Modulation of the Chemical Microenvironment at the Hematite-Based Photoanode Interface with a Covalent Triazine Framework for Efficient Photoelectrochemical Water Oxidation

Abstract

Transferring charge between the hematite photoanode and cocatalyst interface for efficient photoelectrochemical water oxidation is a big obstacle. However, the chemical microenvironment at the interface plays an important role in this field. Here, we used a covalent triazine framework (CTF-BTh) containing a bithiophene moiety to modulate the microenvironment at the interface between Gd-doped hematite and the cobalt silicate cocatalyst. Detailed studies show that Gd doping increases the donor density and reduces the charge transfer resistance. Furthermore, the coordination bonds (N–Co and S–Co) provide pathways for charge flow and also enhance the average oxidation state of Co. The conjugated system of the CTF-BTh is also easy for electron delocalization. These modifications effectively passivate the surface state and synergistically suppress the electron–hole recombination. This work provides a universal strategy for improving the interfacial microenvironment to achieve efficient water oxidation.

Revealing the Essential Role of Iron Phosphide and its Surface‐Evolved Species in the Photoelectrochemical Water Oxidation by Gd‐Doped Hematite Photoanode

“…The binding energy of Fe 2p of FeP is much lower than the binding energy of Fe 3+ and Fe 2+ , which is also side evidence that FeP has a metal‐like electronic state, and thus FeP has better electrical conductivity. As shown in Figure 2b, all samples except α‐Fe 2 O 3 display similar Gd 4d XPS spectra, in which the Gd 4d 5/2 peaks (143–145 eV) are ascribed to Gd 3+ , which proves the successful incorporation of Gd [3a] . Furthermore, the O 1 s XPS spectra (Figure 2c) of all photoanodes can be fitted with two or three deconvolution peaks, corresponding to Fe−O (529.4–529.6 eV), surface‐adsorbed hydroxy oxygen (530.7–531.0 eV), and P−O (532.4–532.6 eV), respectively [3a,13b] .…”

“…As shown in Figure 2b, all samples except α‐Fe 2 O 3 display similar Gd 4d XPS spectra, in which the Gd 4d 5/2 peaks (143–145 eV) are ascribed to Gd 3+ , which proves the successful incorporation of Gd [3a] . Furthermore, the O 1 s XPS spectra (Figure 2c) of all photoanodes can be fitted with two or three deconvolution peaks, corresponding to Fe−O (529.4–529.6 eV), surface‐adsorbed hydroxy oxygen (530.7–531.0 eV), and P−O (532.4–532.6 eV), respectively [3a,13b] . The larger the peak of the surface‐adsorbed hydroxy group, the better the wettability of the sample, which is favorable for the adsorption of OER reactants.…”

“…Based on our previous work, the α‐Fe 2 O 3 and Gd−Fe 2 O 3 photoanodes were fabricated by a hydrothermal‐annealing two‐step method [3a] . Subsequently, the pre‐prepared Gd−Fe 2 O 3 was placed in a tube furnace for phosphating in an N 2 flow at 300 °C, and a pure FeP/Gd−Fe 2 O 3 shell‐core structure photoanode was successfully obtained (Figure 1a) [11d] .…”

“…The photoelectrochemical (PEC) performance of all photoanodes is evaluated by comparing a range of metrics such as photocurrent density, stability, applied bias photon‐to‐current conversion efficiency (ABPE), incident photon‐to‐current conversion efficiency (IPCE), surface charge‐injection efficiency ( η surface ), bulk‐phase charge‐separation efficiency ( η bulk ), and so on [3a] . In the linear sweep voltammetry (LSV) curve (Figure 3a), the FeP/Gd−Fe 2 O 3 photoanode shows an excellent photocurrent of 2.56 mA cm −2 at 1.23 V versus reversible hydrogen electrode (V RHE ), which is a 4‐fold improvement compared to pristine α‐Fe 2 O 3 (0.64 mA cm −2 ).…”

“…For this reason, a structure of α‐Fe 2 O 3 nanorod arrays has been developed and widely used, the height of which ensures good light absorption while the small diameter of the nanorods allows a short diffusion distance of holes in the radial direction, which makes holes reach the surface to participate in water oxidation easily [6a] . Second, Gd doping is successfully used to improve the conductivity of α‐Fe 2 O 3 based on our previous work [3a] . α‐Fe 2 O 3 is a typical n‐type semiconductor, and its conductive carriers are mainly electrons that are excited into the conduction band, belonging to the electron conduction type [7] .…”