Dual-photon-driven hydrogen evolution in copper-based photocatalysts under near-infrared and visible light

Abstract

While the dynamic restructuring of Cu-metalated metal-organic frameworks for photocatalysis has recently been explored, its effect on electronic excitation remains under-investigated. For better mechanistic understanding, we study the light-induced activity of Cu-metalated UiO-66(COOH)₂ metal-organic framework, known as UiO-66(COOH)₂-Cu under varied irradiations, using gas-phase formic acid dehydrogenation at ambient conditions as a model reaction. A photocatalytic logic gate behavior is observed. The UiO-66(COOH)₂-Cu remains photo-catalytically OFF under visible (>390-720 nm) or near-infrared (>700 nm) light alone, but shows high H₂ production of 6.1 mmol·g⁻¹·h⁻¹ (ON state) when both are applied (≥390 nm). Operando Fourier transform infrared spectroscopy and X-ray absorption spectroscopy demonstrate that both visible and near-infrared irradiations are required for metalated Cu^2+/1+ restructuring inside the framework to form photoactive Cu⁰/Cu⁺ binary center, and thereafter for the photocatalytic dehydration of formic acid. X-ray absorption spectroscopic analysis suggests a distinct initiation behavior of Cu⁺ and Cu⁰ species under visible and near-infrared irradiation, respectively. However, operando Fourier-transform infrared spectroscopy reveals a cascade mechanism requiring both irradiations to progress the catalytic reaction. This photocatalytic logic gate behavior also appears in bare Cu⁰/Cu₂O system used as reference. These findings provide insights into dual-photon-driven photocatalysis and aid advanced hydrogen catalyst design.

Fig. 1. Comparative characterization of the as-synthesized and Cu-metalated UiO-66(COOH)₂ MOFs.

A Structure of UiO-66(COOH)₂ highlighting the possible Cu-anchoring sites in the vicinity of free carboxylate groups and missing linker defects. Color coding: green polyhedra = Zr, dark gray = C, red = O, gold = Cu and light gray = H. B SEM images of UiO-66(COOH)₂ and UiO-66(COOH)₂-Cu, C PXRD patterns of UiO-66(COOH)₂ and UiO-66(COOH)₂-Cu compared to the simulated pattern, D N₂ sorption isotherms before and after Cu-metalation.

Fig. 2. Dependence of the post-metalated MOF’s surface chemistry and photocatalytic H₂ generation on the type of irradiation used.

A The irradiance spectra of light sources, B the corresponding operando FTIR surface spectra of the UiO-66(COOH)₂-Cu in the anhydride region during the dehydrogenation of the FAc under various irradiation conditions. Spectra are subtracted from the spectrum after activation. The arrows (1, 2 and 3) indicate the evolution of anhydride bands under visible, NIR and visible + NIR irradiations respectively, C possible anhydride formation step and D H₂ production rate (mmol·g⁻¹·h⁻¹) and (mmol·cm⁻²·h⁻¹) from FAc dehydrogenation at the steady state under each particular irradiation condition. [Experimental conditions: total flow rate = 25 cm³·min⁻¹ (Ar, 2400 ppm HCOOH), T = 25 °C; catalyst ~ 20 mg (self-supported pellet, 1.6 cm² exposed area)].

Fig. 3. Comparative operando XAS analysis of UiO-66(COOH)₂-Cu under the reaction’s conditions and different light irradiations.

The operando XAS spectra of UiO-66(COOH)₂-Cu during photocatalytic H₂ production from FAc. A, C, E, G denote CuK-edge XANES spectra of pristine MOF, under the reaction conditions with visible (>390–720 nm), NIR (>700 nm) and (visible + NIR) (≥390 nm) irradiation, respectively. B, D, F, H denote their corresponding Fourier Transform of EXAFS data. (A, inset) shows the featured peaks for Cu (II). (C, inset, D) show the initial evolution of Cu (I) from Cu (II) (denoted by arrows) under visible light, whereas (F) shows formation of Cu (0) under NIR with peak evolution at 2.16 Å. (G, H) show the formation of Cu(0)/Cu(I) active species from Cu (II) (denoted by arrows) under reaction condition (visible + NIR). [Experimental conditions: total flow rate = 25 cm³·min⁻¹ (Ar, 2400 ppm HCOOH), T = 25 °C; catalyst ~ 20 mg (self-supported pellet, 1.6 cm² exposed area), LC8 Hamamatsu Xe lamp (visible, 60 mW·cm⁻²) (visible + NIR, 70 mW·cm⁻²), MAX-303 (NIR, 170 mW·cm⁻²)].

Fig. 4. Tracking anhydride formation under NIR/visible periodic irradiations.

Schematic illustration of the successive 4 min. cycles (n = 6) of A visible (>390–720 nm), C NIR ( > 700 nm), and E complete visible + NIR ( ≥ 390 nm) irradiation cycles. The corresponding graph showing the evolution of the anhydride characteristic bands of UiO-66(COOH)₂-Cu in the 1900–1780 cm^-1 region, as monitored by operando FTIR. B During half-cycle of visible irradiation (each spectrum is subtracted from the previous one before visible irradiation in the dark). Inset top left: their corresponding cumulative spectra and inset top right: possible hydration process of the anhydride, D during half-cycle of NIR irradiation (each spectrum is subtracted from the previous one before NIR irradiation in the dark). Inset top left: their corresponding cumulative spectra and inset top right: possible anhydride bridge formation step and F during full visible + NIR irradiation cycle (spectra are subtracted from the reference spectrum after activation). The arrows denote evolution of the corresponding anhydride bands. [Experimental conditions: total flow rate = 25 cm³·min⁻¹ (Ar, 2400 ppm HCOOH), T = 25 °C; catalyst ~ 20 mg (self-supported pellet, 1.6 cm² exposed area), LC8 Hamamatsu Xe lamp (visible, 60 mW·cm⁻²) (visible + NIR, 70 mW·cm⁻²), MAX-303 (NIR, 170 mW·cm⁻²)].

Fig. 5. Unraveling the effect of visible and NIR irradiations on photocatalytic H₂ production rates and mechanism.

A Irradiance spectra of combined light sources, where Z-axis represents the used light sources as numbered in Fig. S6B, B corresponding photocatalytic H₂ production of both UiO-66(COO)₂-Cu and Cu⁰/Cu₂O photocatalysts. [Experimental conditions: total flow rate = 25 cm³·min⁻¹ (Ar, 2400 ppm HCOOH), T = 25 °C; catalyst ~ 20 mg (self-supported pellet for UiO-66(COOH)₂-Cu and 1:1 (SiO₂: Cu/Cu₂O) mixture for composite, 1.6 cm² exposed area).] C Absorption spectrum of UiO-66(COOH)₂-Cu after photocatalytic reaction (red spectrum showing schematic subtraction of light source 7 and 5). H₂ production rate as a function of D visible light irradiance and E NIR light irradiance, respectively. Error bars represent the standard error, estimated as 10% of the measured value. Absent error bars fall within the symbols.

Fig. 6. Investigation of the charge carrier dynamics.

A The photocurrent–time curves for UiO-66(COOH)₂ and UiO-66(COOH)₂-Cu samples at a potential of 1.12 V vs RHE upon irradiation with visible light (> 390-720 nm, Xe lamp, 60 mW·cm⁻²) and B Electrochemical impedance spectroscopy (EIS) Nyquist plots for UiO-66(COOH)₂ and UiO-66(COOH)₂-Cu electrodes. Experimental data are shown as symbols, and the solid lines represent the fitted curves obtained using the equivalent electrical circuit model R₁ + Q₁/R₂ + Q₂/R₃ + Ma4, as shown in the bottom inset. Top inset: a magnified view of the high-frequency region. The fitted resistance values (R₁, R₂, and R₃) are 51.10, 1.66 × 10⁵, and 24.46 Ω for UiO-66(COOH)₂, and 92.15, 1.14 × 10⁴, and 0.055 Ω for UiO-66(COOH)₂-Cu, respectively. [EIS conditions: 1 × 1 cm² of thin film of catalyst deposited on FTO; measurement done in 0.5 M Na₂SO₄ at -0.38 V vs RHE. The voltage used for EIS and transient photocurrent is not iR-corrected]. 3D plots of the PL emission maps of C UiO-66(COOH)₂ and D UiO-66(COOH)₂-Cu upon excitation from 320 to 650 nm respectively (arrow showing the emission peak maximum). E The PL spectra of the samples upon excitation at 340 nm (inset: their corresponding normalized intensities), F their corresponding decay lifetime at 430 nm upon 343 nm excitation. G The proposed charge carrier mechanism over UiO-66(COOH)₂-Cu.

Fig. 7. Proposed in-situ restructuring and light-harvesting mechanisms.

A Plausible mechanism of Cu restructuring inside UiO-66(COOH)₂-Cu in presence of FAc and Visible + NIR irradiations and B schematic illustration of the photocatalytic production of CO₂ and H₂ at the steady state. BTCA is 1,2,4,5-benzenetetracarboxylic acid organic linker.