Conjugated Acetylenic Polymers Grafted Cuprous Oxide as an Efficient Z‐Scheme Heterojunction for Photoelectrochemical Water Reduction

Abstract

Up to now, the field of PEC water splitting is dominated by inorganic semiconductors consisting of earth abundant elements (e.g., silicon, [7-12] metal oxides, [13-16] metal sulfides, [17,18] dichalcogenide, [19] etc.). However, the high cost and poor stability of noble metal (e.g., Pt) as cocatalysts seriously restrict the practical applications of inorganic semiconductors for the PEC hydrogen evolution reaction (HER). Compared with traditional inorganic semiconductors, organic semiconductors (e.g., graphitic carbon nitride [g-C 3 N 4 or polyheptazine], [20-23] polythiophene, [24,25] conjugated covalent organic frameworks [COFs], [26] conjugated acetylenic polymers [CAPs] [27-29]) have attracted increasing attentions benefitting from their tunable bandgaps, engineered band edge positions, and molecular-level desirable active centers. [6] However, the PEC HER performance of current organic photocathodes falls far behind inorganic counterparts mainly due to their severe recombination of photoinduced holes and electrons. Recently, diverse strategies have been explored for promoting the charge separation of organic semiconductors, mainly focusing on the heterojunction or homojunction As attractive materials for photoeletrochemical hydrogen evolution reaction (PEC HER), conjugated polymers (e.g., conjugated acetylenic polymers [CAPs]) still show poor PEC HER performance due to the associated serious recombination of photogenerated electrons and holes. Herein, taking advantage of the in situ conversion of nanocopper into Cu 2 O on copper cellulose paper during catalyzing of the Glaser coupling reaction, a general strategy for the construction of a CAPs/Cu 2 O Z-scheme heterojunction for PEC water reduction is demonstrated. The as-fabricated poly(2,5-diethynylthieno[3,2-b]thiophene) (pDET)/Cu 2 O Z-scheme heterojunction exhibits a carrier separation efficiency of 16.1% at 0.3 V versus reversible hydrogen electrode (RHE), which is 6.7 and 1.4-times higher respectively than those for pDET and Cu 2 O under AM 1.5G irradiation (100 mW cm −2) in the 0.1 m Na 2 SO 4 aqueous solution. Consequently, the photocurrent of the pDET/Cu 2 O Z-scheme heterojunction reaches ≈520 µA cm −2 at 0.3 V versus RHE, which is much higher than pDET (≈80 µA cm −2), Cu 2 O (≈100 µA cm −2), and the state-of-the-art cocatalyst-free organic or organicsemiconductor-based heterojunctions/homojunctions photocathodes (1-370 µA cm −2). This work advances the design of polymer-based Z-scheme heterojunctions and high-performance organic photoelectrodes.

Engineering Nanostructure–Interface of Photoanode Materials Toward Photoelectrochemical Water Oxidation

“…[144][145][146] Compared with Z-scheme photocatalyst, it is considered that the reduction and oxidation reaction will take place at a lower potential for general type-II heterojunction, leading to a reduced reaction activity (Figure 11a). [122,147] Moreover, since the electron/electron or hole/hole electrostatic repulsion, it will make the photo-induced carrier migration difficult. [148] In comparison, Z-scheme heterojunction shows similar energy band alignment to the type-II heterojunction, but with a different charge carrier transfer mechanism (Figure 11b).…”

“…[149] The Z-scheme heterojunction can not only achieve efficient carrier separation but also retain the intrinsic photo redox potential for the oxidation/reduction reaction. [147] For instance, Lu et al reported a g-C 3 N 4 -based type-II and Z-scheme heterojunction anodes. [150] By changing the growth sequence of g-C 3 N 4 and WO 3 , different WO 3 /g-C 3 N 4 type-II heterojunction and g-C 3 N 4 /WO 3 Z-scheme heterojunction were obtained.…”

Interfacial Chemical Bond‐Modulated Z‐Scheme Charge Transfer for Efficient Photoelectrochemical Water Splitting

“…Alternatively, Z‐scheme heterojunction has been proposed to address the problem of type II heterojunction. [ 11–13 ] Actually, it has the same band alignment with type II heterojunction, but exhibits an opposite direction of charge transfer as well as superior redox capability. [ 14,15 ] Typically, redox electron mediators such as I − /IO 3 − and Fe 2+ /Fe 3+ , as well as solid electron mediators such as Au and graphene, are required to facilitate Z‐scheme charge transfer.…”

Natural Cellulose Substance Based Energy Materials

“…Taking the flexible and biocompatible advantages of the natural cellulose substance, it was used as the electrode matrix for the growth of active materials for the application of PEC water splitting [80] . For instance, a conjugated acetylenic polymers (CAPs) grafted cuprous oxide (Cu 2 O) Z‐scheme heterostructured composites were in situ prepared on the Cu cellulose paper (Cu−CP), yielding the flexible CAPs/Cu 2 O/Cu−CP material [80] .…”

“…Taking the flexible and biocompatible advantages of the natural cellulose substance, it was used as the electrode matrix for the growth of active materials for the application of PEC water splitting [80] . For instance, a conjugated acetylenic polymers (CAPs) grafted cuprous oxide (Cu 2 O) Z‐scheme heterostructured composites were in situ prepared on the Cu cellulose paper (Cu−CP), yielding the flexible CAPs/Cu 2 O/Cu−CP material [80] . The cellulose paper was deposited with nanocopper by a seed‐mediated growth method with gold clusters as the nucleation centers to give Cu−CP, and CAPs were in situ grown on the Cu−CP, followed with the partial oxidation of nanocopper into cuprous oxide via a simple hydrothermal process.…”