Enhancing the Lifetime of Photoinduced Charge Carriers in CuFeO₂ Nanoplates by Hydrothermal Doping of Mg for Photoelectrochemical Water Reduction

Abstract

Highly active and stable photocathodes are desirable for efficient photoelectrochemical water reduction. However, most photocathode materials suffer from the short lifetime of photoinduced charge carriers resulting in low energy conversion efficiency. Herein, the authors present a novel strategy to enhance the lifetime of photoinduced charge carriers for hexagonal CuFeO 2 platelets based photocathode. The enhancement is realized by simply hydrothermal doping of Mg into CuFeO 2 . Quantitative phase composition analysis and characterizations are carried out for the Mg doped CuFeO 2 materials with varied doping contents. This doping route not only induces to more acceptor-type states to increase the p-type carrier concentration, but also improves conductivity of CuFeO 2 and facilitates easier carriers transport, which are the key factors for the lifetime enhancement. After systematical tuning of the doping amount, the authors find that when the Mg content reaches 0.1%, the CuFeO 2 presents the longest lifetime for the photoinduced charge carriers (0.8485 s) referring to an enhancement of 94% in the lifetime compared with pure CuFeO 2 .

Electronic Structure and Performance Bottlenecks of CuFeO2 Photocathodes

“…[9][10][11][12][13] Among cuprous delafossites, CuFeO 2 possesses the smallest bandgap (E g ~ 1.5 eV), consists of earth-abundant elements, and can be prepared by various scalable and low cost deposition methods. [13][14][15][16][17][18][19][20][21] The PEC stability of CuFeO 2 is also significantly improved compared with Cu 2 O, which can be explained by its conduction band edge character. According to recent first principles results, the lowest optical transition in CuFeO 2 is associated with Cu 3d + O 2p à Fe 3d charge transfer, thus greatly reducing the probability of Cu self-reduction.…”

Doping‐Dependent Phase Fractions in Hydrothermally Synthesized Mn‐Doped CuFeO2

“…[ 27 ] Moreover, rather strong discrepancies between nominal and actual doping levels were also found for Mg‐ and Ca‐doped . [ 26,32 ] Two interesting questions result from these observations, which, however, cannot be comprehensively answered using the present data. First, there is the question concerning the apparent doping limit of about 5% and, second, the one concerning the difference between nominal and actual doping.…”

“…Moreover, a strong dependency of the 2H phase fraction on the Mn content is observed (see Figure 2) with the maximal phase fraction of about 27% for CuFe 0.99 Mn 0.01 O 2 . In general, the formation of both phases in doped and undoped samples is frequently observed following hydro‐ or solvothermal synthesis [ 19,25–27 ] under comparable basic conditions (2.2 mol L −1 NaOH in the presented case), whereas pure 2H formation requires very high basicity (more than 15 mol L −1 NaOH used by Jin and Chumanov [ 20 ] ). Investigations of Ca‐, Mg‐, and Ni‐doped and hydrothermally produced [ 19,26,27 ] are compatible with the presented results on 2H fractions though the latter studies were limited either to relatively low or high doping levels.…”

“…In general, the formation of both phases in doped and undoped samples is frequently observed following hydro‐ or solvothermal synthesis [ 19,25–27 ] under comparable basic conditions (2.2 mol L −1 NaOH in the presented case), whereas pure 2H formation requires very high basicity (more than 15 mol L −1 NaOH used by Jin and Chumanov [ 20 ] ). Investigations of Ca‐, Mg‐, and Ni‐doped and hydrothermally produced [ 19,26,27 ] are compatible with the presented results on 2H fractions though the latter studies were limited either to relatively low or high doping levels. Xiong et al observed no more 2H in Ca‐doped delafossite at a doping level of 5%, which, however, was the lowest doping level studied except for the undoped compound.…”

“…Mn‐doped samples were prepared by a hydrothermal method previously used for the synthesis of pure delafossite. [ 19,26 ] For undoped , 7.5 mmol of both H 2 O and H 2 O as well as 110 mmol of NaOH were dissolved in 50 mL deionized water and magnetically stirred for 10 min. Afterward, the solution was poured into a 100 mL polytetrafluoroethylene (PTFE) lined autoclave and stored at 160 °C for 13 h. For , H 2 O was added in appropriate amounts to the mixture before stirring and the amount of FeSO 4 × 7H 2 O was reduced accordingly.…”