Nitrogen source-dependent modulation of titanium carbide MXene@titanium dioxide heterojunctions: Suppressing macroscopic photothermal contributions for intrinsic carbon dioxide photoreduction.

Abstract

Unveiling the regulatory roles of different nitrogen sources on Ti3C2Tx MXene@TiO2 (MT) heterojunctions remains a critical challenge. Herein, urea, ammonia, and thiourea were selected to dope MT under identical conditions and fabricated into aramid nanofiber composite films. To suppress macroscopic photothermal contributions under controlled low-temperature conditions, CO2 reduction tests were performed at 5 °C, complemented by independent photothermal/electro-heating controls and temperature-dependent experiments. Urea doping (MT-0.2 U) retained the intact Ti3C2Tx skeleton while introducing oxygen vacancies and amide groups, achieving the highest total CO2 reduction rate (240.6 ± 9.5 μmol·g-1·h-1, 1.97 times that of pristine MT), charge separation efficiency (11.2 ± 0.5 μA·cm-2), and carrier lifetime (4.86 ± 0.05 ns). Thiourea doping (MT-0.1 T) realized N, S co-doping, where reduced sulfur species stabilized C1 intermediates, affording a superior CH4 electron selectivity of 73.0 ± 0.7%. In contrast, ammonia doping (MT-0.2 A) severely etched the Ti3C2Tx skeleton, compromising conductivity and leading to the lowest activity. Notably, an activity order inversion was observed between the biased photocurrent screening and unbiased photocatalytic evaluation, revealing that bulk conductivity and surface active sites contribute with different weights to practical catalysis. This work elucidates the trade-off between "conductive skeleton preservation and surface functionalization" dictated by nitrogen source selection, offering a rationale for precursor screening in MXene-based photocatalysts.

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