Van Der Waals gap-rich BiOCl atomic layers realizing efficient, pure-water CO2-to-CO photocatalysis

Abstract

Photocatalytic CO₂ reduction (PCR) is able to convert solar energy into chemicals, fuels, and feedstocks, but limited by the deficiencies of photocatalysts in steering photon-to-electron conversion and activating CO₂, especially in pure water. Here we report an efficient, pure water CO₂-to-CO conversion photocatalyzed by sub-3-nm-thick BiOCl nanosheets with van der Waals gaps (VDWGs) on the two-dimensional facets, a graphene-analog motif distinct from the majority of previously reported nanosheets usually bearing VDWGs on the lateral facets. Compared with bulk BiOCl, the VDWGs-rich atomic layers possess a weaker excitonic confinement power to decrease exciton binding energy from 137 to 36 meV, consequently yielding a 50-fold enhancement in the bulk charge separation efficiency. Moreover, the VDWGs facilitate the formation of VDWG-Bi-V_O^••-Bi defect, a highly active site to accelerate the CO₂-to-CO transformation via the synchronous optimization of CO₂ activation, *COOH splitting, and *CO desorption. The improvements in both exciton-to-electron and CO₂-to-CO conversions result in a visible light PCR rate of 188.2 μmol g^-1 h^-1 in pure water without any co-catalysts, hole scavengers, or organic solvents. These results suggest that increasing VDWG exposure is a way for designing high-performance solar-fuel generation systems.

Fig. 1. Characterizations of VDWGs-rich structure of BiOCl atomic layers.

a TEM image, b dark-filed STEM image, c–e elemental mappings, f AFM image, g theoretical crystalline structures, h aberration-corrected HAADF-STEM image, i three-dimensional topographic color-coded intensity image (taken from h), and j intensity profiles (taken along the lines of h) of BOC-VDWGs-AL. k PAS, l EPR, m Bi L₃-edge EXAFS, and n O K-edge XANES of BOC-VDWGs-AL, CBOC-VDWGs, and BOC-VDWGs-76. The inset in k is the simulation result of the positron density distribution of VDWG-Bi-V_O^••-Bi.

Fig. 2. Pure-water PCR performances of VDWGs-rich BiOCl atomic layers.

a CO yields of four parallel experiments of pure-water PCRs. The first used BOC-VDWGs-AL under visible light in the presence of CO₂, the second did not involve BOC-VDWGs-AL but still using visible light and CO₂, the third involved BOC-VDWGs-AL and CO₂ but under dark, and the fourth employed BOC-VDWGs-AL and visible light but fed with argon. b Cycling tests of pure-water PCR performances over BOC-VDWGs-AL for 50 hours. c Mass spectra of the product generated from pure-water PCR over visible-light-irradiated BOC-VDWGs-AL fed with ¹³CO₂. The inset in panel c shows the corresponding GC pattern. d Yield of O₂ detected during pure-water PCR photocatalyzed by BOC-VDWGs-AL, as well as the calculated yield ratio of CO to O₂. This set of data were obtained from an online PCR system. e Statistical result for the CO/O₂ yield ratio. f Comparison of CO-evolving rate of BOC-VDWGs-AL, CBOC-VDWGs, and BOC-VDWGs-76, all under visible light in the presence of pure-water and CO₂. g Comparison of pure-water, CO-evolving rate of BOC-VDWGs-AL and some representative layered photocatalysts. The nanosheet photocatalysts with VDWGs on lateral facets were obtained by conventional exfoliation method, while VDWGs-dominated nanosheets were synthesized by our developed exfoliation strategy. The error bars derived from triplicate experiments.

Fig. 3. Promoting effect of VDWGs on exciton binding energy and, consequently, on electron–hole separation.

a, b PL of BOC-VDWGs-AL (a) and BOC-VDWGs-76 (b) as a function of operation temperature. c Comparisons of E_b and VDWG coverage percentage of BOC-VDWGs-AL, CBOC-VDWGs, and BOC-VDWGs-76. d Comparisons of E_b of BOC-VDWGs-AL and some representative layered photocatalysts. e Time-resolved PL of BOC-VDWGs-AL and BOC-VDWGs-76. f, g Transient photocurrent responses (f) and bulk charge separation efficiencies (g) of BOC-VDWGs-AL, CBOC-VDWGs, and BOC-VDWGs-76.

Fig. 4. Promoting effect of VDWGs-associated defect on CO₂-to-CO catalysis.

a C–O bond length and O–C–O bond angle of free CO₂ (a1), CO₂ adsorbed on VDWG-Bi-O-Bi (a2), and CO₂ adsorbed on VDWG-Bi-V_O^••-Bi (a3). b1 Optimized geometric structures (parts containing charge density differences) and b2 schematic illustration of intermediates generated during CO₂-to-CO catalysis over VDWG-Bi-V_O^••-Bi. c Gibbs free energy diagrams of intermediates generated during CO₂-to-CO catalysis over VDWG-Bi-O-Bi and VDWG-Bi-V_O^••-Bi. d CO₂-TPD of BOC-VDWGs-AL and BOC-VDWGs-AL-O₂. e In situ FTIR of intermediates generated during pure-water PCR over BOC-VDWGs-AL. f Comparison of FTIR signal intensity of *CO intermediates from BOC-VDWGs-AL and BOC-VDWGs-AL-O₂-driven pure-water PCR. g CO-TPD of BOC-VDWGs-AL and BOC-VDWGs-AL-O₂. The VDWG-Bi-O-Bi was constructed by the calcination of VDWG-Bi-V_O^••-Bi (BOC-VDWGs-AL) under O₂ atmonsphere.