.
Upon illumination, the semiconductor will generate light-induced charges, and the Fermi level of the semiconductor will split into separate quasi-Fermi levels of electrons and holes. The quasi-Fermi level splitting produces an open-circuit photovoltage that is equal to the difference between the two quasi-Fermi levels. Therefore, the magnitude of the photovoltage is directly related to the extent of separation between quasi-Fermi levels.
In our measurements of photovoltage, the illumination intensity of visible and NIR light is 100 mW cm -2 . However, from the analysis of UV-vis-IR absorption spectrum, it can be concluded that BST-MHs have a better absorption capacity for visible light.
Therefore, although NIR light has the same illumination intensity as visible light, the BST-MH photoanode absorbs more photons from the visible region to generate lightinduced charges, resulting in a larger difference between the quasi-Fermi levels.
Based on the above analysis, the photovoltage of BST-MHs should be different under the irradiation of different light sources, which is mainly affected by the difference in the absorption capacity of BST-MH to light of different wavelengths ( Supplementary Fig. 15f).

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To quantitatively compare the carrier lifetime in the photoelectrodes, we study the transient decay time τ through the logarithmic plot of the parameter D. The value of D is calculated as: where It represents the photocurrent at time t(s), and Ist is the steady-state current. We compare the transient decay time τ at lnD = -1.
As shown in Fig To further suppress carrier recombination, we sought to accelerate the hole-

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In order to better prove the function of a lattice-matched heterojunction, we further grew another kind of Bi-based chalcogenides (Bi2S3 nanosheets) and 2D semiconductor (MoS2 nanosheets) on the BiSeTe nanotubes ( Supplementary Fig. 23-24). Large differences in lattice spacing and crystalline orientation can be found from HRTEM images at interfaces (Supplementary Fig. 25-28). Specifically, FFTs of the interfacial regions show two diffraction patterns that can be assigned to nanotubes and nanosheets, indicating the existence of lattice mismatch at interfaces. Then, we examined their PEC performances under the same conditions as the BST-MHs ( Supplementary Fig. 29).
Under both NIR irradiation and visible light irradiation, the photo-current densities of BST/Bi2S3 are slightly higher than that of plain nanotubes, but much lower than that of lattice-matched BST-MHs. We attribute this result to the somewhat impeded charge separation due to interfacial defects, which act as recombination centers, and therefore  To quantify the changes in the transport impedance of the BST material, we fitted the impedance spectra using Zview (Supplementary Table 4 The decrease in transmission impedance proves that the formation of a latticematched heterojunction is conducive to the migration of carriers from the bulk to the surfacegreatly mitigating carrier recombination in the bulk phase, and increasing the number of surface-reaching carriers.