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

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 Cu2O on copper cellulose paper during catalyzing of the Glaser coupling reaction, a general strategy for the construction of a CAPs/Cu2O Z‐scheme heterojunction for PEC water reduction is demonstrated. The as‐fabricated poly(2,5‐diethynylthieno[3,2‐b]thiophene) (pDET)/Cu2O 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 Cu2O under AM 1.5G irradiation (100 mW cm−2) in the 0.1 m Na2SO4 aqueous solution. Consequently, the photocurrent of the pDET/Cu2O Z‐scheme heterojunction reaches ≈520 µA cm−2 at 0.3 V versus RHE, which is much higher than pDET (≈80 µA cm−2), Cu2O (≈100 µA cm−2), and the state‐of‐the‐art cocatalyst‐free organic or organic‐semiconductor‐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.

construction. [6,23,24,28,[30][31][32] Among them, the formation of a type-II heterojunction, attributable to the effective separation of photoinduced electron-hole pairs through the band bending between two semiconductors, is one of the most facile ways for enhancing the PEC performance (Figure 1a). [23,30,32] Unfortunately, for the type-II heterojunction, the reduction and oxidation reactions occur under lower potentials, respectively, resulting in a significantly reduced redox activity. [33,34] In addition, due to electrostatic repulsion among electron-electron or hole-hole, it is difficult for the migration of photoinduced electrons/holes between two semiconductors. [35] Therefore, the separation efficiency of photoinduced electrons and holes is still inferior even in the conjugated-polymer-based type-II heterojunctions or homojunctions with continuous type-II aligns (<2%). [6,28,29] Compared with type-II heterojunction, the Z-scheme heterojunction has a similar band structure but different charge-carrier migration mechanism (chargecarrier migration pathway resembles the letter "Z"). [36,37] The Z-scheme heterojunction can not only achieve complementary light absorption and efficient separation of photogenerated carriers, but also reserve their strong photo redox properties for catalytic reactions (Figure 1b). Moreover, the charge-carrier migration in the Z-scheme heterojunction is physically more feasible than that for the type-II heterojunction due to the electrostatic attraction between the electron and hole. [35] However, owing to the embarrassment in constructions of the "Z" like charge-carrier migration pathway between two organic semiconductors or one organic and one inorganic semiconductors, the fabrication of organic-semiconductor-based Z-scheme heterojunction remains unexplored for the PEC water reduction.
In this work, we report a general approach for the onestep fabrication of CAPs/Cu 2 O Z-scheme heterojunction on Cu cellulose paper (CP) for the first time. The Cu 2 O in such Z-scheme heterojunction is converted from the nanocopper which can be readily oxidized during the catalytic Glaser-coupling of acetylenic monomers. The charge carriers separation efficiency measurement reveals that the as-synthesized poly-(2,5-diethynylthieno[3,2-b]thiophene (pDET)/Cu 2 O Z-scheme heterojunction (noted as pDET/Cu 2 O Z-scheme heterojunction) effectively boosts the separation of photoinduced electrons and holes in contrast to either pure pDET or Cu 2 O, which is attributed to the large energy difference between VB of pDET and CB of Cu 2 O. As a result, in 0.1 m Na 2 SO 4 aqueous solution, the pDET/Cu 2 O Z-scheme heterojunction exhibits a substantially enhanced photocurrent of 520 µA cm −2 at 0.3 V versus RHE, which is ≈6.5 and ≈5.2 times higher than those for pure pDET and Cu 2 O, respectively, as well as superior to the state-of-the-art cocatalyst-free organic semiconductors or organic-semiconductor-based heterojunctions photocathodes (1-370 µA cm −2 ), [6,[20][21][22][23][26][27][28][29]31] and even many inorganic photocathodes such as Cu 2 O, [15] NiO, [16] CuInS 2 , [16] and WSe 2 . [19] The CP was first pretreated by chloroauric acid solution (2 mg mL −1 ) for 2 h and reduced in sodium borohydride solution to create the nucleation centers for Cu. Then Cu CP was prepared by seed-mediated growth method in an aqueous solution containing copper sulfate (30 mg mL −1 ), sodium hydroxide (40 mg mL −1 ), potassium sodium tartrate (120 mg mL −1 ), and formaldehyde (100 µL mL −1 ) for 5 h at room temperature. [38] After the metallic Cu was deposited on the cellulose nanofibrils (Cu content: ≈10.8 mg cm −2 ), it exhibited towerlike morphology (100-200 nm height) (Figure 2a-c). As disclosed in the X-ray diffraction (XRD) pattern of Cu CP, diffraction peaks appeared at 43.3° and 50.3°, which are assigned to the characteristic (111) and (200) facets of the Cu (JCPDS No. 04-0836), respectively ( Figure S1, Supporting Information). The formation of nanocopper is mostly due to the growth confinement by the nanogaps between the cellulose nanofibrils, preventing the further growth from copper nanocrystal to bulk copper ( Figure S2, Supporting Information). [39,40] In view of the crystal growth, the tower-like nanocopper structure on the Cu CP is self-formable and dynamically stable. [41] Next, the pDET ( Figure S3, Supporting Information), was in situ grown on a Cu CP (1 × 3 cm 2 ) via a 3 h-Glaser polycondensation at 60 °C in a pyridine solution containing DET monomers (0.25 mg mL −1 ) and piperidine (4 µL mL −1 ) (Figure 1a). The Cu CP here functioned both as a catalyst and a conductive substrate. The CC coupling of DET occurred at the Cu-liquid interface where various Cu species (Cu I and Cu II triggered by piperidine) were released, yielding a pDET film deposited directly on the Cu CP. [27][28][29] A film (≈300 nm thick) composed of dense, vertically-aligned, and inter-connected nanosheets was formed on the Cu surface ( Figure 2d). The thickness and size of the nanosheets were <20 nm and 400-800 nm, respectively (Figure 2d,e). Similar morphology of pDET nanosheets was observed on the Cu foil ( Figure S4, Supporting Information). In comparison with pDET on bulk Cu substrates (e.g., Cu foil), the appearance of diffraction peak at 36.3° in the XRD pattern of pDET/Cu CP demonstrated the presence of Cu 2 O (JCPDS No. 75-1531) ( Figure S5, Supporting Information). Furthermore, high-resolution transmission electron microscopy (HRTEM) images and the electron diffraction pattern showed that the interplanar spacing was determined to be 0.209 nm in pDET/Cu CP heterostructure, which also evidenced the formation of Cu 2 O (JCPDS No. 75-1531) (Figure 2f,g). Transmission electron microscopy (TEM) images and energy-dispersive X-ray spectro scopy mapping also indicated that both pDET and Cu 2 O were grown on the nanocopper on the Cu CP (Figure 2e,f; Figure S6, Supporting Information).
The density functional theory calculations demonstrate that nanocopper has a higher tendency for oxidation than the bulk copper, ascertaining the reason for the formation of Cu 2 O in pDET/Cu CP ( Figures S7,S8, Supporting Information).  [29] and Cu 2 O (E c : ≈−3.2 eV; E v : ≈−5.3 eV), [5] as well as the fact that both pDET and Cu 2 O are grown on the copper layer on the Cu CP, the metallic copper in the pDET/Cu CP can function as mediate to migrate the photoinduced electron from the CB of Cu 2 O to the VB of pDET, thus realizing the construction of Z-scheme heterojunction between pDET and Cu 2 O (pDET/Cu 2 O Z-scheme heterojunction).
The chemical composition and bonding information of pDET/ Cu 2 O Z-scheme heterojunction are analyzed using Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The characteristic Raman peak and XPS binding energy for CC triple bonds of pDET/Cu foil and pDET/Cu 2 O Z-scheme  Figures S9-S11, Supporting Information), respectively. [27][28][29]42,43] However, compared with the Raman spectrum of pDET/Cu foil, the decreased intensity at 2185 cm −1 and the increased intensity at 1910 cm −1 of pDET/Cu 2 O Z-scheme heterojunction arise from the strong interaction between Cu substrate and CC triple bonds in the pDET/Cu 2 O Z-scheme heterojunction. [44,45] Meanwhile, to clarify the electron transfer direction between pDET and Cu 2 O experimentally and whether it complies with the Z-scheme or type-II heterojunction pathway, high-resolution in situ XPS characterizations for Cu2p of pDET/Cu CP were conducted. The positive shift binding energy (≈0.1 eV) of Cu2p under UV-light irradiation was due to the electron density reduction of Cu 2 O. [46][47][48] These results showed the photogenerated electrons migration from Cu 2 O to pDET, evidencing its Z-scheme pathway ( Figure S12, Supporting Information).
In order to investigate the photoinduced electron-hole separation behavior in pDET/Cu 2 O Z-scheme heterojunction, the carrier separation efficiency of photoelectrodes was evaluated according to the following equation: [28,29,49] j ph = j abs × η sep × η cat (j ph : total photocurrent density; j abs : photon absorption rate expressed as current density; η sep : carrier separation efficiency; η cat : catalytic efficiency for water reduction.) At 0.3 V versus RHE, the carrier-separation efficiency of pDET/Cu 2 O Z-scheme heterojunction was about 16.1% (Figure 3a), which was 6.7 and 1.4 times higher than 2.4% for pDET on Cu foil (pDET/Cu foil) and 11.6% for Cu 2 O on the Cu foil (Cu 2 O/Cu foil, Figures S13,S14, Supporting Information), respectively. These results evidence that the Z-scheme heterojunction of pDET and Cu 2 O significantly improves its separation efficiency of photoinduced electrons and holes, which stand in contrast with pure pDET and Cu 2 O.
The long-term PEC HER stability of pDET/Cu 2 O Z-scheme heterojunction was further evaluated at 0 V versus RHE ( Figures S19-S23, Supporting Information). The gaseous product generated from the pDET/Cu 2 O Z-scheme heterojunction was synchronously analyzed using a gas chromatograph (GC). Experimentally detected H 2 amount reached ≈25 µmol with a Faradic efficiency of ≈90% after 2 h PEC reaction. Under the solar light irradiation, the photocurrent (≈600 µA cm −2 ) of pDET/Cu 2 O Z-scheme heterojunction showed no decrease ( Figure S19, Supporting Information). After the PEC HER stability test, the morphology and chemical composition of pDET/Cu 2 O Z-scheme heterojunction were examined. Obviously, scanning electron microscopy (SEM) images revealed that the sheet-like morphology of pDET was well maintained ( Figure S20, Supporting Information). The Raman spectra and XRD pattern further reveled that no changes of chemical composition and bonding information of pDET/Cu 2 O Z-scheme heterojunction occurred during the PEC HER process (Figures S21,S22, Supporting Information), confirming the excellent electrochemical stability of pDET/Cu 2 O Z-scheme heterojunction photocathode against the photocorrosion. [50][51][52] To unveil the generality of the synthesis approach to construct the CAPs/Cu 2 O Z-scheme heterojunction, three other  were also synthesized on the Cu CP using the Cu-mediated Glaser coupling method ( Figures S24-S26, Supporting Information). The similar nanosheet-like morphologies of pDEB, pDEN, and pDTT were generated on both Cu foil and Cu CP ( Figures S27-S32, Supporting Information). XRD and Raman measurements demonstrated the successful conversion of nanocopper to Cu 2 O and the strong interaction between CC triple bonds and Cu substrate in Cu CP for all samples ( Figures S33-S38, Supporting Information). We found that the PEC performance of these CAPs/Cu 2 O heterojunctions depended significantly on the band structure of CAPs, especially for the energy difference between E v of CAP and E c of Cu 2 O (ΔE). As shown in the Figure 4 and Figures S39-S41, Supporting Information, the larger ΔE results in a larger electrostatic attraction between the photoinduced electrons from Cu 2 O and photoinduced holes from CAPs. As presented above, for the pDET/Cu 2 O Z-scheme heterojunction, the ΔE is ≈2.0 eV and its photocurrent at 0.3 V versus RHE can reach 520 µA cm −2 with an ≈7.5-fold enhancement in comparison with pure pDET. Upon decreasing ΔE down to ≈1.9 eV for pDTT and ≈1.4 eV for pDEB, their photocurrents at 0.3 V versus RHE are 270 and 110 µA cm −2 with an ≈4.5and ≈2.9-fold enhancement with respect to the pristine polymers, respectively. However, for pDEN/Cu 2 O heterostructure, the ΔE further decreases to ≈0.4 eV; and no obvious promotion of photocurrent compared with pure pDEN was observed. Thereby, our results highlight the generality of one-step integration of the Z-scheme CAPs/Cu 2 O heterojunctions on Cu CP.
In summary, we have presented a novel strategy for the construction of organic/inorganic Z-scheme heterojunction based on the in situ conversion of nanocopper into Cu 2 O during the growth of CAPs. The PEC HER performance of the achieved CAPs/Cu 2 O heterostructure depends on the energy difference between E v of CAPs and E c of Cu 2 O. The photoinduced electrons and holes in pDET/Cu 2 O Z-scheme heterojunction are more efficiently separated in comparison with pure pDET and Cu 2 O. As a result, the pDET/Cu 2 O Z-scheme heterojunction photocathode presents a benchmark photocurrent density (≈520 µA cm −2 ) that substantially exceeds the reported organic semiconductors and organic-semiconductor-based heterojunctions or homojunctions (1-370 µA cm −2 ). Therefore, our design strategy of CAPs/Cu 2 O Z-scheme heterojunctions sheds light on exploring high-performance organic photoelectrodes, which holds promising applications in the fields of solar-to-fuel conversions such as water splitting, CO 2 reduction, N 2 reduction, and artificial photosynthesis.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.